Dual optical frequency comb source based on injection locking

Abstract

This application discloses a dual optical frequency comb source based on injection locking, comprising a laser driver, three semiconductor lasers, three waveguide beamsplitters, a waveguide filter, and a waveguide beam combiner. The laser driver drives the three semiconductor lasers, each corresponding to a waveguide beamsplitter. The three waveguide beamsplitters, waveguide filters, and waveguide beam combiner are integrated on a single optical chip. One semiconductor laser operates in continuous wave mode, continuously outputting injected continuous light. After passing through the waveguide filter and waveguide beamsplitter, the injected continuous light is split into two groups of injected continuous light with specific frequencies, which are injected into the other two semiconductor lasers with different operating frequencies. These two semiconductor lasers operate in gain-switching mode under the action of the laser driver. Based on the filtered injected continuous light, these two semiconductor lasers operate in injection-locked mode and generate optical frequency combs respectively. The two optical frequency combs are combined to form a dual optical frequency comb.

Description

Technical Field

[0001] This application belongs to the field of optical frequency comb sources, and more specifically, relates to a dual optical frequency comb source based on injection locking. Background Technology

[0002] An optical frequency comb (OFC) refers to a spectrum composed of a series of uniformly spaced frequency components with a stable coherent phase relationship. In the frequency domain, an optical frequency comb appears as a spectral sequence with equal frequency intervals, while in the time domain, it appears as an ultrashort pulse sequence. The frequency intervals in the frequency domain and the pulse widths in the time domain strictly follow a Fourier transform relationship. Because the distribution shape of an optical frequency comb in the frequency domain closely resembles a comb used in everyday life, it is figuratively called an "optical frequency comb," or simply "optical frequency comb." Dual optical frequency combs have important applications in environmental monitoring, advanced manufacturing, and national defense, and their application scenarios are expanding from advanced metrology laboratories to various industrial field applications.

[0003] In dual-comb spectroscopy, one optical comb with a repetition frequency (f) serves as the signal, while another optical comb with a repetition frequency (f+Δf) (Δf << f) samples the signal. A data acquisition card acquires the time-domain signal from the dual optical combs, and Fourier transform is used to reconstruct the frequency domain information of the object under test. Traditionally, dual optical combs are generated by two independently operating femtosecond mode-locked lasers or two mutually frequency-locked ultrashort pulse mode-locked lasers. A slight difference in the laser cavity lengths of the two mode-locked lasers results in different repetition frequencies. In the frequency domain, a precisely defined frequency difference exists between adjacent comb teeth. This difference is detected by a photodetector, yielding a radio frequency (RF) comb with a frequency interval of Δf. This converts the signal frequency from the optical frequency to the RF domain, making it easy to measure using existing electrical equipment (such as oscilloscopes, spectrum analyzers, and data acquisition cards). Compared to traditional Fourier spectrometers, dual optical comb measurement offers higher accuracy, sensitivity, and speed, leading to its rapid development in recent years.

[0004] However, the method of generating optical frequency combs based on mode-locked lasers is limited by the laser cavity length, and its repetition rate is usually only a few megahertz to tens of megahertz. Optical modulation technology in mode-locked lasers can generate optical frequency combs with frequency intervals of tens of megahertz. However, its frequency interval is limited by optoelectronic modulators and radio frequency signal generators, and its cost will increase sharply with the increase of frequency, which is not conducive to large-scale application.

[0005] Microcavity optical frequency combs, which have been developed in recent years, have an inherent advantage in repetition rate, thus making up for the shortcomings of traditional optical frequency comb technology. The reported microcavity Kerr dual optical frequency combs all use two micro-ring resonators to generate an optical frequency comb, and then couple the two optical frequency combs together to form an optical frequency comb. Such dual optical frequency combs require frequency locking of the pump light of the two optical frequency combs and simultaneous stabilization of the working environment of the two micro-rings. Therefore, the structure is complex, the cost is high, and the system stability is poor. Summary of the Invention

[0006] To address the aforementioned problems, this application provides a dual-frequency comb source based on injection locking. Continuous light generated by one semiconductor laser is used as the external injection light for two other semiconductor lasers, achieving injection locking of these two lasers and generating corresponding optical frequency combs. The two generated optical frequency combs are then combined using a waveguide combiner to form a dual-frequency comb. The specific scheme is as follows:

[0007] This application discloses a dual optical frequency comb source based on injection locking, including a laser driver, a first semiconductor laser, a second semiconductor laser, a third semiconductor laser, a first waveguide beamsplitter, a second waveguide beamsplitter, a third waveguide beamsplitter, a waveguide filter, and a waveguide beam combiner. The first waveguide beamsplitter, the second waveguide beamsplitter, the third waveguide beamsplitter, the waveguide filter, and the waveguide beam combiner are integrated on a single optical chip.

