Monolithic integrated dual-wavelength quantum cascade laser and preparation method thereof

Abstract

The invention provides a monolithic integrated dual-wavelength quantum cascade laser and a preparation method thereof. The laser comprises a substrate; the lower waveguide and the lower limiting layer are sequentially grown on the upper surface of the substrate, the lower limiting layer sequentially comprises a laser region, a passive phase region and a gain amplification region in the cavity length direction, and electrical isolation trenches are etched between the laser region and the passive phase region and between the passive phase region and the gain amplification region; the active layer grows on the upper surface of the lower limiting layer of the laser area and the gain amplification area, and the passive layer grows on the upper surface of the lower limiting layer of the passive phase area; the upper limiting layer grows on the upper surface of the active layer; the upper waveguide covers the upper surfaces of the upper limiting layer and the passive layer; and the semi-insulating layer comprises a ridge-shaped waveguide, and the semi-insulating layer grows on the two sides of the ridge-shaped waveguide.

Description

Technical Field

[0001] The present disclosure relates to the field of semiconductor lasers, and particularly to a monolithic integrated dual-wavelength quantum cascade laser and a preparation method thereof. Background Art

[0002] As an efficient coherent light source in the mid-infrared band, the Quantum Cascade Laser (QCL) is irreplaceable in the fields of trace gas detection and on-line monitoring of industrial processes because it covers the "fingerprint region" of the characteristic absorption of gas molecules. Traditional external cavity tunable lasers rely on mechanical gratings or filter films to achieve dual-wavelength switching, which have defects such as large volume and poor long-term stability, and are difficult to meet the miniaturization requirements of industrial on-site detection. Although the monolithic integrated dual-DFB (Distributed Feedback)-QCL scheme with a frequency interval of the GHz order can reduce the volume, the two DFB gratings cause mode competition due to sharing the gain medium, resulting in an imbalance in the output power of the dual wavelengths, and its wavelength interval is determined by the fixed grating period and cannot be dynamically adjusted to match the shift of the gas absorption peak. In addition, the traditional monolithic scheme lacks an effective temperature drift compensation mechanism, and the change in the device operating temperature will cause a large wavelength drift, seriously restricting the long-term detection reliability of the Differential Absorption Spectroscopy (DAS) system. Summary of the Invention

[0003] (I) Technical Problems to be Solved

[0004] In view of the above problems, the present disclosure provides a monolithic integrated dual-wavelength quantum cascade laser and a preparation method thereof to at least partially solve the technical problems of low precision, poor long-term stability, and large volume of current traditional monolithic integrated lasers.

[0005] (II) Technical Solutions

[0006] The present disclosure provides a monolithic integrated dual-wavelength quantum cascade laser, including: a substrate; a lower waveguide and a lower confinement layer, which are sequentially grown on the upper surface of the substrate. Wherein, along the cavity length direction on the lower confinement layer, there are sequentially a laser region, a passive phase region, and a gain amplification region, and electrical isolation trenches are etched between the laser region and the passive phase region, and between the passive phase region and the gain amplification region; an active layer and a passive layer, the active layer is grown on the upper surface of the lower confinement layer of the laser region and the gain amplification region, and the passive layer is grown on the upper surface of the lower confinement layer of the passive phase region; an upper confinement layer, which is grown on the upper surface of the active layer; an upper waveguide, which covers the upper surfaces of the upper confinement layer and the passive layer; a semi-insulating layer, including a ridge waveguide, and the semi-insulating layer is grown on both sides of the ridge waveguide.

[0007] According to an embodiment of the present disclosure, it further includes: a front electrode and a back electrode; the front electrode is located on the upper surface of the upper waveguide; the back electrode is located on the lower surface of the substrate and is opposite to the position of the lower waveguide.

[0008] According to an embodiment of the present disclosure, a first-order distributed feedback Bragg grating is fabricated on the upper confinement layer in the laser region for realizing laser output.

[0009] According to an embodiment of the present disclosure, the active layer is a repeating period InGaAs / InAlAs stacked layer, and the number of repeating periods is 30 - 50.

[0010] According to an embodiment of the present disclosure, the passive phase region includes an intrinsic InGaAs layer with a doping concentration of 1×10 15 -5×10 15 cm -3 , with a thickness of 1.6 - 3.0 μm and a length of 200 - 800 μm.

[0011] According to an embodiment of the present disclosure, the length of the gain amplification region is 500 - 1500 μm.

