Linear-cavity narrow-linewidth thulium-doped fiber laser

Abstract

The invention provides a narrow-linewidth thulium-doped fiber laser with a linear cavity, and relates to the technical field of lasers. A first input end of a beam combiner in the laser is connected with a laser diode, a second input end of the beam combiner is connected with a fiber bragg grating through an input end and a straight-through end of a first optical fiber coupler, and an output end of the beam combiner is connected with a composite annular cavity through a thulium-doped optical fiber and an input end and a straight-through end of a second optical fiber coupler in sequence; the coupling end of the second optical fiber coupler is connected with the straight-through end of the third optical fiber coupler; the coupling end of the first optical fiber coupler is connected with the input end of the third optical fiber coupler through a delay optical fiber; and the coupling end of the third optical fiber coupler forms the output end of the linear cavity narrow linewidth thulium-doped optical fiber laser. The fiber bragg grating, the composite ring cavity and self-injection locking are combined, the optimal noise suppression and line width compression effects are achieved, meanwhile, the device is simple, the manufacturing cost is low, and the system is compact.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more particularly to a linear cavity narrow linewidth thulium-doped fiber laser. Background Technology

[0002] Narrow-linewidth single-mode fiber lasers, with their strong coherence, excellent anti-interference performance, and ease of integration, are widely used in fields such as surgery, coherent optical communication, distributed fiber optic sensing, spectral analysis, and lidar. In particular, eye-safe single-mode fiber lasers operating in the 2μm band have shown great potential in applications such as laser medicine and free-space optical communication.

[0003] In existing technologies, narrow-linewidth single-longitudinal-mode fiber lasers suffer from high laser noise, high manufacturing costs, and poor system compactness, failing to meet practical application requirements. Summary of the Invention

[0004] This invention provides a linear cavity narrow linewidth thulium-doped fiber laser to solve the problems of high laser noise, high manufacturing cost, and poor system compactness of existing narrow linewidth single-longitudinal-mode fiber lasers.

[0005] In a first aspect, embodiments of the present invention provide a linear cavity narrow linewidth thulium-doped fiber laser, comprising: a laser diode, a beam combiner, a thulium-doped fiber, a delay fiber, a fiber Bragg grating, a composite ring cavity, a first fiber coupler, a second fiber coupler, and a third fiber coupler. The first input end of the beam combiner is connected to the laser diode, the second input end of the beam combiner is connected to the fiber Bragg grating through the input end and the straight end of the first fiber coupler, and the output end of the beam combiner is connected to the composite ring cavity through the thulium-doped fiber and the input end and the straight end of the second fiber coupler in sequence. The coupling end of the second fiber coupler is connected to the straight-through end of the third fiber coupler; The coupling end of the first fiber coupler is connected to the input end of the third fiber coupler via a delay fiber. The coupling end of the third fiber coupler forms the output end of a linear cavity narrow-linewidth thulium-doped fiber laser.

[0006] Optionally, the first fiber optic coupler is a 1×2 type coupler; The input end of the first fiber coupler is connected to the second input end of the combiner, and the through end of the first fiber coupler is connected to the fiber Bragg grating.

[0007] Optionally, the coupling ratio of the first fiber coupler is 90:10.

[0008] Optionally, the second fiber optic coupler is a 1×2 type coupler; The input end of the second fiber coupler is connected to the composite ring cavity, and the through end of the second fiber coupler is connected to the thulium-doped fiber.

[0009] Optionally, the coupling ratio of the second fiber coupler is 60:40.

[0010] Optionally, the first fiber optic coupler is a 1×2 type coupler; The through end of the first fiber coupler is connected to the second input end of the combiner, and the input end of the first fiber coupler is connected to the fiber Bragg grating.

[0011] Optionally, the second fiber optic coupler is a 1×2 type coupler; The through end of the second fiber coupler is connected to the composite ring cavity, and the input end of the second fiber coupler is connected to the thulium-doped fiber.

