Patent Publication Number: US-11664641-B2

Title: Light source device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 16/367,121, filed on Mar. 27, 2019, which claims priority to Japanese Patent Application No. 2018-061647, filed on Mar. 28, 2018. The contents of these applications are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a light source device that applies wavelength beam combining (WBC) to emit high-output laser light. 
     2. Description of Related Art 
     Demand for light source devices to emit high-output laser light has increased in various fields including laser processing such as laser soldering. One example of light source devices to emit high-output laser light is a wavelength beam combining device (hereinafter referred to as a “WBC device”). U.S. Pat. No. 6,192,062 describes an example of a WBC device. The WBC device described in U.S. Pat. No. 6,192,062 includes: a light source unit, such as a laser diode (LD) bar, in which a plurality of laser sources (for example, LDs) are arranged and each of the laser sources are configured to emit light with a predetermined gain spectral bandwidth; a collimating lens that collimates laser beams emitted from the laser sources; a condenser lens that condenses the laser beams passing through the collimating lens; a grating on which the laser beams condensed by the condenser lens is incident; and a partially reflecting mirror that is disposed in an optical path of diffracted beams diffracted by the grating. The partially reflecting mirror and each of the plurality of laser sources form an external cavity, and lights resonated in external cavities are each transmitted through partially reflecting mirrors and are combined. 
     SUMMARY OF THE INVENTION 
     In the WBC device, portions of the diffracted beams from the grating are reflected by the partially reflecting mirror and are returned back into the laser source. However, the light source device with the above structure has a great external cavity length between the laser source and the partially reflecting mirror. In one example, the external cavity length is approximately 1 meter. With a great external cavity length, even a slight deviation in position of an optical element constituting the external cavity due to vibration causes a great deviation of the light beam. This may result in a failure of external resonance, and thus the WBC device may not work. 
     One object of the present invention is to provide a light source device in which the external cavity length can be shortened. 
     A light source device according to one embodiment of the present invention includes: a plurality of laser sources each configured to emit light having a peak wavelength in a range of 350 nm to 550 nm with a predetermined gain bandwidth; collimating parts each configured to collimate light emitted from a corresponding one of the laser sources into a substantially parallel light; a combining grating configured to diffract, at an identical diffraction angle, lights that have passed through the collimating part and are incident on the combining grating at different incident angles, to combine the diffracted lights; and plane transmission gratings each disposed in an optical path between a corresponding one of the collimating parts and the combining grating, each of the plane transmission gratings is adjustable so as to allow selection of a wavelength of the light incident on the combining grating. Each of the plane transmission gratings is configured to diffract a portion of light emitted from a corresponding one of the laser sources back to the corresponding one of the laser sources, to cause external resonance between the corresponding one of the laser sources and each of the plane transmission gratings. 
     A light source device according to another embodiment of the present invention includes: a plurality of laser sources each configured to emit light having a peak wavelength in a range of 350 nm to 550 nm with a predetermined gain bandwidth; collimating parts each configured to collimate the light emitted from a corresponding one of the laser sources into a substantially parallel light; a combining grating configured to diffract, at an identical diffraction angle, lights that have passed through the collimating part and are incident on the combining grating at a different incident angle to combine the diffracted light; and volume holographic gratings each disposed in an optical path between a corresponding one of the laser sources and the combining grating, each of the volume holographic gratings determines a wavelength of the light incident on the combining grating. Each of the volume holographic gratings is configured to diffract a portion of the light emitted from a corresponding one of the laser sources back to the laser source, to cause external resonance between the corresponding one of the laser sources and each of the volume holographic gratings. 
