Mid-infrared high repetition rate raman fiber laser based on h-shaped nanosecond mode-locked seed

Abstract

The application discloses a kind of mid-infrared high-repetition-rate Raman pulse fiber lasers based on h-shaped nanosecond mode-locked source, comprising: h-shaped mode-locked pulse seed source, thulium-doped fiber amplifier and fluorotellurite fiber;H-shaped mode-locked pulse seed source generates 2 μm waveband h-shaped nanosecond mode-locked pulse, by regulating and controlling intracavity nonlinear phase shift amount and power clamping state, output has the MHz and above high-repetition-rate characteristics characteristics of fundamental frequency or harmonic mode-locked h-shaped pulse;Thulium-doped fiber amplifier receives the h-shaped pulse generated by h-shaped mode-locked pulse seed source and promotes h-shaped pulse power level, does not change pulse form;Fluorotellurite fiber is used as Raman gain medium, based on single-pass structure form and thulium-doped fiber amplifier output end are connected or fused coupling, utilize h-shaped pulse pumping excitation Raman frequency shift.The application can avoid unnecessary pump consumption in intermediate spectral region, optimize output spectral quality, with universality and expandability, can be applied to other waveband pulse laser generation.

Description

Technical Field

[0001] This invention belongs to the field of fiber lasers and relates to a mid-infrared high repetition rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked pulse signal that can efficiently convert its energy to long wavelengths. Background Technology

[0002] In the field of laser technology, the mid-infrared band (2μm–5μm) encompasses most of the molecular "fingerprint" region and multiple atmospheric transmission windows, and is characterized by strong water absorption. Therefore, mid-infrared lasers, especially mid-infrared nanosecond pulsed lasers, have become a focus of research both domestically and internationally in recent years due to their unique operating wavelength, high energy, and short pulse width characteristics. Furthermore, with the increasing maturity and sophistication of related material preparation processes and laser technology, long-wavelength mid-infrared nanosecond pulsed lasers have demonstrated enormous application value and potential in scientific research experiments, optical precision measurement, and materials processing.

[0003] Generally, based on different wavelength conversion mechanisms, mid-infrared fiber lasers are broadly classified into two categories: rare-earth ion-doped fiber lasers and Raman fiber lasers. The former primarily obtains mid-infrared pulsed lasers with corresponding wavelengths and pulse widths by selecting a suitable pump source and matching the energy level transition characteristics of dopant ions (such as thulium, holmium, and erbium ions) in the gain fiber, utilizing different optical switching techniques such as Q-switching and gain modulation. However, this method is strictly dependent on the emission characteristics and optical switching performance of rare-earth materials, and has significant limitations in further extending the laser wavelength; and due to the limitation of superimposed optical switching elements, the maximum repetition frequency of short pulses above 2.1 μm (nanosecond and microsecond) obtained based on Q-switching or gain modulation techniques currently does not exceed 300 kHz. Raman fiber lasers, on the other hand, can use existing rare-earth ion-doped fiber pulsed laser sources as signals, and through stimulated Raman scattering of different types of fibers, achieve output conversion to longer wavelengths. Theoretically, relying on this method, as long as the pump and fiber combination is appropriate, flexible output of mid-infrared pulsed lasers of any wavelength within the fiber transmission window can be achieved. This provides a reliable technical approach to compensate for the shortcomings of rare-earth ion-doped fiber lasers and effectively extend the wavelength of mid-infrared pulsed lasers.

[0004] Currently, in mid-infrared Raman pulsed fiber lasers, researchers from various countries mostly choose active Q-switched lasers and laser diodes with wavelengths close to 1.5μm or 2μm and requiring external control elements as seed sources due to pulse pumping requirements. Meanwhile, Raman gain fibers are mainly composed of high-concentration GeO2-doped silica-based fibers and mid-infrared soft glass fibers with wide long-wavelength transmission windows.

