Optical frequency comb generation device and generation method based on 3*3 optical fiber coupler

Abstract

The invention discloses an optical frequency comb generation device and method based on a 3 * 3 optical fiber coupler, and the device comprises a single-frequency microwave source, a phase shifter, a continuous wave laser, an intensity modulator, a phase modulator, a first connection optical fiber part, and a second connection optical fiber part. The microwave signal output end is connected with the intensity modulator or the phase modulator; the continuous wave laser comprises a laser signal output end, and the laser signal output end is connected with the intensity modulator; the intensity modulator and the phase modulator are connected with the input end of the non-linear optical annular mirror through the first connecting optical fiber part, the output end of the non-linear optical annular mirror is connected with the second connecting optical fiber part, and the non-linear optical annular mirror comprises a 3 * 3 optical fiber coupler and a non-linear optical fiber which are coupled; a 3 * 3 optical fiber coupler is adopted to form a nonlinear optical annular mirror, ultra-short pulse shaping is achieved through the interference effect between two paths of reverse transmission light fields, continuous wave background noise and pulse base side lobes are effectively eliminated, and through the design of an optical fiber coupling mode and a loop port connection mode and the selection of an input end and an output end, ultra-short pulse shaping is achieved. And the nonlinear chirp part of the pulse waveform is greatly inhibited, and the linear characteristic of the nonlinear chirp part is kept, so that the ultra-wideband optical frequency comb light source with remarkable spectral balance is realized in the high nonlinear parameter mixing process.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical frequency comb light sources, and particularly relates to an optical frequency comb generating device and a generating method based on a 3×3 fiber coupler. Background Art

[0002] In the past rapid development, optical frequency combs have greatly revolutionized research fields such as precision measurement, frequency standards, optical communication, and spectroscopy, making them a fundamental tool for precise time and frequency transmission. By making full use of the inherent nonlinear characteristics of ultrashort pulses, optical frequency combs can synthesize a broadband quasi-continuous spectrum covering from microwave to extreme ultraviolet bands, greatly improving their versatility and being widely used in various fields.

[0003] However, current advanced applications have strict performance requirements for the comb source, such as spectral flatness, bandwidth, repetition frequency, amplitude, and phase noise. An effective optical frequency comb generation scheme requires precise phase matching conditions and strict power specifications to ensure significant linear chirping of the time-domain pulses and prevent spectral fluctuations. During the generation process of broadband optical frequency combs, it is usually necessary to precisely adjust the time-domain waveform of optical pulses, which mainly includes optimizing some non-linear related parameters, such as non-linear coefficient, optical power, and effective interaction length.

[0004] Since the generation of optical frequency comb signals will inevitably introduce higher-order phases, that is, non-linear frequency chirping, the interference of such non-ideal components in the time-domain waveform often leads to the imbalance of the comb spectrum, thus restricting its effective application, especially in ultra-wideband related applications. Therefore, how to simply and efficiently suppress the influence of non-linear frequency chirping in time-domain pulses and realize an ultra-flat broadband optical frequency comb light source remains a key problem to be solved urgently.

[0005] In view of the above problems, the present invention discloses an ultra-flat broadband optical frequency comb generating device and a generating method based on a 3×3 fiber coupler. Summary of the Invention

[0006] The present invention discloses an optical frequency comb generating device and a generating method based on a 3×3 fiber coupler. A non-linear optical loop mirror is formed by using a 3×3 fiber coupler. Ultra-short pulse shaping is realized through the interference effect between two counter-propagating optical fields, effectively eliminating the continuous wave background noise and the sidelobes of the pulse pedestal. By designing the fiber coupling method, the loop port connection method, and the selection of the input end and the output end, the non-linear chirping part of the pulse waveform is greatly suppressed, and its linear characteristics are retained, so as to realize an ultra-wideband optical frequency comb light source with significant spectral balance during the high non-linear parameter mixing process.

