2μm band tunable single-frequency fiber laser

Abstract

A 2μm band tunable single-frequency fiber laser, a ring cavity laser, includes a pump source, a wavelength division multiplexer, a thulium-holmium co-doped fiber, a mode field adapter, a composite sub-ring filter, a tunable fiber filter, an isolator, and a coupler. The pump source is a 1950nm band holmium ion co-band pump source, which pumps the thulium-holmium co-doped fiber. When the 1950nm band holmium ion co-band pump source pumps the thulium-holmium co-doped fiber, it forms a two-level system structure, which weakens the thulium ion cross-relaxation intensity while improving the energy conversion efficiency, thereby achieving suppression of self-pulses.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and in particular to a 2μm band tunable single-frequency fiber laser. Background Technology

[0002] Tunable single-frequency fiber lasers in the 2μm band, with their superior characteristics such as tunable wavelength, narrow spectral linewidth, and long coherence length, have broad application prospects in cutting-edge fields such as atmospheric trace gas composition measurement, high-precision laser detection, and biomedical diagnosis and treatment requiring specific wavelength matching. 2μm band fiber lasers typically use thulium-doped fiber or thulium-holmium co-doped fiber as the gain medium. Compared to pure thulium-doped fiber, thulium-holmium co-doped fiber, as a gain medium optimized for 2μm band lasers, can extend the wavelength to a longer band (2.05μm~2.15μm). The main emission peak of thulium-doped fiber is located at ~1.94 μm, making it difficult to break through the 2.05μm band; while thulium-holmium co-doped fiber can cover 2.05μm~2.15μm, filling the long-wavelength gap that thulium-doped fiber cannot efficiently cover. This band is not only safe for human eyes, but also has atmospheric transmission windows (such as low water vapor absorption at 2.1 μm), and has important application prospects in fields such as lidar, space communication and mid-infrared pump sources.

[0003] However, thulium-holmium co-doped gain fibers exhibit various complex energy conversion mechanisms, including cross-relaxation, inter-ion energy transfer, and energy upconversion, which can easily lead to uneven gain or absorption distribution within the laser cavity, thereby inducing self-pulse phenomena. When pumping thulium-holmium co-doped fibers with commonly used 793nm semiconductor lasers, the high quantum defect results in insufficient cross-relaxation between thulium ions and energy transfer between thulium and holmium ions in the excited three-level structure. The competition between these ions produces a modulation effect similar to saturable absorption, leading to uneven gain and thus self-pulse effects. Self-pulse degrades the stability of laser output power and mode stability, affecting not only the long-term reliability of the laser but also reducing its lifespan.

[0004] Currently, the commonly used self-pulse suppression methods for 2μm band tunable single-frequency fiber lasers mainly include composite cavity structure, grating spectral shaping, and phase modulation.

[0005] Composite cavity structures, by employing multiple gratings to form resonant cavity mirrors, can construct external cavity feedback to a certain extent, improve the longitudinal mode satisfaction condition of oscillation, extend photon lifetime, avoid giant pulse generation, and thus suppress self-pulse effects. Examples include the thulium-doped QCW fiber laser based on a composite cavity structure disclosed in publication number CN118213838A and the 2-micron all-fiber laser with cascaded grating cavity mirrors disclosed in publication number CN221126526U. However, this relatively complex composite cavity structure not only increases the matching difficulty of laser oscillation initiation and reduces laser efficiency, but also exacerbates the system's environmental sensitivity, making it susceptible to mode hopping due to vibration, temperature, and other environmental factors.

[0006] Grating spectral shaping reduces the intensity of self-pulses and the probability of strong pulses in the output timing by designing and adjusting the reflection spectrum of the output coupling grating, thereby modulating the amplitude of each longitudinal mode of the oscillator. However, these output coupling gratings with special reflection spectra often have very broad reflection spectra, making it difficult to support single-longitudinal-mode laser output. The principle of self-pulse suppression through adjusting the amplitude of each longitudinal mode also indicates that this approach is difficult to achieve self-pulse suppression for single-longitudinal-mode lasers. Phase modulation techniques control the fine structure characteristics of the modulated spectrum by controlling the modulation frequency and modulation depth, suppressing random self-pulse phenomena in continuous-wave narrow-linewidth fiber amplifier systems, and avoiding the generation of random self-pulses. Examples include a narrow-linewidth fiber laser spectral broadening device and its usage method disclosed in CN111509536B, and a seed source spectral broadening system and method driven by a binary multi-frequency signal disclosed in CN113991409A. However, this technique generates lasers with many frequency components through phase modulation, disrupting the single-longitudinal-mode characteristics of a single-frequency laser.

