External Cavity Laser Module

Abstract

To provide an external resonant laser module enabling size reduction, suppression in power consumption and reduction in a spectral linewidth.SOLUTION: An external resonant laser module includes: a quantum cascade laser; a MEMS diffraction grating 51 including a movable portion 63 capable of swinging around an axis A and a diffraction grating portion 64 formed on the movable portion 63; and a lens. The diffraction grating portion 64 includes a plurality of grating grooves 64a aligned in a first direction D1. Each of the grating grooves 64a extends in a second direction D2 perpendicular to the first direction D1. The MEMS diffraction grating 51 is arranged such that a normal line of the diffraction grating portion 64 tilts with respect to an end face of the quantum cascade laser and, when viewed from a direction perpendicular to the end face, the first direction D1 is along a lamination direction of the lamination structure of the quantum cascade laser. A length L1 of the diffraction grating portion 64 in the first direction D1 is longer than a length L2 of the diffraction grating portion 64 in the second direction D2.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an external cavity laser module equipped with a MEMS (Micro Electro Mechanical Systems) diffraction grating. [Background technology]

[0002] A known external cavity laser module includes a quantum cascade laser, an oscillating diffraction grating, and a lens disposed between the quantum cascade laser and the diffraction grating (see, for example, Patent Document 1). In such an external cavity laser module, light from the quantum cascade laser is diffracted and reflected by the diffraction grating, and light of a specific wavelength among the light is fed back to the quantum cascade laser. As a result, an external cavity is formed by the end face of the quantum cascade laser and the diffraction grating, and the light of the specific wavelength is amplified and output to the outside. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-036577 Summary of the Invention [Problem to be solved by the invention]

[0004] The external cavity laser module described above is required to reduce power consumption. In particular, when using a quantum cascade laser, the beam diameter of the light emitted from the quantum cascade laser is large, so the area of ​​the diffraction grating needs to be large. This requires a large driving force, which tends to increase power consumption. Furthermore, the external cavity laser module is required to have a narrow spectral linewidth to increase the purity of the output light, and is also required to be compact.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an external cavity laser module that can be made smaller, consume less power, and have a narrower spectral linewidth. [Means for solving the problem]

[0006] The external cavity laser module of the present invention comprises a quantum cascade laser having a laminated structure and emitting light from an end face, a movable part that can swing around a predetermined axis, and a diffraction grating part formed on the movable part, and is equipped with a MEMS diffraction grating that diffracts and reflects the light emitted from the quantum cascade laser by the diffraction grating part and feeds back a portion of the light to the quantum cascade laser, and a lens arranged between the quantum cascade laser and the MEMS diffraction grating, wherein the diffraction grating part includes a plurality of grating grooves aligned in a first direction, and each of the plurality of grating grooves extends in a second direction perpendicular to the first direction, and the MEMS diffraction grating is arranged so that the normal of the diffraction grating part is inclined with respect to the end face and the first direction is along the laminated direction of the laminated structure when viewed from a direction perpendicular to the end face, and the length of the diffraction grating part in the first direction is longer than the length of the diffraction grating part in the second direction.

[0007] In this external cavity laser module, the MEMS diffraction grating is arranged so that the normal to the diffraction grating portion is inclined with respect to the end face of the quantum cascade laser, and the length of the diffraction grating portion in a first direction, which is the arrangement direction of the grating grooves, is longer than the length of the diffraction grating portion in a second direction perpendicular to the first direction. Because the length of the diffraction grating portion in the first direction is long, even when the diffraction grating portion is arranged at an angle, the diffraction grating portion can effectively receive light from the quantum cascade laser. Furthermore, because the length of the diffraction grating portion in the second direction is reduced, the module can be made more compact, and an increase in the size of the diffraction grating portion can be suppressed, thereby suppressing an increase in power consumption. Furthermore, in this external cavity laser module, the MEMS diffraction grating is arranged so that the first direction is aligned with the stacking direction of the stacked structure when viewed from a direction perpendicular to the end face of the quantum cascade laser. By arranging the MEMS diffraction grating so that the first direction is aligned with the stacking direction, the extension direction (second direction) of the grating grooves in the diffraction grating portion can be made perpendicular to the polarization direction of the light emitted from the quantum cascade laser. Furthermore, by making the first direction, which is the arrangement direction of the grating grooves, the longitudinal direction of the MEMS diffraction grating, the number of grating grooves located within the irradiation beam diameter in the tilted diffraction grating portion can be increased. As a result, the wavelength resolution of the MEMS diffraction grating can be ensured to be high, and the spectral linewidth of the output light can be narrowed. Therefore, this external cavity laser module can be made compact, consume less power, and have a narrower spectral linewidth.

