Patent Publication Number: US-2021167568-A1

Title: Gas laser device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of U.S. patent application Ser. No. 16/232,637 filed on Dec. 26, 2018 which is a continuation application of International Application No. PCT/JP2016/073081 filed on Aug. 5, 2016. The content of the application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a discharge excitation gas laser device. 
     2. Related Art 
     Along with the miniaturization and high integration of a semiconductor integrated circuit, improvement of resolution is demanded in a semiconductor exposure device. Hereinafter, the semiconductor exposure device is simply referred to as an “exposure device.” Accordingly, shortening of the wavelength of light emitted from a light source for exposure has been sought. As the light source for exposure, a discharge excitation gas laser device is in use in place of a conventional mercury lamp. As a laser device for exposure, a KrF excimer laser device that emits ultraviolet rays of a wavelength of 248 nm and an ArF excimer laser device that emits ultraviolet rays of a wavelength of 193.4 nm are currently employed. 
     As a current exposure technology, liquid immersion exposure has been used in practice, in which a gap between a projection lens on an exposure device side and a wafer is filled with a liquid to change the refractive index of the gap, thereby shortening the apparent wavelength of the light source for exposure. In the liquid immersion exposure using the ArF excimer laser device as the light source for exposure, ultraviolet rays having a wavelength of 134 nm in water is applied to the wafer. This technology is called ArF liquid immersion exposure. The ArF liquid immersion exposure is also referred to as ArF liquid immersion lithography. 
     The spectrum line width in natural oscillations of the KrF and ArF excimer laser devices is so wide, about 350 to 400 pm, that a color aberration occurs in the laser light (ultraviolet rays) as projected in a reduced size on the wafer through the projection lens on the exposure device side, and the resolution is degraded. Therefore, it is necessary to narrow the spectrum line width of the laser light emitted from the gas laser device to the extent that the color aberration can be ignored. Accordingly, a line narrowing module having a line narrowing element is provided in a laser resonator of the gas laser device. This line narrowing module is used to achieve narrowing of the spectrum line width. The line narrowing element may be an etalon, a grating, and the like. The laser device with a spectrum line width narrowed in this way is called a narrowband laser device. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-Open No. 06-283787 
         Patent Literature 2: International Publication No. WO 2014/156818 
         Patent Literature 3: Published Japanese Translations of PCT International Publication for Patent Applications No. 2005-512333 
         Patent Literature 4: Japanese Patent Application Laid-Open No. 2009-194063 
         Patent Literature 5: Japanese Patent Application Laid-Open No. 11-177168 
         Patent Literature 6: International Publication No. WO 2015/190012 
       
    
     SUMMARY 
     A discharge excitation gas laser device according to one aspect of the present disclosure may include (A) first and second discharge electrodes, (B) a plurality of peaking capacitors, (C) a charger, (D) a plurality of pulse power modules, (E) a plurality of output pulse sensors, and (F) a control unit. 
     (A) The first and second discharge electrodes may be disposed to face each other. 
     (B) The peaking capacitors may be connected to the first discharge electrode. 
     (D) Each one of the pulse power modules may include (D1) a charging capacitor, 
     (D2) a pulse compression circuit, and (D3) a switch.
         (D1) A charged voltage may be applied to the charging capacitor from the charger.   (D2) The pulse compression circuit may pulse-compress electrical energy stored in the charging capacitor, and output the pulse-compressed electrical energy as an output pulse to a corresponding peaking capacitor of the peaking capacitors.   (D3) The switch may be disposed between the charging capacitor and the pulse compression circuit.       

     (E) Each one of the output pulse sensors may detect an output pulse output by a corresponding one of the pulse power modules. 
     (F) The control unit may be configured to control, based on a detection result of each of the output pulse sensors, a timing of a switch signal to be input to a corresponding switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the present disclosure will be described as an example below with reference to the accompanying drawings. 
         FIG. 1  is a diagram schematically illustrating a configuration of a laser device according to a comparative example; 
         FIG. 2  is a cross-sectional view of a gas laser device as viewed in a Z direction; 
         FIG. 3  is a circuit diagram illustrating configurations of a PPM( 1 ) to a PPM(n); 
         FIG. 4  is a graph showing a relationship between a charged voltage and a required time from a time of inputting a switch signal to PPM(k) to a time of applying a voltage to a discharge electrode; 
         FIG. 5  is a block diagram illustrating a configuration of a synchronization control unit; 
         FIG. 6  is a timing chart illustrating a relationship among an external trigger signal, an internal trigger signal, and a switch signal; 
         FIG. 7  is a flowchart illustrating a process performed by a laser control unit; 
         FIG. 8  is a flowchart illustrating a process performed by a trigger correction unit; 
         FIG. 9  is a timing chart in the gas laser device according to the comparative example; 
         FIG. 10  is a timing chart for explaining problems in the gas laser device according to the comparative example; 
         FIG. 11  is a diagram schematically illustrating a configuration of a gas laser device according to a first embodiment; 
         FIG. 12  is a circuit diagram illustrating configurations of a PPM( 1 ) to a PPM(n); 
         FIG. 13  is a block diagram illustrating a configuration of a synchronization control unit; 
         FIG. 14  is a timing chart illustrating a relationship among an external trigger signal, an internal trigger signal, a switch signal and a detection signal; 
         FIG. 15  is a flowchart illustrating a process performed by a trigger correction unit; 
         FIG. 16  is a diagram schematically illustrating a configuration of a gas laser device according to a second embodiment; 
         FIG. 17  is a circuit diagram illustrating configurations of a PPM( 1 ) to a PPM(n); 
         FIG. 18  is a block diagram illustrating a configuration of a synchronization control unit; 
         FIG. 19  is a flowchart illustrating a calculation process of time difference data; 
         FIG. 20  is a flowchart illustrating a calculation process of a delay time; 
         FIG. 21  is a timing chart in the gas laser device according to the second embodiment; 
         FIG. 22  is a diagram schematically illustrating a configuration of a gas laser device according to a third embodiment; 
         FIG. 23  is a block diagram illustrating a configuration of a synchronization control unit; 
         FIG. 24  is a flowchart illustrating a calculation process of time difference data and a charged voltage; 
         FIG. 25  is a diagram schematically illustrating a configuration of a gas laser device according to a fourth embodiment; 
         FIG. 26  is a block diagram illustrating a configuration of a synchronization control unit; 
         FIG. 27  is a timing chart illustrating a relationship among an external trigger signal, an internal trigger signal, a switch signal and a detection signal; 
         FIG. 28  is a flowchart illustrating a correction process of a delay time by a delay time correction unit; 
         FIG. 29  is a diagram illustrating a specific example of an output pulse sensor in a current detection system; 
         FIG. 30  is a diagram illustrating a specific example of an output pulse sensor for detecting a charging timing based on a waveform of a current flowing through a peaking capacitor; 
         FIG. 31  is a graph showing an operation of a comparator; 
         FIG. 32  is a diagram illustrating a specific example of an output pulse sensor in a current detection system; 
         FIG. 33  is a diagram illustrating a specific example of an output pulse sensor for detecting a charging timing based on a waveform of a voltage applied to the peaking capacitor; 
         FIG. 34  is a graph showing an operation of a comparator; 
         FIG. 35  is a diagram illustrating a specific example of an optical sensor included in a discharge sensor; 
         FIG. 36  is a graph showing an operation of a comparator. 
     
    
    
     EMBODIMENTS 
     &lt;Contents&gt; 
     1. Comparative Example 
     1.1 Configuration
         1.1.1 Overview of gas laser device   1.1.2 Pulse power module   1.1.3 Synchronization control unit       

     1.2 Operation
         1.2.1 Processing in laser control unit   1.2.2 Processing in trigger correction unit   1.2.3 Overall operation of gas laser device       

     1.3 Problem 
     2. First Embodiment 
     2.1 Configuration 
     2.2 Operation
         2.2.1 Processing in laser control unit   2.2.2 Processing in trigger correction unit   2.2.3 Overall operation of gas laser device       

     2.3 Effect 
     3. Second Embodiment 
     3.1 Configuration 
     3.2 Operation
         3.2.1 Calculation process of time difference data   3.2.2 Calculation process of delay time   3.2.3 Generation process of internal trigger signal   3.2.4 Overall operation of gas laser device       

     3.3 Effect 
     4. Third Embodiment 
     4.1 Configuration 
     4.2 Operation
         4.2.1 Calculation process of time difference data and charged voltage   4.2.2 Processing in trigger correction unit   4.2.3 Overall operation of gas laser device       

     4.3 Effect 
     5. Fourth Embodiment 
     5.1 Configuration 
     5.2 Operation
         5.2.1 Correction process of delay time of internal trigger signal to external signal   5.2.2 Overall operation of gas laser device       

