High-speed self-detection quantum cascade laser optical frequency comb based on partitioned coplanar waveguide structure and preparation method thereof

Abstract

The invention discloses a high-speed self-detection quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure and a preparation method thereof. Comprising the following steps: preparing an epitaxial wafer on a substrate; growing a SiO2 layer on the surface of the epitaxial wafer; preparing a semi-insulating material growth window on the surface of the SiO2 layer, and corroding to the lower waveguide layer; filling an insulating material, and removing the residual SiO2 layer; preparing a coplanar waveguide structure; after insulation treatment, preparing a first metal electrode layer, and carrying out annealing treatment and electroplating thickening; preparing a second metal electrode layer on the back surface of the substrate, and performing annealing treatment and cleavage to obtain an optical frequency comb chip; and the optical frequency comb chip is subjected to high-heat-conduction sinking, face-down welding alignment and sintering, and the optical frequency comb is manufactured. According to the invention, by designing the partitioned coplanar waveguide structure, high-bandwidth packaging and efficient RF injection locking of the optical frequency comb of the quantum cascade laser are realized, coherence between comb teeth is enhanced, and system noise is reduced. The optical frequency comb multi-heterodyne self-detection function of the quantum cascade laser is endowed with the high broadband characteristic, an external photoelectric detector is not needed, and a current double-optical-comb system is simplified.

Description

Technical Field

[0001] The present application relates to the field of optical frequency comb technology, and in particular to a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure and a preparation method thereof. Background Art

[0002] An optical frequency comb consists of coherent radiation, whose spectrum is characterized by a series of equally spaced peaks, creating a precise comb scale in the frequency domain. Dual-comb spectroscopy utilizes asynchronous optical sampling from two frequency combs with a small repetition rate difference, enabling parallel detection of multiple comb frequencies without the need for mechanical scanning or dispersion elements.

[0003] Mid-infrared dual-combs have been realized using a variety of light sources, including nonlinear frequency conversion, Kerr microcavities, mid-infrared fiber mode-locked lasers, and quantum cascade lasers. Compared to other light sources, quantum cascade lasers (QCLs) have attracted considerable attention due to their high brightness, gigahertz repetition rates, and electrical pumping. The intersubband transitions within the conduction band and the ultrafast gain recovery mechanism of QCLs impart powerful nonlinearities, several orders of magnitude greater than those of conventional nonlinear crystals. Under certain group velocity dispersion conditions, these strong nonlinear effects can lock the phase relationship between longitudinal modes, thereby achieving optical frequency comb output.

[0004] Compared with other spectroscopic techniques, mid-infrared QCL dual-comb exhibits the following advantages: a microsecond-level wide spectral refresh rate (a million times faster than traditional FTIR) can analyze the dynamic evolution of gases in transient scenarios such as combustion and explosion in real time, providing a new tool for ultrafast chemical reaction kinetics research; an electrically pumped direct radiation source combined with dual-comb autocorrelation noise suppression technology can achieve spectral resolution far higher than FTIR; a chip-level dual-comb light source and optical sampling structure abandon the mechanical external cavity structure, maintaining excellent stability in complex industrial vibration and wide temperature range scenarios, and promoting the evolution of detection systems towards miniaturization. These technical advantages make mid-infrared QCL dual-comb a tool for precise tracing of greenhouse gas emission sources, real-time warning of industrial gas leaks, and high-throughput screening of clinical exhaled breath markers. It is an integrated solution of fast response, high resolution, and miniaturization that meets major national needs and provides technical support for the implementation of environmental governance and precision medicine strategies.

[0005] Currently, the time-frequency coherence of free-running QCL dual-combs is subject to interference from noise such as mechanical vibration, temperature changes, and control circuits, which can cause the heterodyne signal linewidth to broaden to the MHz level, severely limiting the ability to identify high-precision gas molecule fingerprints. Furthermore, current QCL dual-comb systems use fast mercury cadmium telluride detectors to reference and detect multiple heterodyne RF comb signals, which not only increases system cost and complexity but also impacts performance due to the additional noise and bandwidth limitations introduced by the detectors (the -3dB bandwidth is typically less than 2.5GHz). Furthermore, when photodetectors convert optical signals into electrical signals, the overall signal-to-noise ratio of the electrical signal decreases as the number of optical frequency comb teeth increases. Even if the power of a single optical comb tooth remains unchanged, the signal-to-noise ratio of a single comb tooth decreases as the number of comb teeth increases.

[0006] In summary, we are currently facing problems such as complex systems and high noise, and we need to develop a compact, self-detection, and low-noise mid-infrared QCL dual-comb. Summary of the Invention

[0007] To address the aforementioned deficiencies in this field, this application aims to provide a high-speed, self-detecting quantum cascade laser frequency comb based on a partitioned coplanar waveguide structure and a method for its fabrication. By designing a partitioned coplanar waveguide structure, the quantum cascade laser frequency comb achieves high-bandwidth packaging and efficient RF injection locking, enhancing inter-comb coherence and reducing system noise. Its high-bandwidth characteristics enable multi-heterodyne self-detection capabilities in the quantum cascade laser frequency comb, eliminating the need for external photodetectors and significantly simplifying current dual-comb systems.

