Narrow-linewidth compact tunable light source based on Brillouin random fiber laser

Abstract

The invention discloses a narrow linewidth compact tunable light source based on Brillouin random fiber laser. The narrow linewidth compact tunable light source comprises an outer ring tunable pump light generation module and an inner ring semi-open random laser cavity. The outer ring tunable pump light module comprises an active gain, a tunable filter, an optical isolator, an optical circulator and an optical coupler, the working wavelength of the filter is controlled by controlling the input voltage of the filter, and then tunable laser output in a large wavelength range is achieved. The inner-ring half-open Brillouin random laser cavity utilizes a high-nonlinearity optical fiber and a random feedback enhancement optical fiber to respectively provide ultra-narrow-band Brillouin gain and distributed random feedback capable of working at any wavelength, so that random laser oscillation is formed, the laser line width is greatly compressed, the noise level is suppressed, and high-coherence and large-bandwidth tunable laser output is realized. The method has a relatively high application space in the fields of distributed optical fiber sensing, coherence tomography, laser radar and the like.

Description

Technical Field

[0001] The invention relates to the field of tunable fiber lasers, and in particular to a narrow-linewidth compact tunable light source based on Brillouin random fiber lasers. Background Art

[0002] Narrow linewidth tunable fiber lasers refer to lasers that output in a single longitudinal mode and in a tunable wavelength state, and have many advantages such as narrow output linewidth, low frequency noise level, and good stability. Narrow linewidth means that the laser has excellent coherence performance and extremely low frequency noise level; tunability means that the output frequency of the laser has adjustable characteristics within a certain range, that is, different wavelengths can be output using one laser light source, and thus the external information carried by the light wave can be acquired and sensed, which is one of the hot topics in the field of laser research. The characteristics of tunable laser sources (including tuning bandwidth and laser linewidth) essentially determine parameters such as spatial resolution and maximum detection range of practical applications.

[0003] There are currently two main ways to achieve tunable lasers, namely internal modulation and external modulation. Internal modulation changes the laser oscillation parameters including the resonant cavity length and gain spectrum position during the laser establishment process. A typical solution is to use current injection into a distributed feedback semiconductor laser or to install a tunable filter in a fiber laser to tune the output wavelength. However, due to the inherent relaxation oscillation characteristics of the laser and the competition between the modes in the cavity, the line width of the laser based on internal modulation technology is wide and the coherence is poor. External modulation technology refers to the use of external optical modulation devices and high-performance signal sources to modulate a stable continuous optical frequency after the laser is established. After the modulated signal frequency changes continuously and is loaded into the optical signal, the frequency of the optical sideband signal generated will also change in the same way. For example, a single-sideband modulator and a tunable microwave source are used to modulate the output frequency of the laser, but it is difficult to achieve large-scale tuning due to the limited working bandwidth of the device. The performance of the tunable laser directly determines the performance of the detection system. For example, for the optical frequency domain reflectometer, the frequency sweep range of the laser determines the spatial resolution of the sensor / lidar system, its coherence and frequency sweep period directly affect the maximum detectable distance of the system, and the dynamic refresh rate of the system is determined by the frequency sweep speed. For optical coherence tomography technology, the larger the frequency sweep range of the tunable light source, the higher the axial resolution of the system, and its instantaneous line width determines the imaging depth of the system. The laser output power has a great relationship with the sensitivity in the optical coherence tomography system. Therefore, in order to improve the performance of signal detection or imaging systems, it is necessary to use existing laser modulation methods and other advanced technical solutions to realize a tunable laser with narrow line width, wide tuning range and low frequency noise. Random fiber lasers provide an excellent design idea for the design of tunable lasers.

[0004] Brillouin random fiber laser technology provides a feasible solution for realizing advanced ultra-narrow linewidth tunable fiber laser sources. Without the need for complex optoelectronic feedback structures, Brillouin random fiber laser technology uses distributed weak feedback as the feedback mechanism to obtain cavity-free mode laser emission, which is provided by numerous Rayleigh scattering centers formed by the inevitable refractive index inhomogeneity in the optical fiber. This feedback has a wide operating wavelength range. The linewidth compression and noise suppression effects of Brillouin random fiber lasers have been theoretically and experimentally studied in previous literature. Combined with SBS gain with arbitrary wavelength working characteristics, Brillouin random fiber laser technology has great advantages in realizing low-cost, broadband continuously tunable high-coherence fiber laser emission. Summary of the invention

[0005] In order to solve the problems of limited coherence, small tunable bandwidth, poor stability, and high implementation cost caused by the line width difference of existing tunable lasers, the present invention proposes a narrow line width compact tunable light source based on Brillouin random fiber laser. The laser can achieve continuous tunable narrow line width laser output in a large wavelength range, and ensure the compactness and stability of the light source system. The parameters such as tuning rate and tuning range are all set manually, and the operation is simple.