[0008] The movable part is formed in a shape having four corners when viewed in a direction parallel to the normal, and the four corners may be rounded, which can reduce the moment of inertia of the movable part and increase the speed of oscillation of the movable part.

[0009] The light emitted from the quantum cascade laser and incident on the MEMS diffraction grating via the lens may have a beam shape whose length in the first direction is longer than its length in the second direction at the position of the diffraction grating portion. Since the length of the diffraction grating portion in the first direction is longer than the length of the diffraction grating portion in the second direction, even in such a case, the light from the quantum cascade laser can be well received by the diffraction grating portion.

[0010] The MEMS diffraction grating may be arranged so that all of the light emitted from the quantum cascade laser and transmitted through the lens is incident on the diffraction grating section, regardless of the swing angle around the axis of the movable section. In this case, it is possible to prevent a situation in which part of the light from the quantum cascade laser does not enter the diffraction grating section and becomes stray light. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide an external cavity laser module that can be made smaller, consume less power, and have a narrower spectral linewidth. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view of a laser module according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 2 is a perspective view of a diffraction grating unit. [Figure 4] FIG. 1 is a view of a MEMS diffraction grating viewed from the normal direction. [Figure 5] FIG. 1 is a view of a MEMS diffraction grating viewed from the normal direction. [Figure 6] FIG. 6 is a cross-sectional view of the diffraction grating portion taken along line VI-VI in FIG. 5. [Figure 7] FIG. [Figure 8] (a) is a plan view of the magnet, and (b) is a side view of the magnet. [Figure 9]10(a) and 10(b) are plan views of MEMS diffraction gratings according to first and second modified examples. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention will be described in detail below with reference to the drawings. In the following description, the same or equivalent elements will be designated by the same reference numerals, and duplicated explanations will be omitted. For convenience, the X, Y, and Z directions are set as shown in each drawing.

[0014] As shown in FIGS. 1 and 2, an external cavity laser module 1 (hereinafter referred to as "laser module 1") includes a housing 2, a mount member 3, a quantum cascade laser 4 (hereinafter referred to as "QCL4"), a diffraction grating unit 5, and lenses 6 and 7. The housing 2 accommodates the mount member 3, the QCL 4, the diffraction grating unit 5, and the lenses 6 and 7. The housing 2 forms, for example, a butterfly package. As an example, the length of each side of the housing 2 is 30 mm or less. The housing 2 is formed in a box shape and includes a main body 21 having an opening 21a and a lid 22 that covers the opening 21a. The housing 2 has an exit window 2a for outputting the output light L of the laser module 1 to the outside.

[0015] The mount member 3 is fixed to the bottom surface of the housing 2. The QCL 4, the diffraction grating unit 5, and the lenses 6 and 7 are fixed to the mount member 3. More specifically, the mount member 3 has a main body 31 and a protrusion 32 that protrudes from the main body 31 in the Z direction. The QCL 4 is fixed to the top surface of the protrusion 32. An arrangement hole 33 is formed in the main body 31. The diffraction grating unit 5 is fixed to the main body 31 with a portion of a yoke 53 (described later) positioned within the arrangement hole 33. The lenses 6 and 7 are held by lens holders 11 and 12, respectively. The lens 6 is fixed to the main body 31 via the lens holder 11 and is positioned between the QCL 4 and the MEMS diffraction grating 51 of the diffraction grating unit 5. The lens 7 is fixed to the main body 31 via the lens holder 12 and is positioned between the QCL 4 and the exit window 2a. The exit window 2a, the lens 7, the QCL 4, the lens 6, and the MEMS diffraction grating 51 are arranged in this order in the Y direction.

[0016] The QCL 4 has a first end face 4a and a second end face 4b opposite the first end face 4a, and emits light in the mid-infrared region (for example, 4 μm to 12 μm) from each of the first end face 4a and the second end face 4b. The first end face 4a and the second end face 4b are, for example, flat surfaces perpendicular to the Y direction. An anti-reflective coating is applied to the first end face 4a.

