Narrow-linewidth double-frequency laser generation method and device and photo-production microwave method and device

Abstract

The invention discloses a narrow-linewidth double-frequency laser generation method, and belongs to the technical field of microwave photons. Two paths of optical signals generated by two semiconductor lasers with different wavelengths are injected into the same optical microcavity, the two paths of optical signals enter a self-injection locking state in different modes of the optical microcavity, and the optical microcavity can output narrow-linewidth double-frequency laser. The invention further discloses a narrow-linewidth double-frequency laser generating device and a photo-production microwave method and device. Two double-frequency lasers with different frequencies are locked on one optical microcavity through an original double-self-injection locking technology, the problem of phase noise of a double-frequency light source is effectively solved while the linewidth of the input laser is narrowed, common-mode noise caused by mode difference of the two lasers in a locking state is eliminated, and the dual-frequency light source has the advantages of being simple in structure and convenient to operate. And the traditional servo locking technology is not needed, so that the complexity of the system is simplified.

Description

Technical Field

[0001] The invention relates to a method and a device for generating a narrow-linewidth dual-frequency laser, belonging to the technical field of microwave photons. Background Art

[0002] Signal generation is one of the core units of modern electronic systems such as radar, electronic warfare, communication and remote sensing measurement. Its typical functions include providing low phase noise, low spurious, wide-band local oscillator signals in frequency conversion units, providing high-quality complex waveform signals (such as wide-bandwidth linear frequency modulation signals) in transmission units, and providing high-precision, low-phase noise clock signals in system high-speed data acquisition units. As a core component of modern electronic systems, signal generation units are driven by the needs of system multi-function, wide frequency band and miniaturization, and are developing towards millimeter waves and higher frequency bands, lower phase noise, larger modulation bandwidth, higher frequency agility speed, and smaller size, weight and power consumption.

[0003] Therefore, the miniaturization and multifunctionality of electronic systems are the themes of the development of national defense electronic technology. In particular, with the development of miniaturized aerospace combat systems such as future drones, the military has increasingly higher requirements for relevant technical indicators such as the sensitivity and observation capabilities of the entire system, and has put forward requirements for the next generation of local oscillator signal sources such as integration, low cost, high stability, large dynamic range, and low phase noise. The generation of microwave signals based on microwave photonic technology (i.e., photogenerated microwaves) has the advantages of high frequency, low phase noise, and compact structure, and can solve the electronic bottleneck problem faced by the generation of microwave signals in the electrical domain. Therefore, it is increasingly valued by researchers and is increasingly used in optical wireless communication systems, radar systems, satellite communications and other fields. The existing photogenerated microwave implementation technologies can be roughly divided into the following categories:

[0004] The first method is a microwave signal generation system based on optical heterodyne beat frequency technology [1-3], which is also the most basic optical microwave technology. Its core is to use the square law detection principle of the electro-optical detector (PD) to convert two dual-frequency light source signals with the same polarization and different wavelengths into microwave signals with a frequency equal to the difference frequency of the two light wavelengths. Using two independently working laser light sources for heterodyne beat frequency can generate microwave signals up to hundreds of GHz. However, due to the wavelength drift problem of the independently working laser light sources, the spectrum jitter of the output microwave signal is serious and the phase noise is extremely high, making it difficult to be practical.

[0005] Another method is to generate low phase noise microwave signals based on an electro-optical oscillator (OEO), which is another technology of light-generated microwaves. OEO is an oscillator that uses low-loss, wide-bandwidth optical fiber as an energy storage element. Its basic structure is an optical-electric hybrid resonant cavity [4]. In OEO, only modes that meet certain phase conditions and whose loop gain is greater than the loss can oscillate in the cavity. At the same time, there are usually mode-selective elements in the oscillator cavity, such as optical / electrical filters, to select signals of specific frequencies and suppress other modes. Electro-optical oscillators can generate microwave signals from hundreds of MHz to hundreds of GHz. The characteristic of this technology is that with the help of electro-optical / electro-optical conversion, optical filters with extremely high Q values ​​or optical fibers with extremely low losses are introduced into the microwave resonant cavity to form an electro-optical resonant cavity with ultra-high Q values, thereby achieving the generation of low phase noise signals. Thanks to the broadband consistency characteristics of electro-optical modulation technology and the low loss characteristics of optical fiber, electro-optical oscillators have a high quality factor in a broadband range. Therefore, the electro-optical oscillator still has the ability to generate extremely low phase noise and highly stable signals in the high-frequency bands of microwave and millimeter waves [5,6]. However, the OEO side mode suppression ratio and spurious problems have not been solved in this system, and the quality of the output signal is difficult to further improve.

[0006] Another method is to use high-quality dual-frequency light sources to generate adjustable broadband high-frequency low-phase noise microwave sources through electro-optical frequency division[7,8]. The core idea is to use an optical microcavity to first generate two ultra-narrow linewidth Brillouin lasers, and then use a cascade modulator to perform electro-optical frequency division based on this. At the same time, a voltage-controlled oscillator (VCO) is used to form a phase lock loop (PPL), and the principle of optical frequency division is used to reduce phase noise[9]. Each time the microwave signal is divided, its electrical signal phase noise is optimized by 6dB. The system achieves a maximum frequency division of 1 / 148, and its phase noise performance can be as low as -135dBc / Hz@1kHz and -153dBc / Hz@10kHz when the output signal is 20GHz. At the same time, controlling the frequency division number can also achieve a wide range of adjustable microwave signals.

[0007] However, the existing dual-frequency light source electro-optical frequency division photogenerated microwave scheme has the following problems:

[0008] 1. In terms of the generation of dual-frequency light sources, there is a problem that the performance of the dual-frequency light source cannot be taken into account while miniaturization. For example, for the method of using an optical microcavity or a fiber ring cavity to generate a Brillouin laser to generate a high-quality dual-frequency light source, the method of generating the Brillouin laser requires a large amount of servo control for frequency stabilization and locking during the experiment, which further increases the complexity of the system for generating a high-quality dual-frequency light source. If no additional frequency stabilization and locking operation is performed, although the generated dual-frequency light source can have a wide range of frequency tunability, this method greatly reduces the performance of the dual-frequency light source, and due to the wavelength drift of the laser and the influence of the external experimental environment, the frequency noise of the dual-frequency light source will be further deteriorated.