[0008] The laser driver is connected to the first semiconductor laser, the second semiconductor laser, and the third semiconductor laser, respectively, and is used to drive the first semiconductor laser, the second semiconductor laser, and the third semiconductor laser. The first semiconductor laser operates in continuous wave mode and is used to continuously output externally injected continuous light. The second semiconductor laser and the third semiconductor laser both operate in gain-switching mode, outputting optical frequency comb pulses based on the externally injected continuous light. The operating frequencies of the second semiconductor laser and the third semiconductor laser are different. The first waveguide beamsplitter, the second waveguide beamsplitter, and the third waveguide beamsplitter each include a first upper port, a second upper port, a first lower port, and a second lower port. One end of the waveguide filter is connected to the first semiconductor laser, and the other end is connected to the first upper port or the first lower port of the first waveguide beamsplitter. The second upper port of the first waveguide beamsplitter is connected to the second lower port of the second waveguide beamsplitter. The second lower port of the first waveguide beamsplitter is connected to the second upper port of the third waveguide beamsplitter. The first upper port or the first lower port of the second waveguide beamsplitter is connected to the second semiconductor laser. The second upper port of the second waveguide beamsplitter is connected to one input port of the waveguide beam combiner. The first upper port or the first lower port of the third waveguide beamsplitter is connected to the third semiconductor laser. The second lower port of the third waveguide beamsplitter is connected to the other input port of the waveguide beam combiner.

[0009] Preferably, the beam splitting ratio of the first waveguide beam splitter is 50:50, and the beam splitting ratios of the second and third waveguide beam splitters are both (90+N):(10-N), where N is a positive integer less than 10.

[0010] Furthermore, the dual optical frequency comb source also includes a waveguide attenuator integrated on the optical chip for attenuating the intensity of the received externally injected continuous light. One end of the waveguide attenuator is connected to the waveguide filter, and the other end is connected to the first upper port or the first lower port of the first waveguide beam splitter.

[0011] Furthermore, the dual optical frequency comb source also includes an adjustable optical delay chip integrated on the optical chip. The adjustable optical delay chip is disposed on the connection path between the second waveguide beamsplitter or the third waveguide beamsplitter and the waveguide beam combiner. It is used to delay the optical frequency comb pulse output from the second upper port of the second waveguide beamsplitter or the optical frequency comb pulse output from the second lower port of the third waveguide beamsplitter, so that the optical frequency comb pulse output from the second upper port of the second waveguide beamsplitter and the optical frequency comb pulse output from the second lower port of the third waveguide beamsplitter arrive at the waveguide beam combiner simultaneously.

[0012] Preferably, the waveguide attenuator comprises a fourth waveguide beamsplitter, a first transmission waveguide, a second transmission waveguide, a fifth waveguide beamsplitter, and a phase modulator. Both the fourth and fifth waveguide beamsplitters include a third upper port and a third lower port. The two ends of the first transmission waveguide are respectively connected to the third upper port of the fourth waveguide beamsplitter and the third upper port of the fifth waveguide beamsplitter. The two ends of the second transmission waveguide are respectively connected to the third lower port of the fourth waveguide beamsplitter and the third lower port of the fifth waveguide beamsplitter. The phase modulator is disposed on the second transmission waveguide.

[0013] Preferably, the first semiconductor laser is a distributed feedback laser, and the second and third semiconductor lasers are both distributed feedback lasers or distributed Bragg reflector lasers.

[0014] Preferably, the laser driver includes a microcontroller, a first driving circuit module, a second driving circuit module, and a third driving circuit module. The microcontroller is connected to the first driving circuit module, the second driving circuit module, and the third driving circuit module, respectively, and is used to control the first driving circuit module, the second driving circuit module, and the third driving circuit module. The first driving circuit module is used to drive the first semiconductor laser, the second driving circuit module is used to drive the second semiconductor laser, and the third driving circuit module is used to drive the third semiconductor laser.

[0015] Preferably, the waveguide filter is an MZ unequal arm interferometer type filter or a micro-ring resonator type optical filter.

[0016] Preferably, the MZ unequal arm interferometer filter is a single-stage MZ unequal arm interferometer or is composed of multiple cascaded MZ unequal arm interferometers.

[0017] Preferably, the micro-ring resonant cavity optical filter consists of a ring waveguide and a coupled straight waveguide connected end to end, wherein the coupled straight waveguide receives externally injected continuous light and couples the externally injected continuous light to the ring waveguide.

[0018] In summary, compared with the prior art, the above-described technical solutions conceived in this application can achieve the following beneficial effects:

[0019] This application provides a dual-frequency comb source based on injection locking, comprising a laser driver, three semiconductor lasers, three waveguide beamsplitters, a waveguide filter, and a waveguide beam combiner. The laser driver drives the three semiconductor lasers, each corresponding to a waveguide beamsplitter. The three waveguide beamsplitters, waveguide filters, and waveguide beam combiner are integrated on a single optical chip. One semiconductor laser operates in continuous-wave mode under the action of the laser driver, continuously outputting externally injected continuous light. After passing through the waveguide filter and waveguide beamsplitter, the externally injected continuous light is split into two sets of externally injected continuous light of specific frequencies, which are injected into the other two semiconductor lasers with different operating frequencies. These two semiconductor lasers operate in gain-switching mode under the action of the laser driver. Based on the filtered externally injected continuous light, these two semiconductor lasers operate in injection-locked mode, each generating an optical frequency comb. The two optical frequency combs are combined to form a dual-frequency comb. Two semiconductor lasers operating in gain-switching mode are locked by the externally injected continuous light from the same semiconductor laser output. Therefore, the center frequencies of the optical frequency combs output by the two semiconductor lasers are the same. However, the operating frequencies of the two semiconductor lasers operating in gain-switching mode are different, so the optical frequency combs output by these two semiconductor lasers have a stable frequency difference, thus forming a dual optical frequency comb.

