Middle-infrared high-repetition-frequency Raman pulse fiber laser based on h-shaped nanosecond mode locking source

Abstract

The invention discloses a mid-infrared high-repetition-frequency Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source. The mid-infrared high-repetition-frequency Raman pulse fiber laser comprises an h-shaped mode-locked pulse seed source, a thulium-doped fiber amplifier and a fluorotellurite fiber, the h-shaped mode-locked pulse seed source generates h-shaped nanosecond mode-locked pulses with the waveband of 2 micrometers, and fundamental frequency or harmonic mode-locked h-shaped pulses with the high repetition frequency characteristic of MHz or above are output by regulating and controlling the nonlinear phase shift amount and the power clamping state in a cavity. The thulium-doped optical fiber amplifier receives the h-shaped pulse generated by the h-shaped mode-locked pulse seed source and improves the power level of the h-shaped pulse without changing the pulse form; the fluorotellurite optical fiber serves as a Raman gain medium and is in butt joint or fusion welding coupling with the output end of the thulium-doped optical fiber amplifier based on a single-pass structural form, and Raman frequency shift is excited through h-shaped pulse pumping. According to the invention, unnecessary pumping consumption in the middle spectral region can be avoided, the quality of the output spectrum is optimized, and the method has universality and expandability and can be applied to generation of pulse laser of other wavebands.

Description

Technical Field

[0001] The invention belongs to the field of optical fiber lasers and relates to a mid-infrared high-repetition-rate Raman pulse optical fiber laser which is based on an H-shaped nanosecond mode-locked pulse signal and can realize efficient energy conversion to a long wave. Background Art

[0002] In the field of laser technology, the mid-infrared band (2μm to 5μm) covers most of the molecular "fingerprint" regions and multiple atmospheric transmission windows, and has the characteristics of strong water absorption. Therefore, mid-infrared lasers, especially mid-infrared nanosecond pulse lasers, have become the focus of research at home and abroad in recent years due to their special working bands and high energy and short pulse width characteristics. In addition, with the increasing maturity and perfection of related material preparation processes and laser technology, long-wave mid-infrared nanosecond pulse lasers have also shown great application value and potential in scientific research experiments, optical precision measurement, and material processing.

[0003] Generally speaking, according to the different wavelength conversion mechanisms, mid-infrared fiber lasers are roughly divided into two categories, namely: rare earth ion doped fiber lasers and Raman fiber lasers. The former mainly selects a suitable pump light source to excite, matches the energy level transition characteristics of doped ions (such as thulium, holmium, erbium ions, etc.) in the gain fiber, and uses different optical switching technologies such as Q-switching and gain modulation to obtain mid-infrared pulse lasers corresponding to different wavelengths and pulse widths. However, this method strictly depends on the emission characteristics of rare earth materials and the performance of optical switches, and there are still great limitations in achieving further expansion of laser wavelengths; and under the superposition limitation of optical switch elements, the maximum repetition frequency of nanosecond and microsecond short pulses above 2.1μm obtained based on Q-switching or gain modulation technology does not exceed 300kHz. Raman fiber lasers can use the existing rare earth ion doped fiber pulse laser source as a signal, and use the stimulated Raman scattering of different types of optical fibers to realize its conversion output to a longer wavelength. In theory, relying on this method, as long as the pump and optical fiber are properly combined, the flexible output of mid-infrared pulse lasers of any wavelength in the optical fiber transmission window can be achieved. This provides a reliable technical approach to make up for the shortcomings of rare earth ion-doped fiber lasers and effectively expand the wavelength of mid-infrared pulsed lasers.

[0004] At present, in mid-infrared Raman pulse fiber lasers, researchers from various countries mostly use active Q-switched lasers and laser diodes with wavelengths close to 1.5μm or 2μm and external control elements as seed light sources due to pulse pumping requirements, while Raman gain fiber is made of high-concentration GeO doped with a wide long-wave transmission window. 2 Mainly quartz-based optical fiber and mid-infrared soft glass optical fiber.