[0017] The QCL 4 has a semiconductor substrate 41 and a stacked structure 42 formed on the semiconductor substrate 41. The stacked structure 42 includes an active layer and a pair of cladding layers sandwiching the active layer. The active layer includes, for example, multiple quantum well layers made of InGaAs and multiple quantum barrier layers made of InAlAs. The cladding layers are made of, for example, InP. The active layer and cladding layers are formed on the semiconductor substrate 41 by crystal growth. During crystal growth, the active layer and cladding layers are formed on the semiconductor substrate 41 along the Z direction (stacking direction, growth direction). The growth direction is the thickness direction of the active layer. Light emitted from the QCL 4 is linearly polarized light parallel to the stacking direction. Note that the stacked structure 42 may include multiple active layers and a pair of cladding layers having different center wavelengths.

[0018] Lenses 6 and 7 are aspherical lenses made of, for example, zinc selenide (ZnSe). Anti-reflective coatings are applied to the surfaces of lenses 6 and 7. Lens 6 is disposed on the first end face 4a side of QCL 4 and collimates the light emitted from the first end face 4a. Lens 7 is disposed on the second end face 4b side of QCL 4 and collimates the light emitted from the second end face 4b. The light collimated by lens 7 passes through exit window 2a of housing 2 and is output to the outside as output light L.

[0019] The light collimated by the lens 6 is incident on the MEMS diffraction grating 51 of the diffraction grating unit 5. The MEMS diffraction grating 51 diffracts and reflects the incident light, thereby feeding back light of a specific wavelength from the incident light to the first end face 4a of the QCL 4. In the laser module 1, the MEMS diffraction grating 51 and the second end face 4b form a Littrow-type external resonator. This allows the laser module 1 to amplify light of a specific wavelength and output it externally.

[0020] Furthermore, as will be described later, the MEMS diffraction grating 51 can rapidly change the orientation of the diffraction grating portion 64 that diffracts and reflects incident light. This allows the wavelength of the light returning from the MEMS diffraction grating 51 to the first end facet 4a of the QCL 4 to be tunable, thereby tunable the wavelength of the output light L of the laser module 1. By changing the wavelength of the output light L, it is possible to sweep the wavelength within the gain band of the QCL 4, for example.

[0021] The diffraction grating unit 5 includes a MEMS diffraction grating 51, a magnet 52, and a yoke 53. The MEMS diffraction grating 51 is formed in a substantially plate shape. The magnet 52 is disposed on the opposite side of the MEMS diffraction grating 51 from the QCL 4. The MEMS diffraction grating 51 is fixed to the yoke 53, and the magnet 52 is housed within the yoke 53. As a result, the MEMS diffraction grating 51, the magnet 52, and the yoke 53 are integrated and form a single unit.

[0022] 3 to 6, the MEMS diffraction grating 51 includes a support portion 61, a pair of connecting portions 62, a movable portion 63, a diffraction grating portion 64, and a pair of coils 65 and 66. The MEMS diffraction grating 51 is configured as a MEMS device that oscillates the movable portion 63 around an axis A. The MEMS diffraction grating 51 is formed by processing a semiconductor substrate using MEMS technology (patterning, etching, etc.).

[0023] The support part 61 is a flat frame body that has a rectangular shape in a plan view (when viewed from the normal direction DN, which will be described later). The support part 61 supports the movable part 63 via a pair of connecting parts 62. Each connecting part 62 is a flat member that has a rectangular rod shape in a plan view and extends straight along the axis A. Each connecting part 62 connects the movable part 63 to the support part 61 on the axis A so that the movable part 63 can swing freely around the axis A.

[0024] The movable portion 63 is located inside the support portion 61. As described above, the movable portion 63 is capable of swinging around the axis A. The movable portion 63 is a flat member having a substantially rectangular shape in a plan view and has four corners 63a. Each corner 63a of the movable portion 63 is chamfered in an R-shape, and each corner 63a is formed to be rounded. For example, each corner 63a is curved in an arc shape in a plan view. This reduces the moment of inertia of the movable portion 63 and increases the swing speed of the movable portion 63. In this example, the movable portion 63 is formed in a substantially rectangular shape with its long sides parallel to the first direction D1, and the length of the movable portion 63 in the first direction D1 is longer than the length of the movable portion 63 in the second direction D2. As an example, the length of the movable portion 63 in the first direction D1 is approximately 4 mm, the length of the movable portion 63 in the second direction D2 is approximately 3 mm, and the thickness is approximately 30 μm. The support portion 61, the connecting portion 62, and the movable portion 63 are integrally formed by being built into, for example, a single SOI (Silicon on Insulator) substrate.