[0020] Furthermore, the frequency of the filtered externally injected continuous light matches a certain frequency of the spontaneous emission pulses from the two semiconductor lasers operating in gain-switching mode. Therefore, when the externally injected continuous light is fed back into these two semiconductor lasers, the linewidth of the output optical pulses is significantly narrowed, effectively reducing the phase noise of the optical pulses. Each pulse output by these two semiconductor lasers is based on external injection locking, rather than random spontaneous emission. Therefore, the output optical pulses have clearer comb lines and distinguishable comb line spacing, as well as higher comb flatness and coherence. The dual-frequency comb source provided in this application can be implemented based on traditional semiconductor lasers and traditional silicon-based chip processes, making it easy to integrate on-chip. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in this embodiment or the prior art, the drawings used in the description of the embodiment or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 A schematic diagram of a dual optical frequency comb source based on injection locking is provided for an embodiment of this application;

[0023] Figure 2This is a schematic diagram of the structure of the first waveguide beamsplitter, the second waveguide beamsplitter, and the third waveguide beamsplitter in this application;

[0024] Figure 3 A schematic diagram of a dual-optical frequency comb source based on injection locking is provided for another embodiment of this application;

[0025] Figure 4 This is a schematic diagram of the waveguide attenuator structure in one embodiment of this application;

[0026] Figure 5 This application is based on Figure 3 A schematic diagram of a dual-optical frequency comb source based on injection locking is provided;

[0027] Figure 6 This is a schematic diagram of the pulse spectrum of the dual optical frequency comb formed in this application;

[0028] Figure 7 This is a schematic diagram of the structure of the laser driver in this application;

[0029] Figure 8 This is a schematic diagram of the structure of the second and third drive circuit modules in the laser driver of this application;

[0030] Figure 9 This is a schematic diagram of the structure of the MZ unequal-arm interferometer in this application;

[0031] Figure 10 This is a schematic diagram of the structure of an MZ unequal-arm interferometer filter in one embodiment of this application;

[0032] Figure 11 This is a schematic diagram of the structure of the micro-ring resonant cavity type optical filter in the embodiments of this application. Detailed Implementation

[0033] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. 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.

[0034] Many specific details are set forth in the following description in order to provide a full understanding of this application. However, this application may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0035] To facilitate understanding and explanation of the technical solutions provided in the embodiments of this application, the background technology of this application will be described first.

[0036] Dual-frequency combs have important applications in environmental monitoring, advanced manufacturing, and national defense, and their application scenarios have expanded from advanced metrology laboratories to various industrial field applications.

[0037] In dual-comb spectroscopy, one optical comb with a repetition frequency (f) serves as the signal, while another optical comb with a repetition frequency (f+Δf) (Δf << f) samples the signal. A data acquisition card acquires the time-domain signal from the dual optical combs, and Fourier transform is used to reconstruct the frequency domain information of the object under test. Traditionally, dual optical combs are generated by two independently operating femtosecond mode-locked lasers or two mutually frequency-locked ultrashort pulse mode-locked lasers. A slight difference in the laser cavity lengths of the two mode-locked lasers results in different repetition frequencies. In the frequency domain, a precisely defined frequency difference exists between adjacent comb teeth. This difference is detected by a photodetector, yielding a radio frequency (RF) comb with a frequency interval of Δf. This converts the signal frequency from the optical frequency to the RF domain, making it easy to measure using existing electrical equipment (such as oscilloscopes, spectrum analyzers, and data acquisition cards). Compared to traditional Fourier spectrometers, dual optical comb measurement offers higher accuracy, sensitivity, and speed, leading to its rapid development in recent years.

[0038] However, the method of generating optical frequency combs based on mode-locked lasers is limited by the laser cavity length, and its repetition rate is usually only a few megahertz to tens of megahertz. Optical modulation technology in mode-locked lasers can generate optical frequency combs with frequency intervals of tens of megahertz. However, its frequency interval is limited by optoelectronic modulators and radio frequency signal generators, and its cost will increase sharply with the increase of frequency, which is not conducive to large-scale application.

[0039] Microcavity optical frequency combs, which have been developed in recent years, have an inherent advantage in repetition rate, thus making up for the shortcomings of traditional optical frequency comb technology. The reported microcavity Kerr dual optical frequency combs all use two micro-ring resonators to generate an optical frequency comb, and then couple the two optical frequency combs together to form an optical frequency comb. Such dual optical frequency combs require frequency locking of the pump light of the two optical frequency combs and simultaneous stabilization of the working environment of the two micro-rings. Therefore, the structure is complex, the cost is high, and the system stability is poor.