[0008] According to one aspect of the present application, a method for preparing a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure is provided, comprising:

[0009] A lower 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 the first surface of the substrate to prepare an epitaxial wafer, wherein the epitaxial wafer includes a regular region and a coplanar waveguide region;

[0010] Growing a SiO2 layer on the surface of the ohmic contact layer of the epitaxial wafer;

[0011] Performing a first wet etching on the surface of the SiO2 layer to prepare two semi-insulating material growth windows, and performing a second wet etching along the semi-insulating material growth windows to the lower waveguide layer;

[0012] Filling the second wet-etched region with a semi-insulating material through a semi-insulating material growth window and removing a residual SiO2 layer;

[0013] A coplanar waveguide structure is prepared in the coplanar waveguide region by adopting a third wet etching method;

[0014] An insulating layer is prepared on the surface of the coplanar waveguide structure, and a first metal electrode window is prepared on the surface of the insulating layer;

[0015] preparing a first metal electrode layer through a first metal electrode window, and performing a first annealing treatment and electroplating thickening on the first metal electrode layer;

[0016] After thinning and polishing the second surface of the substrate opposite to the first surface, a second metal electrode layer is prepared, and a second annealing treatment and cleavage are performed to obtain a chip;

[0017] The chip and the high thermal conductivity heat sink are aligned and sintered to produce a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure.

[0018] According to some embodiments of the present application, the substrate material is InP, and the doping concentration is 0.1-2×10 18 cm -3 ;

[0019] The lower waveguide layer is an InP layer with a doping concentration of 1-10×10 16 cm -3 , thickness 2-5μm;

[0020] The lower confinement layer is an InGaAs layer with a doping concentration of 1-6×10 16 cm -3 , thickness 0.1-0.5μm;

[0021] The upper confinement layer is an InGaAs layer with a doping concentration of 1-6×10 16 cm -3 , thickness 0.1-0.5μm;

[0022] The upper waveguide layer is an InP layer with a doping concentration of 2-10×10 16 cm -3 , thickness 2-4 μm;

[0023] The ohmic contact layer is an InP layer with a doping concentration of 5-10×10 18 cm -3 , thickness 0.3-1μm;

[0024] The active region is a superlattice structure of alternating InGaAs and InAlAs with a thickness of 1.5-3μm.

[0025] According to some embodiments of the present application, the etching solution of the first wet etching is HF: NH4F: H2O = 1:2: (2-4), and the etching temperature is 30-50° C.;

[0026] The second wet etching solution is HBr:HNO3:H2O=1:(1-2):(5-20), and the etching temperature is 20-40°C;

[0027] The etching solution of the third wet etching is HBr:HCl:H2O:H2O2=20:10:(50-500):2, and the etching temperature is 30-40°C.

[0028] According to some embodiments of the present application, the width of the semi-insulating material growth window is 10-50 μm; the semi-insulating material is Fe-doped semi-insulating InP material.

[0029] According to some embodiments of the present application, the thickness of the SiO2 layer is 200-600 nm;

[0030] According to some embodiments of the present application, the insulating layer is a SiO2 insulating layer; the thickness of the SiO2 insulating layer is 300-1000 nm.

[0031] According to some embodiments of the present application, the first metal electrode layer is an alloy of Au and In or an alloy of Au and Sn;

[0032] According to some embodiments of the present application, the first metal electrode layer is a 20-50 nm Ti layer and a 200-1000 nm Au layer;

[0033] According to some embodiments of the present application, the second metal electrode layer is Ge / Au / Ni / Au, and has a thickness of 10-40 nm / 20-100 nm / 5-50 nm / 100-1000 nm.

[0034] According to some embodiments of the present application, the temperature of the first annealing treatment and the second annealing treatment is 340-380° C.; the annealing time is 20-80 seconds;

[0035] According to some embodiments of the present application, the first annealing treatment and the second annealing treatment are performed under an inert atmosphere.

[0036] According to some embodiments of the present application, before preparing the second metal electrode layer, the substrate is thinned;

[0037] The thickness of the substrate after thinning is 100-200 μm.

[0038] According to some embodiments of the present application, the high thermal conductivity heat sink includes an AlN heat sink, a diamond heat sink, a Si3N4 heat sink, and a SiC heat sink;

[0039] According to some embodiments of the present application, the high thermal conductivity heat sink is an AlN heat sink.

[0040] According to another aspect of the present application, there is also provided a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure, which is prepared using the above-mentioned preparation method;

[0041] The epitaxial wafer includes a common region and a coplanar waveguide region;

[0042] The coplanar waveguide area occupies 1 / 5-1 / 20 of the epitaxial wafer;

[0043] The coplanar waveguide region includes a coplanar waveguide structure, and the coplanar waveguide structure includes a light-emitting ridge and auxiliary ridges located on both sides of the light-emitting ridge.

[0044] According to another aspect of the present application, a self-detecting dual optical comb system is also provided, comprising the above-mentioned high-speed self-detecting quantum cascade laser optical frequency comb based on the partitioned coplanar waveguide structure.