[0012] According to an embodiment of the present disclosure, the material of the upper confinement layer is an n-type doped InGaAs layer with a doping concentration of 1×10 15 -5×10 15 cm -3 .

[0013] According to an embodiment of the present disclosure, the material of the front electrode is Ti / Au, and the material of the back electrode is Ge / Au / / Ni / Au.

[0014] According to an embodiment of the present disclosure, the depth of the electrical isolation trench is 1.0 - 1.5 μm, and the width is 50 - 100 μm.

[0015] Another aspect of the present disclosure provides a method for fabricating a monolithic integrated dual-wavelength quantum cascade laser, including: sequentially growing a lower waveguide and a lower confinement layer on the upper surface of a substrate, wherein the lower confinement layer includes, along the cavity length direction, a laser region formed by etching, a passive phase region epitaxially grown by butt-joint growth technology, and a gain amplification region formed by etching in sequence; etching electrical isolation trenches between the laser region and the passive phase region, and between the passive phase region and the gain amplification region; growing an active layer on the upper surface of the lower confinement layer of the laser region and the gain amplification region, and growing a passive layer on the upper surface of the lower confinement layer of the passive phase region; growing an upper confinement layer on the upper surface of the active layer; epitaxially growing an upper waveguide on the upper surfaces of the upper confinement layer and the passive layer by metal-organic chemical vapor deposition; wet-etching ridge-shaped mesa on both sides of the upper waveguide, and epitaxially growing a semi-insulating layer on both sides of the ridge waveguide by metal-organic chemical vapor deposition.

[0016] (III) Beneficial Effects

[0017] The monolithic integrated dual-wavelength quantum cascade laser and the fabrication method thereof provided by the present disclosure have at least the following technical effects:

[0018] 1. By using butt-joint growth technology, smooth on-chip butt-joint between the active layer and the passive layer is realized. The initial single-mode wavelength is provided by the laser region, and through the synergistic effect of the feedback light intensity adjustment in the gain amplification region and the thermo-optic phase tuning in the passive phase region, dual-wavelength high-stability output is achieved.

[0019] 2. The laser region provides initial wavelength-selective feedback through conventional first-order buried distributed feedback, forms a closed-loop resonant cavity with the passive phase region and the gain amplification region, and realizes dual-wavelength phase locking and light intensity equalization.

[0020] 3. The passive phase region changes the local refractive index of the passive phase region through the thermo-optic effect, dynamically adjusts the dual-wavelength interval and compensates for environmental temperature drift, and ensures the long-term matching accuracy between the dual wavelengths and the target gas absorption peak.

[0021] 4. By adjusting the carrier injection level in the gain amplification region, the light feedback intensity is dynamically changed, enabling the laser to operate in the multi-mode lasing region. The gain saturation effect is used to suppress cavity mode competition, and finally the dual longitudinal modes (wavelength interval Δλ = 0.01 - 0.1 μm) are locked as the main oscillation modes, with the output power difference < 5%, fundamentally avoiding the mode competition problem caused by the shared gain medium in traditional dual DFB-QCL.

[0022] 5. Through the electrical isolation trenches between the laser region, the passive phase region and the gain amplification region, power can be applied to each region separately, independently controlling the generation and tuning of the dual wavelengths, breaking through the wavelength fixed limitation, and improving the flexibility and accuracy of the system.

[0023] 6. The process of the monolithic integrated dual-wavelength quantum cascade laser provided by the present disclosure highly coincides with the existing QCL standard buried heterostructure process. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] To more fully understand the present disclosure and its advantages, reference will now be made to the following description in conjunction with the accompanying drawings, in which:

[0025] Figure 1 Schematically shows the structural schematic diagram of the monolithic integrated dual-wavelength quantum cascade laser provided by the embodiment of the present disclosure;

[0026] Figure 2 Schematically shows the cross-sectional scanning electron microscope image (SEM) of the butt joint interface between the active layer and the passive layer after butt-joint growth of the passive layer provided by the embodiment of the present disclosure;

[0027] Figure 3 Schematically shows the loss curves of InGaAs layers with three doping concentrations at different wavelengths calculated by the Drude-Lorentz model provided by the embodiment of the present disclosure;

[0028] Figure 4 Schematically shows the lasing mode diagram of the monolithic integrated dual-wavelength quantum cascade laser provided by the embodiment of the present disclosure;

[0029] Figure 5 Schematically shows the typical spectrum test diagram of the monolithic integrated dual-wavelength quantum cascade laser provided by the embodiment of the present disclosure.

