Light detection module and beat frequency spectrometer

Abstract

The present disclosure provides a light detection module (1) having a light detector (10) and a fixing member (50). The light detector (10) has: a semiconductor substrate (11); a mesa portion (12); a first contact layer (13); a second contact layer (14); and a first electrode (15) formed in a planar shape on a main surface (11a) of the semiconductor substrate (11) and electrically connected to one of the first contact layer (13) and the second contact layer (14). The fixing member (50) has: an insulating substrate (51); and a first wiring (52) formed in a planar shape on a main surface (51a) of the insulating substrate (51). A recess (54) is formed in the main surface of the insulating substrate (51), and at least a portion of the mesa portion (12) is disposed in the recess (54). The first electrode (15) is electrically connected to the first wiring (52) in a state of being in surface contact with the first wiring (52).

Description

TECHNICAL FIELD

[0001] One aspect of the present disclosure relates to an optical detection module and a beat spectrometer. BACKGROUND

[0002] A quantum cascade detector is disclosed in Patent Literature 1. In the quantum cascade detector, light is detected by using inter-subband transition (inter-subband absorption) in a quantum well structure.

[0003] Prior Art Documents

[0004] Patent Literature 1: Japanese Patent Application Publication No. 2017-147428 SUMMARY

[0005] Problems to be Solved by the Invention

[0006] In the optical detector as described above, a high speed of response speed is required. In addition, it is also required to ensure reliability at the same time. Therefore, an object of one aspect of the present disclosure is to provide an optical detection module capable of achieving a high speed of response speed and ensuring reliability, and a beat spectrometer capable of expanding a wavelength scanning range.

[0007] Technical Solution for Solving the Problem

[0008] The optical detection module according to one aspect of the present disclosure includes an optical detector, and a fixing member that fixes the optical detector. The optical detector includes a semiconductor substrate having a main surface, a mesa portion including a semiconductor region that generates an electric signal according to an amount of incident light, and formed on the main surface of the semiconductor substrate, a first contact layer formed on a surface on the opposite side of the semiconductor substrate in the mesa portion, a second contact layer formed between the main surface of the semiconductor substrate and the mesa portion, and a first electrode formed in a planar shape on the main surface of the semiconductor substrate, and electrically connected to one of the first contact layer and the second contact layer. The fixing member includes an insulating substrate having a main surface, and a first wiring formed in a planar shape on the main surface of the insulating substrate, a recess portion is formed on the main surface of the insulating substrate, at least a part of the mesa portion is disposed in the recess portion, and the first electrode is electrically connected to the first wiring in a state of being in surface contact with the first wiring.

[0009] In the light detection module, the first electrode on the semiconductor substrate in the photodetector is electrically connected to the first wiring on the insulating substrate in a state of being in surface contact with the first wiring. Thus, compared to a case where the first electrode and the first wiring are connected by wire bonding, for example, inductance due to a wire can be avoided, and the inductance can be reduced. Therefore, the response speed can be made high speed. Further, by connecting the first electrode and the first wiring in a state of being in surface contact, the photodetector and the fixing member can be securely fixed, and reliability can be ensured. Furthermore, in the light detection module, at least a part of the mesa portion of the photodetector is disposed in a recess formed in the insulating substrate. Thus, the thin mesa portion can be protected, and reliability can be ensured. Therefore, according to the light detection module, high speed of the response speed can be achieved and reliability can be ensured.

[0010] The semiconductor region of the mesa portion can include an active layer in which an absorption region that absorbs the detection light by inter-sub-band absorption and a transport region that transports electrons excited by the inter-sub-band absorption are alternately laminated. In the case where the mesa portion has such an active layer, reduction of inductance becomes particularly important from the viewpoint of high speed of the response speed. In this regard, in the light detection module, as described above, the inductance can be reduced, and high speed of the response speed can be achieved.

[0011] One end surface of the mesa portion in a direction parallel to the main surface of the semiconductor substrate can serve as an incident surface of the detection light. In this case, the light can be efficiently absorbed in the mesa portion, and the intensity of the output signal can be ensured.

[0012] One end surface of the mesa portion in a direction parallel to the main surface of the semiconductor substrate is the same surface as a side surface of the semiconductor substrate. In this case, the end surface of the mesa portion can be easily used as the incident surface of the detection light.

[0013] The light detection module according to one aspect of the present disclosure further includes a lens disposed so as to be opposed to the end surface of the mesa portion, and concentrates the detection light toward the end surface of the mesa portion. In this case, the width (length in a direction perpendicular to the light waveguide direction) of the end surface of the mesa portion can be narrowed, and the area of the mesa portion in a plan view can be reduced. As a result, the parasitic capacitance can be reduced, and the response speed can be further made high speed. Further, assuming that the area of the mesa portion is constant, by narrowing the width of the mesa portion, the length of the mesa portion in the light waveguide direction can be made long. Therefore, the light can be efficiently absorbed in the mesa portion, and the intensity of the output signal can be ensured.

[0014] The recess can be open at a side surface of the insulating substrate. In this case, the detection light can be easily made incident to the end surface of the mesa portion. Alternatively, the recess can also be constituted by a hole formed in the main surface of the insulating substrate in such a manner as to be apart from the outer edge of the main surface.

[0015] The light detector can further have a connection wiring electrically connecting one of the first electrode, the first contact layer, and the second contact layer, at least a part of the connection wiring being disposed in the recess. In this case, the connection wiring can be protected, and reliability can be ensured.

[0016] The connection wiring can be an air bridge wiring in a sheet shape. In this case, by being disposed in the recess, the fine air bridge wiring can be protected, and reliability can be ensured.

[0017] The light detector can further have a second electrode formed in a planar shape on the main surface of the semiconductor substrate and electrically connected to the other of the first contact layer and the second contact layer. The fixing member can further have a second wiring formed in a planar shape on the main surface of the insulating substrate. The second electrode can be electrically connected to the second wiring in a state of being in surface contact with the second wiring. In this case, compared to a case where the second electrode and the second wiring are connected by wire bonding, for example, the inductance can be reduced, and the response speed can be further increased. Further, by connecting the second electrode and the second wiring in a state of being in surface contact, the light detector and the fixing member can be firmly fixed, and reliability can be ensured.

[0018] The second contact layer can have, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion. The second electrode can be formed on the second portion of the second contact layer. In this case, the area of the second electrode can be ensured to be large, and the contact area between the second electrode and the second wiring can be ensured to be large.

[0019] The first electrode can be in surface contact with the first wiring via a solder layer, and the second electrode can be in surface contact with the second wiring via a solder layer. In this case, the first electrode and the first wiring can be electrically and mechanically firmly connected, and the second electrode and the second wiring can be electrically and mechanically firmly connected. Further, when the first electrode and the first wiring are connected and the second electrode and the second wiring are connected, even in a case where there is a height difference between the first electrode and the second electrode, the height difference can be absorbed by the solder layer, and good surface contact can be achieved at each connection portion.

[0020] The light detector can have a height adjustment layer formed in a planar shape on the main surface of the semiconductor substrate and electrically separated from the second contact layer. The first electrode can be formed on the height adjustment layer. In this case, when the first electrode and the first wiring are connected and the second electrode and the second wiring are connected, even in the case where there is a height difference between the first electrode and the second electrode, the height difference between the first electrode and the second electrode can be reduced by the height adjustment layer, and good surface contact can be achieved at each connection portion.

[0021] The first wiring can have a length of 1 / 4 or less of the wavelength of the electrical signal propagating in the first wiring. In this case, impedance matching can be achieved.

[0022] Both side surfaces of the mesa portion in a direction perpendicular to the direction of the optical waveguide can be exposed. In this case, generation of parasitic capacitance can be suppressed, and the response speed can be further increased.

[0023] The mesa portion can be separated from the inner surface of the recess. In this case, the mesa portion can be effectively protected.

[0024] A beat spectrometer according to one aspect of the present disclosure includes a wavelength-fixed light source, a wavelength-variable light source, and a light detection module described above, detects light from the wavelength-fixed light source and light from the wavelength-variable light source as detection light, changes the wavelength of the light from the wavelength-variable light source in a manner that scans the frequency of a beat signal having a frequency corresponding to the wavelength difference between the light from the wavelength-fixed light source and the light from the wavelength-variable light source, and detects the light from the wavelength-fixed light source and the light from the wavelength-variable light source by the light detector. In the light detection module provided with the beat spectrometer, the response speed is increased based on the reasons described above. Therefore, in the beat spectrometer, the wavelength scanning range in beat spectroscopy can be expanded.

