阅读说明：本技术 一种宽带相干光频梳重频调谐方法 (Method for tuning repetition frequency of broadband coherent optical frequency comb ) 是由 刘彦丹 李少波 梁晓东 张磊 于 2021-08-30 设计创作，主要内容包括：本发明公开了一种宽带相干光频梳重频调谐方法,属于微波光子学及超宽带信号处理领域。本发明首先根据调谐频率和重频差需求分解设计级联调制器的调制频率；然后对选定的种子梳齿进行注入锁定；最后通过非线性光纤、单模光纤及色散平坦光纤的组合模块对注入锁定后的梳齿进行梳齿复制和展宽。本发明中种子梳齿是从同一主激光器生成的且调制信号共参考源,从而保证相干性,得到高重频多梳齿的相干光频梳,解决综合一体化、大容量卫星通信等应用对微波光子信道化中相干光频梳带宽越来越大的迫切需求。本发明能够产生高重频多梳齿的相干光频梳,并可以根据需求进行调谐光频梳的重频频率,在保证两个光频梳高相干性的同时,调节相干频梳的重频差。(The invention discloses a method for tuning a broadband coherent optical frequency comb repetition frequency, and belongs to the field of microwave photonics and ultra-wideband signal processing. Firstly, decomposing and designing the modulation frequency of a cascade modulator according to the requirements of a tuning frequency and a repetition frequency difference; then injecting and locking the selected seed comb teeth; and finally, copying and widening the comb teeth after injection locking through a combined module of the nonlinear optical fiber, the single-mode optical fiber and the dispersion flat optical fiber. The seed comb teeth are generated from the same main laser and the modulation signal is a common reference source, so that the coherence is ensured, the coherent optical frequency comb with high repetition frequency and multiple comb teeth is obtained, and the urgent need that the bandwidth of the coherent optical frequency comb is larger and larger in microwave optical sub-channelization in the applications of comprehensive integration, large-capacity satellite communication and the like is solved. The invention can generate the coherent optical frequency comb with high repetition frequency and multiple comb teeth, can tune the repetition frequency of the optical frequency comb according to requirements, and can adjust the repetition frequency difference of the coherent optical frequency comb while ensuring the high coherence of the two optical frequency combs.)

1. A method for tuning a comb repetition frequency of a broadband coherent optical frequency is characterized by comprising the following steps:

step 1, acquiring the repetition frequency FSR of the needed coherent optical frequency comb₁And FSR₂Frequency difference of repetition Δ f, number of comb teeth x, where Δ f | (FSR)₁-FSR₂)|；

Step 2, decomposing the indexes of the coherent optical frequency comb according to the requirement, and decomposing the repeated frequency of the primary optical frequency comb into f₁＝FSR₁A and f₂＝FSR₂A is an even number and a is not less than 4; the modulated RF signals for designing the cascade modulator are respectively f₁And f₂The amplitude of the radio frequency signal is higher than 10dBm, and the number of generated comb teeth is more than b, b>a + 1; the modulation radio frequency signal of the cascade modulator is the same reference source, and the optical carrier comes from the same laser, thus guarantee the coherence of the optical frequency comb;

step 3, filtering out two groups of four comb teeth at corresponding intervals as seed comb teeth by wavelength division demultiplexing, wherein the intervals of the two groups of comb teeth are FSR respectively₁And FSR₂The two comb teeth in each group respectively correspond to the 1 st comb tooth and the a +1 st comb tooth of the primary optical frequency comb;

step 4, carrying out light injection locking on the seed comb teeth through a circulator and a pump laser, and improving the comb tooth power by more than 10dB on the premise of ensuring narrow line width;

and 5, after injection locking, injecting the signal optical frequency comb and the local oscillator optical frequency comb into the locked seed comb teeth, respectively multiplexing the seed comb teeth through a wavelength division multiplexer, synthesizing the seed comb teeth into two groups of seed comb teeth with repetition frequency meeting the requirement, wherein the interval between the two groups of seed comb teeth is FSR (frequency selective reflection) respectively₁And FSR₂；

