Patent Publication Number: US-2021194215-A1

Title: Laser system and electronic device manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Application No. PCT/JP2018/039341, filed on Oct. 23, 2018, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a laser system and an electronic device manufacturing method. 
     2. Related Art 
     Improvement in resolution of semiconductor exposure apparatuses (hereinafter simply referred to as “exposure apparatuses”) has been desired due to miniaturization and high integration of semiconductor integrated circuits. For this purpose, exposure light sources that output light with shorter wavelengths have been developed. As the exposure light source, a gas laser apparatus is used in place of a conventional mercury lamp. As a gas laser apparatus for exposure, a KrF excimer laser apparatus that outputs ultraviolet light having a wavelength of 248 nm and an ArF excimer laser apparatus that outputs ultraviolet light having a wavelength of 193 nm are currently used. 
     As current exposure technology, immersion exposure is practically used in which a gap between a projection lens of an exposure apparatus and a wafer is filled with a liquid and a refractive index of the gap is changed to reduce an apparent wavelength of light from an exposure light source. When the immersion exposure is performed using the ArF excimer laser apparatus as the exposure light source, the wafer is irradiated with ultraviolet light having a wavelength of 134 nm in the water. This technology is referred to as ArF immersion exposure (or ArF immersion lithography). 
     The KrF excimer laser apparatus and the ArF excimer laser apparatus have a large spectral line width of about 350 to 400 pm in spontaneous oscillation. Thus, chromatic aberration of a laser beam (ultraviolet light), which is reduced and projected on a wafer by a projection lens of an exposure apparatus, occurs to reduce resolution. Then, a spectral line width (also referred to as a spectral width) of a laser beam output from the gas laser apparatus needs to be narrowed to the extent that the chromatic aberration can be ignored. For this purpose, a line narrowing module (LNM) having a line narrowing element is provided in a laser resonator of the gas laser apparatus to narrow the spectral width. The line narrowing element may be etalon, grating, or the like. A laser apparatus with such a narrowed spectral width is referred to as a line narrowing laser apparatus. 
     LIST OF DOCUMENTS 
     Patent Documents 
     Patent Document 1: US Published Patent Application No. 2013/0215916 
     Patent Document 2: US Published Patent Application No. 2004/0012844 
     Patent Document 3: U.S. Pat. No. 9,929,529 
     Patent Document 4: Japanese Unexamined Patent Application Publication No. 2011-187521 
     Patent Document 5: Japanese Unexamined Patent Application Publication No. 2011-249399 
     SUMMARY 
     A laser system according to one aspect of the present disclosure includes a first solid-state laser device configured to output a first pulse laser beam; a wavelength conversion system configured to wavelength-convert the first pulse laser beam output from the first solid-state laser device; an excimer amplifier configured to amplify a second pulse laser beam wavelength-converted by the wavelength conversion system; and a control unit configured to control at least a center wavelength or a spectral line width of an excimer laser beam output from the excimer amplifier, the first solid-state laser device including a first multiple semiconductor laser system, a first semiconductor optical amplifier configured to pulse-amplify a laser beam output from the first multiple semiconductor laser system, and a first fiber amplifier including a first optical fiber configured to amplify the pulse laser beam output from the first semiconductor optical amplifier, the first multiple semiconductor laser system including a plurality of first semiconductor lasers configured to perform continuous wave oscillation in a single longitudinal mode with different wavelengths, a first beam combiner configured to combine laser beams output from the first semiconductor lasers and output a laser beam having a first multiline spectrum including a plurality of peak wavelengths, and a first spectrum monitor configured to receive part of the continuous wave laser beams output from the first semiconductor lasers and measure a wavelength and light intensity of each of the laser beams output from the first semiconductor lasers, the control unit controlling an oscillation wavelength and light intensity of each line of the first multiline spectrum generated by the first semiconductor lasers to obtain an excimer laser beam having at least a target center wavelength or a target spectral line width instructed by an external device. 
     An electronic device manufacturing method according to another aspect of the present disclosure includes generating an excimer laser beam with a laser system, the laser system including a first solid-state laser device configured to output a first pulse laser beam, a wavelength conversion system configured to wavelength-convert the first pulse laser beam output from the first solid-state laser device, an excimer amplifier configured to amplify a second pulse laser beam wavelength-converted by the wavelength conversion system, and a control unit configured to control at least a center wavelength or a spectral line width of an excimer laser beam output from the excimer amplifier, the first solid-state laser device including a first multiple semiconductor laser system, a first semiconductor optical amplifier configured to pulse-amplify a laser beam output from the first multiple semiconductor laser system, and a first fiber amplifier including a first optical fiber configured to amplify the pulse laser beam output from the first semiconductor optical amplifier, the first multiple semiconductor laser system including a plurality of first semiconductor lasers configured to perform continuous wave oscillation in a single longitudinal mode with different wavelengths, a first beam combiner configured to combine laser beams output from the first semiconductor lasers and output a laser beam having a first multiline spectrum including a plurality of peak wavelengths, and a first spectrum monitor configured to receive part of the continuous wave laser beams output from the first semiconductor lasers and measure a wavelength and light intensity of each of the laser beams output from the first semiconductor lasers, the control unit controlling an oscillation wavelength and light intensity of each line of the first multiline spectrum generated by the first semiconductor lasers to obtain an excimer laser beam having at least a target center wavelength or a target spectral line width instructed by an external device; outputting the excimer laser beam to an exposure apparatus; and exposing the excimer laser beam onto a photosensitive substrate within the exposure apparatus to manufacture an electronic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the accompanying drawings, some embodiments of the present disclosure will be described below merely by way of example. 
         FIG. 1  shows a typical spectral shape of a laser beam. 
         FIG. 2  illustrates a definition of a spectral line width of an excimer laser beam. 
         FIG. 3  illustrates a definition of a spectral line width and a center wavelength of multiline. 
         FIG. 4  shows an example of a spectral shape of the multiline having the same light intensity. 
         FIG. 5  schematically shows an exemplary configuration of a laser system. 
         FIG. 6  is a flowchart of an example of processing of a laser control unit. 
         FIG. 7  is a flowchart of an example of an initial setting subroutine of the laser system. 
         FIG. 8  is a flowchart of an example of a control subroutine of a solid-state laser system. 
         FIG. 9  is a flowchart of an example of a control subroutine of the laser system. 
         FIG. 10  is a flowchart of an example of processing of a solid-state laser system control unit. 
         FIG. 11  is a flowchart of an example of an initial setting subroutine of the solid-state laser system. 
         FIG. 12  is a flowchart of an example of a control subroutine of a first semiconductor laser system. 
         FIG. 13  is a flowchart of an example of a control subroutine of a second semiconductor laser system. 
         FIG. 14  is a flowchart of an example of a subroutine for calculating a target center wavelength λ 2   ct  of the second semiconductor laser system. 
         FIG. 15  is a flowchart of an example of an energy control subroutine of the solid-state laser system. 
         FIG. 16  schematically shows an exemplary configuration of a semiconductor laser system. 
         FIG. 17  shows a laser spectrum output from a distributed feedback semiconductor laser. 
         FIG. 18  is a flowchart of an example of processing of a first semiconductor laser control unit. 
         FIG. 19  is a flowchart of an example of processing of a second semiconductor laser control unit. 
         FIG. 20  schematically shows a configuration of a laser system according to Embodiment 1. 
         FIG. 21  is a flowchart of an example of processing of a laser control unit. 
         FIG. 22  is a flowchart of an example of a control subroutine ( 2 ) of a solid-state laser system. 
         FIG. 23  is a flowchart of an example of processing for calculating a target spectral line width Δλ 1   mt  of a first multiple semiconductor system. 
         FIG. 24  is a graph showing an example of a function representing a relationship between a spectral line width Δλ of an excimer beam and a spectral line width Δλ 1   m  of the first multiple semiconductor laser system. 
         FIG. 25  is a block diagram of Control example 1 of the first multiple semiconductor laser system. 
         FIG. 26  shows an example of a multiline spectrum detected by a first spectrum monitor in Control example 1 in  FIG. 25 . 
         FIG. 27  shows an example of a multiline spectrum obtained when a center wavelength of the multiline is fixed and control to change a spectral line width of the multiline is performed in a spectral shape in  FIG. 26 . 
         FIG. 28  is a block diagram of a control example of a second multiple semiconductor laser system. 
         FIG. 29  shows an example of a multiline spectrum detected by a second spectrum monitor in Control example 1 in  FIG. 28 . 
         FIG. 30  shows an example of a multiline spectrum obtained when a spectral line width of the multiline is fixed and control to change a center wavelength of the multiline is performed in a spectral shape in  FIG. 29 . 
         FIG. 31  is a flowchart of an example of processing of a solid-state laser system control unit. 
         FIG. 32  is a flowchart of an example of an initial setting subroutine ( 2 ) of a solid-state laser system. 
         FIG. 33  is a flowchart of an example of a control subroutine of a first multiple semiconductor laser system. 
         FIG. 34  is a flowchart of an example of a control subroutine of a second multiple semiconductor laser system. 
         FIG. 35  is a flowchart of an example of processing for calculating a target center wavelength λ 2   mct  of the second multiple semiconductor laser system. 
         FIG. 36  is a flowchart of an example of processing of a first multiline control unit. 
         FIG. 37  is a flowchart of an example of processing for calculating a target oscillation wavelength of each semiconductor laser of the first multiple semiconductor laser system. 
         FIG. 38  is a flowchart of an example of a control subroutine of each semiconductor laser DFB 1 ( k ). 
         FIG. 39  is a flowchart of an example of processing for calculating and determining a spectral line width Δλ 1   m  and a center wavelength λ 1   mc  of the first multiple semiconductor laser system. 
         FIG. 40  is a flowchart of an example of processing of a second multiline control unit. 
         FIG. 41  is a flowchart of an example of processing for calculating a target wavelength of each semiconductor laser of the second multiple semiconductor laser system. 
         FIG. 42  is a flowchart of an example of a control subroutine of each semiconductor laser DFB 2 ( k ). 
         FIG. 43  is a flowchart of an example of processing for calculating and determining a spectral line width Δλ 2   m  and a center wavelength λ 2   mc  of the second multiple semiconductor laser system. 
         FIG. 44  is a block diagram of Control example 2 of the first multiple semiconductor laser system. 
         FIG. 45  shows an example of a multiline spectrum detected by a first spectrum monitor in Control example 2 in  FIG. 44 . 
         FIG. 46  shows an example of a multiline spectrum obtained when a spectral line width of the multiline is fixed and control to change a center wavelength of the multiline is performed in a spectral shape in  FIG. 45 . 
         FIG. 47  is a block diagram of Control example 2 of the second multiple semiconductor laser system. 
         FIG. 48  shows an example of a multiline spectrum detected by a second spectrum monitor in Control example 2 in  FIG. 47 . 
         FIG. 49  shows an example of a multiline spectrum obtained when a center wavelength of the multiline is fixed and control to change a spectral line width of the multiline is performed in a spectral shape in  FIG. 48 . 
         FIG. 50  is a block diagram of Control example 3 of the first multiple semiconductor laser system. 
         FIG. 51  shows an example of a multiline spectrum detected by the first spectrum monitor. 
         FIG. 52  is a block diagram of Control example 3 of the second multiple semiconductor laser system. 
         FIG. 53  shows an example of a multiline spectrum detected by the second spectrum monitor in Control example 3 in  FIG. 52 . 
         FIG. 54  shows an example of a multiline spectrum when control to change a center wavelength and a spectral line width of the multiline is performed in a spectral shape in  FIG. 53 . 
         FIG. 55  is a block diagram of Control example 4 of the first multiple semiconductor laser system. 
         FIG. 56  shows an example of a multiline spectrum detected by the first spectrum monitor in Control example 4 in  FIG. 55 . 
         FIG. 57  shows an example of a multiline spectrum when control to change a center wavelength and a spectral line width of the multiline is performed in a spectral shape in  FIG. 56 . 
         FIG. 58  is a block diagram of Control example 4 of the second multiple semiconductor laser system. 
         FIG. 59  shows an example of a multiline spectrum detected by the second spectrum monitor in Control example 4 in  FIG. 58 . 
         FIG. 60  is a block diagram of Variant 1 of the multiple semiconductor laser system. 
         FIG. 61  shows an example of a multiline spectrum detected by the first spectrum monitor in a control example of a configuration in  FIG. 60 . 
         FIG. 62  is a flowchart of an example of a control subroutine of the semiconductor laser DFB 1 ( k ). 
         FIG. 63  is a block diagram of Variant 2 of the multiple semiconductor laser system. 
         FIG. 64  shows an example of a multiline spectrum detected by the first spectrum monitor in Variant 2 in  FIG. 63 . 
         FIG. 65  is a flowchart of an example of a control subroutine of the semiconductor laser DFB 1 ( k ). 
         FIG. 66  schematically shows an exemplary configuration of a spectrum monitor. 
         FIG. 67  schematically shows another exemplary configuration of the spectrum monitor. 
         FIG. 68  illustrates detection of a beat signal using a heterodyne interferometer and calculation of a wavelength and light intensity. 
         FIG. 69  schematically shows an exemplary configuration of an excimer amplifier. 
         FIG. 70  schematically shows an exemplary configuration of an excimer amplifier using a ring resonator. 
         FIG. 71  schematically shows an exemplary configuration of a spectrum monitor using an etalon spectrometer. 
         FIG. 72  shows an example of a spectrum of a laser beam. 
         FIG. 73  schematically shows an example of a beam combiner including an optical fiber. 
         FIG. 74  schematically shows an example of a beam combiner including a half mirror and a high reflective mirror. 
         FIG. 75  schematically shows an example of a multiple semiconductor laser system using a single longitudinal mode external cavity semiconductor laser. 
         FIG. 76  is a block diagram of an example of a CW oscillation reference laser beam source. 
         FIG. 77  is a block diagram of another example of the CW oscillation reference laser beam source. 
         FIG. 78  schematically shows an example of a multi-longitudinal mode CW oscillation semiconductor laser. 
         FIG. 79  shows an example of a spectrum of a laser beam output from the semiconductor laser in  FIG. 78 . 
         FIG. 80  shows an example of a waveform of a value of current passed through a DFB laser. 
         FIG. 81  is a graph showing a wavelength change of a laser beam output from the DFB laser by modulation current. 
         FIG. 82  schematically shows an exemplary configuration of a semiconductor optical amplifier. 
         FIG. 83  schematically shows an example of a laser system according to Embodiment 3. 
         FIG. 84  is a block diagram of a control example of a third multiple semiconductor laser system. 
         FIG. 85  shows an example of a multiline spectrum detected by a third spectrum monitor in the control example in  FIG. 84 . 
         FIG. 86  shows an example of a multiline spectrum when control to change a center wavelength and a spectral line width of the multiline is performed in a spectral shape in  FIG. 85 . 
         FIG. 87  schematically shows an exemplary configuration of an exposure apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Contents&gt; 
     1. Terms 
     1.1 Definition of spectral line width Δλ of excimer laser beam 
     1.2 Definition of spectral line width Δλm and center wavelength λmc of multiline 
     2. Overview of laser system 
     2.1 Configuration 
     2.2 Operation 
     2.3 Example of processing of laser control unit 
     2.4 Example of processing of solid-state laser system control unit 
     2.5 Example of semiconductor laser system 
     2.5.1 Configuration 
     2.5.2 Operation 
     2.6 Example of processing of first semiconductor laser control unit 
     2.7 Example of processing of second semiconductor laser control unit 
     3. Problem 
     4. Embodiment 1 
     4.1 Configuration 
     4.2 Operation 
     4.3 Example of processing of laser control unit 
     4.4 Control example 1 of first multiple semiconductor laser system 
     4.5 Control example 1 of second multiple semiconductor laser system 
     4.6 Example of processing of solid-state laser system control unit 
     4.7 Example of processing of first multiline control unit 
     4.8 Example of processing of second multiline control unit 
     4.9 Effect 
     4.10 Variant 
     4.10.1 Control example 2 of first multiple semiconductor laser system 
     4.10.2 Control example 2 of second multiple semiconductor laser system 
     5. Embodiment 2 
     5.1 Configuration 
     5.2 Operation 
     5.2.1 Control example 3 of first multiple semiconductor laser system 
     5.2.2 Control example 3 of second multiple semiconductor laser system 
     5.3 Effect 
     5.4 Variant 
     5.4.1 Control example 4 of first multiple semiconductor laser system 
     5.4.2 Control example 4 of second multiple semiconductor laser system 
     6. Variant 1 of multiple semiconductor laser system 
     6.1 Configuration 
     6.2 Operation 
     7. Variant 2 of multiple semiconductor laser system 
     7.1 Configuration 
     7.2 Operation 
     8. Specific example of spectrum monitor 
     8.1 Example of spectrum monitor using spectrometer and reference laser beam source 
     8.1.1 Configuration 
     8.1.2 Operation 
     8.2 Example of spectrum monitor using heterodyne interferometer 
     8.2.1 Configuration 
     8.2.2 Operation 
     8.2.3 Example of beat signal 
     8.2.4 Variant 
     9. Example of excimer amplifier 
     9.1 Multipath amplification 
     9.2 Amplification with ring resonator 
     10. Example of spectrum monitor using etalon spectrometer
 
11. Example of beam combiner
 
     11.1 Beam combiner including optical fiber 
     11.2 Beam combiner including half mirror and high reflective mirror 
     12. Another example of single longitudinal mode semiconductor laser 
     12.1 Configuration 
     12.2 Operation 
     12.3 Others 
     13. Example of CW oscillation reference laser beam source 
     13.1 CW oscillation reference laser beam source of wavelength region of 1030 nm 
     13.2 CW oscillation reference laser beam source of wavelength region of 1554 nm 
     14. Example of multi-longitudinal mode CW oscillation semiconductor laser
 
15. SBS suppression by chirping
 
16. Example of semiconductor optical amplifier
 
     16.1 Configuration 
     16.2 Operation 
     17. Embodiment 3 
     17.1 Configuration 
     17.2 Operation 
     17.3 Control example of third multiple semiconductor laser system 
     17.4 Effect 
     17.5 Variant 
     18. Electronic device manufacturing method 
     19. Others 
     Now, with reference to the drawings, embodiments of the present disclosure will be described in detail. The embodiments described below illustrate some examples of the present disclosure, and do not limit contents of the present disclosure. Also, all configurations and operations described in the embodiments are not necessarily essential as configurations and operations of the present disclosure. The same components are denoted by the same reference numerals, and overlapping descriptions are omitted. 
     1. Terms 
     1.1 Definition of Spectral Line Width Δλ of Excimer Laser Beam 
     A line width of 95% of total spectral width area of an excimer laser beam is herein defined as a spectral line width Δλ of the excimer laser beam. Generally, as shown in  FIG. 1 , a spectral line width refers to a full width of a spectral waveform of a laser beam at a light amount threshold. For example, a half of a peak value is referred to as a line width threshold of 0.5. A full width W 1 / 2  of the spectral waveform at the line width threshold of 0.5 is particularly referred to as a full width at half maximum (FWHM). Only with the full width at half maximum of the spectrum of the excimer laser beam, however, reflection of resolving power of a projection lens is difficult. 
     Then, for example, as shown in  FIG. 2 , a spectral line width Δλ that reflects resolving power of a projection lens is a full width W 95 % that is 95% of total spectrum energy with a wavelength λ 0  at a center, and Expression (1) below is satisfied. 
     
       
         
           
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     The spectral line width Δλ of the excimer laser beam may be herein a spectral line width that reflects resolving power of a projection lens, not limited to the line width of 95% of the full spectral width area. For example, a spectral line width may be evaluated with resolving power calculated by inputting a wavelength and light intensity distribution in accordance with data of a projection lens used and a spectral waveform of the excimer laser beam. The excimer laser beam is herein sometimes referred to as “excimer beam”. 
     1.2 Definition of Spectral Line Width Δλm and Center Wavelength λmc of Multiline 
     The term “multiline” herein refers to a spectrum representing distribution of light intensity of each wavelength and including a plurality of peak wavelengths as illustrated in  FIGS. 3 and 4 , and is synonymous with “multiline spectrum”. The term “multiline” sometimes refers to a laser beam having a multiline spectrum. The multiline can be obtained, for example, by combining a plurality of laser beams output from a plurality of single longitudinal mode semiconductor lasers having different wavelengths. In this case, a wavelength (peak wavelength) of each line of the multiline corresponds to an oscillation wavelength of each semiconductor laser. 
     A spectral line width Δλm and a center wavelength λmc of multiline are defined as below. As shown in  FIG. 3 , a spectral line width Δλm of n-line multiline is defined as a difference between a maximum wavelength λ(n) and a minimum wavelength λ( 1 ). The number n is an integer equal to or larger than 2, and  FIG. 3  shows an example of n being 5. 
       Δλ m =λmax−λmin=λ( n )−λ(1)  (2)
 
     The center wavelength λmc of the multiline is defined as a wavelength of a spectrum centroid as in Expression (3) below. 
       [Expression 3] 
       λ mc=Σ   k=1   n   I ( k )·λ( k )/Σ k=1   n   I ( k )  (3)
 
     As shown in  FIG. 4 , when the n-line multiline have the same light intensity and adjacent lines of the n-line multiline have the same wavelength interval Δλp therebetween, the center wavelength λmc is an average value of wavelengths of the n-line multiline from Expression 4. 
       [Expression 4] 
       λ mc=Σ   k=1   n λ( k )/ n   (4)
 
