Fiber laser amplifier with high nonlinear threshold

Abstract

The invention discloses an optical fiber laser amplifier with a high non-linear threshold value, the amplifier comprises a laser seed source, a pre-amplification stage, an optical modulator, a main amplification stage and a focusing lens, after the pre-amplification stage improves the optical power of the laser seed source, the optical modulator based on phase modulation is arranged between the pre-amplification stage and the main amplification stage; the wavefront shaping stage is used for carrying out wavefront shaping on the laser output by the pre-amplification stage, and after single-transverse-mode laser is converted into coherent multi-transverse-mode laser, the multi-transverse-mode laser is coupled into a large-mode-field optical fiber of the main amplification stage for power boosting; multi-transverse-mode laser is injected into a fiber core, the phase of each mode of the multi-transverse-mode laser is regulated and controlled through an optical modulator, all the modes meet at the target position of the emergent end of the large-mode-field optical fiber in the main amplification stage, the phase difference is 2kpi (k is an integer), and a focusing lens is used in a combined mode to obtain a focusing light spot with high beam quality. According to the high-power fiber laser, the threshold value of each nonlinear effect in the high-power fiber laser is effectively improved, and high-power and high-beam-quality fiber laser is output.

Description

Technical Field

[0001] The invention relates to the field of lasers, and in particular to a fiber laser amplifier with a high nonlinear threshold. Background Art

[0002] When using a fiber amplifier to increase the laser power, nonlinear effects are very likely to occur due to the continuous increase in the laser power density in the gain fiber core. [1,2] , which has a negative impact on the amplification system. The nonlinear effects in fiber amplifiers mainly include stimulated Brillouin scattering, stimulated Raman scattering, self-phase modulation, etc., which hinder power increase, broaden the output spectrum, and there is also a risk of damaging the previous fiber system, so certain measures need to be taken to suppress it. The use of large mode field step-index fiber can reduce the core power density, but the laser will be in a few-mode operation state, and it is difficult to ensure high beam quality. Large mode field microstructure fiber with a specially designed cladding structure, although it supports single transverse mode operation of the laser, is difficult to prepare the fiber, and most of them cannot be coiled, making it difficult to be widely used.

[0003] In addition, applying a temperature gradient or stress gradient to the gain fiber can reduce the effective gain coefficient of nonlinear effects such as stimulated Brillouin scattering, but it is difficult to control and costly, and it will also shorten the service life of the gain fiber and increase the complexity of the laser system.

[0004] Therefore, it is particularly important to explore efficient and easy-to-implement methods to suppress nonlinear effects in fiber amplifiers.

[0005] References

[0006] [1] C.Shi, H.Tian, ​​S.Fu, Q.Sheng, Z.Shi, P.Jiang, W.Shi, and J.Yao, "High-energy single-frequency pulsed fiber MOPA at 1064nm based on a hybrid active-fiber," Opt.Express30(9),15575-15582(2022).

[0007] [2] Y.Xu, Q.Sheng, P.Wang, X.Cui, Y.Zhao, H.Xu, 60(13),3740-3746(2021). Summary of the invention

[0008] The present invention provides a fiber laser amplifier with a high nonlinear threshold. The present invention breaks through the limitations of traditional nonlinear effect suppression methods and solves the technical problem that conventional step-type large mode field active optical fibers are difficult to output high beam quality lasers. By introducing an optical modulator based on phase modulation, a single transverse mode laser is converted into a coherent multi-transverse mode laser, which can not only reduce the laser intensity in the active optical fiber to increase the nonlinear threshold, but also each coherent laser mode can be focused into a laser spot with high beam quality after high-power amplification in a large mode field gain optical fiber. See the following description for details:

[0009] A fiber laser amplifier with a high nonlinear threshold, the amplifier comprising: a laser seed source, a pre-amplification stage, an optical modulator, a main amplifier stage and a focusing lens,

[0010] After the pre-amplifier boosts the optical power of the laser seed source, an optical modulator based on phase modulation is placed between the pre-amplifier and the main amplifier to perform wavefront shaping on the laser output from the pre-amplifier. After the single transverse mode laser is converted into a coherent multi-transverse mode laser, it is coupled into the large mode field optical fiber of the main amplifier for power boosting.

