A method to improve the flatness of the output spectrum of a Brillouin laser cavity soliton optical frequency comb

Abstract

The present invention discloses a method for improving the flatness of the output spectrum of a Brillouin laser cavity soliton optical frequency comb, which belongs to the technical field of optical frequency combs. A method of intracavity dispersion control based on a Brillouin laser cavity is adopted to introduce a positive dispersion optical fiber into a Brillouin laser cavity with full negative dispersion, and the lengths of the positive and negative dispersion optical fibers are adjusted to make the net dispersion of the entire laser cavity a near-zero negative value. When positive and negative dispersion and nonlinear effects, gain and loss reach a double balance, wide pulse width time domain bright solitons with a flat-top envelope are generated, and the spectral sidebands of the soliton optical frequency comb are amplified by the modulation instability gain, so as to improve the flatness of the output spectrum of the soliton frequency comb. The generated flat-top bright solitons have a large time domain overlap with the dual-wavelength Brillouin laser due to their wide pulse width, and the corresponding flat-top soliton frequency comb has a high conversion efficiency. By changing the frequency interval of the dual-wavelength laser source, flat-top bright soliton pulses and flat-top soliton frequency combs with tunable repetition frequency are obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of optical frequency combs, and in particular relates to a method for improving the flatness of an output spectrum of an optical frequency comb based on a Brillouin laser cavity soliton. Background Art

[0002] An optical frequency comb is a pulsed light source with equally spaced frequency components in the frequency domain and a fixed repetition frequency in the time domain. It effectively realizes the interconnection between optical frequency and microwave frequency, promotes the development of many disciplines such as precision spectroscopy, optical measurement technology, coherent optical communication, optical clock, etc., and is the cutting-edge technology of laser and time frequency disciplines today. At present, the most mainstream way to generate optical frequency combs is to obtain soliton optical frequency combs based on Kerr nonlinear optical cavities. The optical frequency combs it generates have smooth spectral envelopes, high coherence and low noise. In addition, the corresponding time-domain cavity solitons are ultrashort pulse lasers that can keep the pulse shape unchanged during transmission. Due to these characteristics, it has been widely used in soliton dual-comb spectroscopy, massively parallel coherent optical communication, soliton micro-comb ranging and other fields. Although the research and application of soliton frequency combs based on Kerr microcavities have made significant progress, there are still shortcomings in the time-domain cavity solitons and soliton frequency combs generated by Kerr nonlinear optical cavities.

[0003] First, most of the time-domain cavity solitons generated by Kerr optical cavities currently work in the anomalous dispersion region. When the nonlinear effect in the cavity reaches a double balance with the anomalous dispersion, gain and loss, a soliton pulse that satisfies the hyperbolic secant function will be generated, and its corresponding frequency domain spectrum also has a hyperbolic secant envelope. At a position far away from the pump light frequency, the intensity of the comb teeth decays rapidly and the spectrum flatness is poor. In the fields of optical communication, lidar ranging, microwave photon signal processing, etc., an optical frequency comb with flat spectral characteristics is required; secondly, the soliton pulses generated by the Kerr optical cavity with anomalous dispersion characteristics have a hyperbolic secant envelope, a narrow pulse width, and a small time domain overlap with the single-wavelength continuous light used as the driving light. , the conversion efficiency of the corresponding soliton frequency comb is low, which limits the wide application of this type of frequency comb; in addition, although a flat-top optical frequency comb with high conversion efficiency has been achieved in a Kerr microcavity with normal dispersion characteristics, the generation of this frequency comb relies on complex excitation means to induce modulation instability, and its spectral bandwidth is relatively small compared with the soliton frequency comb generated in a microcavity with anomalous dispersion; and the repetition frequency of the time-domain cavity soliton and soliton frequency comb generated based on a single-wavelength pumped Kerr optical cavity is limited by the cavity length and is difficult to tune over a large range. Frequency combs with tunable repetition frequency over a large range have very important applications in optical communications, optical sensing, spectroscopy, terahertz wave generation, etc. Summary of the invention

[0004] In order to overcome the above-mentioned shortcomings of the current optical frequency comb, the present invention provides a method for improving the flatness of the output spectrum of a soliton optical frequency comb based on a Brillouin laser cavity. The method adopts a method of intracavity dispersion control based on a Brillouin laser cavity, introduces an optical fiber with positive dispersion (normal dispersion) into an optical fiber Brillouin laser cavity originally composed of a full negative dispersion (abnormal dispersion) optical fiber, and optimizes the lengths of the negative dispersion and positive dispersion optical fibers in the cavity to control the net dispersion of the entire Kerr nonlinear cavity to a near-zero negative value, thereby obtaining a bright soliton pulse with a flat-topped envelope in the time domain. At the same time, by utilizing periodic positive and negative dispersion modulation in the cavity, modulation instability gain is generated, thereby enhancing the comb tooth strength at the corresponding frequency of the soliton optical frequency comb, and thereby improving the flatness of the output spectrum of the soliton frequency comb.

