High-power integrated optical frequency comb chip and optical frequency comb generation method

Abstract

The invention provides a high-power integrated optical frequency comb chip and an optical frequency comb generation method. The high-power integrated optical frequency comb chip comprises a runway-shaped resonant cavity, a coupling waveguide, a radio frequency modulation electrode and a micro heater. The runway-shaped resonant cavity serves as an active gain medium and outputs light to the coupling waveguide through evanescent field coupling. A physical process of quantum walking is excited by applying a radio frequency signal which is overlapped with the longitudinal mode frequency adjacent to the runway-shaped resonant cavity to the radio frequency modulation electrode, so that a frequency modulation type optical frequency comb is generated in the cavity. Compared with a traditional Kerr frequency comb or mode-locked laser, the Kerr frequency comb mode-locked laser based on the quantum walking mechanism and the monolithic integrated semiconductor technology has the advantages of high output power, high beam quality, wide spectrum coverage and high electro-optical bandwidth. A more compact, efficient and reliable frequency comb solution is provided for on-chip spectroscopy, high-speed optical communication and quantum information processing.

Description

Technical Field

[0001] This invention relates to the field of information photonics, specifically to a high-power integrated optical frequency comb chip and a method for generating the optical frequency comb. Background Technology

[0002] Optical frequency combs can generate a series of equally spaced, coherent frequency components across the spectrum, making them key light sources for precision spectral measurement, optical atomic clocks, and high-speed optical communication. Traditional frequency comb sources, such as mode-locked femtosecond lasers, while offering excellent performance, are typically complex, bulky, and expensive, making chip-level integration and application difficult.

[0003] In recent years, the development of integrated photonics has provided new technological pathways for the generation of on-chip frequency combs. Among them, the Kerr optical frequency comb scheme based on the third-order nonlinear effect in microring resonators has attracted widespread attention due to its compact structure. However, this type of scheme usually requires a high pump power threshold, and the conversion efficiency from pump light to frequency comb is generally low, resulting in limited output power and restricting its application in scenarios requiring high signal-to-noise ratio.

[0004] Meanwhile, in the field of semiconductor lasers, generating frequency combs via direct electric pumping has become a highly attractive approach, especially for quantum cascade lasers, which themselves can serve as ideal active media in the mid-infrared band. Early implementations were primarily based on spontaneous mode-locking of Fabry-Perot cavities, resulting in frequency combs that are typically amplitude-modulated and exhibit poor control over mode stability and beam quality. Furthermore, these devices face challenges in packaging and coupling, making it difficult to maintain good beam quality while outputting high power, thus increasing the complexity of subsequent optical systems.

[0005] Quantum walks are a generalization of classical random walks in the quantum world, describing the evolution of a particle in a discrete lattice or continuous space. Unlike the random path selection of classical walks, particles in quantum walks, due to the property of quantum superposition, can propagate along all possible paths simultaneously, and their spatial expansion speed is much higher than that of classical walks due to quantum interference effects. However, combining quantum walks with high-performance, mass-producible integrated processes to achieve stable output of high power and high beam quality at room temperature, while simultaneously possessing high electro-optic modulation bandwidth to support high-speed applications, remains a significant challenge for current technology. Summary of the Invention

[0006] The purpose of this invention is to provide a high-power integrated optical frequency comb chip and an optical frequency comb generation method to overcome the shortcomings of existing integrated optical frequency combs in terms of output power and beam quality.

[0007] This invention is implemented as follows: A high-power integrated optical frequency comb chip includes a substrate and a racetrack-shaped resonant cavity formed on the substrate. The racetrack-shaped resonant cavity includes two semi-circular curved waveguides with opposite openings and a straight waveguide segment connecting one opposite end of the two semi-circular curved waveguides. The other opposite ends of the two semi-circular curved waveguides are free ends. A coupling waveguide is disposed outside the straight waveguide segment of the racetrack-shaped resonant cavity. The coupling waveguide is used to couple laser light from the racetrack-shaped resonant cavity through evanescent field coupling. A radio frequency modulation electrode is disposed between the free ends of the two semi-circular curved waveguides of the racetrack-shaped resonant cavity. A radio frequency signal can be injected into the racetrack-shaped resonant cavity through the radio frequency modulation electrode to excite the quantum walk process, thereby generating a frequency-modulated optical frequency comb.

