Laser module

Abstract

To provide a laser module which is miniaturized and is packaged in such an embodiment that loss of light contributing to the generation of a terahertz wave can be suppressed.SOLUTION: A laser module 1A comprises: a QCL element 2; a diffraction lattice unit 3 including a movable diffraction lattice 31; a first lens 4 which allows light pass through, wherein the light is outgoing light from an end surface 12b of the QCL element 2 and the light feedbacking from the movable diffraction lattice 31 to the QCL element 2; a second lens 6 which allows terahertz wave pass through, wherein the terahertz wave is exited from the QCL element 2; a first holder 7; and a package 8. The first holder 7 includes: a support surface 7a on which the QCL element 2 is mounted; and a side surface 7e which is connected to the support surface 7a, and is opposite to the first lens 4 in a resonance direction. A distance d1 from a cross point 7i in which the side surface 7e along the resonance direction and the support surface 7a intersect each other to the end surface 12b is shorter than a distance d2 of a cross point 7i along the resonance direction and the first lens 4.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This disclosure relates to a laser module. [Background technology]

[0002] Conventionally, difference frequency generation terahertz quantum cascade lasers (DFG-THz-QCL) are known (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6893591 [Patent Document 2] U.S. Patent Application Publication No. 2015 / 0311665 [Overview of the project] [Problems that the invention aims to solve]

[0004] In the terahertz quantum cascade laser described above, there is a need for a product that can be miniaturized and packaged in a way that suppresses the loss of light that contributes to the generation of terahertz waves, from the viewpoint of user convenience. However, the above-mentioned Patent Documents 1 and 2 do not disclose a method for realizing such packaging.

[0005] Therefore, one aspect of this disclosure aims to provide a laser module packaged in a manner that enables miniaturization and suppresses the loss of light that contributes to the generation of terahertz waves. [Means for solving the problem]

[0006] A laser module relating to one aspect of the present disclosure is a quantum cascade laser element comprising: a substrate having a main surface and a back surface opposite to the main surface; a first cladding layer provided on the main surface; an active layer provided on the side of the first cladding layer opposite to the substrate; and a second cladding layer provided on the side of the active layer opposite to the first cladding layer, wherein the active layer has a first end face and a second end face facing each other in a second direction orthogonal to a first direction which is the stacking direction of the substrate, the first cladding layer, the active layer, and the second cladding layer, the first end face constituting a resonator for emitting light of a first frequency and light of a second frequency, and the active layer generates terahertz waves at the difference frequency between the first frequency and the second frequency; a diffraction grating unit including a movable diffraction grating which constitutes an external resonator for light of the first frequency; and a quantum cascade laser element disposed between the quantum cascade laser element and the movable diffraction grating, which emits light from the second end face and the movable diffraction grating. The quantum cascade laser element comprises a first lens that allows light returning to the cascade laser element to pass through, a second lens positioned opposite the first end face that allows terahertz waves emitted from the quantum cascade laser element to pass through, a first holder that holds the quantum cascade laser element, and a package that houses the quantum cascade laser element, a diffraction grating unit, the first lens, and the first holder, and in which the optical path between the light incident surface of the second lens and the movable diffraction grating is arranged, wherein the first holder has a support surface on which the quantum cascade laser element is mounted, and a first side surface connected to the support surface and facing the first lens in a second direction, and when viewed from a direction perpendicular to the first and second directions, the first distance from the intersection of the first side surface and the support surface along the second direction to the second end face, when the direction from the first lens toward the quantum cascade laser element along the second direction is taken as the positive direction, is smaller than the second distance from the intersection point along the second direction to the first lens.

[0007] The above laser module packages a configuration for oscillating light of a first frequency and light of a second frequency necessary for generating terahertz waves by difference frequency generation, namely a quantum cascade laser element, a first holder for holding the quantum cascade laser element, a movable diffraction grating (diffraction grating unit), and a first lens positioned between the quantum cascade laser element and the movable diffraction grating. Furthermore, the first distance from the intersection point between the first side surface and the support surface of the first holder along the second direction (i.e., the resonance direction) where both end faces (first and second end faces) of the quantum cascade laser element face each other, with the direction from the first lens towards the quantum cascade laser element being defined as the positive direction, is made smaller than the second distance between the intersection point along the second direction and the first lens. By arranging the quantum cascade laser element and the first holder in this way, the quantum cascade laser element (second end face) can be brought as close as possible to the first lens. This suppresses the increase in size of the first lens and the movable diffraction grating, and allows for miniaturization of the entire package. Furthermore, by setting the first distance to a small value as described above, interference between the light emitted from the second end face and the first holder (i.e., a portion of the emitted light being blocked by the first holder and not reaching the first lens) can be suppressed. This reduces the loss of light that contributes to the generation of terahertz waves. As a result, a laser module can be obtained that is miniaturized and packaged in a manner that suppresses the loss of light that contributes to the generation of terahertz waves.

[0008] The first distance may be 0 or less. With the above configuration, that is, a configuration in which the second end face is flush with the first side surface, or a configuration in which the second end face protrudes further toward the first lens than the first side surface, interference between the light emitted from the second end face and the first holder can be prevented more reliably, and thus the loss of light that contributes to the generation of terahertz waves can be reduced more effectively.

[0009] The first distance may be 0. With the above configuration, that is, the configuration in which the second end face is flush with the first side surface, the entire portion of the quantum cascade laser element extending to the second end face can be brought into contact with the first holder (support surface), thereby increasing the heat dissipation efficiency from the quantum cascade laser element to the first holder.

[0010] In the quantum cascade laser element, the resonant axis passing through the first and second end faces may intersect the optical axis of the second lens, provided that the support surface is inclined to intersect the optical axis of the second lens, and the first end face may be spaced apart from the optical incident surface of the second lens. With the above configuration, it is possible to space the first end face, which constitutes the resonator for first-frequency and second-frequency light for generating terahertz waves, apart from the optical incident surface of the second lens, while bringing the quantum cascade laser element (for example, the side surface of the substrate continuous with the first end face) into contact with or close to the optical incident surface of the second lens. This makes it possible to ensure the efficiency of extracting terahertz waves from the quantum cascade laser element to the second lens, while preventing the reflectivity of the first end face, which is important in the oscillation of first-frequency and second-frequency light, from being affected by the second lens.

[0011] The first lens and diffraction grating unit may be fixed to the first holder. With the above configuration, these components can be fixed to the first holder and their positions adjusted before housing the quantum cascade laser element, the first lens, and the diffraction grating unit in the package. This improves work efficiency compared to adjusting the positions of each component within the package.

[0012] The second lens may be a silicon lens formed in the shape of a hemisphere or hyperhemispheric region. Silicon lenses are resistant to scratches. Furthermore, even if scratches or dirt occur, terahertz waves, which have longer wavelengths than visible light and near-infrared light, are less affected, making it possible to position the second lens so that it is exposed on the outside of the package.

[0013] The first holder may be connected to the support surface and may have a second side surface facing the second lens in the second direction, and the first end surface may be located closer to the second lens than the second side surface in the second direction. According to the above configuration, by protruding the first end surface of the quantum cascade laser element toward the second lens side beyond the second side surface of the first holder, the first end surface of the quantum cascade laser element can be brought into contact with or close to the second lens for terahertz wave output while suppressing interference between the first holder and the second lens.

[0014] The package may have a side wall facing the first end surface, the side wall may have a through hole penetrating in a third direction perpendicular to the side wall, the through hole includes a first hole portion opening to the inside of the package, a second hole portion that includes the first hole portion when viewed from the third direction, is larger than the first hole portion and opens to the outside of the package, and an annular counterbore surface that connects the first hole portion and the second hole portion and extends along a plane intersecting the third direction, and an outer edge portion of the light incident surface of the second lens may be inserted into the second hole portion from the outside of the package, brought into surface contact with the counterbore surface and fixed thereto. According to the above configuration, since the second lens for terahertz wave output can be attached to the side wall from the outside of the package, the attachment work of the second lens can be easily performed. Further, the second lens can be used as a window material that blocks the through hole provided in the side wall. As a result, it is possible to reduce the manufacturing cost by reducing the number of parts and reduce the size of the entire package. Furthermore, since light loss (attenuation of terahertz waves that are output light) caused by providing a window material separate from the second lens can be avoided, higher output of terahertz waves can also be achieved.

[0015] The first end surface may be located inside the first hole portion. According to the above configuration, by advancing the first end surface of the quantum cascade laser element to the inside of the first hole portion, the substrate of the quantum cascade laser element can be brought into contact with or close to the light incident surface of the second lens. Thereby, the extraction efficiency of terahertz waves can be improved, and the size of the entire package can be reduced.

[0016] The first holder may comprise a main body portion including a portion on the first lens side of the support surface and a first side surface, and a protruding portion connected to the main body portion, the protruding portion including a portion on the second lens side of the support surface, wherein at least a part of the protruding portion may be located inside the first hole. According to the above configuration, by extending the first holder (protruding portion) into the first hole, the contact area between the quantum cascade laser element and the first holder (support surface) can be increased. As a result, the heat dissipation efficiency from the quantum cascade laser element to the first holder can be improved.

[0017] The laser module may further comprise a second holder that holds the second lens, the second lens and the second holder may be housed in a package, the package may have a side wall facing the first end face, and the side wall may be provided with a light exit window for allowing light that has been emitted from the first end face and passed through the second lens to pass through. According to the above configuration, the second lens for outputting terahertz waves can be housed in the package in the same manner as the first lens, so damage and contamination of the second lens can be prevented.