[0025] A diffraction grating unit 64 is provided on the surface of the movable unit 63 facing the QCL 4. The diffraction grating unit 64 has multiple grating grooves 64a and diffracts and reflects light emitted from the QCL 4. The diffraction grating unit 64 includes, for example, a resin layer provided on the surface of the movable unit 63 and having a diffraction grating pattern formed thereon, and a metal layer provided on the surface of the resin layer so as to conform to the diffraction grating pattern. Alternatively, the diffraction grating unit 64 may be formed solely by a metal layer provided on the movable unit 63 and having a diffraction grating pattern formed thereon. In this example, the diffraction grating pattern is a blazed grating with a sawtooth cross section to improve diffraction efficiency, but it may also be a binary grating with a rectangular cross section or a holographic grating with a sinusoidal cross section. 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 formed by vapor deposition.

[0026] 5, the grating grooves 64a are arranged at equal intervals in a first direction D1. Each grating groove 64a extends straight in a second direction D2 perpendicular to the first direction D1. The second direction D2 is parallel to the axis A. The repetition period d of the grating grooves 64a in the first direction D1 (the distance between adjacent grating grooves 64a) is, for example, 4 μm to 10 μm. The angle θ (blazed angle) of the grating grooves 64a with respect to a normal direction DN parallel to the normal N of the diffraction grating portion 64 (a straight line perpendicular to the grating surface S) is, for example, 20 degrees to 35 degrees.

[0027] The diffraction grating section 64 is formed to be slightly smaller than the movable section 63 in a plan view, and the outer edge of the diffraction grating section 64 extends along the outer edge of the movable section 63 at a fixed distance from the outer edge of the movable section 63. In this example, the diffraction grating section 64 is formed in a substantially rectangular shape similar to the movable section 63 in a plan view. That is, the diffraction grating section 64 is formed in a substantially rectangular shape with its long sides parallel to the first direction D1, and the length L1 of the diffraction grating section 64 in the first direction D1 is longer than the length L2 of the diffraction grating section 64 in the second direction D2.

[0028] The coils 65 and 66 are made of a metal material such as copper, and have a damascene structure embedded in grooves formed in the surface of the movable part 63. In a plan view, the coil 65 is disposed on one side of the axis A (upper side in FIG. 4), and the coil 66 is disposed on the other side of the axis A (lower side in FIG. 4).

[0029] Each of the coils 65, 66 is wound multiple times in a spiral shape in a plan view. The outer end of the coil 65 is electrically connected to an electrode pad 71 provided on the support part 61 via a wiring 72. The wiring 72 extends across the support part 61, one of the connecting parts 62, and the movable part 63. The outer end of the coil 66 is electrically connected to an electrode pad 73 provided on the support part 61 via a wiring 74. The wiring 74 extends across the support part 61, the other connecting part 62, and the movable part 63.

[0030] The inner end of coil 65 is electrically connected to the inner end of coil 66. In this example, the inner ends of coils 65, 66 are electrically connected to each other by multilayer wiring 67. Multilayer wiring 67 can also be considered to constitute part of coils 65, 66, and in MEMS diffraction grating 51, one coil wiring (multilayer wiring) can also be considered to extend so as to be folded back in a figure-eight shape in plan view, thereby constituting a pair of coils 65, 66. Note that coils 65, 66 may be formed integrally with each other.

[0031] The coil 65 has an inner portion 65a extending along the axis A and an outer portion 65b extending along the outer edge of the movable portion 63. The inner portion 65a and the outer portion 65b extend linearly and parallel to each other along the second direction D2. The inner portion 65a extends along the axis A at a fixed distance from the axis A. The outer portion 65b extends along the outer edge of the movable portion 63 at a fixed distance from the outer edge of the movable portion 63.

[0032] The coil 66 has an inner portion 66a extending along the axis A and an outer portion 66b extending along the outer edge of the movable portion 63. The inner portion 66a and the outer portion 66b extend linearly and parallel to each other along the second direction D2. The inner portion 66a extends along the axis A at a fixed distance from the axis A. The outer portion 66b extends along the outer edge of the movable portion 63 at a fixed distance from the outer edge of the movable portion 63.