     5.3 Effect 
     6. Specific Example of Output Pulse Sensor 
     6.1 Output pulse sensor in current detection system 
     6.2 Output pulse sensor in voltage detection system 
     7. Specific Example of Discharge Sensor 
     8. Modification Example of Pulse Power Module 
     8.1 Configuration 
     8.2 Effect 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The embodiments described below illustrate some examples of the present disclosure, and do not limit the contents of the present disclosure. Further, all of the configurations and the operations described in the embodiments are not always indispensable as configurations and operations of the present disclosure. The same constituent elements are denoted by the same reference signs, and redundant description is omitted. 
     1. Comparative Example 
     1.1 Configuration 
     1.1.1 Overview of Gas Laser Device 
       FIG. 1  and  FIG. 2  each schematically illustrate a configuration of a gas laser device  2  according to a comparative example.  FIG. 1  schematically illustrates the configuration of the gas laser device  2 .  FIG. 2  is a cross-sectional view of the gas laser device  2  illustrated in  FIG. 1  as viewed in a Z direction. The gas laser device  2  is a discharge excitation gas laser device such as an excimer laser device. 
     In  FIG. 1 , the Z direction is defined as a traveling direction of pulse laser light PL emitted from the gas laser device  2 . A V direction is defined as a direction of electric discharge between first and second discharge electrodes  20   a  and  20   b  which are described later. An H direction is defined as a direction which is perpendicular to both of the Z direction and the V direction. 
     In  FIG. 1 , the gas laser device  2  includes, a laser chamber  10 , a charger  11 , and a plurality of pulse power modules (PPMs)  12 . The gas laser device  2  further includes a rear mirror  14 , an output coupling mirror  15 , a pulse energy measurement unit  16 , a synchronization control unit  17 , and a laser control unit  18 . 
     First and second discharge electrodes  20   a  and  20   b  as main electrodes, a ground plate  21 , wires  22 , a fan  23 , and a heat exchanger  24  are provided in the laser chamber  10 . The laser chamber  10  may be provided with a preliminary electrode (not illustrated) therein. 
     Laser gas serving as a laser medium is enclosed in the laser chamber  10 . The laser gas contains, for example, rare gas such as argon gas, krypton gas, or xenon gas, buffer gas such as neon gas or helium gas, and halogen gas such as chlorine gas or fluorine gas, etc. 
     An opening is formed at the laser chamber  10 . An electric insulation plate  26  in which a plurality of feedthroughs  25  are embedded is provided to plug the opening. A plurality of peaking capacitors (Cp)  27  and a holder  28  which holds these peaking capacitors  27  are disposed on this electric insulation plate  26 . The plurality of PPMs  12  are disposed on this holder  28 . In addition, the laser chamber  10  is provided with windows  21   a  and  21   b.    
     The first and second discharge electrodes  20   a  and  20   b  are disposed to face each other in the laser chamber  10  as electrodes for exciting the laser medium by a discharge. The first discharge electrode  20   a  and the second discharge electrode  20   b  are disposed such that their discharge surfaces face each other. A space between the discharge surface of the first discharge electrode  20   a  and the discharge surface of the second discharge electrode  20   b  is referred to as a “discharge space.” A surface opposite to the discharge surface of the first discharge electrode  20   a  is supported on the electric insulation plate  26 . A surface opposite to the discharge surface of the second discharge electrode  20   b  is supported on the ground plate  21 . 
     The feedthroughs  25  are connected to the first discharge electrode  20   a . As illustrated in  FIG. 2 , the feedthrough  25  is connected to a pair of peaking capacitors  27  through a connecting part  29 , the peaking capacitors  27  being held to the holder  28 . The connecting part  29  is a member for connecting the peaking capacitors  27  with the other constituent element. 
     Walls  28   a  which form an internal space of the holder  28  are formed of a metal material such as aluminum metal. The peaking capacitors  27 , the connecting part  29 , and a high voltage terminal  12   b  of the PPM  12  are disposed in the holder  28 . The peaking capacitors  27  each are a capacitor for supplying electrical energy to the first and second discharge electrodes  20   a  and  20   b . The pair of peaking capacitors  27  receive the electrical energy from the corresponding PPM  12  to accumulate the electrical energy therein, and then discharge the accumulated electrical energy to the first and second discharge electrodes  20   a  and  20   b.    
     The pair of peaking capacitors  27  are disposed in the H direction. A plurality of peaking capacitors  27  may be disposed in the Z direction. One electrode  27   a  of the peaking capacitor  27  is connected to the high voltage terminal  12   b  and the feedthrough  25  through the connecting part  29 . The other electrode  27   b  of the peaking capacitor  27  is connected to a wall  28   a  of the holder  28  through the connecting part  29 . 
     The connecting part  29  includes a connecting plate  29   a , and connecting terminals  29   b  and  29   c . The connecting plate  29   a  is made up of a conductive plate having a U-shaped cross section, and is connected to the high voltage terminal  12   b  and the feedthrough  25 . 
     The ground plate  21  is connected to the laser chamber  10  through the wires  22 . The laser chamber  10  is connected to ground. The ground plate  21  is maintained at a ground potential through the wires  22 . The ends of the ground plate  21  in the Z direction are fixed to the laser chamber  10 . 
     The fan  23  is a crossflow fan to circulate the laser gas in the laser chamber  10 . The fan  23  is disposed such that its longitudinal direction is approximately parallel to the Z direction. The fan  23  is disposed opposite to the discharge space with respect to the ground plate  21 . The fan  23  is rotationally driven by a motor  23   a  which is connected to the laser chamber  10 , to generate the flow of the laser gas. 
     The laser gas blown out of the fan  23  flows into the discharge space. The direction of the laser gas flowing into the discharge space is approximately parallel to the H direction. The laser gas flown out of the discharge space may be drawn into the fan  23  through the heat exchanger  24 . The heat exchanger  24  exchanges heat between a refrigerant supplied into the heat exchanger  24  and the laser gas. 
     The windows  21   a  and  21   b  are provided at the ends of the laser chamber  10 . The light generated in the laser chamber  10  is emitted to the outside of the laser chamber  10  through the windows  21   a  and  21   b.    
     The rear mirror  14  and the output coupling mirror  15  constitutes an optical resonator. The laser chamber  10  is provided on an optical path of the optical resonator. The rear mirror  14  includes a substrate formed of calcium fluoride (CaF 2 ) or the like which transmits the pulse laser light PL, and a high reflective film is formed on the substrate. The output coupling mirror  15  includes a substrate formed of calcium fluoride (CaF 2 ) or the like which transmits the pulse laser light PL, and a partially reflective film is formed on the substrate. The reflectance of the partially reflective film of the output coupling mirror  15  is in a range of 8% to 15%. 
     The light emitted from the laser chamber  10  makes round trips between the rear mirror  14  and the output coupling mirror  15 , and is amplified every time the light passes through the discharge space. A part of the amplified light is emitted through the output coupling mirror  15 , as the pulse laser light PL. 
     The pulse energy measurement unit  16  is provided on the optical path of the pulse laser light PL emitted through the output coupling mirror  15 . The pulse energy measurement unit  16  includes a beam splitter  16   a , a focusing optical system  16   b , and an optical sensor  16   c.    
     The beam splitter  16   a  transmits a part of the pulse laser light PL at high transmittance, and reflects the remaining part of the pulse laser light PL toward the focusing optical system  16   b . The focusing optical system  16   b  concentrates the light reflected by the beam splitter  16   a  on a light reception surface of the optical sensor  16   c . The optical sensor  16   c  detects pulse energy of the light concentrated on the light reception surface and outputs data on the detected pulse energy to the laser control unit  18 . 
     The charger  11  is a DC (direct current) power supply device for charging a charging capacitor C 0  (described later) included in each PPM  12  at a constant charged voltage. Each PPM  12  includes a switch  12   a  controlled by the laser control unit  18 . The switch  12   a  includes an insulated gate bipolar transistor (IGBT). When the switch  12   a  is turned from OFF to ON, the PPM  12  generates a high-voltage pulse using the electrical energy in the charging capacitor C 0  so that the high-voltage pulse is applied to the first discharge electrode  20   a.    
     The plurality of PPMs  12  are arranged in the Z direction on the holder  28 . At least one peaking capacitor  27  is electrically connected to each PPM  12 . In this comparative example, two peaking capacitors  27  are connected in parallel to one PPM  12 . The total number of the plurality of PPMs  12  is denoted by n. Hereinafter, each PPM  12  is referred to as a PPM(k). Here, k is 1, 2, . . . , or n. One or two or more peaking capacitors  27  which are connected to the PPM(k) are referred to as a Cp(k). 
     The laser control unit  18  transmits and receives various signals to and from an external device control unit  3  included in an external device such as an exposure device (not illustrated). For example, the laser control unit  18  receives an external trigger signal TR as a light emission trigger, and data on the target pulse energy Et from the external device control unit  3 . The laser control unit  18  receives a pulse energy value measured by the pulse energy measurement unit  16 . The external device may not be the exposure device. The external device may be a processing laser device, a laser annealing device, or a laser doping device. 
     The laser control unit  18  calculates a charged voltage V to be set at the charger  11  with reference to the data on the target pulse energy Et received from the external device control unit  3  and the measured pulse energy value received from the pulse energy measurement unit  16 . The laser control unit  18  is connected to the synchronization control unit  17  to transmit the external trigger signal TR and a setting value of the charged voltage V to the synchronization control unit  17 . 
     The synchronization control unit  17  is connected to the laser control unit  18 , the charger  11 , and the PPM( 1 ) to PPM(n). The charger  11  receives the setting value of the charged voltage V through the synchronization control unit  17 , and charges the charging capacitor C 0  included in each PPM(k) based on the setting value of the charged voltage V. 
     The synchronization control unit  17  generates n switch signals S( 1 ) to S(n) based on the external trigger signal TR received from the laser control unit  18 . The switch signal S(k) is input to the switch  12   a  included in the PPM(k). 
     1.1.2 Pulse Power Module 
       FIG. 3  illustrates configurations of the PPM( 1 ) to PPM(n) illustrated in  FIG. 1 . The PPM( 1 ) to PPM(n) have the same configurations with one another, and the configuration of one PPM(k) will be described. The PPM(k) includes the charging capacitor C 0 , the switch  12   a , a pulse transformer PT, a plurality of magnetic switches MS 1  and MS 2 , and a plurality of capacitors C 1  and C 2 . The pulse transformer PT, a plurality of magnetic switches MS 1  and MS 2  and the plurality of capacitors C 1  and C 2  form a pulse compression circuit. 
     The magnetic switches MS 1  and MS 2  each include a saturable reactor. Each of the magnetic switches MS 1  and MS 2  is switched to a low impedance state when the time integral of the voltage applied across the magnetic switch becomes a predetermined threshold determined by the properties of the magnetic switch. 
     The switch  12   a  in the PPM(k) receives a switch signal S(k) from the synchronization control unit  17 . When the switch  12   a  receives the switch signal S(k), and is turned ON, electric current flows from the charging capacitor C 0  to a primary side of the pulse transformer PT. 
     The electric current flowing through the primary side of the pulse transformer PT causes electromagnetic induction to generate reverse electric current through a secondary side of the pulse transformer PT. The reverse electric current flowing through the secondary side of the pulse transformer PT causes a current pulse to flow in a capacitor C 1  to charge the capacitor C 1 . At this time, the time integral of the voltage applied to the magnetic switch MS 1  reaches the threshold. When the time integral of the voltage applied to the magnetic switch MS 1  reaches the threshold, the magnetic switch MS 1  is magnetically saturated and closed. 
     When the magnetic switch MS 1  is closed, the current pulse may flow from the capacitor C 1  to a capacitor C 2  to charge the capacitor C 2 . At this time, the current pulse flowing through the capacitor C 2  has a shorter pulse width than the current pulse flowing through the capacitor C 1 . Charging the capacitor C 2  allows the magnetic switch MS 2  to be magnetically saturated and closed. 
     When the magnetic switch MS 2  is closed, the current pulse flows from the capacitor C 2  to the Cp(k) which is the peaking capacitor  27  connected to the PPM(k), to charge the Cp(k). At this time, the current pulse flowing through the Cp(k) has a shorter pulse width than the current pulse flowing through the capacitor C 2 . As described above, the current pulse sequentially flows from the capacitor C 1  to the capacitor C 2  and then from the capacitor C 2  to the Cp(k), so that the pulse width of the current pulse is compressed. Thus, compressing the pulse width of the current pulse is referred to as pulse compression. 
     When the voltage across the Cp(k) reaches a breakdown voltage of the laser gas, the laser gas is dielectrically broken down between the first and second discharge electrodes  20   a  and  20   b . Thus, the laser gas is excited, and the ultraviolet laser light is emitted when the excited state returns to the ground state. Such a discharge operation is repeated with the switching operation of the switch  12   a , resulting in the pulse laser light PL being emitted at a predetermined oscillation frequency. 
       FIG. 4  is a graph showing a relationship between the charged voltage V of the PPM(k) and a required time F(V) from the time of inputting the switch signal S(k) to the PPM(k) to the time of applying the high voltage to the first discharge electrode  20   a . The PPM(k) includes the pulse compression circuit (magnetic compression circuit), and the relationship between the required time F(V) and the charged voltage (V) is represented by the following formula (1). 
         F ( V )= K/V   (1)
 