[0040] Based on this, this application provides a dual-frequency comb source based on injection locking, such as... Figure 1As shown, it includes a laser driver, a first semiconductor laser, a second semiconductor laser, a third semiconductor laser, a first waveguide beamsplitter, a second waveguide beamsplitter, a third waveguide beamsplitter, a waveguide filter, and a waveguide beam combiner, all of which are integrated on a single optical chip.

[0041] In one embodiment of the present invention, when the optical chip is made of silicon-based silicon dioxide, three semiconductor lasers and a laser driver are integrated with the optical chip through a hybrid packaging method. In another embodiment of the present invention, when the optical chip is made of a group III-V material such as indium phosphide (InP) or gallium arsenide (GaAs), the three semiconductor lasers can be directly integrated onto the optical chip.

[0042] The laser driver is connected to a first semiconductor laser, a second semiconductor laser, and a third semiconductor laser, respectively, to drive these lasers. The first semiconductor laser operates in continuous wave mode, continuously outputting externally injected continuous light. Both the second and third semiconductor lasers operate in gain-switching mode, outputting optical frequency comb pulses based on the externally injected continuous light. The operating frequencies of the second and third semiconductor lasers are different. The first, second, and third waveguide beamsplitters each include a first upper port, a second upper port, a first lower port, and a second lower port, as shown below. Figure 2 As shown; one end of the waveguide filter is connected to the first semiconductor laser, and the other end is connected to the first upper port or the first lower port of the first waveguide beamsplitter. The second upper port of the first waveguide beamsplitter is connected to the second lower port of the second waveguide beamsplitter. The second lower port of the first waveguide beamsplitter is connected to the second upper port of the third waveguide beamsplitter. The first upper port or the first lower port of the second waveguide beamsplitter is connected to the second semiconductor laser. The second upper port of the second waveguide beamsplitter is connected to one input port of the waveguide beam combiner. The first upper port or the first lower port of the third waveguide beamsplitter is connected to the third semiconductor laser. The second lower port of the third waveguide beamsplitter is connected to the other input port of the waveguide beam combiner.

[0043] In this application, the first, second, and third semiconductor lasers are all driven by a laser driver. Under the drive of the laser driver, the first semiconductor laser operates in continuous wave mode, continuously outputting externally injected continuous light; the second and third semiconductor lasers both operate in gain-switching mode under the drive of the laser driver, outputting optical frequency comb pulses based on the externally injected continuous light. The operating frequencies of the second and third semiconductor lasers are different, exhibiting a slight frequency difference.

[0044] The externally injected continuous light output from the first semiconductor laser is filtered by a waveguide filter to narrow the center spectral line of the input continuous light, preferably to less than 100 MHz. To achieve external injection locking of the second and third semiconductor lasers, a crucial condition is that the wavelength of the light pulses injected into the second and third semiconductor lasers is close to the wavelength of the light pulses they can emit. Therefore, a waveguide filter is used to filter the externally injected continuous light. In this application, the waveguide filter is an MZ unequal-arm interferometer type filter or a micro-ring resonator type optical filter.

[0045] After filtering, the externally injected continuous light is split into two sets of externally injected continuous light by the first waveguide beam splitter. One set of externally injected continuous light is input to the second semiconductor laser through the second waveguide beam splitter, serving as the externally injected light of the second semiconductor laser. The other set of externally injected continuous light is input to the third semiconductor laser through the third waveguide beam splitter, serving as the externally injected light of the third semiconductor laser. The second and third semiconductor lasers achieve injection locking based on the externally injected light and output optical frequency comb pulses respectively. The optical frequency comb pulses output by the second and third semiconductor lasers are combined by the waveguide beam combiner to form a dual optical frequency comb.

[0046] The first, second, and third waveguide beamsplitters in this application can be directional couplers, multimode interferometers (MMI), or MZ interferometers. Those skilled in the art can choose different types of beamsplitters as needed. Specifically, the beam splitting ratio of the first waveguide beamsplitter is set to 50:50, splitting the filtered externally injected continuous light into two, which are then input to the second and third semiconductor lasers respectively. To ensure that the majority of the energy of the optical frequency comb pulses output from the second and third semiconductor lasers is input to the waveguide beam combiner, and that only a small amount of externally injected continuous light is input to the second and third semiconductor lasers, the beam splitting ratio of both the second and third waveguide beamsplitters is set to (90+N):(10-N), where N is a positive integer less than 10. That is, the second and third waveguide beamsplitters can split the input optical pulses or continuous light into beams at a ratio of (90+N):(10-N). Specifically, the beam splitting ratio of the second waveguide beam splitter and the third waveguide beam splitter can be 99:1 or 95:5, etc.

[0047] Both the second and third semiconductor lasers are locked by externally injected continuous light from the output of the first semiconductor laser. Therefore, the center frequencies of the optical frequency combs output by the second and third semiconductor lasers are the same. However, the operating frequencies of the second and third semiconductor lasers are different. Thus, the optical frequency combs output by the second and third semiconductor lasers have a stable frequency difference between the comb teeth of adjacent frequency combs (one frequency comb is output by the second semiconductor laser and the other frequency comb is output by the third semiconductor laser) in each pulse cycle, which can form a dual optical frequency comb.