[0012] Optionally, the composite ring cavity includes: a circulator, a fourth fiber coupler, a fifth fiber coupler and a sixth fiber coupler, a first fiber, a second fiber and a third fiber; The first input terminal of the fourth fiber coupler is connected to the third terminal of the circulator, the second input terminal of the fourth fiber coupler is connected to the second output terminal of the sixth fiber coupler, and the first output terminal of the fourth fiber coupler is connected to the first input terminal of the fifth fiber coupler through the first fiber. The second input terminal of the fifth fiber coupler is connected to the first output terminal of the sixth fiber coupler through the third fiber, the first output terminal of the fifth fiber coupler is connected to the first input terminal of the sixth fiber coupler through the second fiber, and the second output terminal of the fifth fiber coupler is connected to the first terminal of the circulator. The second end of the circulator is connected to the second fiber optic coupler; The circulator conducts unidirectionally in the order of the first end, the second end, and the third end.

[0013] Optionally, the coupling ratio of the fourth fiber coupler is 90:10, and the coupling ratios of the fifth and sixth fiber couplers are both 80:20. The first optical fiber is 1.05m long, the second optical fiber is 1.7m long, and the third optical fiber is 0.63m long.

[0014] Optionally, the length of the delay fiber is 100m.

[0015] This invention provides a linear cavity narrow linewidth thulium-doped fiber laser, comprising: a laser diode, a combiner, a thulium-doped fiber, a delay fiber, a fiber Bragg grating, a composite ring cavity, a first fiber coupler, a second fiber coupler, and a third fiber coupler. The first input end of the combiner is connected to the laser diode; the second input end of the combiner is connected to the fiber Bragg grating via the input end and the through end of the first fiber coupler; the output end of the combiner is connected to the composite ring cavity via the thulium-doped fiber, the input end of the second fiber coupler, and the through end of the second fiber coupler; the coupling end of the second fiber coupler is connected to the through end of the third fiber coupler; the coupling end of the first fiber coupler is connected to the input end of the third fiber coupler via the delay fiber; and the coupling end of the third fiber coupler forms the output end of the linear cavity narrow linewidth thulium-doped fiber laser. In this application, the fiber Bragg grating and the composite ring cavity work together to ensure single-mode output; simultaneously, combined with self-injection locking technology, the linewidth is compressed by the delay fiber to reduce noise, achieving low-noise, narrow-linewidth, single-mode output, with a simple structure, low cost, and compact system. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of a linear cavity narrow linewidth thulium-doped fiber laser provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of another linear cavity narrow linewidth thulium-doped fiber laser provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of another linear cavity narrow linewidth thulium-doped fiber laser provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of four self-injected locking feedback source ports provided in the embodiments of the present invention; Figure 5 This is provided by the embodiments of the present invention. Figure 4 A comparison chart of the output spectra corresponding to the four connection methods shown; Figure 6 This is a comparison of the output spectra under two injection paths provided in the embodiments of the present invention; Figure 7 This is a comparison diagram of the output spectra of the second fiber coupler provided in this embodiment of the invention at different coupling ratios; Figure 8 This is a comparison diagram of the output power of the second fiber coupler provided in this embodiment of the invention under different coupling ratios; Figure 9 This is a schematic diagram of a composite annular cavity provided in an embodiment of the present invention; Figure 10 This is a diagram showing the RIN variation for different delay fiber lengths provided in an embodiment of the present invention; Figure 11The frequency fluctuation power spectral density and linewidth under conditions without self-injection locking and different delay fiber lengths; Figure 12 This is a comparison chart of laser spectra with and without self-injection locking. Detailed Implementation

[0017] To enable those skilled in the art to better understand this solution, the technical solutions in the embodiments of this solution will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this solution, not all of them. Based on the embodiments of this solution, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this solution.

[0018] The term "comprising" and any other variations thereof in the specification, claims, and accompanying drawings of this invention mean "including but not limited to," and are intended to cover a non-exclusive inclusion, not limited to the examples listed herein. Furthermore, the terms "first" and "second," etc., are used to distinguish different objects, not to describe a specific order.

