A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective subcavity

Abstract

A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity comprises a semiconductor laser, a wavelength division multiplexer, a rare-earth ion doped fiber, an optical isolator, a polarization controller, two optical circulators, an unpumped rare-earth ion doped fiber, a fiber Bragg grating, two 1x2 optical couplers, two displacement stages, two 2x2 optical couplers, a fiber mirror and an output coupler. The two 1x2 optical couplers form a Mach-Zehnder structure; the two 2x2 optical couplers form a fiber loop structure. The two structures are combined with the fiber mirror to form a reflective sub-ring cavity, achieving single-longitudinal-mode narrow-linewidth output. The arm of the Mach-Zehnder structure is fixed at both ends on the two displacement stages, and the arm length is changed by fixing one displacement stage and moving the other displacement stage to apply tension, so as to tune the central wavelength of the laser. The present application can effectively reduce the filtering bandwidth, has wavelength tunability, and is suitable for long-distance sensing, precise spectroscopy and laser radar fields.

Description

Technical Field

[0001] This invention relates to a tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity, which is a tunable single-longitudinal-mode narrow-linewidth fiber laser based on sub-ring cavity filtering of fiber mirrors, fiber Mach-Zehnder structures and fiber ring structures, and belongs to the field of fiber optic communication. Background Technology

[0002] The commercialization of low-loss single-mode fiber and the emergence of fiber doping technology have propelled the development of fiber lasers. Compared to traditional solid-state lasers, fiber lasers possess unique shielding structures and better optical immunity; their collimation can be achieved simply through fusion splicing or flange connections, and they offer better beam regulation and constraint, resulting in higher output beam quality. Furthermore, fiber lasers offer numerous advantages such as excellent heat dissipation, ease of integration, low cost, and high conversion efficiency, making them widely used in lidar, laser ranging, optical sensing, and spectroscopy.

[0003] The traveling-wave ring cavity structure of a single-longitudinal-mode narrow-linewidth fiber laser can effectively suppress multi-longitudinal-mode oscillations caused by the spatial hole-burning effect. Currently, when using silicon-based micro-ring filters to realize a single-longitudinal-mode narrow-linewidth fiber laser, fiber-chip coupling increases alignment complexity, and the filtering effect is limited by the micro-ring device fabrication process; while using a traditional fiber cascaded double-ring structure will result in limited filtering bandwidth.

[0004] In view of the above-mentioned technical problems, an improved technical solution is proposed. Summary of the Invention

[0005] The purpose of this invention is to design the arm length of the Mach-Zehnder structure, the cavity length of the fiber ring, the reflectivity of the fiber mirror, and the coupling ratio of each coupler in a reflective sub-ring cavity to achieve single-longitudinal-mode narrow-linewidth filtering in a fiber laser. Utilizing the principle of Mach-Zehnder interference spectrum generation, the arm length difference of one arm of the Mach-Zehnder structure is changed by pulling it, thereby modulating the peak position of the interference spectrum, ultimately achieving tunable single-longitudinal-mode narrow-linewidth erbium-doped fiber laser output.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity is provided. The ring cavity fiber laser consists of a semiconductor laser, a wavelength division multiplexer, a rare-earth-doped fiber, an optical isolator, a polarization controller, a first optical circulator, a second optical circulator, an unpumped rare-earth-doped fiber, a fiber Bragg grating, a first 1×2 optical coupler, a second 1×2 optical coupler, a first displacement stage, a second displacement stage, a first 2×2 optical coupler, a second 2×2 optical coupler, a fiber optic mirror, and an output coupler.