[0008] This invention can excite a frequency-modulated optical comb by injecting a radio frequency signal that matches its free spectral range into a racetrack-shaped resonant cavity, which has the advantages of wide spectrum and good coherence.

[0009] Preferably, a micro heater is provided on the outer side of the racetrack-shaped resonant cavity away from the straight waveguide section. The micro heater is used to tune the refractive index of the racetrack-shaped resonant cavity by heating to achieve frequency modulation.

[0010] Preferably, the width of both the racetrack-shaped resonant cavity and the coupling waveguide is 10 μm, and the distance between the straight waveguide section of the racetrack-shaped resonant cavity and the coupling waveguide is 0.8 μm.

[0011] Preferably, the radius of the semi-circular curved waveguide of the racetrack-shaped resonant cavity is 500 μm, and the length of the straight waveguide section is 1300 μm.

[0012] Preferably, the RF modulation electrode is a GSG electrode; an insulating layer is disposed between the RF modulation electrode and the substrate. The insulating layer, for example, can be a silicon nitride layer, which is used to reduce parasitic capacitance and improve the RF modulation bandwidth. The insulating layer is relatively thick, approximately 1 μm.

[0013] This invention employs a thick insulating layer design and a low parasitic capacitance electrode structure to achieve stable operation under high-frequency modulation. By optimizing the bias conditions of the resonant cavity, unidirectional laser operation can be achieved, thereby concentrating all optical power to the coupled waveguide output. This invention overcomes the bottlenecks of low output power and poor beam quality of traditional on-chip frequency combs, and possesses excellent near-circular beam quality.

[0014] Preferably, the high-power integrated optical frequency comb chip of the present invention further includes bias electrodes connected to the racetrack-shaped resonant cavity, the coupling waveguide, and the microheater, respectively. Each bias electrode is independent. A bias current is applied to the racetrack-shaped resonant cavity through the bias electrode connected to it. Adjusting this bias current controls the propagation direction of the laser within the cavity, achieving unidirectional operation and concentrating the optical power to the coupling waveguide with a single output port.

[0015] Preferably, the racetrack-shaped resonant cavity is a quantum cascade laser structure that operates in the mid-infrared band.

[0016] Preferably, the substrate material is InP; the racetrack-shaped resonant cavity includes an InP upper cladding, an InP lower cladding, and an InGaAs / AllnAs multilayer structure located between the InP upper cladding and the InP lower cladding; the portion of the coupling waveguide opposite to the straight waveguide section of the racetrack-shaped resonant cavity is an InGaAs / AllnAs multilayer structure, and the remaining portion is an InP waveguide.

[0017] The high-power integrated optical frequency comb chip in this invention, operating in room temperature continuous wave mode, achieves an output power greater than 80mW from a single-output coupled waveguide, with a maximum continuous wave output power greater than 100mW. The optical frequency comb generated by this invention possesses a Hermite-Gaussian spectral envelope with a spectral width greater than 30 cm⁻¹. -1 The laser output from the coupled waveguide has a near-circular far-field spot distribution.

[0018] The specific method for generating a high-power integrated optical frequency comb using the aforementioned high-power integrated optical frequency comb chip is as follows: (1) Apply a bias current to the racetrack-shaped resonant cavity to generate laser oscillation and enable the laser to move unidirectionally within the cavity; (2) Apply an RF signal whose frequency coincides with the longitudinal mode frequency adjacent to the racetrack-shaped resonant cavity to the RF modulation electrode to excite the quantum walk mechanism and generate a frequency-modulated optical frequency comb. (3) The laser in the racetrack-shaped resonant cavity is coupled into the coupled waveguide through evanescent field coupling, and the coupled waveguide outputs a high-power, wide-spectrum optical frequency comb.

[0019] The high-power integrated optical frequency comb chip provided by this invention can be used in optical systems, making it possible to apply it in on-chip dual-comb spectroscopy, gas sensing, high-speed optical communication or quantum information processing.