[0030] DESCRIPTION OF THE REFERENCE NUMERALS:

[0031] 1 - Substrate;

[0032] 2 - Lower waveguide;

[0033] 3 - Lower confinement layer;

[0034] 4 - Active layer;

[0035] 5 - Upper confinement layer;

[0036] 6 - Upper waveguide;

[0037] 7 - Semi-insulating layer;

[0038] 8 - Insulating layer;

[0039] 9 - Front electrode;

[0040] 10 - Passive waveguide layer;

[0041] 11 - Antireflection film;

[0042] 12 - High-reflection film;

[0043] 13 - Laser region;

[0044] 14 - Passive phase region;

[0045] 15 - Gain amplification region;

[0046] 16 - Sub - heat sink;

[0047] 17 - Back electrode;

[0048] 18 - Heat sink. Detailed implementation manners

[0049] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the following detailed description, for the sake of explanation, numerous specific details are set forth in order to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is obvious that one or more embodiments can also be implemented without these specific details. In addition, in the following description, descriptions of well - known structures and technologies are omitted to avoid unnecessarily obscuring the concepts of the present disclosure.

[0050] The terms used herein are merely for describing specific embodiments and are not intended to limit the present disclosure. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0051] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0052] The inventors' research found that in the related art, the strong absorption characteristics of molecular vibration-rotation energy levels in the mid-infrared band enable QCL to achieve ultra-high sensitivity detection of gas concentrations at the ppb (one billionth) level, especially suitable for real-time monitoring of gas concentrations in complex environments such as high temperature and high dust in industrial process monitoring. However, environmental interferences in industrial sites (such as aerosol scattering and background radiation noise) will significantly reduce the signal-to-noise ratio of traditional single-wavelength absorption spectroscopy. The sensing technology based on Differential Absorption Spectroscopy (DAS) can effectively suppress such interferences through dual-wavelength differential measurement: its core requirement is that the light source outputs two wavelengths, one precisely locked to the absorption peak of the target gas, and the other in the low-absorption reference area beside the absorption peak (with an interval of about 17 GHz). By calculating the difference value of the signals at the two wavelengths in real time, the common-mode noise can be eliminated. To achieve this technical advantage, the light source needs to simultaneously meet the requirements of adjustable dual-wavelength interval at the GHz level, long-term wavelength stability, and switching rate at the kHz level under the condition of monolithic integration to dynamically track the shift of the gas absorption peak and adapt to the real-time control requirements of industrial systems. In addition, the embedded deployment of industrial equipment requires the volume of the laser to be less than 1 cm³, and the inherent temperature drift problem of QCL needs to be overcome. Therefore, how to achieve high-precision and high-stability dynamic regulation of dual wavelengths in a monolithic integration architecture has become the core proposition for breaking through the application bottleneck of DAS technology.

[0053] In view of this, the embodiments of the present disclosure provide a monolithic integrated dual-wavelength quantum cascade laser and a preparation method thereof. The laser and the preparation method will be introduced below with reference to the accompanying drawings.

[0054] Figure 1 The structural schematic diagram of the monolithic integrated dual-wavelength quantum cascade laser provided by the embodiments of the present disclosure is schematically shown.

[0055] As Figure 1 shown, the laser may include: a substrate 1, a lower waveguide 2, a lower confinement layer 3, an active layer 4, an upper confinement layer 5, an upper waveguide 6, a semi-insulating layer 7, an insulating layer 8, a front electrode 9, a passive waveguide layer 10, an antireflection film 11, a high-reflection film 12, a laser region 13, a passive phase region 14, a gain amplification region 15, a sub-heat sink 16, a back electrode 17, and a heat sink 18.

[0056] Specifically, in the embodiments of the present disclosure, the substrate 1 may be an n-type InP substrate.

[0057] The lower waveguide 2 is grown on the upper surface of the substrate 1, and n-type doped indium phosphide (n-InP) can be used as the waveguide material.

[0058] The lower confinement layer 3 is grown on the upper surface of the lower waveguide 2, and the material can be InGaAs (indium gallium arsenide). Along the cavity length direction on the lower confinement layer 3, there are successively a laser region 13, a passive phase region 14, and a gain amplification region 15. An electrical isolation trench is etched between the laser region 13 and the passive phase region 14, and between the passive phase region 14 and the gain amplification region 15.

