CN109755850B - Intermediate infrared Raman ultrafast fiber laser oscillator based on micro-cavity - Google Patents

Abstract

The invention discloses a microcavity-based mid-infrared Raman ultrafast fiber laser oscillator, which belongs to the fields of laser technology and nonlinear optics. The laser oscillator adopts a linear cavity structure, which mainly includes a mode-locked element saturable absorber, a pump source, a fiber combiner, a gain fiber for generating 2-5 micron laser light, a dispersion compensation element, a polarization controller, a high-Q The microcavity with a high Q value is a whispering gallery mode optical microcavity, and its quality factor Q value is not lower than 10 ⁶ . The invention utilizes the saturable absorption effect of the saturable absorption crystal and the Raman scattering effect of the high-Q-value microcavity to replace the traditional fiber with a length of kilometer to increase the nonlinearity in the cavity, and can generate coherent mode-locked pulses laser, and directly realize long-wave Raman frequency shift through intra-cavity oscillation to obtain a high repetition rate, high power ultrafast Raman laser of 2-5 microns; the invention can be applied to basic research, national defense, communication sensing, biomedical and material processing and other fields.

Description

A Microcavity-Based Mid-Infrared Raman Ultrafast Fiber Laser Oscillator

technical field

The invention belongs to the field of laser technology and nonlinear optics, in particular to a mid-infrared Raman ultrafast fiber laser oscillator based on a microcavity.

Background technique

Due to the flexible wavelength characteristics, Raman fiber lasers have always attracted the attention of researchers. At present, the wavelength range of Raman lasers has covered the visible light to mid-infrared band, and the maximum output power in the near-infrared band can exceed kilowatts. The existing development of high-power Raman fiber laser technology has the following characteristics: 1. Amplification of power through multiple ultrafast seed sources to achieve high-power Raman fiber lasers, this method requires multiple pump sources and isolators, so The cavity structure is complex, the cost is high and the reliability is low; 2 Although in the near-infrared band, Yb ³⁺ ion-doped fiber lasers and Er ³⁺ ion-doped fiber lasers can be used to form oscillators with long fibers on the order of kilometers. Thus, the Raman ultrafast laser output is directly realized in the cavity; however, the long fiber on the order of kilometer leads to a very high threshold of Raman laser, and the obtained pulse repetition frequency is on the order of tens of KHz, which is lower than that of the usual mode-locked fiber laser. two magnitudes. This is the reason for the lack of Raman ultrafast fiber laser oscillators in the mid-infrared band.

计算得出。其中f为脉冲重复频率，L表示光在谐振腔内往还一次的光程，c表示光速。由上述公式可知脉冲重复频率与腔长成反比，因此要想获得高重复频率，应尽量减短腔长。There are no reports on the realization of Raman fiber laser oscillators in the 2-5 micron band. There are two main reasons: one is that the nonlinear intensity of the interaction between light and matter is inversely proportional to the wavelength of the light (proportional to the photon energy), so The Raman threshold increases as the wavelength of light increases; second, as the wavelength approaches 2 microns, the intrinsic loss of the silica fiber to the laser increases in this band, and the Raman threshold is more difficult to achieve, so 2 microns are achieved based on long fibers. Band Raman ultrafast laser has high threshold and complex structure. Because the repetition rate of the pulse can be determined by

Calculated. Among them, f is the pulse repetition frequency, L is the optical path of the light in the resonator, and c is the speed of light. It can be known from the above formula that the pulse repetition frequency is inversely proportional to the cavity length, so in order to obtain a high repetition frequency, the cavity length should be shortened as much as possible.

SUMMARY OF THE INVENTION

In order to solve the problem of high threshold and low repetition frequency caused by realizing mid-infrared Raman ultrafast laser output through a long cavity, the present invention provides a mid-infrared Raman ultrafast fiber laser oscillator based on a microcavity.