[0045] According to some embodiments of the present application, a self-detection dual-comb system includes:

[0046] At least two of the above-mentioned high-speed self-detected quantum cascade laser optical frequency combs based on the partitioned coplanar waveguide structure; and

[0047] Driving power supply, DC bias tee (Bias-T), temperature controller, signal generator, high-speed oscilloscope, circulator, lens, reflector, gas cell to be tested and multiple high-speed cables.

[0048] Compared with the prior art, this application has at least the following beneficial effects:

[0049] The present application provides a method for preparing a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure. Through the partitioned coplanar waveguide design, active mode locking is achieved by relying on radio frequency injection to prepare a wide-spectrum and highly coherent mid-infrared optical frequency comb source, and a compact, high-resolution self-detecting dual-comb system is constructed based on this.

[0050] In this application's fabrication method, efficient RF injection is achieved through a high-speed, zoned coplanar waveguide packaging design, addressing device impedance mismatch. A coplanar flip-chip solder joint is employed, promoting longitudinal heat dissipation from the core through direct contact between the active area and the heat sink. This application optimizes the process steps, eliminating the cost pressures and low device yields associated with complex processes. Furthermore, the fabrication method adheres to standard semiconductor process flows, is simple and efficient, and is suitable for industrial mass production.

[0051] The present application provides a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure. QCL, as a fast gain medium, has a sub-picosecond gain recovery time. When appropriate waveguide and packaging designs are adopted, QCL can achieve broadband optical response, which provides a basis for its application in self-detection technology. By adopting self-detection technology, the present application can directly use the optical frequency comb itself as a detector, thereby avoiding these limitations of external detectors. The mutual coupling of the optical radiation between the QCL optical frequency combs in the device will generate multiple heterodyne radio frequency signals. The signal is extracted from the device using a high-speed packaging circuit, and this heterodyne signal in the radio frequency domain can achieve fast and high-precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a schematic cross-sectional view of the entire area of ​​the epitaxial wafer according to an exemplary embodiment of the present application.

[0053] Figure 2 Schematic diagram of the cross section of the entire area where the SiO2 layer is grown on the surface of the epitaxial wafer.

[0054] Figure 3 This is a schematic cross-sectional view of the entire area where a window filled with semi-insulating InP:Fe material is etched on the surface of the SiO2 layer.

[0055] Figure 4 Schematic diagram of the entire cross-section of the deep trench filled with semi-insulating InP:Fe material.

[0056] Figure 5 Schematic diagram of the cross section of the entire area filled with semi-insulating InP:Fe material.

[0057] Figure 6 Schematic diagram of the cross section of the entire area after the remaining SiO2 layer is removed.

[0058] Figure 7 Schematic diagram of the cross section of the coplanar waveguide area of ​​the corroded coplanar waveguide structure.

[0059] Figure 8 Schematic diagram of the cross section of the coplanar waveguide region where the SiO2 layer is grown.

[0060] Figure 9 Schematic diagram of the cross section of the coplanar waveguide area of ​​the front metal electrode window.

[0061] Figure 10 Schematic diagram of the cross section of the coplanar waveguide region where the Ge / Au / Ni / Au first metal electrode layer is deposited.

[0062] Figure 11 Schematic diagram of the cross section of the coplanar waveguide region where the Ti / Au first metal electrode layer is deposited.

[0063] Figure 12 This is a schematic cross-sectional view of the coplanar waveguide region where the first metal electrode layer is thickened by electroplating.

[0064] Figure 13 Schematic diagram of the cross-section of the coplanar waveguide region after backside thinning and polishing.

[0065] Figure 14 Schematic diagram of the cross section of the coplanar waveguide region where the Ge / Au / Ni / Au second metal electrode layer is deposited.

[0066] Figure 15 Schematic diagram of chip sintering to AlN heat sink.

[0067] Figure 16 This is the power-voltage-current test diagram.

[0068] Figure 17 This is a comparison data chart of the normalized microwave rectification between the partitioned coplanar waveguide QCL optical frequency comb and the traditional QCL optical frequency comb.

[0069] Figure 18 For the beat frequency diagram that changes with current, set the resolution bandwidth to 300kHz and the video resolution bandwidth to 3kHz.

[0070] Figure 19 It is the first-order beat signal and high-order harmonic signal.

[0071] Figure 20 The evolution of the beat frequency with the injection frequency. (a)f RF ≈f beatnote (b)f RF ≈2*f beatnote (c)f RF ≈3*f beatnote .

[0072] Figure 21 (a) Spectra of RF injection with varying injection intensity. (b) Spectra of RF injection with varying injection frequency. (c) Spectral broadening versus injection intensity and detuning frequency. (d) RF injection leads to significant mode changes and the generation of additional modes. (e) Comparison of spectra at different injection frequencies.

[0073] Figure 22 The beat frequencies of QCL optical frequency comb 1 and QCL optical frequency comb 2 measured by a spectrum analyzer. The linewidths of both combs are less than 1 kHz.

[0074] Figure 23 (a) Multiheterodyne signals from two QCL optical frequency combs with and without RF injection. (b) Without RF injection, the linewidth of a typical dual-comb multiheterodyne signal is 109.4 kHz. With RF injection, the linewidth of a typical dual-comb multiheterodyne signal is 10 kHz.

[0075] Figure 24 Schematic diagram of the high-speed packaged QCL optical frequency comb of this application.

