A quantum cascade laser optical frequency comb based on on-chip dispersion control and a preparation method thereof

Abstract

The application discloses a kind of quantum cascade laser light frequency comb based on on-chip dispersion regulation and preparation method thereof, comprising: sequentially growing lower waveguide layer, passive waveguide layer, low-doped waveguide layer, high-doped waveguide layer, lower limiting layer, active region, upper limiting layer, upper waveguide layer, ohmic contact layer on substrate;SiO2 layer is grown on the surface of ohmic contact layer, and double-groove strip-shaped ridge structure is formed on the surface, and the etching depth reaches the lower waveguide layer;Fill half-insulating InP:Fe in double groove, remove residual SiO2 layer;Insulating layer is grown on the surface of ohmic contact layer;Surface of insulating layer is prepared front metal electrode window;Insulating layer surface and inside front metal electrode window, front metal electrode layer is grown;Thicken front metal electrode layer;Substrate is thinned and polished, back metal electrode layer is grown, annealing treatment, packaging, and quantum cascade laser light frequency comb is obtained.The dispersion problem of mid-infrared quantum cascade laser is solved, the yield is high, the cost is low, the process is simple, and the manufacturing demand of large-scale application is met.

Description

Technical Field

[0001] This application relates to the field of optical frequency comb technology, specifically to an optical frequency comb based on on-chip dispersion-controlled quantum cascade laser and its fabrication method, which couples the basic optical mode of the active region waveguide into the passive waveguide below the active region to form an antisymmetric mode for directional control of its dispersion. Background Technology

[0002] Optical frequency combs (OFCs), as ultrashort pulse sources, consist of a series of coherent spectral lines evenly spaced in the frequency domain. They exhibit characteristics such as a wide spectral range, strong coherence, low phase noise, and narrow linewidth per tooth, leading to their widespread application in sensing, communication, radar, and high-precision measurement. Especially in the mid-infrared (3-24 μm) frequency range, where many gas and biomolecule fingerprint absorption regions are distributed, the development of corresponding optical frequency comb light sources is crucial for advancing basic science, astrophysics, precise ranging, communication, frequency metrology, and precision spectroscopy.

[0003] Quantum cascade lasers (QCLs) are semiconductor laser frequency combs based on conduction band and subband transitions. Relying on their powerful nonlinear effect—four-wave mixing—they can achieve phase locking between different longitudinal modes. Due to their advantages such as tunable wavelength, high power, electric pumping, and on-chip integration, they have been proven to be an ideal light source for generating frequency combs in the mid-infrared region.

[0004] Currently, the high group velocity dispersion (GVD) within ordinary optical frequency comb devices makes it difficult for them to operate as optical frequency combs. Over a wide emission range, dispersion causes the spacing between adjacent longitudinal modes to vary with frequency, preventing optical frequency comb devices from guaranteeing a single beat frequency signal and resulting in non-optical frequency comb operation. When dispersion is uncontrollable, the further the mode is from the center frequency, the greater the frequency shift. Current mid-infrared quantum cascade lasers, without dispersion compensation, often suffer from high internal dispersion, leading to a limited operating range for the optical frequency comb, beat frequency linewidths ranging from tens to thousands of kHz, and extremely poor stability, among other problems, making it difficult to meet the precision measurement requirements of future applications. Summary of the Invention

[0005] To address the aforementioned shortcomings in this field, this application aims to provide an optical frequency comb based on on-chip dispersion-controlled quantum cascade lasers and its fabrication method. This solves the dispersion problem in mid-infrared quantum cascade lasers, achieving high yield, low cost, and simple operation, thus meeting the needs of large-scale application manufacturing.

[0006] According to one aspect of this application, a method for fabricating an optical frequency comb based on an on-chip dispersion-controlled quantum cascade laser is provided, comprising:

[0007] According to the device dispersion compensation requirements, a certain thickness and doping concentration of the following layers are sequentially grown on the substrate: lower waveguide layer, passive waveguide layer, low-doped waveguide layer, high-doped waveguide layer, lower confinement layer, active region, upper confinement layer, upper waveguide layer, and ohmic contact layer.

[0008] A SiO2 layer is grown on the surface of the ohmic contact layer;

[0009] A semi-insulating material growth window was prepared on the surface of the SiO2 layer and then wet-etched down to the lower InP waveguide layer.

[0010] The semi-insulating material is filled into the semi-insulating material window, and the residual SiO2 layer is removed;

[0011] An insulating layer is grown on the surface of the ohmic contact layer;

[0012] A front-side metal electrode window is prepared on the surface of the insulating layer;

[0013] A front metal electrode layer is grown on the surface of the insulating layer and within the front metal electrode window;

[0014] Thicken the front metal electrode layer;

[0015] After thinning and polishing the substrate, a back metal electrode layer is grown, and then annealed and packaged to obtain the optical frequency comb of the quantum cascade laser.