[0025] Effects of the Invention

[0026] According to one aspect of the present disclosure, a light detection module capable of achieving an increase in response speed and ensuring reliability and a beat spectrometer capable of expanding the wavelength scanning range can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a front view of the light detection module of the embodiment.

[0028] Figure 2 is a perspective view of the light detector.

[0029] Figure 3 is a plan view of the light detector.

[0030] Figure 4is a front view of the light detector.

[0031] Figure 5 is a table showing the structure of the light detector.

[0032] Figure 6 is a plan view of the fixing member.

[0033] Figure 7 is a perspective view showing the mounting state of the light detection module.

[0034] Figure 8 is a plan view showing the mounting state of the light detection module.

[0035] Figure 9 is a front view showing the mounting state of the light detection module.

[0036] Figure 10 is a graph showing the response characteristic of the light detector.

[0037] Figure 11 is a graph showing an example of the output signal in the high frequency region.

[0038] Figure 12 (a) of FIG. 1 is a front view of a light detector of an embodiment, Figure 12 (b) of FIG. 1 is a front view of a light detector of a first modification example.

[0039] Figure 13 is a cross-sectional view of a light detection module according to a second modification example.

[0040] Figure 14 is a plan view of a fixing member according to the second modification example.

[0041] Figure 15 is a plan view of a light detector according to a third modification example.

[0042] Figure 16 is a diagram showing the structure of a beat frequency spectrometer.

[0043] Figure 17 is a graph showing the sensitivity characteristic of the light detector and the oscillation wavelength of the wavelength-fixed light source.

[0044] Figure 18 is a graph showing the oscillation wavelengths of the wavelength-fixed light source and the wavelength-variable light source.

[0045] Figure 19 is a graph showing the relationship between the injection current and the oscillation wavelength of the wavelength-variable light source.

[0046] Figure 20 is a graph showing the relationship between the injection current and the beat frequency of the wavelength-variable light source.

[0047] Figure 21 is a graph showing an example of beat signal.

[0048] Figure 22 is a graph showing a comparison result of response characteristics.

[0049] Figure 23 is a graph showing a measurement result of beat spectroscopy.

[0050] Figure 24 is a graph showing a measurement result of beat spectroscopy.

[0051] Figure 25 is a graph showing a measurement result of beat spectroscopy. DETAILED DESCRIPTION

[0052] Hereinafter, one embodiment of the present application will be explained in detail with reference to the attached drawings. In the following explanation, the same or corresponding elements are denoted with the same reference numerals, and repetitive explanation is omitted.

[0053] [Light detection module]

[0054] As shown in Figure 1 , the light detection module 1 is provided with a light detector 10, and a fixing member (sub-mount) 50 that fixes the light detector 10. The light detector 10 is, for example, a Quantum Cascade Detector (QCD) that detects a detection light DL using inter-subband transition (inter-subband absorption) in a quantum well structure.

[0055] [Light detector]

[0056] As shown in Figure 2 , Figure 3 , and Figure 4 , the light detector 10 is provided with a semiconductor substrate 11, a mesa portion 12, a first contact layer 13, a second contact layer 14, a first electrode 15, an air-bridge wiring 16, and a second electrode 17.

[0057] The semiconductor substrate 11 is formed, for example, in a rectangular flat plate shape, and has a flat main surface 11a. The semiconductor substrate 11 is, for example, a semi-insulating InP substrate. Hereinafter, a width direction, a depth direction, and a thickness direction of the semiconductor substrate 11 will be explained as an X direction, a Y direction, and a Z direction, respectively. The X direction, the Y direction, and the Z direction are orthogonal to each other. Further, in Figures 2-4 , each structure of the light detector 10 is arranged in reverse with respect to the X direction, compared to Figure 1 . Actually, each structure of the light detector 10 is arranged in reverse with respect to the X direction, compared to Figures 2-4The illustrated configuration is a reverse configuration with respect to the X direction. In summary, as described later, each element can be configured in a manner in which the first electrode 15 is connected to the first wiring 52 and the second electrode 17 is connected to the second wiring 53.

[0058] The mesa portion 12 is formed on the main surface 11a of the semiconductor substrate 11 in a manner extending in the optical waveguide direction A. The optical waveguide direction A is parallel to the Y direction. The mesa portion 12 is formed on the main surface 11a via the second contact layer 14, and protrudes in the Z direction from the semiconductor substrate 11. A pair of side surfaces 12a of the mesa portion 12 in the X direction are each exposed. That is, each side surface 12a is not covered by another element. The side surface 12a is a surface extending in a manner intersecting the X direction, and is a flat surface perpendicular to the X direction in this example.

[0059] The mesa portion 12 includes the active layer 21. As Figure 5 illustrated, the active layer 21 includes a plurality of absorption regions 22 and a plurality of transport regions 23 alternately stacked in the Z direction (a direction perpendicular to the main surface 11a of the semiconductor substrate 11), and has a quantum cascade structure. In the active layer 21, a unit stack 24 composed of a pair of the absorption region 22 and the transport region 23 is repeatedly stacked. In this example, the active layer 21 is configured in a manner in which the peak of the sensitivity wavelength is 4.5 μm, and includes 90 unit stacks 24 of periods. The number of periods of the unit stack 24 may, for example, also be 10 or more and 150 or less.

[0060] The absorption region 22 includes a barrier layer 261 and a well layer 271, and absorbs the detection light DL by inter-subband absorption. The transport region 23 includes a plurality of barrier layers 262 to 267 and a plurality of well layers 272 to 277, and transports an electron excited by inter-subband absorption in the absorption region 22 to the absorption region 22 of the next period. One example of the composition, layer thickness, and doping state of the barrier layers 261 to 267 and the well layers 272 to 277 is Figure 5 illustrated.

[0061] If the detection light DL is incident on the active layer 21, the detection light DL is absorbed in the active layer 21. More specifically, by repeating, in a plurality of unit stacks 24, excitation of an electron according to inter-subband absorption, relaxation of the excited electron, transport, and extraction of the electron to the unit stack 24 of the next period, cascaded light absorption occurs in the active layer 21. In the light detection module 1, a current generated by this light absorption is taken out as an electric signal, and the detection light DL is detected by measuring the amount of the current. That is, the active layer 21 functions as a semiconductor region that generates an electric signal according to the amount of incidence of the detection light DL.

[0062] The mesa portion 12 is formed in a long strip shape in the Y direction, for example, in a rectangular shape in which the long side is parallel to the Y direction in a plan view (when viewed in the Z direction). That is, in the plan view, the length LI of the mesa portion 12 in the Y direction (optical waveguide direction A) is longer than the length (width) L2 of the mesa portion 12 in the X direction (direction perpendicular to the optical waveguide direction A). The length LI is, for example, 50 μm or more. The length LI is, for example, in the order of 50 μm to 3000 μm, and in this example, 100 μm. The length L2 is, for example, in the order of 10 μm to 1000 μm, and in this example, 25 μm. The aspect ratio (ratio of the length LI to the length L2) of the mesa portion 12 in the plan view is 1 to 100. The aspect ratio of the mesa portion 12 can be 1.5 to 50, and preferably 2 to 10. The lengths LI and L2 can also be 10 μm or more. If the length LI is less than 50 μm, it can be difficult to ensure the strength of the output signal. If the length L2 is less than 10 μm, it can be difficult to form the air-bridge wiring 16. If the length LI is more than 3000 μm or the length L2 is more than 1000 μm, the element size becomes large, and it can be difficult to achieve a high cut-off.

[0063] The first contact layer 13 is an upper contact layer formed on the surface 12b of the mesa portion 12 on the opposite side of the semiconductor substrate 11. The second contact layer 14 is a lower contact layer formed between the main surface 11a of the semiconductor substrate 11 and the mesa portion 12. An example of the composition, layer thickness, and doping state of the first contact layer 13 and the second contact layer 14 is shown in Table 1. Figure 5

[0064] The second contact layer 14 is formed in a rectangular shape in which the length of each side is longer than the mesa portion 12, and has a portion exposed from the mesa portion 12. That is, in the plan view, the second contact layer 14 has a first portion 14a between the main surface 11a of the semiconductor substrate 11 and the mesa portion 12, and a second portion 14b outside the mesa portion 12.