[0018] The second holder may be fixed to the package independently of the first holder. According to the above configuration, since the first holder and the second holder are independent of each other, before housing each member in the package, the work of attaching the quantum cascade laser element to the first holder and the work of attaching the second lens to the second holder can be performed in parallel. Thereby, the work efficiency of assembling the laser module can be improved.

[0019] The first lens and the diffraction grating unit may be fixed to the first holder, and the second holder may be formed integrally with the first holder or fixed to the first holder. According to the above configuration, a structure in which the first holder and the second holder are integrated can be prepared outside the package, so that the position adjustment of the quantum cascade laser element, the first lens, the diffraction grating unit, and the second lens can be performed outside the package. Thereby, the work efficiency can be improved compared to the case where position adjustment is performed inside the package.

[0020] The second holder may have a through-hole penetrating in a third direction perpendicular to the side wall. The through-hole may have a first hole opening toward the quantum cascade laser element, a second hole that, when viewed from the third direction, includes the first hole and is larger than the first hole and opens toward the side wall, and an annular counterbore surface connecting the first and second holes and extending along a plane intersecting in the second direction. The outer edge of the light incident surface of the second lens may be inserted through the second hole from the side wall and fixed in surface contact with the counterbore surface. With the above configuration, the second lens for terahertz wave output can be attached to the second holder from the outside of the second holder (opposite the side where the quantum cascade laser element is located), thus facilitating the attachment of the second lens.

[0021] The first end face may be located inside the first hole. With the above configuration, the first end face of the quantum cascade laser element can be extended into the first hole, bringing the substrate of the quantum cascade laser element into contact with or close proximity to the light incident surface of the second lens. This improves the efficiency of terahertz wave extraction and allows for miniaturization of the entire package.

[0022] The first holder may have a main body portion including a portion on the support surface on the first lens side and a first side surface, and a protruding portion connected to the main body portion and including a portion on the support surface on the second lens side, and at least a part of the protruding portion may be located inside the first hole. With the above configuration, by extending the first holder (protruding portion) into the interior of the first hole, the contact area between the quantum cascade laser element and the first holder (support surface) can be increased. As a result, the heat dissipation efficiency from the quantum cascade laser element to the first holder can be increased.

[0023] The width of the protrusion in the fourth direction, which is perpendicular to the first and second directions, may be smaller than the width of the main body in the fourth direction. With the above configuration, by making the width of the protrusion smaller than the width of the main body, interference between the first holder and the package or the second holder can be prevented, while ensuring the volume of the first holder that functions as a heat bath.

[0024] The protrusion may be formed in a shape such that its width gradually decreases as it approaches the second lens along the second direction. With the above configuration, when housing and positioning the first holder on which the quantum cascade laser is mounted within the package, the risk of interference (contact) between the protrusion and the side wall (mainly the inner surface of the first hole) can be effectively reduced, and the volume of the first holder, which functions as a heat bath, can be made sufficiently large. [Effects of the Invention]

[0025] According to one aspect of this disclosure, it is possible to provide a laser module packaged in a manner that enables miniaturization while suppressing the loss of light that contributes to the generation of terahertz waves. [Brief explanation of the drawing]

[0026] [Figure 1] Figure 1 is a perspective view of a laser module according to the first embodiment. [Figure 2] Figure 2 is a perspective view of the laser module from a different angle than in Figure 1. [Figure 3] Figure 3 is a cross-sectional view of the laser module along the line III-III in Figure 2. [Figure 4] Figure 4 is a cross-sectional view of a quantum cascade laser device. [Figure 5] Figure 5 shows the positional relationship between the quantum cascade laser element and the second lens. [Figure 6] Figure 6 is a side view showing the arrangement of the first lens, second lens, quantum cascade laser element, first holder, and diffraction grating unit housed within the package. [Figure 7] Figure 7 is a plan view showing the arrangement of the first lens, lens holder, second lens, quantum cascade laser element, first holder, and diffraction grating unit housed within the package. [Figure 8] Figure 8 is a cross-sectional view of the laser module according to the second embodiment. [Figure 9] Figure 9 is a schematic perspective view of the laser module according to the second embodiment. [Figure 10] Figure 10 is a cross-sectional view of a laser module according to the third embodiment. [Figure 11] Figure 11 is a schematic perspective view of the laser module according to the third embodiment. [Modes for carrying out the invention]

[0027] Hereinafter, one embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same reference numerals will be used for the same or equivalent elements, and redundant explanations will be omitted. Also, terms such as "upper" and "lower" are for convenience based on the state shown in the drawings.

[0028] [First Embodiment] Referring to Figures 1 to 7, a laser module 1A according to the first embodiment will be described. Laser module 1A is a small terahertz light source module smaller than hand-top size. As an example, laser module 1A is configured to be single-mode, wavelength tunable, and room-temperature operable using a quantum cascade laser element 2 (hereinafter referred to as "QCL element 2") which is a difference frequency generation type terahertz quantum cascade laser (DFG-THz-QCL). Laser module 1A comprises the QCL element 2, a diffraction grating unit 3, a first lens 4 (lens for external resonator), a lens holder 5, a second lens 6 (lens for terahertz wave output), a first holder 7, and a package 8.

[0029] Package 8 is a housing that hermetically houses the QCL element 2, the diffraction grating unit 3, the first lens 4, the lens holder 5, and the first holder 7. The optical path between the light incident surface 61 of the second lens 6 and the movable diffraction grating 31 (diffraction grating unit 3) is also located within Package 8. Other components (for example, a temperature sensor for measuring the temperature inside Package 8, a temperature control element such as a Peltier element, etc.) may also be placed inside Package 8.

[0030] In this embodiment, as an example, package 8 is a butterfly package. Package 8 has a bottom wall 81, side walls 82, and a top wall 83. Note that the top wall 83 is not shown in Figure 2.

[0031] The bottom wall 81 is a rectangular plate-shaped member. The bottom wall 81 is formed of a metallic material such as copper tungsten. The bottom wall 81 is a base member on which the first holder 7 is mounted. In this embodiment, the first holder 7 is placed directly on the bottom wall 81, but a separate member such as a heat dissipation member (e.g., a Peltier element) may be placed between the bottom wall 81 and the first holder 7. That is, the first holder 7 may be placed on the bottom wall 81 via a separate member. For convenience, in this specification, the longitudinal direction of the bottom wall 81 is represented as the X-axis direction, the short direction of the bottom wall 81 is represented as the Y-axis direction, and the direction perpendicular to the bottom wall 81 (i.e., the direction perpendicular to the X-axis direction and the Y-axis direction) is represented as the Z-axis direction.

[0032] The side wall 82 is erected on the bottom wall 81. When viewed from the Z-axis direction, the side wall 82 is formed in an annular shape so as to surround the internal space in which the QCL element 2 and the like are housed. In this embodiment, the side wall 82 is formed in a rectangular cylindrical shape. The side wall 82 is made of a metallic material such as Kovar. For example, the side wall 82 is a Kovar frame with Ni / Au plating. In this embodiment, the side wall 82 is provided in the central part of the bottom wall 81 in the longitudinal direction (X-axis direction). The width of the side wall 82 along the short direction (Y-axis direction) is the same as the width of the bottom wall 81 in the short direction, and the width of the side wall 82 along the longitudinal direction (X-axis direction) is shorter than the width of the bottom wall 81 in the longitudinal direction. That is, protruding portions 81a are formed on both sides of the bottom wall 81 in the longitudinal direction, extending outward from the side wall 82. Screw holes 81b for attaching the package 8 (bottom wall 81) to other components are provided in the portions of the protruding portion 81a corresponding to the four corners of the bottom wall 81.

[0033] The top wall 83 is a member that closes the opening on the side wall 82 opposite to the bottom wall 81. The top wall 83 has a rectangular plate shape. The outer shape of the top wall 83 (width in the longitudinal and transverse directions) as viewed from the Z-axis direction is approximately the same as the outer shape of the side wall 82. The top wall 83 is formed of the same metal material as the side wall 82 (e.g., Kovar). The top wall 83 is hermetically joined to the end 82a of the side wall 82 opposite to the bottom wall 81 by, for example, seam welding, while the inside of the package 8 is vacuumed or filled with nitrogen.

[0034] A pair of side walls 821 extending along the longitudinal direction (X-axis direction) of the side wall 82 (i.e., the portion intersecting the short direction (Y-axis direction)) have multiple lead terminals 9 inserted through them (in this embodiment, 14 in total, 7 on each side in the short direction) for supplying current to components such as the QCL element 2 housed in the package 8. Each lead terminal 9 is a flat conductive member extending in the Y-axis direction.

[0035] Furthermore, each of the pair of side walls 821 is provided with a protruding wall 84 that extends outwards from both the outer surface (the outer surface of the package 8) and the inner surface (the inner surface of the package 8). The protruding wall 84 is an overhang-like member that extends along the X-axis direction above the center position of the side wall 821 in the Z-axis direction (towards the top wall 83). Each lead terminal 9 is arranged on the upper surface of the protruding wall 84 at approximately equal intervals along the X-axis direction.

[0036] The portion of the lead terminal 9 located outside the package 8 is electrically connected to the power supply for the QCL element 2, the power supply for the movable diffraction grating 31 (a power supply for supplying current to the coil 315, described later), and so on. On the other hand, the portion of the lead terminal 9 located inside the package 8 functions as an electrode terminal for supplying power to each component inside the package 8 (for example, the QCL element 2, the movable diffraction grating 31, etc.). That is, the electrode terminal and each component inside the package 8 are electrically connected via conductive wires (not shown), so that power is supplied to each component from an external power supply via the lead terminal 9 and the wires. Furthermore, if the temperature sensor, temperature control element, etc., described above are placed inside the package 8, these components are also electrically connected to the electrode terminals.

