Repetition frequency synchronous and continuously adjustable three-wavelength ultrafast fiber laser

Abstract

The invention discloses a repetition frequency synchronous and continuously adjustable three-wavelength ultrafast fiber laser, which is generated by using a cascade nonlinear optical gain modulation technology. The laser comprises a gain switching diode, a rare earth doped optical fiber amplifier, a single-frequency continuous laser, an optical fiber coupling element, a light splitting element, a light filtering element and the like. The gain switch diode generates repetition-frequency continuously adjustable picosecond laser, the picosecond laser is divided into two paths after being amplified, one path is reserved, the other path serves as a first-order Raman scattering pumping source and provides nonlinear optical gain for single-frequency continuous laser, first-order Stokes light pulses are generated, the light pulses are divided into two paths, one path is reserved, and the other path serves as a second-order Raman scattering pumping source and provides nonlinear optical gain for single-frequency continuous laser. And the other part is used as a second-order Raman scattering pumping source to provide nonlinear optical gain for the other single-frequency continuous laser and generate a second-order Stokes light pulse. Through the cascading process, the system can output three-wavelength ultrafast laser with synchronous and continuously adjustable repetition frequency, and is suitable for the fields of nonlinear microscopic imaging, micromachining, terahertz generation and the like.

Description

Technical Field

[0001] The invention relates to the technical field of ultrafast fiber lasers, in particular to a repetition frequency synchronized and continuously adjustable three-wavelength ultrafast fiber laser. Background Art

[0002] The rapid development of ultrashort pulse generation technology has driven tremendous progress in scientific research and industry. As an extension of single-wavelength light sources, multi-wavelength ultrashort pulse lasers with synchronized repetition rates and large wavelength differences are important tools in ultrafast pump-probe technology, coherent Raman scattering spectroscopy, and nonlinear frequency conversion.

[0003] Currently, methods for generating multi-wavelength ultrashort pulses with synchronized repetition rates in fiber lasers mainly include active synchronous mode locking, passive synchronous mode locking, and optical parametric oscillation or amplification. Active synchronous mode locking systems require a precise servo system to achieve repetition rate locking of multiple different-wavelength mode-locked fiber lasers, resulting in a complex structure (see K. Nose, Y. Ozeki, T. Kishi, et al., "Sensitivity enhancement of fiber-laser-based stimulated Raman scattering microscopy by collinear balanced detection technique," Opt. Express 20, 13958-13965 (2012)). Although the passive synchronous mode-locked system achieves pulse synchronization by using the cross-phase modulation effect through the shared partial resonant cavity structure, it is limited by the coupling of fiber dispersion and nonlinear effects, and its cavity length matching tolerance is limited to the centimeter level, which significantly increases the difficulty of system assembly (see J. Zeng, B. Li, Q. Hao, et al., "Passively synchronized dual-color mode-locked fiber lasers based on nonlinear amplifying loop mirrors," Opt. Lett. 44, 5061-5064 (2019)). At the same time, for mode-locked fiber lasers, the pulse repetition rate is difficult to adjust due to the structure of the resonant cavity, and the output wavelength is also limited to the emission spectrum range of rare-earth-doped optical fibers. Although optical parametric oscillation or amplification technology based on nonlinear crystals can break through the spectral width limitation of the gain medium, it requires the introduction of spatial optical elements for phase matching, causing the system to lose the core advantages of the all-fiber structure in terms of stability and integration.

[0004] The Raman amplifier based on nonlinear optical gain modulation technology uses a single-frequency laser as a seed source. The nonlinear optical gain provided by the ultrafast laser pump can modulate the single-frequency continuous laser into a pulsed laser with the same repetition rate as the ultrafast pump. This technology circumvents the resonant cavity configuration and, combined with the gain switching diode technology, can generate Raman lasers with arbitrarily adjustable repetition rates (see Z. Cheng, J. Zhou, X. Cao, et al., "Repetition-Rate and Wavelength Flexible Femtosecond Laser Pulse Generation". Laser Photonics Rev. 18, 2400788 (2024)). However, there is still no device that uses a Raman amplifier based on nonlinear optical gain modulation technology to generate multi-wavelength ultrafast lasers with synchronized repetition rates. Summary of the Invention

[0005] In order to solve the problems of high system complexity, limited wavelength range, difficulty in adjusting the repetition rate, and difficulty in achieving an all-fiber structure in the technology of generating multi-wavelength ultrashort pulses with synchronized repetition rates, the present invention provides a three-wavelength ultrafast fiber laser with synchronized and continuously adjustable repetition rates. The purpose is to simply and stably obtain synchronized three-wavelength ultrafast lasers with continuously adjustable repetition rates.

