Optical frequency comb source based on self-injection locking

Abstract

The application discloses an optical frequency comb source based on self-injection locking, which comprises a semiconductor laser, a laser driver, a first waveguide beam splitter, a second waveguide beam splitter and a waveguide filter, wherein the first waveguide beam splitter, the second waveguide beam splitter and the waveguide filter are integrated on an optical chip. The semiconductor laser works in a gain-switched mode under the action of the laser driver to output optical pulses. After being split by the first waveguide beam splitter and the second waveguide beam splitter, part of the optical pulses is output to outside the optical chip, and the other part of the optical pulses is processed by the waveguide filter to form a self-injection optical signal with a specific frequency. The self-injection optical signal is fed back to the semiconductor laser to realize self-injection locking of the semiconductor laser and generation of an effective optical frequency comb. In addition, the optical frequency comb source provided by the application can be realized based on a conventional semiconductor laser and a conventional silicon-based chip process, and is easy to be integrated on a chip.

Description

Technical Field

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

[0002] An optical frequency comb (OFC) refers to a spectrum composed of a series of uniformly spaced frequency components with stable coherent phase relationships. In the frequency domain, an OFC appears as a spectral sequence with uniform frequency spacing; in the time domain, it appears as a sequence of ultrashort pulses. The frequency spacing in the frequency domain and the pulse width in the time domain strictly follow a Fourier transform relationship. Because the distribution shape of an OFC 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." Having numerous frequency components, an OFC can be used as a precise measuring tool for optical frequencies, much like a ruler with a fine scale. It is the most accurate optical frequency measurement tool to date. OFCs provide ideal research tools for precise optical frequency measurement, atomic ion transition energy level measurement, remote signal clock synchronization, and satellite navigation. In recent years, with the rapid development of optical communication technology, OFCs have also been increasingly widely used in dense wavelength division multiplexing, multi-wavelength ultrashort pulse generation, and arbitrary optical waveform generation.

[0003] Generating optical frequency combs using mode-locked pulsed lasers is a traditional approach. Mode-locked pulsed lasers refer to periodically changing pulses generated by locking the phase between the longitudinal modes of a laser. In the frequency domain, this manifests as a spectrum with a series of longitudinal modes, thus producing an optical frequency comb. The longitudinal mode spacing of the laser is equivalent to the comb tooth spacing of the optical frequency comb. However, methods based on mode-locked lasers are limited by the laser cavity length, and their repetition rate is typically only a few megahertz to tens of megahertz. Optical modulation techniques within mode-locked lasers can generate optical frequency combs with frequency intervals of tens of megahertz; however, this frequency interval is limited by the optoelectronic modulator and RF signal generator, and the cost increases dramatically with increasing frequency, hindering large-scale applications.

[0004] Microcavity optical frequency combs, which have emerged in recent years, have an inherent advantage in repetition rate, thus compensating for the shortcomings of traditional optical frequency comb technology. Utilizing microcavity structures to generate optical frequency combs is a relatively efficient solution. Microcavity structures can generate large-spaced optical frequency combs with intervals exceeding 100 GHz, and these combs exhibit very low noise, a signal-to-noise ratio of up to 40 dB, and good coherence between the comb teeth. However, due to the immaturity of current industrial-scale microcavity fabrication technology, the cost of these optical frequency combs is relatively high. Furthermore, the high temperature sensitivity of microcavities necessitates continuous control, resulting in a complex and expensive final optical comb device structure, difficulty in on-chip integration, and poor system stability. Summary of the Invention

[0005] To address the aforementioned problems, this application provides a self-injection-locked optical frequency comb source, which generates an effective optical frequency comb through self-injection, offering advantages such as simple structure, stable spectrum, and ease of on-chip integration. The specific solution is as follows:

[0006] This application discloses an optical frequency comb source based on self-injection locking, including a semiconductor laser, a laser driver, a first waveguide beam splitter, a second waveguide beam splitter, and a waveguide filter, wherein the first waveguide beam splitter, the second waveguide beam splitter, and the waveguide filter are integrated on a single optical chip;