[0005] The present invention proposes a method for improving the flatness of the output spectrum of the Brillouin laser cavity soliton optical frequency comb. The principle is as follows:

[0006] By introducing positive dispersion fiber into the Brillouin laser cavity originally composed of all-negative dispersion fiber, the Brillouin laser pulse in the cavity is compressed when transmitted in the negative dispersion fiber and broadened when transmitted in the positive dispersion fiber; by optimizing the length of the positive and negative dispersion fibers in the cavity and adjusting the net dispersion of the entire fiber Brillouin laser cavity to a net zero negative value, the positive and negative dispersion and nonlinear effects, gain and loss will reach a new double balance, generating a wide pulse width time domain bright soliton pulse with a flat-top envelope; and, as the pump power in the cavity increases, the pulse will continuously circulate in the cavity due to the alternating excitation of modulation instability by positive and negative dispersion, forming oscillations at the top of the pulse of the flat-top bright soliton and being locked by the switching wave effect; at the same time, this modulation instability will also generate gain on both sides of the pump light of the soliton frequency comb generated by the dual-wavelength Brillouin laser cascade four-wave mixing effect, so that the comb teeth intensity on both sides of the pump light is enhanced, and a soliton frequency comb with a flat spectrum is generated. In addition, since the generated wide-pulse flat-top bright solitons have a larger temporal overlap with the dual-wavelength Brillouin laser pulses, the corresponding flat-top soliton frequency comb has a higher conversion efficiency.

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

[0008] A method for improving the flatness of an output spectrum of a Brillouin laser cavity soliton optical frequency comb comprises the following steps:

[0009] A positive dispersion fiber is introduced into the Brillouin laser cavity originally composed of all-negative dispersion fiber. The Brillouin laser pulse in the cavity is compressed when transmitted in the negative dispersion fiber and broadened when transmitted in the positive dispersion fiber. By optimizing the length of the positive and negative dispersion fibers in the cavity and adjusting the net dispersion of the entire fiber Brillouin laser cavity to a net zero negative value, the gain and loss will reach a new double balance due to the positive and negative dispersion and nonlinear effects, generating a wide pulse width time domain bright soliton pulse with a flat-top envelope. With the increase of pump power, the flat-top bright soliton pulse will excite modulation instability due to the alternation of positive and negative dispersion when circulating in the cavity, forming oscillation at the top of the flat-top bright soliton pulse and being locked by the switching wave effect. At the same time, the modulation instability will generate gain on both sides of the pump light of the soliton frequency comb generated by the dual-wavelength Brillouin laser cascade four-wave mixing effect, so that the comb tooth intensity on both sides of the pump light is enhanced, thereby improving the flatness of the output spectrum of the soliton frequency comb.

[0010] Furthermore, the intracavity Brillouin laser pulse is injected into the Brillouin laser cavity through a dual-wavelength laser. When the power of the dual-wavelength laser exceeds the stimulated Brillouin threshold, a dual-wavelength Brillouin laser with equal frequency intervals opposite to its transmission direction is generated. The generated Brillouin laser is always located at the cavity resonance wavelength, which is used as the intracavity pump light to drive the laser cavity.

[0011] Furthermore, the wavelengths of the dual-wavelength laser are 1560.20 nm and 1561.02 nm respectively, the frequency interval is 100 GHz, and the average power is 2280 mW.

[0012] Furthermore, the wavelengths of the dual-wavelength laser are 1560.20 nm and 1562.64 nm respectively, the frequency interval is 300 GH, and the average power is 2280 mW.

[0013] Furthermore, the full negative dispersion optical fiber specifically refers to a composition of a commercial high nonlinear optical fiber with a length of 20m and a single-mode optical fiber with a length of 2.04m, and the nonlinear coefficient of the commercial high nonlinear optical fiber is 10W -1 km -1 , the nonlinear coefficient of the single-mode optical fiber is 1W -1 km -1 The positive dispersion fiber introduced is a dispersion-compensating fiber with a length of 0.8 m and a nonlinear coefficient of 1.5 W. -1 km -1 , which is a negative value close to zero.