[0065] The mesa portion 12, the first contact layer 13, and the second contact layer 14 are formed on the main surface 11a of the semiconductor substrate 11, for example, by crystal growth. After the crystal growth, the mesa portion 12, the first contact layer 13, and the second contact layer 14 are formed by etching to the surface of the second contact layer 14 or inside the second contact layer 14 using a photolithography technique and a dry etching technique. At the time of manufacture, a plurality of elements are formed at once, for example, by forming a plurality of portions corresponding to the mesa portion 12, the first contact layer 13, and the second contact layer 14 on a wafer, and then cutting the wafer.

[0066] ​In this example, one end surface 12c of the mesa portion 12 in the Y direction (optical waveguide direction A) (direction parallel to the main surface 11a of the semiconductor substrate 11) becomes an incident surface (light receiving surface) of the detection light DL. The detection light DL incident from the end surface 12c travels in the optical waveguide direction A inside the mesa portion 12. The end surface 12c is the same surface as the side surface 11b of the semiconductor substrate 11. That is, the end surface 12c and the side surface 11b are located on the same plane. The side surface 11b is a surface extending in a manner intersecting the Y direction, and is a flat surface perpendicular to the Y direction in this example. In this example, the side surface 11b and the end surface 12c are cut surfaces formed by dicing of a wafer.

[0067] The first electrode 15 is formed in a planar shape on the main surface 11a of the semiconductor substrate 11. The first electrode 15 is composed of, for example, gold, and is formed in a square shape by patterning. The first electrode 15 has a connection portion 15a located on one side in the X direction with respect to the mesa portion 12. In this example, the connection portion 15a is the entirety of the first electrode 15. The first electrode 15 is electrically connected to the first contact layer 13 via the air-bridge wiring 16. The first electrode 15 is provided in order to take out the current output from the active layer 21 to the outside. The area of the first electrode 15 is, for example, 10000 μm 2 The above. In this case, the first electrode 15 and the first wiring 52 described later can be brought into good surface contact. In this example, the size of the first electrode 15 is 400 μm x 400 μm.

[0068] The air-bridge wiring 16 is a connection wiring electrically connected to the first contact layer 13 and the first electrode 15. The air-bridge wiring 16 is drawn out from the first contact layer 13 to one side in the X direction, and is erected between the first contact layer 13 and the first electrode 15. In this example, the air-bridge wiring 16 is drawn out from the first contact layer 13 in a direction parallel to the X direction. In a plan view, the air-bridge wiring 16 is drawn out from the end portion of the first contact layer 13 (mesa portion 12) in the X direction. The air-bridge wiring 16 is an air wiring (three-dimensional wiring) having a bridge portion 16a extending in the air. The bridge portion 16a is electrically connected to the first electrode 15 (connection portion 15a) via the plating layer 151 described later. The air-bridge wiring 16 further has a planar portion 16b formed on the first contact layer 13. The bridge portion 16a is integrally formed with the portion 16b.

[0069] The air-bridge wiring 16 is formed, for example, by the following procedure. First, a resist is formed by patterning in a region between the mesa portion 12 and the first electrode 15 on the main surface 11a of the semiconductor substrate 11. Next, a thin film of gold having a thickness of about 5 μm is formed by plating on the resist, and then the resist is removed. Thus, the air-bridge wiring 16 in a sheet shape is formed.

[0070] The bridging portion 16a is formed as a wide sheet (layer). The length (width) L3 of the bridging portion 16a in the Y direction (optical waveguide direction A) is longer than the length L4 of the bridging portion 16a in the X direction. In this example, the bridging portion 16a extends in a curved manner when viewed from the Y direction. In this case, as... Figure 4 As shown, the length L4 of the bridging portion 16a in the X direction is the length (actual length) of the bridging portion 16a along the extension direction. The length L3 is, for example, 50 μm or more. The length L3 is, for example, between 50 μm and 3000 μm, and in this example, it is 80 μm. The length L4 is, for example, between 5 μm and 200 μm, and in this example, it is 30 μm. The aspect ratio of the bridging portion 16a (the ratio of length L3 to length L4) can be 0.25 to 100, preferably 1 to 50. The aspect ratio of the bridging portion 16a can be more preferably 1 to 20, and even more preferably 2 to 10. The thickness of the bridging portion 16a (air bridging wiring 16) in the Z direction is 1 μm or more and 10 μm or less.

[0071] The second electrode 17 is formed in a planar shape on the second portion 14b of the second contact layer 14. In other words, the second electrode 17 is formed in a planar shape on the main surface 11a of the semiconductor substrate 11 via the second contact layer 14. The second electrode 17 is made of, for example, gold and is patterned into a square shape. The second electrode 17 is disposed on the opposite side in the X direction (opposite to the first electrode 15) relative to the mesa 12. The second electrode 17 is electrically connected to the second contact layer 14. The second electrode 17 is provided to extract the current output from the active layer 21 to the outside. The second electrode 17 is electrically disconnected from the first electrode 15. The area of ​​the second electrode 17 is, for example, 10000 μm. 2 That's all. In this case, good ground contact can be achieved between the second electrode 17 and the second wiring 53, which will be described later. In this example, the second electrode 17 has dimensions of 400 μm × 400 μm, the same as the first electrode 15.

[0072] During the electroplating process in the formation of the above-mentioned overhead bridging wiring 16, plating layers can also be formed on the first electrode 15 and the second electrode 17. Figures 2-4 In this designation, these plating layers are represented by symbols 151 and 171. Plating layers 151 and 171 can be considered to constitute the first electrode 15 and the second electrode 17, respectively. In this example, plating layer 151 is formed to be slightly smaller than the first electrode 15 (formed only on a portion of the first electrode 15), but it can also be formed on the entire surface of the first electrode 15. Similarly, plating layer 171 is formed to be slightly smaller than the second electrode 17, but it can also be formed on the entire surface of the second electrode 17. Plating layer 151 can also be considered to constitute the overhead bridging wiring 16. Alternatively, plating layers 151 and 171 may not be formed.

[0073] [Fixed components]

[0074] As shown in Figure 1 and Figure 6 , the fixing member 50 includes an insulating substrate 51, a first wiring 52, and a second wiring 53. In Figure 6 , for ease of understanding, a cross-sectional line is added to the first wiring 52 and the second wiring 53. The insulating substrate 51 is formed, for example, in a rectangular flat plate shape, and has a main surface 51a and a surface 51b on the opposite side of the main surface 51a. In this example, the main surfaces 51a and 51b are flat surfaces parallel to each other. The base material of the insulating substrate 51 is, for example, alumina. In this case, dielectric loss in a high frequency band can be reduced. The base material of the insulating substrate 51 can also be SiC or ceramic, which are materials having high insulation.

[0075] A recess 54 is formed on the main surface 51a of the insulating substrate 51. In this example, the recess 54 is a groove extending in the Y direction (optical waveguide direction A) and opens at a side surface 51c of the insulating substrate 51. That is, the recess 54 extends to the side surface 51c. The side surface 51c is a surface extending in a direction intersecting the Y direction, and in this example, is a flat surface perpendicular to the Y direction. As one example, the recess 54 is rectangular in a plan view and is substantially semicircular in a cross section perpendicular to the Y direction.

[0076] The first wiring 52 is formed in a planar shape on the main surface 51a of the insulating substrate 51. The first wiring 52 is composed of, for example, gold and is formed in a prescribed shape by patterning. The first wiring 52 is disposed on one side in the X direction with respect to the recess 54. The first wiring 52 is formed in a rectangular shape with a long side parallel to the Y direction and extends across between the side surfaces 51c and 51d of the insulating substrate 51. The side surface 51d is a surface of the insulating substrate 51 on the opposite side of the side surface 51c.

[0077] The second wiring 53 is formed in a planar shape on the main surface 51a of the insulating substrate 51. The second wiring 53 is electrically separated from the first wiring 52. The second wiring 53 is composed of, for example, gold and is formed in a prescribed shape by patterning. The second wiring 53 is formed in such a manner as to reach the main surface 51b from the main surface 51a of the insulating substrate 51 through the side surface 51e (in a manner of winding in). That is, the second wiring 53 has a first portion 53a disposed on the main surface 51a and a second portion 53b disposed on the main surface 51b and the side surface 51e. The first portion 53a is disposed on the other side in the X direction (on the opposite side of the first wiring 52) with respect to the recess 54. The first portion 53a has a portion 53a1 formed in a rectangular shape with a long side parallel to the X direction and a portion 53a2 formed in a rectangular shape with a long side parallel to the Y direction. The portion 53a2 extends from the portion 53a1 to the side surface 51d of the insulating substrate 51. The second portion 53b is formed on the entire surface of the main surface 51b and the side surface 51e.