[0006] The solution of the present invention is as follows:

[0007] A three-wavelength ultrafast fiber laser with synchronized repetition rate and continuous tunability, the core architecture of which includes a gain switching diode, a rare-earth-doped fiber amplifier, single-frequency continuous lasers of different wavelengths, and fiber coupling, splitting and filtering elements. When the fiber laser system is in operation, the gain switching diode first generates picosecond laser pulses with continuously adjustable repetition rate. After the laser pulses are power-enhanced by the first rare-earth-doped fiber amplifier, they are divided into two paths by the first splitting element: one path is directly output to the first port, and the other path is coupled with the first single-frequency continuous laser generated by the first single-frequency continuous laser, wherein the central wavelength of the first single-frequency continuous laser is within the first-order Raman gain spectrum; then, the laser pulses are transmitted through the second rare-earth-doped fiber amplifier and the first passive fiber. During this process, the laser pulses generated by the gain switching diode are further amplified by the second rare-earth-doped fiber amplifier and used as a pump source for first-order Raman scattering. The energy is transferred to the first single-frequency continuous laser through the stimulated Raman scattering effect in the first passive fiber, forming a first-order Stokes light pulse. The key to this process is that the first single-frequency continuous laser obtains nonlinear optical gain only in the interval that overlaps with the pump laser in time domain, so its final output is a single-frequency continuous laser. The first-order Stokes light pulse maintains a completely consistent repetition rate characteristic with the pump laser. After extracting the first-order Stokes light component, the beam is split again into two paths by a second beam splitter: one path is directly output to the second port, and the other path serves as a pump source for second-order Raman scattering and couples with a second single-frequency continuous laser generated by a second single-frequency continuous laser. The central wavelength of the second single-frequency continuous laser lies within the second-order Raman gain spectrum and is transmitted through a second passive optical fiber. During this process, the first-order Stokes light pulse pump laser transfers energy to the second single-frequency continuous laser through stimulated Raman scattering in the second passive optical fiber. The second single-frequency continuous laser only obtains nonlinear optical gain in the interval that overlaps with the first-order Stokes light pulse pump laser in the temporal domain. This gain mechanism ultimately modulates the second single-frequency continuous laser into second-order Stokes light pulses with the same repetition rate as the first-order Stokes light pulse pump laser. The second single-frequency continuous laser is finally extracted by the second filter element and output to the third port. Through this structural design, the system can synchronously output three pulse lasers of different wavelengths, and the repetition rate of all output laser pulses is strictly consistent with the gain switching diode, that is, the repetition rate is synchronized and continuously adjustable.

[0008] To adjust the average power of lasers of different wavelengths, the average output power of the first rare-earth-doped fiber amplifier can be adjusted during the initial signal amplification process. By varying parameters such as pump power, pump wavelength, or pumping mode, the amplifier's amplification of the input signal can be precisely controlled, thereby influencing the power level of the first-order Stokes light pulses generated by the subsequent first-order Raman scattering process. This ensures that the pulsed laser output to the first port and the subsequent lasers of different wavelengths generated by Raman scattering have a controllable power foundation at the initial stage, approaching the desired final power balance.

[0009] The first-order Raman scattering process can adjust the average output power of the second rare-earth-doped fiber amplifier. By varying parameters such as pump power, pump wavelength, or pumping mode, the power level of the first-order Stokes light can be precisely controlled. Simultaneously, the length of the first passive fiber can be adjusted, as the pump pulse laser and the first single-frequency continuous laser interact within the fiber. This fiber length affects the efficiency of stimulated Raman scattering, and thus the power of the first-order Stokes light. Furthermore, the splitting ratio of the first beam splitter can be adjusted based on actual needs to rationally distribute the optical power across different paths, ensuring that the laser power generated by the first-order Raman scattering process reaches the desired level.