[0037] Of the side walls 82, one of the pair of side walls 822 extending along the shorter direction (Y-axis direction) (i.e., the portion intersecting the longer direction (X-axis direction)) is provided with a through hole 85 in the side wall 822 facing one end face (end face 2a) of the QCL element 2, which penetrates in a direction perpendicular to the side wall 822 (i.e., the X-axis direction) (third direction). The second lens 6 is attached to the through hole 85.

[0038] Next, we will describe the configuration of each part housed in package 8.

[0039] [Configuration of a quantum cascade laser element] The QCL element 2 is a terahertz light source configured to output terahertz waves at room temperature. The configuration of the QCL element 2 will be described in detail, mainly with reference to Figures 3 and 4. Figure 4 shows the cross-sectional structure of the QCL element 2 along a plane passing through the center of the Y-axis and parallel to the Z-axis. The QCL element 2 has a rod-like shape. The QCL element 2 can be formed as a ridge-stripe laser element by a general semiconductor process. For example, the QCL element 2 can be obtained by forming InGaAs / InAlAs on an InP substrate (substrate 21, described later) by epitaxial growth.

[0040] The QCL element 2 has end faces 2a and 2b facing each other along its longitudinal direction, and emits broadband light in the mid-infrared region (e.g., 3 μm to 20 μm) from each of the end faces 2a and 2b. The end faces 2a and 2b are, for example, cleavage planes formed by cleavage. End face 2a is the surface facing the second lens 6. End face 2b is the surface facing the first lens 4. The end face 2b may be provided with a low-reflection coating such that the reflectivity is 5% or less with respect to the wavelength that is the gain peak of the QCL element 2. In order to emit such broadband light, the QCL element 2 may have a structure in which multiple active layers having different central wavelengths are stacked, or it may have a structure consisting of a single active layer.

[0041] As shown in Figure 4, the QCL element 2 comprises a substrate 21 and a semiconductor layer 10. The semiconductor layer 10 comprises a lower cladding layer 11 (first cladding layer), an active layer 12, and an upper cladding layer 13 (second cladding layer). In this embodiment, in addition to the above, the semiconductor layer 10 also comprises an upper guide layer 14, a lower guide layer 15, an upper contact layer 16, and a lower contact layer 17.

[0042] The substrate 21 has a main surface 21a and a back surface 21b opposite to the main surface 21a. The main surface 21a is the surface on which the active layer 12 is placed. The back surface 21b is the surface facing the support surface 7a of the first holder 7 (see Figures 2 and 3). The substrate 21 is, for example, a rectangular plate-shaped InP single crystal substrate (semi-insulating substrate: a high-resistance semiconductor substrate that is not doped with impurities). The length, width, and thickness of the substrate 21 are approximately several hundred μm to several mm, several hundred μm to several mm, and several hundred μm, respectively. Terahertz waves generated by difference frequency generation inside the active layer 12 are mainly extracted to the outside (second lens 6) via the substrate 21. From the viewpoint of increasing the efficiency of extracting terahertz waves from the substrate 21 to the outside, the substrate 21 is a semi-insulating substrate as described above, or 1 × 10⁻⁶ 17 cm -3 The substrate is preferably one having the following carrier density.

[0043] The substrate 21 has a side surface 21c that connects the main surface 21a and the back surface 21b. The side surface 21c faces the light incident surface 61 of the second lens 6. In order to prevent total internal reflection of terahertz waves propagating inside the substrate 21 at the cleaved substrate edge surface (side surface 21c) and to improve the efficiency of extracting terahertz waves to the outside, the side surface 21c is polished to be inclined at an angle θ1 as shown in Figure 4. Furthermore, in order to avoid degradation of the oscillation characteristics of mid-infrared light, the polished inclined surface is configured not to reach the epitaxial growth layer (i.e., semiconductor layer 10) containing the active layer 12. More specifically, the side surface 21c has a first surface 21d which is an inclined surface formed by polishing, and a second surface 21e which remains unpolished.

[0044] The first surface 21d is connected to the back surface 21b and extends from the back surface 21b toward the main surface 21a. The first surface 21d is inclined with respect to the main surface 21a and the back surface 21b. The first surface 21d is inclined so that it moves away from the end surface 2b as it moves from the back surface 21b toward the main surface 21a. The angle θ1 between the first surface 21d and the main surface 21a is, for example, about 30° to 80°. The first surface 21d is, for example, a polished surface formed by polishing a rectangular plate-shaped semiconductor substrate. Between the first surface 21d and the back surface 21b, a corner portion 21f is formed that connects the first surface 21d and the back surface 21b.

[0045] The second surface 21e is connected to the end of the first surface 21d on the main surface 21a side and to the main surface 21a. The second surface 21e is inclined with respect to the first surface 21d. The second surface 21e is substantially perpendicular to the main surface 21a and the back surface 21b. Between the second surface 21e and the main surface 21a, a corner portion 21g is formed that connects the second surface 21e and the main surface 21a. Between the first surface 21d and the second surface 21e, a corner portion 21h is formed that connects the first surface 21d and the second surface 21e. Because the first surface 21d is an inclined surface as described above, the corner portion 21f is located closer to the end surface 2b than the corner portion 21h.

[0046] The semiconductor layer 10 is provided on the main surface 21a of the substrate 21. The semiconductor layer 10 has a side surface 10a. The side surface 10a is part of the end surface 2a of the QCL element 2. The side surface 10a is a flat surface. The side surface 10a is substantially perpendicular to the main surface 21a and the back surface 21b of the substrate 21. That is, in this embodiment, the side surface 10a is located on the same plane as the second surface 21e of the substrate 21. The thickness of the semiconductor layer 10 is about 10 μm to 20 μm.

[0047] The lower contact layer 17 is, for example, an InGaAs layer (Si doped: 1.5 × 10⁻¹⁶) with a thickness of about 400 nm. 18 cm -3 The lower cladding layer 11 is, for example, an InP layer (Si doped: 1.5 × 10⁻¹⁰) with a thickness of about 5 μm. 16 cm -3 The lower cladding layer 11 is provided on the surface of the lower contact layer 17. In other words, the lower cladding layer 11 is provided on the main surface 21a of the substrate 21 via the lower contact layer 17. The lower guide layer 15 is, for example, an InGaAs layer (Si doped: 1.5 × 10¹⁵ nm thick) with a thickness of about 250 nm. 16 cm -3 ) and is provided on the surface of the lower cladding layer 11.

[0048] The active layer 12 is a layer having a quantum cascade structure and is provided on the surface of the lower guide layer 15. In other words, the active layer 12 is provided on the side of the lower cladding layer 11 opposite to the substrate 21. The active layer 12 has end faces 12a (first end face) and 12b (second end face) that face each other in a direction D2 (second direction) perpendicular to direction D1 (first direction), which is the stacking direction of the substrate 21 and the semiconductor layer 10. End face 12a of the active layer 12 is part of the end face 2a of the QCL element 2 and the side surface 10a of the semiconductor layer 10. End face 12b of the active layer 12 is part of the end face 2b of the QCL element 2. As an example, the active layer 12 has a structure in which multiple InGaAs layers and InAlAs layers are stacked alternately along direction D1.

[0049] The upper guide layer 14 is, for example, an InGaAs layer with a thickness of about 450 nm (Si doped: 1.5×10 16 cm -3 ) provided on the surface of the active layer 12. The upper clad layer 13 is, for example, an InP layer with a thickness of about 5 μm (Si doped: 1.5×10 16 cm -3 ) provided on the surface of the upper guide layer 14. That is, the upper clad layer 13 is provided on the opposite side of the active layer 12 from the lower clad layer 11. The upper contact layer 16 is, for example, an InP layer with a thickness of about 15 nm (Si doped: 1.5×10 18 cm -3 ) provided on the surface of the upper clad layer 13.

[0050] To obtain single-mode terahertz waves, it is necessary to simultaneously oscillate two different wavelengths of single-mode light (both mid-infrared light) inside the QCL element 2. In this embodiment, a single mode at one wavelength (the wavelength corresponding to the second frequency ω2) is oscillated by a diffraction grating provided inside the QCL element 2 (to be described in detail later), and a single mode at the other wavelength (the wavelength corresponding to the first frequency ω1 different from the second frequency ω2) is oscillated by an external resonator (to be described in detail later). In order to oscillate the single mode at the one wavelength, a diffraction grating layer 14a that functions as a distributed feedback (DFB: distributed feedback) structure is formed in the upper guide layer 14 along the direction D2 in which the end face 12a and the end face 12b face each other (that is, the resonance direction). The diffraction grating layer 14a oscillates light of a wavelength (the wavelength corresponding to the second frequency ω2) deviated from the wavelength that is the gain peak of the QCL element 2 in a single mode. The QCL element 2 is in a state where the DFB single mode and the Fabry-Perot mode have gain at the same time. Note that both the light of the first frequency ω1 (hereinafter referred to as "first light") and the light of the second frequency ω2 (hereinafter referred to as "second light") are mid-infrared light.

[0051] The end faces 12a and 12b of the active layer 12 constitute a resonator for oscillating the second light. On the other hand, the end face 12a of the active layer 12 and the movable diffraction grating 31, which is an external resonator, constitute a resonator for oscillating the first light. As a result of the oscillation of the first and second lights by this configuration, the active layer 12 generates a terahertz wave with a difference frequency ω3 (=|ω1-ω2|) between the first frequency ω1 of the first light and the second frequency ω2 of the second light by difference frequency generation through Cherenkov phase matching.