     A light source device according to yet another embodiment of the present invention includes: a plurality of laser sources each configured to emit light having a peak wavelength in a range of 350 nm to 550 nm with a predetermined gain bandwidth; collimating parts each configured to collimate an incident light into a substantially parallel light; a combining grating configured to diffract, at an identical diffraction angle, lights each having passed through a corresponding one of the collimating parts and incident on the combining grating at different incident angles, to combine the diffracted light; and fiber Bragg gratings each disposed in an optical path between a corresponding one of the laser sources and a corresponding one of the collimating parts, each of the fiber Bragg gratings determines a wavelength of a light incident on the combining grating. Each of the fiber Bragg gratings is configured to diffract a portion of the light emitted from a corresponding one of the laser sources back to the corresponding laser source, to cause external resonance between the corresponding one of the laser sources and each of the fiber Bragg gratings. 
     According to certain embodiments, a light source device having a short external cavity length can be provided. 
     Problems, configurations, and effects other than the above will become apparent from the following description of certain embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view of a light source device according to a first embodiment of the present invention. 
         FIG.  2    is a schematic view illustrating the incident angle of incident light entering a combining grating and the diffraction angle of diffracted light. 
         FIG.  3    is a schematic view of a light source device according to a second embodiment of the present invention. 
         FIG.  4    is a schematic view of a light source device according to a third embodiment of the present invention. 
         FIG.  5    is a schematic view of a light source device according to a fourth embodiment of the present invention. 
         FIG.  6    is a schematic view of a light source device according to a fifth embodiment of the present invention. 
         FIG.  7    is a schematic view of a light source device according to a sixth embodiment of the present invention. 
         FIG.  8    is a schematic view of a light source device according to a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Certain embodiments of the present invention will be described below with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a schematic view of a light source device according to a first embodiment of the present invention. A light source device  10  of the present embodiment includes a plurality of external cavity modules  100  and a combining grating  110 . Each of the external cavity modules  100  includes a laser source  102 , a collimating part  104 , a plane transmission grating  106 , and a stage  108 . In each external cavity module  100 , the laser source  102 , the collimating part  104 , and the plane transmission grating  106  are disposed at the same stage  108 , so that the external cavity module  100  can be moved integrally. In this case, the laser source  102  is preferably disposed in contact with the stage  108  to allow the laser source  102  to be cooled by cooling the stage  108 . 
     An x axis, a y axis, and a z axis, which are orthogonal to one another, are shown in  FIG.  1    for the convenience of description. In  FIG.  1   , light beams are schematically represented in dashed lines and denoted as light beams  120 . Each of the light beams  120  shown in dashed lines is emitted from the laser source  102 , passes through the collimating part  104  and the plane transmission grating  106 , and then enters the combining grating  110 . Although a light beam has a divergence angle or a width, only the optical axis of the beam is shown in dashed lines in  FIG.  1    for the convenience of description. 
     All of the light beams  120  have their optical axes on a single x-z plane. 
     The laser source  102  may be a laser diode (hereinafter referred to as “LD”) configured to emit light having a peak wavelength in a range of 350 nm to 550 nm with a predetermined gain bandwidth. For example, the laser source  102  may be an LD including a nitride semiconductor configured to emit light having a center wavelength of 410 nm with a gain bandwidth Δλ of 20 nm. In this case, the wavelength of light emitted from the laser source  102  is in a range of 400 nm to 420 nm. 
     When the LD serving as the laser source  102  has a front side from which light is emitted and a rear side opposite to the front side, it is preferable that the front side is provided with an antireflective coating to have a reflectance reduced to nearly 0%, for example, approximately in a range of 0.1% to 2.0%. The rear mirror preferably has a reflectance of nearly 100%, for example, 85% to 99.9%. Because the LD to emit light having a wavelength in a range of 350 nm to 550 nm tends to be deteriorated in the air, the laser source  102  is preferably hermetically sealed. For example, an LD in a can-type package may be used for the laser source  102 . Using the can-type package for the laser source  102  allows for further having an effect of cooling and an effect of electrostatic and electromagnetic shielding. 