[0008] The connections between the components are as follows: the semiconductor laser is connected to one input port of the wavelength division multiplexer (WDM); the output port of the WDM is connected to one end of the rare-earth-doped fiber; the input port of the optical isolator is connected to one remaining port of the WDM; the input of the polarization controller is connected to the output of the optical isolator; the first port of the first optical circulator is connected to the output of the squeezed polarization controller; the second port of the first optical circulator is connected to one end of the unpumped rare-earth-doped fiber; the other end of the unpumped rare-earth-doped fiber is connected to the fiber Bragg grating; the third port of the first optical circulator is connected to the first port of the second optical circulator; the second port of the second optical circulator is connected to the input of the first 1×2 optical coupler; the two outputs of the first 1×2 optical coupler are respectively connected to the two inputs of the second 1×2 optical coupler to form a Mach-Zehnder structure; one arm of the Mach-Zehnder structure uses a first displacement stage and a second... The displacement stage is fixed and can be stretched by moving the second displacement stage. The output end of the second 1×2 optical coupler is connected to the input end of the first 2×2 optical coupler. The two output ends of the first 2×2 optical coupler are respectively connected to the two input ends of the second 2×2 optical coupler to form an optical fiber ring structure. The output end of the second 2×2 optical coupler is connected to the optical fiber mirror. The third port of the second optical circulator is connected to the input end of the output coupler. The first output end of the output coupler is connected to the other end of the rare earth ion-doped optical fiber. The second output end of the output coupler serves as the total output end of the laser, outputting a single longitudinal mode narrow linewidth laser. Among them, the free spectral range and spectral width of the interference spectrum of a single Mach-Zehnder structure are inversely proportional to the difference in arm length, and the free spectral range and spectral width of the interference spectrum of a single optical fiber ring structure are inversely proportional to the ring length. Therefore, it is difficult to achieve a single longitudinal mode and narrow linewidth of the fiber laser using a single optical fiber filter structure. This invention expands the free spectral range and achieves side-mode suppression by cascading a single fiber ring structure using a Mach-Zehnder structure. However, the side-mode suppression and linewidth compression capabilities of cascading two filter structures are limited, and multiple cascaded structures not only suffer from size mismatch but also significant energy loss. Therefore, this invention doubles the number of filters using fiber mirrors, effectively improving the main peak linewidth compression. Mechanical tuning of the reflective sub-ring cavity in this invention enables stable, wide-spectral-range, continuously tunable fiber laser output, which is advantageous for long-distance sensing.

[0009] Furthermore, the light guiding direction of the optical isolator described in this invention is opposite to the propagation direction of the pump light of the semiconductor laser, so as to ensure the purity of the output spectrum of the tunable single-longitudinal-mode narrow-linewidth erbium-doped fiber laser.

[0010] Furthermore, the free spectral ranges of the Mach-Zehnder structure output spectrum and the fiber ring structure output spectrum should not differ too much, so as to achieve the effect of suppressing side modes by the vernier effect.

[0011] Furthermore, the reflectivity of the fiber optic mirror described in this invention should be as high as possible to avoid energy loss due to reflection, which would reduce the output slope efficiency of the tunable single-mode narrow-linewidth erbium-doped fiber laser. To avoid energy loss due to reflection, the reflectivity of the fiber optic mirror should be close to 1.

[0012] Furthermore, the rare-earth-doped fiber described in this invention should have an appropriate length to avoid insufficient inversion particle number due to excessive length and excessive intrinsic loss due to excessive length, which would ultimately cause the single-longitudinal-mode narrow-linewidth erbium-doped fiber laser to have difficulty outputting laser light.

[0013] Furthermore, the polarization controller described in this invention can actively compensate for random birefringence effects within the cavity, stabilize the polarization state at the start of oscillation, and suppress mode competition related to polarization.

[0014] Furthermore, the intracavity / extracavity output coupling ratio of the output coupler described in this invention should be appropriate to ensure that the laser oscillation gain is greater than the loss threshold condition.

[0015] Furthermore, the light guiding direction of the first and second optical circulators of the present invention is the same as that of the optical isolator, both being counterclockwise.

[0016] Furthermore, the fiber Bragg grating described in this invention can achieve high reflectivity narrowband reflection across the gain spectrum wavelength range; the unpumped rare-earth-doped fiber, as a saturable absorber, achieves anti-saturable absorption of the light intensity distribution corresponding to the standing wave effect, thereby enhancing the output stability of single-longitudinal-mode narrow-linewidth lasers.