[0014] Compared with the prior art, the advantages of the present invention are as follows:

[0015] 1. The method of the present invention improves the flatness of the output spectrum of the soliton optical frequency comb based on the Brillouin laser cavity. The fiber Brillouin laser cavity is used as the Kerr nonlinear cavity for generating the soliton frequency comb. By introducing the Brillouin gain in the cavity, the generated dual-wavelength Brillouin laser is always located at the cavity resonance wavelength, which is used as the intra-cavity pump light to drive the laser cavity. When the intra-cavity laser power increases, due to the Kerr effect, the generated dual-wavelength Brillouin laser is automatically located at the cavity redshift detuning frequency, and the soliton frequency comb can be spontaneously generated without artificial fine tuning of the pump light frequency.

[0016] 2. The present invention introduces positive dispersion optical fiber into the Brillouin laser cavity with full negative dispersion, optimizes the length of positive and negative dispersion optical fiber in the cavity, and makes the net dispersion of the entire cavity close to zero negative value, without complex excitation means (such as inter-mode coupling effect, pump light modulation, etc.). When the positive and negative dispersion and nonlinear effects, gain and loss in the cavity reach a new double balance, time-domain bright soliton pulses with flat-top envelope and spectrally flat soliton frequency combs will be generated;

[0017] 3. Compared with the hyperbolic secant time-domain cavity solitons generated by the traditional single-wavelength continuous light-driven full-negative dispersion Kerr optical cavity, the wide pulse width flat-top bright solitons generated by the present invention have a larger time-domain overlap with the dual-wavelength Brillouin laser pulses, and the corresponding flat-top soliton frequency comb has a higher conversion efficiency;

[0018] 4. The flat-top bright soliton pulse and flat-top soliton frequency comb generated by the present invention can achieve large-scale tuning of the repetition frequency by changing the frequency interval of the dual-wavelength laser source. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0020] Figure 1 : Schematic diagram of the method for improving the flatness of the output spectrum of the Brillouin laser cavity soliton optical frequency comb according to the present invention; (a) Schematic diagram of the experimental device for generating flat-top bright solitons and flat-top soliton frequency comb; (b) Schematic diagram of the generation of flat-top bright solitons after the positive and negative dispersion and nonlinear effects, the gain and loss provided by the dual-wavelength pump light reach a double balance;

[0021] Figure 2 : Relationship between the group velocity dispersion (GVD) of the three different optical fibers constituting the Brillouin laser cavity described in Example 1 of the present invention and the variation with wavelength;

[0022] Figure 3: The method for improving the flatness of the output spectrum of the Brillouin laser cavity soliton optical frequency comb described in Example 1 of the present invention, when the three optical fibers constituting the Brillouin laser cavity, the high nonlinear fiber length is 20m, the single-mode fiber length is 2.04m, the dispersion compensation fiber length is 0.8m, the dual-wavelength laser average power is 2300mW, and the repetition frequency is 100GHz, the numerical simulation (ignoring high-order dispersion) obtains (a) the output time domain pulse and frequency domain spectrum, (b) the relationship between the parametric gain and the frequency shift relative to the pump wavelength and the intracavity power, and (c) the parametric gain spectrum when the intracavity power is 2860mW in Figure (a);

[0023] Figure 4 : The flat-top bright solitons and flat-top soliton frequency combs based on the fiber Brillouin laser cavity described in Example 1 of the present invention, when the average power of the dual-wavelength laser is 2280mW and the repetition frequency is 100GHz, the (a) spectrum test results and (b) time domain simulation results of the output, and when the power of the dual-wavelength laser is reduced to an average power of 1100mW, the (c) time domain pulses and simulation results measured experimentally;

[0024] Figure 5 : When the average power of the dual-wavelength laser in Example 1 of the present invention is 2280mW and the repetition rate is 300GHz, the output (a) spectrum test results and (b) time domain simulation results. DETAILED DESCRIPTION

[0025] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0026] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0027] Example 1