[0048] The frequency of the filtered externally injected continuous light matches a certain frequency of the spontaneous emission pulses from the second and third semiconductor lasers. Therefore, when the externally injected continuous light is fed back into the second and third semiconductor lasers, the linewidth of the output light pulses from these two lasers is significantly narrowed, effectively reducing the phase noise of the light pulses. Since each pulse output by the second and third semiconductor lasers is based on external injection locking rather than random spontaneous emission, the output light pulses have clearer comb lines and distinguishable comb line spacing, as well as higher comb flatness and coherence. The dual-frequency comb source provided in this application can be implemented based on traditional semiconductor lasers and traditional silicon-based chip processes, making it easy to integrate on-chip.

[0049] In another embodiment of the present invention, the dual optical frequency comb source further includes a waveguide attenuator integrated on the optical chip for attenuating the intensity of the received externally injected continuous light. One end of the waveguide attenuator is connected to a waveguide filter, and the other end is connected to the first upper port or the first lower port of the first waveguide beam splitter. The dual optical frequency comb source structure based on this embodiment is as follows: Figure 3 As shown.

[0050] To ensure successful injection locking of the second and third semiconductor lasers, another crucial condition is that the intensity of the externally injected continuous light fed back to them cannot be too high; otherwise, the externally injected continuous light will affect the stability of the output optical pulses of the second and third semiconductor lasers. Therefore, a waveguide attenuator is used to attenuate the intensity of the externally injected continuous light, thus adjusting its intensity.

[0051] In one embodiment of this application, the waveguide attenuator comprises a fourth waveguide beamsplitter, a first transmission waveguide, a second transmission waveguide, a fifth waveguide beamsplitter, and a phase modulator, as shown in the following structure. Figure 4As shown, both the fourth and fifth waveguide beamsplitters include a third upper port and a third lower port. The two ends of the first transmission waveguide are connected to the third upper port of the fourth waveguide beamsplitter and the third upper port of the fifth waveguide beamsplitter, respectively. The two ends of the second transmission waveguide are connected to the third lower port of the fourth waveguide beamsplitter and the third lower port of the fifth waveguide beamsplitter, respectively. The phase modulator is disposed on the second transmission waveguide.

[0052] Specifically, the beam splitting ratio of the fourth and fifth waveguide beam splitters in the waveguide attenuator is 50:50. The phase modulator modulates the phase of the externally injected continuous light input to the second transmission waveguide, forming a phase difference with the externally injected continuous light transmitted on the first transmission waveguide. After the phase-modulated externally injected continuous light on the second transmission waveguide meets the externally injected continuous light on the first transmission waveguide, they interfere and cancel each other out, thus achieving intensity attenuation of the externally injected continuous light.

[0053] In this embodiment, the specific signal transmission process is as follows:

[0054] The externally injected continuous light output from the first semiconductor laser is transmitted to a waveguide filter. After filtering, it outputs externally injected continuous light of a specific frequency. This specific frequency externally injected continuous light is then input to the fourth waveguide beam splitter of the waveguide attenuator to form two sets of externally injected continuous light of specific frequencies. One set of externally injected continuous light of specific frequencies is input to the first transmission waveguide through the third upper port, and the other set is input to the second transmission waveguide through the third lower port. A phase modulator located on the second transmission waveguide modulates the phase of this set of externally injected continuous light of specific frequencies. The specific frequency externally injected continuous light transmitted through the first transmission waveguide... A fixed-frequency externally injected continuous light and a phase-modulated externally injected continuous light of a specific frequency on the second transmission waveguide meet and interfere destructively at the fifth waveguide beam splitter, achieving attenuation of the intensity of the externally injected continuous light of the specific frequency. The attenuated externally injected continuous light of the specific frequency is then split by the first waveguide beam splitter and input to the second and third waveguide beam splitters, respectively. It is then fed back into the second semiconductor laser through the second waveguide beam splitter and correspondingly fed back into the third semiconductor laser through the third waveguide beam splitter, thus achieving external injection locking between the second and third semiconductor lasers.

[0055] based on Figure 3In another embodiment of the present invention, the optical frequency comb source further includes an adjustable optical delay chip integrated on the optical chip. The adjustable optical delay chip is disposed on the connection path between the second waveguide beamsplitter and the waveguide beam combiner or on the connection path between the third waveguide beamsplitter and the waveguide beam combiner. It is used to delay the optical frequency comb pulse output from the second upper port of the second waveguide beamsplitter or the optical frequency comb pulse output from the second lower port of the third waveguide beamsplitter, so that the optical frequency comb pulse output from the second upper port of the second waveguide beamsplitter and the optical frequency comb pulse output from the second lower port of the third waveguide beamsplitter arrive at the waveguide beam combiner simultaneously.

