CN112968347B - Method for inhibiting stimulated Raman scattering, high-power optical fiber laser and all-solid-state microstructure optical fiber - Google Patents
Method for inhibiting stimulated Raman scattering, high-power optical fiber laser and all-solid-state microstructure optical fiber Download PDFInfo
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
Abstract
The invention provides a method for inhibiting stimulated Raman scattering, a high-power optical fiber laser and an all-solid-state microstructure optical fiber, wherein the all-solid-state microstructure optical fiber comprises a fiber core and a microstructure cladding, the fiber core is a solid fiber core, the microstructure cladding is wrapped on the periphery of the fiber core, the microstructure cladding is positioned on germanium rods and a solid substrate in a regular hexagonal lattice, multiple layers are distributed on the regular hexagonal lattice from inside to outside, the central distances of any two adjacent points in the regular hexagonal lattice are equal, the fiber core is positioned at the central position of the regular hexagonal lattice, the multiple germanium rods are distributed on each layer of the regular hexagonal lattice in a sparse structure, and the solid substrate is filled between the germanium rods. Under the condition that the propagation constant is certain, the diameter and the refractive index of the germanium rod are adjusted, so that when the high-power optical fiber laser is transmitted by the all-solid-state microstructure optical fiber, the high-power laser quasi-single-mode long-distance transmission can be realized, and the stimulated Raman scattering can be inhibited.
Description
Technical Field
The invention mainly relates to the technical field of laser fibers, in particular to a method for inhibiting stimulated Raman scattering, a high-power fiber laser and an all-solid-state microstructure fiber.
Background
The high average power laser has been widely applied to various fields such as metal cutting, material cladding, laser welding, laser ignition and the like by virtue of natural advantages such as energy concentration, flexible conversion, small heat affected zone and the like. In recent years, due to the development of high-brightness pump sources, key optical components, laser materials, pump coupling, beam combining and other technologies, various types of lasers such as high-power fiber lasers, gas lasers, solid-state lasers, semiconductor lasers and the like have made remarkable progress in power enhancement. Taking High Power Fiber Laser (HPFL) as an example, the technology of kilowatt-level fiber oscillators and amplifiers at home and abroad is mature day by day, and the kilowatt-level fiber laser test prototype and industrial products are also successively released and gradually step into the market.
In a High Power Laser (HPL) application process, the energy transmitting fiber provides the possibility of flexible single mode transmission of the high power laser beam from the laser source to the work platform. In conventional laser machining applications, for example, a high power laser beam is typically required to transmit several meters, tens of meters, or even hundreds of meters from a light source to a work platform. However, due to the increase of optical power density and the limited area of the optical fiber mode field, high-brightness optical beams in the hundreds of watts or kilowatts inevitably induce nonlinear effects such as Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS), etc. when transmitted over long distances. In general, the spectral broadening caused by SBS is the first problem to be solved by narrow-linewidth lasers, and the fluctuation of signal light power and the reduction of efficiency caused by SRS stokes shift are common problems faced by all laser oscillators and amplifiers. Commercial continuous wave HPLs typically have a wide output spectral bandwidth, and thus the primary obstacle limiting their energy transmission fiber length is stimulated raman scattering.
The length and the mode field area of the optical fiber directly determine the threshold power of the SRS, and the requirements are met
Wherein A iseffIs the area of the mode field, gR(Ω) is the Raman gain coefficient, LeffIs the length of the optical fiber. To overcome SRS to achieve long distance transmission of high power laser beams typically requires increasing the core size of the fiber, the so-called Large Mode Area (LMA) fiber. However, the use of LMA fibers inevitably results in fiber multimode operation leading to a reduction in output beam quality. In order to ensure the single-mode operation characteristics of the optical fiber as much as possible, the Numerical Aperture (NA) of the optical fiber is generally required to be reduced with the increase of the core size, and an excessively small NA may result in a high degree of sensitivity of the mode area of the optical fiber to bending, which is not favorable for practical application. Meanwhile, the appearance of part of microstructure optical fibers such as photonic crystal optical fibers, leakage channel optical fibers, multi-core optical fibers and hollow-core optical fibers can enable the optical fibers to have the characteristics of large mode field area, low NA and constant and stable single-mode operation, but the optical fibers are high in preparation cost, large in transmission loss and bending sensitivity and generally not suitable for flexible transmission in industrial application. With further power boost of HPL, the mode characteristics, conduction characteristics, spectral characteristics and mechanical properties of the energy transmitting fiber itself are also subject to greater examination.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a method for inhibiting stimulated Raman scattering, a high-power optical fiber laser and an all-solid-state microstructure optical fiber, which can realize high-power laser quasi-single-mode long-distance transmission and can inhibit stimulated Raman scattering.
