Tunable Ultra-High Repetition Rate Ultra-Short Pulse Fiber Laser with Two-Dimensional Materials and M-Z Synergy

Abstract

The present invention proposes an adjustable ultra-high repetition rate ultra-short pulse fiber laser in cooperation with two-dimensional materials and an M-Z sub-cavity. The fiber laser includes: a pump source, a wavelength division multiplexer, a single-mode fiber, a gain fiber, a saturable absorber, and an M-Z sub-cavity. The single-mode fiber, the gain fiber, the M-Z sub-cavity, and the saturable absorber are connected to form a ring-shaped fiber laser cavity. The pump source is coupled to the ring-shaped fiber laser cavity through the wavelength division multiplexer. The saturable absorber is made of a two-dimensional thin film material. The M-Z sub-cavity and the saturable absorber cooperate to generate an ultra-high repetition rate laser output greater than 0.1 THz. The present invention realizes for the first time a high repetition rate with a repetition rate above 11.11 THz and an ultra-short pulse laser output with a pulse width of 31.81 fs, which is one order of magnitude higher than the currently known highest repetition rate. Moreover, the overall device structure is simple to manufacture and has a low cost, having wide popularization and application value.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber lasers, and particularly relates to a fiber laser for generating tunable ultra-high repetition rate and ultra-short pulse fiber laser based on the synergistic effect of two-dimensional materials and M-Z sub-cavities, and a method for generating continuously tunable ultra-high repetition rate and ultra-short pulse fiber laser based on the fiber laser. Background Art

[0002] With the rapid development of fiber lasers and laser technologies, people have higher and higher requirements for the performance of fiber lasers. In order to meet the growing needs in different fields, fiber lasers have gradually formed different research directions, mainly divided into high-power fiber lasers, multi-wavelength fiber lasers, ultra-short pulse fiber lasers, wide-spectrum fiber lasers, and high-repetition rate fiber lasers. At present, we have conducted relatively in-depth research on high-power fiber lasers and achieved good results. However, for high-repetition rate fiber lasers, there is still a large room for development. Continuous ultra-high-frequency pulse sequences play an irreplaceable important role in many fields. For example, it can be used in ultra-high bit rate optical communication networks, all-optical logic and signal processing circuits, as well as test and measurement systems, microwave photonics and nonlinear optics, arbitrary optical waveform generation, time-resolved dual optical comb spectroscopy, ultrafast asynchronous optical sampling, high-precision time-frequency metrology, astronomical optical comb spectrometer calibration, biological imaging and other fields. More importantly, for future computer processors with clock frequencies exceeding the electronic limit, how to obtain higher and higher repetition frequencies becomes particularly important. Currently, there are mainly two methods for generating high-repetition rate lasers, namely inserting a Fabry-Perot (F-P) sub-cavity and a fiber micro-ring sub-cavity in a fiber laser. Using an F-P sub-cavity to generate high-repetition rate laser, the repetition rate can be adjusted by adjusting the distance between the F-P cavity mirrors. The closer the distance between the cavity mirrors, the higher the generated repetition rate. However, continuously shortening the distance between the cavity mirrors will also increase the power density when light oscillates back and forth between them. To obtain high-power laser output, it is very easy to damage the cavity mirrors. Therefore, this method cannot generate high-repetition rate lasers with relatively high power. Using a fiber micro-ring sub-cavity to generate high-repetition rate laser, the smaller the radius of the micro-ring, the higher the obtained repetition rate. However, since the radius of the micro-ring cannot be adjusted after it is fabricated, the obtained laser repetition rate cannot be adjusted, and the higher the repetition rate, the smaller the required radius of the micro-ring, and the corresponding manufacturing difficulty increases. Therefore, the devices for generating ultra-high repetition rate and ultra-short pulse lasers in the prior art all have certain defects, which limit their practical applications. Summary of the Invention

[0003] Based on the above existing technical problems, the present invention innovatively proposes a high-repetition-rate laser generating device and method based on a Mach-Zehnder (M-Z) sub-cavity, and firstly proposes a fiber laser for generating tunable ultra-high-repetition-rate and ultra-short pulse fiber lasers through the synergistic effect of two-dimensional materials and the M-Z sub-cavity, as well as a method for generating continuously tunable ultra-high-repetition-rate and ultra-short pulse fiber lasers based on the said fiber laser. The present invention generates high-repetition-rate lasers by synergistically using a saturable absorber based on two-dimensional materials and the M-Z sub-cavity, enabling the repetition rate of the generated laser to be adjusted only by adjusting the length difference between the two arms of the M-Z sub-cavity. The smaller the length difference, the higher the obtainable repetition rate. Moreover, even when the power is relatively high, the entire system can operate stably without causing device damage, and for the first time, a high-repetition-rate laser output with a repetition rate above 11.11 THz and an ultra-short pulse laser with a pulse width of 31.81 fs is achieved, which is one order of magnitude higher than the currently known highest repetition rate. In addition, the overall device structure is simple to fabricate and has a low cost, possessing extensive popularization and application value.