     2. Overview of Laser System 
     2.1 Configuration 
       FIG. 5  schematically shows an exemplary configuration of a laser system  1 . The laser system  1  includes a solid-state laser system  10 , a first high reflective mirror  11 , a second high reflective mirror  12 , an excimer amplifier  14 , a monitor module  16 , a synchronization control unit  17 , and a laser control unit  18 . 
     The solid-state laser system  10  includes a first solid-state laser device  100 , a second solid-state laser device  200 , a wavelength conversion system  300 , a first pulse energy monitor  330 , a synchronization circuit unit  340 , and a solid-state laser system control unit  350 . 
     The first solid-state laser device  100  includes a first semiconductor laser system  110  that outputs a laser beam having a wavelength of about 1030 nm, a first semiconductor optical amplifier  120 , a first dichroic mirror  130 , a first pulse excitation light source  132 , a first fiber amplifier  140 , a second dichroic mirror  142 , a second pulse excitation light source  144 , and a solid-state amplifier  150 . 
     The first semiconductor laser system  110  includes a first semiconductor laser  111  that performs continuous wave (CW) oscillation in a single longitudinal mode to output a laser beam having a wavelength of about 1030 nm, a first wavelength monitor  112 , a first semiconductor laser control unit  114 , and a first beam splitter  116 . The term “CW” refers to continuous wave, and the term “CW oscillation” refers to continuous wave oscillation. 
     The first semiconductor laser  111  may be, for example, a distributed feedback (DFB) semiconductor laser, and can change an oscillation wavelength near the wavelength of 1030 nm by current control and/or temperature control. The distributed feedback semiconductor laser is referred to as “DFB laser”. 
     The first beam splitter  116  is arranged to reflect part of the laser beam output from the first semiconductor laser  111  and cause the laser beam to enter the first wavelength monitor  112 . The first wavelength monitor  112  monitors a spectrum of the incident laser beam and detects the oscillation wavelength of the first semiconductor laser  111 . 
     The first semiconductor laser control unit  114  is connected to the first wavelength monitor  112  and the solid-state laser system control unit  350 , and controls operation of the first semiconductor laser  111 . 
     The first semiconductor optical amplifier  120  is arranged in an optical path of the laser beam having passed through the first beam splitter  116 . The first semiconductor optical amplifier  120  pulse-amplifies the laser beam output from the first semiconductor laser system  110 . 
     The first dichroic mirror  130  is coated with a film that highly transmits the laser beam output from the first semiconductor optical amplifier  120  and highly reflects excitation light output from the first pulse excitation light source  132 . The first dichroic mirror  130  is arranged such that the pulse laser beam output from the first semiconductor optical amplifier  120  and the excitation light output from the first pulse excitation light source  132  enter the first fiber amplifier  140 . 
     The first fiber amplifier  140  may be a Yb fiber amplifier that uses an optical fiber doped with ytterbium (Yb). The Yb-doped optical fiber is an example of “first optical fiber” in the present disclosure. The second dichroic mirror  142  is coated with a film that highly transmits the laser beam output from the first fiber amplifier  140  and highly reflects excitation light output from the second pulse excitation light source  144 . The second dichroic mirror  142  is arranged such that the pulse laser beam output from the first fiber amplifier  140  and the excitation light output from the second pulse excitation light source  144  enter the solid-state amplifier  150 . 
     The solid-state amplifier  150  may include, for example, a Yb-doped crystal or ceramics. The pulse laser beam amplified by the solid-state amplifier  150  enters the wavelength conversion system  300 . The pulse laser beam output from the first solid-state laser device  100  may be the pulse laser beam amplified by the solid-state amplifier  150 . The pulse laser beam output from the first solid-state laser device  100  is referred to as a first pulse laser beam LP 1 . The first pulse laser beam LP 1  wavelength-converted by the wavelength conversion system  300  and output from the wavelength conversion system  300  is referred to as a second pulse laser beam LP 2 . 
     The second solid-state laser device  200  includes a second semiconductor laser system  210  that outputs a laser beam having a wavelength of about 1554 nm, a second semiconductor optical amplifier  220 , a third dichroic mirror  230 , a third pulse excitation light source  232 , and a second fiber amplifier  240 . 
     The second semiconductor laser system  210  includes a second semiconductor laser  211  that performs CW oscillation in a single longitudinal mode to output a laser beam having a wavelength of about 1554 nm, a second wavelength monitor  212 , a second semiconductor laser control unit  214 , and a second beam splitter  216 . 
     The second semiconductor laser  211  may be, for example, a DFB laser, and can change an oscillation wavelength near the wavelength of 1554 nm by current control and/or temperature control. 
     The second beam splitter  216  is arranged to reflect part of the laser beam output from the second semiconductor laser  211  and cause the laser beam to enter the second wavelength monitor  212 . The second wavelength monitor  212  monitors a spectrum of the incident laser beam and detects the oscillation wavelength of the second semiconductor laser  211 . 
     The second semiconductor laser control unit  214  is connected to the second wavelength monitor  212  and the solid-state laser system control unit  350 , and controls operation of the second semiconductor laser  211 . 
     The second semiconductor optical amplifier  220  is arranged in an optical path of the laser beam having passed through the second beam splitter  216 . The second semiconductor optical amplifier  220  pulse-amplifies the laser beam output from the second semiconductor laser system  210 . 
     The third dichroic mirror  230  is coated with a film that highly transmits the pulse laser beam output from the second semiconductor optical amplifier  220  and highly reflects excitation light output from the third pulse excitation light source  232 . The third dichroic mirror  230  is arranged such that the pulse laser beam output from the second semiconductor optical amplifier  220  and the excitation light output from the third pulse excitation light source  232  enter the second fiber amplifier  240 . 
     The second fiber amplifier  240  may be an erbium (Er) fiber amplifier that uses an optical fiber doped with Er. The Er-doped optical fiber is an example of “second optical fiber” in the present disclosure. The pulse laser beam amplified by the second fiber amplifier  240  enters the wavelength conversion system  300 . The pulse laser beam output from the second solid-state laser device  200  may be the pulse laser beam amplified by the second fiber amplifier  240 . The pulse laser beam output from the second solid-state laser device  200  is referred to as a third pulse laser beam LP 3 . 
     The wavelength conversion system  300  includes an LBO (LiB 3 O 5 ) crystal  310  and a first CLBO (CsLiB 6 O 10 ) crystal  312  that are nonlinear crystals, a fourth dichroic mirror  314 , a second CLBO crystal  316 , a fifth dichroic mirror  318 , a third CLBO crystal  320 , a sixth dichroic mirror  322 , a third high reflective mirror  324 , a fourth high reflective mirror  326 , and a beam splitter  328 . 
     The LBO crystal  310  and the first CLBO crystal  312  are arranged in an optical path of the first pulse laser beam LP 1  having the wavelength of about 1030 nm such that the first pulse laser beam LP 1  is wavelength-converted into a fourth pulse laser beam LP 4  (wavelength of about 257.5 nm) that is fourth harmonic light. 
     The third high reflective mirror  324  is arranged to highly reflect a third pulse laser beam LP 3  (wavelength of about 1554 nm) output from the second solid-state laser device  200  and cause the third pulse laser beam LP 3  to enter the fourth dichroic mirror  314 . 
     The fourth dichroic mirror  314  is coated with a film that highly transmits the fourth pulse laser beam LP 4  and highly reflects the third pulse laser beam LP 3 . The fourth dichroic mirror  314  is arranged in an optical path between the first CLBO crystal  312  and the second CLBO crystal  316  such that the third pulse laser beam LP 3  and the fourth pulse laser beam LP 4  enter the second CLBO crystal  316  with their optical path axes being aligned. 
     The second CLBO crystal  316 , the fifth dichroic mirror  318 , the third CLBO crystal  320 , and the sixth dichroic mirror  322  are arranged in this order in the optical path of the pulse laser beam including the fourth pulse laser beam LP 4 . 
     The second CLBO crystal  316  generates a fifth pulse laser beam LP 5  (wavelength of about 220.9 nm) that is a sum frequency of the third pulse laser beam LP 3  and the fourth pulse laser beam LP 4 . The fifth dichroic mirror  318  is coated with a film that highly reflects the fourth pulse laser beam LP 4  (wavelength of about 257.5 nm) having passed through the second CLBO crystal  316  and highly transmits the third pulse laser beam LP 3  (wavelength of about 1554 nm) and the fifth pulse laser beam LP 5  (wavelength of about 220.9 nm). 
     The third CLBO crystal  320  generates a pulse laser beam (wavelength of about 193.4 nm) that is a sum frequency of the third pulse laser beam LP 3  and the fifth pulse laser beam LP 5 . The pulse laser beam having the wavelength of about 193.4 nm output from the third CLBO crystal  320  is the second pulse laser beam LP 2 . 
     The sixth dichroic mirror  322  is coated with a film that highly transmits the third pulse laser beam LP 3  (wavelength of about 1554 nm) and the fifth pulse laser beam LP 5  (wavelength of about 220.9 nm) having passed through the third CLBO crystal  320  and highly reflects the pulse laser beam having the wavelength of about 193.4 nm (second pulse laser beam LP 2 ). 
     The fourth high reflective mirror  326  is arranged such that the pulse laser beam having the wavelength of about 193.4 nm is output from the wavelength conversion system  300 . 
     The beam splitter  328  is arranged in an optical path of a reflected beam from the fourth high reflective mirror  326  such that the partially reflected laser beam enters the first pulse energy monitor  330 . 
     The solid-state laser system control unit  350  is connected to the first semiconductor laser control unit  114 , the second semiconductor laser control unit  214 , the synchronization circuit unit  340 , the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232 . The solid-state laser system control unit  350  includes an internal trigger generator  351 . 
     The synchronization circuit unit  340  has signal lines to receive delay data and a trigger signal Tr 1  from the solid-state laser system control unit  350  and input a trigger signal delayed by a predetermined time to each of the first semiconductor optical amplifier  120 , the second semiconductor optical amplifier  220 , the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232 . 
     The first pulse energy monitor  330  is a detector that detects pulse energy of ultraviolet light, and is, for example, a pulse energy sensor including a photodiode or a pyroelectric element. 
     The excimer amplifier  14  includes an amplifier control unit  400 , a charger  402 , a trigger corrector  404 , a pulse power module (PPM)  408  including a switch  406 , and a chamber  410 . 
     The chamber  410  contains, for example, ArF laser gas containing Ar gas, F 2  gas, and Ne gas. A pair of discharge electrodes  412 ,  413  are arranged in the chamber  410 . The discharge electrodes  412 ,  413  are connected to an output terminal of the PPM  408 . 
     Two windows  415 ,  416  that transmit a laser beam having a wavelength of around 193.4 nm is arranged in the chamber  410 . 
     The monitor module  16  includes a beam splitter  600  and a second pulse energy monitor  602 . The beam splitter  600  is arranged in an optical path of the pulse laser beam (excimer laser beam) output from the excimer amplifier  14  such that the pulse laser beam reflected by the beam splitter  600  enters the second pulse energy monitor  602 . 
     The second pulse energy monitor  602  is a detector that detects pulse energy of ultraviolet light, and is, for example, a pulse energy sensor including a photodiode or a pyroelectric element. Information detected by the second pulse energy monitor  602  is transmitted to the laser control unit  18 . 
     The laser control unit  18  is connected to the solid-state laser system control unit  350 , the synchronization control unit  17 , the amplifier control unit  400 , and an exposure control unit  22  of an exposure apparatus  20 . The laser control unit  18  includes an internal trigger generator  19 . 
     In the present disclosure, a control device that functions as the first semiconductor laser control unit  114 , the second semiconductor laser control unit  214 , the solid-state laser system control unit  350 , the amplifier control unit  400 , the synchronization control unit  17 , the laser control unit  18 , the exposure control unit  22 , and other control units may be realized by a combination of hardware and software of one or more computers. Software is synonymous with program. Programable controllers are included in the concept of computers. The computer may include a central processing unit (CPU) and a memory. The CPU included in the computer is an example of a processor. 
     Some or all of processing functions of the control device may be realized using an integrated circuit represented by a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). 
     Functions of a plurality of control devices may be realized by one control device. Further, in the present disclosure, the control devices may be connected to each other via a communication network such as a local area network or the Internet. In a distributed computing environment, program units may be stored in both local and remote memory storage devices. In  FIG. 5  and thereafter, for example, “pulse energy monitor  1 ” and “SOA #2” with numerical values represent a first pulse energy monitor and a second semiconductor optical amplifier (SOA), respectively. “SOA” is an abbreviation for “semiconductor optical amplifier”. 
     2.2 Operation 
     The laser control unit  18  of the laser system  1  in  FIG. 5  receives a light emission trigger signal Tr and data of target pulse energy Et and a target center wavelength λct from the exposure control unit  22  of the exposure apparatus  20 . The laser control unit  18  transmits and receives data to and from the exposure control unit  22  as required, and notifies the exposure control unit  22  of an exposure NG signal or an exposure OK signal. 
     The light emission trigger signal Tr is input to the synchronization control unit  17  via the laser control unit  18 . The synchronization control unit  17  outputs a first trigger signal Tr 1  and a second trigger signal Tr 2  at timing with delay times set such that the pulse laser beam output from the solid-state laser system  10  is synchronously discharged and amplified when passing through the excimer amplifier  14 . 
     The first trigger signal Tr 1  is input to the synchronization circuit unit  340  via the solid-state laser system control unit  350 . The second trigger signal Tr 2  is input to the trigger corrector  404  via the amplifier control unit  400 , and output from the trigger corrector  404  is input to the switch  406  of the PPM  408 . 
     The solid-state laser system control unit  350  receives data of the target center wavelength λct from the laser control unit  18 . The solid-state laser system control unit  350  transmits, to the first semiconductor laser control unit  114  and the second semiconductor laser control unit  214 , an instruction to cause CW oscillation of the first semiconductor laser  111  and the second semiconductor laser  211 . The solid-state laser system control unit  350  also transmits data of target center wavelengths λ 1   t , λ 2   t  to the first semiconductor laser control unit  114  and the second semiconductor laser control unit  214 . 
     The first semiconductor laser control unit  114  controls a current value A 1  and/or a temperature T 1  of the first semiconductor laser  111  such that a difference δλ 1  between a center wavelength λ 1   c  measured by the first wavelength monitor  112  and a target center wavelength λ 1   ct  is brought close to 0. 
     Similarly, the second semiconductor laser control unit  214  controls a current value A 2  and/or a temperature T 2  of the second semiconductor laser  211  such that a difference δλ 2  between a center wavelength λ 2   c  measured by the second wavelength monitor  212  and a target center wavelength λ 2   ct  is brought close to 0. 
     The first semiconductor laser control unit  114  and the second semiconductor laser control unit  214  determine whether or not the differences δλ 1 , δλ 2  from the target center wavelengths are within their allowable ranges. When the differences are within the allowable ranges, the first semiconductor laser control unit  114  and the second semiconductor laser control unit  214  notify the solid-state laser system control unit  350  of wavelength OK signals. 
     When receiving the wavelength OK signals from both the first semiconductor laser control unit  114  and the second semiconductor laser control unit  214 , the solid-state laser system control unit  350  causes the internal trigger generator  351  to generate a first trigger signal Tr 1  having a predetermined repetition frequency. The internal trigger generator  351  can generate the first trigger signal Tr 1  irrespective of the first trigger signal Tr 1  from the synchronization control unit  17 . The first trigger signal Tr 1  generated by the internal trigger generator  351  among the first trigger signals Tr 1  is hereinafter particularly referred to as “internal trigger signal Tr 1 ”. The first trigger signal Tr 1  is input to the synchronization circuit unit  340 . 
     The synchronization circuit unit  340  outputs a pulse excitation trigger signal to each of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232  with a predetermined delay time synchronously with the first trigger signal Tr 1 . Then, the synchronization circuit unit  340  outputs a signal indicating amplification timing with predetermined delay times to each of the first semiconductor optical amplifier  120  and the second semiconductor optical amplifier  220 . 
     Pulse excitation timing of each of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232  is set such that pulse seed light passing therethrough can be sufficiently amplified. 
     Trigger timing of each of the first semiconductor optical amplifier  120  and the second semiconductor optical amplifier  220  is set such that the first pulse laser beam LP 1  output from the first solid-state laser device  100  and the third pulse laser beam LP 3  output from the second solid-state laser device  200  enter the second CLBO crystal  316  at the same timing. 
     Here, a specific example will be described in which the target center wavelength λct of the laser system  1  is 193.4 nm, the target center wavelength λ 1   ct  of the first solid-state laser device  100  is 1030 nm, and the target center wavelength λ 2   ct  of the second solid-state laser device  200  is 1554 nm. 
     The first semiconductor laser system  110  in the first solid-state laser device  100  outputs a CW oscillation laser beam (hereinafter referred to as “first CW laser beam”) having a center wavelength of 1030 nm. 
     The first semiconductor optical amplifier  120  pulse-amplifies the first CW laser beam and outputs a pulse laser beam. The pulse laser beam output from the first semiconductor optical amplifier  120  is amplified by the first fiber amplifier  140  and the solid-state amplifier  150 . The first pulse laser beam LP 1  amplified by the first fiber amplifier  140  and the solid-state amplifier  150  enter the LBO crystal  310  in the wavelength conversion system  300 . 
     In the second solid-state laser device  200 , the second semiconductor laser system  210  outputs a CW oscillation laser beam (hereinafter referred to as “second CW laser beam”) having a center wavelength of 1554 nm. 
     The second semiconductor optical amplifier  220  pulse-amplifies the second CW laser beam and outputs a pulse laser beam. The pulse laser beam output from the second semiconductor optical amplifier  220  is amplified by the second fiber amplifier  240 . The third pulse laser beam LP 3  amplified by the second fiber amplifier  240  enters the third high reflective mirror  324  in the wavelength conversion system  300 . 
     The first pulse laser beam LP 1  (wavelength of 1030 nm) having entered the wavelength conversion system  300  is converted into fourth harmonic light by the LBO crystal  310  and the first CLBO crystal  312  to generate a fourth pulse laser beam LP 4  (wavelength of 257.5 nm). 
     The fourth pulse laser beam LP 4  enters the second CLBO crystal  316  via the fourth dichroic mirror  314 . 
     The third pulse laser beam LP 3  (wavelength of 1554 nm) output from the second solid-state laser device  200  enters the second CLBO crystal  316  via the third high reflective mirror  324  and the fourth dichroic mirror  314 . 
     With the fourth dichroic mirror  314 , the third pulse laser beam LP 3  and the fourth pulse laser beam LP 4  substantially simultaneously enter the second CLBO crystal  316 , and overlap each other on the second CLBO crystal  316 . As a result, the second CLBO crystal  316  generates a fifth pulse laser beam LP 5  having a center wavelength of 220.9 nm, which is a sum frequency of the third pulse laser beam LP 3  (wavelength of 1554 nm) and the fourth pulse laser beam LP 4  (wavelength of 257.5 nm). 
     The fifth dichroic mirror  318  highly reflects the fourth pulse laser beam LP 4  having the center wavelength of 257.5 nm, and highly transmits both the third pulse laser beam LP 3  having the wavelength of about 1554 nm and the fifth pulse laser beam LP 5  having the wavelength of about 220.9 nm. 
     The pulse laser beams having passed through the fifth dichroic mirror  318  enter the third CLBO crystal  320 . The third CLBO crystal  320  generates a second pulse laser beam LP 2  having a center wavelength of about 193.4 nm, which is a sum frequency of the fifth pulse laser beam LP 5  (wavelength of 220.9 nm) and the third pulse laser beam LP 3  (wavelength of 1554 nm). 
     The sixth dichroic mirror  322  highly transmits the fifth pulse laser beam LP 5  and the third pulse laser beam LP 3  output from the third CLBO crystal  320 . The sixth dichroic mirror  322  highly reflects the second pulse laser beam LP 2  (wavelength of 193.4 nm) output from the third CLBO crystal  320 , and the second pulse laser beam LP 2  is output from the wavelength conversion system  300  via the fourth high reflective mirror  326  and the beam splitter  328 . 
     The pulse laser beam reflected by the beam splitter  328  enters the first pulse energy monitor  330 . The first pulse energy monitor  330  measures pulse energy Es of the pulse laser beam reflected by the beam splitter  328 . Information obtained by the first pulse energy monitor  330  is transmitted to the solid-state laser system control unit  350 . 
     The solid-state laser system control unit  350  calculates a difference ΔEs between pulse energy Es after wavelength conversion by the wavelength conversion system  300  and target pulse energy Est. 
     The solid-state laser system control unit  350  controls output of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232  such that ΔEs is brought close to 0. 
     The solid-state laser system control unit  350  determines whether ΔEs is within an allowable range. When it is OK, the solid-state laser system control unit  350  stops output of an internal trigger signal Tr 1  and notifies the laser control unit  18  of a solid-state laser system control OK signal. 
     Then, the laser control unit  18  generates an internal trigger signal Tr having a predetermined repetition frequency. As a result, the second pulse laser beam LP 2  having the center wavelength of 193.4 nm output from the solid-state laser system  10  enters the excimer amplifier  14  via the first high reflective mirror  11  and the second high reflective mirror  12 . 
     The excimer amplifier  14  forms inverted distribution by discharge synchronously with entering of the second pulse laser beam LP 2  having the wavelength of 193.4 nm. The trigger corrector  404  adjusts timing of the switch  406  of the PPM  408  such that the second pulse laser beam LP 2  is efficiently amplified by the excimer amplifier  14 . Thus, the excimer amplifier  14  outputs an amplified pulse laser beam LP 6 . 
     The pulse laser beam LP 6  amplified by the excimer amplifier  14  enters the monitor module  16 . The beam splitter  600  causes part of the pulse laser beam to enter the second pulse energy monitor  602 , which measures pulse energy E of the pulse laser beam. 
     The laser control unit  18  obtains information on the pulse energy E from the second pulse energy monitor  602 . The laser control unit  18  calculates a difference ΔE between the pulse energy E measured by the second pulse energy monitor  602  and target pulse energy Et. 
     The laser control unit  18  controls charging voltage Vhv of the charger  402  via the amplifier control unit  400  such that ΔE is brought close to 0. 
     The laser control unit  18  determines whether ΔE is within an allowable range. When it is OK, the laser control unit  18  stops output of an internal trigger signal Tr and notifies the exposure control unit  22  of a laser system OK signal (exposure OK signal). When receiving the laser system OK signal, the exposure control unit  22  transmits a light emission trigger signal Tr to the laser control unit  18 . 
     As a result, the laser system  1  outputs a pulse laser beam within the allowable ranges of the target center wavelength λt of 193.4 nm and the target pulse energy Et. The pulse laser beam (excimer beam) output from the laser system  1  enters the exposure apparatus  20 , which performs an exposure process. 
     When receiving data of a new target center wavelength λt from the exposure control unit  22 , the laser control unit  18  transmits the data to the solid-state laser system control unit  350 . 
     The solid-state laser system control unit  350  controls the first semiconductor laser system  110  and the second semiconductor laser system  210  such that the internal trigger generator  351  generates the internal trigger signal Tr 1  and the new target center wavelength λt is reached even if the solid-state laser system control unit  350  does not receive a trigger signal Tr 1  from the synchronization control unit  17 . 
     2.3 Example of Processing of Laser Control Unit 
       FIG. 6  is a flowchart of an example of processing of the laser control unit  18 . The processing and the operation in the flowchart in  FIG. 6  are realized by, for example, a processor that functions as the laser control unit  18  executing a program. 
     In step S 11 , the laser control unit  18  performs an initial setting subroutine of the laser system. After step S 11 , the laser control unit  18  performs a control subroutine of the solid-state laser system  10  (step S 12 ) and a control subroutine of the laser system  1  (step S 13 ). The processes in steps S 12  and S 13  may be performed in parallel or concurrently. 
     The control of the solid-state laser system  10  in step S 12  is constantly performed. In particular, wavelength control of the first semiconductor laser system  110  and the second semiconductor laser system  210  is performed irrespective of whether or not the trigger signal Tr 1  is input. The control of the laser system  1  in step S 13  is mainly feedback control of pulse energy of an excimer laser beam amplified by the excimer amplifier  14 . 
     In step S 14 , the laser control unit  18  determines whether or not to stop control of the laser system  1 . When the determination result in step S 14  is No, the laser control unit  18  returns to step S 12  and step S 13 . When the determination result in step S 14  is Yes, the laser control unit  18  goes to step S 15 . 
     In step S 15 , the laser control unit  18  notifies the exposure control unit  22  of stop of the laser system  1 , and finishes the flowchart in  FIG. 6 . 
       FIG. 7  is a flowchart of an example of the initial setting subroutine of the laser system  1 . The flowchart in  FIG. 7  is applied to step S 11  in  FIG. 6 . 
     In step S 21  in  FIG. 7 , the laser control unit  18  transmits a pulse energy NG signal of an excimer beam to the exposure control unit  22 . Pulse energy of the excimer beam is previously set to NG in initial setting, and in the process in step S 21 , the laser control unit  18  transmits the pulse energy NG signal to the exposure control unit  22  according to the initial setting. 
     In step S 22 , the laser control unit  18  transmits a spectrum NG signal to the exposure control unit  22 . A center wavelength of the excimer beam is previously set to NG in initial setting, and in the process in step S 22 , the laser control unit  18  transmits the spectrum NG signal to the exposure control unit  22  according to the initial setting. 
     In step S 23 , the laser control unit  18  sets charging voltage Vhv of the excimer amplifier  14  to an initial value Vhv 0 . 
     In step S 24 , the laser control unit  18  sets target pulse energy Et of the laser system  1  to an initial value Et 0 . The laser control unit  18  sets a predetermined standard initial value Et 0  before receiving data of the target pulse energy Et from the exposure apparatus  20 . 
     In step S 25 , the laser control unit  18  sets delay times of the first trigger signal Tr 1  and the second trigger signal Tr 2  from the light emission trigger signal Tr. The laser control unit  18  sets the delay times such that the pulse laser beam output from the solid-state laser system  10  discharges at timing when entering the excimer amplifier  14 . The delay times may be fixed values. The laser control unit  18  transmits data of the delay times to the synchronization control unit  17 . 
       FIG. 8  is a flowchart of an example of the control subroutine of the solid-state laser system  10 . The flowchart in  FIG. 8  is applied to step S 12  in  FIG. 6 . 
     In step S 31  in  FIG. 8 , the laser control unit  18  determines whether or not data of a new target center wavelength has been received from the exposure control unit  22 . When the determination result in step S 31  is Yes, the laser control unit  18  goes to step S 32 . 
     In step S 32 , the laser control unit  18  reads the data of the target center wavelength λct. Then, in step S 33 , the laser control unit  18  transmits the data of the target center wavelength λct to the solid-state laser system control unit  350 . 
     After step S 33 , the laser control unit  18  goes to step S 40 . When the determination result in step S 31  is No, the laser control unit  18  skips step S 32  and step S 33  and goes to step S 40 . 
     In step S 40 , the laser control unit  18  checks values of a flag F 1  and a flag F 2 , and determines whether or not the flag F 1  being 1 and the flag F 2  being 1 is satisfied. The flag F 1  indicates whether the first semiconductor laser system  110  is in an OK state or an NG state. The flag F 2  indicates whether the second semiconductor laser system  210  is in an OK state or an NG state. The value “1” of the flags indicates OK and the value “0” indicates NG. In other words, the laser control unit  18  determines whether or not both the first semiconductor laser system  110  and the second semiconductor laser system  210  are in the OK state. 
     When the determination result in step S 40  is Yes, the laser control unit  18  goes to step S 41 . In step S 41 , the laser control unit  18  transmits a spectrum OK signal to the exposure control unit  22 . 
     In step S 42 , the laser control unit  18  determines whether or not an energy OK signal has been received from the solid-state laser system  10 . For example, the laser control unit  18  checks a value of a flag Fs, and determines whether or not the flag Fs is 1. The flag Fs indicates whether the pulse energy output from the solid-state laser system  10  is in an OK state or an NG state. The value “1” of the flag Fs indicates OK and the value “0” indicates NG. The laser control unit  18  determines whether or not the pulse energy from the solid-state laser system  10  is in the OK state in accordance with the value of the flag Fs. When the determination result in step S 42  is Yes, the laser control unit  18  goes to step S 43 . 
     In step S 43 , the laser control unit  18  transmits an energy OK signal of the solid-state laser system  10  to the exposure control unit  22 . When the determination result in step S 42  is No, the laser control unit  18  goes to step S 44 . 
     In step S 44 , the laser control unit  18  transmits an energy NG signal of the solid-state laser system  10  to the exposure control unit  22 . 
     When the determination result in step S 40  is No, the laser control unit  18  goes to step S 45  and transmits a spectrum NG signal to the exposure control unit  22 . 
     After step S 43 , step S 44 , or step S 45 , the laser control unit  18  finishes the flowchart in  FIG. 8  and returns to the flowchart in  FIG. 6 . 
       FIG. 9  is a flowchart of an example of the control subroutine of the laser system  1 . The flowchart in  FIG. 9  is applied to step S 13  in  FIG. 6 . 
     In step S 51  in  FIG. 9 , the laser control unit  18  determines whether or not data of new target pulse energy has been received from the exposure control unit  22 . When the determination result in step S 51  is Yes, the laser control unit  18  goes to step S 52 . 
     In step S 52 , the laser control unit  18  reads the data of the target pulse energy Et. After step S 52 , the laser control unit  18  goes to step S 53 . When the determination result in step S 51  is No, the laser control unit  18  skips step S 52  and goes to step S 53 . 
     In step S 53 , the laser control unit  18  determines whether or not a light emission pulse of an excimer beam has been detected. The laser control unit  18  determines whether or not pulse energy of the pulse laser beam (excimer beam) output to the exposure apparatus  20  has been detected in accordance with a signal obtained from the monitor module  16 . When the determination result in step S 53  is Yes, the laser control unit  18  goes to step S 54 . 
     In step S 54 , the laser control unit  18  obtains data of the pulse energy E of the excimer beam detected by the monitor module  16 . 
     In step S 55 , the laser control unit  18  calculates a difference ΔE between the pulse energy E and the target pulse energy Et. 
     In step S 56 , the laser control unit  18  controls the charging voltage Vhv of the excimer amplifier  14  such that ΔE is brought close to 0. 
     Then, in step S 57 , the laser control unit  18  determines whether or not an absolute value of ΔE is equal to or smaller than an allowable upper limit value Etr which indicates an allowable range. When the determination result in step S 57  is Yes, the laser control unit  18  goes to step S 58 , and transmits a pulse energy OK signal of the excimer beam to the exposure control unit  22 . 
     When the determination result in step S 57  is No, the laser control unit  18  goes to step S 59 , and transmits a pulse energy NG signal of the excimer beam to the exposure control unit  22 . 
     After step S 58  or S 59 , the laser control unit  18  finishes the flowchart in  FIG. 9  and returns to the flowchart in  FIG. 6 . 
     When the determination result in step S 53  in  FIG. 9  is No, the laser control unit  18  skips steps S 54  to S 59 , finishes the flowchart in  FIG. 9 , and returns to the flowchart in  FIG. 6 . 
     2.4 Example of Processing of Solid-State Laser System Control Unit 
       FIG. 10  is a flowchart of an example of processing of the solid-state laser system control unit  350 . The processing and the operation in the flowchart in  FIG. 10  are realized by, for example, a processor that functions as the solid-state laser system control unit  350  executing a program. 
     In step S 61 , the solid-state laser system control unit  350  performs an initial setting subroutine of the solid-state laser system  10 . 
     After step S 61 , the solid-state laser system control unit  350  performs a control subroutine of the first semiconductor laser system  110  (step S 62 ), a control subroutine of the second semiconductor laser system  210  (step S 63 ), and an energy control subroutine of the solid-state laser system  10  (step S 64 ). The processes of the subroutines in steps S 62 , S 63 , and S 64  may be performed in parallel or concurrently. 
     In step S 65 , the solid-state laser system control unit  350  determines whether or not to stop control of the solid-state laser system  10 . 
     When the determination result in step S 65  is No, the solid-state laser system control unit  350  returns to steps S 62 , S 63 , and S 64 . When the determination result in step S 65  is Yes, the solid-state laser system control unit  350  goes to step S 66 . 
     In step S 66 , the solid-state laser system control unit  350  notifies the laser control unit  18  of stop of the solid-state laser system  10 , and finishes the flowchart in  FIG. 10 . 
       FIG. 11  is a flowchart of an example of the initial setting subroutine of the solid-state laser system  10 . The flowchart in  FIG. 11  is applied to step S 61  in  FIG. 10 . 
     In step S 71  in  FIG. 11 , the solid-state laser system control unit  350  sets the state of the first semiconductor laser system  110  to NG. In other words, the solid-state laser system control unit  350  sets the value of the flag F 1  to “0”. 
     In step S 72 , the solid-state laser system control unit  350  sets the state of the second semiconductor laser system  210  to NG. In other words, the solid-state laser system control unit  350  sets the value of the flag F 2  to “0”. 
     In step S 73 , the solid-state laser system control unit  350  sets the energy state of the solid-state laser system  10  to NG. In other words, the solid-state laser system control unit  350  sets the value of the flag Fs to “0”. 
     In step S 74 , the solid-state laser system control unit  350  sets a target center wavelength λ 1   ct  of the first semiconductor laser system  110  to an initial value λ 1   c   0 . The initial value λ 1   c   0  may be, for example, 1030 nm. 
     In step S 75 , the solid-state laser system control unit  350  sets a target center wavelength λ 2   ct  of the second semiconductor laser system  210  to an initial value λ 2   c   0 . The initial value λ 2   c   0  may be, for example, 1554 nm. 
     In step S 76 , the solid-state laser system control unit  350  sets an initial value of pulse energy from each of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232 . The initial values of the pulse energy from the pulse excitation light sources may be different. 
     In step S 77 , the solid-state laser system control unit  350  sets target pulse energy Est of the solid-state laser system  10  to an initial value Es 0 . The initial value Es 0  is a predetermined fixed value at which amplified spontaneous emission (ASE) can be suppressed in the excimer amplifier  14 . 
     In step S 78 , the solid-state laser system control unit  350  sets a delay time of each trigger signal in the synchronization circuit unit  340 . The delay time of the first trigger signal Tr 1  is set in the synchronization circuit unit  340  as described below. 
     Pulse excitation timing of each of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232  is set such that pulse seed light passing therethrough can be sufficiently amplified. Trigger timing of each of the first semiconductor optical amplifier  120  and the second semiconductor optical amplifier  220  is set such that the first pulse laser beam output from the first solid-state laser device  100  and the second pulse laser beam output from the second solid-state laser device  200  enter the second CLBO crystal  316  at the same timing. 
     In step S 79 , the solid-state laser system control unit  350  sets current values and temperatures of the first semiconductor laser  111  and the second semiconductor laser  211  to initial values to cause CW oscillation. In other words, the solid-state laser system control unit  350  controls the first semiconductor laser  111  such that current values and temperatures at which an oscillation wavelength of the first semiconductor laser  111  is brought close to λ 1   c   0  are initial values, and causes CW oscillation of the first semiconductor laser  111 . Similarly, the solid-state laser system control unit  350  controls the second semiconductor laser  211  such that current values and temperatures at which an oscillation wavelength of the second semiconductor laser  211  is brought close to λ 2   c   0  are initial values, and causes CW oscillation of the second semiconductor laser  211 . 
     After step S 79 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 11  and returns to the flowchart in  FIG. 10 . 
       FIG. 12  is a flowchart of an example of the control subroutine of the first semiconductor laser system  110 . The flowchart in  FIG. 12  is applied to step S 62  in  FIG. 10 . 
     In step S 81  in  FIG. 12 , the solid-state laser system control unit  350  transmits data of the target center wavelength λ 1   ct  to the first semiconductor laser control unit  114 . 
     In step S 82 , the solid-state laser system control unit  350  determines whether or not an OK signal of the first semiconductor laser system  110  has been received from the first semiconductor laser control unit  114 . When the determination result in step S 82  is Yes, that is, when the flag F 1  is 1, the solid-state laser system control unit  350  goes to step S 83 . 
     In step S 83 , the solid-state laser system control unit  350  transmits the OK signal of the first semiconductor laser system to the laser control unit  18 . Specifically, the solid-state laser system control unit  350  transmits a flag signal of F 1  being 1 to the laser control unit  18 . 
     When the determination result in step S 82  is No, that is, when the flag F 1  is 0, the solid-state laser system control unit  350  goes to step S 84 . 
     In step S 84 , the solid-state laser system control unit  350  transmits an NG signal of the first semiconductor laser system to the laser control unit  18 . Specifically, the solid-state laser system control unit  350  transmits a flag signal of F 1  being 0 to the laser control unit  18 . 
     After step S 83  or S 84 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 12  and returns to the flowchart in  FIG. 10 . 
       FIG. 13  is a flowchart of an example of the control subroutine of the second semiconductor laser system  210 . The flowchart in  FIG. 13  is applied to step S 63  in  FIG. 10 . 
     In step S 91  in  FIG. 13 , the solid-state laser system control unit  350  determines whether or not an instruction to change a target center wavelength has been received from the exposure control unit  22  via the laser control unit  18 . When the determination result in step S 91  is Yes, the solid-state laser system control unit  350  goes to step S 92 . 
     In step S 92 , the solid-state laser system control unit  350  transmits a wavelength NG signal to the laser control unit  18 . When the target center wavelength is changed, a wavelength NG state (F 2 =0) is reached because the wavelength needs to be adjusted. 
     In step S 93 , the solid-state laser system control unit  350  reads data of a new target center wavelength λct. 
     In step S 94 , the solid-state laser system control unit  350  calculates a target center wavelength λ 2   ct  of the second semiconductor laser system  210 . The processing in step S 94  will be described later with reference to  FIG. 14 . The solid-state laser system control unit  350  calculates the target center wavelength λ 2   ct  according to a wavelength conversion formula described later. 
     In step S 95  in  FIG. 13 , the solid-state laser system control unit  350  transmits data of the target center wavelength λ 2   ct  to the second semiconductor laser control unit  214 . After step S 95 , the solid-state laser system control unit  350  goes to step S 96 . 
     When the determination result in step S 91  is No, that is, when the instruction to change the target center wavelength has not been received from the exposure control unit  22 , the solid-state laser system control unit  350  skips steps S 92  to S 95  and goes to step S 96 . 
     In step S 96 , the solid-state laser system control unit  350  determines whether or not an OK signal of the second semiconductor laser system  210  has been received from the second semiconductor laser control unit  214 . When the determination result in step S 96  is Yes, the solid-state laser system control unit  350  goes to step S 97 . 
     In step S 97 , the solid-state laser system control unit  350  transmits the OK signal of the second semiconductor laser system  210  to the laser control unit  18 . Specifically, the solid-state laser system control unit  350  transmits a flag signal of F 2  being 1 to the laser control unit  18 . 
     When the determination result in step S 96  is No, that is, when the flag F 2  is 0, the solid-state laser system control unit  350  goes to step S 98 . 
     In step S 98 , the solid-state laser system control unit  350  transmits an NG signal of the second semiconductor laser system  210  to the laser control unit  18 . Specifically, the solid-state laser system control unit  350  transmits a flag signal of F 2  being 0 to the laser control unit  18 . 
     After step S 97  or S 98 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 13  and returns to the flowchart in  FIG. 10 . 
       FIG. 14  is a flowchart of an example of a subroutine for calculating the target center wavelength λ 2   ct  of the second semiconductor laser system  210 . The flowchart in  FIG. 14  is applied to step S 94  in  FIG. 13 . 
     In step S 101  in  FIG. 14 , the solid-state laser system control unit  350  converts the target center wavelength λ 1   ct  of the first semiconductor laser system  110  into a frequency fit. 
     The conversion formula is expressed by 
         f 1 t=C/λ 1 ct    
     where C is light speed. 
     In step S 102 , the solid-state laser system control unit  350  converts the target center wavelength λct after wavelength conversion by the wavelength conversion system  300  into a frequency ft. 
     The conversion formula is expressed by 
     
