CN114530751A - High-order Raman suppression method based on walk-off effect - Google Patents
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- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
- H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
- H01S3/094042—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a fibre laser
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Abstract
The invention provides a high-order Raman suppression method based on walk-off effect, in a Raman fiber laser, a pulse laser pumping source array is used for outputting pulse laser with single wavelength as pumping light, continuous laser is used as signal light, the signal light and the pumping light are input into the Raman fiber, in the Raman fiber, the walk-off effect on a time domain can be generated between pulse lasers with different wavelengths, high-order Raman gain can be reduced, high-order Raman is suppressed, and high-power pulse low-order Raman fiber laser output is finally obtained.
Description
Technical Field
The invention belongs to the technical field of fiber laser, and particularly relates to a high-order Raman suppression method based on a walk-off effect.
Background
The fiber laser has the advantages of compact structure, high efficiency, good beam quality, stability, easy use and the like, and is widely applied to the fields of industry, national defense, medical treatment, basic research and the like. The Raman fiber laser generates laser gain by utilizing the stimulated Raman scattering effect in the fiber, has no limitation on the pump wavelength, is not influenced by gain saturation, amplified spontaneous radiation, photon darkening effect and the like, is widely applied to important fields of optical communication, supercontinuum generation, special waveband light source acquisition, medical treatment and the like, has the greatest characteristic and advantage of wide laser emitting wavelength range, and can generate laser in any wavelength in the fiber transparent range as long as pump laser with proper wavelength exists. Meanwhile, the ultrashort pulse laser has huge application prospect in the fields of basic research, biological medical treatment, industrial processing, optical communication and the like. Pulsed laser synchronous pumping can achieve high raman gain per unit length and therefore may be a more optimal means to achieve high performance ultrafast raman fiber laser output. But pulsed laser synchronous pumping techniques require long-term locking of the pulse repetition frequency to the raman cavity and any slight deviations will translate into laser output noise. Although a high-energy and high-efficiency ultrafast raman laser output can be obtained, if a low-noise output is to be obtained, the system complexity is high and the practicability is insufficient.
Thanks to the development of high brightness pumping technology, raman fiber amplifiers have achieved continuous laser output in the kilowatt range. Currently, the main factor limiting the further power increase of the raman fiber amplifier comes from the generation of higher-order raman, i.e. the first-order stokes light corresponding to the signal light. This is because when the power of the signal wavelength laser reaches the pumping threshold power of the higher-order raman, the signal wavelength laser converts the power to the higher-order raman wavelength as pumping light, resulting in a rise or even a drop in the output power.
At present, researchers in the aspect of suppressing stimulated raman scattering adopt different technical schemes: the gain optical fiber with wavelength selectivity is adopted, such as W-type optical fiber and the like, but the optical fiber has the disadvantages of complex drawing process, high cost, large transmission loss (7.5 dB/km @ lambda is 1 mu m) and difficult realization of an all-optical fiber structure; spectral filtering optical devices, such as spatial structure low pass filters, long period gratings and tilted gratings, are used to couple the core mode to the cladding mode, polarization maintaining fiber 45-degree dislocation fusion technology, and the like. It has the following disadvantages: the spatial structure low-pass filter has coupling loss, is not in an all-fiber structure, and has low system stability. Long period gratings and tilted gratings are currently used for single seed implantation only, and have limited suppression capability. The polarization maintaining fiber 45-degree dislocation welding technology is only suitable for a linear polarization Raman fiber laser; by adopting the cascade pumping mixed gain, the signal light and the Raman light with the same mode are injected at the same time to serve as seeds, but only backward stimulated Raman scattering can be inhibited, and the generation of forward stimulated Raman scattering cannot be inhibited or even promoted.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a high-order Raman suppression method based on a walk-off effect, which is used for suppressing the generation of high-order Raman and finally obtaining the laser output of a high-power low-order Raman pulse fiber.
