Patent Publication Number: US-2023163558-A1

Title: Semiconductor laser device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a semiconductor laser device. 
     BACKGROUND ART 
     In the related art, a semiconductor laser device (semiconductor laser module) that accommodates a semiconductor laser element in a package has been known (for example, Patent Document 1). In the semiconductor laser device, a light-emitting window for extracting laser light emitted from the semiconductor laser element to the outside of the package is provided on a side wall of the package. 
     Citation List 
     Patent Document 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2003-315633 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the semiconductor laser device described above, in order to improve the extraction efficiency of the laser light, an anti-reflection film may be formed on the light-emitting window. When the heat-resistant temperature of the anti-reflection film is relatively low, when the light-emitting window is brazed to the package using a brazing material having a relatively high melting point, the anti-reflection film might be damaged by heat generated when brazing is performed, which is a problem. In addition, in the semiconductor laser device described above, since the inside of the package needs to be set to a vacuum state or an inert gas atmosphere, the package requires airtightness. 
     Therefore, an object of the present disclosure is to provide a semiconductor laser device capable of suppressing damage to an anti-reflection film provided on a light-emitting window, and of securing high airtightness of a package. 
     Solution to Problem 
     A semiconductor laser device according to one aspect of the present disclosure includes: a semiconductor laser element; and a package that airtightly accommodates the semiconductor laser element. The package includes a bottom wall, a side wall standing on the bottom wall and being formed in an annular shape to surround a region in which the semiconductor laser element is accommodated, when viewed in a direction perpendicular to the bottom wall, and a top wall that closes an opening on an opposite side of the side wall from a bottom wall side. A light-emitting window through which laser light emitted from the semiconductor laser element passes is provided on the side wall. The light-emitting window includes a first hole portion that opens to an inside of the package in an optical axis direction along an optical axis of the laser light, a second hole portion that opens to an outside of the package, and that includes the first hole portion and is larger than the first hole portion when viewed in the optical axis direction, a counterbore surface having an annular shape that connects the first hole portion and the second hole portion and that extends along a plane intersecting the optical axis direction, and a window member disposed inside the second hole portion. The window member includes an incident surface on which the laser light is incident, an emitting surface that is a surface opposite to the incident surface, and that emits the laser light that has transmitted through the window member, to the outside of the package, and a side surface that connects the incident surface and the emitting surface, and that extends along the optical axis direction. The incident surface includes a first region which includes a central portion of the incident surface and in which a first anti-reflection film is provided, and a second region metallized and formed in an annular shape to be separated from the first region and to surround the first region. The second region is joined to the counterbore surface through a solder member. 
     In the semiconductor laser device, the light-emitting window is joined to the side wall of the package through the solder member having a lower melting point than that of a brazing material. Accordingly, compared to when the brazing material is used, the window member and the counterbore surface can be brought into close contact with each other while suppressing damage to the window member (particularly, the anti-reflection film) caused by heat. In addition, the first region in which the anti-reflection film (first anti-reflection film) is provided and the second region to which the solder member is joined are separated from each other on the incident surface of the window member. Accordingly, stress generated in the second region when the solder member is melted or solidified is prevented from being transmitted to the anti-reflection film on the first region. As a result, damage (crack, peeling, or the like) to the anti-reflection film caused by the stress is suppressed. As described above, according to the semiconductor laser device, damage to the anti-reflection film provided on the light-emitting window can be suppressed, and high airtightness of the package can be secured. 
     The side surface may include a third region metallized to be continuous with the second region, and at least a part of the side surface may be joined to at least a part of an inner surface of the second hole portion through the solder member. According to this configuration, since a region that is continuous from the second region to the side surface of the window member (third region) is metallized, when solder joining is performed, some of the solder member suitably wet-spreads to a third region side. As a result, the solder member can be interposed between the side surface of the window member and the inner surface of the second hole portion, and the airtightness of the package can be suitably improved. 
     A length of the first hole portion along the optical axis direction may be shorter than a length of the second hole portion along the optical axis direction. According to this configuration, the light-emitting window can be brought closer to the semiconductor laser element compared to when the length of the first hole portion is equal to or longer than the length of the second hole portion. Accordingly, even when the beam radiation angle of the laser light emitted from the semiconductor laser element is large, the laser light can be incident on the light-emitting window while a degree of the spread of the laser light is reduced. As a result, the size of the light-emitting window can be reduced, and the size of the package can be reduced. 
     The emitting surface may include a fourth region in which a second anti-reflection film is provided, and the fourth region may include the first region and be larger than the first region when viewed in the optical axis direction. When laser light that is divergent light is incident on the window member, a region through which the laser light passes on the incident surface of the window member is smaller than a region through which the laser light passes on the emitting surface of the window member. Therefore, as in this configuration, a region corresponding to a difference between the fourth region and the first region can be secured as the second region by making the first anti-reflection film on an incident surface side smaller than the second anti-reflection film on an emitting surface side (namely, by making the first region smaller than the fourth region). In such a manner, since the sizes of the first region, the second region, and the fourth region are designed in consideration of the beam radiation angle of the laser light, the size of the window member can be reduced, and the size of the package can be reduced. 
     The semiconductor laser device may further include a lens disposed on an outer side of the package to concentrate or collimate the laser light that has transmitted through the light-emitting window. According to this configuration, since the lens is a member to be externally attached that is disposed on the outer side of the package, the disposition, replacement, and the like of the lens can be flexibly performed. 
     A wavelength of the laser light may be included in a range of 4 µm to 12 µm. Generally, the anti-reflection film corresponding to light having a wavelength of 4 µm to 12 µm has a low heat-resistant temperature, but in the semiconductor laser device, since the solder member having a relatively low melting point is used as a joining material, the window member on which the anti-reflection film is provided can be attached to the side wall by solder joining while suppressing damage to the anti-reflection film caused by heat. 
     The emitting surface of the window member may further protrude to the outside of the package than an outer surface of the side wall of the package on which the light-emitting window is provided. According to this configuration, the workability when the window member is joined to the side wall from the outside of the package can be improved. In addition, when the lens to be externally attached is attached to the emitting surface of the window member, the workability of lens attachment can also be improved. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide the semiconductor laser device capable of suppressing damage to the anti-reflection film provided on the light-emitting window, and of securing high airtightness of the package. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view of a quantum cascade laser device of one embodiment. 
         FIG.  2    is a side cross-sectional view of the quantum cascade laser device shown in  FIG.  1   . 
         FIG.  3    is a plan view of the quantum cascade laser device shown in  FIG.  1   . 
         FIG.  4    is a graph showing a typical example of a relationship between the radiation angle (horizontal axis) and the radiation intensity (vertical axis) of laser light emitted by a quantum cascade laser. 
         FIG.  5    is a partial enlarged view of the quantum cascade laser device shown in  FIG.  1   . 
         FIG.  6    is a front view of a portion of a side wall including a small-diameter hole and a large-diameter hole. 
       (A) of  FIG.  7    is a view showing an incident surface of a window member, (B) of  FIG.  7    is a cross-sectional view of the window member, and (C) of  FIG.  7    is a view showing an emitting surface of the window member. 
       (A) of  FIG.  8    is a top view of a Peltier module, and (B) of  FIG.  8    is a side view of the Peltier module. 
       (A) of  FIG.  9    is a top view of a heat spreader, (B) of  FIG.  9    is a side view of the heat spreader, and (C) of  FIG.  9    is a bottom view of the heat spreader. 
       (A) of  FIG.  10    is a top view of a heat sink on which each element is mounted, and (B) of  FIG.  10    is a side view of the heat sink on which each element is mounted. 
         FIG.  11    is a perspective view of a lens holder. 
         FIG.  12    is a front view of the lens holder. 
       (A) of  FIG.  13    is a top view of the lens holder, (B) of  FIG.  13    is a bottom view of the lens holder, and (C) of  FIG.  13    is a side cross-sectional view of the lens holder. 
         FIG.  14    is a view schematically showing a cross section taken along line XIV-XIV in  FIG.  12   . 
         FIG.  15    is a view schematically showing a positional relationship between a lens and a lens holder when the lens holder according to a comparative example is used. 
         FIG.  16    is a view showing a part of an assembly procedure of the quantum cascade laser device shown in  FIG.  1   . 
       (A) of  FIG.  17    is a front view of a lens holder of a first modification example, and (B) of  FIG.  17    is a cross-sectional view of the lens holder taken along line B-B of (A) of  FIG.  17   . 
       (A) of  FIG.  18    is a front view of a lens holder of a second modification example, and (B) of  FIG.  18    is a cross-sectional view of the lens holder taken along line B-B of (A) of  FIG.  18   . 
       (A) of  FIG.  19    is a front view of a lens holder of a third modification example, and (B) of  FIG.  19    is a cross-sectional view of the lens holder taken along line B-B of (A) of  FIG.  19   . 
         FIG.  20    is a side cross-sectional view of a quantum cascade laser device of a modification example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the drawings, the same or equivalent portions are denoted by the same reference signs, and a duplicated description will not be repeated. Incidentally, in the drawings, some portions may be exaggerated for an easy-to-understand description of configurations according to the embodiment, and have dimensions different from actual dimensions. In addition, in the following description, the terms “up”, “down”, and the like are for convenience based on states shown in the drawings. 
