Abstract:
The present invention provides a deep ultraviolet laser apparatus exhibiting high robustness which can generate laser beams in a wavelength region of wavelengths of from 198.3 to 198.8 nm, further may be loaded on a variety of apparatuses as a light source for lighting, and is practicable and a size of the whole structure of thereof is reduced. The deep ultraviolet laser apparatus is arranged in such that laser beams having a wavelength of from 1064 to 1065 nm pulse-output from a first light source is a first fundamental wave; fourth harmonic obtained by wavelength-converting the first fundamental wave by means of a first wave-length conversion means is a second fundamental wave; laser beams having a wavelength of from 1560 to 1570 nm pulse-output from a second light source is a third fundamental wave; second harmonic obtained by wavelength-converting the third fundamental wave by means of a second wave-length conversion means is a fourth fundamental wave; and laser beams having a wavelength of from 198.3 to 198.8 nm which are a sum-frequency light of the second fundamental wave and the fourth fundamental wave are generated by means of a sum-frequency wave generation means.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a deep ultraviolet laser apparatus, and more particularly to a deep ultraviolet laser apparatus for generating deep ultraviolet laser beam by utilizing nonlinear optical effects.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, a micropattern used for manufacturing semiconductors and the like is recognized and inspected by the use of a laser beam wherein a continuous light source is used frequently as a light source applied in lighting for such recognition and inspection.  
         [0005]     Recently, with the progress of further intensifying the micropattern used for inspection and the like, a development directing to making the wavelength shorter in the light source used for lighting the inspection and the like has been made in order to improve optical resolution.  
         [0006]     In these circumstances, such a manner that a laser beam of long wavelength laser oscillating continuously is wavelength-converted to the side of a shorter wavelength by using the long wavelength laser as its fundamental harmonic is carried out for obtaining a continuous light source of a short wavelength. More specifically, the manner is such that a long wavelength laser beam is made to be a shorter wavelength laser beam by means of sum-frequency generation with the use of the laser beam having a longer wavelength than a desired wavelength as its fundamental harmonic while maintaining continuous oscillation of the laser beam of such longer wavelength than the desired wavelength.  
         [0007]     Such wavelength conversion as mentioned above is in a nonlinear process utilizing nonlinear optical effects; and a high electric field is required for improving the conversion efficiency therefor.  
         [0008]     In this respect, however, continuous oscillation of a long wavelength laser beam provides essentially a low electric field; so that it is pointed out that a special conversion technique is required for implementing the wavelength conversion.  
         [0009]     As the special conversion technique, for example, it is proposed to use a nonlinear crystal being a nonlinear medium inside a resonator having a structure for confining fundamental harmonic in order to improve the electric field intensity in a nonlinear medium.  
         [0010]     As the structure of a resonator for sum-frequency generation being an example of such proposal as mentioned above, a manner according to an internal resonator wherein a laser amplification medium is located in the resonator; and a manner according to an external resonator wherein a fundamental harmonic generation source is allowed to be independent from a resonator for sum-frequency generation are known (for example, see Japanese Patent Application Laid-Open No. 10-341054 submitted as a patent literary document 1).  
         [0011]     However, there is such a problem that when a resonator device having the structure as described above is introduced into a part of a light source for generating a fundamental harmonic (a laser beam of a long wavelength), the whole structure of the apparatus becomes extremely jumboized. Besides there is a problem of being easily affected by disturbance, and in addition, there is a problem of requiring much time for the maintenance because of its complicated structure of the apparatus. Moreover, there are such problem that a run length of the apparatus during which no maintenance is required is restricted; and the like problems.  
         [0012]     More than that described above, when, for example, an argon laser or the like is used for a part of a light source for generating a fundamental harmonic (a laser beam of a long wavelength), there is such a problem that the whole structure of the apparatus becomes further jumboized.  
         [0013]     On the other hand, a short wavelength laser light source wherein pulsed laser radiation by which a high electric field is easily obtained has heretofore been also proposed. In the research paper submitted as non-patent literary document 1 and Japanese Patent Application Laid-Open No. 2001-83557 submitted as patent literary document 2, a short wavelength laser beam source for generating pulsed laser radiation of 193 nm wavelength by means of higher harmonics of a fundamental wave is disclosed.  