     Here, K is a constant value. 
     Accordingly, a time difference ΔTV(k) represented by the following formula (2) is generated between the required time F(V) when the charged voltage set at the PPM(k) is V and the required time F(V 0 ) when the charged voltage V is a reference voltage V 0 . 
       Δ TV ( k )= F ( V   0 )− F ( V )  (2)
 
     Specifically, when the charged voltage V set at the PPM(k) is larger than the reference voltage V 0 , the required time F(V) is shorter than the required time F(V 0 ) when the charged voltage V is a reference voltage V 0 , by the time difference ΔTV(k). 
     1.1.3 Synchronization Control Unit 
       FIG. 5  illustrates a configuration of the synchronization control unit  17  illustrated in  FIG. 1 . The synchronization control unit  17  includes an internal trigger signal generation unit  30 , and a plurality of trigger correction units (TCS)  31 . Each of the trigger correction units  31  includes a processing unit  32  and a delay circuit  33 . 
     The trigger correction unit  31  is provided for each PPM  12 . In other words, the total number of trigger correction units  31  is n. Hereinafter, the trigger correction unit  31  corresponding to the PPM(k) is referred to as a TCS(k). The TCS( 1 ) to TCS(n) have the same configurations with one another. 
     The internal trigger signal generation unit  30  is connected to the laser control unit  18  and the TCS( 1 ) to TCS(n). Upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  30  generates an internal trigger signal TR(k) and inputs the internal trigger signal TR(k) to the delay circuit  33  in each TCS(k). 
     As illustrated in  FIG. 6 , the internal trigger signal generation unit  30  outputs the internal trigger signal TR(k) after a delay time Trd(k) has passed since the time of receiving the external trigger signal TR. Here, all of the delay times Trd( 1 ) to Trd(n) have a reference delay time Trd0, and therefore are the same value. In other words, the internal trigger signal TR(k) is input to the delay circuit  33  in each TCS(k) at the same time. 
     The processing unit  32  in each TCS(k) is connected to the laser control unit  18  and the delay circuit  33  in the TCS(k). The processing unit  32  calculates the delay time Td(k) for delaying the internal trigger signal TR(k) based on the setting value of the charged voltage V received from the laser control unit  18 , and inputs the calculated delay time Td(k) to the delay circuit  33 . Specifically, the processing unit  32  determines the time difference ΔTV(k) based on the above-described formula (2). The processing unit  32  may store a function representing the required time F(V) as table data, and determine the time difference ΔTV(k) based on the table data. 
     The processing unit  32  determines the time difference ΔTV(k), and then calculates the delay time Td(k) based on the following formula (3). 
         Td ( k )= Td 0( k )+Δ TV ( k )  (3)
 
     Here, Td0(k) is a reference delay time when the charged voltage V is a reference voltage V 0 . In other words, the delay time Td(k) results from adding the correction time ΔTV(k) determined based on the above-described formula (2) to the reference delay time Td0(k). 
     The delay circuit  33  acquires and holds the data on the delay time Td(k) calculated by the processing unit  32 . As illustrated in  FIG. 6 , upon reception of the internal trigger signal TR(k) from the internal trigger signal generation unit  30 , the delay circuit  33  inputs a signal obtained by delaying the internal trigger signal TR(k) by the delay time Td(k) as a switch signal S(k) to the corresponding PPM  12 . Thereby, the required time from the timing when the laser control unit  18  receives the external trigger signal TR to the timing when the PPM(k) applies the high voltage to the first discharge electrode  20   a  is approximately constant. 
     1.2 Operation 
     The operation of the gas laser device  2  according to the comparative example will be described with reference to  FIG. 7  to  FIG. 9 . 
     1.2.1 Processing in Laser Control Unit 
       FIG. 7  is a flowchart illustrating a process performed by the laser control unit  18 . The laser control unit  18  calculates a charged voltage V to be set at the charger  11  based on the target pulse energy Et through the following process. 
     First, in step S 101 , the laser control unit  18  sets a setting value of the charged voltage V to a reference voltage V 0  as an initial value. Next, in step S 102 , the laser control unit  18  reads the data on the target pulse energy Et transmitted from the external device control unit  3 . 
     Next, in step S 103 , upon reception of an external trigger signal TR from the external device control unit  3 , the laser control unit  18  transmits the external trigger signal TR to the synchronization control unit  17 , and determines whether the gas laser device  2  has performed laser oscillation. If the gas laser device  2  has not performed laser oscillation (S 103 : NO), the laser control unit  18  waits until the gas laser device  2  performs laser oscillation. If the gas laser device  2  has performed laser oscillation (S 103 : YES), the laser control unit  18  proceeds to step S 104 . 
     In step S 104 , the laser control unit  18  detects pulse energy E of the pulse laser light PL emitted from the gas laser device  2 . The pulse energy E is measured by the pulse energy measurement unit  16 . 
     Next, in step S 105 , the laser control unit  18  calculates a difference ΔE between the measured pulse energy E and the target pulse energy Et by the following formula (4). 
       Δ E=E−Et   (4)
 
     Next, in step S 106 , the laser control unit  18  calculates a change amount ΔV in the setting value of the charged voltage V based on the difference ΔE by the following formula (5). 
       Δ V=H·ΔE   (5)
 