     As shown in  FIGS.  1  to  3   , a quantum cascade laser device  1  (semiconductor laser device) includes a quantum cascade laser element (hereinafter, “QCL element”)  2  and a package  3  that airtightly accommodates the QCL element  2 . 
     The QCL element  2  is one type of semiconductor laser element. The QCL element  2  has an end surface  2   a  (emitting surface) intersecting one direction (in the present embodiment, an X-axis direction), and is configured to emit laser light L having a broadband wavelength (for example, 4 µm to 12 µm) in a mid-infrared region from the end surface  2   a . In the present embodiment, an optical axis of the laser light L emitted from the QCL element  2  is disposed along the X-axis direction. The QCL element  2  has, for example, a structure in which a plurality of active layers having different center wavelengths are stacked in a stack, and can emit the above-described broadband light. However, the QCL element  2  may have a structure including a single active layer, and in this case as well, can emit the above-described broadband light. As shown in  FIG.  4   , the laser light L emitted from the QCL element  2  has a very large beam radiation angle (divergence angle) as per the principle of a quantum cascade laser compared to a laser diode or the like. 
     The package  3  is a so-called butterfly package. The package  3  includes a bottom wall  31 , a side wall  32 , and a top wall  33 . In  FIG.  3   , the top wall  33  of the package  3  is not shown. 
     The bottom wall  31  is a rectangular plate-shaped member. The bottom wall  31  is made of, for example, a metal material such as copper-tungsten. The bottom wall  31  is a base member on which various members such as a Peltier module  4  to be described later are to be mounted. In this specification, for convenience, a longitudinal direction of the bottom wall  31  is referred to as the X-axis direction, a lateral direction of the bottom wall  31  is referred to as a Y-axis direction, and a direction perpendicular to the bottom wall  31  (namely, a direction orthogonal to the X-axis direction and to the Y-axis direction) is referred to as a Z-axis direction. As described above, the X-axis direction is also a direction along the optical axis of the laser light L emitted from the QCL element  2  (optical axis direction). 
     The side wall  32  stands on the bottom wall  31 . When viewed in the Z-axis direction, the side wall  32  is formed in an annular shape to surround a region (internal space S) in which the QCL element  2  and the like are accommodated. In the present embodiment, the side wall  32  is a rectangular tubular member that surrounds the internal space S. The side wall  32  is made of a metal material such as Kovar. The side wall  32  is, for example, a Kovar frame to which Ni/Au plating is applied. In the present embodiment, the side wall  32  is provided on a central portion of the bottom wall  31  in the longitudinal direction (X-axis direction). A width of the side wall  32  in the lateral direction (Y-axis direction) is equal to a width of the bottom wall  31  in the lateral direction, and a width of the side wall  32  in the longitudinal direction (X-axis direction) is shorter than a width of the bottom wall  31  in the longitudinal direction. Namely, protrusion portions  31   a  protruding and extending outward from the side wall  32  are formed on both respective sides of the bottom wall  31  in the longitudinal direction. Screw holes  31   b  for attaching the package  3  (bottom wall  31 ) to another member are provided in respective portions of the protrusion portions  31   a  corresponding to four corners of the bottom wall  31 . 
     The top wall  33  is a member that closes an opening on an opposite side of the side wall  32  from a bottom wall  31  side. The top wall  33  has a rectangular plate shape. An outer shape (widths in the longitudinal direction and in the lateral direction) of the top wall  33  viewed in the Z-axis direction substantially coincides with an outer shape of the side wall  32 . The top wall  33  is made of, for example, the same metal material (for example, Kovar or the like) as that of the side wall  32 . 
     A plurality (in the present embodiment, seven on each of both sides in the lateral direction, for a total of  14 ) of lead pins  10  for allowing a current to flow to members such as the QCL element  2  accommodated in the package  3  are inserted into portions  321  of the side wall  32  extending along the longitudinal direction (X-axis direction) (namely, portions intersecting the lateral direction (Y-axis direction)). 
     A light-emitting window  11  through which the laser light L emitted from one end surface  2   a  of the QCL element  2  passes is provided on one of portions  322  of the side wall  32  extending along the lateral direction (Y-axis direction) (namely, portions intersecting the longitudinal direction (X-axis direction)). 
     As shown in  FIGS.  5  and  6   , the light-emitting window  11  includes a small-diameter hole  12  (first hole portion), a large-diameter hole  13  (second hole portion), a counterbore surface  14 , which are formed by the side wall  32  (portion  322 ), and a window member  15 . Incidentally, anti-reflection films  151  and  152  and a metal film  153  to be described later are provided on the window member  15  (refer to  FIG.  7   ). Since these members are very thin compared to a main body of the window member  15 , these members are not shown in the drawings other than  FIG.  7   . 
     The small-diameter hole  12  opens to the inside (namely, the internal space S) of the package  3  in the optical axis direction along the optical axis of the laser light L (namely, the X-axis direction). The large-diameter hole  13  opens to the outside of the package  3 . The large-diameter hole  13  is shaped to include the small-diameter hole  12  and to be larger than the small-diameter hole  12  when viewed in the X-axis direction. Each of the small-diameter hole  12  and the large-diameter hole  13  extends in the X-axis direction. The small-diameter hole  12  and the large-diameter hole  13  connected to each other by the counterbore surface  14  forms a through-hole penetrating through the side wall  32  in the X-axis direction. In the present embodiment, each of the small-diameter hole  12  and the large-diameter hole  13  is formed in a circular shape, and a diameter d2 of the large-diameter hole  13  is larger than a diameter d1 of the small-diameter hole  12  (d2 &gt; d1). In addition, a central axis of the small-diameter hole  12  and a central axis of the large-diameter hole  13  may coincide with the optical axis of the laser light L emitted from the QCL element  2 . The counterbore surface  14  is an annular surface that connects the small-diameter hole  12  and the large-diameter hole  13 , and that extends along a plane intersecting the X-axis direction (Y-Z plane). More specifically, the counterbore surface  14  connects an end portion on a large-diameter hole  13  side of the small-diameter hole  12  and an end portion on a small-diameter hole  12  side of the large-diameter hole  13 . The large-diameter hole  13  and the counterbore surface  14  can be formed by performing counterbore processing from the outside of the package  3 . Incidentally, in the present embodiment, the counterbore surface  14  is formed in a continuous annular shape, but the counterbore surface  14  may be formed in a discontinuous annular shape. For example, a cutout may be formed at a part of an inner wall surface of the small-diameter hole  12  to divide the counterbore surface  14  at the portion at which the cutout is formed. 
     In the present embodiment, the diameter d1 of the small-diameter hole  12  is 3.8 mm, the diameter d2 of the large-diameter hole  13  is 5.7 mm, and a width of the counterbore surface  14  in a radial direction ((d2 - d1)/2) is 0.95 mm. In addition, a length w1 of the small-diameter hole  12  along the X-axis direction is shorter than a length w2 of the large-diameter hole  13  along the X-axis direction. In the present embodiment, a thickness t (length along the X-axis direction) of the side wall  32  is 1 mm, the length w1 of the small-diameter hole  12  is 0.23 mm, and the length w2 of the large-diameter hole  13  is 0.77 mm. 
     The window member  15  is made of a material (for example, germanium or the like) that transmits the laser light L having a wavelength in the mid-infrared region. The window member  15  is formed in a disk shape, and is disposed inside the large-diameter hole  13 . The window member  15  has an incident surface  15   a , an emitting surface  15   b , and a side surface  15   c . The incident surface  15   a  and the emitting surface  15   b  are surfaces intersecting the X-axis direction, and are formed in a circular shape. The incident surface  15   a  is a surface on an internal space S side, and is a surface on which the laser light L (in the present embodiment, the laser light L collimated by a lens  8 ) is incident. The emitting surface  15   b  is a surface opposite to the incident surface  15   a  (namely, an outer surface of the package  3 ), and a surface that emits the laser light L that has transmitted through the window member  15 , to the outside of the package  3 . The side surface  15   c  is a surface that connects the incident surface  15   a  and the emitting surface  15   b , and that extends along the X-axis direction. In the present embodiment, a diameter of the window member  15  (the incident surface  15   a  or the emitting surface  15   b ) is 5.4 mm, and a thickness (length along the X-axis direction) of the window member  15  is 0.7 mm. 
     As shown in (A) and (B) of  FIG.  7   , the incident surface  15   a  includes a first region A 1  and a second region A 2 . The first region A 1  is a region which includes a central portion of the incident surface  15   a  and in which the anti-reflection film  151  (first anti-reflection film) is provided. The anti-reflection film  151  is a film member having a function of suppressing the reflection of the laser light L having a wavelength in the mid-infrared region on the incident surface  15   a . The anti-reflection film  151  is made of, for example, a high refractive index material such as germanium (Ge) or silicon (Si), an intermediate refractive index material such as zinc sulfide (ZnS) or zinc selenide (ZnSe), or a low refractive index material such as yttrium fluoride (YF3), or is a dielectric multilayer film in which a plurality of substances having different refractive indexes that transmit mid-infrared light are alternately stacked. The anti-reflection film  151  is formed in a circular shape. A thickness (length along the X-axis direction) of the anti-reflection film  151  is determined according to the design of a transmission wavelength of the anti-reflection film  151  (wavelength of the laser light L that is transmitted). The thickness of the anti-reflection film  151  is, for example, 1.0 µm to 3.0 µm. For example, when a design value of the transmission wavelength is 5.2 µm, the thickness of the anti-reflection film  151  is set to, for example, 1.4 µm. In addition, in the present embodiment, a diameter of the anti-reflection film  151  (namely, a diameter of the first region A 1 ) is 4.2 mm. 
