quantum cascade laser device

Abstract

Quantum cascade laser device comprising a hollow housing (2) with an exit window (8) for emitting laser light (L) to the outside; the housing (2) contains: a heat sink (4); a submount (5) that is attached to the heat sink (4); a quantum cascade laser element (6) mounted on the submount (5); and a cover element (7) opposite the submount (5) which is to be arranged between an emission end surface (6a) of the quantum cascade laser element (6) and the exit window (8) and which has a light absorption property for the laser light (L) emitted by one emission end surface (6a) and another emission end surface (6b) of the quantum cascade laser element (6); wherein the emission end surface (6a) of the quantum cascade laser element (6) and a surface opposite the cover element (7) in the submount (5) are flush with each other; the cover element (7) has an opening (18) which is arranged at a position opposite one emission end surface (6a) in order to direct laser light (L) through it in the direction of the exit window (8); the opening (18) has: a cone-shaped first opening part (21), the diameter of which increases from the side of one emission end surface (6a) to the side of the exit window (8), a second opening part (22) with a fixed diameter that is not smaller than the smallest diameter of the first opening part (21) on the side of one emission end surface (6a) of the first opening part (21), and a third opening part (23) with a fixed diameter for positioning a lens (24), wherein the diameter of the third opening part (23) is larger than the largest diameter of the first opening part (21), wherein the third opening part (23) is located on the side of the exit window (8) and wherein a lens (24) is inserted into the third opening part (23) so that the laser light (L) emitted from the emission end surface (6a) and passing through the opening (18) exits the housing (2) through the exit window (8).

Description

Technical field

[0001] The present invention relates to a quantum cascade laser device. background

[0002] Mid-infrared light (e.g., where the wavelength is on the order of 5 µm to 30 µm) is an important wavelength range, for example, in the field of spectroscopic measurement. Attention has focused on quantum cascade lasers (QCLs) as high-power semiconductor light sources in such a wavelength range (see, for example, the published documents of the Japanese patent applications JP H08 - 279 647 A, JP 2008 - 177 366 A, JP 2008 - 60 396 A and JP H10 - 4 242 A).

[0003] Quantum cascade laser elements are monopolar laser elements that utilize a layered structure formed by subbands in a semiconductor quantum well to generate light according to the electronic transitions between the subbands. The quantum cascade laser element achieves extremely efficient, high power output through the cascaded connection of multiple stages of light-emitting quantum well layers, each consisting of a quantum well structure and serving as the active region. The cascaded connection of the light-emitting quantum well layers is achieved by employing electron injection layers to inject electrons to higher emission levels and by alternating the stacking of the light-emitting quantum well layers and the injection layers. Summary

[0004] When using a quantum cascade laser element as a single-mode laser for continuous wave operation with distributed feedback for, for example, spectroscopic measurement, it is common to connect a submount mounted on the quantum cascade laser element with a heat sink and a temperature control element and to arrange it in a nitrogen-filled housing to form an assembly and to stabilize the oscillation wavelength.

[0005] Such an assembly uses a structure in which laser light emitted from one end face of the quantum cascade laser element exits through an exit window provided in the housing. If laser light from the other end face of the quantum cascade laser element is accidentally reflected within the housing, causing stray light to escape, noise can be generated during the spectroscopic measurement. While this noise can be suppressed by operating the quantum cascade laser element in pulsed mode, interference from the stray light can compromise the basis of the measurement results and reduce the measurement sensitivity in the case of continuous-wave operation.

[0006] Therefore, prior art experiments have focused on structures in which the elements within the housing are coated with a black layer to prevent the emission of scattered light, or on structures in which a cover element with a pinhole is attached to the quantum cascade laser element. However, in practice, it is difficult to apply the black coating to all elements within the housing in the first case, while in the second case, the radiation angles of laser light in the mid-infrared range cannot be managed, thus blocking the laser light that is intended to escape.