Step 6, enabling the two groups of seed comb teeth obtained in the step 5 to pass through a section of first high nonlinear optical fiber with longitudinal strain, causing nonlinear phase shift through a nonlinear Kerr effect in the first high nonlinear optical fiber, generating positive chirp, and expanding signals to obtain expanded optical pulses;

step 7, inputting the expanded optical pulse into a single-mode optical fiber, thereby compressing the time domain waveform of the optical pulse, improving the peak power of the optical pulse, and keeping the frequency domain unchanged;

and 8, inputting the optical pulse processed in the step 7 into a section of second high nonlinear optical fiber with flat dispersion to further realize spectrum spreading and copying, and finally obtaining the coherent optical frequency comb required in the step 1.

2. The method as claimed in claim 1, wherein in step 6, the length L of the first highly nonlinear optical fiber is greater than L₀Satisfies 80m<L₀<120m, dispersion slope S₀Satisfies 8ps/nm²/km<S₀<12ps/nm²Km, zero dispersion point D₀Satisfies 1560nm<D₀<1565nm, and nonlinear coefficient gamma₀Satisfies 4e^-20W^-1/km<γ₀<5e^-20W^-1/km。

3. The method as claimed in claim 2, wherein in step 7, the length of the single-mode fiber is estimated by a nonlinear schrodinger equation to obtain the length L of the single-mode fiber₁Satisfies 10m<L₁<50m。

4. The method as claimed in claim 3, wherein in step 8, the second highly nonlinear fiber is a normal near-zero dispersion flat highly nonlinear fiber with a length L₂Satisfy 180m<L₂<220m, dispersion slope S₂Satisfies 8ps/nm²/km<S₂<13ps/nm²Km, nonlinear coefficient γ₂Satisfies 4e^-20W^-1/km<γ₂<5e^-20W^-1(ii) km; when the optical pulse is transmitted in the second high nonlinear optical fiber, the time domain waveform of the pulse is broadened and the peak power is reduced, the frequency spectrum also shows broadening trend in the distance of the optical fiber, and the flatness of the frequency spectrum is along with the transmission distanceThe increase is also more and more flat, resulting in the coherent optical frequency comb required in step 1.

5. The method as claimed in claim 4, wherein in step 1, FSR is used₁＝80GHz，FSR₂81.2GHz,. DELTA.f ═ 1.2GHz, and x ═ 20; in the step 2, a is 4, b is 6, and the flatness of the comb teeth generated by the cascade modulator reaches 3 dB; in step 6, L₀＝100m，S₀＝10ps/nm²/km，D₀＝1562nm，γ₀＝4.93e^-20W^-1(ii) km; in step 7, L₁40 m; in step 8, L₂＝200m，S₂＝10ps/nm²/km，γ₂＝3.53e^-20W^-1Km, Abbe number-3 e^-6μs/m²。

Technical Field

The invention belongs to the fields of microwave photonics, satellite communication, integrated ultra-wideband signal processing and the like, and particularly relates to a method for tuning a comb repetition frequency of a broadband coherent optical frequency.

Background

The optical frequency comb technology is used as a bridge for connecting an optical domain and microwaves, the microwave fine flexibility and the characteristics of optical wave broadband coarse grains can be fully fused, and the typical advantages of the optical frequency comb mainly comprise: the multi-carrier structure, the fixed interval of the tooth trace frequency, the good coherence among the comb teeth and the like have potential advantages in ultra wide band signal processing, cross-frequency band multi-carrier frequency conversion and the like, and are widely applied to the fields of high-capacity satellite communication, wide-frequency band sparse signal perception receiving, integrated comprehensive radio frequency, channelized receiving, precise measurement and the like. The optical frequency comb technology can effectively break through the problems that the traditional electronic technology needs multi-stage frequency conversion when processing broadband or across octave signals, and the nonlinearity and noise accumulation are brought, the size, the weight and the power consumption are increased, and the like. In wideband channelization applications, one key issue for implementing channelization using signal multicasting is the generation of coherent optical frequency comb with stable frequency spacing, large number of wavelengths, and high flatness.

