Compact tunable narrow-linewidth fiber laser light source

Abstract

The invention discloses a compact tunable narrow-linewidth fiber laser light source, and belongs to the field of fiber lasers. The structure comprises a Brillouin pump and a Brillouin semi-open annular random fiber cavity. The Brillouin pump adopts a tunable laser and provides pump light for the Brillouin semi-open annular random optical fiber cavity. The erbium-doped optical fiber in the cavity serves as a nonlinear Brillouin gain medium and a linear gain medium at the same time, the narrowband gain of Stokes light is achieved through the stimulated Brillouin scattering effect in the optical fiber, laser output of different wavelengths can be achieved by adjusting the wavelength of Brillouin pump light and matching the center wavelength of the tunable filter, and the laser output of different wavelengths can be achieved. Random distributed feedback is introduced through the random fiber bragg grating to form random laser oscillation, the cavity length is greatly shortened, the line width is deeply compressed, and ultra-narrow line width tunable laser output is achieved while it is guaranteed that the structure of the laser light source is compact. According to the invention, the application of the tunable narrow linewidth laser in the aspects of optical fiber communication, biomedical imaging and the like is expanded.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fiber lasers, in particular to a compact tunable narrow linewidth fiber laser source. TECHNICAL BACKGROUND

[0002] Since the fiber laser was invented, it has been widely used in the fields of communication, sensing and medical treatment due to its high efficiency, compactness and good beam quality. Traditional fiber lasers include Brillouin fiber lasers (BFL) and Brillouin erbium-doped fiber lasers (BEFL), which have significant advantages in realizing ultra-narrow linewidth laser output. The Brillouin erbium-doped fiber laser is developed to overcome the harsh pump coupling resonance condition and high pump threshold of the Brillouin fiber laser. This laser combines the linear gain of the erbium-doped fiber and the Brillouin gain of the single-mode fiber, and has the advantages of ultra-narrow linewidth, low threshold and high power. However, due to the small Brillouin gain coefficient of the single-mode fiber, the Brillouin erbium-doped fiber laser needs a long single-mode fiber to ensure sufficient Brillouin gain. The long single-mode fiber leads to a Brillouin gain spectrum much larger than the free spectral range of the cavity, and there are many longitudinal mode patterns in the gain spectrum. Although the uniform broadening characteristics of the Brillouin gain can ensure that the laser mainly operates in a single longitudinal mode state, it is inevitable to be affected by environmental disturbances and jump modes.

[0003] The compact Brillouin erbium-doped fiber laser based on a few meters of ordinary erbium-doped fiber breaks through the limitation that traditional Brillouin erbium-doped fiber lasers cannot have both ultra-short cavities and low thresholds. The ultra-narrow linewidth output of the compact Brillouin erbium-doped fiber laser is achieved under the condition of a relatively large Brillouin pump linewidth, which has strong linewidth compression characteristics. It can convert low-coherence Brillouin pump light into high-coherence Brillouin Stokes laser. The compact Brillouin erbium-doped fiber laser overcomes the harsh pump coupling resonance condition of the Brillouin laser, while retaining the advantages of low threshold and relatively high output power of the traditional Brillouin erbium-doped fiber laser. It does not require additional laser frequency stabilization technology or special temperature control and vibration isolation conditions. The compact Brillouin erbium-doped fiber laser can achieve ultra-narrow linewidth, ultra-low phase noise and high-coherence laser output. Its tuning stability is better than that of the current ultra-narrow linewidth semiconductor laser, and its stable center frequency is better than that of the ultra-narrow linewidth erbium-doped fiber laser based on a saturable absorber. These excellent laser characteristics make it have a very broad application prospect in many coherent optical fields.