[0007] Semiconductor laser devices with a specific aperture structure for suppressing stray light are known from US 2008 / 0116473A1 and JP H01-151285A. Furthermore, DE 102011081417A1, JP 2005-340807A, and US 6012853A deal with semiconductor laser devices in which light scattering is reduced by blackening the interior of the housing.

[0008] To solve the problems mentioned above, one objective of the present invention is to provide a quantum cascade laser device that can prevent the escape of scattered light while maintaining the emission efficiency of the laser light.

[0009] To solve the above-mentioned problems, a quantum cascade laser device according to claims 1 and 3 is provided.

[0010] In these quantum cascade laser devices, the light-absorbing cover element between one emission end face of the quantum cascade laser element and the exit window absorbs stray laser light within the housing, thus preventing stray light from escaping. The cover element has an opening located opposite one emission end face. Because the opening has a conical first part, the cover element prevents the laser light from being blocked, even if the laser light emitted from one emission end face has a large radiation angle, thus preserving the emission efficiency of the laser light exiting the window.Since the second aperture section, with a fixed diameter no smaller than the smallest diameter of the first aperture section, is located on the side of one emission end face of the first aperture section, that emission end face can be positioned closer to the aperture without coming into contact with the cover element. Even if the aperture diameter of the first aperture section is reduced, the cover element is prevented from blocking the laser light, thus suppressing the outward escape of stray light while maintaining the emission efficiency of the laser light.

[0011] The cover element can be in contact with the surface opposite the cover element in the submount, while one emission end face of the quantum cascade laser element is located at one end of the second aperture portion in the cover element. In this case, one emission end face can be positioned closer to the aperture portion. This allows for even more reliable prevention of stray light escaping.

[0012] The housing can consist of a lower main part and a lid part with the exit window, the lid part having a black inner surface. In this case, the inner surface of the lid part absorbs stray light, thus preventing the escape of stray light even more reliably.

[0013] The entire inner surface of the housing can be black. In this case, the entire inner surface of the housing absorbs stray light, thus preventing the escape of stray light even more reliably.

[0014] The housing can be filled with dry nitrogen. This prevents the formation of condensation inside the housing.

[0015] The quantum cascade laser element can be a distributed feedback continuous-wave laser. When the quantum cascade laser element is used for spectroscopic measurements, interference from scattered light can compromise the basis of the measurement results and thus reduce the measurement sensitivity. The structure described above prevents scattered light from escaping, thereby advantageously suppressing interference from scattered light during the spectroscopic measurement.

[0016] As explained above, one aspect of the present invention can prevent stray light from escaping while maintaining the emission efficiency of the laser light. Brief description of the drawings Fig. Figure 1 is a sectional view of an embodiment of a quantum cascade laser device; Fig. Figure 2 is a graph of an example of the radiation angles of the laser light emitted by a quantum cascade laser element; Fig. 3 is a diagram of the transmittance characteristics of synthetic resin materials in a mid-infrared range; Fig. Figure 4 is an enlarged cross-sectional view of a main part of the quantum cascade laser device made of Fig. 1; Fig. 5 is a diagram of the results of an experiment on scattered light emission; and Fig. Figure 6 is a sectional view of a modified example of the quantum cascade laser device. Detailed description

[0017] Preferred embodiments of the quantum cascade laser device according to the present invention are described below with reference to the drawings.

[0018] Fig. Figure 1 is a sectional view of an embodiment of the quantum cascade laser device. As shown in this drawing, the quantum cascade laser device 1 comprises a housing 2, a temperature control element 3, a heat sink 4, a submount 5, a quantum cascade laser element 6, and a cover element 7. The quantum cascade laser device 1 serves, for example, as a light source for spectroscopic measurement and uses a structure in which laser light L (see Figure 1) is directed onto the quantum cascade laser. Fig. 4), which is emitted by the quantum cascade laser element 6, exits from an exit window 8 of the housing 2.