[0052] The radiation direction A1 of the terahertz waves generated in this way is at a radiation angle θ with respect to the direction from end face 12b to end face 12a (rightward direction in Figure 4) along the resonance direction (direction D2), which is along the direction from end face 12b to end face 12a. C It tilts downward (towards the substrate 21) by (Cherenkov radiation angle). More specifically, the terahertz waves generated in the active layer 12 are emitted at the radiation angle θ shown in equation (1) below. C The wave propagates within the substrate 21 as a plane wave (i.e., in phase). In equation (1) below, n MIR n is the refractive index of the substrate 21 for mid-infrared light, and THz This is the refractive index of the substrate 21 with respect to terahertz waves.

[0053] θ C =cos -1 (n MIR / n THz )...(1)

[0054] [Configuration of the terahertz wave output lens (second lens)] The second lens 6 is a lens for outputting terahertz waves generated by the QCL element 2 to the outside. The second lens 6 allows the light (terahertz wave L1) emitted from the QCL element 2 to pass through. The second lens 6 is, for example, a silicon lens formed in the shape of a hemisphere or hyperhemispheric. Silicon lenses are resistant to scratches. Furthermore, even if scratches or dirt occur, terahertz waves, which have longer wavelengths compared to visible light, near-infrared light, etc., are less affected, so it is possible to position the second lens 6 so that it is exposed on the outside of the package 8.

[0055] In this embodiment, the second lens 6 is a superhemispherical lens. The second lens 6 has a flat light incident surface 61 facing the end face 2a of the QCL element 2, and a curved light emission surface 62 that emits the output light, a terahertz wave L1, to the outside of the package 8. The lens diameter (diameter) of the second lens 6 is not limited to a specific range, but if the second lens 6 is a hemispherical or superhemispherical lens, the lens diameter is directly reflected in the thickness of the lens. For this reason, from the viewpoint of achieving miniaturization of the laser module 1A, the lens diameter of the second lens 6 is preferably 20 mm or less. Furthermore, the material of the second lens 6 is not limited to silicon. For example, the second lens 6 may be formed from other materials that transmit terahertz waves, such as Tsurupica. In addition, the light emission surface 62 of the second lens 6 may be provided with a low-reflection coating to reduce the reflectivity of terahertz waves in order to improve the efficiency of terahertz wave extraction.

[0056] The second lens 6 is mounted in a through-hole 85 provided in the side wall 822 of the package 8. The details of the mounting structure of the second lens 6 to the through-hole 85 will be described below.

[0057] As shown in Figure 3, the through-hole 85 has a small-diameter hole 85a (first hole), a large-diameter hole 85b (second hole), and a counterbore surface 85c. The small-diameter hole 85a opens to the inside of the package 8 in the optical axis direction (i.e., the X-axis direction) of the second lens 6. The large-diameter hole 85b opens to the outside of the package 8 in the X-axis direction. When viewed from the X-axis direction, the large-diameter hole 85b has a shape that includes the small-diameter hole 85a and is larger than the small-diameter hole 85a. The small-diameter hole 85a and the large-diameter hole 85b each extend in the X-axis direction. In this embodiment, each of the small-diameter hole 85a and the large-diameter hole 85b is formed in a circular shape, and the diameter of the large-diameter hole 85b is larger than the diameter of the small-diameter hole 85a. As an example, the central axis of the small-diameter hole 85a and the central axis of the large-diameter hole 85b substantially coincide with the optical axis AX2 of the second lens 6 (see Figure 6).

[0058] The counterbore surface 85c connects the small-diameter hole 85a and the large-diameter hole 85b and is an annular surface extending along a plane (YZ plane) that intersects in the X-axis direction. More specifically, the counterbore surface 85c connects the end of the small-diameter hole 85a on the large-diameter hole 85b side to the end of the large-diameter hole 85b on the small-diameter hole 85a side. The large-diameter hole 85b and the counterbore surface 85c can be formed by counterboring the side wall 822 from the outside of the package 8. In this embodiment, the counterbore surface 85c is formed as a continuous annular shape, but the counterbore surface 85c may be formed as a discontinuous annular shape. For example, the counterbore surface 85c may be divided at the portion where a notch is formed by forming a notch in a part of the inner wall surface of the small-diameter hole 85a. A through hole 85 is formed in the X-axis direction by a small-diameter hole 85a and a large-diameter hole 85b connected by a counterbore surface 85c.

[0059] The outer edge of the light incident surface 61 of the second lens 6 is inserted through a large-diameter hole 85b from the outside of the package 8 and fixed in surface contact with the counterbore surface 85c. The outer edge of the light incident surface 61 is fixed to the counterbore surface 85c by, for example, a fixing resin or adhesive. The second lens 6, thus attached to the through hole 85 of the side wall 822, receives the terahertz wave L1 output from the QCL element 2 at the light incident surface 61 and functions to collimate the terahertz wave L1 into a beam and extract it to the outside of the package 8 from the light emission surface 62. The second lens 6 also has the function of maintaining hermetically sealed interior of the package 8. With the above configuration, the second lens 6 for terahertz wave output can be attached to the side wall 822 from the outside of the package 8, making the installation of the second lens 6 easy. In addition, the second lens 6 can be used as a window material to close the through hole 85 provided in the package 8 (side wall 822). As a result, manufacturing costs can be reduced by decreasing the number of parts, and the overall package can be made smaller. Furthermore, since it is possible to avoid optical loss (i.e., attenuation of terahertz waves L1 due to passing through a window material other than the second lens 6) caused by providing a separate window material from the second lens 6, it is possible to achieve higher output of terahertz waves L1. In addition, with the above method of mounting the second lens 6, a hemispherical or hyperhemispherical second lens 6 that does not have edge thickness for holding the lens from the side can be easily and accurately fixed.

[0060] [Positional relationship between the QCL element and the second lens] Next, the positional relationship between the QCL element 2 and the second lens 6 will be described. As shown in Figures 3 and 5, the QCL element 2 and the second lens 6 are arranged such that a portion of the side surface 21c of the substrate 21 of the QCL element 2 contacts approximately the center of the light incident surface 61 of the second lens 6. For this purpose, the QCL element 2 is positioned at an angle with respect to the optical axis direction (X-axis direction) of the second lens 6. As a result, the QCL element 2 and the second lens 6 are positioned such that the end face 12a of the active layer 12 and the light incident surface 61 are separated, while only a portion of the side surface 21c of the substrate 21 contacts the light incident surface 61. For example, as shown in Figure 5, the corner 21h of the side surface 21c is in line contact with the light incident surface 61. Alternatively, the first surface 21d, which is an inclined surface of the side surface 21c, may be in surface contact with the light incident surface 61. By separating the end face 12a of the active layer 12 from the light incident surface 61 in this way, it is possible to prevent the oscillation characteristics of mid-infrared light at the end face 12a from being affected. Furthermore, the tilt angle θ of the QCL element 2 with respect to the optical axis direction (X-axis direction) of the second lens 6 is... Q (See Figure 5) For example, the radiation angle θ of the terahertz wave L1 generated inside the QCL element 2. C The settings are adjusted to roughly coincide with (see Figure 4). For example, the QCL element 2 can be tilted relative to the second lens 6 so that the radiation direction A1 (see Figure 4) of the terahertz wave L1 generated inside the QCL element 2 roughly coincides with the optical axis direction (X-axis direction) of the second lens 6.

[0061] The resonance axis (axis along the resonance direction (direction D2)) passing through the end faces 12a and 12b of the QCL element 2 intersects the optical axis AX2 of the second lens 6 because the support surface 7a is inclined to intersect the optical axis AX2 of the second lens 6 (i.e., because the QCL element 2 is placed on the inclined support surface 7a). The end face 12a is spaced apart from the light incident surface 61 of the second lens 6, and the first lens 4 is positioned so that its optical axis AX1 substantially coincides with the resonance axis. With this configuration, it is possible to space the end face 12a, which constitutes the resonator for the first and second light for generating terahertz waves L1, apart from the light incident surface 61 of the second lens 6, while bringing the QCL element 2 (for example, the side surface 21c of the substrate 21 continuous with the end face 12a) into contact with or close to the light incident surface 61 of the second lens 6. This prevents the reflectivity of the end face 12a, which is important in the oscillation of the first and second light, from being affected by the second lens 6 (i.e., the reflection characteristics of the end face 12a from changing due to the end face 12a contacting the light incident surface 61 of the second lens 6), while ensuring the efficiency of extracting the terahertz wave L1 from the QCL element 2 to the second lens 6.

[0062] From the viewpoint of efficiently introducing terahertz waves L1 into the second lens 6, it is preferable that the focal point of the second lens 6 be located inside the QCL element 2. By partially contacting the second lens 6 and the QCL element 2 (substrate 21) in this way, the effect of the refractive index of the air interposed between them (i.e., light loss due to reflection at the interface with the air) is reduced. As a result, terahertz waves L1 emitted from the QCL element 2 can be efficiently introduced into the second lens 6, and the output power of terahertz waves L1 can be increased. In addition, the spreading of terahertz waves L1 radiated by the second lens 6 can be suppressed, and an output with an appropriate beam shape can be obtained.

[0063] [External resonator configuration] The first lens 4 and the movable diffraction grating 31 (diffraction grating unit) constitute an external resonator that oscillates the first light (light of the first frequency ω1) described above. That is, the first light is amplified by the first light reciprocating between the end face 12a of the active layer 12 and the movable diffraction grating 31 (specifically, the diffraction grating portion 314 described later) via the first lens 4. From the viewpoint of miniaturizing the laser module 1A and reducing light loss, it is preferable to arrange the first lens 4 and the movable diffraction grating 31 as close together as possible, within a range where interference between components within the package 8 does not occur.