[0011] Multi-transverse mode laser is injected into the core of optical fiber, and the phase of each mode of the multi-transverse mode laser is adjusted by optical modulator, so that all modes meet at the target position of the output end of the large mode field optical fiber in the main amplifier stage with a phase difference of 2kπ, where k is an integer. A focusing lens is used in combination to obtain a focused spot with high beam quality.

[0012] Wherein, the main amplifier stage includes: a pump source, a beam combiner, a large mode field gain optical fiber and a cladding light stripper.

[0013] The pump source is a multi-mode semiconductor laser, the cladding light stripper filters out the remaining pump light in the inner cladding of the optical fiber, and the focusing lens focuses the multi-mode laser output by the main amplifier stage to obtain the fiber laser output.

[0014] The laser seed source outputs single-mode laser, the output wavelength is any wavelength in the range from visible light to mid-infrared band, and the laser operation mode is continuous operation, quasi-continuous operation or pulsed operation.

[0015] The pre-amplification stage is a one-stage or multi-stage main oscillator power amplification structure, which is used to amplify the seed light power and output a single-mode light spot.

[0016] The optical modulator converts the input single-mode laser into a coherent multi-mode laser, actively modulates the phase of each mode, and couples the multi-mode laser into the core of the large mode field optical fiber;

[0017] The optical fiber core diameter at the input end of the optical modulator is relatively small, and can transmit single-mode laser, and its mode field matches the mode field of the optical fiber at the output end of the pre-amplifier stage; the optical fiber core diameter at the output end of the optical modulator is relatively large, and can transmit multi-mode laser, and its mode field matches the mode field of the optical fiber at the input end of the main amplifier stage.

[0018] Among them, the pump source in the main amplifier stage is a semiconductor laser, a fiber laser or a solid laser, and its laser mode is a fundamental transverse mode or multiple transverse modes; the pump light emitted by the pump source is coupled into the large mode field active optical fiber through a beam combiner; the absorption band and the emission band of the large mode field active optical fiber correspond to the wavelength of the pump light and the wavelength of the seed light respectively; the large mode field active optical fiber absorbs the pump light to provide gain for high-power amplification of the seed light.

[0019] The beneficial effects of the technical solution provided by the present invention are:

[0020] 1. The technical route proposed by the present invention not only reduces the demand for high-cost large-mode-field single-mode active optical fibers, but also avoids the degradation of the quality of active optical fibers in experiments (for example, although the nonlinear threshold can be increased by changing the stress distribution of active optical fibers with external forces, the service life of the optical fibers will be shortened); in addition, by optimizing the phase applied to each laser mode, a focused light spot with high beam quality can be obtained at the target position at the output end of the main amplifier stage;

[0021] 2. The present invention realizes the conversion of single transverse mode laser to coherent multi-transverse mode laser by introducing an optical modulator into the optical fiber amplification system, which can not only improve the threshold of various nonlinear effects in the optical fiber amplifier, but also obtain high beam quality laser output by focusing at the output end of the amplifier, and cleverly solves the problem of beam quality degradation that is easily caused by using large mode field optical fiber to improve the nonlinear threshold in the traditional method;

[0022] 3. The technical solution proposed by the present invention broadens the selection space of gain optical fiber. In terms of optical fiber size, large-mode-field gain optical fiber with a larger core diameter or even more than 100 μm can be selected; in terms of optical fiber material, in addition to traditional quartz glass, multi-component glass with high solubility of rare earth ions can also be selected, such as silicate glass, phosphate glass and tellurite glass; in terms of optical fiber structure, there is no need to use microstructured optical fiber with high cost and difficulty in preparation, and conventional step-index optical fiber can be used;

[0023] 4. The technical solution proposed in the present invention can be implemented flexibly. According to the medium parameters of the large mode field gain optical fiber to be used in the amplifier, the phase modulation parameters to be applied to each modulation unit in the optical modulator can be optimized, so that the output multi-mode laser energy can be focused into a light spot with high beam quality;

[0024] 5. The technical solution proposed in the present invention does not require external force to change the physical properties of the gain fiber, thus avoiding the complication of the laser system, facilitating the miniaturization integration of the laser system, and broadening the application of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the structure of a fiber laser amplifier with a high nonlinear threshold.