[0076] Figure 25 Schematic diagram of the self-detection dual-comb system of this application.

[0077] Description of reference numerals:

[0078] 1: substrate, 2: lower waveguide layer, 3: lower confinement layer, 4: active region, 5: upper confinement layer, 6: upper waveguide layer, 7: ohmic contact layer, 8: SiO2 layer, 9: semi-insulating material, 10: SiO2 layer (insulating layer), 11: front metal positive electrode layer (Ge / Au / Ni / Au), 12: front metal negative electrode layer (Ti / Au), 13: back metal layer, 14: Au and In conductive layers on AlN patterned heat sink, 15: AlN heat sink. DETAILED DESCRIPTION

[0079] The following will clearly and completely describe the technical solutions of this application in conjunction with the embodiments of this application. Obviously, the embodiments described are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of this application.

[0080] It is particularly important to note that similar substitutions and modifications made with respect to the present application are obvious to those skilled in the art and are considered to be included in the present application. Relevant persons can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of the present application to implement and apply the technology of the present application. Obviously, the embodiments described are only some of the embodiments of the present application, and not all of them.

[0081] If no specific conditions are specified in this application, the preparation shall be carried out in accordance with conventional conditions or the conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, for which the manufacturers are not specified, are all conventional products that can be obtained commercially.

[0082] The following is a detailed description of this application.

[0083] Radio frequency (RF) injection locking technology achieves active mode locking by periodically modulating the QCL's drive current. The injection frequency matches the round-trip frequency of the laser cavity, forcing the phase synchronization of each longitudinal mode and effectively suppressing the inter-mode phase mismatch caused by dispersion. RF injection locking technology can significantly improve the phase noise performance and stability of the optical comb, enhance its coherence, and simultaneously reduce phase noise and amplitude noise, thereby improving the signal-to-noise ratio of spectral measurements. In addition, RF injection can precisely control the repetition frequency of the optical comb, achieve synchronization between the two optical combs, and further improve spectral resolution and measurement accuracy. This technology can also simplify the system structure, reduce the number of optical components, reduce costs, and enhance the robustness of the system in complex environments.

[0084] Most existing RF injection schemes are limited by impedance mismatches caused by device parasitics, resulting in a significant decrease in transmission efficiency. Furthermore, insufficient modulation depth makes it difficult to achieve both phase locking and spectral broadening. Kapsalidis employs a microstrip waveguide geometry to address the impedance mismatch issue in device RF injection, achieving a -3dB bandwidth of approximately 13 GHz and successfully achieving phase locking. However, due to the solder-on packaging method, the device cannot operate at room temperature, with power output less than 100 mW at -5°C and a very low yield rate. Furthermore, its RF injection method, which involves pressing the probe into the tail of the device, is extremely sensitive to vibration and does not meet application requirements. A π-type electrode device developed by the Institute of Semiconductors achieves room-temperature power exceeding 500 mW, but the introduction of gold wire reduces the system's signal transmission efficiency and results in a -3dB bandwidth of only 5.2 GHz. While achieving initial phase locking, it fails to effectively expand the spectral range, failing to meet application requirements. Current RF injection schemes are limited by the output power constraints of the solder-on packaging method and the application challenge of device-circuit co-design.

[0085] The present application solves the above-mentioned problems existing in the prior art through the following technical solutions.

[0086] Example

[0087] The present application provides a method for preparing a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure.

[0088] A lower waveguide layer 2, a lower confinement layer 3, an active region 4, an upper confinement layer 5, an upper waveguide layer 6, and an ohmic contact layer 7 are sequentially grown on a substrate 1 using a molecular beam epitaxy device or a metal organic chemical vapor deposition device to obtain an epitaxial wafer. Figure 1 The figure shows a cross-sectional view of the epitaxial wafer. The substrate 1 is made of InP with a doping concentration of 2×10 18 cm -3 The material of the lower waveguide layer 2 is InP, and the doping concentration is 2×10 16 cm -3 , thickness 2μm; the lower confinement layer 3 is InGaAs with a doping concentration of 2×10 16 cm -3 , thickness 0.1μm; the active region 4 is a superlattice structure of alternating InGaAs and InAlAs, with a thickness of 1.8μm; the upper confinement layer 5 is InGaAs with a doping concentration of 2×10 16 cm -3 , thickness 0.1μm; the material of the upper waveguide layer 6 is InP, with a doping concentration of 2×10 16 cm -3 , thickness 4μm; the material of the ohmic contact layer 7 is InP, with a doping concentration of 5×10 18 cm-3 , thickness 0.5μm.

[0089] A SiO2 layer 8 with a thickness of 300 nm is grown on the surface of the epitaxial wafer ohmic contact layer 7 by PECVD. Figure 2 FIG. 1 is a cross-sectional schematic diagram of growing a SiO2 layer on the surface of an epitaxial wafer.