[0007] The semiconductor laser operates in gain-switching mode to output optical pulses, which are either initial pulses or optical frequency comb pulses. The laser driver is connected to the semiconductor laser and drives it. The first waveguide beam splitter splits the optical pulses output by the semiconductor laser, the optical pulses input by the second waveguide beam splitter, or the optical pulses input by the waveguide filter. The waveguide filter filters the received optical pulses and transmits the filtered optical pulses to the second waveguide beam splitter or the first waveguide beam splitter. The second waveguide beam splitter splits the optical pulses input by the first waveguide beam splitter or the optical pulses input by the waveguide filter. Both the first and second waveguide beam splitters include a first upper port, a second upper port, a first lower port, and a second lower port. The first upper port of the first waveguide beam splitter is connected to the semiconductor laser, and the first upper port of the second waveguide beam splitter is connected to the second upper port of the first waveguide beam splitter. The two ends of the waveguide filter are connected to the second lower port of the first waveguide beam splitter and the second lower port of the second waveguide beam splitter, respectively.

[0008] Preferably, the beam splitting ratio of the first waveguide beam splitter and the beam splitting ratio of the second waveguide beam splitter are both (90+N):(10-N), where N is a positive integer less than 10.

[0009] Furthermore, the optical frequency comb source also includes a waveguide attenuator integrated on the optical chip for attenuating the intensity of the received optical pulses. The two ends of the waveguide attenuator are respectively connected to the waveguide filter and the second lower port of the second waveguide beam splitter.

[0010] Furthermore, the optical frequency comb source also includes a waveguide delay line integrated on the optical chip for delaying the received optical pulses, so that the time for the delayed optical pulses to be transmitted to the semiconductor laser is consistent with the time of one of the optical pulses output by the semiconductor laser. The two ends of the waveguide delay line are respectively connected to the waveguide attenuator and the second lower port of the second waveguide beam splitter.

[0011] Preferably, the waveguide attenuator comprises a third waveguide beamsplitter, a first transmission waveguide, a second transmission waveguide, a fourth waveguide beamsplitter, and a phase modulator. The third and fourth waveguide beamsplitters each 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 third waveguide beamsplitter and the third upper port of the fourth waveguide beamsplitter. The two ends of the second transmission waveguide are respectively connected to the third lower port of the third waveguide beamsplitter and the third lower port of the fourth waveguide beamsplitter. The phase modulator is disposed on the second transmission waveguide.

[0012] Preferably, the semiconductor laser is a distributed feedback laser or a distributed Bragg reflector laser.

[0013] Preferably, the laser driver includes a DC source, an RF signal source, and a bias device. The DC source is used to generate a DC bias current, the RF signal source is used to generate a sinusoidal drive signal, and the bias device 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 semiconductor laser through the bias device.

[0014] Preferably, the waveguide filter is an MZ unequal arm interferometer type filter or a micro-ring resonator type optical filter.

[0015] 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.

[0016] 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 optical pulses and couples the optical pulses to the ring waveguide.

[0017] In summary, compared with the prior art, the above-described technical solutions conceived in this application can achieve the following beneficial effects:

[0018] This application provides a self-injection locked optical frequency comb source, including a semiconductor laser, a laser driver, a first waveguide beamsplitter, a second waveguide beamsplitter, and a waveguide filter. The first waveguide beamsplitter, the second waveguide beamsplitter, and the waveguide filter are integrated on a single optical chip. The semiconductor laser operates in gain-switching mode under the action of the laser driver, outputting optical pulses. After being split by the first and second waveguide beamsplitters, a portion of the output optical pulses is output outside the optical chip, while the other portion is processed by the waveguide filter to form a self-injected optical signal of a specific frequency. This self-injected optical signal is then fed back into the semiconductor laser, achieving self-injection locking of the semiconductor laser and generating an effective optical frequency comb. The self-injected optical signal, after being filtered by the waveguide filter, forms a self-injected optical signal with a frequency consistent with a certain frequency of the spontaneously emitted optical pulses from the semiconductor laser. When the self-injected optical signal is fed back into the semiconductor laser, the linewidth of the output optical pulses is significantly narrowed, effectively reducing the phase noise of the optical pulses. After the self-injected optical signal is fed back into the semiconductor laser, each pulse output by the semiconductor laser is based on self-injection locking, rather than random spontaneous emission. Therefore, the optical pulses output by the semiconductor laser have the same phase difference, which can produce clearer comb lines and distinguishable comb line spacing, while improving the flatness and coherence of the generated optical frequency comb. In addition, the optical frequency comb source provided in this application can be implemented based on conventional semiconductor lasers and conventional silicon-based chip processes, making it easy to integrate on a chip. Attached Figure Description

[0019] 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.