[0005] In 2008, Rakich et al. at MIT injected an amplified 1.53 μm gain-modulated pulse into a commercially available germanium-doped silica fiber with a concentration of approximately 58 wt.%, achieving a fifth-order Raman nanosecond pulse output with a maximum wavelength of 2.41 μm using a spontaneous cascaded Raman amplification mechanism. In 2015, Jiang et al. at the Shanghai Institute of Optics and Fine Mechanics, addressing the issue of low output power of the 2.41 μm Raman pulse laser reported above, used the same mechanism and fiber, employing actively Q-switched pulse seeds with wavelengths of 2.008 μm and 2.04 μm, a repetition frequency of 8 kHz, and a pulse width of 100 ns, to obtain second-order Raman pulses with wavelengths of 2.43 μm and 2.48 μm, respectively. However, spectral results show that due to Raman gain saturation and fiber nonlinear spectral broadening, there is significant radiation noise between the cascaded Raman signals. Figure 1 and Figure 2 Furthermore, due to the influence of the seed source, this type of laser usually requires an additional control system to adjust the pump parameters, resulting in a complex overall system structure and high cost; at the same time, due to the limitations of the available seed source parameters, its pulse repetition frequency is generally not high.

[0006] During this period, numerous experimental and theoretical research reports emerged internationally on obtaining long-wavelength Raman lasers using different types of soft glass fibers, such as chalcogenide, fluoride, and telluride fibers. However, no reports were found on obtaining mid-infrared nanosecond Raman pulses using fluoride and telluride glass fibers. Among reports on chalcogenide glass fiber Raman lasers, although as early as 2014, Bernier et al. at Laval University in Canada had already achieved a breakthrough in Raman pulse wavelength recording to 3.77 μm in an As2S3 single-mode fiber cascaded Raman laser based on a nested double-grating FP cavity structure, its total output power was less than 10 mW and it was in a quasi-continuous state. In 2021, Wang et al. from Northeastern University in China, using a self-made As2S5 glass fiber and a single-pass Raman structure, observed a six-stage Raman frequency shift from 2.149 μm to 3.425 μm using a 0.1 W, 10 kHz 2 μm nanosecond pulse source as the pump. Leveraging the high Raman gain of this fiber, they experimentally observed this shift. Unfortunately, due to the fiber's damage threshold, the incident pump light signal was weak, and the multi-stage frequency shift resulted in excessive power consumption. The report failed to provide the average power, energy, and efficiency of the system's output Raman light. In 2022, Jiao et al. from Jilin University in China used a self-made novel glass fiber (fluorotellurate fiber) as the Raman gain medium. Pumped by a 1.55 μm nanosecond pulse with a repetition rate of 80 kHz, they observed a three-stage Raman frequency shift, with a maximum wavelength, corresponding average power, and conversion efficiency of 2.438 μm, 31.6 mW, and 6.43%, respectively. Figure 3Because the pump pulse wavelength is relatively short and the intermediate frequency shift process consumes too much pump energy, the power level of the third-order Raman pulse has not yet shown the significant advantages of fluorotellurate fiber in terms of large frequency shift and high Raman gain compared with the output results of the nanosecond Raman pulse laser in the same band shown above. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] In Raman fiber lasers, the following technical problems are solved by utilizing H-shaped mode-locked pulse seeds combined with fiber amplifiers and fluorotellurate fibers:

[0009] a. Solve the problem of limited output pulse repetition rate in mid-infrared Raman pulsed fiber lasers caused by traditional Q-switched sources or electrically gained modulated pulse sources;

[0010] b. Solve the problems of excessive pump energy consumption, poor spectral quality, and low Raman conversion efficiency in mid-infrared Raman pulsed fiber lasers due to the short pump pulse wavelength and multiple frequency shift orders;

[0011] c. Solve the problem of generally low long-wavelength output power in mid-infrared soft glass fiber Raman pulse lasers.

[0012] (II) Technical Solution

[0013] To address the aforementioned technical problems, this invention provides a mid-infrared high repetition rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source, comprising: an h-shaped mode-locked pulse seed source I, a thulium-doped fiber amplifier II, and a fluorotellurate fiber III; the h-shaped mode-locked pulse seed source I generates h-shaped nanosecond mode-locked pulses in the 2μm band, and outputs fundamental or harmonic mode-locked h-shaped pulses with high repetition rate characteristics of MHz and above by adjusting the intracavity nonlinear phase shift and power clamping state; the thulium-doped fiber amplifier II receives the h-shaped pulses generated by the h-shaped mode-locked pulse seed source I and enhances the power level of the h-shaped pulses without changing the pulse shape; the fluorotellurate fiber III serves as the Raman gain medium, and is docked or fused to the output end of the thulium-doped fiber amplifier II based on a single-pass structure, using the h-shaped pulse to pump and excite the Raman frequency shift.