[0090] The photoresist is coated on the surface of the SiO2 layer 8 of the sample, and the window pattern is transferred to the sample by photolithography, exposure, and development. The photoresist used is S1805 with a thickness of 0.5μm. Then, the SiO2 layer 8 at the window is removed by SiO2 etching solution HF: NH4F: H2O = 3:6:9 to prepare the required two semi-insulating InP:Fe material windows. Figure 3 Figure 1 shows a cross-sectional schematic diagram of etching a window on the surface of the SiO2 layer to fill it with semi-insulating InP:Fe material. The SiO2 layer 8 serves as a mask for the subsequent MOCVD selective epitaxial growth of the semi-insulating InP:Fe material 9. Because InP material cannot grow epitaxially on the SiO2 layer, InP molecules that land on the SiO2 layer migrate toward the area without the silicon dioxide mask, resulting in epitaxial growth only in the window without the SiO2 layer 8, filling the window to the ohmic contact layer 7.

[0091] Then, two grooves filled with semi-insulating InP:Fe material are etched with an etching solution of HBr:HCl:H2O:H2O2=20:10:100:2. The etching depth reaches the lower waveguide layer 2 and the groove width is 30 μm. The area between the two grooves is the light-emitting ridge device structure to be prepared, such as Figure 4 FIG. 1 is a cross-sectional schematic diagram of etching a deep trench filled with semi-insulating InP:Fe material;

[0092] Then use MOCVD to grow semi-insulating InP:Fe material 9 to fill the two trenches. Figure 5 Schematic diagram of the cross section of the semi-insulating InP:Fe material filled;

[0093] Use etching solution HF: NH4F: H2O = 3:6:9 to remove the remaining SiO2 layer 8, such as Figure 6 FIG. 1 is a schematic cross-sectional view of removing the remaining SiO2 layer.

[0094] The photoresist is coated on the surface of the sample, and the coplanar waveguide pattern is transferred to the sample in a specific area through photolithography, exposure, and development. The photoresist used is AZ6130 with a thickness of 3μm. The required coplanar waveguide structure is etched with an etching solution of HBr:HCl:H2O:H2O2=20:10:100:2 to a depth of substrate 1 and a width of 100μm. Figure 7 FIG. 1 is a schematic cross-sectional view of the coplanar waveguide region of the corroded coplanar waveguide structure.

[0095] The SiO2 layer 10 is covered on the surface of the sample by PECVD for insulation treatment. Figure 8 As shown, it is a schematic cross-sectional view of the coplanar waveguide region where the SiO2 layer is grown.

[0096] The photoresist is coated on the surface of the sample, and the window pattern is transferred to the sample through photolithography, exposure, and development. The photoresist used is AZ6130 with a thickness of 5μm. Then, the SiO2 layer 10 at the window is removed by SiO2 etching solution HF: NH4F: H2O = 3:6:9 to prepare the required front metal electrode window, such as Figure 9 Shown is a schematic cross-sectional view of the coplanar waveguide region of the front metal electrode window.

[0097] The photoresist is coated on the surface of the sample, and the Ge / Au / Ni / Au front metal electrode pattern is transferred to the sample by photolithography, exposure, and development. The photoresist used is NR9 with a thickness of 5μm. The Ge / Au / Ni / Au layer 11 with a thickness of 26nm / 54nm / 15nm / 200nm is grown on the sample by electron beam evaporation. The Ge / Au / Ni / Au layer outside the pattern is removed by adhesive stripping technology. Figure 10 , which is a schematic cross-sectional view of the coplanar waveguide region where the Ge / Au / Ni / Au first metal electrode layer is deposited.

[0098] Then, a thermal annealing treatment was performed at a temperature of 380°C in a nitrogen environment for 40 seconds. A photoresist was coated on the surface of the sample, and the Ti / Au front metal electrode pattern was transferred to the sample through photolithography, exposure, and development. The photoresist used was NR9 with a thickness of 5μm. A Ti / Au layer 12 with a thickness of 20nm / 300nm was grown on the sample by electron beam evaporation. Finally, the Ti / Au layer outside the pattern was removed using a tape stripping technique, as shown in FIG. Figure 11 , which is a schematic cross-sectional view of the coplanar waveguide region where the Ti / Au first metal electrode layer is deposited.

[0099] The photoresist is coated on the surface of the sample, and the electroplated thickened front metal electrode pattern is transferred to the sample through photolithography, exposure, and development. The photoresist used is AZ6130 with a thickness of 5μm; then a plasma stripper is used to remove the glue for 15s, and a step meter is used to measure the height difference between the glue and the first metal electrode layer. Under the condition of constant temperature of 30℃ and constant speed stirring, constant current electroplating is carried out with a current of 1mA. The electroplating time is determined by the thickness to be electroplated, which is generally 5μm. After the electroplating is completed, the height difference is measured with a step meter to determine the electroplating thickness. The Au layer is electroplated to 5μm on the front Ti / Au metal layer evaporated by electron beam of the device, which enhances the heat dissipation characteristics of the device. Then, the excess electroplated Au layer is removed by stripping with glue in acetone, such as Figure 12, which is a schematic cross-sectional view of the coplanar waveguide region after the first metal electrode layer is thickened by electroplating.

[0100] The thickness of the sample substrate was thinned to 150 μm by mechanical thinning and physical and chemical polishing. Figure 13 FIG. 4 shows a schematic cross-sectional view of a coplanar waveguide region after backside thinning and polishing. FIG.