With the outstanding technical advantages and the urgent need of application, various optical frequency comb generation methods are emerging, including a mode-locked laser method, an electro-optical modulation method, a nonlinear medium method, a micro-ring resonant cavity method and other various methods for generating coherent optical frequency combs. The method for generating the coherent optical frequency comb by using the mode-locked laser combines the photonic crystal fiber with the mode-locked femtosecond laser, so that the generated spectral lines are more in number and better in stability, but the repetition frequency of the optical frequency comb is limited by the length of a resonant cavity of the laser, and the defects that the frequency interval is not easy to tune and the stability is difficult to control exist; the method for generating the coherent optical frequency comb by using the cascade modulator method is to modulate an RF signal carrying information onto an optical carrier by using a modulator, the central wavelength and spectral line interval are adjustable, the spectral line is stable, but the defects of small number of comb teeth, poor flatness and the like exist. An optical frequency comb is generated by using an optical nonlinear effect method, and the spectral line bandwidth is increased by using a nonlinear medium phase modulation technology, but the problems of poor comb flatness, uneven energy distribution and large frequency comb signal jitter exist. The micro-ring waveguide structure is utilized to generate the coherent optical frequency comb, so that the system tends to be miniaturized, the integration level is high, the cost is low, the stability is good, but the problems of complex structural design, long period, poor spectral line flatness and the like exist, and therefore a tuning and generating method of the broadband, multi-comb-tooth and high-flatness coherent optical frequency comb needs to be designed.

Disclosure of Invention

The invention aims to provide a method for tuning the repetition frequency of a broadband coherent optical frequency comb, which realizes the repetition frequency tuning of the broadband coherent optical frequency comb by utilizing the nonlinear effect of optical fibers and can generate the coherent optical frequency comb with large repetition frequency, more comb teeth and high flatness.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a method for tuning a comb repetition frequency of a broadband coherent optical frequency comprises the following steps:

step 1, acquiring the repetition frequency FSR of the needed coherent optical frequency comb₁And FSR₂Frequency difference of repetition Δ f, number of comb teeth x, where Δ f | (FSR)₁-FSR₂)|；

Step 2, decomposing the indexes of the coherent optical frequency comb according to the requirement, and decomposing the repeated frequency of the primary optical frequency comb into f₁＝FSR₁A and f₂＝FSR₂A is an even number and a is not less than 4; the modulated RF signals for designing the cascade modulator are respectively f₁And f₂The amplitude of the radio frequency signal is higher than 10dBm, and the number of generated comb teeth is more than b, b>a + 1; the modulation radio frequency signal of the cascade modulator is the same reference source, and the optical carrier comes from the same laser, thus guarantee the coherence of the optical frequency comb;

step 3, filtering out two groups of four comb teeth at corresponding intervals as seed comb teeth by wavelength division demultiplexing, wherein the intervals of the two groups of comb teeth are FSR respectively₁And FSR₂The two comb teeth in each group respectively correspond to the 1 st comb tooth and the a +1 st comb tooth of the primary optical frequency comb;

step 4, carrying out light injection locking on the seed comb teeth through a circulator and a pump laser, and improving the comb tooth power by more than 10dB on the premise of ensuring narrow line width;

and 5, after injection locking, injecting the signal optical frequency comb and the local oscillator optical frequency comb into the locked seed comb teeth, respectively multiplexing the seed comb teeth through a wavelength division multiplexer, synthesizing the seed comb teeth into two groups of seed comb teeth with repetition frequency meeting the requirement, wherein the interval between the two groups of seed comb teeth is FSR (frequency selective reflection) respectively₁And FSR₂；

Step 6, enabling the two groups of seed comb teeth obtained in the step 5 to pass through a section of first high nonlinear optical fiber with longitudinal strain, causing nonlinear phase shift through a nonlinear Kerr effect in the first high nonlinear optical fiber, generating positive chirp, and expanding signals to obtain expanded optical pulses;

step 7, inputting the expanded optical pulse into a single-mode optical fiber, thereby compressing the time domain waveform of the optical pulse, improving the peak power of the optical pulse, and keeping the frequency domain unchanged;

and 8, inputting the optical pulse processed in the step 7 into a section of second high nonlinear optical fiber with flat dispersion to further realize spectrum spreading and copying, and finally obtaining the coherent optical frequency comb required in the step 1.