[0019] The implementation of the present invention will be described in detail below with reference to the accompanying drawings: Figures 1-3 This is a schematic diagram of a linear cavity, narrow linewidth, thulium-doped fiber TDF laser provided as an embodiment of the present invention. (Refer to...) Figures 1-3 The linear cavity narrow linewidth thulium-doped fiber TDF laser includes: a laser diode (LD), a beam combiner (FC), a thulium-doped fiber (TDF), a delay fiber, a fiber Bragg grating (UFBG), a composite ring cavity, a first fiber coupler (OC1), a second fiber coupler (OC2), and a third fiber coupler (OC3). The first input end of the combiner FC is connected to the laser diode LD. The second input end of the combiner FC is connected to the fiber Bragg grating UFBG through the input end and the through end of the first fiber coupler OC1. The output end of the combiner FC is connected to the composite ring cavity through the thulium-doped fiber TDF and the input end and the through end of the second fiber coupler OC2 in sequence. The coupling end of the second fiber coupler OC2 is connected to the through end of the third fiber coupler OC3; The coupling end of the first fiber coupler OC1 is connected to the input end of the third fiber coupler OC3 through a delay fiber. The coupling end of the third fiber coupler OC3 forms the output end of the linear cavity narrow linewidth thulium-doped fiber TDF laser.

[0020] A laser diode (LD) (e.g., 793 nm) is used as a pump source to inject pump light into a thulium-doped fiber (TDF) through a beam combiner (FC). After absorbing the pump light energy, the thulium-doped fiber (TDF) forms a population inversion, providing a gain basis for laser generation.

[0021] A fiber Bragg grating (UFBG) and a composite ring cavity form a linear resonant cavity. The UFBG is the core wavelength selection element, exhibiting high reflectivity only for specific wavelengths of light, while other wavelengths experience transmission loss, thus locking the laser oscillation wavelength. The amplified spontaneous emission light, after being amplified in a thulium-doped fiber (TDF), enters the composite ring cavity via a second fiber coupler (OC2). Part of the light, after mode selection within the ring cavity, is fed back to the through end of the second fiber coupler (OC2) to continue participating in the main resonance of the linear cavity. Simultaneously, the optical signal in the main resonant cavity enters the first fiber coupler (OC1) via the UFBG. A portion of the light split off from its coupling end undergoes dispersion modulation and phase matching via a delay fiber, then passes through the through end of the third fiber coupler (OC3) and merges with the light from the coupling end of the second fiber coupler (OC2). Finally, the light is fed back to the combiner (FC) and, together with the pump light from the laser diode (LD), is injected again into the incident end of the thulium-doped fiber (TDF), forming a linear cavity resonant cycle.

[0022] The composite ring cavity can introduce mode interference effect, suppress multi-longitudinal mode oscillation, and only allow longitudinal modes that meet the ring cavity phase matching condition to participate in resonance, thus compressing the laser linewidth.

[0023] The introduction of delay fiber can adjust the intracavity dispersion characteristics. On the one hand, it can compensate for the dispersion of thulium-doped fiber (TDF) and avoid pulse broadening or waveform distortion. On the other hand, by extending the photon lifetime, it can further filter out side modes, improve the longitudinal mode rejection ratio, and achieve narrow linewidth output.

[0024] This application combines the wavelength locking of fiber Bragg gratings (UFBG), the mode selection of a composite ring cavity, and the synergistic effect of delay fiber to effectively suppress multi-longitudinal-mode oscillations and achieve narrow-linewidth laser output.

[0025] In this application, the fiber Bragg grating (UFBG) and the composite ring cavity work together for filtering to ensure single longitudinal mode output. At the same time, the self-injection locking technology is combined with the delay fiber to compress the linewidth and reduce noise, so as to achieve low noise, narrow linewidth, single longitudinal mode output, and simple structure, low cost and compact system.

[0026] In one possible implementation, refer to Figure 1 The first fiber optic coupler OC1 is a 1×2 type coupler; The input end of the first fiber coupler OC1 is connected to the second input end of the combiner FC, and the through end of the first fiber coupler OC1 is connected to the fiber Bragg grating UFBG.