[0020] The core working mechanism of the high-power integrated optical frequency comb chip of this invention lies in the following: by applying an RF signal matching the frequency of the adjacent longitudinal mode of the resonant cavity to the RF modulation electrode, a physical process of "quantum walk" is excited within the cavity. This process causes the optical field to expand in the frequency domain, thereby directly generating a frequency-modulated optical frequency comb, rather than a traditional amplitude-modulated optical comb. By setting independent bias electrodes for the racetrack-shaped resonant cavity and precisely adjusting its bias current, the laser can be forced to travel unidirectionally within the ring cavity, thereby concentrating all optical power into the coupled waveguide, significantly improving output power and efficiency.

[0021] The chip features a meticulously optimized structure. A thick silicon nitride insulating layer is fabricated between the RF modulation electrode and the semiconductor substrate. This design significantly reduces the parasitic capacitance of the GSG electrode, enabling the device to achieve a 3 dB electro-optic modulation cutoff frequency exceeding 10 GHz, laying the foundation for high-speed frequency comb applications. Simultaneously, the coupled waveguide design ensures that the laser can be coupled out of the racetrack-shaped cavity in a low-loss, high-efficiency manner, forming a near-circular, excellent far-field beam with beam quality comparable to traditional Fabry-Perot cavity lasers.

[0022] The present invention has the following beneficial effects: 1. High output power: Thanks to the unidirectional operating mechanism and efficient evanescent field output coupling design, the device can achieve a maximum output power of over 100 mW on a single end face under continuous wave operation at room temperature. The output power of the quantum walk frequency comb also reaches 80.9 mW, which is far higher than most on-chip frequency comb solutions.

[0023] 2. Excellent overall performance: This device also possesses broad spectral coverage (>30 cm⁻¹). -1 It features high frequency modulation characteristics, high modulation bandwidth (>10 GHz), and near-diffraction-limited beam quality, solving the problem of balancing high power and high performance.

[0024] 3. Technological advancement and practicality: By combining the advanced quantum walk physics mechanism with mature and mass-producible semiconductor dry etching technology, a compact, stable, and easy-to-package on-chip high-power frequency comb solution is provided, which greatly promotes its practical application in fields such as chip-level dual-comb spectroscopy and high-speed optical communication. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the high-power integrated optical frequency comb chip of the present invention.

[0026] Figure 2 This is a schematic diagram of laser oscillation and final unidirectional traveling wave output in the racetrack-shaped resonant cavity of the present invention.

[0027] Figure 3 This is a schematic diagram of the racetrack-shaped resonant cavity and coupled waveguide of the present invention, which are coupled through evanescent field and output light.

[0028] Figure 4 This is a spectrum diagram of the optical frequency comb generated in an embodiment of the present invention.

[0029] Figure 5 This is a power-current-voltage characteristic curve of the high-power integrated optical frequency comb chip of the present invention.

[0030] Figure 6This is the far-field spot pattern of the output laser in an embodiment of the present invention. Detailed Implementation

[0031] This invention provides a high-power integrated optical frequency comb chip based on semiconductor technology and quantum walk mechanism. For example... Figure 1 As shown, the high-power integrated optical frequency comb chip structure includes a substrate 1 and a racetrack-shaped resonant cavity 2 (RT), a coupling waveguide 3 (WG), an RF modulation electrode 4, and a microheater 5 (HT) on the substrate 1. The substrate 1 is a semiconductor substrate, specifically an InP semiconductor substrate. The racetrack-shaped resonant cavity 2 is located on the substrate 1 and is formed by a dry etching process. The racetrack-shaped resonant cavity 2 includes two semi-circular curved waveguides with opposite openings. One end of the two semi-circular curved waveguides is connected by a straight waveguide segment, and no straight waveguide segment is provided between the other end of the two semi-circular curved waveguides (this other end is a free end). Therefore, compared to a complete racetrack, the racetrack-shaped resonant cavity 2 in this invention is a racetrack-shaped resonant cavity 2 lacking a "straight track".