[0016] According to some embodiments of this application, the thickness of the lower waveguide layer is 1-5 μm; the thickness of the passive waveguide layer is 0.8-3 μm;

[0017] The thickness of the low-doped waveguide layer is 0.5-2 μm; the thickness of the high-doped waveguide layer is 0.5-1.5 μm.

[0018] The thickness of the lower confinement layer is 0.1-0.5 μm; the thickness of the active region is 1.5-3 μm; and the thickness of the upper confinement layer is 0.1-0.5 μm.

[0019] The thickness of the upper waveguide layer is 2-4 μm; the thickness of the ohmic contact layer is 0.3-1 μm.

[0020] According to some embodiments of this application, the substrate material is InP with a doping concentration of 0.1-2 × 10⁻⁶. 18 cm -3 ;

[0021] The lower waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ;

[0022] The passive waveguide layer is made of InGaAs or InAlAs, with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ;

[0023] The low-doped waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ;

[0024] The highly doped waveguide layer is made of InP with a doping concentration of 0.5-5×10⁻⁶. 17 cm -3 ;

[0025] The lower confinement layer material is InGaAs with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ;

[0026] The upper confinement layer material is InGaAs, with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ;

[0027] The upper waveguide layer is made of InP with a doping concentration of 2×10⁻⁶. 16 -10×10 16 cm -3 ;

[0028] The ohmic contact layer is made of InP with a doping concentration of 5 × 10⁻⁶. 18 -10×10 18 cm -3 ;

[0029] According to some embodiments of this application, the active region is a superlattice structure in which InGaAs and InAlAs are grown alternately.

[0030] According to some embodiments of this application, the methods for preparing the semi-insulating material window and the method for preparing the front metal electrode window include: photolithography and wet etching.

[0031] The method for growing the front metal electrode layer includes: photolithography, electron beam evaporation, and adhesive stripping.

[0032] According to some embodiments of this application, the thickness of the SiO2 layer is 300-1000 nm;

[0033] Preferably, the insulating layer material is selected from SiO2, Si3N4, AlN, or InP;

[0034] More preferably, the thickness of the insulating layer is 300-1000 nm.

[0035] According to some embodiments of this application, the semi-insulating material window is a double-groove strip ridge structure with a width of 10-50 μm;

[0036] Preferably, the semi-insulating material includes: Fe-doped semi-insulating InP material, or other matching high thermal conductivity materials.

[0037] According to some embodiments of this application, the front metal electrode layer includes: a Ti layer of 20-50 nm and an Au layer of 200-1000 nm;

[0038] Preferably, the back metal electrode layer is Ge / Au / Ni / Au, with a thickness of 10-40nm / 20-100nm / 5-50nm / 100-1000nm.

[0039] According to some embodiments of this application, the thickened front metal electrode layer includes: after removing the photolithographic sample by plasma stripping for 15 seconds, performing constant current electroplating at a current of 0.5-5mA under constant temperature and constant speed stirring conditions, with an electroplating thickness of 3-10μm.

[0040] According to some embodiments of this application, the annealing temperature is 340-380°C;

[0041] Preferably, the annealing process is carried out under an inert atmosphere for 30-60 seconds.

[0042] According to some embodiments of this application, the substrate is thinned to a thickness of 100-200 μm.

[0043] According to another aspect of this application, an optical frequency comb based on on-chip dispersion-controlled quantum cascade laser is provided, comprising, from bottom to top: a back metal electrode layer, a substrate, a lower waveguide layer, a passive waveguide layer, a low-doped waveguide layer, a high-doped waveguide layer, a lower confinement layer, an active region, an upper confinement layer, an upper waveguide layer, an ohmic contact layer, a semi-insulating material layer, an insulating layer, and a front metal electrode layer;

[0044] According to some embodiments of this application, the substrate thickness is 100-200 μm;

[0045] The thickness of the lower waveguide layer is 1-5 μm; the thickness of the passive waveguide layer is 0.8-3 μm.

[0046] The thickness of the low-doped waveguide layer is 0.5-2 μm; the thickness of the high-doped waveguide layer is 0.5-1.5 μm.

[0047] The thickness of the lower confinement layer is 0.1-0.5 μm; the thickness of the active region is 1.5-3 μm; and the thickness of the upper confinement layer is 0.1-0.5 μm.

[0048] The thickness of the upper waveguide layer is 2-4 μm; the thickness of the ohmic contact layer is 0.3-1 μm.

[0049] The thickness of the insulating layer is 300-1000 nm;

[0050] The thickness of the front metal electrode layer is 220-1050 nm;

[0051] The material of the back metal electrode layer is Ge / Au / Ni / Au, and the thickness is 10-40nm / 20-100nm / 5-50nm / 100-1000nm.