In order to achieve the above objects, the technical solutions adopted in the present invention are as follows: a mid-infrared Raman ultrafast fiber laser oscillator based on a microcavity adopts a linear cavity structure, and mainly includes a mode-locking element saturable absorber, a lens I, and a lens II. , pump source, fiber combiner, gain fiber, dispersion compensation element, polarization controller, microcavity with high Q value, the microcavity with high Q value is a whispering gallery mode optical microcavity, and its quality factor Q value is different less than 10 ⁶ ;

Cut an oblique angle with an angle α of 8° to 20° on the side of the signal input end of the fiber combiner, and the outgoing light of the oblique angle is perpendicular to the center of the lens, that is, the angled end of the oblique angle is focused to the lock by a lens with a suitable focal length. On the saturable absorber of the mode element, the flat end of the oblique angle is connected to the signal input end of the fiber combiner to form resonance, the pump source is connected to the pump end of the fiber combiner, and one end of the gain fiber is connected to the signal output of the fiber combiner. The other end of the gain fiber is connected to one end of the dispersion compensation element, the other end of the dispersion compensation element is connected to one end of the polarization controller, the high-Q microcavity is connected to the back of the polarization controller, or the high-Q microcavity is connected to the fiber combiner and Between gain fibers, or a high-Q microcavity is connected between the gain fiber and the dispersion compensating element.

Preferably, the high-Q microcavity is one of a microsphere type, a microcolumn type, a microdisk type, a microbubble type, a microtubule type or a microring type.

Preferably, the material of the high-Q microcavity is one of low-loss quartz in a 2-micron waveband or a mid-infrared low-loss material of silicon, germanium, calcium fluoride, and zinc selenide with low-loss in a 3-micron waveband.

Preferably, the coupling mode of the high-Q microcavity adopts one of taper fiber coupling or prism-to-free space coupling.

Preferably, the mode-locking element saturable absorber is one of a semiconductor saturable absorber mirror, a graphene saturable absorber mirror, a carbon nanotube saturable absorber mirror or a graphene oxide saturable absorber mirror, and its working range is Covering the 2-3 micron waveband, the value range of the modulation depth ΔR is 8%≤ΔR≤30%.

Preferably, the pump source is a semiconductor laser pump source, the center wavelength λ is 793 nm or 976 nm, and the corresponding rare-earth ion-doped gain fiber is excited to generate laser light in the wavelength range of 2-5 microns.

Preferably, the gain fiber is a rare earth singly doped Tm ³⁺ silica fiber or a rare earth ion Tm ³⁺ and Ho ³⁺ co-doped silica fiber, which is used to generate a laser of 1.6-2.2 microns, or Er ³⁺ doped silica fiber. The fluoride fiber is used to generate a laser of about 3 microns, and then a first-order Stokes laser and a high-order Stokes laser are obtained through Raman frequency shift, which can generate mid-infrared ultrafast with a wavelength of 2 to 5 microns. laser.

Preferably, the dispersion compensation element is a dispersion compensation fiber or a chirped fiber grating.

Further, the microcavity-based mid-infrared Raman ultrafast fiber laser oscillator further includes a band-pass filter, and the band-pass filter is connected outside the resonant cavity.

Compared with the prior art, the present invention has the following beneficial effects:

1. Good wavelength expansion characteristics, simple and compact structure, and easy wavelength switching. By exciting rare earth singly doped Tm ³⁺ silica fiber or rare earth ion Tm ³⁺ , Ho ³⁺ co-doped silica fiber to provide gain to generate a laser of about 2 microns, and by increasing or decreasing the length of the gain fiber to change the center wavelength and intra-cavity gain, Through the Raman scattering effect of the microcavity element, the Raman frequency shift is realized in the cavity. By increasing the pump power, the cascaded Raman can be realized. The Raman frequency shift makes the laser move to the long-wave direction, so that the output of 2-3 Micron Raman Laser. Similarly, by exciting Er ³⁺ doped fluoride fiber to provide gain to generate a laser of about 3 microns, a Raman laser of 3 to 5 microns can be generated by Raman frequency shift.

2. Femtosecond laser with high repetition frequency (nearly 100 MHz) and high power laser (watt level) can be realized. The present invention adopts the microcavity as the main component for providing the Raman effect. The microcavity has a very high Q value and a very low optical nonlinear threshold, which greatly reduces the difficulty of mid-infrared Raman laser realization, and can directly realize long-wave Raman frequency shift through intracavity oscillation, and obtain a high frequency of tens of megahertz. Repetition rate, watt-level high-power 2-5 micron ultrafast Raman laser. In addition, the microcavity can be made of common materials such as quartz or silicon, and the cost is low and the process is simple, which can meet the needs of practical applications.