       
      
       ft=C/λct.  
      
     
     In step S 103 , the solid-state laser system control unit  350  calculates a target frequency f 2   t  of the second semiconductor laser system  210  from Expression (5) of wavelength conversion mentioned below. 
     The symbol “·” in the expression represents a multiplication operator. 
         f= 4· f 1+2· f 2  (5)
 
     f: frequency of laser beam wavelength-converted by sum frequency 
     f 1 : frequency of laser beam of first solid-state laser device 
     f 2 : frequency of laser beam of second solid-state laser device 
     In the example in  FIG. 5 , the frequency f is the frequency of the laser beam having the wavelength of about 193.4 nm. The frequency f 1  is the frequency of the laser beam having the wavelength of about 1030 nm. The frequency f 2  is the frequency of the laser beam having the wavelength of about 1554 nm. Thus, Expression (5) is converted with f being ft, f 1  being f 1   t , and f 2  being f 2   t  to obtain Expression (6) below that is a conversion formula applied to step S 103 . 
         f 2 t =(½)· ft− 2· f 1 t   (6)
 
     In step S 104 , the solid-state laser system control unit  350  converts the target frequency f 2   t  into the target center wavelength λ 2   ct . The conversion formula is expressed by 
       λ2 ct=C/f 2 t.  
 