[0052] According to some embodiments of this application, the substrate material is InP with a doping concentration of 0.1-2 × 10⁻⁶. 18 cm -3 ;

[0053] The lower waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ;

[0054] The passive waveguide layer is made of InGaAs or InAlAs, with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ;

[0055] The low-doped waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ;

[0056] The highly doped waveguide layer is made of InP with a doping concentration of 0.5-5×10⁻⁶. 17 cm -3 ;

[0057] The lower confinement layer material is InGaAs with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ;

[0058] The upper confinement layer material is InGaAs, with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ;

[0059] The upper waveguide layer is made of InP with a doping concentration of 2×10⁻⁶. 16 -10×10 16 cm -3 ;

[0060] The ohmic contact layer is made of InP with a doping concentration of 5 × 10⁻⁶. 18 -10×10 18 cm -3 ;

[0061] The active region is a superlattice structure with alternating growth of InGaAs and InAlAs.

[0062] Compared with the prior art, this application has at least the following beneficial effects:

[0063] This application provides a quantum cascade laser optical frequency comb, which integrates a passive waveguide into the mid-infrared quantum cascade laser optical frequency comb device structure to form a strip-coupled dual waveguide structure. The basic optical mode of the active region waveguide is coupled to the passive waveguide below the active region to form an antisymmetric mode, which is used to control its dispersion.

[0064] This application provides a method for fabricating a quantum cascade laser optical frequency comb. By using a one-time epitaxy method, the entire structure is grown in a single operation using epitaxial equipment, avoiding the effects of secondary epitaxy and improving the device yield and consistency. Simultaneously, this method optimizes the process steps, eliminating the cost pressures and low device yield associated with complex processes.

[0065] The quantum cascade laser optical frequency comb of this application is filled with semi-insulating InP:Fe material, which not only restricts the lateral diffusion of current, but also greatly improves the lateral heat dissipation performance of the quantum cascade laser optical frequency comb, and ensures that the light-emitting ridge is not subjected to excessive pressure from the soft solder, thereby improving the yield.

[0066] The fabrication method of the optical frequency comb based on the dispersion-controlled quantum cascade laser of the monolithically integrated passive waveguide in this application can successfully fabricate dispersion-compensated optical frequency comb devices. The fabrication method conforms to the standard semiconductor process flow, is simple and efficient to operate, and is suitable for industrial mass production. Attached Figure Description

[0067] Figure 1 This is a schematic cross-sectional view of the epitaxial wafer in an example embodiment of this application.

[0068] Figure 2 This is a schematic cross-sectional view of a SiO2 layer grown on the surface of an epitaxial wafer.

[0069] Figure 3 This is a schematic diagram of a cross-section showing a window filled with semi-insulating InP:Fe material etched onto the surface of a SiO2 layer.

[0070] Figure 4 A schematic diagram of a cross-section showing the deep trenches etched to create a semi-insulating InP:Fe material.

[0071] Figure 5A schematic diagram of a cross-section filled with semi-insulating InP:Fe material.

[0072] Figure 6 A schematic diagram of the cross-section after the remaining SiO2 layer has been removed.

[0073] Figure 7 This is a schematic diagram of the cross-section of the growing insulating layer.

[0074] Figure 8 A schematic cross-sectional view of the front metal electrode window deposited by wet etching to form luminescent ridges on the surface of the insulating layer.

[0075] Figure 9 This is a schematic diagram of the cross-section of the deposited front metal electrode layer.

[0076] Figure 10 A schematic diagram of the cross-section of the electroplated thickened front metal electrode layer.

[0077] Figure 11 This is a schematic diagram of the cross-section of the back side after thinning and polishing.

[0078] Figure 12 This is a schematic diagram of the cross-section of the deposited back metal electrode layer.

[0079] Figure 13 This is a schematic diagram of the packaged device.

[0080] Figure 14 The complete solution for the device modes is (a) an antisymmetric mode and (b) a symmetric mode.

[0081] Figure 15 (a) is the effective refractive index, and (b) is the group velocity dispersion as a function of wavenumber, where the active region (double dotted line), passive waveguide (dotted line), antisymmetric (dotted line), and symmetric (short dotted line) modes are represented.

[0082] Figure 16 For a 4.6 μm wavelength device, the dispersion is directionally modulated by changing the thickness of the passive waveguide. (a) is the effective refractive index, and (b) is the group velocity dispersion as a function of the wave number.

[0083] Figure 17 For a 4.6 μm wavelength device, the dispersion is directionally modulated by changing the width of the passive waveguide. (a) is the effective refractive index, and (b) is the group velocity dispersion as a function of the wavenumber.

[0084] Figure 18 For a 4.6 μm wavelength device, the dispersion is directionally modulated by changing the thickness of InP between the active region and the InGaAs waveguide. (a) is the effective refractive index, and (b) is the group velocity dispersion as a function of wavenumber.

[0085] Figure 19For a device with a wavelength of 8 μm, the dispersion is directionally modulated by changing the thickness of the passive waveguide. (a) is the effective refractive index, and (b) is the group velocity dispersion as a function of the wave number.