[0004] The technical solutions adopted by the present invention to solve the above technical problems are as follows:

[0005] An adjustable ultra-high-repetition-rate and ultra-short pulse fiber laser, comprising: a pump source, a wavelength division multiplexer, a single-mode fiber, a gain fiber, a saturable absorber, and an M-Z sub-cavity. The single-mode fiber, the gain fiber, the M-Z sub-cavity, and the saturable absorber are connected to form a ring fiber laser cavity. The pump source is coupled to the ring fiber laser cavity through the wavelength division multiplexer. The saturable absorber is made of a two-dimensional thin film material, and the M-Z sub-cavity and the saturable absorber synergistically generate an ultra-high-repetition-rate laser output greater than 0.1 THz.

[0006] Furthermore, for the adjustable ultra-high-repetition-rate and ultra-short pulse fiber laser according to the present invention, the saturable absorber is a graphene thin film, the thickness of the graphene thin film is 3 - 8 microns, the light transmittance is 60 - 90%, and the number of graphene layers contained is 1 - 30 layers.

[0007] Furthermore, for the adjustable ultra-high-repetition-rate and ultra-short pulse fiber laser according to the present invention, the preparation method of the graphene thin film includes the following steps:

[0008] Step (1): Weigh a certain amount of graphene powder, measure a certain volume of deionized water, dissolve the graphene powder in the deionized water and stir evenly to form a graphene aqueous solution, and the concentration of the graphene aqueous solution is 0.02 - 0.1 mg / ml.

[0009] Step (2): Pour the obtained graphene aqueous solution into a small beaker, use an ultrasonic crusher to crush for a predetermined time, and ensure that the temperature of the graphene aqueous solution does not exceed 50°C during the crushing process.

[0010] Step (3): After the crushing is completed, place a magnetic stir bar into a small beaker containing the graphene aqueous solution, and place the small beaker on a heating magnetic stirrer. Set the temperature of the stirrer to 75 - 100 °C, start heating the graphene solution, and keep the magnetic stir bar stirring the solution while heating.

[0011] Step (4): Weigh a predetermined amount of polyvinyl alcohol powder, add the polyvinyl alcohol powder into the graphene solution, and control its concentration to be 0.02 - 0.1 g / ml. Continue heating to maintain the solution temperature, and in addition, increase the stirring intensity and keep stirring until the solution becomes uniform and viscous, then stop heating and stirring, and then cool the solution to room temperature.

[0012] Step (5): After the solution is cooled, use a Pasteur pipette to suck a small amount of the solution and evenly coat it on the surface of a glass slide. Fix the center of the glass slide on a spin coater, and set the spin coater to rotate at a speed exceeding 50 r / min for a predetermined time.

[0013] Step (6): After the rotation is completed, place the glass slide in a vacuum drying oven at a temperature above 60 °C and bake it for a predetermined time to obtain a graphene film with uniform thickness.

[0014] Furthermore, for the tunable ultra-high repetition rate ultra-short pulse fiber laser according to the present invention, the graphene film is connected to the ring fiber laser cavity in the following manner: Cut off the single-mode fiber in the ring fiber laser cavity and fuse a first fiber pigtail and a second fiber pigtail at both ends respectively. Use tweezers to pick up a piece of graphene film and place it on the output end face of the first fiber pigtail, and then connect the first fiber pigtail and the second fiber pigtail with a connection flange. After the connection is completed, the end face of the single-mode fiber inside the first fiber pigtail is closely attached to one side surface of the graphene film, and the end face of the single-mode fiber inside the second fiber pigtail is closely attached to the other side surface of the graphene film. The first fiber pigtail and the second fiber pigtail are tightly connected by the connection flange.

[0015] Further, for the tunable ultra-high-repetition-rate ultra-short pulse fiber laser according to the present invention, the M-Z sub-cavity includes a first optical coupler 7, a second optical coupler 10, a first optical fiber arm, a second optical fiber arm, a first polarization controller 8, and an optical delay line 9. The first optical coupler 7 has a first input end, a first output end, and a second output end. The second optical coupler 10 has a first input end, a second input end, a first output end, and a second output end. The first input end of the first optical coupler 7 is connected to the single-mode optical fiber of the ring-shaped optical fiber laser cavity. The first output end of the first optical coupler 7 is connected to one end of the first optical fiber arm. The other end of the first optical fiber arm is connected to the first input end of the second optical coupler 10. The first polarization controller 8 is disposed on the first optical fiber arm. The second output end of the first optical coupler 7 is connected to one end of the second optical fiber arm. The other end of the second optical fiber arm is connected to the second input end of the second optical coupler 10. The optical delay line 9 is disposed on the second optical fiber arm. The first output end of the second optical coupler 10 is connected to the single-mode optical fiber of the ring-shaped optical fiber laser cavity. The second output end of the second optical coupler 10 serves as the output end of the ring-shaped optical fiber laser cavity.

[0016] Further, for the tunable ultra-high-repetition-rate ultra-short pulse fiber laser according to the present invention, the first optical coupler 7 is a 1×2 optical coupler, and the output coupling ratios of its first output end and second output end are 40%:60%; the second optical coupler 7 is a 2×2 optical coupler, and the input coupling ratios of its first input end and second input end and the output coupling ratios of its first output end and second output end are all 50%:50%. The first optical fiber arm and the second optical fiber arm are both single-mode optical fibers.