     The calculation is not limited to the procedure described in steps S 101  to S 104  in  FIG. 14 , but may be performed using table data or the like that provides similar conversion results. 
     After step S 104 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 14  and returns to the flowchart in  FIG. 13 . 
       FIG. 15  is a flowchart of an example of the energy control subroutine of the solid-state laser system  10 . The flowchart in  FIG. 15  is applied to step S 64  in  FIG. 10 . 
     In step S 111  in  FIG. 15 , the solid-state laser system control unit  350  checks the values of the flag F 1  and the flag F 2 , and determines whether or not the flag F 1  being 1 and the flag F 2  being 1 is satisfied. In other words, the solid-state laser system control unit  350  determines whether or not OK signals have been received from both the first semiconductor laser system  110  and the second semiconductor laser system  210 . 
     When the determination result in step S 111  is No, the solid-state laser system control unit  350  repeats the process in step S 111 . When the determination result in step S 111  is Yes, the solid-state laser system control unit  350  goes to step S 112 . 
     In step S 112 , the solid-state laser system control unit  350  determines whether or not the first pulse energy monitor  330  has detected pulse energy of the pulse laser beam. The solid-state laser system control unit  350  makes a determination in accordance with a signal obtained from the first pulse energy monitor  330 . 
     When the determination result in step S 112  is No, the solid-state laser system control unit  350  repeats the process in step S 112 . When the determination result in step S 112  is Yes, the solid-state laser system control unit  350  goes to step S 113 . 
     In step S 113 , the solid-state laser system control unit  350  reads a value of the pulse energy Es detected by the first pulse energy monitor  330 . 
     In step S 114 , the solid-state laser system control unit  350  calculates a difference ΔEs between the pulse energy Es and the target pulse energy Est. 
     In step S 115 , the solid-state laser system control unit  350  controls pulse energy from each of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232  such that ΔEs is brought close to 0. 
     Then, in step S 116 , the solid-state laser system control unit  350  determines whether or not an absolute value of ΔEs is equal to or smaller than an allowable upper limit value ΔEstr which indicates an allowable range. When the determination result in step S 116  is Yes, the solid-state laser system control unit  350  goes to step S 117 . 
     In step S 117 , the solid-state laser system control unit  350  transmits, to the laser control unit  18 , a pulse energy OK signal of the solid-state laser system  10 , that is, a flag signal of Fs being 1. 
     When the determination result in step S 116  is No, the solid-state laser system control unit  350  goes to step S 118 , and transmits, to the laser control unit  18 , a pulse energy NG signal of the solid-state laser system  10 , that is, a flag signal of Fs being 0. 
     After step S 117  or S 118 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 15  and returns to the flowchart in  FIG. 10 . 
     2.5 Example of Semiconductor Laser System 
     2.5.1 Configuration 
       FIG. 16  schematically shows an exemplary configuration of a semiconductor laser system  30 . The semiconductor laser system  30  in  FIG. 16  can be applied to the first semiconductor laser system  110  and the second semiconductor laser system  210  in  FIG. 5 . 
     The semiconductor laser system  30  includes a single longitudinal mode DFB laser  31 , a wavelength monitor  32 , a semiconductor laser control unit  34 , and a beam splitter  36 . The DFB laser  31  includes a semiconductor element  40 , a Peltier element  50 , a temperature sensor  52 , a current control unit  54 , and a temperature control unit  56 . The semiconductor element  40  includes a first cladding layer  41 , an active layer  42 , a second cladding layer  43 , and a grating  44  at a boundary between the active layer  42  and the second cladding layer  43 . 
     2.5.2 Operation 
     An oscillation wavelength of the DFB laser  31  can be changed by changing a current value A and/or a set temperature T of the semiconductor element  40 . The current value A may be, for example, a direct current (DC) value. The current value A is changed to change the oscillation wavelength within a narrow range at high speed. The set temperature T is changed to significantly change the oscillation wavelength. 
       FIG. 17  shows an example of a spectral waveform of a laser beam output from the DFB laser  31 . As shown in  FIG. 17 , the laser beam output from the DFB laser  31  has a single-line spectral shape with a narrow spectral line width due to single longitudinal mode oscillation. 
     2.6 Example of Processing of First Semiconductor Laser Control Unit 
       FIG. 18  is a flowchart of an example of processing of the first semiconductor laser control unit  114 . The processing and the operation in the flowchart in  FIG. 18  are realized by, for example, a processor that functions as the first semiconductor laser control unit  114  executing a program. 
     In step S 121 , the first semiconductor laser control unit  114  sets a current value and a temperature of the first semiconductor laser  111  to initial values to cause CW oscillation. For example, the first semiconductor laser control unit  114  reads the current value and the temperature of the first semiconductor laser set to the initial values in step S 79  in  FIG. 11  to cause CW oscillation of the first semiconductor laser  111 . 
     In step S 122 , the first semiconductor laser control unit  114  reads data of a target center wavelength λ 1   ct.    
     In step S 123 , the first semiconductor laser control unit  114  uses the wavelength monitor  32  to measure an oscillation center wavelength λ 1   c.    
     In step S 124 , the first semiconductor laser control unit  114  calculates a difference δλ 1   c  between the oscillation center wavelength λ 1   c  and the target center wavelength λ 1   ct.    
     In step S 125 , the first semiconductor laser control unit  114  determines whether or not an absolute value of δλ 1   c  is equal to or smaller than an allowable upper limit value δλ 1   ctr  which indicates an allowable range. When the determination result in step S 125  is No, the first semiconductor laser control unit  114  goes to step S 126 , and transmits a flag signal of F 1  being 0 to the solid-state laser system control unit  350 . 
     Then, in step S 127 , the first semiconductor laser control unit  114  determines whether or not the absolute value of δλ 1   c  is an allowable upper limit value δ 1   catr  or lower, which is a range in which a wavelength can be controlled by current control. When the determination result in step S 127  is Yes, the first semiconductor laser control unit  114  goes to step S 129 , and controls the current value A 1  of the first semiconductor laser  111  such that δλ 1   c  is brought close to 0. 
     When the determination result in step S 127  is No, the first semiconductor laser control unit  114  goes to step S 130 , and controls the temperature T 1  of the first semiconductor laser  111  such that δλ 1   c  is brought close to 0. 
     When the determination result in step S 125  is Yes, the first semiconductor laser control unit  114  goes to step S 128 , and transmits a flag signal of F 1  being 1 to the solid-state laser system control unit  350 . After step S 128 , the first semiconductor laser control unit  114  goes to step S 129 . 
     After step S 129  or S 130 , the first semiconductor laser control unit  114  goes to step S 131 . In step S 131 , the first semiconductor laser control unit  114  determines whether or not to stop control of the first semiconductor laser system  110 . When the determination result in step S 131  is No, the first semiconductor laser control unit  114  returns to step S 123 , and repeats the processes in steps S 123  to S 131 . 
     When the determination result in step S 131  is Yes, the first semiconductor laser control unit  114  finishes the flowchart in  FIG. 18 . 
     2.7 Example of Processing of Second Semiconductor Laser Control Unit 
       FIG. 19  is a flowchart of an example of processing of the second semiconductor laser control unit  214 . The processing and the operation in the flowchart in  FIG. 19  are realized by, for example, a processor that functions as the second semiconductor laser control unit  214  executing a program. 
     In step S 151 , the second semiconductor laser control unit  214  sets a current value and a temperature of the second semiconductor laser  211  to initial values to cause CW oscillation. For example, the second semiconductor laser control unit  214  reads the current value and the temperature of the second semiconductor laser  211  set to the initial values in step S 79  in  FIG. 11  to cause CW oscillation of the second semiconductor laser  211 . 
     In step S 152 , the second semiconductor laser control unit  214  determines whether or not a target center wavelength of the second semiconductor laser system  210  has been changed by the solid-state laser system control unit  350 . When the determination result in step S 152  is Yes, the second semiconductor laser control unit  214  goes to step S 153 , and transmits, to the solid-state laser system control unit  350 , an NG signal indicating that the second semiconductor laser system  210  is in an NG state. Specifically, the second semiconductor laser control unit  214  transmits a flag signal of F 2  being 0 to the solid-state laser system control unit  350 . 
     In step S 154 , the second semiconductor laser control unit  214  reads data of a target center wavelength λ 2   ct . After step S 154 , the second semiconductor laser control unit  214  goes to step S 155 . 
     When the determination result in step S 152  is No, the second semiconductor laser control unit  214  skips steps S 153  and S 154  and goes to step S 155 . 
     In step S 155 , the second semiconductor laser control unit  214  uses the second wavelength monitor  212  to measure an oscillation center wavelength λ 2   c.    
     In step S 156 , the second semiconductor laser control unit  214  calculates a difference δλ 2   c  between the oscillation center wavelength λ 2   c  and the target center wavelength λ 2   ct.    
     In step S 157 , the second semiconductor laser control unit  214  determines whether or not an absolute value of δλ 2   c  is equal to or smaller than an allowable upper limit value δλ 2   ctr  which indicates an allowable range. When the determination result in step S 157  is No, the second semiconductor laser control unit  214  goes to step S 158 , and transmits a flag signal of F 2  being 0 to the solid-state laser system control unit  350 . 
     Then, in step S 159 , the second semiconductor laser control unit  214  determines whether or not the absolute value of δλ 2   c  is an allowable upper limit value δ 2   catr  or lower, which is a range in which a wavelength can be controlled by current control. When the determination result in step S 159  is Yes, the second semiconductor laser control unit  214  goes to step S 161 , and controls the current value A 2  of the second semiconductor laser  211  such that δλ 2   c  is brought close to 0. 
     When the determination result in step S 159  is No, the second semiconductor laser control unit  214  goes to step S 162 , and controls the temperature T 2  of the second semiconductor laser  211  such that a 2   c  is brought close to 0. 
     When the determination result in step S 157  is Yes, the second semiconductor laser control unit  214  goes to step S 160 , and transmits a flag signal of F 2  being 1 to the solid-state laser system control unit  350 . After step S 160 , the second semiconductor laser control unit  214  goes to step S 161 . 
     After step S 161  or S 162 , the second semiconductor laser control unit  214  goes to step S 163 . In step S 163 , the second semiconductor laser control unit  214  determines whether or not to stop control of the second semiconductor laser system  210 . When the determination result in step S 163  is No, the second semiconductor laser control unit  214  returns to step S 152 , and repeats the processes in steps S 152  to S 163 . 
     When the determination result in step S 163  is Yes, the second semiconductor laser control unit  214  finishes the flowchart in  FIG. 19 . 
     3. Problem 
     Using the semiconductor lasers that oscillate in a single longitudinal mode as the first semiconductor laser  111  and the second semiconductor laser  211  in  FIG. 5  have the following problems. 
     [Problem 1] 
     If a seed laser beam is pulse-amplified using a fiber amplifier so as to have high pulse energy, stimulated Brillouin scattering (SBS) that is a nonlinear phenomenon in an optical fiber occurs due to a narrow spectral line width, which may damage the solid-state laser device. Thus, it is difficult to increase the pulse energy of the pulse laser beam by pulse amplification using the fiber amplifier. 
     [Problem 2] 
     In order to realize a desired exposure process by the exposure apparatus  20 , the spectral line width of the pulse laser beam (excimer beam) entering the exposure apparatus  20  needs to be controlled. However, it is difficult to change the spectral line width of the laser beam in the semiconductor laser that oscillates in the single longitudinal mode, and thus it is difficult to control the spectral line width of the excimer beam wavelength-converted by the wavelength conversion system  300  and amplified. 
     [Problem 3] 
     If a semiconductor laser that oscillates in a multi-longitudinal mode (not shown) is used in the solid-state laser system  10 , SBS can be suppressed, but it is difficult to control the spectral line width to a target spectral line width with high accuracy. 
     [Problem 4] 
     For a configuration in which output of the laser system  1  is controlled using a measurement result from the monitor module  16 , the wavelength and the spectral line width cannot be measured without the pulse laser beam output from the excimer amplifier  14 , which may reduce control speed. In order to increase the control speed, it is desirable to control the output of the solid-state laser system  10  such that a desired target center wavelength and a desired target spectral line width can be realized even without the pulse laser beam output from the excimer amplifier  14 . 
     4. Embodiment 1 
     4.1 Configuration 
       FIG. 20  schematically shows a configuration of a laser system  1 A according to Embodiment 1. Differences from  FIG. 5  will be described. The laser system  1 A according to Embodiment 1 in  FIG. 20  includes a first multiple semiconductor laser system  160  and a second multiple semiconductor laser system  260  in place of the first semiconductor laser system  110  and the second semiconductor laser system  210  in  FIG. 5 . 
     In the laser system  1 A, the first multiple semiconductor laser system  160  can perform variable control of a spectral line width, and the second multiple semiconductor laser system  260  can perform variable control of a wavelength. 
     The first multiple semiconductor laser system  160  includes a plurality of semiconductor lasers  161  that perform CW oscillation in a single longitudinal mode with different oscillation wavelengths, a first beam combiner  163 , a first beam splitter  164 , a first spectrum monitor  166 , and a first multiline control unit  168 . 
     The semiconductor lasers  161  may be, for example, distributed feedback semiconductor lasers. An example using five semiconductor lasers  161  is shown herein, but not limited to this example, the number of the semiconductor lasers  161  may be two or more. In  FIG. 20 , the semiconductor lasers  161  included in the first multiple semiconductor laser system  160  are denoted as DFB 1 ( 1 ) to DFB 1 ( 5 ). The semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) are set to perform CW oscillation with different wavelengths near a wavelength of about 1030 nm. The semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) are examples of “a plurality of first semiconductor lasers” in the present disclosure. The semiconductor laser  161  is sometimes referred to as the first semiconductor laser  161 . 
     The second multiple semiconductor laser system  260  includes a plurality of semiconductor lasers  261  that perform CW oscillation in a single longitudinal mode with different oscillation wavelengths, a second beam combiner  263 , a second beam splitter  264 , a second spectrum monitor  266 , and a second multiline control unit  268 . 
     The semiconductor lasers  261  may be, for example, distributed feedback semiconductor lasers. An example using five semiconductor lasers  261  is shown herein, but not limited to this example, the number of the semiconductor lasers  261  may be appropriately two or more. In  FIG. 20 , the semiconductor lasers  261  included in the second multiple semiconductor laser system  260  are denoted as DFB 2 ( 1 ) to DFB 2 ( 5 ). The semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) are set to perform CW oscillation with different wavelengths near a wavelength of about 1554 nm. The semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) are examples of “a plurality of second semiconductor lasers” in the present disclosure. The semiconductor laser  261  is sometimes referred to as the second semiconductor laser  261 . 
     The monitor module  16  in  FIG. 20  further includes a beam splitter  604  and a spectrum monitor  606 . The spectrum monitor  606  may include, for example, an etalon spectrometer that measures a spectral line width of an ArF laser beam (excimer beam) as shown in  FIG. 71  described later. 
     The exposure control unit  22  has a signal line for transmitting data of a target spectral line width ΔXt of an excimer beam to the laser control unit  18 . 
     4.2 Operation 
     When receiving the data of the target spectral line width Δλt of the excimer beam from the exposure control unit  22 , the laser control unit  18  of the laser system  1 A in  FIG. 20  calculates a target spectral line width Δλ 1   mt  of multiline of the first multiple semiconductor laser system  160  such that the target spectral line width Δλt is reached. The multiline output from the first multiple semiconductor laser system  160  is referred to as “first multiline”. The target spectral line width Δλ 1   mt  of the first multiline is also referred to as “target spectral line width Δλ 1   mt  of the first multiple semiconductor laser system  160 ”. The target spectral line width Δλ 1   mt  may be a difference between a shortest wavelength (minimum wavelength) and a longest wavelength (maximum wavelength) of oscillation wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). 
     For the laser control unit  18  to calculate the target spectral line width Δλ 1   mt  from the target spectral line width Δλ of the excimer beam, a correlation between Δλt and Δλm may be previously stored as table data or a function in a memory unit such as a memory. The data indicating the correlation between Δλt and Δλm is an example of “relationship data specifying a relationship between the spectral line width of the excimer laser beam and the first multiline spectrum” in the present disclosure. Such relationship data may be updated as the laser system  1 A operates. 
     The laser control unit  18  transmits, to a solid-state laser system control unit  350 , data of the target spectral line width Δλ 1   mt  of the first multiple semiconductor laser system  160 . 
     When receiving the target spectral line width Δλ 1   mt  of the first multiple semiconductor laser system  160 , the solid-state laser system control unit  350  calculates target oscillation wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ), and transmits, to the first multiline control unit  168 , data of the target oscillation wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). 
     The first multiline control unit  168  calculates the target oscillation wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ), and control current values A 1  and temperatures T 1  of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) such that the oscillation wavelengths have the same predetermined light intensity. Here, a center wavelength λ 1   mc   0  of the first multiline calculated from the oscillation wavelengths and the light intensity of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) is controlled to be, for example, 1030 nm. 
     The multiline obtained by combining laser beams output from the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) in the first multiple semiconductor laser system  160  is an example of “first multiple spectrum” in the present disclosure. 
     When receiving data of a target center wavelength λct from the exposure control unit  22 , the laser control unit  18  calculates a target center wavelength λ 2   mct  of multiline of the second multiple semiconductor laser system  260  such that the target center wavelength λct is reached, and transmits the target center wavelength λ 2   mct  to the solid-state laser system control unit  350 . The multiline output from the second multiple semiconductor laser system  260  is referred to as “second multiline”. The target center wavelength λ 2   mct  of the second multiline is also referred to as “target center wavelength λ 2   mct  of the second multiple semiconductor laser system  260 ”. 
     The solid-state laser system control unit  350  transmits data of the target center wavelength λ 2   mt  of the second multiple semiconductor laser system  260  to the second multiline control unit  268 . 
     The second multiline control unit  268  controls, in accordance with the target center wavelength λ 2   mct , current values A 2  and temperatures T 2  of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) such that the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) have oscillation wavelengths to suppress SBS and that the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) have the same predetermined light intensity. Here, a center wavelength of the second multiline calculated from the oscillation wavelengths and the light intensity of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) is controlled to reach the target center wavelength λ 2   mct . The multiline obtained by combining laser beams output from the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) in the second multiple semiconductor laser system  260  is an example of “second multiline spectrum” in the present disclosure. 
     4.3 Example of Processing of Laser Control Unit 
       FIG. 21  is a flowchart of an example of processing of the laser control unit  18 . The flowchart in  FIG. 21  can be applied in place of the flowchart in  FIG. 6 . Differences from  FIG. 6  will be described. 
     The flowchart in  FIG. 21  includes step S 12 A in place of step S 12  in  FIG. 6 . In step S 12 A, the laser control unit  18  performs a control subroutine ( 2 ) of the solid-state laser system  10 . 
       FIG. 22  is a flowchart of an example of the control subroutine ( 2 ) of the solid-state laser system. The flowchart in  FIG. 22  is applied to step S 12 A in  FIG. 21 . Differences between the flowcharts in  FIG. 22  and  FIG. 8  will be described. 
     The flowchart in  FIG. 22  includes steps S 35  to S 38  between step S 33  and step S 40 . 
     When a determination result in step S 31  is No or after step S 33 , the laser control unit  18  goes to step S 34 . 
     In step S 34 , the laser control unit  18  determines whether or not data of a target spectral line width has been received from the exposure control unit  22 . When the determination result in step S 34  is No, the laser control unit  18  goes to step S 40 . 
     When the determination result in step S 34  is Yes, that is, when data of a new target spectral line width has been received from the exposure control unit  22 , the laser control unit  18  goes to step S 35 , and reads data of a target spectral line width Δλt. 
     Then, in step S 36 , the laser control unit  18  calculates a target spectral line width Δλ 1   mt  of the first multiple semiconductor laser system  160  from the target spectral line width Δλt. 
     Then, in step S 38 , the laser control unit  18  transmits data of the target spectral line width Δλ 1   mt  to the solid-state laser system control unit  350 . 
     After step S 38 , the laser control unit  18  goes to step S 40 . When the determination result in step S 34  is No, the laser control unit  18  skips steps S 35  to S 38  and goes to step S 40 . The processes in step S 40  and thereafter are as described for the flowchart in  FIG. 8 . 
       FIG. 23  is a flowchart of an example of processing for calculating the target spectral line width Δλ 1   mt  of the first multiple semiconductor system. The flowchart in  FIG. 23  is applied to step S 36  in  FIG. 22 . 
     In step S 171  in  FIG. 23 , the laser control unit  18  calls a function of Δλ 1   m  being f(Δλ) representing a relationship between a spectral line width Δλ of the excimer beam and the spectral line width Δλ 1   m  of the first multiple semiconductor laser system. 
       FIG. 24  shows an example of the function of Δλ 1   m  being f(Δλ).  FIG. 24  is a graph showing an example of the function representing the relationship between the spectral line width Δλ of the excimer beam and the spectral line width Δλ 1   m  of the first multiple semiconductor laser system. Such a function is obtained by, for example, previously measuring data of the spectral line width Δλ of the pulse laser beam amplified by the excimer amplifier  14  and the spectral line width Δλm of the multiline generated by the first multiple semiconductor laser system  160  and calculating an approximate function from the measurement result. 
     The laser control unit  18  can call the approximate function as shown in  FIG. 24  from a memory to calculate Δλ 1   mt  from Δλt. 
     In step S 172  in  FIG. 23 , the laser control unit  18  uses the called function to calculate the target spectral line width Δλ 1   mt  of the first multiple semiconductor laser system  160  from the target spectral line width Δλt of the excimer beam. 
     After step S 172 , the laser control unit  18  finishes the flowchart in  FIG. 23  and returns to the flowchart in  FIG. 22 . 
     Instead of the function as shown in  FIG. 24 , table data may be stored in the memory and called to calculate Δλ 1   mt  from Δλt. 
     4.4 Control Example 1 of First Multiple Semiconductor Laser System 
       FIG. 25  is a block diagram of Control example 1 of the first multiple semiconductor laser system  160 . Here, an example is shown in which a target center wavelength λ 1   mct  and light intensity I 1   st  of the first multiline are fixed and control to change the spectral line width Δλ 1   m  is performed. 
     The solid-state laser system control unit  350  transmits, to the first multiline control unit  168 , data of the target spectral line width Δλ 1   mt , the target center wavelength λ 1   mc   0 , and target light intensity I 1   s   0  of the first multiline. The first multiline control unit  168  controls current values A 1 ( 1 ) to A 1 ( 5 ) and temperatures T 1 ( 1 ) to T 1 ( 5 ) of the semiconductor lasers DFB 1 ( 1 ) to DFB( 5 ). Wavelengths of laser beams output from the semiconductor lasers DFB 1 ( 1 ) to DFB( 5 ) are denoted as λ 1 ( 1 ) to λ 1 ( 5 ). The plurality of laser beams are combined by the first beam combiner  163 . 
     The multiline laser beam output from the first beam combiner  163  enters the first beam splitter  164 . The laser beam having passed through the first beam splitter  164  enters the first semiconductor optical amplifier  120 . The laser beam reflected by the first beam splitter  164  enters the first spectrum monitor  166 . Part of the CW laser beam output from the first beam combiner  163  enters the first spectrum monitor  166 . 
       FIG. 26  shows an example of a multiline spectrum detected by the first spectrum monitor  166  in Control example 1 in  FIG. 25 . Here, an example of multiline is shown obtained when the target center wavelength λ 1   mct  is λ 1   mc   0  and the target spectral line width Δλ 1   mt  is Δλ 1   m.    
     In  FIG. 26 , the wavelengths of the multiline are λ 1 ( 1 ) to λ 1 ( 5 ), and the center wavelength is λ 1   mc   0 . A wavelength interval Δλ 1   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 1   m . Further, the lines of the wavelengths λ 1 ( 1 ) to λ 1 ( 5 ) have the same light intensity I 1   s   0 . The wavelength interval Δλ 1   p  of the multiline is an interval between the oscillation wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). 
       FIG. 27  shows an example of a multiline spectrum obtained when the center wavelength of the multiline is fixed and control to change the spectral line width of the multiline is performed in a spectral shape in  FIG. 26 . In  FIG. 27 , as compared to  FIG. 26 , the target spectral line width Δλ 1   mt  is changed to Δλ 1   ma . Thus, the wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) are changed to λ 1 ( 1 ) a  to λ 1 ( 5 ) a.    
     In  FIG. 27 , a wavelength interval Δλ 1   pa  of the multiline is generally constant and one fourth of the spectral line width Δλ 1   ma . On the other hand, the center wavelength of the multiline is still λ 1   mc   0  as in  FIG. 26 , and the lines of the wavelengths λ 1 ( 1 ) a  to λ 1 ( 5 ) a  have the same light intensity I 1 s 0  as in  FIG. 26 . 
     4.5 Control Example 1 of Second Multiple Semiconductor Laser System 
       FIG. 28  is a block diagram of a control example of the second multiple semiconductor laser system. Here, an example is shown in which a spectral line width Δλ 2   mt  and light intensity I 2   st  of the second multiline are fixed and control to change the target center wavelength λ 2   mct  is performed. 
     The solid-state laser system control unit  350  transmits, to the second multiline control unit  268 , data of a target spectral line width Δλ 2   m   0 , the target center wavelength λ 2   mct , and target light intensity I 2   s   0  of the second multiline. The second multiline control unit  268  controls current values A 2 ( 1 ) to A 2 ( 5 ) and temperatures T 2 ( 1 ) to T 2 ( 5 ) of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ). Wavelengths of laser beams output from the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) are denoted as λ 2 ( 1 ) to λ 2 ( 5 ). The plurality of laser beams are combined by the second beam combiner  263 . 
     The multiline laser beam output from the second beam combiner  263  enters the second beam splitter  264 . The laser beam having passed through the second beam splitter  264  enters the second semiconductor optical amplifier  220 . The laser beam reflected by the second beam splitter  264  enters the second spectrum monitor  266 . Part of the CW laser beam output from the second beam combiner  263  enters the second spectrum monitor  266 . 
       FIG. 29  shows an example of a multiline spectrum detected by the second spectrum monitor  266  in Control example 1 in  FIG. 28 . Here, an example of multiline is shown obtained when the target center wavelength λ 2   mct  is λ 2   mc  and the target spectral line width Δλ 2   mt  is Δλ 2   m   0 . In  FIG. 29 , the wavelengths of the multiline are λ 2 ( 1 ) to λ 2 ( 5 ) and the center wavelength is λ 2   mc . A wavelength interval Δλ 2   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 2   m   0 . Further, the lines of the wavelengths λ 2 ( 1 ) to λ 2 ( 5 ) have the same light intensity I 2   s   0 . 
       FIG. 30  shows an example of a multiline spectrum obtained when the spectral line width of the multiline is fixed and control to change a center wavelength of the multiline is performed in a spectral shape in  FIG. 29 . In  FIG. 30 , as compared to  FIG. 29 , the target center wavelength λ 2   mct  of the multiline is changed to λ 2   mca . Thus, the wavelengths of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) are changed to λ 2 ( 1 ) a  to λ 2 ( 5 ) a . On the other hand, the target spectral line width Δλ 2   mt  of the multiline is still Δλ 2   m   0 , and the lines of the multiline have the same light intensity I 2   s   0  as in  FIG. 29 . A variable range of the target center wavelength of the multiline generated by the second multiple semiconductor laser system  260  may be, for example, 1548 nm to 1557 nm. 
     4.6 Example of Processing of Solid-State Laser System Control Unit 
       FIG. 31  is a flowchart of an example of processing of the solid-state laser system control unit  350 . The flowchart in  FIG. 31  can be applied in place of the flowchart in  FIG. 10 . Differences from  FIG. 10  will be described. 
     The flowchart in  FIG. 31  includes step S 61 A, step S 62 A, and step S 63 A in place of step S 61 , step S 62 , and step S 63  in  FIG. 10 . 
     In step S 61 A, the solid-state laser system control unit  350  performs an initial setting subroutine ( 2 ) of the solid-state laser system. 
     In step S 62 A, the solid-state laser system control unit  350  performs a control subroutine of the first multiple semiconductor laser system  160 . 
     In step S 63 A, the solid-state laser system control unit  350  performs a control subroutine of the second multiple semiconductor laser system  260 . 
       FIG. 32  is a flowchart of an example of the initial setting subroutine ( 2 ) of the solid-state laser system. The flowchart in  FIG. 32  is applied to step S 61 A in  FIG. 31 . 
     In step S 171  in  FIG. 32 , the solid-state laser system control unit  350  sets a flag signal indicating a state of the first multiple semiconductor laser system  160  to NG. In other words, the solid-state laser system control unit  350  sets a value of a flag F 1  to “0”. 
     In step S 172 , the solid-state laser system control unit  350  sets a flag signal indicating a state of the second multiple semiconductor laser system  260  to NG. In other words, the solid-state laser system control unit  350  sets a value of a flag F 2  to “0”. 
     In step S 173 , the solid-state laser system control unit  350  sets a flag signal indicating a state of energy of the solid-state laser system  10  to NG. In other words, the solid-state laser system control unit  350  sets a value of a flag Fs to “0”. 
     In step S 174 , the solid-state laser system control unit  350  sets the target center wavelength λ 1   mct  of the first multiple semiconductor laser system  160  to an initial value λ 1   mc   0 . The initial value λ 1   mc   0  may be, for example, 1030 nm. 
     In step S 175 , the solid-state laser system control unit  350  sets the target center wavelength λ 2   mct  of the second multiple semiconductor laser system  260  to an initial value λ 2   mc   0 . The initial value λ 2   mc   0  may be, for example, 1554 nm. 
     In step S 176 , the solid-state laser system control unit  350  sets the target spectral line width Δλ 1   mt  of the first multiple semiconductor laser system  160  to an initial value Δλ 1   m   0 . Here, the target spectral line width Δλ 1   mt  is set to the initial value Δλ 1   m   0  at which SBS is suppressed in the first fiber amplifier  140 . 
     In step S 177 , the solid-state laser system control unit  350  sets the target spectral line width Δλ 2   mt  of the second multiple semiconductor laser system  260  to an initial value Δλ 2   m   0 . Here, the target spectral line width Δλ 2   mt  is set to the initial value Δλ 2   m   0  at which SBS is suppressed in the second fiber amplifier  240 . 
     In step S 178 , the solid-state laser system control unit  350  sets the target light intensity I 1   st  of the multiline generated by the first multiple semiconductor laser system  160  to an initial value I 1   s   0 . 
     In step S 179 , the solid-state laser system control unit  350  sets the target light intensity I 2   st  of the multiline generated by the second multiple semiconductor laser system  260  to an initial value I 2   s   0 . 
     Steps S 180  to S 183  are similar to steps S 77  to S 79  in  FIG. 11 . 
     In step S 180  in  FIG. 32 , the solid-state laser system control unit  350  sets an initial value of pulse energy from each of the first pulse excitation light source  132 , the second pulse excitation light source  144 , and the third pulse excitation light source  232 . 
     In step S 181 , the solid-state laser system control unit  350  sets target pulse energy Est of the solid-state laser system  10  to an initial value Es 0 . 
     In step S 182 , the solid-state laser system control unit  350  sets a delay time of each trigger signal in the synchronization circuit unit  340 . 
     In step S 183 , the solid-state laser system control unit  350  sets current values and temperatures of the first semiconductor laser  161  and the second semiconductor laser  261  to initial values to cause CW oscillation. 
     After step S 183 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 32  and returns to the flowchart in  FIG. 31 . 
       FIG. 33  is a flowchart of an example of the control subroutine of the first multiple semiconductor laser system  160 . The flowchart in  FIG. 33  is applied to step S 62 A in  FIG. 31 . 
     In step S 201  in  FIG. 33 , the solid-state laser system control unit  350  determines whether or not data of the target center wavelength of the multiline has been transmitted to the first multiline control unit  168 . When the determination result in step S 201  is No, the solid-state laser system control unit  350  goes to step S 202 , and transmits data of the target center wavelength λ 1   mct  to the first multiline control unit  168 . In this case, the target center wavelength λ 1   mct  is a fixed value (initial value) λ 1   mc   0 , which is, for example, 1030 nm. 
     After step S 202 , the solid-state laser system control unit  350  goes to step S 203 . When the determination result in step S 201  is Yes, the solid-state laser system control unit  350  skips step S 202  and goes to step S 203 . 
     In step S 203 , the solid-state laser system control unit  350  determines whether or not the target spectral line width has been changed. When the determination result in step S 203  is Yes, that is, when the target spectral line width has been changed, the solid-state laser system control unit  350  goes to step S 204  and transmits, to the laser control unit  18 , a flag signal of F 1  being 0 indicating that the first multiple semiconductor laser system  160  is in an NG state. 
     Then, in step S 205 , the solid-state laser system control unit  350  reads data of the target spectral line width Δλ 1   mt.    
     In step S 206 , the solid-state laser system control unit  350  transmits the data of the target spectral line width Δλ 1   mt  to the first multiline control unit  168 . 
     After step S 206 , the solid-state laser system control unit  350  goes to step S 208 . When the determination result in step S 203  is No, that is, when the exposure control unit  22  does not request the change of the target spectral line width, the solid-state laser system control unit  350  skips steps S 204  to S 206  and goes to step S 208 . 
     In step S 208 , the solid-state laser system control unit  350  determines whether or not an OK signal has been received from the first multiple semiconductor laser system  160 . 
     When the determination result in step S 208  is Yes, the solid-state laser system control unit  350  goes to step S 209 , and transmits a flag signal of F 1  being 1 to the laser control unit  18 . 
     When the determination result in step S 208  is No, the solid-state laser system control unit  350  goes to step S 210 , and transmits a flag signal of F 1  being 0 to the laser control unit  18 . 
     After step S 209  or S 210 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 33  and returns to the flowchart in  FIG. 31 . 
       FIG. 34  is a flowchart of an example of the control subroutine of the second multiple semiconductor laser system  260 . The flowchart in  FIG. 34  is applied to step S 63 A in  FIG. 31 . 
     In step S 221  in  FIG. 34 , the solid-state laser system control unit  350  determines whether or not data of the target spectral line width of the multiline has been transmitted to the second multiline control unit  268 . When the determination result in step S 221  is No, the solid-state laser system control unit  350  goes to step S 222 , and transmits data of the target spectral line width Δλ 2   mt  to the second multiline control unit  268 . In this case, the target spectral line width Δλ 2   mt  is a fixed value (initial value) Δλ 2   m   0 . 
     After step S 222 , the solid-state laser system control unit  350  goes to step S 223 . When the determination result in step S 221  is Yes, the solid-state laser system control unit  350  skips step S 222  and goes to step S 223 . 
     In step S 223 , the solid-state laser system control unit  350  determines whether or not the target center wavelength has been changed. When the determination result in step S 223  is Yes, that is, when the target center wavelength has been changed, the solid-state laser system control unit  350  goes to step S 224  and transmits, to the laser control unit  18 , a flag signal of F 2  being 0 indicating that the second multiple semiconductor laser system  260  is in an NG state. 
     Then, in step S 225 , the solid-state laser system control unit  350  reads data of the target center wavelength λct designated by the exposure control unit  22 . 
     In step S 226 , the solid-state laser system control unit  350  calculates, from the target center wavelength λct, the target center wavelength λ 2   mct  of the multiline of the second multiple semiconductor laser system  260 . 
     In step S 227 , the solid-state laser system control unit  350  transmits data of the target center wavelength λ 2   mct  to the second multiline control unit  268 . 
     After step S 227 , the solid-state laser system control unit  350  goes to step S 228 . When the determination result in step S 223  is No, that is, when the exposure control unit  22  does not request the change of the target center wavelength, the solid-state laser system control unit  350  skips steps S 224  to S 227  and goes to step S 228 . 
     In step S 228 , the solid-state laser system control unit  350  determines whether or not an OK signal has been received from the second multiple semiconductor laser system  260 . 
     When the determination result in step S 228  is Yes, the solid-state laser system control unit  350  goes to step S 229 , and transmits a flag signal of F 2  being 1 to the laser control unit  18 . 
     When the determination result in step S 228  is No, the solid-state laser system control unit  350  goes to step S 230 , and transmits a flag signal of F 2  being 0 to the laser control unit  18 . 
     After step S 229  or S 230 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 34  and returns to the flowchart in  FIG. 31 . 
       FIG. 35  is a flowchart of an example of processing for calculating the target center wavelength λ 2   mct  of the second multiple semiconductor laser system  260 . The flowchart in  FIG. 35  is applied to step S 226  in  FIG. 34 . A calculation method in the flowchart in  FIG. 35  is the same as in the flowchart in  FIG. 14 . 
     In step S 241  in  FIG. 35 , the solid-state laser system control unit  350  converts, into a frequency f 1   mt , the target center wavelength λ 1   mct  of the first multiline from the first multiple semiconductor laser system  160 . 
     The conversion formula is expressed by 
         f 1 mt=C/λ 1 mct    
     where C is light speed. 
     In step S 242 , the solid-state laser system control unit  350  converts the target center wavelength λct after wavelength conversion by the wavelength conversion system  300  into a frequency ft. 
     The conversion formula is expressed by 
     