[0006] In order to achieve the purpose of the above invention, the present invention adopts the following inventive concept:

[0007] The present invention provides a narrow-linewidth compact tunable light source based on Brillouin random fiber laser, comprising an outer ring tunable pump light module and an inner ring semi-open ring random laser cavity;

[0008] The outer ring tunable pump light module is used to generate tunable Brillouin pump light and transmit the Brillouin pump light to the inner ring half-open ring random laser cavity; the inner ring half-open ring random laser cavity is used to convert the Brillouin pump light into reverse-transmitting Stokes light to achieve linewidth compression and noise suppression.

[0009] As a preferred embodiment of the present invention, the outer ring tunable pump optical module comprises a driving module and active gain, a tunable filter, a first optical isolator, a polarization controller, a first optical circulator, a highly nonlinear optical fiber, an optical coupler, and a third optical circulator connected in sequence along the optical path; the driving module is used to drive the tunable filter to select light of a preset wavelength from the wide-spectrum background light of the active gain; the light sequentially passes through the first optical isolator, the polarization controller, the first optical circulator, the highly nonlinear optical fiber, the optical coupler, and the third optical circulator to realize unidirectional laser oscillation output in the annular cavity of the outer ring, and when the central wavelength of the tunable filter changes, the laser output wavelength will also change; the optical fiber coupler is used for light beam coupling, and the optical fiber coupler is connected to an external spectrometer to observe the characteristics of the output light.

[0010] As a preferred embodiment of the present invention, the inner ring semi-open ring random laser cavity comprises Rayleigh enhanced feedback fiber, a second optical isolator and a second optical circulator; the high nonlinear optical fiber, the optical coupler, the third optical circulator, the second optical circulator and the first optical circulator are connected in sequence along the optical path; the Rayleigh enhanced feedback fiber is connected to two ports of the second optical circulator, and the Rayleigh enhanced feedback fiber is also connected to the second optical isolator.

[0011] As a preferred solution of the present invention, the light transmitted in the optical circulator can only pass unidirectionally from port one to port two and from port two to port three, so in the inner ring semi-open ring random laser cavity, the Stokes light can only be transmitted from port two to port three of the first optical circulator to the second optical circulator, and the Stokes light is unidirectionally transmitted to the Rayleigh enhanced feedback fiber after port one to port two of the second optical circulator, and the feedback light after Rayleigh enhancement can only return to the high nonlinear optical fiber from port two to port three of the second optical circulator and from port one to port two of the third optical circulator.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] 1) In the present invention, stimulated Brillouin scattering in a highly nonlinear optical fiber is used to provide ultra-narrowband gain, and the length of the gain optical fiber is reduced to achieve a compact system. Preliminary linewidth compression and frequency noise suppression are achieved for the laser at any pump wavelength, and highly coherent Stokes light whose wavelength follows the wavelength of the pump light is obtained.

[0014] 2) The present invention further deeply compresses the linewidth by introducing a broadband enhanced random distributed feedback structure, further suppresses laser frequency noise, and greatly reduces the length of the Rayleigh fiber. The Brillouin frequency shift in the fiber ensures the fast tuning and following of the laser and the pump light, while the enhanced random feedback design effectively eliminates the limitation of the fixed cavity mode on the laser tuning, effectively overcoming the technical bottleneck of the limited laser tuning range of the conventional fiber cavity structure.

[0015] 3) The present invention combines the outer ring tunable pump source and the inner ring half-open random laser cavity in the same structure, which has a compact structure and high integration. It also uses high nonlinear optical fiber and random feedback enhancement technology to further ensure the miniaturization of the entire system and improve stability. When the pump light is tuned at a certain speed, the narrow linewidth and highly coherent random laser will be tuned and output at the same speed. After polarization state management, the interference resistance is stronger, the structure is simple and reliable, and the cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a diagram showing a narrow-linewidth compact tunable light source based on Brillouin random fiber laser in an embodiment of the present invention.