[0017] Further, for the tunable ultra-high-repetition-rate ultra-short pulse fiber laser according to the present invention, the optical delay line includes a light input end 21, a light output end 22, and a mirror 23. The light input end 21 and the light output end 22 are connected to the second optical fiber arm. The position of the mirror 23 can be translated, and the light beam incident through the light input end is reflected towards the light output end. By adjusting the position of the mirror 23, the optical path difference between the first optical fiber arm and the second optical fiber arm is changed.

[0018] Further, for the tunable ultra-high-repetition-rate ultra-short pulse fiber laser according to the present invention, the pump source includes a first pump source 1 and a second pump source 5. The wavelength division multiplexer includes a first wavelength division multiplexer 2 and a second wavelength division multiplexer 4. The first pump source 1 is coupled to the ring-shaped optical fiber laser cavity through the first wavelength division multiplexer. The second pump source 5 is coupled to the ring-shaped optical fiber laser cavity through the second wavelength division multiplexer. The gain fiber is disposed between the first wavelength division multiplexer 2 and the second wavelength division multiplexer 4.

[0019] Furthermore, for the tunable ultra-high repetition rate ultra-short pulse fiber laser according to the present invention, the ring fiber laser cavity further includes an optical isolator 6 and a second polarization controller 12. The optical isolator 6 is disposed between the second wavelength division multiplexer and the first optical coupler 7 of the M-Z sub-cavity. The saturable absorber 11 is disposed between the first wavelength division multiplexer and the second optical coupler 10 of the M-Z sub-cavity. The second polarization controller 12 is disposed between the saturable absorber 11 and the second optical coupler 10.

[0020] A method for generating ultra-high repetition rate ultra-short pulse laser based on the tunable ultra-high repetition rate ultra-short pulse fiber laser according to the present invention includes the following steps:

[0021] Step 1: Connect the second output end of the second optical coupler of the M-Z sub-cavity to a spectrometer 14 and an autocorrelator 15.

[0022] Step 2: Turn on the first pump source and the second pump source simultaneously, and use the spectrometer and the autocorrelator to monitor the spectrum and waveform respectively.

[0023] Step 3: Continuously adjust the optical delay line in the M-Z sub-cavity to change the optical path difference ΔL between the first fiber arm and the second fiber arm of the M-Z sub-cavity until a comb-shaped spectrum image appears on the spectrometer.

[0024] Step 4: When the comb-shaped spectrum appears, carefully adjust the first polarization controller and the second polarization controller to change the polarization states in the M-Z sub-cavity and the ring fiber laser cavity until a high repetition rate laser trajectory is observed on the autocorrelator.

[0025] Step 5: Then, keep the pump power unchanged and continuously adjust the optical delay line to gradually reduce the optical path difference ΔL, and continue to adjust the first polarization controller and the second polarization controller until an autocorrelation curve and a comb-shaped spectrum are obtained.

[0026] Through the technical solution of the present invention, at least the following innovative technical effects can be achieved:

[0027] 1). For the first time, the present invention uses two-dimensional materials (such as graphene, black phosphorus, MoS2, etc. and SESAM can also be used) as the saturable absorber and combines with the M-Z sub-cavity structure to generate ultra-high repetition rate and ultra-short pulse laser output based on the synergistic effect in the ring fiber laser resonator, and proposes a brand-new structure of ultra-high repetition rate ultra-short pulse laser generating device.

[0028] 2). The present invention creatively introduces two-dimensional materials as the saturable absorber into the ring fiber laser to generate ultra-short optical pulses and high repetition rate lasers, and also creatively proposes a manufacturing method for such two-dimensional materials applied to fiber lasers.

[0029] 3), For the first time, the present invention combines a Mach-Zehnder (M-Z) sub-cavity with a ring fiber laser cavity, and adjusts the repetition frequency of the generated laser by adjusting the length difference between the two arms of the M-Z sub-cavity. The smaller the length difference, the higher the repetition frequency that can be obtained, and the repetition frequency is adjustable between 0.1 and 11.11 THz. Moreover, even when the power is relatively high, the entire system can operate stably without causing damage to the device. For the first time, high repetition frequency above 11.11 THz and ultra-short pulse laser output with a pulse width of 31.81 fs are realized, which is one order of magnitude higher than the currently known highest repetition frequency;

[0030] 4) The overall structure of the device of the present invention is simple to manufacture and low in cost, and can be used as a THz radiation excitation source for generating tunable wavelengths, having broad prospects for popularization and application. Description of the Drawings

[0031] Appendix Figure 1 It is a SEM scanning electron microscope image of the two-dimensional material - graphene applied to the fiber laser of the present invention;

[0032] Appendix Figure 2 It is a schematic structural diagram of the fiber laser of the present invention;

[0033] Appendix Figure 3 It is a schematic structural diagram of the optical delay line in the fiber laser of the present invention;

[0034] Appendix Figure 4 It is a schematic diagram of the insertion structure of the two-dimensional material as a saturable absorber in the ring fiber laser cavity;

[0035] Appendix Figure 5 It is an autocorrelation curve and a comb-shaped spectrogram obtained based on the fiber laser of the present invention;

[0036] Appendix Figure 6 For Appendix Figure 5 It is a schematic diagram of pulse width characterization obtained by fitting the autocorrelation curve.