       
      
       ft=C/λct.  
      
     
     In step S 243 , the solid-state laser system control unit  350  calculates a target frequency f 2   mt  of the second multiple semiconductor laser system  260  from Expression (5) of wavelength conversion. The target frequency f 2   mt  can be calculated by Expression (7) below. 
         f 2 mt =(½)· ft− 2· f 1 mt   (7)
 
     In step S 244 , the solid-state laser system control unit  350  converts the target frequency f 2   mt  into the target center wavelength λ 2   mct . The conversion formula is expressed by 
       λ2 mct=C/f 2 mt.  
 
     The calculation is not limited to the procedure described in steps S 241  to S 244  in  FIG. 35 , but may be performed using table data or the like that provides similar conversion results. 
     After step S 244 , the solid-state laser system control unit  350  finishes the flowchart in  FIG. 35  and returns to the flowchart in  FIG. 34 . 
     4.7 Example of Processing of First Multiline Control Unit 
       FIG. 36  is a flowchart of an example of processing of the first multiline control unit  168 . The processing and the operation in the flowchart in  FIG. 36  are realized by, for example, a processor that functions as the first multiline control unit  168  executing a program. 
     In step S 251 , the first multiline control unit  168  reads data of the target center wavelength λ 1   mct  of the multiline (first multiline) of the first multiple semiconductor laser system  160 . Here, the target center wavelength λ 1   mct  is a fixed value (initial value) λ 1   mc   0 , which is, for example, 1030 nm. 
     In step S 252 , the first multiline control unit  168  reads data of target light intensity I 1   st  of the first multiline. Here, the target light intensity I 1   st  is a fixed value (initial value) I 1   s   0 . 
     In step S 253 , the first multiline control unit  168  determines whether or not the target spectral line width has been changed. When the determination result in step S 253  is Yes, the first multiline control unit  168  goes to step S 254 , and reads data of the target spectral line width Δλ 1   mt.    
     Then, in step S 255 , the first multiline control unit  168  calculates a target oscillation wavelength λ 1 ( k )t of each semiconductor laser DFB 1 ( k ) of the first multiple semiconductor laser system  160  in accordance with the target center wavelength λ 1   mct  and the target spectral line width Δλ 1   mt . The letter k represents an index number for identifying each of the semiconductor lasers. The letter k represents an integer equal to or larger than 1 and equal to or smaller than n. The letter n represents the number of semiconductor lasers  161  included in the first multiple semiconductor laser system  160 . 
     In  FIG. 36 , after step S 255 , the first multiline control unit  168  goes to step S 256 ( 1 ), step S 256 ( 2 ) . . . step S 256 ( k ) . . . step S 256 ( n ). Hereinafter, step S 256 ( k ) will be descried as a representative of steps S 256 ( 1 ) to S 256 ( n ). 
     In step S 256 ( k ), the first multiline control unit  168  performs a control subroutine of the semiconductor laser DFB 1 ( k ) such that a wavelength and light intensity of the semiconductor laser DFB 1 ( k ) are brought close to a target center wavelength and target light intensity. Steps S 256 ( k ) for k of 1, 2 . . . n may be performed in parallel or concurrently. After step S 256 ( k ), the first multiline control unit  168  goes to step S 258 . 
     In step S 258 , the first multiline control unit  168  calculates a spectral line width Δλ 1   m  and a center wavelength λ 1   mc  of the multiline generated by the first multiple semiconductor laser system  160 , and determines whether or not differences from the target values are within allowable ranges. 
     Then, in step S 259 , the first multiline control unit  168  determines whether or not to stop control of the first multiple semiconductor laser system  160 . When the determination result in step S 259  is No, the first multiline control unit  168  returns to step S 253  and repeats the processes in steps S 253  to S 259 . 
     When the determination result in step S 259  is Yes, the first multiline control unit  168  finishes the flowchart in  FIG. 36 . 
       FIG. 37  is a flowchart of an example of processing for calculating a target oscillation wavelength of each semiconductor laser of the first multiple semiconductor laser system  160 . The flowchart in  FIG. 37  is applied to step S 255  in  FIG. 36 . 
     In step S 271  in  FIG. 37 , the first multiline control unit  168  calculates a wavelength interval Δλ 1   p  of the multiline from the target spectral line width λ 1   mt . With the number of semiconductor lasers  161  included in the first multiple semiconductor laser system  160  being n, the wavelength interval Δλ 1   p  of the multiline can be calculated by Expression (8) below. 
       Δλ1 p =Δλ1 mt /( n− 1)  (8)
 
     In step S 272 , the first multiline control unit  168  calculates a target oscillation wavelength λ 1 ( k )t of each semiconductor laser DFB 1 ( k ). 
     The target oscillation wavelength λ 1 ( k )t can be calculated from the target center wavelength λ 1   mct  and the wavelength interval Δλ 1   p  by Expression (9) below. 
       λ1( k ) t =λ1 mct −{( n− 2 k+ 1)/2}·Δλ1 p   (9)
 
     After step S 272 , the first multiline control unit  168  finishes the flowchart in  FIG. 37  and returns to the flowchart in  FIG. 36 . 
       FIG. 38  is a flowchart of an example of the control subroutine of each semiconductor laser DFB 1 ( k ). The flowchart in  FIG. 38  is applied to step S 256 ( k ) in  FIG. 36 . 
     In step S 281  in  FIG. 38 , the first multiline control unit  168  reads data of the target oscillation wavelength λ 1 ( k )t and target light intensity I 1 ( k )t of each semiconductor laser DFB 1 ( k ). 
     In step S 282 , the first multiline control unit  168  uses the first spectrum monitor  166  to measure an oscillation wavelength λ 1 ( k ) and light intensity I 1 ( k ) of the semiconductor laser DFB 1 ( k ). 
     In step S 283 , the first multiline control unit  168  calculates a difference ΔI 1 ( k ) between the light intensity I 1 ( k ) and the target light intensity I 1   st.    
     In step S 284 , the first multiline control unit  168  determines whether or not an absolute value of AI 1 ( k ) is equal to or smaller than an allowable upper limit value ΔI 1   tr  which indicates an allowable range. When the determination result in step S 284  is Yes, the first multiline control unit  168  goes to step S 285 , and calculates a difference δλ 1 ( k ) between the oscillation wavelength λ 1 ( k ) and the target oscillation wavelength λ 1 ( k )t. 
     In step S 286 , the first multiline control unit  168  determines whether or not an absolute value of Δλ 1 ( k ) is equal to or smaller than an allowable upper limit value δλ 1   tr  which indicates an allowable range. When the determination result in step S 286  is No, the first multiline control unit  168  goes to step S 287 . 
     In step S 287 , the first multiline control unit  168  controls a temperature T 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. After step S 287 , the first multiline control unit  168  finishes the flowchart in  FIG. 38  and returns to the flowchart in  FIG. 36 . 
     When the determination result in step S 286  in  FIG. 38  is Yes, the first multiline control unit  168  skips step S 287 , finishes the flowchart in  FIG. 38 , and returns to the flowchart in  FIG. 36 . 
     When the determination result in step S 284  in  FIG. 38  is No, the first multiline control unit  168  goes to step S 288 . In step S 288 , the first multiline control unit  168  controls a current value A 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that ΔI 1 ( k ) is brought close to 0. After step S 288 , the first multiline control unit  168  finishes the flowchart in  FIG. 38  and returns to the flowchart in  FIG. 36 . 
       FIG. 39  is a flowchart of an example of processing for calculating and determining a spectral line width Δλ 1   m  and a center wavelength λ 1   mc  of the first multiple semiconductor laser system  160 . The flowchart in  FIG. 39  is applied to step S 258  in  FIG. 36 . 
     In step S 291  in  FIG. 39 , the first multiline control unit  168  calculates a spectral line width Δλ 1   m  of the multiline of the first multiple semiconductor laser system  160  from a spectrum measured by the first spectrum monitor  166 . The spectral line width Δλ 1   m  can be obtained by calculating a difference between a minimum wavelength and a maximum wavelength of oscillation wavelengths λ 1 ( 1 ) to λ 1 ( n ) of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). 
       Δλ1 m =λ1( n )−λ1(1)  (10)
 
     In step S 292 , the first multiline control unit  168  calculates a center wavelength λ 1   mc  of the multiline from the spectrum measured by the first spectrum monitor  166 . For example, the first multiline control unit  168  calculates a centroid of the measured multiline spectrum as the center wavelength λ 1   mc . The spectrum centroid is calculated from each oscillation wavelength and light intensity by Expression (11) below. 
       [Expression 11] 
       λ1 mc=Σ   k=1   n /1( k )·λ1( k )/Σ k=1   n   /I 1( k )  (11)
 
     Then, in step S 293 , the first multiline control unit  168  calculates a difference ΔΔλ 1   m  between the spectral line width Δλ 1   m  obtained in step S 291  and the target spectral line width Δ 1   mt  of the multiline of the first multiple semiconductor laser system  160 . 
       ΔΔλ1 m =Δλ1 m −Δλ1 mt   (12)
 
     In step S 294 , the first multiline control unit  168  calculates a difference δλ 1   mc  between the center wavelength λ 1   mc  obtained in step S 292  and the target center wavelength λ 1   mct  of the first multiple semiconductor laser system  160 . 
     Then, in step S 295 , the first multiline control unit  168  determines whether or not an absolute value of ΔΔλ 1   m  is equal to or smaller than an allowable upper limit value ΔΔλ 1   mtr  which indicates an allowable range and an absolute value of δλ 1   mc  is equal to or smaller than an allowable upper limit value δλ 1   mctr  which indicates an allowable range. When the determination result in step S 295  is Yes, the first multiline control unit  168  goes to step S 296 , and transmits, to the solid-state laser system control unit  350 , a flag signal of F 1  being 1 indicating that the first multiple semiconductor laser system  160  is in an OK state. 
     When the determination result in step S 295  is No, the first multiline control unit  168  goes to step S 297 , and transmits, to the solid-state laser system control unit  350 , a flag signal of F 1  being 0 indicating that the first multiple semiconductor laser system  160  is in an NG state. 
     After step S 296  or S 297 , the first multiline control unit  168  finishes the flowchart in  FIG. 39  and returns to the flowchart in  FIG. 36 . 
     4.8 Example of Processing of Second Multiline Control Unit 
       FIG. 40  is a flowchart of an example of processing of the second multiline control unit  268 . The processing and the operation in the flowchart in  FIG. 40  are realized by, for example, a processor that functions as the second multiline control unit  268  executing a program. 
     In step S 351 , the second multiline control unit  268  reads data of the target spectral line width Δλ 2   mt  of the multiline (second multiline) of the second multiple semiconductor laser system  260 . Here, the target spectral line width Δλ 2   mt  is a fixed value (initial value) Δλ 2   m   0 . 
     In step S 352 , the second multiline control unit  268  reads data of target light intensity I 2   st  of the second multiline. Here, the target light intensity I 2   st  is a fixed value (initial value) I 2   s   0 . 
     In step S 353 , the second multiline control unit  268  determines whether or not the target center wavelength has been changed. When the determination result in step S 353  is Yes, the second multiline control unit  268  goes to step S 354  and reads data of the target center wavelength λ 2   mct.    
     Then, in step S 355 , the second multiline control unit  268  calculates a target oscillation wavelength λ 2 ( k )t of each semiconductor laser DFB 2 ( k ) of the second multiple semiconductor laser system  260  in accordance with the target center wavelength λ 2   mct . After step S 355 , the second multiline control unit  268  goes to step S 356 ( 1 ), step S 356 ( 2 ) . . . step S 356 ( k ) . . . step S 356 ( n ). The letter k represents an integer equal to or larger than 1 and equal to or smaller than n. The letter n represents the number of semiconductor lasers  261  included in the second multiple semiconductor laser system  260 .  FIG. 20  shows an example of n being 5. Here, the number of semiconductor lasers  161  included in the first multiple semiconductor laser system  160  and the number of semiconductor lasers  261  included in the second multiple semiconductor laser system  260  are the same and five (n is 5), but may be different. 
     Step S 356 ( k ) will be descried as a representative of steps S 356 ( 1 ) to S 356 ( n ). 
     In step S 356 ( k ), the second multiline control unit  268  performs a control subroutine of the semiconductor laser DFB 2 ( k ) such that a wavelength and light intensity of the semiconductor laser DFB 2 ( k ) are brought close to a target oscillation wavelength and target light intensity. Steps S 356 ( k ) for k of 1, 2 . . . n may be performed in parallel or concurrently. After step S 356 ( k ), the second multiline control unit  268  goes to step S 358 . 
     In step S 358 , the second multiline control unit  268  calculates a spectral line width Δλ 2   m  and a center wavelength λ 2   mc  of the second multiline generated by the second multiple semiconductor laser system  260 , and determines whether or not differences from target values are within allowable ranges. 
     Then, in step S 359 , the second multiline control unit  268  determines whether or not to stop control of the second multiple semiconductor laser system  260 . When the determination result in step S 359  is No, the second multiline control unit  268  returns to step S 353  and repeats the processes in steps S 353  to S 359 . 
     When the determination result in step S 359  is Yes, the second multiline control unit  268  finishes the flowchart in  FIG. 40 . 
       FIG. 41  is a flowchart of an example of processing for calculating a target oscillation wavelength of each semiconductor laser of the second multiple semiconductor laser system  260 . The flowchart in  FIG. 41  is applied to step S 355  in  FIG. 40 . 
     In step S 371  in  FIG. 41 , the second multiline control unit  268  calculates a wavelength interval Δλ 2   p  of the multiline from the target spectral line width Δλ 2   mt . With the number of semiconductor lasers included in the second multiple semiconductor laser system  260  being n, the wavelength interval Δλ 2   p  of the multiline can be calculated by Expression (13) below. 
       Δλ2 p=Δλ 2 mt /( n− 1)  (13)
 
     In step S 372 , the second multiline control unit  268  calculates a target oscillation wavelength λ 2 ( k )t of each semiconductor laser DFB 2 ( k ). 
     The target oscillation wavelength λ 2 ( k )t can be calculated from the target center wavelength λ 2   mct  and the wavelength interval Δλ 2   p  of the multiline by Expression (14) below. 
       λ2( k ) t=λ 2 mct −{( n− 2 k+ 1)/2}·Δλ2 p   (14)
 