[0033] The magnet 52 generates a magnetic field (magnetic force) that acts on the coils 65 and 66. As shown in FIGS. 2, 7, and 8, the magnet 52 is a neodymium magnet (permanent magnet) formed in a substantially rectangular parallelepiped shape and has a surface 54. The surface 54 is the surface on the MEMS diffraction grating 51 side and faces the MEMS diffraction grating 51 in the normal direction DN. The magnet 52 is made of a single member having different magnetic poles on one side and the other side in the normal direction DN. In other words, the magnet 52 is not a magnet formed by combining multiple members, but is an integrated bulk magnet magnetized in the thickness direction. In this example, the magnet 52 has an N-pole portion 57 on the MEMS diffraction grating 51 side and an S-pole portion 58 on the opposite side from the MEMS diffraction grating 51. In FIG. 8, the boundary between the N-pole portion 57 and the S-pole portion 58 is indicated by a two-dot chain line.

[0034] The surface 54 includes a pair of inclined surfaces 55. When viewed from the second direction D2, each inclined surface 55 is inclined so that the further away from the axis A the inclined surface 55 is, the further away from the movable portion 63 of the MEMS diffraction grating 51 it is. The inclination angle of the inclined surface 55 (the angle with respect to a plane perpendicular to the normal direction DN) is set to half the maximum mechanical inclination angle of the movable portion 63 of the MEMS diffraction grating 51. This prevents the movable portion 63 from interfering with the magnet 52 when the movable portion 63 oscillates around the axis A, thereby increasing the oscillation angle of the movable portion 63. For example, when the maximum mechanical inclination angle is 10 degrees, the inclination angle of the inclined surface 55 is set to 5 degrees. As shown in FIG. 8(b), when viewed from the second direction D2, imaginary extensions of the pair of inclined surfaces 55 intersect at an intersection point B on a line that passes through the axis A and is parallel to the normal direction DN.

[0035] A recess 56 is formed in the surface 54. In this example, the recess 56 is a groove extending straight along the second direction D2. The recess 56 has, for example, a uniform rectangular cross section in the second direction D2 and a flat bottom surface 56a perpendicular to the normal direction DN. In the first direction D1, the recess 56 is disposed between a pair of inclined surfaces 55 and is continuous with the pair of inclined surfaces 55. In the second direction D2, the recess 56 extends to both ends of the magnet 52. The space within the recess 56 is an air gap, but may be filled with a non-magnetic material. The recess 56 is formed in the N-pole portion 57 so as to stop at the N-pole portion 57, but does not reach the S-pole portion 58. This is because if the recess 56 were formed to reach the S-pole portion 58 or if the recess 56 were a through hole, the magnetic field lines would become complex, potentially hindering stable operation.

[0036] 4, the positional relationship between the MEMS diffraction grating 51 and the magnet 52 when viewed from the normal direction DN will be described. The length of the MEMS diffraction grating 51 in the first direction D1 is longer than the length of the magnet 52 in the first direction D1. The length of the magnet 52 in the first direction D1 is approximately equal to the length of the diffraction grating portion 64 in the first direction D1. The length of the MEMS diffraction grating 51 in the second direction D2 is approximately equal to the length of the magnet 52 in the second direction D2. The length of the magnet 52 in the second direction D2 is longer than the length of the diffraction grating portion 64 in the second direction D2. By adopting such a positional relationship, it is possible to make the magnet 52 as large as possible while preventing light from the QCL 4 from being incident on the magnet 52 when the movable portion 63 oscillates around the axis A.

[0037] At least a portion of the recess 56 (bottom surface 56a) overlaps with the inner portions 65a, 66a of the coils 65, 66 when viewed from the normal direction DN. In this example, a portion of the recess 56 on the axis A side overlaps with the entire inner portions 65a, 66a. That is, when viewed from the normal direction DN, the width W1 of the recess 56 is wider than the width W2 of the inner portions 65a, 66a. The widths W1, W2 are widths in the first direction D1. When viewed from the normal direction DN, the recess 56 is located closer to the axis A than the outer portions 65b, 66b of the coils 65, 66, and does not overlap with the outer portions 65b, 66b. As an example, the length of the magnet 52 in the first direction D1 is 4 mm, and the length of the magnet 52 in the second direction D2 is 6 mm. The width W1 of the recess 56 is approximately 1 mm to 2 mm, e.g., 2 mm. The depth of the recess 56 (the distance between the bottom surface 56a and the intersection point B) is about 1 mm, and the thickness of the magnet 52 in the normal direction DN is 3 mm to 3.5 mm.