3. Adopt simple linear structure, high damage threshold and strong maintainability. In the present invention, a free space linear resonance structure formed by rare earth ion-doped optical fiber and an oblique angle is adopted, and the linear cavity structure does not need a complicated isolation device, thereby reducing the cost. The resonant structure greatly increases the energy required for Raman laser generation in the cavity, thereby improving the output power and efficiency of mid-infrared Raman laser oscillators. In view of the damage characteristics of the saturable absorber mirror, the free space structure adopted by the saturable absorber mirror makes the position of the saturable absorber mirror adjustable, so that the mid-infrared Raman laser oscillator has good maintenance and long life.

4. The present invention adopts passive mode locking technology to generate ultra-short pulse laser, does not need external additional modulation source, and has a simple structure. The invention adopts the saturable absorber as the mode-locking device, and the mode-locking performance is relatively stable.

Description of drawings

1 is a basic schematic diagram of a microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to Embodiment 1 of the present invention;

2 is a diagram of a tapered optical fiber preparation device;

Figure 3 is a diagram of a device for preparing a microsphere whispering gallery mode optical microcavity;

Figure 4 is an image of the optical microcavity microscope image of the prepared microsphere whispering gallery mode with an optical fiber handle;

Figure 5 is a microscope view of the microsphere-shaped whispering gallery mode optical microcavity coupled with a taper fiber;

FIG. 6 is a diagram of the expected mode-locked pulse sequence obtained in Embodiment 1 of the present invention;

Fig. 7 is the Raman spectrogram expected to be obtained in Example 1 of the present invention;

FIG. 8 is a basic schematic diagram of a microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to Embodiment 2 of the present invention;

Fig. 9 is the imaging image of the prepared microdisk-type whispering gallery mode optical microcavity microscope: (a) top view, (b) front view;

FIG. 10 is a diagram of a mode-locked pulse sequence expected to be obtained in Embodiment 2 of the present invention;

FIG. 11 is a Raman spectrum expected to be obtained in Example 2 of the present invention.

In the figure, 1-mode-locked element saturable absorber, 2-lens I, 3-lens II, 4-oblique angle, 5-pump source, 6-fiber combiner, 7-gain fiber, 801-dispersion compensation Fiber, 802-chirped fiber grating, 9-polarization controller, 101-microsphere whispering gallery mode optical microcavity, 102-microdisk whispering gallery mode optical microcavity, 11-bandpass filter, 12-single-mode fiber , 13-stepping motor, 14-computer, 15-motor drive, 16-camera, 17-three-dimensional mobile platform, 18-carbon dioxide laser, 19-focusing lens, 20-fiber handle, 21-pulled fiber.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

Example 1 Generation of 2-3 micron Raman mode-locked laser based on 2 micron wavelength pumping

The structure of a mid-infrared Raman ultrafast fiber laser oscillator based on microcavity is shown in Figure 1, including:

The saturable absorber 1 of the mode-locked element is a broadband semiconductor saturable absorber mirror (SESAM) in the 2μm band. The modulation depth of SESAM is in the range of 8%≤ΔR≤30%. The pulse energy in the cavity is controlled by increasing the pump power, so that the When the pulse energy flux reaches 3 to 5 times the saturation flux, the three-dimensional spatial position of the SESAM is properly adjusted to achieve mode locking;

Lens Ⅰ2, focal length is 50mm;

Lens II3, focal length is 100mm;

Bevel 4, use a fiber bevel cutter to cut a bevel with an angle of 8° to 20° to prevent laser feedback and avoid F-P effect from destroying the mode-locked state.

The pump source 5 is a semiconductor laser pump source with a center wavelength of 793nm, with a maximum output power of 30W, which can excite the corresponding gain fiber to generate laser light in the 2-micron band;

Fiber combiner 6, select (2+1)×1 quartz fiber combiner, the pump end of the combiner and the LD pigtail are connected by fusion;

The gain fiber 7 is selected from rare earth singly doped Tm ³⁺ silica fiber or rare earth ion Tm ³⁺ and Ho ³⁺ co-doped silica fiber;

Dispersion compensation element, UHNA4 dispersion compensation fiber 801 is selected to adjust the relative balance between nonlinearity and dispersion in the resonator cavity, so as to realize the tunable pulse width;