[0004] Random fiber laser has the characteristics of simple structure, low cost, high stability, ultra-narrow linewidth, arbitrary operating wavelength, anti-vibration and wide application. It realizes laser oscillation through distributed random feedback, does not need the traditional fixed resonant cavity structure, simplifies the design and manufacturing process, and does not need precise resonant cavity mirror, and the overall cost is low. Its good phase noise characteristics and stability make it suitable for long-time stable operation. By using the effect of Brillouin scattering, the random fiber laser can realize ultra-narrow linewidth laser output, and is suitable for high-precision measurement and communication. The distributed feedback mechanism such as Rayleigh scattering makes it have the advantage of arbitrary operating wavelength, and is suitable for ultra-wideband wavelength tuning. In addition, it is not sensitive to external vibration due to the absence of fixed cavity mirror structure, and is suitable for harsh environments. These characteristics make the random fiber laser have broad application prospects in the fields of optical fiber communication, optical fiber sensing and biomedical imaging.

[0005] The present application combines erbium-doped fiber and random fiber grating to realize the laser output of the Brillouin erbium-doped random fiber laser with hybrid gain mechanism and compact structure, and has the functions of single longitudinal mode laser output, narrow linewidth and tunability. SUMMARY

[0006] The present application mainly aims at the incompatibility between the compactness of the laser structure and the intensity of the random distributed feedback, and provides a compact tunable narrow linewidth fiber laser source. The erbium-doped fiber is used as the medium of nonlinear Brillouin gain and linear gain, which greatly reduces the cavity length and realizes the hybrid gain mechanism and compact laser structure. A random fiber grating is used to provide random distributed feedback, instead of the distributed feedback provided by Rayleigh scattering accumulated in the traditional long standard single-mode fiber, so that the laser has the characteristics of compact structure and low threshold. The source can not only realize stable laser output, but also has the characteristics of ultra-narrow gain bandwidth, simple structure, low cost, environmental disturbance resistance and the like, and has good application prospects in optical fiber sensing and optical communication transmission.

[0007] In order to achieve the above-mentioned purposes, the present application adopts the following inventive concept:

[0008] The basic idea of the present application is to realize the laser output with ultra-narrow linewidth and tunable by utilizing the Brillouin scattering effect and the distributed random feedback mechanism. The Brillouin scattering is a kind of nonlinear effect in the optical fiber, and the ultra-narrow linewidth laser can be generated through the stimulated Brillouin scattering (SBS). The light source utilizes the Brillouin scattering effect as the laser gain mechanism, and can realize the ultra-narrow linewidth laser output with the order of Hz. The traditional fiber laser relies on the fixed mirror feedback type optical resonant cavity for mode selection, while the light source adopts the distributed random feedback mechanism, and realizes the laser oscillation through the distributed feedback such as Rayleigh scattering in the optical fiber. The design not only simplifies the structure, but also significantly reduces the cost. The light source has the tunable ability, and can realize the wavelength tuning and sweep in the ultra-wideband range. Through the distributed feedback mechanism such as Rayleigh scattering, the light source can realize the stable laser output at any working wavelength, and is suitable for the application scenarios which need to frequently adjust the wavelength. Since there is no fixed cavity mirror structure, the light source is not sensitive to the external vibration, and is suitable for the harsh environment. In addition, through the optimization design, the central frequency of the light source has good stability, and is suitable for the long time stable operation. The basic structure includes the Brillouin pump and the Brillouin semi-open ring random fiber cavity.

[0009] The Brillouin pump is a tunable laser, and is used for providing the Brillouin pump light, and the output optical power of the Brillouin pump light is adjustable.

[0010] The Brillouin semi-open ring random fiber cavity contains a hybrid gain medium, a random fiber grating and a tunable filter.

[0011] The working process of the light source is as follows: the Brillouin pump light is injected into the hybrid gain medium, and the reverse transmission Stokes light is generated after the stimulated Brillouin scattering threshold is reached, and the Brillouin pump light is automatically separated; the reverse transmission Stokes light forms a one-way circulation path in the ring random fiber cavity: the Stokes light is reflected by the random fiber grating in the form of distributed feedback after the center wavelength is selected by the tunable filter, and then returns to the hybrid gain medium to form a cycle.

[0012] The Stokes light continuously receives the Brillouin gain and the linear gain amplification in the ring random fiber cavity, and when the gain exceeds the cavity loss, the Stokes light realizes the output through the port of the random fiber grating.

[0013] As a preferred embodiment of the present application, the hybrid gain medium adopts the erbium-doped optical fiber.