[0056] Specifically, in one embodiment, such as Figure 5 As shown, the tunable optical delay chip is placed on the connection path between the second waveguide beamsplitter and the waveguide beam combiner. The tunable optical delay chip delays the optical frequency comb pulses output from the second upper port of the second waveguide beamsplitter, so that the optical frequency comb pulses output from the second semiconductor laser and the third semiconductor laser arrive at the waveguide beam combiner simultaneously, ensuring that the optical frequency comb pulses output from the second semiconductor laser and the third semiconductor laser have a high pulse overlap rate, forming a complete dual optical frequency comb.

[0057] In this application, the first semiconductor laser is preferably a distributed feedback laser, and the second and third semiconductor lasers are both distributed feedback lasers or distributed Bragg reflector lasers.

[0058] Semiconductor lasers have advantages such as small size, wide wavelength coverage, mature manufacturing process, and easy integration. Operating in gain-switching mode, semiconductor lasers can generate ultrashort optical pulses. Furthermore, distributed feedback lasers or distributed Bragg reflector lasers have a periodic ripple structure, which has mode selectivity and can achieve dynamic single-mode operation.

[0059] The first semiconductor laser operates in continuous-wave mode, preferably a distributed feedback laser. Distributed feedback lasers operate in continuous-wave mode, allowing for precise adjustment of the output continuous-wave power and wavelength by regulating the operating temperature and drive current, exhibiting excellent wavelength and power stability. The second and third semiconductor lasers operate in gain-switching mode, and can be either distributed feedback lasers or distributed Bragg reflector lasers. The resonant cavities of distributed feedback and distributed Bragg reflector lasers possess multiple oscillation modes and broad gain curves. When externally injected continuous light is transmitted into the resonant cavity, changes occur in the cavity carriers and the refractive index of the cavity medium. This suppresses the oscillation modes that operate freely within the cavity, altering the oscillation frequency. The oscillation modes with the frequency of the externally injected continuous light are enhanced. Due to mode competition, other modes are suppressed until a single longitudinal mode output pulse is achieved, thus achieving external injection locking.

[0060] Assume the frequency of the injected continuous light output from the first semiconductor laser, after being filtered by a waveguide filter, is f1 (i.e., the waveguide filter frequency is f1). The operating frequencies of the second and third semiconductor lasers are f2 and f3, respectively. f2 and f3 are not equal, having a small frequency difference, and both f2 and f3 are greater than 10 GHz. After the filtered injected continuous light is input to the second and third semiconductor lasers, based on the input of the injected continuous light, the second and third semiconductor lasers, operating in gain-switching mode, reach injection-locked state and output optical frequency comb pulses respectively. The two optical frequency comb pulses are combined in a waveguide combiner to form a dual optical frequency comb, with the spectrum as shown below. Figure 6 As shown, the solid line represents the optical frequency comb pulse output by the second semiconductor laser, and the dashed line represents the optical frequency comb pulse output by the third semiconductor laser. The center frequency of both optical frequency comb pulses is determined by the waveguide filter frequency, f1. The comb tooth width of the optical frequency comb pulse output by the second semiconductor laser is determined by the operating frequency f2 of the second semiconductor laser, and the comb tooth width of the optical frequency comb pulse output by the third semiconductor laser is determined by the operating frequency f3 of the third semiconductor laser. (Reference) Figure 6For ease of understanding, let the center frequency be denoted as comb pair 0 (two overlapping comb teeth), and sequentially labeled to the right as comb pair 1, comb pair 2, comb pair 3, and so on. Then, the frequency difference for comb pair 1 is (f2-f3), the frequency difference for comb pair 2 is 2(f2-f3), the frequency difference for comb pair 3 is 3(f2-f3), and so on. Since both the second and third semiconductor lasers use injection-locked externally injected continuous light from the first semiconductor laser, there is a stable frequency difference between the comb teeth of adjacent second and third semiconductor laser outputs within each pulse period. Furthermore, since the second and third semiconductor lasers operate in injection-locked mode, the pulses they generate are based on stimulated emission of externally injected continuous light. Therefore, the phase difference between the optical frequency comb pulses output by the second and third semiconductor lasers is stable, and thus the dual optical frequency comb formed based on these two pulses has a stable phase.

[0061] In this application, the laser driver includes a microcontroller, a first driving circuit module, a second driving circuit module, and a third driving circuit module, such as... Figure 7 As shown, the microcontroller is connected to the first driving circuit module, the second driving circuit module, and the third driving circuit module respectively, and is used to control the first driving circuit module, the second driving circuit module, and the third driving circuit module. The first driving circuit module is used to drive the first semiconductor laser, the second driving circuit module is used to drive the second semiconductor laser, and the third driving circuit module is used to drive the third semiconductor laser.

[0062] In this application, the first semiconductor laser operates in continuous wave mode; therefore, the first driving circuit module driving the first semiconductor laser is a conventional driving module, which will not be described in detail here. The second and third semiconductor lasers both operate in gain-switching mode. The second and third driving circuit modules are identical, each including a DC source, an RF signal source, and a bias circuit, with the structure as follows: Figure 8 As shown, the DC source is used to generate the DC bias current, the RF signal source is used to generate the sinusoidal drive signal, and the biaser consists of a feed inductor and a blocking capacitor. The feed inductor is connected to the DC source, and the blocking capacitor is connected to the RF signal source. The DC bias current and the sinusoidal drive signal are injected into the second semiconductor laser or the third semiconductor laser through the biaser.