[0032] Coupled waveguide 3 is a strip waveguide located outside the straight waveguide section of the racetrack-shaped resonant cavity 2. The distance between coupled waveguide 3 and the straight waveguide section of the racetrack-shaped resonant cavity 2 (i.e., the coupling gap width) is 0.8 μm. (See...) Figure 3 Both the coupling waveguide 3 and the racetrack-shaped resonant cavity 2 have a waveguide width of 10 μm. In the racetrack-shaped resonant cavity 2, the radius of the semi-circular curved waveguide is 500 μm, and the length of the straight waveguide section is 1300 μm. The structure of the racetrack-shaped resonant cavity 2 includes an InP upper cladding, an InP lower cladding, and an InGaAs / AllnAs multilayer structure located between the InP upper and lower cladding. The portion of the coupling waveguide 3 opposite to the straight waveguide section of the racetrack-shaped resonant cavity 2 also adopts an InGaAs / AllnAs multilayer structure, while the remaining portion is an InP waveguide. Light circulates and resonates in the racetrack-shaped resonant cavity 2, maintaining the characteristics of a ring resonant cavity while providing a straight waveguide section for efficient evanescent wave coupling with the coupling waveguide 3. The coupling waveguide 3 is used to couple the light from the racetrack-shaped resonant cavity 2 to the output; coupling is achieved through the evanescent field between the two waveguides. The coupling waveguide 3 is an "active coupler" and can be independently electrically controlled.

[0033] The radio frequency modulation electrode 4 is positioned at the missing "straight path" of the racetrack-shaped resonant cavity 2, that is, the radio frequency modulation electrode 4 is positioned opposite to and away from the straight waveguide section of the racetrack-shaped resonant cavity 2. In this invention, the radio frequency modulation electrode 4 is specifically a ground-signal-ground (GSG) electrode, or simply a GSG electrode. A relatively thick (approximately 1 μm) insulating layer separates the GSG electrode from the substrate 1. The insulating layer is made of Si3N4 material, and the GSG electrode is made of gold. The insulating layer is used to reduce parasitic capacitance and increase the radio frequency modulation bandwidth. By applying a radio frequency signal to the radio frequency modulation electrode 4, the laser is driven into a "quantum walk" frequency comb state.

[0034] The microheater 5 is located outside the "straight track" missing in the racetrack-shaped resonant cavity 2, and the microheater 5 includes a straight segment and two arc segments connecting the straight segment. The two arc segments of the microheater 5 are located outside the semi-circular curved waveguide of the racetrack-shaped resonant cavity 2.

[0035] The racetrack-shaped resonant cavity 2, the coupling waveguide 3, and the microheater 5 are each connected to an independent bias electrode. The bias electrode corresponding to the racetrack-shaped resonant cavity 2 is used to control the bias current input to the racetrack-shaped resonant cavity 2, adjusting the gain and resonant frequency. The bias electrode corresponding to the coupling waveguide 3 is used to control the bias current input to the coupling waveguide 3, adjusting the coupling strength or acting as an amplifier to reduce losses and output a high-quality beam. The bias electrode corresponding to the microheater 5 is used to control the current on the microheater 5, achieving modulation of the refractive index of the racetrack-shaped resonant cavity 2, further realizing frequency tuning.

[0036] Combination Figure 2 When the current injected into the racetrack-shaped resonant cavity (RT) exceeds its laser threshold, the gain overcomes the loss, and laser oscillation begins. Initially, light waves in both clockwise (CW) and counterclockwise (CCW) directions oscillate simultaneously. Due to nonlinear effects (mode competition, scattering), the clockwise (CW) direction becomes dominant and suppresses the counterclockwise (CCW) oscillation. The system eventually stabilizes in a unidirectional traveling wave state.

[0037] A radio frequency signal (frequency f) is injected into the racetrack-shaped resonant cavity (RT) through the GSG electrodes. RF The signal is precisely matched to the longitudinal mode spacing of the racetrack-shaped resonator. This radio frequency signal modulates the phase of the racetrack-shaped resonator through the electro-optic effect. Unlike amplitude modulation, which directly "switches" the laser, it periodically "pushes" the phase of the light. In the time domain, this is equivalent to periodically changing the frequency of the light. In the frequency domain, a simple single-frequency light (e.g., with frequency f0) after sinusoidal phase modulation will produce a series of equally spaced sidebands with frequencies of f0 ± n × f. RF , where n is an integer.