[0086] Figure 20 For an 8μm wavelength device, the dispersion is directionally modulated by changing the width of the passive waveguide. (a) is the effective refractive index, and (b) is the group velocity dispersion as a function of the wave number.

[0087] Figure 21 For an 8μm wavelength device, the dispersion (a) effective refractive index and (b) group velocity dispersion as a function of wavenumber were directionally modulated by changing the thickness of InP between the active region and the InGaAs waveguide.

[0088] Figure 22 (a) Device cavity surface view; (b) Test pattern of the light spot emitted by the device.

[0089] Figure 23 This is a power-voltage-current test diagram.

[0090] Figure 24 This is the laser spectrum of the device.

[0091] Figure 25 This is a beat frequency test diagram of the device.

[0092] Explanation of reference numerals in the attached figures:

[0093] 1: Substrate, 2: Lower waveguide layer, 3: Passive waveguide layer, 4: Low-doped waveguide layer, 5: High-doped waveguide layer, 6: Lower confinement layer, 7: Active region, 8: Upper confinement layer, 9: Upper waveguide layer, 10: Ohmic contact layer, 11: SiO2 layer, 12: Semi-insulating material, 13: Insulating layer, 14: Front metal electrode layer, 15: Back metal electrode layer. Detailed Implementation

[0094] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0095] It should be particularly noted that similar substitutions and modifications made to this application are obvious to those skilled in the art, and they are all considered to be included in this application. Those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this application to implement and apply the technology of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0096] Unless otherwise specified, this application is conducted under standard conditions or conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, whose manufacturers are not specified, are all conventional products that can be obtained commercially.

[0097] The following is a detailed description of this application.

[0098] The lack of dispersion compensation in mid-infrared quantum cascade lasers (QCLs) leads to a series of problems, including a limited operating range for the optical frequency comb, a beat frequency linewidth ranging from tens to thousands of kHz, and extremely poor stability, making it difficult to meet the precision measurement requirements of future applications. Current solutions mainly focus on fabricating dispersion compensation structures. Since the active region of QCLs provides gain, dispersion compensation structures are often designed directly onto the waveguide using processes such as etching. For example, a dielectric material is grown on the end face of the QCL to form a Gires-Tournois interferometer (GTI) to achieve dispersion compensation; or a double-chirped mirror (DCM) structure is etched onto the QCL to achieve dispersion compensation.

[0099] Compared to the DCM structure, the GTI structure offers greater design flexibility and simplicity. However, the GTI structure provides a smaller range and degree of dispersion compensation, and has a lower yield. While the DCM structure can provide a large amount of dispersion compensation, it sacrifices significant device performance, is less convenient for later production, and has a complex manufacturing process with low production efficiency. Currently used technologies are suitable for long-wavelength bands and are not compatible with all mid-infrared bands.

[0100] To address the aforementioned issues and ensure device dispersion control and compatibility across all mid-infrared bands, this application provides a method for fabricating a quantum cascade laser optical frequency comb based on dispersion control using a monolithically integrated passive waveguide, comprising:

[0101] (1) Using molecular beam epitaxy or metal-organic chemical vapor deposition, a lower waveguide layer 2, a passive waveguide layer 3, a lightly doped waveguide layer 4, a heavily doped waveguide layer 5, a lower confinement layer 6, an active region 7, an upper confinement layer 8, an upper waveguide layer 9, and an ohmic contact layer 10 are sequentially grown on substrate 1 to obtain an epitaxial wafer. For example... Figure 1 The figure shown is a schematic diagram of the cross-section of the epitaxial wafer.

[0102] The substrate 1 is made of InP with a doping concentration of 2×10⁻⁶. 18 cm -3 The material of the lower waveguide layer 2 is InP, with a doping concentration of 2×10⁻⁶. 16 cm -3 Passive waveguide layer 3 is InGaAs with a doping concentration of 2×10⁻⁶. 16 cm -3The low-doped waveguide layer 4 is made of InP with a doping concentration of 5 × 10⁻⁶. 16 cm -3 The highly doped waveguide layer 5 is made of InP with a doping concentration of 1×10⁻⁶. 17 cm -3 The lower confinement layer 6 is InGaAs with a doping concentration of 2×10⁶. 16 cm -3 The active region 7 is a superlattice structure with alternating growth of InGaAs and InAlAs; the upper confinement layer 8 is InGaAs with a doping concentration of 2×10⁻⁶. 16 cm -3 The upper waveguide layer 9 is made of InP with a doping concentration of 2 × 10⁻⁶. 16 cm -3 The ohmic contact layer 10 is made of InP with a doping concentration of 5 × 10⁻⁶. 18 cm -3 .

[0103] (2) A SiO2 layer 11 is grown on the surface of the ohmic contact layer 10 of the epitaxial wafer using PECVD. For example... Figure 2 The diagram shows a cross-sectional view of a SiO2 layer grown on the surface of an epitaxial wafer, with a thickness of 300-1000 nm. In some embodiments, the corresponding SiO2 layer thickness is 450 nm.