Further, in step 6, the length L of the first highly nonlinear optical fiber₀Satisfies 80m<L₀<120m, dispersion slope S₀Satisfies 8ps/nm²/km<S₀<12ps/nm²Km, zero dispersion point D₀Satisfies 1560nm<D₀<1565nm, and nonlinear coefficient gamma₀Satisfies 4e^-20W^-1/km<γ₀<5e^-20W^-1/km。

Further, in step 7, the length of the single-mode fiber is estimated through a nonlinear Schrodinger equation, and the estimated length L of the single-mode fiber is obtained₁Satisfies 10m<L₁<50m。

Further, in step 8, the second highly nonlinear fiber is a normal near-zero dispersion flat highly nonlinear fiber with a length L₂Satisfy 180m<L₂<220m, dispersion slope S₂Satisfies 8ps/nm²/km<S₂<13ps/nm²Km, nonlinear coefficient γ₂Satisfies 4e^-20W^-1/km<γ₂<5e^-20W^-1(ii) km; when the optical pulse is transmitted in the second high nonlinear optical fiber, the time domain waveform of the pulse is broadened, the peak power is reduced, the spectrum also shows a broadening trend in the distance of the optical fiber, the flatness of the spectrum is more and more flat along with the increase of the transmission distance, and finally the coherent optical frequency comb required in the step 1 is obtained.

Further, in step 1, FSR₁＝80GHz，FSR₂81.2GHz,. DELTA.f ═ 1.2GHz, and x ═ 20; in the step 2, a is 4, b is 6, and the flatness of the comb teeth generated by the cascade modulator reaches 3 dB; in step 6, L₀＝100m，S₀＝10ps/nm²/km，D₀＝1562nm，γ₀＝4.93e^-20W^-1(ii) km; in step 7, L₁40 m; in step 8, L₂＝200m，S₂＝10ps/nm²/km，γ₂＝3.53e^-20W^-1Km, Abbe number-3 e^-6μs/m²。

The invention has the beneficial effects that:

1. the invention utilizes the cascade modulator to generate the coherent optical frequency comb with lower repetition frequency, selects the seed comb which meets the system requirement, adopts a nonlinear method to perform frequency spectrum broadening after the optical injection locking, and can generate the coherent optical frequency comb with large repetition frequency, more comb teeth and high flatness.

2. The invention realizes the multiple frequency tuning of the broadband coherent optical frequency comb by utilizing the fiber nonlinear effect, and can realize the multi-comb high-multiple frequency and high-flatness coherent optical frequency comb for broadband channelized reception sensing.

Drawings

FIG. 1 is a flow chart of coherent optical comb re-frequency tuning in an embodiment of the present invention.

Fig. 2 is a schematic diagram of a cascaded modulator-generated optical comb used in an embodiment of the present invention.

FIG. 3 is a schematic diagram of injection locking of a seed optical comb used in an embodiment of the present invention.

Fig. 4 is a schematic diagram of a nonlinear re-frequency tuning method used in an embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

A method for tuning a comb repetition frequency of a broadband coherent optical frequency comprises the steps of firstly decomposing and designing the modulation frequency of a cascade modulator according to the requirements of the tuning frequency and the repetition frequency difference; and finally, the comb teeth after injection locking are copied and widened through a combined module of the nonlinear optical fiber, the single-mode optical fiber and the dispersion flat optical fiber. The method specifically comprises the following steps:

step 1, acquiring the repetition frequency FSR of the needed coherent optical frequency comb₁And FSR₂Frequency difference of repetition Δ f, number of comb teeth x, where Δ f | (FSR)₁-FSR₂)|；