[0026] In the accompanying drawings, the components represented by the reference numerals are listed as follows:

[0027] 1: Laser seed source; 2: Pre-amplification stage;

[0028] 3: optical modulator; 4: pump source;

[0029] 5: beam combiner; 6: large mode field active optical fiber;

[0030] 7: Cladding light stripper; 8: Focusing lens. DETAILED DESCRIPTION

[0031] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0032] The embodiment of the present invention provides a fiber laser amplifier with a high nonlinear threshold. After the seed light power is increased by the pre-amplifier stage, an optical modulator based on phase modulation is placed between the pre-amplifier stage and the main amplifier stage, and the laser output by the pre-amplifier stage is wavefront shaped. After the single transverse mode laser is converted into a coherent multi-transverse mode laser, it is coupled into the large mode field optical fiber of the main amplifier stage for power increase. The multi-transverse mode laser is injected into the optical fiber core, which reduces the laser intensity in the core and increases the threshold of various nonlinear effects in the optical fiber. Taking the suppression of stimulated Brillouin scattering as an example, such a method can also broaden the Brillouin gain spectrum, reduce the Brillouin gain peak, and effectively increase the stimulated Brillouin scattering threshold, so that the multi-transverse mode laser can be amplified to a higher power. Finally, the phase of each mode of the multi-transverse mode laser is regulated by an optical modulator, so that all modes meet at the target position of the output end of the large mode field optical fiber in the main amplifier stage and the phase difference is 2kπ (k is an integer), which not only meets the constructive interference condition, but also uses a focusing lens to obtain a focused light spot with high beam quality. The specific technical solutions of the embodiment of the present invention are as follows:

[0033] Example 1

[0034] An embodiment of the present invention provides a fiber laser amplifier with a high nonlinear threshold, which includes: a laser seed source 1, a pre-amplifier stage 2, an optical modulator 3, a main amplifier stage (including: a pump source 4, a combiner 5, a large mode field gain fiber 6, a cladding light stripper 7), and a focusing lens 8.

[0035] Among them, the continuous laser center wavelength output by the laser seed source 1 is 1064nm, the output power is 30mW, and the output fiber pigtail size is 6 / 125μm (that is, the core size is 6μm, the cladding size is 125μm, the same meaning below, no longer repeated); the pre-amplification stage 2 includes one or more amplifiers based on core-pumped ytterbium-doped single-clad optical fiber, and the input and output fiber pigtail sizes are both 6 / 125μm; the optical modulator 3 can operate in the wavelength range of 430nm to 1600nm, with a resolution of 1920×1200 and an adjustable phase range of 0-2π. Its input fiber pigtail matches the specifications of the pre-amplification stage output fiber pigtail, and its output fiber pigtail matches the input fiber pigtail of the main amplifier stage; the pump source 4 is a 976nm stable wavelength multi-mode mode semiconductor laser, with a maximum output power of 60W and an output fiber pigtail size of 105 / 125μm; the fiber pigtail sizes of the input / output signal ports of the combiner 5 are both 50 / 400μm, and the fiber pigtail size of the pump port is 105 / 125μm; the active fiber 6 is an ytterbium-doped double-clad active fiber with a fiber size of 50 / 400μm and a fiber length of 1m; the fiber pigtail size of the cladding light stripper 7 is 50 / 400μm, which can filter out the remaining pump light in the inner cladding of the optical fiber, and the output end is cut at an angle to ensure stable and safe laser output; the focusing lens 8 is a quartz substrate plano-convex lens with an aperture of 25mm and a focal length of 50mm, which can focus the multi-mode laser output by the main amplifier stage to obtain high-power, high-beam-quality fiber laser output.

[0036] After determining the focal position of the output laser, the phase of each mode applied to the multi-transverse mode laser can be optimized through the optical modulator 3, and combined with the use of a focusing lens, all laser modes can meet at the target position with a phase difference of 2kπ (k is an integer), that is, the constructive interference condition is satisfied, thereby achieving high-power, high-beam-quality laser output at this position.