In order to achieve the technical purpose, the technical scheme of the invention is as follows:
a method of suppressing stimulated raman scattering, comprising:
designing an all-solid-state microstructure optical fiber, wherein the all-solid-state microstructure optical fiber comprises a fiber core and a microstructure cladding, the fiber core is a solid fiber core, the microstructure cladding is wrapped on the periphery of the fiber core, the microstructure cladding comprises germanium rods and a solid substrate, the germanium rods are positioned in a regular hexagonal lattice, multiple layers are distributed on the regular hexagonal lattice from inside to outside, the central distance Lambda between any two adjacent points in the regular hexagonal lattice is equal, the fiber core is positioned at the central position of the regular hexagonal lattice, the germanium rods are distributed on each layer of regular hexagonal lattice in a sparse structure, and the solid substrate is filled between the germanium rods;
welding the all-solid-state microstructure optical fiber on an output tail fiber of a high-power optical fiber laser to serve as a long-distance transmission optical fiber; when the all-solid-state microstructure fiber transmits high-power fiber laser, the germanium rod in the microstructure cladding generates a photonic band gap effect due to antiresonant coupling, the multiple interference among scattered light in a specific frequency range weakens the intensity of the light and prevents the light from passing through the microstructure cladding, and finally the light is limited to be transmitted in the fiber core, wherein the specific frequency range is called as a photonic band gap frequency range. For signal wavelength laser, multiple interference exists between scattered light falling in a photonic band gap frequency range, so that the intensity of the scattered light is weakened and the scattered light cannot pass through a microstructure cladding, and finally the scattered light is limited in a fiber core for stable transmission, so that high-efficiency single-mode low-loss transmission of the signal wavelength laser is realized; meanwhile, the Stokes wavelength of Stimulated Raman Scattering (SRS) falling outside the photonic band gap frequency range is high in loss and cannot be stably transmitted, and the all-solid-state microstructure optical fiber can inhibit the stimulated Raman scattering effect in the long-distance transmission process of high-power optical fiber laser.
As a further improvement of the invention, the regular hexagonal lattice has four layers, namely a first layer of regular hexagonal lattice, a second layer of regular hexagonal lattice, a third layer of regular hexagonal lattice and a fourth layer of regular hexagonal lattice from inside to outside.
As a further improvement of the invention, 12 points are arranged on the first layer of regular hexagon lattice, germanium rods are uniformly distributed on all the points, 18 points are arranged on the second layer of regular hexagon lattice, germanium rods are uniformly distributed on two points positioned in the middle of each edge, 24 points are arranged on the third layer of regular hexagon lattice, germanium rods are uniformly distributed on four points positioned at two ends of each edge, 30 points are arranged on the fourth layer of regular hexagon lattice, and germanium rods are uniformly distributed on two points positioned at two ends of each edge and two points positioned in the center of each edge. Namely, no germanium rod is arranged on the point at the hexagonal vertex in the second layer of regular hexagonal lattice; no germanium rod is arranged at the center of each edge in the second layer of regular hexagonal lattice; and the second point and the fourth point of each side in the fourth layer of regular hexagon lattice are not provided with germanium rods. The positions of the points where the germanium rods are not arranged and the gaps between the germanium rods are filled by the solid quartz substrate. Therefore, the low loss of the signal laser fundamental mode is ensured, and the bending loss of a high-order mode is increased so as to realize the quasi-single mode operation of the optical fiber.
As a further improvement of the invention, the core material is solid silica. The solid substrate is a solid quartz substrate.
As a further improvement of the invention, the structure and the size of all the germanium rods are the same, and the germanium rods comprise a high-refractive-index germanium-doped region in the center and a low-refractive-index cladding region in the outer layer, wherein the refractive index of the high-refractive-index germanium-doped region is in parabolic graded or step-type refractive index distribution. The high refractive index germanium-doped region and the low refractive index cladding region have certain core-to-core ratio, and the ratio is arbitrarily set according to requirements, for example, set to be 1: 2.5.
As a further improvement of the invention, the high refractive index germanium-doped region and the low refractive index cladding region of the germanium rod have a refractive index difference delta, and the refractive index difference delta is satisfied as (n ═ n)high 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive index of the high refractive index germanium-doped region is the maximum of the refractive index and the refractive index of the low refractive index cladding region is the refractive index.
As a further improvement of the invention, the refractive index difference delta is more than or equal to 1.5% and less than or equal to 3%, when the refractive index difference delta is determined, the distribution of the photonic band gap is only related to the diameter d of the germanium rods, but not related to the arrangement mode of the germanium rods, and the suppression of the stimulated Raman scattering effect can be realized by increasing or decreasing the diameter d of the germanium rods.
As a further improvement of the present invention, the low-refractive-index cladding region of the germanium rod may be made of a low-refractive-index material such as solid quartz or Fluorine (Fluorine) -doped solid quartz.
The invention provides a high-power optical fiber laser which comprises a laser generating unit, wherein the laser generating unit is used for generating high-power optical fiber laser, and an all-solid-state microstructure optical fiber is welded on an output tail fiber of the laser generating unit to serve as a long-distance transmission optical fiber; the all-solid-state microstructure optical fiber comprises a fiber core and a microstructure cladding, wherein the fiber core is a solid fiber core, the microstructure cladding is coated on the periphery of the fiber core, the microstructure cladding comprises germanium rods and a solid substrate, the germanium rods are located in a regular hexagonal lattice, multiple layers of layers are distributed on the regular hexagonal lattice from inside to outside, the central distance lambada between any two adjacent points in the regular hexagonal lattice is equal, the fiber core is located in the central position of the regular hexagonal lattice, the multiple germanium rods are distributed on each layer of regular hexagonal lattice in a sparse structure, and the solid substrate is filled between the germanium rods. Various further optimization design schemes related to the all-solid-state microstructure optical fiber are the same as those described above, and are not described herein again. When the all-solid-state microstructure fiber is used as a long-distance transmission fiber of a high-power fiber laser and transmits high-power fiber laser, a germanium rod in a microstructure cladding generates a photonic band gap effect due to anti-resonance coupling, light beams entering the microstructure cladding generate multiple scattering, multiple interference exists between scattered light falling in a photonic band gap frequency range, the intensity of the scattered light is weakened and the scattered light cannot pass through the microstructure cladding, the stable transmission in a fiber core is limited, and the high-efficiency single-mode transmission of signal wavelength laser is realized; meanwhile, the all-solid-state microstructure optical fiber can inhibit the stimulated Raman scattering effect in the long-distance transmission process of high-power optical fiber laser.
