A high-power polarization-maintaining all-fiber mode-locked oscillator

Abstract

The present invention belongs to an ultrashort pulse laser mode-locked oscillator. In order to solve the problem of how to achieve high-power mode-locked output of the current all-fiber all-polarization-maintaining NPR mode-locked oscillator, there is no report on the research results in this area. The realization of high-power mode-locked output is limited by the technical problems of nonlinear accumulation and pump power. The present invention proposes a high-power polarization-maintaining all-fiber mode-locked oscillator, including a fiber reflector, a polarization beam splitter, a beam combiner, a pump laser, a fiber filter and a Faraday reflector. The output end of the fiber reflector is connected to the incident end of the polarization beam splitter, and the transmission end of the polarization beam splitter is connected to the input end of the beam combiner; the angle between the polarization-maintaining optical axis of the polarization beam splitter and the beam combiner is greater than 0° and less than 45°, the pump end of the beam combiner is connected to the pump laser, the output end is connected to the input end of the fiber filter through the gain fiber, and the output end of the fiber filter is connected to the incident end of the Faraday reflector.

Description

Technical Field

[0001] The invention belongs to an ultrashort pulse mode-locked oscillator, and in particular relates to a high-power polarization-maintaining all-fiber mode-locked oscillator. Background Art

[0002] Fiber mode-locked oscillators are the seed sources of high-power femtosecond lasers. Seed sources with excellent performance are the basis for ultrashort pulse laser amplification and the development of high-performance ultrashort pulse lasers. In fiber mode-locked oscillators, the traditional mode-locking methods are: inherent saturable absorber mode-locking and artificial saturable absorber mode-locking (nonlinear fiber loop mirror, nonlinear amplifying loop mirror, nonlinear polarization rotation (NPR)). Compared with the use of inherent saturable absorber mode-locking, the NPR mode-locking in artificial saturable absorber mode-locking has a higher damage threshold and faster response time. However, the traditional NPR mode-locking cannot achieve a polarization-maintaining all-fiber structure, so it is easily affected by ambient temperature fluctuations, vibration, stress, etc. and is unstable. This has led to the development of an all-fiber fully polarization-maintaining NPR (PM-NPR) mode-locked oscillator, which avoids the use of space devices, maximizes the advantages of fiber lasers, and has good environmental stability. Therefore, this type of mode-locked oscillator has broad application prospects.

[0003] All-fiber fully polarization-maintaining NPR mode locking has inherent stability in terms of mechanism, and the optical path can be built using only mature and stable common optical fiber components. “Nonlinear polarization evolution of ultrashort pulses in polarizationmaintaining fibers” published by J. Szczepanek et al. in Optics Express in May 2018, “Ultrafast laser mode-locked using Nonlinear Polarization Evolution inPolarization Maintaining fibers” published in Conference on Lasers and Electro-Optics in May 2017, “Fiberoscillator mode-locked using a novel scheme for Nonlinear Polarization Evolution in Polarization Maintaining fibers” published in Optics Express in 2019, and “Dissipative-soliton generation with nonlinear-polarization-evolution in a polarization maintaining fiber” published by J. Zhou et al. in Optics Express in February 2018 are all research results based on all-fiber full polarization-maintaining NPR mode-locking. They theoretically analyzed and experimentally verified the mode-locking mechanism from various aspects, and proposed a variety of methods to improve the mode-locking state. Since the damage threshold of the device used in all-fiber fully polarization-maintaining NPR mode-locking is high, it is expected to achieve high-power mode-locked output, but the research results in this regard have not been reported. In addition, in addition to the device damage threshold, the main limiting factors for achieving high-power output are nonlinear accumulation and pump power. Nonlinear accumulation is because too much nonlinear phase shift will accumulate in the fiber resonant cavity during high-power output, resulting in pulse splitting. Pump power is because the maximum power of single-mode pump output of fiber coupling is only a few hundred milliwatts. Summary of the invention

[0004] The present invention provides a high-power polarization-maintaining all-fiber mode-locked oscillator to solve the technical problem that the high-power mode-locked output of the current all-fiber fully polarization-maintaining NPR mode-locked is limited by nonlinear accumulation and pump power.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A high-power polarization-maintaining all-fiber mode-locked oscillator, which is special in that it includes a fiber reflector, a polarization beam splitter, a beam combiner, a pump laser, a fiber filter and a Faraday reflector;

[0007] The output end of the optical fiber reflector is connected to the incident end of the polarization beam splitter, and the reflective end of the polarization beam splitter is used for mode-locked output;

[0008] The transmission end of the polarization beam splitter is connected to the input end of the beam combiner; the angle between the polarization-maintaining optical axes of the polarization beam splitter and the beam combiner is greater than 0° and less than 45°;

[0009] The pump end of the beam combiner is connected to the pump laser, and the output end is connected to the input end of the optical fiber filter through the gain optical fiber;

[0010] The output end of the optical fiber filter is connected to the incident end of the Faraday reflector;