[0104] (3) Using photolithography and wet etching, windows filled with semi-insulating InP:Fe material were fabricated on the surface of the SiO2 layer 11. Double trenches were etched using an etching solution, and then filled with semi-insulating InP:Fe material. Combined with... Figures 3-6As shown, in one embodiment, photoresist is first coated onto the surface of the SiO2 layer 11 of the sample, and then the window pattern is transferred to the sample by exposure and development. The photoresist used is AZ6130 with a thickness of 1.5 μm. Then, the excess SiO2 layer 11 is removed using an etching solution HF:NH4F:H2O = 3:6:9 to prepare the two required windows. Then, two trenches are etched using an etching solution HBr:HCl:H2O:H2O2 = 20:10:100:2, with an etching depth exceeding the lower InP waveguide layer 2 and a trench width of 30 μm. The area between the trenches is the structure of the light-emitting ridge device to be fabricated. Finally, two trenches are filled with a semi-insulating material grown by MOCVD. This semi-insulating material is a semi-insulating InP:Fe material 12. The SiO2 layer 11 serves as a mask for the next step of MOCVD selective epitaxy of the semi-insulating InP:Fe material 12. Because InP material cannot be epitaxially grown on the SiO2 layer, InP molecules falling on the SiO2 layer migrate to the region without the silicon dioxide mask, thus allowing epitaxial growth only in the selected region, filling to the ohmic contact layer 10. The remaining SiO2 layer 11 is removed using an etching solution HF:NH4F:H2O = 3:6:9.

[0105] (4) Use PECVD to cover the surface of the sample with an insulating layer 13 for insulation treatment, and wet etch out the deposited front metal electrode window with a light-emitting ridge on the surface of the insulating layer.

[0106] (5) The front metal electrode pattern is fabricated using photolithography. For example... Figure 9 The diagram shows a cross-sectional view of the deposited front metal electrode layer. In some embodiments, photoresist is first coated on the sample surface, and the front metal electrode pattern is transferred onto the sample by exposure and development. The photoresist used is NR9 with a thickness of 5 μm. Ti / Au = 20-50 nm / 200-1000 nm is grown on the sample using electron beam evaporation, and finally the Ti / Au layer outside the pattern is removed using a stripping technique.

[0107] (6) The front metal electrode pattern is thickened by photolithography and electroplating. For example... Figure 10The diagram shows a cross-sectional view of the electroplated thickened front metal electrode layer. In some embodiments, photoresist is first coated onto the sample surface, and the electroplating pattern is transferred onto the sample through exposure and development. The photoresist used is AZ6130 with a thickness of 5 μm. Subsequently, the photoresist is removed using a plasma stripper for 15 seconds, and the height difference between the photoresist and the front metal electrode layer is measured using a profilometer. Under constant temperature (30°C) and constant speed stirring conditions, constant current electroplating is performed at a current of 0.5-5 mA. The electroplating time depends on the desired thickness, typically 3-10 μm. After electroplating, the height difference is measured using a profilometer to determine the electroplating thickness. Electroplating an Au layer thickened to more than 5 μm on the front Ti / Au metal layer evaporated by electron beam enhances the heat dissipation characteristics of the device. Then, a stripping process with the photoresist is performed in acetone to remove the excess electroplated Au layer.

[0108] (7) Figures 11-12 As shown, in some embodiments, the substrate thickness is reduced to 100-200 μm through mechanical thinning and physicochemical polishing. Then, metal electrodes (Ge / Au / Ni / Au: 10-40 nm / 20-100 nm / 5-50 nm / 100-1000 nm) are deposited on the back of the substrate using electron beam evaporation. Thermal annealing is then performed at a specific temperature and under nitrogen atmosphere; optionally, the annealing process is carried out at 340-380°C under nitrogen atmosphere for 30-60 seconds.

[0109] (8) Perform sample cleaving. Divide the entire epitaxial wafer into individual dies along the reserved cavity surface and lateral cleaving channels to obtain the chip body.

[0110] (9) A front metal electrode pattern is fabricated on the surface of the diamond heat sink material, wherein the front metal electrode pattern is a mirror image of the front metal electrode pattern of the sample; the chip body is then inverted and sintered onto the diamond heat sink material 16. In some embodiments, a front metal electrode pattern mirroring the front metal electrode pattern of the sample is first fabricated on the surface of the diamond heat sink material, and Au and In are evaporated as conductive channels. Then, at a temperature of 270°C, a single laser die is inverted and sintered onto the diamond heat sink material 16 under heating conditions, and the leads for the positive and negative electrodes are made through the Au layer isolated on the substrate.