[0007] The present invention is realized through the following technical solutions:

[0008] An optical frequency comb generation device based on a 3×3 fiber coupler, comprising a single-frequency microwave source, a phase shifter, a continuous wave laser, an intensity modulator, a phase modulator, a first connecting fiber section, and a second connecting fiber section. The single-frequency microwave source includes a plurality of groups of microwave signal output terminals, and the microwave signal output terminals are connected to the intensity modulator or the phase modulator; the continuous wave laser includes a laser signal output terminal, and the laser signal output terminal is connected to the intensity modulator; the intensity modulator and the phase modulator are connected to the input end of a nonlinear optical loop mirror through the first connecting fiber section, the output end of the nonlinear optical loop mirror is connected to the second connecting fiber section, the nonlinear optical loop mirror includes a coupled 3×3 fiber coupler and a nonlinear optical fiber, the three optical fibers in the 3×3 fiber coupler are arranged symmetrically in a coplanar manner, and the transmission slope of the nonlinear optical loop mirror is the ratio of the maximum output intensity to the minimum output intensity of the nonlinear optical loop mirror at different splitting ratios.

[0009] To better implement the present invention, further, the nonlinear optical loop mirror includes a group of 3×3 fiber couplers and a group of nonlinear optical fibers. The 3×3 fiber coupler includes an input end, a central output end, and a side output end, and the central output end is connected to the side output end through the nonlinear optical fiber.

[0010] To better implement the present invention, further, the side output end is directly connected to the input end, and the central output end is connected to the side output end through the nonlinear optical fiber.

[0011] To better implement the present invention, further, the splitting ratio of the loop is 60:40.

[0012] To better implement the present invention, further, the side output end is not directly connected to the input end, and the central output end is connected to the side output end through the nonlinear optical fiber.

[0013] To better implement the present invention, further, the splitting ratio of the loop is 40:60.

[0014] To better implement the present invention, further, the first connecting fiber section includes a first dispersion compensation fiber and a first erbium-doped fiber amplifier connected in series. The input end of the first dispersion compensation fiber is connected to the phase modulator, and the output end of the first erbium-doped fiber amplifier is connected to the nonlinear optical loop mirror; the second connecting fiber section includes a second erbium-doped fiber amplifier and a second nonlinear optical fiber connected in series, and the input end of the second erbium-doped fiber amplifier is connected to the nonlinear optical loop mirror.

[0015] To better implement the present invention, further, the output end of the single-frequency microwave source is connected to the intensity modulator or the phase modulator through a microwave amplifier; the continuous wave laser is connected to the intensity controller through a polarization controller; and a power splitter is provided at the output end of the single-frequency microwave source.

[0016] Optical frequency comb generation method based on a 3×3 fiber coupler, comprising the following steps:

[0017] Step 1: Provide an electrical signal through a single-frequency microwave source, provide an optical signal through a continuous-wave laser, and obtain an initial optical comb through cascaded modulation of an intensity modulator and a phase modulator;

[0018] Step 2: Eliminate the nonlinear chirp introduced by phase modulation through the first connecting fiber section, and perform pulse compression on the optical comb;

[0019] Step 3: Suppress the low-power components of the pulses through the response of different powers in the time-domain waveform of the optical comb by a nonlinear optical loop mirror to achieve the best shaping effect of the optical comb;

[0020] Step 4: Improve the spectral bandwidth of the optical comb through the second connecting fiber section.

[0021] To better implement the present invention, further, specifically included in the said step 3 are:

[0022] Step 3.1: Calculate the conversion relationship curve of the input power and output power of the nonlinear optical loop mirror composed of a 3×3 fiber coupler;

[0023] Step 3.2: Calculate the transmission slope of the loop of the 3×3 fiber coupler at different splitting ratios based on the conversion relationship curve of the input power and output power;

[0024] Step 3.3: Select the splitting ratio of the loop when the transmission slope reaches the maximum value as the optimal splitting ratio.

[0025] To better implement the present invention, further, when the input end of the 3×3 fiber coupler is directly connected to the side output end, and the central output end is connected to the side output end to form a loop, the instantaneous input power P in and the conversion relationship formula of the instantaneous output power P out1 is:

[0026]

[0027] where: T1 represents the input / output power conversion equation when the central output end is connected to the directly connected side output end; P4 represents the output power of the directly connected side output end; P5 represents the output power of the central output end; P6 represents the output power of the non-directly connected side output end; φ1 = γL(P5 - P4), γ represents the nonlinear coefficient of the nonlinear optical fiber, and L represents the length of the nonlinear optical fiber;

[0028] When the input end of the 3×3 fiber coupler is not directly connected to the side output end and the center output end is connected to the side output end to form a loop, the instantaneous input power P of the non-linear optical loop mirror in and the instantaneous output power P out2 The calculation formula of the conversion relationship is:

[0029]

[0030] Where: T2 represents the input / output power conversion equation when the center output end is connected to the non-directly connected side output end; P4 represents the output power of the directly connected side output end; P5 represents the output power of the center output end; P6 represents the output power of the non-directly connected side output end; γ represents the non-linear coefficient of the non-linear optical fiber, and L represents the length of the non-linear optical fiber.