         [0014]     In the meantime, the inventors of this application has found that the shortest wavelength which is the most suitable for using transmission optics as a short wavelength light source for inspection is around 197 nm. The main reasons of that the wavelength of around 197 nm is preferred are in that quartz components may be used as the optical components, and that absorption of air may be ignored. As described in the research paper submitted as a non-patent literary document 2, a part of the inventor realizes a light source having 199 nm wavelength as to continuous light.  
         [0015]     As described above, although the shortest wavelength which is the most suitable for using transmission optics as a short wavelength light source for inspection is around 197 nm, a short wavelength is realized by high harmonic generation of a fundamental wave in a conventional short wavelength laser beam source wherein pulsed laser radiation is used. Accordingly, there is such a problem that no generated wavelength is obtained in a wavelength region of the shortest wavelength of around 197 nm which is the most suitable for using the transmission optics as the short wavelength light source for inspection.  
         [0016]     Moreover, a variety of all the conventional light source apparatuses as described above have a large size; so that it is difficult to unite them with a section to be lit as a lighting source, and as a result, there is such a problem that a good deal of effort must be expensed for operation and maintenance of the optical axis.  
         [0017]     Patent literary document 1: Japanese Patent Application Laid-Open No. 10-341054  
         [0018]     Patent literary document 2: Japanese Patent Application Laid-Open No. 2001-83557  
         [0019]     Non-patent literary document 1: The 23rd Annual Conference on Lasers and Electro-Optics (CLEO 2003) and the 11th Quantum Electronics and Laser Science Conference (QELS 2003) Research Paper No. CTuT4  
         [0020]     Non-patent literary document 2: OSA TOPS83 (2003) 380-383  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0021]     The present invention has been made in view of the above-described problems involved in the prior art, and an object of the invention is to provide a deep ultraviolet laser apparatus exhibiting high robustness which can generate laser beams in a wavelength region of wavelengths of from 198.3 to 198.8 nm, further may be loaded on a variety of apparatuses as a light source for lighting, and is practicable and a size of the whole structure thereof is reduced.  
         [0022]     In order to achieve the above-described object, the deep ultraviolet laser apparatus according to the present invention is arranged to generate laser beams having 198.3 nm to 198.8 nm wavelengths by means of sum-frequency mixing of fourth harmonic of laser beams having 1064 to 1065 nm wavelengths and second harmonic of laser beams having 1560 to 1570 nm wavelengths.  
         [0023]     Namely, the present invention may be a deep ultraviolet laser apparatus including laser beams having a wavelength of from 1064 to 1065 nm pulse-output from a first light source being a first fundamental wave; fourth harmonic obtained by wavelength-converting the first fundamental wave by means of a first wave-length conversion means being a second fundamental wave; laser beams having a wavelength of from 1560 to 1570 nm pulse-output from a second light source being a third fundamental wave; second harmonic obtained by wavelength-converting the third fundamental wave by means of a second wave-length conversion means being a fourth fundamental wave; and laser beams having a wavelength of from 198.3 to 198.8 nm which are a sum-frequency light of the second fundamental wave and the fourth fundamental wave being generated by means of a sum-frequency wave generation means.  
         [0024]     Furthermore, in the deep ultraviolet laser apparatus according to the present invention, the first light source and the first wavelength conversion means may be installed in casings independent from one another wherein the first light source is connected to the first wavelength conversion means through a first optical fiber for transmission; and the second light source and the second wavelength conversion means may be installed in casings independent from one another wherein the second light source is connected to the second wavelength conversion means through a second optical fiber for transmission.  
         [0025]     Moreover, in the deep ultraviolet laser apparatus according to the present invention, the first light source may be composed of a first semiconductor laser pulse-outputting laser beams having a wavelength of from 1064.0 to 1065.0 nm by means of current modulation, and the first optical fiber amplifier for amplifying the laser beams having the wavelength of 1064.0 to 1065.0 nm output from the first semiconductor laser; and the second light source may be composed of a second semiconductor laser pulse-outputting laser beams having a wavelength of from 1560 to 1570 nm by means of current modulation, and the second optical fiber amplifier for amplifying the laser beams having the wavelength of 1560 to 1570 nm output from the second semiconductor laser.  