[0101] A Ge / Au / Ni / Au metal layer 13 with a thickness of 26nm / 54nm / 15nm / 200nm was deposited on the back of the sample by electron beam evaporation. A thermal annealing treatment was performed at a temperature of 380°C in a nitrogen environment for 40 seconds. Figure 14 , which is a schematic cross-sectional view of the coplanar waveguide region where the Ge / Au / Ni / Au second metal electrode layer is deposited.

[0102] Cleavage the sample: Divide the entire epitaxial wafer into independent tube cores along the reserved cavity surface and the lateral cleavage channel to obtain the QCL optical frequency comb chip body. Make a front metal electrode pattern that is a mirror image of the front metal electrode pattern of the sample on the surface of the AlN heat sink material, and evaporate the Au and In layers 14 as conductive channels. Then, at a temperature of 320℃, sinter the single chip upside down on the AlN heat sink material 15, and connect the positive and negative electrodes through the isolated electrode layer on the substrate, as shown in the figure. Figure 15 The figure shows the schematic diagram of the chip sintering to the AlN heat sink. The commercial SMP connector is then sintered to the AlN heat sink with solder to complete the high-speed packaging of the QCL optical frequency comb. Figure 25 Figure 2 shows a schematic diagram of a high-speed packaged QCL optical frequency comb.

[0103] The present application also provides a self-detecting dual optical comb system, comprising: two QCL optical frequency combs and their driving power supplies, Bias-T, temperature controller, signal generator, high-speed oscilloscope, circulator, lens, reflector, gas cell to be measured, and high-speed cable. The system is as follows: Figure 25 Figure 2 shows a schematic diagram of a self-detecting dual-comb system.

[0104] A set DC signal provided by the driver power supply is stably applied to QCL frequency combs 1 and 2 via the DC port of the Bias-Tee. After being collimated by a lens, QCL frequency combs 1 and 2 are reflected by a reflector and transmitted along a set optical path, optically injecting into each other. A signal generator outputs a specific RF signal, which is then injected into QCL frequency combs 1 and 2 via the AC port of the Bias-Tee for RF injection locking. After undergoing a series of complex physical processing within QCL frequency combs 1 and 2, the signal is output again from the AC port and, through a circulator, loaded onto a high-speed oscilloscope for analysis. The heterodyne signal extracted from QCL frequency comb 2 serves as the reference signal, while the heterodyne signal extracted from QCL frequency comb 1, after absorption by the gas in the absorption cell under test, serves as the sample signal. After data processing, the spectral absorption signal is obtained. The synchronization between the reference and sample signals ensures the accuracy and integrity of data acquisition during each scan cycle. The signal is used to calculate the type and concentration of the gas under test.

[0105] Application Examples

[0106] like Figure 16 The figure shows the power-voltage-current test diagram of the optical frequency comb of the above embodiment. After high-speed packaging of the QCL optical frequency comb, it is driven in continuous wave mode using a driving power supply within a heat sink temperature range of 10 to 45°C to characterize its power-voltage-current characteristics. The QCL optical frequency comb is installed on a temperature controller using a thermoelectric cooler, and a thermistor is placed as close as possible to perform real-time temperature feedback and adjustment. The emitted optical power is measured by a calibrated thermopile detector placed in front of the laser cavity. At 10°C, the threshold current of the device is 0.6A, and the maximum CW output power is about 700mW. For a device with a width of 6.5μm and a length of 4.5mm, the CW threshold current density is 2.05kA·cm -2 , the peak current density is 3.07 kA·cm -2 .

[0107] like Figure 17 The figure shows the normalized microwave rectified signal at a constant RF power of 10 dBm. The high-frequency characteristics of the QCL optical frequency comb based on a partitioned coplanar waveguide structure were measured using microwave rectification technology. It can be seen that the -3dB bandwidth of the QCL optical frequency comb based on the partitioned coplanar waveguide structure is 16.2 GHz, while the -3dB bandwidth of the traditional inverted solder QCL optical frequency comb is only 0.7 GHz. More importantly, the cutoff frequencies at -15 dB are 40 GHz and 3 GHz, respectively. The observed -3dB bandwidth and -15dB cutoff frequency of our designed QCL optical frequency comb based on the partitioned coplanar waveguide structure are comparable to those of the most advanced microstrip waveguide designs. The inverted solder process also greatly improves heat dissipation, maintaining high power characteristics at room temperature while also achieving high-speed characteristics.

[0108] like Figure 18 As shown, the beat frequency diagram changes with current, with the resolution bandwidth set to 300kHz and the video resolution bandwidth to 3kHz. The high-speed packaged QCL optical frequency comb uses a Bias-Tee to extract the beat frequency signal generated inside the device. In this example, the beat frequency signal of the 4.5mm cavity length QCL optical frequency comb is around 10GHz. The narrow single beat frequency signal indicates that the QCLs exhibit optical frequency comb operation. At this time, the beat frequency is the repetition frequency generated by the equally spaced Fabry-Perot cavity. The repetition frequency can be written as a function of the effective refractive index, device length, and speed of light, that is, The single beat signal lasts from 0.67 A to 0.92 A, exceeding 80% of the full dynamic range. The 40 GHz cutoff frequency allows us to observe high-order harmonic signals.

[0109] like Figure 19 When I = 0.882A, due to the broadband waveguide design and high-speed packaging, not only the first-order beat signal at 10.05 GHz but also the high-order harmonic signals at 20.10, 30.15, and 40.20 GHz are observed.