     Here, H is a proportional constant. The change amount ΔV represents a change amount in the setting value of the charged voltage V which is set to make the difference ΔE zero. The laser control unit  18  calculates a next setting value by adding the change amount ΔV to the present setting value of the charged voltage V. 
     Next, in step S 107 , the laser control unit  18  transmits the setting value of the charged voltage V which has been calculated in step S 106 , to the charger  11  and the plurality of trigger correction units  31 . 
     Next, in step S 108 , the laser control unit  18  determines whether the target pulse energy Et transmitted from the external device control unit  3  has been changed. If the target pulse energy Et has been changed (S 108 : YES), the laser control unit  18  returns to step S 102 . If the target pulse energy Et has not been changed (S 108 : NO), the laser control unit  18  returns to step S 103 . The above-described process is repeatedly performed. 
     1.2.2 Processing in Trigger Correction Unit 
       FIG. 8  is a flowchart illustrating a process performed by a trigger correction unit  31 . Each of the trigger correction units  31  calculates, in the following process, the delay time Td(k) to correct the internal trigger signal TR(k) when the setting value of the charged voltage V has been transmitted from the laser control unit  18  in step S 107  illustrated in  FIG. 7 . 
     First, in step S 201 , the processing unit  32  included in each TCS(k) reads the setting value of the charged voltage V transmitted from the laser control unit  18 . Next, in step S 202 , the processing unit  32  calculates the correction time ΔTV(k) based on the above-described formula (1) and formula (2). Next, in step S 203 , the processing unit  32  calculates the delay time Td(k) based on the above-described formula (3). In step S 204 , the processing unit  32  transmits the data on the calculated delay time Td(k) to the delay circuit  33 . Then, the processing unit  32  returns to step S 201 . The above-described process is repeatedly performed. 
     Upon reception of the internal trigger signal TR(k) from the internal trigger signal generation unit  30 , the delay circuit  33  delays the internal trigger signal TR(k) by the delay time Td(k), and inputs the delayed internal trigger signal TR(k), as a switch signal S(k), to the PPM(k). 
     1.2.3 Overall Operation of Gas Laser Device 
       FIG. 9  is a timing chart in the gas laser device  2  according to the comparative example. The overall operation of the gas laser device will be described with reference to  FIG. 9 . 
     Upon reception of the data on the target pulse energy Et from the external device control unit  3 , the laser control unit  18  calculates the setting value of the charged voltage V so that the pulse energy E of the pulse laser light PL approaches the target pulse energy Et, and transmits the calculated setting value of the charged voltage V to the charger  11  through the synchronization control unit  17 . 
     In the synchronization control unit  17 , the processing unit  32  in each TCS(k) calculates the delay time Td(k) based on the setting value of the charged voltage V, and transmits the data on the delay time Td(k) to the delay circuit  33 . Upon reception of the external trigger signal TR from the external device control unit  3  through the laser control unit  18 , the internal trigger signal generation unit  30  in the synchronization control unit  17  generates the internal trigger signal TR(k) to input to the delay circuit  33  in each TCS(k). The internal trigger signal TR(k) input to the delay circuit  33  in each TCS(k) is delayed by the delay time Td(k), and is input to the switch  12   a  in the PPM(k) as a switch signal S(k). 
     As illustrated in  FIG. 9 , the switch signals S( 1 ) to S(n) are input to the respective switches  12   a  in the PPM( 1 ) to PPM(n) at approximately the same time, so that the respective switches  12   a  are turned ON at approximately the same time. The Cp( 1 ) to Cp(n) are charged by the current pulses pulse-compressed by the PPM( 1 ) to PPM(n) at approximately the same time, and apply the high voltage to the first discharge electrode  20   a  at approximately the same time. 
     As a result, dielectric breakdown occurs in the laser gas, and pulse discharge is generated in the discharge space. This pulse discharge results in excitation of the laser gas, and the ultraviolet laser light is emitted when the excited state returns to the ground state. The ultraviolet laser light is subjected to laser oscillation by the optical resonator, and the pulse laser light PL is emitted from the output coupling mirror  15 . The pulse energy E of the emitted pulse laser light PL is measured by the pulse energy measurement unit  16 . 
     The laser control unit  18  reads the pulse energy E of the pulse laser light measured by the pulse energy measurement unit  16 , and calculates the setting value of the charged voltage V so that the pulse energy E of the pulse laser light PL approaches the target pulse energy Et. The above-described steps are repeated. 
     As described above, the synchronization control unit  17  controls the timings of turning ON the switches  12   a  in the PPM( 1 ) to PPM(n) based on the charged voltage V of the charger  11  so that the timings of charging the Cp( 1 ) to Cp(n) approximately coincide with one another. 
     1.3 Problem 
     In the gas laser device  2  according to the comparative example, the timings of charging the Cp( 1 ) to Cp(n) are so controlled as to approximately coincide with one another, but even if the control is thus performed, the timings of charging the Cp( 1 ) to Cp(n) may be shifted from one another as illustrated in  FIG. 10 . When the charging timings are shifted from one another, the timings of applying the high voltage to the first discharge electrode  20   a  from the PPM( 1 ) to PPM(n) are shifted from one another, thereby reducing the discharge intensity. As a result, the light emission intensity of the pulse laser light PL is reduced. To prevent the light emission intensity of the pulse laser light PL from being reduced, the timings of charging the Cp( 1 ) to Cp(n) need to coincide with one another with an accuracy of several nanoseconds or less. 
     The shifts in the charging timing of about several nanoseconds may be caused by individual difference, temperature difference, or the like in the PPM( 1 ) to PPM(n). For example, the charging timing depends on the temperature of each constituent element of the PPM( 1 ) to PPM(n), and therefore the shifts in the timings of charging the Cp( 1 ) to Cp(n) are caused by the temperature difference. If the temperature of each constituent element of the PPM( 1 ) to PPM(n) can be directly measured, or the temperature changes can be accurately predicted, the shifts in the charging timing can be reduced to some extent, but it is practically difficult to directly measure or predict the temperature. It is also difficult to eliminate the individual difference among the PPM( 1 ) to PPM(n). 
     Accordingly, there are problems in that in the gas laser device  2  according to the comparative example, the shifts in the charging timing of Cp( 1 ) to Cp(n) cannot be suppressed, and the resulting reduction in the light emission intensity of the pulse laser light PL cannot be suppressed. 
     2. First Embodiment 
     A gas laser device according to a first embodiment of the present disclosure will be described below. The gas laser device according to the first embodiment has the same configuration as the gas laser device  2  according to the comparative example except that the gas laser device according to the first embodiment includes an output pulse sensor, and the trigger correction unit having a different configuration from that according to the comparative example. Hereinafter, the constituent elements that are the same as the constituent elements of the gas laser device  2  according to the comparative example are denoted by the same reference signs, and the description thereof is appropriately omitted. 
     2.1 Configuration 
       FIG. 11  schematically illustrates a configuration of a gas laser device  2   a  according to the first embodiment.  FIG. 12  illustrates configurations of a PPM( 1 ) to a PPM(n) which are illustrated in  FIG. 11 . In the first embodiment, an output pulse sensor  40  is provided between the PPM  12  and the peaking capacitor  27 . The output pulse sensor  40  is provided for each PPM  12 . Hereinafter, the output pulse sensor  40  disposed between the PPM(k) and the Cp(k) is referred to as an A(k). 
     In the first embodiment, the output pulse sensor A(k) is a current sensor for detecting a current pulse as an output pulse. The output pulse sensor A(k) is connected between the magnetic switch MS 2  and the peaking capacitor  27 . Upon detection of the current pulse, the output pulse sensor A(k) inputs a detection signal D 1 ( k ) to a synchronization control unit  50 . 
       FIG. 13  illustrates a configuration of a synchronization control unit  50  according to the first embodiment. The synchronization control unit  50  includes an internal trigger signal generation unit  30 , and a plurality of trigger correction units (TCS)  51 . The synchronization control unit  50  controls the timings of switch signals S( 1 ) to S(n) to be input to the PPM( 1 ) to PPM(n), respectively. All of or part of the synchronization control unit  50  is composed of an FPGA (Field Programmable Gate Array) enabling a high speed processing operation. Hereinafter, the trigger correction unit  51  corresponding to the PPM(k) is referred to as a TCS(k). 
     The internal trigger signal generation unit  30  has the same configuration as the internal trigger signal generation unit  30  according to the comparative example. Upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  30  generates an internal trigger signal TR(k) and inputs the generated internal trigger signal TR(k) to the TCS(k). 
     Each TCS(k) includes a processing unit  52 , a delay circuit  53 , and a timer  54 . The internal trigger signal TR(K) is input to the delay circuit  53  and the timer  54  from the internal trigger signal generation unit  30 , at the same time. The detection signal D 1 ( k ) is input to the timer  54  in the TCS(k) from the output pulse sensor A(k). 
     The timer  54  starts clocking upon input of the internal trigger signal TR(k) and stops clocking upon input of the detection signal D 1 ( k ). In other words, the timer  54  measures a time Tdm(k) required from the input of the internal trigger signal TR(k) to the input of the detection signal D 1 ( k ), as illustrated in  FIG. 14 . The timer  54  inputs the data on the measured time Tdm(k) to the processing unit  52 . 
     The processing unit  52  in the TCS(k) calculates the delay time Td(k) for delaying the internal trigger signal TR(k) based on the setting value of the charged voltage V received from the laser control unit  18 , and inputs the calculated delay time Td(k) to the delay circuit  53 . Specifically, the processing unit  52  determines the time difference ΔTV(k) based on the above-described formula (2). The processing unit  52  determines the time difference ΔTV(k), and then calculates the delay time Td(k) based on the above-described formula (3). 
     The processing unit  52  in the TCS(k) corrects the delay time Td(k) based on the data on the measured time Tdm(k) input from the timer  54 . Thereby, the timing of the switch signal S(k) is corrected. 
     As described above, the synchronization control unit  50  and the laser control unit  18  constitute a control unit for controlling the timing of the switch signal S(k) based on the detection result of the output pulse sensor A(k). 
     2.2 Operation 
     2.2.1 Processing in Laser Control Unit 
     The process performed by the laser control unit  18  in the first embodiment is similar to that described using the flowchart illustrated in  FIG. 7 , and the description thereof is omitted. 
     2.2.2 Processing in Trigger Correction Unit 
       FIG. 15  is a flowchart illustrating a process performed by each TCS(k). Each TCS(k) calculates the delay time Td(k) to correct the internal trigger signal TR(k) in the following process when the setting value of the charged voltage V has been transmitted from the laser control unit  18  in step S 107  illustrated in  FIG. 7 . 
     First, in step S 300 , the processing unit  52  in each TCS(k) sets a reference delay time Td0(k) to an initial value as follows. 
         Td 0( k )= Tdt−F ( V   0 ) 
     Here, Tdt is a target value of the measured time Tdm(k). F(V 0 ) is the above-described required time F(V) when the charged voltage V is the reference voltage V 0 . The relationship among the reference delay time Td0(k), the target value Tdt, and the required time F(V 0 ) is illustrated in  FIG. 14 . 
     Next, in step S 301 , the processing unit  52  resets a variable as follows. 
         J= 0 
         Tdmsum ( k )=0 
     Here, J is a counter for counting the number of oscillation pulses. Tdmsum(k) is a total value for calculating the average value of the measured time Tdm(k) measured by the timer  54 . 
     Next, in step S 302 , the processing unit  52  reads the setting value of the charged voltage V transmitted from the laser control unit  18 . Next, in step S 303 , the processing unit  52  calculates the correction time ΔTV(k) based on the above-described formula (1) and formula (2). Next, in step S 304 , the processing unit  52  calculates the delay time Td(k) based on the above-described formula (3). Next, in step S 305 , the processing unit  52  transmits the data on the calculated delay time Td(k) to the delay circuit  53 . 
     Next, in step S 306 , the processing unit  52  determines whether the gas laser device  2   a  has performed laser oscillation. Whether the gas laser device  2   a  has performed laser oscillation is determined based on whether the timer  54  has received the detection signal D 1 ( k ) from the output pulse sensor A(k). If the gas laser device  2   a  has performed laser oscillation (S 306 : YES), the processing unit  52  proceeds to step S 307 . If the gas laser device  2   a  has not performed laser oscillation (S 306 : NO), the processing unit  52  waits until the gas laser device  2   a  performs laser oscillation. 
     In step S 307 , the processing unit  52  adds 1 to the present value of the counter J to update the value of J. Next, in step S 308 , the processing unit  52  receives the data on the measured time Tdm(k) from the timer  54 . Next, in step S 309 , the processing unit  52  adds the measured time Tdm(k) to the present total value Tdmsum(k) to update the total value Tdmsum(k). 
     Next, in step S 310 , the processing unit  52  determines whether the value of the counter J has reached a predetermined value Jmax representing the number of samples. If the value of the counter J has not reached the predetermined value Jmax (S 310 : NO), the processing unit  52  returns to step S 302 . If the value of the counter J has reached the predetermined value Jmax (S 310 : YES), the processing unit  52  proceeds to step S 311 . 
     In step S 311 , the processing unit  52  calculates the difference ΔTd(k) between the average value of the measured time Tdm(k) and the target value Tdt. The difference ΔTd(k) is calculated by the following formula (6). 
       Δ Td ( k )= Tdmsum ( k )/ J max− Tdt   (6)
 