[0007] Furthermore, most of the aforementioned methods involve external technical structural adjustments, which can increase the overall complexity of the device and lead to problems such as reduced output power. More importantly, they do not address the fundamental nature of fiber laser self-pulse generation, thus their core mechanisms and key technologies have not yet achieved effective breakthroughs. Additionally, these solutions are all designed for manipulating single-wavelength lasers and are difficult to apply to tunable fiber lasers.

[0008] In summary, for 2μm band tunable single-frequency fiber lasers, there is still a lack of simple, direct, stable and reliable solutions for self-pulse suppression. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention proposes a 2μm band tunable single-frequency fiber laser.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] On one hand, this invention provides a 2μm band tunable single-frequency fiber laser, which is a ring cavity laser, including a pump source, a wavelength division multiplexer, a thulium-holmium co-doped fiber, a mode field adapter, a composite sub-ring filter, a tunable fiber filter, an isolator, and a coupler. The pump source is a 1950nm band holmium ion co-band pump source, which pumps the thulium-holmium co-doped fiber. When the 1950nm band holmium ion co-band pump source pumps the thulium-holmium co-doped fiber, it forms a two-level system structure, which weakens the thulium ion cross-relaxation intensity while improving the energy conversion efficiency, thereby achieving suppression of self-pulses.

[0012] Furthermore, the output of the 1950nm band holmium ion co-pump source is connected to the pump input of the wavelength division multiplexer; the output of the wavelength division multiplexer is connected to one end of the thulium-holmium co-doped fiber; the other end of the thulium-holmium co-doped fiber is connected in sequence to a composite sub-loop filter and a tunable fiber filter via a mode field adapter, and mode selection and filtering are performed through the composite sub-loop filter and the tunable fiber filter; the output of the tunable fiber filter is connected to the input of an isolator, and the output of the isolator is connected to the input of a coupler; the coupler has two output ports, one of which is connected to the input of the wavelength division multiplexer to form an oscillation circuit, and the other output port of the coupler serves as the output port of a laser for outputting single-frequency laser light.

[0013] Furthermore, it also includes a temperature control unit for temperature control of the composite sub-loop filter.

[0014] Furthermore, the pump light output from the 1950nm band holmium ion co-band pump source is coupled into the thulium-holmium co-doped gain fiber through a wavelength division multiplexer to generate signal light. The signal light is coupled into the composite sub-loop filter through the mode field adapter. The composite sub-loop filter selects a single longitudinal mode. The mode-selected signal light enters the tunable fiber filter. The output laser wavelength of the tunable fiber filter is controlled by changing the loading voltage of the tunable fiber filter 7. The output laser of the tunable fiber filter passes through an isolator and a coupler. A portion of the laser power input to the coupler is input to the input of the wavelength division multiplexer through one output port of the coupler to form an oscillation circuit. The other portion of the laser power input to the coupler is output through the other output port of the coupler, resulting in a single-frequency laser output.

[0015] Furthermore, the two output ports of the coupler are an 80% output port and a 20% output port, respectively. 80% of the laser power in the input coupler is output through the 80% output port, and 20% of the laser power is output through the 20% output port. The 80% output port is connected to the input terminal of the wavelength division multiplexer to form an oscillation circuit, and the 20% output port serves as the output port of the laser for outputting single-frequency laser.

[0016] Furthermore, the center wavelength of the 1950nm band holmium ion co-band pump source is any wavelength within the range of 1900nm to 2000nm.

[0017] Furthermore, the thulium-holmium co-doped fiber is a single-clad or double-clad gain fiber co-doped with thulium and holmium, with a length of 1m to 50m. Its matrix material and core glass composition are one or any two or more of quartz, phosphate, silicate, tellurate, fluoride or sulfide in any proportion, and its spontaneous emission spectrum range matches the filtering range of the tunable fiber filter.