         [0026]     Still further, in the deep ultraviolet laser apparatus according to the present invention, the sum-frequency generation means may contain a nonlinear optical crystal; and the second fundamental wave and the fourth fundamental wave may be axially input to the nonlinear optical crystal.  
         [0027]     Yet further, in the deep ultraviolet laser apparatus according to the present invention, the second fundamental wave and the fourth fundamental wave may be input to the nonlinear optical crystal through a single condensation system.  
         [0028]     Besides, in the deep ultraviolet laser apparatus according to the present invention, a current modulation frequency of the first semiconductor laser and the second semiconductor laser may be 100 kHz or more.  
         [0029]     Since the present invention is constituted as described above, it provides such an excellent advantageous effect that the resulting deep ultraviolet apparatus exhibiting high robustness can generate laser beams in a wavelength region of wavelengths of from 198.3 to 198.8 nm, further may be loaded on a variety of apparatuses as a light source for lighting, and is practicable and a size of the whole structure thereof is reduced.  
         [0030]     The present invention as mentioned above is applicable in case of manufacturing pulse width variable laser apparatuses in laser apparatus makers, implementing experiments wherein a pulsed laser is used in a variety of laser experimental facilities, or in the like cases.  
         [0031]     Furthermore, the present invention is applicable for a light source of lighting used in recognition or inspection of micropattern, a system for generating the light therefor and the like in semiconductor manufacturing fields and the like. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0032]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0033]      FIG. 1  is a conceptual constitutional explanatory view showing a deep ultraviolet laser apparatus according to an example of a manner of practice of the present invention; and  
         [0034]      FIG. 2  is a schematic constitutional explanatory diagram showing optics being the substantial part of the deep ultraviolet laser apparatus shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     In the following, an example of a manner of practice of the deep ultraviolet laser apparatus according to the present invention will be described in detail by referring to the accompanying drawings.  
         [0036]      FIG. 1  is a conceptual constitutional explanatory view showing a deep ultraviolet laser apparatus according to an example of a manner of practice of the present invention, and  FIG. 2  is a schematic constitutional explanatory diagram showing optics being the substantial part of the deep ultraviolet laser apparatus shown in  FIG. 1 .  
         [0037]     Referring appropriately to  FIGS. 1 and 2 , the deep ultraviolet laser apparatus  10  according to the present invention is composed of a first fundamental wave light source  12  set up on a usual floor and for generating a first fundamental wave, a second fundamental wave light source  14  set up on a usual floor and for generating a second fundamental wave, a wavelength conversion section  20  connected to the first fundamental wave light source  12  through a first optical fiber  16  for transmission and further connected to the second fundamental wave light source  14  through a second optical fiber  18  for transmission, a scanning section  22  connected to the wavelength conversion section  20  to irradiate a deep ultraviolet laser beam output from the wavelength conversion section  20  onto a lighting object (not shown) disposed inside a section to be lit  24  (which will be mentioned later) while scanning the lighting object, and the section to be lit  24  set up on a vibration-free floor, containing the lighting object (not shown) inside thereof, and connected to a scanning section  22 .  
         [0038]     The first fundamental wave light source  12 , the second fundamental wave light source  14 , the wavelength conversion section  20 , the scanning section  22 , and the section to be lit  24  are placed respectively in an independent casing.  
         [0039]     The first fundamental wave light source  12  is composed of a first semiconductor laser  102  for outputting laser beams having 1064 to 1065 nm wavelengths, a first optical fiber amplifier  106  for amplifying the laser beams having 1064 to 1065 nm wavelengths output from the first semiconductor laser  102 , and a first pulse current source  110  for driving the first semiconductor laser  102 .  
         [0040]     In the manner of practice, a quartz fiber to which Yb is added as an active substance is used for the first optical fiber amplifier  106 .  
         [0041]     Furthermore, since exclusive parts which have been heretofore well known as optical communication parts may be used in case of connecting respective optics constituting the above-described first fundamental wave light source  12 , explanations for the detailed constitutions and functions thereof are omitted.  
         [0042]     Moreover, all the polarized phases of the respective optics constituting the first fundamental wave light source  12  are conserved.  
         [0043]     The output from the first fundamental wave light source  12  is transmitted to the wavelength conversion section  20  through the first optical fiber  16  for transmission connected to the end of the first optical fiber amplifier  106 .  