[0027] In one possible implementation, refer to Figure 2 The first fiber optic coupler OC1 is a 1×2 type coupler; The through end of the first fiber coupler OC1 is connected to the second input end of the combiner FC, and the input end of the first fiber coupler OC1 is connected to the fiber Bragg grating UFBG.

[0028] The first fiber coupler OC1 is a 1×2 type coupler, with the through end and the coupling end on one side. The first fiber coupler OC1 can be arranged in either a forward or reverse orientation. The laser at the input end of the third fiber coupler OC3 can come from four output ports (Output1~Output4). (See reference...) Figure 4 : Second port (Output2): The input end of the first fiber coupler OC1 is connected to the second input end of the combiner FC, and the straight end is connected to the fiber Bragg grating UFBG. The coupling port serves as the second port. Fourth port (Output4): The through end of the first fiber coupler OC1 is connected to the second input end of the combiner FC, and the input end is connected to the fiber Bragg grating UFBG. The coupling port serves as the fourth port. The two ports on the left side of the fiber Bragg grating (UFBG) are designated as the first port (Output1) and the third port (Output3).

[0029] The noise characteristics of the four connection methods were quantified. Under the same pump power, the output spectrum of each port was measured using an optical spectrum analyzer (OSA), and its noise floor and optical signal-to-noise ratio (OSNR) were extracted. The results are as follows: Figure 5 Experimental results show that the first port (Output1) and the third port (Output3) on the left side of the fiber Bragg grating (UFBG) have high noise and are unsuitable as feedback source locations for self-injection locking. Meanwhile, the second port (Output2) at the front of the fiber Bragg grating (UFBG) (i.e., the location described in this application)... Figure 1 The embodiment shown has the lowest noise floor and the highest OSNR (>70.25 dB), and is therefore preferably a self-injection locked feedback source. After being re-injected into the main cavity, the laser from this path can re-enter the composite annular cavity for secondary filtering, thereby further suppressing ASE noise and mode competition, and significantly improving the spectral purity and stability of the output laser.

[0030] In one possible implementation, the coupling ratio of the first fiber coupler OC1 is 90:10.

[0031] Of this, 10% of the laser output is used for self-injection locking, injected into the third fiber coupler OC3, and then re-injected into the main cavity through the second fiber coupler OC2. The 10% feedback laser power is sufficient to support the normal operation of the reference optical path, ensuring effective calibration without the inability to achieve the desired result due to insufficient reference optical power.

[0032] In one possible implementation, refer to Figure 1 The second fiber optic coupler OC2 is a 1×2 type coupler; The input end of the second fiber coupler OC2 is connected to the composite ring cavity, and the through end of the second fiber coupler OC2 is connected to the thulium-doped fiber TDF.

[0033] In one possible implementation, refer to Figure 3 The second fiber optic coupler OC2 is a 1×2 type coupler; The through end of the second fiber coupler OC2 is connected to the composite ring cavity, and the input end of the second fiber coupler OC2 is connected to the thulium-doped fiber TDF.

[0034] The third fiber coupler OC3 can be arranged in either the forward or reverse direction, see reference. Figure 1 and Figure 3 After the laser exits from the third fiber coupler OC3, it is re-injected into the main cavity. It can be injected from the left side or the right side of the third fiber coupler OC3.

[0035] Will Figure 1 The path shown is defined as Path1, and... Figure 3 The path shown is defined as Path2. Figure 6 The output spectra are shown for two injection paths. Figure 6 It can be seen that the laser spectral intensity and optical signal-to-noise ratio corresponding to Path1 are higher than those of Path2. This is because the laser under this path can re-enter the annular cavity after being injected into the main cavity, thereby achieving further suppression of noise.

[0036] In one possible implementation, the coupling ratio of the second fiber coupler OC2 can be 60:40.