[0018] Furthermore, the composite sub-loop filter is composed of five 2×2 couplers connected together, namely coupler #1, coupler #2, coupler #3, coupler #4, and coupler #5. The first port of coupler #1 serves as the input terminal of the composite sub-loop filter. The second and fourth ports of coupler #1 are connected to the third and first ports of coupler #2, respectively, to form the first sub-loop. The fourth port of coupler #2 is connected to the first port of coupler #3. The second and fourth ports of coupler #3 are connected to the third and first ports of coupler #4, respectively, to form the second sub-loop. The second port of coupler #4 is connected to the first port of coupler #5. The second and third ports of coupler #5 are connected to form the third sub-loop, and the fourth port of coupler #5 serves as the output terminal of the composite sub-loop filter. The third ports of couplers #1, #2, #3, and #4 are all angled at an octave. There is a difference in length between the first and second sub-loops. Preferably, the difference between the length of the first sub-ring and the length of the second sub-ring is in the range of 0.1cm to 1cm.

[0019] Furthermore, the free spectral range of the composite sub-ring filter satisfies the expression:

[0020]

[0021] in FSR DCR The free spectral range of the composite sub-loop filter. c At the speed of light, n eff The effective refractive index of the optical fiber core. This is the difference between the length of the first sub-ring and the length of the second sub-ring;

[0022] By controlling This allows the free spectral range of the composite sub-ring filter to be much larger than that of the tunable fiber filter, and the 3dB bandwidth to be smaller than the longitudinal mode spacing of the ring cavity. This effectively eliminates the dense longitudinal modes caused by the excessive length of the ring cavity, and enables single-frequency laser output.

[0023] Furthermore, the tunable fiber filter is a dielectric thin-film filter, a Fabry-Perot tunable fiber filter, or an acousto-optic filter, with an operating range covering 1950nm~2200nm, insertion loss <3dB, and 3dB bandwidth <1nm. Under different loading voltages, the tunable fiber filter 7 exhibits different transmittances for different wavelengths. By changing the loading voltage of the tunable fiber filter 7, the output laser wavelength of the tunable fiber filter is controlled, and the output wavelength is continuously calibrated, thereby reducing the tuning error range.

[0024] Furthermore, the isolator has an isolation level greater than 20dB, ensuring unidirectional operation of the optical path, suppressing the generation of return light, and further reducing the probability of self-pulse induction.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] Self-pulse oscillations originate from non-uniform gain or absorption within the laser. Traditional 793nm pump sources pumping thulium-holmium co-doped fibers form a three-level excitation structure. The cross-relaxation between thulium ions and the energy transfer between thulium and holmium ions are insufficient. Competition among multiple energy conversion processes generates a modulation effect similar to saturable absorption, leading to non-uniform gain and a tendency for self-pulses. To address these problems in existing technologies, this invention provides a 2μm band tunable single-frequency fiber laser that uses a 1950nm band holmium ion co-pump source to pump thulium-holmium co-doped fibers. This fundamentally reduces energy competition between thulium and holmium ions during laser excitation while improving energy conversion efficiency, thus suppressing self-pulses.

[0027] This invention employs a 1950nm band holmium ion co-pumping method to suppress self-pulse generation. Simultaneously, through the combined effect of mode selection by the composite sub-ring filter and filtering by the tunable fiber filter, a stable single-frequency laser output with wide-range self-pulse-free tunability is achieved. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the structure of a 2μm band tunable single-frequency fiber laser in one embodiment;

[0030] Figure 2 This is a tunable output laser spectrum based on an electrically tunable dielectric thin-film filter in one embodiment;

[0031] Figure 3 This is a schematic diagram of the structure of a composite sub-loop filter in one embodiment;

[0032] Figure 4 This is a comparison of the time-domain output effects of a laser pumped by a 1950nm band holmium ion co-band pump source and a laser pumped by a direct 793nm pump source in one embodiment.

[0033] Marked in the image:

[0034] 1. 1950nm band holmium ion co-band pump source; 2. Wavelength division multiplexer; 3. Thulium-holmium co-doped fiber; 4. Mode field adapter; 5. Composite sub-ring filter; 6. Temperature control unit; 7. Tunable fiber optic filter; 8. Isolator; 9. Coupler;

[0035] Coupler 5-1, 1#; Coupler 5-2, 2#; Coupler 5-3, 3#; Coupler 5-4, 4#; Coupler 5-5, 5#. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0037] One embodiment provides a 2μm band tunable single-frequency fiber laser, which is a ring cavity laser, including a 1950nm band holmium ion co-band pump source 1, a wavelength division multiplexer 2, a thulium-holmium co-doped fiber 3, a mode field adapter 4, a composite sub-ring filter 5, a tunable fiber filter 7, an isolator 8, and a coupler 9. By using a 1950nm band holmium ion co-band pump source to pump the thulium-holmium co-doped fiber in a two-level system structure, the cross-relaxation intensity of thulium ions is weakened while the energy conversion efficiency is improved, thereby achieving suppression of self-pulses.