[0019] The housing 2 has a substantially rectangular, parallel-flat shape, made of, for example, a metal. The housing 2 consists of a lower main part 9 and a cover part 10. The main part 9 has a thick, flat bottom part 11 and a side part 12 that rises from the bottom part 11.

[0020] The lower end of the side part 12 is, for example, permanently connected to a surface of the base part 11 by welding. Feed-through terminals, conductors, and the like, used to control the temperature control element 3 and the quantum cascade laser element 6, are arranged on the side part 12 as needed. The base part 11 is slightly larger than the cover part 10 and has a projection extending outwards from four surfaces of the side part 12 in the top view of the housing 2. This ensures the mounting stability of the quantum cascade laser device 1.

[0021] The cover part 10 has a flat shape and is thinner than the base part 11. An edge portion of the cover part 10 is firmly connected to the front end of the side part 12, for example, by welding. The housing 2 has an interior S, which is hermetically sealed by the main part 9 and the cover part 10. The interior S is filled, for example, with dry nitrogen, which prevents condensation from forming in the housing 2. Near the center of the cover part 10, the round exit window 8 is formed to allow the laser light L emitted by the quantum cascade laser element 6 to exit the housing 2. Examples of materials that can be used for the exit window 8 include Ge and ZnSe.

[0022] The inner surfaces of the housing 2, i.e., the part facing the interior S in one surface of the base 11, the inner surface of the side 12, and the inner surface of the lid 10, are each black. This black finish can be achieved, for example, by applying a black resin coating containing carbon. Such a black finish gives the inner surfaces of the housing 2 light-absorbing properties for the laser light L emitted by the quantum cascade laser element 6.

[0023] The aforementioned temperature control element 3, the heat sink 4, the submount 5, the quantum cascade laser element 6 and the cover element 7 are contained in the interior S of the housing 2.

[0024] The temperature control element 3 is a component that controls the temperature of the quantum cascade laser element 6 according to an external control signal in order to stabilize the oscillation wavelength of the quantum cascade laser element 6. For example, a Peltier element is used as the temperature control element 3. A surface 3a of the temperature control element 3 is attached, for example, by soldering to the base 11 of the housing 2.

[0025] The heat sink 4, also known as a package heat sink, is a component through which the heat generated in the quantum cascade laser element 6 is dissipated towards the temperature control element 3. The heat sink 4 is made of a material with excellent thermal conductivity, such as copper. One surface 4a of the heat sink 4 is attached, for example, by soldering to the other surface 3b of the temperature control element 3. The heat sink 4 has a mounting part 13 to which the submount 5 is mounted, a positioning part 14 for positioning the submount 5 on the mounting part 13, and an extension 15 extending from the mounting part 13.

[0026] The mounting part 13 has a mounting surface 13a that is substantially parallel to a surface 4a of the heat sink 4. The positioning part 14 rises substantially parallel to the side part 12 of the housing 2 from the lower end of the mounting surface 13a and has a contact surface 14a against which an end surface 5a of the submount 5 rests. The extension 15 has an opposing surface 15a that faces an emission end surface 6b (which will be explained later) of the quantum cascade laser element 6 at a position one step lower than the front end of the mounting surface 13h. The opposing surface 15a is inclined at an obtuse angle to the emission end surface 6b (the optical axis of the laser light L from the emission end surface 6b), such that it is further away from the optical axis of the emission end surface 6b with increasing distance from the emission end surface 6b.Preferably, the opposite surface 15a, as well as the inner surfaces of the housing 2, are black.

[0027] The submount 5, also known as the laser heat sink, is a component to which the quantum cascade laser element 6 is attached and which dissipates the heat generated in the quantum cascade laser element 6 towards the heat sink 4. The submount 5 has an essentially rectangular, parallel-flat shape made of a material with excellent thermal conductivity, such as copper. The submount 5 is mounted on the mounting surface 13a, such that an end surface 5a rests against the contact surface 14a, and is securely fixed to the heat sink 4, for example, by screws.