[0110] like Figure 20 As shown, the beat frequency evolution diagram with the injection frequency. (a)f RF ≈f beatnote (b)f RF ≈2*f beatnote (c)f RF ≈3*f beatnote . The beat signal (also called repetition frequency) of the QCL optical frequency comb can be injection-locked to an external RF oscillator while maintaining full inter-mode coherence. This allows for fully electrical stability similar to a phase-locked loop, with increased comb operating range and stability to optical feedback. With an injection current of 0.882A, an RF signal is injected into the QCL optical frequency comb and the frequency of the signal is gradually increased to evaluate the evolution of the beat signal. The beat spectrum measured when the injection frequency is swept through three harmonic state frequencies is shown. When the frequency of the injected signal approaches the beat signal, the beat signal is pulled towards it. The comb beat signal can be fully controlled by an external RF oscillator, and the locking range is Due to the different injection efficiencies at different frequencies, at the same power RF injection (f RF =15dBm), the locking bandwidth decreases with increasing frequency.

[0111] like Figure 21Shown are (a) RF injection spectra with varying injection intensity; (b) RF injection spectra with varying injection frequency; (c) relationship between spectral broadening, injection intensity, and detuning frequency; (d) RF injection leading to significant mode changes and the generation of additional modes; (e) comparison of spectra at different injection frequencies. The injection locking effect was further verified by comparing the lasing spectra of the QCL optical frequency comb with and without RF injection. The spectral evolution at different injection modulation powers is shown. By injecting an RF signal resonant with the beat frequency into the device, side modes were excited under 30 dBm RF modulation, and the spectrum coverage was extended to ~75 cm -1 , a new distribution of energy across the spectrum was also observed. Furthermore, weak injection preserved the underlying comb state, while strong injection fundamentally altered the operating state. When the QCL optical frequency comb operated at 0.882A, the spectrum broadened to 75cm when RF modulation was enabled and tuned to resonance. -1 , the spectrum envelope shows lobes on both sides and an almost flat center. We also found that if the RF signal is appropriately far away from the beat note (within the locking range of ~3.4MHz), the spectrum broadening result can also be obtained, but the effect is reduced. Therefore, the bandwidth of the QCL comb can be continuously adjusted by changing the RF injection frequency or power. Similarly, the RF injection and Figure 21 (e) The second or third harmonic resonance. Due to the limited RF source power, only a small spectral broadening is observed, but the new energy distribution in the spectrum is clearly visible. RF injection is achieved using a partitioned coplanar waveguide device, allowing for the observation of highly coherent spectral broadening in the ground state or high-harmonic states. This is of great significance for obtaining stable broad-comb sources and promoting the application of self-detecting dual-combs.

[0112] like Figure 22 As shown, the beat frequencies of QCL optical frequency comb 1 and QCL optical frequency comb 2 measured by a spectrum analyzer are both less than 1 kHz. The optical frequency comb generation inside the QCL is very sensitive to optical feedback, and optical isolators are usually used to reduce optical feedback. Under coherent RF injection locking, the QCL optical frequency comb still shows strong robustness despite the presence of optical feedback. Therefore, high-speed packaged QCL optical frequency combs can be transformed into high-speed photodetectors, making them an ideal choice for self-detection technology. In the experiment, we demonstrated a mid-infrared self-detecting dual-comb system based on a high-speed QCL injection-locked at λ~4.6μm. The two QCL optical frequency combs operate at a temperature of 15°C with bias currents of 882mA and 780mA, respectively. Figure 22 The beat signals of the two QCL optical frequency combs are both less than 1 kHz, and the frequency spacing is tuned to 6.83 MHz.

[0113] like Figure 23Shown are (a) the multi-heterodyne signals of two QCL optical frequency combs with and without RF injection. (b) Without RF injection, the linewidth of a typical dual-comb multi-heterodyne signal is 109.4 kHz. With RF injection, the linewidth of a typical dual-comb multi-heterodyne signal is 10 kHz. The dual-comb multi-heterodyne spectrum of the QCL optical frequency comb covers a range of ~1.4 GHz and contains a total of ~200 comb lines, which corresponds to 68 cm -1 The spectrum span is similar to the spectrum range under RF injection (75cm -1 ) and shows that the device operates as an optical frequency comb over the entire spectral bandwidth, comparable to the dual-comb multiheterodyne spectrum without injection ( Figure 23 Compared with (a), there is obvious spectral broadening. Figure 23 (b) shows the linewidth of the dual-comb multi-heterodyne signal without RF injection, which is 109.4 kHz. With RF injection, the linewidth of the dual-comb multi-heterodyne signal is 10 kHz. The narrow and sharp dual-comb teeth demonstrate the well-defined phase relationship between the different modes in the system. Furthermore, a significant broadening of the dual-comb spectrum with RF injection can be observed, demonstrating that RF injection increases the spectral bandwidth of the heterodyne signal and enhances its coherence.