[0037] The output performance of the second fiber coupler OC2 was tested sequentially at coupling ratios of 90:10, 80:20, 70:30, 60:40, and 50:50 between port 1 and port 2. The output spectra at each coupling ratio were recorded using a high-resolution spectrometer, and the output power was measured simultaneously. The results are as follows: Figure 7 and Figure 8As shown in the figure. Experiments revealed that when the coupling ratio is 60:40, the 3dB bandwidth of the output laser is the narrowest, and the noise floor is the lowest. Simultaneously, the output power reaches its maximum at this coupling ratio, resulting in optimal system stability. Further reducing the coupling ratio of port 1 (e.g., 50:50) leads to a significant deterioration in laser stability. Therefore, based on a comprehensive evaluation of spectral characteristics, noise level, and output power, the optimal coupling ratio for the second fiber coupler OC2 was ultimately determined to be 60:40. This optimization process ensures the best balance between noise suppression and power output in the self-injection locking system.

[0038] In one possible implementation, refer to Figure 1 The composite ring cavity may include: a circulator CIR, a fourth fiber coupler OC4, a fifth fiber coupler OC5 and a sixth fiber coupler OC6, a first fiber L1, a second fiber L2 and a third fiber L3. The first input terminal of the fourth fiber coupler OC4 is connected to the third terminal of the circulator CIR, the second input terminal of the fourth fiber coupler OC4 is connected to the second output terminal of the sixth fiber coupler OC6, and the first output terminal of the fourth fiber coupler OC4 is connected to the first input terminal of the fifth fiber coupler OC5 through the first fiber L1. The second input terminal of the fifth fiber coupler OC5 is connected to the first output terminal of the sixth fiber coupler OC6 through the third fiber L3. The first output terminal of the fifth fiber coupler OC5 is connected to the first input terminal of the sixth fiber coupler OC6 through the second fiber L2. The second output terminal of the fifth fiber coupler OC5 is connected to the first terminal of the circulator CIR. The second end of the circulator CIR is connected to the second fiber coupler OC2; The circulator CIR conducts unidirectionally in the order of the first end, the second end, and the third end.

[0039] Figure 9 The schematic diagram of the composite ring cavity is shown, which consists of three couplers forming two rings.

[0040] The first sub-loop consists of the path 5-11-9-7-5, and the second sub-loop consists of the path 2-4-6-8-10-12-2.

[0041] In one possible implementation, the coupling ratio of the fourth fiber coupler OC4 can be 90:10, and the coupling ratios of the fifth fiber coupler OC5 and the sixth fiber coupler OC6 can both be 80:20. The first optical fiber L1 can be 1.05m, the second optical fiber L2 can be 1.7m, and the third optical fiber L3 can be 0.63m.

[0042] In one possible implementation, the length of the delay fiber can be 100m.

[0043] The output light of the fiber laser provided in this application is converted into an electrical signal by a photodetector and then connected to a signal analyzer and an oscilloscope with an impedance of 50Ω to obtain the relative intensity noise. Figure 10 The relative intensity noise curves of the fiber laser are shown under the same pump power, without self-injection locking, and with delay fiber lengths of 20m, 60m, and 100m. Without self-injection locking, the laser's relaxation oscillation frequency is 71.2kHz. As the delay fiber length increases, the relaxation oscillation frequency shifts towards lower frequencies, while the relative intensity noise gradually decreases. When the delay fiber length increases from 60m to 100m, the low-frequency shift of the relaxation oscillation frequency exhibits saturation characteristics. Experimental results show that further increasing the delay fiber length gradually weakens the modulation effect on the relaxation oscillation frequency. When the delay fiber length is 100m, the relaxation oscillation frequency can be reduced to 10.4 kHz, and in the frequency range above 1MHz, the lower limit of the relative intensity noise drops to approximately -139.2 dB / Hz.