[0038] The yoke 53 amplifies the magnetic force of the magnet 52 and forms a magnetic circuit together with the magnet 52. As shown in FIG. 2, the yoke 53 has an inclined surface 53a. The inclined surface 53a extends flat and perpendicular to the normal direction DN and is inclined with respect to the first end face 4a of the QCL 4. By fixing the MEMS diffraction grating 51 on the inclined surface 53a, the normal N of the diffraction grating portion 64 of the MEMS diffraction grating 51 can be inclined with respect to the first end face 4a. In this example, the diffraction grating portion 64 is inclined so as to face one side in the Z direction (the side of the lid 22 of the housing 2). However, the diffraction grating portion 64 may be inclined so as to face the other side in the Z direction (the side of the bottom surface of the housing 2). The inclination angle of the inclined surface 53a (the angle with respect to the first end face 4a) is set depending on the oscillation wavelength of the QCL 4, the number of grating grooves 64a in the diffraction grating portion 64, and the angle θ. For example, when the oscillation wavelength is in the 7 μm band and the number of grooves is 150 / mm, the inclination angle of the inclined surface 53a is set to about 30 degrees.

[0039] The yoke 53 is formed in a roughly U-shape (inverted C-shape) when viewed from the X direction, and defines an arrangement space SP that opens to an inclined surface 53a. The magnet 52 is arranged in this arrangement space SP, and the magnet 52 is housed within the yoke 53. When viewed from the X direction, the yoke 53 surrounds the magnet 52. The MEMS diffraction grating 51 is fixed to the inclined surface 53a at the edge of the support portion 61 so as to cover the opening of the arrangement space SP. When viewed from the Y direction, the MEMS diffraction grating 51 is arranged so that the first direction D1 is along (parallel to) the Z direction (the stacking direction of the stacked structure 42 of the QCL4) and the second direction D2 (axis A) is parallel to the X direction.

[0040] QCL 4 emits light having a beam shape that is elongated in the stacking direction of stacked structure 42. In laser module 1, light emitted from QCL 4 and incident on MEMS diffraction grating 51 via lens 6 has an elliptical beam shape at the position of diffraction grating section 64, with the length in first direction D1 being longer than the length in second direction D2. Furthermore, MEMS diffraction grating 51 is positioned so that all of the light emitted from QCL 4 and transmitted through lens 6 is incident on diffraction grating section 64, regardless of the swing angle of movable section 63 about axis A. In other words, even when movable section 63 is swung around axis A up to the maximum mechanical tilt angle, all of the light is incident on diffraction grating section 64.

[0041] As shown in FIG. 3, the magnet 52 and the yoke 53 generate a magnetic field M that passes through the MEMS diffraction grating 51. In this example, the magnetic field M is generated so as to cross the movable part 63 from the axis A side toward the edge of the movable part 63. The yoke 53 is formed from an iron material with added carbon, which facilitates the processing of the inclined surface 53a. The surface of the yoke 53 may be protected by zinc plating or the like to prevent deterioration.

[0042] In the MEMS diffraction grating 51, when a current flows through the coils 65 and 66, a magnetic field M formed by the magnet 52 and the yoke 53 generates a Lorentz force in a predetermined direction on electrons flowing through the coils 65 and 66. As a result, the coil 65 receives a force in a predetermined direction. Therefore, by controlling the direction or magnitude of the current flowing through the coil 65, the movable part 63 (diffraction grating part 64) can be oscillated around the axis A. Furthermore, by passing a current having a frequency corresponding to the resonant frequency of the movable part 63 through the coils 65 and 66, the movable part 63 can be oscillated at high speed at the resonant frequency level (for example, at a frequency of 1 kHz or higher). In this way, the coils 65 and 66, the magnet 52, and the yoke 53 function as an actuator that oscillates the movable part 63.