[0038] Traditional 793nm pump sources pumping thulium-holmium co-doped fibers employ a three-level excitation structure. Due to insufficient cross-relaxation between thulium ions and insufficient energy transfer between thulium and holmium ions, competition among multiple energy conversion processes generates a modulation effect similar to saturable absorption, leading to uneven gain and a tendency for self-pulses. This invention utilizes a 1950nm band holmium ion co-band pump source pumping thulium-holmium co-doped fibers, resulting in a two-level system structure. This weakens the cross-relaxation intensity of thulium ions while improving energy conversion efficiency, effectively avoiding the modulation effect of the three-level system and thus suppressing self-pulses. Preferably, the center wavelength of the 1950nm band holmium ion co-band pump source is in the range of 1900nm to 2000nm, and can be any wavelength within this range. Compared with 793nm semiconductor pump sources, holmium ion co-band pump sources with center wavelengths in the range of 1900nm~2000nm can improve the energy transfer efficiency of thulium-holmium co-doped fibers, reduce energy competition between thulium and holmium ions, suppress pulse modulation of the output laser by the three-level laser system, promote the flattening of the gain curve, and obtain stable continuous output laser without self-pulses.

[0039] The output of the 1950nm band holmium ion co-band pump source 1 is connected to the pump input of the wavelength division multiplexer 2; the output of the wavelength division multiplexer 2 is connected to one end of the thulium-holmium co-doped fiber 3; the other end of the thulium-holmium co-doped fiber 3 is connected in sequence to a composite sub-loop filter 5 and a tunable fiber filter 7 via a mode field adapter 4, and mode selection and filtering are performed through the composite sub-loop filter 5 and the tunable fiber filter 7; the output of the tunable fiber filter 7 is connected to the input of an isolator 8, which ensures unidirectional laser transmission; the output of the isolator 8 is connected to the input of a coupler 9; the coupler 9 has two output ports, one of which is connected to the input of the wavelength division multiplexer 2 to form an oscillation circuit, and the other output port of the coupler 9 serves as the output port of the laser for outputting single-frequency laser.

[0040] Preferably, the coupler 9 has two output ports, an 80% output port and a 20% output port. 80% of the laser power input to the coupler 9 is output through the 80% output port, and 20% of the laser power is output through the 20% output port. The 80% output port is connected to the input of the wavelength division multiplexer 2 to form an oscillation circuit, and the 20% output port serves as the output port of the laser for outputting single-frequency laser light.

[0041] The pump light output from the 1950nm band holmium ion co-band pump source 1 is coupled into the thulium-holmium co-doped gain fiber 3 through wavelength division multiplexer 2 to generate signal light. The signal light is coupled into the composite sub-ring filter 5 through mode field adapter 4 to reduce the loss caused by fiber mismatch. The composite sub-ring filter 5 selects a single longitudinal mode. The signal light after mode selection enters the tunable fiber filter 7. The output laser wavelength of the tunable fiber filter 7 is controlled by changing the device loading voltage of the tunable fiber filter 7. The output laser of the tunable fiber filter 7 passes through isolator 8 and coupler 9. Isolator 8 controls the unidirectional propagation of the laser to reduce return light interference. The laser finally enters the input of the wavelength division multiplexer through the 80% output port of coupler 9 to form a ring cavity oscillation. The 20% output port of coupler 9 is used as the external output.

[0042] The thulium-holmium co-doped fiber 3 described in this invention can be a single-clad gain fiber or a double-clad gain fiber co-doped with thulium and holmium, with a length of 1m to 50m. Its matrix material and core glass composition are one or any two or more of quartz, phosphate, silicate, tellurate, fluoride or sulfide in any proportion, and its spontaneous emission spectrum range matches the filtering range of the tunable fiber filter.

[0043] The mode field adapter 4 described in this invention effectively connects thulium-holmium co-doped optical fibers with mismatched core sizes to passive transmission optical fibers, achieving a coupling efficiency of >95% and significantly reducing fiber fusion splicing loss.