[0011] The gain fiber generates random fluorescence emission, which is polarized by the polarization beam splitter and then returned through the fiber reflector to become linearly polarized light. Due to the angular fusion between the polarization beam splitter transmission pigtail and the combiner input pigtail, two polarization components are orthogonally decomposed, which enter the combiner through the fast axis and slow axis of the combiner pigtail respectively, and are power amplified in the gain fiber, filtered by the fiber filter, and then reflected by the Faraday reflector, and then filtered by the fiber filter and amplified by the gain fiber in turn, and enter the polarization beam splitter again. The laser pulse part with high peak power passes through the polarization beam splitter and is returned by the fiber reflector again. After multiple cycles, a stable mode-locked state is reached to achieve mode-locked output.

[0012] Furthermore, the pigtail fiber and gain fiber of the combiner are both double-clad fibers.

[0013] Furthermore, the pump laser is a multi-mode pump laser.

[0014] Furthermore, the optical fiber filter is a biaxial optical fiber filter.

[0015] Furthermore, the fiber reflector, polarization beam splitter, beam combiner, fiber filter, Faraday reflector and pigtail of gain fiber are all polarization-maintaining fibers.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The high-power polarization-maintaining all-fiber mode-locked oscillator of the present invention realizes an all-fiber, stable and reliable high-power mode-locked oscillator, which greatly improves the output power of the all-fiber full polarization-maintaining NPR mode-locked oscillator. At the same time, the output power can be adjusted by adjusting the angle of the polarization-maintaining optical axis between the polarization beam splitter and the beam combiner. Combined with the adjustment of parameters such as pump power, cavity length, and filter bandwidth, a wide spectrum output can be obtained to achieve a narrower compressed pulse, and the cavity structure can be used in multiple bands.

[0018] 2. The pigtails of all devices constituting the mode-locked resonant cavity in the present invention are polarization-maintaining optical fibers, and the gain optical fiber and the combiner pigtail are double-clad optical fibers, so that a high-power multi-mode pump laser can be used for pumping. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the structure of the high-power polarization-maintaining all-fiber mode-locked oscillator of the present invention.

[0020] Among them: 1-fiber reflector, 2-polarization beam splitter, 3-beam combiner, 4-pump laser, 5-fiber filter, 6-Faraday reflector, 7-gain fiber. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0022] The present invention overcomes the problem of low output power of the existing all-fiber fully polarization-maintaining NPR mode-locked oscillator, gives full play to the advantages of the all-fiber fully polarization-maintaining NPR mode-locked technology, provides a low-nonlinear, high-power pumped mode-locked seed source cavity structure, and finally realizes the output of high-power mode-locked laser.

[0023] like Figure 1As shown, a high-power polarization-maintaining all-fiber mode-locked oscillator includes a fiber reflector 1, a polarization beam splitter 2, a beam combiner 3, a pump laser 4, a fiber filter 5 and a Faraday reflector 6. The fiber pigtail of the fiber reflector 1 is fused with the fiber pigtail of the polarization beam splitter 2, and the transmission fiber pigtail of the polarization beam splitter 2 is fused with the signal input fiber pigtail of the beam combiner 3. When fusion is performed, the polarization-maintaining optical axes of the two should form a certain angle, which is not limited and can be adjusted as needed. The reflection fiber pigtail of the polarization beam splitter 2 is used for laser output, the pump end of the beam combiner 3 is fused with the pump laser 4, the output end of the beam combiner 3 is fused with a large-mode-field double-clad gain fiber 7, the other end of the gain fiber 7 is fused with a biaxial fiber filter, and the other end of the fiber filter 5 is fused with a Faraday reflector 6.

[0024] In the above optical path, the pigtails of all devices are polarization-maintaining large mode field fibers, wherein the pigtail of the combiner 3 and the gain fiber 7 are both double-clad fibers to implement the cladding pumping method, so that a high-power multimode pump laser can be used for pumping.

[0025] The mode locking principle of the high-power polarization-maintaining all-fiber mode-locked laser of the present invention is as follows: the fiber reflector 1 and the Faraday reflector 6 are used as two cavity mirrors of the resonant cavity, the polarization beam splitter 2 and the tail fiber of the beam combiner 3 are angularly fused, and the polarization rotation of the Faraday reflector 6 is coordinated to form an equivalent fast saturable absorber to achieve mode locking, and the cavity loss and output power can be controlled by controlling the angle of the fast and slow axis fusion.