[0028] The other end part of the submount 5 is a mounting surface 5b to which the quantum cascade laser element 6 is attached. The mounting surface 5b is flush with a step-side surface 13b between the mounting surface 13a and the opposite surface 15a at a position corresponding to the exit window 8. The surface opposite the mounting surface 13a in the submount 5 is an opposing surface 5c, which is opposite the cover element 7. The opposing surface 5c is flush with a front end surface 14b of the positioning part 14. The electrode pads used to control the quantum cascade laser element 6, which are connected to the electrode pads, and other components not shown, are also attached to the mounting surface 5b.

[0029] The quantum cascade laser element 6 is a monopolar laser element that utilizes a plane structure formed by subbands in a semiconductor quantum well structure to generate light according to the electronic transitions between the subbands. The quantum cascade laser element 6 operates in continuous-wave mode and features a distributed feedback structure that includes a diffraction grating to generate a longitudinal single-mode spectrum as a light source for spectroscopic measurement.

[0030] The quantum cascade laser element 6 is attached to the mounting surface 5b of the submount 5, such that one emission end surface 6a and the other emission end surface 6b face the exit window 8 and the opposite surface 15a of the heat sink 4, respectively. The emission end surface 6a is flush with the front end surface 14b of the positioning part 14 in the heat sink 4 and the opposite surface 5c of the submount 5. "Flush" here means that the laser light L emitted from the emission end surface 6a does not strike the mounting surface 5b of the submount 5.

[0031] In the distributed feedback structure, only one wavelength corresponding to the period of the diffraction grating is selectively fed back to achieve a single-mode oscillation. In such a structure, an antireflective coating is applied to one emission end surface 6a (or to both emission end surfaces 6a and 6b) of the quantum cascade laser element 6 to prevent the generation of further Fabry-Perot modes. A highly reflective coating can be applied to the other emission end surface 6b to suppress scattered laser light L within the housing 2.

[0032] However, if the reflectivity of the emission end surface 6b is increased by the high-reflectivity coating, competition with other modes occurs more readily, so that stable single-mode oscillation may not be achieved. Therefore, the emission end surface 6b of the quantum cascade laser element 6 is left as a split end surface without the high-reflectivity coating.

[0033] The quantum cascade laser element 6 also features a refractive index guide structure in which a light-emitting layer is positioned between cladding layers, while light propagating along the waveguide is emitted into free space from the element's end faces (i.e., the emission end faces 6a, 6b). Since this refractive index guide structure can be considered a slit where the thickness portion of the active layer is an opening, the laser light L emitted into free space has a fixed radiation angle due to a diffraction effect, similar to the case when light emerges from a tiny slit.

[0034] The diffraction effect of light becomes even more pronounced with longer wavelengths. For spectroscopic measurements, the restriction to the transverse single mode exists partly to improve the refraction effect of the light. The emission angle, for example, is approximately 30° (±15° if the perpendicular to the end surface is taken as 0°) in the near-infrared range, which serves as a communication wavelength band, but is 100° or more in the mid-infrared range with a wavelength of 3 µm or longer. Fig. Figure 2 is a diagram of a far-field pattern in one growth direction of a distributed feedback quantum cascade laser element with a wavelength of 7.2 µm. The diagram shows data obtained by fitting actual measured values ​​with a Gaussian function, indicating that the emission angle extends beyond 140°.

[0035] To obtain the laser light L from the emission end face 6a of the quantum cascade laser element 6, it is therefore necessary to design the interior of the housing 2 such that the laser light with a wide emission angle is not blocked. The quantum cascade laser element 6, on the other hand, emits the laser light L from the emission end face 6b with an emission angle similar to that of the emission end face 6a. Therefore, the laser light L emitted from the emission end face 6b can be randomly reflected within the housing 2, allowing stray light to escape from the exit window 8 and cause noise at the time of spectroscopic measurement. Thus, a structure is needed that can prevent the escape of stray light while simultaneously maintaining the emission efficiency of the laser light L from the exit window 8.