     After step S 372 , the second multiline control unit  268  finishes the flowchart in  FIG. 41  and returns to the flowchart in  FIG. 40 . 
       FIG. 42  is a flowchart of an example of the control subroutine of each semiconductor laser DFB 2 ( k ). The flowchart in  FIG. 42  is applied to step S 356 ( k ) in  FIG. 40 . 
     In step S 381  in  FIG. 42 , the second multiline control unit  268  reads data of the target oscillation wavelength λ 2 ( k )t and target light intensity I 2 ( k )t of each semiconductor laser DFB 2 ( k ). 
     In step S 382 , the second multiline control unit  268  uses the second spectrum monitor  266  to measure an oscillation wavelength λ 2 ( k ) and light intensity I 2 ( k ) of the semiconductor laser DFB 2 ( k ). 
     In step S 383 , the second multiline control unit  268  calculates a difference ΔI 2 ( k ) between the light intensity I 2 ( k ) and the target light intensity I 2   st.    
     In step S 384 , the second multiline control unit  268  determines whether or not an absolute value of ΔI 2 ( k ) is equal to or smaller than an allowable upper limit value ΔI 2   tr  which indicates an allowable range. When the determination result in step S 384  is Yes, the second multiline control unit  268  goes to step S 385  and calculates a difference δλ 2 ( k ) between the oscillation wavelength λ 2 ( k ) and the target oscillation wavelength λ 2 ( k )t. 
     In step S 386 , the second multiline control unit  268  determines whether or not an absolute value of Δλ 2 ( k ) is equal to or smaller than an allowable upper limit value δλ 2   tr  which indicates an allowable range. When the determination result in step S 386  is No, the second multiline control unit  268  goes to step S 387 . 
     In step S 387 , the second multiline control unit  268  controls a temperature T 1 ( k ) of the semiconductor laser DFB 2 ( k ) such that δλ 2 ( k ) is brought close to 0. After step S 387 , the second multiline control unit  268  finishes the flowchart in  FIG. 42  and returns to the flowchart in  FIG. 40 . 
     When the determination result in step S 386  in  FIG. 42  is Yes, the second multiline control unit  268  skips step S 387 , finishes the flowchart in  FIG. 42 , and returns to the flowchart in  FIG. 40 . 
     When the determination result in step S 384  in  FIG. 42  is No, the second multiline control unit  268  goes to step S 388 . In step S 388 , the second multiline control unit  268  controls a current value A 2 ( k ) of the semiconductor laser DFB 2 ( k ) such that ΔI 2 ( k ) is brought close to 0. After step S 388 , the second multiline control unit  268  finishes the flowchart in  FIG. 42  and returns to the flowchart in  FIG. 40 . 
       FIG. 43  is a flowchart of an example of processing for calculating and determining a spectral line width Δλ 2   m  and a center wavelength λ 2   mc  of the multiline of the second multiple semiconductor laser system. The flowchart in  FIG. 43  is applied to step S 358  in  FIG. 40 . 
     In step S 391  in  FIG. 43 , the second multiline control unit  268  calculates a spectral line width Δλ 2   m  of the multiline of the second multiple semiconductor laser system  260  from a spectrum measured by the second spectrum monitor  266 . The spectral line width Δλ 2   m  of the multiline can be obtained by calculating a difference between a minimum wavelength and a maximum wavelength of oscillation wavelengths λ 2 ( 1 ) to λ 2 ( n ) of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ). 
       Δλ2 m =λ2( n )−λ2(1)  (15)
 
     In step S 392 , the second multiline control unit  268  calculates a center wavelength λ 2   mc  of the multiline from the spectrum measured by the second spectrum monitor  266 . For example, the second multiline control unit  268  calculates a centroid of the measured multiline spectrum as the center wavelength λ 2   mc.    
       [Expression 16] 
       λ2  mc=Σ   k=1   n   I 2( k )·λ2( k )/Σ k=1   n   I 2( k )  (16)
 
     Then, in step S 393 , the second multiline control unit  268  calculates a difference ΔΔλ 2   m  between the spectral line width Δλ 2   m  obtained in step S 391  and the target spectral line width Δλ 2   mt  of the multiline of the second multiple semiconductor laser system  260 . 
       ΔΔλ2 m =Δλ2 m −Δλ2 mt   (17)
 
     In step S 394 , the second multiline control unit  268  calculates a difference δλ 2   mc  between the center wavelength λ 2   mc  obtained in step S 392  and the target center wavelength λ 2   mct  of the multiline of the second multiple semiconductor laser system  260 . 
     Then, in step S 395 , the second multiline control unit  268  determines whether or not an absolute value of ΔΔλ 2   m  is equal to or smaller than an allowable upper limit value ΔΔλ 2   mtr  which indicates an allowable range and an absolute value of δλ 2   mc  is equal to or smaller than an allowable upper limit value δλ 2   mct r which indicates an allowable range. When the determination result in step S 395  is Yes, the second multiline control unit  268  goes to step S 396 , and transmits, to the solid-state laser system control unit  350 , a flag signal of F 2  being 1 indicating that the second multiple semiconductor laser system  260  is in an OK state. 
     When the determination result in step S 395  is No, the second multiline control unit  268  goes to step S 397 , and transmits, to the solid-state laser system control unit  350 , a flag signal of F 2  being 0 indicating that the second multiple semiconductor laser system is in an NG state. 
     After step S 396  or S 397 , the second multiline control unit  268  finishes the flowchart in  FIG. 43  and returns to the flowchart in  FIG. 40 . 
     4.9 Effect 
     The laser system  1 A according to Embodiment 1 provides the following effects. 
     [1] The oscillation wavelength intervals of the semiconductor lasers of the multiline generated by the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) included in the first multiple semiconductor laser system  160  can be constantly controlled to control the spectral line width of the pulse-amplified excimer laser beam with high accuracy. 
     [2] The solid-state laser system  10  can constantly control the spectral line width and the center wavelength in accordance with the data of the target center wavelength λct and the target spectral line width Δλt irrespective of whether or not the excimer amplifier  14  generates a pulse laser beam (excimer beam). Thus, the spectral line width and the center wavelength can be controlled with high accuracy irrespective of a laser operation load (repetition frequency) and burst operation of the laser system  1 A. Specifically, when receiving the data of the target spectral line width Δλt, the laser control unit  18  can control the first solid-state laser device  100  before pulse amplification, thereby increasing control speed of the spectral line width. 
     [3] The oscillation wavelength intervals of the semiconductor lasers of the multiline generated by the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) of the first multiple semiconductor laser system  160  are controlled to suppress SBS in the first fiber amplifier  140 . This can suppress damage to the first fiber amplifier  140  and the first multiple semiconductor laser system  160 . Similarly, the oscillation wavelength intervals of the semiconductor lasers of the multiline generated by the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) of the second multiple semiconductor laser system  260  are controlled to suppress SBS in the second fiber amplifier  240 . This can suppress damage to the second fiber amplifier  240  and the second multiple semiconductor laser system  260 . 
     [4] In Embodiment 1, the target center wavelength λ 1   mct  of the first multiline generated by the first multiple semiconductor laser system  160  is fixed. Thus, there is no need to control an incident angle for phase matching of an LBO crystal  310  and a first CLBO crystal  312  for generating fourth harmonic light in the wavelength conversion system  300 . 
     [5] For significantly changing the target center wavelength λ 2   mct  of the second multiline generated by the second multiple semiconductor laser system  260  according to the change in the target center wavelength λct, it is only necessary to control an incident angle for phase matching of a second CLBO crystal  316  and a third CLBO crystal  320  that generate a sum frequency in the wavelength conversion system  300 . 
     The laser control unit  18  of the laser system  1 A according to Embodiment 1 described above is an example of “control unit” in the present disclosure. The exposure apparatus  20  including the exposure control unit  22  is an example of “external device” in the present disclosure. The LBO crystal  310  and the first CLBO crystal  312  are examples of “first nonlinear crystal” and “second nonlinear crystal” in the present disclosure. The second CLBO crystal  316  is an example of “third nonlinear crystal” in the present disclosure. The third CLBO crystal  320  is an example of “fourth nonlinear crystal” in the present disclosure. 
     4.10 Variant 
     As a variant of Embodiment 1, the first multiple semiconductor laser system  160  may have a configuration in which the spectral line width of the multiline is fixed and the center wavelength is variable. The second multiple semiconductor laser system  260  may have a configuration in which the center wavelength of the multiline is fixed and the spectral line width is variable. 
     4.10.1 Control Example 2 of First Multiple Semiconductor Laser System 
       FIG. 44  is a block diagram of Control example 2 of the first multiple semiconductor laser system  160 . Here, an example is shown in which the spectral line width Δλ 1   mt  and the light intensity I 1   st  of the first multiline are fixed and control to change the target center wavelength is performed. As shown in  FIG. 44 , in the first multiple semiconductor laser system  160 , a spectral line width Δλ 1   m   0  of the first multiline may be fixed at a spectral line width at which SBS is suppressed and a target center wavelength λ 1   mct  may be variable. For example, a variable wavelength range of λ 1   mct  may be 1028.5 nm to 1030.7 nm. In this case, for significantly changing the target center wavelength, it is necessary to rotate the second CLBO crystal  316  and the third CLBO crystal  320  in the wavelength conversion system  300  and also the LBO crystal  310  and the first CLBO crystal  312  for generating fourth harmonic light to suppress a reduction in wavelength conversion efficiency. 
     The solid-state laser system control unit  350  transmits, to the first multiline control unit  168 , data of a target spectral line width Δλ 1   mt  (=Δλ 1   m   0 ), the target center wavelength λ 1   mct , and the target light intensity I 1   st  (=I 1   s   0 ) of the multiline. The first multiline control unit  168  controls the current values A 1 ( 1 ) to A 1 ( 5 ) and the temperatures T 1 ( 1 ) to T 1 ( 5 ) of the semiconductor lasers DFB 1 ( 1 ) to DFB( 5 ). 
       FIG. 45  shows an example of a multiline spectrum detected by the first spectrum monitor  166  in Control example 2 in  FIG. 44 . Here, an example of multiline is shown obtained when the target center wavelength λ 1   mct  is λ 1   mc  and the target spectral line width Δλ 1   mt  is Δλ 1   m   0 . 
     In  FIG. 45 , the wavelengths of the multiline are λ 1 ( 1 ) to λ 1 ( 5 ) and the center wavelength is λ 1   mc . A wavelength interval Δλ 1   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 1   m   0 . Further, the lines of the wavelengths λ 1 ( 1 ) to λ 1 ( 5 ) have the same light intensity I 1   s   0 . 
       FIG. 46  shows an example of a multiline spectrum obtained when the spectral line width of the multiline is fixed and control to change the center wavelength of the multiline is performed in a spectral shape in  FIG. 45 . In  FIG. 46 , as compared to  FIG. 45 , the target center wavelength λ 1   mct  of the multiline is changed to λ 1   mca . Thus, the wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) are changed to λ 1 ( 1 ) a  to λ 1 ( 5 ) a . On the other hand, the target spectral line width Δλ 1   mt  of the multiline is still Δλ 1   m   0 , and the lines of the multiline have the same light intensity I 1   s   0  as in  FIG. 45 . 
     4.10.2 Control Example 2 of Second Multiple Semiconductor Laser System 
       FIG. 47  is a block diagram of Control example 2 of the second multiple semiconductor laser system  260 . Here, an example is shown in which the target center wavelength λ 2   mct  and the light intensity I 2   st  of the multiline are fixed and control to change the spectral line width Δλ 2   mt  is performed. In the second multiple semiconductor laser system  260 , the center wavelength of the multiline may be fixed at, for example, 1554 nm and the spectral line width Δλ 2   mt  may be variable. 
       FIG. 48  shows an example of a multiline spectrum detected by the second spectrum monitor  266  in Control example 2 in  FIG. 47 . Here, an example of multiline is shown obtained when the target center wavelength λ 2   mct  is λ 2   mc   0  and the target spectral line width Δλ 2   mt  is Δλ 2   m.    
     In  FIG. 48 , the wavelengths of the multiline are λ 2 ( 1 ) to λ 2 ( 5 ) and the center wavelength is λ 2   mc   0 . A wavelength interval Δλ 2   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 2   m . Further, the lines of the wavelengths λ 2 ( 1 ) to λ 2 ( 5 ) have the same light intensity I 2   s   0 . 
       FIG. 49  shows an example of a multiline spectrum obtained when the center wavelength of the multiline is fixed and control to change the spectral line width of the multiline is performed in a spectral shape in  FIG. 48 . In  FIG. 49 , as compared to  FIG. 48 , the target spectral line width Δλ 2   mt  is changed to Δλ 2   ma . Thus, the wavelengths of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) are changed to λ 2 ( 1 ) a  to λ 2 ( 5 ) a.    
     In  FIG. 49 , a wavelength interval Δλ 2   pa  of the multiline is generally constant and one fourth of the spectral line width Δλ 2   ma . On the other hand, the center wavelength of the multiline is still λ 2   mc   0  as in  FIG. 48 , and the lines of the wavelengths λ 2 ( 1 ) a  to λ 2 ( 5 ) a  have the same light intensity I 2   s   0  as in  FIG. 48 . 
     5. Embodiment 2 
     5.1 Configuration 
     A laser system according to Embodiment 2 may have the same configuration as in Embodiment 1 in  FIG. 20 . 
     5.2 Operation 
     One of the first multiple semiconductor laser system  160  and the second multiple semiconductor laser system  260  may perform control to fix the center wavelength and the spectral line width, and the other may perform variable control of the center wavelength and the spectral line width. 
     For example, in the first multiple semiconductor laser system  160 , the center wavelength of the multiline may be fixed at 1030 nm, and the spectral line width may be also fixed at a spectral line width at which SBS is suppressed in the first fiber amplifier  140 . In this case, the center wavelength of the laser beam output from the first solid-state laser device  100  is fixed, and thus there is no need to change an incident angle on the LBO crystal  310  and the first CLBO crystal  312  for generating fourth harmonic light. 
     5.2.1 Control Example 3 of First Multiple Semiconductor Laser System 
       FIG. 50  is a block diagram of Control example 3 of the first multiple semiconductor laser system  160 . The solid-state laser system control unit  350  transmits, to the first multiline control unit  168 , data of a target spectral line width Δλ 1   mt  (=Δλ 1   m   0 ), a target center wavelength λ 1   mct  (=λ 1   mc   0 ), and target light intensity I 1   st  (=I 1   s   0 ) of the multiline. The first multiline control unit  168  controls current values A 1 ( 1 ) to A 1 ( 5 ) and temperatures T 1 ( 1 ) to T 1 ( 5 ) of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). 
       FIG. 51  shows an example of a multiline spectrum detected by the first spectrum monitor  166 . Here, an example of multiline is shown obtained when the target center wavelength λ 1   mct  is λ 1   mc   0  and the target spectral line width Δλ 1   mt  is Δλ 1   m   0 . 
     In  FIG. 51 , wavelengths of the multiline are λ 1 ( 1 ) to λ 1 ( 5 ) and a center wavelength is λ 1   mc   0 . A wavelength interval Δλ 1   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 1   m   0 . Further, the lines of the wavelengths λ 1 ( 1 ) to λ 1 ( 5 ) have the same light intensity I 1   s   0 . 
     5.2.2 Control Example 3 of Second Multiple Semiconductor Laser System 
       FIG. 52  is a block diagram of Control example 3 of the second multiple semiconductor laser system. In the second multiple semiconductor laser system  260 , the center wavelength of the multiline may be variable and the spectral line width may be also variable. 
     The solid-state laser system control unit  350  transmits, to the second multiline control unit  268 , a target spectral line width Δλ 2   mt , a target center wavelength λ 2   mct , and target light intensity I 2   s   0  of the multiline. The second multiline control unit  268  controls current values A 2 ( 1 ) to A 2 ( 5 ) and temperatures T 2 ( 1 ) to T 2 ( 5 ) of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ). 
       FIG. 53  shows an example of a multiline spectrum detected by the second spectrum monitor  266  in Control example 3 in  FIG. 52 . Here, an example of multiline is shown obtained when the target center wavelength λ 2   mct  is λ 2   mc  and the target spectral line width Δλ 2   mt  is Δλ 2   m . In  FIG. 53 , wavelengths of the multiline are λ 2 ( 1 ) to λ 2 ( 5 ) and a center wavelength is λ 2   mc . A wavelength interval Δλ 2   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 2   m . Further, the lines of the wavelengths λ 2 ( 1 ) to λ 2 ( 5 ) have the same light intensity I 2   s   0 . 
       FIG. 54  shows an example of a multiline spectrum when control to change the center wavelength and the spectral line width of the multiline is performed in a spectral shape in  FIG. 53 . In  FIG. 54 , as compared to  FIG. 53 , the target center wavelength λ 2   mct  of the multiline is changed to λ 2   mca . Further, the target spectral line width Δλ 2   mt  is changed to Δλ 2   ma.    
     Thus, the wavelengths of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ) are changed to λ 2 ( 1 ) a  to λ 2 ( 5 ) a . In  FIG. 54 , the wavelength interval Δλ 2   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 2   ma . The lines of the wavelengths λ 2 ( 1 ) a  to λ 2 ( 5 ) a  of the multiline have the same light intensity I 2   s   0  as in  FIG. 53 . 
     5.3 Effect 
     Embodiment 2 provides the same effects as Embodiment 1. Also, the first multiple semiconductor laser system  160  does not need to perform the variable control of the center wavelength and the spectral line width, thereby simplifying the wavelength conversion control by the wavelength conversion system  300 . 
     The second multiple semiconductor laser system  260  in Embodiment 2 is an example of “first multiple semiconductor laser system” in the present disclosure. The second solid-state laser device  200  in Embodiment 2 is an example of “first solid-state laser device” in the present disclosure. 
     5.4 Variant 
     As a variant of Embodiment 2, in the second multiple semiconductor laser system  260 , for example, the center wavelength of the multiline may be fixed at 1554 nm and the spectral line width may be also fixed at a spectral line width at which SBS is suppressed in the second fiber amplifier  240 . In the first multiple semiconductor laser system  160 , the center wavelength of the multiline may be variable and the spectral line width may be variable. 
     In a configuration with such a combination, for significantly changing the center wavelength of the first multiple semiconductor laser system  160 , it is necessary to rotate the second CLBO crystal  316  and the third CLBO crystal  320  in the wavelength conversion system  300  and also the LBO crystal  310  and the first CLBO crystal  312  for generating fourth harmonic light to suppress a reduction in wavelength conversion efficiency. 
     5.4.1 Control Example 4 of First Multiple Semiconductor Laser System 
       FIG. 55  is a block diagram of Control example 4 of the first multiple semiconductor laser system  160 . The solid-state laser system control unit  350  transmits, to the first multiline control unit  168 , data of the target spectral line width Δλ 1   mt , the target center wavelength λ 1   mct , and the target light intensity list (=I 1   s   0 ) of the multiline. The first multiline control unit  168  controls the current values A 1 ( 1 ) to A 1 ( 5 ) and the temperatures T 1 ( 1 ) to T 1 ( 5 ) of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). 
       FIG. 56  shows an example of a multiline spectrum detected by the first spectrum monitor  166  in Control example 4 in  FIG. 56 . Here, an example of multiline is shown obtained when the target center wavelength λ 1   mct  is λ 1   mc  and the target spectral line width Δλ 1   mt  is Δλ 1   m . In  FIG. 56 , the wavelength interval Δλ 1   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 1   m . Further, the lines of the wavelengths λ 1 ( 1 ) to λ 1 ( 5 ) have the same light intensity I 1   s   0 . 
       FIG. 57  shows an example of a multiline spectrum when control to change the center wavelength and the spectral line width of the multiline is performed in a spectral shape in  FIG. 56 . In  FIG. 57 , as compared to  FIG. 56 , the target center wavelength λ 1   mct  of the multiline is changed to λ 1   mca . Further, the target spectral line width Δλ 1   mt  is changed to Δλ 1   ma.    
     Thus, the wavelengths of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) are changed to λ 1 ( 1 ) a  to λ 1 ( 5 ) a . In  FIG. 57 , the wavelength interval Δλ 1   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 1   ma . The lines of the wavelengths λ 1 ( 1 ) a  to λ 1 ( 5 ) a  of the multiline have the same light intensity I 1 s 0  as in  FIG. 56 . 
     5.4.2 Control Example 4 of Second Multiple Semiconductor Laser System 
       FIG. 58  is a block diagram of Control example 4 of the second multiple semiconductor laser system. In the second multiple semiconductor laser system  260 , the center wavelength and the spectral line width of the multiline may be fixed. 
     The solid-state laser system control unit  350  transmits, to the second multiline control unit  268 , data of the target spectral line width Δλ 2   mt  (=Δλ 2   m   0 ), the target center wavelength λ 2   mct  (=λ 2   mc   0 ), and the target light intensity I 2   st  (=I 2   s   0 ) of the multiline. The second multiline control unit  268  controls the current values A 2 ( 1 ) to A 2 ( 5 ) and the temperatures T 2 ( 1 ) to T 2 ( 5 ) of the semiconductor lasers DFB 2 ( 1 ) to DFB 2 ( 5 ). 
       FIG. 59  shows an example of a multiline spectrum detected by the second spectrum monitor  266  in Control example 4 in  FIG. 58 . Here, an example of multiline is shown obtained when the target center wavelength λ 2   mct  is λ 2   mc   0  and the target spectral line width Δλ 2   mt  is Δλ 2   m   0 . 
     In  FIG. 59 , the wavelengths of the multiline are λ 2 ( 1 ) to λ 2 ( 5 ) and the center wavelength is λ 2   mc   0 . The wavelength interval Δλ 2   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 2   m   0 . Further, the lines of the wavelengths λ 2 ( 1 ) to λ 2 ( 5 ) have the same light intensity I 2   s   0 . 
     6. Variant 1 of Multiple Semiconductor Laser System 
     6.1 Configuration 
       FIG. 60  is a block diagram of Variant 1 of the multiple semiconductor laser system.  FIG. 60  shows an example of the first multiple semiconductor laser system  160 , but the second multiple semiconductor laser system  260  may have the same configuration as in  FIG. 60 . 
     Differences between the configurations in  FIG. 60  and  FIG. 25  will be described. In the first multiple semiconductor laser system  160  in  FIG. 60 , a semiconductor optical amplifier  162  is arranged in an optical path of a laser beam between each semiconductor laser DFB 1 ( k ) and the first beam combiner  163 . A value of current AA 1 ( k ) passed through each semiconductor optical amplifier SOA 1 ( k ) can be controlled to control light intensity of each wavelength λ 1 ( k ) at high speed with high accuracy. 
     6.2 Operation 
     In  FIG. 60 , the solid-state laser system control unit  350  transmits, to the first multiline control unit  168 , data of the target spectral line width Δλ 1   mt , the target center wavelength λ 1   mct , and the target light intensity I 1   st  of the multiline. The first multiline control unit  168  controls the current values A 1 ( 1 ) to A 1 ( 5 ) and the temperatures T 1 ( 1 ) to T 1 ( 5 ) of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ). The first multiline control unit  168  also controls current values AA 1 ( 1 ) to AA 1 ( 5 ) of semiconductor optical amplifiers SOA 1 ( 1 ) to SOA 1 ( 5 ). 
       FIG. 61  shows an example of a multiline spectrum detected by the first spectrum monitor  166  in a control example of the configuration in  FIG. 60 . Here, an example is shown in which control is performed such that the lines of the multiline have the same light intensity. As shown in  FIG. 61 , the current values A 1 ( 1 ) to A 1 ( 5 ) and the temperatures T 1 ( 1 ) to T 1 ( 5 ) of the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) and the current values AA 1 ( 1 ) to AA 1 ( 5 ) of the semiconductor optical amplifiers SOA 1 ( 1 ) to SOA 1 ( 5 ) may be controlled such that the lines of the multiline have the same light intensity. 
       FIG. 62  is a flowchart of an example of a control subroutine of the semiconductor laser DFB 1 ( k ). The flowchart in  FIG. 62  can be applied in place of the flowchart in  FIG. 38 . 
     In step S 401  in  FIG. 62 , the first multiline control unit  168  reads data of a target oscillation wavelength λ 1 ( k )t and the target light intensity list of the semiconductor laser DFB 1 ( k ). 
     In step S 402 , the first multiline control unit  168  uses the first spectrum monitor  166  to measure an oscillation wavelength λ 1 ( k ) of each semiconductor laser DFB 1 ( k ). 
     In step S 403 , the first multiline control unit  168  calculates a difference δλ 1 ( k ) between the oscillation wavelength λ 1 ( k ) measured in step S 402  and the target oscillation wavelength λ 1 ( k )t. 
       δλ1( k )=λ1( k )−λ1( k ) t   (18)
 