[0037] Example 2

[0038] In the above-mentioned embodiment 1, the wavelength output by the laser seed source 1 can be any wavelength in the range from visible light to mid-infrared, and the laser operation mode can be continuous operation, quasi-continuous operation or pulse operation, which is not limited in this embodiment of the present invention.

[0039] Correspondingly, the emission bands of the single-clad active optical fiber used in the pre-amplification stage 2 and the double-clad active optical fiber 6 in the main amplifier stage only need to match the output wavelength of the seed source, and this embodiment of the present invention does not impose any limitation on this.

[0040] Accordingly, the wavelength of the pump laser emitted by the single-mode pump source used in the pre-amplification stage 2 and the pump source 4 in the main amplifier stage only needs to be within the absorption band of the active optical fiber, which is not limited in the embodiment of the present invention.

[0041] Except for special instructions on the models of various components in the embodiments of the present invention, the models and specifications of other components, including: the size, numerical aperture, length and doping concentration of the optical fiber, and the aperture and focal length of the lens, are not specifically limited, and any device that can perform the above functions may be used.

[0042] Those skilled in the art will appreciate that the accompanying drawing is only a schematic diagram of a preferred embodiment, and the serial numbers of the embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0043] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A fiber laser amplifier with a high nonlinear threshold, characterized in that: The amplifier comprises: a laser seed source, a pre-amplification stage, an optical modulator, a main amplifier stage and a focusing lens, After the pre-amplifier boosts the optical power of the laser seed source, an optical modulator based on phase modulation is placed between the pre-amplifier and the main amplifier to perform wavefront shaping on the laser output from the pre-amplifier. After the single transverse mode laser is converted into a coherent multi-transverse mode laser, it is coupled into the large mode field optical fiber of the main amplifier for power boosting. Multi-transverse mode laser is injected into the core of optical fiber, and the phase of each mode of the multi-transverse mode laser is adjusted by optical modulator, so that all modes meet at the target position of the output end of the large mode field optical fiber in the main amplifier stage with a phase difference of 2kπ, where k is an integer. A focusing lens is used in combination to obtain a focused spot with high beam quality.

2. The fiber laser amplifier with high nonlinear threshold according to claim 1, characterized in that: The main amplifier stage includes: a pump source, a beam combiner, a large mode field gain optical fiber and a cladding light stripper. The pump source is a multi-mode semiconductor laser, the cladding light stripper filters out the remaining pump light in the inner cladding of the optical fiber, and the focusing lens focuses the multi-mode laser output by the main amplifier stage to obtain the fiber laser output.

3. The fiber laser amplifier with high nonlinear threshold according to claim 1, characterized in that: The laser seed source outputs single-mode laser, the output wavelength is any wavelength in the range from visible light to mid-infrared band, and the laser operation mode is continuous operation, quasi-continuous operation or pulse operation.

4. The fiber laser amplifier with high nonlinear threshold according to claim 1, characterized in that: The pre-amplification stage is a one-stage or multi-stage main oscillation power amplification structure, which is used to amplify the seed light power and output a single-mode light spot.

5. The fiber laser amplifier with high nonlinear threshold according to claim 1, characterized in that: The optical modulator converts the input single-mode laser into a coherent multi-mode laser, actively modulates the phase of each mode, and couples the multi-mode laser into the core of the large mode field optical fiber; The optical fiber core diameter at the input end of the optical modulator is relatively small, and can transmit single-mode laser, and its mode field matches the mode field of the optical fiber at the output end of the pre-amplifier stage; the optical fiber core diameter at the output end of the optical modulator is relatively large, and can transmit multi-mode laser, and its mode field matches the mode field of the optical fiber at the input end of the main amplifier stage.

6. The fiber laser amplifier with high nonlinear threshold according to claim 2, characterized in that: The pump source in the main amplifier stage is a semiconductor laser, a fiber laser or a solid laser, and its laser mode is a fundamental transverse mode or multiple transverse modes; the pump light emitted by the pump source is coupled into the large mode field active optical fiber through a beam combiner; the absorption band and the emission band of the large mode field active optical fiber correspond to the wavelength of the pump light and the wavelength of the seed light respectively; the large mode field active optical fiber absorbs the pump light and provides gain for high-power amplification of the seed light.

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