Step 2, decomposing the indexes of the coherent optical frequency comb according to the requirement, and decomposing the repeated frequency of the primary optical frequency comb into f₁＝FSR₁A and f₂＝FSR₂A is an even number and a is not less than 4; the modulated RF signals for designing the cascade modulator are respectively f₁And f₂The amplitude of the radio frequency signal is higher than 10dBm, and the number of generated comb teeth is more than b, b>a + 1; the modulation radio frequency signal of the cascade modulator is the same reference source, and the optical carrier comes from the same laser, thus guarantee the coherence of the optical frequency comb;

step 3, filtering out two groups of four comb teeth at corresponding intervals as seed comb teeth by wavelength division demultiplexing, wherein the intervals of the two groups of comb teeth are FSR respectively₁And FSR₂The two comb teeth in each group respectively correspond to the 1 st comb tooth and the a +1 st comb tooth of the primary optical frequency comb;

step 4, carrying out light injection locking on the seed comb teeth through a circulator and a pump laser, and improving the comb tooth power by more than 10dB on the premise of ensuring narrow line width;

and 5, after injection locking, injecting the signal optical frequency comb and the local oscillator optical frequency comb into the locked seed comb teeth, respectively multiplexing the seed comb teeth through a wavelength division multiplexer, synthesizing the seed comb teeth into two groups of seed comb teeth with repetition frequency meeting the requirement, wherein the interval between the two groups of seed comb teeth is FSR (frequency selective reflection) respectively₁And FSR₂；

Step 6, enabling the two groups of seed comb teeth obtained in the step 5 to pass through a section of first high nonlinear optical fiber with longitudinal strain, causing nonlinear phase shift through a nonlinear Kerr effect in the first high nonlinear optical fiber, generating positive chirp, and expanding signals to obtain expanded optical pulses;

step 7, inputting the expanded optical pulse into a single-mode optical fiber, thereby compressing the time domain waveform of the optical pulse, improving the peak power of the optical pulse, and keeping the frequency domain unchanged;

and 8, inputting the optical pulse processed in the step 7 into a section of second high nonlinear optical fiber with flat dispersion to further realize spectrum spreading and copying, and finally obtaining the coherent optical frequency comb required in the step 1.

Further, in step 6, the length L of the first highly nonlinear optical fiber₀Satisfies 80m<L₀<120m, dispersion slope S₀Satisfies 8ps/nm²/km<S₀<12ps/nm²Km, zero dispersion point D₀Satisfies 1560nm<D₀<1565nm, and nonlinear coefficient gamma₀Satisfies 4e^-20W^-1/km<γ₀<5e^-20W^-1/km。

Further, in step 7, the length of the single-mode fiber is estimated through a nonlinear Schrodinger equation, and the estimated length L of the single-mode fiber is obtained₁Satisfies 10m<L₁<50m。

Further, in step 8, the second highly nonlinear fiber is a normal near-zero dispersion flat highly nonlinear fiber with a length L₂Satisfy 180m<L₂<220m, dispersion slope S₂Satisfies 8ps/nm²/km<S₂<13ps/nm²Km, nonlinear coefficient γ₂Satisfies 4e^-20W^-1/km<γ₂<5e^-20W^-1(ii) km; when the optical pulse is transmitted in the second high nonlinear optical fiber, the time domain waveform of the pulse is broadened, the peak power is reduced, the spectrum also shows a broadening trend in the distance of the optical fiber, the flatness of the spectrum is more and more flat along with the increase of the transmission distance, and finally the coherent optical frequency comb required in the step 1 is obtained.

Further, in step 1, FSR₁＝80GHz，FSR₂81.2GHz,. DELTA.f ═ 1.2GHz, and x ═ 20; in the step 2, a is 4, b is 6, and the flatness of the comb teeth generated by the cascade modulator reaches 3 dB; in step 6, L₀＝100m，S₀＝10ps/nm²/km，D₀＝1562nm，γ₀＝4.93e^-20W^-1(ii) km; in step 7, L₁40 m; in step 8, L₂＝200m，S₂＝10ps/nm²/km，γ₂＝3.53e^-20W^-1Km, Abbe number-3 e^-6μs/m²。

In the method, the seed comb teeth are generated from the same main laser and the modulation signal is used as a reference source, so that the coherence is ensured, the coherent optical frequency comb with high repetition frequency and multiple comb teeth is obtained, and the urgent requirement that the bandwidth of the coherent optical frequency comb is larger and larger in microwave optical sub-channelization in the applications of comprehensive integration, large-capacity satellite communication and the like is solved.