[0078] A solder layer 55 is formed on the first wiring 52, and a solder layer 56 is formed on the second wiring 53. The solder layer 56 is formed on a portion 53a1 of the second wiring 53. Each solder layer 55 and 56 is formed into a square shape from a metallic material, for example, and has a thickness of 1 μm to 10 μm.

[0079] like Figure 1 As shown, the photodetector 10 is fixed to the fixing member 50 with the main surface 11a of the semiconductor substrate 11 facing the main surface 51a of the insulating substrate 51. In the fixed state where the photodetector 10 is fixed to the fixing member 50, at least a portion of the mesa 12 is disposed within the recess 54. In this example, the front end of the mesa 12 (the end on the opposite side of the semiconductor substrate 11 in the Y direction) is disposed within the recess 54. The mesa 12 moves away from the inner surface 54a of the recess 54. The long side direction of the mesa 12 is parallel to the long side direction of the recess 54. The end face 12c of the mesa 12 is exposed from the recess 54 when viewed from the Y direction. In the fixed state, a portion of the overhead bridging cable 16 (bridging portion 16a) is also disposed within the recess 54 in a manner that moves away from the inner surface 54a. Thus, the connection between the overhead bridging cable 16 and the first contact layer 13 is disposed within the recess 54.

[0080] The photodetector 10 is connected to the first wiring 52 via the first electrode 15 and the second electrode 17 is connected to the second wiring 53, and is fixed to the fixing member 50. The first electrode 15 is fused to the first wiring 52 using a solder layer 55, and is electrically connected to the first wiring 52 when it is in contact with the first wiring 52 via the surface of the solder layer 55. The second electrode 17 is fused to the second wiring 53 using a solder layer 56, and is electrically connected to the second wiring 53 when it is in contact with the second wiring 53 via the surface of the solder layer 56. The contact area between the first electrode 15 and the first wiring 52, and the contact area between the second electrode 17 and the second wiring 53, are both 10000 μm. 2 That's all. In the fixed state, in order to enable the terminals 81 of the connector 80 (described later) to connect, a portion of the first wiring 52 is exposed ( Figure 7 , Figure 8 ).

[0081] [Installation status of the optical detection module]

[0082] like Figure 7 , Figure 8 and Figure 9As shown, the optical detection module 1 is used, for example, by connecting to connector 80. Connector 80 is, for example, an SMA connector, having terminals 81 that are electrically connected to the signal lines of an SMA cable. Terminals 81 are mechanically connected to and electrically connected to the first wiring 52 of the fixing member 50. Alternatively, terminals 81 can also be soldered to the first wiring 52. Thus, in this example, the first wiring 52 is the signal side wiring electrically connected to the signal lines.

[0083] The connector 80 further includes: a main body component 82; and a support component 83 integrally formed with the main body component 82. The main body component 82 and the support component 83 are electrically connected to the grounding wire of the SMA cable. The photodetector module 1 is fixed to the connector 80 by connecting the fixing component 50 to the support component 83. For example, the second portion 53b of the second wiring 53 of the fixing component 50 is soldered to a gold layer formed on the surface of the support component 83. The gold layer is formed, for example, in a manner that covers the entire surface of the support component 83. Alternatively, the main body component 82 may also be separately constructed from the support component 83. In this case, the support component 83, to which the fixing component 50 is connected, may also be fixed to the main body component 82 by screws or soldering. Through the above connection, the second wiring 53 is electrically connected to the grounding wire. That is, in this example, the second wiring 53 is a grounding side wiring electrically connected to the grounding wire. In this example, the electrical connection (wireless connection) between the photodetector 10 and the connector 80 is achieved without using bonding leads. Thus, the generation of inductance caused by bonding leads can be avoided. Furthermore, when using connecting leads, care must be taken to handle the device as the leads may be damaged by impacts or snagging. In contrast, in this example, since connecting leads are not used, the device can be easily handled.

[0084] The shape (wiring pattern) of the first wiring 52 is set with impedance matching in mind. In this example, the first wiring 52 is configured as a micro-strip line. As an example, the thickness of the insulating substrate 51 is 0.5 mm, the relative permittivity of the insulating substrate 51 is 9.8, the thickness of the first wiring 52 is 0.8 μm, and with a matching resistance of 50 Ω at 20 GHz, the width W of the first wiring 52 is set to 0.5 mm or less, and the length L5 of the first wiring 52 is set to 1.45 mm or less. Figure 6 Therefore, the length L5 of the first wiring 52 is less than 1 / 4 of the wavelength (electrical length) of the electrical signal propagating in the first wiring 52. In this way, by making the length L5 of the first wiring 52 less than 1 / 4 of the electrical length calculated based on the design parameters of the first wiring 52, impedance matching in the desired frequency band can be achieved. Furthermore, the first wiring 52 can also be configured as a coplanar line instead of a microstrip line.

[0085] likeFigure 8 As shown, the light detection module 1 (the light detector 10) further has a lens 40. The lens 40 is disposed in opposition to an end face 12c of the mesa portion 12 that is a light incident face, and focuses the detection light DL toward the end face 12c, and condenses the detection light DL on the end face 12c. The numerical aperture (NA) of the lens 40 is, for example, 0.4 or more, and the condensing diameter is several μm to several tens of μm. On both faces of the lens 40 and the end face 12c, a low reflection layer including a dielectric multilayer film can also be formed. In this case, the transmittance of light with respect to the sensitivity wavelength range of the light detector 10 on these surfaces can be 95% or more.

[0086] [Effects]

[0087] In the light detection module 1, the first electrode 15 on the semiconductor substrate 11 of the light detector 10 is electrically connected (surface-connected) to the first wiring 52 in a state of surface-contacting the first wiring 52 on the insulating substrate 51 of the fixing member 50. Thereby, compared to a case where the first electrode 15 and the first wiring 52 are connected by wire bonding, for example, inductance due to a wire can be avoided, and the inductance can be reduced. As a result, the response speed can be made high speed. Further, by connecting the first electrode 15 and the first wiring 52 in a surface-contacting state, the light detector 10 and the fixing member 50 can be firmly fixed, and the reliability can be ensured. Furthermore, in the light detection module 1, at least a part of the mesa portion 12 of the light detector 10 is disposed inside the recessed portion 54 formed in the insulating substrate 51. Thereby, the thin mesa portion 12 can be protected, and the reliability can be ensured. For example, the mesa portion 12 can be protected from breakage (mechanical damage due to external factors) caused by contact from the outside, attachment of floating matter, wind pressure, and the like. Therefore, according to the light detection module 1, the high speed of the response speed can be achieved and the reliability can be ensured.

[0088] Figure 10 is a graph showing the response characteristic of the light detector 10, Figure 11 is a graph showing an example of the output signal in a high frequency region. As Figure 10 shown, in the light detector 10, the cut-off frequency that is the frequency at which the signal intensity is reduced by 3 dB is 20 GHz or more. For example, in an MCT (HgCdTe) sensor used as an electrical detector of mid-infrared light, only a cut-off frequency of several hundred MHz or so (at most 1 GHz or so) can be achieved. In contrast, in the light detector 10, the cut-off frequency can be increased, and the response speed can be made high speed. Further, as Figure 11 shown, in a frequency band region exceeding 26 GHz, a high signal intensity exceeding 30 dB is obtained.

[0089] The mesa portion 12 includes an active layer 21 in which absorption regions 22 that absorb detection light by inter-subband absorption and transport regions 23 that transport electrons excited by inter-subband absorption are alternately laminated. In the case where the mesa portion 12 has such an active layer 21, it is particularly important to reduce the inductance from the viewpoint of the high speed of the response speed. In this regard, in the light detection module 1, as described above, it is possible to reduce the inductance, and it is possible to achieve the high speed of the response speed.

[0090] One end surface 12c of the mesa portion 12 in the Y direction (a direction parallel to the main surface 11a of the semiconductor substrate 11) (the light waveguide direction A) becomes an incident surface of the detection light DL. Thereby, it is possible to efficiently absorb light in the mesa portion 12, and it is possible to ensure the intensity of the output signal.