[0037] The meanings of the reference numerals in the drawings are as follows:

[0038] 1 - First pump source, 2 - First wavelength division multiplexer, 3 - Gain fiber, 4 - Second wavelength division multiplexer, 5 - Second pump source, 6 - Optical isolator, 7 - First optical coupler, 8 - First polarization controller, 9 - Optical delay line, 10 - Second optical coupler, 11 - Saturable absorber, 12 - Second polarization controller, 13 - Third optical coupler, 14 - Spectrometer, 15 - Autocorrelator, 16 - First fiber jumper head, 17 - Second fiber jumper head, 18 - Connection flange, 19 - Graphene film, 20 - Single-mode fiber, 21 - Light input end, 22 - Light output end, 23 - Mirror. Detailed Embodiments

[0039] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can understand the present invention more clearly, but this does not limit the protection scope of the present invention.

[0040] The present invention introduces two-dimensional materials as saturable absorbers into a ring fiber laser to generate ultrashort optical pulses and high-repetition-rate lasers. First, the two-dimensional materials used as saturable absorbers in the present invention are described. In the present invention, two-dimensional materials are used as saturable absorbers and placed in a fiber laser with an M-Z structure as a sub-cavity to generate high-repetition-rate lasers. Two-dimensional materials have excellent optical, electrical, and chemical properties and are widely used in the fields of ultrafast photonics and nonlinear photonics. Based on the saturable absorption characteristics and nonlinear optical characteristics of two-dimensional materials, placing them in a laser resonator can achieve high-repetition-rate modulation of the laser and generate ultrashort pulses with an ultra-high repetition rate. Among two-dimensional materials, graphene has a unique linear band structure with a zero bandgap, which enables photons of any wavelength to be absorbed by graphene. In addition, the strong interband transition of carriers and the ultrafast relaxation process make graphene an ideal saturable absorber for generating ultrashort optical pulses and high-repetition-rate lasers.

[0041] In a specific embodiment of the present invention, graphene two-dimensional materials are preferably used as saturable absorbers. The graphene two-dimensional materials therein have a thin film structure, with a film thickness of 3-8 microns, preferably 5 μm, a light transmittance of the film of 60-90%, preferably 62.8%, and the number of graphene layers contained in the film is 1-30 layers, preferably 20 layers of graphene.

[0042] The specific manufacturing process of the graphene two-dimensional material film of the present invention is as follows:

[0043] First, weigh 2 mg of graphene powder using an electronic balance, and then measure 40 ml of deionized water with a measuring cylinder and dissolve the graphene in it and stir evenly. The concentration of the graphene aqueous solution is 0.05 mg / ml.

[0044] Next, pour the obtained graphene aqueous solution into a small beaker, and crush the solution with an ultrasonic crusher for 30 min. During the crushing, place the small beaker in a container filled with ice water and replace the ice water every 10 minutes to ensure that the temperature of the graphene solution does not exceed 50°C.

[0045] Then, after the crushing is completed, since the graphene in the solution is in a dispersed state and has not started to deposit, at this time, place a magnetic stir bar into the beaker and place the beaker on a heating magnetic stirrer, set the temperature of the stirrer to 75°C and start heating the graphene solution. Start the stirrer while heating to make the magnetic stir bar continuously stir the solution, and insert a thermometer into the solution to monitor the real-time temperature of the solution.

[0046] Next, weigh 2 g of polyvinyl alcohol (PVA) powder. Since PVA becomes a colloidal substance when the temperature is higher than 75 °C, it can be used to prepare a colloidal film. When the solution temperature reaches 75 °C, add the PVA powder to the graphene solution and control the PVA concentration at 0.05 g / ml. At this time, continue heating to maintain the solution temperature, and increase the stirring intensity and stir continuously for about 20 min until the solution becomes uniform and viscous, then stop heating and stirring and wait for it to cool to room temperature.

[0047] After cooling, use a Pasteur pipette to suck a small amount of the solution and evenly coat the surface of a 2 cm * 2 cm glass slide, and then fix the center of the glass slide on a spin coater. The spin coater can make the colloidal solution on the glass slide form a film with a uniform thickness through high-speed rotation. Set the spin coater to run at a low speed of 50 r / min for 5 s first, and then run at a high speed of 500 r / min for 10 s.

[0048] After the rotation is completed, place the glass slide in a vacuum drying oven at 60 °C and dry it for 20 min. At this time, a graphene two-dimensional material film with uniform thickness is prepared, and a part of the film can be torn off when in use.

[0049] Through testing, the transmittance of this film is 62.8%, while the transmittance of a single layer of graphene is 97.7%. From this, it can be concluded that there are approximately 20 layers of graphene in this film. Attached Figure 1 (a) is the result obtained by SEM scanning of the graphene in the solution, Figure 1 (b) is the SEM scanning of the side of the prepared film, and it can be seen that the thickness of the film is about 5 μm.