The invention provides an all-solid-state microstructure optical fiber which comprises a fiber core and a microstructure cladding, wherein the fiber core is a solid fiber core, the microstructure cladding is coated on the periphery of the fiber core, the microstructure cladding comprises germanium rods and a solid substrate, the germanium rods are positioned in a regular hexagonal lattice, a plurality of layers of the regular hexagonal lattice are distributed from inside to outside, the central distance Lambda between any two adjacent points in the regular hexagonal lattice is equal, the fiber core is positioned at the central position of the regular hexagonal lattice, the germanium rods are distributed on each layer of the regular hexagonal lattice in a sparse structure, the solid substrate is filled between the germanium rods, and the structures and the sizes of all the germanium rods are the same.
The microstructure cladding provided by the invention generates a photonic band gap effect due to antiresonant coupling, light beams entering the microstructure cladding generate multiple scattering under the action of the microstructure, and the multiple interference among scattered light in a specific frequency range weakens the intensity of the light beams and cannot pass through the microstructure cladding, so that the light beams are limited to be transmitted in a fiber core finally. Under the condition that the propagation constant is certain, the relative position distribution of the photonic band gap along with the wavelength change is controlled by adjusting the diameter and the refractive index of the germanium rod, so that the Stimulated Raman Scattering (SRS) Stokes wavelength band (1110 nm-1130nm) falling outside the photonic band gap frequency range has high loss and cannot be stably transmitted, the light of the signal wavelength band (1060 nm-1080 nm) falling within the photonic band gap frequency range is repelled by the cladding and has low loss, so that the light is well limited in the stable transmission of the fiber core, and a certain spectrum filtering effect is macroscopically shown.
Furthermore, the fiber core in the all-solid-state microstructure fiber consists of a single quartz rod or is formed by a plurality of quartz rods which are arranged in a periodical and compact mode. The core type is defined as n-cell core, and the core diameter is 4 Λ -d, depending on the number n of quartz rods used to form the core.
Compared with the prior art, the invention has the beneficial effects that at least:
1. the all-solid-state microstructure optical fiber disclosed by the invention has the advantages that the photonic band gap effect generated by the microstructure cladding structure of the all-solid-state microstructure optical fiber directly shows low limiting loss in a laser signal band, and shows high limiting loss at the Stokes wavelength of stimulated Raman scattering, and the all-solid-state microstructure optical fiber can have a natural stimulated Raman scattering inhibition effect without other technical means such as bending, writing of tilted gratings and the like.
2. The all-solid-state microstructure fiber has larger fiber core diameter and mode field area than the traditional Step Index Fiber (SIF), improves the nonlinear effect threshold value in principle, and has certain inhibition effect on various high-power laser nonlinear effects including stimulated Raman scattering, stimulated Brillouin scattering and the like.
3. According to the all-solid-state microstructure optical fiber, the germanium rods are arranged on each layer of regular hexagonal lattice in a sparse structure, namely, the germanium rods are not arranged on some points on the regular hexagonal lattice, and the positions where the germanium rods are not arranged and gaps among the germanium rods are filled by the solid-state quartz substrate. Therefore, the low loss of the signal laser fundamental mode is ensured, and the bending loss of a high-order mode is increased so as to realize the quasi-single mode operation of the optical fiber. Specifically, except for the fact that a circle of germanium rods are strictly arranged according to a regular hexagon on each point in the first layer of regular hexagon lattice close to the fiber core, the germanium rods in the other layers of regular hexagon lattices are arranged in a sparse structure, namely, the germanium rods are selectively not arranged on some points on the other layers of regular hexagon lattices, and the positions where the germanium rods are not arranged and gaps among the germanium rods are filled by the solid quartz substrate. Therefore, the bending loss of a high-order mode is far larger than that of a basic mode, and the high-power laser can be guaranteed to operate in a quasi-single mode in the transmission process under the condition of a certain range of coiling diameter.
4. By reasonably setting the structural parameters of the germanium rod, the Stimulated Raman Scattering (SRS) Stokes wavelength band (1110 nm-1130nm) falling outside the band gap can be high-loss and cannot be stably transmitted, and the light of the signal wavelength band (1060 nm-1080 nm) falling in the photonic band gap is rejected by the cladding layer to be low-loss, so that the light is well limited to the stable transmission of the fiber core, and macroscopically has a certain spectrum filtering effect.
5. Compared with other types of complex microstructure optical fibers such as hollow optical fibers, air hole photonic crystal optical fibers and the like, the microstructure optical fiber is of an all-solid-state structure, the difficulty in preparing a prefabricated rod and drawing the optical fiber is relatively low, the difficulty in welding with a laser output tail fiber is relatively low in the using process, and the high-power laser transmission application is facilitated.
Drawings
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 obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the structures shown in the drawings without creative efforts.