[0017] The working principle of the tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity described in this invention is as follows: A semiconductor laser, acting as a pump source, enters the ring cavity through a wavelength division multiplexer, causing the rare-earth-doped fiber to absorb the pump light, generating population inversion and providing optical amplification. An optical isolator provides a counterclockwise propagation direction for this optical amplification, and a polarization controller stabilizes the polarization state and suppresses mode competition related to polarization. The first circulator guides light counterclockwise, and the fiber Bragg grating achieves efficient reflection of light of a specific wavelength. The unpumped rare-earth-doped fiber acts as a saturable absorber, enhancing the output stability of the single-longitudinal-mode narrow-linewidth laser. The second circulator guides light counterclockwise, allowing the amplified light to enter the reflective sub-ring cavity for filtering. The reflective sub-ring cavity includes a Mach-Zehnder structure, a fiber ring structure, and a fiber mirror, which are cascaded sequentially. The vernier effect expands the free spectral range of the filtering structure, and the fiber mirror doubles the number of times the amplified light passes through the Mach-Zehnder and fiber ring filters, suppressing side modes with low reflection loss. This optical amplification is continuously transmitted within the ring cavity until a balance is reached between saturation gain and total loss per turn, ultimately achieving single-longitudinal-mode narrow-linewidth laser output via an output coupler. The first displacement stage is fixed, and the arm length difference of the Mach-Zehnder structure can be increased by moving the second displacement stage, thereby altering the filtering center wavelength of the reflective sub-ring cavity and achieving tunable single-longitudinal-mode narrow-linewidth laser center wavelength.

[0018] The advantages and positive effects of the tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity provided by this invention compared with the prior art are as follows:

[0019] A reflective sub-cavity filtering method is proposed, which consists of a Mach-Zehnder structure, an optical fiber ring structure, and an optical fiber mirror structure. Simulations were performed on the optimized Mach-Zehnder structure and the optical fiber ring structure, considering the coupling ratio of each coupler and the fiber length of each part, achieving a good filtering effect.

[0020] Mechanical tuning of the Mach-Zehnder structure in the reflective sub-ring cavity not only ensures high stability of the system, but also guarantees the narrow linewidth characteristics of the single longitudinal mode of the output fiber laser and the continuous tunability of the wavelength.

[0021] The fiber Bragg grating used has high reflectivity, and its reflection bandwidth and center wavelength were optimized through simulation. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity.

[0023] Figure 2 This is a schematic diagram of a reflective sub-ring cavity structure.

[0024] Figure 3 The electric field distribution of the fundamental transverse mode in a single-mode optical fiber.

[0025] Figure 4 This describes the single-frequency output principle of a tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity.

[0026] Figure 5 This represents the reflection spectrum of a fiber Bragg grating.

[0027] Figure 6 This is the filtered transmission spectrum of the reflective sub-ring cavity.

[0028] Figure 7 This is a comparison of the main peaks of the transmission spectra of a reflective sub-ring cavity, a Mach-Zehnder structure, and a fiber ring structure cascaded together.

[0029] Figure 8 To adjust the change of the main peak of the transmission spectrum of the reflective sub-ring cavity when the arm length difference of the Mach-Zehnder structure is adjusted.

[0030] Figure 9 The relationship between the position of the main peak in the transmission spectrum of the reflective sub-ring cavity and different arm length differences in the Mach-Zehnder structure.

[0031] In the diagram: 1. Semiconductor laser; 2. Wavelength division multiplexer; 3. Rare-earth-doped fiber; 4. Optical isolator; 5. Polarization controller; 6. First optical circulator; 7. Unpumped rare-earth-doped fiber; 8. Fiber Bragg grating; 9. Second optical circulator; 10. First 1×2 optical coupler; 11. Second 1×2 optical coupler; 12. First 2×2 optical coupler; 13. Second 2×2 optical coupler; 14. Fiber optic mirror; 15. First displacement stage; 16. Second displacement stage; 17. Output coupler.