     Each collimating part  104  is configured to collimate individual light emitted from a corresponding one of the laser sources  102  into substantially parallel light. Each collimating part  104  may be, for example, a collimating lens disposed opposing a corresponding one of the laser sources  102 . Each collimating part  104  corresponding to a corresponding one of the laser sources  102  may be a single lens or a plurality of lenses in combination. 
     Each plane transmission grating  106  is disposed in an optical path between the collimating part  104  and the combining grating  110  of the same external cavity module  100 . Each plane transmission grating  106  is configured to diffract a portion of light emitted from the laser source  102  of the same external cavity module  100  back to the laser source  102 , to cause external resonance between the laser source  102  and the corresponding plane transmission grating  106 . In more detail, the external resonance occurs between the rear side of the LD that serves as the laser source  102  and the plane transmission grating  106 . That is, a combination of the laser source  102 , the collimating part  104 , and the plane transmission grating  106  forms a single external cavity. The cavity length of the external cavity is defined by the distance between the laser source  102  and the plane transmission grating  106 . The external cavity may have a Littrow configuration. The “Littrow configuration” refers to a configuration that allows light to be diffracted at the same angle as an incident angle and the diffracted light being reflected is fed back to the laser diode along the same route as the incident light. 
     The plane transmission grating  106  having grating grooves is disposed such that the orientation of the grating grooves is parallel to the y axis as shown in  FIG.  1   . The plane transmission grating  106  has a rotation axis parallel to the y axis and is disposed on the stage  108  so as to be rotated about the rotation axis with respect to the stage  108 . When the plane transmission grating  106  is rotated about the rotation axis, the incident angle at which the light emitted from the laser source  102  enters the plane transmission grating  106  changes, so that the wavelength of light resonating in a corresponding external cavity may be selected. Accordingly, the plane transmission grating determines the wavelength of light passing through the plane transmission grating  106  and incident on the combining grating  110 . That is, the plane transmission grating  106  serves as a wavelength selecting element for the external cavity. Rotation of the plane transmission grating  106  may be controlled by a known driving unit in the art, such as a stepper motor. 
     As shown in  FIG.  1   , the light passing through the plane transmission grating  106  of the external cavity module  100  enters the combining grating  110  at a different incident angle. In  FIG.  1   , the light beam  120  is shown in the dashed line extending from the plane transmission grating  106  to the combining grating  110 . 
     For the convenience of description, the light beam  120  is also referred to as the light emitted from the external cavity module  100  in the present specification. The incident angle of each light beam  120  entering the combining grating  110  is defined by the mounting angle of a corresponding external cavity in which the external resonance occurs. More specifically, the incident angle of each light beam  120  can be set and adjusted by selecting the position and the angle of a corresponding one of the external cavity modules  100  relative to the combining grating  110 . For example, the angle of the external cavity module  100  as a whole can be adjusted by rotating the stage  108 , so that the incident angle of the light entering the combining grating  110  can be adjusted. Such an adjustment can be performed by a known device in the art, such as a stepper motor. 
     The combining grating  110  has grating grooves and is disposed such that the orientation of the grating grooves is parallel to the y axis shown in  FIG.  1   . The combining grating  110  is configured to diffract the light beams  120 , each of which passes through the collimating part  104  and the plane transmission grating  106  of a corresponding external cavity module  100  and which enter the combining grating  110  at different incident angles, toward the same diffraction angle to combine the light beam  120  so as to form a combined beam  130 . The combined beam  130  has an optical axis on the x-z plane, on which the optical axes of the light beams  120  are also located. Next, diffracting of the combining grating  110  will be described with reference to  FIG.  2   . 
       FIG.  2    is a schematic view illustrating the incident angle of the light entering the combining grating  110  and the diffraction angle of the diffracted light. In  FIG.  2   , a solid line N-N indicates a normal of the combining grating  110 . Assuming the light beam  120  enters the combining grating  110  at an incident angle of a and the diffracted beam (the combined beam  130 ) is diffracted by the combining grating  110  at a diffraction angle of β, the relationship expressed in Formula (1) is satisfied.
 