The polarization controller 9 is a manual polarization controller or an electric polarization controller. In this embodiment, a manual polarization controller is used, and the optical fiber is pulled by rotating the polarization control plate to change the non-polarization state in the cavity. The cost is low and the operation is flexible. It is easy to fix after adjusting to the appropriate polarization state, thereby enhancing the stability of the system;

For the microcavity with high Q value, the microsphere whispering gallery mode optical microcavity 101 is selected;

Cut a bevel 4 with an angle of 8° to 20° at the signal input end of the (2+1)×1 fiber combiner 6 to prevent laser feedback and prevent the F-P effect from destroying the mode-locking state; the bevel 4 has an angled end in turn Through lens II3 with a focal length of 100mm and lens I2 with a focal length of 50mm, it is focused on the semiconductor saturable absorber mirror 1, so that the light spot energy incident on the semiconductor saturable absorber mirror 1 is larger, which is conducive to reaching the mode locking threshold; The flat angle end is connected to the signal input end of the (2+1)×1 fiber combiner 6 to form resonance, the semiconductor laser pump source 5 is connected to the pump end of the (2+1)×1 fiber combiner 6, and the gain fiber One end of 7 is connected to the signal output end of the (2+1)×1 fiber combiner 6, and the other end of the gain fiber 7 is connected to the dispersion compensation fiber 801, the polarization controller 9 and the micro-spherical whispering gallery mode optical microcavity 101 in sequence. , at the output end of the microsphere whispering gallery mode optical microcavity 101, the direct output of the fiber end face is adopted, so that a mid-infrared Raman ultrafast fiber laser oscillator based on the microcavity can output a high-power Raman laser. A bandpass filter 11 with a wavelength of 2 microns is connected outside the resonator to obtain the output of the Raman laser with a specific wavelength. The optical fibers between all optical components are connected by ordinary fusion splicing.

Wherein, the preparation method of the microsphere whispering gallery mode optical microcavity is a high temperature melting and cooling method, and the specific steps are as follows:

First, the tapered optical fiber is prepared by the flame taper method: after removing the coating layer of a section of single-mode optical fiber 12 and wiping it with alcohol, it is fixed on a pair of stepper motors 13 with a clamp, and the stepper motors 13 are controlled by a computer 14. Motor drive 15 , a flame nozzle is fixed directly under the single-mode optical fiber 12. At the beginning of the experiment, adjust the flame size so that the optical fiber is in the outer flame, set the stepping motor 13 to move away from each other at the same speed, and drive the optical fiber to stretch to both ends, gradually forming a cone The camera 16 feeds back image information to the computer 14. In the experiment, the whole process is controlled by controlling the step length and moving speed of the stepping motor 13, and the taper fiber with various structural parameters is prepared; the drawing of the taper fiber preparation device is shown in Figure 2.

After the tapered fiber is prepared by the flame taper method, it is cut, and the obtained single tapered fiber is fixed on the three-dimensional mobile platform 17. The laser light emitted by the carbon dioxide laser 18 is focused by the focusing lens 19 to the end to be cut for heating. Under the action of surface tension, the optical fiber will automatically curl upward to form a spherical shape. After cooling, a microsphere-shaped microcavity 101 with an optical fiber handle 20 is obtained. The camera 16 feeds back image information to the computer 14 . The spherical microcavity prepared by this method has good uniformity and smooth surface, and has a section of optical fiber handle 20, which is easy to handle. Figure 3 shows a diagram of a microsphere-type whispering gallery mode optical microcavity fabrication device. Fig. 4 shows the microscopic image of the optical microcavity microcavity microcavity microsphere with optical fiber handle prepared experimentally, the diameter is 50 μm, and the measured Q value is 10 ⁶ .

The microsphere-type microcavity coupling method in Example 1 is to use a taper fiber for coupling, and the core diameter of the taper fiber 21 is pulled to 2 microns to ensure that enough light is coupled from the taper fiber 21 to the microsphere. In the optical microcavity 101 of the type whispering gallery mode, the coupling ratio is controlled by controlling the distance between the optical microcavity 101 of the microsphere type whispering gallery mode and the tapered fiber. This method has high coupling efficiency and simple operation. The coupled microscope image is shown in Figure 5.