[0064] (Configuration of the lens for the external resonator (first lens)) The first lens 4 is a lens for the external resonator and transmits mid-infrared light. The first lens 4 can be formed from, for example, zinc selenide (ZnSe). The first lens 4 is, for example, an aspherical lens with a working distance of 3 mm or less. From the viewpoint of increasing the efficiency as an external resonator, it is preferable that the first lens 4 is configured with a working distance of 1 mm or less and an numerical aperture of 0.6 or more. The first lens 4 is placed between the QCL element 2 and the movable diffraction grating 31 and passes through the light emitted from the end face 12b (mid-infrared light) and the light returning from the movable diffraction grating 31 to the QCL element 2 (mid-infrared light).

[0065] The first lens 4 has a first lens surface 41 and a second lens surface 42. The first lens 4 is, for example, an aspherical lens. The first lens surface 41 is the surface facing the end surface 2b of the QCL element 2 (the end surface 12b of the active layer 12). The first lens surface 41 may be a flat surface or a non-flat surface (for example, a curved surface that is convex toward the QCL element 2). The second lens surface 42 is a curved surface on the opposite side of the first lens surface 41 that faces the movable diffraction grating 31. The first lens surface 41 and the second lens surface 42 may be provided with a low-reflection coating such that the reflectivity is 5% or less with respect to the wavelength at which the gain peak of the QCL element 2 occurs. Furthermore, the material of the first lens 4 may be a material that transmits mid-infrared light, such as Ge or CaF2.

[0066] The first lens 4 is fixed to the lens holder 5 such that the optical axis AX1 of the first lens 4 (see Figure 6) and the resonance axis of the QCL element 2 (i.e., the axis parallel to the direction D2 passing through the end faces 12a and 12b of the active layer 12, and the optical axis of the mid-infrared light emitted by the QCL element 2) substantially coincide. In other words, the first lens 4 and the movable diffraction grating 31 are arranged to be tilted with respect to the horizontal plane (XY plane) in the same way as the QCL element 2. That is, the radiation direction A1 of the terahertz wave L1 and the resonance direction (direction D2) of the external resonator are not on the same line, but intersect.

[0067] The first lens 4 is held by a lens holder 5. For example, the lens holder 5 has a roughly rectangular parallelepiped shape. The lens holder 5 has a small diameter hole 5a, a large diameter hole 5b, and a counterbore surface 5c. The small diameter hole 5a opens towards the QCL element 2 in the optical axis direction (direction D2) of the first lens 4. The large diameter hole 5b opens towards the movable diffraction grating 31 in direction D2. When viewed from direction D2, the large diameter hole 5b has a shape that includes the small diameter hole 5a and is larger than the small diameter hole 5a. The small diameter hole 5a and the large diameter hole 5b each extend in direction D2. Each of the small diameter hole 5a and the large diameter hole 5b is formed in a circular shape, and the diameter of the large diameter hole 5b is larger than the diameter of the small diameter hole 5a. For example, the central axis of the small diameter hole 5a and the central axis of the large diameter hole 5b substantially coincide with the optical axis AX1 of the first lens 4. The counterbore surface 5c is an annular surface that connects the small-diameter hole 5a and the large-diameter hole 85b and extends along a plane intersecting direction D2. More specifically, the counterbore surface 5c connects the end of the small-diameter hole 5a on the large-diameter hole 5b side to the end of the large-diameter hole 5b on the small-diameter hole 5a side. The outer edge of the first lens surface 41 of the first lens 4 is inserted through the large-diameter hole 5b and fixed in surface contact with the counterbore surface 5c. The outer edge of the first lens surface 41 is fixed to the counterbore surface 5c by, for example, a fixing resin, adhesive, etc.

[0068] (Configuration of the diffraction grating unit) The diffraction grating unit 3 is positioned on the side opposite to the QCL element 2 relative to the first lens 4 (lens holder 5). The diffraction grating unit 3 comprises a movable diffraction grating 31, a magnet 32, and a yoke 33. The movable diffraction grating 31 is formed in a substantially plate shape. The movable diffraction grating 31 is a MEMS diffraction grating fabricated using, for example, a MEMS process. The movable diffraction grating 31 is not limited to a specific configuration. For example, the movable diffraction grating 31 may be an electrostatically driven type that can fix the diffraction grating angle at any angle, or an electromagnetically driven type that can perform high-frequency angle modulation at the resonant frequency. The magnet 32 ​​is positioned on the side opposite to the QCL element 2 relative to the movable diffraction grating 31. The movable diffraction grating 31 is fixed to the yoke 33, and the magnet 32 ​​is housed within the yoke 33. In this way, the movable diffraction grating 31, the magnet 32, and the yoke 33 are integrated and constitute a single unit.

[0069] Light collimated by the first lens 4 is incident on the movable diffraction grating 31. The movable diffraction grating 31 diffracts and reflects this incident light, thereby returning light of a specific wavelength from the incident light to the end face 2b of the QCL element 2 (end face 12b of the active layer 12) via the first lens 4. In this embodiment, the movable diffraction grating 31 and the end face 2b constitute a Littrow-type external resonator.

[0070] Furthermore, the movable diffraction grating 31 allows for rapid changes in the orientation of the diffraction grating portion 314 (see Figure 7) that diffracts and reflects incident light. This makes the wavelength of the light returning from the movable diffraction grating 31 to the end face 12b of the QCL element 2 (i.e., the wavelength corresponding to the first frequency ω1) variable. In other words, the wavelength of the terahertz wave L1 generated by the difference frequency generation between the first frequency ω1 and the second frequency ω2 can be changed. This allows for wavelength sweeping within the gain band range of the QCL element 2.

[0071] As shown in Figure 7, the movable diffraction grating 31 comprises a support portion 311, a pair of connecting portions 312, a movable portion 313, a diffraction grating portion 314, and a coil 315. The movable diffraction grating 31 is configured as a MEMS device that swings the movable portion 313 around an axis A passing through the pair of connecting portions 312. Axis A is an axis parallel to the Y-axis direction.

[0072] The support portion 311 is a flat, plate-shaped frame that has a rectangular shape in plan view. The support portion 311 supports the movable portion 313 via a pair of connecting portions 312. Each connecting portion 312 is a flat, plate-shaped member that has a rectangular rod shape in plan view and extends straight along axis A. Each connecting portion 312 connects the movable portion 313 to the support portion 311 on axis A so that the movable portion 313 can swing freely around axis A.

[0073] The movable part 313 is located inside the support part 311. As described above, the movable part 313 is pivotable around axis A. The movable part 313 is a flat plate-shaped member that is substantially rectangular in plan view. The support part 311, the connecting part 312, and the movable part 313 are integrally formed, for example, by being fabricated on a single SOI (Silicon on Insulator) substrate.

[0074] A diffraction grating portion 314 is provided on the surface (mirror surface) of the movable portion 313 facing the QCL element 2. The diffraction grating portion 314 has a plurality of grating grooves (not shown) and diffracts and reflects the light emitted from the QCL element 2. The diffraction grating unit 3 is positioned such that the optical axis AX1 of the first lens 4 (i.e., the optical axis of the mid-infrared light collimated by the first lens 4) substantially coincides with the center of the diffraction grating portion 314, and the light incident on the diffraction grating portion 314 is diffracted along the same optical axis AX1 in the opposite direction to the incident direction.

[0075] The diffraction grating section 314 includes, for example, a resin layer on which a diffraction grating pattern is formed on the surface of the movable section 313, and a metal layer provided on the surface of the resin layer along the diffraction grating pattern. Alternatively, the diffraction grating section 314 may consist only of a metal layer provided on the movable section 313 and on which a diffraction grating pattern is formed. As the diffraction grating pattern, for example, a blazed grating with a sawtooth cross-section, a binary grating with a rectangular cross-section, a holographic grating with a sinusoidal cross-section, etc., can be used. The diffraction grating pattern is formed on the resin layer by, for example, nanoimprint lithography. The metal layer is, for example, a metal reflective film made of gold, and is formed by vapor deposition. The period and depth of the grating grooves in the diffraction grating section 314 are configured, for example, to maximize the diffraction efficiency with respect to the wavelength at which the gain peak of the QCL element 2 occurs. Here, diffraction efficiency refers to the efficiency when light incident on the diffraction grating section 314 is diffracted in opposite directions on the same optical axis. From the viewpoint of miniaturizing the laser module 1A, the size of the diffraction grating portion 314 (mirror surface) is preferably such that it includes a circular area with a diameter of 1 mm to 15 mm.

[0076] The coil 315 is made of a metal material such as copper and has a damascene structure embedded in a groove formed on the surface of the movable part 313. The coil 315 is a drive coil that carries an electric current to drive the movable diffraction grating 31 (i.e., to oscillate the movable part 313).

[0077] The magnet 32 ​​generates a magnetic field (magnetic force) that acts on the coil 315. The magnet 32 ​​is a neodymium magnet (permanent magnet) formed in a roughly rectangular parallelepiped shape.

[0078] The yoke 33 amplifies the magnetic force of the magnet 32 ​​and forms a magnetic circuit together with the magnet 32. The surface of the yoke 33 is blackened, for example by zinc plating. The yoke 33 has an inclined surface 33a, a lower surface 33b, and a protruding portion 33c.

[0079] The inclined surface 33a is inclined with respect to the end face 2b of the QCL element 2. By fixing the movable diffraction grating 31 on such an inclined surface 33a, the normal direction of the diffraction grating portion 314 of the movable diffraction grating 31 can be inclined with respect to the end face 2b. The inclination angle of the inclined surface 33a (angle with respect to the end face 2b of the QCL element 2) is set according to the oscillation wavelength of the QCL element 2, as well as the number of grooves and blazed angle of the grating grooves in the diffraction grating portion 314.