[0005] In 2008, Rakich et al. from the Massachusetts Institute of Technology injected the amplified 1.53μm gain-modulated pulse into a section of commercial germanium-doped quartz fiber with a concentration of about 58wt.%, and used the spontaneous cascade Raman amplification mechanism to take the lead in achieving the output of a 5th-order cascaded Raman nanosecond pulse with a maximum wavelength of 2.41μm. In 2015, Jiang et al. from the Shanghai Institute of Optics and Fine Mechanics used the same mechanism and optical fiber to address the problem of low output power of 2.41μm Raman pulse lasers reported above, using an actively Q-switched pulse seed with a wavelength of 2.008μm and 2.04μm, a repetition frequency of 8kHz, and a pulse width of 100ns, to obtain second-order Raman pulses with wavelengths of 2.43μm and 2.48μm, respectively. However, judging from the spectral results, due to the Raman gain saturation effect and the nonlinear broadening of the optical fiber spectrum, there is a lot of radiation noise between the cascaded Raman signals ( Figure 1 and Figure 2 ). In addition, due to the influence of the seed source, this type of laser usually requires an additional control system to adjust the pump parameters, which makes the overall structure of the system complex and expensive; at the same time, due to the parameters of the available seed source, its pulse repetition frequency is generally not high.

[0006] During this period, many experimental and theoretical research reports on the use of different types of soft glass fibers such as chalcogenides, fluorides, and tellurides to obtain long-wave Raman lasers have appeared internationally. However, there have been no reports on the use of fluoride glass fibers and telluride glass fibers to obtain mid-infrared nanosecond Raman pulses. In the reports on chalcogenide glass fiber Raman lasers, as early as 2014, Bernier et al. at Laval University in Canada had already used a double grating nested FP cavity structure in As 2 S 3 In the single-mode fiber cascade Raman laser, the wavelength record of Raman pulses was first broken to 3.77μm. However, its total output power is less than 10mW and is in a quasi-continuous state. In 2021, Wang et al. from Northeastern University in China used their homemade As 2 S 5The glass optical fiber and single-pass Raman structure used a 0.1W, 10kHz 2μm nanosecond pulse source as a pump. Relying on the high Raman gain characteristics of the optical fiber, a 6-order cascade Raman frequency shift from 2.149μm to 3.425μm was experimentally observed. Unfortunately, affected by the damage threshold of the optical fiber, the incident pump light signal was weak, and the multi-order cascade frequency shift resulted in excessive consumption. The report failed to provide relevant characteristics such as the average power, energy, and efficiency of the system output Raman light. In 2022, Jiao et al. from Jilin University in China selected a new type of self-made glass optical fiber (fluorotellurite optical fiber) as the Raman gain medium. By pumping with a 1.55μm nanosecond pulse with a repetition rate of 80kHz, they observed a third-order cascade Raman frequency shift. The maximum wavelength and corresponding average power and conversion efficiency were 2.438μm, 31.6mW, and 6.43%, respectively ( Figure 3 ). Due to the short wavelength of the pump pulse and the excessive consumption of pump energy in the intermediate frequency shift process, the power level of the third-order Raman pulse is compared with the output results of the nanosecond Raman pulse laser in the same band shown above, and has not yet demonstrated the significant advantages of fluorotellurite fiber in terms of large frequency shift and high Raman gain. Summary of the invention

[0007] 1. Technical issues to be resolved

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

[0009] a. Solve the problem that the output pulse repetition rate of mid-infrared Raman pulse fiber laser is limited due to traditional Q-switched source or electrical gain modulated pulse source;

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

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

[0012] (II) Technical solution

[0013] In order to solve the above technical problems, the present invention provides a mid-infrared high repetition rate Raman pulse fiber laser based on an h-shaped nanosecond mode-locked source, which comprises: an h-shaped mode-locked pulse seed source I, a thulium-doped fiber amplifier II and a fluorotellurite fiber III; the h-shaped mode-locked pulse seed source I generates an h-shaped nanosecond mode-locked pulse in the 2 μm band, and outputs a fundamental frequency or harmonic mode-locked h-shaped pulse with a high repetition rate characteristic of MHz or above by regulating the nonlinear phase shift amount and the 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 improves the power level of the h-shaped pulse without changing the pulse morphology; the fluorotellurite fiber III is used as a Raman gain medium, and is docked or fusion-coupled with the output end of the thulium-doped fiber amplifier II based on a single-pass structure, and the Raman frequency shift is excited by the h-shaped pulse pump.