[0010] The second-order Raman scattering process allows the length of the second passive fiber to be adjusted. Simultaneously, the splitting ratio of the second beam splitter can be appropriately adjusted. If the second-order Stokes light power is too high, the splitting ratio of that path can be increased to distribute more optical signals to another path, thereby reducing the output power. Conversely, the splitting ratio can be reduced to increase the output power. By adjusting these parameters in concert, the average power of the second-order Stokes light can be brought close to that of the lasers at other wavelengths, achieving optimal power balance across the entire three-wavelength ultrafast fiber laser system.

[0011] The gain switch diode generates a picosecond laser with continuously adjustable repetition frequency, the repetition frequency range of which is 10kHz to 1GHz, and the pulse width range of which is 1ps to 500ps.

[0012] The gain media of the first rare-earth-doped fiber amplifier and the second rare-earth-doped fiber amplifier are ytterbium-doped fibers.

[0013] The first beam splitting element and the second beam splitting element are both optical couplers.

[0014] The line widths of the first and second single-frequency continuous lasers should be less than 10 MHz, and the center wavelength λ0 of the gain switch diode, the wavelength λ1 of the first single-frequency continuous laser, and the wavelength λ2 of the second single-frequency continuous laser should satisfy the following relationship:

[0015]

[0016]

[0017] Where Δν is the frequency shift corresponding to the peak of the nonlinear gain coefficient, and c is the speed of light in a vacuum.

[0018] The first coupling element and the second coupling element are both wavelength division multiplexers.

[0019] The first optical filtering element is a filter or a wavelength division multiplexer, and the second optical filtering element is a filter or a wavelength division multiplexer.

[0020] The repetition frequency synchronized and continuously adjustable three-wavelength ultrafast fiber laser is an all-fiber structure, and all devices have polarization-maintaining characteristics.

[0021] Compared with the prior art, the technical effects of the present invention are:

[0022] 1) The present invention generates three-wavelength ultrafast pulses based on nonlinear optical gain modulation technology, providing a method for generating synchronized three-wavelength pulses without the need for additional electrical feedback. Through the stimulated Raman scattering effect of the pump pulse in the optical fiber, nonlinear optical gain modulation is applied to a single-frequency continuous laser whose central wavelength falls within the Raman gain spectrum, converting it into a first-order Stokes optical pulse with the same repetition rate as the pump pulse. A portion of the laser light from the first-order Stokes optical pulse is used as the pump laser to generate second-order Stokes optical pulses with the same repetition rate using the same method. This invention generates synchronized three-wavelength ultrafast pulses without the need for additional electrical synchronization devices or complex cavity length matching mechanisms, thus reducing system complexity.

[0023] 2) The repetition rate of the generated three-wavelength ultrafast pulses is continuously adjustable over a wide range. The pulses generated by the gain-switching diode can achieve continuous adjustment of the repetition rate. The repetition rate of the generated Stokes light pulses is the same as that of the gain-switching diode, thus achieving synchronized three-wavelength ultrafast pulse output with continuously adjustable repetition rate over a wide range.

[0024] 3) The three-wavelength synchronized ultrafast pulses generated by the present invention through nonlinear optical gain modulation technology are suitable for nonlinear microscopy, micromachining, terahertz generation and other fields. They have a wide range of application scenarios and significant market value. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 1 is a schematic diagram of the laser structure of an embodiment of the present invention;

[0026] Figure 2 The spectrum of the three-wavelength ultrafast laser of 1065nm, 1120nm and 1178nm at a repetition rate of 40MHz in the embodiment of the present invention;

[0027] Figure 3 1065nm, 1120nm, and 1178nm three-wavelength ultrafast laser autocorrelation curves at a repetition rate of 40MHz in an embodiment of the present invention;

[0028] Figure 4 The spectra of the three-wavelength ultrafast laser at different repetition rates when the single pulse energies of the 1065nm, 1120nm, and 1178nm three-wavelength lasers are 25nJ, 25nJ, and 37.5nJ, respectively, output by an embodiment of the present invention are shown.