[0009] 2. In terms of the experimental method of electro-optical frequency division, the traditional electro-optical frequency division method requires a large number of optoelectronic devices, the system structure is complex, and the cost of building the system is very high. It can neither be put into harsh actual combat environments for field experiments nor meet the practical requirements of miniaturization and low cost of airborne equipment.

[0010] References:

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[0012] [2]Doi Y, Fukushima S, Ohno T, et al. Frequency stabilization of millimeter-wave subcarrier using laser heterodyne source and optical delayline. IEEE Photonics Technology Letters, 2002, 13(9): 1002-1004.

[0013] [3]Kittlaus EA,Eliyahu D,Ganji S,et al.Alow-noise photonicheterodyne synthesizer and its application to millimeter-wave radar.NatureCommunications,2021,12(1):1-10.

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[0015] [5] Liu Shifeng, Xu Xiaorui, Zhang Fangzheng, et al. Research on ultra-low phase noise optoelectronic oscillator and its frequency synthesis technology [J]. Journal of Radars, 2019, 8(2): 243–250. doi:10.12000 / JR19029.

[0016] [6]Xu XR, Zhang FZ, and Pan SL, "Low-phase-noise frequency synthesizer based on optoelectronic oscillator" in the International Conference on Optical Communications and Networks (ICOCN 2017), Wuzheng, China, Aug. 7-10, 2017.

[0017] [7]Li J,Vahala K.Stabilized microwave-frequency source:USPatent 10,523,214[P].2019-12-31.

[0018] [8]Li J,Vahala K.Ultra-low phase-noise microwave oscillator based on electro-optical frequency division[C] / / 2017Joint Conference of the EuropeanFrequency and Time Forum and IEEE International Frequency Control Symposium(EFTF / IFCS).IEEE,2017:265-266.

[0019] [9]Liang W,Ilchenko VS,Eliyahu D,et al.Ultralow noise miniatureexternal cavity semiconductor laser[J].Nature communications,2015,6(1):1-6.

[0020]

[10] Liang W,Eliyahu D,Ilchenko VS,et al.High spectral purity Kerrfrequency comb radio frequency photonic oscillator[J].Nature communications,2015,6(1):1-8.

[0021]

[11] Shen B,Chang L,Liu J,et al.Integrated turnkey soliton microcombs[J].Nature,2020,582(7812):365-369. Summary of the invention

[0022] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing photogenerated microwave technology based on electro-optical frequency division of a dual-frequency light source, and provide a narrow-linewidth dual-frequency laser generation method. Through the original dual self-injection locking technology, two dual-frequency lasers with different frequencies are locked on an optical microcavity. While achieving the narrowing of the input laser linewidth, the phase noise problem of the dual-frequency light source is effectively improved, and the common-mode noise caused by the mode difference of the two lasers in the locked state is eliminated. No traditional servo locking technology is required, which simplifies the system complexity.

[0023] The technical solution proposed by the present invention is as follows:

[0024] A method for generating narrow-linewidth dual-frequency lasers is disclosed. Two optical signals generated by two semiconductor lasers with different wavelengths are injected into the same optical microcavity, and the two optical signals are respectively put into a self-injection locking state in different modes of the optical microcavity. The optical microcavity can then output narrow-linewidth dual-frequency lasers.

[0025] Preferably, when the line width / phase noise of the optical microcavity output light signal or its beat frequency signal is significantly reduced, it is determined that the two optical signals enter the self-injection locking state in different modes of the optical microcavity respectively.

[0026] Based on the same inventive concept, the following technical solutions can also be obtained:

[0027] A narrow linewidth dual-frequency laser generating device, comprising:

[0028] Two semiconductor lasers are used to generate two optical signals with different wavelengths;

[0029] An optical microcavity is used to inject the two optical signals of different wavelengths into the optical microcavity, and make the two optical signals enter the self-injection locking state respectively in different modes of the optical microcavity, so that the optical microcavity can output narrow linewidth dual-frequency laser.

[0030] Preferably, the narrow linewidth dual-frequency laser generating device further comprises:

[0031] The spectrum observation module is used to judge according to the spectrum of the optical microcavity output light signal or its beat frequency signal: when the line width / phase noise of the optical microcavity output light signal or its beat frequency signal is significantly reduced, it is judged that the two optical signals enter the self-injection locking state respectively in different modes of the optical microcavity.

[0032] A method for photogenerating microwaves, based on a narrow-linewidth dual-frequency laser, uses an electro-optical frequency division method to generate an adjustable broadband high-frequency low-phase noise microwave signal; the narrow-linewidth dual-frequency laser is generated using the method described in any of the above technical solutions.

[0033] As a further improved technical solution, the electro-optical frequency division method is specifically as follows:

[0034] The narrow linewidth dual-frequency laser is cascaded electro-optically modulated by using a radio frequency signal output by a voltage-controlled oscillator, and pairs of optical frequency combs with a plurality of sidebands are generated by using the narrow linewidth dual-frequency laser as a carrier respectively;

[0035] Broadening the optical frequency comb to increase the number of sidebands;

[0036] In the optical domain, two groups of sideband comb teeth with two wavelengths adjacent to each other in the widened optical frequency comb are filtered out;

[0037] Beat the two filtered sideband comb teeth to obtain the microwave signal after electro-optical frequency division;

[0038] The microwave signal after the electro-optical frequency division is phase-detected with the reference microwave signal, and the voltage-controlled oscillator is servo-feedback-controlled according to the error signal generated by the phase-detection to achieve the locking of the output frequency of the voltage-controlled oscillator;

[0039] The radio frequency signal output by the voltage-controlled oscillator is divided into two paths, one of which is used to perform cascade electro-optical modulation on the narrow-linewidth dual-frequency laser after phase shifting, and the other is output as a photogenerated microwave signal.