[0050] The fiber laser based on the synergistic effect of two-dimensional materials and M-Z sub-cavities to generate tunable ultra-high repetition rate ultra-short pulse lasers described in the present invention is specifically as attached Figure 2 shown, including a first pump source 1, a first wavelength division multiplexer 2, a gain fiber 3, a second wavelength division multiplexer 4, a second pump source 5, an optical isolator 6, a first optical coupler 7, a first polarization controller 8, an optical delay line 9, a second optical coupler 10, a saturable absorber 11, a second polarization controller 12, a third optical coupler 13, a spectrometer 14 and an autocorrelator 15, as Figure 2For the optical path diagram shown, the present invention uses a common single-mode fiber (SMF) to prepare a ring fiber laser cavity, and then adds an M-Z sub-cavity into the ring cavity to obtain high-repetition-rate laser. The total length of the ring laser cavity is about 8 m, and the length of the M-Z sub-cavity needs to be much greater than the working wavelength. Since the working wavelength of ordinary lasers is in the micrometer or sub-micrometer range, the length of the M-Z sub-cavity generally meets this condition in actual operation. The first and second pump sources (Pump) 1 and 5 respectively use continuous lasers. As a preferred embodiment of the present invention, two continuous lasers with a working wavelength preferably of 980 nm are used as the first and second pump sources (Pump) 1 and 5, and the pump lasers are coupled into the ring fiber laser cavity through two wavelength division multiplexers (WDM), namely the first wavelength division multiplexer 2 and the second wavelength division multiplexer 4. A gain fiber 3 with a length preferably of 1.4 m is added between the two WDMs. The gain fiber 3 is preferably a highly doped erbium-doped fiber (EDF) with a working wavelength of 1550 nm. The 980-nm pump laser coupled into the ring cavity pumps and excites the EDF to generate fiber laser oscillation. Preferably, the working wavelength of the highly doped erbium-doped fiber at 1550 nm is matched with the working wavelength of the corresponding pump source at 980 nm. The working wavelengths of the first and second wavelength division multiplexers are selected as 980 nm / 1550 nm. Further, an optical isolator (ISO) 6 with a working wavelength of 1550 nm is added into the ring laser cavity to ensure unidirectional light transmission. The present invention innovatively introduces a Mach-Zehnder sub-cavity, i.e., an M-Z sub-cavity, into the ring fiber laser cavity. Specifically, the M-Z sub-cavity is composed of a first optical coupler 7, a second optical coupler 10, and two sections of single-mode fibers. The first optical coupler 7 is a 1×2 optical coupler (OC) with a coupling ratio of the two output ends of 40%:60%; the second optical coupler 10 is a 2×2 optical coupler OC with a coupling ratio of 50%:50%. The 40% coupling end of the first optical coupler 7 is connected to the first coupling input end of the second optical coupler 10 through a first single-mode fiber, and a first polarization controller 8 is arranged on the first single-mode fiber, jointly constituting the first arm of the M-Z sub-cavity; the 60% coupling end of the first optical coupler 7 is connected to the second coupling input end of the second optical coupler 10 through a second single-mode fiber, and an optical delay line 9 is connected to the second single-mode fiber, jointly constituting the second arm of the M-Z sub-cavity. The optical delay line 9 is used to change the optical path difference between the first arm and the second arm of the M-Z sub-cavity to meet the working conditions of the M-Z sub-cavity. To match the working wavelength of the preferably highly doped erbium-doped fiber at 1550 nm, the working wavelengths of the first optical coupler 7 and the second optical coupler 10 are both near 1550 nm.The present invention is provided with an optical time delay line 9 (OTDL) on the second arm of the M-Z sub-cavity. Since the optical time delay line OTDL has an insertion loss of approximately 1.8 dBm near the 1550 nm wavelength band, in order to ensure that the optical intensities of the two arms of the M-Z sub-cavity are close, it is selected to connect the second arm to the 60% coupling output end of the first optical coupler 7, so as to compensate for the insertion loss of the optical time delay line OTDL through 60% coupling of this arm. The present invention changes the optical path of light in this arm through the optical time delay line, and further changes the optical path difference between the first arm and the second arm. The structure of the optical time delay line is as attached. Figure 3 As shown, it includes a light input end 21, a light output end 22 and a mirror 23. The mirror 23 is a plane mirror and its position can be translated. No matter which position the mirror is translated to, the light beam incident from the light input end is reflected by the mirror 23 and then output to the light output end. The light input end 21 and the light output end 22 are respectively coupled to the single-mode optical fibers of the second arm of the M-Z sub-cavity. After the light from the single-mode optical fiber is incident from the light input end, it is reflected by the mirror and then output from the light output end 22 and coupled to the single-mode optical fiber. By adjusting the position of the mirror 23, the transmission distance of light in it can be changed, and further the optical path of light in the second arm of the M-Z sub-cavity can be changed. In addition, the adjustment range of the used OTDL is 10 cm. Therefore, the length difference between the optical fibers of the two arms of the M-Z sub-cavity should be within the optical path of 0 - 10 cm to ensure that the OTDL can achieve the required adjustment effect. The polarization controller (PC) added to the first arm of the M-Z sub-cavity can be used to adjust the polarization state of this arm. The coupling output ratio of the two output ends of the second optical coupler 10 is also 50%:50%. One coupling output end leads out of the ring cavity and is connected to a spectrometer 14 and / or an autocorrelator 15 to monitor spectral signals or pulse signals. The other end is coupled and connected to the single-mode optical fiber in the ring fiber laser cavity, and a second polarization controller 12 is arranged on the single-mode optical fiber in this ring laser cavity. The second polarization controller 12 and the first polarization controller 8 in the M-Z sub-cavity respectively adjust the polarization states in the ring fiber laser cavity and the M-Z sub-cavity.