[0028] A method of improving the flatness of the output spectrum of the Brillouin laser cavity soliton optical frequency comb in this embodiment is as follows: Figure 1(a) shows the principle diagram of the flat-top soliton frequency comb generated based on the Brillouin laser cavity of the present invention; the Brillouin laser cavity is composed of different optical fibers with positive and negative dispersions, and the lengths of the different optical fibers are adjusted to make the net dispersion of the entire fiber Brillouin laser cavity a near-zero negative value. A dual-wavelength laser is injected into the fiber Brillouin laser cavity. When the power of the dual-wavelength laser exceeds the stimulated Brillouin threshold, a dual-wavelength Brillouin laser with equal frequency intervals opposite to its transmission direction is generated. The generated Brillouin laser is always located at the cavity resonance wavelength, which is used as the intra-cavity pump light to drive the laser cavity. When the Brillouin laser pulse is transmitted in the cavity, it is compressed in the negative dispersion fiber and broadened in the positive dispersion fiber; when the intra-cavity laser power increases, due to the Kerr effect, the generated dual-wavelength Brillouin laser is automatically located at the cavity redshift detuning frequency, and the necessary conditions for generating time-domain cavity solitons are achieved without artificial fine tuning of the pump light frequency. As the pulse continues to circulate in the Kerr nonlinear cavity, as shown in FIG. Figure 1 As shown in (b), when the positive and negative dispersion and nonlinear effects in the cavity, and the gain and loss provided by the dual-wavelength Brillouin laser reach a new double balance, time-domain bright soliton pulses with a flat-top envelope and a flat-top soliton frequency comb will be generated.

[0029] By using the dispersion control method in the Brillouin laser cavity, when the wavelengths of the dual-wavelength lasers are 1560.20nm and 1561.02nm, the frequency interval is 100GHz, and the average power is 2280mW, they are injected into the dispersion-controlled fiber Brillouin laser cavity with a cavity length of 22.84m to obtain flat-top bright solitons and flat-top soliton frequency combs. The dispersion-controlled fiber Brillouin laser cavity is composed of three different optical fibers. The relationship between their group velocity dispersion and wavelength is shown in the figure. Figure 2 The commercial high nonlinear optical fiber with a length of 20m has negative dispersion at the working wavelength and is nearly flat near the working wavelength. It is used to generate Brillouin gain and parametric gain of four-wave mixing effect, and its nonlinear coefficient is 10W. -1 km -1 ; The single-mode optical fiber with a length of 2.04m is the pigtail of optical devices such as circulators and output couplers. It also has negative dispersion at the working wavelength and its nonlinear coefficient is 1W -1 km -1 The positive dispersion fiber used to compensate for negative dispersion is a 0.8m long dispersion compensation fiber with a nonlinear coefficient of 1.5W. -1 km -1 The net dispersion of the entire fiber Brillouin laser cavity is -2.171×10 -3 ps 2 , which is a negative value close to zero.

[0030] Figure 3 is the theoretical simulation result of this embodiment without considering high-order dispersion, where Figure 3(a) is the output result of time domain pulse and frequency domain spectrum when the dual-wavelength frequency interval is 100 GHz. It can be seen that the output time domain pulse is a wide pulse width flat-top bright soliton with an oscillation on the top. The top oscillation is caused by the modulation instability excited by the continuous circulation of the flat-top bright soliton pulse in the positive and negative dispersion optical fiber, and is locked by the switching wave effect; and the modulation instability will also generate gain on both sides of the pump light of the soliton frequency comb generated by the dual-wavelength Brillouin laser cascade four-wave mixing effect, so that the comb teeth intensity on both sides of the pump light is enhanced to form a soliton frequency comb with a flat spectrum. This modulation instability gain can be quantitatively calculated, as shown in Figure 3 (b) shows the relationship between the parameter gain obtained by theoretical calculation and the frequency shift and the intracavity power. Figure 3 (c) Figure 3 (a) The corresponding parametric gain spectrum when the intracavity power is 2860 mW. It can be seen that the calculated frequency shift of the modulation instability gain peak relative to the pump light (5 THz) is Figure 3 The peak value of the flat-top region of the spectrum shown in (a) matches the frequency shift of the pump light, proving that the gain generated by modulation instability will enhance the comb strength on both sides of the pump light, thereby improving the flatness of the output spectrum of the soliton frequency comb. In addition, since the generated wide pulse width flat-top bright solitons have a greater temporal overlap with the dual-wavelength Brillouin laser pulses, the corresponding flat-top soliton frequency comb has a higher conversion efficiency of 12.3%.