[0044] like Figure 3As shown, the composite sub-loop filter 5 is composed of five 2×2 couplers connected together. Each 2×2 coupler has four ports: port a, port b, port c, and port d. The five 2×2 couplers are designated as coupler 5-1 (1#), coupler 5-2 (2#), coupler 5-3 (3#), coupler 5-4 (4#), and coupler 5-5 (5#). Coupler 5-1 and coupler 5-2 are connected to form the first sub-loop, and coupler 5-3 and coupler 5-4 are connected to form the second sub-loop. Couplers 5-1, 5-2, 5-3, and 5-4 have the same coupling ratio, which can be any value between 50:50 and 10:90. Specifically, the first port a of coupler 1# 5-1 serves as the input of the composite sub-loop filter 5. The second port b and the fourth port d of coupler 1# 5-1 are connected to the third port c and the first port a of coupler 2# 5-2, respectively, to form the first sub-loop. The fourth port d of coupler 2# 5-2 is connected to the first port a of coupler 3#. The second port b and the fourth port d of coupler 3# 5-3 are connected to the third port c and the first port a of coupler 4# 5-4, respectively, to form the second sub-loop. The second port b of coupler 4# 5-4 is connected to the first port a of coupler 5# 5-5. The second port b and the third port c of coupler 5# 5-5 are connected to form the third sub-loop. The fourth port d of coupler 5# 5-5 serves as the output of the composite sub-loop filter 5. The third port c of coupler 1# 5-1, the second port b of coupler 2# 5-2, the third port c of coupler 3# 5-3, and the second port b of coupler 4# 5-4 are all angled at an octave. The length difference between the first sub-ring and the second sub-ring is maintained within the range of 0.1 to 1 cm. This length difference, which is the difference between the fiber length of the first sub-ring and the fiber length of the second sub-ring, can be adjusted by using a passive fiber shearing device.

[0045] Preferably, couplers 5-1 (1#), 5-2 (2#), 5-3 (3#), and 5-4 (4#) are four 2×2 couplers with a coupling ratio of 70:30, 60:40, or 50:50, while coupler 5-5 (5#) is a 2×2 coupler with a coupling ratio of 50:50. By adjusting the lengths of the first and second sub-rings, the free spectral range of the composite sub-ring filter is made much larger than that of the tunable fiber filter. Only laser modes meeting specific frequencies can oscillate, effectively eliminating the dense longitudinal modes caused by excessive ring cavity length and achieving single-frequency laser output.

[0046] In one embodiment, the specific structure of the composite sub-loop filter 5 is as follows: Figure 3As shown, couplers 1# (5-1), 2# (5-2), 3# (5-3), and 4# (5-4) are 2×2 couplers with a coupling ratio of 70:30, and coupler 5# (5-5) is a 2×2 coupler with a coupling ratio of 50:50. The connection relationships between couplers 1# (5-1), 2# (5-2), 3# (5-3), 4# (5-4), and 5# (5-5) are as follows: Figure 3 As shown in the above embodiments, the details have been described in detail and will not be repeated here.

[0047] The length difference between the first and second sub-rings is in the range of 0.1cm to 1cm. This length difference, i.e., the difference between the fiber lengths of the first and second sub-rings, can be achieved by cutting the passive fiber using a shearing device. When the laser propagates in the composite sub-ring filter 5, the free spectral ranges of the different sub-rings of the composite sub-ring filter are different, forming a structure similar to periodic peaks or valleys, which can achieve the effect of a narrowband filter. The free spectral range of the composite sub-ring filter 5 satisfies the expression:

[0048]

[0049] in FSR DCR The free spectral range of the composite sub-loop filter. c At the speed of light, n eff The effective refractive index of the optical fiber core. The difference between the length of the first sub-ring and the length of the second sub-ring is within the range of 0.1cm to 1cm.

[0050] By controlling This allows the free spectral range of the composite sub-ring filter to be much larger than that of the tunable fiber filter, and the 3dB bandwidth to be smaller than the longitudinal mode spacing of the ring cavity. This effectively eliminates the dense longitudinal modes caused by the excessive length of the ring cavity, and enables single-frequency laser output.