[0044] Furthermore, using an unbalanced Michelson interferometer based on a 3×3 coupler, the frequency fluctuation power spectral density of the output laser was measured under different delay fiber lengths. Combined with the β-segmentation method, the linewidth values ​​at different integration times were calculated, and the results are as follows: Figure 11 As shown. During the measurement process, system physical vibration, low-frequency signal interference, and thermal noise introduced by cladding pump all affect the results, and the extension of the integration time also leads to a corresponding increase in the measured linewidth. Therefore, the linewidth measured at a shorter integration time is closer to the inherent linewidth of the laser. This application selects the linewidth data at an integration time of 0.001s for illustration. Under the same pump power conditions, the linewidth measured without self-injection locking is 8.82kHz; with self-injection locking technology, and with delay fiber lengths of 20m, 60m, and 100m, the corresponding linewidth values ​​are 3.88 kHz, 2.31 kHz, and 1.40 kHz, respectively. These experimental results clearly show that as the delay fiber length increases, the compression effect of self-injection locking technology on the laser linewidth becomes more significant.

[0045] However, this compression effect tends to saturate as the length of the delay fiber increases further. When the length of the delay fiber reaches 100m, the linewidth compression ratio can reach -7.99 dB. Figure 12The spectral characteristics of a self-injected locked laser with a 100m delay fiber were compared with those of a conventional laser without self-injection locking. It can be seen that after applying self-injection locking technology, the laser's noise floor is significantly suppressed, and the spectral bandwidth is narrowed, while the center wavelength and peak power fluctuation levels of the laser remain essentially the same as those of the laser without self-injection locking.

[0046] From the above, it can be seen that, for reference Figure 1 The preferred embodiment shown in this application provides a linear cavity narrow linewidth thulium-doped fiber TDF laser with the following advantages: 1. Ultra-high optical signal-to-noise ratio: The output laser has an ultra-high optical signal-to-noise ratio of 70.25dB, which is far higher than that of similar 2μm band lasers, making it suitable for high-sensitivity detection and communication systems.

[0047] 2. Extremely low intensity noise: In the frequency range of 0~1 MHz, the relative intensity noise is reduced to -139.2 dB / Hz, which is more than 15 dB lower than the state without self-injection lock-in, and is superior to the existing technology.

[0048] 3. Narrow linewidth output: The laser linewidth is compressed from 8.82 kHz to 1.40 kHz (integration time 0.001 s), with a compression ratio of -7.99 dB. The linewidth narrowing effect is significant and superior to most reported 2μm lasers (such as 3.21 kHz and 9.1 MHz).

[0049] 4. Stable single longitudinal mode operation: Through the synergistic effect of fiber Bragg grating (UFBG) and composite ring cavity, stable single longitudinal mode output is achieved with wavelength fluctuation less than 0.02 nm and power fluctuation less than 0.683 dB, making it suitable for long-term stable working environment.

[0050] 5. Simple structure and low cost: It adopts an all-fiber linear cavity structure, eliminating the need for complex external cavities or expensive filters. The system is compact, easy to integrate, and suitable for industrial and practical deployment.

[0051] The linear cavity narrow-linewidth thulium-doped fiber TDF laser provided in this application is a single-longitudinal-mode laser with narrow linewidth and low relative intensity noise, making it ideal for spectral analysis, especially in high-precision spectral measurements. For example, the laser source can be used as the laser light source in a spectrometer for accurately measuring the absorption and emission spectra of substances. This laser exhibits high frequency stability and low noise, providing more accurate spectral linewidths and clearer spectra, thereby improving the accuracy of spectral analysis.

[0052] In fiber optic communication systems, the linear cavity narrow-linewidth thulium-doped fiber TDF laser provided in this application can be used as a light source for long-distance data transmission. Narrow-linewidth lasers can improve the bandwidth and signal quality of communication systems, especially in high-speed optical communications where low noise and high stability are critical, such as quantum communication and long-distance fiber optic communication.

[0053] The linear cavity narrow-linewidth thulium-doped fiber TDF laser provided in this application can be used in lidar systems, especially in fields such as autonomous driving, terrain surveying, and environmental monitoring. Its narrow linewidth and low relative intensity noise characteristics help improve the ranging accuracy and resolution of lidar systems, particularly in applications requiring high-precision measurement and long-range detection.

[0054] In the medical field, particularly in laser surgery and treatment, the linear cavity narrow-linewidth thulium-doped fiber TDF laser provided in this application is highly suitable for use in medical devices such as laser therapy and laser cutting due to its low noise and stability. Because of its excellent stability and narrow linewidth characteristics, this laser can provide a high-precision laser beam, reducing tissue damage and improving surgical outcomes.