[0014] As a preferred embodiment of the present application, the 980nm pump and the wavelength division multiplexer are further included, the pump light emitted by the 980nm pump is coupled to the erbium-doped optical fiber through the wavelength division multiplexer, and is used for providing the linear gain amplification of the Stokes light.

[0015] As the preferred of the present application, the unidirectional circulation path is realized based on the first optical circulator and the second optical circulator; the first port of the first optical circulator is used for receiving the Brillouin pump light, the third port of the first optical circulator is connected with the first port of the second optical circulator through the tunable filter, the second port of the first optical circulator is connected with the first port of the wavelength division multiplexer, the second port of the wavelength division multiplexer is used for receiving the pump light emitted by the 980nm pump, the third port of the wavelength division multiplexer is connected with the third port of the second optical circulator through the erbium-doped optical fiber, and the second port of the second optical circulator is connected with the random fiber grating; in each optical circulator, the input light can only be transmitted from the first port to the second port and from the second port to the third port.

[0016] As the preferred of the present application, the polarization controller is further arranged between the tunable laser and the first port of the first optical circulator.

[0017] As the preferred of the present application, the random fiber grating is made by carving 5000-10000 refractive index modulation points in a standard single-mode fiber core plane by all.

[0018] As the preferred of the present application, the center wavelength of the tunable filter matches the wavelength of the Brillouin pump light, the laser output tuning is realized by adjusting the wavelength of the Brillouin pump light, and the tuning range covers the Brillouin frequency shift amount of the fiber material.

[0019] As the preferred of the present application, the output end of the random fiber grating is further provided with the optical isolator.

[0020] As the preferred of the present application, the line width of the finally output Stokes light is compressed to the order of Hz, and the output Stokes spectrum satisfies the Brillouin frequency shift amount of the fiber material under the frequency downshift of the pump light.

[0021] Compared with the prior art, the present application has the following obvious and substantial characteristics and advantages:

[0022] (1) The present application uses the erbium-doped optical fiber as the medium of the nonlinear Brillouin gain and the linear gain, and the mixed gain mechanism can make the laser have the characteristics of the ultrashort cavity and the low threshold, can realize the effect of the ultranarrow line width compression, and can make the laser work at any wavelength by adjusting the wavelength of the Brillouin pump and matching the center wavelength of the corresponding filter, so that the output Stokes spectrum satisfies the Brillouin frequency shift amount of the fiber material under the frequency downshift of the pump light, and the output Stokes tunable narrow line width laser has excellent coherent performance.

[0023] (2) The application adopts random fiber grating to provide random distributed feedback, which can eliminate the longitudinal mode of fixed cavity and realize single longitudinal mode laser output. Due to the small Brillouin gain coefficient and Rayleigh backscattering coefficient, the prerequisite for using intrinsic Rayleigh scattering as random distributed feedback is that the long gain fiber accumulates sufficient gain and feedback intensity, resulting in a bulky laser structure. The random fiber grating can replace the intrinsic Rayleigh scattering to provide random distributed feedback, which can greatly shorten the cavity length and make the laser structure compact. The random distributed feedback can work at any wavelength, and can realize wide-range tunable laser output.

[0024] (3) The application adopts a semi-open ring random cavity structure built by optical fibers. The structure fully utilizes the characteristics of reverse transmission of Stokes light, realizes a large sweep range by adjusting the pump light wavelength and matching the center wavelength of the filter, and excites high-coherence Stokes random laser with reverse transmission without additional narrowband filter for residual pump light filtering operation, so that the structure is simple and compact, and the cost is low. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a display diagram of a compact tunable narrow linewidth fiber laser source implemented by the application.

[0026] Figure 2 It is the reflection spectrum of a random fiber grating and a standard single-mode optical fiber.

[0027] Figure 3 It is the output spectrum of a Brillouin erbium-doped random fiber laser.

[0028] Figure 4 It is the output power variation of a Brillouin erbium-doped random fiber laser.

[0029] Figure 5 It is the wavelength tuning characteristics of a Brillouin erbium-doped random fiber laser.