[0040] As another further improved technical solution, the electro-optical frequency division method is specifically as follows:

[0041] The narrow linewidth dual-frequency laser is coupled with a reference optical frequency comb and then divided into two links;

[0042] The optical signals of the two links are filtered respectively, wherein one filter selects the comb teeth of the negative order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filter selects the comb teeth of the positive order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser;

[0043] The two filtered comb teeth are beat and mixed to obtain a frequency-divided electrical signal;

[0044] The frequency-divided electrical signal is phase-detected with the reference microwave signal, and the error signal generated by the phase-detection is used as a feedback control signal to perform locking feedback control on the reference optical frequency comb;

[0045] The reference optical frequency comb is separated into a beat frequency path and the beat frequency signal is output as a photogenerated microwave signal.

[0046] As another further improved technical solution, the electro-optical frequency division method is specifically as follows:

[0047] Electro-optically modulating a reference optical signal using a radio frequency signal output by a voltage-controlled oscillator to generate an optical frequency comb, and broadening the generated optical frequency comb to increase the number of sidebands, thereby generating a reference optical frequency comb;

[0048] The narrow linewidth dual-frequency laser is coupled with a reference optical frequency comb and then divided into two links;

[0049] The optical signals of the two links are filtered respectively, wherein one filter selects the comb teeth of the negative order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filter selects the comb teeth of the positive order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser;

[0050] The two filtered comb teeth are beat and mixed to obtain a frequency-divided electrical signal;

[0051] The divided frequency electrical signal is phase-compared with the reference microwave signal, and the voltage-controlled oscillator is servo-feedback-controlled according to the error signal generated by the phase-comparison to achieve locking of the output frequency of the voltage-controlled oscillator;

[0052] The radio frequency signal output by the voltage-controlled oscillator is divided into two paths, one of which is used to electro-optically modulate the reference optical signal after phase shifting, and the other is output as a light-generated microwave signal.

[0053] A photogenerated microwave device comprises a narrow-linewidth dual-frequency laser generating device and an electro-optical frequency division device, which is used to generate an adjustable broadband high-frequency low-phase noise microwave signal based on the narrow-linewidth dual-frequency laser generated by the narrow-linewidth dual-frequency laser generating device by using an electro-optical frequency division method; the narrow-linewidth dual-frequency laser generating device is the narrow-linewidth dual-frequency laser generating device as described in any of the above technical solutions.

[0054] As a further improved technical solution, the electro-optical frequency division device comprises:

[0055] An electro-optical modulation module, used to perform cascade electro-optical modulation on the narrow-linewidth dual-frequency laser using a radio frequency signal output by a voltage-controlled oscillator, and to generate a pair of optical frequency combs with a plurality of sidebands using the narrow-linewidth dual-frequency laser as a carrier; a comb-tooth widening module, used to widen the optical frequency comb to increase the number of sidebands;

[0056] An optical filter module is used to filter out two sideband comb teeth with two wavelengths adjacent to each other in two groups of widened optical frequency combs in the optical domain;

[0057] The photoelectric detection module is used to beat the two filtered sideband comb teeth to obtain the microwave signal after electro-optical frequency division;

[0058] A mixer, used for phase detection between the microwave signal after electro-optical frequency division and the reference microwave signal;

[0059] A servo control module, used for performing servo feedback control on the voltage controlled oscillator according to the error signal generated by the phase detector to achieve locking of the output frequency of the voltage controlled oscillator;

[0060] The voltage-controlled oscillator outputs a radio frequency signal in two paths, one of which is used to perform cascade electro-optical modulation on the narrow-linewidth dual-frequency laser after phase shifting, and the other is output as a photogenerated microwave signal.

[0061] As another further improved technical solution, the electro-optical frequency division device comprises:

[0062] A reference optical frequency comb module, used to generate a reference optical frequency comb;

[0063] An optical coupler is used to couple the narrow-linewidth dual-frequency laser with a reference optical frequency comb and then divide the two links into two links; two optical filter modules are used to filter the optical signals of the two links respectively, one of which filters out the comb teeth of the negative-order part of the reference optical frequency comb that are closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filters out the comb teeth of the positive-order part of the reference optical frequency comb that are closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser;

[0064] The first and second photoelectric detection modules are used to perform beat frequency on the two comb teeth filtered out respectively;

[0065] A first mixer is used to mix the two beat frequency signals to obtain a frequency-divided electrical signal;

[0066] A second mixer is used for phase detection between the divided frequency electrical signal and a reference microwave signal;

[0067] A servo control module, used for performing locking feedback control on the reference optical frequency comb module by using the error signal generated by the phase detector as a feedback control signal;

[0068] The third photoelectric detection module is used to separate a reference optical frequency comb into a beat frequency signal and output the beat frequency signal as a photogenerated microwave signal.

[0069] As another further improved technical solution, the electro-optical frequency division device comprises:

[0070] An electro-optical modulation module, used for electro-optically modulating a reference optical signal using a radio frequency signal output by a voltage-controlled oscillator to generate an optical frequency comb;

[0071] A comb-tooth broadening module, used for broadening the optical frequency comb to increase the number of sidebands, thereby generating a reference optical frequency comb;

[0072] An optical coupler is used to couple the narrow-linewidth dual-frequency laser with a reference optical frequency comb and then divide the combined signals into two links; two optical filter modules are used to filter the optical signals of the two links respectively, one of which filters out the comb teeth of the negative-order part of the reference optical frequency comb that are closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filters out the comb teeth of the positive-order part of the reference optical frequency comb that are closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser;

[0073] Two photoelectric detection modules are used to beat the two comb teeth filtered out respectively;

[0074] A first mixer is used to mix the two beat frequency signals to obtain a frequency-divided electrical signal;

[0075] A second mixer is used for phase detection between the divided frequency electrical signal and a reference microwave signal;

[0076] A servo control module, used for performing servo feedback control on the voltage controlled oscillator according to the error signal generated by the phase detector to achieve locking of the output frequency of the voltage controlled oscillator;

[0077] The voltage-controlled oscillator outputs a radio frequency signal in two paths, one of which is input into the electro-optical modulation module after phase shifting to perform electro-optical modulation on the reference optical signal, and the other is output as a light-generated microwave signal.