In order to achieve the technical purpose, the technical scheme provided by the invention is as follows:
the invention provides a high-order Raman suppression method based on walk-off effect, in a Raman fiber laser, a pulse laser pumping source array is used for outputting pulse laser with single wavelength as pumping light, continuous laser is used as signal light, the signal light and the pumping light are input into the Raman fiber, in the Raman fiber, the walk-off effect on a time domain can be generated between pulse lasers with different wavelengths, high-order Raman gain can be reduced, high-order Raman is suppressed, and high-power pulse low-order Raman fiber laser output is finally obtained.
Furthermore, the Raman fiber laser comprises a pulse laser pumping source array, wherein the pulse laser pumping source array consists of pulse laser pumping sources, and pulse lasers are output by the pulse laser pumping source array.
Further, the pulse laser pump source is a semiconductor pulse laser or a solid pulse laser or a fiber pulse laser.
Further, the pulse laser pumping source is a mode-locked pulse laser or a Q-switched pulse laser.
Furthermore, the pulse laser pumping source is a semiconductor pulse laser, and the wavelength range is 915nm-976 nm.
Furthermore, the pulse laser pump source is a fiber pulse laser with a wavelength range of 1018 and 1080 nm.
Furthermore, the pulse repetition frequency of the pulse laser pumping source is continuously adjustable from 100KHz to 500KHz, and the pulse width is less than 10 ps.
Furthermore, the Raman fiber laser is a Raman fiber amplifier and comprises a pulse laser pumping source array, a seed source, a pumping signal combiner, a Raman fiber and an output end cap; the seed source is a laser source with signal light wavelength, and an output arm of the seed source is connected with a central signal arm of the pumping signal beam combiner; the output arm of each pulse laser pumping source in the pulse laser pumping source array is respectively connected to the pumping input arm of the pumping signal beam combiner; the output end of the pumping signal beam combiner is connected with the Raman fiber, the Raman fiber is spirally bent to form an amplifier structure, the pumping light transmits energy and amplifies continuous optical signals in the Raman fiber through a stimulated Raman scattering effect to obtain pulse Raman fiber laser output of the signal light, and the other end of the Raman fiber is connected with the output end cap and outputs the Raman laser through the output end cap.
Furthermore, the raman fiber amplifier further comprises an isolator, an isolator is arranged between the output arm of each pulse laser pumping source in the pulse laser pumping source array and the pumping input arm of the pumping signal combiner, and an isolator is arranged between the output arm of the seed source and the central signal arm of the pumping signal combiner.
The high-order Raman suppression method based on the walk-off effect is not only suitable for an amplifier structure but also suitable for an oscillator structure, namely the Raman fiber laser can be a Raman fiber oscillator.
Compared with the prior art, the invention has the advantages that:
the invention aims to inhibit the generation of high-order Raman in a Raman fiber laser, improve the conversion effect of pumping power and improve the output power of Raman laser. The invention uses the pulse laser output by the pulse laser pump source array as the pump light, uses the continuous laser as the signal light, when the signal light and the pump light are transmitted in the Raman fiber, the signal light can be amplified into the pulse signal light, when the power of the amplified pulse signal light is increased to a certain degree, the pulse laser of the high-order Raman can be excited, the pulse signal light of the low-order Raman and the pulse laser of the high-order Raman have different laser wavelengths, the propagation speed in the Raman fiber is different, and further the walk-off effect on the time domain is generated between different pulse lasers. For higher-order raman, because the generated raman frequency shift is larger relative to the pump wavelength, and the timing difference between the pump wavelength and the wavelength of the lower-order raman signal light is larger, the lower the gain of the higher-order raman is, the higher the pump power that can be output is, the longer the length of the raman fiber that is used is, the higher the raman conversion efficiency is, and further, the higher the power of the raman laser is output.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It is to be understood that the drawings in the following description are merely exemplary of the invention and that other drawings may be derived from the structure shown in the drawings by those skilled in the art without the exercise of inventive faculty.