[0091] One end surface 12c of the mesa portion 12 in the Y direction is the same surface as the side surface 11b of the semiconductor substrate 11. Thereby, it is possible to easily utilize the end surface 12c of the mesa portion 12 as the incident surface of the detection light DL.

[0092] The light detection module 1 is configured in a manner opposing the end surface 12c of the mesa portion 12, and has a lens 40 that converges the detection light DL toward the end surface 12c of the mesa portion 12. Thereby, it is possible to narrow the width (the length in the X direction) of the end surface 12c of the mesa portion 12, and it is possible to make the area of the mesa portion 12 in a plan view small. Therefore, it is possible to reduce the parasitic capacitance, and it is possible to further speed up the response speed. Further, assuming that the area of the mesa portion 12 is constant, by narrowing the width of the mesa portion 12, it is possible to extend the length of the mesa portion 12 in the light waveguide direction A. Therefore, it is possible to more efficiently absorb light in the mesa portion 12, and it is possible to practically ensure the intensity of the output signal. By making the active layer 21 thick by increasing the number of periods of the unit laminate 24 in the active layer 21, it is possible to improve the absorption efficiency. The increase in the thickness of the active layer 21 also contributes to the reduction of the parasitic capacitance, and the reduction of the noise due to the increase in the element resistance. On the other hand, if the number of periods is too large, it can lead to a decrease in the quantum efficiency, and therefore it is preferable that the number of periods is in the order of 10 or more and 150 or less.

[0093] The recess 54 is opened at the side surface 51c of the insulating substrate 51. Thereby, it is possible to easily cause the detection light to be incident to the end surface 12c of the mesa portion 12 in the Y direction.

[0094] The light detector 10 has an air-bridge wiring 16 (connection wiring) electrically connected to the first electrode 15 and the first contact layer 13, and a part of the air-bridge wiring 16 is disposed in the recess 54. Thereby, it is possible to protect the thin air-bridge wiring 16, and it is possible to practically ensure the reliability.

[0095] The second electrode 17 is electrically connected (surface-connected) to the second wiring 53 in a state of surface-contacting the second wiring 53. Thus, compared to a case where the second electrode 17 and the second wiring 53 are connected by wire bonding, for example, it is possible to reduce the inductance, and it is possible to further speed up the response speed. Further, by connecting the second electrode 17 and the second wiring 53 in a state of surface-contacting, it is possible to firmly fix the photodetector 10 and the fixing member 50, and it is possible to surely ensure the reliability.

[0096] In a plan view, the second contact layer 14 has a first portion 14a located between the main surface 11a of the semiconductor substrate 11 and the mesa portion 12, and a second portion 14b located outside the mesa portion 12, on which the second electrode 17 is formed. Thus, it is possible to ensure that the second electrode 17 has a large area, and it is possible to ensure that the contact area between the second electrode 17 and the second wiring 53 is large.

[0097] The first electrode 15 is surface-contacted to the first wiring 52 via a solder layer 55, and the second electrode 17 is surface-contacted to the second wiring 53 via a solder layer 56. Thus, it is possible to firmly electrically and mechanically connect the first electrode 15 and the first wiring 52, and it is possible to firmly electrically and mechanically connect the second electrode 17 and the second wiring 53. Further, when the first electrode 15 and the first wiring 52 are connected, and the second electrode 17 and the second wiring 53 are connected, even in a case where there is a difference in height between the first electrode 15 and the second electrode 17, it is possible to absorb the difference in height by the solder layers 55, 56, and it is possible to achieve good surface-contacting at each connection site.

[0098] The first wiring 52 has a length of 1 / 4 or less of the wavelength of an electric signal propagating on the first wiring 52. Thus, it is possible to achieve impedance matching.

[0099] The side surface 12a of the mesa portion 12 in the X direction is exposed. Thus, it is possible to suppress the generation of parasitic capacitance, and it is possible to further speed up the response speed. Hereinafter, with reference to Figure 12 This point will be described.

[0100] Figure 12 (a) of FIG. 1 is a front view of the photodetector 10, Figure 12 (b) of FIG. 1 is a front view of a first modified example of the photodetector 10A. As Figure 12 As shown in (a) of FIG. 1, in the photodetector 10, both side surfaces 12a of the mesa portion 12 in the X direction are exposed. In contrast, as shown in (b) of FIG. 1, in the photodetector 10A, only one side surface 12a of the mesa portion 12 in the X direction is exposed. Figure 12In the light detector 10A, as shown in (b), the both side surfaces 12a of the mesa portion 12 are covered with an insulating layer 90. The insulating layer 90 is, for example, a silicon nitride (SiN) film. The insulating layer 90 enters between the 1st contact layer 13 and the metal layer on the 1st contact layer 13 (a portion 16b of the air-bridge wiring 16). Since this portion has a structure in which an insulator is sandwiched by metal, in the case where, for example, an alignment deviation occurs at the time of forming the insulating layer 90, a slight parasitic capacitance is likely to occur in this portion. At this point, in the light detector 10, since the both side surfaces 12a of the mesa portion 12 are exposed, it is possible to suppress the occurrence of such a parasitic capacitance.

[0101] The mesa portion 12 is apart from the inner face of the recessed portion 54. Thereby, it is possible to surely protect the mesa portion 12. In the case where the light detector 10 is a detector that detects the detection light DL using inter-subband absorption in a quantum well structure, it is possible to perform detection without applying an external voltage, and thus it is difficult to generate heat in the mesa portion 12. Therefore, it is possible to make the mesa portion 12 apart from the insulating substrate 51.

[0102] In the light detector 10, the 1st contact layer 13 and the 1st electrode 15 are connected by the air-bridge wiring 16. Thereby, compared with the case where the 1st contact layer 13 and the 1st electrode 15 are connected by wire bonding, for example, it is possible to make the wiring length shorter, and it is possible to make the area of the mesa portion 12 in a plan view smaller. In the case of wire bonding, the area required for the wiring is determined by the area required for pressing the leading end of the wire, and an area of about 100 μm x 100 μm is required, for example. By making the wiring length shorter, it is possible to reduce the inductance, and it is possible to make the response speed high. Further, by making the area of the mesa portion 12 smaller, it is possible to reduce the parasitic capacitance, and thereby it is also possible to make the response speed high. On the other hand, if only the area of the mesa portion 12 is made smaller, there is a concern that the strength of the output signal decreases. At this point, in the light detector 10, when viewed from the Z direction (a direction perpendicular to the main face 11a of the semiconductor substrate 11), the length LI of the mesa portion 12 in the light waveguide direction A (Y direction) is longer than the length L2 of the mesa portion 12 in the X direction (a direction perpendicular to the light waveguide direction A). Thereby, it is possible to make the path of the detection light DL traveling within the mesa portion 12 longer, and it is possible to efficiently absorb light in the mesa portion 12. Therefore, even in the case where the area of the mesa portion 12 is made smaller, it is possible to secure the strength of the output signal. Further, in the light detector 10, the air-bridge wiring 16 is drawn out from the 1st contact layer 13 to one side in the X direction, and is erected between the 1st contact layer 13 and the 1st electrode 15. Thereby, it is possible to secure the width (length in the light waveguide direction A) of the air-bridge wiring 16. In the case where the wiring width is wide, by the cross-sectional area becoming large, it is possible to further reduce the inductance, and it is possible to secure the strength of the air-bridge wiring 16, and to secure the reliability.

[0103] The first electrode 15 has a connection portion 15a on one side of the platform surface 12 in the X direction. An overhead bridging cable 16 extends from the first contact layer 13 to one side in the X direction and is connected to the connection portion 15a of the first electrode 15. Thus, the overhead bridging cable 16 can be extended from the first contact layer 13 to one side in the X direction, and the width of the overhead bridging cable 16 can be reliably ensured.

[0104] The overhead bridging cable 16 has a bridging portion 16a extending in the air, and the length L3 of the bridging portion 16a in the optical waveguide direction A is longer than the length L4 of the bridging portion 16a in the X direction. This ensures the width of the bridging portion 16a (overhead bridging cable 16).

[0105] The length L1 of the mesa 12 in the optical waveguide direction A and the length L3 of the bridging portion 16a in the optical waveguide direction A are both 50 μm or more. This ensures the width of the bridging portion 16a. Furthermore, it ensures the length of the mesa 12 in the optical waveguide direction A and enables more efficient light absorption within the mesa 12.