[0038] Initially, the laser may oscillate in a single longitudinal mode (e.g., f0). Phase modulation produces sidebands f0±n×f. RF The newly generated sideband frequencies are f0 ± n × f RF The longitudinal mode frequencies adjacent to the racetrack-shaped resonant cavity coincide. These sidebands are located at the resonant frequency of the racetrack-shaped resonant cavity and are strongly amplified by the gain medium inside the cavity, unlike in the non-resonant case where they are lost. These amplified sidebands themselves become new "carrier waves." They are also subject to phase modulation of the radio frequency signal, generating their own sidebands (f0±2×f). RF These secondary sidebands resonate with and are amplified by other longitudinal modes. This process of "generating sidebands, resonating and amplifying, and then generating new sidebands" causes the frequency of photons to "walk" across the spectrum step by step. Its mathematical description is consistent with the "quantum walk" model, and the final result is the generation of a frequency comb covering a very wide spectrum.

[0039] The racetrack-shaped resonant cavity 2 serves as the active gain medium, preferably employing a quantum cascade laser structure and operating in the mid-infrared band. By applying a radio frequency modulation signal matching its free spectral range to the racetrack-shaped resonant cavity 2, the quantum walk physical process is excited, achieving efficient generation of a frequency-modulated optical frequency comb. The coupling waveguide 3 is coupled to the racetrack-shaped resonant cavity 2 via an evanescent field, efficiently outputting the generated frequency comb.

[0040] The high-performance integrated optical frequency comb chip provided by this invention is based on a racetrack-shaped resonant cavity structure and a quantum walk dynamics mechanism. The racetrack-shaped resonant cavity is designed with optimized quality factor and dispersion characteristics, precisely matched to the frequency of the applied radio frequency (RF) signal. When an RF signal is applied through an RF modulation electrode, the electro-optic effect of the semiconductor material within the resonant cavity introduces strong coupling between adjacent resonant modes. The mode coupling rate is proportional to the amplitude of the applied RF voltage. After a DC bias current is injected into the resonant cavity to bring it to a lasing state, applying an RF modulation signal excites a quantum walk process within the cavity, causing the optical field to expand in the frequency domain, thus transforming into a series of equally spaced comb-shaped spectra.

[0041] This invention achieves high-power frequency comb output by optimizing the waveguide structure and coupling parameters of the resonant cavity. Specifically, by controlling the ratio between the coupling rate between the coupled waveguide and the resonant cavity and the intrinsic loss of the resonant cavity, efficient energy transfer and output are achieved. Figure 4 The output spectrum of a device according to an embodiment of the present invention is shown. Under suitable bias current and RF signal driving conditions, the output frequency comb spectrum exhibits a broadband Hermite-Gaussian envelope with a spectral width exceeding 30 cm⁻¹. -1 The output state and spectral characteristics of the frequency comb can be dynamically controlled by adjusting the bias current and RF signal drive parameters. For example... Figure 5As shown, under optimized high-power pumping conditions and continuous wave operation at room temperature, the intracavity circulating power obtained from the coupled waveguide output port is high, and continuous wave output power exceeding 100 mW can be achieved.

[0042] The present invention also characterized the beam quality of the generated frequency comb. For example... Figure 6 As shown, measurements were taken at approximately 1 meter from the output cross-section of the coupled waveguide. Testing and data analysis confirmed that the far-field beam output from the chip is nearly circular, highly collimated, and possesses excellent beam quality, making it easy to couple with external optical systems. This result fully demonstrates that this device successfully achieves high-power, high-beam-quality frequency comb output, and can serve as a practical integrated mid-infrared frequency comb light source, directly serving fields such as spectroscopy, sensing, and communication.