[0051] Furthermore, the present invention innovatively inserts a saturable absorber composed of two-dimensional materials into the ring fiber laser cavity. The two-dimensional materials are thin film materials formed based on graphene, black phosphorus, MoS2, etc. Preferably, the two-dimensional material is a graphene thin film. The two-dimensional material is combined with the ring fiber laser cavity in the manner as attached Figure 4 As shown, first, the two-dimensional material is combined with the single-mode optical fiber segment in the ring fiber laser cavity located between the first wavelength division multiplexer and the second polarization controller. The specific combination method is: truncate this segment of single-mode optical fiber and splice fiber pigtails at both ends respectively, as attached Figure 4The first fiber optic jumper head 16 and the second fiber optic jumper head 17 shown, each having a single-mode optical fiber 20 fixedly disposed therein. Use tweezers to pick up a small piece of the previously prepared graphene film 19 and place it on the output end face of one of the jumper heads, ensuring that the end face of the single-mode optical fiber can be completely covered by the graphene film. Then carefully connect the two fiber optic jumper heads with the connecting flange 18 to prevent the graphene film from falling off. After the connection is completed, the end face of the single-mode optical fiber inside the first fiber optic jumper head 16 is closely attached to one side surface of the graphene film, and the end face of the single-mode optical fiber inside the second fiber optic jumper head 17 is closely attached to the other side surface of the graphene film. The first fiber optic jumper head and the second fiber optic jumper head are tightly connected through the connecting flange 18. Preferably, external threads are formed on the outer sides of the front ends of the first fiber optic jumper head and the second fiber optic jumper head, and the flange has a sleeve structure and internal threads are formed on the inner side of the sleeve. Through this innovative structure, the present invention combines the graphene film, which is a two-dimensional material, into the ring fiber laser cavity, and the graphene film serves as the saturable absorber (SA) of the ring fiber laser cavity.

[0052] The present invention further provides a method for generating tunable ultra-high repetition rate ultra-short pulse laser by a fiber laser based on the tunable ultra-high repetition rate ultra-short pulse laser. The specific process is as follows: After the optical path shown is set up, turn on the two first and second pump sources 1 and 5 simultaneously, and set the pump power to 300 mW each. Use the spectrometer 14 and the autocorrelator 15 to monitor the spectrum and waveform respectively. The M-Z sub-cavity mainly functions as a spectral filter. Continuously adjust the position of the mirror in the optical delay line 9 (OTDL) to change the optical path difference (ΔL) between the two arms of the M-Z sub-cavity. When ΔL is within the adjustable 10 cm optical path of the optical delay line 9, an image of a comb spectrum can appear on the spectrometer. This is because the wavelengths that satisfy λ = ΔL / 2m form the peaks of the spectrum, while the wavelengths that satisfy λ = ΔL / (2n + 1) form the valleys of the spectrum, where m and n are both arbitrary integers. Arbitrarily take two adjacent integers m and m + 1, and the difference Δλ between the corresponding λ1 and λ2 is approximately Δλ ≈ λ Figure 2 / ΔL, where λ = (λ1 + λ2) / 2. From this, it can be seen that the adjacent peak interval Δλ of the comb spectrum can be changed by changing ΔL, that is, it can be controlled by the optical delay line 9. When the comb spectrum appears, carefully adjust the first polarization controller 8 and the second polarization controller 12 to change the polarization states in the M-Z sub-cavity and the fiber ring cavity. When the polarization states of the light in the two arms of the M-Z sub-cavity reach consistency and interference occurs, and then the laser longitudinal mode characteristics are modulated through the saturable absorption characteristics of the graphene film in the fiber ring cavity to improve the side mode suppression ratio of the output longitudinal mode of the laser, and the high repetition rate laser trajectory can be clearly observed on the autocorrelator. Then, while keeping the pump power unchanged, continuously adjust the optical delay line 9 to gradually reduce ΔL, and continue to adjust the first polarization controller 8 and the second polarization controller 12 to obtain as 2 / ΔL, where λ = (λ1 + λ2) / 2. It can be seen from this that the adjacent peak interval Δλ of the comb spectrum can be changed by changing ΔL, that is, it can be controlled by the optical delay line 9. When the comb spectrum appears, carefully adjust the first polarization controller 8 and the second polarization controller 12 to change the polarization states in the M-Z sub-cavity and the fiber ring cavity. When the polarization states of the light in the two arms of the M-Z sub-cavity reach consistency and interference occurs, and then the laser longitudinal mode characteristics are modulated through the saturable absorption characteristics of the graphene film in the fiber ring cavity to improve the side mode suppression ratio of the output longitudinal mode of the laser, and the high repetition rate laser trajectory can be clearly observed on the autocorrelator. Then, while keeping the pump power unchanged, continuously adjust the optical delay line 9 to gradually reduce ΔL, and continue to adjust the first polarization controller 8 and the second polarization controller 12 to obtain as Figure 5To characterize the autocorrelation curve and comb spectrum of the waveform. The repetition frequency Δυ of the laser can be deduced from the peak-to-peak distance ΔT of the autocorrelation curve. By adjusting ΔL, the magnitude of Δυ can also be changed. The specific relationship is Δυ = c / ΔL, where c is the speed of light in a vacuum. It can be seen that as ΔL decreases, Δυ will continuously increase. When the optical delay line 9 is adjusted to increase Δλ to 6.06 nm, the two pump powers are both set to 400 mW at this time to generate a laser with a higher repetition frequency. According to the above formula, when Δλ = 6.06 nm, Δυ should be 740 GHz, but the actual Δυ is 15 times the calculated value. This is because the energy of the fundamental frequency laser cannot be increased infinitely. When the power increases to a certain amount, some energy will be dispersed to other modes, forming new pulses, which causes each previous independent peak to split into 15 small peaks. Figure 5 (a)-(d) are the autocorrelation curves when Δυ is 110.25 GHz, 196.08 GHz, 425.53 GHz, and 11.11 THz respectively; (e)-(h) are the comb spectra collected in the states of (a)-(d) respectively, and the corresponding ΔL are 2.72 mm, 1.53 mm, 0.71 mm, and 0.404 mm. We Figure 5 take one pulse of the lasers in (a)-(d) respectively for fitting and measure their pulse widths. The obtained results are as Figure 6 shown. It can be seen that when the repetition frequency is 11.11 THz, we can obtain an ultrashort pulse output with a pulse width of 31.81 fs.