[0078] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0079] The present invention innovatively introduces dual self-injection locking technology, which locks two dual-frequency lasers of different frequencies on an optical microcavity at the same time through self-injection locking technology. While achieving the narrowing of the input laser linewidth, it does not require traditional servo locking technology, thereby simplifying the system complexity. This method promotes the miniaturization of the dual-frequency light source in the cavity locking mechanism, and effectively improves the phase noise problem of the dual-frequency light source, eliminating the common-mode noise caused by the mode difference of the two lasers in the locked state. By adjusting the pump current of the laser and the coupling spacing of the optical microcavity, the locking frequency, locking range and phase can be dynamically controlled, thereby achieving the generation of a miniaturized, low-phase-noise, narrow-linewidth dual-frequency light source.

[0080] Based on the dual self-injection locked dual-frequency light source, the present invention further makes multiple improvements to the existing electro-optical frequency division architecture to purify the spectrum of the generated photogenerated microwave signal, achieve significant reduction in noise and spurious suppression, and obtain tunability, while achieving the comprehensive technical requirements of miniaturization, low cost, high stability, large dynamic range and low phase noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 Schematic diagram of laser self-injection locking structure;

[0082] Figure 2 This is a schematic diagram of the structural principle of a narrow-linewidth dual-frequency laser generating device based on double self-injection locking proposed by the present invention;

[0083] Figure 3 Schematic diagram of the self-injection locking structure of the crystal optical microcavity;

[0084] Figure 4 Schematic diagram of the self-injection locking structure of the optical microcavity integrated with silicon nitride waveguide chip;

[0085] Figure 5 This is a schematic diagram of the structure of double self-injection locking using the combined light coupling method;

[0086] Figure 6 A schematic diagram of the structural principle of the direct electro-optical frequency division method proposed in the present invention;

[0087] Figure 7 A schematic diagram of the structural principle of the reference optical frequency comb electro-optical frequency division method proposed in the present invention;

[0088] Figure 8 This is a schematic diagram of the structural principle of the reference electro-optical comb frequency division method proposed in the present invention. DETAILED DESCRIPTION

[0089] In view of the shortcomings of the existing photogenerated microwave technology based on electro-optical frequency division of dual-frequency light sources, the solution of the present invention is to lock two dual-frequency lasers with different frequencies on an optical microcavity (to distinguish it from the existing self-injection locking technology, this article refers to it as dual self-injection locking), so as to achieve the narrowing of the input laser linewidth while effectively improving the phase noise problem of the dual-frequency light source, eliminating the common-mode noise caused by the mode difference of the two lasers in the locked state, and simplifying the system complexity without the need for traditional servo locking technology.

[0090] The technical solution proposed by the present invention is as follows:

[0091] A method for generating narrow-linewidth dual-frequency lasers is disclosed. Two optical signals generated by two semiconductor lasers with different wavelengths are injected into the same optical microcavity, and the two optical signals are respectively put into a self-injection locking state in different modes of the optical microcavity. The optical microcavity can then output narrow-linewidth dual-frequency lasers.

[0092] A narrow linewidth dual-frequency laser generating device, comprising:

[0093] Two semiconductor lasers are used to generate two optical signals with different wavelengths;

[0094] An optical microcavity is used to inject the two optical signals of different wavelengths into the optical microcavity, and make the two optical signals enter the self-injection locking state respectively in different modes of the optical microcavity, so that the optical microcavity can output narrow linewidth dual-frequency laser.

[0095] To facilitate public understanding, the technical solution of the present invention is described in detail below with reference to the accompanying drawings:

[0096] The basic structure and principle of the existing laser self-injection locking structure are as follows: Figure 1 As shown in the figure, the self-injection locking process of this structure can be achieved by simply coupling the laser to the optical microcavity through the end face. The locking mechanism is based on the existence of the laser pumping optical microcavity, while the self-injection locking relies on the Rayleigh scattering phenomenon of the light wave inside the microcavity. Due to the feedback effect of the backscattered light generated by Rayleigh scattering, the output frequency of the laser can be locked to the optical pumping mode corresponding to the Rayleigh scattering. In this way, the backscattered light generated by Rayleigh scattering of a certain power realizes feedback control of the laser frequency, thereby compressing the line width of the laser.

[0097] The efficiency ratio η of the compression linewidth of self-injection locking can be described by the solution of formula (1):

[0098]

[0099] Where P r is the power reflected back into the laser mode, Q and Q LDis the quality factor of the pump laser and the optical microcavity. The higher the quality factor of the optical microcavity, the higher the suppression ratio η of the noise suppression equation (1). And due to the extremely strong optical feedback from the optical microcavity to the laser, the optical feedback not only stabilizes the frequency but also slightly reduces the threshold of the laser.