[0017] Figure 21 is a characteristic curve of the working characteristic voltage and filtering center wavelength of the tunable optical filter in the embodiment of the present invention.

[0018] Figure 3 It is a dynamic frequency sweep characteristic curve of the tunable optical filter in the embodiment of the present invention.

[0019] In the figure: 1-high nonlinear optical fiber, 2-optical coupler, 3-third optical circulator, 4-active gain, 5-tunable filter, 6-driving module, 7-first optical isolator, 8-polarization controller, 9-first optical circulator, 10-second optical circulator, 11-Rayleigh enhanced feedback optical fiber, 12-second optical isolator. DETAILED DESCRIPTION

[0020] The following reference description introduces the preferred embodiments of the present invention in combination with the accompanying drawings to make its technical content clearer and easier to understand. The present invention can be implemented through many different forms of examples. It should be understood that the specific embodiments described here are only used to explain this embodiment, and the protection scope of the present invention is not limited to the embodiments mentioned in the text.

[0021] In the same structural framework, tunable pump light generation is achieved through an outer ring cavity composed of an electrically controlled tunable filter, active gain and high nonlinear fiber. The pump light excites the Stokes laser in the inner semi-open ring cavity, and performs corresponding polarization management. Under the joint action of Brillouin gain and random feedback, tunable narrow linewidth and low-noise random laser output is achieved. Considering the miniaturization and compactness of the system, in the framework of the internal and external composite structure, high nonlinear fiber and distributed random feedback enhancement structure are used to greatly reduce the length and weight of the optical fiber used, and improve the working stability of the laser system.

[0022] Thanks to the ultra-narrow gain bandwidth of stimulated Brillouin scattering in optical fiber, Brillouin fiber lasers naturally exhibit unique advantages such as low frequency noise, narrow linewidth, arbitrary operating wavelength and high optical signal-to-noise ratio compared to fiber lasers based on other gain mechanisms. After the Brillouin pump light energy is transferred to Brillouin Stokes light, a Brillouin frequency shift is generated accordingly, and both the phase noise and linewidth are greatly reduced, with the characteristics of ultra-narrow linewidth tunability. At the same time, combined with distributed random feedback with a large working bandwidth as a feedback mechanism, single-frequency laser oscillation without cavity mode can be obtained, and the laser linewidth can be compressed and phase noise can be suppressed in one step, realizing broadband tunable laser output with ultra-narrow linewidth. Considering the miniaturization and anti-interference of the laser system, high nonlinear fiber and Rayleigh enhanced fiber are used to provide Brillouin gain and distributed random feedback to reduce the length of the optical fiber used.

[0023] Its basic structure consists of an outer ring tunable pump light module and an inner ring semi-open ring random laser cavity, which is relatively compact. First, the outer ring tunable pump light module generates tunable Brillouin pump light, which then enters the inner ring semi-open random cavity to achieve deep compression of the tunable laser line width and noise suppression.

[0024] The semi-open random cavity mainly includes a high nonlinear optical fiber, a Rayleigh enhanced feedback optical fiber, a first optical circulator, a second optical circulator, a third optical circulator and a second optical isolator. The high nonlinear optical fiber, the optical coupler, the first optical circulator, the second optical circulator and the third optical circulator are connected in sequence; when the pump power reaches the stimulated Brillouin scattering threshold, the Stokes light with a slight wavelength shift compared to the pump laser can be excited by Brillouin scattering, and the polarization states of the two are matched. In the process of propagation along the high nonlinear optical fiber, the coherence time of the Stokes light is extended in the stimulated amplification process. After being injected into the Rayleigh feedback medium, it is scattered by multiple "equivalent scattering centers" in the optical fiber to generate distributed backscattered Stokes light with random scattering intensity of the same frequency, and is fed back to the high nonlinear optical fiber to realize single longitudinal mode Brillouin random laser oscillation. The laser output end eliminates the end face scattering at the end of the single-mode optical fiber through the second optical isolator to avoid strong reflection points caused by end face scattering. Since the Brillouin gain and random distributed feedback provided by the highly nonlinear polarization-maintaining fiber have excellent tuning performance, when the pump light is tuned at a certain speed, the ultra-narrow linewidth random laser will be tuned and output at the same speed, with an extremely high coherence length and a large tuning range. It is also more compact in structure, does not require sophisticated and complex optoelectronic feedback structures, and is easy to operate.