[0106] [Variation Example]

[0107] exist Figure 13 and Figure 14 In the second modified example of the photodetector module 1 shown, the detection light DL travels within the semiconductor substrate 11 and is incident on the mesa 12. In this second modified example, the side surface 11b of the semiconductor substrate 11 is an inclined surface tilted relative to the Z direction. As an example, when viewed from the X direction, the tilt angle θ of the side surface 11b relative to the Z direction is 45 degrees. The side surface 11b is, for example, a polished surface formed by grinding. The detection light DL is incident on the side surface 11b from a direction perpendicular to the side surface 11b, travels within the semiconductor substrate 11, and is incident on the surface 12d of the mesa 12 via the second contact layer 14. The surface 12d is the semiconductor substrate 11 side surface of the mesa 12. That is, in this example, the side surface 11b is the incident surface of the detection light DL. The end surface 12c of the mesa 12 is not the same as the side surface 11b of the semiconductor substrate 11. The detection light DL incident on the mesa 12 undergoes multiple reflections within the mesa 12, and the electric field vibration component parallel to the Z direction in the detection light DL is absorbed in the active layer 21. The detection light DL undergoes multiple reflections within the semiconductor substrate 11, allowing it to be incident on the surface 12d of the mesa 12 multiple times. Furthermore, in Figure 13 In the text, the first electrode 15, etc., are omitted, and the section lines representing the cross-section are also omitted.

[0108] like Figure 14In the second modification, the recess 54 is constituted by a hole formed in the main surface 51a of the insulating substrate 51. The recess 54 is not opened on the side surface 51c of the insulating substrate 51, and is apart from the outer edge of the main surface 51a. The recess 54 is substantially semicircular in a cross section perpendicular to the X direction. In the second modification, at least a part of the mesa portion 12 is also disposed in the recess 54. The mesa portion 12 is formed in a shape capable of being disposed in the recess 54. According to such a second modification, as in the above-described embodiment, a high speed of response can be achieved and reliability can be ensured. Further, since the recess 54 is constituted by a hole not opened on the side surface 51c of the insulating substrate 51, the mesa portion 12 can be more securely protected.

[0109] In Figure 15 In the third modification, the light detector 10 has a height adjustment layer 19. The height adjustment layer 19 is formed in a planar shape on the main surface 11a of the semiconductor substrate 11. The height adjustment layer 19 is formed on the main surface 11a apart from the second contact layer 14, and is electrically separated from the second contact layer 14. The height adjustment layer 19 is formed at the same time as the second contact layer 14, and has the same structure as the second contact layer 14. That is, the height adjustment layer 19 has the same thickness as the second contact layer 14. The first electrode 15 is formed on the height adjustment layer 19. According to such a third modification, as in the above-described embodiment, a high speed of response can be achieved and reliability can be ensured. Further, when the first electrode 15 and the first wiring 52 are connected and the second electrode 17 and the second wiring 53 are connected, even in a case where there is a height difference between the first electrode 15 and the second electrode 17, the height difference between the first electrode 15 and the second electrode 17 can be reduced by the height adjustment layer 19, and good surface contact can be achieved at each connection site.

[0110] The present application is not limited to the above-described embodiment. For example, the materials and shapes of the structures are not limited to the above-described materials and shapes, and various materials and shapes can be employed. The length L3 of the bridge portion 16a in the direction A of the optical waveguide can also be not longer than the length L4 of the bridge portion 16a in the X direction. The lens 40 can also be omitted. The second contact layer 14 can also not have the second portion 14b, and have only the first portion 14a. The mesa portion 12 can further include an upper cladding layer disposed between the active layer 21 and the first contact layer 13, and a lower cladding layer disposed between the active layer 21 and the second contact layer 14.

[0111] The mesa portion 12 can also not be formed in a long strip shape in the Y direction, and can be, for example, a square or a circle in a plan view, or the like. The entire mesa portion 12 can also be disposed in the recess 54. The entire air bridge wiring 16 (connection wiring) can also be disposed in the recess 54. The connection wiring connecting the first electrode 15 and the first contact layer 13 can also be a bonding wire.

[0112] At least one of the pair of first electrodes 15 and first wiring 52, and the pair of second electrodes 17 and second wiring 53, can be connected to each other in a surface-to-surface contact state. The first electrode 15 and first wiring 52 may not be surface-to-surface connected; for example, they can be connected via bonding leads. The second electrode 17 and second wiring 53 may not be surface-to-surface connected; for example, they can be connected via bonding leads. The first electrode 15 or the second electrode 17 may also be formed on the surface of the semiconductor substrate 11 opposite to the main surface 11a. Solder layers 55 and 56 may also be omitted. That is, the first electrode 15 may be in surface contact with the first wiring 52 via a solder layer or plating, or it may be in direct surface contact with the first wiring 52. Similarly, the second electrode 17 may be in surface contact with the second wiring 53 via a solder layer or plating, or it may be in direct surface contact with the second wiring 53.

[0113] Alternatively, the first electrode 15, the second electrode 17, the first wiring 52, and the second wiring 53 in the above embodiments can be regarded as the second electrode, the first electrode, the second wiring, and the first wiring, respectively. That is, the first electrode can be connected to the second contact layer 14, and the second electrode can be connected to the first contact layer 13.

[0114] In the above embodiment, the photodetector 10 is configured as a quantum cascade detector, but it can also be configured as another photodetector such as a quantum well infrared photodetector (QWIP). A quantum well infrared photodetector is an infrared detector that detects light by utilizing inter-subband absorption in a quantum well structure. The mesa 12 only needs to include a semiconductor region that generates an electrical signal based on the incident amount of the detection light DL; it does not necessarily need to include the active layer 21. For example, the photodetector 10 can also be configured as a photodiode, in which case the mesa 12 includes a semiconductor region forming a PN junction.

[0115] [Beat frequency beam splitter]

[0116] like Figure 16 As shown, the beat frequency splitter 100 includes: a first light source (fixed wavelength light source) 101, a second light source (variable wavelength light source) 102, a beam splitter 103, a photodetector module 1, a spectrum analyzer 104, and a gas cell 105. In the beat frequency splitter 100, the wavelength of light P2 is changed by scanning a beat frequency signal having a frequency corresponding to the wavelength difference between light P1 from the first light source 101 and light P2 from the second light source 102, and the wavelengths of P1 and P2 are detected by the photodetector module 1. This enables heterodyne detection and splitting.

[0117] The first light source 101 and the second light source 102 output light P1, P2 of oscillation wavelengths included in the sensitivity wavelength range of the light detection module 1 and close to each other. The first light source 101 and the second light source 102 are, for example, Distributed Feedback Quantum Cascade Lasers (DFB-QCL). In use, the oscillation wavelength of the first light source 101 is fixed, and the oscillation wavelength of the second light source 102 is modulated. For example, the oscillation wavelength of the second light source 102 can be modulated by changing the injection current. That is, the first light source 101 is a wavelength fixed light source whose output light P1 is fixed in wavelength, and the second light source 102 is a wavelength variable light source whose output light P2 varies in wavelength. The operation of the second light source 102 is controlled by a control section composed of a computer, for example.

[0118] In Figure 17 , the sensitivity characteristic of the light detector 10 is indicated by the symbol R1, and the oscillation wavelength of the second light source 102 is indicated by the symbol R2. As Figure 17 indicated, the oscillation wavelength of the second light source 102 is included in the sensitivity wavelength range of the light detector 10. Figure 18 is a graph indicating the oscillation wavelengths of the first light source 101 and the second light source 102. The symbol F indicates the oscillation wavelength of the first light source 101 in the case where the injection current is 780 mA. The symbols T1 to T4 respectively indicate the oscillation wavelengths of the second light source 102 in the cases where the injection current is 780 mA, 810 mA, 830 mA, and 850 mA. In this way, the oscillation wavelength of the second light source 102 can be modulated by changing the injection current.

[0119] The light P1 from the first light source 101 is reflected by the mirrors 111, 112 after passing through the lens 108 to be incident on the beam splitter 103. The light P2 from the second light source 102 is reflected by the mirror 113 after passing through the lens 109 to be incident on the beam splitter 103. The lights P1, P2 are combined by the beam splitter 103. The combined lights P1, P2 are reflected by the mirror 116 after passing through the aperture members (apertures) 114, 115 to be incident on the lens 40, and on the light detector 10. The spectrum analyzer 104 is connected to the light detector 10. The gas chamber 105 in which a gas of a measurement object is housed is arranged between the second light source 102 and the mirror 113. The light P2 from the second light source 102 is incident on the mirror 113 after passing through the gas chamber 105. In the case where the wavelength modulation range of the light P2 from the second light source 102 straddles an absorption line of the gas, the light of a specific wavelength in the light P2 is absorbed.