[0014] Among them, the h-shaped mode-locked pulse seed source I includes a laser diode 1 and a pump & 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 polarization controller 10 connected in sequence from beginning to end, and the pump & signal fiber combiner 2 is arranged on the light output side of the laser diode 1.

[0015] Among them, 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 is used as its pump light source. The pump light emitted is coupled into the inner cladding of the thulium-doped optical fiber 3 as the gain medium through the pump & signal optical fiber combiner 2. Under the excitation of the pump light, the thulium ions absorb energy and radiate 2μm band optical signals through energy level transition. After the optical signal is transmitted through the ring cavity surrounded by the various devices of the h-shaped mode-locked pulse seed source I for one cycle, it passes through the pump & signal optical fiber combiner 2 and enters the thulium-doped optical fiber 3 again to be amplified until the loss and gain in the cavity reach a balanced state, and then a stable laser signal is output through the output coupler 5.

[0016] Among them, the thulium-doped fiber amplifier II adopts a forward pumping mode, including a polarization-independent isolator 11, a laser diode 12, a pump & signal fiber combiner 13 and a thulium-doped fiber 14, the output end of the output coupler 5, the polarization-independent isolator 11, the pump & signal fiber combiner 13 and the thulium-doped fiber 14 are connected in sequence, the laser diode 12 is connected to the input end of the pump & signal fiber combiner 13, and the output end of the thulium-doped fiber 14 is connected to the fluorotellurite fiber III.

[0017] The fluorotellurite 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 -12 m / W, frequency shift is 785cm -1 .

[0018] Among them, the laser diode 1 and the laser diode 2 12 are pump light sources with an output wavelength of 793nm and a maximum light output power of 20W; the pump & signal fiber combiner 1 2 and the pump & signal fiber combiner 2 13 are used for combining 793nm pump and 2μm band laser signals.

[0019] Among them, the thulium-doped optical fiber 13 and the thulium-doped optical fiber 214 are both double-clad single-mode optical fibers. In the h-shaped mode-locked pulse seed source I, the thulium-doped optical fiber 3 is 3.6 m long, 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 optical fiber amplifier II, the thulium-doped optical fiber 214 is 5 m long, and the core / cladding diameter ratio is 10 / 130.

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

[0021] The coupling ratio of the output coupler 5 is 50:50, and the output coupler 5 includes two 50% output ports, which are respectively connected to the polarization-independent isolator 1 6 and the polarization-independent isolator 2 11 .

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

[0023] (III) Beneficial effects

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

[0025] (1) The present invention adopts a 2 μm h-shaped pulse thulium-doped fiber mode-locked laser to replace the traditional Q-switched source or electrical 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 device, and the cost is lower. The pulse shape and characteristic parameters such as repetition rate and pulse width can be flexibly changed by only adjusting the nonlinear phase shift of the light wave in the cavity. The operation method and structure are simpler.

[0026] (2) Compared with the most common noise-like mode-locked nanosecond pulses in fiber lasers (composed of many split femtosecond sub-pulses), the h-shaped nanosecond pulses provided by the present invention do not have the phenomenon of light wave splitting in the time domain envelope. Therefore, the influence of harmful nonlinearity in the system can be better avoided, and its power level can be efficiently improved without obvious distortion of the time domain morphology during the amplification process, which is conducive to suppressing the influence of Raman gain saturation and spectrum broadening on the radiation noise in the single-pass structure.

[0027] (3) The present invention uses amplified h-shaped pulses as pumps. With the inherent high repetition rate characteristics of mode-locked pulses and the high energy characteristics of h-shaped pulses, it can ensure that Raman pulses also obtain high repetition rate characteristics. At the same time, the leading edge peak of the h-shaped pulse can quickly stimulate the Raman frequency shift effect. Combined with the high energy ratio per unit time on the base, it can promote Raman gain accumulation, thereby achieving efficient conversion of pump energy to long waves. Through experimental comparison of Raman pulse lasers in the same band based on Q-switched pulse pumping, it is found that when using h-shaped pulses with a repetition rate of kHz to pump, the Raman conversion efficiency of the laser can be increased by more than 1.5 times.