[0032] 61. First optical circulator port I; 62. First optical circulator port II; 63. First optical circulator port III; 91. Second optical circulator port I; 92. Second optical circulator port II; 93. Second optical circulator port III. Detailed Implementation

[0033] The following are specific embodiments of the present invention, further illustrating the invention. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms fall within the scope defined by the appended claims.

[0034] like Figure 1As shown, this embodiment provides a tunable single-longitudinal-mode narrow-linewidth erbium-doped fiber laser with a reflective sub-ring cavity, comprising a semiconductor laser 1, a wavelength division multiplexer 2, a section of rare-earth-doped fiber 3, an optical isolator 4, a polarization controller 5, a first optical circulator 6, a second optical circulator 9, a section of unpumped rare-earth-doped fiber 7, a section of high-reflectivity fiber Bragg grating 8, a first 1×2 optical coupler 10, a second 1×2 optical coupler 11, a first displacement stage 15, a second displacement stage 16, a first 2×2 optical coupler 12, a second 2×2 optical coupler 13, a fiber optic mirror 14, and an output coupler 17.

[0035] Semiconductor laser 1 is connected to one port of wavelength division multiplexer 2 to provide pump for the ring cavity fiber laser; the output port of wavelength division multiplexer 2 is connected to one end of rare-earth-doped fiber 3; the input port of optical isolator 4 is connected to one remaining port of wavelength division multiplexer 2; the input of squeezed polarization controller 5 is connected to the output of optical isolator 4; the first port 61 of the first optical circulator 6 is connected to the output of polarization controller 5; the second port 62 of the first optical circulator 6 is connected to one end of unpumped rare-earth-doped fiber 7; the other end of unpumped rare-earth-doped fiber 7 is connected to high-reflectivity fiber Bragg grating 8; the third port 63 of the first optical circulator 6 is connected to the first port 91 of the second optical circulator 9; and the second port 9... Port 92 is connected to the input of the first 1×2 optical coupler 10. The two outputs of the first 1×2 optical coupler 10 are respectively connected to the two inputs of the second 1×2 optical coupler 11. The output of the second 1×2 optical coupler 11 is connected to the input of the first 2×2 optical coupler 12. The two outputs of the first 2×2 optical coupler 12 are respectively connected to the two inputs of the second 2×2 optical coupler 13. The output of the second 2×2 optical coupler 13 is connected to the fiber optic mirror 14. The third port 93 of the second optical isolator 9 is connected to the input of the output coupler 17. The first output of the output coupler 17 is connected to the other end of the rare earth-doped fiber 3. The second output of the output coupler 17 serves as the total output of the laser, outputting a single longitudinal mode narrow linewidth laser.

[0036] This embodiment uses an erbium-doped fiber laser as an example, where both the rare-earth-doped fiber 3 and the unpumped rare-earth-doped fiber 7 are erbium-doped fibers. The center wavelength of the output laser is around 1550 nm. The erbium-doped fiber laser achieves single-longitudinal-mode narrow-linewidth laser output based on a reflective sub-ring cavity filter composed of a Mach-Zehnder structure, a fiber ring structure, and a fiber mirror. A schematic diagram of the reflective sub-ring cavity is shown below. Figure 2As shown, the two input and output ports of the first 1×2 optical coupler 10 and the second 1×2 optical coupler 11 are connected to form a Mach-Zehnder structure. One arm of the Mach-Zehnder structure is fixed by the first displacement stage 15 and the second displacement stage 16, and this arm can be stretched by moving the second displacement stage 16. The two input and output ports of the first 2×2 optical coupler 12 and the second 2×2 optical coupler 13 are connected to form an optical fiber ring.

[0037] It should be clarified that the semiconductor laser 1 of the present invention is not limited to the above-mentioned erbium-doped fiber laser, and its output wavelength is variable. For example, in the erbium-doped fiber laser, its output wavelength can be 915nm or 976nm; in the ytterbium-doped fiber laser, its output wavelength can be 980nm or 1480nm; and in the thulium-doped fiber laser, its output wavelength can be 793nm or 1569nm. The wavelength division multiplexer has a wavelength division multiplexing function of "semiconductor laser output wavelength / fiber laser output wavelength".