sin α+sin β= N·m·λ   Formula (1)
 
     In Formula (1), a indicates the incident angle, β indicates the diffraction angle, N indicates the number of grooves in the combining grating per 1 mm, m indicates an order of diffraction, and λ indicates the wavelength of the light beam. 
     For example, provided that each of the laser sources  102  is configured to emit a laser beam with a center wavelength of 410 nm and a wavelength in range of 400 nm to 420 nm and the combining grating has 2,222 grooves per 1 mm, a first order diffraction occurs such that, when each of the light beams having wavelengths shown in Table 1 enters the combining grating at the corresponding incident angle α, the light beams are diffracted at the same diffraction angle β, so as to form the combined beam. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 λ (nm) 
                 α (degree) 
                 β (degree) 
               
               
                   
               
             
            
               
                 400.53 
                 43.32 
                 11.77 
               
               
                 405.30 
                 44.16 
                 11.77 
               
               
                 410.00 
                 45.00 
                 11.77 
               
               
                 414.63 
                 45.84 
                 11.77 
               
               
                 419.19 
                 46.68 
                 11.77 
               
               
                   
               
            
           
         
       
     
     That is, with the plane transmission grating  106 , a wavelength of a light emitted from each of the external cavity modules  100  is selected to be a corresponding one of 400.53 nm, 405.30 nm, 410.00 nm, 414.63 nm, or 419.19 nm, which are in the column of the wavelength “λ” in Table 1. 
     Then, the position of the external cavity module  100  is adjusted such that light enters the combining grating  110  at the corresponding incident angle shown in the column of the incident angle “α” in Table 1, namely, 43.32 degrees, 44.16 degrees, 45.00 degrees, 45.84 degrees, or 46.68 degrees. Accordingly, the lights emitted from the external cavity modules  100  are diffracted by the combining grating  110  at the same diffraction angle of 11.77 degrees, so that the high-output combined beam  130  is formed. 
       FIG.  1    illustrates an example in which a reflection grating is used for the combining grating  110 ; however, a transmission grating may be used for the combining grating  110 . The transmission grating absorbs less light compared with the reflection grating, and thus is less easily damaged. With the reflection grating, only a zero-order reflecting light and a first-order diffracted light is obtained, which allows the stray light to be reduced. 
     In the present embodiment, the external cavity is formed between the laser source  102  and the plane transmission grating  106  in each external cavity module  100 , so that a single longitudinal mode oscillation at the selected wavelength is obtained. The length of the external cavity is defined by a distance between the laser source  102  and the plane transmission grating  106 . Reduction in the distance between the laser source  102  and the plane transmission grating  106  increases a tolerance of deviation of the position of optical elements that constitute the external cavity is, which enables the light source device  10  to have a high vibration resistance. For example, along with increase in output of a light source device, a water-cooling system is required for the light source device; however, the operation of the external cavity may become unstable due to the pulsation of cooling water. Also, an external cavity with a long cavity length requires precise adjustment during initial assembly, which leads to the difficulty in the assembly. According to the present embodiment, such a high precision adjustment is not required during initial assembly, so that the light source device can be easily assembled. 
     Second Embodiment 
       FIG.  3    is a schematic view of a light source device according to a second embodiment of the present invention. A light source device  20  in the present embodiment is a modification of the light source device  10 . In the present embodiment, members, portions, components, and elements having the same functions as the first embodiment are denoted by the same reference numerals as those in the first embodiment, and duplicative description thereof may be omitted. 
     The light source device  20  includes a plurality of laser sources  102  as in the light source device  10 . Each of the laser sources  102  includes a collimating part  104  and a plane transmission grating  106 . The plane transmission grating  106  is configured to diffract a portion of light emitted from the corresponding laser source  102  back to the laser source  102  to cause external resonance between the laser source  102  and the plane transmission grating  106 . That is, the laser source  102 , the collimating part  104 , and the plane transmission grating  106  form an external cavity. 