[0080] The yoke 33 is formed in a roughly U-shape (inverted C-shape) when viewed from the Y-axis direction, defining a placement space SP that opens onto the inclined surface 33a. The magnet 32 ​​is placed within this placement space SP, and the magnet 32 ​​is housed within the yoke 33. The movable diffraction grating 31 is fixed to the inclined surface 33a at the edge of the support portion 311 so as to cover the opening of the placement space SP.

[0081] In the movable diffraction grating 31, when current flows through the coil 315, the magnetic field formed by the magnet 32 ​​and the yoke 33 generates a Lorentz force in a predetermined direction on the electrons flowing through the coil 315. As a result, the coil 315 is subjected to a force in a predetermined direction. Therefore, by controlling the direction or magnitude of the current flowing through the coil 315, the movable part 313 (diffraction grating part 314) can be oscillated around axis A. Furthermore, by flowing a current with a frequency corresponding to the resonant frequency of the movable part 313 through the coil 315, the movable part 313 can be oscillated at high speed at the resonant frequency level (for example, at a frequency of 1 kHz or higher).

[0082] [Composition of the first holder] Next, the configuration of the first holder 7 will be described, mainly with reference to Figures 3 and 6. The first holder 7 is a component that holds (supports) the QCL element 2 within the package 8. The first holder 7 also functions as a heat bath to suppress the heat generated by the QCL element 2 when it is driven. The first holder 7 can be formed from a material that has good thermal conductivity, allows for precise machining, and has sufficient hardness and rigidity to maintain its shape. The first holder 7 can be formed from a metallic material such as copper tungsten, similar to the bottom wall 81.

[0083] In this embodiment, in addition to the QCL element 2, the first lens 4 (lens holder 5) and the diffraction grating unit 3 are also fixed to the first holder 7. That is, all the components for forming an external resonator for oscillating the first light are supported by the first holder 7. With the above configuration, these components can be fixed to the first holder 7 and their positions adjusted (positioned) can be performed before housing the QCL element 2, the first lens 4 (lens holder 5), and the diffraction grating unit 3 in the package 8. In other words, the QCL element 2, the first lens 4 (lens holder 5), and the diffraction grating unit 3 can be mounted on the first holder 7 while performing the above position adjustment, and then the first holder 7 can be housed in the package 8. This improves work efficiency compared to when the positions of each component are adjusted within the package 8.

[0084] The first holder 7 is fixed to the bottom wall 81 within the package 8. For example, the bottom surface 7b of the first holder 7 is fixed to the bottom wall 81 with an adhesive or the like. However, as mentioned above, another component such as a Peltier element may be interposed between the first holder 7 and the bottom wall 81. Also, the first holder 7 may be attached to a part other than the bottom wall 81 (for example, the side wall 82).

[0085] The first holder 7 has, in order from the side where the second lens 6 is positioned, a support surface 7a on which the QCL element 2 is placed, a support surface 7c on which the lens holder 5 is placed, and a support surface 7d on which the diffraction grating unit 3 is placed, along direction D2. These support surfaces 7a, 7c, and 7d are inclined so as to be parallel to the resonance direction (direction D2) of the external resonator. By arranging each component (QCL element 2, lens holder 5, and diffraction grating unit 3) on each of these inclined support surfaces 7a, 7c, and 7d, the positional relationship between each of the components constituting the external resonator can be appropriately determined. With respect to the upper surface of the bottom wall 81, support surface 7a is higher than support surface 7c, and support surface 7c is higher than support surface 7d. Steps are formed between support surface 7a and support surface 7c, and between support surface 7c and support surface 7d.

[0086] The first holder 7 has a side surface 7e (first side surface) connecting support surface 7a and support surface 7c, and a side surface 7f connecting support surface 7c and support surface 7d. Side surfaces 7e and 7f are surfaces perpendicular to direction D2. Side surface 7e is the surface facing the first lens 4 (lens holder 5) in direction D2. The lens holder 5 is placed on the support surface 7c so as to be spaced apart from side surface 7e. Side surface 7f abuts against the side surface of the yoke 33 on the lens holder 5 side and has the function of positioning the yoke 33. A recess 7g is formed in the support surface 7d into which a projection 33c provided on the lower surface 33b of the yoke 33 is inserted. The diffraction grating unit 3 is fixed to the support surface 7d by the projection 33c being fitted into the recess 7g.

[0087] Furthermore, the bottom surface 5d of the lens holder 5 is fixed to the support surface 7c via an adhesive layer B1. The adhesive layer B1 is, for example, a photocurable resin. The lens holder 5 is fixed to the support surface 7c such that the optical axis AX1 of the first lens 4 and the resonance axis of the QCL element 2 (the optical axis of the mid-infrared light emitted from the end surface 12b) coincide. For example, the height position of the lens holder 5 relative to the support surface 7c can be finely adjusted by adjusting the amount of adhesive layer B1, the strength with which the lens holder 5 is pressed against the adhesive layer B1, etc. However, the method of fixing the lens holder 5 to the support surface 7c is not limited to the above. For example, the lens holder 5 may be fixed to the support surface 7c by screwing or the like.

[0088] From the end face (end face 12b) on the first lens 4 side of the QCL element 2, mid-infrared light with a relatively large divergence angle is emitted. Therefore, in order to suppress the shielding of such mid-infrared light by the first holder 7 (mainly the support surface 7a) and avoid a decrease in the coupling efficiency of the external resonator, the laser module 1A is configured as follows. That is, when viewed from the Y-axis direction (directions perpendicular to directions D1 and D2), the distance d1 (first distance) from the intersection point 7i of the side surface 7e and the support surface 7a along direction D2 to the end face 12b, when the direction from the first lens 4 to the QCL element 2 along direction D2 is considered the positive direction, is smaller than the distance d2 (second distance) from the intersection point 7i along direction D2 to the first lens 4 (first lens surface 41). In this embodiment, the end face 2b of the QCL element 2, including the end face 12b of the active layer 12, is flush with the side surface 7e. That is, the distance d1 is 0. If, in direction D2, the end face 2b is located on the second lens 6 side of the intersection 7i, the distance d1 is greater than 0, and if, in direction D2, the end face 2b protrudes on the first lens 4 side of the intersection 7i, the distance d1 is less than 0. Furthermore, if the first lens surface 41 is a non-flat surface (i.e., the distance between the intersection 7i and the first lens surface 41 changes along the Y-axis), the distance d2 is the minimum distance between the intersection 7i and the first lens surface 41 (i.e., the distance between the part of the first lens surface 41 closest to the intersection 7i along direction D2 and the intersection 7i).

[0089] The first holder 7 is connected to the support surface 7a and has a side surface 7h (second side surface) that faces the light incident surface 61 of the second lens 6 in direction D2. The end surface 2a of the QCL element 2 on the second lens 6 side (end surface including the end surface 12a of the active layer 12) is located on the second lens 6 side of the side surface 7h in direction D2. With the above configuration, by making the end surface 12a of the QCL element 2 protrude on the second lens 6 side of the side surface 7h of the first holder 7, interference between the first holder 7 and the second lens 6 can be suppressed while the end surface 12a of the QCL element 2 can be brought into contact with or close to the second lens 6 for terahertz wave output.

[0090] In this embodiment, the first holder 7 has a main body portion 71 including a portion 71a (see Figure 7) on the first lens 4 side of the support surface 7a and a side surface 7e, and a protruding portion 72 connected to the main body portion 71. In this embodiment, as shown in Figure 7, the protruding portion 72 is a portion formed in a tapered shape toward the second lens 6 side in a plan view. The protruding portion 72 includes the portion 72a on the second lens 6 side of the support surface 7a, as well as the side surface 7h described above.

[0091] As shown in Figure 3, the end face 12a of the active layer 12 of the QCL element 2 is located inside the small-diameter hole 85a of the side wall 822. With this configuration, the end face 12a of the QCL element 2 can be extended into the small-diameter hole 85a, allowing the substrate 21 of the QCL element 2 to contact or be in close proximity (in this embodiment) to the light incident surface 61 of the second lens 6. This improves the extraction efficiency of terahertz waves L1 and allows for miniaturization of the entire package. More specifically, by bringing the QCL element 2 and the second lens 6 closer together, the size of the second lens 6 required to collimate the terahertz waves L1 can be reduced, thus allowing for miniaturization of the entire laser module 1A.

[0092] Furthermore, at least a portion of the aforementioned protrusion 72 is located inside the small-diameter hole 85a. That is, the protrusion 72 is partially inserted into the small-diameter hole 85a. With the above configuration, by extending the first holder 7 (protrusion 72) into the small-diameter hole 85a, the contact area between the QCL element 2 and the first holder 7 (support surface 7a) can be increased. As a result, the heat dissipation efficiency from the QCL element 2 to the first holder 7 can be improved.

[0093] Furthermore, the first width of the projection 72 in the width direction (i.e., the Y-axis direction) (fourth direction) perpendicular to directions D1 and D2 is smaller than the width of the main body 71 in the width direction. As shown in Figure 7, the projection 72 is formed in a shape in which the width of the projection 72 gradually decreases as it approaches the second lens 6 along direction D2. In this embodiment, as an example, the projection 72 is formed in a tapered shape that narrows when viewed from above. With the above configuration, by making the width of the projection 72 smaller than the width of the main body 71, interference between the first holder 7 and the package 8 (side wall 822) can be prevented, while ensuring the volume of the first holder 7 that functions as a heat bath. Furthermore, by forming the protrusion 72 in the tapered shape described above, the risk of interference (contact) between the protrusion 72 and the side wall 822 (mainly the inner surface of the small-diameter hole 85a) when housing and positioning the first holder 7 on which the QCL element 2 is mounted within the package 8 can be effectively reduced, and the volume of the first holder 7 that functions as a heat bath (the volume of the portion of the first holder 7 that is close to the QCL element 2) can be made sufficiently large.