[0020] Figure 1 A schematic diagram of a self-injection-locked optical frequency comb source provided in an embodiment of this application;

[0021] Figure 2 This is a schematic diagram of the structure of the first waveguide beamsplitter and the second waveguide beamsplitter in this application;

[0022] Figure 3 A schematic diagram of a self-injection-locked optical frequency comb source is provided for another embodiment of this application;

[0023] Figure 4 This is a schematic diagram of the waveguide attenuator structure in one embodiment of this application;

[0024] Figure 5 This application is based on Figure 3 A schematic diagram of a self-injection-locked optical frequency comb source is provided;

[0025] Figure 6 This is a schematic diagram of the optical frequency comb pulse spectrum formed in this application;

[0026] Figure 7 This is a schematic diagram of the structure of the laser driver in this application;

[0027] Figure 8 This is a schematic diagram of the structure of the MZ unequal-arm interferometer in this application;

[0028] Figure 9 This is a schematic diagram of the structure of an MZ unequal-arm interferometer filter in one embodiment of this application;

[0029] Figure 10 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

[0030] 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.

[0031] 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.

[0032] 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.

[0033] In recent years, with the rapid development of optical communication technology, optical frequency combs have been increasingly widely used in fields such as dense wavelength division multiplexing, multi-wavelength ultrashort pulse generation, and optical arbitrary waveform generation.

[0034] Generating optical frequency combs using mode-locked pulsed lasers is a traditional approach. Mode-locked pulsed lasers refer to periodically changing pulses generated by locking the phase between the longitudinal modes of a laser. In the frequency domain, this manifests as a spectrum with a series of longitudinal modes, thus producing an optical frequency comb. The longitudinal mode spacing of the laser is equivalent to the comb tooth spacing of the optical frequency comb. However, methods based on mode-locked lasers are limited by the laser cavity length, and their repetition rate is typically only a few megahertz to tens of megahertz. Optical modulation techniques within mode-locked lasers can generate optical frequency combs with frequency intervals of tens of megahertz; however, this frequency interval is limited by the optoelectronic modulator and RF signal generator, and the cost increases dramatically with increasing frequency, hindering large-scale applications.

[0035] Microcavity optical frequency combs, which have emerged in recent years, have an inherent advantage in repetition rate, thus compensating for the shortcomings of traditional optical frequency comb technology. Utilizing microcavity structures to generate optical frequency combs is a relatively efficient solution. Microcavity structures can generate large-spaced optical frequency combs with intervals exceeding 100 GHz, and these combs exhibit very low noise, a signal-to-noise ratio of up to 40 dB, and good coherence between the comb teeth. However, due to the immaturity of current industrial-scale microcavity fabrication technology, the cost of these optical frequency combs is relatively high. Furthermore, the high temperature sensitivity of microcavities necessitates continuous control, resulting in a complex and expensive final optical comb device structure, difficulty in on-chip integration, and poor system stability.

[0036] Based on this, this application provides an optical frequency comb source based on self-injection locking, such as... Figure 1 As shown, it includes a semiconductor laser, a laser driver, a first waveguide beam splitter, a second waveguide beam splitter, and a waveguide filter, with the first waveguide beam splitter, the second waveguide beam splitter, and the waveguide filter integrated on a single optical chip.

[0037] In one embodiment of the present invention, when the optical chip is made of silicon-based silicon dioxide, the semiconductor laser and the 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 semiconductor laser can be directly integrated onto the optical chip.

[0038] In this application, the semiconductor laser operates in gain-switching mode to output optical pulses, and the laser driver is connected to the semiconductor laser to drive the semiconductor laser. The optical pulses output by the semiconductor laser are initial pulses or optical frequency comb pulses.

[0039] Specifically, when the semiconductor laser starts operating under the drive of the laser driver, it outputs an initial pulse. Under the beam-splitting effect of the first and second waveguide beam splitters and the filtering effect of the waveguide filter, a portion of the initial pulse becomes a self-injected optical signal of a specific frequency. This self-injected optical signal is fed back into the semiconductor laser, which then outputs an optical frequency comb pulse based on it. The current optical frequency comb pulse continues to be split by the first and second waveguide beam splitters and filtered by the waveguide filter, and a portion of the current optical frequency comb pulse becomes a self-injected optical signal of a specific frequency, which is then fed back into the semiconductor laser. The semiconductor laser continues to output an optical frequency comb pulse based on this current self-injected optical signal. This process is a cyclical and continuous process; in summary, the generation of each optical frequency comb pulse originates from the self-injection of the optical frequency comb pulse from the previous cycle.