FIG. 1 is a schematic structural view of example 1 of the present invention;
FIG. 2 is a schematic structural diagram of example 2 of the present invention;
FIG. 3 is a schematic structural diagram according to embodiment 3 of the present invention;
FIG. 4 is a schematic diagram of pulses based on the walk-off effect in example 4 of the present invention;
reference numbers in the figures:
1. a seed source; 2. a pulsed laser pumping source array; 3. a beam combiner; 4. a Raman fiber; 5. an output end cap; 6. an isolator; 7. a highly reflective fiber grating; 8. low reflection fiber grating.
Detailed Description
For the purpose of promoting a clear understanding of the objects, aspects and advantages of the embodiments of the invention, reference will now be made to the drawings and detailed description, wherein there are shown in the drawings and described below specific embodiments of the invention, in which modifications and variations can be made by one skilled in the art without departing from the spirit and scope of the invention. The exemplary embodiments of the present invention and the description thereof are provided to explain the present invention and not to limit the present invention.
In one embodiment, a high-order raman suppression method based on a walk-off effect is provided, in a raman fiber laser, a pulsed laser pump source array is used for outputting pulsed laser with a single wavelength as pump light, continuous laser is used as signal light, the signal light and the pump light are input into the raman fiber, and in the raman fiber, the walk-off effect in a time domain occurs between the pulsed laser with different wavelengths, so that high-order raman gain can be reduced, and high-order raman is suppressed. The Raman fiber laser comprises a pulse laser pumping source array, wherein the pulse laser pumping source array consists of pulse laser pumping sources, and pulse lasers are output by the pulse laser pumping source array.
The pulse laser pump source array outputs pulse laser with a single wavelength as pump light, continuous laser is used as signal light, when the signal light and the pump light are transmitted in a Raman fiber, the signal light can be amplified into pulse signal light, when the power of the amplified pulse signal light is increased to a certain degree, high-order Raman can be excited, however, because the wavelength of the high-order Raman is longer, a time domain walk-off effect can occur among the pulse lasers with different wavelengths (the pump light, the pulse signal light and the high-order Raman laser are pulse lasers with different wavelengths), the time sequence difference between the pulse waveform of the high-order Raman and the amplified pulse signal light is much different, the time sequence is overlapped very little, and no gain exists, so that the power improvement of the high-order Raman can be inhibited.
The walk-off effect is classified into a spatial walk-off effect (spatial walk-off) and a temporal walk-off effect (temporal walk-off). Whereas the time domain walk-off effect is a misalignment in the pulse time domain due to group velocity mismatch. The nonlinear frequency conversion efficiency of ultrashort pulses is high because these pulses still have high peak power even though the average power is low. However, the effective interaction length in nonlinear crystals is limited due to the effect of the so-called time-domain walk-off effect: the related pulses with different frequencies have different group velocities, and after propagating for a certain distance, the optical waves with larger frequency difference are not overlapped in the time domain, so that the effect has an important influence on limiting the conversion efficiency between the optical waves with larger frequency difference. For the higher-order raman gain, because the generated raman frequency shift is larger, and the difference between the wavelength timing sequence of the pump and the wavelength timing sequence of the low-order raman signal light is larger, the higher-order raman gain is correspondingly weaker, the pumpable pump power is higher, and the output raman laser power is further improved.
In addition to ultra-short pulses with a pulse width of about 10fs, the raman response time is generally much smaller than the typical pulse width, so the interaction between the pump pulses of a raman pulse is described by two coupled amplitude equations, which include group velocity dispersion, self-phase modulation, cross-phase modulation, pulse walk-off and pump consumption, the fiber used in most experiments is shorter, the effect due to fiber loss is negligible, and with the coordinate system moving with the pump pulses as a reference system, the pulse transmission equation is as follows:
in the formula
Ap，AsSlowly varying envelopes, v, of pump pulses and Raman pulses, respectivelygjIs the group velocity, fRRepresenting fractional raman contributions.