[0111] The diamond heat sink material with the chip body is sintered onto the copper heat sink material to obtain the dispersion-compensated mid-infrared quantum cascade laser frequency comb. In some embodiments, indium is plated onto the oxygen-free copper surface by electroplating. The thickness of the indium layer on the oxygen-free copper has a significant impact on the yield of the laser frequency comb; the thickness is optionally 1.5-3 μm, preferably 2 μm. If the indium layer is too thin, the indium cannot fill the gap between the die surface and the oxygen-free copper heat sink well after sintering, reducing the device's heat dissipation capacity, and may even result in the die and ceramic sheet not being able to be sintered onto the heat sink. If the indium layer is too thick, the indium can easily creep along the front and rear cavity surfaces of the die through the SiO2 insulating layer during flip soldering, causing device leakage or even short circuits. Finally, the diamond heat sink material with the chip body is sintered onto the copper heat sink material to complete the die encapsulation.

[0112] According to some embodiments of this application, the high thermal conductivity heat sink is preferably a diamond heat sink, followed by AlN heat sink, Si3N4 heat sink, and SiC heat sink.

[0113] The technical solution of this application will be further described below with reference to specific embodiments.

[0114] Example

[0115] Fabrication of the quantum cascade laser optical frequency comb of this application:

[0116] An epitaxial wafer is obtained by sequentially growing a lower InP waveguide layer, a passive InGaAs waveguide layer, a lightly doped InP waveguide layer, a heavily doped InP waveguide layer, a lower confinement layer, an active region, an upper confinement layer, an upper InP waveguide layer, and an ohmic contact layer on an n-InP substrate using a molecular beam epitaxy apparatus.

[0117] A 450 nm thick SiO2 layer was grown on the surface of the ohmic contact layer of an epitaxial wafer using PECVD.

[0118] Photoresist was coated onto the SiO2 layer surface of the sample, and then the two window patterns were transferred onto the sample by exposure and development. The photoresist used was AZ6130 with a thickness of 1.5 μm. Then, the excess SiO2 layer was removed with an etching solution HF:NH4F:H2O = 3:6:9 to prepare the two required windows. Then, two trenches were etched with an etching solution HBr:HCl:H2O:H2O2 = 20:10:100:2, with an etching depth exceeding the lower InP waveguide layer and a trench width of 30 μm. The area between the two trenches is the light-emitting ridge device structure to be prepared. The two trenches were filled with semi-insulating InP:Fe material grown by MOCVD.

[0119] The SiO2 insulating layer was coated onto the sample surface using PECVD for insulation treatment, and the front metal electrode window with light-emitting ridges was deposited by wet etching on the surface of the insulating layer.

[0120] Photoresist was coated on the sample surface, and the front metal electrode pattern was transferred onto the sample by exposure and development. The photoresist used was NR9 with a thickness of 5 μm. Ti / Au = 20 nm / 300 nm was grown on the sample by electron beam evaporation, and finally the Ti / Au layer outside the pattern was removed by stripping technique.

[0121] Photoresist was coated onto the sample surface, and the electroplating pattern was transferred onto the sample through exposure and development. The photoresist used was AZ6130, with a thickness of 5 μm. Subsequently, the photoresist was removed using a plasma stripper for 15 seconds, followed by constant current electroplating at 1.2 mA under constant temperature and constant speed stirring conditions at 30℃, with a plating thickness of 5 μm. An Au layer was then electroplated onto the Ti / Au metal layer to thicken it to more than 5 μm, and then a stripping process with the photoresist was performed in acetone.

[0122] The substrate thickness was reduced to 120 μm by mechanical thinning and physicochemical polishing. Then, metal electrodes Ge / Au / Ni / Au: 26 nm / 54 nm / 15 nm / 200 nm were deposited on the back of the sample using electron beam evaporation.

[0123] The entire epitaxial wafer is divided into individual dies along the reserved cavity surface and lateral cleaving channels to obtain the chip body.

[0124] First, a mirror image of the front metal electrode pattern of the sample is fabricated on the surface of the diamond heat sink material, and Au and In are evaporated as conductive channels. Then, at a temperature of 270°C, a single laser die is inverted and sintered on the diamond heat sink material 16 under heating conditions, and the positive and negative electrode leads are made through the Au layer isolated on the substrate.

[0125] Indium is plated to a thickness of 2 μm on the oxygen-free copper surface using electroplating. Finally, the diamond heat sink material containing the chip body is sintered onto the copper heat sink material to complete the die packaging.

[0126] Experimental Example

[0127] Two-dimensional optical simulations of the optical frequency comb structure of Example 1 were performed using the nonlinear finite element method in COMSOL software. The ridge width of the active region was set to 7 μm, where waveguide loss significantly affects the performance of QCLs. Therefore, accurate calculation of the optical cavity characteristics is crucial. In doped media, losses arise from the absorption of free carriers. Based on the Drude-Lorentz model, the real and imaginary parts of the refractive index of each doped material were calculated as functions of the intrinsic refractive index. A MATLAB program was written and embedded into COMSOL software to calculate the waveguide loss of the laser.