     In step S 404 , the first multiline control unit  168  determines whether or not an absolute value of δλ 1 ( k ) is within a predetermined range. The first multiline control unit  168  determines whether or not |δλ 1 ( k )| is equal to or smaller than δλ 1   catr , where δλ 1   catr  is an allowable upper limit value of the predetermined range. 
     When the determination result in step S 404  is Yes, the first multiline control unit  168  goes to step S 405 . 
     In step S 405 , the first multiline control unit  168  controls the current value A 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. In other words, when the absolute value of the difference δλ 1 ( k ) between the oscillation wavelength λ 1 ( k ) and the target oscillation wavelength λ 1 ( k ) is within the predetermined range, the first multiline control unit  168  controls the current value A 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. 
     When the determination result in step S 404  is No, the first multiline control unit  168  goes to step S 406 . 
     In step S 406 , the first multiline control unit  168  controls the temperature T 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. In other words, when the absolute value of the difference δλ 1 ( k ) between the oscillation wavelength λ 1 ( k ) and the target oscillation wavelength λ 1 ( k )t exceeds the predetermined range, the first multiline control unit  168  controls the temperature T 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. 
     After step S 405  or S 406 , the first multiline control unit  168  goes to step S 407 . 
     In step S 407 , the first spectrum monitor  166  measures light intensity I 1   s (k) of the semiconductor laser DFB 1 ( k ). 
     In step S 408 , the first multiline control unit  168  calculates a difference SI 1 ( k ) between the light intensity I 1   s (k) measured in step S 407  and the target light intensity I 1   st.    
       δ I 1 s ( k )= I 1 s ( k )− I 1 st   (19)
 
     In step S 409 , the first multiline control unit  168  controls the current value AA 1 ( k ) of the semiconductor optical amplifier SOA 1 ( k ) such that δI 1   s (k) is brought close to 0. 
     After step S 409 , the first multiline control unit  168  finishes the flowchart in  FIG. 62  and returns to the flowchart in  FIG. 36 . 
     7. Variant 2 of Multiple Semiconductor Laser System 
     7.1 Configuration 
       FIG. 63  is a block diagram of Variant 2 of the multiple semiconductor laser system.  FIG. 63  shows an example of the first multiple semiconductor laser system  160 , but the second multiple semiconductor laser system  260  may have the same configuration as in  FIG. 63 . Differences between the configurations in  FIG. 63  and  FIG. 60  will be described. 
     In  FIG. 63 , the semiconductor optical amplifier SOA 1 ( k ) is arranged in the optical path of the laser beam between each semiconductor laser DFB 1 ( k ) and the first beam combiner  163  as in  FIG. 60 . The first multiple semiconductor laser system  160  in  FIG. 63  can freely control each light intensity of multiline. 
     7.2 Operation 
     In  FIG. 63 , the solid-state laser system control unit  350  transmits, to the first multiline control unit  168 , data of the target spectral line width Δλ 1   mt , the target center wavelength λ 1   mct , and target light intensities I 1   s ( 1 ) t  to I 1   s ( 5 ) t  of respective wavelengths λ 1 ( k ) of the multiline. 
     The first multiline control unit  168  controls a current value AA 1 ( k ) of each semiconductor optical amplifier SOA 1 ( k ) such that the light intensities of the oscillation wavelengths λ 1 ( k ) are the target light intensities I 1   s ( 1 ) t  to I 1   s ( 5 ) t.    
       FIG. 64  shows an example of a multiline spectrum detected by the first spectrum monitor  166  in Variant 2 in  FIG. 63 . As shown in  FIG. 64 , the light intensity of each wavelength λ 1 ( k ) of the multiline can be freely controlled, and thus this example can be also applied to control of a spectral waveform. 
     The second multiple semiconductor laser system  260  may have the same configuration as in  FIG. 63 . 
       FIG. 65  is a flowchart of an example of a control subroutine of the semiconductor laser DFB 1 ( k ). The flowchart in  FIG. 65  can be applied in place of the flowchart in  FIG. 38 . 
     In step S 421  in  FIG. 65 , the first multiline control unit  168  reads data of a target oscillation wavelength λ 1 ( k )t and target light intensity I 1   s (k)t of the semiconductor laser DFB 1 ( k ). The target light intensity I 1   s (k)t may be set to a different value for each target oscillation wavelength λ 1 ( k )t. 
     In step S 422 , the first multiline control unit  168  uses the first spectrum monitor  166  to measure an oscillation wavelength λ 1 ( k ) of each semiconductor laser DFB 1 ( k ). 
     In step S 423 , the first multiline control unit  168  calculates a difference a 1 ( k ) between the measured oscillation wavelength λ 1 ( k ) and the target oscillation wavelength λ 1 ( k )t. 
       δλ1( k )=λ1( k )−λ1( k ) t   (20)
 
     In step S 424 , the first multiline control unit  168  determines whether or not an absolute value of δλ 1 ( k ) is within a predetermined range. The first multiline control unit  168  determines whether or not |δλ 1 ( k )| is equal to or smaller than δλ 1   catr , where δλ 1   catr  is an upper limit value of the predetermined range. 
     When the determination result in step S 424  is Yes, the first multiline control unit  168  goes to step S 425 . 
     In step S 425 , the first multiline control unit  168  controls the current value A 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. In other words, when the absolute value of the difference δλ 1 ( k ) between the oscillation wavelength λ 1 ( k ) and the target oscillation wavelength λ 1 ( k )t is within the predetermined range, the first multiline control unit  168  controls the current value A 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. 
     When the determination result in step S 424  is No, the first multiline control unit  168  goes to step S 426 . 
     In step S 426 , the first multiline control unit  168  controls the temperature T 1 ( k ) of the semiconductor laser DFB 1 ( k ) such that δλ 1 ( k ) is brought close to 0. In other words, when the absolute value of the difference δλ 1 ( k ) between the oscillation wavelength λ 1 ( k ) and the target oscillation wavelength λ 1 ( k )t exceeds the predetermined range, the temperature T 1 ( k ) of the semiconductor laser DFB 1 ( k ) is controlled such that δλ 1 ( k ) is brought close to 0. 
     After step S 425  or S 426 , the first multiline control unit  168  goes to step S 427 . 
     In step S 427 , the first multiline control unit  168  uses the first spectrum monitor  166  to measure light intensity I 1   s (k) of the semiconductor laser DFB 1 ( k ). 
     In step S 428 , the first multiline control unit  168  calculates a difference δI 1 ( k ) between the light intensity I 1   s (k) measured in step S 427  and the target light intensity I 1   s (k)t. 
       δ I 1 s ( k )= I 1 s ( k )− I 1 s ( k ) t   (21)
 
     In step S 429 , the first multiline control unit  168  controls the current value AA 1 ( k ) of the semiconductor optical amplifier SOA 1 ( k ) such that δI 1   s (k) is brought close to 0. 
     After step S 429 , the first multiline control unit  168  finishes the flowchart in  FIG. 62  and returns to the flowchart in  FIG. 36 . 
     8. Specific Example of Spectrum Monitor 
     8.1 Example of Spectrum Monitor Using Spectrometer and Reference Laser Beam Source 
     8.1.1 Configuration 
       FIG. 66  schematically shows an exemplary configuration of the spectrum monitor.  FIG. 66  shows an example of the first spectrum monitor  166 , but the second spectrum monitor  266  may have the same configuration as in  FIG. 66 . 
     The first spectrum monitor  166  in  FIG. 66  may include a spectrometer  702  including a grating  700 , a line sensor  703 , a spectrum analyzer  704 , a CW oscillation reference laser beam source  706 , and a beam splitter  708 . 
     The spectrometer  702  includes an entrance slit  710 , a collimator lens  712 , and a high reflective mirror  714 . The CW oscillation reference laser beam source  706  is a reference light source that outputs a laser beam having a reference wavelength by CW oscillation. Here, the laser beam having a reference wavelength output from the CW oscillation reference laser beam source  706  is referred to as “reference laser beam”. The laser beam output from each semiconductor laser DFB 1 ( k ) is referred to as “semiconductor laser beam”. 
     8.1.2 Operation 
     In  FIG. 66 , part of the laser beam reflected by the first beam splitter  164  passes through the beam splitter  708 . The reference laser beam output from the CW oscillation reference laser beam source  706  is reflected by the beam splitter  708  and overlapped with the multiline laser beam having passed through the beam splitter  708 . 
     The laser beam overlapped with the reference laser beam by the beam splitter  708  enters the spectrometer  702  through the entrance slit  710 . The laser beam having passed through the entrance slit  710  enters the grating  700  via the collimator lens  712  and is dispersed by the grating  700 . A peak position and peak intensity of a slit image of each semiconductor laser beam and the reference laser beam formed on the line sensor  703  via the collimator lens  712  and the high reflective mirror  714  can be measured to measure an absolute wavelength and light intensity of each semiconductor laser. 
     In  FIG. 66 , the example of the spectrometer  702  including the grating  700  is shown, but an etalon spectrometer in  FIG. 71  described later may be used. The CW oscillation reference laser beam source  706  is an example of “first reference laser beam source” in the present disclosure. The spectrometer  702  is an example of “first spectrometer” in the present disclosure. 
     8.2 Example of Spectrum Monitor Using Heterodyne Interferometer 
     8.2.1 Configuration 
       FIG. 67  schematically shows another exemplary configuration of the spectrum monitor.  FIG. 67  shows an example of the first spectrum monitor  166 . As shown in  FIG. 67 , the first spectrum monitor  166  may include a heterodyne interferometer. The first multiple semiconductor laser system  160  in  FIG. 67  includes beam splitters  164 ( 1 ) to  164 ( 5 ) in optical paths of laser beams between the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) and the first beam combiner  163 . 
     The first spectrum monitor  166  includes a CW oscillation reference laser beam source  706 , a plurality of beam splitters  708 ( 1 ) to  708 ( 5 ), a plurality of light intensity sensors  720 ( 1 ) to  720 ( 5 ), and a spectrum analyzer  704 . 
     As shown in  FIG. 67 , the beam splitter  164 ( k ) is arranged in the optical path of the laser beam output from the semiconductor laser DFB 1 ( k ). The beam splitter  708 ( k ) is arranged in the optical path between the beam splitter  164 ( k ) and the light intensity sensor  720 ( k ). The beam splitter  708 ( k ) is arranged such that a reference laser beam output from the CW oscillation reference laser beam source  706  and overlapped with part of the laser beam output from the semiconductor laser DFB 1 ( k ) enters the light intensity sensor  720 ( k ). 
     8.2.2 Operation 
     The first spectrum monitor  166  in  FIG. 67  uses the light intensity sensor  720 ( k ) to measure a change in light intensity of the reference laser beam output from the CW oscillation reference laser beam source  706  and overlapped with part of the laser beam output from each semiconductor laser DFB 1 ( k ). 
     The spectrum analyzer  704  can analyze a beat signal detected by each light intensity sensor  720 ( k ) to measure a frequency difference between the laser beam of each semiconductor laser DFB 1 ( k ) and the reference laser beam and the light intensity. A wavelength difference can be calculated from the frequency difference. 
     In  FIG. 67 , the example is shown in which the beat signals of the reference laser beam and the laser beam of each semiconductor laser are detected, but not limited to this. For example, beat signals of the CW oscillation reference laser beam source and the semiconductor laser DFB 1 ( 1 ) may be detected, and beat signals of the semiconductor lasers DFB 1 ( 1 ) and DFB 1 ( 2 ), beat signals of the semiconductor lasers DFB 1 ( 2 ) and DFB 1 ( 3 ), beat signals of the semiconductor lasers DFB 1 ( 3 ) and DFB 1 ( 4 ), and beat signals of the semiconductor lasers DFB 1 ( 4 ) and DFB 1 ( 5 ) may be detected, thereby detecting a wavelength and light intensity of each semiconductor laser. 
     Not limited to the first spectrum monitor  166 , the second spectrum monitor  266  (see  FIG. 20 ) may have the same configuration as in  FIG. 67 . 
     8.2.3 Example of Beat Signal 
       FIG. 68  illustrates detection of a beat signal using the heterodyne interferometer and calculation of a wavelength and light intensity. The top waveform in  FIG. 68  is a waveform of a signal indicating intensity of the reference laser beam. The horizontal axis represents time and the longitudinal axis represents light intensity. Here, an example is shown in which the reference laser beam has a wavelength of 1553.000 nm. 
     The middle waveform in  FIG. 68  is a waveform of a signal indicating intensity of a laser beam as a beam to be detected output from the semiconductor laser. Here, an example is shown in which the laser beam has a wavelength of 1553.001 nm. 
     The bottom waveform in  FIG. 68  is a waveform of a beat signal caused by interference between the reference laser beam and the beam to be detected (semiconductor laser beam). A frequency difference 1/T0 between the reference laser beam and the beam to be detected can be measured from a beat cycle of the beat signal. Also, light intensity I of the beam to be detected can be measured in accordance with a maximum amplitude value Imax of beat of the beat signal. 
     The light intensity I of the beam to be detected can be calculated by Expression (22) below. 
         I=I max 2 /(2· Is )  (22)
 
     where Is is light intensity of the reference laser beam. 
     As described above, the heterodyne interferometer can detect even a wavelength difference of 0.001 nm with high accuracy. 
     8.2.4 Variant 
     When the heterodyne interferometer is used to measure the wavelength and the light intensity of each semiconductor laser, a beat signal between part of the laser beam after being combined and the reference laser beam may be detected, and the beat signal may be analyzed using a fast Fourier transform (FFT) algorithm to measure the wavelength and the light intensity of each semiconductor laser. 
     9. Example of Excimer Amplifier 
     9.1 Multipath Amplification 
       FIG. 69  schematically shows an exemplary configuration of the excimer amplifier  14 . The excimer amplifier  14  in  FIG. 69  passes three times seed light having a wavelength of 193.4 nm through a discharge space between the discharge electrodes  412 ,  413  for amplification. Here, the seed light having the wavelength of 193.4 nm is the second pulse laser beam LP 2  output from the solid-state laser system  10 . 
     In  FIG. 69 , the excimer amplifier  14  includes a convex mirror  420  and a concave mirror  422  in an optical path of the seed light outside the chamber  410 . The convex mirror  420  and the concave mirror  422  are arranged such that their focal points FP are substantially aligned with each other. 
     The seed light having entered the excimer amplifier  14  is reflected by the convex mirror  420  and the concave mirror  422  to pass three times through the discharge space between the discharge electrodes  412 ,  413 . This expands and amplifies the seed light, which is output toward the exposure apparatus  20 . 
     9.2 Amplification with Ring Resonator 
       FIG. 70  shows an example in which a ring resonator is used as the excimer amplifier  14 . The ring resonator includes an output coupling mirror  430  and high reflective mirrors  431  to  433 . The excimer amplifier  14  may further include a high reflective mirror (not shown) that guides seed light having a wavelength of 193.4 nm to the ring resonator, and a high reflective mirror (not shown) that guides a pulse laser beam output from the ring resonator to the exposure apparatus  20 . 
     The chamber  410  includes the windows  415 ,  416 . The discharge electrodes  412 ,  413  are arranged in the chamber  410 . In  FIG. 70 , the discharge electrodes  412 ,  413  are arranged to face each other in a direction perpendicular to the sheet surface. A discharge direction is perpendicular to the sheet surface. 
     In the excimer amplifier  14 , the seed light repeatedly travels through the output coupling mirror  430 , the high reflective mirror  431 , the discharge space between the discharge electrodes  412 ,  413 , the high reflective mirror  432 , the high reflective mirror  433 , and the discharge space between the discharge electrodes  412 ,  413  in this order, and is amplified. 
     10. Example of Spectrum Monitor Using Etalon Spectrometer 
       FIG. 71  schematically shows an exemplary configuration of a spectrum monitor using an etalon spectrometer. An etalon spectrometer  606 A in  FIG. 71  can be applied to the spectrum monitor  606  (see  FIG. 20 ) that measures a spectrum of an excimer laser beam. The etalon spectrometer  606 A is an example of “spectrometer” in the present disclosure. 
     As shown in  FIG. 71 , the etalon spectrometer  606 A includes a diffusing element  610 , an etalon  612 , a light condensing lens  614 , and an image sensor  616 . The image sensor  616  may be, for example, one-dimensional or two-dimensional photodiode array. 
     The laser beam first enters the diffusing element  610 . The diffusing element  610  may be a transmissive optical element having many irregularities on its surface. The diffusing element  610  transmits the laser beam having entered the diffusing element  610  as scattered light. The scattered light enters the etalon  612 . The etalon  612  may be an air gap etalon including two partially reflective mirrors having predetermined reflectance. In the air gap etalon, the two partially reflective mirrors face each other with an air gap of a predetermined distance therebetween and are bonded together via a spacer. 
     Depending on an incident angle θ of the light having entered the etalon  612 , an optical path difference differs between light passing through the etalon  612  without reciprocating between the two partially reflective mirrors and light reciprocating once between the two partially reflective mirrors and then passing through the etalon  612 . When the optical path difference is an integer multiple of the wavelength, the light having entered the etalon  612  passes through the etalon  612  with high transmittance. 
     The light having passed through the etalon  612  enters the light condensing lens  614 . The laser beam having passed through the light condensing lens  614  enters the image sensor  616  arranged in a position corresponding to a focal length f of the light condensing lens  614 . Specifically, the transmitted light condensed by the light condensing lens  614  forms an interference pattern on a focal plane of the light condensing lens  614 . 
     The image sensor  616  is arranged on the focal plane of the light condensing lens  614 . The image sensor  616  receives the light having passed through the light condensing lens  614  and detects the interference pattern. The square of the radius of the interference pattern may be proportional to the wavelength of the laser beam. Thus, a spectral line width (spectrum profile) and a center wavelength of the entire laser beam are detected from the detected interference pattern. 
     The spectral line width and the center wavelength may be calculated from the detected interference pattern by an information processing device (not shown) or calculated by the laser control unit  18 . 
     A relationship between the radius r of the interference pattern and the wavelength λ is approximated by Expression (23) below. 
       Wavelength λ=λ c+α·r   2   (23)
 