[0030] Figure 6 In (a), it is the delay self-heterodyne beat spectrum of BP and a Brillouin erbium-doped random fiber laser, and (b) is the delay self-heterodyne beat spectrum of a Brillouin erbium-doped random fiber laser.

[0031] In the drawings, the reference numerals specifically represent: 1, tunable laser; 2, polarization controller; 3, first optical circulator; 4, wavelength division multiplexer; 5, 980 nm pump; 6, erbium-doped fiber; 7, tunable filter; 8, second optical circulator; 9, random fiber grating; 10, optical isolator. DETAILED DESCRIPTION

[0032] The preferred examples of the present application will be described below in conjunction with the accompanying drawings, which make the technical content clearer and easier to understand. The specific examples described below are only used to explain the present application, but the present application is not limited to the specific embodiments described below. The present application can also be implemented in many different forms of examples without departing from the purpose of the present application and the scope protected by the claims.

[0033] The compact tunable narrow linewidth fiber laser source provided by the present application combines a wide-spectrum tunable laser as the Brillouin pump light and a Brillouin semi-open ring random fiber cavity to excite high-coherence Stokes tunable laser output, which can ensure that the output laser has the characteristics of narrow linewidth and high stability. Those skilled in the art can further optimize the length of the gain fiber, the type of the fiber, and the type of the feedback mechanism in the Brillouin semi-open random fiber cavity to adjust the power, threshold, efficiency, linewidth, noise, and other performance of the output laser of the present application.

[0034] The above scheme will be further described below in conjunction with specific examples.

[0035] Referring to Figure 1 A compact tunable narrow linewidth fiber laser source mainly includes a Brillouin pump and a Brillouin semi-open ring random fiber cavity structure.

[0036] The Brillouin pump is a tunable laser 1, which is used to provide Brillouin pump light, and the output optical power of which is adjustable.

[0037] The Brillouin semi-open ring random fiber cavity contains a hybrid gain medium, a random fiber grating 9, and a tunable filter 7. The hybrid gain medium simultaneously serves as the medium of Brillouin gain and linear gain.

[0038] In an embodiment, a 10m-long erbium-doped fiber is used as the hybrid gain medium of nonlinear Brillouin gain and linear gain. Random distributed feedback is provided by the random fiber grating 9, and the low-coherence Brillouin pump light is converted into high-coherence Brillouin Stokes laser by combining stimulated Brillouin scattering and laser gain provided by the erbium-doped fiber, thereby achieving significant compression of the linewidth of the Stokes laser.

[0039] The random fiber grating 9 is made by using a femtosecond laser to engrave about 9600 refractive index modulation points in the fiber core of a 3 cm long standard single-mode fiber (SSMF) plane by plane, can eliminate the longitudinal mode of the fixed cavity, and realizes single longitudinal mode laser output. Since the Brillouin gain coefficient and the Rayleigh backscattering coefficient are very small, using intrinsic Rayleigh scattering as a random distributed feedback requires a long gain fiber to accumulate sufficient gain and feedback strength, resulting in a bulky laser structure. Using a random fiber grating to replace intrinsic Rayleigh scattering to provide random distributed feedback can not only provide high-intensity distributed feedback, but also greatly shorten the fiber length and realize a compact laser source structure. Using the characteristic of random distributed feedback that can work at any wavelength, a wide range of tunable laser output can be realized.

[0040] In one specific embodiment of the present application, a wide-spectrum tunable laser 1 is used as a Brillouin pump, and the output optical power of the Brillouin pump can be adjusted according to the actual requirements of the laser source. The pump light passes through a 10 m erbium-doped fiber 6 to provide nonlinear Brillouin gain for Stokes laser. At the same time, a 980 nm pump 5 is used as a pump source, and the pump light emitted by the 980 nm pump 5 is coupled to the erbium-doped fiber 6 through a wavelength division multiplexer 4 to provide linear gain amplification for Stokes light. The bandwidth of the tunable filter is 0.15 nm, which is used to select the center wavelength of the Stokes light and filter out the residual amplified spontaneous emission. By adjusting the wavelength of the Brillouin pump and matching the center wavelength of the tunable filter, tunable output of random laser in a wide range can be realized.