     The second region A 2  is a region formed in an annular shape to be separated from the first region A 1  and to surround the first region A 1 . The second region A 2  is metallized by the metal film  153 . The metal film  153  is made of a material suitable for solder joining (namely, a material having good compatibility with a solder member  16  to be described later). The metal film  153  is made of, for example, Cr/Ni/Au (0.2 µm/0.5 µm/0.5 µm). In the present embodiment, an inner diameter of the metal film  153  (namely, an inner diameter of the second region A 2 ) formed on the incident surface  15   a  is 4.5 mm. Namely, in the present embodiment, an annular region having a width of 0.15 mm in which the incident surface  15   a  (germanium base material) is exposed is formed between an outer edge of the first region A 1  and an inner edge of the second region A 2 . 
     As shown in (B) of  FIG.  7   , the side surface  15   c  includes a third region A 3  metallized to be continuous with the second region A 2 . Namely, the metal film  153  is continuously provided from the second region A 2  to the side surface  15   c . 
     As shown in (B) and (C) of  FIG.  7   , the emitting surface  15   b  includes a fourth region A 4  in which the anti-reflection film  152  (second anti-reflection film) is provided. The anti-reflection film  152  is a film member having a function of suppressing the reflection of the laser light L having a wavelength in the mid-infrared region on the emitting surface  15   b . The anti-reflection film  152  is formed in a circular shape from the same material as that of the anti-reflection film  151 . In the present embodiment, a diameter of the anti-reflection film  152  (namely, a diameter of the fourth region A 4 ) is 4.6 mm. Namely, the fourth region A 4  is a region that includes the first region A 1  and that is larger than the first region A 1  when viewed in the X-axis direction. 
     The window member  15  is directly joined to the side wall  32  (portion  322 ). Specifically, the second region A 2  of the incident surface  15   a  (namely, a region metallized by the metal film  153 ) is joined to the counterbore surface  14  through the solder member  16  formed in an annular shape. The solder member is a joining material having a melting point of 450° C. or lower. The solder member  16  is made of, for example, a SnAgCu-based solder material having a melting point of 220° C. The solder member  16  is a sheet-shaped member that is originally formed in an annular shape (refer to  FIG.  16   ). 
     In the present embodiment, a thickness of the solder member  16  before soldering (namely, in the state of an annular sheet) is 0.1 mm, an outer diameter thereof is 5.5 mm, and an inner diameter thereof is 4.2 mm. Namely, in the present embodiment, the inner diameter (4.2 mm) of the solder member  16  is equal to the diameter (4.2 mm) of the first region A 1 , and the first region A 1  and the solder member  16  do not overlap each other. In addition, the first region A 1  in which the anti-reflection film  151  is formed and the second region A 2  in which the metal film  153  is formed are separated from each other. In such a manner, since the first region A 1  and the second region A 2  are completely separated from each other, and the region in which the base material (in the present embodiment, germanium base material) of the window member  15  is exposed exists between the first region A 1  and the second region A 2 , during soldering, the solder member  16  is unlikely to flow onto the first region A 1  (onto the anti-reflection film  151 ). On the other hand, the solder member  16  is likely to wet-spread on the metal film  153  having high compatibility with the solder member  16 . Accordingly, stress generated when the solder member  16  is melted or solidified during soldering is unlikely to be transmitted to the anti-reflection film  151  on the first region A 1 . 
     In addition, as described above, since the solder member  16  wet-spreads on the metal film  153 , some of the solder member  16  also wraps around onto the third region A 3  (refer to  FIG.  5   ). Namely, some of the solder member  16  enters a gap between the side surface  15   c  and an inner surface of the large-diameter hole  13 . Namely, at least a part of the side surface  15   c  of the window member  15  is joined to at least a part of the inner surface of the large-diameter hole  13  through the solder member  16 . Accordingly, the airtightness of the package  3  at an attachment portion of the window member  15  is effectively enhanced. 
     In the present embodiment, the emitting surface  15   b  of the window member  15  is substantially flush with an outer surface  32   a  of the side wall  32  (outer surface of the package  3 ) on which the light-emitting window  11  is provided. Namely, the length w2 of the large-diameter hole  13  (namely, a depth of counterbore processing) is adjusted such that the emitting surface  15   b  is substantially flush with the outer surface  32   a  of the side wall  32 . 
     Next, each member accommodated in the package  3  will be described. The internal space S formed by the package  3  mainly accommodates the Peltier module  4 , a heat spreader  5 , a heat sink  6 , a submount  7 , the lens  8 , a lens holder  9 , and a temperature sensor T (refer to  FIG.  10   ) in addition to the QCL element  2 . 
     The Peltier module  4  is a temperature control element that adjusts the temperature of the QCL element  2 . Specifically, the Peltier module  4  has a cooling and heating function of keeping the temperature of the QCL element  2  at a temperature corresponding to the oscillation wavelength of the QCL element  2 . Temperature control by the Peltier module  4  is performed based on the temperature of the QCL element  2  measured by the temperature sensor T (refer to  FIG.  10   ) to be mounted on the heat sink  6 . 
     As shown in  FIG.  8   , the Peltier module  4  includes a plurality of Peltier elements  41  that are thermoelectric semiconductor elements, and a pair of ceramic substrates  42  and  43  that sandwich the plurality of Peltier elements  41  therebetween from above and below. The ceramic substrate  42  is provided on a top wall  33  side with respect to the Peltier elements  41 , and the ceramic substrate  43  is provided on the bottom wall  31  side with respect to the Peltier elements  41 . Each of the ceramic substrates  42  and  43  is made of, for example, alumina. An outer surface of each of the ceramic substrates  42  and  43  (surface opposite to a Peltier element  41  side) is a metallized surface on which a metal film  44  made of Cu/Ni/Au or the like is formed by plating. An In foil  45  that is a solder member is provided on the outer surface of each of the ceramic substrates  42  and  43  with the metal film  44  interposed therebetween. The ceramic substrate  42  is solder joined to the heat spreader  5  through the In foil  45 . On the other hand, the ceramic substrate  43  is solder joined to an upper surface  31   c  (surface facing the top wall  33 ) of the bottom wall  31  of the package  3  through the In foil  45 . Two lead wires  20  for allowing a direct current to flow to the Peltier module  4  are electrically connected to one ceramic substrate (in the present embodiment, the ceramic substrate  43 ). The two lead wires  20  are connected to different respective lead pins  10 . 
     The heat spreader  5  is a member to be mounted on the Peltier module  4 , and dissipates heat generated by the QCL element  2 , to a Peltier module  4  side. The heat spreader  5  is made of, for example, a material having good thermal conductivity such as copper. As shown in  FIG.  9   , the heat spreader  5  has a bottom surface  51  to be solder joined to the Peltier module  4  through the In foil  45  (refer to  FIG.  8   ) provided on the ceramic substrate  42 ; a first upper surface  52  on which the heat sink  6  and the submount  7  are to be mounted; and a second upper surface  53  (second attachment surface) on which the lens holder  9  is to be mounted. 
     Here, a thermal expansion coefficient of copper (approximately 17 × 10 -6 /K) is larger than a thermal expansion coefficient of alumina (approximately  7  × 10 -6 /K). For this reason, when the ceramic substrate  42  of the Peltier module  4  is made of alumina and the heat spreader  5  is made of copper, if the entirety of the bottom surface  51  of the heat spreader  5  is joined to the ceramic substrate  42  through the In foil  45 , cracks might occur in the Peltier elements  41  because of a large difference in temperature or the like between upper surfaces and lower surfaces of the Peltier elements  41  during the long-term use, temperature control, or the like of the quantum cascade laser device  1 , which is a problem. 
     Therefore, in the present embodiment, groove portions  51   a  that divide a surface to be joined to the Peltier module  4  into a plurality of segments are formed in the bottom surface  51  of the heat spreader  5 . In the present embodiment, as one example, two groove portions  51   a  extend along the lateral direction (Y-axis direction) at positions where the bottom surface  51  is divided into three segments in the longitudinal direction (X-axis direction). The surface to be joined to the Peltier module  4  is substantially evenly into three segments by the two groove portions  51   a . In such a manner, since the surface to be joined to the Peltier module  4  is divided into a plurality (here, three) of segments, the stress caused by a difference in thermal expansion coefficient between the material (alumina) of the ceramic substrate  42  and the material (copper) of the heat spreader  5  is reduced, and the occurrence of cracks in the Peltier elements  41  described above is suppressed. 
     In addition, four corners (vertexes) of the Peltier module  4  are weak particularly in mechanical strength. Therefore, in the present embodiment, cutout grooves  51   b  are formed at four corners of the bottom surface  51  of the heat spreader  5 . Accordingly, the ceramic substrate  42  and the bottom surface  51  of the heat spreader  5  can be prevented from being joined to each other at portions corresponding to the four corners of the Peltier module  4 , and the stress on the four corners of the Peltier module  4  caused by the difference in thermal expansion coefficient can be effectively reduced. 