[0120] In the light detector 10, a beat signal (beat note) having a frequency according to a wavelength difference between the light P1 from the first light source 101 and the light P2 from the second light source 102 is detected. It is difficult to directly measure the frequency of light. For example, in the case where the wavelength is 4 μm, the response speed of the light detector needs to be above around 75 THz. In contrast, in the beat spectroscopy method using the beat signal, for example, in the case where the wavelength of the light P1 is 4.000 μm and the wavelength of the light P2 is 4.001 μm, the response speed of the light detector 10 only needs to be above around 18 GHz. As described above, in the light detection module 1, a cut-off frequency of 20 GHz or more is obtained. Therefore, by using the light detection module 1 for beat spectroscopy, it is possible to perform spectroscopy on a wide wavelength range.

[0121] As described above, in the beat spectroscopy apparatus 100, the wavelength of the light P2 is changed in a manner of scanning the frequency of the beat signal, and the lights P1, P2 are detected by the light detection module 1. Figure 19 is a graph showing the relationship between the injection current of the second light source 102 and the oscillation wavelength. As shown in Figure 19 , if the current of the second light source 102 is set to x and the wave number of the light P2 from the second light source 102 is set to y, the relationship y = 0.0147x + 2202.1 holds. The broken line B indicates the oscillation wavelength of the first light source 101 in the case where the injection current is 750 mA. In this example, the operating temperature of the first light source 101 and the second light source 102 is 20°C, and both are driven by CW (Continuous Wave). The symbol C indicates the difference in the oscillation wavelength of the first light source 101 and the second light source 102 at a certain injection current. The faster the response speed of the light detector 10, the more the difference can be increased, and the wavelength scanning range can be expanded. Figure 20 is a graph showing the relationship between the injection current to the second light source 102 and the beat frequency. As shown in Figure 20 , the beat frequency linearly increases as the injection current increases.

[0122] Figure 21 is a graph showing an example of the beat signal. Figure 22 is a graph showing the comparison result of the response characteristics. In Figure 22 , the symbol S1 indicates the frequency response characteristic of the light detection module 1, and the symbol S2 indicates the frequency response characteristic of the light detector which is the comparison object. As is apparent from Figure 22 , in the light detection module 1, the cut-off frequency is 20 GHz or more, and the response speed is made high compared to the light detector which is the comparison object.

[0123] Figure 23 , Figure 24 and Figure 25 are graphs showing the measurement result of the beat spectroscopy.Figure 23 This is a graph showing the output signal of the spectrum analyzer 104. Figure 24 It is Figure 23 The horizontal axis of the graph shown is transformed into a wavelength graph. Figure 25 It is a graph of the output signal minus the background light signal.

[0124] In this example, the gas being measured was set to carbon monoxide, and a wavenumber of 2190 cm⁻¹ was observed. -1 Nearby absorption lines. In cases where the wavelength scan range crosses the absorption lines of the gas being measured, such as... Figure 23 As indicated by arrow D, wavelength-dependent gas absorption is observed. This is because, when the wavelength of light P2 from the second light source 102 coincides with the absorption wavelength of the gas, the light intensity decreases due to absorption. The intensity of the beat frequency signal is proportional to the product of the electric field amplitude of light P1 from the first light source 101 and the electric field amplitude of light P2 from the second light source 102. Therefore, a dip, as indicated by arrow D, is observed due to the decrease in the intensity of light P2. Figure 24 In the diagram, the output signal when carbon monoxide is sealed within the gas chamber 105 is represented by the symbol SG, and the output signal when carbon monoxide is absent from the gas chamber 105 is represented by the symbol BG. Furthermore, the absorption spectrum of carbon monoxide is represented by dashed lines. The symbol FT represents an example of the resolution range of a Fourier Transform Infrared Spectrometer (FTIR). The resolution of FTIR is 3 GHz (0.1 cm⁻¹). -1 In contrast, the beat frequency splitter 100 can achieve a resolution of approximately 20 MHz. This is more than 150 times the resolution of FTIR. Thus, the beat frequency splitter 100 can expand the wavelength scanning range in beat frequency splitting.

[0125] Furthermore, in the beat frequency splitter 100, the first light source 101 and the second light source 102 are quantum cascade lasers, and the photodetector 10 is a quantum cascade detector. Since the output light of the quantum cascade laser is linearly polarized light parallel to the growth direction of the active layer, by arranging the first light source 101, the second light source 102, and the photodetector 10 in such a way that the polarization directions of the first light source 101 and the second light source 102 are aligned with the polarization direction of the photodetector 10, the light P1 and P2 from the first light source 101 and the second light source 102 can be effectively absorbed into the active layer 21 of the photodetector 10. Furthermore, in this case, by having the polarization characteristics of the first light source 101 and the second light source 102 and the polarization characteristics of the photodetector 10 function as polarization filters, the influence of background light with random polarization can be suppressed, thus improving the SN ratio.

[0126] Explanation of reference signs

[0127] 1 … light detection module, 10, 10A … light detector, 11 … semiconductor substrate, 11a … main surface, 11b … side surface, 12 … mesa portion, 12a … side surface, 12c … end surface, 13 … 1st contact layer, 14 … 2nd contact layer, 14a … 1st portion, 14b … 2nd portion, 15 … 1st electrode, 16 … air bridge wiring (connection wiring), 17 … 2nd electrode, 19 … height adjustment layer, 21 … active layer, 22 … absorption region, 23 … transport region, 40 … lens, 50 … fixing member, 51 … insulating substrate, 51a … main surface, 51c … side surface, 52 … 1st wiring, 53 … 2nd wiring, 54 … recessed portion, 54a … inner surface, 55, 56 … solder layer, 100 … beat spectrometer, 101 … 1st light source (wavelength fixed light source), 102 … 2nd light source (wavelength variable light source), A … optical waveguide direction, DL … detection light, P1 … light from 1st light source, P2 … light from 2nd light source.

Claims

1. A light detection module, comprising: Photodetector; and The fixing component that secures the photodetector The photodetector has: Semiconductor substrate, having a main surface; The platform surface includes a semiconductor region that generates an electrical signal corresponding to the incident amount of detection light, and is formed on the main surface of the semiconductor substrate; The first contact layer is formed on the surface of the mesa, on the opposite side of the semiconductor substrate; A second contact layer is formed between the main surface and the mesa surface of the semiconductor substrate; and The first electrode is formed in a planar shape on the main surface of the semiconductor substrate and is electrically connected to one of the first contact layer and the second contact layer. The fixing component has: An insulating substrate having a main surface; and The first wiring is formed in a planar shape on the main surface of the insulating substrate. A recess is formed on the main surface of the insulating substrate. At least a portion of the table surface is disposed within the recess. The first electrode is electrically connected to the first wiring when it is in surface contact with the first wiring. The photodetector further comprises: a connection wiring electrically connected to the first electrode and one of the first contact layer and the second contact layer. At least a portion of the connecting wiring is disposed within the recess.

2. The optical detection module according to claim 1, wherein, The semiconductor region of the platform includes an active layer consisting of an absorption region that absorbs detection light through inter-subband absorption and a transport region that transports electrons excited by the inter-subband absorption, which are alternately stacked.

3. The optical detection module according to claim 1, wherein, One end face of the mesa, in a direction parallel to the main surface of the semiconductor substrate, serves as the incident surface of the detection light.

4. The optical detection module according to claim 2, wherein, One end face of the mesa, in a direction parallel to the main surface of the semiconductor substrate, serves as the incident surface of the detection light.

5. The optical detection module according to claim 1, wherein, One end face of the mesa, in a direction parallel to the main surface of the semiconductor substrate, is the same as the side surface of the semiconductor substrate.

6. The optical detection module according to claim 2, wherein, One end face of the mesa, in a direction parallel to the main surface of the semiconductor substrate, is the same as the side surface of the semiconductor substrate.

7. The optical detection module according to claim 3, wherein, One end face of the mesa, in a direction parallel to the main surface of the semiconductor substrate, is the same as the side surface of the semiconductor substrate.