[0114] The above embodiments are only intended to help understand the method and core concept of the present application. It should be noted that, for those skilled in the art, without departing from the principles of the present application, several improvements and modifications may be made to the present application, and such improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for preparing a high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure, characterized in that: include: A lower 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 first surface of the substrate to prepare an epitaxial wafer, wherein the epitaxial wafer includes a regular region and a coplanar waveguide region; growing a SiO2 layer on the surface of the ohmic contact layer of the epitaxial wafer; Performing a first wet etching on the surface of the SiO2 layer to prepare two semi-insulating material growth windows, and performing a second wet etching along the semi-insulating material growth windows to the lower waveguide layer; Filling the second wet-etched region with a semi-insulating material through the semi-insulating material growth window and removing the remaining SiO2 layer; A coplanar waveguide structure is prepared in the coplanar waveguide region by using a third wet etching method; preparing an insulating layer on the surface of the coplanar waveguide structure, and preparing a first metal electrode window on the surface of the insulating layer; preparing a first metal electrode layer through the first metal electrode window, and performing a first annealing treatment and electroplating thickening on the first metal electrode layer; After the electroplating thickening is completed, the second surface of the substrate opposite to the first surface is thinned and polished, a second metal electrode layer is prepared on the second surface, and a second annealing treatment and cleavage are performed to obtain a chip; The chip and the high thermal conductivity heat sink are aligned and sintered to obtain the high-speed self-detection quantum cascade laser optical frequency comb based on the partitioned coplanar waveguide structure.

2. The preparation method according to claim 1, characterized in that The substrate is made of InP with a doping concentration of 0.1-2×10 18 cm -3 ; The lower waveguide layer is an InP layer with a doping concentration of 1-10×10 16 cm -3 , thickness 2-5μm; The lower confinement layer is an InGaAs layer with a doping concentration of 1-6×10 16 cm -3 , thickness 0.1-0.5μm; The upper confinement layer is an InGaAs layer with a doping concentration of 1-6×10 16 cm -3 , thickness 0.1-0.5μm; The upper waveguide layer is an InP layer with a doping concentration of 2-10×10 16 cm -3 , thickness 2-4 μm; The ohmic contact layer is an InP layer with a doping concentration of 5-10×10 18 cm -3 , thickness 0.3-1μm; The active region is a superlattice structure of alternating InGaAs and InAlAs, with a thickness of 1.5-3 μm.

3. The preparation method according to claim 1, characterized in that The etching solution of the first wet etching is HF: NH4F: H2O = 1:2: (2-4), and the etching temperature is 30-50°C; The etching solution of the second wet etching is HBr:HNO3:H2O=1:(1-2):(5-20), and the etching temperature is 20-40°C; The etching solution of the third wet etching is HBr:HCl:H2O:H2O2=20:10:(50-500):2, and the etching temperature is 30-40°C.

4. The preparation method according to claim 1, characterized in that The semi-insulating material growth window width is 10-50 μm; the semi-insulating material is Fe-doped semi-insulating InP material.

5. The preparation method according to claim 1, characterized in that The SiO2 layer has a thickness of 200-600 nm; Optionally, the insulating layer is a SiO2 insulating layer; the thickness of the SiO2 insulating layer is 300-1000 nm.

6. The preparation method according to claim 1, characterized in that The first metal electrode layer is an alloy of Au and In or an alloy of Au and Sn; Optionally, the first metal electrode layer is a Ti layer of 20-50 nm and an Au layer of 200-1000 nm; Further optionally, the second metal electrode layer is Ge / Au / Ni / Au, and has a thickness of 10-40 nm / 20-100 nm / 5-50 nm / 100-1000 nm.

7. The preparation method according to claim 1, characterized in that The temperature of the first annealing treatment and the second annealing treatment is 340-380° C.; the annealing time is 20-80 seconds; Optionally, the first annealing treatment and the second annealing treatment are performed under an inert atmosphere.

8. The preparation method according to claim 1, characterized in that Before preparing the second metal electrode layer, the method further includes thinning the substrate; The thickness of the substrate after the thinning process is 100-200 μm.

9. The preparation method according to claim 1, characterized in that The high thermal conductivity heat sink includes AlN heat sink, diamond heat sink, Si3N4 heat sink, SiC heat sink; Optionally, the high thermal conductivity heat sink is an AlN heat sink.

10. A high-speed self-detecting quantum cascade laser optical frequency comb based on a partitioned coplanar waveguide structure, characterized in that: Prepared by the preparation method according to any one of claims 1 to 9; Wherein, the epitaxial wafer includes a common area and a coplanar waveguide area; The coplanar waveguide region occupies 1 / 5-1 / 20 of the epitaxial wafer; The coplanar waveguide region includes the coplanar waveguide structure, and the coplanar waveguide structure includes a light-emitting ridge and auxiliary ridges located on both sides of the light-emitting ridge.

11. A self-detecting dual-comb system, characterized in that: Including the high-speed self-detecting quantum cascade laser optical frequency comb based on the partitioned coplanar waveguide structure as described in claim 10.

12. The self-detection dual-comb system according to claim 11, characterized in that: include: At least two high-speed self-detecting quantum cascade laser optical frequency combs based on a partitioned coplanar waveguide structure according to claim 10; as well as Driving power supply, DC bias tee, temperature controller, signal generator, high-speed oscilloscope, circulator, lens, reflector, gas cell to be tested and high-speed cable.

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