The walk-off parameter d represents the group velocity mismatch between the pump pulse and the raman pulse. Group velocity dispersion parameter beta of pump pulse and Raman pulse2jOf a non-linear parameter gammajAnd Raman gain coefficient gj(j ═ p or s, p for pump light and s for signal light) are slightly different because the raman shift between their carriers is about 13 Thz. The relationship between these parameters of the pump pulse and Raman pulse can be defined by the wavelength ratio lambdap/λsRepresents:
the relative importance of the terms in equations (1) and (2) can be measured by introducing 4 length scales. For pulse width T0And peak power of P0These quantities are defined as:
length of dispersionLDThe walk-off length, the nonlinear length and the raman gain length each represent a length scale beyond which the group velocity dispersion, the pulse walk-off, the raman gain effect of the nonlinearity (self-phase modulation and cross-phase modulation) are important, so that the shortest one of these length scales will play a major role. From equation (5), for T0<10ps, the pulse walk-off effect plays a major role.
Referring to the embodiment shown in fig. 1, the raman fiber laser is a raman fiber amplifier, and includes a pulse laser pump source array 2, a seed source 1, a beam combiner 3, a raman fiber 4 and an output end cap 5. The combiner 3 is a pump signal combiner.
The seed source 1 is a laser source with signal light wavelength, and an output arm of the seed source 1 is connected with a central signal arm of the pumping signal beam combiner; the output arm of each pulse laser pumping source in the pulse laser pumping source array 3 is respectively connected to the pumping input arm of the pumping signal beam combiner; the output end of the pumping signal combiner is connected with a Raman fiber 4, the Raman fiber 4 is spirally bent to form an amplifier structure, pumping light transmits energy and amplifies continuous optical signals in the Raman fiber 4 through a stimulated Raman scattering effect, meanwhile, in the Raman fiber 4, a walk-off effect in a time domain can occur among pulse lasers with different wavelengths, high-order Raman gain can be reduced, high-order Raman is inhibited, low-order Raman fiber laser output of the signal light is obtained, the other end of the Raman fiber 4 is connected with an output end cap 5, and high-power pulse low-order Raman fiber laser is output through the output end cap 5.
In the transmission process of the pulse laser, the propagation speeds of the pulse lasers with different wavelengths in the optical fiber are different, a walk-off effect on a time domain can occur between the pulse lasers with different wavelengths, and the pulses are not overlapped in a time sequence. In the invention, the used pump source array is a pulse laser pump source array, the instantaneous gain process is performed in the low-order Raman gain process, the pump gain is extracted when the time sequence is overlapped, and for the high-order Raman gain, the generated Raman frequency shift is larger, and the difference between the wavelength time sequence of the pump and the wavelength time sequence of the low-order Raman signal light is larger, so that the high-order Raman gain is correspondingly weaker, the pump power capable of being pumped is higher, and the output Raman laser power is further improved.
In the above embodiment, the pulse repetition frequency of the pulse laser pumping source array is continuously adjustable from 100KHz to 500KHz, and the pulse width is less than 10 ps.
From the formulas (4) and (5), for T0<10ps, the pulse walk-off effect plays a main role, and the pulse repetition frequency of 100KHz-500KHz can be adjusted, so that the overlapping size between pulses on the time sequence is convenient to adjust, and the high-order Raman gain is better inhibited.
In the above embodiments, the types of the pulse laser pumping sources in the pulse laser pumping source array are not limited, and the output wavelengths are not limited. The pulse laser pump source can be a semiconductor pulse laser, and can also be a solid pulse laser, a fiber pulse laser and the like. If a semiconductor pulsed laser is used, the wavelength range is preferably 915nm to 976 nm. If a fiber pulse laser is selected, the preferred wavelength range of the fiber pulse laser is 1018-1080 nm. In this wavelength range, semiconductor pulse lasers and fiber pulse lasers can provide optimum output power at an optimum price, thereby outputting laser light of higher power while reducing costs.
In the above embodiment, each pulse laser pump source in the pulse laser pump source array may be a mode-locked pulse laser, or may be other pulse lasers, such as a Q-switched pulse laser.