[0063] The DC bias current generated by the DC source and the sinusoidal drive signal generated by the RF signal source are combined in the bias circuit and then injected into the second or third semiconductor laser to drive the generation of periodic optical pulses. The feed inductor in the bias circuit is used to add DC bias current to prevent the sinusoidal drive signal generated by the RF signal source from leaking into the DC source system; the blocking capacitor in the bias circuit is used to input the sinusoidal drive signal and at the same time blocks the bias current from being input to the RF signal source.

[0064] In this application, the MZ unequal arm interferometer type filter is specifically a single-stage MZ unequal arm interferometer or composed of multiple cascaded MZ unequal arm interferometers.

[0065] The MZ unequal-arm interferometer consists of a first 3dB directional coupler, a short transmission arm, a long transmission arm, and a second 3dB directional coupler connected together, as shown in the structure below. Figure 9 As shown, when externally injected continuous light enters from port 1, it is split into two sets of optical signals with the same energy and a phase difference of π / 2 by the first 3dB directional coupler. One set of optical signals is transmitted along the short transmission arm, and the other set is transmitted along the long transmission arm. Based on the phase difference caused by the two 3dB directional couplers and the path difference between the short and long transmission arms, the two sets of optical signals interfere with each other at port 3 and with each other at port 4. According to the settings, the optical frequency that satisfies the wavelength-specific relationship is output from port 3, and the optical frequency that satisfies the other wavelength-specific relationship is output from port 4. The optical frequency output from port 4 is set to be discarded, thus achieving the filtering function.

[0066] In another embodiment, the MZ unequal-arm interferometer type filter is composed of multiple cascaded MZ unequal-arm interferometers, with port 3 of the current MZ unequal-arm interferometer connected to port 1 of the next-stage MZ unequal-arm interferometer, as shown in the structure. Figure 10 As shown, interference effects are formed on each MZ unequal arm interferometer, and the cumulative interference effects eventually achieve the filtering function.

[0067] Furthermore, in this application, the waveguide filter can also be a micro-ring resonant cavity optical filter. Specifically, the micro-ring resonant cavity optical filter consists of a ring waveguide connected end-to-end and a coupled straight waveguide. The coupled straight waveguide receives the externally injected continuous light and couples the externally injected continuous light to the ring waveguide, as shown in the structure. Figure 11As shown, the injected continuous light is coupled into the ring waveguide through a coupled straight waveguide, where it propagates in a ring and resonates under certain conditions. When the phase of light at a specific wavelength around the ring waveguide satisfies the constructive interference condition, it reaches a resonant state. The light energy at the resonant wavelength balances the losses in the ring waveguide and is not output. Light energy at wavelengths that do not satisfy the resonant state condition cannot be stored in the ring waveguide and is output from the port of the coupled straight waveguide. Based on the performance of the micro-ring resonant cavity, filtering is achieved. To achieve better filtering, multiple micro-ring resonant cavity optical filters can be cascaded. However, as the number of cascaded structures increases, the manufacturing difficulty also increases. Therefore, two to three cascaded structures are preferred to achieve spectral modulation of the injected continuous light.

[0068] Based on the above explanation and elaboration of this application, the first semiconductor laser operates in continuous wave mode, continuously outputting injected continuous light under the drive of the first driving circuit module. The second and third semiconductor lasers operate in gain-switching mode, respectively, under the drive of the second and third driving circuit modules. The injected continuous light, after filtering and attenuation, is split into two groups by the first waveguide beam splitter. One group is injected into the second semiconductor laser via the second waveguide beam splitter, and the other group is injected into the third semiconductor laser via the third waveguide beam splitter. The second and third semiconductor lasers achieve injection locking and generate effective optical frequency comb pulses based on the filtered and attenuated injected continuous light. The frequency of the injected continuous light after passing through the waveguide filter is consistent with a certain frequency of the spontaneously emitted optical pulses of the second and third semiconductor lasers. Based on the injected continuous light, the linewidth of the output optical pulses of the second and third semiconductor lasers is significantly narrowed, effectively reducing the phase noise of the optical pulses. Since each pulse output by the second and third semiconductor lasers is based on external injection locking rather than random spontaneous emission, the output light pulses have clearer comb lines and distinguishable comb line spacing, as well as higher comb flatness and coherence.

[0069] The various embodiments in this specification are described in a progressive, parallel, or combined manner. Each embodiment focuses on the differences from other embodiments, and the same or similar parts between the various embodiments can be referred to each other.

[0070] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or apparatus comprising a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the article or apparatus that includes the aforementioned element.