[0025] Specifically, the outer ring cavity adjustable pump source includes active gain, electrically controlled adjustable filter and its driving module, high nonlinear optical fiber, circulator and coupler, etc. The narrowband electrically controlled filter selects a specific wavelength from the wide spectrum background light of the active gain, and realizes unidirectional laser oscillation output in the outer ring cavity circulation after passing through the optical fiber isolator and circulator. When the central wavelength of the filter changes, the laser output wavelength will also change. The characteristics of its output light are observed by an optical fiber coupler, thereby realizing wavelength-tuned Brillouin pump light and injecting it into the semi-open random cavity of the inner ring.

[0026] Specifically, the inner circle semi-open annular random cavity, the semi-open annular random cavity structure includes the first, second and third optical circulators, the optical circulators have unidirectional conduction characteristics, light is transmitted therein, and can only be transmitted from one port to two ports in one direction, and from two ports to three ports, after the Stokes light is generated, it can only be transmitted from the two ports to the three ports of the first circulator to the feedback optical fiber in one direction, and similarly, the Rayleigh feedback light can only be returned to the gain medium from the two ports to the three ports of the second circulator, and from the one port to the two ports of the third circulator to realize cavity-free mode laser oscillation output.

[0027] Specifically, the inner ring semi-open ring random cavity, the semi-open ring random laser cavity contains a section of polarization-maintaining high nonlinearity to provide Brillouin gain. When the power of the incident light continues to increase until it exceeds the stimulated Brillouin scattering threshold, most of the energy of the incident light will be transferred to the reverse-transmitting Stokes light. The wavelength interval between the pump light and the Stokes light in ordinary single-mode optical fiber is between 0.08nm and 0.09nm. Combined with the linewidth depth compression effect of random feedback, the Brillouin random fiber laser linewidth compression formula can be expressed as:

[0028]

[0029] L r It can be equivalent to the length L of the gain fiber g Adding the effective length of the Rayleigh scattered fiber Δv P is the pump line width. Considering that the distributed random feedback has a certain average effect on the laser frequency noise, the laser line width can be further compressed. Therefore, it is necessary to introduce the line width compression factor β related to Rayleigh scattering. At the same time, the feedback factor in the Brillouin fiber laser needs to be expressed as the Rayleigh feedback coefficient R r c represents the speed of light propagation in vacuum, n is the refractive index of the optical fiber, α is the line width correction factor, β is the Rayleigh scattering line width narrowing factor, ln is the natural logarithm, Δυ B Represents the spectral width of the intrinsic Brillouin gain spectrum in the optical fiber (~20 MHz).

[0030] Specifically, in the semi-open annular random cavity, the random laser output end of the semi-open annular cavity structure is connected to an optical isolator to isolate the interference caused by Fresnel reflection at the optical fiber joint.

[0031] Specifically, the active gain in the outer ring adjustable pump light can be provided by a semiconductor optical amplifier or a rare earth active doped optical fiber.

[0032] The above scheme is further described below in conjunction with specific examples, and the preferred embodiments of the present invention are described in detail as follows:

[0033] Embodiment 1:

[0034] In this embodiment, see Figure 1 A narrow-linewidth compact tunable light source based on Brillouin random fiber laser includes a composite structure consisting of an outer ring cavity tunable pump light generation module and an inner half-open random laser cavity. In this example, a tunable pump light is generated by an outer ring cavity and injected into the inner half-open random cavity to achieve a tunable narrow-linewidth Stokes laser.

[0035] Embodiment 2:

[0036] This embodiment is basically the same as the first embodiment, except that:

[0037] In this embodiment, the tunable pump light utilizes the amplified background light of the active gain 4, selects a specific working wavelength range through the electrically controlled tunable filter 5 and the driving module 6, and realizes tunable laser output in a specific wavelength range in the ring laser resonant cavity of the outer circle. The transmission direction is determined by the first optical isolator 7, and then the tunable pump light is formed by the polarization controller and enters the semi-open random laser cavity of the inner circle through port 1 of the first circulator 9.