In the above embodiment, the type of the laser used by the seed source is not limited, and the seed source laser may be a conductor laser, or may be other lasers, such as a solid laser, a fiber laser, and the like.
In the above embodiments, the optical fiber substrate material used for the input arm, the output arm, and the raman fiber of each component device is a silica-based optical fiber, or a phosphate optical fiber, a fluoride optical fiber, or a germanium-doped optical fiber. For example, in a quartz fiber, the position with the central maximum frequency shift of 13.2THz can be selected, and other frequency shift amounts can also be selected as the raman frequency shift of the pulsed laser pump source array, and other raman fibers can also select different raman frequency shift amounts to obtain raman laser outputs with different wavelengths.
Referring to fig. 2, the raman fiber laser is a raman fiber amplifier, and includes a pulse laser pump source array 2, a seed source 1, a beam combiner 3, a raman fiber 4, an output end cap 5 and an isolator 6. The combiner 3 is a pump signal combiner. The difference in structure from the embodiment shown in fig. 1 is that: an isolator 6 is arranged between the output arm of each pulse laser pumping source in the pulse laser pumping source array 2 and the pumping input arm of the pumping signal beam combiner, and an isolator 6 is arranged between the output arm of the seed source 1 and the central signal arm of the pumping signal beam combiner. The purpose of the isolator is to prevent back-lighting and protect the pulsed laser pump source array and the seed source. The preferred scheme of selection and parameters of each component device in the embodiment shown in fig. 2 is the same as that of the corresponding device in embodiment 1, and has been described in detail above, and will not be described again here.
The high-order Raman suppression method based on the walk-off effect is not only suitable for an amplifier structure but also suitable for an oscillator structure, namely the Raman fiber laser can be a Raman fiber oscillator. Referring to fig. 3, the raman fiber laser in the illustrated embodiment may be a raman fiber oscillator including a pulse laser pump source array 2, a beam combiner 3, a raman fiber 4, an output end cap 5, an isolator 6, a high fiber-reflecting grating 7, and a low fiber-reflecting grating 8. The beam combiner 3 is a pumping beam combiner. The output arm of each pulse laser pumping source in the pulse laser pumping source array 2 is connected with an isolator 6 and is connected with the pumping input arm of the beam combiner 3 through the isolator 6, the output arm of the beam combiner 3 is connected with the input end of the high-reflection fiber grating 7, the high-reflection fiber grating 7 is connected with one end of the Raman fiber 4, the other end of the Raman fiber 4 is connected with the low-reflection fiber grating 8, and an oscillation cavity is formed between the high-reflection fiber grating 7 and the low-reflection fiber grating 8. The output end of the low-reflection fiber grating 8 is connected with the output end cap 5, and high-power pulse low-order Raman fiber laser is output through the output end cap 5.
The reflectivity of the high-reflection fiber grating 7 is more than 95%, and the emissivity of the low-reflection fiber grating 8 is 4-50%. Meanwhile, the high-reflectivity fiber grating 7 and the low-reflectivity fiber grating 8 can use a plurality of grating pairs (namely, the high-reflectivity grating and the low-reflectivity grating are used in a matching way) according to actual needs.
In an embodiment, a high-order raman suppression method based on a walk-off effect is provided, in a raman fiber laser, a pulse laser with a single wavelength output by a pulse laser pump source array is used as a pump light, the pump light output by the pulse laser pump source array is input into a raman fiber after a beam combiner, oscillation occurs in a resonant cavity formed by a high-reflection fiber grating 7 and a low-reflection fiber grating 8, first-order stokes raman light is generated after the pump power reaches a raman threshold, pulse laser of high-order raman can be generated when the first-order stokes light power reaches the second-order stokes raman threshold, the walk-off effect on a time domain can occur between pulse lasers with different wavelengths in the raman fiber, high-order raman gain can be reduced, high-order raman is suppressed, and high-power pulse low-order raman fiber laser output is finally obtained.