[0031] Figure 4 (a) Spectral test results after injection into the dispersion-controlled Brillouin laser cavity when the average power of the dual-wavelength laser is 2280 mW and the repetition frequency is 100 GHz as described in this embodiment. As can be seen from the figure, the spectrum has a flat-top envelope, and the flat spectral region covers 1537 nm to 1588 nm. In addition, the spectrum contains more than 200 comb teeth, and the spectral range is 1460 nm-1640 nm, which has a wide spectral bandwidth. Figure 4 The dashed envelope in (a) is the theoretical simulation result after considering high-order dispersion and Raman effect, which is close to the experimentally measured spectrum. The corresponding time domain pulse is shown in Figure 4 (b) shows a wide pulse width flat-top bright soliton with oscillation at the top. Due to the influence of high-order dispersion and Raman effect, the oscillation at the top of the pulse is not symmetrical. Figure 4 (c) is the flat-top bright soliton pulse measured when the average power of the dual-wavelength laser is reduced to 1100 mW in this embodiment. When the pump power is relatively low, the modulation instability in the cavity has not yet been excited, and no oscillation will appear at the top of the pulse. The dotted line is the result obtained by numerical simulation, which is consistent with the experimental test results.

[0032] Figure 5(a) The flat-top soliton frequency comb is obtained when the wavelengths of the dual-wavelength pump light are changed to 1560.20nm and 1562.64nm, the frequency interval is 300GHz, and the average power is 2280mW. The wavelengths are injected into the dispersion-controlled fiber Brillouin laser cavity. On both sides of the pump light, the spectrum has a flat-top envelope, and the flat spectrum region covers 1533nm to 1590nm. Figure 5 The dashed envelope in (a) is the theoretical simulation result after considering high-order dispersion and Raman effect, which is consistent with the experimental spectrum. The corresponding time domain pulse is shown in Figure 5 (b) shows a wide pulse width flat-top bright soliton with oscillation on the top. Figure 5 The results show that the flat-top soliton frequency comb generated based on the Brillouin laser cavity can achieve a wide range of tunable repetition frequency by changing the frequency interval of the dual-wavelength laser.

[0033] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0034] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0035] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method to improve the flatness of the output spectrum of a Brillouin laser cavity soliton optical frequency comb. It is characterized in that The specific steps include: A positive dispersion fiber is introduced into the Brillouin laser cavity originally composed of all-negative dispersion fiber. The Brillouin laser pulse in the cavity is compressed when transmitted in the negative dispersion fiber and broadened when transmitted in the positive dispersion fiber. By optimizing the length of the positive and negative dispersion fibers in the cavity and adjusting the net dispersion of the entire fiber Brillouin laser cavity to a net zero negative value, the gain and loss will reach a new double balance due to the positive and negative dispersion and nonlinear effects, generating a wide pulse width time domain bright soliton pulse with a flat-top envelope. With the increase of pump power, the flat-top bright soliton pulse will excite modulation instability due to the alternation of positive and negative dispersion when circulating in the cavity, forming oscillations at the top of the flat-top bright soliton pulse and being locked by the switching wave effect. At the same time, the modulation instability will generate gain on both sides of the pump light of the soliton frequency comb generated by the dual-wavelength Brillouin laser cascade four-wave mixing effect, thereby enhancing the comb tooth intensity on both sides of the pump light, thereby improving the flatness of the output spectrum of the soliton frequency comb.

2. A method for improving the flatness of the output spectrum of a Brillouin laser cavity soliton optical frequency comb according to claim 1, It is characterized in that The wavelengths of the dual-wavelength laser are 1560.20 nm and 1561.02 nm respectively, the frequency interval is 100 GHz, and the average power is 2280 mW.

3. A method for improving the flatness of the output spectrum of a Brillouin laser cavity soliton optical frequency comb according to claim 1, It is characterized in that The wavelengths of the dual-wavelength laser are 1560.20 nm and 1562.64 nm respectively, the frequency interval is 300 GH, and the average power is 2280 mW.

4. A method for improving the flatness of the output spectrum of a Brillouin laser cavity soliton optical frequency comb according to claim 1, It is characterized in that The full negative dispersion optical fiber specifically refers to a commercial high nonlinear optical fiber with a length of 20m and a single-mode optical fiber with a length of 2.04m. The nonlinear coefficient of the commercial high nonlinear optical fiber is 10W. -1 km -1 , the nonlinear coefficient of the single-mode optical fiber is 1W -1 km -1 The positive dispersion fiber introduced is a dispersion-compensating fiber with a length of 0.8 m and a nonlinear coefficient of 1.5 W. -1 km -1 , which is a negative value close to zero.

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