FIG. 1 is a schematic cross-sectional view of an embodiment;
FIG. 2 is a schematic diagram of the arrangement of germanium rods in FIG. 1, wherein dotted circles in a regular hexagonal lattice represent the positions of the points where no germanium rods are arranged, and solid circles represent the positions of the points where the germanium rods are arranged;
FIG. 3 is a schematic cross-sectional view of a germanium rod used in one embodiment;
FIG. 4 is a graph showing the refractive index profile of a germanium rod used in example 1;
FIG. 5 is a schematic diagram of the intensity distribution of the eigenmodes of the signal band of the all-solid-state microstructured optical fiber provided in example 1, wherein (a) is the LP of the all-solid-state microstructured optical fiber at 1070nm signal light01Intensity distribution diagram corresponding to mode, (b) LP of signal light of 1070nm of all-solid-state microstructure fiber11oIntensity distribution diagram corresponding to mode, (c) LP of all-solid microstructure fiber at 1070nm11eThe intensity distribution diagram corresponding to the mode;
FIG. 6 is a schematic diagram of wavelength dependent loss of an all-solid-state microstructured optical fiber provided in example 1 in a straight fiber and in a bent state;
FIG. 7 is a graph of fundamental mode losses at different bend radii for the all-solid-state microstructured optical fiber signal band wavelengths provided in example 1;
FIG. 8 is a schematic representation of the fundamental mode and minimum-loss higher-order mode bending losses of the all-solid-state microstructured optical fiber provided in example 1;
FIG. 9 is a graph showing the refractive index profile of a germanium rod used in example 2;
FIG. 10 is a schematic diagram of the wavelength dependent loss and the signal band loss at different bending radii of the all-solid-state microstructured optical fiber provided in example 2 in a straight state;
FIG. 11 is a schematic diagram showing the change of the mode field area of the signal band at different bending radii of the all-solid-state microstructured optical fiber provided in example 2;
FIG. 12 is a schematic representation of the fundamental mode and minimum-loss higher-order mode bending losses of an all-solid-state microstructured optical fiber provided in example 2;
FIG. 13 is a schematic structural view of example 4.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, the invention provides an all-solid-state microstructure optical fiber, which comprises a fiber core and a microstructure cladding, wherein the fiber core 1 is a solid fiber core, the microstructure cladding is coated on the periphery of the fiber core 1, the microstructure cladding comprises germanium rods 2 and a solid substrate 3, the germanium rods 2 are positioned in a regular hexagonal lattice, the regular hexagonal lattice is provided with a plurality of layers from inside to outside, the central distance Λ between any two adjacent points in the regular hexagonal lattice is equal, the fiber core 2 is positioned at the central position of the regular hexagonal lattice, the germanium rods 2 are distributed on each layer of the regular hexagonal lattice in a sparse structure, and the solid substrate 3 is filled between the germanium rods 2. Wherein, the materials of the fiber core 1 and the solid substrate 3 are quartz (Silica).
Referring to fig. 1 and 2, the regular hexagonal lattice has four layers, namely, a first layer of regular hexagonal lattice, a second layer of regular hexagonal lattice, a third layer of regular hexagonal lattice and a fourth layer of regular hexagonal lattice from the inside to the outside. Except for the fact that the germanium rods are arranged on each point in the first layer of regular hexagonal lattice close to the fiber core strictly according to the regular hexagon, the germanium rods in the other layers of regular hexagonal lattices are arranged in a sparse structure, namely, the germanium rods are not selectively arranged on some points on the other layers of regular hexagonal lattices, and the positions where the germanium rods are not arranged and gaps among the germanium rods are filled by the solid quartz substrate. Therefore, the bending loss of the high-order mode is far larger than that of the basic mode, so that the high-power laser can be ensured to operate in a quasi-single mode in the transmission process under the condition of a coiling diameter within a certain range. Specifically, 12 points are arranged on the first layer of regular hexagonal lattice, germanium rods 2 are uniformly distributed on all the points, 18 points are arranged on the second layer of regular hexagonal lattice, germanium rods 2 are uniformly distributed on two points located in the middle of each side, 24 points are arranged on the third layer of regular hexagonal lattice, germanium rods 2 are uniformly distributed on four points located at two ends of each side, 30 points are arranged on the fourth layer of regular hexagonal lattice, two points located at two ends of each side and two points located in the center of each side are uniformly distributed with germanium rods 2. Namely, no germanium rod is arranged on the point at the hexagonal vertex in the second layer of regular hexagonal lattice; no germanium rod is arranged at the center point of each side in the second layer of regular hexagonal lattice; and no germanium rod is arranged on the second point and the fourth point of each side in the fourth layer of regular hexagon lattice, as shown in fig. 2, wherein dotted line dot circles in the regular hexagon lattice represent the point positions where no germanium rod is arranged, and solid line dot circles represent the point positions where the germanium rod is arranged. The arrangement mode of the germanium rods in the microstructure cladding ensures that the bending loss of a basic mode (BL <0.1dB/m) of the optical fiber in a signal wavelength band (1060 nm-1080 nm) is small on one hand, and ensures that the bending loss of a high-order mode (BL >5dB/m) is large on the other hand.
Example 1:
the structure of the all-solid-state microstructure optical fiber provided in an embodiment of the present invention is shown in fig. 1, and the parameters thereof include: the diameter d of the germanium rod is 4.8 mu m, the central distance Lambda between any two adjacent points in the regular hexagonal lattice is 12 mu m, and the characteristic parameters of the optical fiber
0.4 and the core diameter 4 Λ -d is 43.2 μm. Wherein the fiber core is formed by 7 quartz rods which are periodically and closely arranged.