     Incidentally, the groove portions  51   a  and the cutout grooves  51   b  described above also function as escape routes of air layers (voids) that are mixed when the ceramic substrate  42  and the bottom surface  51  are solder joined to each other through the In foil  45 . Accordingly, the quality of joining and the thermal conductivity between the Peltier module  4  (ceramic substrate  42 ) and the heat spreader  5  can be improved. 
     In addition, since a soft solder material such as In or InSn (in the present embodiment, In) is used as a solder member that joins the Peltier module  4  (ceramic substrate  42 ) and the heat spreader  5 , the stress of expansion or contraction by heat can be suitably absorbed, and the reliability of the quantum cascade laser device  1  can be improved. 
     The first upper surface  52  is located at a position higher than that of the second upper surface  53  (top wall  33  side). In the present embodiment, as one example, the first upper surface  52  is provided with two screw holes  52   a  for screwing the heat sink  6  and with a protrusion portion  52   b  protruding upward (top wall  33  side). The protrusion portion  52   b  extends along the lateral direction (Y-axis direction) at an end portion of the first upper surface  52  in the longitudinal direction (X-axis direction) (end portion opposite to a second upper surface  53   side). The protrusion portion  52   b  is a portion that comes into contact with an end portion of the heat sink  6  to position the heat sink  6 . 
     The second upper surface  53  is provided with a plurality (in the present embodiment, four) of protrusions  53   a  (second protrusions) formed in an island shape. The four protrusions  53   a  are portions to be joined to the lens holder  9  to be described later. 
     The heat sink  6  is a member to be mounted on the first upper surface  52  of the heat spreader  5 . Similarly to the heat spreader  5 , the heat sink  6  is made of, for example, a material having good thermal conductivity such as copper. The heat sink  6  is formed in a substantially rectangular parallelepiped shape. For example, a width of the heat sink  6  along the X-axis direction is 5 mm, and a width of the heat sink  6  along the Y-axis direction is 6 mm. As shown in  FIG.  10   , the submount  7  on which the QCL element  2  is mounted, the temperature sensor T that measures a temperature of the QCL element  2 , and ceramic patterns SP for wiring wires are mounted on an upper surface  6   a  (surface on the top wall  33  side) of the heat sink  6 . A lower surface  6   b  of the heat sink  6  is in contact with the first upper surface  52  of the heat spreader  5 . In order to enable the QCL element  2  to be easily replaced when a defect occurs in the QCL element  2 , the heat sink  6  is fixed to the heat spreader  5  with screws. Screw holes  6   c  penetrating through the heat sink  6  in the Z-axis direction, and counterbore grooves 6d formed around the screw holes  6   c  are formed in the heat sink  6  for such screwing. The screw holes  6   c  are provided at positions corresponding to the screw holes  52   a  (refer to  FIG.  9   ) of the heat spreader  5  described above. For example, the heat sink  6  is fixed to the heat spreader  5  by inserting screw members (not shown) into the screw holes  6   c  and into the screw holes  52   a , screw tips of the screw members being coated with a screw locking agent (adhesive agent for preventing the loosening of screws). Each of the counterbore grooves 6d is a groove portion provided to accommodate a head of the screw member. Incidentally, as the screw locking agent, a thermosetting resin adhesive agent that does not generate outgas (for example, epoxy resin or the like) is suitably used. 
     The temperature sensor T and the ceramic patterns SP are electrically connected to predetermined lead pins  10  through wires (not shown). In addition, the QCL element  2  is electrically connected to a predetermined lead pin  10  through the ceramic patterns SP and through wires (not shown). Accordingly, electric power is supplied from an external power supply device to the QCL element  2  and to the temperature sensor T through the lead pins  10 . 
     The submount  7  is a rectangular plate-shaped member on which the QCL element  2  is to be placed. The QCL element  2  is placed on the submount  7  such that the optical axis of the laser light L emitted from the end surface  2   a  coincides with a center of the light-emitting window  11  (namely, the central axes of the small-diameter hole  12  and the large-diameter hole  13 ). The submount  7  is made of a material having a thermal expansion coefficient close to that of the QCL element  2  (for example, aluminum nitride or the like). The QCL element  2  and the submount  7  are joined to each other through, for example, an AnSn-based solder material. In addition, the submount  7  and the heat sink  6  are joined to each other through, for example, a SnAgCuNiGe-based solder material. 
     Subsequently, the lens  8  and the lens holder  9  will be described with reference to  FIGS.  11  to  15   . As described above, the laser light L has a relatively large beam radiation angle as per the principle of a quantum cascade laser (refer to  FIG.  4   ). For this reason, in order to effectively use the laser light L, it is necessary to perform beam shaping (concentrating, collimating, or the like) of the laser light L using an optical element such as a lens. On the other hand, since the laser light L having an oscillation wavelength in the mid-infrared region is invisible, an expensive beam monitor, detector, or the like having sensitivity in the mid-infrared region is required to perform an alignment (position alignment) of the lens for the beam shaping. Therefore, in the present embodiment, the lens  8  for performing the beam shaping is built in inside the package  3  in advance. This configuration has an advantage that it is not necessary to perform an alignment of a lens (lens to be externally attached) on a user side. 
     The lens  8  is a member that concentrates or collimates the laser light L emitted from the QCL element  2 . The lens holder  9  is a member that holds the lens  8 . The lens  8  is disposed to face the end surface  2   a  that is the emitting surface of the QCL element  2  that emits the laser light L. 
     The lens  8  is, for example, an aspherical lens made of ZnSe. As shown in  FIGS.  11  and  14   , the lens  8  has an incident surface  8   a , a side surface  8   b , and an emitting surface  8   c . The incident surface  8   a  is a surface on which the laser light L is incident. In the present embodiment, the incident surface  8   a  is a flat surface. The side surface  8   b  is a surface extending from an edge portion of the incident surface  8   a  along the optical axis direction (namely, the X-axis direction) of the laser light L. The emitting surface  8   c  is a surface that emits the laser light L that has passed through the lens  8 . In the present embodiment, the emitting surface  8   c  is formed in a curved aspherical surface shape. In the present embodiment, a diameter of the lens  8  (diameter of the incident surface  8   a ) is 5 mm, and an effective diameter of the lens  8  is 4.5 mm. The effective diameter of the lens  8  is a diameter of an incident beam capable of satisfying optical characteristics of the lens on a plane (incident surface  8   a ) orthogonal to the optical axis direction (X-axis direction) of the laser light L (when the lens  8  is a collimating lens, a diameter of incident light that can be transmitted and collimated, as a specification of the lens). In addition, a region within the effective diameter of the lens  8  is referred to as an effective region. 
     As shown in  FIG.  11   , the lens holder  9  is a member having a substantially rectangular parallelepiped outer shape. The lens holder  9  is made of, for example, aluminum to which black alumite surface treatment is applied. A through-hole penetrating through the lens holder  9  along the X-axis direction is provided in a central portion of the lens holder  9  when viewed in the X-axis direction. The lens holder  9  has a small-diameter hole  9   a  and a large-diameter hole  9   b  that form the through-hole. Each of the small-diameter hole  9   a  and the large-diameter hole  9   b  extends in the X-axis direction. The large-diameter hole  9   b  is provided at a position farther from the QCL element  2  than the small-diameter hole  9   a . Namely, the small-diameter hole  9   a  is provided on a QCL element  2  side with respect to the large-diameter hole  9   b . The large-diameter hole  9   b  is shaped to include the small-diameter hole  9   a  and to be larger than the small-diameter hole  9   a  when viewed in the X-axis direction. The large-diameter hole  9   b  is formed in a larger size than the outer shape of the lens  8  such that the lens  8  can be accommodated inside the large-diameter hole  9   b . In the present embodiment, each of the small-diameter hole  9   a  and the large-diameter hole  9   b  is formed in a circular shape. The small-diameter hole  9   a  and the large-diameter hole  9   b  are connected to each other by a counterbore surface  9   c  having an annular shape and extending along a plane intersecting the X-axis direction (Y-Z plane). More specifically, the counterbore surface  9   c  connects an end portion on a large-diameter hole  9   b  side of the small-diameter hole  9   a  and an end portion on a small-diameter hole  9   a  side of the large-diameter hole  9   b . Incidentally, in the present embodiment, the counterbore surface  9   c  is formed in a continuous annular shape, but the counterbore surface  9   c  may be formed in a discontinuous annular shape. For example, a cutout may be formed at a part of an inner wall surface of the small-diameter hole  9   a  to divide the counterbore surface  9   c  at the portion at which the cutout is formed. 
     As shown in  FIGS.  11  and  12   , a groove portion  9   d  (recess) that extends from an end portion on an opposite side of the lens holder  9  from the QCL element  2  side to reach the counterbore surface  9   c  is formed in an inner surface of the large-diameter hole  9   b  along the X-axis direction. In the present embodiment, a pair of the groove portions  9   d  facing each other along one diagonal line of the lens holder  9  having a rectangular shape when viewed in the X-axis direction are formed. 
     As shown in  FIGS.  12  and  14   , a central axis AX 1  of the small-diameter hole  9   a  does not coincide with a central axis AX 2  of the large-diameter hole  9   b . Namely, the central axis AX 1  of the small-diameter hole  9   a  is eccentric from the central axis AX 2  of the large-diameter hole  9   b . Incidentally, in  FIG.  14   , the pair of groove portions  9   d  are ignored for easy understanding of the description. Namely,  FIG.  14    is a view in which the large-diameter hole  9   b  is not provided with the pair of groove portions  9   d , and schematically shows a cross-sectional structure taken along line XIV-XIV of  FIG.  12   . 