[0038] In this embodiment, the inner ring semi-open random laser cavity, the semi-open ring cavity structure includes the first optical circulator, the second optical circulator and the third optical circulator. Due to the characteristics of the optical circulator, the light transmitted therein can only be unidirectionally conducted from one port to two ports and from two ports to three ports, so the excited Stokes laser cannot form a traditional closed resonant cavity through the three ports to one port of the second optical circulator and the three ports to two ports of the second optical circulator 10. The Stokes light is transmitted unidirectionally to the Rayleigh enhanced feedback fiber 11 after the one port to the two port of the second circulator 10, and the feedback light after Rayleigh enhancement can only return to the high nonlinear optical fiber from the two ports to the three ports of the second circulator 10 and from the one port to the two ports of the third circulator 3.

[0039] In this embodiment, the semi-open annular random cavity, the semi-open annular cavity structure includes a section of high nonlinear optical fiber 1 as a Brillouin gain optical fiber, and the pump light injected therein undergoes stimulated Brillouin scattering effect after reaching the Brillouin threshold, generating reverse-propagating Stokes light, thereby achieving preliminary linewidth compression and noise suppression.

[0040] In this embodiment, the semi-open annular random cavity, the semi-open annular cavity structure includes a random feedback enhancement feedback structure to provide distributed random feedback, and after providing backward random feedback, it is fed back to the high nonlinear optical fiber through the second to third ports of the second optical circulator 10 and the first to second ports of the first optical circulator, deeply compressing the laser line width, realizing Stokes laser oscillation and outputting narrow line width tunable laser. The random laser line width satisfies Formula 1. In this embodiment, the semi-open annular random cavity, the semi-open annular laser cavity structure includes an optical isolator 12 at the output end to reduce the interference of the output end face reflection on the random laser and avoid multi-longitudinal mode laser oscillation.

[0041] Embodiment three:

[0042] In this embodiment, the composite structure of the overall tunable light source device is as follows: Figure 1As shown. The background light gain is generated by the active gain 4, and then the working wavelength is selected by the electrically controlled tunable filter 5, and is unidirectionally transmitted to the polarization controller 8 through the isolator 7 to form a tunable pump light. The pump light is injected into the semi-open annular random cavity through the 1 to 2 ports of the first circulator 9. The semi-open annular random cavity is composed of a short section of highly nonlinear optical fiber as the Brillouin gain optical fiber. When the pump power reaches the stimulated Brillouin scattering threshold, the Stokes light with a slight wavelength shift compared to the pump laser can be excited through Brillouin scattering. In the process of propagating along the highly nonlinear optical fiber, the Stokes light is amplified through the highly nonlinear optical fiber, and its coherence time is extended in the stimulated amplification process. Stokes light is incident into the random feedback enhanced Rayleigh enhanced feedback fiber 11 through port 1 to port 2 of the second circulator 10, generating distributed backscattered Stokes light with random scattering intensity of the same frequency. The distributed backscattered Stokes light passes through port 1 to port 2 of the third optical circulator 3 and is fed back into the high nonlinear fiber to realize Brillouin random laser oscillation. The introduction of random feedback fully suppresses the cavity mode oscillation to realize cavity mode-free narrow linewidth tunable laser output with a wide wavelength range. The output parameters of the tunable laser can be set by the AC driving electrical signal loaded on the tunable optical filter. By editing the frequency, DC bias and peak-to-peak value parameters of the AC driving electrical signal, the sweep speed, spectral center wavelength and sweep range of the narrow linewidth laser can be set.

[0043] The central wavelength of an electrically tunable optical filter is directly related to the driving voltage injected into it. When the driving voltage changes, the cavity length of the filter changes, and then its central wavelength changes accordingly. Figure 2 As shown in the figure, the curve in the figure represents the variation of the output center wavelength of the tunable optical filter with the driving voltage. From this figure, it can be seen that within a larger wavelength range, the center wavelength of the filter has a good linear relationship with the input driving voltage, maintaining good optical properties.

[0044] In this embodiment, an AC scanning electrical signal with a DC bias is loaded to the tunable filter, which can realize dynamic scanning of the output wavelength. The scanning speed is determined by the frequency of the AC signal, the DC bias signal determines the central wavelength of the scanning spectrum, and the peak-to-peak value of the AC signal determines the wavelength range of the scanning spectrum. Figure 3 As shown, after loading an AC signal of 2kHz and 2V peak-to-peak, the output wavelength of the electrically controlled tunable optical filter changes continuously, and it has good dynamic scanning characteristics.