The microcavity-based mid-infrared Raman ultrafast fiber laser oscillator of the present invention can realize the tuning of the center wavelength by increasing or decreasing the length of the gain fiber and adjusting the rare-earth ion doping concentration of the gain fiber, and the adjustment range is 1.8-2 microns; The Raman scattering effect of the microcavity element can realize the Raman frequency shift in the cavity; by increasing the pump power and the coupling efficiency of the microsphere cavity, the Raman intensity can be controlled, so that the first-order Raman to the higher-order Raman can be realized. Mann conversion, so that a 2-3 micron Raman laser can be obtained. Adding a 2-micron band-pass filter to the cavity can obtain the required Raman laser in a specific band.

计算出拉曼光纤激光器第i阶斯托克斯激光输出波长λ_i，其中λ_i-1为入射光波长，Δs为所用光纤的拉曼频移量，且石英光纤的拉曼频移为13.2THz(440cm^-1)。实施例1中预期在光谱仪上2150nm波段可观察到拉曼激光，预期获得的光谱图如图7所示。同时，使用自相关仪进一步测量获得的超快脉冲的脉冲宽度和子结构。The micro-cavity-based mid-infrared Raman ultrafast fiber laser oscillator of the present invention can observe the mode-locked pulse diagram through an oscilloscope (the detection speed is 1 GHz). The mode-locked pulse diagram is shown in FIG. 16MHz. The Raman spectrum can be observed by a spectrometer (measurement band is 1600nm-3400nm). The Raman frequency shift generated by different fiber materials is different, but the Raman frequency shift value of the same material is fixed and has nothing to do with the wavelength change of the incident light, which can be determined according to the formula

Calculate the output wavelength λ _i of the i-th Stokes laser of the Raman fiber laser, where λ _i-1 is the wavelength of the incident light, Δs is the Raman frequency shift of the fiber used, and the Raman frequency shift of the silica fiber is 13.2 THz (440cm ^-1 ). In Example 1, it is expected that Raman laser can be observed in the 2150 nm band on the spectrometer, and the expected spectrum is shown in FIG. 7 . Meanwhile, the pulse width and substructure of the obtained ultrafast pulses were further measured using an autocorrelator.

Example 2 Generation of 3-5 μm Raman Laser Based on 3 μm Wavelength Pumping

The structure of a microcavity-based mid-infrared Raman ultrafast fiber laser oscillator is shown in Figure 8, including:

Mode-locking element saturable absorber 1, a broadband semiconductor saturable absorber mirror (SESAM) in the 3μm band (2000nm ~ 3400nm) is selected. The modulation depth of SESAM is in the range of 8%≤ΔR≤30%. The cavity is controlled by increasing the pump power. Internal pulse energy, when the pulse energy flux reaches 3 to 5 times the saturation flux, the three-dimensional spatial position of the SESAM is properly adjusted to achieve mode locking;

Lens Ⅰ2, focal length is 50mm;

Lens II3, focal length is 100mm;

Bevel 4, use a fiber bevel cutter to cut a bevel with an angle of 8° to 20° to prevent laser feedback and avoid F-P effect from destroying the mode-locked state.

The pump source 5 is a semiconductor laser pump source with a center wavelength of 976nm, and the maximum output power is 30W, which can excite the corresponding gain fiber to generate a laser in the 3-micron waveband;

Fiber combiner 6, select (2+1)×1 fluoride fiber combiner, wherein the pump end of the combiner and the LD pigtail are connected by mechanical splicing;

The gain fiber 7 is made of rare earth doped Er ³⁺ fluoride fiber, and the molar concentration range of Er ³⁺ doping can be 7% to 20%. In this embodiment, a molar doping concentration of 7% is selected;

For the dispersion compensation element, the chirped fiber grating 802 engraved with fluoride fiber is used to compensate the dispersion in the cavity, so as to adjust the relative balance between nonlinearity and dispersion in the resonator cavity, and realize the tunable pulse width;

The polarization controller 9 is a manual polarization controller or an electric polarization controller. In this embodiment, a manual polarization controller is used, and the optical fiber is pulled by rotating the polarization control plate to change the non-polarization state in the cavity. The cost is low and the operation is flexible. It is easy to fix after adjusting to the appropriate polarization state, thereby enhancing the stability of the system;

The microcavity with high Q value is a microdisk-type whispering gallery mode optical microcavity made of calcium fluoride crystal.