[0028] (4) The present invention combines the 2μm band h-shaped pulse pumping scheme with the high Raman gain and large frequency shift characteristics of fluorotellurite glass fiber, which can avoid unnecessary pump consumption in the intermediate spectral region and optimize the output spectrum quality. In experimental studies, when an h-shaped pulse with a power of nearly 1.74W is used to pump a fluorotellurite fiber with a length of about 6.75m, Raman pulses with a wavelength of up to 2.4μm can be obtained only through the first-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 reach ~2.9μm through the second-order Raman frequency shift.

[0029] (5) The proposed method for efficiently generating mid-infrared high-repetition-rate nanosecond Raman pulses is universal and scalable, and can be applied to pulse laser generation in other bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 and Figure 2 It is a schematic diagram of the structure and output laser spectrum of a 2.2μm&2.4μm second-order cascaded Raman laser based on 2μm nanosecond Q-switched pulse pumping in the background technology.

[0031] Figure 3 The third-order cascade Raman laser spectrum is obtained by amplifying a 1.55 μm electrically modulated nanosecond seed pulse and pumping a fluorotellurite optical fiber in the background technology. The wavelength of the third-order Raman pulse is about 2.44 μm, corresponding to a power of 31.6 mW.

[0032] Figure 4The structure diagram of the laser of the present invention is shown in the figure, and the marks in the figure are respectively represented as: 1, laser diode 1; 2, pump & signal fiber combiner 1; 3, thulium-doped fiber 1; 4, dispersion compensation fiber; 5, output coupler; 6, polarization-independent isolator 1; 7, polarization controller 1; 8, polarizer; 9, polarization-maintaining fiber; 10, polarization controller 2; 11, polarization-independent isolator 2; 12, laser diode 2; 13, pump & signal fiber combiner 2; 14, thulium-doped fiber 2; 15, fluorotellurate fiber. The rotation arrow in the figure indicates the optical path circulation direction in the ring cavity mode-locked fiber laser.

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

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

[0035] In order to make the purpose, content and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below in conjunction with the drawings and examples.

[0036] The present invention aims to study a scheme for efficiently generating mid-infrared high-repetition-rate Raman pulses based on a 2μm-band h-shaped nanosecond mode-locked signal and a fluorotellurite optical fiber. The Raman pulse fiber laser does not require an external modulation device, and the pulse width and repetition rate can be regulated by changing the seed source pump power or the mode-locked state, respectively. The 2μm h-shaped seed signal is amplified and then pumped into the fluorotellurite optical fiber, which can effectively suppress the intermediate process pump consumption, promote energy conversion, and achieve higher-power long-wave pulse laser output.

[0037] like Figure 4As shown, the mid-infrared high repetition rate Raman pulse fiber laser based on the h-shaped nanosecond mode-locked source in this embodiment includes: an h-shaped mode-locked pulse seed source I, a thulium-doped fiber amplifier II and a fluorotellurite fiber III; the h-shaped mode-locked pulse seed source I generates an h-shaped nanosecond mode-locked pulse in the 2 μm band, and outputs a fundamental frequency or harmonic mode-locked h-shaped pulse with a high repetition rate characteristic of MHz or above by regulating the nonlinear phase shift amount and the power clamping state in the cavity, 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 pulse generated by the h-shaped mode-locked pulse seed source I and improves the power level of the h-shaped pulse without changing the pulse morphology; the fluorotellurite fiber III is used as a Raman gain medium, and is docked or fusion-coupled with the output end of the thulium-doped fiber amplifier II based on a single-pass structure, and the h-shaped pulse pump is used to excite the Raman frequency shift. The laser mainly relies on the inherent characteristics of H-shaped mode-locked pulses and the advantages of fluorotellurite optical fiber to reduce the order of intermediate frequency shifts and ineffective pump consumption, optimize spectral quality, promote energy conversion during frequency shift, and achieve higher-power mid-infrared high-repetition-rate nanosecond pulse output.