     Next, in step S 312 , the processing unit  52  calculates a new reference delay time Td0(k) which is a value obtained by subtracting the difference ΔTd(k) from the reference delay time Td0(k). Thus, after correcting the reference delay time Td0(k), the processing unit  52  returns to step S 301 . The above-described process is repeated. 
     Upon reception of the internal trigger signal TR(k) from the internal trigger signal generation unit  30 , the delay circuit  53  in the TCS(k) delays the internal trigger signal TR(k) by the delay time Td(k), and inputs the delayed internal trigger signal TR(k) to the PPM(k) as a switch signal S(k). 
     As described above, in steps S 302  to S 305 , a first correction process (jitter correction process) for correcting the timing of the switch signal S(k) is performed based on the charged voltage V. In steps S 306  to S 312 , a second correction process (drift correction process) for correcting the timing of the switch signal S(k) is performed based on the detection result of the output pulse sensor A(k). 
     It is preferable that the number of samples Jmax is 200 or more and 10,000 or less. In other words, it is preferable that the frequency of the second correction process is lower than the frequency of the first correction process. 
     2.2.3 Overall Operation of Gas Laser Device 
     Hereinafter, the overall operation of the gas laser device  2   a  according to the first embodiment will be described. Upon reception of the data on the target pulse energy Et from the external device control unit  3 , the laser control unit  18  calculates the setting value of the charged voltage V so that the pulse energy E of the pulse laser light PL approaches the target pulse energy Et, and transmits the calculated setting value of the charged voltage V to the charger  11  through the synchronization control unit  50 . 
     In the synchronization control unit  50 , the processing unit  52  in each TCS(k) calculates the delay time Td(k) based on the setting value of the charged voltage V and the reference delay time Td0(k), and transmits the data on the delay time Td(k) to the delay circuit  53 . 
     Upon reception of the external trigger signal TR from the external device control unit  3  through the laser control unit  18 , the internal trigger signal generation unit  30  in the synchronization control unit  50  generates the internal trigger signal TR(k) to input to the delay circuit  53  and the timer  54  in each TCS(k). Upon reception of the internal trigger signal TR(k), the timer  54  is reset and starts clocking. The internal trigger signal TR(k) input to the delay circuit  53  in each TCS(k) is delayed by the delay time Td(k), and is input to the switch  12   a  in the PPM(k) as a switch signal S(k). 
     The switch signals S( 1 ) to S(n) are input to the respective switches  12   a  in the PPM( 1 ) to PPM(n) at approximately the same time, so that the respective switches  12   a  are turned ON at approximately the same time. The current pulse pulse-compressed by the PPM(k) is output to the Cp(k) as an output pulse. 
     At this time, the output pulse from the PPM(k) is detected by the output pulse sensor A(k) provided in a subsequent state of the PPM(k). Upon detection of the output pulse, the output pulse sensor A(k) transmits the detection signal D 1 ( k ) to the timer  54  in the TCS(k). Upon reception of the detection signal D 1 ( k ), the timer  54  stops clocking, and inputs the measured time Tdm(k) from the input of the internal trigger signal TR(k) to the input of the detection signal D 1 ( k ) to the processing unit  52 . Upon reception of the measured time Tdm(k), the processing unit  52  performs the above-described process, calculates the difference ΔTd(k) between the average value of the measured time Tdm(k) and the target value Tdt, and corrects the reference delay time Td0(k). 
     The Cp(k) is charged by the current pulse, resulting in the high voltage being applied between the first discharge electrode  20   a  and the second discharge electrode  20   b . As a result, dielectric breakdown occurs in the laser gas, and pulse discharge is generated in the discharge space. This pulse discharge results in excitation of the laser gas, and the ultraviolet laser light is emitted when the excited state returns to the ground state. The ultraviolet laser light is subjected to laser oscillation by the optical resonator, and the pulse laser light PL is emitted from the output coupling mirror  15 . The pulse energy E of the emitted pulse laser light PL is measured by the pulse energy measurement unit  16 . 
     The laser control unit  18  reads the pulse energy E of the pulse laser light measured by the pulse energy measurement unit  16 , and calculates the setting value of the charged voltage V so that the pulse energy E of the pulse laser light PL approaches the target pulse energy Et. The above-described steps are repeated. 
     2.3 Effect 
     In the first embodiment, the reference delay time Td0(k) is corrected based on the difference ΔTd(k) between the average value of the measured time Tdm(k) and the target value Tdt, so that the delay time Td(k) calculated in the next cycle is corrected by the difference ΔTd(k). Thereby, the measured time Tdm(k) approaches the target value Tdt. The above-described process is individually performed by each TCS(k), so that the measured time Tdm(k) measured by each timer  54  is approximately the same. 
     As a result, the timings of detecting the output pulse by the output pulse sensors A( 1 ) to A(n) approximately coincide with one another, thereby suppressing the shifts in the timings of charging the Cp( 1 ) to Cp(n). Accordingly, according to the first embodiment, the reduction in the light emission intensity of the pulse laser light PL caused by the shifts in the timings of charging the Cp( 1 ) to Cp(n) can be suppressed. 
     The gas laser device  2   a  includes n PPMs  12 , thereby increasing the output energy by a factor of n. For example, if the output energy of one PPM  12  is 10 J, the gas laser device  2   a  has performance equivalent to that of the gas laser device which includes a high output PPM having the output energy of n×10 J. 
     In the first embodiment, a plurality of PPMs  12  are connected in parallel with only one charger  11 , so that the charged voltage V applied to the plurality of PPMs  12  is approximately the same. Thus, the difference in the charged voltage V between the plurality of PPMs  12  is small, so that the influence on the charging timing is small. However, if a large number of PPMs  12  causes too large output of the charger  11 , a plurality of chargers may be provided, so that the charged voltage V can be supplied to each PPM  12  from each of the chargers. 
     3. Second Embodiment 
     A gas laser device according to a second embodiment of the present disclosure will be described below. The gas laser device according to the second embodiment enables a pulse width of the pulse laser light to be controlled with high accuracy by making the timing of the switch signal different for each PPM. Hereinafter, the constituent elements that are the same as the constituent elements of the gas laser device  2   a  according to the first embodiment are denoted by the same reference signs, and the description thereof is appropriately omitted. 
     3.1 Configuration 
       FIG. 16  schematically illustrates a configuration of a gas laser device  2   b  according to the second embodiment.  FIG. 17  illustrates configurations of a PPM( 1 ) to a PPM(n) which are illustrated in  FIG. 16 . In the second embodiment, in addition to the external trigger signal TR and the data on the target pulse energy Et, the data on the target pulse width Dt is transmitted to the laser control unit  18  from the external device control unit  3 . 
     The second embodiment is different from the first embodiment in that a plurality of first discharge electrodes  20   a   1  to  20   a   n  and a plurality of second discharge electrodes  20   b   1  to  20   b   n  are provided in the laser chamber  10 . To the PPM(k), the first discharge electrode  20   a   k  and the second discharge electrode  20   b   k  are provided. This is because the first discharge electrode  20   a   k  connected to the PPM(k) individually discharges. Here, k is 1, 2, . . . , or n. 
     All of the second discharge electrodes  20   b   1  to  20   b   n  are ground electrodes, and therefore it is not necessary that the gas laser device  2   b  is provided with the plurality of second discharge electrodes, and it is merely required to provide one second discharge electrode  20   b  like the gas laser device  2   a  according to the first embodiment. 
     The PPM  12  has the same configuration as the first embodiment. The PPM(k) is connected to the corresponding Cp(k) through the output pulse sensor A(k). The Cp(k) is connected to the first and second discharge electrodes  20   a   k ,  20   b   k . 
     In the second embodiment, the laser control unit  18  calculates time difference data ΔT( 1 ) to ΔT(n) for determining the timings of the switch signals S( 1 ) to S(n) based on the data on the target pulse width Dt input from the external device control unit  3 , and transmits the calculated time difference data to a synchronization control unit  60 . 
       FIG. 18  illustrates a configuration of a synchronization control unit  60  according to the second embodiment. The synchronization control unit  60  includes a delay time calculation unit  61 , an internal trigger signal generation unit  62 , and a plurality of trigger correction units  51 . The trigger correction unit  51  has the same configuration as the first embodiment. The delay time calculation unit  61  calculates delay times Trd( 1 ) to Trd(n) based on the time difference data ΔT( 1 ) to ΔT(n) input from the laser control unit  18 , and inputs the calculated delay times to the internal trigger signal generation unit  62 . 
     Upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  62  generates an internal trigger signal TR(k) and inputs the internal trigger signal TR(k) to the TCS(k). The internal trigger signal generation unit  62  generates the internal trigger signal TR(k) obtained by delaying the external trigger signal TR according to the delay time Trd(k) input from the delay time calculation unit  61 . 
     The other configurations of the gas laser device  2   b  according to the second embodiment are the same as those of the gas laser device  2   a  according to the first embodiment. 
     3.2 Operation 
     3.2.1 Calculation Process of Time Difference Data 
     In the second embodiment, the laser control unit  18  performs a calculation process of the time difference data ΔT(k) illustrated in  FIG. 19 , in addition to the setting process of the charged voltage V illustrated in  FIG. 7  in the comparative example. Hereinafter, the calculation process of the time difference data ΔT(k) will be described with reference to a flowchart illustrated in  FIG. 19 . 
     First, in step S 401 , the laser control unit  18  receives the data on the target pulse width Dt from the external device control unit  3 . Next, in step S 402 , the laser control unit  18  calculates a charging time interval ΔTch required for the pulse width of the pulse laser light PL to be the target pulse width Dt based on the following formula (7). This charging time interval ΔTch refers to a charging timing difference between the Cp(k−1) and the Cp(k) which are adjacent to each other. 
       Δ Tch =( Dt−D 0)/( n− 1)  (7)
 
     Here, D0 is a pulse width of the pulse laser light PL when all of the Cp( 1 ) to Cp(n) have been charged at the same time. D0 is determined in advance experimentally and theoretically. 
     Next, in step S 403 , the laser control unit  18  calculates the time difference data ΔT(k) based on the following formula (8). 
       Δ T ( k )=( k− 1)·Δ Tch   (8)
 