[0100] The narrow linewidth dual-frequency laser generating device of the dual self-injection locking structure proposed by the present invention is as follows Figure 2 As shown, it includes two main parts: one part is the dual-frequency laser part, where two semiconductor lasers with different wavelengths can be used as the basic structure of the dual-frequency light source; the other part is the optical microcavity part, and the optical microcavity used includes but is not limited to integrated chip optical microcavity, various crystal optical microcavities, Fabry-Perot optical microcavity (abbreviated as FP cavity) and various fiber ring cavities. Figure 2 The discrete locking structure in which two semiconductor lasers are locked independently is used as an example for explanation. Two semiconductor lasers with different wavelengths are fixed on a three-axis translation stage. By adjusting the translation stage, the emission light of the laser is coupled with the optical microcavity, so that the output laser can be coupled into the optical microcavity; then the wavelengths of the two lasers are adjusted respectively. According to the needs of subsequent electro-optical frequency division, the required wavelength is adjusted and the output laser of the wavelength is allowed to enter two different modes of the optical microcavity to achieve dual self-injection locking of the two lasers based on a single optical microcavity. The locked dual-frequency laser is input into a spectrum analyzer for observation after beating. When the line width of the beat signal is found to be significantly narrowed and the phase noise of the beat signal is significantly reduced from the spectrum analyzer, it can be determined that the dual self-injection locking is successful, indicating that the generation of a narrow linewidth dual-frequency light source has been achieved. Therefore, whether the double self-injection locking is successful can be judged by observing the linewidth change or phase noise change of the beat frequency signal of the optical microcavity output light signal; in addition, the linewidth change or phase noise change of the optical microcavity output light signal can also be directly observed, but considering that the existing spectral measurement technology is difficult to achieve, it is preferred to use the method of observing the beat frequency signal for judgment.

[0101] The coupling method between the semiconductor laser and the optical microcavity needs to be determined according to the selected optical microcavity. Figure 3 The self-injection locking structure using a crystal optical microcavity is shown (for simplicity, the figure only shows the coupling structure of a semiconductor laser and an optical microcavity); the crystal optical microcavity used for locking is usually densely packaged on an optical prism coupling temperature-controlled platform. The semiconductor laser for pump input is generally installed on a base with integrated three-axis displacement. The light output from the optical fiber is converted into spatial light output through the input prism, and then the pump laser is focused onto the inner surface of the total internal reflection prism through the prism on the coupling platform. Figure 3As shown, a crystal optical microcavity is placed near the focal point to couple the end face of the prism with the end face of the optical microcavity. The lockable optical mode is excited by adjusting the wavelength and the distance of the displacement stage to meet the locking conditions. Finally, by monitoring the line width or phase noise reduction of the beat signal on the spectrum analyzer after the PD beat, it is determined whether the dual self-injection locking state is successfully entered to achieve the output of narrow linewidth dual-frequency laser. Figure 4 It shows the self-injection locking structure of an optical microcavity with an integrated silicon nitride waveguide chip (again, only the coupling structure of a semiconductor laser and an optical microcavity is shown). The semiconductor laser is fixed on a three-axis translation stage, and the light-emitting end face is directly close to the chip waveguide for end face coupling. The light power is coupled into the corresponding optical microcavity by adjusting the coupling distance between the laser light-emitting end face and the chip, and the lockable optical mode is excited by adjusting the wavelength and the distance of the translation stage to meet the locking conditions. Finally, by monitoring the line width or phase noise reduction of the beat signal on the spectrum analyzer after the PD beat, it is determined whether the dual self-injection locking state is successfully entered to achieve the output of narrow linewidth dual-frequency laser.

[0102] The above is an example of a discrete locking structure in which two semiconductor lasers are locked independently. In fact, the following can also be used: Figure 5 The combined beam locking structure shown uses a fiber combiner to combine the pump light outputs of two semiconductor lasers with different wavelengths, and then couples the combined laser output to the optical microcavity to excite the corresponding lockable optical mode. Next, the power of the pump laser is adjusted to meet the threshold and conditions of self-injection locking to achieve a double self-injection locking state. This method is suitable for a standard microcavity coupling platform with a relatively simple structure. Its advantage is that by using a combiner to first combine the dual-frequency lasers and then perform optical microcavity coupling, the crosstalk between the dual-frequency lasers can be optimized, so that when selecting the locking state in different modes, the desired experimental results can be obtained more conveniently.

[0103] From a theoretical perspective, the narrow-linewidth dual-frequency light source generated by dual self-injection locking has advantages over the self-injection locking light source generated by two single self-injection locking light sources in parallel (hereinafter referred to as the parallel scheme). The locking range tuning coefficient of dual self-injection locking is described by formula (2):

[0104]

[0105] In the parallel scheme, the tuning coefficient is Since dual self-injection locking uses only a single optical microcavity, nonlinear coupling occurs between the two lasers. This nonlinear coupling is achieved through the same optical microcavity. Therefore, in the case of dual self-injection locking, this technical method can reduce the mutual interference between the two lasers, improve the frequency stability of the lasers, and achieve the effect of optimizing system performance. In addition, the nonlinear coupling relationship caused by locking in the same optical microcavity makes it possible to establish a real-time feedback mechanism between two lasers with different output frequencies, and can dynamically adjust the crosstalk level of the dual-frequency light source to ensure that it works stably in the expected locking state. This nonlinear coupling mechanism caused by locking in a single optical microcavity not only improves the frequency stability of the system, but also provides an effective control method for the frequency tuning of the dual-frequency light source, achieving precise control of the frequency while maintaining the independence of the dual-frequency lasers.

[0106] From an experimental perspective, the use of a single optical microcavity to implement dual self-injection locking technology can enable two lasers to be locked through different modes of the same microcavity. Compared with the parallel solution, this scheme promotes the miniaturization of the dual-frequency light source in the cavity locking mechanism, and effectively improves the frequency noise problem of the dual-frequency light source, while eliminating the common-mode noise caused by the mode difference between the two lasers in the locked state. In addition, the dual-frequency light source based on dual self-injection locking technology performs better in line width and frequency noise than the Brillouin laser based on the optical microcavity and the fiber laser based on the high-precision stable optical cavity phase-locked loop. It is worth noting that the laser using self-injection locking technology can achieve high frequency stability during operation without the need for additional vacuum pumps or vibration isolation devices, which further simplifies the system design and improves practicality.

[0107] Based on the narrow-linewidth dual-frequency laser generation scheme proposed in the present invention, an adjustable broadband high-frequency low-phase noise microwave signal can be generated using the electro-optical frequency division method, and the generated microwave signal has lower noise, better tunability, and higher system integration. For example, a photogenerated microwave device is formed by combining the above-mentioned narrow-linewidth dual-frequency laser generation device with the electro-optical frequency division scheme disclosed in reference 7 or 8.