[0059] The electrical isolation trench can be formed by dry etching a highly doped layer and then depositing a SiO2 passivation layer to make it a current isolation region, thereby achieving mutual isolation between the electrodes of the three regions and realizing partitioned electrical injection. The depth of the electrical isolation trench can be 1.0 - 1.5 μm, and the width can be 50 - 100 μm.

[0060] The active layer 4 (the active layer of the laser region 13 and the gain amplification region 15) is grown on the upper surface of the lower confinement layer 3 of the laser region 13 and the gain amplification region 15. The active layer 4 is a stacked layer of InGaAs / InAlAs with a repeating period, and the number of repeating periods is 30 - 50. The thickness of the active layer is 1.6 - 3.0 μm, and its lasing wavelength is 4 - 20 μm.

[0061] The passive layer ( Figure 1 not shown in the figure) is grown on the upper surface of the lower confinement layer 3 of the passive phase region 14.

[0062] The upper confinement layer 5 is grown on the upper surface of the active layer 4, that is, only on the active layer 4 of the laser region 13 and the gain amplification region 15. The material of the upper confinement layer 5 is an n - type doped InGaAs layer, and the doping concentration is 1×10 15 - 5×10 15 cm -3 , and the thickness is 300 nm.

[0063] A first - order distributed feedback Bragg grating is fabricated on the upper confinement layer 5 in the laser region 13. This grating can be a buried grating, thereby realizing laser output. The internal part of the laser region 13 realizes single - wavelength lasing through a gain - coupled grating.

[0064] In some exemplary embodiments, a first - order DFB grating can be fabricated on the upper confinement layer 5 of the laser region 13 by holographic exposure. Based on the distributed feedback characteristics of the first - order DFB grating, initial single - mode laser is provided to realize initial single - mode wavelength selection.

[0065] It should be noted that the grating depth can be less than the thickness of the upper confinement layer 5. The grating period is determined by the target wavelength according to the Bragg formula Λ = λ / 2n eff where Λ is the period of the first - order DFB grating, λ is the target wavelength, and n eff is the effective mode refractive index of the DFB laser region.

[0066] The upper waveguide 6 covers the upper surfaces of the upper confinement layer 5 and the passive layer, and the material can be n-type doped indium phosphide (n-InP).

[0067] The semi-insulating layer 7 can be formed by doping iron (Fe) atoms in indium phosphide (InP) material. The semi-insulating layer can include a ridge waveguide and grow on both sides of the ridge waveguide, that is, on the upper surface of the insulating layer 8, as well as on both sides of the lower waveguide 2, the lower confinement layer 3, the active layer 4, the upper confinement layer 5, and the upper waveguide 6.

[0068] Furthermore, the upper waveguide 6, the cover layer, etc. are consistent with the active region structure, and the subsequent processes of butt-joint growth are compatible with the existing QCL buried heterostructure process. The whole forms a ridge waveguide through wet etching to achieve fundamental transverse mode output. At the same time, after the active region of the upper waveguide 6 is wet-etched to form a ridge waveguide, the semi-insulating layer 7 is filled on both sides of the ridge, that is, the semi-insulating layer 7 is filled on the sidewalls of the ridge waveguide, which is used to improve the heat dissipation and insulation of the laser, ensuring that the monolithic integrated dual-wavelength quantum cascade laser can achieve continuous lasing at room temperature.

[0069] The material of the insulating layer 8 can be SiO2, and it is prepared between the semi-insulating layer 7 and the front electrode 9.

[0070] The front electrode 9 is located on the upper surface of the upper waveguide 6, and the material is Ti / Au; the back electrode 17 is fabricated on the back surface of the substrate 1, that is, on the lower surface of the substrate 1, opposite to the position of the lower waveguide 2, and the material is Ge / Au / / Ni / Au.

[0071] The passive waveguide layer 10 can be a low-doped InGaAs passive waveguide layer. The passive waveguide layer 10 in the passive phase region can be formed by first etching the upper confinement layer 5 and the active layer 4 and then selectively butt-joint growing a low-doped InGaAs bulk material.

[0072] Using the butt-joint growth technology, the passive waveguide layer 10 is secondarily epitaxially grown to smoothly butt-joint with the QCL active layer 4, thereby designing a monolithic integrated optical feedback type dual-wavelength QCL for the laser region 13, the passive phase region 14, and the gain amplification region 15.