     Next, in step S 404 , the laser control unit  18  transmits the calculated time difference data ΔT(k) to the delay time calculation unit  61  in the synchronization control unit  60 . Next, in step S 405 , the laser control unit  18  determines whether a change signal of the target pulse width Dt has been received from the external device control unit  3 . If the change signal has not been received (S 405 : NO), the laser control unit  18  waits until the change signal is received. If the change signal has been received (S 405 : YES), the laser control unit  18  returns to step S 401 . The above-described process is repeatedly performed. 
     3.2.2 Calculation Process of Delay Time 
       FIG. 20  illustrates a calculation process of the delay time Trd(k) performed by the delay time calculation unit  61 . First, in step S 501 , the delay time calculation unit  61  receives the time difference data ΔT(k) transmitted from the laser control unit  18 . 
     Next, in step S 502 , the delay time calculation unit  61  calculates the delay time Trd(k) based on the following formula (9). 
         Trd ( k )= Trd 0+Δ T ( k )  (9)
 
     Here, Trd0 is a reference delay time, and is a constant value. 
     Next, in step S 503 , the delay time calculation unit  61  transmits the calculated delay time Trd(k) to the internal trigger signal generation unit  62 , and returns to step S 501 . The above-described process is repeatedly performed. 
     3.2.3 Generation Process of Internal Trigger Signal 
     The internal trigger signal generation unit  62  receives and holds the delay time Trd(k) transmitted from the delay time calculation unit  61 , and upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  62  generates the internal trigger signal TR(k) obtained by delaying the external trigger signal TR based on the formula (10). 
         TR ( k )= TR+Trd ( k )  (10)
 
     The internal trigger signal generation unit  62  inputs the generated internal trigger signal TR(k) to the TCS(k). There is the time difference ΔTch between TR(k−1) and T(k). 
     3.2.4 Overall Operation of Gas Laser Device 
       FIG. 21  is a timing chart in the gas laser device  2   b  according to the second embodiment. The overall operation of the gas laser device  2   b  will be described with reference to  FIG. 21 . 
     Upon reception of the data on the target pulse energy Et from the external device control unit  3 , the laser control unit  18  calculates the setting value of the charged voltage V so that the pulse energy E of the pulse laser light PL approaches the target pulse energy Et, and transmits the calculated setting value of the charged voltage V to the charger  11  through the synchronization control unit  60 . 
     In the synchronization control unit  60 , the processing unit  52  in each TCS(k) calculates the delay time Td(k) based on the setting value of the charged voltage V and the reference delay time Td0(k), and transmits the data on the delay time Td(k) to the delay circuit  53 . 
     Upon reception of the data on the target pulse width Dt from the external device control unit  3 , the laser control unit  18  calculates the time difference data ΔT(k), and transmits the calculated time difference data to the delay time calculation unit  61  in the synchronization control unit  60 . The delay time calculation unit  61  calculates the delay time Trd(k) based on the above-described formula (9), and inputs the calculated delay time to the internal trigger signal generation unit  62 . 
     Upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  62  generates the internal trigger signal TR(k) based on the above-described formula (10) to input to the delay circuit  53  and the timer  54  in the TCS(k). As illustrated in  FIG. 21 , there is the time difference among the internal trigger signals TR( 1 ) to TR(n). 
     Upon reception of the internal trigger signal TR(k), the timer  54  in each TCS(k) is reset and starts clocking. The internal trigger signal TR(k) input to each delay circuit  53  is delayed by the delay time Td(k), and is input to the switch  12   a  in the PPM(k) as a switch signal S(k). 
     As illustrated in  FIG. 21 , the switch signals S( 1 ) to S(n) are input to the respective switches  12   a  in the PPM( 1 ) to PPM(n) with time differences thereamong. The respective switches  12   a  in the PPM( 1 ) to PPM(n) are turned ON sequentially for each time difference ΔTch. The pulse-compressed current pulse is output from the PPM(k) to the Cp(k) as an output pulse. As a result, the Cp( 1 ) to Cp(n) are charged sequentially for each time difference ΔTch. 
     The output pulse from each PPM(k) is detected by the output pulse sensor A(k), and the detection signal D 1 ( k ) is transmitted to the timer  54  in the TCS(k). Upon reception of the detection signal D 1 ( k ), the timer  54  stops clocking, and inputs the measured time Tdm(k) from the input of the internal trigger signal TR(k) to the input of the detection signal D 1 ( k ) to the processing unit  52 . Upon reception of the measured time Tdm(k), the processing unit  52  calculates the difference ΔTd(k) between the average value of the measured time Tdm(k) and the target value Tdt, and corrects the reference delay time Td0(k). 
     The Cp(k) is charged by the current pulse, resulting in the high voltage being applied to the first discharge electrode  20   a   k , and the pulse discharge being generated in the discharge space between the first discharge electrode  20   a   k  and the second discharge electrode  20   b   k . As illustrated in  FIG. 21 , the pulse discharge is generated sequentially for each time difference ΔTch. Each pulse discharge results in laser oscillation, and the pulse laser light PL is emitted from the output coupling mirror  15 . The pulse laser light PL is light on which the laser light output for each time difference ΔTch is superimposed, and therefore the pulse width becomes almost target pulse width Dt. 
     The pulse energy E of the pulse laser light PL output from the output coupling mirror  15  is measured by the pulse energy measurement unit  16 . The laser control unit  18  reads the pulse energy E of the pulse laser light measured by the pulse energy measurement unit  16 , and calculates the setting value of the charged voltage V so that the pulse energy E of the pulse laser light PL approaches the target pulse energy Et. The above-described steps are repeated. 
     3.3 Effect 
     In the second embodiment, similarly to the first embodiment, the measured time Tdm(k) from the input of the internal trigger signal TR(k) to each TCS(k) to the output of the output pulse from the PPM(k) is controlled to approach the target value Tdt. Thus, the timings of the internal trigger signals TR( 1 ) to TR(n) are controlled, thereby enabling the charging timings of the Cp( 1 ) to Cp(n) to be controlled with high accuracy. Accordingly, in the second embodiment, the pulse width of the pulse laser light PL can be controlled with high accuracy to approach the target pulse width Dt. 
     In the second embodiment, when Dt is set to D0, and ΔTch is set to zero, the timings of charging the Cp( 1 ) to Cp(n) can coincide with one another as with the first embodiment. 
     In the second embodiment, the pulse energy measurement unit  16  may include a PIN photodiode, or an ultraviolet photoelectric tube such as a biplanar tube, instead of the optical sensor  16   c . In this case, the pulse energy measurement unit  16  can measure the pulse waveform in addition to the pulse energy of the pulse laser light PL. The laser control unit  18  may determine the pulse width based on the pulse waveform measured by the pulse energy measurement unit  16 , and correct the time difference ΔTch so that this pulse width approaches the target pulse width Dt. 
     4. Third Embodiment 
     A gas laser device according to a third embodiment of the present disclosure will be described below. The gas laser device according to the third embodiment enables a pulse waveform of the pulse laser light to be controlled with high accuracy by making the timing of the switch signal and the charged voltage different for each PPM. Hereinafter, the constituent elements that are the same as the constituent elements of the gas laser device  2   b  according to the second embodiment are denoted by the same reference signs, and the description thereof is appropriately omitted. 
     4.1 Configuration 
       FIG. 22  schematically illustrates a configuration of a gas laser device  2   c  according to the third embodiment. In the third embodiment, the external device control unit  3  transmits, to the laser control unit  18 , the data on the target pulse waveform Ft in addition to the external trigger signal TR and the data on the target pulse energy Et. 
     The third embodiment is different from the second embodiment in that the gas laser device  2   c  includes a plurality of chargers  70 . The charger  70  is provided for each PPM(k). In other words, the total number of chargers  70  is n. Hereinafter, the charger  70  corresponding to the PPM(k) is referred to as a CG(k). 
     In the third embodiment, the laser control unit  18  calculates the time difference data ΔT( 1 ) to ΔT(n) described later and the data on the charged voltages V( 1 ) to V(n) based on the data on the target pulse waveform Ft input from the external device control unit  3 , and transmits the calculated time difference data and the data on the charged voltages to a synchronization control unit  60   a.    
       FIG. 23  illustrates a configuration of the synchronization control unit  60   a  according to the third embodiment. The synchronization control unit  60   a  has the same configuration as the synchronization control unit  60  according to the second embodiment except that the data on the charged voltage V(k) received from the laser control unit  18  is input to the processing unit  52  in the corresponding TCS(k). 
     The delay time calculation unit  61  calculates delay times Trd( 1 ) to Trd(n) based on the time difference data ΔT( 1 ) to ΔT(n) input from the laser control unit  18 , and inputs the calculated delay times to the internal trigger signal generation unit  62 . 
     The processing unit  52  in the TCS(k) calculates the delay time Td(k) based on the data on the charged voltage V(k) to input to the delay circuit  53 . The data on the charged voltage V(k) is input to the CG(k) through the processing unit  52  in the TCS(k). 
     The other configurations of the gas laser device  2   c  according to the third embodiment are the same as those of the gas laser device  2   b  according to the second embodiment. 
     4.2 Operation 
     4.2.1 Calculation Process of Time Difference Data and Charged Voltage 
       FIG. 24  illustrates a calculation process of time difference data ΔT(k) and a charged voltage V(k) which is performed by the laser control unit  18  of the third embodiment. 
     First, in step S 601 , the laser control unit  18  receives the data on the target pulse waveform Ft from the external device control unit  3 . Next, in step S 602 , the laser control unit  18  calculates the time difference data ΔT(k) corresponding to the width of the target pulse waveform Ft based on the data on the target pulse waveform Ft. Next, in step S 603 , the laser control unit  18  calculates the charged voltage V(k) corresponding to an intensity distribution of the target pulse waveform Ft. 
     Next, in step S 604 , the laser control unit  18  transmits the calculated time difference data ΔT(k) to the delay time calculation unit  61  in the synchronization control unit  60   a . Next, in step S 605 , the laser control unit  18  transmits the data on the calculated charged voltage V(k) to the processing unit  52  in the TCS(k). 
     Next, in step S 606 , the laser control unit  18  determines whether a change signal of the target pulse waveform Ft has been received from the external device control unit  3 . If the change signal has not been received (S 606 : NO), the laser control unit  18  waits until the change signal is received. If the change signal has been received (S 606 : YES), the laser control unit  18  returns to step S 601 . The above-described process is repeatedly performed. 
     In the third embodiment, the laser control unit  18  controls an attenuator not illustrated without changing the setting value of the charged voltage V(k), so that the pulse energy E measured by the pulse energy measurement unit  16  approaches the target pulse energy Et. In other words, in the third embodiment, the attenuator not illustrated is controlled instead of S 106  and S 107  in the flowchart illustrated in  FIG. 7 . 
     4.2.2 Processing in Trigger Correction Unit 
     In the third embodiment, each TCS(k) performs the similar process to the process illustrated in the flowchart of  FIG. 15 . In the third embodiment, a different charged voltage V(k) for each TCS(k) is input, and therefore the following formula (2′) is used instead of the formula (2). 
       Δ TV ( k )= F ( V   0 )− F [ V ( k )]  (2′)
 