[0051] 3dB bandwidth of composite sub-loop filter 5 Satisfying the expression:

[0052]

[0053] in This represents the 3dB bandwidth of the composite sub-loop filter. c At the speed of light, The transmission loss of the laser light input into the composite sub-ring filter 5 after passing through the longest sub-ring among the three sub-rings of the composite sub-ring filter 5 for one revolution is given. n eff The effective refractive index of the fiber core; the composite sub-ring filter 5 contains three sub-rings, namely the first sub-ring, the second sub-ring, and the third sub-ring. This is the length of the longest sub-ring among the three sub-rings in the composite sub-ring filter 5.

[0054] The longitudinal mode spacing of a ring cavity laser satisfies the following formula:

[0055]

[0056] in FSR L The longitudinal mode spacing of the ring cavity laser. c At the speed of light, n The refractive index of the fiber core. L This is the total cavity length of the ring cavity laser.

[0057] The free spectral range of the composite sub-loop filter 5 is obtained by applying the above expression and the vernier effect principle. FSR DCR With 3dB bandwidth Adjusting the sub-ring length ratio between the first and second sub-rings allows for a wider free spectral range for the composite sub-ring filter 5. FSR DCR It has a bandwidth of 0.5 to 1 times that of the tunable fiber optic filter 7 and a bandwidth of 3 dB. The longitudinal mode spacing is 0.5 to 1 times that of the ring cavity laser, so that only lasers meeting a certain frequency can oscillate. The structure can effectively eliminate the dense longitudinal modes caused by excessively long ring cavity length and realize single-frequency laser output.

[0058] Reference Figure 1 The invention also includes a temperature control unit for temperature control of the composite sub-loop filter 5. The temperature control unit 6 stabilizes the temperature of the composite sub-loop filter 5 to maintain the cavity length difference of each sub-loop structure within the composite sub-loop filter, thereby improving mode stability. Because the composite sub-loop filter of the present invention is highly flexible but easily affected by the external environment, a temperature control unit is provided below the composite sub-loop filter to strictly control the temperature change range, reduce cavity length changes, and ensure output stability.

[0059] The tunable fiber optic filter 7 of this invention adjusts the transmittance of the tunable fiber optic filter to different wavelengths of light by adjusting the voltage of the tuning driver, thereby achieving high-precision tuning over a wide wavelength range. Specifically, the tunable fiber optic filter 7 is a dielectric thin-film filter with a tuning range up to 200nm and a tuning precision of 0.1nm. Under different applied voltages, the transmittance of the dielectric thin-film filter to different wavelengths varies. By precisely adjusting the applied voltage to change the output wavelength of the dielectric thin-film filter, high-precision continuous tunable laser output is achieved. The tuned output spectrum is measured by a spectrometer as follows: Figure 2 As shown, the tuning range is limited by the pump power.

[0060] In summary, the temporal output performance of lasers pumped by a 1950nm holmium ion co-band pump source and a direct 793nm pump source is compared as follows: Figure 4 As shown, the output changes from pulsed to continuous, with simple operation and significant effects. Compared with existing technologies, this invention changes the pumping method by using a 1950nm band holmium ion co-pump source to pump a thulium-holmium co-doped fiber, weakening the thulium ion cross-relaxation intensity, improving energy conversion efficiency, and promoting gain flattening to suppress self-pulses. Simultaneously, a tunable filter is used for filtering, and mode selection via a composite sub-loop filter ensures stable single-longitudinal mode output. The stability of the single-longitudinal mode further enhances the time-domain output stability, achieving 2μm band wide-tunable continuous laser output. This invention provides a self-pulse suppression method for 2μm band tunable single-frequency fiber lasers, characterized by simplicity, directness, stability, reliability, and a wide tuning range, providing a high-quality light source for fields such as lidar and atmospheric detection.

[0061] Matters not covered in this invention are common knowledge.

[0062] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0063] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these modifications and improvements all fall within the protection scope of this application.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

A 1.2μm band tunable single-frequency fiber laser, characterized in that, This is a ring cavity laser, comprising a pump source, a wavelength division multiplexer (WDM), a thulium-holmium co-doped fiber, a mode field adapter, a composite sub-ring filter, a tunable fiber filter, an isolator, and a coupler. The pump source is a 1950nm band holmium ion co-band pump source, which pumps the thulium-holmium co-doped fiber. The output of the 1950nm band holmium ion co-band pump source is connected to the pump input of the WDM. The output of the WDM is connected to one end of the thulium-holmium co-doped fiber. The other end of the fiber is connected in sequence to a composite sub-loop filter and a tunable fiber filter via a mode field adapter. Mode selection and filtering are performed through the composite sub-loop filter and the tunable fiber filter. The output end of the tunable fiber filter is connected to the input end of an isolator, and the output end of the isolator is connected to the input end of a coupler. The coupler has two output ports. One output port is connected to the input end of a wavelength division multiplexer to form an oscillation circuit. The other output port of the coupler serves as the output port of a laser for outputting single-frequency laser light. When a 1950nm band holmium ion co-pump source pumps a thulium-holmium co-doped fiber, the 2μm band tunable single-frequency fiber laser has a two-level system structure, which weakens the cross-relaxation intensity of thulium ions while improving the energy conversion efficiency, thereby achieving suppression of self-pulses.