[0128] Integrating passive InGaAs waveguides into the optical frequency comb structure of a mid-infrared quantum cascade laser to form a strip-coupled dual waveguide structure, such as...Figure 13 As shown, the basic optical modes of the active waveguide are coupled into the passive InGaAs waveguide below the active region to form antisymmetric modes, which are used to modulate its dispersion. The complete solutions for the two different GVD modes are shown below. Figure 14 As shown.

[0129] For the obtained characteristic solution, the optical field mode is selected as an antisymmetric mode. The corresponding group velocity dispersion is calculated based on the refractive index of the antisymmetric mode. The simulation results are as follows: Figure 15 As shown. In this case, a large frequency shift occurs between the anti-crossing and coupling mode GVD peaks of the propagation vector. This means that dispersion compensation can be achieved in the antisymmetric mode GVD(-) at the specified wavelength. If the gain medium of this specific wavelength is placed in the waveguide containing the antisymmetric mode GVD(-), and a passive material is placed in another waveguide, a strong mode selection mechanism occurs, which is crucial for the application of dual waveguides in semiconductor laser dispersion compensation. The overall structure meets the design requirements and can provide negative dispersion to compensate for intracavity dispersion.

[0130] At this point, the coupled dual waveguide structure can provide -2500 fs. 2 The compensation amount of approximately / mm satisfies the dispersion compensation requirements of the optical frequency comb in a 4.6μm quantum cascade laser. Furthermore, only one of the two waveguides contains active gain, enabling single-mode operation of the desired antisymmetric mode.

[0131] This monolithically integrated passive InGaAs waveguide coupling method for dispersion modulation can cover the entire 3-12μm mid-infrared band, meeting current dispersion modulation requirements for mid-infrared quantum cascade laser frequency combs. Dispersion can be directionally modulated based on the device's inherent positive dispersion value. This case study illustrates fine dispersion modulation around wavelengths near 4.6μm and 8μm. For the 4.6μm device, changing the thickness and width of the passive InGaAs waveguide adjusts the dispersion compensation range. Figures 16-17 The dispersion compensation amount can be modulated by changing the thickness of InP between the active region and the InGaAs waveguide. Figure 18 The same applies to devices with a wavelength of 8μm. Figures 19-21 ).

[0132] Figure 22 The optical field distribution during lasing of the optical frequency comb of this application is shown, which conforms to the coupled dual waveguide structure of this application, that is, the basic optical mode of the active region waveguide is coupled into the passive InGaAs waveguide below the active region to form an antisymmetric mode.

[0133] Figure 23The power-current-voltage characteristics of the optical frequency comb of this application are shown. At 20°C, the continuous wave threshold current is 0.67 A, the peak current is 1.05 A, and the peak power is 778 mW. For a device with a width of 7.395 μm and a length of 5 mm, the threshold current density is 1.812 kA·cm⁻¹. -2 The maximum current density is 2.839 kA·cm. -2 The current dynamic range is 1.57, and the optical frequency comb operating range accounts for more than 92% of the light emission range.

[0134] Figure 24 The optical frequency comb of this application shows a continuous emission spectrum up to 97 cm⁻¹. -1 .

[0135] Figure 25 As can be seen, when the intermodal beat frequency test is performed using a spectrum analyzer, the narrowest beat frequency linewidth is 1.8kHz at 0.9A.

[0136] Meanwhile, devices fabricated in the same batch of processes also exhibited similar performance, with good uniformity and stability, as shown in Table 1:

[0137] Table 1 Test Data

[0138]

[0139]

[0140] As can be seen from the data in the table above, the preparation process of this application can guarantee uniformity and stability. It does not require multiple secondary epitaxy cycles, the process is simple, and the yield is high.

[0141] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of this application. It should be noted that, for those skilled in the art, several improvements and modifications can be made to this application without departing from the principles of this application, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A method for fabricating an optical frequency comb based on an on-chip dispersion-controlled quantum cascade laser, characterized in that, include: A lower waveguide layer, a passive waveguide layer, a low-doped waveguide layer, a high-doped waveguide layer, a lower confinement layer, an active region, an upper confinement layer, an upper waveguide layer, and an ohmic contact layer are sequentially grown on a substrate. A SiO2 layer is grown on the surface of the ohmic contact layer; the thickness of the SiO2 layer is 300-1000 nm. A semi-insulating material growth window is prepared on the surface of the SiO2 layer and then wet-etched to the lower waveguide layer; The semi-insulating material is filled into the growth window of the semi-insulating material, and the residual SiO2 layer is removed; An insulating layer is grown on the surface of the ohmic contact layer; the thickness of the insulating layer is 300-1000 nm. A front-side metal electrode window is prepared on the surface of the insulating layer; A front metal electrode layer is grown on the surface of the insulating layer and within the front metal electrode window; Thicken the front metal electrode layer; After thinning and polishing the substrate, a back metal electrode layer is grown, and then annealed and packaged to obtain the quantum cascade laser optical frequency comb. The insulating layer material is selected from SiO2, Si3N4, AlN or InP; The growth window of the semi-insulating material is a double-groove strip-ridge structure with a width of 10-50 μm; The semi-insulating material includes: Fe-doped semi-insulating InP material; The passive waveguide layer has a thickness of 0.8-3 μm; the material of the passive waveguide layer is InGaAs or InAlAs, and the doping concentration is 1-10×10⁻⁶. 16 cm -3 .