[0036] The cover element 7 is a part that suppresses stray light in the housing 2. As shown in Fig. As shown in Figure 1, the cover element 7 is located opposite the submount 5, such that it is positioned between an emission end face 6a of the quantum cascade laser element 6 and the exit window 8. The cover element 7 has a planar main part 16, which is provided with an opening 18 through which the laser light L can pass towards the exit window 8, and a planar extension part 17, which is bent from an end part of the main part 16 at substantially right angles to the main part 16.

[0037] The main part 16 is arranged such that it comes into contact with a front end surface 14b of the positioning part 14 in the heat sink 4 and with the opposite surface 5c of the submount 5 at a position where the opening 18 is opposite one emission end surface 6a. The heat sink 4 has a mounting surface that is aligned with the opposite surface 5c of the submount 5 in the depth direction. Fig. 1 is flush with the outer surface of the positioning part 14 in the heat sink 4, while the main part 16 is firmly attached to the mounting surface of the heat sink 4 by screws or adhesive. One end surface 16a of the main part 16 is flush with the outer surface of the positioning part 14 in the heat sink 4, whereas the other end surface 16b of the main part 16 protrudes from the mounting surface 5b of the submount 5, so that it is located on one side of the extension 15 of the heat sink 4 beyond the position of the outlet window 8.

[0038] The extended part 17 extends from the other end face of the main part 16 to the opposite surface 15a of the heat sink 4, essentially parallel to the mounting surface 5b of the submount 5, but separated from the quantum cascade laser element 6. A front end face 17a of the extended part 17 is located on the side of the opposite surface 15a of the heat sink 4 behind the position of the other emission end face 6b of the quantum cascade laser element 6. The extended part 17 may be thicker than the main part 16.

[0039] Preferably, a material with light absorption properties for wavelengths in the mid-infrared range is used for the cover element 7. Fig. Figure 3 is a diagram of the transmission characteristics of synthetic resin materials in the mid-infrared range. The examples shown in the diagram, whose abscissa and ordinate respectively indicate the wavenumber (inverse of the wavelength) and transmittance, plot the transmission characteristics of acrylic (Graph A), polyoxymethylene (POM) (Graph B), and polyetheretherketone (PEEK) (Graph C).

[0040] The results in the diagram suggest that the POM or PEEK resin is preferably selected as the material for forming the cover element 7 in the mid-infrared range (where the wavelengths range, for example, from 3 µm to 11 µm), provided that the transmittance is sufficient across the entire wavelength range. The PEEK resin is particularly preferred with regard to heat resistance, chemical resistance, mechanical strength, electrical insulation, ease of processing, and the like. Preferably, the front surface of the cover element 7, as well as the inner surfaces of the housing 2, are black.

[0041] Not only synthetic resins but also ceramic materials can be used as materials for forming the cover element 7. In this case, the cover element 7b can be made of Al₂O₃ or Al₃, and an antireflective coating can subsequently be applied to the front surface.

[0042] As in Fig. As shown in Figure 4, the opening 18 in the main part 16 has, in detail, a first opening part 21, a second opening part 22, and a third opening part 23. The first opening part 21 is located in the middle in the thickness direction of the main part and has a conical shape, the diameter of which extends from an emission end surface 6a towards the side of the exit window 8 (see Figure 4). Fig. 1) becomes larger.

[0043] The cone angle of the first opening section 21 is determined according to the emission angle of the laser light L emitted from the emission end surface 6a. For example, if the emission angle of the laser light L is 140°, the cone angle of the first opening section 21 is set to 140° (±70° if the direction perpendicular to the emission end surface 6a is taken as 0°) or greater. If the thickness of the main part 16 of the cover element 7 is 0.8 mm, the smallest diameter of the first opening section (the diameter on the side of the emission end surface 6a) is set to approximately 1.5 mm ±0.5 mm. The length of the first opening section 21 in the thickness direction of the main part 16 (the direction perpendicular to the emission end surface 6a) is set to approximately 0.3 mm.