[0014] The h-shaped mode-locked pulse seed source I includes a laser diode-1 and a pump and signal fiber combiner-2, a thulium-doped fiber-3, a dispersion compensation fiber-4, an output coupler-5, a polarization-independent isolator-6, a polarization controller-7, a polarizer-8, a polarization-maintaining fiber-9, and a second polarization controller-10 connected in sequence. The pump and signal fiber combiner-2 is arranged on the light-emitting side of the laser diode-1.

[0015] In the h-shaped mode-locked pulse seed source I, a laser diode-1 with a wavelength of 793nm and a maximum optical power of 20W serves as its pump source. The pump light emitted is coupled into the inner cladding of the thulium-doped fiber-3, which serves as the gain medium, through the pump and signal fiber combiner-2. Under the excitation of the pump light, the thulium ions absorb energy and radiate a 2μm band optical signal through energy level transitions. This optical signal is transmitted through the ring cavity formed by the various devices of the h-shaped mode-locked pulse seed source I for one cycle, and then re-enters the thulium-doped fiber-3 through the pump and signal fiber combiner-2 to be amplified until the cavity loss and gain reach a balance. Finally, a stable laser signal is output through the output coupler 5.

[0016] The thulium-doped fiber amplifier II is a forward-pumped amplifier, comprising a polarization-independent isolator II 11, a laser diode II 12, a pump and signal fiber combiner II 13, and a thulium-doped fiber II 14. The output end of the output coupler 5, the polarization-independent isolator II 11, the pump and signal fiber combiner II 13, and the thulium-doped fiber II 14 are connected in sequence. The laser diode II 12 is connected to the input end of the pump and signal fiber combiner II 13, and the output end of the thulium-doped fiber II 14 is connected to the fluorotellurate fiber III.

[0017] The fluorotellurate fiber III is a Raman gain fiber with a transmission loss of 0.4 dB / m at a wavelength of 2 μm and a peak Raman gain coefficient of 1.28 × 10⁻⁶. -12 m / W, frequency shift of 785cm -1 .

[0018] Laser diode 1 and laser diode 2 are pump light sources with an output wavelength of 793nm and a maximum output power of 20W; pump & signal fiber combiner 2 and pump & signal fiber combiner 2 are used to combine 793nm pump and 2μm band laser signals.

[0019] Specifically, both the thulium-doped fiber 13 and the thulium-doped fiber 14 are double-clad single-mode fibers. In the h-shaped mode-locked pulse seed source I, the length of the thulium-doped fiber 13 is 3.6m, the core / cladding diameter ratio is 10 / 128, and the inner cladding is octagonal with a corresponding numerical aperture of 0.45. In the thulium-doped fiber amplifier II, the length of the thulium-doped fiber 14 is 5m, and the core / cladding diameter ratio is 10 / 130.

[0020] The group velocity dispersion of the dispersion-compensating fiber 4 is +89 ps. 2 / km, length ≥20m.

[0021] The output coupler 5 has a coupling ratio of 50:50 and includes two 50% output ports, which are respectively connected to polarization-independent isolator 6 and polarization-independent isolator 11.

[0022] The polarization-maintaining fiber 9 is located between the polarizer 8 and the polarization controller 10, and has a length of 0.5m; the polarizer 8 is a polarization-dependent isolator.

[0023] (III) Beneficial Effects

[0024] The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source provided by the above technical solution has the following beneficial effects:

[0025] (1) This invention uses a 2μm h-shaped pulse thulium-doped fiber mode-locked laser instead of a traditional Q-switched source or electrically gain modulated pulse source as the seed signal. It can easily achieve high repetition rate nanosecond pulse output in the MHz or even GHz range without any modulation or external shaping devices, and the cost is lower. The pulse shape and characteristic parameters such as repetition rate and pulse width can be flexibly changed by adjusting the nonlinear phase shift of the optical wave in the cavity. The operation method and structure are simpler.