         [0044]     Next, the second fundamental wave light source  14  is composed of a second semiconductor laser  104  for outputting laser beams having 1560 to 1570 nm wavelengths, a second optical fiber amplifier  108  for amplifying the laser beams having 1560 to 1570 nm wavelengths output from the second semiconductor laser  104 , and a second pulse current source  112  for driving the second semiconductor laser  104 . In the manner of practice, a quartz fiber to which Er is added as an active substance is used for the second optical fiber amplifier  108 .  
         [0045]     Furthermore, since exclusive parts which have been heretofore well known as optical communication parts may be used in case of connecting respective optics constituting the above-described second fundamental wave light source  14 , explanations for the detailed constitutions and functions thereof are omitted.  
         [0046]     Moreover, all the polarized phases of the respective optics constituting the second fundamental wave light source  14  are conserved.  
         [0047]     The output from the second fundamental wave light source  14  is transmitted to the wavelength conversion section  20  through the second optical fiber  18  for transmission connected to the end of the second optical fiber amplifier  108 .  
         [0048]     Next, the wavelength conversion section  20  is composed of a first condenser lens  114  for condensing laser beams having 1064 to 1065 nm wavelengths output from the end of the first optical fiber  16  for transmission connected to the first optical fiber amplifier  106 , a first nonlinear optical crystal  118  for inputting laser beams having 1064 to 1065 nm output from the first condenser lens  114  to output laser beams having 532 to 532.5 nm wavelengths as second harmonic, a second nonlinear optical crystal  120  for inputting the laser beams having 532 to 532.5 nm wavelengths output from the first nonlinear optical crystal  118  to output laser beams having 266 to 266.25 nm wavelengths as fourth harmonic of the laser beams having 1064 nm to 1065 nm wavelengths, a second condenser lens  116  for condensing the laser beams having 1560 to 1570 mm wavelengths output from the end of the second optical fiber  18  for transmission connected to the second optical fiber amplifier  108 , a third nonlinear optical crystal  122  for inputting the laser beams having 1560 to 1570 nm wavelengths output from the second condenser lens  116  to output laser beams having 780 to 785 nm wavelengths as the second harmonic, a reflection mirror  124  for reflecting the laser beams having 266 to 266.25 nm wavelengths output from the second nonlinear optical crystal  120 , a coupling mirror  126  for coupling the laser beams having 266 to 266.25 nm wavelengths reflected by the reflection mirror  124  with the laser beams having 780 to 785 nm wavelengths output from the third nonlinear optical crystal  122 , a matching lens system  128  for matching the light beams output from the coupling mirrors  126 , a sum-frequency generation nonlinear optical crystal  130  for generating laser beams having 198.3 to 198.8 nm wavelengths by means of wavelength conversion due to sum-frequency generation of the laser beams having 266 to 266.25 nm wavelengths output from the matching lens system  128  and laser beams having 780 to 785 nm wavelengths, and a collimator lens  132  for outputting the laser beams having 198.3 to 198.8 nm wavelengths output from the sum-frequency generation optical crystal  130  as parallel lights.  
         [0049]     In the manner of practice, a KTP crystal is used as the first nonlinear optical crystal  118 , a BBO crystal is used as the second nonlinear optical crystal  120 , a LBO crystal is used as the third nonlinear optical crystal  122 , and a BBO crystal is used as the fourth nonlinear optical crystal  130 .  
         [0050]     In the above-described constitution, operations of the deep ultraviolet laser apparatus  10  will be described wherein a semiconductor laser having 1064 nm laser oscillation wavelength and exhibiting 100 mW average output in case of continuous operation is used as the first semiconductor laser  102 , while another semiconductor laser having 1562 nm laser oscillation wavelength and exhibiting 80 mW average output in case of continuous operation is used as the second semiconductor laser  104 .  
         [0051]     In order to laser-oscillate the above-described first semiconductor laser  102 , when the first pulse current source  110  is driven in such that the driving current of the first semiconductor laser  102  becomes 2 MHz and the pulse width of its driving current waveform is made to be 2 ns, a laser pulse having 1.5 ns pulse width is obtained as a laser beam having 1064 nm wavelength output from the first semiconductor laser  102 ; and the average output obtained at that time was 0.3 mW. When the output of the first semiconductor laser  102  is introduced into the first optical fiber amplifier  106 , 5 W average output is obtained, and the laser beams thus amplified are transmitted to the wavelength conversion section  20  through the first optical fiber  16  for transmission.  