[0044] The second opening section 22 is located on the side of the emission end surface 6a and has a fixed diameter that is not smaller than the smallest diameter of the first opening section 21. For example, the diameter of the second opening section 22 is approximately 2.6 mm. The length of the second opening section 22 in the thickness direction of the main section 16 is, for example, set to approximately 0.2 mm.

[0045] The third opening section 23 is located on the side of the exit window 8 and has a fixed diameter that is larger than the largest diameter of the second opening section 22 (the diameter on the side of the exit window 8). The third opening section 23 serves to position a lens, in this embodiment a plano-convex lens 24 being inserted therein. The laser light L from the emission end surface 6a passes through the opening 18 at a predetermined emission angle, is then aligned by the plano-convex lens 24 to parallel light, and exits the housing 2 through the exit window 8. The surface of the plano-convex lens 24 on the side of the cover element 7 is preferably provided with an antireflective coating. The length of the third opening section 23 in the thickness direction of the main part 16 is, for example, set to approximately 0.3 mm.

[0046] As explained above, the light-absorbing cover element 7 is arranged between an emission end face 6a of the quantum cascade laser element 6 and the exit window 8 in the quantum cascade laser device 1. As shown in Fig. As shown in Figure 4, the cover element 7 absorbs scattered light caused by the reflection of the laser light L at the opposite surface 15a of the heat sink 4 after it has been emitted from the other emission end surface 6b, and scattered light caused by the reflection of the laser light L at the inner surfaces of the housing 2, thereby preventing the escape of scattered light. By suppressing the escape of scattered light, the problem of scattered light interference affecting the basis of the measurement results and thus reducing the measurement sensitivity is advantageously eliminated when the quantum cascade laser element 6 of the distributed feedback continuous-wave laser type is used for spectroscopic measurement.

[0047] The cover element 7 is provided with an opening 18 at a position opposite an emission end surface 6a. Since the opening 18 is provided with the conical first opening part 21, it is possible to prevent the cover element 7 from blocking the laser light L, even if the laser light L emitted from the exit window 8 has a large radiation angle, so that the emission efficiency of the laser light L from the exit window 8 can be maintained.

[0048] In contrast, the second opening section 22, which has a fixed diameter no smaller than the smallest diameter of the first opening section 21, is located on the side of the emission end surface 6a of the first opening section 21. This allows the emission end surface 6a to be positioned close to the opening 18 without coming into contact with the cover element 7. Even if the opening diameter of the first opening section 21 is reduced, the cover element 7 is prevented from blocking the laser light L, thus suppressing the outward emission of stray light and maintaining the emission efficiency of the laser light L. Furthermore, the second opening section 22, whose diameter is no smaller than the smallest diameter of the first opening section 21, prevents the emission end surface 6a from coming into contact with the cover element 7 when the cover element 7 is mounted on the submount 5.

[0049] In the quantum cascade laser device 1, the main part 16 of the cover element 7 is in contact with the opposite surface 5c of the submount 5, while an end surface 6a of the quantum cascade laser element 6 is arranged at the opening end of the second opening part 22 in the cover element 7. This allows one emission end surface 6a to be positioned closer to the opening 18.

[0050] Fig. Figure 5 is a diagram of the results of an experiment on stray light emission. In this experiment, a highly reflective coating was applied to an emission end face 6a of the quantum cascade laser element 6, such that the laser light L was emitted only from the other emission end face 6b, and the intensity of the laser light L emitted by the exit window 8 was measured as stray light emission while the aperture diameter (the smallest diameter) of the first aperture portion 21 was changed. The results in the diagram show that the stray light emission, which was 26 mW when the cover element 7 was not attached, gradually decreased when the aperture diameter of the first aperture portion 21 was narrowed by attaching the cover element 7.