[0026] (2) The h-shaped nanosecond pulse provided by this invention, compared with the most common noise-like mode-locked nanosecond pulse in fiber lasers (composed of numerous split femtosecond sub-pulses), does not exhibit optical wave splitting within its time-domain envelope. Therefore, it can better avoid the influence of harmful nonlinearity within the system, and its power level can be efficiently improved without significant distortion in the time-domain morphology during amplification. This is beneficial for suppressing the influence of Raman gain saturation and spectral broadening on radiated noise in single-pass structures.

[0027] (3) This invention uses an amplified h-shaped pulse as the pump. By leveraging the inherent high repetition rate of the mode-locked pulse and the high energy of the h-shaped pulse, the Raman pulse can also achieve a high repetition rate. Simultaneously, the leading edge of the h-shaped pulse can rapidly excite the Raman frequency shift effect. Combined with the high energy ratio per unit time on the base, this promotes Raman gain accumulation, thereby achieving efficient conversion of pump energy to long wavelengths. Experimental comparisons with Q-switched pulse-pumped Raman pulse lasers in the same band show that, when using an h-shaped pulse with a repetition rate on the order of kHz for pumping, the Raman conversion efficiency of the laser can be increased by more than 1.5 times.

[0028] (4) This invention combines the 2μm band h-shaped pulse pumping scheme with the high Raman gain and large frequency shift characteristics of fluorotellurate glass fiber, which can avoid unnecessary pump consumption in the middle spectral region and optimize the output spectral quality. In experimental studies, when using an h-shaped pulse with a power of nearly 1.74W to pump a fluorotellurate fiber with a length of about 6.75m, a Raman pulse with a wavelength of up to 2.4μm can be obtained by only one-order frequency shift; its power level can exceed 300mW, which is nearly 10 times that of the current mid-infrared soft glass fiber nanosecond pulse Raman laser; if the pump is sufficient and the nonlinear suppression is good, the wavelength can be made to reach ~2.9μm by second-order Raman frequency shift.

[0029] (5) The high-efficiency generation scheme for mid-infrared high repetition rate nanosecond Raman pulse proposed in this invention has universality and scalability, and can be applied to the generation of pulsed lasers in other bands. Attached Figure Description

[0030] Figure 1 and Figure 2 The diagram below shows the structure and output laser spectrum of a 2.2μm & 2.4μm two-stage Raman laser based on 2μm nanosecond Q-switched pulse pumping.

[0031] Figure 3 The background technique describes the amplification of a 1.55 μm electrically modulated nanosecond seed pulse and its subsequent pumping of a fluorotellurate fiber to obtain a third-order Raman laser spectrum. The wavelength of the third-order Raman pulse is approximately 2.44 μm, corresponding to a power of ~31.6 mW.

[0032] Figure 4 This is a schematic diagram of the laser structure of the present invention. The markings in the diagram represent: 1. Laser diode one; 2. Pump & signal fiber combiner one; 3. Thulium-doped fiber one; 4. Dispersion compensation fiber; 5. Output coupler; 6. Polarization-independent isolator one; 7. Polarization controller one; 8. Polarizer; 9. Polarization-maintaining fiber; 10. Polarization controller two; 11. Polarization-independent isolator two; 12. Laser diode two; 13. Pump & signal fiber combiner two; 14. Thulium-doped fiber two; 15. Fluorotellurate fiber. The rotating arrows in the diagram indicate the optical path circulation direction in the ring cavity mode-locked fiber laser.

[0033] Figure 5 This describes the incident light power and transmittance corresponding to the change in the total phase shift of the optical signal in a ring cavity mode-locked fiber laser.

[0034] Figure 6 and Figure 7 The Raman pulse spectrum and conversion efficiency results obtained in the experiment based on h-shaped nanosecond pulse pumping of fluorotellurate fiber are used to prove that h-shaped pulses have a positive effect on improving the spectral quality and conversion efficiency of Raman fiber lasers. Detailed Implementation

[0035] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0036] This invention aims to study an efficient generation scheme for mid-infrared high repetition rate Raman pulses based on a 2μm band h-shaped nanosecond mode-locked signal and fluorotellurate fiber. The Raman pulse fiber laser does not require external modulation devices, and the pulse width and repetition rate can be controlled by changing the seed source pump power or the mode-locking state, respectively. The 2μm h-shaped seed signal is amplified and then used to pump the fluorotellurate fiber, which can effectively suppress pump consumption in the intermediate process, promote energy conversion, and realize high-power long-wavelength pulse laser output.