Cut a bevel 4 with an angle of 8° to 20° at the signal input end of the (2+1)×1 fiber combiner 6 to prevent laser feedback and prevent the F-P effect from destroying the mode-locking state; the bevel 4 has an angled end in turn Through lens II3 with a focal length of 100mm and lens I2 with a focal length of 50mm, it is focused on the semiconductor saturable absorber mirror 1, so that the light spot energy incident on the semiconductor saturable absorber mirror 1 is larger, which is conducive to reaching the mode locking threshold; The flat end and the signal input end of the (2+1)×1 fiber combiner 6 are connected by welding to form resonance. The semiconductor laser pump source 5 and the pump end of the (2+1)×1 fiber combiner 6 Connected by mechanical splicing, one end of the gain fiber 7 is connected with the signal output end of the (2+1)×1 fiber combiner 6 by welding, and the other end of the gain fiber 7 is connected in sequence by welding the dispersion compensation chirp The chirped fiber grating 802, the polarization controller 9 by mechanical splicing and the micro-disk whispering gallery mode optical microcavity 102 are connected by mechanical splicing, and the optical fiber end face is directly output at the output end of the microdisk whispering gallery mode optical microcavity 102. , so that a microcavity-based mid-infrared Raman ultrafast fiber laser oscillator can output high-power Raman laser. A bandpass filter 11 with a wavelength of 3 microns is connected outside the resonator to obtain a Raman laser output of a specific wavelength.

The calcium fluoride microdisk-type whispering gallery mode optical microcavity 102 is prepared by using high-purity single-crystal calcium fluoride as the basic material, and is prepared through the steps of cutting, grinding, and polishing. The microscope imaging diagram of the prepared microdisk-type whispering gallery mode optical microcavity 102 is shown in FIG. 9 , it can be seen that the diameter is 5 mm, the thickness is 1 mm, and the Q value is 10 ⁶ .

The micro-disk-type micro-cavity coupling method in the second embodiment of the micro-cavity-based mid-infrared Raman ultrafast fiber laser oscillator of the present invention is a taper fiber coupling. Here, the taper fiber 23 is a fluoride fiber. The core diameter of the optical fiber 23 is pulled to about 2 microns to ensure that enough light is coupled from the tapered optical fiber 23 into the microdisk-type whispering gallery mode optical microcavity 102. By controlling the microdisk-type whispering gallery mode optical microcavity 102 and the cone The distance of the shaped optical fiber 23 is used to control the coupling ratio, which has high coupling efficiency and simple operation.

The mid-infrared Raman ultrafast fiber laser oscillator based on the microcavity of the invention can realize the tuning of the center wavelength by increasing or decreasing the length of the gain fiber and adjusting the Er ³⁺ doping concentration of the gain fiber, and the tuning range is 2.7-2.9 microns. The fibers of different materials are connected by mechanical splicing.

The mid-infrared Raman ultrafast fiber laser oscillator based on the microcavity of the present invention can realize the Raman frequency shift in the cavity through the Raman scattering effect of the microcavity element. By increasing the pump power and the coupling efficiency of the microsphere cavity, the Raman intensity can be controlled, so as to realize the conversion of the first-order Raman to the high-order Raman, and obtain a Raman laser with a wavelength of 3 microns and longer. Adding a band-pass filter 11 with a 3-micron wavelength band at the output end can obtain the required Raman laser of a specific wavelength band.

计算得出。其中f为脉冲重复频率，L表示光在谐振腔内往返一次的光程，c表示光速，实施例2中为30MHz。可通过光谱仪观察到拉曼光谱，其中氟化物光纤的拉曼频移值为17.4THz(580cm^-1)。实施例2中预期在光谱仪上3430nm附近观察到一阶拉曼激光图，预期获得的光谱图如图11所示。同时，使用自相关仪可进一步测量获得的超快脉冲的脉冲宽度和子结构。The micro-cavity-based mid-infrared Raman ultrafast fiber laser oscillator of the present invention can observe the mode-locked pulse diagram through an oscilloscope. The expected mode-locked pulse diagram is shown in Figure 10. The pulse mode is stable, and the repetition frequency is determined by the formula

Calculated. Among them, f is the pulse repetition frequency, L represents the optical path of the light going back and forth once in the resonant cavity, and c represents the speed of light, which is 30 MHz in Example 2. The Raman spectrum can be observed by a spectrometer, where the Raman frequency shift value of the fluoride fiber is 17.4 THz (580 cm ^-1 ). In Example 2, a first-order Raman laser pattern is expected to be observed near 3430 nm on the spectrometer, and the expected spectral pattern is shown in FIG. 11 . Meanwhile, the pulse width and substructure of the obtained ultrafast pulses can be further measured using an autocorrelator.