[0108] Considering the following problems with the electro-optical frequency division schemes of references 7 and 8: first, the comb teeth are not further widened under cascade modulation, resulting in a small frequency division multiple, which leads to limited frequency tunability of the photogenerated microwave signal; second, no additional reference optical signal part is introduced to further improve the quality of the photogenerated microwave signal, making its applicability in microwave photonic applications relatively narrow; third, a large number of optoelectronic devices are required, the system structure is complex, the cost of building the system is very high, and it is difficult to realize application in miniaturization.

[0109] To this end, the present invention further improves it and proposes the following three improved electro-optical frequency division schemes.

[0110] (a) Direct electro-optical frequency division method:

[0111] The structure of the improved photogenerated microwave device based on direct electro-optical frequency division is as follows: Figure 6 First, the narrow-linewidth dual-frequency laser (the frequencies of the dual-frequency laser are v1 and v2, respectively, where v2>v1) generated by the narrow-linewidth dual-frequency laser generating device of the present invention passes through the electro-optic modulation module and the frequency f set by the voltage-controlled oscillator is m The RF input is cascaded electro-optically modulated, and a pair of optical frequency combs with several sidebands are generated using dual-frequency lasers as carriers (as shown in the waveform at a in the figure); then the optical frequency comb generated by the electro-optical frequency division module is further widened by the comb tooth widening module (for example, by using nonlinear effects to widen it), increasing the number of sidebands. The comb tooth frequencies after widening can be expressed as v1+N1f m and v2-N2f m (The waveform is shown at b in the figure); then it passes through the optical filter module to filter out two sideband combs with two wavelengths adjacent to each other in the optical domain (the waveform is shown at c in the figure); then the two adjacent comb teeth are beat through the photoelectric detection module to generate a beat frequency of f beat =(v2-N2f m )-(v1+N1f m )=v2-v1-(N1+N2)×f m The optical microwave signal is the electrical signal after the electro-optical frequency division, and the frequency division multiple is N1+N2; finally, the frequency division signal is combined with the frequency f r The reference source enters the mixer for comparison and phase detection, generating an error signal of relative phase error. The error signal is fed into the voltage controlled oscillator (VCO) through the feedback signal output by the servo control module to lock the output frequency of the voltage controlled oscillator, further improving the frequency stability of the VCO and achieving spectrum purification of the output microwave signal; the output of the voltage controlled oscillator is divided into two paths, one of which is connected to the electro-optical modulation module after passing through the electrically controlled phase shift module to form a locking feedback loop, forming a phase-locked loop; the other path is used to output the optically generated microwave signal. This method can successfully achieve the generation of optically generated microwave signals with low phase noise and high frequency stability.

[0112] (II) Reference optical frequency comb electro-optical frequency division method:

[0113] The structure of the improved photogenerated microwave device based on the reference optical frequency comb electro-optical frequency division method is as follows: Figure 7 The reference optical frequency comb module generates the reference optical frequency comb required for frequency division (such as Figure 7 The frequency of each comb tooth can be expressed as: v0±nf m, wherein n is an integer, the optical frequency comb can be a microcavity optical frequency comb generated by an additional optical microcavity, or a modulated electro-optical frequency comb generated by a cascade modulator or a thin-film lithium niobate microcavity; the reference optical frequency comb and the narrow-linewidth dual-frequency laser generated by the narrow-linewidth dual-frequency laser generating device of the present invention (such as Figure 7 The optical fiber is coupled through an optical coupler (as shown in the waveform at b in the figure) (the frequencies of the dual-frequency lasers are v1 and v2, respectively, where v2>v1); then it is divided into two links, which are filtered by optical filter module 1 and optical filter module 2 respectively. The optical filter module 1 on the upper branch optical link filters out the negative order part of the reference optical frequency comb (i.e., the comb tooth part on the left) which is closest to the narrow linewidth laser v1. The frequency of the filtered comb tooth is expressed as: v0-N1f m , where N1 is an integer; similarly, the optical filter module 2 on the lower branch optical link filters out the positive order part of the reference optical frequency comb (i.e. the right comb tooth part) closest to the narrow linewidth laser v2, and its frequency is expressed as: v0+N2f m , where N2 is an integer, and the two comb teeth filtered out are as follows Figure 7 As shown in the waveforms at c and d in the figure, these two optical signals are beat by the photoelectric detection module 1 and the photoelectric detection module 2 respectively, and the electrical signals output after the beat are: f beat1 =|v1-(v0-N1f m )|、f beat2 =|v2-(v0+N2f m )|; After the two beat frequency electrical signals are compared and phase-detected by mixer 1, an electrical signal can be obtained: f diff =f beat2 -f beat1 =|v2-v1-(N2+N1)×f m |, the electrical signal is also the frequency division signal of the whole structure, the frequency division multiple is N2+N1; the frequency division signal and the reference source f r After passing through mixer 2, comparison and phase detection are performed to obtain an error signal to be fed back. After passing through the servo control module, the error signal obtains a feedback control signal, and the feedback control signal is fed into the reference optical frequency comb module to form a locked feedback loop, thereby realizing the spectrum purification of the optical frequency comb beat signal; the optical link part of the reference optical frequency comb module is led out to the photoelectric detection module 3 for beat frequency, and the microwave signal obtained by the beat frequency at this time is the stable low phase noise optical microwave signal generated after spectrum purification. The feedback of the error signal not only reduces the phase noise of the optical frequency comb generated by the reference optical frequency comb module, but also further improves the frequency stability of the optical frequency comb. This method can also successfully achieve the generation of low phase noise, high frequency and stable optical microwave signals.