[0037] like Figure 4 As shown, this embodiment of the mid-infrared high repetition rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source includes: an h-shaped mode-locked pulse seed source I, a thulium-doped fiber amplifier II, and a fluorotellurate fiber III. The h-shaped mode-locked pulse seed source I generates h-shaped nanosecond mode-locked pulses in the 2μm band. By adjusting the intracavity nonlinear phase shift and power clamping state, it outputs fundamental frequency or harmonic mode-locked h-shaped pulses with high repetition rate characteristics of MHz and above, thereby ensuring that the Raman pulse fiber laser outputs nanosecond pulses with the same repetition rate. The thulium-doped fiber amplifier II receives the h-shaped pulses generated by the h-shaped mode-locked pulse seed source I and enhances the power level of the h-shaped pulses without changing the pulse shape. The fluorotellurate fiber III serves as the Raman gain medium and is docked or fused to the output end of the thulium-doped fiber amplifier II based on a single-pass structure, using h-shaped pulse pumping to excite the Raman frequency shift. This laser primarily utilizes the inherent characteristics of h-shaped mode-locked pulses and the advantages of fluorotellurate fiber to reduce intermediate frequency shift order and ineffective pump consumption, optimize spectral quality, promote energy conversion during frequency shift, and achieve high-power mid-infrared high-repetition-rate nanosecond pulse output.

[0038] The ring resonant cavity of the h-shaped mode-locked pulse seed source I adopts a typical nonlinear polarization rotation structure with a simple and compact layout and strong damage resistance threshold. The subtle difference lies in the insertion of a polarization-maintaining fiber between the polarizer and the second polarization controller to assist in the harmonic mode-locking of the h-shaped pulse. The h-shaped mode-locked pulse seed source I includes a laser diode-1 and, in sequence, a pump and signal fiber combiner-2, a thulium-doped fiber-3, a dispersion compensation fiber-4, an output coupler-5, a polarization-independent isolator-6, a polarization controller-7, a polarizer-8, a polarization-maintaining fiber-9, and a second polarization controller-10. The pump and signal fiber combiner-2 is arranged on the output side of the laser diode-1.

[0039] For the h-shaped mode-locked pulse seed source I: a laser diode-1 with a wavelength of 793nm and a maximum optical power of 20W serves as its pump source. The emitted pump light is coupled into the inner cladding of a thulium-doped fiber-3, which serves as the gain medium, via a pump and signal fiber combiner-2. Under the excitation of the pump light, thulium ions absorb energy and radiate a 2μm band optical signal through energy level transitions. This signal is transmitted through the ring cavity once and then re-enters the thulium-doped fiber-3 via the pump and signal fiber combiner-2 for amplification until the cavity loss and gain reach equilibrium. At this point, the h-shaped mode-locked pulse seed source I outputs a stable laser signal through the output coupler-5. At this time, the laser's operating mode is generally in continuous mode. The dispersion compensation fiber-4 is mainly used to adjust the net group velocity dispersion value within the cavity and increase the nonlinear value, thereby reducing the generation threshold of the fundamental frequency mode-locked pulse. The polarization-independent isolator-6 is used to achieve stable unidirectional transmission of the laser signal while avoiding laser echo damage. The polarization-maintaining fiber 9 is responsible for controlling the comb-shaped transmission spectrum state (such as bandwidth and period) within the seed source cavity, which helps generate h-shaped pulses and harmonic mode-locking phenomena. If high-repetition-rate pulses are not required, this fiber can be removed. Polarization controller 7, polarizer 8, and polarization controller 10 are the core mechanisms of the seed source (nonlinear polarization rotation structures), used for polarization rotation and filtering selection of the optical signal, realizing mode-locked pulse generation and narrowing. Polarization controller 7 and polarization controller 10 are mainly responsible for applying stress to the clamped fiber, changing the polarization direction of the optical signal by inducing a weak birefringence effect; polarizer 8 uses a polarization-dependent isolator, combining polarization, analysis, and optical signal echo suppression functions.