8. The optical detection module according to claim 4, wherein, One end face of the mesa, in a direction parallel to the main surface of the semiconductor substrate, is the same as the side surface of the semiconductor substrate.

9. The optical detection module according to claim 3, wherein, It further includes a lens configured to face the end face of the stage surface, such that the detection light converges toward the end face of the stage surface.

10. The optical detection module according to claim 4, wherein, It further includes a lens configured to face the end face of the stage surface, such that the detection light converges toward the end face of the stage surface.

11. The optical detection module according to claim 5, wherein, It further includes a lens configured to face the end face of the stage surface, such that the detection light converges toward the end face of the stage surface.

12. The optical detection module according to claim 6, wherein, It further includes a lens configured to face the end face of the stage surface, such that the detection light converges toward the end face of the stage surface.

13. The optical detection module according to claim 7, wherein, It further includes a lens configured to face the end face of the stage surface, such that the detection light converges toward the end face of the stage surface.

14. The optical detection module according to claim 8, wherein, It further includes a lens configured to face the end face of the stage surface, such that the detection light converges toward the end face of the stage surface.

15. The optical detection module according to claim 1, wherein, The recess opens on the side of the insulating substrate.

16. The optical detection module according to claim 2, wherein, The recess opens on the side of the insulating substrate.

17. The optical detection module according to claim 3, wherein, The recess opens on the side of the insulating substrate.

18. The optical detection module according to claim 4, wherein, The recess opens on the side of the insulating substrate.

19. The optical detection module according to claim 5, wherein, The recess opens on the side of the insulating substrate.

20. The optical detection module according to claim 6, wherein, The recess opens on the side of the insulating substrate.

21. The optical detection module according to claim 7, wherein, The recess opens on the side of the insulating substrate.

22. The optical detection module according to claim 8, wherein, The recess opens on the side of the insulating substrate.

23. The optical detection module according to claim 9, wherein, The recess opens on the side of the insulating substrate.

24. The optical detection module according to claim 10, wherein, The recess opens on the side of the insulating substrate.

25. The optical detection module according to claim 11, wherein, The recess opens on the side of the insulating substrate.

26. The optical detection module according to claim 12, wherein, The recess opens on the side of the insulating substrate.

27. The optical detection module according to claim 13, wherein, The recess opens on the side of the insulating substrate.

28. The optical detection module according to claim 14, wherein, The recess opens on the side of the insulating substrate.

29. The optical detection module according to claim 1, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

30. The optical detection module according to claim 2, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

31. The optical detection module according to claim 3, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

32. The optical detection module according to claim 4, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

33. The optical detection module according to claim 5, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

34. The optical detection module according to claim 6, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

35. The optical detection module according to claim 7, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

36. The optical detection module according to claim 8, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

37. The optical detection module according to claim 9, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

38. The optical detection module according to claim 10, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

39. The optical detection module according to claim 11, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

40. The optical detection module according to claim 12, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

41. The optical detection module according to claim 13, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

42. The optical detection module according to claim 14, wherein, The recess is formed by a hole on the main surface that is separated from the outer edge of the main surface of the insulating substrate.

43. The light detection module according to any one of claims 1 to 42, wherein, The connection wiring is a sheet-like overhead bridging wiring.

44. The light detection module according to any one of claims 1 to 42, wherein, The photodetector further comprises: a second electrode, which is planar on the main surface of the semiconductor substrate and electrically connected to the other side of the first contact layer and the second contact layer. The fixing component further includes: a second wiring formed in a planar shape on the main surface of the insulating substrate. The second electrode is electrically connected to the second wiring when it is in surface contact with the second wiring.

45. The optical detection module according to claim 43, wherein, The photodetector further comprises: a second electrode, which is planar on the main surface of the semiconductor substrate and electrically connected to the other side of the first contact layer and the second contact layer. The fixing component further includes: a second wiring formed in a planar shape on the main surface of the insulating substrate. The second electrode is electrically connected to the second wiring when it is in surface contact with the second wiring.

46. ​​The optical detection module according to claim 44, wherein, The second contact layer, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, has: a first portion located between the main surface and the mesa portion of the semiconductor substrate; and a second portion located outside the mesa portion. The second electrode is formed on the second portion of the second contact layer.

47. The optical detection module according to claim 45, wherein, The second contact layer, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, has: a first portion located between the main surface and the mesa portion of the semiconductor substrate; and a second portion located outside the mesa portion. The second electrode is formed on the second portion of the second contact layer.

48. The optical detection module according to claim 46, wherein, The first electrode contacts the first wiring via a solder layer. The second electrode is in contact with the second wiring via a solder layer.

49. The optical detection module according to claim 47, wherein, The first electrode contacts the first wiring via a solder layer. The second electrode is in contact with the second wiring via a solder layer.

50. The optical detection module according to claim 46, wherein, The photodetector has a height adjustment layer, which is planar on the main surface of the semiconductor substrate and electrically separated from the second contact layer. The first electrode is formed on the height adjustment layer.

51. The optical detection module according to claim 47, wherein, The photodetector has a height adjustment layer, which is planar on the main surface of the semiconductor substrate and electrically separated from the second contact layer. The first electrode is formed on the height adjustment layer.

52. The optical detection module according to claim 48, wherein, The photodetector has a height adjustment layer, which is planar on the main surface of the semiconductor substrate and electrically separated from the second contact layer. The first electrode is formed on the height adjustment layer.

53. The optical detection module according to claim 49, wherein, The photodetector has a height adjustment layer, which is planar on the main surface of the semiconductor substrate and electrically separated from the second contact layer. The first electrode is formed on the height adjustment layer.

54. The light detection module according to any one of claims 1 to 42, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

55. The optical detection module according to claim 43, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

56. The optical detection module according to claim 44, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

57. The optical detection module according to claim 45, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

58. The optical detection module according to claim 46, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

59. The optical detection module according to claim 47, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

60. The optical detection module according to claim 48, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

61. The optical detection module according to claim 49, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

62. The optical detection module according to claim 50, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

63. The optical detection module according to claim 51, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

64. The optical detection module according to claim 52, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

65. The optical detection module according to claim 53, wherein, The first wiring has a length of less than 1 / 4 of the wavelength of the electrical signal propagating in the first wiring.

66. The light detection module according to any one of claims 1 to 42, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

67. The optical detection module according to claim 43, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

68. The optical detection module according to claim 44, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

69. The optical detection module according to claim 45, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

70. The optical detection module according to claim 46, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

71. The optical detection module according to claim 47, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

72. The optical detection module according to claim 48, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

73. The optical detection module according to claim 49, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

74. The optical detection module according to claim 50, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

75. The optical detection module according to claim 51, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

76. The optical detection module according to claim 52, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

77. The optical detection module according to claim 53, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

78. The optical detection module according to claim 54, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

79. The optical detection module according to claim 55, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

80. The optical detection module according to claim 56, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

81. The optical detection module according to claim 57, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

82. The optical detection module according to claim 58, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

83. The optical detection module according to claim 59, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

84. The optical detection module according to claim 60, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

85. The optical detection module according to claim 61, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

86. The optical detection module according to claim 62, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

87. The optical detection module according to claim 63, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

88. The optical detection module according to claim 64, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

89. The optical detection module according to claim 65, wherein, The two sides of the platform are exposed in the direction perpendicular to the optical waveguide.

90. The light detection module according to any one of claims 1 to 42, wherein, The surface of the table moves away from the inner surface of the recess.

91. The optical detection module according to claim 43, wherein, The surface of the table moves away from the inner surface of the recess.

92. The optical detection module according to claim 44, wherein, The surface of the table moves away from the inner surface of the recess.

93. The optical detection module according to claim 54, wherein, The surface of the table moves away from the inner surface of the recess.

94. The optical detection module according to claim 66, wherein, The surface of the table moves away from the inner surface of the recess.

95. The light detection module according to any one of claims 45-53, 55-65, and 67-89, wherein, The surface of the table moves away from the inner surface of the recess.

96. A beat frequency splitting device, have: Fixed wavelength light source; Variable wavelength light source; and According to any one of claims 1 to 95, the light detection module uses light from the fixed-wavelength light source and light from the variable-wavelength light source as the detection light for detection. The wavelength of the light from the wavelength-variable light source is varied by scanning a beat signal having a frequency corresponding to the wavelength difference between the light from the fixed wavelength light source and the light from the variable wavelength light source, and the light from the fixed wavelength light source and the light from the variable wavelength light source are detected by the photodetector.

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