[0073] Figure 2 Schematically shows a cross-sectional scanning electron microscope image (SEM) of the butt-joint interface between the active layer and the passive layer after butt-joint growing the passive layer provided by the embodiment of the present disclosure.

[0074] As Figure 2 shown, high-quality butt-joint growth realizes the smooth butt-joint (without voids) between the active layer 4 and the passive waveguide layer 10, reducing the interface loss.

[0075] Furthermore, the antireflection film 11 on the cavity surface of the laser region 13 and the high-reflection film 12 on the cavity surface of the gain amplification region 15 can effectively suppress the FP cavity optical mode in the monolithic integrated laser cavity, which is beneficial to the stability of optical feedback and realizes single-sided optical power output.

[0076] The laser region 13 (front electrode layer) can be a DFB laser region, and the length of the DFB laser region is 1.5 mm - 2.0 mm.

[0077] The structure of the passive phase region 14 (front electrode layer) can be n-InP / n-InGaAs / n-InP, including an intrinsic (lightly doped) InGaAs layer with low loss, and the doping concentration is 1×10 15 -5×10 15 cm -3 , the thickness is 1.6 - 3.0 μm, and the length is 200 - 800 μm. The passive phase region 14 adjusts the phase of the feedback light by changing the injected current, enabling low-loss transmission of light in the waveguide. At the same time, through the thermo-optic tuning effect, the refractive index of the material is changed.

[0078] In the embodiment of the present disclosure, the refractive index of the material of the passive phase region 14 is about 3.3, while the refractive index of the material of the QCL active layer is about 3.2, and the two are similar. Therefore, according to the Fresnel formula, the reflectivity of the two passive-active docking interfaces is about 1%. In addition, the difference between the thickness of the passive layer of the passively grown passive phase region 14 and the thickness of the active layer 4 can be controlled within ±200 nm to ensure high coupling efficiency of light between regions.

[0079] Figure 3 Schematically shows the loss curves of InGaAs layers with three doping concentrations at different wavelengths calculated by the Drude-Lorentz model provided in the embodiment of the present disclosure.

[0080] As Figure 3 shown, according to the Drude-Lorentz model, it can be calculated that when the doping concentration of In 0.53 Ga 0.47 As is lower than 5×10 15 cm -3 , within the wavelength range of 3 - 12 μm, the loss of the material is lower than 2.5 dB / cm, which is much lower than the loss of the active region material and is beneficial to reducing the propagation loss of light in the waveguide.

[0081] The length of the gain amplification region 15 (front electrode layer) is 500 - 1500 μm. The gain amplification region 15 has the same active layer as the laser region 13, and the gain amplification region 15 is used to amplify and adjust the intensity of the feedback light.

[0082] Exemplarily, by injecting current into the passive phase region 14, the refractive index of the material can be effectively changed using the thermo-optic effect, enabling rapid dynamic tuning of the phase of the feedback light at the kHz level. As Figure 4 , when the lengths of the laser region 13, the passive phase region 14, and the gain amplification region 15 are set to 2 mm, 1 mm, and 1.2 mm respectively, the reflectivity of the high-reflection film 12 is 0.99, and other parameters are reasonably set, when the initial single-mode laser generated by the laser region 13 is fed back after the phase adjustment of the passive phase region 14 and the intensity adjustment of the gain amplification region 15, there are multiple steady-state solutions for the lasing mode in the cavity. When considering feedback phase matching, there are three steady-state solutions for this monolithic integrated dual-wavelength laser ( ), and their threshold gains are simultaneously modulated by feedback. At this time, by reasonably controlling the magnitude of the injected current, two modes with lower and closer threshold gains ( ) can be preferentially lasing, realizing the monolithic integrated QCL dual-wavelength function. As Figure 5 , this is a typical spectral test diagram of the monolithic integrated dual-wavelength QCL. In addition, by adjusting the feedback phase through the thermo-optic effect and changing the gain magnitude to adjust the feedback intensity, the frequency interval and peak intensity of the dual wavelengths can be regulated.

[0083] There is electrical isolation between the laser region 13, the passive phase region 14, and the gain amplification region 15. After the laser is flip-chip bonded to the sub-heatsink 16, power can be applied separately to the three electrodes, thereby enabling zonal regulation of the monolithic integrated dual-wavelength quantum cascade laser and improving the flexibility and precision of the system.