[0040] Since the nonlinear polarization rotation structure formed by polarization controller 7, polarizer 8, and polarization controller 10 is equivalent to a saturable absorber, and the transmittance curve of the optical signal in the cavity exhibits a sinusoidal periodic variation characteristic with the total phase shift, such as... Figure 5 As shown. Furthermore, as the total phase shift increases, the transmittance of the optical signal gradually decreases in the anti-saturation absorption region (regions B and D). At this point, once the intensity of a light pulse component exceeds its maximum transmittable intensity, the absorber acts as a power equalizer, limiting the intensity of each component of the light pulse. Combined with its periodic filtering characteristics, increasing the additional phase shift of the polarization controller shifts the peak position of the transmission spectrum towards longer wavelengths. Therefore, by adjusting polarization controller 7 and polarization controller 10, the position of the laser's transmission spectrum can be changed simultaneously. When the frequency component corresponding to the leading edge of the light pulse in the time domain is located near the transmission peak, and its intensity is too low, it is located... Figure 5In region A of the fiber, this component is rapidly amplified by the effective gain within the cavity, leading to incomplete clamping and resulting in a high-amplitude leading-edge spike in the mode-locked pulse, exhibiting an overall h-shaped distribution. Building upon this, further increasing the power intensity injected from laser diode-1 into thulium-doped fiber-3 allows the repetition rate of the h-shaped mode-locked pulse to increase by an integer multiple of the fundamental frequency due to long-range pulse interactions caused by intracavity modulation instability, gain loss, and recovery mechanisms. The seed source fundamental frequency is determined by the seed source cavity length; to obtain a fundamental frequency pulse in the MHz range, the cavity length cannot exceed 206.9 m.

[0041] The thulium-doped fiber amplifier II is a typical master oscillator power amplifier, employing a forward-pumped configuration. It consists of a polarization-independent isolator 11, a laser diode 12, a pump & signal fiber combiner 13, and a thulium-doped fiber 14. The output of the output coupler 5, the polarization-independent isolator 11, the pump & signal fiber combiner 13, and the thulium-doped fiber 14 are connected sequentially. The laser diode 12 is connected to the input of the pump & signal fiber combiner 13, and the output of the thulium-doped fiber 14 is connected to the fluorotellurate fiber III. The pump-gain structure of the thulium-doped fiber amplifier II is consistent with that of the h-shaped mode-locked pulse seed source I, differing only slightly in the specifications and length of the thulium-doped fiber 14. For higher-power h-shaped pulses, a multi-stage amplifier cascade structure is used. Only a single-stage amplifier is shown here.

[0042] For fluorotellurate fiber III, Figure 4 In this embodiment, the primary function of component 15 is to stimulate stimulated Raman scattering via pulse pumping, thereby promoting the conversion of pump energy to longer wavelengths (redshift). Its pump source is an amplified h-shaped pulsed laser, employing a simple single-pass Raman structure design that minimizes Raman threshold and avoids bandwidth limitations. The laser signal is directly input from one end of the fiber and output from the other. Due to the high-intensity leading edge of the h-shaped pulse, Raman scattering is rapidly excited upon entering the fiber, and the energy within the pulse is quickly transferred to the first-order Raman signal. The selected fluorotellurate fiber 15 can be mechanically connected or directly fused to the output fiber of the thulium-doped fiber amplifier II. However, considering the higher complexity of fusion coupling, this embodiment uses mechanical connection, achieving a coupling efficiency of no less than 73%. Because the fluorotellurate fiber 15 exhibits a large frequency shift, when using a 2μm band h-shaped pulse pump, it helps reduce the Raman frequency shift order, directly shifting the pump wavelength to around 2.4μm, thus avoiding unnecessary pump consumption. Meanwhile, among common mid-infrared soft glass fibers, fluorotellurate fiber 15 has a high damage resistance threshold and a large Raman gain coefficient. Therefore, it can compensate for the loss introduced by fiber coupling to a certain extent and has the value of obtaining long-wavelength high-power Raman laser output.