[0029] Figure 1: Gain switching diode 1, first ytterbium-doped fiber amplifier 2, first optical coupler 3, first single-frequency continuous laser 4, first wavelength division multiplexer 5, second ytterbium-doped fiber amplifier 6, first passive fiber 7, second wavelength division multiplexer 8, second optical coupler 9, second single-frequency continuous laser 10, third wavelength division multiplexer 11, second passive fiber 12, fourth wavelength division multiplexer 13, first port A1, second port A2, third port A3. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to an example and accompanying drawings, but this should not limit the scope of protection of the present invention.

[0031] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0033] It should be understood that when an element is referred to herein as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may exist. Conversely, when an element is referred to herein as being "directly connected" or "directly coupled" to another element, this does not indicate the presence of intervening elements.

[0034] This example provides a repetition frequency synchronized and continuously adjustable three-wavelength ultrafast fiber laser. The system structure is similar to Figure 1The same. The 1065nm band gain switching diode 1 is used to generate a picosecond laser with continuously adjustable repetition rate. The first ytterbium-doped fiber amplifier 2 amplifies the picosecond laser. The first optical coupler 3 with a splitting ratio of 30:70 splits the 1065nm band ultrafast laser into two paths. One path accounts for 70% of the optical power and is output to the first port A1. The 1064 / 1120nm first wavelength division multiplexer 5 is used to couple the 1065nm band ultrafast laser and the 1120nm single-frequency continuous laser generated by the first single-frequency continuous laser 4. The 1065nm band ultrafast laser output from the 30% port is further amplified in the ytterbium-doped fiber amplifier. The 1120nm single-frequency continuous laser is nonlinearly gain modulated into a first-order Strange laser in the 1120nm band in the second ytterbium-doped fiber amplifier 6 and the first passive fiber 7. Stokes light pulses. A 1064 / 1120nm second wavelength division multiplexer 8 filters out the generated first-order Stokes light. A second optical coupler 9, with a splitting ratio of 30:70, splits the 1120nm ultrafast laser into two paths, one of which accounts for 30% of the optical power and is output to the second port A2. A 1120 / 1178nm third wavelength division multiplexer 11 couples the 1120nm ultrafast laser at the 70% port with the 1178nm single-frequency continuous laser generated by the second single-frequency continuous laser 10. The 1178nm single-frequency continuous laser is nonlinearly gain modulated in the second passive fiber 12 into second-order Stokes light pulses in the 1178nm band. The second-order Stokes light is filtered out by a 1120 / 1178nm fourth wavelength division multiplexer 13 and output to the third port A3. The system uses a fully polarization-maintaining fiber structure.

[0035] The gain switch diode 1 can output picosecond laser with continuously adjustable repetition rate. In this embodiment, the adjustment range is 20 MHz to 50 MHz, the output pulse width is 19.0 ps, and the central wavelength is 1064.97 nm.

[0036] The first single-frequency continuous laser 4 and the second single-frequency continuous laser 10 are both semiconductor lasers, with central wavelengths of 1120.47 nm and 1178.88 nm, and average powers of 15 mW and 20 mW, respectively.

[0037] The first passive optical fiber 7 and the second passive optical fiber 12 are both PM980 optical fibers, with lengths of 0.5 m and 4 m respectively.