The cross-sectional structure of the germanium rod used in this embodiment is schematically shown in FIG. 3, and the germanium rod 2 includes a central high refractive index germanium-doped region 4 and an outer low refractive index cladding region 5, wherein the refractive index of the high refractive index germanium-doped region 4 is a parabolic graded index profile, as shown in FIG. 4. The outer low-refractive-index cladding region 5 is a quartz cladding. The high refractive index germanium-doped region 4 and the low refractive index cladding region 5 of the germanium rod have a refractive index difference delta, and the refractive index difference delta is (n ═ n)high 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive indices of the high index germanium-doped region and the low index cladding region are the maximum and low index, respectively. In this embodiment, the refractive index difference between the central high refractive index germanium-doped region and the outer low refractive index cladding region of the germanium rod is 2%, and the ratio of the diameter of the central high refractive index germanium-doped region to the outer diameter of the outer low refractive index cladding region is 1: 2.5.
The intrinsic mode field distribution of the all-solid-state microstructured optical fiber provided in this embodiment in the signal band is shown in fig. 5, in which (a) is LP of the all-solid-state microstructured optical fiber in 1070nm signal light01Intensity distribution diagram corresponding to mode, (b) LP of all-solid microstructure fiber at 1070nm11oIntensity distribution diagram corresponding to mode, (c) LP of all-solid microstructure fiber at 1070nm11eThe intensity distribution of the corresponding mode is shown schematically. The confinement loss of the fiber and the mode field area of the fundamental mode corresponding to each eigenmode at this wavelength are shown in Table 1 below, which shows that the fiber has very low confinement loss to the fundamental mode and a mode field area of 567 μm2Therefore, the optical fiber has to have a larger mode field area and a capability of stably transmitting signal wavelength laser, and meets the basic application requirements of the energy transmission optical fiber.
TABLE 1 effective refractive index and mode field area of fundamental mode for each eigenmode
Specifically, the wavelength-dependent loss of the all-solid-state microstructured optical fiber provided by the above embodiment in the straight optical fiber and 20cm bent state is shown in fig. 6. The loss of a fundamental mode at a signal band can be smaller than 0.1dB/m no matter in a straight optical fiber or a bent state, the loss of the fundamental mode at the SRS Thosk wavelength (1120nm-1130nm) is larger than 20dB/m, and the capability of effective inhibition at the SRS wavelength is ensured.
Specifically, the fundamental mode losses at different bending radii (15-20cm) when signal band wavelength lasers (1060nm and 1070nm for example) are transmitted through the all-solid-state microstructured optical fiber provided in the above example are shown in FIG. 7. The bend loss increases slightly with decreasing bend radius, but remains below 0.1dB/m at all times, ensuring lower bend sensitivity of the signal strip.
Specifically, according to the practical application of the all-solid-state microstructured optical fiber provided by the present embodiment, in addition to the wavelength-dependent loss, the mode-dependent bending loss during the transmission of the signal band wavelength laser is also considered. Only the wavelength-dependent loss of the optical fiber is controlled well, the optical fiber can have the SRS restraining function; quasi-single mode transmission of high power lasers can only be achieved if the mode dependent bending losses are controlled well. If the quasi-single mode operation of the optical fiber is to be realized, a larger high-order mode bending loss and a smaller fundamental mode bending loss are required to be ensured at the same time, and the high-order mode possibly existing in the optical fiber is filtered by a bending mode selection method. As shown in fig. 8, the all-solid-state microstructured optical fiber provided in this embodiment can ensure that the bending loss of the fundamental mode of the signal band is less than 0.1dB/m and the bending loss of the higher-order mode is greater than 5dB/m under a bending radius of 20cm, wherein the higher-order mode loss corresponds to the higher-order mode with the minimum calculated loss, and therefore, the quasi-single-mode transmission of the high-power laser can be ensured to some extent.
Example 2:
the structure of the all-solid-state microstructured optical fiber provided in an embodiment of the present invention is shown in fig. 1, and the parameters thereof include: the diameter d of the germanium rod is 5.3 mu m, the center distance Lambda between any two adjacent points in the regular hexagonal lattice is 13.95 mu m, and the characteristic parameters of the optical fiber
0.38 and a core diameter 4 Λ -d of 50.5 μm. Wherein the fiber core is formed by 7 quartz rods which are periodically and closely arranged.
Specifically, the sectional structure and the refractive index profile of the germanium rod used in this embodiment are shown in fig. 3 and fig. 9, respectively, and the germanium rod 2 includes a high refractive index germanium-doped region 4 located at the center and a low refractive index cladding region 5 located at the outer layer, wherein the refractive index of the high refractive index germanium-doped region 4 has a parabolic graded index profile. The low refractive index cladding region 5 of the outer layer is quartz clad. The high refractive index germanium-doped region 4 and the low refractive index cladding region 5 of the germanium rod have a refractive index difference delta which satisfies the condition that delta is (n)high 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive indices of the high index germanium-doped region and the low index cladding region are the maximum and low index, respectively. In this embodiment, the refractive index difference between the central high refractive index germanium-doped region 5 of the germanium rod and the outer low refractive index cladding region 6 is 1.7%, and the ratio of the outer diameters of the high refractive index germanium-doped region 4 and the low refractive index cladding region 5 of the germanium rod is 1: 2.5.
Specifically, the all-solid-state microstructured optical fiber provided by the present embodiment has a straight fiber wavelength-dependent loss and a 25cm bending state, and the signal band loss is shown in fig. 10. The loss of a fundamental mode at a signal band can be less than 0.1dB/m and the loss of the fundamental mode at the SRS Toxoks wavelength (1120nm-1130nm) is more than 40dB/m no matter in a straight optical fiber or a bent state, so that the capability of effectively inhibiting at the SRS wavelength is ensured.