[0031] To better implement the present invention, further, the calculation formula of the transmission slope is:

[0032] ΔT = ΔP out / ΔP in ;

[0033] Where: ΔT represents the transmission slope of the non-linear optical loop mirror; ΔP out represents the difference in output power corresponding to the instantaneous input power of 20 dBW and 0 dBW; ΔP in represents the difference in input power of 20 dB.

[0034] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0035] The ultra-flat broadband optical frequency comb generation scheme based on the 3×3 fiber coupler of the present invention utilizes the phase bias advantage brought by the structure of the 3×3 fiber coupler itself. The non-linear optical loop mirror formed has greater flexibility in terms of coupling method and port selection; by designing and optimizing the architecture and coupling strength of the optical loop mirror, the ideal pulse shaping effect can be achieved more efficiently; under the same input optical field conditions, compared with the traditional 2×2 non-linear optical loop mirror, the scheme can improve the pulse sideband suppression ratio by 15.9 dB and enhance the flatness of the central comb spectrum by 11.6 dB, reflecting its superior pulse shaping characteristics. The beneficial effects provided by the present invention are particularly important for ultra-wideband applications that focus on accuracy and efficiency. Description of the Drawings

[0036] Figure 1 It is a schematic structural diagram of an ultra-flat broadband optical frequency comb generation device;

[0037] Figure 2 It is a schematic diagram of the directly connected output end connected to the center end through a non-linear optical fiber;

[0038] Figure 3 Schematic diagram of the non-direct output end connected to the central end through a non-linear optical fiber;

[0039] Figure 4 Schematic diagram of the optical frequency comb output by a 3×3 non-linear optical loop mirror;

[0040] Figure 5 Schematic diagram of the optical frequency comb output by a 2×2 non-linear optical loop mirror;

[0041] Figure 6 Schematic diagram of the flatness of the broadband optical frequency comb under the action of a 3×3 non-linear optical loop mirror;

[0042] Figure 7 Schematic diagram of the flatness of the broadband optical frequency comb under the action of a 2×2 non-linear optical loop mirror;

[0043] Figure 8 Schematic diagram of the transmission slope corresponding to different splitting ratios under the action of a 2×2 non-linear optical loop mirror;

[0044] Figure 9 Schematic diagram of the conversion curve of the input power and the output power under the action of a 2×2 non-linear optical loop mirror;

[0045] Figure 10 Schematic diagram of the transmission slope corresponding to different splitting ratios under the action of a 3×3 non-linear optical loop mirror;

[0046] Figure 11 Schematic diagram of the conversion curve of the input power and the output power under the action of a 3×3 non-linear optical loop mirror.

[0047] Wherein: 1 - single-frequency microwave source; 2 - power divider; 3 - microwave amplifier; 4 - phase shifter; 5 - continuous wave laser; 6 - polarization controller; 7 - intensity controller; 8 - phase modulator; 9 - first dispersion compensation optical fiber; 10 - first erbium-doped optical fiber amplifier; 11 - 3×3 fiber coupler; 12 - non-linear optical fiber; 13 - second erbium-doped optical fiber amplifier; 14 - second non-linear optical fiber. Specific implementation mode

[0048] Example 1:

[0049] The optical frequency comb generating device based on a 3×3 fiber coupler in this embodiment is as follows Figure 1As shown in the figure, it includes a single-frequency microwave source 1, a phase shifter 4, a continuous-wave laser 5, an intensity modulator 7, a phase modulator 8, a first connecting optical fiber part, and a second connecting optical fiber part. It is characterized in that the single-frequency microwave source 1 includes several groups of microwave signal output terminals, and the microwave signal output terminals are connected to the intensity modulator 7 or the phase modulator 8; the continuous-wave laser 5 includes a laser signal output terminal, and the laser signal output terminal is connected to the intensity modulator 7; the intensity modulator 7 and the phase modulator 8 are connected to the input end of the nonlinear optical loop mirror through the first connecting optical fiber part, the output end of the nonlinear optical loop mirror is connected to the second connecting optical fiber part, the nonlinear optical loop mirror includes a coupled 3×3 optical fiber coupler 11 and a nonlinear optical fiber 12, the three optical fibers in the 3×3 optical fiber coupler 11 are arranged symmetrically in the same plane, and the transmission slope of the nonlinear optical loop mirror is the ratio of the maximum output intensity to the minimum output intensity of the nonlinear optical loop mirror at different splitting ratios.