     In the present embodiment, the central axis AX 1  of the small-diameter hole  9   a  is offset with respect to the central axis AX 2  of the large-diameter hole  9   b  in a direction D. The direction D is a direction from one groove portion  9   d  toward the other groove portion  9   d  when viewed in the X-axis direction. In addition, a diameter d3 of the small-diameter hole  9   a  is the same as the effective diameter of the lens  8  and is 4.5 mm, and a diameter d4 of the large-diameter hole  9   b  is 5.15 mm. In addition, as described above, since the central axis AX 1  is eccentric with respect the central axis AX 2 , as shown in  FIGS.  12  and  14   , a width of the counterbore surface  9   c  on a central axis AX 1  side (here, a portion excluding the groove portion  9   d ) on a straight line passing through the central axis AX 1  and through the central axis AX 2  is a minimum width wmin of the counterbore surface  9   c . In addition, a width of the counterbore surface  9   c  on a central axis AX 2  side (here, a portion excluding the groove portion  9   d ) on the straight line passing through the central axis AX 1  and through the central axis AX 2  is a maximum width wmax of the counterbore surface  9   c . In the present embodiment, the minimum width wmin is 0.25 mm, the maximum width wmax is 0.4 mm, and a distance d between the central axis AX 1  and the central axis AX 2  is 0.075 mm. 
     An edge portion of the incident surface  8   a  of the lens  8  is in contact with the counterbore surface  9   c . In addition, in the lens  8 , the side surface  8   b  of the lens  8  is positioned with respect to the inner surface of the large-diameter hole  9   b  along the direction D from the central axis AX 2  of the large-diameter hole  9   b  toward the central axis AX 1  of the small-diameter hole  9   a . Specifically, the side surface  8   b  of the lens  8  is abutted against the inner surface of the large-diameter hole  9   b  along the direction D. Accordingly, a central axis AX 3  of the lens  8  is disposed at a position closer to the central axis AX 1  of the small-diameter hole  9   a  than to the central axis AX 2  of the large-diameter hole  9   b . In the present embodiment, the diameter (5 mm) and the effective diameter (4.5 mm) of the lens  8 , the diameter d3 (4.5 mm) of the small-diameter hole  9   a , the diameter d4 (5.15 mm) of the large-diameter hole  9   b , and the distance d (0.075 mm) between the central axis AX 1  and the central axis AX 2  are set as described above. Accordingly, the central axis AX 3  of the lens  8  substantially coincides with the central axis AX 1  of the small-diameter hole  9   a . Namely, when viewed in the X-axis direction, the entirety of the effective region of the lens  8  overlaps the small-diameter hole  9   a . In other words, the entirety of the effective region of the lens  8  is exposed to the QCL element  2  side through the small-diameter hole  9   a . Accordingly, it is possible to make the most use of the effective region of the lens  8 . 
     Next, a method for fixing the lens  8  to the lens holder  9  will be described. As shown in  FIG.  14   , at least a part of the side surface  8   b  of the lens  8  is fixed to the inner surface of the large-diameter hole  9   b  through a resin adhesive agent B 1  in a state where the lens  8  is positioned with respect to the lens holder  9  as described above. The resin adhesive agent B 1  is made of, for example, a thermosetting resin such as epoxy resin. For example, the resin adhesive agent B 1  is poured into the groove portions  9   d , so that the resin adhesive agent B 1  that has entered the groove portions  9   d  also pours into a gap between the side surface  8   b  of the lens  8  and the inner surface of the large-diameter hole  9   b  on peripheries of the groove portions  9   d  because of the capillary phenomenon. In addition, the resin adhesive agent B 1  also flows into a gap between the incident surface  8   a  of the lens  8  and the counterbore surface  9   c  because of the capillary phenomenon. The resin adhesive agent B 1  is cured and the lens  8  is fixed to the lens holder  9  by performing the bake processing of the lens holder  9  in this state. A process of pouring the resin adhesive agent B 1  into the groove portion  9   d  is performed, for example, by inserting a needle member for coating the resin adhesive agent B 1  into the groove portion  9   d , and by injecting the resin adhesive agent B 1  from a tip of the needle member toward the counterbore surface  9   c  in the groove portion  9   d . In this case, the groove portions  9   d  may be formed in such a size that the needle member can be inserted thereinto. 
     Effects obtained by a structure in which the central axis AX 1  of the small-diameter hole  9   a  is eccentric with respect to the central axis AX 2  of the large-diameter hole  9   b  in the direction D and the lens  8  is positioned along the direction D (hereinafter, referred to as a “eccentric structure”) will be described in detail with reference to  FIG.  15   .  FIG.  15    is a view schematically showing a positional relationship between the lens  8  and a lens holder  900  when the lens holder  900  according to a comparative example is used. In the lens holder  900 , the central axis AX 1  of the small-diameter hole  9   a  is not eccentric with respect to the central axis AX 2  of the large-diameter hole  9   b . Namely, the central axis AX 1  and the central axis AX 2  coincide with each other. In this case, in order to make the most use of the effective diameter of the lens  8 , as shown in a left part of  FIG.  15   , it is necessary to cause the central axis AX 3  of the lens  8  to coincide with a central axis (namely, the central axes AX 1  and AX 2 ) of the lens holder  900 . No problem occurs as long as such a relationship between the lens  8  and the lens holder  900  is maintained. However, in reality, when the lens  8  is installed at a center of the lens holder  900  in such a manner, and the side surface  8   b  of the lens  8  and the inner surface of the large-diameter hole  9   b  are joined to each other through the resin adhesive agent B 1 , as shown in a right part of  FIG.  15   , when bake processing is performed, the central axis AX 3  of the lens  8  might be offset from the central axis (central axes AX 1  and AX 2 ) of the lens holder  9  because of the surface tension of the resin adhesive agent B 1 , which is a problem. Specifically, since the amount of the resin adhesive agent B 1  with which the gap between the side surface  8   b  of the lens  8  and the large-diameter hole  9   b  is filled is not always uniform, a phenomenon can occur in which the lens  8  moves in a direction in which the surface tension of the resin adhesive agent B 1  acts strongly. When such movement (positional offset) of the lens  8  occurs, a part of the effective region of the lens  8  overlaps the counterbore surface  9   c , and it is not possible to make the most use of the effective region. Namely, as shown in the right part of  FIG.  15   , when the central axis AX 3  of the lens  8  and the optical axis of the laser light L emitted from the QCL element  2  are disposed to coincide with each other, light fluxes of the laser light L cannot be captured in a portion of the effective region of the lens  8  overlapping the counterbore surface  9   c . 
     On the other hand, according to the eccentric structure shown in  FIGS.  12  and  14   , the side surface  8   b  of the lens  8  can be brought into close contact with the inner surface of the large-diameter hole  9   b  along the direction D in advance, the inner surface serving as an installation end of the lens  8 . Then, the closer the distance between the side surface  8   b  of the lens  8  and the inner surface of the large-diameter hole  9   b  at portions (namely, portions in close contact and a periphery thereof) is, the stronger the surface tension of the resin adhesive agent B 1  acts. For this reason, even when bake processing is performed, the lens  8  is not pulled back opposite to the direction D with respect to the lens holder  9 . Namely, before and after the bake processing, a state where the side surface  8   b  of the lens  8  is abutted against the inner surface of the large-diameter hole  9   b  along the direction D (refer to  FIGS.  12  and  14   ) is maintained. Therefore, according to the eccentric structure, it is possible to make the most use of the effective region of the lens  8  by adjusting dimensions such that the effective region of the lens  8  and the small-diameter hole  9   a  coincide with each other in a state where the lens  8  is positioned as described above. 
     Incidentally, a portion of the side surface  8   b  of the lens  8  and the inner surface of the large-diameter hole  9   b  do not necessarily need to be in direct contact with each other, the portion being abutted against the inner surface of the large-diameter hole  9   b . Namely, as shown in  FIG.  14   , the resin adhesive agent B 1  that has slightly entered a gap between the portion of the side surface  8   b  of the lens  8  and the inner surface of the large-diameter hole  9   b  because of the capillary phenomenon may be interposed therebetween, the portion being abutted against the inner surface of the large-diameter hole  9   b . 
     As shown in  FIGS.  11 ,  12   , and (B) of  13 , a wall portion  90  (namely, a tubular portion extending along the X-axis direction) forming the large-diameter hole  9   b  of the lens holder  9  includes a bottom wall portion  92  (first wall portion) facing the second upper surface  53  of the heat spreader  5 . The bottom wall portion  92  has a lower surface  92   a  (first attachment surface) facing the second upper surface  53 . A plurality (in the present embodiment, four) of protrusions  92   b  (first protrusions) protruding to a heat spreader  5  side are formed on the lower surface  92   a . The four protrusions  92   b  are provided at positions corresponding to the four protrusions  53   a  (refer to (A) of  FIG.  9   ) provided on the second upper surface  53  of the heat spreader  5 . The protrusions  92   b  are joined to the respective protrusions  53   a  through an adhesive layer B 2  (refer to  FIG.  2   ) made of a UV curable resin (photocurable resin). 