Specifically, the all-solid-state microstructured optical fiber provided by the present embodiment has the mode field area change of the signal band (1040nm-1070nm) under different bending radii (50cm and 25cm) as shown in fig. 11. Under the bending radius larger than 25cm, the all-solid-state microstructure optical fiber of the embodiment can ensure that the signal band laser is 740 micrometers2The above mode field area.
Specifically, according to the practical application of the all-solid-state microstructured optical fiber provided by the present embodiment, in addition to the wavelength-dependent loss, the mode-dependent bending loss during the transmission of the signal band wavelength laser is also considered. Only if the wavelength-dependent loss of the optical fiber is well controlled, the optical fiber can have the SRS restraining function; the quasi-single mode transmission of high power laser can only be achieved if the mode dependent bending losses are controlled well. If the quasi-single mode operation of the optical fiber is to be realized, a larger high-order mode bending loss and a smaller fundamental mode bending loss are required to be ensured at the same time, and the high-order mode possibly existing in the optical fiber is filtered by a bending mode selection method. As shown in fig. 12, the all-solid-state microstructured optical fiber provided in this embodiment can ensure that the bending loss of the fundamental mode of the signal band is less than 0.1dB/m and the bending loss of the higher-order mode is greater than 5dB/m under a bending radius of 20cm, wherein the higher-order mode loss corresponds to the higher-order mode with the minimum calculated loss, and therefore, the quasi-single-mode transmission of the high-power laser can be ensured to some extent.
The microstructure cladding designed by the invention generates a photonic band gap effect due to anti-resonance coupling, and the optical fiber structure designed by the invention enables the wavelength of signal light to be positioned on a photonic band gap and the Stokes wavelength to be positioned outside the photonic band gap, so that the all-solid-state microstructure optical fiber has the characteristics of stably transmitting signal laser and highly inhibiting stimulated Raman scattering; through the arrangement design of the outer-layer germanium rod sparse structure, the bending loss of a high-order mode is increased while the low loss of a signal laser fundamental mode is ensured, so that the all-solid-state microstructure optical fiber has the characteristic of effectively inhibiting the high-order mode, and the quasi-single-mode operation in the high-power laser transmission process is ensured. In conclusion, the invention can meet the application requirement of high-power laser quasi-single-mode long distance, and can further ensure the application effect of high-power laser in the fields of medical treatment, industry, national defense and the like.
Example 3:
the present embodiment provides a method for suppressing stimulated raman scattering, including:
the method comprises the steps of designing the all-solid-state microstructure optical fiber, wherein the structure of the all-solid-state microstructure optical fiber is shown in figure 1 and comprises a fiber core and a microstructure cladding, the fiber core is a solid-state fiber core, the microstructure cladding is wrapped on the periphery of the fiber core, the microstructure cladding comprises germanium rods and a solid-state substrate, the germanium rods are located in a regular hexagonal lattice, multiple layers of layers are distributed on the regular hexagonal lattice from inside to outside, the central distance Lambda of any two adjacent points in the regular hexagonal lattice is equal, the fiber core is located in the central position of the regular hexagonal lattice, the multiple germanium rods are distributed on each layer of the regular hexagonal lattice in a sparse structure, and the solid-state substrate is filled between the germanium rods. Specifically, the regular hexagon lattice has four layers, including a first layer of regular hexagon lattice, a second layer of regular hexagon lattice, a third layer of regular hexagon lattice and a fourth layer of regular hexagon lattice from inside to outside. Except for the fact that the germanium rods are arranged on each point in the first layer of regular hexagonal lattice close to the fiber core strictly according to the regular hexagon, the germanium rods in the other layers of regular hexagonal lattices are arranged in a sparse structure, namely, the germanium rods are not selectively arranged on some points on the other layers of regular hexagonal lattices, and the positions where the germanium rods are not arranged and gaps among the germanium rods are filled by the solid quartz substrate. Therefore, the bending loss of a high-order mode is far larger than that of a basic mode, and the high-power laser can be guaranteed to operate in a quasi-single mode in the transmission process under the condition of a certain range of coiling diameter. Specifically, 12 points are arranged on the first layer of regular hexagonal lattice, germanium rods 2 are uniformly distributed on all the points, 18 points are arranged on the second layer of regular hexagonal lattice, germanium rods 2 are uniformly distributed on two points located in the middle of each side, 24 points are arranged on the third layer of regular hexagonal lattice, germanium rods 2 are uniformly distributed on four points located at two ends of each side, 30 points are arranged on the fourth layer of regular hexagonal lattice, two points located at two ends of each side and two points located in the center of each side are uniformly distributed with germanium rods 2.
Welding the all-solid-state microstructure optical fiber on an output tail fiber of a high-power optical fiber laser to serve as a long-distance transmission optical fiber; when the all-solid-state microstructure optical fiber transmits high-power optical fiber laser, the germanium rod in the microstructure cladding generates photonic band gap effect due to anti-resonant coupling. The light beam entering the microstructure cladding generates multiple scattering, and the multiple interference between the scattered light in a specific frequency range causes the light beam to weaken the intensity of the light beam and not pass through the microstructure cladding, and finally the light beam is limited to be transmitted in the fiber core, wherein the specific frequency range is called a photonic band gap frequency range. (ii) a For signal wavelength laser, multiple interference exists between scattered light falling in a photonic band gap frequency range, so that the intensity of the scattered light is weakened and the scattered light cannot pass through a microstructure cladding, and finally the scattered light is limited to be stably transmitted in a fiber core, so that high-efficiency single-mode low-loss transmission of the signal wavelength laser is realized; meanwhile, the stimulated Raman scattering Stokes wavelength falling outside the photonic band gap frequency range has high loss and cannot be stably transmitted, and the all-solid-state microstructure fiber can inhibit the stimulated Raman scattering effect in the long-distance transmission process of high-power fiber laser.