     The light source device  20  is different from the light source device  10  mainly in the aspects described below. A deflector-condenser lens  210  is disposed in an optical path between the external cavities and a combining grating  110 . Light beams  120 , which pass through the plane transmission gratings  106  and are emitted from the external cavities, propagate in the z direction as shown in  FIG.  3   , in parallel to one another until the light beams  120  reach the deflector-condenser lens  210 . The incident angles of the light beams  120  incident on the combining grating  110  are defined by the deflector-condenser lens  210 . That is, with the deflector-condenser lens  210 , the light beams  120  emitted from the external cavities are incident on the combining grating  110  at appropriate incident angles. The light beams that have entered the combining grating  110  are then diffracted at the same diffraction angle and are combined by the combining grating  110 . 
     The laser sources  102  are preferably hermetically sealed as in the first embodiment. For example, an LD in a can-type package may be used for the laser sources  102 . Each laser source  102 , a corresponding one of collimating parts  104 , and a corresponding one of plane transmission gratings  106  may be provided at the same stage to form a module, as in the first embodiment. Additionally, in the present embodiment, the plurality of the laser sources  102  are arranged parallel to the x-axis direction shown in  FIG.  3   , so that all of the laser sources  102  may be disposed on the same stage. In this case, a lens array corresponding to the arrangement of the laser sources  102  may be used for the collimating part  104 . Also, an LD bar in which a plurality of LDs are arranged on the same semiconductor substrate may be used for the plurality of the laser sources  102 . In the present embodiment, as in the first embodiment, a cavity length of the external cavity is reduced, which enables the light source device  20  to have a high vibration resistance. Further, according to the present embodiment, the light source device  20  can be easily assembled. 
     Third Embodiment 
       FIG.  4    is a schematic view of a light source device according to a third embodiment of the present invention. A light source device  30  in the present embodiment is a modification of the light source device  20  in the second embodiment. In the present embodiment, members, portions, components, and elements having the same functions as those in the second embodiment are denoted by the same reference numerals as the second embodiment, and duplicative description thereof may be omitted. 
     The light source device  30  is different from the light source device  20  mainly in that prisms  310  are used instead of a deflector-condenser lens  210 . As shown in  FIG.  4   , five external cavities are provided in the light source device  30 , and prisms  310  are disposed each corresponding to a corresponding one of external cavities other than the external cavity at the center. The incident angle of light beam  120  emitted from the external cavity and entering a combining grating  110  is defined by the prism  310  disposed in the optical path between the external cavity and the combining grating  110 . That is, with the prisms  310 , corresponding ones of the light beams  120  each emitted from a corresponding one of the external cavities are incident on the combining grating  110  at appropriate incident angles. An orientation of the optical path of the light beam  120  emitted from the central external cavity of the plurality of external cavities is not needed to be changed until reaching the combining grating  110 , and thus the prism  310  is not disposed on the optical path of the light beam  120  emitted from the central external cavity. The light beams incident on the combining grating  110  at respective incident angles are then diffracted at the same diffraction angle to be combined by the combining grating  110 . 
     In the present embodiment, each single prism  310  is provided with respect to a corresponding one of external cavities except the central external cavity, so that, if an alignment error or the like of the external cavity occurs, a position of the corresponding prism can be individually adjusted to correct an incident position on the combining grating  110 . For example, in the case in which a position of a single external cavity is slightly deviated from a predetermined position along the x axis, without correction, the light beam  120  emitted from the external cavity at the deviated position is incident on the combining grating at a different incident angle. This increases a beam parameter product (BPP) of the combined beam  130 , which is formed by diffraction and combination on the combining grating  110 , resulting in deterioration of quality of the combined beam  130 . When such a deviation in position occurs, by adjusting the position of the corresponding prism  310  along the z axis, the incident position of the light beam  120  refracted by the prism  310  incident on the combining grating  110  can be adjusted while maintaining the incident angle. 
     Thus, the deviation of the incident position of the light beam  120  can be corrected. 
     Fourth Embodiment 