     4.2.3 Overall Operation of Gas Laser Device 
     The overall operation of the gas laser device  2   c  according to the third embodiment will be described. First, upon reception of the data on the target pulse waveform Ft from the external device control unit  3 , the laser control unit  18  calculates the time difference data ΔT(k) and the setting value of the charged voltage V(k), so that the pulse waveform of the pulse laser light PL approaches the target pulse waveform Ft, and transmits the calculated time difference data and setting value of the charged voltage V(k) to the synchronization control unit  60   a.    
     In the synchronization control unit  60   a , the processing unit  52  in each TCS(k) calculates the delay time Td(k) based on the charged voltage V(k) and the reference delay time Td0(k), and transmits the data on the delay time Td(k) to the delay circuit  53 . In the synchronization control unit  60   a , the delay time calculation unit  61  calculates the delay time Trd(k) for each TCS(k) based on the above-described formula (9), and inputs the calculated delay time to the internal trigger signal generation unit  62 . 
     Upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  62  generates the internal trigger signal TR(k) based on the above-described formula (10) to input to the delay circuit  53  and the timer  54  in the TCS(k). Upon reception of the internal trigger signal TR(k), the timer  54  in each TCS(k) is reset and starts clocking. The internal trigger signal TR(k) input to each delay circuit  53  is delayed by the delay time Td(k), and is input to the switch  12   a  in the PPM(k) as a switch signal S(k). 
     The switch signals S( 1 ) to S(n) are input to the respective switches  12   a  in the PPM( 1 ) to PPM(n) with time differences thereamong. The respective switches  12   a  in the PPM( 1 ) to PPM(n) are turned ON sequentially. The pulse-compressed current pulse is output from each PPM(k) to the Cp(k) as an output pulse. As a result, the Cp( 1 ) to Cp(n) are charged sequentially. 
     The output pulse from each PPM(k) is detected by the output pulse sensor A(k), and the detection signal D 1 ( k ) is transmitted to the timer  54  in the TCS(k). Upon reception of the detection signal D 1 ( k ), the timer  54  stops clocking, and inputs the measured time Tdm(k) from the input of the internal trigger signal TR(k) to the input of the detection signal D 1 ( k ) to the processing unit  52 . Upon reception of the measured time Tdm(k), the processing unit  52  calculates the difference ΔTd(k) between the average value of the measured time Tdm(k) and the target value Tdt, and corrects the reference delay time Td0(k). 
     The Cp(k) is charged by the current pulse, resulting in the high voltage being applied to the first discharge electrode  20   a   k , and the pulse discharge being generated in the discharge space between the first discharge electrode  20   a   k  and the second discharge electrode  20   b   k . The voltage applied to the first discharge electrode  20   a   k  varies depending on the charged voltage V(k). 
     Each pulse discharge results in laser oscillation, and the pulse laser light PL is emitted from the output coupling mirror  15 . The pulse laser light PL is light on which a plurality of laser lights generated by the discharge timing corresponding to the time difference data ΔT(k) and the excitation intensity corresponding to the charged voltage V(k) are superimposed, and therefore the pulse waveform becomes almost target pulse waveform Ft. 
     The pulse energy E of the pulse laser light PL output from the output coupling mirror  15  is measured by the pulse energy measurement unit  16 . The laser control unit  18  reads the pulse energy E of the pulse laser light measured by the pulse energy measurement unit  16 , and controls an attenuator not illustrated so that the pulse energy E of the pulse laser light approaches the target pulse energy Et. The above-described steps are repeated. 
     4.3 Effect 
     In the third embodiment, the timings of the internal trigger signals TR( 1 ) to TR(n) and the charged voltages V( 1 ) to V(n) are controlled, thereby enabling the charging timings of the Cp( 1 ) to Cp(n) and the excitation intensity to be controlled with high accuracy. Accordingly, in the third embodiment, the pulse waveform of the pulse laser light PL can be controlled with high accuracy to approach the target pulse waveform Ft. 
     In the third embodiment, the pulse energy measurement unit  16  may include a PIN photodiode, or an ultraviolet photoelectric tube such as a biplanar tube, instead of the optical sensor  16   c . In this case, the pulse energy measurement unit  16  can measure the pulse waveform in addition to the pulse energy of the pulse laser light PL. The laser control unit  18  may determine the difference between the pulse waveform measured by the pulse energy measurement unit  16  and the target pulse waveform Ft, and correct the time difference data ΔT(k) and the charged voltage V(k) so that the difference becomes smaller. 
     5. Fourth Embodiment 
     In the first embodiment, the timing of charging the peaking capacitor is detected, but the time period from when the peaking capacitor is charged to when the discharge is practically generated in the discharge space may vary. This is caused by the variation in gas pressure of the laser gas, for example. The gas laser device according to the fourth embodiment enables variation in time period from the input of the external trigger signal to the practical generation of discharge to be suppressed. Hereinafter, the constituent elements that are the same as the constituent elements of the gas laser device  2   a  according to the first embodiment are denoted by the same reference signs, and the description thereof is appropriately omitted. 
     5.1 Configuration 
       FIG. 25  schematically illustrates a configuration of a gas laser device  2   d  according to the fourth embodiment. In the fourth embodiment, a discharge sensor  80  is provided on the side opposite to the laser chamber  10  with respect to the rear mirror  14 . The discharge sensor  80  includes a focusing optical system  80   a , and an optical sensor  80   b . The optical sensor  80   b  is a sensor sensitive to visible light, and includes a photodiode, or a photoelectric tube. 
     The rear mirror  14  is configured of a substrate coated with a multilayer film which allows the visible light to pass therethrough at high transmittance and allows the pulse laser light to be reflected at high reflectivity. The discharge light generated in the discharge space includes ultraviolet laser light and visible light. The focusing optical system  80   a  focuses the visible light which is emitted from the inside of the laser chamber  10  through the window  21   b  and is transmitted through the rear mirror  14  on the light collecting face of the optical sensor  80   b . Upon detection of the visible light, the optical sensor  80   b  transmits the detection signal D 2  to the synchronization control unit  60   b.    
       FIG. 26  illustrates a configuration of the synchronization control unit  60   b  according to the fourth embodiment. The synchronization control unit  60   b  includes a delay time correction unit  81  and a timer  82 , in addition to the configuration of the synchronization control unit  60  according to the first embodiment. The external trigger signal TR is input to the timer  82  from the laser control unit  18 . The detection signal D 2  is input to the timer  82  from the optical sensor  80   b.    
     The timer  82  starts clocking upon input of the external trigger signal TR and stops clocking upon input of the detection signal D 2 . In other words, as illustrated in  FIG. 27 , the timer  82  measures the time Trdm required from the time of inputting the external trigger signal TR to the time of inputting the detection signal D 2 . The timer  82  inputs the data on the measured time Trdm to the delay time correction unit  81 . 
     The delay time correction unit  81  calculates the delay time Trd(k) based on the data on the measured time Trdm input from the timer  82 . The delay time Trd(k) represents a time period from when the internal trigger signal generation unit  62  receives the external trigger signal TR to when the internal trigger signal generation unit  62  outputs the internal trigger signal TR(k), in other words, the delay time of the internal trigger signal TR(k) to the external trigger signal TR. 
     The other configurations of the gas laser device  2   d  according to the fourth embodiment are the same as those of the gas laser device  2   a  according to the first embodiment. 
     5.2 Operation 
     5.2.1 Correction Process of Delay Time of Internal Trigger Signal to External Signal 
       FIG. 28  is a flowchart illustrating a correction process of a delay time Trd(k) by the delay time correction unit  81 . The delay time correction unit  81  corrects the delay time Trd(k) by the following process. 
     First, in step S 701 , the delay time correction unit  81  resets variables as follows. 
         I= 0 
         Trdmsum= 0 
     Here, I is a counter for counting the number of oscillation pulses. Trdmsum is a total value for calculating the average value of the measured time Trdm measured by the timer  82 . 
     Next, in step S 702 , the delay time correction unit  81  sets all of delay times Trd( 1 ) to Trd(n) to a reference delay time Trd0. Next, in step S 703 , the delay time correction unit  81  transmits the data on the delay time Trd(k) to the internal trigger signal generation unit  62 . 
     Next, in step S 704 , the delay time correction unit  81  determines whether the gas laser device  2   d  has performed laser oscillation. Whether the gas laser device  2   d  has performed laser oscillation is determined based on whether the timer  82  has received the detection signal D 2  from the optical sensor  80   b . If the gas laser device  2   d  has performed laser oscillation (S 704 : YES), the delay time correction unit  81  proceeds to step S 705 . If the gas laser device  2   d  has not performed laser oscillation (S 704 : NO), the delay time correction unit  81  waits until the gas laser device  2   d  performs laser oscillation. 
     In step S 705 , the delay time correction unit  81  adds 1 to the present value of the counter I to update the value of I. Next, in step S 706 , the delay time correction unit  81  receives the data on the measured time Trdm from the timer  82 . Next, in step S 707 , the delay time correction unit  81  adds the measured time Trdm to the present total value Trdmsum to update the total value Trdmsum. 
     Next, in step S 708 , the delay time correction unit  81  determines whether the value of the counter I has reached a predetermined value Imax representing the number of samples. If the value of the counter I has not reached the predetermined value Imax (S 708 : NO), the delay time correction unit  81  returns to step S 704 . If the value of the counter I has reached the predetermined value Imax (S 708 : YES), the delay time correction unit  81  proceeds to step S 709 . 
     In step S 709 , the delay time correction unit  81  calculates the difference ΔTrd between the average value of the measured time Trdm and the target value Trdt. The difference ΔTrd is calculated by the following formula (11). 
       Δ Trd=Trdmsum ( k )/ I max− Trdt   (11)
 