[0040] The first waveguide beamsplitter is used to split the optical pulses output from the semiconductor laser, the optical pulses input to the second waveguide beamsplitter, or the optical pulses input to the waveguide filter. The waveguide filter is used to filter the received optical pulses and transmit the filtered optical pulses to the second waveguide beamsplitter or the first waveguide beamsplitter. The second waveguide beamsplitter is used to split the optical pulses input to the first waveguide beamsplitter or the optical pulses input to the waveguide filter. Both the first and second waveguide beamsplitters include a first upper port, a second upper port, a first lower port, and a second lower port. Figure 2 As shown, the first upper port of the first waveguide beam splitter is connected to the semiconductor laser, the first upper port of the second waveguide beam splitter is connected to the second upper port of the first waveguide beam splitter, and the two ends of the waveguide filter are respectively connected to the second lower port of the first waveguide beam splitter and the second lower port of the second waveguide beam splitter.

[0041] In this application, the optical pulses output from the semiconductor laser are transmitted to the first waveguide beam splitter for beam splitting. Most of the split optical pulses are transmitted to the second waveguide beam splitter for further beam splitting, while a small portion of the optical pulses are transmitted to the waveguide filter. For ease of distinction and representation, this small portion of the optical pulses is named the reverse optical pulse. After being filtered by the waveguide filter, the reverse optical pulses are sequentially fed back into the semiconductor laser through the second and first waveguide beam splitters. After the optical pulses transmitted from the first waveguide beam splitter to the second waveguide beam splitter are split, most of the optical pulses are transmitted outside the optical chip, while a small portion of the optical pulses are transmitted to the waveguide filter. This small portion of the optical pulses is named the forward optical pulse. The forward optical pulses are sequentially fed back into the semiconductor laser after passing through the waveguide filter and the first waveguide beam splitter. Here, the reverse and forward optical pulses arrive at the semiconductor laser at the same time.

[0042] The first and second waveguide beamsplitters can be directional couplers, multimode interferometers (MMIs), or MZ interferometers; those skilled in the art can choose different types of beamsplitters as needed. To ensure that only a small number of optical pulses are self-injected into the semiconductor laser, the splitting ratio of both the first and second waveguide beamsplitters is set to (90+N):(10-N), where N is a positive integer less than 10. That is, the first and second waveguide beamsplitters can split the input optical pulses according to a (90+N):(10-N) energy ratio, resulting in two separate optical pulses. Specifically, the splitting ratio of the first and second waveguide beamsplitters can be 99:1, 95:5, etc.

[0043] The waveguide filter filters the input optical pulses, narrowing the center spectral line of the input pulses, preferably typically to less than 100 MHz. For self-injection locking of a semiconductor laser, a crucial condition is that the wavelength of the self-injected optical pulse is close to the wavelength of the optical pulse that the semiconductor laser can emit. Therefore, a waveguide filter is used to filter the self-injected optical pulse. In this application, the waveguide filter is an MZ unequal-arm interferometer type filter or a micro-ring resonator type optical filter.

[0044] In this embodiment of the invention, the semiconductor laser operates in gain-switching mode under the action of a laser driver, outputting optical pulses. After being split by a first and second waveguide beam splitter, most of the optical pulses are output outside the optical chip. A small portion of the optical pulses (including inverse and forward pulses) are processed by a waveguide filter to form a self-injected optical signal of a specific frequency. This self-injected optical signal is then fed back into the semiconductor laser, achieving self-injection locking and the generation of an effective optical frequency comb. The small portion of the optical pulses output by the semiconductor laser, after being filtered by the waveguide filter, forms a self-injected optical signal. The frequency of this self-injected optical signal is consistent with a certain frequency of the spontaneously emitted optical pulses from the semiconductor laser. When the self-injected optical signal is fed back into the semiconductor laser, the linewidth of the output optical pulses is significantly narrowed, effectively reducing the phase noise of the optical pulses. After the self-injected optical signal is fed back into the semiconductor laser, each pulse output by the semiconductor laser is based on self-injection locking, rather than random spontaneous emission. Therefore, the optical pulses output by the semiconductor laser have the same phase difference, which can produce clearer comb lines and distinguishable comb line spacing, while improving the flatness and coherence of the generated optical frequency comb. In addition, the optical frequency comb source provided in this application can be implemented based on conventional semiconductor lasers and conventional silicon-based chip processes, making it easy to integrate on a chip.