[0041] The working process of the light source is as follows: the Brillouin pump light is injected into the mixed gain medium, and after the stimulated Brillouin scattering threshold is reached, the reverse transmission Stokes light is generated and automatically separated from the Brillouin pump light; the reverse transmission Stokes light forms a unidirectional circulation path in the annular random fiber cavity: after the residual amplified spontaneous emission and the central wavelength of the Stokes light are filtered by the tunable filter 7, the Stokes light is reflected by the random fiber grating 9 in the form of distributed feedback, and then returns to the mixed gain medium to form a circulation. In the embodiment, the unidirectional circulation path is realized based on the first optical circulator 3 and the second optical circulator 8, the first port of the first optical circulator 3 is used for receiving the Brillouin pump light, the third port of the first optical circulator 3 is connected to the first port of the second optical circulator 8 through the tunable filter 7, the second port of the first optical circulator 3 is connected to the first port of the wavelength division multiplexer 4, the second port of the wavelength division multiplexer 4 is used for receiving the pump light emitted by the 980 nm pump 5, the third port of the wavelength division multiplexer 4 is connected to the third port of the second optical circulator 8 through the erbium-doped fiber 6, and the second port of the second optical circulator 8 is connected to the random fiber grating 9. Due to the characteristics of the optical circulator, the light transmitted therein can only be transmitted unidirectionally from the ① port to the ② port and from the ② port to the ③ port, therefore, the Stokes laser in the random cavity cannot be transmitted from the ③ port of the second optical circulator 8 to the ① port, nor can it be transmitted from the ③ port of the first optical circulator to the ② port, so it cannot form a traditional closed annular resonant cavity.

[0042] The Brillouin semi-open annular random fiber cavity is realized by the random distributed feedback of the random fiber grating 9, the erbium-doped fiber 6 serving as the medium of the Brillouin gain and the linear gain, the tunable filter 7 matching the central wavelength, and the first optical circulator 3 and the second optical circulator 8.

[0043] The semi-open annular cavity structure mentioned in the application utilizes the characteristic that the Stokes light is transmitted in the opposite direction relative to the pump light in the stimulated Brillouin scattering effect. The pump light injected into the gain fiber occurs the stimulated Brillouin effect after reaching the Brillouin threshold, and the Stokes light is generated and transmitted in the opposite direction to the pump light, and the frequency difference between them is described as the Brillouin frequency shift; the Stokes light is continuously amplified by the Brillouin gain and the linear gain in the annular cavity, and when the gain exceeds the cavity loss, the Stokes laser can be output through the port of the random fiber grating. Since the Stokes laser propagates in the opposite direction to the pump light, it can be automatically separated from the input pump light, and the pump light does not resonate, so it does not need to satisfy the precise frequency regulation and can be arbitrarily tuned, thus meeting the tunable demand, and after the Stokes laser is output, no additional filter device is needed to filter the residual pump light, and the device structure is compact, simple and easy to operate.

[0044] The output end of the Brillouin semi-open annular random fiber cavity comprises an optical isolator 10 to reduce the interference of the Fresnel reflection of the output end surface on the random cavity and form a single longitudinal mode laser.

[0045] The complete principle of the present application is as follows:

[0046] The erbium-doped fiber 6 is used as the medium of Brillouin gain and linear gain, the tunable laser 1 is used as the Brillouin pump, the polarization state of the Brillouin pump light emitted by the tunable laser 1 is controlled by the polarization controller 2, and then the Brillouin pump light enters the wavelength division multiplexer through the ① to ② ports of the first optical circulator 3, the pump light emitted by the 980 nm pump 5 enters the wavelength division multiplexer 4, and the two kinds of light are coupled to the erbium-doped fiber 6. The Brillouin pump light transmitted along the 10 m erbium-doped fiber 6 is amplified, and after reaching the Brillouin threshold, the stimulated Brillouin effect occurs, generating Stokes light transmitted in the opposite direction of the pump light (defined as backward transmission). The backward transmission Stokes first enters the tunable filter 7 through the ② to ③ ports of the first optical circulator 3, then passes through the ① to ② ports of the second optical circulator 8 after selecting the center wavelength by the tunable filter 7, is reflected in the form of distributed feedback by the random fiber grating 9, and then circulates back to the erbium-doped fiber 6 through the ② to ③ ports of the second optical circulator 8. In this way, the Stokes light can continuously obtain nonlinear Brillouin gain from the interaction with the Brillouin pump light and linear gain from the power boost of the 980 nm laser pumped erbium-doped fiber 6, realizing the random laser oscillation of Stokes. When the gain is greater than the cavity loss, the Stokes light can pass through the port of the random fiber grating 9, and the output is realized through the optical isolator 10. The optical isolator 10 can reduce the influence of the Fresnel reflection of the output fiber end face on the random cavity, so as to avoid the formation of multi-longitudinal-mode laser oscillation.