[0038] In this embodiment, the cladding refractive index n of the single-mode fiber is set. clad The core refractive index n is 1.444. core The core diameter is d = 1.467. core It is 9μm. For example... Figure 3 As shown, the effective refractive index n of the fundamental transverse mode propagating in the optical fiber at a center wavelength of 1550 nm is obtained by solving COMSOL software. eff It is 1.457.

[0039] like Figure 4 The diagram shows the single-frequency output principle of a tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity. The explanation is as follows: Rare-earth-doped fiber can achieve broadband gain, meaning that without further filtering of the ring cavity laser, the final output laser will contain multiple longitudinal modes. After adding a fiber grating for filtering, the 3dB bandwidth of the fiber grating's reflection spectrum is smaller than the gain curve bandwidth but larger than the longitudinal mode spacing, thus reducing the number of multiple longitudinal modes output by the ring cavity laser. When a sub-ring cavity is added, the 3dB bandwidth of the sub-ring cavity's transmission spectrum is smaller than the longitudinal mode spacing, thus achieving single-longitudinal-mode output. Furthermore, during the above filtering process, the filter spectral lines at each stage are not flat, therefore the single-longitudinal-mode linewidth is also narrowed step by step. To obtain a single-longitudinal-mode narrow-linewidth output in the kHz to Hz range, unpumped rare-earth-doped fiber is needed as a saturable absorber to suppress the low-intensity frequency portion of the single-longitudinal-mode light.

[0040] Assuming the main cavity length L of the ring cavity fiber laser is 10m, the dn can be obtained in COMSOL software. eff / dλ is 1×10 - 5 nm -1According to the longitudinal mode spacing formula (1), the longitudinal mode spacing Δν is 20.37MHz.

[0041] (1)

[0042] According to the Bragg condition formula for optical fiber (2):

[0043] (2)

[0044] Formula (3) for the center wavelength reflectivity of a fiber Bragg grating (FBG):

[0045] (3)

[0046] FBG 3dB bandwidth formula (4):

[0047] (4)

[0048] Where λ is the center wavelength, ˄ is the grating period, p is the refractive index modulation depth, and L B The grating length is 10 mm; with a grating period of 531.9 nm, a grating length of 10 mm, and a refractive index modulation depth of 2 × 10⁻⁶. -4 The reflection spectrum of the FBG was calculated as follows: Figure 5 As shown, a reflectivity close to 1 can be achieved at 1550nm. The 3dB bandwidth of the FBG is 0.25nm, corresponding to a bandwidth of 31.23GHz. This means that the longitudinal modes of multiple ring cavity lasers can be reflected by the FBG. Therefore, further filtering is needed to achieve single longitudinal mode output.

[0049] Furthermore, to ensure that the FBG filter channel contains only one sub-ring cavity with an effective filter passband and a dominant main resonance peak, the designed free spectral range (FSR) of the sub-ring cavity needs to be between 0.5 and 1 times the 3dB bandwidth of the FBG filter spectrum. To guarantee that the longitudinal mode spacing of the single-mode filtered ring cavity laser is greater than half the bandwidth of the main resonance peak of the sub-ring cavity filter, the designed free spectral range of the sub-ring cavity filter spectrum needs to be between 0.125 nm and 0.25 nm, and the bandwidth of the main resonance peak of the sub-ring cavity filter spectrum needs to be less than 10.185 MHz (0.0816 nm).

[0050] The transmission spectrum of the Mach-Zehnder structure is shown in equation (5):

[0051] (5)

[0052] Where a1 is the strength coupling loss factor of the 1×2 coupler, b1 is the fusion splicing loss factor between the two 1×2 couplers, k1 and k2 are the coupling coefficients of the two 1×2 couplers, σ1 is the fiber attenuation coefficient, δ=2πneff / λ is the fiber transmission coefficient, and h1 and h2 are the lengths of the two arms of the Mach-Zehnder structure.