The technical features of the above embodiments can be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the above embodiments are not described, but should be considered as the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is specific and detailed, but not to be understood as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, and these are all within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (10)
1. A high-order Raman suppression method based on walk-off effect is characterized in that in a Raman fiber laser, a pulse laser pump source array is used for outputting pump light, continuous laser is used as signal light and the pump light is input into the Raman fiber, and in the Raman fiber, the walk-off effect on a time domain can occur among pulse lasers with different wavelengths, so that high-order Raman gain can be reduced, and high-order Raman is suppressed.
2. The walk-off effect based higher order raman suppression method of claim 1, wherein the raman fiber laser comprises a pulsed laser pump source array that outputs pulsed laser as pump light.
3. The walk-off effect based higher order raman suppression method according to claim 2, wherein the pulsed laser pump source is a semiconductor pulsed laser or a solid state pulsed laser or a fiber pulsed laser.
4. The walk-off effect based higher order raman suppression method according to claim 2, wherein the pulsed laser pump source is a mode-locked pulsed laser or a Q-switched pulsed laser.
5. The method of claim 2, wherein the pulsed laser pump source is a semiconductor pulsed laser with a wavelength range of 915nm-976 nm.
6. The method as claimed in claim 2, wherein the pulsed laser pump source is a fiber pulsed laser with a wavelength range of 1018-1080 nm.
7. A higher order Raman suppression method based on walk-off effect according to claim 2, 3, 4, 5 or 6, wherein pulse repetition frequency of the pulse laser pumping source is continuously adjustable from 100KHz to 500KHz, and pulse width is less than 10 ps.
8. The walk-off effect based higher order raman suppression method according to claim 7, wherein the raman fiber laser is a raman fiber amplifier comprising a pulsed laser pump source array, a seed source, a pump signal combiner, a raman fiber and an output end cap; the seed source is a laser source with signal light wavelength, and an output arm of the seed source is connected with a central signal arm of the pumping signal beam combiner; the output arm of each pulse laser pumping source in the pulse laser pumping source array is respectively connected to the pumping input arm of the pumping signal beam combiner; the output end of the pumping signal beam combiner is connected with the Raman fiber, the Raman fiber is spirally bent to form an amplifier structure, the pumping light transmits energy and amplifies continuous optical signals in the Raman fiber through a stimulated Raman scattering effect to obtain pulse Raman fiber laser output of the signal light, and the other end of the Raman fiber is connected with the output end cap and outputs the Raman laser through the output end cap.
9. The method of claim 8, wherein the raman fiber amplifier further comprises an isolator disposed between the output arm of each pulse laser pump source in the pulse laser pump source array and the pump input arm of the pump signal combiner, and an isolator disposed between the output arm of the seed source and the central signal arm of the pump signal combiner.
10. The walk-off effect based higher order raman suppression method according to claim 7, wherein the raman fiber laser is a raman fiber oscillator.
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Citations (2)
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|---|---|---|---|---|
| US20060210269A1 (en) * | 2003-02-12 | 2006-09-21 | Mahmoud Farhadiroushan | Method and apparatus for generation and transmission of high energy optical pulses for long range measurements |
| CN112003116A (en) * | 2020-08-24 | 2020-11-27 | 中国科学院上海光学精密机械研究所 | Ultrashort pulse Raman fiber amplifier |
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Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20060210269A1 (en) * | 2003-02-12 | 2006-09-21 | Mahmoud Farhadiroushan | Method and apparatus for generation and transmission of high energy optical pulses for long range measurements |
| CN112003116A (en) * | 2020-08-24 | 2020-11-27 | 中国科学院上海光学精密机械研究所 | Ultrashort pulse Raman fiber amplifier |
Non-Patent Citations (2)
| Title |
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| 刘毓等: "WDM 系统中色散走离条件下的拉曼串话研究" * |
| 王劲松等: "光纤色散对 WDM 系统中受激喇曼散射串扰的影响" * |
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