The method can generate the coherent optical frequency comb with high repetition frequency and multiple comb teeth, can tune the repetition frequency of the optical frequency comb according to requirements, and can adjust the repetition frequency difference of the coherent optical frequency comb while ensuring the high coherence of the two optical frequency combs.

The following is a more specific example:

referring to fig. 1, a process of re-tuning a wideband coherent optical frequency comb includes the following steps:

step 1, determining information such as coherent optical frequency comb repetition frequency, comb tooth number and the like: optical comb 1 repetition frequency FSR in coherent optical frequency comb₁The frequency is 80GHz, the repetition frequency of the optical comb 2 is 81.2GHz, and the difference between the repetition frequencies is 1.2 GHz;

step 2, referring to fig. 2, the primary coherent optical frequency comb is generated: the method comprises the steps that a primary coherent optical frequency comb is generated through a cascade modulator, an optical carrier generated by a laser is divided into two seed lights through an optical coupler, two cascade phase modulators and an MZM are adopted to generate a signal seed frequency comb, and two cascade phase modulators are used to generate a primary local oscillator seed frequency comb.

The frequency of a signal modulated by the signal optical frequency comb is 20GHz, the frequency of a signal modulated by the local oscillator optical frequency comb is 20.3GHz, and the two radio frequency synthesizers are referenced by a common reference signal; the two paths of radio frequency signals are subjected to phase matching through a radio frequency phase shifter, and the phase difference of the two radio frequency signals input into the modulator is guaranteed to be zero.

And 3, filtering out 5 comb teeth with the interval of the repetition frequency in the two optical combs through a wavelength division demultiplexer according to the repetition frequency of the optical frequency comb to be tuned, namely selecting two comb teeth with the frequency interval of 80GHz and the frequency interval of 81.2GHz for generating the signal optical frequency comb, wherein two groups of four comb teeth are used for injection locking from the laser.

And 4, referring to fig. 3, when the seed comb teeth are injected and locked, a circulator and a high-power slave laser are utilized, wherein the filtered seed comb teeth are input into a port 1 of the circulator, a port 2 of the circulator is connected with the high-power slave laser, and a port 3 of the circulator is an output optical signal after injection and locking. The 4 filtered comb teeth respectively correspond to the injection locking structures.

And 5, multiplexing signals of 2 signal optical combs into an optical signal by a wavelength division multiplexer after the optical signal after injection locking is obtained, multiplexing two injection locking signals of the local oscillator optical comb into an optical signal by the same wavelength division multiplexer, and carrying out a subsequent nonlinear process on the two optical signals.

Step 6, the first stage of the nonlinear process is composed of a section of high nonlinear optical fiber 1(HNLF1) with applied longitudinal strain, the nonlinear Kerr effect in HNLF1 can cause nonlinear phase shift, generate positive chirp and expand signals, the time domain waveform of the pulse is unchanged, and due to the SPM effect, the frequency domain of the pulse can generate a plurality of frequency components when the pulse is transmitted in the section of optical fiber, and the pulse spectrum is widened;

step 7, when the obtained chirp signal propagates in a dispersion medium single-mode fiber (SMF) with positive dispersion, the sinusoidal intensity distribution is compressed, so that an optical pulse with high peak power is generated in a time domain; when an optical pulse is transmitted in SMF, the time domain waveform of the pulse is compressed, the pulse peak power is improved, the frequency domain is not changed, in order to realize better dispersion matching, the length of a single-mode optical fiber can be determined by utilizing a nonlinear Schrodinger equation (NLSE) to estimate:

where T is a time measure of the movement of the pulses in the reference frame at the group velocity and A (z, T) is the slowly varying amplitude of the envelope of the pulses; β 2 is the Group Velocity Dispersion (GVD) parameter of the fiber; gamma is the nonlinear coefficient of the optical fiber; α is the fiber loss. The right side of the formula corresponds to the fiber loss, dispersion effect and nonlinear effect of the optical pulse during fiber transmission respectively. Two length scales were introduced: dispersion length LD-T₀ ²/| β 2| and nonlinear length L_NL＝1/γP₀。