[0045] In another embodiment of the present invention, the optical frequency comb source further includes a waveguide attenuator integrated on the optical chip for attenuating the intensity of the received optical pulse. The two ends of the waveguide attenuator are respectively connected to the second lower port of the waveguide filter and the second waveguide beam splitter. The optical frequency comb source structure based on this embodiment is as follows: Figure 3 As shown.

[0046] To ensure successful self-injection locking of a semiconductor laser, another crucial condition is that the intensity of the self-injected optical pulse fed back to the laser cannot be too high; otherwise, the self-injected pulse will affect the stability of the output optical pulse. Therefore, a waveguide attenuator is used to attenuate the intensity of the self-injected optical pulse, thus adjusting its intensity.

[0047] In one embodiment of this application, the waveguide attenuator comprises a third waveguide beamsplitter, a first transmission waveguide, a second transmission waveguide, a fourth waveguide beamsplitter, and a phase modulator, as shown in the following structure. Figure 4 As shown, both the third and fourth 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 third waveguide beamsplitter and the third upper port of the fourth waveguide beamsplitter, respectively. The two ends of the second transmission waveguide are connected to the third lower port of the third waveguide beamsplitter and the third lower port of the fourth waveguide beamsplitter, respectively. The phase modulator is disposed on the second transmission waveguide.

[0048] Specifically, the beam splitting ratio of the third and fourth waveguide beam splitters in the waveguide attenuator is 50:50. The phase modulator modulates the phase of the optical pulse input to the second transmission waveguide, forming a phase difference with the optical pulse transmitted on the first transmission waveguide. After the phase-modulated optical pulse on the second transmission waveguide meets the optical pulse on the first transmission waveguide, they interfere and cancel each other out, thus achieving intensity attenuation of the optical pulse.

[0049] In this embodiment, the specific signal transmission process is as follows:

[0050] The optical pulses output from the semiconductor laser are transmitted to the first waveguide beam splitter for beam splitting. Most of the split optical pulses are transmitted to the second waveguide beam splitter for further beam splitting, while a small portion of the optical pulses (reverse optical pulses) are transmitted to the waveguide filter. After being filtered by the waveguide filter, the reverse optical pulses are input to the third waveguide beam splitter of the waveguide attenuator for further beam splitting. One set of reverse optical pulses is input to the first transmission waveguide through the third upper port, and another set of reverse optical pulses is input to the second transmission waveguide through the third lower port. The phase modulator located on the second transmission waveguide modulates the phase of this set of reverse optical pulses. The reverse optical pulses transmitted through the first transmission waveguide and the phase-modulated reverse optical pulses on the second transmission waveguide meet and merge at the fourth waveguide beam splitter, resulting in destructive interference and attenuation of the reverse optical pulse intensity. The attenuated reverse optical pulses are then fed back into the semiconductor laser through the second and first waveguide beam splitters in sequence.

[0051] After the optical pulses transmitted from the first waveguide beamsplitter to the second waveguide beamsplitter are split, most of the optical pulses are transmitted outside the optical chip, while a small portion of the optical pulses (forward optical pulses) are transmitted to the waveguide attenuator. The forward optical pulses are then input to the fourth waveguide beamsplitter of the waveguide attenuator for further splitting. One set of forward optical pulses after splitting is input to the first transmission waveguide through the third upper port, and another set of reverse optical pulses is input to the second transmission waveguide through the third lower port. The phase modulator located on the second transmission waveguide modulates the phase of this set of forward optical pulses. The forward optical pulses transmitted through the first transmission waveguide and the phase-modulated forward optical pulses on the second transmission waveguide meet and merge at the third waveguide beamsplitter, resulting in destructive interference and attenuation of the intensity of the reverse optical pulses. The attenuated forward optical pulses are then fed back into the semiconductor laser through the waveguide filter and the first waveguide beamsplitter in sequence.

[0052] based on Figure 3 In another embodiment of the present invention, the optical frequency comb source further includes a waveguide delay line integrated on the optical chip for delaying the received optical pulses, so that the time for the delayed optical pulses to reach the semiconductor laser is consistent with the time of one of the optical pulses output by the semiconductor laser. The two ends of the waveguide delay line are respectively connected to the second lower port of the waveguide attenuator and the second waveguide beam splitter, as shown below. Figure 5 As shown.