     α: constant of proportionality 
     r: radius of interference pattern 
     λc: wavelength when light intensity at middle of interference pattern is maximum 
     From Expression (23), the spectral line width Δλ may be calculated after conversion into a spectral waveform representing the relationship between the light intensity and the wavelength as shown in  FIG. 72 . The spectral line width Δλ may be a width (E95) containing 95% of total energy. 
     11. Example of Beam Combiner 
     11.1 Beam Combiner Including Optical Fiber 
       FIG. 73  schematically shows an example of a beam combiner including an optical fiber. Here, the first beam combiner  163  is exemplified, but the second beam combiner  263  may have the same configuration as in  FIG. 73 . As the first beam combiner  163 , a beam combiner including a plurality of optical fibers  630  may be arranged. The optical fibers  630  that transmit the laser beams output from the semiconductor lasers DFB 1 ( 1 ) to DFB 1 ( 5 ) are connected by fusion. 
     11.2 Beam Combiner Including Half Mirror and High Reflective Mirror 
       FIG. 74  schematically shows an example of a beam combiner including a half mirror and a high reflective mirror. Here, the first beam combiner  163  is exemplified, but the second beam combiner  263  may have the same configuration as in  FIG. 74 . 
     The first beam combiner  163  in  FIG. 74  includes a combination of a plurality of half mirrors  641 ,  642 ,  643 ,  644  and a plurality of high reflective mirrors  651 ,  652 ,  653 ,  654 . The mirrors are arranged as shown. Part or all of the mirrors may be formed on a silicon chip. 
     12. Another Example of Single Longitudinal Mode Semiconductor Laser 
     12.1 Configuration 
       FIG. 75  schematically shows an example of a multiple semiconductor laser system using a single longitudinal mode external cavity (EC) semiconductor laser. Here, the first multiple semiconductor laser system  160  is exemplified, but the second multiple semiconductor laser system  260  may have the same configuration as in  FIG. 75 . As a semiconductor laser that oscillates in a single longitudinal mode, the external cavity semiconductor laser may be used, not limited to the DFB laser. 
     In  FIG. 75 , a plurality of external cavity semiconductor lasers  800  included in the first multiple semiconductor laser system  160  are denoted as EC 1 ( 1 ) to EC 1 ( 5 ). The external cavity semiconductor lasers EC 1 ( 1 ) to EC 1 ( 5 ) are set to perform CW oscillation with different wavelengths near a wavelength of about 1030 nm. The external cavity semiconductor lasers EC 1 ( 1 ) to EC 1 ( 5 ) are examples of “a plurality of first semiconductor lasers” in the present disclosure.  FIG. 75  shows a configuration of the external cavity semiconductor laser EC 1 ( 1 ), but the other external cavity semiconductor lasers EC 1 ( 2 ) to EC 1 ( 5 ) each have the same configuration. 
     The external cavity semiconductor laser EC 1 ( k ) includes a semiconductor laser control unit  810 , a semiconductor laser element  820 , the Peltier element  50 , the temperature sensor  52 , the current control unit  54 , the temperature control unit  56 , a collimator lens  830 , a grating  831 , and a grating holder  832 . The external cavity semiconductor laser EC 1 ( k ) further includes a rotation stage  833 , a piezoelectric element  834 , a micrometer  835 , a pin  836 , a reaction spring  837 , a piezoelectric power source  838 , and a steering mirror  840 . 
     The semiconductor laser element  820  includes a layer structure including a first semiconductor layer  821 , an active layer  822 , and a second semiconductor layer  823 . The Peltier element  50  and the temperature sensor  52  are fixed to the semiconductor laser element  820 . The semiconductor laser control unit  810  includes a signal line for receiving, from the first multiline control unit  168 , data of a difference δλ 1 ( k ) between a target oscillation wavelength λ 1 ( k )t and an oscillation wavelength λ 1 ( k ). 
     The piezoelectric power source  838  includes a signal line for receiving, from the semiconductor laser control unit  810 , data of a value of voltage V 1  applied to the piezoelectric element  834 . The current control unit  54  includes a signal line for receiving, from the semiconductor laser control unit  810 , data of a current value A 1 . The temperature control unit  56  includes a signal line for receiving, from the semiconductor laser control unit  810 , data of a set temperature T 1 . 
     The grating  831  is arranged on an output side of the semiconductor laser element  820  via the collimator lens  830  in a Littrow configuration in which a diffraction angle of first order diffracted light coincides with an incident angle. The grating  831  is fixed to the rotation stage  833  via the grating holder  832  such that an incident angle on the grating  831  changes. 
     The steering mirror  840  is arranged via a holder (not shown) such that its mirror surface is substantially parallel to a diffraction plane of the grating  831 . 
     12.2 Operation 
     When receiving the data of δλ 1 ( k ), the semiconductor laser control unit  810  controls the set temperature T 1 , the current value A 1 , and the incident angle on the grating  831  so as to suppress mode hopping, and thus can control an oscillation wavelength in a single longitudinal mode such that δλ 1 ( k ) is brought close to 0. 
     The semiconductor laser control unit  810  previously controls a rotation angle of the rotation stage  833  and a temperature of the semiconductor laser element  820  so as to cause laser oscillation in a fine wavelength region. The semiconductor laser control unit  810  previously stores, as table data, a relationship between δλ 1 ( k ), the value of current A 1  passed through the semiconductor laser element  820 , and the value of voltage V 1  applied to the piezoelectric element  834  such that mode hopping does not occur. 
     When receiving the data of δλ 1 ( k ) from the first multiline control unit  168 , the semiconductor laser control unit  810  calculates, from the table data, the value of current A 1  passed through the semiconductor laser element  820 , and the value of voltage V 1  applied to the piezoelectric element  834 . 
     The semiconductor laser control unit  810  transmits, to the current control unit  54 , data of the value of current A 1  passed through the semiconductor laser element  820 . The semiconductor laser control unit  810  also transmits, to the piezoelectric power source  838 , data of the value of voltage V 1  applied to the piezoelectric element  834  that controls a rotation angle of the grating  831 . 
     The piezoelectric element  834  changes the incident angle on the grating  831 , and the current passed through the semiconductor laser element  820  changes a refractive index of the active layer  822  of the semiconductor laser element  820 . As a result, the oscillation wavelength of the semiconductor laser is brought close to the target oscillation wavelength λ 1 ( k )t at high speed with mode hopping being suppressed. Then, zeroth order light of the grating  831  is output, and the steering mirror  840  outputs a CW laser beam to the outside. 
     12.3 Others 
     Examples of other semiconductor lasers that oscillate in a single longitudinal mode include a distributed Bragg reflector (DBR) laser, a volume holographic grating (VHG) laser, a discrete mode (DM) laser, and the like. 
     13. Example of CW Oscillation Reference Laser Beam Source 
     13.1 CW Oscillation Reference Laser Beam Source of wavelength region of 1030 nm 
       FIG. 76  is a block diagram of an example of a CW oscillation reference laser beam source. A CW oscillation reference laser beam source  750  includes a first reference semiconductor laser  751 , a beam splitter  754 , a high reflective mirror  755 , a nonlinear crystal  756 , an iodine absorption cell  757 , a first light intensity sensor  758 , and a first reference laser control unit  761 . 
     The first reference semiconductor laser  751  performs CW oscillation of a laser beam in a wavelength region of 1030 nm. The laser beam reflected by the beam splitter  754  enters the nonlinear crystal  756  via the high reflective mirror  755 . The nonlinear crystal  756  generates second harmonic light to obtain a laser beam having a wavelength of about 515 nm. The laser beam having the wavelength of about 515 nm enters the iodine absorption cell  757 . 
     The iodine absorption cell  757  contains iodine gas. A specific absorption line of iodine in the iodine absorption cell  757  includes, for example, an absorption line of 514.581 nm. The laser beam having passed through the iodine absorption cell  757  enters the first light intensity sensor  758 . 
     The first reference laser control unit  761  controls an oscillation wavelength of the first reference semiconductor laser  751  in accordance with a detected signal from the first light intensity sensor  758  such that the absorption line of the iodine absorption cell  757  coincides with the wavelength of the second harmonic light. 
     The CW oscillation reference laser beam source  750  can be applied as the CW oscillation reference laser beam source  706  of the first spectrum monitor  166  shown in  FIGS. 66 and 67 . 
     The iodine absorption cell  757  is an example of “first absorption cell” in the present disclosure. The absorption line of iodine is an example of “first absorption line” in the present disclosure. The CW oscillation reference laser beam source  750  is an example of “first reference laser beam source” in the present disclosure. 
     13.2 CW Oscillation Reference Laser Beam Source of Wavelength Region of 1554 nm 
       FIG. 77  is a block diagram of another example of the CW oscillation reference laser beam source. A CW oscillation reference laser beam source  770  includes a second reference semiconductor laser  772 , a beam splitter  774 , a high reflective mirror  775 , a hydrogen cyanide isotope absorption cell  777 , a second light intensity sensor  778 , and a second reference laser control unit  782 . 
     The second reference semiconductor laser  772  performs CW oscillation of a laser beam in a wavelength region of 1554 nm. The laser beam reflected by the beam splitter  774  enters the hydrogen cyanide isotope absorption cell  777  via the high reflective mirror  775 . 
     The absorption cell  777  contains isotope hydrogen cyanide gas. A specific absorption line of the hydrogen cyanide isotope includes, for example, an absorption line of 1553.756 nm. The absorption cell  777  is an example of “second absorption cell” in the present disclosure. The absorption line of the hydrogen cyanide isotope is an example of “second absorption line” in the present disclosure. 
     An acetylene isotope absorption cell may be used as the absorption cell of this wavelength region. Specifically, an absorption cell containing isotope acetylene gas may be used in place of the hydrogen cyanide isotope absorption cell  777 . 
     The laser beam having passed through the hydrogen cyanide isotope absorption cell  777  enters the second light intensity sensor  778 . 
     The second reference laser control unit  782  controls an oscillation wavelength of the second reference semiconductor laser  772  in accordance with a detected signal from the second light intensity sensor  778  such that the absorption line of the hydrogen cyanide isotope absorption cell  777  coincides with the wavelength of the laser beam of the second reference semiconductor laser  772 . 
     The CW oscillation reference laser beam source  770  can be applied as the CW oscillation reference laser beam source of the second spectrum monitor  266 . The CW oscillation reference laser beam source  770  is an example of “second reference laser beam source” in the present disclosure. 
     14. Example of Multi-Longitudinal Mode CW Oscillation Semiconductor Laser 
     The multiple semiconductor laser system including a plurality of single longitudinal mode CW oscillation semiconductor lasers has been described, but a multi-longitudinal mode CW oscillation semiconductor laser may be used as a semiconductor laser for outputting a spectrum at which SBS is suppressed. 
     The multi-longitudinal mode CW oscillation semiconductor laser can be used in place of either the first multiple semiconductor laser system  160  or the second multiple semiconductor laser system  260 . For example, the multi-longitudinal mode CW oscillation semiconductor laser may be used in place of the first multiple semiconductor laser system  160  in Embodiment 2. 
       FIG. 78  schematically shows an example of a multi-longitudinal mode CW oscillation semiconductor laser.  FIG. 78  shows an example of a semiconductor laser including a chirped grating. In a semiconductor laser  870  in  FIG. 78 , an optical fiber  872  includes gratings having refractive index distribution with different pitches corresponding to wavelengths λ 1  to λ 5 , and is connected to a rear side of a semiconductor laser element  860 . Specifically, the optical fiber  872  includes a plurality of gratings with pitches corresponding to a plurality of oscillation wavelengths. Each grating has a high refractive index part and a low refractive index part periodically formed. 
     The semiconductor laser element  860  includes a layer structure including a first cladding layer  861 , an active layer  862 , and a second cladding layer  863 , and a light output side is coated with a partially reflective film  866 . 
     When current is passed through the semiconductor laser element  860  via an electrode (not shown), a laser beam is output having a plurality of wavelengths corresponding to the pitches of the refractive index distribution formed in the optical fiber  872 . 
       FIG. 79  shows an example of a spectrum of the laser beam output from the semiconductor laser  870  in  FIG. 78 . As shown in  FIG. 79 , multiline output is obtained from the semiconductor laser  870 . 
     15. SBS Suppression by Chirping 
     In a semiconductor laser system using a single longitudinal mode DFB laser, chirping may be caused by passing current of a sum of a DC component and a high frequency AC component through the single longitudinal mode DFB laser, thereby suppressing SBS. The configuration of the single longitudinal mode DFB laser may be that shown in  FIG. 16 . In this case, the semiconductor laser control unit  34  provides an instruction for current modulation to the current control unit  54 . 
       FIG. 80  shows an example of a waveform of a value of current passed through the DFB laser.  FIG. 81  is a graph showing a wavelength change of a laser beam output from the DFB laser by modulation current. 
     A relationship between a cycle Ta of the AC component of the current passed through the DFB laser and an amplification pulse width D of the semiconductor optical amplifier is preferably as expressed below. 
         D=m·Ta   (24)
 
     where m is an integer of one or more. 
     As shown in  FIG. 81 , the current is controlled to obtain a wavelength change width at which SBS is suppressed. 
     Also, chirping may be caused by passing pulse current through the single longitudinal mode DFB laser, thereby suppressing SBS. Wavelength chirping is caused by high-speed current modulation of the semiconductor laser element  860 . 
     16. Example of Semiconductor Optical Amplifier 
     16.1 Configuration 
       FIG. 82  schematically shows an exemplary configuration of a semiconductor optical amplifier. Here, the first semiconductor optical amplifier  120  is exemplified, but other semiconductor optical amplifiers such as the second semiconductor optical amplifier  220  may have the same configuration as in  FIG. 82 . 
     The first semiconductor optical amplifier  120  includes a semiconductor element  500  and a current control unit  520 . The semiconductor element  500  includes a P-type semiconductor element  501 , an active layer  502 , an N-type semiconductor element  503 , a first electrode  511 , and a second electrode  512 . The current control unit  520  is connected to the first electrode  511  and the second electrode  512 . 
     16.2 Operation 
     When current is passed from the first electrode  511  to the second electrode  512 , the active layer  502  is excited. When seed light enters the excited active layer  502  and passes through the active layer  502 , the seed light is amplified. 
     Pulse current is passed with the CW seed light entering the active layer  502 , and thus the seed light having passed through the active layer  502  is output as a pulse laser beam. 
     As a result, for example, the current control unit  520  controls a value of current passing through the semiconductor element  500  in accordance with a control signal from an external control unit  540 , and thus the seed light is amplified to light intensity of a laser beam according to the current value. 
     Pulse current is passed through the first semiconductor optical amplifier  120  and the second semiconductor optical amplifier  220  in  FIG. 20 , and thus CW seed light is amplified in a pulse shape. 
     Also, as in the case of SOA 1 ( k ) in  FIGS. 60 and 63 , DC current may be controlled to amplify seed light. 
     17. Embodiment 3 
     17.1 Configuration 
       FIG. 83  schematically shows an example of a laser system according to Embodiment 3. Here, only a solid-state laser system  910  is shown. The solid-state laser system  910  in  FIG. 82  may be applied in place of the solid-state laser system  10  in Embodiments 1 and 2 described with reference to  FIG. 20 . 
     The solid-state laser system  910  includes a third solid-state laser device  920 , a second wavelength conversion system  302 , the first pulse energy monitor  330 , the synchronization circuit unit  340 , and the solid-state laser system control unit  350 . In the solid-state laser system  910 , the third solid-state laser device  920  outputs a pulse laser beam LP 31  having a wavelength of about 1547.2 nm, and the second wavelength conversion system  302  wavelength-converts the pulse laser beam LP 31  into eighth harmonic light to obtain a pulse laser beam having a wavelength of about 193.4 nm. 
     The third solid-state laser device  920  has the same configuration as the second solid-state laser device  200  in  FIG. 20 , and a center wavelength of multiline is about 1547.2 nm. The center wavelength of the multiline of the third solid-state laser device  920  may be changed within a range of 1544 nm to 1548 nm. 
     The third solid-state laser device  920  includes a third multiple semiconductor laser system  930 , a third semiconductor optical amplifier  950 , a dichroic mirror  960 , a fourth pulse excitation light source  962 , and a third fiber amplifier  970 . 
     The third multiple semiconductor laser system  930  includes a plurality of semiconductor lasers  931 , a third beam combiner  933 , a third beam splitter  934 , a third spectrum monitor  936 , and a third multiline control unit  938 . 
     The semiconductor lasers  931  are distributed feedback semiconductor lasers that perform CW oscillation in a single longitudinal mode, and five semiconductor lasers  931  are shown herein. In  FIG. 83 , the semiconductor lasers  931  included in the third multiple semiconductor laser system  930  are denoted as DFB 3 ( 1 ) to DFB 3 ( 5 ). The DFB 3 ( 1 ) to DFB 3 ( 5 ) are set to oscillate near a wavelength of about 1554 nm. 
     The third fiber amplifier  970  is an Er fiber amplifier. 
     The second wavelength conversion system  302  wavelength-converts, using a nonlinear crystal, fundamental wave light having a wavelength of about 1547.2 nm output from the third solid-state laser device  920  into eighth harmonic light to generate ultraviolet light having a wavelength of about 193.4 nm. 
     As shown in  FIG. 83 , the second wavelength conversion system  302  includes a first LBO crystal  1301 , a second LBO crystal  1302 , a third LBO crystal  1303 , a fourth CLBO crystal  1304 , a fifth CLBO crystal  1305 , dichroic mirrors  1311 ,  1312 ,  1313 ,  1314 ,  1315 , high reflective mirrors  1321 ,  1322 ,  1323 , and a beam splitter  1328 . 
     17.2 Operation 
     Operation of the third solid-state laser device  920  in  FIG. 83  is the same as that of the second solid-state laser device  200  described with reference to  FIG. 20 . Operation of the third multiple semiconductor laser system  930  may be, for example, the same as that of the second multiple semiconductor laser system  260  described in the second embodiment. 
     In the second wavelength conversion system  302 , the first LBO crystal  1301  converts the pulse laser beam LP 31  (wavelength of about 1547.2 nm) output from the third solid-state laser device  920  into second harmonic light (wavelength of about 773.6 nm). 
     The second LBO crystal  1302  generates third harmonic light (wavelength of about 515.78 nm) that is a sum frequency of the second harmonic light (wavelength of about 776.7 nm) and the fundamental wave light (wavelength of about 1547.2 nm). The third harmonic light is split by the dichroic mirror  1311 , and one enters the third LBO crystal  1303  and the other enters the fourth CLBO crystal  1304  via the high reflective mirror  1322  and the dichroic mirror  1313 . 
     The third LBO crystal  1303  performs wavelength conversion into fourth harmonic light (wavelength of about 386.8 nm). The fourth harmonic light output from the third LBO crystal  1303  enters the fourth CLBO crystal  1304  and the fifth CLBO crystal  1305  via the dichroic mirror  1312 . 
     The fourth CLBO crystal  1304  performs wavelength conversion into seventh harmonic light (wavelength of about 221.01 nm) that is a sum frequency of the fourth harmonic light (wavelength of about 386.8 nm) and the third harmonic light (wavelength of about 515.78 nm). 
     The fifth CLBO crystal  1305  performs wavelength conversion into eighth harmonic light (wavelength of about 193.4 nm) that is a sum frequency of the seventh harmonic light (wavelength of about 221.01 nm) and the fundamental wave light (wavelength of about 1547.2 nm). 
     The operation of the second wavelength conversion system  302  will be further described in detail. Second harmonic generation when the fundamental wave light having the wavelength of about 1547.2 nm (frequency ( 0 ) output from the third solid-state laser device  920  passes through the first LBO crystal  1301  leads to generation of second harmonic light having a frequency  2   w  (wavelength of about 773.6 nm). Non-critical phase matching (NCPM), which is a method of adjusting a temperature of an LBO crystal, is used for phase matching for wavelength-converting the fundamental wave light into the second harmonic light. 
     The fundamental wave light having passed through the first LBO crystal  1301  and the second harmonic light generated by the wavelength conversion with the first LBO crystal  1301  enter the second LBO crystal  1302 . The second LBO crystal  1302  uses NCPM at a temperature different from that of the first LBO crystal  1301 . 
     The second LBO crystal  1302  generates the sum frequency of the fundamental wave light and the second harmonic light to generate the third harmonic light (wavelength of about 515.73 nm). 
     The third harmonic light obtained by the second LBO crystal  1302  and the fundamental wave light and the second harmonic light having passed through the second LBO crystal  1302  are split by the dichroic mirror  1311 . The third harmonic light (wavelength of about 515.73 nm) reflected by the dichroic mirror  1311  enters the fourth CLBO crystal  1304  via the high reflective mirror  1322  and the dichroic mirror  1313 . 
     The fundamental wave light and the second harmonic light having passed through the dichroic mirror  1311  enter the third LBO crystal  1303 . The fundamental wave light passes through the third LBO crystal  1303  without being wavelength-converted, while the second harmonic light is converted into the fourth harmonic light (wavelength of about 386.8 nm) in the third LBO crystal  1303  because of second harmonic generation. The fourth harmonic light obtained by the third LBO crystal  1303  and the fundamental wave light having passed through the third LBO crystal  1303  are split by the dichroic mirror  1312 . 
     The fourth harmonic light reflected by the dichroic mirror  1312  is coaxially combined with the third harmonic light by the dichroic mirror  1313  and enters the fourth CLBO crystal  1304 . 
     The fundamental wave light having passed through the dichroic mirror  1312  is reflected by the high reflective mirror  1321  and enters the fifth CLBO crystal  1305  via the dichroic mirror  1314 . 
     The fourth CLBO crystal  1304  generates the sum frequency of the third harmonic light and the fourth harmonic light to obtain the seventh harmonic light (wavelength of about 221.02 nm). The seventh harmonic light obtained by the fourth CLBO crystal  1304  is coaxially combined with the fundamental wave light by the dichroic mirror  1314  and enters the fifth CLBO crystal  1305 . 
     The fifth CLBO crystal  1305  generates the sum frequency of the fundamental wave light and the seventh harmonic light to obtain the eighth harmonic light (wavelength of about 193.4 nm). 
     The eighth harmonic light obtained by the fifth CLBO crystal  1305  and the fundamental wave light and the seventh harmonic light having passed through the fifth CLBO crystal  1305  are split by the dichroic mirror  1315 . 
     The eighth harmonic light (wavelength of about 193.4 nm) reflected by the dichroic mirror  1315  is output from the second wavelength conversion system  302  via the high reflective mirror  1323  and the beam splitter  1328 . In this manner, the eighth harmonic light output from the second wavelength conversion system  302  is input to the excimer amplifier  14  via the first high reflective mirror  11  and the second high reflective mirror  12  in  FIG. 20 . 
     17.3 Control Example of Third Multiple Semiconductor Laser System 
       FIG. 84  is a block diagram of a control example of the third multiple semiconductor laser system  930 . In the third multiple semiconductor laser system  930 , the center wavelength of the multiline may be variable and the spectral line width may be also variable. A multiline spectrum obtained by the third multiple semiconductor laser system  930  is referred to as “third multiline”. Here, an example is shown in which control is performed to change a target center wavelength λ 3   mct  and a target spectral line width Δλ 3   mt  of the third multiline. 
     The solid-state laser system control unit  350  transmits, to the third multiline control unit  938 , data of the target spectral line width Δλ 3   mt , the target center wavelength λ 3   mct , and target light intensity list (=I 1   s   0 ) of the third multiline. The third multiline control unit  938  controls current values A 3 ( 1 ) to A 3 ( 5 ) and temperatures T 3 ( 1 ) to T 3 ( 5 ) of semiconductor lasers DFB 3 ( 1 ) to DFB 3 ( 5 ). Wavelengths of laser beams output from the semiconductor lasers DFB 3 ( 1 ) to DFB 3 ( 5 ) are denoted as λ 3 ( 1 ) to λ 3 ( 5 ). The plurality of laser beams having different wavelengths are combined by the third beam combiner  933 . 
     The multiline laser beam output from the third beam combiner  933  enters the third beam splitter  934 . The laser beam having passed through the third beam splitter  934  enters the third semiconductor optical amplifier  950 . The laser beam reflected by the third beam splitter  934  enters the third spectrum monitor  936 . 
       FIG. 85  shows an example of a multiline spectrum detected by the third spectrum monitor  936  in the control example in  FIG. 84 . Here, an example is shown in which the target center wavelength λ 3   mct  of the multiline is set to λ 3   mc  and the target spectral line width Δλ 3   mt  is set to Δλ 3   m . A wavelength interval Δλ 3   p  of the multiline is generally constant and one fourth of the spectral line width Δλ 3   m.    
       FIG. 86  shows an example of a multiline spectrum when control to change the center wavelength and the spectral line width of the multiline is performed in a spectral shape in  FIG. 85 . In  FIG. 86 , as compared to  FIG. 85 , the target center wavelength λ 3   mct  is changed to λ 3   mca  and the target spectral line width Δλ 3   mt  is changed to Δλ 3   ma . As a result, the wavelengths of the semiconductor lasers are changed to λ 3 ( 1 ) a  to λ 3 ( 5 ) a , and the wavelength interval Δλ 3   p  is one fourth of the spectral line width Δλ 3   ma . In the multiline in  FIG. 86 , the lines of the wavelengths λ 3 ( 1 ) a  to λ 3 ( 5 ) a  have the same light intensity I 3   s   0  as in  FIG. 85 . 
     17.4 Effect 
     According to Embodiment 3, the oscillation wavelength intervals of the semiconductor lasers can be constantly controlled while the multiline spectrum generated by the semiconductor lasers in the third multiple semiconductor laser system  930  is being monitored, thereby controlling the spectral line width of the pulse-amplified excimer laser beam with high accuracy. 
     Also, the oscillation wavelength intervals of the semiconductor lasers of the multiline generated by the semiconductor lasers are controlled to suppress SBS in the third fiber amplifier  970 . This can suppress damage to the third fiber amplifier  970  and the third multiple semiconductor laser system. 
     The third solid-state laser device  920  in Embodiment 3 is an example of “first solid-state laser device” in the present disclosure. The pulse laser beam LP 31  output from the third solid-state laser device  920  is an example of “first pulse laser beam” in the present disclosure. The third multiple semiconductor laser system  930  is an example of “first multiple semiconductor laser system” in the present disclosure. The semiconductor lasers  931  are examples of “a plurality of first semiconductor lasers” in the present disclosure. 
     17.5 Variant 
     As the third multiple semiconductor laser system  930 , the same configuration as in  FIG. 60  or  FIG. 67  may be applied in place of the configuration in  FIG. 84 . 
     18. Electronic Device Manufacturing Method 
       FIG. 87  schematically shows an exemplary configuration of the exposure apparatus  20 . In  FIG. 87 , the exposure apparatus  20  includes an illumination optical system  24  and a projection optical system  25 . The illumination optical system  24  illuminates, with a laser beam incident from the laser system  1 , a reticle pattern on a reticle stage RT. The projection optical system  25  reduces and projects the laser beam having passed though the reticle and forms an image thereof on a workpiece (not shown) arranged on a workpiece table WT. The workpiece is a photosensitive substrate such as a semiconductor wafer coated with photoresist. The exposure apparatus  20  synchronously translates the reticle stage RT and the workpiece table WT to expose the laser beam reflecting the reticle pattern onto the workpiece. Through the exposure process as described above, a device pattern is transferred onto the semiconductor wafer, thereby manufacturing a semiconductor device. 
     19. Others 
     The description above is intended to be illustrative and the present disclosure is not limited thereto. Therefore, it would be obvious to those skilled in the art that various modifications to the embodiments of the present disclosure would be possible without departing from the spirit and the scope of the appended claims. Further, it would be also obvious for those skilled in the art that embodiments of the present disclosure would be appropriately combined. 
     The terms used throughout the present specification and the appended claims should be interpreted as non-limiting terms. For example, terms such as “comprise”, “include”, “have”, and “contain” should not be interpreted to be exclusive of other structural elements. Further, indefinite articles “a/an” described in the present specification and the appended claims should be interpreted to mean “at least one” or “one or more.” Further, “at least one of A, B, and C” should be interpreted to mean any of A, B, C, A+B, A+C, B+C, and A+B+C as well as to include combinations of the any thereof and any other than A, B, and C.