[0045] For the purpose of illustration and description, the present invention provides the aforementioned illustrative examples of the present invention. This is not intended to be an exhaustive description of the present invention or to limit the present invention to the precise form described, and modifications and changes can be made based on the above description. The embodiments are selected and described to explain the principles of the present invention and as a practical application of the present invention, so that those skilled in the art can use the present invention in various embodiments and make various modifications for specific purposes. All variations that can be directly derived or associated with the content disclosed by a person of ordinary skill in the art should be considered to be within the scope of protection of the present invention.

Claims

1. A narrow-linewidth compact tunable light source based on Brillouin random fiber laser, characterized by: It includes an outer ring tunable pump optical module and an inner ring semi-open ring random laser cavity; The outer ring tunable pump light module is used to generate tunable Brillouin pump light and transmit the Brillouin pump light to the inner ring half-open ring random laser cavity; the inner ring half-open ring random laser cavity is used to convert the Brillouin pump light into reverse-transmitting Stokes light to achieve linewidth compression and noise suppression.

2. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 1, characterized in that: The outer ring tunable pump optical module comprises a driving module (6) and an active gain (4), a tunable filter (5), a first optical isolator (7), a polarization controller (8), a first optical circulator (9), a high nonlinear optical fiber (1), an optical coupler (2), and a third optical circulator (3) connected in sequence along an optical path; the driving module (6) is used to drive the tunable filter (5) to select a light beam of a preset wavelength from the wide spectrum background light of the active gain (4); the light beam sequentially passes through the first optical isolator (7), the polarization controller (8), the first optical circulator (9), the high nonlinear optical fiber (1), the optical coupler (2), and the third optical circulator (3) to realize unidirectional laser oscillation output in the outer ring cavity; when the central wavelength of the tunable filter (5) changes, the laser output wavelength will also change; the optical fiber coupler is used for light beam coupling, and the optical fiber coupler is connected to an external spectrometer to observe the characteristics of the output light.

3. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 2, characterized in that: The active gain (4) provides gain through a semiconductor optical amplifier or a rare earth active doped optical fiber.

4. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 2, characterized in that: The inner ring semi-open ring random laser cavity comprises a Rayleigh enhanced feedback optical fiber (11), a second optical isolator and a second optical circulator (10); the high nonlinear optical fiber (1), an optical coupler (2), a third optical circulator (3), the second optical circulator (10) and the first optical circulator (9) are connected in sequence along an optical path; the Rayleigh enhanced feedback optical fiber (11) is connected to two ports of the second optical circulator (10), and the Rayleigh enhanced feedback optical fiber (11) is also connected to the second optical isolator.

5. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 4, characterized in that: The Rayleigh enhanced feedback optical fiber (11) adopts polarization-maintaining fiber grating or highly doped and highly scattering special optical fiber.

6. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 4, characterized in that: The output end of the Rayleigh enhanced feedback optical fiber (11) is connected to a second optical isolator (12), and the second optical isolator (12) is used to isolate the interference caused by Fresnel reflection at the optical fiber joint to achieve cavity-free mode laser oscillation output.

7. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 4, characterized in that: The formula for Brillouin random fiber laser linewidth compression in Rayleigh enhanced feedback fiber (11) is: Where Δv P is the pump linewidth, L r Equivalent to the length of the gain fiber L g Adding the effective length of the Rayleigh scattered fiber represents the speed of light propagation in vacuum, n is the refractive index of the optical fiber, R r is the Rayleigh feedback coefficient, Δυ B represents the spectral width of the intrinsic Brillouin gain spectrum in the optical fiber, is the line width of the Brillouin random fiber laser, α is the line width correction coefficient, β is the Rayleigh scattering line width narrowing factor, and ln is the natural logarithm.

8. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 4, characterized in that: Light transmitted in the optical circulator can only pass unidirectionally from port one to port two and from port two to port three; the Stokes light in the inner ring semi-open ring random laser cavity can only be transmitted from port two to port three of the first optical circulator to the second optical circulator, the Stokes light is unidirectionally transmitted to the Rayleigh enhanced feedback optical fiber (11) after port one to port two of the second optical circulator, and the feedback light after Rayleigh enhancement can only return to the high nonlinear optical fiber (1) from port two to port three of the second optical circulator and from port one to port two of the third optical circulator.

9. The narrow-linewidth compact tunable light source based on Brillouin random fiber laser according to claim 4, characterized in that: After the pump light injected into the highly nonlinear optical fiber (1) reaches the Brillouin threshold, a stimulated Brillouin scattering effect occurs, generating Stokes light that is transmitted in the opposite direction to the pump light.

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