[0047] Due to the unidirectional transmission characteristics of the optical circulator, the Stokes light transmitted in the cavity cannot pass through the ③ port to the ① port of the second optical circulator 8, and cannot pass through the ③ port to the ② port of the first optical circulator 3 to form a traditional closed ring resonant cavity, avoiding multi-longitudinal-mode oscillation. The laser light source proposed in the present application has strong linewidth compression characteristics, and can convert low-coherence pump light into high-coherence Brillouin Stokes laser, thereby realizing narrow-linewidth laser output.

[0048] In order to verify the performance of the laser light source, broadband amplified spontaneous emission is injected by using an erbium-doped fiber amplifier, and the reflection spectrum of the random fiber grating and the 25 km SSMF is characterized, and the results are shown in Figure 2 The average reflectivity of the random fiber grating in the wavelength range of 1545 nm to 1555 nm is about -20 dB, which is about 12 dB higher than that of the 25 km SSMF. Near 1549 nm, the reflectivity of the random fiber grating can reach about -10 dB, indicating that the random fiber grating can provide efficient distributed feedback.

[0049] The output spectrum of the Brillouin Erbium-doped random fiber laser at different 980-nm pump powers was measured by using a spectrum analyzer, as shown in FIG. 2. Figure 3 The output spectrum contains the Rayleigh scattering and the Brillouin Stokes light induced by the BP. With the increase of the 980-nm pump power, the intensity of the Rayleigh scattering light is basically unchanged, while the intensity of the Brillouin Stokes light gradually increases from zero. The wavelength of the BP is 1548.920 nm, and the wavelength of the Brillouin Stokes light is 1549.004 nm, with a difference of 0.084 nm, corresponding to the Brillouin frequency shift of the EDF.

[0050] The output power of the Brillouin Erbium-doped random fiber laser at different BP powers was measured, as shown in FIG. 3. Figure 4 After exceeding the pump threshold, the output power of the Brillouin Erbium-doped random fiber laser linearly increases with the 980-nm pump power. The threshold of the laser is low, and when the BP power is 1.58 mW, the threshold is about 170 mW. With the increase of the BP power, the output optical power of the laser decreases, the threshold of the 980-nm pump increases, but the laser efficiency basically remains unchanged, about 2.34%.

[0051] The wavelength tuning characteristics of the Brillouin Erbium-doped random fiber laser were measured, as shown in FIG. 4. Figure 5 By changing the wavelength of the BP, the output laser wavelength of the Brillouin Erbium-doped random fiber laser changes, and the laser has good wavelength tuning and sweeping ability.

[0052] The line width of the BP and the Brillouin Erbium-doped random fiber laser was measured by using the delay heterodyne beat frequency method. The measured heterodyne beat frequency spectrum is shown in FIG. 5. Figure 6 (a) is the delay heterodyne beat frequency spectrum of the BP and the Brillouin Erbium-doped random fiber laser, and (b) is the delay heterodyne beat frequency spectrum of the Brillouin Erbium-doped random fiber laser. The 3dB line width of the BP is 215.1 kHz, and the 3dB line width of the Brillouin Erbium-doped random fiber laser is 892 Hz. Due to the combined effects of the narrow-band Brillouin gain spectrum, the linear gain of the EDF and the distributed random feedback, the line width of the Brillouin Erbium-doped random fiber laser is compressed by two orders of magnitude compared with the BP.