[0084] The second aspect of the present disclosure provides a method for fabricating a monolithic integrated dual-wavelength quantum cascade laser, which can be applied to a monolithic integrated dual-wavelength quantum cascade laser, including: sequentially growing a lower waveguide 2 and a lower confinement layer 3 on the upper surface of a substrate 1, wherein the lower confinement layer 3 sequentially includes a laser region 13 formed by etching, a passive phase region 14 epitaxially grown by butt-joint growth technology, and a gain amplification region 15 formed by etching along the cavity length direction; etching electrical isolation trenches between the laser region 13 and the passive phase region 14, and between the passive phase region 14 and the gain amplification region 15; growing an active layer 4 on the upper surface of the lower confinement layer 3 of the laser region 13 and the gain amplification region 15, and growing a passive layer on the upper surface of the lower confinement layer 3 of the passive phase region 14; growing an upper confinement layer 5 on the upper surface of the active layer 4; epitaxially growing an upper waveguide 6 on the upper surfaces of the upper confinement layer 5 and the passive layer by metal-organic chemical vapor deposition; wet etching ridge-shaped mesa on both sides of the upper waveguide 6, and epitaxially growing a semi-insulating layer 7 on both sides of the ridge waveguide by metal-organic chemical vapor deposition.

[0085] Exemplarily, at 0 °C, the waveguide 6 on n-InP is etched with hydrochloric acid, then SiO2 is deposited on the upper confinement layer 5. After photolithography, SiO2 is etched with BOE, and the active layer 4 of the DFB laser region 13 and the gain amplification region 15 is defined by the remaining SiO2 on the surface. The exposed upper confinement layer 5 and the active layer 4 are wet-etched with an etchant (H3PO4:H2O2:H2O = 1:1:3) until the n-InP lower waveguide 2. After the epitaxial wafer is strictly cleaned, a low-doped InGaAs passive waveguide layer 10, i.e., the passive phase region 14, is grown by metalorganic chemical vapor deposition (MOCVD) for secondary selective area butt joint. Then, the surface SiO2 is removed with BOE, and SiO2 is deposited again, and the grating fabrication area is defined by photolithography. A first-order DFB grating is fabricated on the upper confinement layer 5 in the DFB laser region 13 by holographic exposure and wet etching, and the surface SiO2 is removed with BOE. After strict surface cleaning, the n-InP upper waveguide 6 is epitaxially grown three times by MOCVD. SiO2 is deposited again. After photolithography, a ridge mesa is wet-etched (the etching depth should be at least more than the active layer 4), and then an InP:Fe semi-insulating layer 7 is epitaxially grown four times on both sides of the ridge by MOCVD. The surface SiO2 is removed with BOE, and SiO2 is deposited again. After photolithographically etching SiO2, the surface InP highly doped layer is dry-etched to form an electrical isolation trench between the regions. Furthermore, an SiO2 insulating layer 8 is deposited on the surface. After photolithography, the SiO2 in the center of the ridge top is etched with BOE, and an electrical injection window is formed by electron beam evaporation of the Ti / Au layer. Gold is electroplated to increase the thickness of the front gold layer to form the Ti / Au front electrode 9. The back surface of the substrate is thinned to 150 μm and polished smoothly, and a 1Ge / Au / Ni / Au back electrode 17 is formed by electron beam evaporation of Ge / Au / Ni / Au. The die is cleaved into dies with different cavity lengths, and a high-reflection film 12 is deposited on the cavity surface of the DFB laser region 13 by electron beam evaporation. At the same time, an antireflection film 11 is deposited on the cavity surface of the gain amplification region 15. Finally, the die is flip-chip bonded to a special submount 16 (such as a diamond submount) and finally sintered to a heat sink 18 (such as a Cu heat sink).

[0086] The monolithic integrated dual-wavelength quantum cascade laser provided by the embodiments of the present disclosure can be applied in the mid-infrared band. The laser is composed of three parts: a laser region, a passive phase region, and a gain amplification region. By fabricating a first-order distributed feedback Bragg grating on the upper confinement layer, single-mode operation is achieved. In addition, using the butt-joint growth technology, a smooth direct butt-joint between the active region of the quantum cascade laser and the low-loss passive waveguide is realized. At the same time, the phase region and the amplification region jointly form an on-chip integrated external cavity. By changing the electrical injection conditions respectively, the phase and intensity of the feedback light are modulated simultaneously. Using the principle of on-chip optical feedback, dual-wavelength lasing with a frequency interval of GHz - 100 GHz is realized. The monolithic integrated feedback quantum cascade laser does not require an external optical path and alignment system, has extremely high stability, can be used for differential absorption spectroscopy, effectively cancels the common-mode noise, significantly improves the signal-to-noise ratio, and can achieve fast and highly sensitive detection of a single target gas.