[0114] (III) Reference electro-optical comb frequency division method:

[0115] The structure of the improved photogenerated microwave device based on the reference electro-optical comb frequency division method is as follows: Figure 8 As shown, its essence is the combination of direct electro-optic frequency division method and reference optical frequency comb electro-optic frequency division method. The difference from the direct electro-optic modulation method in (I) is the introduction of a reference light source. Although the introduction of the reference light source increases the complexity of the overall structure and adds a set of optical filter modules and electro-optic detection modules, it can further reduce the phase noise of the output microwave signal and improve its frequency stability. The implementation steps of this method are as follows: the reference light source v0 is used as a pump laser and passes through a voltage-controlled oscillator with a frequency set to f m The RF input is modulated by the electro-optical modulation module to generate Figure 8 The reference electro-optical frequency comb with the waveform shown in a in the figure is then passed through the comb broadening module to obtain a comb frequency spacing of f m The reference electro-optical frequency comb with wide spectrum and multiple comb teeth, the frequency of each comb tooth can be expressed as: v0±

[0116] nf m , where n is an integer. The reference electro-optical frequency comb is then coupled through an optical coupler to a narrow-linewidth dual-frequency laser (the frequencies of the dual-frequency lasers are v1 and v2, where v2>v1) generated by two pumped semiconductor lasers after a dual self-injection locking structure. It is then divided into two links and filtered through different optical filter modules. Among them, the filter on the first optical link will filter out the comb teeth closest to the narrow-linewidth laser v1 in the negative order part of the reference electro-optical frequency comb (i.e. the comb teeth on the left), and the frequency of the filtered comb teeth is expressed as: v0-N1f m , where N1 is an integer. Similarly, the filter on the second optical link will filter out the positive order part of the reference electro-optical frequency comb (i.e. the right comb part) that is closest to the narrow linewidth laser v2, and its frequency is expressed as: v0+N2 f m , where N2 is an integer. The principle of selecting the filter comb teeth is as follows Figure 8 The waveforms shown at d and e in the figure. These two paths pass through different electro-optical detection modules respectively, and the electrical signals output after the beat frequency are: beat1 =|v1-(v0-N1f m )|、f beat2 =|v2-(v0+N2f m )|. After the two beat frequency electro-optical signals are compared by mixer 1, an electrical differential signal can be obtained: f diff =f beat2 -f beat1 =|v2-v1-(N2+N1)×f m |; This signal is also the electro-optical frequency division signal of the entire structure, and the frequency division multiple is N2+N1. The subsequent voltage-controlled oscillator output part is similar to the steps of the direct electro-optical frequency division method: the obtained frequency division signal f diff With frequency f rThe reference microwave source is compared with the mixer 2, and the voltage-controlled oscillator is locked by using the electrical error signal of the relative phase error between its frequency division signal and the reference microwave source signal, so that it generates a frequency of f m The microwave output is divided into two paths, one of which is received by the electro-optical modulation module as the RF input of the cascade electro-optical modulation after passing through the servo control module, and forms a negative feedback loop with the entire electro-optical frequency division structure, playing the role of a phase-locked loop. Therefore, the other path can be used as the output of the microwave source of the entire structure, that is, the low phase noise, high frequency and stable microwave signal after the electro-optical frequency division.

Claims

1. A method for generating a narrow linewidth dual-frequency laser, characterized in that: Two optical signals generated by two semiconductor lasers with different wavelengths are injected into the same optical microcavity, and the two optical signals are made to enter the self-injection locking state in different modes of the optical microcavity respectively, so that the optical microcavity can output narrow linewidth dual-frequency laser.

2. The narrow linewidth dual-frequency laser generation method according to claim 1, characterized in that: When the line width / phase noise of the optical microcavity output light signal or its beat frequency signal is significantly reduced, it is determined that the two optical signals enter the self-injection locking state in different modes of the optical microcavity respectively.

3. A narrow linewidth dual-frequency laser generating device, characterized in that: include: Two semiconductor lasers are used to generate two optical signals with different wavelengths; An optical microcavity is used to inject the two optical signals of different wavelengths into the optical microcavity, and make the two optical signals enter the self-injection locking state respectively in different modes of the optical microcavity, so that the optical microcavity can output narrow linewidth dual-frequency laser.

4. The narrow linewidth dual-frequency laser generating device as claimed in claim 3, characterized in that: Also includes: The spectrum observation module is used to judge according to the spectrum of the optical microcavity output light signal or its beat frequency signal: when the line width / phase noise of the optical microcavity output light signal or its beat frequency signal is significantly reduced, it is judged that the two optical signals enter the self-injection locking state respectively in different modes of the optical microcavity.

5. A method of photogenerating microwaves, based on a narrow linewidth dual-frequency laser, uses an electro-optical frequency division method to generate an adjustable broadband high-frequency low-phase noise microwave signal; characterized in that: The narrow linewidth dual-frequency laser is generated using the method as claimed in claim 1 or 2.

6. The method for photogenerating microwaves according to claim 5, characterized in that: The electro-optical frequency division method is specifically as follows: The narrow linewidth dual-frequency laser is cascaded electro-optically modulated by using a radio frequency signal output by a voltage-controlled oscillator, and pairs of optical frequency combs with a plurality of sidebands are generated by using the narrow linewidth dual-frequency laser as a carrier respectively; Broadening the optical frequency comb to increase the number of sidebands; In the optical domain, two groups of sideband comb teeth with two wavelengths adjacent to each other in the widened optical frequency comb are filtered out; Beat the two filtered sideband comb teeth to obtain the microwave signal after electro-optical frequency division; The microwave signal after the electro-optical frequency division is phase-detected with the reference microwave signal, and the voltage-controlled oscillator is servo-feedback-controlled according to the error signal generated by the phase-detection to achieve the locking of the output frequency of the voltage-controlled oscillator; The radio frequency signal output by the voltage-controlled oscillator is divided into two paths, one of which is used to perform cascade electro-optical modulation on the narrow-linewidth dual-frequency laser after phase shifting, and the other is output as a photogenerated microwave signal.