[0053] The above describes the embodiments of the present application with reference to the drawings, but the present application is not limited to the above embodiments, and can be changed in many ways according to the purpose of the present application. Any change, modification, replacement, combination or simplification made according to the spirit and principle of the present application shall be an equivalent replacement, as long as it meets the purpose of the present application and does not deviate from the technical principle and concept of the present application.

Claims

1. A compact tunable narrow linewidth fiber laser source, characterized in that, The light source comprises a Brillouin pump and a Brillouin semi-open ring random fiber cavity. The Brillouin pump is a tunable laser (1) for providing Brillouin pump light, the output light power of which is adjustable. The Brillouin semi-open ring random fiber cavity comprises a hybrid gain medium, a random fiber grating (9) and a tunable filter (7), and the hybrid gain medium simultaneously serves as a medium for Brillouin gain and linear gain. In the working process of the light source, the Brillouin pump light is injected into the hybrid gain medium, and after reaching the threshold of stimulated Brillouin scattering, the reverse transmission Stokes light is automatically separated from the Brillouin pump light; the reverse transmission Stokes light forms a one-way circulation path in the ring random fiber cavity: the Stokes light is selected by the tunable filter (7) in terms of center wavelength, is reflected by the random fiber grating (9) in the form of distributed feedback, and then returns to the hybrid gain medium to form a circulation. The Stokes light continuously receives the amplification of the Brillouin gain and the linear gain in the ring random fiber cavity, and when the gain exceeds the cavity loss, the Stokes light is output through the port of the random fiber grating (9).

2. The compact tunable narrow linewidth fiber laser source of claim 1, wherein, The hybrid gain medium adopts an erbium-doped fiber (6).

3. The compact tunable narrow linewidth fiber laser source of claim 2, wherein, The light source further comprises a 980nm pump (5) and a wavelength division multiplexer (4), the pump light emitted by the 980nm pump (5) is coupled to the erbium-doped fiber (6) through the wavelength division multiplexer (4), and is used for providing linear gain amplification of the Stokes light.

4. The compact tunable narrow linewidth fiber laser source of claim 3, wherein, The one-way circulation path is realized based on a first optical circulator (3) and a second optical circulator (8); the first port of the first optical circulator (3) is used for receiving the Brillouin pump light, the third port of the first optical circulator (3) is connected to the first port of the second optical circulator (8) through the tunable filter (7), the second port of the first optical circulator (3) is connected to the first port of the wavelength division multiplexer (4), the second port of the wavelength division multiplexer (4) is used for receiving the pump light emitted by the 980nm pump (5), the third port of the wavelength division multiplexer (4) is connected to the third port of the second optical circulator (8) through the erbium-doped fiber (6), and the second port of the second optical circulator (8) is connected to the random fiber grating (9); in each optical circulator, the input light can only be transmitted from the first port to the second port and from the second port to the third port.

5. The compact tunable narrow linewidth fiber laser source of claim 4, wherein, A polarization controller (2) is further arranged between the tunable laser (1) and the first port of the first optical circulator (3).

6. The compact tunable narrow linewidth fiber laser source of claim 1, wherein, The random fiber grating (9) is made by plane-by-plane scribing 5000-10000 refractive index modulation points in a standard single-mode fiber core through a femtosecond laser.

7. The compact tunable narrow linewidth fiber laser source of claim 1, wherein, The center wavelength of the tunable filter (7) matches the wavelength of the Brillouin pump light, the laser output is tuned by adjusting the wavelength of the Brillouin pump light, and the tuning range covers the Brillouin frequency shift of the fiber material.

8. The compact tunable narrow linewidth fiber laser source of claim 1, wherein, An optical isolator (10) is further arranged at the output end of the random fiber grating (9).

9. The compact tunable narrow linewidth fiber laser source of claim 1, wherein, The line width of the finally output Stokes light is compressed to the order of Hz, and the output Stokes spectrum satisfies the Brillouin frequency shift of the fiber material under the frequency downshift of the pump light.

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