[0043] In this embodiment, after the h-shaped mode-locked pulse seed source I, the thulium-doped fiber amplifier II includes a polarization-independent isolator II 11, a laser diode II 12, and a pump & signal fiber combiner II 13. Its specifications are basically the same as the corresponding devices in the h-shaped mode-locked pulse seed source I, but the thulium-doped fiber II 14 is different.

[0044] Following the thulium-doped fiber amplifier, the fluorotellurate fiber is a Raman gain fiber developed and provided by Jilin University, with a length of approximately 6.75m, a transmission loss of ~0.4dB / m at a wavelength of 2μm, and a peak Raman gain coefficient of 1.28×10⁻⁶. - 12 m / W, frequency shift of 785cm -1 .

[0045] In this embodiment, laser diode 1 and laser diode 2 are pump light sources with an output wavelength of 793nm and a maximum output power of 20W; pump & signal fiber combiner 2 and pump & signal fiber combiner 2 are used to combine 793nm pump and 2μm band laser signals.

[0046] Thulium-doped fiber 13 and Thulium-doped fiber 24 are double-clad single-mode fibers. In the h-shaped mode-locked pulse seed source I, Thulium-doped fiber 13 has a length of 3.6 m, a core / cladding diameter ratio of 10 / 128, and an octagonal inner cladding with a corresponding numerical aperture of 0.45. In the Thulium-doped fiber amplifier II, Thulium-doped fiber 24 has a length of 5 m and a core / cladding diameter ratio of 10 / 130.

[0047] The dispersion-compensating fiber 4 is a normal dispersion fiber with a group velocity dispersion value of +89 ps. 2 / km, length ≥20m, the length can be adjusted according to the net dispersion and nonlinearity requirements in the cavity.

[0048] The output coupler 5 has a coupling ratio of 50:50 and is used for beam splitting inside and outside the laser cavity. It includes two 50% output ports, which are respectively connected to the polarization-independent isolator inside the annular cavity and the polarization-independent isolator at the amplifier input.

[0049] The polarization-independent isolator 6 and polarization-independent isolator 11 are used to maintain stable unidirectional laser transmission and avoid laser echo damage.

[0050] The polarization controller 7 and polarization controller 10 work together with the polarizer to achieve the nonlinear polarization rotation effect of the light wave and promote the generation of mode-locked pulses.

[0051] The polarization-maintaining fiber 9 is located between the polarizer 8 and the polarization controller 10, and its length is approximately 0.5m.

[0052] The polarizer 8 is a polarization-dependent isolator, which can also be replaced by a 45° tilted fiber optic grating.

[0053] The proposed solution utilizes an h-shaped pulse-pumped fluorotellurate fiber. Figure 6 and Figure 7 As shown, some experimental results have demonstrated that this concept helps improve the performance of Raman fiber lasers. When an h-shaped pulse with a wavelength of 2μm and a repetition rate of 100kHz is injected into a 6μm core diameter, ~2.7m length fluorotellurate fiber 15, a weak Raman laser signal with a wavelength of 2.38μm can be detected. After changing the fiber length to 6.75m to promote Raman gain accumulation, the total output power of the laser is 0.84W under a 1.74W pump excitation, and no obvious radiation noise is observed between the pump and Raman signal spectra. Among them, the optical power corresponding to the 2.38μm Raman pulse is 306mW with a conversion efficiency of 17.6%, which is the highest power level of mid-infrared soft glass fiber Raman pulse laser, and is nearly 10 times the power of similar lasers in the same wavelength band currently available internationally.

[0054] In summary, this invention enables efficient generation of mid-infrared high-repetition-rate nanosecond Raman pulse fiber lasers. While improving the performance of Raman fiber lasers, it can also obtain high-repetition-rate nanosecond pulse output in the mid-infrared band by utilizing the repetition rate characteristics of the mode-locked seed source. This is beneficial for applications such as high-speed optical sampling and precision detection that require high-repetition-rate, short-pulse-width light sources.