2. The 2μm band tunable single-frequency fiber laser according to claim 1, characterized in that, It also includes a temperature control unit for temperature control of the composite sub-loop filter.

3. The 2μm band tunable single-frequency fiber laser according to claim 1 or 2, characterized in that, Pump light from a 1950nm holmium ion co-pump source is coupled into a thulium-holmium co-doped gain fiber via a wavelength division multiplexer to generate signal light. The signal light is then coupled into a composite sub-loop filter via a mode field adapter. The composite sub-loop filter selects a single longitudinal mode. The mode-selected signal light then enters a tunable fiber filter. The output laser wavelength of the tunable fiber filter is controlled by changing its loading voltage. The output laser of the tunable fiber filter passes through an isolator and a coupler. A portion of the laser power input to the coupler is input to the input of the wavelength division multiplexer via one output port of the coupler, forming an oscillation loop. The other portion of the laser power input to the coupler is output via the other output port of the coupler, resulting in a single-frequency laser output.

4. The 2μm band tunable single-frequency fiber laser according to claim 3, characterized in that, The coupler has two output ports, namely the 80% output port and the 20% output port. The laser with 80% power input to the coupler is output through the 80% output port, and the laser with 20% power is output through the 20% output port. The 80% output port is connected to the input terminal of the wavelength division multiplexer to form an oscillation circuit, and the 20% output port serves as the output port of the laser for outputting single-frequency laser.

5. The 2μm band tunable single-frequency fiber laser according to claim 3, characterized in that, The 1950nm band holmium ion co-band pump source is a fiber laser pump source, a semiconductor laser pump source, or a solid-state laser coupled fiber pump source, with a center wavelength of any wavelength in the range of 1900nm to 2000nm.

6. The 2μm band tunable single-frequency fiber laser according to claim 1, 2, 4, or 5, characterized in that, The thulium-holmium co-doped fiber is a single-clad or double-clad gain fiber with a length of 1~50 m. Its matrix material and core glass composition are one or any two or more of the following: quartz, phosphate, silicate, tellurate, fluoride or sulfide, in any proportion. Its spontaneous emission spectrum range matches the filtering range of the tunable fiber filter.

7. The 2μm band tunable single-frequency fiber laser according to claim 1, 2, 4, or 5, characterized in that, The composite sub-loop filter is composed of five 2×2 couplers connected together, namely coupler #1, coupler #2, coupler #3, coupler #4, and coupler #5. The first port of coupler #1 serves as the input terminal of the composite sub-loop filter. The second and fourth ports of coupler #1 are connected to the third and first ports of coupler #2, respectively, to form the first sub-loop. The fourth port of coupler #2 is connected to the first port of coupler #3. The second and fourth ports of coupler #3 are connected to the third and first ports of coupler #4, respectively, to form the second sub-loop. The second port of coupler #4 is connected to the first port of coupler #5. The second and third ports of coupler #5 are connected to form the third sub-loop. The fourth port of coupler #5 serves as the output terminal of the composite sub-loop filter. The third port of coupler #1, the second port of coupler #2, the third port of coupler #3, and the second port of coupler #4 are all cut at an octave angle. There is a difference between the length of the first sub-ring and the length of the second sub-ring.

8. The 2μm band tunable single-frequency fiber laser according to claim 7, characterized in that, The difference between the length of the first sub-ring and the length of the second sub-ring is in the range of 0.1cm to 1cm.

9. The 2μm band tunable single-frequency fiber laser according to claim 7, characterized in that, The free spectral range of the composite sub-loop filter satisfies the expression: in FSR DCR The free spectral range of the composite sub-loop filter. c At the speed of light, n eff The effective refractive index of the optical fiber core. This represents the difference between the length of the first sub-ring and the length of the second sub-ring.

Images