[0038] The ring resonator of the h-shaped mode-locked pulse seed source I adopts a typical nonlinear polarization rotation structure with a simple and compact layout and a strong anti-damage threshold. The slight difference is that a section of polarization-maintaining fiber is inserted between the polarizer and the polarization controller 2 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 a pump & 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 polarization controller 10 connected in sequence. The pump & signal fiber combiner 2 is arranged on the light-emitting 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 is used as its pump light source. The emitted pump light is coupled into the inner cladding of the thulium-doped fiber 3 as the gain medium through the pump & signal fiber combiner 2. Under the excitation of the pump light, the thulium ions absorb energy and radiate 2μm band optical signals through energy level transition. After the signal is transmitted through the ring cavity for one cycle, it passes through the pump & signal fiber combiner 2 and enters the thulium-doped fiber 3 again to be amplified until the loss and gain in the cavity reach a balanced state. The h-shaped mode-locked pulse seed source I outputs a stable laser signal through the output coupler 5. At this time, the working mode of the laser is generally in a continuous state. The dispersion compensation fiber 4 is mainly used to adjust the net group velocity dispersion value in the cavity and increase the nonlinear value, so as to reduce the generation threshold of the fundamental frequency mode-locked pulse. The role of the polarization-independent isolator 6 is to achieve stable unidirectional transmission of the laser signal and avoid laser echo damage. Polarization-maintaining fiber 9 is responsible for regulating the comb-shaped transmission spectrum state (such as bandwidth, period, etc.) in the seed source cavity, which is helpful for the generation of h-shaped pulses and harmonic mode-locking phenomena. If high repetition rate pulses are not required, the fiber can be removed. Polarization controller 1 7, polarizer 8 and polarization controller 2 10 are the core mechanisms of the seed source (nonlinear polarization rotation structure), which are used for polarization rotation and filtering selection of optical signals to achieve mode-locked pulse generation and narrowing. Among them, polarization controller 1 7 and polarization controller 2 10 are mainly responsible for applying stress to the clamped optical fiber to change the polarization direction of the optical signal by inducing a weak birefringence effect; polarizer 8 uses a polarization-dependent isolator, which has the triple functions of polarization, polarization detection and optical signal echo suppression.

[0040] Since the nonlinear polarization rotation structure formed by the polarization controller 1 7, the polarizer 8 and the polarization controller 2 10 is equivalent to a saturable absorber, and the transmittance curve of the optical signal in the cavity shows a sinusoidal periodic variation characteristic with the total phase shift, such as Figure 5 As shown. And as the total phase shift increases, the transmittance of the optical signal will gradually decrease in the anti-saturation absorption region (region B, D). At this time, once the intensity of the optical pulse component exceeds its maximum transmittance, the absorber will act as a power equalizer to limit the intensity of each component of the optical pulse. Coupled with its periodic filtering characteristics, increasing the additional phase shift of the polarization controller, the peak position of the transmission spectrum will move toward the long wave. Therefore, by adjusting the polarization controller 17 and the polarization controller 2 10, the position of the laser transmission spectrum can be changed at the same time. When the frequency component corresponding to the front edge of the optical pulse in the time domain is located near the transmission peak, and is located due to the low intensity. Figure 5When the A region is reached, this component will be rapidly amplified under the effective gain in the cavity, resulting in incomplete clamping, so that the mode-locked pulse has a high amplitude front peak and an overall h-shaped distribution. On this basis, the power intensity injected into the thulium-doped fiber 3 by the laser diode 1 is further increased. Due to the long-range interaction of pulses caused by intracavity modulation instability, gain loss and recovery mechanism, the repetition rate of the h-shaped mode-locked pulse can increase by an integer multiple of the fundamental frequency. The fundamental frequency of the seed source is determined by the seed source cavity length. To obtain MHz-level fundamental frequency pulses, the cavity length cannot exceed 206.9m.