       FIG.  5    is a schematic view of a light source device according to a fourth embodiment of the present invention. A light source device  40  in the present embodiment is a modification of the light source device  20  in the second embodiment. In the present embodiment, members, portions, components, and elements having the same functions as those in the second embodiment are denoted by the same reference numerals as those in the second embodiment, and the duplicative description thereof may be omitted. 
     The light source device  40  is different from the light source device  20  mainly in that a condenser lens  402 , a diaphragm  404 , and a collimating lens  406  are further disposed in this order in an optical path between an external cavity and a combining grating  110 . The condenser lens  402 , the diaphragm  404 , and the collimating lens  406  form an afocal optical system. In the present embodiment, more specifically, the afocal optical system is disposed between a plane transmission grating  106  and a deflector-condenser lens  210 . In order to describe the effects of the afocal optical system, each light beam  120  is indicated by a beam having a width in  FIG.  5   . In  FIG.  5   , f 1  indicates a focal length of a collimating part  104 , f 2  indicates a focal length of the deflector-condenser lens  210 , and f 3  indicates focal lengths of the condenser lens  402  and the collimating lens  406 . 
     As shown in  FIG.  5   , the collimating part  104  is disposed spaced apart from a laser source  102  at a distance of f 1 , the plane transmission grating  106  is disposed spaced apart from the collimating part  104  at a distance of f 1 , the deflector-condenser lens  210  is disposed spaced apart from the plane transmission grating  106  at a distance of f 2 , and the combining grating  110  is disposed spaced apart from the deflector-condenser lens  210  at a distance of f 2 . The afocal optical system that includes the condenser lens  402  and other optical elements is disposed within the front focal length f 2  of the deflector-condenser lens  210 . 
     The light emitted from the laser source  102  has a divergence angle within a certain range. The collimating part  104  collimates the light emitted from the laser source  102  into a substantially parallel light. In order to perform such collimation, a distance along the z axis between the laser source  102  and the collimating part  104  needs to be precisely controlled. As described above, in the case in which the laser source  102  includes the LD configured to emit light having a peak wavelength in a range of 350 nm to 550 nm with a predetermined gain bandwidth, the laser source  102  is preferably hermetically sealed. In view of this, the collimating part  104  can be provided by, for example, with the use of a can-type package, joining a lens to be the collimating part  104  to the can-type package, in which precise control of the distance between the collimating part  104  and the laser source  102  is difficult. Thus, the deviation of position of the collimating part  104  may cause the light beam  120  after passing through the collimating part  104  to be in an undesirable condition. 
     The afocal optical system disposed between the collimating part  104  and the deflector-condenser lens  210  can correct the condition of the light beam  120  that has passed through the collimating part  104 . That is, even the undesirable condition of the light beam  120  that has passed through the collimating part  104  can be converted to the desirable condition by the afocal optical system. Accordingly, a combined beam  130  with a higher quality can be obtained. In other words, the afocal optical system can increase the tolerance of the deviation of position of the collimating part  104 . 
     In the afocal optical system, the light beam  120  is once condensed at the rear focal point by the condenser lens  402 . The diaphragm  404  is disposed at the rear focal point. The diaphragm  404  blocks a portion of light that is not well focused by the condenser lens  402 , which increases the quality of the light beam  120  that passes through the afocal optical system. Thus, the combined beam  130  with a higher quality and a low BPP can be obtained. Instead of the diaphragm  404 , a pinhole may be employed. 
     Fifth Embodiment 
       FIG.  6    is a schematic view of a light source device according to a fifth embodiment of the present invention. A light source device  50  in the present embodiment is a modification of the light source device  20  in the second embodiment. In the present embodiment, members, portions, components, and elements having the same functions as those in the second embodiment are denoted by the same reference numerals as those in the second embodiment, and duplicative description thereof may be omitted. 