[0026] The specific working process is that after the pump laser 4 is added, the gain fiber 7 generates random fluorescence emission, which is polarized by the polarization beam splitter 2 and then returned through the fiber reflector 1 to become linearly polarized light. Due to the angular fusion between the transmission pigtail of the polarization beam splitter 2 and the input pigtail of the combiner 3, two polarization components of orthogonal decomposition are formed, which enter the combiner 3 through the fast axis and slow axis of the pigtail of the combiner 3 respectively, and are power amplified in the gain fiber 7, filtered by the fiber filter 5, and then reflected by the Faraday reflector 6. The polarization components of the fast axis and slow axis of the returned laser are rotated 90° respectively, and then filtered by the fiber filter 5 and amplified by the gain fiber 7 in turn, and finally enter the polarization beam splitter 2 again. For lasers with relatively low peak power, since the polarization of the returned light is rotated by 90° compared to the original incident light, it cannot pass through the polarization beam splitter 2 along the original path and be reflected back into the cavity again, but will be output from the other port of the polarization beam splitter 2 and cannot form laser oscillation. However, for lasers with relatively high peak power, due to the NPR effect, the polarization rotation of the pulse will occur when it is transmitted in the optical fiber. Therefore, a part of the laser will pass through the polarization beam splitter 2 and be reflected by the optical fiber 1 and return again. When the intracavity gain, nonlinearity, and dispersion meet certain conditions, after multiple cycles, a stable mode-locked state will be reached, and mode-locked output will be achieved.

[0027] The following is a specific embodiment of the present invention:

[0028] The pigtail of the optical fiber reflector 1 is fused with the pigtail of the polarization-maintaining polarization beam splitter 2 with a core diameter of 10 μm, and the transmission pigtail of the polarization beam splitter 2 is fused with the signal pigtail of the combiner 3 at a polarization axis angle of 35°. The core diameters of the input and output pigtails of the combiner 3 are both 10 μm, and the cladding diameters are both 125 μm. The pump end of the combiner 3 is fused with a multimode pump laser 4 with a core diameter of 105 μm, a cladding diameter of 125 μm, a maximum power of 9 W, and a center wavelength of 976 nm. The output end of the combiner 3 is fused with a large-mode double-clad Yb-doped gain fiber 7 with a core diameter of 10 μm and a cladding diameter of 125 microns. The other end of the gain fiber 7 is fused with a dual-axis optical fiber filter 5 with a bandwidth of 4 nm and a center wavelength of 1030 nm. The other end of the fiber filter 5 is fused with a polarization-maintaining fiber Faraday reflector 6, the fiber core diameter is 10 μm, and the central wavelength is 1030 nm. When the pump power is 1.5 W, a stable mode-locked laser output with a power of 102 mW, a repetition frequency of 11.36 MHz, and a pulse width of 2.8 ps is obtained.

[0029] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high-power polarization-maintaining all-fiber mode-locked oscillator, characterized in that: It comprises a fiber reflector (1), a polarization beam splitter (2), a beam combiner (3), a pump laser (4), a fiber filter (5) and a Faraday reflector (6); The output end of the optical fiber reflector (1) is connected to the input end of the polarization beam splitter (2), and the reflection end of the polarization beam splitter (2) is used for mode-locked output; The transmission end of the polarization beam splitter (2) is connected to the input end of the beam combiner (3); the angle between the polarization-maintaining optical axes of the polarization beam splitter (2) and the beam combiner (3) is greater than 0° and less than 45°, and is used to control cavity loss and output power; The pump end of the beam combiner (3) is connected to the pump laser (4), and the output end is connected to the input end of the optical fiber filter (5) through the gain optical fiber (7); The output end of the optical fiber filter (5) is connected to the incident end of the Faraday reflector (6); The gain optical fiber (7) generates random fluorescence emission, which is polarized by the polarization beam splitter (2) and then returned through the optical fiber reflector (1) to become linearly polarized light. The linearly polarized light forms two polarization components that are orthogonally decomposed through the polarization-maintaining optical axis between the polarization beam splitter (2) and the beam combiner (3). The two polarization components enter the beam combiner (3) through the fast axis and the slow axis of the pigtail of the beam combiner (3) respectively, and are power-amplified in the gain optical fiber (7), filtered by the optical fiber filter (5), and then reflected by the Faraday reflector (6) and the polarization direction is rotated by 90 degrees. Then, the light is filtered by the optical fiber filter (5) and amplified by the gain optical fiber (7) in sequence, and enters the polarization beam splitter (2) again, partially passes through the polarization beam splitter (2), and is returned by the optical fiber reflector (1) again. After multiple cycles, a stable mode-locked state is reached, and mode-locked output is realized.

2. A high-power polarization-maintaining all-fiber mode-locked oscillator as claimed in claim 1, characterized in that: The beam combiner (3) and the gain optical fiber (7) are both double-clad optical fibers.

3. A high-power polarization-maintaining all-fiber mode-locked oscillator as claimed in claim 1, characterized in that: The pump laser is a multi-mode pump laser.

4. A high-power polarization-maintaining all-fiber mode-locked oscillator as claimed in claim 3, characterized in that: The fiber optic reflector (1), polarization beam splitter (2), beam combiner (3), fiber optic filter (5), Faraday reflector (6) and pigtail fibers of the gain optical fiber (7) are all polarization-maintaining optical fibers.

Images