2. The preparation method according to claim 1, characterized in that, The thickness of the lower waveguide layer is 1-5 μm; The thickness of the low-doped waveguide layer is 0.5-2 μm; the thickness of the high-doped waveguide layer is 0.5-1.5 μm. The thickness of the lower confinement layer is 0.1-0.5 μm; the thickness of the active region is 1.5-3 μm; and the thickness of the upper confinement layer is 0.1-0.5 μm. The thickness of the upper waveguide layer is 2-4 μm; the thickness of the ohmic contact layer is 0.3-1 μm.

3. The preparation method according to claim 2, characterized in that, The substrate is made of InP with a doping concentration of 0.1-2×10⁻⁶. 18 cm -3 ; The lower waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ; The low-doped waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ; The highly doped waveguide layer is made of InP with a doping concentration of 0.5-5×10⁻⁶. 17 cm -3 ; The lower confinement layer material is InGaAs with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ; The upper confinement layer material is InGaAs, with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ; The upper waveguide layer is made of InP with a doping concentration of 2×10⁻⁶. 16 -10×10 16 cm -3 ; The ohmic contact layer is made of InP with a doping concentration of 5 × 10⁻⁶. 18 -10×10 18 cm -3 .

4. The preparation method according to claim 2, characterized in that, The active region is a superlattice structure with alternating growth of InGaAs and InAlAs.

5. The preparation method according to claim 1, characterized in that, The methods for preparing the semi-insulating material growth window and the front metal electrode window include: photolithography and wet etching. The method for growing the front metal electrode layer includes: photolithography, electron beam evaporation, and adhesive stripping.

6. The preparation method according to claim 1, characterized in that, The front metal electrode layer includes a 20-50nm Ti layer and a 200-1000nm Au layer.

7. The preparation method according to claim 6, characterized in that, The back metal electrode layer is Ge / Au / Ni / Au, with a thickness of 10-40nm / 20-100nm / 5-50nm / 100-1000nm.

8. The preparation method according to claim 1, characterized in that, The annealing temperature is 340-380℃; the annealing time is 30-60s.

9. The preparation method according to claim 8, characterized in that, The annealing process is performed under an inert atmosphere.

10. The preparation method according to claim 1, characterized in that, The substrate is thinned to a thickness of 100-200 μm.

11. A frequency comb for an on-chip dispersion-controlled quantum cascade laser, prepared by any one of the preparation methods described in claims 1-10, characterized in that, From bottom to top, the layers are: back metal electrode layer, substrate, lower waveguide layer, passive waveguide layer, low-doped waveguide layer, high-doped waveguide layer, lower confinement layer, active region, upper confinement layer, upper waveguide layer, ohmic contact layer, semi-insulating material layer, insulating layer, and front metal electrode layer. The substrate has a thickness of 100-200 μm; The thickness of the lower waveguide layer is 1-5 μm; the thickness of the passive waveguide layer is 0.8-3 μm. The thickness of the low-doped waveguide layer is 0.5-2 μm; the thickness of the high-doped waveguide layer is 0.5-1.5 μm. The thickness of the lower confinement layer is 0.1-0.5 μm; the thickness of the active region is 1.5-3 μm; and the thickness of the upper confinement layer is 0.1-0.5 μm. The thickness of the upper waveguide layer is 2-4 μm; the thickness of the ohmic contact layer is 0.3-1 μm. The thickness of the insulating layer is 300-1000 nm; The thickness of the front metal electrode layer is 220-1050 nm; The material of the back metal electrode layer is Ge / Au / Ni / Au, and the thickness is 10-40nm / 20-100nm / 5-50nm / 100-1000nm.

12. The quantum cascade laser optical frequency comb according to claim 11, characterized in that, The substrate is made of InP with a doping concentration of 0.1-2×10⁻⁶. 18 cm -3 ; The lower waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ; The passive waveguide layer is made of InGaAs or InAlAs, with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ; The low-doped waveguide layer is made of InP with a doping concentration of 1-10×10⁻⁶. 16 cm -3 ; The highly doped waveguide layer is made of InP with a doping concentration of 0.5-5×10⁻⁶. 17 cm -3 ; The lower confinement layer material is InGaAs with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ; The upper confinement layer material is InGaAs, with a doping concentration of 1-6×10⁻⁶. 16 cm -3 ; The upper waveguide layer is made of InP with a doping concentration of 2×10⁻⁶. 16 -10×10 16 cm -3 ; The ohmic contact layer is made of InP with a doping concentration of 5 × 10⁻⁶. 18 -10×10 18 cm -3 ; The active region is a superlattice structure with alternating growth of InGaAs and InAlAs.

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