     In the present embodiment, the four protrusions  92   b  are disposed at four corners of the bottom wall portion  92  in a well-balanced manner. Namely, the four protrusions  92   b  are disposed such that a center of the four protrusions  92   b  substantially coincides with a center of the bottom wall portion  92  when viewed in the Z-axis direction. Accordingly, the lens holder  9  can be stably fixed onto the second upper surface  53  of the heat spreader  5 , and a structure that is resistant to impact, vibration, and the like can be realized. 
     As shown in  FIG.  11    and (A) of  13 , the wall portion  90  includes a top wall portion  91  (second wall portion) facing the top wall  33  of the package  3 . The top wall portion  91  faces the bottom wall portion  92  through the large-diameter hole  9   b . A cutout  91   a  is formed at an end portion on an opposite side of the top wall portion  91  from the small-diameter hole  9   a  side. In addition, a cutout  92   c  is formed at an end portion on an opposite side of the bottom wall portion  92  from the small-diameter hole  9   a  side. When viewed in the Z-axis direction, the cutout  91   a  and the cutout  92   c  include portions overlapping each other. Namely, the top wall portion  91  is formed not to overlap at least a part of the cutout  92   c  of the bottom wall portion  92  in a direction in which the bottom wall portion  92  and the top wall portion  91  face each other (Z-axis direction). A part of the second upper surface  53  of the heat spreader  5  can be visually recognized from above the top wall portion  91  through a portion at which the cutout  91   a  and the cutout  92   c  overlap each other. Namely, the second upper surface  53  of the heat spreader  5  can be irradiated with light from above the top wall portion  91  through the portion. According to such a configuration, UV light can be suitably guided to a space between the lower surface  92   a  of the lens holder  9  and the second upper surface  53  of the heat spreader  5  by placing the lens holder  9  on the second upper surface  53  of the heat spreader  5  such that the positions of the four protrusions  53   a  and the positions of the four protrusions  92   b  are aligned with each other, and then by irradiating the second upper surface  53  with the UV light from above the lens holder  9 . Accordingly, the adhesive layer B 2  provided between each of the protrusions  92   b  and the corresponding protrusion  53   a  can be appropriately cured. 
     In addition, since locations to be coated with the adhesive layer B 2  are defined by each of the protrusions  92   b  and each of the protrusions  53   a  formed in an island shape, the locations to be coated with the adhesive layer B 2  and the coating amount of the adhesive layer B 2  can be equalized among a plurality of products (quantum cascade laser devices  1 ). In addition, there is a limit to the depth by which UV light penetrates into a UV curable resin. For this reason, if the entirety of the lower surface  92   a  is coated with the UV curable resin without providing the protrusions  92   b  and the protrusions  53   a , a problem that the UV light does not reach the inside of the UV curable resin (center side) and the UV curable resin cannot be completely cured can occur. Such a problem can be avoided by defining the locations to be coated with the adhesive layer B 2  in an island shape as described above. In addition, since the protrusions  92   b  and the protrusions  53   a  are provided in an island shape, a sufficient space for the passing of the UV light can be formed between the lower surface  92   a  and the second upper surface  53  at positions where the protrusions  92   b  and the protrusions  53   a  do not overlap each other. Accordingly, the UV light that has entered the space can be reflected by valleys (portions at which the protrusions  92   b  and the protrusions  53   a  are not formed) of each of the lower surface  92   a   and the second upper surface  53 , and the adhesive layer B 2  on each of the protrusions  53   a  can be irradiated with the UV light. 
     Next, a method for manufacturing the quantum cascade laser device  1  (assembly method) will be described. As shown in  FIG.  16   , first, the package  3  before the top wall  33  is joined to the side wall  32  is prepared. Subsequently, the window member  15  (window member  15  on which the anti-reflection films  151  and  152  and the metal film  153  are provided in advance (refer to  FIG.  7   )) is joined to the side wall  32 . Specifically, the solder member  16  that is an annular sheet member molded in a washer shape is sandwiched between the counterbore surface  14  and the window member  15 . Then, a load is applied to the window member  15  from the outside of the package  3  to push the window member  15  against the counterbore surface  14 . In this state, the window member  15  and the counterbore surface  14  are joined to each other through the solder member  16  by using, for example, a vacuum soldering device (vacuum soldering furnace). At this time, a jig for aligning a center of the window member  15  with the central axes of the counterbore openings (the small-diameter hole  12  and the large-diameter hole  13 ) may be used. 
     Subsequently, the heat spreader  5  is placed on the Peltier module  4  to the top and the bottom of which the In foils  45  that are solder members are affixed, and these members are disposed on the bottom wall  31  at a predetermined position using a jig. Then, a load is applied from above the heat spreader  5  to push these members against the bottom wall  31 . In this state, the bottom wall  31 , the Peltier module  4 , and the heat spreader  5  are joined to each other through the In foils  45  disposed between these members, by using, for example, a vacuum soldering device. Subsequently, as shown in  FIG.  2   , the lead wires  20  of the Peltier module  4  are solder joined to the lead pins  10 . 
     Subsequently, the heat sink  6  on which elements such as the QCL element  2 , the submount  7 , the temperature sensor T, and the ceramic patterns SP are mounted in advance is fixed to the first upper surface  52  of the heat spreader  5 . Specifically, the heat sink  6  is screwed to the heat spreader  5  by inserting screw members (not shown) into the screw holes  6   c  of the heat sink  6  (refer to  FIG.  10   ) and into the screw holes  52   a  of the heat spreader  5 . In addition, the temperature sensor T and the ceramic patterns SP are electrically connected to the predetermined lead pins  10  by wires (not shown). 
     Subsequently, the lens holder  9  on which the lens  8  is mounted as described above is fixed to the second upper surface  53  of the heat spreader  5 . Specifically, each of the protrusions  53   a  formed on the second upper surface  53  of the heat spreader  5  is coated with a UV curable resin (adhesive layer B 2 ) in advance. Then, the lens holder  9  is vacuum-chucked using, for example, Convum (vacuum generator) or the like, and is moved into the package  3 . Then, the QCL element  2  is driven to emit the laser light L, and an active alignment is performed to align the optical axis of the laser light L and the central axis of the lens  8  with each other while observing the laser light L using a beam monitor. 
     Subsequently, the lens holder  9  is fixed to the heat spreader  5  in a state where the positions of the optical axis of the laser light L and the central axis of the lens  8  are aligned with each other. Specifically, in a state where the optical axis of the laser light L and the central axis of the lens  8  are aligned with each other, the second upper surface  53  of the heat spreader  5  is irradiated with UV light from above the lens holder  9  through the cutout  91   a  and the cutout  92   c  of the lens holder  9 . Accordingly, each of the protrusions  92   b  of the lens holder  9  and the corresponding protrusion  53   a  of the heat spreader  5  are joined to each other through the adhesive layer B 2 . 
     Here, the position of each of the protrusions  92   b  is designed so as to overlap the corresponding protrusion  53   a  of the heat spreader  5  in a state where the optical axis of the laser light L and the central axis of the lens  8  are aligned with each other. In addition, the height dimension (length along the Z-axis direction) of each of the protrusions  53   a  and each of the protrusions  92   b  is designed such that a gap of approximately several hundreds of µm smaller than the thickness of the UV curable resin (adhesive layer B 2 ) coated on each of the protrusions  53   a  in advance is formed between each of the protrusions  53   a  of the heat spreader  5  and the corresponding protrusion  92   b  of the lens holder  9  in a state where the optical axis of the laser light L and the central axis of the lens  8  are aligned with each other. Accordingly, when the lens holder  9  is moved with respect to the heat spreader  5  to align the optical axis of the laser light L and the central axis of the lens  8  with each other, an adjustment is made such that each of the protrusions  92   b  of the lens holder  9  and the adhesive layer B 2  on the corresponding protrusion  53   a  of the heat spreader  5  come into contact with each other. In other words, the height dimension of each of the protrusions  53   a  and each of the protrusions  92   b  and the thickness of the adhesive layer B 2  are designed such that the optical axis of the laser light L and the central axis of the lens  8  are aligned with each other in a state where the lens holder  9  is pushed against the UV curable resin (adhesive layer B 2 ) coated on the heat spreader  5  in advance. 
     Subsequently, an upper end portion of the side wall  32  of the package  3  (end portion opposite to the bottom wall  31  side) is joined to the top wall  33 . As described above, the quantum cascade laser device  1  shown in  FIG.  1    is obtained. 
     In the quantum cascade laser device  1  described above, the light-emitting window  11  is joined to the side wall  32  of the package  3  by the solder member  16  (in the present embodiment, an SnAgCu-based solder material having a melting point of 220° C.) having a lower melting point than that of a brazing material (melting point is 450° C. or higher). Accordingly, compared to when the brazing material is used, the window member  15  and the counterbore surface  14  can be brought into close contact with each other while suppressing damage to the window member  15  and the like (particularly, the anti-reflection films  151  and  152 ) caused by heat. In addition, the first region A 1  in which the anti-reflection film  151  is provided and the second region A 2  to which the solder member  16  is joined are separated from each other on the incident surface  15   a  of the window member  15  (refer to  FIG.  7   ). Accordingly, stress generated in the second region A 2  when the solder member  16  is melted or solidified is prevented from being transmitted to the anti-reflection film  151  on the first region A 1 . As a result, damage (crack, peeling, or the like) to the anti-reflection film  151  caused by the stress is suppressed. As described above, according to the quantum cascade laser device  1 , damage to the anti-reflection film  151  provided on the light-emitting window  11  can be suppressed, and high airtightness of the package  3  can be secured. 