7. The method for photogenerating microwaves according to claim 5, characterized in that: The electro-optical frequency division method is specifically as follows: The narrow linewidth dual-frequency laser is coupled with a reference optical frequency comb and then divided into two links; The optical signals of the two links are filtered respectively, wherein one filter selects the comb teeth of the negative order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filter selects the comb teeth of the positive order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser; The two filtered comb teeth are beat and mixed to obtain a frequency-divided electrical signal; The frequency-divided electrical signal is phase-detected with the reference microwave signal, and the error signal generated by the phase-detection is used as a feedback control signal to perform locking feedback control on the reference optical frequency comb; The reference optical frequency comb is separated into a beat frequency path and the beat frequency signal is output as a photogenerated microwave signal.

8. The method for photogenerating microwaves according to claim 5, characterized in that: The electro-optical frequency division method is specifically as follows: Electro-optically modulating a reference optical signal using a radio frequency signal output by a voltage-controlled oscillator to generate an optical frequency comb, and broadening the generated optical frequency comb to increase the number of sidebands, thereby generating a reference optical frequency comb; The narrow linewidth dual-frequency laser is coupled with a reference optical frequency comb and then divided into two links; The optical signals of the two links are filtered respectively, wherein one filter selects the comb teeth of the negative order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filter selects the comb teeth of the positive order part of the reference optical frequency comb which are closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser; The two filtered comb teeth are beat and mixed to obtain a frequency-divided electrical signal; The divided frequency electrical signal is phase-compared with the reference microwave signal, and the voltage-controlled oscillator is servo-feedback-controlled according to the error signal generated by the phase-comparison to achieve locking of the output frequency of the voltage-controlled oscillator; The radio frequency signal output by the voltage-controlled oscillator is divided into two paths, one of which is used to electro-optically modulate the reference optical signal after phase shifting, and the other is output as a light-generated microwave signal.

9. A photogenerated microwave device, comprising a narrow linewidth dual-frequency laser generating device and an electro-optical frequency division device, for generating an adjustable broadband high-frequency low-phase noise microwave signal based on the narrow linewidth dual-frequency laser generated by the narrow linewidth dual-frequency laser generating device by using an electro-optical frequency division method; characterized in that: The narrow linewidth dual-frequency laser generating device is the narrow linewidth dual-frequency laser generating device as claimed in claim 3 or 4.

10. The photogenerated microwave device according to claim 9, characterized in that: The electro-optical frequency division device comprises: An electro-optical modulation module, used for performing cascade electro-optical modulation on the narrow linewidth dual-frequency laser using a radio frequency signal output by a voltage-controlled oscillator, and generating paired optical frequency combs with a plurality of sidebands using the narrow linewidth dual-frequency laser as a carrier respectively; A comb broadening module, used for broadening the optical frequency comb to increase the number of sidebands; An optical filter module is used to filter out two sideband comb teeth with two wavelengths adjacent to each other in two groups of widened optical frequency combs in the optical domain; The photoelectric detection module is used to beat the two filtered sideband comb teeth to obtain the microwave signal after electro-optical frequency division; A mixer, used for phase detection between the microwave signal after electro-optical frequency division and the reference microwave signal; A servo control module, used for performing servo feedback control on the voltage controlled oscillator according to the error signal generated by the phase detector to achieve locking of the output frequency of the voltage controlled oscillator; The voltage-controlled oscillator outputs a radio frequency signal in two paths, one of which is used to perform cascade electro-optical modulation on the narrow-linewidth dual-frequency laser after phase shifting, and the other is output as a photogenerated microwave signal.

11. The photogenerated microwave device according to claim 9, characterized in that: The electro-optical frequency division device comprises: A reference optical frequency comb module, used to generate a reference optical frequency comb; An optical coupler, used to couple the narrow-linewidth dual-frequency laser with a reference optical frequency comb and then divide it into two links; Two optical filter modules, used for filtering the optical signals of the two links respectively, one of which filters out the comb teeth of the negative order part of the reference optical frequency comb closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filters out the comb teeth of the positive order part of the reference optical frequency comb closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser; The first and second photoelectric detection modules are used to perform beat frequency on the two comb teeth filtered out respectively; A first mixer is used to mix the two beat frequency signals to obtain a frequency-divided electrical signal; A second mixer is used for phase detection between the divided frequency electrical signal and a reference microwave signal; A servo control module, used for performing locking feedback control on the reference optical frequency comb module by using the error signal generated by the phase detector as a feedback control signal; The third photoelectric detection module is used to separate a reference optical frequency comb into a beat frequency signal and output the beat frequency signal as a photogenerated microwave signal.

12. The photogenerated microwave device according to claim 9, characterized in that: The electro-optical frequency division device comprises: An electro-optical modulation module, used for electro-optically modulating a reference optical signal using a radio frequency signal output by a voltage-controlled oscillator to generate an optical frequency comb; A comb-tooth broadening module, used for broadening the optical frequency comb to increase the number of sidebands, thereby generating a reference optical frequency comb; An optical coupler is used to couple the narrow linewidth dual-frequency laser with a reference optical frequency comb and then divide the coupled laser into two links; Two optical filter modules, used for filtering the optical signals of the two links respectively, one of which filters out the comb teeth of the negative order part of the reference optical frequency comb closest to the narrow-linewidth laser with a lower frequency in the narrow-linewidth dual-frequency laser, and the other filters out the comb teeth of the positive order part of the reference optical frequency comb closest to the narrow-linewidth laser with a higher frequency in the narrow-linewidth dual-frequency laser; Two photoelectric detection modules are used to beat the two comb teeth filtered out respectively; A first mixer is used to mix the two beat frequency signals to obtain a frequency-divided electrical signal; A second mixer is used for phase detection between the divided frequency electrical signal and a reference microwave signal; A servo control module, used for performing servo feedback control on the voltage controlled oscillator according to the error signal generated by the phase detector to achieve locking of the output frequency of the voltage controlled oscillator; The voltage-controlled oscillator outputs a radio frequency signal in two paths, one of which is input into the electro-optical modulation module after phase shifting to perform electro-optical modulation on the reference optical signal, and the other is output as a light-generated microwave signal.

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