The maximum amount of pulse compression required (i.e., SMF length) is determined based on the parameters of the device used in the experimental protocol, and the actual SMF length is then adjusted experimentally by maximizing the peak power of the optical pulses.

And 8, forming a section of the high-nonlinearity fiber 2(HNLF2) with flat dispersion so as to further realize spectrum spreading and copying. The compressed waveform enters the second nonlinear stage with higher peak power, so that the second mixer operates with a correspondingly larger quality factor, but the presence of chirp can cause the phase of an optical signal to change, and therefore, in order to reduce the influence of frequency chirp on a system, a positive dispersion medium, namely a single-mode optical fiber, needs to be introduced to counteract the positive chirp generated by the phase modulator, and the positive dispersion medium, namely the single-mode optical fiber and the single-mode optical fiber are matched with each other to better realize the initial compression of pulses. When the optical pulse is transmitted in the third section of normal near-zero dispersion flat high-nonlinearity optical fiber, the time domain waveform of the pulse is broadened, the peak power is reduced, the spectrum also shows a broadening trend in the distance, and the flatness of the spectrum shows a trend of being flatter and flatter with the increase of the transmission distance.

The principle of the invention is as follows:

firstly, information such as the repetition frequency, the repetition frequency difference, the number of comb teeth and the like of a required coherent optical frequency comb is obtained, then the coherent optical frequency comb is generated by utilizing a cascade modulator, an optical carrier generated by a laser is divided into two seed lights by an optical coupler, the cascade modulator is adopted to generate a signal seed frequency comb, two paths of radio frequency signals are subjected to phase matching through a radio frequency phase shifter, the phase difference of the two radio frequency signals input into the modulator is ensured to be zero, similarly, the local oscillator optical frequency comb generates a primary local oscillator seed frequency comb by utilizing the two cascade modulators, and the two radio frequency synthesizers ensure good coherence by referring to a common reference signal.

Then, according to the specific range of the required frequency band, the repetition frequency of the secondary optical frequency comb is tuned, the distance between the comb teeth of the two optical combs entering the frequency mixer, namely the repetition frequency, is determined, and the interval between the two frequencies of the optical frequency comb generating the signal is selected as FSR₁And the frequency interval of the local oscillator optical comb is FSR₂For injection locking from the laser.

Injecting and locking to obtain seed comb teeth: the single longitudinal mode continuous laser with narrow line width, good frequency and power stability is injected into the slave laser with high output energy, the frequency characteristic, the phase characteristic and the space characteristic of the slave laser are controlled by the master laser, and laser output with the characteristics of narrow line width, single longitudinal mode, high energy and the like is obtained.

The injection locked seed comb passes through a section of high nonlinear optical fiber 1(HNLF1) with applied longitudinal strain, and the nonlinear Kerr effect in HNLF1 can cause nonlinear phase shift, generate positive chirp and expand signals;

when the obtained chirp signal propagates in a dispersion medium Single Mode Fiber (SMF) having positive dispersion, the sinusoidal intensity distribution will be compressed, thereby generating optical pulses with high peak power in the time domain;

the finally obtained time domain compressed signal is formed by a section of dispersion flat high nonlinear optical fiber 2(HNLF2) to further realize the spectrum expansion and copy, and finally the tuned coherent optical frequency comb is obtained.

In conclusion, the invention realizes the repetition frequency tuning method of the coherent optical frequency comb, and can realize the coherent optical frequency comb with wide repetition frequency, multiple comb teeth and high flatness after tuning according to requirements.

The above description is only a specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.