[0053] The timing of the injection of both the reverse and forward optical pulses into the semiconductor laser, after the delay effect of the waveguide delay line, coincides precisely with the timing of the semiconductor laser's output optical pulse. Specifically, the reverse optical pulse is injected into the semiconductor laser sequentially through a waveguide filter, a waveguide attenuator, a waveguide delay line, a second waveguide beam splitter, and a first waveguide beam splitter. Similarly, the forward optical pulse is injected into the semiconductor laser sequentially through a waveguide delay line, a waveguide attenuator, a waveguide filter, and a first waveguide beam splitter. Both the reverse and forward optical pulses are injected into the semiconductor laser simultaneously. By setting the waveguide delay line, the timing of the injected optical pulse reaching the semiconductor laser is ensured to correspond to the timing of the semiconductor laser's output optical pulse.

[0054] In this application, the preferred semiconductor laser is a distributed feedback laser or a distributed Bragg reflector laser.

[0055] Semiconductor lasers possess advantages such as small size, wide wavelength coverage, mature fabrication technology, and ease of 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 selects modes and enables dynamic single-longitudinal-mode operation. In this application, a portion of the optical pulses generated by the semiconductor laser is processed and then fed back into the semiconductor laser, achieving self-injection of the optical pulse. The resonant cavity of the semiconductor laser has multiple oscillation modes and a wide gain curve. When the self-injected optical pulse returns to the resonant cavity of the semiconductor laser, it causes changes in the charge carriers and the refractive index of the cavity medium. The oscillation modes that operate freely are suppressed, and the oscillation frequency changes. The oscillation modes with the frequency of the self-injected optical pulse are enhanced. Due to mode competition, other modes are suppressed until finally, a single-longitudinal-mode laser pulse is output, achieving injection locking.

[0056] Assuming the modulation frequency of the semiconductor laser is f2, and the frequency of the optical pulse passing through the waveguide filter is f1, the initial output pulse is coupled to the optical chip. After transmission and processing on the optical chip, a portion of the initial pulse (self-injected optical pulse) is fed back into the semiconductor laser. Under the injection lock of the self-injected optical pulse, the semiconductor laser reaches a self-injection locked state and outputs optical frequency comb pulses. In the subsequent process, the generation of each optical frequency comb pulse originates from the self-injection of the optical frequency comb pulse of the previous cycle. The resulting optical frequency comb pulse spectrum is as follows: Figure 6 As shown, the center frequency of the spectrum is determined by f1, and the width of the comb teeth of the spectrum is determined by the modulation frequency f2 of the semiconductor laser.

[0057] In this application, the laser driver includes a DC source, an RF signal source, and a biaser, with the structure as follows: Figure 7As 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 semiconductor laser through the biaser.

[0058] 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 semiconductor laser to drive the semiconductor laser to periodically generate light 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.

[0059] 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.

[0060] 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 8 As shown, when an optical pulse is input 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 arm of the transmission, and the other set is transmitted along the long arm. Based on the phase difference caused by the two 3dB directional couplers and the path difference between the short and long arms, the two sets of optical signals interfere with each other at port 3 and cancel each other out 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 discarded, thus achieving the filtering function.

[0061] 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 9 As shown, interference effects are formed on each MZ unequal arm interferometer, and the cumulative interference effects eventually achieve the filtering function.

[0062] 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 optical pulses and couples the optical pulses to the ring waveguide, as shown in the structure. Figure 10As shown, an optical pulse is coupled into a ring waveguide via a 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 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 optical pulse.

[0063] Based on the above explanation and elaboration of this application, it can be understood that the semiconductor laser operates in gain-switching mode to output optical pulses under the action of the laser driver. Under the action of the waveguide beam splitter, a small portion of these optical pulses are filtered, attenuated, and delayed before being fed back into the semiconductor laser, achieving self-injection locking of the semiconductor laser and the generation of an effective optical frequency comb. After being filtered by the waveguide filter, the frequency of the optical signal self-injected into the semiconductor laser is consistent with a certain frequency of the optical pulse spontaneously emitted by the semiconductor laser. Based on the self-injected optical signal, the linewidth of the optical pulse output by the semiconductor laser is significantly narrowed, effectively reducing the phase noise of the optical pulse. After the self-injected optical signal is fed back into the semiconductor laser, each pulse output by the semiconductor laser is based on self-injection locking, rather than random spontaneous emission. Therefore, the optical pulses output by the semiconductor laser have the same phase difference, which can generate clearer comb lines and distinguishable comb line spacing, while improving the flatness and coherence of the generated optical frequency comb. In addition, the optical 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.