[0094] The laser module 1A is manufactured, for example, as follows. First, the second lens 6 is mounted in the through-hole 85 of the package 8 before the top wall 83 is attached. Next, the QCL element 2, the first lens 4 (lens holder 5), and the diffraction grating unit 3 are mounted on the first holder 7 on the outside of the package 8. At this time, the positions of the QCL element 2, the first lens 4 (lens holder 5), and the diffraction grating unit 3 are adjusted so that the resonant axis of the QCL element 2 coincides with the optical axis AX1 of the first lens 4, and the optical axis AX1 passes through the center of the diffraction grating portion 314. Next, the first holder 7, with each component mounted as described above, is housed inside the package 8. The first holder 7 is fixed inside the package 8 such that a part of the side surface 21c of the substrate 21 of the QCL element 2 is in contact with (or close to) the approximate center of the light incident surface 61 of the second lens 6. Next, the components housed in the package 8 that require electrode supply (such as the QCL element 2) and the electrode terminals (lead terminals 9) are electrically connected by wires or the like (not shown). After the arrangement and wiring of each component within the package 8 are completed, the inside of the package 8 is replaced with a vacuum or nitrogen, and the top wall 83 is hermetically joined to the end 82a of the side wall 82 opposite to the bottom wall 81. The laser module 1A described above is thus obtained.

[0095] In the laser module 1A described above, the components necessary for generating the first and second beams of light required for generating the terahertz wave L1 by difference frequency generation are packaged together: a QCL element 2, a first holder 7 for holding the QCL element 2, a movable diffraction grating 31 (diffraction grating unit 3), and a first lens 4. Furthermore, when viewed from the Y-axis direction, the distance d1 (0 in this embodiment) from the intersection point 7i of the side surface 7e and support surface 7a of the first holder 7 to the end surface 12b, when the direction from the first lens 4 towards the QCL element 2 along the direction D2 (i.e., the resonance direction) where both end faces (end faces 12a and end faces 12b) of the QCL element 2 are facing each other is set to be smaller than the distance d2 from the intersection point 7i along direction D2 to the first lens 4 (first lens surface 41) (see Figure 6). Here, if the distance from the end face 12b from which mid-infrared light is emitted to the first lens 4 (first lens surface 41) increases, the size of the first lens 4 (first lens surface 41) required to allow a sufficient amount of mid-infrared light with a divergence angle to be incident increases. In addition, the range of mid-infrared light collimated by the first lens 4 also increases, so the area of ​​the movable diffraction grating 31 (diffraction grating portion 314) that receives the collimated mid-infrared light needs to be increased accordingly. As a result, the movable diffraction grating 31 also becomes larger. On the other hand, by arranging the QCL element 2 and the first holder 7 in this way, the QCL element 2 (end face 12b) can be brought as close as possible to the first lens 4. This suppresses the increase in size of the first lens 4 and the movable diffraction grating 31, and makes the overall package 8 smaller. Furthermore, by setting the distance d1 to a small value as described above, interference between the light emitted from the end face 12b (mid-infrared light) and the first holder 7 can be suppressed (i.e., a portion of the emitted light is blocked by the first holder 7 and does not reach the first lens 4). This reduces the loss of light that contributes to the generation of terahertz waves. Thus, a laser module 1A can be obtained that is miniaturized and packaged in a manner that suppresses the loss of light that contributes to the generation of terahertz waves. For example, 10000 mm 3A laser module 1A can be obtained that is compacted to a size of less than [specify size]. In the laser module 1A, a single-mode, wavelength-variable terahertz wave L1 can be obtained by injecting a current above a threshold into the QCL element 2 and changing the angle of the movable diffraction grating 31 by external control.

[0096] Furthermore, in the laser module 1A, the distance d1 may be 0 or less. With the above configuration, that is, a configuration in which the end face 12b is flush with the side surface 7h (the configuration of this embodiment) or a configuration in which the end face 12b protrudes further toward the first lens 4 than the side surface 7h, interference between the light emitted from the end face 12b and the first holder 7 can be prevented more reliably, and thus the loss of light that contributes to the generation of terahertz waves can be reduced even more effectively.

[0097] Furthermore, in a configuration where the distance d1 is equal to 0, as in this embodiment (i.e., the end face 12b and the side surface 7h are flush), the entire portion of the QCL element 2 extending to the end face 12b can be brought into contact with the first holder 7 (support surface 7a), thereby increasing the heat dissipation efficiency from the QCL element 2 to the first holder 7.

[0098] [Second Embodiment] The laser module 1B according to the second embodiment will be described with reference to Figures 8 and 9. In Figure 9, the top wall 83, protruding wall 84, and lead terminals 9 are omitted from the illustration in order to clearly show the positional relationship of the main components housed in the package 8.

[0099] Laser module 1B differs from laser module 1A mainly in that the terahertz wave output lens (second lens 6) is housed inside the package 8. Laser module 1B includes a second holder 100 that holds the second lens 6 inside the package 8. In other words, in laser module 1B, the second lens 6 and the second holder 100 are housed inside the package 8. In addition, instead of the second lens 6, a light emission window 200, which is a different window material from the second lens 6, is provided in the through hole 85 of the side wall 822.

[0100] The second holder 100 has a roughly rectangular parallelepiped shape. The second holder 100 is fixed to the bottom wall 81 of the first holder 7 between the side wall 822, which has a through hole 85, and the first holder 7, by means of a fixing resin, adhesive, screws, etc. However, the shape and fixing method of the second holder 100 are not limited to the above. For example, the second holder 100 may be fixed to the side wall 82 (for example, a pair of side walls 821 located on both sides of the second holder 100) instead of the bottom wall 81. The second holder 100 may be made of a material that can be precisely machined and has sufficient hardness and rigidity to maintain its shape. The second holder 100 may be made of a metal such as aluminum, stainless steel, or copper, or a hard plastic, for example.

[0101] The second holder 100 is provided with a through hole 101 that penetrates the side wall 822 in a direction perpendicular to it (i.e., the X-axis direction) (third direction). The second lens 6 is mounted in the through hole 101. The through hole 101 has a small diameter hole 101a (first hole portion), a large diameter hole 101b (second hole portion), and a counterbore surface 101c. The small diameter hole 101a opens towards the QCL element 2 side in the optical axis direction (i.e., the X-axis direction) of the second lens 6. The large diameter hole 101b opens towards the side wall 822 side where the through hole 85 is provided in the X-axis direction. When viewed from the X-axis direction, the large diameter hole 101b has a shape that includes the small diameter hole 101a and is larger than the small diameter hole 101a. The small diameter hole 101a and the large diameter hole 101b each extend in the X-axis direction. In this embodiment, the small-diameter hole 101a and the large-diameter hole 101b are each formed in a circular shape, and the diameter of the large-diameter hole 101b is larger than the diameter of the small-diameter hole 101a. As an example, the central axis of the small-diameter hole 101a and the central axis of the large-diameter hole 101b substantially coincide with the optical axis AX2 of the second lens 6 (see Figure 6).

[0102] The counterbore surface 101c connects the small-diameter hole 101a and the large-diameter hole 101b and is an annular surface extending along a plane (YZ plane) that intersects in the X-axis direction. More specifically, the counterbore surface 101c connects the end of the small-diameter hole 101a on the large-diameter hole 101b side to the end of the large-diameter hole 101b on the small-diameter hole 101a side. The large-diameter hole 101b and the counterbore surface 101c can be formed by counterboring the second holder 100 from the side opposite to the QCL element 2 side of the second holder 100. In this embodiment, the counterbore surface 101c is formed as a continuous annular shape, but the counterbore surface 101c may be formed as a discontinuous annular shape. For example, the counterbore surface 101c may be divided at the portion where a notch is formed by forming a notch in a part of the inner wall surface of the small-diameter hole 101a. A through hole 101 is formed by a small-diameter hole 101a and a large-diameter hole 101b connected by a counterbore surface 101c, which penetrate in the X-axis direction.

[0103] The outer edge of the light incident surface 61 of the second lens 6 is inserted through the large-diameter hole 101b from the side of the side wall 822 where the through hole 85 is provided, and is fixed in surface contact with the counterbore surface 101c. The outer edge of the light incident surface 61 is fixed to the counterbore surface 101c by, for example, a fixing resin or adhesive. With the above configuration, the second lens 6 for terahertz wave output can be attached to the second holder 100 from the outside of the second holder 100 (the side opposite to the side where the QCL element 2 is placed), making the installation of the second lens 6 easy.

[0104] The height position of the through-hole 101 in the second holder 100 is set such that the optical axis AX2 of the second lens 6 attached to the through-hole 101 and the center of the light emission window 200 provided in the through-hole 85 substantially coincide (i.e., coincide within the range of mechanical precision).

[0105] In the examples shown in Figures 8 and 9, the height (length in the Z-axis direction) and width (length in the Y-axis direction) of the second holder 100 are smaller than the internal dimensions (height and width) of the package 8. However, the height and width of the second holder 100 may be set to approximately match the internal dimensions of the package 8. That is, the second holder 100 may be configured to partition the package 8 into a first space on the side wall 822 side where the through-hole 85 is provided, and a second space on the QCL element 2 side of the second holder 100. This prevents the scattering component of mid-infrared light generated in the second space (a component that does not contribute to external resonance) from entering the first space by being reflected inside the package 8. As a result, the generation of the above-mentioned scattering component (stray light) emitted to the outside of the package 8 through the light emission window 200 can be suppressed.