All the germanium rods have the same structure and size, and each germanium rod comprises a high-refractive-index germanium-doped region positioned in the center and a low-refractive-index cladding region positioned on the outer layer, wherein the refractive index of the high-refractive-index germanium-doped region is in parabolic graded-index distribution. The high refractive index germanium-doped region and the low refractive index cladding region of the germanium rod have a refractive index difference delta, and the refractive index difference delta is (n)high 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive indices of the high index germanium-doped region and the low index cladding region are the maximum and low index, respectively. Difference in refractive indexDelta is more than or equal to 1.5 percent and less than or equal to 3 percent, when the refractive index difference Delta is determined, the distribution of the photonic band gap is only related to the diameter d of the germanium rod and is not related to the arrangement mode of the germanium rod, and the suppression of the stimulated Raman scattering effect can be realized by increasing or decreasing the diameter d of the germanium rod. Wherein the low index cladding region of the germanium rod is made of solid quartz or fluorine doped solid quartz.
In the case of the example 4, the following examples are given,
referring to fig. 13, the present embodiment provides a high power fiber laser, which includes a laser generating unit 301, where the laser generating unit is configured to generate a high power fiber laser, an output pigtail 302 of the laser generating unit is welded with an all-solid-state microstructure fiber 303 as a long-distance transmission fiber, and finally, a signal laser is transmitted to a collimator 305 through a relay fiber 304 and output; wherein the all-solid-state microstructured optical fiber is the all-solid-state microstructured optical fiber described in any of the embodiments above.
When the all-solid-state microstructure fiber transmits high-power fiber laser, a germanium rod in a microstructure cladding generates a photonic band gap effect due to antiresonant coupling, light beams entering the microstructure cladding generate multiple scattering, the intensity of the scattered light is weakened due to multiple interference among scattered light in a specific frequency range, the scattered light cannot pass through the microstructure cladding, and finally transmission in a fiber core is limited, and the specific frequency range is called a photonic band gap frequency range. For signal wavelength laser, multiple interference exists between scattered light falling in a photonic band gap frequency range, so that the intensity of the scattered light is weakened and the scattered light cannot pass through a microstructure cladding, and finally the scattered light is limited in a fiber core for stable transmission, so that high-efficiency single-mode low-loss transmission of the signal wavelength laser is realized; meanwhile, the Stokes wavelength of Stimulated Raman Scattering (SRS) falling outside the photonic band gap frequency range is high in loss and cannot be stably transmitted, and the all-solid-state microstructure optical fiber can inhibit the stimulated Raman scattering effect in the long-distance transmission process of high-power optical fiber laser.
In summary, although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention.
Claims (6)
1. A method of suppressing stimulated raman scattering, comprising:
designing an all-solid-state microstructure optical fiber, wherein the all-solid-state microstructure optical fiber comprises a fiber core and a microstructure cladding, the fiber core is a solid fiber core, the microstructure cladding is wrapped on the periphery of the fiber core, the microstructure cladding comprises a germanium rod and a solid substrate which are positioned in a regular hexagonal lattice, four layers of the regular hexagonal lattice are distributed from inside to outside, and a first layer of the regular hexagonal lattice, a second layer of the regular hexagonal lattice, a third layer of the regular hexagonal lattice and a fourth layer of the regular hexagonal lattice are respectively arranged from inside to outside; the central distance Lambda between any two adjacent points in the regular hexagonal lattice is equal, the fiber core is positioned at the central position of the regular hexagonal lattice, 12 points are arranged on the first layer of regular hexagonal lattice, germanium rods are uniformly distributed on all the points, 18 points are arranged on the second layer of regular hexagonal lattice, germanium rods are uniformly distributed on two points positioned in the middle of each edge, 24 points are arranged on the third layer of regular hexagonal lattice, germanium rods are uniformly distributed on four points positioned at two ends of each edge, 30 points are arranged on the fourth layer of regular hexagonal lattice, two points positioned at two ends of each edge and two points positioned at the center of each edge are uniformly distributed with germanium rods, and a solid substrate is filled between the germanium rods; the high refractive index germanium-doped region and the low refractive index cladding region of the germanium rod have a refractive index difference delta which satisfies that delta ═ nhigh 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive index difference delta is between the refractive index maximum value of the high-refractive-index germanium-doped region and the refractive index of the low-refractive-index cladding region, respectively, the refractive index difference delta is between 1.5% and delta 3%, when the refractive index difference delta is determined, the distribution of the photonic band gap is only related to the diameter d of the germanium rod, but is not related to the arrangement mode of the germanium rod, and the inhibition on the stimulated Raman scattering effect can be realized by increasing or reducing the diameter d of the germanium rod;
welding the all-solid-state microstructure optical fiber on an output tail fiber of a high-power optical fiber laser to serve as a long-distance transmission optical fiber; when the all-solid-state microstructure fiber transmits high-power fiber laser, a germanium rod in a microstructure cladding generates a photonic bandgap effect due to anti-resonant coupling; for signal wavelength laser, multiple interference exists between scattered light falling in a photonic band gap frequency range, so that the intensity of the scattered light is weakened and the scattered light cannot pass through a microstructure cladding, and finally the scattered light is limited to be stably transmitted in a fiber core, so that high-efficiency single-mode low-loss transmission of the signal wavelength laser is realized; meanwhile, the stimulated Raman scattering Stokes wavelength falling outside the photonic band gap frequency range has high loss and cannot be stably transmitted, and the all-solid-state microstructure fiber can inhibit the stimulated Raman scattering effect in the long-distance transmission process of high-power fiber laser.