[0050] Further, the first connecting optical fiber part includes a first dispersion compensation optical fiber 9 and a first erbium-doped optical fiber amplifier 10 connected in series. The input end of the first dispersion compensation optical fiber 9 is connected to the phase modulator 8, and the output end of the first erbium-doped optical fiber amplifier 10 is connected to the nonlinear optical loop mirror; the second connecting optical fiber part includes a second erbium-doped optical fiber amplifier 13 and a second nonlinear optical fiber 14 connected in series. The input end of the second erbium-doped optical fiber amplifier 13 is connected to the nonlinear optical loop mirror.

[0051] Further, the output terminal of the single-frequency microwave source 1 is connected to the intensity modulator 7 or the phase modulator 8 through a microwave amplifier 3; the continuous-wave laser 5 is connected to the intensity controller 7 through a polarization controller 6; a power splitter 2 is arranged at the output terminal of the single-frequency microwave source 1.

[0052] An optical signal is provided by the continuous-wave laser 5, and an electrical signal is provided by the single-frequency microwave source 1. After being modulated by the intensity modulator 7 and the phase modulator 8, a basic optical comb is formed. Among them, the center frequencies of the optical signal and the electrical signal respectively determine the center frequency and the repetition frequency of the basic optical comb. The dual-channel balance of the intensity modulator 7 and the modulation depth of the phase modulator 8 are respectively used to improve the flatness of the basic optical comb and the initial number of comb teeth. Then, the chirp introduced by the microscopic modulation is eliminated through the first connecting optical fiber part to realize the compression of the time-domain pulse. An optical interference process is designed through the optical fiber amplifier and the nonlinear optical loop mirror to suppress the low-power base component of the pulse and simultaneously retain the high-power pulse peak.

[0053] A 3×3 fiber coupler 11 and a nonlinear optical fiber 12 are used to form a nonlinear optical loop mirror. The coupling method of the three optical fibers adopts a coplanar symmetric type. One side port of the 3×3 fiber coupler 11 is selected as the input port, and the central port adjacent to the input port is used as the output port of the nonlinear optical loop mirror. The other side of the 3×3 fiber coupler forms a loop through the connection port of the nonlinear optical fiber 12. Moreover, an optical fiber amplifier and the nonlinear optical fiber 12 are used for parametric mixing to greatly broaden the spectral range based on the self-phase modulation and four-wave mixing effects, and then generate an ideal broadband optical frequency comb.

[0054] An initial optical frequency comb is generated by cascading an intensity modulator 7 and a phase modulator 8. Among them, the drive power and bias voltage of the intensity modulator 7 need to be jointly regulated to improve the flatness of the optical frequency comb. The drive microwave signal of the phase modulator 8 is amplified to the saturation drive power by a microwave amplifier 3 to increase the number of comb teeth. The first dispersion compensation fiber 9 is used to eliminate the nonlinear chirp introduced by the phase modulation to achieve pulse compression. The pulse shaping part is composed of an erbium-doped fiber amplifier 10 and a 3×3 nonlinear optical loop mirror. Among them, the erbium-doped fiber amplifier 10 is used to control the average power entering the optical loop mirror, and the low-power components of the pulse are suppressed by the response of the nonlinear optical loop mirror to different powers in the time-domain waveform to achieve the best shaping effect. The spectral broadening part is composed of a second erbium-doped fiber amplifier 13 and a second nonlinear optical fiber 14. Among them, the second erbium-doped fiber amplifier 13 is used to amplify the power to achieve the best nonlinear effect, and the spectral bandwidth of the optical frequency comb is greatly increased through the self-phase modulation and four-wave mixing effects in the second nonlinear optical fiber 14. The optical frequency comb output by the 3×3 nonlinear optical loop mirror as shown in Figure 4 is obtained. Compared with the optical frequency comb output by the 2×2 nonlinear optical loop mirror as shown in Figure 5 , the 3×3 nonlinear optical loop mirror increases the peak-to-sideband ratio of the time-domain pulse by 15.9 dB and significantly enhances the uniformity of the central spectrum of the optical frequency comb by 11.6 dB. The result of the optical frequency comb after high-nonlinearity parametric mixing is as shown in Figure 6 . For the broadband optical frequency comb under the action of the 3×3 nonlinear optical loop mirror, the comb spectrum flatness is maintained within 5 dB in the range of 6 THz. In contrast, as shown in Figure 7 , the broadband optical frequency comb output by the traditional structure has a flatness exceeding 20 dB.