[0055] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source, characterized in that, include: h-shaped mode-locked pulse seed source (I), thulium-doped fiber amplifier (II), and fluorotellurate fiber (III); h-shaped mode-locked pulse seed source (I) generates h-shaped nanosecond mode-locked pulses in the 2μm band, and outputs fundamental or harmonic mode-locked h-shaped pulses with high repetition rate characteristics of MHz and above by adjusting the nonlinear phase shift and power clamping state in the cavity. The thulium-doped fiber amplifier (II) receives the h-shaped pulse generated by the h-shaped mode-locked pulse seed source (I) and increases the power level of the h-shaped pulse without changing the pulse shape; Fluorotellurate fiber (III) is used as the Raman gain medium. It is fused to the output end of thulium-doped fiber amplifier (II) based on a single-pass structure and excited by the Raman frequency shift using h-shaped pulse pump. The h-shaped mode-locked pulse seed source (I) includes a laser diode (1) and, in sequence, a pump and signal fiber combiner (2), a thulium-doped fiber (3), a dispersion compensation fiber (4), an output coupler (5), a polarization-independent isolator (6), a polarization controller (7), a polarizer (8), a polarization-maintaining fiber (9), and a second polarization controller (10). The pump and signal fiber combiner (2) is located on the output side of the laser diode (1). The group velocity dispersion of the dispersion compensation fiber (4) is +89 ps. 2 / km, length ≥20m; the polarization-maintaining fiber (9) is located between the polarizer (8) and the second polarization controller (10), with a length of 0.5m, and is used to assist in the harmonic mode-locking of the h-shaped pulse; the polarizer (8) adopts a polarization-dependent isolator.

2. The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source as described in claim 1, characterized in that, In the h-shaped mode-locked pulse seed source (I), a laser diode (1) with a wavelength of 793nm and a maximum optical power of 20W serves as its pump source. The pump light emitted is coupled into the inner cladding of the thulium-doped fiber (3) which serves as the gain medium through the pump and signal fiber combiner (2). Under the excitation of the pump light, the thulium ions absorb energy and radiate a 2μm band optical signal through energy level transition. The optical signal is transmitted through the ring cavity formed by the various devices of the h-shaped mode-locked pulse seed source (I) for one cycle and then enters the thulium-doped fiber (3) again through the pump and signal fiber combiner (2) to be amplified until the cavity loss and gain reach a balance. Then, a stable laser signal is output through the output coupler (5).

3. The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source as described in claim 2, characterized in that, The thulium-doped fiber amplifier (II) is forward-pumped and includes a polarization-independent isolator II (11), a laser diode II (12), a pump & signal fiber combiner II (13), and a thulium-doped fiber II (14). The output end of the output coupler (5), the polarization-independent isolator II (11), the pump & signal fiber combiner II (13), and the thulium-doped fiber II (14) are connected in sequence. The laser diode II (12) is connected to the input end of the pump & signal fiber combiner II (13), and the output end of the thulium-doped fiber II (14) is connected to the fluorotellurate fiber (III).

4. The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source as described in claim 3, characterized in that, The fluorotellurate fiber (III) is a Raman gain fiber with a transmission loss of 0.4 dB / m at a wavelength of 2 μm and a peak Raman gain coefficient of 1.28 × 10⁻⁶. -12 m / W, frequency shift of 785cm -1 .

5. The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source as described in claim 4, characterized in that, The laser diode one (1) and laser diode two (12) are pump light sources with an output wavelength of 793nm and a maximum output power of 20W; the pump & signal fiber combiner one (2) and the pump & signal fiber combiner two (13) are for combining 793nm pump and 2μm band laser signals.

6. The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source as described in claim 5, characterized in that, Both the thulium-doped fiber one (3) and the thulium-doped fiber two (14) are double-clad single-mode fibers. In the h-shaped mode-locked pulse seed source (I), the length of the thulium-doped fiber one (3) is 3.6m, the core / cladding diameter ratio is 10 / 128, the inner cladding is octagonal, and the corresponding numerical aperture is 0.

45. In the thulium-doped fiber amplifier (II), the length of the thulium-doped fiber two (14) is 5m, and the core / cladding diameter ratio is 10 / 130.

7. The mid-infrared high-repetition-rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source as described in claim 6, characterized in that, The output coupler (5) has a coupling ratio of 50:50 and includes two 50% output ports, which are respectively connected to polarization-independent isolator one (6) and polarization-independent isolator two (11).

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