     The light source device  50  is different from the light source device  20  mainly in that a volume holographic grating  506  is used instead of the plane transmission grating  106  as a wavelength determining element. For the convenience of description, the “volume holographic grating” is also referred to as “VHG” in the present specification. The VHG  506  is disposed in an optical path between a laser source  102  and a combining grating  110 . More specifically, the VHG  506  is disposed in the optical path between a collimating part  104  and a deflector-condenser lens  210 . The VHG  506  is configured to diffract a portion of light emitted from the laser source  102  back to the laser source  102  to cause external resonance between the laser source  102  and the VHG  506 . That is, a combination of the laser source  102 , the collimating part  104 , and the VHG  506  forms a single external cavity. The cavity length of the external cavity is defined by the distance between the laser source  102  and the VHG  506 . 
     The VHG  506  is configured to diffract only light with the designed wavelength. Accordingly, the wavelength of light that resonates in an external cavity and the wavelength of light that passes through the VHG  506  and that is incident on the combining grating  110  are determined by the VHG  506 , to be the designed wavelength of the VHG  506 . Light beams  120  each emitted from a corresponding one of the external cavities pass through the deflector-condenser lens  210  and are incident on the combining grating  110  at different incident angles. As described above, in order to allow all of the lights incident on the combining grating  110  to be diffracted at the same diffraction angle, each light beam  120  emitted from a corresponding one of the external cavities needs to have a wavelength corresponding to a corresponding incident angle (refer to Table 1). Thus, the external cavity is provided with VHG  506  having the required designed wavelength. According to the present embodiment, as in the first to fourth embodiments, a cavity length of the external cavity can be reduced, which allows for increasing vibration resistance of the light source device  50 . Also, a high precision adjustment is not required during initial assembly, so that the light source device can be easily assembled. 
     Sixth Embodiment 
       FIG.  7    is a schematic view of a light source device according to a sixth embodiment of the present invention. A light source device  60  in the present embodiment is a modification of the light source device  50  in the fifth embodiment. In the present embodiment, members, portions, components, and elements having the same functions as those in the fifth embodiment are denoted by the same reference numerals as those in the fifth embodiment, and duplicative description thereof may be omitted. 
     The light source device  60  is different from the light source device  50  mainly in that a VHG  506  is disposed between a laser source  102  and a collimating part  104 . According to the present embodiment, as in the first to sixth embodiments, a cavity length of the external cavity can be reduced, which allows for increasing vibration resistance of the light source device  60 . Further, the distance between the VHG  506  and the laser source  102  is shorter than that in the light source device  50 , which allows for increasing the tolerance of angular deviation of the laser source  102  on the x-z plane. 
     Seventh Embodiment 
       FIG.  8    is a schematic view of a light source device according to a seventh embodiment of the present invention. A light source device  70  in the present embodiment is a modification of the light source device  60  in the sixth embodiment. In the present embodiment, members, portions, components, and elements having the same functions as those in the sixth embodiment are denoted by the same reference numerals as those in the sixth embodiment, and duplicative description thereof may be omitted. In  FIG.  8   , a light beam  120  is shown as a beam having a width for the convenience of description. 
     The light source device  70  is different from the light source device  60  mainly in that a fiber Bragg grating  706  is used instead of the volume holographic grating  506  as the wavelength determining element. For the convenience of description, the “fiber Bragg grating” is also referred to as a “FBG” in the present specification. The FBG  706  is disposed in an optical path between a laser source  102  and a collimating part  104 . A condenser lens  702  is disposed between the laser source  102  and the FBG  706 . 
     The light emitted from the laser source  102  is condensed by the condenser lens  702  and enters the FBG  706 . A portion of light emitted from the laser source  102  is diffracted by the FBG  706  back to the laser source  102 , and thus external resonance occurs between the laser source  102  and the FBG  706 . That is, a combination of the laser source  102 , the condenser lens  702 , and the FBG  706  forms a single external cavity. The cavity length of the external cavity is defined by the distance between the laser source  102  and the FBG  706 . 