[0087] Those skilled in the art can understand that the features recited in the various embodiments and / or claims of the present disclosure can be combined or combined in various ways, even if such combinations or combinations are not explicitly recited in the present disclosure. In particular, without departing from the spirit and teachings of the present disclosure, the features recited in the various embodiments and / or claims of the present disclosure can be combined and combined in various ways. All such combinations and / or combinations fall within the scope of the present disclosure.

[0088] Although the present disclosure has been shown and described with reference to specific exemplary embodiments of the present disclosure, those skilled in the art should understand that various changes in form and detail can be made to the present disclosure without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents. Therefore, the scope of the present disclosure should not be limited to the above embodiments, but should be determined not only by the appended claims but also by the equivalents of the appended claims.

Claims

1. A monolithic integrated dual-wavelength quantum cascade laser, characterized in that, Comprising: A substrate; A lower waveguide and a lower confinement layer, which are sequentially grown on the upper surface of the substrate. Wherein, along the cavity length direction on the lower confinement layer, there are sequentially included a laser region, a passive phase region and a gain amplification region. An electrical isolation trench is etched between the laser region and the passive phase region, and between the passive phase region and the gain amplification region; An active layer and a passive layer. The active layer is grown on the upper surface of the lower confinement layer in the laser region and the gain amplification region, and the passive layer is grown on the upper surface of the lower confinement layer in the passive phase region; An upper confinement layer, which is grown on the upper surface of the active layer; An upper waveguide, which covers the upper surfaces of the upper confinement layer and the passive layer; A semi-insulating layer, including a ridge waveguide. The semi-insulating layer is grown on both sides of the ridge waveguide.

2. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, wherein Further comprising: A front electrode and a back electrode; The front electrode is located on the upper surface of the upper waveguide; The back electrode is located on the lower surface of the substrate, opposite to the position of the lower waveguide.

3. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, wherein A first-order distributed feedback Bragg grating is fabricated on the upper confinement layer in the laser region for realizing laser output.

4. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, wherein The active layer is a repetitive periodic InGaAs / InAlAs stacked layer, and the number of repetitive periods is 30 - 50.

5. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, characterized in that, The passive phase region includes an intrinsic InGaAs layer with a doping concentration of 1×10 15 -5×10 15 cm -3 , a thickness of 1.6 - 3.0 μm, and a length of 200 - 800 μm.

6. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, wherein The length of the gain amplification region is 500 - 1500 μm.

7. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, wherein The material of the upper confinement layer is an n-type doped InGaAs layer with a doping concentration of 1×10 15 -5×10 15 cm -3 .

8. The monolithic integrated dual-wavelength quantum cascade laser according to claim 2, characterized in that, The material of the front electrode is Ti / Au, and the material of the back electrode is Ge / Au / / Ni / Au.

9. The monolithic integrated dual-wavelength quantum cascade laser according to claim 1, wherein The depth of the electrical isolation trench is 1.0 - 1.5 μm, and the width is 50 - 100 μm.

10. A method for fabricating a monolithic integrated dual-wavelength quantum cascade laser, characterized in that, Comprising: Growing a lower waveguide and a lower confinement layer sequentially on the upper surface of the substrate. Wherein, along the cavity length direction on the lower confinement layer, there are sequentially included a laser region formed by etching, a passive phase region epitaxially grown by butt-joint growth technology, and a gain amplification region formed by etching; Etching an electrical isolation trench between the laser region and the passive phase region, and between the passive phase region and the gain amplification region; Growing an active layer on the upper surface of the lower confinement layer in the laser region and the gain amplification region, and growing a passive layer on the upper surface of the lower confinement layer in the passive phase region; Growing an upper confinement layer on the upper surface of the active layer; Epitaxially growing an upper waveguide on the upper surfaces of the upper confinement layer and the passive layer by metalorganic chemical vapor deposition; Wet etching a ridge mesa on both sides of the upper waveguide, and epitaxially growing a semi-insulating layer on both sides of the ridge waveguide by metalorganic chemical vapor deposition.

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