     Next, in step S 710 , the delay time correction unit  81  calculates a new reference delay time Trd0 which is a value obtained by subtracting the difference ΔTrd from the reference delay time Trd0. Thus, after correcting the reference delay time Trd0, the delay time correction unit  81  returns to step S 701 . The above-described process is repeated. 
     Upon reception of the external trigger signal TR from the laser control unit  18 , the internal trigger signal generation unit  62  generates the internal trigger signal TR(k) obtained by delaying the external trigger signal TR by the delay Trd(k), and inputs the generated internal trigger signal to the TCS(k). 
     As described above, in steps S 704  to S 709 , a third correction process (drift correction process) for correcting the timing of the switch signal S(k) is performed based on the detection result of the optical sensor  80   b.    
     It is preferable that the number of samples Imax is larger than the above-described number of samples Jmax, and particularly, is 2,000 or more and 100,000 or less. In other words, it is preferable that the frequency of the third correction process is lower than the frequency of the second correction process. 
     5.2.2 Overall Operation of Gas Laser Device 
     Hereinafter, the overall operation of the gas laser device  2   d  according to the fourth embodiment will be described. Upon reception of the external trigger signal TR from the external device control unit  3 , the laser control unit  18  inputs the external trigger signal TR to the timer  82  and the internal trigger signal generation unit  62 . Upon reception of the external trigger signal TR, the timer  82  is reset and starts clocking. 
     Upon reception of the external trigger signal TR, the internal trigger signal generation unit  62  generates the internal trigger signal TR(k) obtained by delaying the external trigger signal TR by the delay Trd(k) input from the delay time correction unit  81 , and inputs the generated internal trigger signal to the TCS(k). After this, the operation similar to the first embodiment is performed, and pulse discharge is generated in the discharge space in the laser chamber  10 . At this time, the ultraviolet laser light is emitted, and the visible light is emitted. A part of this visible light is transmitted through the rear mirror  14 , and is detected by the optical sensor  80   b.    
     Upon detection of the visible light, the optical sensor  80   b  transmits the detection signal D 2  to the timer  82 . Upon reception of the detection signal D 2 , the timer  82  stops clocking, and inputs the measured time Trdm from the input of the external trigger signal TR to the input of the detection signal D 2  to the delay time correction unit  81 . Upon reception of the measured time Trdm, the delay time correction unit  81  performs the above-described process, calculates the difference ΔTrd between the average value of the measured time Trdm and the target value Trdt, and corrects the reference delay time Trd0. 
     The other operations of the gas laser device  2   d  according to the fourth embodiment are the same as those of the gas laser device  2   a  according to the first embodiment. 
     5.3 Effect 
     In the fourth embodiment, the reference delay time Trd0 of the internal trigger signal TR(k) to the external trigger signal TR is corrected based on the difference ΔTrd between the average value of the measured time Trdm and the target value Trdt. The difference ΔTrd calculated in the next cycle is corrected by the difference ΔTrd. Thus, the timing of the switch signal S(k) is corrected, so that the measured time Trdm approaches the target value Trdt. 
     Thus, the synchronization control unit  60   b  performs the third correction process for correcting the timing of the switch signal S(k) based on the detection result of the discharge timing by the optical sensor  80   b . As a result, variation in time period from the input of the external trigger signal TR to the gas laser device  2   d  to the practical generation of discharge can be suppressed. 
     When the optical resonator of the free-run oscillation including the rear mirror  14  and the output coupling mirror  15  is used as with the fourth embodiment, the loss is small. Therefore, the timing of discharge approximately coincides with the timing of the pulse laser light PL output from the output coupling mirror  15 . Thus, in the fourth embodiment, accuracy in synchronization between the external trigger signal TR and the pulse laser light PL is improved, thereby improving the accuracy of the partial processing when the gas laser device  2   d  is applied to the processing laser device, and the accuracy of the laser irradiation when the gas laser device  2   d  is applied to the laser annealing device. 
     In the fourth embodiment, the discharge sensor  80 , the delay time correction unit  81 , and the timer  82  are added to the gas laser device  2   a  according to the first embodiment. These may be added to the gas laser device  2   b  according to the second embodiment or the gas laser device  2   c  according to the third embodiment, so that the reference delay time Trd0 is corrected as with the fourth embodiment. 
     In the fourth embodiment, the discharge timing is detected by the discharge sensor  80  which is disposed on a back surface side of the rear mirror  14 , and the discharge timing may be detected by the optical sensor  16   c  included in the pulse energy measurement unit  16 . 
     6. Specific Example of Output Pulse Sensor 
     Hereafter, a specific example of the output pulse sensor  40  for detecting the charging timing of the peaking capacitor  27  will be described. The output pulse sensor  40  includes two types being a current detection system and a voltage detection system. 
     6.1 Output Pulse Sensor in Current Detection System 
       FIG. 29  illustrates a specific example of an output pulse sensor in the current detection system. In  FIG. 29 , the output pulse sensor  40   a  is a current sensor including a magnetic core  91 , a coil  92 , and a voltmeter  93 . A wire connecting the magnetic switch MS 2  and the peaking capacitor  27  is inserted into a hollow portion of the magnetic core  91 . The coil  92  is wound around a part of the magnetic core  91 , and both ends of the coil  92  are connected to the voltmeter  93 . The voltmeter  93  detects an induced voltage which is generated in the magnetic core  91  when the current pulse flows in the above-described wire. The detected voltage of the induced voltage is transmitted to the timer  54  as the above-described detection signal D 1 ( k ). 
     The output pulse sensor may be a current sensor including Rogosky coils. The output pulse sensor may be a hall element current sensor in which a hall element is arranged in a gap part in the magnetic core. 
       FIG. 30  illustrates a specific example of an output pulse sensor for detecting a charging timing based on a waveform of a current flowing through the peaking capacitor  27 . In  FIG. 30 , the output pulse sensor  40   b  includes a current sensor  94 , an amplifier  95 , and a comparator  96 . The current sensor  94  is disposed between the magnetic switch MS 2  and the peaking capacitor  27 , and detects a current flowing through the peaking capacitor  27  to output the current to the amplifier  95 . The amplifier  95  converts the current input from the current sensor  94  into the voltage Vcplm to output the voltage to the comparator  96 . 
     As shown in  FIG. 31 , the comparator  96  compares the voltage Vcplm input from the amplifier  95  with the reference voltage Vcpls, and outputs a constant voltage Vcplp when the voltage Vcplm is lower than the reference voltage Vcpls. This voltage Vcplp is in a pulse form, and is transmitted to the timer  54  as the above-described detection signal D 1 ( k ). 
     The reference voltage Vcpls is set to a negative value close to zero to detect the rising and falling timings of the voltage Vcplm. It is preferable that the timer  54  detects the rising timing of the voltage Vcplp. In this case, the charging start timing of the peaking capacitor  27  can be detected. 
     The timer  54  may detect the falling timing of the voltage Vcplp. In this case, the charging completion timing of the peaking capacitor  27  can be detected. Since the charging completion timing is close to the discharge timing in the discharge space, the detection of the charging completion timing enables the discharge timing to be detected with high accuracy. 
     6.2 Output Pulse Sensor in Voltage Detection System 
       FIG. 32  illustrate a specific example of an output pulse sensor in the voltage detection system. In  FIG. 32 , the output pulse sensor  40   c  includes a voltmeter  100  which is connected in parallel with the peaking capacitor  27 . The voltmeter  100  detects the voltage applied to the peaking capacitor  27  from the PPM  12 . This detection voltage is transmitted to the timer  54  as the above-described detection signal D 1 ( k ). 
       FIG. 33  illustrates a specific example of an output pulse sensor for detecting a charging timing based on a waveform of a voltage applied to the peaking capacitor  27 . In  FIG. 33 , the output pulse sensor  40   d  includes an amplifier  101 , and a comparator  102 . The amplifier  101  is connected to the wire between the magnetic switch MS 2  and the peaking capacitor  27 . The voltage applied to the peaking capacitor  27  is input to the amplifier  101 . The amplifier  101  converts the voltage applied to the peaking capacitor  27  into the voltage Vcpm to output the converted voltage to the comparator  102 . 
     As shown in  FIG. 34 , the comparator  102  compares the voltage Vcpm input from the amplifier  101  with the reference voltage Vcps, and outputs a constant voltage Vcpp when the voltage Vcpm is lower than the reference voltage Vcps. This voltage Vcpp is in a pulse form, and is transmitted to the timer  54  as the above-described detection signal D 1 ( k ). 
     The reference voltage Vcps is set to a negative value close to zero to detect the rising and falling timings of the voltage Vcpm. The timer  54  detects the rising timing of the voltage Vcpp or the falling timing of the voltage Vcpp. 
     7. Specific Example of Discharge Sensor 
       FIG. 35  illustrates a specific example of an optical sensor  80   b  included in a discharge sensor  80 . The optical sensor  80   b  includes a photodiode  110 , an amplifier  111 , and a comparator  112 . The photodiode  110  is sensitive to visible light, and outputs the current corresponding to the light intensity of the received visible light to the amplifier  111 . The amplifier  111  converts the current input from the photodiode  110  into the voltage Vpm to output the converted voltage to the comparator  112 . 
     As shown in  FIG. 36 , the comparator  112  compares the voltage Vpm input from the amplifier  111  with the reference voltage Vps, and outputs a constant voltage Vpp when the voltage Vpm is higher than the reference voltage Vps. This voltage Vpp is in a pulse form, and is transmitted to the timer  82  as the above-described detection signal D 2 . 
     The reference voltage Vps is set to a positive value close to zero to detect the rising and falling timings of the voltage Vpm. It is preferable that the timer  82  detects the rising timing of the voltage Vpp. In this case, the discharge timing in the discharge space can be detected with high accuracy. 
     The above-described embodiments and specific examples may be combined unless any contradiction occurs. The descriptions provided above are intended to provide just examples without any limitations. Accordingly, it will be obvious to those skilled in the art that change can be made to the embodiments of the present disclosure without departing from the scope of the accompanying claims. 
     The terms used in the present specification and in the entire scope of the accompanying claims should be construed as terms “without limitations.” For example, a term “including” or “included” should be construed as “not limited to that described to be included.” A term “have” should be construed as “not limited to that described to be held.” Moreover, a modifier “a/an” described in the present specification and in the accompanying claims should be construed to mean “at least one” or “one or more.”