In addition to the microsphere type and microdisk type optical microcavity mentioned in the above embodiment, the optical microcavity can also be selected from one of the microcolumn type, the microbubble type, the microtubule type or the microring type, as long as the microcavity is guaranteed The quality factor Q value should not be lower than 10 ⁶ . The diameter of the micro-ring type optical micro-cavity should not be less than 500 microns, so as to ensure that the micro-cavity has a large number of modes in the gain band.

Claims (9)

1. A mid-infrared Raman ultrafast fiber laser oscillator based on a microcavity, characterized in that it adopts a linear cavity structure and mainly comprises a mode-locking element saturable absorber (1), a lens I (2), a lens II ( 3), pump source (5), fiber combiner (6), gain fiber (7), dispersion compensation element, polarization controller (9), high-Q microcavity, the high-Q microcavity It is a whispering gallery mode optical microcavity, and its quality factor Q value is not less than 10 ⁶ ; Cut an oblique angle (4) with an angle α of 8° to 20° on the side of the signal input end of the fiber combiner (6), and the outgoing light at the oblique angle (4) is perpendicular to the center of the lens, that is, the oblique angle (4) ) The angled end is focused on the mode-locked element saturable absorber (1) through two lenses of suitable focal length, and the flat end of the oblique angle (4) is connected to the signal input end of the fiber combiner (6) to form resonance, the pump The pump source (5) is connected to the pump end of the fiber combiner (6), one end of the gain fiber (7) is connected to the signal output end of the fiber combiner (6), and the other end of the gain fiber (7) is connected to one end of the dispersion compensation element, The other end of the dispersion compensation element is connected to one end of the polarization controller (9), the high-Q microcavity (10) is connected behind the polarization controller (9), or the high-Q microcavity (10) is connected to the fiber combiner ( 6) and the gain fiber (7), or a high-Q microcavity (10) is connected between the gain fiber (7) and the dispersion compensation element. 2. a kind of mid-infrared Raman ultrafast fiber laser oscillator based on microcavity according to claim 1, is characterized in that, the microcavity of described high Q value is microsphere type, microcolumn type, microdisk type , one of microbubble, microtubule or microring type. 3. A kind of mid-infrared Raman ultrafast fiber laser oscillator based on microcavity according to claim 1 or 2, it is characterized in that, the material of described microcavity with high Q value is 2 micron waveband low loss Quartz or one of the low-loss mid-infrared low-loss materials of silicon, germanium, calcium fluoride, and zinc selenide in the 3-micron band. 4. The microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to claim 1 or 2, wherein the coupling mode of the high-Q microcavity adopts tapered fiber coupling or One of the free-space couplings of prism pairs. 5. A microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to claim 1 or 2, wherein the mode-locking element saturable absorber (1) is a semiconductor saturable absorber One of the mirror, graphene saturable absorption mirror, carbon nanotube saturable absorption mirror or graphene oxide saturable absorption mirror, its working range covers the 2-3 micron waveband, and the value range of its modulation depth ΔR is 8 %≤∆R≤30%. 6. A microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to claim 1 or 2, wherein the pump source (3) is a semiconductor laser pump source, and the center wavelength λ is 793nm or 976nm. 7. A microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to claim 1 or 2, wherein the gain fiber (4) is a rare earth single-doped Tm ³⁺ silica fiber Or rare earth ion Tm ³⁺ , Ho ³⁺ co-doped silica fiber, or Er ³⁺ doped fluoride fiber. 8. A microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to claim 1 or 2, wherein the dispersion compensation element is a dispersion compensation fiber (801) or a chirped fiber grating (802). 9. A microcavity-based mid-infrared Raman ultrafast fiber laser oscillator according to claim 1 or 2, characterized in that, further comprising a bandpass filter (11), the bandpass filter (11) ) is connected outside the resonator.

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