[0053] Innovatively, an M-Z sub-cavity is added to the ring fiber laser cavity in the present invention, and a graphene film is used as a saturable absorber, and finally an output of a high-repetition-frequency ultrashort pulse laser is obtained. The research results through multiple repeated experiments show that in the technical solution of the present invention, the repetition frequency of the laser can be continuously adjusted within a large range by adjusting the optical delay line OTDL. And as the repetition frequency increases, the laser pulse width is also continuously decreasing, which is an advantage not possessed by the prior art of using a micro-ring sub-cavity and an F-P sub-cavity to generate a high-repetition-frequency laser. Finally, based on the technical solution of the present invention, by changing the optical path difference between the two arms of the M-Z sub-cavity through the optical delay line OTDL, an ultrashort pulse laser with a high repetition frequency of up to 11.11 THz and a pulse width of 31.81 fs can be obtained, which has a broad application prospect for popularization.

[0054] The above only describes the preferred embodiments of the present invention and does not limit the technical solutions of the present invention thereto. Any well-known deformation made by those skilled in the art on the basis of the main technical concept of the present invention belongs to the technical scope to be protected by the present invention. The specific protection scope of the present invention shall be subject to the records in the claims.

Claims

1. An adjustable ultra-high repetition rate ultra-short pulse fiber laser, characterized in that, Including: A pump source, a wavelength division multiplexer, a single-mode optical fiber, a gain optical fiber, a saturable absorber, and an M-Z sub-cavity. The single-mode optical fiber, the gain optical fiber, the M-Z sub-cavity, and the saturable absorber are connected to form a ring-shaped fiber laser cavity. The pump source is coupled to the ring-shaped fiber laser cavity through the wavelength division multiplexer. The saturable absorber is made of a two-dimensional thin film material. The M-Z sub-cavity and the saturable absorber cooperate to generate an ultra-high repetition rate laser output greater than 0.1 THz. The saturable absorber is a graphene thin film. The thickness of the graphene thin film is 3 - 8 microns, the light transmittance is 60 - 90%, and the number of graphene layers contained is 1 - 30 layers. The preparation method of the graphene thin film includes the following steps: Step (1): Weigh a certain amount of graphene powder, measure a certain volume of deionized water, dissolve the graphene powder in the deionized water and stir evenly to form a graphene aqueous solution, and the concentration of the graphene aqueous solution is 0.02 - 0.1 mg / ml. Step (2): Pour the obtained graphene aqueous solution into a small beaker, use an ultrasonic crusher to crush for a predetermined time, and ensure that the temperature of the graphene aqueous solution does not exceed 50 °C during the crushing. Step (3): After the crushing is completed, place a magnetic stir bar into the small beaker containing the graphene aqueous solution and place the small beaker on a heating magnetic stirrer. Set the temperature of the stirrer to 75 - 100 °C, start heating the graphene solution, and keep the magnetic stir bar stirring the solution while heating. Step (4): Weigh a predetermined amount of polyvinyl alcohol powder, add the polyvinyl alcohol powder to the graphene solution, and control its concentration to be 0.02 - 0.1 g / ml. Continue heating to maintain the solution temperature, and increase the stirring intensity and continue stirring until the solution becomes uniform and viscous, then stop heating and stirring, and then cool the solution to room temperature. Step (5): After the solution is cooled, use a Pasteur pipette to suck a small amount of the solution and evenly coat it on the surface of a glass slide. Fix the center of the glass slide on a spin coater and set the spin coater to rotate at a speed exceeding 50 r / min for a predetermined time. Step (6): After the rotation is completed, place the glass slide in a vacuum drying oven at a temperature above 60 °C and bake it for a predetermined time to obtain a graphene thin film with uniform thickness.