2. The method of suppressing stimulated raman scattering according to claim 1, wherein: the fiber core material is solid quartz, and the solid substrate is a solid quartz substrate.
3. The method of suppressing stimulated raman scattering according to claim 2, wherein: the germanium rods have the same structure and size, and comprise a high-refractive-index germanium-doped region in the center and a low-refractive-index cladding region in the outer layer, wherein the refractive index of the high-refractive-index germanium-doped region is in parabolic graded or step-type refractive index distribution.
4. The method of suppressing stimulated raman scattering of claim 1, 2 or 3, wherein: the low index cladding region of the germanium rod is made of solid quartz or fluorine doped solid quartz.
5. The high-power optical fiber laser is characterized by comprising a laser generating unit, wherein the laser generating unit is used for generating high-power optical fiber laser, and an all-solid-state microstructure optical fiber is welded on an output tail fiber of the laser generating unit to be used as a long-distance transmission optical fiber; the all-solid-state microstructure optical fiber comprises a fiber core and a microstructure cladding, wherein the fiber core is a solid-state fiber core, the microstructure cladding is coated on the periphery of the fiber core, the microstructure cladding comprises a germanium rod and a solid-state substrate, the germanium rod is positioned in a regular hexagonal lattice, four layers are distributed on the regular hexagonal lattice from inside to outside, and a first layer of the regular hexagonal lattice, a second layer of the regular hexagonal lattice and a third layer of the regular hexagonal lattice are respectively arranged on the regular hexagonal lattice from inside to outsideThe hexagonal lattice and the fourth layer of regular hexagonal lattice; the central distance Lambda between any two adjacent points in the regular hexagon lattice is equal, the fiber core is positioned at the central position of the regular hexagon lattice, 12 points are arranged on the first layer of regular hexagon lattice, germanium rods are uniformly distributed on all the points, 18 points are arranged on the second layer of regular hexagon lattice, germanium rods are uniformly distributed on two points positioned in the middle of each edge, 24 points are arranged on the third layer of regular hexagon lattice, germanium rods are uniformly distributed on four points positioned at two ends of each edge, 30 points are arranged on the fourth layer of regular hexagon lattice, germanium rods are uniformly distributed on two points positioned at two ends of each edge and two points positioned at the center of each edge, a solid substrate is filled between the germanium rods, a high-refractive-index germanium-doped region and a low-refractive-index region of the germanium rods have refractive index difference Lambda, and the requirement that Lambda (n) is methigh 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive index difference delta is between the refractive index maximum value of the high-refractive-index germanium-doped region and the refractive index of the low-refractive-index cladding region, delta is between 1.5% and 3%, when the refractive index difference delta is determined, the distribution of photonic band gaps is only related to the diameter d of the germanium rods, but not related to the arrangement mode of the germanium rods, and the suppression of the stimulated Raman scattering effect can be realized by increasing or decreasing the diameter d of the germanium rods.
6. An all-solid-state microstructured optical fiber, comprising: the optical fiber comprises a fiber core and a microstructure cladding, wherein the fiber core is a solid fiber core, the microstructure cladding is coated on the periphery of the fiber core, the microstructure cladding comprises a germanium rod and a solid substrate which are positioned in a regular hexagonal lattice, four layers of the regular hexagonal lattice are distributed from inside to outside, and a first layer of the regular hexagonal lattice, a second layer of the regular hexagonal lattice, a third layer of the regular hexagonal lattice and a fourth layer of the regular hexagonal lattice are respectively arranged from inside to outside; the central distance Lambda of any two adjacent points in the regular hexagon lattice is equal, the fiber core is positioned at the central position of the regular hexagon lattice, 12 points are arranged on the first layer of regular hexagon lattice, germanium rods are uniformly distributed on all the points, 18 points are arranged on the second layer of regular hexagon lattice, germanium rods are uniformly distributed on two points positioned in the middle of each edge, 24 points are arranged on the third layer of regular hexagon latticeGermanium rods are uniformly distributed on four points at two ends of each edge, 30 points are arranged on the fourth layer of regular hexagonal lattice, two points at two ends of each edge and two points at the center of each edge are uniformly distributed with germanium rods, a solid substrate is filled between the germanium rods, the structures and the sizes of all the germanium rods are the same, and a high-refractive-index germanium-doped region and a low-refractive-index cladding region of each germanium rod have a refractive index difference delta which satisfies the condition that delta is equal to (n is equal to)high 2-nlow 2)/(2×nhigh 2) Wherein n ishigh、nlowThe refractive index difference delta is between 1.5% and 3%, when the refractive index difference delta is determined, the distribution of the photonic band gap is only related to the diameter d of the germanium rod, but not related to the arrangement mode of the germanium rod, and the inhibition of the stimulated Raman scattering effect can be realized by increasing or reducing the diameter d of the germanium rod.
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