[0053] The transmission spectrum of the fiber optic ring structure is shown in equation (6):

[0054] (6)

[0055] Where a2 is the strength coupling loss factor of the 2×2 coupler, b2 is the fusion splicing loss factor of the fiber ring, k3 and k4 are the coupling coefficients of the two 2×2 couplers, σ2 is the fiber attenuation coefficient, and h3 and h4 are the lengths of the left and right parts of the fiber ring structure, respectively.

[0056] The values ​​of each parameter used in the calculation are shown in the table below:

[0057] <![CDATA[a1]]> 0.02 <![CDATA[a2]]> 0.02 <![CDATA[b1]]> 0.01 <![CDATA[b2]]> 0.01 <![CDATA[k1]]> 0.5 <![CDATA[b2]]> 0.5 <![CDATA[k2]]> 0.5 <![CDATA[k4]]> 0.5 <![CDATA[σ1]]> 0.2dB / km <![CDATA[σ2]]> 0.2dB / km <![CDATA[h1]]> 0.51m <![CDATA[h3]]> 0.25 <![CDATA[h2]]> 0.5m <![CDATA[h4]]> 0.25

[0058] Since the reflectivity of fiber optic mirrors on the market is generally close to 1, such as the fiber optic mirrors produced by Xiaoxiao Photonics Technology Co., Ltd. with a reflectivity of up to 0.995, we set the reflectivity to 0.99 in the calculation.

[0059] At this point, the transmission spectrum of the reflective sub-ring cavity is as shown in formula (7):

[0060] (7)

[0061] like Figure 6 As shown, the dark curve represents the transmission spectrum of a reflective sub-ring cavity composed of a Mach-Zehnder structure, a fiber ring structure, and a fiber mirror structure. In this case, the transmission spectrum of the sub-ring cavity composed of the Mach-Zehnder structure and the fiber ring structure has the same FSR as that of the sub-ring cavity, which is 0.16 nm.

[0062] like Figure 7 As shown, when the transmission spectrum peak at the center wavelength of 1550nm is further magnified, the 3dB width of the transmission spectrum peak of the reflective sub-ring cavity is 0.45pm, and the 3dB width of the transmission spectrum peak of the sub-ring cavity composed of Mach-Zehnder structure and fiber ring structure is 0.68pm. At this time, the reflective sub-ring cavity structure can compress the linewidth of the ring cavity laser to a greater extent.

[0063] For a handheld displacement stage with a total displacement length of 2.5cm, its movement accuracy can be guaranteed to be within 0.01mm. Figure 8As shown, the positions of the main peak of the transmission spectrum of the reflective sub-ring cavity were further calculated when the second displacement stage moved by 0.2 mm, 0.4 mm, 0.6 mm, and 0.8 mm. As the length of the upper arm of the Mach-Zehnder structure increases with the movement of the second displacement stage, the position of the main peak of the transmission spectrum redshifts accordingly.

[0064] like Figure 9 As shown, the relationship between the position of the main peak of the transmission spectrum and the moving distance of the displacement stage is correlated, and it is found that they conform to a linear relationship. Under the conditions of this embodiment, the slope is 0.08198. It can be found that the total moving length of the peak position is less than 0.08 nm, which does not exceed half of the 3dB bandwidth of the FBG reflection spectrum. Therefore, in this process, the output laser wavelength of the ring cavity fiber laser can be tunable.

[0065] The above embodiments are used to explain and illustrate the present invention, and not to limit it. Any modifications and changes made to the present invention within the spirit and scope of the claims fall within the protection scope of the present invention.