[0064] 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 embodiments can be referred to each other.

[0065] 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.

[0066] 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. A self-injection locked based optical frequency comb source, characterized in that, The semiconductor laser, the laser driver, the first waveguide beam splitter, the second waveguide beam splitter and the waveguide filter are integrated on one optical chip. The semiconductor laser works in a gain-switched mode to output optical pulses, which are initial pulses or optical frequency comb pulses; the laser driver is connected with the semiconductor laser to drive the semiconductor laser; the first waveguide beam splitter is used to split the optical pulses output by the semiconductor laser, the optical pulses input by the second waveguide beam splitter or the optical pulses input by the waveguide filter; the waveguide filter is used to filter the received optical pulses and transmit the filtered optical pulses to the second waveguide beam splitter or the first waveguide beam splitter; the second waveguide beam splitter is used to split the optical pulses input by the first waveguide beam splitter or the optical pulses input by the waveguide filter; the first waveguide beam splitter and the second waveguide beam splitter each include a first upper port, a second upper port, a first lower port and a second lower port; the first upper port of the first waveguide beam splitter is connected with the semiconductor laser; the first upper port of the second waveguide beam splitter is connected with the second upper port of the first waveguide beam splitter; the waveguide filter is connected with the second lower port of the first waveguide beam splitter and the second lower port of the second waveguide beam splitter respectively. The optical frequency comb source further includes a waveguide attenuator integrated on the optical chip, which is used to attenuate the intensity of the received optical pulses; the waveguide attenuator is connected with the waveguide filter and the second lower port of the second waveguide beam splitter respectively.

2. The self-injection locked optical comb source of claim 1, wherein, The splitting ratio of the first waveguide beam splitter and the splitting ratio of the second waveguide beam splitter are both (90+N):(10-N), wherein N is a positive integer less than 10.

3. The self-injection locked optical comb source of claim 1, wherein, The optical frequency comb source further includes a waveguide delay line integrated on the optical chip, which is used to delay the received optical pulses, so that the time of the delayed optical pulses transmitted to the semiconductor laser is consistent with the time of one of the optical pulses output by the semiconductor laser; the waveguide delay line is connected with the waveguide attenuator and the second lower port of the second waveguide beam splitter respectively.

4. The self-injection locked optical comb source of claim 1, wherein, The waveguide attenuator is composed of a third waveguide beam splitter, a first transmission waveguide, a second transmission waveguide, a fourth waveguide beam splitter and a phase modulator; the third waveguide beam splitter and the fourth waveguide beam splitter each include a third upper port and a third lower port; the first transmission waveguide is connected with the third upper port of the third waveguide beam splitter and the third upper port of the fourth waveguide beam splitter respectively; the second transmission waveguide is connected with the third lower port of the third waveguide beam splitter and the third lower port of the fourth waveguide beam splitter respectively; the phase modulator is arranged on the second transmission waveguide.

5. The self-injection locked optical comb source of any of claims 1 to 4, wherein, The semiconductor laser is a distributed feedback laser or a distributed Bragg reflector laser.

6. The self-injection locked optical comb source of any of claims 1 to 4, wherein, The laser driver comprises a direct current source for generating a direct current bias, an RF signal source for generating a sinusoidal driving signal, and a bias tee, the bias tee comprising a feed inductance connected to the direct current source and a blocking capacitance connected to the RF signal source, the direct current bias and the sinusoidal driving signal being injected into the semiconductor laser through the bias tee.

7. The self-injection locked optical comb source of any of claims 1 to 4, wherein the optical comb source is a semiconductor optical comb source. The waveguide filter is an MZ unequal arm interferometer type filter or a micro ring resonator type optical filter.

8. The self-injection locked optical comb source of claim 7, wherein, 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 self-injection locked optical comb source of claim 7, wherein, The micro ring resonator type optical filter is composed of a ring waveguide connected head to tail and a coupling straight waveguide, the coupling straight waveguide receiving the optical pulse and coupling the optical pulse to the ring waveguide.

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