[0041] For Thulium-doped Fiber Amplifier II, it is a typical master oscillator power amplifier, which adopts forward pumping mode. It is composed of polarization-independent isolator 11, laser diode 12, pump & signal fiber combiner 13 and thulium-doped fiber 14. The output end of output coupler 5, polarization-independent isolator 11, pump & signal fiber combiner 13 and thulium-doped fiber 14 are connected in sequence. Laser diode 12 is connected to the input end of pump & signal fiber combiner 13, and the output end of thulium-doped fiber 14 is connected to fluorotellurite fiber III. The pump-gain structure of Thulium-doped Fiber Amplifier II is consistent with the layout of h-shaped mode-locked pulse seed source I, and is only slightly different in the specifications and length selection of thulium-doped fiber 14. For higher power h-shaped pulses, a multi-stage amplifier cascade structure is adopted. Only one stage amplifier is used for demonstration here.

[0042] For Fluorotellurate Fiber III, Figure 4 The main function of the reference numeral 15 in this embodiment is to stimulate the stimulated Raman scattering effect through pulse pumping and promote the conversion of pump energy to long waves (red shift). Its pump light source is an amplified h-shaped pulse laser, which adopts the simplest single-pass Raman structure design with low Raman threshold and can avoid the limitation of device working bandwidth, that is, the laser signal is directly input from one end of the optical fiber and output from the other end. Due to the high-intensity front peak of the h-shaped pulse, it will quickly stimulate Raman scattering after entering the optical fiber, and the energy in the pulse will be quickly transferred to the first-order Raman signal. The selected fluorotellurite fiber 15 can be mechanically docked or directly fusion-coupled with the output fiber of the thulium-doped fiber amplifier II. However, considering the difficulty of fusion coupling, the mechanical docking method is adopted in the embodiment, and the coupling efficiency is not less than 73%. Since the fluorotellurite fiber 15 has a large frequency shift characteristic, when the h-shaped pulse pumping in the 2μm band is used, it can help reduce the Raman frequency shift order and directly shift the pump wavelength to near 2.4μm, thereby avoiding unnecessary pump consumption. At the same time, among the common mid-infrared soft glass fibers, fluorotellurite fiber 15 has a high damage 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-wave and 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 11, a laser diode 12, and a pump & signal fiber combiner 13, whose specifications are basically the same as the corresponding devices in the h-shaped mode-locked pulse seed source I, but the thulium-doped fiber 14 is different.

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

[0045] In this embodiment, laser diode 1 and laser diode 2 12 are pump light sources with an output wavelength of 793 nm and a maximum optical output power of 20 W; pump & signal fiber combiner 1 2 and pump & signal fiber combiner 2 13 are used for combining 793 nm pump and 2 μm band laser signals.

[0046] Thulium-doped fiber 13 and thulium-doped fiber 214 are double-clad single-mode fibers. In the h-shaped mode-locked pulse seed source I, the thulium-doped fiber 13 is 3.6 m long, 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 thulium-doped fiber 214 is 5 m long, and the core / cladding diameter ratio is 10 / 130.

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

[0048] The coupling ratio of the output coupler 5 is 50:50, and it 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 in the ring cavity and the polarization-independent isolator at the input end of the amplifier.

[0049] The polarization-independent isolator 1 6 and the polarization-independent isolator 2 11 are used to maintain stable unidirectional transmission of laser and avoid laser echo damage.

[0050] The polarization controller 1 7 and the polarization controller 2 10 work together with the polarizer to achieve a nonlinear polarization rotation effect of light waves and promote the generation of mode-locked pulses.

[0051] The polarization-maintaining optical fiber 9 is located between the polarizer 8 and the polarization controller 10, and has a length of about 0.5 m.

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

[0053] Regarding the concept of using h-shaped pulses to pump fluorotellurite fibers, Figure 6 and Figure 7 As shown in the figure, some experimental results have proved that this concept is helpful to 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 fluorotellurite fiber 15 with a core diameter of 6μm and a length of ~2.7m, a weak Raman laser signal with a wavelength of 2.38μm can be found. After the fiber length is changed to 6.75m to promote Raman gain accumulation, under 1.74W pump excitation, the total output power of the laser is 0.84W, and no obvious radiation noise is observed between the pump and Raman signal spectra. Among them, the corresponding optical power of the 2.38μm Raman pulse is 306mW and the conversion efficiency is 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 band in the world.