Claims

1. A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective subcavity, characterized in that, The ring cavity fiber laser comprises the following components: a semiconductor laser, a wavelength division multiplexer, a rare-earth-doped fiber, an optical isolator, a polarization controller, a first optical circulator, a second optical circulator, an unpumped rare-earth-doped fiber, a fiber Bragg grating, a first 1×2 optical coupler, a second 1×2 optical coupler, a first displacement stage, a second displacement stage, a first 2×2 optical coupler, a second 2×2 optical coupler, a fiber optic mirror, and an output coupler. The connections between these components are as follows: A semiconductor laser is connected to one port of a wavelength division multiplexer (WDM) to pump the ring-cavity fiber laser. The output port of the WDM is connected to one end of a rare-earth-doped fiber. The input port of an optical isolator is connected to one remaining port of the WDM. The input port of a polarization controller is connected to the output port of the optical isolator. The first port of a first optical circulator is connected to the output port of the polarization controller. The second port of the first optical circulator is connected to one end of an unpumped rare-earth-doped fiber. The other end of the unpumped rare-earth-doped fiber is connected to a fiber Bragg grating. The third port of the first optical circulator is connected to the first port of a second optical circulator, and the second port of the second optical circulator... The input terminal of the first 1×2 optical coupler is connected, the two output terminals of the first 1×2 optical coupler are respectively connected to the two input terminals of the second 1×2 optical coupler, the output terminal of the second 1×2 optical coupler is connected to the input terminal of the first 2×2 optical coupler, the two output terminals of the first 2×2 optical coupler are respectively connected to the two input terminals of the second 2×2 optical coupler, the output terminal of the second 2×2 optical coupler is connected to the fiber optic mirror, the third port of the second optical isolator is connected to the input terminal of the output coupler, the first output terminal of the output coupler is connected to the other end of the rare earth ion-doped fiber, and the second output terminal of the output coupler serves as the total output terminal of the laser, outputting a single longitudinal mode narrow linewidth laser. The first 1×2 optical coupler and the second 1×2 optical coupler are connected to form a Mach-Zehnder structure. One arm of the Mach-Zehnder structure is fixed by a first displacement stage and a second displacement stage, and the arm can be stretched by moving the second displacement stage. The first 2×2 optical coupler and the second 2×2 optical coupler are connected to form an optical fiber ring. The free spectral range and spectral width of a single Mach-Zehnder structure interference spectrum are inversely proportional to the difference in arm length, while the free spectral range and spectral width of a single fiber ring structure interference spectrum are inversely proportional to the ring length. 2.The tunable single-longitudinal-mode narrow-line-width fiber laser with a reflective subcavity according to claim 1, characterized in that: The wavelength division multiplexer described above has a wavelength division multiplexing function of "semiconductor laser output wavelength / fiber laser output wavelength". 3.The tunable single-longitudinal-mode narrow-line-width fiber laser with a reflective subcavity according to claim 1, characterized in that: The optical isolator's light guiding direction is opposite to the pump light propagation direction of the semiconductor laser to ensure the purity of the output spectrum of the tunable single-longitudinal-mode narrow-linewidth erbium-doped fiber laser. 4.The tunable single-longitudinal-mode narrow-line-width fiber laser with a reflective subcavity according to claim 1, characterized in that: The reflectivity of the fiber optic mirror should be close to 1.

5. A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity according to claim 1, characterized in that: The length of the rare-earth-doped fiber should be sufficient to ensure that a single-mode narrow-linewidth fiber laser can output laser light.

6. A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity according to claim 1, characterized in that: The polarization controller can actively compensate for random birefringence effects within the cavity, stabilize the polarization state at the start of oscillation, and suppress mode competition related to polarization.

7. A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity according to claim 1, characterized in that: The intracavity / extracavity output coupling ratio of the output coupler should be able to ensure that the laser oscillation gain is greater than the loss threshold condition.

8. A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity according to claim 1, characterized in that: The light guiding direction of the first and second optical circulators is the same as that of the optical isolator, both being counterclockwise.

9. A tunable single-longitudinal-mode narrow-linewidth fiber laser with a reflective sub-ring cavity according to claim 1, characterized in that: The fiber Bragg grating enables high reflectivity narrowband reflection across the gain spectrum wavelength range; the unpumped rare-earth-doped fiber, acting as a saturable absorber, achieves anti-saturable absorption of the intensity distribution corresponding to the standing wave effect, thereby enhancing the output stability of single-longitudinal-mode narrow-linewidth lasers.

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