     In addition, the side surface  15   c  of the window member  15  includes the third region A 3  metallized to be continuous with the second region A 2 , and at least a part of the side surface  15   c  is joined to at least a part of the inner surface of the large-diameter hole  13  through the solder member  16  (refer to  FIG.  5   ). According to this configuration, since a region that is continuous from the second region A 2  to the side surface  15   c  of the window member  15  (third region A 3 ) is metallized, when solder joining is performed, some of the solder member  16  suitably wet-spreads to a third region A 3  side. As a result, the solder member  16  can be interposed between the side surface  15   c  of the window member  15  and the inner surface of the large-diameter hole  13 , and the airtightness of the package  3  can be suitably improved. 
     In addition, the wavelength of the laser light L emitted from the QCL element  2  is included within a range of 4 µm to 12 µm. As one example, the heat-resistant temperature of the anti-reflection films  151  and  152  is approximately 260° C. On the other hand, in the quantum cascade laser device  1 , since the solder member  16  having a relatively low melting point is used as a joining material, the window member  15  on which the anti-reflection films  151  and  152  are provided can be attached to the side wall  32  by solder joining while suppressing damage to the anti-reflection films  151  and  152  caused by heat. 
     In addition, in the quantum cascade laser device  1 , the lens holder  9  has the small-diameter hole  9   a  and the large-diameter hole  9   b  of which the central axes AX 1  and AX 2  are eccentric with respect to each other. In addition, the side surface  8   b  of the lens  8  is positioned with respect to the inner surface of the large-diameter hole  9   b  along the direction D from the central axis AX 2  of the large-diameter hole  9   b  toward the central axis AX 1  of the small-diameter hole  9   a . Accordingly, the positional offset of the lens  8  (movement of the lens  8  with respect to the lens holder  9 ) that may be caused by the surface tension of the resin adhesive agent B 1  disposed around the lens  8  in case that the lens  8  is disposed at a central portion of the large-diameter hole  9   b  (for example, refer to the left part of  FIG.  15   ) can be suitably suppressed. Further, in a state where the lens  8  is positioned in such a manner, the central axis AX 3  of the lens  8  is disposed at a position close to the central axis AX 1  of the small-diameter hole  9   a  (in the present embodiment, the central axis AX 3  and the central axis AX 1  coincide with each other). Accordingly, the area of a region in which the effective region of the lens  8  (region within the effective diameter around the central axis AX 3  of the lens) and the counterbore surface  9   c  interfere with (overlap) each other can be reduced. As a result, the effective region of the lens  8  can be efficiently used. In addition, since the effective region of the lens  8  can be efficiently used, the size of the lens  8  can be reduced, and the size of the package  3  can be reduced. 
     In addition, in the present embodiment, the central axis AX 3  of the lens  8  substantially coincides with the central axis AX 1  of the small-diameter hole  9   a , and the effective diameter of the lens  8  substantially coincides with the diameter d3 of the small-diameter hole  9   a . According to this configuration, the entirety of the effective region (region within the effective diameter) of the lens  8  can be exposed through the small-diameter hole  9   a . Accordingly, the size of the small-diameter hole  9   a  is suppressed to its minimum to secure the area of the counterbore surface  9   c , so that it is possible to make the most use of the effective region of the lens  8  while appropriately supporting the edge portion of the incident surface  8   a  of the lens  8 . 
     In addition, the groove portions  9   d  that reach the counterbore surface  9   c  along the X-axis direction are formed in the inner surface of the large-diameter hole  9   b , and the resin adhesive agent B 1  enters the groove portions  9   d . According to this configuration, the resin adhesive agent B 1  can be easily injected into the gap between the side surface  8   b  of the lens  8  and the inner surface of the large-diameter hole  9   b  through the groove portions  9   d . 
     In addition, the lens holder  9  has the lower surface  92   a  on which the plurality (in the present embodiment, four) of protrusions  92   b  protruding to the heat spreader  5  side are formed, and the plurality of protrusions  92   b  are joined to the second upper surface  53  of the heat spreader  5  through the adhesive layer B 2  made of a UV curable resin. In the present embodiment, the plurality of protrusions  53   a  protruding to a lens holder  9  side are formed on the second upper surface  53  at the positions corresponding to the plurality of protrusions  92   b , and the plurality of protrusions  92   b  are joined to the plurality of protrusions  53   a  through the adhesive layer B 2 . According to this configuration, since locations where the adhesive layer B 2  is provided can be dispersed onto the plurality of protrusions  92   b , the adhesive layer B 2  on each of the protrusions  92   b  can be easily and appropriately cured compared to when the adhesive layer B 2  is provided in a wide range on the entire surface. Further, in the present embodiment, the adhesive layer B 2  is disposed at a central portion of the space formed between the lower surface  92   a  and the second upper surface  53  (between the protrusions  92   b  and the protrusions  53   a ). Accordingly, the adhesive layer B 2  can be suitably irradiated with UV light reflected by the lower surface  92   a  and by the second upper surface  53  in the space. As a result, the adhesive layer B 2  can be more appropriately cured, and the lens holder  9  can be stably fixed to the heat spreader  5 . 
     In addition, the bottom wall portion  92  of the lens holder  9  is provided with the cutout  92   c  for guiding light to the second upper surface  53  of the heat spreader  5 . According to this configuration, the second upper surface  53  of the heat spreader  5  can be irradiated with UV light from a side opposite to a side on which the heat spreader  5  is disposed with respect to the lens holder  9  (namely, from above the lens holder  9 ), through the cutout  92   c  provided in the bottom wall portion  92 . Accordingly, light irradiation for curing the adhesive layer B 2  between the lower surface  92   a  and the second upper surface  53  can be easily performed. 
     In addition, the lens holder  9  includes the top wall portion  91  facing the bottom wall portion  92  through the large-diameter hole  9   b . Then, the top wall portion  91  is formed not to overlap at least a part of the cutout  92   c  provided in the bottom wall portion  92  when viewed in the direction in which the bottom wall portion  92  and the top wall portion  91  face each other (Z-axis direction). According to this configuration, the lens  8  disposed in the large-diameter hole  9   b  can be appropriately protected from the outside by the bottom wall portion  92  and the top wall portion  91 . In addition, since the top wall portion  91  is formed not to overlap at least a part of the cutout  92   c  provided in the bottom wall portion  92 , the second upper surface  53  of the heat spreader  5  can be irradiated with light by irradiating the lens holder  9  with the light from the outside of the lens holder  9  (side opposite to the bottom wall portion  92  with the top wall portion  91  sandwiched therebetween). 
     In addition, instead of the cutout  92   c , a through-hole penetrating through the bottom wall portion  92  in the Z-axis direction may be formed in the bottom wall portion  92 . Similarly, instead of the cutout  91   a , a through-hole penetrating through the top wall portion  91  in the Z-axis direction and including a portion overlapping the cutout  92   c  or the through-hole provided in the bottom wall portion  92  may be formed in the top wall portion  91 . Even with such a configuration, light can be guided to the second upper surface  53  of the heat spreader  5  by performing light irradiation from above the lens holder  9 . 
     In addition, the QCL element  2  and the lens holder  9  are mounted on the same heat spreader  5 . Incidentally, the QCL element  2  is mounted on the heat spreader  5  with the submount  7  and the heat sink  6  interposed therebetween. According to this configuration, since a base (heat spreader  5 ) on which the QCL element  2  and the lens holder  9  are placed is shared, when the heat spreader  5  expands or contracts because of heat, a relative movement of the lens holder  9  with respect to the QCL element  2  can be suppressed. As a result, the occurrence of an optical axis offset (offset of the central axis AX 3  of the lens  8  with respect to the optical axis of the laser light L) caused by a temperature change in the package  3  can be suppressed. 
     In addition, the package  3  airtightly accommodates the QCL element  2 , the lens  8 , and the lens holder  9  described above. According to this configuration, since the effective region of the lens  8  disposed in the package  3  can be efficiently used, the size of the lens  8  can be reduced, and the size of the package  3  can be reduced. 
     Modification Examples 
     One embodiment of the present disclosure has been described above; however, the present disclosure is not limited to the above-described embodiment. For example, the material and the shape of each configuration are not limited to the material and the shape described above, and various materials and shapes can be adopted. In addition, some configurations included in the embodiment may be appropriately changed or omitted. 
     The shape of the lens holder is not limited to the shape of the lens holder  9  described above. For example, instead of the lens holder  9  described above, lens holders  9 A to  9 C shown in  FIGS.  17  to  19    may be used. 