[0038] The repetition frequency of the gain switch diode 1 is set to 40 MHz, and the pump powers of the first ytterbium-doped fiber amplifier 2 and the second ytterbium-doped fiber amplifier 6 are adjusted so that the average powers of the 1065 nm pulse output from the A1 port and the 1120 nm pulse output from the A2 port are both 1 W, and the average power of the 1178 nm pulse output from the A3 port is 1.5 W. The single pulse energies of the three ultrafast lasers with different wavelengths are 25 nJ, 25 nJ, and 37.5 nJ, respectively. Figure 2The radio frequency spectrum of the three-wavelength ultrafast laser is 1065nm, 1120nm and 1178nm. The measured repetition frequency is 40MHz, which confirms the synchronization of the repetition frequency among the three. Figure 3 As shown, the pulse widths are 19.0ps, 11.6ps and 9.2ps, respectively, confirming that single-frequency continuous laser can be modulated by nonlinear optical gain into ultrafast laser in the corresponding band. Figure 4 The spectrum of the three-wavelength ultrafast laser at different repetition rates is shown. By adjusting the pump power of the first ytterbium-doped fiber amplifier 2 and the second ytterbium-doped fiber amplifier 6, the energy of the single pulse of the three-wavelength ultrafast laser at 1065nm, 1120nm, and 1178nm remains unchanged. It can be observed that there is no obvious change in the spectrum, which confirms the continuously adjustable repetition rate of the system.

Claims

1. A three-wavelength ultrafast fiber laser with synchronized repetition rate and continuous tunability, characterized in that: include: The gain switching diode, the first rare-earth-doped fiber amplifier, the first beam splitter, the first single-frequency continuous laser, the first coupling element, the second rare-earth-doped fiber amplifier, the first passive fiber, the first filter element, the second beam splitter, the second single-frequency continuous laser, the second coupling element, the second passive fiber and the second filter element. The specific working process is as follows: the picosecond laser pulse with continuously adjustable repetition rate generated by the gain switching diode is amplified by the first rare-earth-doped fiber amplifier and then divided into two paths through the first beam splitter. One path of the laser pulse is directly output to the first port, and the other path of the laser pulse is used as a pump source for first-order Raman scattering and is coupled with the first single-frequency continuous laser generated by the first single-frequency continuous laser and having a central wavelength within the first-order Raman gain spectrum through the first coupling element. After coupling, The laser pulses sequentially pass through a second rare-earth-doped fiber amplifier, a first passive optical fiber, and a first optical filter element to filter out first-order Stokes light pulses. The first-order Stokes light pulses are split into two paths by a second optical splitter element, one of which is output to the second port, and the other is used as a pump source for second-order Raman scattering. The other is coupled to a second single-frequency continuous laser generated by a second single-frequency continuous laser and having a central wavelength within the second-order Raman gain spectrum through a second coupling element. The coupled laser pulses sequentially pass through a second passive optical fiber and a second optical filter element to filter out second-order Stokes light pulses and output to the third port. The repetition frequencies of the three laser pulses output to the first port, the second port, and the third port are synchronized with the repetition frequency of the gain switch diode and are continuously adjustable.

2. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The gain switching diode can generate picosecond laser pulses with continuously adjustable repetition rate, the repetition rate range is 10kHz to 1GHz, and the generated pulse width range is 1ps to 500ps.

3. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The gain media of the first rare-earth-doped fiber amplifier and the second rare-earth-doped fiber amplifier are ytterbium-doped fibers.

4. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The first beam splitting element and the second beam splitting element are optical couplers.

5. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The laser line width of the first single-frequency continuous laser and the second single-frequency continuous laser should both be less than 10 MHz, and the output wavelengths are λ1 and λ2 respectively. The output wavelengths λ1 and λ2 should satisfy the following relationship with the central wavelength λ0 of the gain switch diode: Where Δv is the frequency shift corresponding to the peak value of the nonlinear gain coefficient, and c is the speed of light in a vacuum.

6. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The first coupling element and the second coupling element are both wavelength division multiplexers.

7. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The first optical filtering element is a filter or a wavelength division multiplexer, and the second optical filtering element is a filter or a wavelength division multiplexer.

8. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The laser is an all-fiber structure, and all components have polarization-maintaining characteristics.

9. The repetition rate synchronized and continuously adjustable three-wavelength ultrafast fiber laser according to claim 1, characterized in that: The method of making the average power of three wavelength ultrafast lasers close to each other includes: adjusting the average power output by the first and second rare earth doped fiber amplifiers, the length of the first and second passive optical fibers, the splitting ratio of the first and second splitting elements, etc.

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