         [0052]     On the other hand, for the sake of laser-oscillating the above-described second semiconductor laser  104 , when the second pulse current source  112  is driven in such that the driving current of the second semiconductor laser  104  becomes 2 MHz and the pulse width of its driving current waveform is made to be 2 ns, a laser pulse having 1.5 ns pulse width is obtained as a laser beam having 1562 nm wavelength output from the second semiconductor laser  104 ; and the average output obtained at that time was 0.24 mW. When the output of the second semiconductor laser  104  is introduced into the second optical fiber amplifier  108 , 5 W average output is obtained, and the laser beams thus amplified are transmitted to the wavelength conversion section  20  through the second optical fiber  18  for transmission.  
         [0053]     It was possible to drive the second semiconductor laser  104  simultaneously with the first semiconductor laser  102  and to operate it in jitter of 80 ps or less.  
         [0054]     In accordance with the manner as described above, the laser beam having 1064 nm wavelength transmitted to the wavelength conversion section  20  through the first optical fiber  16  for transmission is input to the first nonlinear optical crystal  118  wherein second harmonic generation is effected, whereby the laser beam having 532 nm wavelength is output from the first nonlinear optical crystal  118 . Then, the laser beam having 532 nm wavelength output from the first nonlinear optical crystal  118  is thereafter input further to the second nonlinear optical crystal  120  wherein second harmonic is further effected, whereby the laser beam having 266 nm wavelength is output from the second nonlinear optical crystal  120 . The resulting laser beam having 266 nm wavelength is fourth harmonic of the laser beam of 1064 nm.  
         [0055]     On one hand, the laser beam having 1562 nm wavelength transmitted to the wavelength conversion section  20  through the second optical fiber  18  for transmission is input to the third nonlinear optical crystal  122  wherein second harmonic is effected, whereby the laser beam having 781 nm wavelength is output from the third nonlinear optical crystal  122 .  
         [0056]     Furthermore, the laser beam having 266 nm wavelength is coupled with the laser beam having 781 nm wavelength following to the above-described wavelength conversion in the wavelength conversion section  20 .  
         [0057]     Namely, the laser beam having 266 nm wavelength output from the second nonlinear optical crystal  120  turns the light path through the reflection mirror  124 , and is input to the coupling mirror  126 .  
         [0058]     On one hand, the laser beam having 781 nm wavelength output from the third nonlinear optical crystal  122  is also input to the coupling mirror  126 , and is coupled with the laser beam of 266 nm wavelength.  
         [0059]     Then, the laser beam having 266 nm wavelength and the laser beam having 781 nm wavelength coupled with the coupling mirror  126  are input to the fourth nonlinear optical crystal  130  being a nonlinear optical crystal for sum-frequency generation after these coupled laser beams were passed through a lens system  128  wherein a ultraviolet laser beam having about 198.4 nm wavelength which is a sum-frequency of 266 nm wavelength and 781 nm wavelength is output from the fourth nonlinear optical crystal  130  as a result of sum-frequency generation.  
         [0060]     In the case where the laser beam of 266 nm wavelength and the laser beam of 781 nm laser beam are input to the fourth nonlinear optical crystal  130 , if the both laser beams are input coaxially to the fourth nonlinear optical crystal  130 , it becomes possible to maintain a phase matching condition.  
         [0061]     According to the experiments by the inventor of this application, when frequencies of the driving current pulses for driving the first semiconductor laser  102  and the second semiconductor laser  104  are changed in the deep ultraviolet laser apparatus  10 , it was possible to change output repetition frequencies of the deep ultraviolet laser beam within a range of from 100 kHz to 10 MHz.  
         [0062]     In these circumstances, the inventor of this application has found that the deep ultraviolet laser apparatus  10  according to the present invention may be applied to the use application in which a continuous light has been used heretofore so far as the condition is in such that an image acquisition is made with a charge storage device in a conventional system wherein the image acquisition is carried out by the use of a continuous light; and the repetition frequency of its light source is 100 kHz or more. In other words, it has been confirmed by the experiments according to the inventor of this application that when a light source repetition frequency of a value equal to the frame rate or more of the image acquisition means is obtained, the deep ultraviolet laser apparatus  10  may be applied as the light source.  