     The FBG  706  diffracts only the light having the Bragg wavelength corresponding to a grating period of the FBG  706 , and thus the wavelength of light that resonates in the external cavity and the wavelength of light that passes through the FBG  706  and that enters a combining grating  110  are determined by the FBG  706 , to be the Bragg wavelength of the FBG  706 . The light beam  120  emitted from the external cavity passes through a deflector-condenser lens  210  and is incident on the combining grating  110  at a different incident angle. As described above, in order to allow all of the lights incident on the combining grating  110  to be diffracted at the same diffraction angle, each light beam  120  emitted from corresponding one of the external cavities needs to have a wavelength corresponding to a corresponding one of the incident angles (refer to Table 1). Thus, each external cavity is provided with the FBG  706  having the required Bragg wavelength. According to the present embodiment, as in the first to sixth embodiments, a cavity length of each external cavity can be reduced, which allows for increasing vibration resistance of the light source device  70 . Also, a high precision adjustment is not required during initial assembly, so that the light source device can be easily assembled. 
     While certain embodiments of the present invention have been described above, it should be understood that the technical scope of the present invention is not limited to the description of those embodiments. Some of the configurations in the described embodiments may be replaced by other configurations, or may be modified. An embodiment based on such changes and modifications may be encompassed in the technical scope of the present invention as clearly indicated in the descriptions of claims. For example, the embodiments described above are described in detail to facilitate understanding of the present invention. 
     However, the present invention is not necessarily limited to the configurations that include all of the structures and steps described in the embodiments described above. 
     A portion of a structure of a certain embodiment may be replaced with or added to a structure of another embodiment. For example, the afocal optical system in the light source device  40  shown in  FIG.  5    according to the fourth embodiment including the condenser lens  402 , the diaphragm  404 , and the collimating lens  406  may also be employed in the other embodiments. For example, the afocal optical system may be disposed between each of the external cavity modules  100  and the combining grating  110  in the light source device  10  shown in  FIG.  1   . In the light source device  30  shown in  FIG.  4   , the afocal optical system may be disposed between each of external cavities except for the external cavity at the center of the plurality of external cavities and the prism  310 , and the afocal optical system may also be disposed between the external cavity at the center and the combining grating  110 . The afocal optical system may be disposed between each of the external cavities and the deflector-condenser lens  210  in the light source device  50  shown in  FIG.  6   . Also, the afocal optical system may be disposed between each of the collimating parts  104  and the deflector-condenser lens  210  in the light source device  60  shown in  FIG.  7   . Furthermore, the afocal optical system may be disposed between each of the collimating parts  104  and the deflector-condenser lens  210  in the light source device  70  shown in  FIG.  8   . 
     Moreover, as in the external cavity modules  100  in the light source device  10 , the laser source  102 , the collimating part  104 , and the VHG  506  in each of the external cavities of the light source device  50  may be formed as a module. Then, in the case in which the light beams  120  each emitted from a corresponding one of the external cavities are incident on the combining grating  110  without using the deflector-condenser lens  210 , the incident angle of each of the light beams  120  on the combining grating  110  can be selected and adjusted by selecting the mounting angle of each of the modules that includes the corresponding external cavity. The laser source  102 , the VHG  506 , and the collimating part  104  in the external cavity of the light source device  60  may be formed as a module. Then, in the case in which the light beams  120  each emitted from a corresponding one of the external cavities are incident on the combining grating  110  without using the deflector-condenser lens  210 , the incident angle of each of the light beams  120  on the combining grating  110  can be selected and adjusted by selecting the mounting angle of each of the modules that includes the corresponding external cavity. The laser source  102 , the condenser lens  702 , the FBG  706 , and the corresponding collimating part  104  in each external cavity of the light source device  70  may be formed as a module. Then, in the case in which the light beams  120  each emitted from a corresponding one of the external cavities are incident on the combining grating  110  without using the deflector-condenser lens  210 , the incident angle of each of the light beams  120  on the combining grating  110  can be selected and adjusted by selecting the mounting angle of each of the modules that includes the corresponding external cavity. 
     In the light source devices  50 ,  60 , and  70 , prisms  310  shown in  FIG.  4    may be used instead of the deflector-condenser lens  210 . With such a configuration, if an alignment error or the like of the external cavities occurs, adjustment of corresponding one or more of prism allows individual correction.