[0051] The stray light output was approximately 16 mW and approximately 9.8 mW when the aperture diameter of the first aperture section 21 was 6 mm and 4 mm, respectively. The stray light output was approximately 4.6 mW and approximately 1.73 mW when the aperture diameter of the first aperture section 21 was 2.6 mm and 1.6 mm, respectively. The stray light output was approximately 1.55 mW when the aperture diameter of the first aperture section 21 was 0.8 mm. The laser light L emitted from one emission end face 6a appears to be blocked by the aperture 18 when the aperture diameter of the first aperture section 21 is less than 1 mm. Therefore, the opening diameter of the first opening part 21 must be in a range of at least 1 mm but not more than 2 mm, and can then advantageously prevent the escape of scattered light to the outside and maintain the emission efficiency of the laser light L.

[0052] In the quantum cascade laser device 1, the aperture 18 further comprises a third aperture section 23 for arranging the plano-convex lens 24 on the side of the exit window 8 of the first aperture section 21. This allows the plano-convex lens 24 to be positioned with a simple structure. The lens positioned by the third aperture section 23 need not necessarily be a plano-convex lens 24, but can be modified according to the specifications of the quantum cascade laser device 1.

[0053] In the quantum cascade laser device 1, the cover element 7 has an extended part 17 that extends along the mounting surface 5b of the submount 5 onto the side of the heat sink 4 of the other emission end face 6b of the quantum cascade laser element 6. The extended part 17 of the cover element 7 can efficiently absorb the laser light L emitted from the other emission end face 6b. This further prevents stray light from escaping.

[0054] In the quantum cascade laser device 1, the heat sink has a surface 15a opposite the other emission end surface 6b of the quantum cascade laser element 6, which is inclined. Because the opposite surface 15a is inclined, it prevents the laser light L emitted from the other emission end surface 6b from being reflected normally towards and returning to the quantum cascade laser element 6 (see Fig. 4) This stabilizes the operation of the quantum cascade laser element 6. In this embodiment, the opposite surface 15a is black and can thus suppress the influence of reflected light even more reliably.

[0055] In the quantum cascade laser device 1, all internal surfaces of the housing 2 are black. This ensures that stray light is absorbed by all internal surfaces of the housing 2 and can be prevented from escaping even more reliably. Alternatively, instead of all internal surfaces of the housing 2, only the inside of the cover part 10 can be black. If the inside of the cover part 10, which contains the exit window, is black, it can efficiently absorb stray light.

[0056] Fig.Figure 6 is a sectional view of the quantum cascade laser device according to a modified embodiment. As shown in this drawing, the quantum cascade laser device 1 according to the modified embodiment differs from the first embodiment in that the cover element 7 is not provided with the extended part 17. More precisely, in the quantum cascade laser device 1, the main part 16 of the cover element 7 projects beyond the mounting surface 5b of the submount 5, so that the other end surface 16b extends beyond the position of the emission window 8 to the position of the front end surface 15b of the extension 15 of the heat sink 4. With such a structure, the laser light emitted from the other emission end surface 6b of the quantum cascade laser element 6 can be efficiently absorbed by the cover element 7, and the emission of scattered light is prevented. Furthermore, this simplifies the shape of the cover element 7. List of reference symbols

[0057] 1: Quantum cascade laser device; 2: Housing; 4: Heat sink; 5: Submount; 5b: Mounting surface; 5c: Opposite surface; 6: Quantum cascade laser element; 6a: One emission end surface; 6b: The other emission end surface; 7: Cover element; 8: Exit window; 9: Main part; 10: Cover part; 15a: Opposite surface; 17: Extended part; 18: Opening; 21: First opening part; 22: Second opening part; 23: Third opening part; L: Laser light.