         [0063]     According to the above-described deep ultraviolet laser apparatus  10 , not only a compact ultraviolet light source having remarkably higher efficiency than that of a conventional light source can be realized, but also the first fundamental wave light source  12  and the second fundamental wave light source  14  may be composed of a semiconductor laser and an optical fiber amplifier being longer lasting devices, whereby it may be intended to make the whole apparatus longer lasting.  
         [0064]     Moreover, according to the deep ultraviolet laser apparatus  10 , since the first fundamental wave light source  12  is connected to the wavelength conversion section  20  through the first optical fiber  16  for transmission, while the second fundamental wave light source  14  is connected to the wavelength conversion section  20  through the second optical fiber  18  for transmission, the wavelength conversion section  20  which is composed of optical parts required for maintenance, respectively, may be contained in a casing independent from the casings for the first and second fundamental wave light sources  12  and  14 , whereby the operation and maintenance of the whole apparatus become easy.  
         [0065]     Furthermore, according to the deep ultraviolet laser apparatus  10 , as the fundamental wave light source  12  and the second fundamental wave light source  14 ; and the wavelength conversion section  20 , the scanning section  22  and the section to be lit  24  are connected through the first optical fiber  16  for transmission, and the second optical fiber  18  for transmission, each group of them may be set out on different floors, respectively, so that a degree of freedom in the constitution may be remarkably improved.  
         [0066]     Still further, in an apparatus which has been used heretofore by the inventor of this application wherein an argon laser is used as the light source, 50 kW electric power and cooling water of 50 liters per minute have been required, but the electric power consumption becomes 500 W which is hundredth part of the former electric power consumption, besides no cooling water is required in the present deep ultraviolet laser apparatus  10 .  
         [0067]     Yet further, in an apparatus which has been used heretofore by the inventor of this application wherein an argon laser is used as the light source, an exclusive place for a space of setting the light source is required, but when only the scanning section  22  and the compact wavelength conversion section  20  are installed in the section to be lit  24 , and when the first fundamental wave light source  12  and the second fundamental wave light source  14  or the like are contained in the same rack as that of a control system of the section to be lit  24 , the place for setting exclusively the light source becomes unnecessary according to the deep ultraviolet laser apparatus  10 .  
         [0068]     Besides, in the case when the section to be lit  24  accompanies oscillations, a vibration-free countermeasure is applied only to the first fundamental wave light source  12  and the second fundamental wave light source  14 , while it is sufficient to apply a usual vibration-free countermeasure to the rest of the components, and thus, it becomes possible to reduce significantly the manufacturing cost.  
         [0069]     It is to be noted that the above-described manners of practice may be modified as described in the following paragraphs (1) to (3).  
         [0070]     (1) In the above-described manners of practice, although the quartz fiber to which Yb is added as an active substance is used as the first optical amplifier  106 , while the other quartz fiber to which Er is added as an active substance is used as the second optical amplifier  108 , the invention is not restricted thereto as a matter of course. Furthermore, a material of the fiber is not limited to quartz, but the other materials may be used as a matter of course, so far as they are transparent materials with respect to a laser beam to be amplified.  
         [0071]     (2) In the above-described manners of practice, although a KTP crystal is used as the first nonlinear optical crystal  118 , a BBO crystal is used as the second nonlinear optical crystal  120 , a LBO crystal is used as the third nonlinear optical crystal  122 , and a BBO crystal is used as the above-described fourth nonlinear optical crystal  130 , the nonlinear optical crystals used in the respective wavelength conversions are not limited thereto as a matter of course, but the other crystals may be properly used, of course, so far as they are transparent in the respective wavelengths in case of the wavelength conversion and they are phase-matched in the respective wavelength conversion processes.  
         [0072]     (3) The above-described manners of practice as well as the modifications described in the above paragraphs (1) and (2) may be properly combined with each other.  
         [0073]     It will be appreciated by those of ordinary skill in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof.  
         [0074]     The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.  
         [0075]     The entire disclosure of Japanese Patent Application No. 2005-271357 filed on Sep. 20, 2005 including specification, claims, drawing and summary are incorporated herein by reference in its entirety.