[0055] According to the matching-mode coupling theorem, the transmission matrix of the 3×3 fiber coupler 11 is:

[0056]

[0057] where A = e -jk / 3, a = e 3jk +2, b = e 3jk -1. As shown in Figure 1 andFigure 2 As shown, select the P1 terminal as the input port, and define the instantaneous input power as P in , three different output powers of the coupler can be obtained, which are respectively:

[0058]

[0059] In the formula, κ is the coupling strength parameter, defined as κ = Kz, where K is the coupling coefficient and z is the coupling length. Figure 1 and Figure 2 respectively represent 3×3 nonlinear optical loop mirrors of two structures. Select the P2 terminal as the output port, and the input / output power conversion equations of the two structures are respectively:

[0060]

[0061]

[0062] In the formula, φ1 = γL(P5 - P4), φ2 = γL(P6 - P5), where γ and L are respectively the nonlinear coefficient and length of the highly nonlinear optical fiber forming the loop. Normalize the input power and fix the values of γ and L, and the loop splitting ratios of the above two 3×3 nonlinear optical loop mirrors when the transmission change slope reaches the maximum value are 60:40 and 40:60 respectively, and the corresponding coupling strength parameters κ are 1.2121 and 1.009 respectively. Compared with other 3×3 coupling methods, coupling strengths and various loop structures, the above two structures can achieve the optimal pulse pedestal sideband suppression effect.

[0063] Example 2:

[0064] This example further optimizes on the basis of Example 1. The nonlinear optical loop mirror includes a group of 3×3 fiber couplers 11 and a group of nonlinear optical fibers 12. The 3×3 fiber coupler 11 includes an input end, a central output end, and a side output end, and the central output end is connected to the side output end through the nonlinear optical fiber 12.

[0065] The first connection method:

[0066] The side output end is directly connected to the input end, and the central output end is connected to the side output end through the nonlinear optical fiber 12. As Figure 2 shown, with P1 as the input end, the side output end is P4, and the central output end P5 is connected to the side output end P4 through the nonlinear optical fiber 12 to form a loop, and output through the port P2.

[0067] The conversion calculation formula of input / output power is:

[0068]

[0069] Where: T1 represents the input / output power conversion equation when the central output end is connected to the directly connected side output end; P4 represents the output power of the directly connected side output end; P5 represents the output power of the central output end; P6 represents the output power of the non-directly connected side output end; φ1 = γL(P5 - P4), γ represents the nonlinear coefficient of the nonlinear optical fiber, and L represents the length of the nonlinear optical fiber.

[0070] The second connection method:

[0071] The side output end is not directly connected to the input end, and the central output end is connected to the side output end through the nonlinear optical fiber 12. As Figure 3 shown, with P1 as the input end, the central output end P5 and the side output end P6 are connected through the nonlinear optical fiber 12 to form a loop.

[0072] The input / output power conversion calculation formula is:

[0073]

[0074] Where: T2 represents the input / output power conversion equation when the central output end is connected to the non-directly connected side output end; P4 represents the output power of the directly connected side output end; P5 represents the output power of the central output end; P6 represents the output power of the non-directly connected side output end; γ represents the nonlinear coefficient of the nonlinear optical fiber, and L represents the length of the nonlinear optical fiber.

[0075] According to the conversion relationship between the input power and the output power, the calculation formula for the transmission slope is obtained as:

[0076] ΔT = ΔP out / ΔP in ;

[0077] Where: ΔT represents the transmission slope of the nonlinear optical loop mirror; ΔP out represents the difference between the output powers corresponding to the instantaneous input powers of 20 dBW and 0 dBW; ΔP in represents the difference in input power of 20 dB.