[0054] In summary, the present invention can realize the efficient generation of mid-infrared high-repetition-rate nanosecond Raman pulse fiber laser, and can promote the performance improvement of Raman fiber laser, while using the repetition-rate characteristics of the mode-locked seed source to obtain high-repetition-rate nanosecond pulse output in the mid-infrared band, which is beneficial to the needs of high-repetition-rate and short-pulse-width light sources for applications such as high-speed optical sampling and precision detection.

[0055] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A mid-infrared high repetition rate Raman pulsed 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 fluorotellurite fiber (III); the h-shaped mode-locked pulse seed source (I) generates h-shaped nanosecond mode-locked pulses in the 2 μm band, and outputs fundamental frequency or harmonic mode-locked h-shaped pulses with high repetition rate characteristics of MHz and above by regulating the nonlinear phase shift amount 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 a Raman gain medium and is butt-jointed or fusion-coupled with the output end of a thulium-doped fiber amplifier (II) based on a single-pass structure, and Raman frequency shift is excited by h-shaped pulse pumping.

2. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source according to claim 1, characterized in that: The h-shaped mode-locked pulse seed source (I) comprises a laser diode (1) and a pump & 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 polarization controller (10) connected in sequence end to end. The pump & signal fiber combiner (2) is arranged on the light-emitting side of the laser diode (1).

3. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source as claimed in claim 2, 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 is used as its pump light source. The pump light emitted is coupled into the inner cladding of a thulium-doped optical fiber (3) as a gain medium through a pump & signal optical fiber combiner (2). After thulium ions absorb energy under the stimulation of the pump light, they radiate a 2μm band optical signal through energy level transition. After the optical signal is transmitted through the annular cavity formed by the components of the h-shaped mode-locked pulse seed source (I) for one cycle, it passes through the pump & signal optical fiber combiner (2) and enters the thulium-doped optical fiber (3) again to be amplified until the loss and gain in the cavity reach a balanced state, and then a stable laser signal is output through an output coupler (5).

4. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source as claimed in claim 3, characterized in that: The thulium-doped fiber amplifier (II) adopts a forward pumping mode, and comprises a second polarization-independent isolator (11), a second laser diode (12), a second pump & signal fiber combiner (13) and a second thulium-doped fiber (14); the output end of the output coupler (5), the second polarization-independent isolator (11), the second pump & signal fiber combiner (13) and the second thulium-doped fiber (14) are connected in sequence; the second laser diode (12) is connected to the input end of the second pump & signal fiber combiner (13); and the output end of the second thulium-doped fiber (14) is connected to the fluorotellurite fiber (III).

5. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source according to claim 4, characterized in that: The fluorotellurate optical fiber (III) is a Raman gain optical 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 -12 m / W, frequency shift is 785cm -1 .

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

7. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source according to claim 6, characterized in that: The thulium-doped optical fiber 1 (3) and the thulium-doped optical fiber 2 (14) are both double-clad single-mode optical fibers. In the h-shaped mode-locked pulse seed source (I), the thulium-doped optical fiber 1 (3) is 3.6 meters long, the core / cladding diameter ratio is 10 / 128, and the inner cladding is octagonal, corresponding to a numerical aperture of 0.45; in the thulium-doped optical fiber amplifier (II), the thulium-doped optical fiber 2 (14) is 5 meters long, and the core / cladding diameter ratio is 10 / 130.

8. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source according to claim 7, characterized in that: The group velocity dispersion value of the dispersion compensating optical fiber (4) is +89ps 2 / km, length ≥20m.

9. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source according to claim 8, characterized in that: The coupling ratio of the output coupler (5) is 50:50, and it comprises two 50% output ports, which are respectively connected to the polarization-independent isolator 1 (6) and the polarization-independent isolator 2 (11).

10. The mid-infrared high repetition rate Raman pulse fiber laser based on an H-shaped nanosecond mode-locked source according to claim 9, characterized in that: The polarization-maintaining optical fiber (9) is located between the polarizer (8) and the second polarization controller (10) and has a length of 0.5 m; the polarizer (8) adopts a polarization-dependent isolator.

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