[0043] 4, the arrows indicate the direction of current flowing through coils 65 and 66. As shown in Fig. 4, in MEMS diffraction grating 51, current flows in the same direction through outer portions 65b and 66b of coils 65 and 66. This makes it possible to ensure a large driving force for oscillating movable part 63. [Action and effect]

[0044] In the laser module 1, the MEMS diffraction grating 51 is disposed such that the normal N of the diffraction grating portion 64 is inclined relative to the first end face 4a of the QCL 4. The length L1 of the diffraction grating portion 64 in the first direction D1, which is the arrangement direction of the grating grooves 64a, is longer than the length L2 of the diffraction grating portion 64 in the second direction D2 perpendicular to the first direction D1. The longer length of the diffraction grating portion 64 in the first direction D1 allows the diffraction grating portion 64 to effectively receive light from the QCL 4, even when the diffraction grating portion 64 is disposed at an angle. For example, this prevents some of the light from the QCL 4 from becoming stray light without entering the diffraction grating portion 64. Furthermore, the reduced length L2 of the diffraction grating portion 64 in the second direction D2 allows for a compact module and prevents the diffraction grating portion 64 from becoming large, thereby suppressing an increase in power consumption. Furthermore, in the laser module 1, the MEMS diffraction grating 51 is arranged such that the first direction D1 is aligned with the stacking direction (Z direction) of the stacked structure 42 when viewed from the Y direction perpendicular to the first end face 4a of the QCL 4. By arranging the MEMS diffraction grating 51 so that the first direction D1 is aligned with the stacking direction, the extension direction (second direction D2) of the grating grooves 64a in the diffraction grating portion 64 can be made perpendicular to the polarization direction of the light emitted from the QCL 4. Furthermore, by aligning the first direction D1, which is the arrangement direction of the grating grooves 64a, with the longitudinal direction of the MEMS diffraction grating 51, the number of grating grooves 64a located within the irradiation beam diameter in the tilted diffraction grating portion 64 can be increased. As a result, the wavelength resolution of the MEMS diffraction grating 51 can be ensured to be high, and the spectral linewidth of the output light can be narrowed. Therefore, the laser module 1 can achieve miniaturization, reduced power consumption, and a narrow spectral linewidth. Note that ensuring the wavelength resolution of the MEMS diffraction grating 51 and narrowing the spectral linewidth is likely to be particularly important when using a light source that emits light with a long mid-infrared wavelength, such as QCL4, whereas the direction and number of grating grooves are less of an issue when using a light with a short wavelength, such as visible light.

[0045] The following describes how increasing the number of grating grooves 64a can increase the wavelength resolution of the MEMS diffraction grating 51 and narrow the spectral linewidth. For two spectral lines with wavelengths λ and λ+Δλ that are close to each other, the index (resolution) of how small Δλ can be to distinguish between the two spectra is expressed by equation (1). λ / Δλ=mN×W …(1) m represents the diffraction order, and N×W represents the number of grating grooves 64a.

[0046] For example, when a laser module 1 with an oscillation wavelength of 8000 nm is constructed using a MEMS diffraction grating 51 with a period d of 7800 nm, a spectral linewidth of approximately 13 nm is obtained as a result of actual measurements. When Δλ is calculated for a MEMS diffraction grating 51 with a period d of 7800 nm and a length L1 of 4.2 mm, it becomes 14.8 nm using the following equation (2), which is a calculation result equivalent to the actually measured spectral linewidth. Δλ=8000nm / 1st order × (total number of 538) = 14.8nm … (2) From the above, it can be seen that by increasing the length L1 of the diffraction grating portion 64, the wavelength resolution of the diffraction grating portion 64 can be increased and the spectral linewidth can be narrowed.

[0047] The four corners 63a of the movable part 63 are rounded, which reduces the moment of inertia of the movable part 63 and allows the movable part 63 to swing at a higher speed.

[0048] The light emitted from QCL 4 and incident on MEMS diffraction grating 51 via lens 6 has a beam shape whose length in first direction D1 is longer than its length in second direction D2 at the position of diffraction grating portion 64. Because length L1 of diffraction grating portion 64 in first direction D1 is longer than length L2 of diffraction grating portion 64 in second direction D2, even in such a case, light from QCL 4 can be well received by diffraction grating portion 64.

[0049] MEMS diffraction grating 51 is positioned so that, regardless of the swing angle of movable portion 63 around axis A, all of the light emitted from QCL 4 and transmitted through lens 6 is incident on diffraction grating portion 64. This makes it possible to prevent a situation in which part of the light from QCL 4 does not enter diffraction grating portion 64 and becomes stray light. [Variations]

[0050] The present invention is not limited to the above-described embodiment and modifications. For example, the materials and shapes of each component are not limited to those described above, and various materials and shapes can be used. In the above-described embodiment, the pair of coils 65, 66 are formed using a single coil wiring connected in series. However, the coils 65, 66 may be formed separately and led out separately. However, forming the coils 65, 66 using a single coil wiring is preferable from the viewpoint of increasing current and improving controllability. In the above-described embodiment, the MEMS diffraction grating 51 is driven by an electromagnetic drive system. However, the MEMS diffraction grating 51 may also be driven by an electrostatic or piezoelectric system. In these cases, for example, electrostatic comb teeth or piezoelectric elements may be provided instead of the coils 65, 66, the magnet 52, and the yoke 53.