[0078] As Figure 8 shown, it is the change curve of the transmission slope and the splitting ratio when using a traditional 2×2 fiber coupler. It can be seen that the transmission slope far from reaches 2.7; as Figure 9 shown, it is the conversion curve of the input power and the output power under different splitting ratios when using a traditional 2×2 fiber coupler. As Figure 10 shown, finally, when the splitting ratio of the loop reaches 60:40, the transmission slope reaches the maximum value of 2.7, which is about 30% higher than the transmission slope when using a 2×2 fiber coupler. As Figure 11As shown, in the case of a splitting ratio of 60:40 or 40:60, the conversion curve of the input power and the output power shows that at the maximum transmission slope, when the instantaneous input power is 0 dBW, the instantaneous output intensity of the output structure using a 3×3 fiber coupler is -36.2 dBW, and the output intensity of the output structure using a 2×2 fiber coupler is -20 dBW. That is, the suppression degree of the low-power component of the output structure using a 3×3 fiber coupler is about 16.2 dB better than that of the output structure using a 2×2 fiber coupler.

[0079] As Figure 4 and Figure 5 shown, the peak-sidelobe suppression ratio of the time-domain pulse of the 3×3 nonlinear optical loop mirror is 29.6 dB, and the peak-valley fluctuation of the central spectrum of the optical frequency comb is about 1.4 dB. The peak-sidelobe suppression ratio of the time-domain pulse of the 2×2 nonlinear optical loop mirror is 13.7 dB, and the peak-valley fluctuation of the central spectrum of the optical frequency comb is about 13 dB.

[0080] That is, the peak of the time-domain pulse of the 3×3 nonlinear optical loop mirror has a peak-sidelobe suppression ratio improvement of about 15.9 dB compared to that of the 2×2 nonlinear optical loop mirror, and the central spectrum equalization value of the optical frequency comb is improved by 11.6 dB. The result of the optical frequency comb after high-nonlinearity parameter mixing is as Figure 6 shown. For the broadband optical frequency comb under the action of the 3×3 nonlinear optical loop mirror, the flatness of the comb spectrum is maintained within 5 dB in the range of 6 THz. In contrast, as Figure 7 shown, for the broadband optical frequency comb output by the traditional 3×3 nonlinear optical loop mirror structure, its flatness exceeds 20 dB.

[0081] Other parts of this embodiment are the same as those of the above-mentioned Embodiment 1, so they will not be elaborated here.

[0082] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any simple modification or equivalent change made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. An optical frequency comb generation device based on a 3×3 optical fiber coupler, comprising a single-frequency microwave source (1), a phase shifter (4), a continuous wave laser (5), an intensity modulator (7), a phase modulator (8), a first connecting optical fiber part, and a second connecting optical fiber part, characterized in that, The single-frequency microwave source (1) includes several groups of microwave signal output terminals, and the microwave signal output terminals are connected to an intensity modulator (7) or a phase modulator (8); the continuous-wave laser (5) includes a laser signal output terminal, and the laser signal output terminal is connected to the intensity modulator (7); the intensity modulator (7) and the phase modulator (8) are connected to the input end of the nonlinear optical loop mirror through a first connecting optical fiber part, the output end of the nonlinear optical loop mirror is connected to a second connecting optical fiber part, the nonlinear optical loop mirror includes a coupled 3×3 fiber coupler (11) and a nonlinear optical fiber (12), the three optical fibers in the 3×3 fiber coupler (11) are arranged symmetrically in a coplanar manner, and the transmission slope of the nonlinear optical loop mirror is the ratio of the maximum output intensity to the minimum output intensity of the nonlinear optical loop mirror under different splitting ratios.

2. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to claim 1, wherein The nonlinear optical loop mirror includes a group of 3×3 fiber couplers (11) and a group of nonlinear optical fibers (12), the 3×3 fiber coupler (11) includes an input end, a central output end, and a side output end, and the central output end is connected to the side output end through the nonlinear optical fiber (12).

3. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to claim 2, wherein, The side output end is directly connected to the input end, and the central output end is connected to the side output end through the nonlinear optical fiber (12).

4. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to claim 3, wherein The splitting ratio of the loop is 60:

40.

5. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to claim 2, wherein The side output end is not directly connected to the input end, and the central output end is connected to the side output end through the nonlinear optical fiber (12).

6. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to claim 5, wherein, The splitting ratio of the loop is 40:

60.

7. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to any one of claims 1-6, characterized in that, The first connecting optical fiber part includes a first dispersion compensation optical fiber (9) and a first erbium-doped optical fiber amplifier (10) connected in series. The input end of the first dispersion compensation optical fiber (9) is connected to the phase modulator (8), and the output end of the first erbium-doped optical fiber amplifier (10) is connected to the nonlinear optical loop mirror; the second connecting optical fiber part includes a second erbium-doped optical fiber amplifier (13) and a second nonlinear optical fiber (14) connected in series, and the input end of the second erbium-doped optical fiber amplifier (13) is connected to the nonlinear optical loop mirror.

8. The optical frequency comb generation device based on a 3×3 optical fiber coupler according to any one of claims 1-6, characterized in that, The output end of the single-frequency microwave source (1) is connected to the intensity modulator (7) or the phase modulator (8) through a microwave amplifier (3); the continuous-wave laser (5) is connected to the intensity controller (7) through a polarization controller (6); an electric power splitter (2) is arranged at the output end of the single-frequency microwave source (1).

9. A method for generating an optical frequency comb based on a 3×3 optical fiber coupler, implemented based on the optical frequency comb generating device according to any one of claims 1-8, characterized in that, It includes the following steps: Step 1: Provide an electrical signal through the single-frequency microwave source (1), provide an optical signal through the continuous-wave laser (5), and obtain an initial optical comb through the cascaded modulation of the intensity modulator (7) and the phase modulator (8); Step 2: Eliminate the nonlinear chirp introduced by phase modulation through the first connecting optical fiber part, and perform pulse compression on the optical comb; Step 3: Suppress the low-power components of the pulses through the response of the nonlinear optical loop mirror to different powers in the time-domain waveform of the optical comb, and achieve the best shaping effect on the optical comb; Step 4: Improve the spectral bandwidth of the optical comb through the second connecting optical fiber part.

10. The method for generating an optical frequency comb based on a 3×3 optical fiber coupler according to claim 9, wherein Specifically included in the said Step 3: Step 3.1: Calculate the conversion relationship curve between the input power and the output power of the nonlinear optical loop mirror formed by the 3×3 fiber coupler (11); Step 3.2: Based on the conversion relationship curve between the input power and the output power, calculate the transmission slope of the loop of the 3×3 fiber coupler (11) at different splitting ratios; Step 3.3: Select the splitting ratio of the loop when the transmission slope reaches the maximum value as the optimal splitting ratio.

11. The optical frequency comb generation method based on a 3×3 fiber coupler according to claim 10, wherein When the input end of the 3×3 fiber coupler (11) is directly connected to the side output end, and the central output end and the side output end are connected to form a loop, the instantaneous input power P of the nonlinear optical loop mirror in and the instantaneous output power P out1 have the following conversion relationship: Wherein: T1 represents the input / output power conversion equation when the central output terminal is connected to the directly connected side output terminal; P4 represents the output power of the directly connected side output terminal; P5 represents the output power of the central output terminal; P6 represents the output power of the non-directly connected side output terminal; γ represents the nonlinear coefficient of the nonlinear optical fiber, and L represents the length of the nonlinear optical fiber; When the input end of the 3×3 fiber coupler (11) is not directly connected to the side output end, and the central output end and the side output end are connected to form a loop, the instantaneous input power P of the nonlinear optical loop mirror in and the instantaneous output power P out have the following conversion relationship: Where: T2 represents the input / output power conversion equation when the central output terminal is connected to the non-directly connected side output terminal; P4 represents the output power of the directly connected side output terminal; P5 represents the output power of the central output terminal; P6 represents the output power of the non-directly connected side output terminal. γ represents the nonlinear coefficient of the nonlinear optical fiber, and L represents the length of the nonlinear optical fiber.

12. The optical frequency comb generation method based on a 3×3 optical fiber coupler according to claim 10, wherein The calculation formula for the said transmission slope is: Where: transmission slope; ΔP out represents the difference in output power corresponding to an instantaneous input power of 20 dBW and 0 dBW; ΔP in represents a difference in input power of 20 dB.

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