2. The tunable ultra-high repetition rate ultra-short pulse fiber laser according to claim 1, characterized in that, The graphene thin film is connected to the ring-shaped fiber laser cavity in the following way: Cut off the single-mode optical fiber in the ring-shaped fiber laser cavity and weld a first fiber pigtail head and a second fiber pigtail head at both ends respectively. Use tweezers to pick up a piece of graphene thin film and place it on the output end face of the first fiber pigtail head. Then use a connection flange to connect the first fiber pigtail head and the second fiber pigtail head. After the connection is completed, the end face of the single-mode optical fiber inside the first fiber pigtail head is closely attached to one side surface of the graphene thin film, and the end face of the single-mode optical fiber inside the second fiber pigtail head is closely attached to the other side surface of the graphene thin film. The first fiber pigtail head and the second fiber pigtail head are tightly connected through the connection flange.

3. The tunable ultra-high repetition rate ultra-short pulse fiber laser according to claim 1, wherein The M-Z sub-cavity includes a first optical coupler (7), a second optical coupler (10), a first optical fiber arm, a second optical fiber arm, a first polarization controller (8), and an optical delay line (9). The first optical coupler (7) has a first input end, a first output end, and a second output end. The second optical coupler (10) has a first input end, a second input end, a first output end, and a second output end. The first input end of the first optical coupler (7) is connected to the single-mode optical fiber of the ring fiber laser cavity. The first output end of the first optical coupler (7) is connected to one end of the first optical fiber arm. The other end of the first optical fiber arm is connected to the first input end of the second optical coupler (10). The first polarization controller (8) is disposed on the first optical fiber arm. The second output end of the first optical coupler (7) is connected to one end of the second optical fiber arm. The other end of the second optical fiber arm is connected to the second input end of the second optical coupler (10). The optical delay line (9) is disposed on the second optical fiber arm. The first output end of the second optical coupler (10) is connected to the single-mode optical fiber of the ring fiber laser cavity. The second output end of the second optical coupler (10) serves as the output end of the ring fiber laser cavity.

4. The tunable ultra-high repetition rate ultrashort pulse fiber laser according to claim 3, characterized in that, The first optical coupler (7) is a 1×2 optical coupler, and the output coupling ratios of its first output end and second output end are 40%:60%. The second optical coupler (7) is a 2×2 optical coupler, and the input coupling ratios of its first input end and second input end and the output coupling ratios of its first output end and second output end are all 50%:50%. Both the first optical fiber arm and the second optical fiber arm are single-mode optical fibers.

5. The tunable ultra-high repetition rate ultra-short pulse fiber laser according to claim 3, characterized in that, The optical delay line includes a light input end (21), a light output end (22), and a reflector (23). The light input end (21) and the light output end (22) are connected to the second optical fiber arm. The position of the reflector (23) can be translated, and the light beam incident through the light input end is reflected towards the light output end. The optical path difference between the first optical fiber arm and the second optical fiber arm is changed by adjusting the position of the reflector (23).

6. The tunable ultra-high repetition rate ultrashort pulse fiber laser according to any one of claims 1-5, characterized in that, The pump source includes a first pump source (1) and a second pump source (5). The wavelength division multiplexer includes a first wavelength division multiplexer (2) and a second wavelength division multiplexer (4). The first pump source (1) is coupled to the ring fiber laser cavity through the first wavelength division multiplexer. The second pump source (5) is coupled to the ring fiber laser cavity through the second wavelength division multiplexer. The gain fiber is disposed between the first wavelength division multiplexer (2) and the second wavelength division multiplexer (4).

7. The tunable ultra-high-repetition-rate ultrashort-pulse fiber laser according to claim 6, wherein The ring fiber laser cavity further includes an optical isolator (6) and a second polarization controller (12). The optical isolator (6) is disposed between the second wavelength division multiplexer and the first optical coupler (7) of the M-Z sub-cavity. The saturable absorber (11) is disposed between the first wavelength division multiplexer and the second optical coupler (10) of the M-Z sub-cavity. The second polarization controller (12) is disposed between the saturable absorber (11) and the second optical coupler (10).

8. A method for generating ultra-high repetition rate ultra-short pulse laser by using the tunable ultra-high repetition rate ultra-short pulse fiber laser according to any one of claims 1-7, characterized in that, Including the following steps: Step 1: Connect the second output end of the second optical coupler of the M-Z sub-cavity to the spectrometer (14) and the autocorrelator (15). Step 2: Turn on the first pump source and the second pump source simultaneously, and use the spectrometer and the autocorrelator to monitor the spectrum and waveform respectively. Step 3: Continuously adjust the optical delay line in the M-Z sub-cavity to change the optical path difference ΔL between the first optical fiber arm and the second optical fiber arm of the M-Z sub-cavity until a comb spectrum image appears on the spectrometer. Step 4: When the comb spectrum appears, carefully adjust the first polarization controller and the second polarization controller to change the polarization states in the M-Z sub-cavity and the ring fiber laser cavity until a high-repetition-rate laser trajectory is observed on the autocorrelator. Step 5: Then, keep the pump power unchanged and continuously adjust the optical delay line to gradually reduce the optical path difference ΔL, and continue to adjust the first polarization controller and the second polarization controller until an autocorrelation curve and a comb spectrum are obtained.

Images