[0051] The shape of the movable portion 63 is not limited to a rectangular shape, and may be, for example, an elliptical shape, a circle, or a square shape. The corners 63a of the movable portion 63 do not have to be rounded, and may be corners where edges intersect at right angles. The shape of the diffraction grating portion 64 is not limited to a rectangular shape, and may be, for example, an elliptical shape. The light emitted from the QCL 4 and incident on the MEMS diffraction grating 51 via the lens 6 may have a substantially circular beam shape at the position of the diffraction grating portion 64. The magnet 52 does not need to have a recess 56.

[0052] The MEMS diffraction grating 51 may be configured as a first modified example shown in FIG. 9( a) or a second modified example shown in FIG. 9( b). In the first and second modified examples, each connecting portion 62 extends in a meandering manner. In the first modified example, the pair of connecting portions 62 are formed symmetrically with respect to a line that passes through the center of the movable portion 63 and is perpendicular to the axis A in a planar view. In the second modified example, the pair of connecting portions 62 are formed asymmetrically with respect to the line in a planar view. Similar to the above embodiment, the first and second modified examples can also achieve miniaturization, reduced power consumption, and narrowed spectral linewidth. Furthermore, the first and second modified examples are suitable for use when the movable portion 63 is operated in a non-resonant mode. When the movable portion 63 is operated in a non-resonant mode, the wavelength of the output light L can be controlled and fixed to any wavelength. [Explanation of symbols]

[0053] 1...external cavity laser module, 4...quantum cascade laser, 4a...first end face, 6...lens, 42...laminated structure, 51...MEMS diffraction grating, 63...movable part, 63a...corner part, 64...diffraction grating part, 64a...grating groove, D1...first direction, D2...second direction, N...normal, L1, L2...length.

Claims

1. a quantum cascade laser having a laminated structure and emitting light from an end face; a MEMS diffraction grating having a movable portion that can swing around a predetermined axis and a diffraction grating portion formed on the movable portion, the MEMS diffraction grating diffracting and reflecting the light emitted from the quantum cascade laser by the diffraction grating portion and feeding back a portion of the light to the quantum cascade laser; a lens disposed between the quantum cascade laser and the MEMS diffraction grating; the diffraction grating portion includes a plurality of grating grooves aligned in a first direction, and each of the plurality of grating grooves extends in a second direction perpendicular to the first direction; the MEMS diffraction grating is disposed such that a normal line of the diffraction grating portion is inclined with respect to the end face, and a projection of the first direction onto a plane having a normal line perpendicular to the end face is aligned along a stacking direction of the stacked structure, a length of the diffraction grating portion in the first direction is longer than a length of the diffraction grating portion in the second direction; The external cavity laser module, wherein the axis is parallel to the second direction.

2. the movable portion is formed in a shape having four corners when viewed in a direction parallel to the normal line, 2. The external cavity laser module according to claim 1, wherein the four corners are rounded.

3. 3. The external cavity laser module according to claim 1, wherein the light emitted from the quantum cascade laser and incident on the MEMS diffraction grating via the lens has a beam shape in which the length in the first direction is longer than the length in the second direction at the position of the diffraction grating portion.

4. An external cavity laser module according to any one of claims 1 to 3, wherein the MEMS diffraction grating is positioned so that all of the light emitted from the quantum cascade laser and transmitted through the lens is incident on the diffraction grating portion, regardless of the rocking angle of the movable portion around the axis.

5. An external cavity laser module described in any one of claims 1 to 4, wherein when viewed from a direction parallel to the normal, the outer edge of the diffraction grating portion extends along the outer edge of the movable portion at a distance from the outer edge of the movable portion.

6. The MEMS diffraction grating further has a support portion surrounding the movable portion and a connecting portion connecting the movable portion to the support portion; An external cavity laser module according to any one of claims 1 to 5, wherein when viewed from a direction parallel to the normal, the distance between the movable part and the support part in the second direction is larger than the distance between the movable part and the support part in the first direction.

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