     As shown in  FIG.  17   , the lens holder  9 A of a first modification example differs from the lens holder  9  in that a large-diameter hole  9 A b  having a quadrangular shape is formed instead of the large-diameter hole  9   b  having a circular shape and the groove portions  9   d  are not formed (large-diameter hole  9 A b  is formed in such a size as to include portions corresponding to the groove portions  9   d ). In such a manner, the large-diameter hole  9 A b  may be formed in such a size as to accommodate the lens  8 , and may not necessarily be formed in a circular shape. According to the large-diameter hole  9 A b , a sufficient space for the filling of the resin adhesive agent B 1  can be secured at four corners of the large-diameter hole  9 A b  without providing the groove portions  9   d . From a different perspective, in the lens holder  9 A, each of the portions corresponding to the four corners of the large-diameter hole  9 A b  functions as a recess corresponding to the groove portion  9   d  of the lens holder  9 . Incidentally, similarly to the large-diameter hole  9 A b , the small-diameter hole  9   a  may also be formed in a shape other than a circular shape (for example, the same quadrangular shape as that of the large-diameter hole  9 A b , a size smaller than the large-diameter hole  9 A b , or the like). 
     As shown in  FIG.  18   , the lens holder  9 B of a second modification example differs from the lens holder  9  in that the lens holder  9 B includes a top wall portion  91 B of which an upper surface is formed in a circular shape (curved surface shape), instead of the top wall portion  91 . In addition, in the lens holder  9 B, since the top wall portion  91 B is adopted, there is no space for forming the groove portion  9   d  provided on a top wall portion  91  side in the lens holder  9 . For this reason, in the lens holder  9 B, a pair of the groove portions  9   d  are formed on both respective sides in the Y-axis direction on a bottom wall portion  92  side. In such a manner, positions where the groove portions  9   d  are formed in the lens holder are not particularly limited. In addition, the number of the groove portions  9   d  is not particularly limited. In addition, in the lens holder  9 B, a length of a portion of a wall portion along the X-axis direction excluding the bottom wall portion  92  is shorter compared to the wall portion  90  of the lens holder  9 , the wall portion forming the large-diameter hole  9   b . Specifically, the length of the portion along the X-axis direction is slightly shorter than a length of the lens  8  along the X-axis direction. According to such a configuration, since a portion that blocks UV light from above the lens holder  9 B can be reduced, the UV light for curing the adhesive layer B 2  can be more suitably guided to the heat spreader  5  side. 
     As shown in  FIG.  19   , the lens holder  9 C of a third modification example differs from the lens holder  9  in that the lens holder  9 C includes a top wall portion  91 C having a shorter length along the X-axis direction than that of the top wall portion  91 , instead of the top wall portion  91 . The length of the top wall portion  91 C along the X-axis direction is approximately half the length of the lens  8  along the X-axis direction. In addition, in the lens holder  9 C, a length of a portion of a wall portion along the X-axis direction excluding the bottom wall portion  92  is the same as that of the top wall portion  91 C, the wall portion forming the large-diameter hole  9   b . Namely, the lens holder  9 C is formed in a substantially L shape when viewed in the Y-axis direction. In the lens holder  9 C, since a portion that blocks UV light from above the lens holder  9 C is smaller than in the lens holder  9 B, the UV light for curing the adhesive layer B 2  can be more suitably guided to the heat spreader  5  side. In addition, as in the lens holder  9 C, the wall portion of the lens holder forming the large-diameter hole  9   b  does not need to surround the entirety of the side surface  8   b  of the lens  8 , and may be configured to surround only a part of an incident surface  8   a  side of the side surface  8   b  of the lens  8 . 
     In addition, as in a quantum cascade laser device  1 A according to a modification example shown in  FIG.  20   , the lens may not necessarily be accommodated in the package  3 . The quantum cascade laser device  1 A differs from the quantum cascade laser device  1  in that the quantum cascade laser device  1 A includes a lens  8 A externally attached to an outer side of the package  3 , instead of including the lens  8  and the lens holder  9  in the package  3 . Namely, the quantum cascade laser device  1 A includes the lens  8 A disposed on the outer side of the package  3  to concentrate or collimate the laser light L that has transmitted through the light-emitting window  11 . In addition, because of the difference, the quantum cascade laser device  1 A differs from the quantum cascade laser device  1 , also in that the quantum cascade laser device  1 A includes a heat spreader  5 A configured such that the QCL element  2  can be disposed at a position close to the light-emitting window  11 , instead of the heat spreader  5 . As described above, the beam radiation angle of the laser light L is very large. For this reason, in order to make the light-emitting window  11  as small as possible while allowing all light fluxes of the laser light L to pass through the light-emitting window  11  to the outside of the package  3 , it is desirable that the light-emitting window  11  and the emitting surface (end surface  2   a ) of the QCL element  2  that emits the laser light L are brought as close as possible to each other. For this reason, the quantum cascade laser device  1 A not including the lens inside the package  3  includes the heat spreader  5 A described above. 
     According to the quantum cascade laser device  1 A, since the lens  8 A is a member to be externally attached that is disposed on the outer side of the package  3 , the disposition, replacement, and the like of the lens  8  can be flexibly performed. Further, as described above, the length w1 of the small-diameter hole  12  along the optical axis direction (X-axis direction) of the laser light L is shorter than the length w2 of the large-diameter hole  13  (refer to  FIG.  5   ). According to this configuration, the light-emitting window  11  can be brought closer to the QCL element  2  compared to when the length w1 of the small-diameter hole  12  is equal to or longer than the length w2 of the large-diameter hole  13 . Accordingly, even when the beam radiation angle of the laser light L emitted from the QCL element  2  is large, the laser light L can be incident on the light-emitting window  11  while a degree of the spread of the laser light L is reduced. As a result, the size of the light-emitting window  11  can be reduced, and the size of the package  3  can be reduced. 
     In addition, as described above, the emitting surface  15   b  of the window member  15  includes the fourth region A 4  in which the anti-reflection film  152  is provided, and the fourth region A 4  includes the first region A 1  and is larger than the first region A 1  when viewed in the X-axis direction. As in the quantum cascade laser device  1 A, when the lens is not provided in the package  3  and the laser light L that is divergent light is incident on the window member  15 , a region through which the laser light L passes on the incident surface  15   a  of the window member  15  is smaller than a region through which the laser light L passes on the emitting surface  15   b  of the window member  15 . Therefore, as in this configuration, a region corresponding to a difference between the fourth region A 4  and the first region A 1  can be secured as the second region A 2  by making the anti-reflection film  151   on an incident surface  15   a  side smaller than the anti-reflection film  152  on an emitting surface  15   b  side (namely, by making the first region A 1  smaller than the fourth region A 4 ). In such a manner, since the sizes of the first region A 1 , the second region A 2 , and the fourth region A 4  are designed in consideration of the beam radiation angle of the laser light L, the size of the window member  15  can be reduced, and the size of the package  3  can be reduced. 
     In addition, in the embodiment, the emitting surface  15   b  of the window member  15  is substantially flush with the outer surface of the side wall  32 , but the emitting surface  15   b  of the window member  15  may further protrude to the outside of the package  3  than the outer surface of the side wall  32 . Namely, the thickness of the window member  15  may be larger than the length w2 of the large-diameter hole  13 . In this case, the workability when the window member  15  is joined to the side wall  32  from the outside of the package  3  can be improved. In addition, as in the quantum cascade laser device  1 A, when the lens  8 A to be externally attached is attached to the emitting surface  15   b  of the window member  15 , the workability of lens attachment can also be improved. In addition, the size of the package  3  can be reduced or the lens  8 A can be reliably disposed close to the window member  15  by reducing the thickness of the side wall  32 . 
     In addition, in the embodiment, as one example of the semiconductor laser element, the quantum cascade laser element (QCL element  2 ) has been exemplified, but as the semiconductor laser element to be accommodated in the package  3 , a laser element other than the quantum cascade laser element may be used. In addition, the semiconductor laser element may be a distributed feedback (DFB) semiconductor laser element in which a diffraction grating structure is provided on an upper portion of an active layer. 
     In addition, in the embodiment, the package  3  that is a butterfly package has been exemplified, but the form of the package is not limited thereto. For example, the package may be a CAN package. 
     Reference Signs List 
       1 ,  1 A: quantum cascade laser device (semiconductor laser device),  2 : quantum cascade laser element (semiconductor laser element),  2   a : end surface (emitting surface),  3 : package,  5 ,  5 A: heat spreader,  8 ,  8 A: lens,  8   a : incident surface,  8   b : side surface,  8   c : emitting surface,  9 ,  9 A,  9 B,  9 C: lens holder,  9   a : small-diameter hole,  9   b : large-diameter hole,  9   c : counterbore surface,  9   d : groove portion (recess),  11 : light-emitting window,  12 : small-diameter hole (first hole portion),  13 : large-diameter hole (second hole portion),  14 : counterbore surface,  15 : window member,  15   a : incident surface,  15   b : emitting surface,  15   c : side surface,  16 : solder member,  31 : bottom wall,  32 : side wall,  33 : top wall,  53 : second upper surface (second attachment surface),  53   a : protrusion (second protrusion),  91 : top wall portion (second wall portion),  92 : bottom wall portion (first wall portion),  92   a : lower surface (first attachment surface),  92   b : protrusion (first protrusion),  92   c : cutout,  151 : anti-reflection film (first anti-reflection film),  152 : anti-reflection film (second anti-reflection film),  153 : metal film, A 1 : first region, A 2 : second region, A 3 : third region, A 4 : fourth region, AX 1 , AX 2 , AX 3 : central axis, B 1 : resin adhesive agent, B 2 : adhesive layer, D: direction, L: laser light.