[0106] The light-emitting window 200 is formed, for example, in a disc shape. The light-emitting window 200 is attached to the through hole 85, for example, similar to the second lens 6 in the first embodiment. That is, the outer edge of the side wall 822 side of the light-emitting window 200 is inserted from the outside of the package 8 into the large-diameter hole 85b and fixed in surface contact with the counterbore surface 85c. The light-emitting window 200 is sealed to the counterbore surface 85c to maintain airtightness inside the package 8. The light-emitting window 200 is formed of any material that transmits terahertz waves, such as silicon, Tsurupica, or polyethylene. Both sides (outer and inner surfaces) of the light-emitting window 200 may be provided with a low-reflection coating to reduce the reflection of terahertz waves.

[0107] As described above, with the laser module 1B, the second lens 6 for terahertz wave output can be housed in the same package as the first lens 4, thus preventing damage and contamination of the second lens 6.

[0108] Furthermore, in the laser module 1B, the second holder 100 is a separate component from the first holder 7 and is fixed to the package 8 independently of the first holder 7. With this configuration, since the first holder 7 and the second holder 100 are independent, the work of attaching the QCL element 2 etc. to the first holder 7 and the work of attaching the second lens 6 to the second holder 100 can be performed in parallel before housing each component in the package 8. This improves the work efficiency of assembling the laser module 1B.

[0109] Furthermore, in laser module 1B, similar to laser module 1A, the end face 12a of the active layer 12 of the QCL element 2 is located inside the small-diameter hole 101a of the second holder 100. With this configuration, the end face 12a of the QCL element 2 can be extended into the small-diameter hole 101a, allowing the substrate 21 of the QCL element 2 to contact or be in close proximity (contact in this embodiment) to the light incident surface 61 of the second lens 6. This improves the extraction efficiency of terahertz waves L1 and allows for miniaturization of the entire package.

[0110] Furthermore, at least a portion of the protrusion 72 of the first holder 7 is located inside the small-diameter hole 101a of the second holder 100. With the above configuration, by extending the first holder 7 (protrusion 72) into the small-diameter hole 101a, the contact area between the QCL element 2 and the first holder 7 (support surface 7a) can be increased. As a result, the heat dissipation efficiency from the QCL element 2 to the first holder 7 can be improved.

[0111] [Third Embodiment] A laser module 1C according to the third embodiment will be described with reference to Figures 10 and 11. In Figure 11, the top wall 83, protruding wall 84, and lead terminals 9 are omitted from the illustration in order to clearly show the positional relationship of the main components housed in the package 8.

[0112] Laser module 1C differs from laser module 1B mainly in that the second holder 100 is fixed to the first holder. Laser module 1C has a first holder 7A instead of the first holder 7. The first holder 7A differs from the first holder 7 in that it has an extended portion 73 that extends along the bottom wall 81 toward the side wall 822, where a through hole 85 is provided, from the lower part of the main body portion 71 of the first holder 7. The second holder 100 is fixed on the extended portion 73 by adhesive, fixing resin, screws, etc. In this embodiment, as an example, the second holder 100 is fixed to the upper surface of the extended portion 73 via an adhesive layer B2. The adhesive layer B2 is, for example, a photocurable resin similar to adhesive layer B1.

[0113] In the laser module 1C, in addition to the QCL element 2, the first lens 4 (lens holder 5) and the diffraction grating unit 3 are fixed to the first holder 7A, similar to the first and second embodiments. The second holder 100 is also fixed to the first holder 7A (extended portion 73). With the above configuration, a structure in which the first holder 7A and the second holder 100 are integrated can be prepared outside the package 8, so that the positions of the QCL element 2, the first lens 4 (lens holder 5), the diffraction grating unit 3, and the second lens 6 can be adjusted outside the package 8. This improves work efficiency compared to when position adjustment is performed inside the package 8. More specifically, the adjustment of the external resonator and the positioning of the holder (a combined member of the first holder 7A and the second holder 100) for emitting terahertz waves L1 in a desired direction while bringing the end face of the QCL element 2 (a part of the side surface 21c of the substrate 21) into contact with (or close to) the light incident surface 61 of the terahertz wave output lens (second lens 6) can be performed outside the package 8. This frees the user from the constraints of workspace when performing alignment of each component housed within the package 8, thereby improving the efficiency and accuracy of the alignment work. The second holder 100 may be formed integrally with the first holder 7A. In this case, a member for attaching the second holder 100 to the first holder 7A (adhesive layer B2 in this embodiment) is not required.

[0114] Furthermore, in the laser module 1C, if a temperature control element such as a Peltier element is placed between the first holder 7A and the bottom wall 81, the contact area between the first holder 7A and the temperature control element can be increased by the amount of the extended portion 73, thereby improving the heat dissipation efficiency compared to when the first holder 7 is used.

[0115] [Differentiation] Although one embodiment of the present disclosure has been described above, the present disclosure is not limited to the above embodiment. The materials and shapes of each component are not limited to those described above, but can be made from a variety of materials and shapes. In addition, some components included in the laser modules 1A, 1B, and 1C according to the above embodiment may be omitted or modified as appropriate. For example, according to the above embodiment, the laser modules 1A, 1B, and 1C can be miniaturized as described above, but the laser modules 1A, 1B, and 1C are not necessarily limited to being hand-top size or smaller. Also, the type of package 8 is not limited to a butterfly package. [Explanation of Symbols]

[0116] 1A, 1B, 1C…Laser module, 2…Quantum cascade laser element, 3…Diffraction grating unit, 4…First lens, 6…Second lens, 7, 7A…First holder, 7a…Support surface, 7e…Side (first side), 7h…Side (second side), 7i…Intersection, 8…Package, 11…Lower cladding layer (first cladding layer), 12…Active layer, 12a…End face (first end face), 12b…End face (second end face), 13…Upper cladding layer (second cladding layer), 21…Substrate, 21a…Main surface, 21b…Back surface, 31…Movable Diffraction grating, 61...light incident surface, 71...main body, 72...protruding part, 71a, 72a...parts, 82, 822...side walls, 85, 101...through holes, 85a, 101a...small diameter holes (first hole), 85b, 101b...large diameter holes (second hole), 85c, 101c...counterbore surface, 100...second holder, 200...light emission window, D1...direction (first direction), d1...distance (first distance), D2...direction (second direction), d2...distance (second distance), L1...terahertz wave, ω1...first frequency, ω2...second frequency, ω3...difference frequency.

Claims

1. A quantum cascade laser element comprising a substrate having a main surface and a back surface opposite to the main surface, a first cladding layer provided on the main surface, an active layer provided on the side of the first cladding layer opposite to the substrate, and a second cladding layer provided on the side of the active layer opposite to the first cladding layer, The active layer has a first end face and a second end face that face each other in a second direction perpendicular to the first direction which is the stacking direction of the substrate, the first cladding layer, the active layer, and the second cladding layer. The first end face constitutes a resonator for emitting light of a first frequency and light of a second frequency. The active layer generates terahertz waves at the difference frequency between the first frequency and the second frequency. The aforementioned quantum cascade laser element, A diffraction grating unit including a movable diffraction grating that constitutes an external resonator for light of the first frequency, A first lens is positioned between the quantum cascade laser element and the movable diffraction grating, allowing light emitted from the second end face and light returning from the movable diffraction grating to the quantum cascade laser element to pass through. A second lens is positioned opposite the first end face and allows the terahertz waves emitted from the quantum cascade laser element to pass through, A first holder for holding the quantum cascade laser element, The package comprises the quantum cascade laser element, the diffraction grating unit, the first lens, and the first holder, and the optical path between the light incident surface of the second lens and the movable diffraction grating. The first holder is, The support surface on which the back surface of the substrate of the quantum cascade laser element is placed, It has a first side surface that is connected to the support surface and faces the first lens in the second direction, In the second direction, the second end face protrudes toward the first lens side beyond the intersection of the first side surface and the support surface. Laser module.

2. The resonance axis passing through the first end face and the second end face of the quantum cascade laser element intersects with the optical axis of the second lens because the support surface is inclined to intersect with the optical axis of the second lens. The first end face is spaced apart from the light incident surface of the second lens. The laser module according to claim 1.

3. The first lens and the diffraction grating unit are fixed to the first holder. The laser module according to claim 1 or 2.

4. The second lens is a silicon lens formed in the shape of a hemisphere or hyperhemispheric. A laser module according to any one of claims 1 to 3.

5. The system further comprises a second holder for holding the second lens, The second lens and the second holder are housed within the package. The package has a side wall facing the first end face, The side wall is provided with a light emission window for allowing light emitted from the first end face and passing through the second lens to pass through. A laser module according to any one of claims 1 to 4.

6. The second holder is fixed to the package independently of the first holder. The laser module according to claim 5.

7. The first lens and the diffraction grating unit are fixed to the first holder. The second holder is formed integrally with the first holder, or is fixed to the first holder. The laser module according to claim 5.

8. The second holder has a through hole that penetrates the side wall in a third direction perpendicular to it, The aforementioned through hole is The first hole opening on the quantum cascade laser element side, A second hole, when viewed from the third direction, includes the first hole and is larger than the first hole and opens towards the side wall, It has an annular counterbore surface that connects the first hole and the second hole and extends along a plane intersecting the second direction, The outer edge of the light incident surface of the second lens is inserted into the second hole from the side wall and fixed in surface contact with the counterbore surface. The laser module according to any one of claims 5 to 7.

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