Claims

[1] Quantum cascade laser device comprising a hollow housing (2) with an exit window (8) for emitting laser light (L) to the outside; the housing (2) contains: a heat sink (4); a submount (5) that is attached to the heat sink (4); a quantum cascade laser element (6) mounted on the submount (5); and a cover element (7) opposite the submount (5) which is to be arranged between an emission end surface (6a) of the quantum cascade laser element (6) and the exit window (8) and which has a light absorption property for the laser light (L) emitted by one emission end surface (6a) and another emission end surface (6b) of the quantum cascade laser element (6); wherein the emission end surface (6a) of the quantum cascade laser element (6) and a surface opposite the cover element (7) in the submount (5) are flush with each other; the cover element (7) has an opening (18) which is arranged at a position opposite one emission end surface (6a) in order to direct laser light (L) through it in the direction of the exit window (8); the opening (18) has: a cone-shaped first opening part (21), the diameter of which increases from the side of one emission end surface (6a) to the side of the exit window (8), a second opening part (22) with a fixed diameter that is not smaller than the smallest diameter of the first opening part (21) on the side of one emission end surface (6a) of the first opening part (21), and a third opening part (23) with a fixed diameter for positioning a lens (24), wherein the diameter of the third opening part (23) is larger than the largest diameter of the first opening part (21), wherein the third opening part (23) is located on the side of the exit window (8) and wherein a lens (24) is inserted into the third opening part (23) so that the laser light (L) emitted from the emission end surface (6a) and passing through the opening (18) exits the housing (2) through the exit window (8). [2] Quantum cascade laser device according to claim 1, wherein the cover element (7) is in contact with the surface opposite the cover element (7) in the submount (5); and wherein one emission end surface (6a) of the quantum cascade laser element (6) is located at an opening end of the second opening part (22) in the cover element (7). [3] Quantum cascade laser device comprising a hollow housing (2) with an exit window (8) for emitting laser light (L) to the outside; the housing (2) contains: a heat sink (4); a submount (5) that is attached to the heat sink (4); a quantum cascade laser element (6) mounted on the submount (5); and a cover element (7) opposite the submount (5) which is to be arranged between an emission end surface (6a) of the quantum cascade laser element (6) and the exit window (8) and which has a light absorption property for the laser light (L) emitted by one emission end surface (6a) and another emission end surface (6b) of the quantum cascade laser element (6); wherein the emission end surface (6a) of the quantum cascade laser element (6) and a surface opposite the cover element (7) in the submount (5) are flush with each other; wherein the heat sink (4) has an opposite surface (15a) which is in an inclined state opposite the other emission end surface (6b) of the quantum cascade laser element (6); wherein the cover element (7) has an opening (18) which is arranged at a position opposite one emission end surface (6a) in order to direct laser light (L) through it in the direction of the exit window (8); wherein the opening (18) has a cone-shaped first opening part (21), the diameter of which increases from the side of one emission end surface (6a) to the side of the exit window (8), and a second opening part (22) with a fixed diameter that is not smaller than the smallest diameter of the first opening part (21) on the side of one emission end surface (6a) of the first opening part (21), wherein the cover element (7) has a planar main part (16) in which the opening (18) is formed, and a planar extended part (17) which is bent substantially perpendicularly to the main part (16) from an end part of the main part (16), wherein the extended part (17) extends substantially parallel to a surface (5b) of the submount (5) for mounting the quantum cascade laser element (6) to the opposite surface (15a) of the heat sink (4) and is separated from the quantum cascade laser element (6), and wherein a front end surface (17a) of the extended part (17) is located on the side of the opposite surface (15a) of the heat sink (4) behind the position of the other emission end surface (6b) of the quantum cascade laser element (6). [4] Quantum cascade laser device according to one of claims 1 to 3, wherein the housing (2) consists of a lower main part (9) and a cover part (10) in which the exit window (8) is arranged; and wherein the cover part (10) has a black interior. [5] Quantum cascade laser device according to claim 4, wherein the entire inner surface of the housing (2) is black. [6] Quantum cascade laser device according to one of claims 1 to 5, wherein the housing (2) is filled with dry nitrogen. [7] Quantum cascade laser device according to any one of claims 1 to 6, wherein the quantum cascade laser element (6) is a distributed feedback continuous wave laser.

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