[0055] In distributed fiber optic sensing applications, such as temperature, stress, and pressure monitoring systems, the linear cavity narrow-linewidth thulium-doped fiber TDF laser provided in this application can serve as a light source to provide a highly stable laser signal. Its low-noise characteristics enable the sensor to detect even weaker signals, improving the sensitivity and accuracy of the sensing system.

[0056] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A linear cavity narrow linewidth thulium-doped fiber laser, characterized in that, include: Laser diode, beam combiner, thulium-doped fiber, delay fiber, fiber Bragg grating, composite ring cavity, first fiber coupler, second fiber coupler, and third fiber coupler; The first input end of the beam combiner is connected to the laser diode, the second input end of the beam combiner is connected to the fiber Bragg grating through the input end and the through end of the first fiber coupler, and the output end of the beam combiner is connected to the composite ring cavity through the thulium-doped fiber and the input end and the through end of the second fiber coupler in sequence. The coupling end of the second fiber coupler is connected to the through end of the third fiber coupler; The coupling end of the first fiber coupler is connected to the input end of the third fiber coupler via the delay fiber; The coupling end of the third fiber coupler forms the output end of the linear cavity narrow linewidth thulium-doped fiber laser.

2. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 1, characterized in that, The first fiber optic coupler is a 1×2 type coupler; The input end of the first fiber coupler is connected to the second input end of the combiner, and the through end of the first fiber coupler is connected to the fiber Bragg grating.

3. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 2, characterized in that, The coupling ratio of the first fiber coupler is 90:

10.

4. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 2, characterized in that, The second fiber optic coupler is a 1×2 type coupler; The input end of the second fiber coupler is connected to the composite ring cavity, and the through end of the second fiber coupler is connected to the thulium-doped fiber.

5. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 4, characterized in that, The coupling ratio of the second fiber coupler is 60:

40.

6. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 1, characterized in that, The first fiber optic coupler is a 1×2 type coupler; The through end of the first fiber coupler is connected to the second input end of the combiner, and the input end of the first fiber coupler is connected to the fiber Bragg grating.

7. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 1, characterized in that, The second fiber optic coupler is a 1×2 type coupler; The through end of the second fiber coupler is connected to the composite ring cavity, and the input end of the second fiber coupler is connected to the thulium-doped fiber.

8. The linear cavity narrow linewidth thulium-doped fiber laser according to any one of claims 1 to 7, characterized in that, The composite ring cavity includes: a circulator, a fourth fiber coupler, a fifth fiber coupler, a sixth fiber coupler, a first fiber, a second fiber, and a third fiber; The first input terminal of the fourth fiber optic coupler is connected to the third terminal of the circulator, the second input terminal of the fourth fiber optic coupler is connected to the second output terminal of the sixth fiber optic coupler, and the first output terminal of the fourth fiber optic coupler is connected to the first input terminal of the fifth fiber optic coupler through the first fiber optic cable. The second input terminal of the fifth fiber coupler is connected to the first output terminal of the sixth fiber coupler through the third fiber, the first output terminal of the fifth fiber coupler is connected to the first input terminal of the sixth fiber coupler through the second fiber, and the second output terminal of the fifth fiber coupler is connected to the first terminal of the circulator. The second end of the circulator is connected to the second optical fiber coupler; The circulator is unidirectionally guided in the order of the first end, the second end, and the third end.

9. The linear cavity narrow linewidth thulium-doped fiber laser as described in claim 8, characterized in that, The coupling ratio of the fourth fiber coupler is 90:10, and the coupling ratios of the fifth and sixth fiber couplers are both 80:

20. The first optical fiber is 1.05m long, the second optical fiber is 1.7m long, and the third optical fiber is 0.63m long.

10. The linear cavity narrow linewidth thulium-doped fiber laser according to any one of claims 1 to 7, characterized in that, The length of the delay fiber is 100m.

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