[0043] The integrated optical frequency comb chip provided by this invention overcomes the bottlenecks of low output power and poor beam quality of traditional on-chip frequency combs by combining an innovative semiconductor racetrack-shaped resonant cavity structure with the quantum walk mechanism. Radio frequency modulation and thermal tuning work in tandem to achieve precise electrical control over the frequency comb generation process and output state. This device also features high output power, excellent beam quality, wide spectral coverage, and high modulation rate.

[0044] This invention effectively suppresses power fluctuations caused by parasitic mode competition through meticulous waveguide dispersion design and mode control, ensuring the stability and long-term reliability of the output spectrum. This highly integrated monolithic design provides a compact, efficient, and high-performance frequency comb solution for next-generation mid-infrared spectroscopy systems, gas sensing, free-space communication, metrology, quantum information processing, and other applications.

[0045] The specific embodiments described above further illustrate the technical solutions and implementation methods of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-power integrated optical frequency comb chip, characterized in that, The system includes a substrate and a racetrack-shaped resonant cavity formed on the substrate. The racetrack-shaped resonant cavity includes two opposing semicircular curved waveguides and a straight waveguide segment connecting one opposing end of the two semicircular curved waveguides. The other opposing end of the two semicircular curved waveguides is a free end. A coupling waveguide is disposed outside the straight waveguide segment of the racetrack-shaped resonant cavity. The coupling waveguide is used to couple laser light from the racetrack-shaped resonant cavity through evanescent field coupling. A radio frequency modulation electrode is disposed between the free ends of the two semicircular curved waveguides of the racetrack-shaped resonant cavity. A radio frequency signal can be injected into the racetrack-shaped resonant cavity through the radio frequency modulation electrode to excite the quantum walk process, thereby generating a frequency-modulated optical frequency comb.

2. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, in A micro heater is provided on the outer side of the racetrack-shaped resonant cavity away from the straight waveguide section. The micro heater is used to tune the refractive index of the racetrack-shaped resonant cavity by heating to achieve frequency modulation.

3. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, The width of both the racetrack-shaped resonant cavity and the coupling waveguide is 10 μm, and the distance between the straight waveguide section of the racetrack-shaped resonant cavity and the coupling waveguide is 0.8 μm.

4. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, The semi-circular curved waveguide of the racetrack-shaped resonant cavity has a radius of 500 μm, and the straight waveguide section has a length of 1300 μm.

5. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, The radio frequency modulation electrode is a GSG electrode; an insulating layer is disposed between the radio frequency modulation electrode and the substrate.

6. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, It also includes a bias electrode connected to the racetrack-shaped resonant cavity; by adjusting the bias current applied to the racetrack-shaped resonant cavity through the bias electrode, the propagation direction of the laser in the cavity can be controlled to achieve unidirectional operation and concentrate the optical power to the output of the coupled waveguide.

7. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, The racetrack-shaped resonant cavity is a quantum cascade laser structure that operates in the mid-infrared band.

8. The high-power integrated optical frequency comb chip according to claim 1, characterized in that, The substrate material is InP; the racetrack-shaped resonant cavity includes upper and lower InP cladding layers and an InGaAs / AllnAs multilayer structure located between the upper and lower InP cladding layers; the part of the coupling waveguide opposite to the straight waveguide section of the racetrack-shaped resonant cavity is an InGaAs / AllnAs multilayer structure, and the remaining part is an InP waveguide.

9. A method for generating a high-power integrated optical frequency comb, characterized in that, The method, employing the high-power integrated optical frequency comb chip according to any one of claims 1 to 8, comprises the following steps: (1) Apply a bias current to the racetrack-shaped resonant cavity to generate laser oscillation and enable the laser to move unidirectionally within the cavity; (2) Apply an RF signal whose frequency coincides with the longitudinal mode frequency adjacent to the racetrack-shaped resonant cavity to the RF modulation electrode to excite the quantum walk mechanism and generate a frequency-modulated optical frequency comb. (3) The laser in the racetrack-shaped resonant cavity is coupled into the coupled waveguide through evanescent field coupling, and the coupled waveguide outputs a high-power, wide-spectrum optical frequency comb.

10. The application of the high-power integrated optical frequency comb chip according to any one of claims 1 to 8 in on-chip dual-comb spectroscopy, gas sensing, high-speed optical communication or quantum information processing.

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