[0071] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An injection-locked dual optical frequency comb source, comprising: The laser driver, the first semiconductor laser, the second semiconductor laser, the third semiconductor laser, the first waveguide beam splitter, the second waveguide beam splitter, the third waveguide beam splitter, the waveguide filter and the waveguide combiner are integrated on one optical chip; The laser driver is connected with the first semiconductor laser, the second semiconductor laser and the third semiconductor laser respectively, and is used for driving the first semiconductor laser, the second semiconductor laser and the third semiconductor laser; The first semiconductor laser works in a continuous wave mode, and is used for continuously outputting external injection continuous light; The second semiconductor laser and the third semiconductor laser both work in a gain-switched mode, and output optical frequency comb pulses based on the external injection continuous light, and the working frequency of the second semiconductor laser is different from that of the third semiconductor laser; the first waveguide beam splitter, the second waveguide beam splitter and the third waveguide beam splitter each include a first upper port, a second upper port, a first lower port and a second lower port, one end of the waveguide filter is connected with the first semiconductor laser, and the other end is connected with the first upper port or the first lower port of the first waveguide beam splitter; the second upper port of the first waveguide beam splitter is connected with the second lower port of the second waveguide beam splitter; the second lower port of the first waveguide beam splitter is connected with the second upper port of the third waveguide beam splitter; the first upper port or the first lower port of the second waveguide beam splitter is connected with the second semiconductor laser; the second upper port of the second waveguide beam splitter is connected with one input port of the waveguide combiner; the first upper port or the first lower port of the third waveguide beam splitter is connected with the third semiconductor laser; and the second lower port of the third waveguide beam splitter is connected with the other input port of the waveguide combiner. The dual optical frequency comb source further includes a waveguide attenuator integrated on the optical chip, which is used for attenuating the intensity of the received external injection continuous light, one end of the waveguide attenuator is connected with the waveguide filter, and the other end is connected with the first upper port or the first lower port of the first waveguide beam splitter.

2. The dual optical frequency comb source based on injection locking according to claim 1, characterized in that, The splitting ratio of the first waveguide beam splitter is 50:50, and the splitting ratios of the second waveguide beam splitter and the third waveguide beam splitter are both (90+N):(10-N), wherein N is a positive integer less than 10.

3. The dual optical frequency comb source based on injection locking of claim 1, wherein, The dual optical frequency comb source further includes an adjustable optical delay chip integrated on the optical chip, which is arranged on the connection path between the second waveguide beam splitter or the third waveguide beam splitter and the waveguide combiner, and is used for performing delay processing on the optical frequency comb pulses output from the second upper port of the second waveguide beam splitter or the optical frequency comb pulses output from the second lower port of the third waveguide beam splitter, so that the optical frequency comb pulses output from the second upper port of the second waveguide beam splitter and the optical frequency comb pulses output from the second lower port of the third waveguide beam splitter reach the waveguide combiner at the same time.

4. The dual optical frequency comb source based on injection locking of claim 1, wherein, The waveguide attenuator is composed of a fourth waveguide beam splitter, a first transmission waveguide, a second transmission waveguide, a fifth waveguide beam splitter and a phase modulator, the fourth waveguide beam splitter and the fifth waveguide beam splitter each include a third upper port and a third lower port, two ends of the first transmission waveguide are connected with the third upper port of the fourth waveguide beam splitter and the third upper port of the fifth waveguide beam splitter respectively, two ends of the second transmission waveguide are connected with the third lower port of the fourth waveguide beam splitter and the third lower port of the fifth waveguide beam splitter respectively, and the phase modulator is arranged on the second transmission waveguide.

5. The injection-locked dual optical comb source of any of claims 1 to 4, wherein the first optical comb source and the second optical comb source are injection-locked by a common injection signal. The first semiconductor laser is a distributed feedback laser, and the second semiconductor laser and the third semiconductor laser are each a distributed feedback laser or a distributed Bragg reflection laser.

6. The injection-locked dual optical comb source of any of claims 1 to 4, wherein the first optical comb source is a mode-locked laser and the second optical comb source is a mode-locked laser. The laser driver includes a microcontroller, a first drive circuit module, a second drive circuit module and a third drive circuit module, the microcontroller is connected with the first drive circuit module, the second drive circuit module and the third drive circuit module respectively, and is used for controlling the first drive circuit module, the second drive circuit module and the third drive circuit module, the first drive circuit module is used for driving the first semiconductor laser, the second drive circuit module is used for driving the second semiconductor laser, and the third drive circuit module is used for driving the third semiconductor laser.

7. The injection-locked dual optical comb source of any of claims 1 to 4, wherein the first optical comb source and the second optical comb source are injection-locked by a common injection signal. The waveguide filter is an MZ unequal arm interferometer type filter or a micro ring resonant cavity type optical filter.

8. The dual optical frequency comb source based on injection locking according to claim 7, characterized in that, The MZ unequal arm interferometer type filter is a single-stage MZ unequal arm interferometer or is composed of a plurality of MZ unequal arm interferometers in cascade.

9. The dual optical frequency comb source based on injection locking of claim 7, wherein, The micro ring resonant cavity type optical filter is composed of a ring waveguide connected in a head-tail mode and a coupling straight waveguide, and the coupling straight waveguide receives external injection continuous light and couples the external injection continuous light to the ring waveguide.

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