CN118712854B - Raman fiber laser system to improve high-order Raman threshold - Google Patents
Raman fiber laser system to improve high-order Raman threshold Download PDFInfo
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- 238000001069 Raman spectroscopy Methods 0.000 title claims abstract description 301
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- 239000013307 optical fiber Substances 0.000 claims description 52
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical group [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 claims description 6
- XHGGEBRKUWZHEK-UHFFFAOYSA-L tellurate Chemical compound [O-][Te]([O-])(=O)=O XHGGEBRKUWZHEK-UHFFFAOYSA-L 0.000 claims description 6
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
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Abstract
The invention provides a Raman fiber laser system for improving a high-order Raman threshold, which comprises a backward multimode pump signal beam combiner, a Raman low-reflection grating, a Raman high-reflection grating and a Raman gain fiber. The invention is based on the experimental structure of the backward multimode pumping signal beam combiner, and can integrate the output end of the signal light with the injection end of the pumping light, thereby achieving the effect of rapidly amplifying the signal light at the injection end of the pumping light, reducing the effective acting distance between the signal light and the high-order Raman, further realizing the effective promotion of the high-order Raman threshold value and realizing the output of the signal light with higher power. In the invention, the signal light output and the injection direction of the pump light are opposite, and the residual pump light and the generated signal light can be effectively separated, so that the use of a light splitting device is reduced, and the experimental operation is simplified.
Description
Technical Field
The invention belongs to the technical field of fiber lasers, in particular to the technical field of Raman fiber lasers, and particularly relates to a Raman fiber laser system for improving a high-order Raman threshold.
Background
The Raman fiber laser is a high-efficiency, flexible and simple-structure laser, utilizes the stimulated Raman scattering effect in the optical fiber to generate high-power lasers with different wavelengths, and has the advantages of strong wavelength conversion capability, high output beam quality, compact structure, good stability, high spectral purity and the like. The laser is widely applied to the fields of optical communication, biomedical treatment, laser processing, optical particle manipulation, super-resolution imaging, special-band light sources and the like, and particularly has great potential and application value in the aspects of generating special beam modes such as vortex beams and serving as a high-power pumping source.
Currently, there are three main techniques for suppressing the generation of higher order raman in raman gain fiber lasers, the first is to use raman gain fiber with wavelength selectivity itself, mainly based on bending loss, such as W-type fiber, etc. However, the fiber drawing process is complex, the cost is high, the transmission loss is large (7.5 dB/km@ λ=1μm), and the high-order Raman gain is sensitive to the bending radius, and the bending radius needs to be strictly controlled. The second is to couple the core mode to the cladding mode using spectrally filtering optics such as low pass filters, long period gratings and tilted gratings, polarization maintaining fiber 45 degree offset fusion techniques, and the like. The low-pass filter is not of an all-fiber structure, has coupling loss and has low system stability. Long period gratings and tilted gratings have limited power bearing capability. The 45-degree dislocation fusion technology of the polarization maintaining fiber is only suitable for linear polarization Raman fiber lasers. The third is to use low noise pump source with stable time sequence, such as super fluorescent fiber light source, super long cavity length laser, single frequency fiber laser, etc. However, such pump sources have a low output power and cannot be used for high order raman suppression above the kw level.
Disclosure of Invention
Aiming at the defects and shortcomings in the prior art, the invention discloses a Raman fiber laser system for improving a high-order Raman threshold. And injecting pump light into the Raman fiber resonant cavity by using the backward multimode pump signal beam combiner, carrying out laser oscillation on the signal light by using the resonant cavity, and finally obtaining Raman fiber laser output by using a signal arm of the backward multimode pump beam combiner.
In order to achieve the technical purpose, the invention adopts the following technical scheme:
The Raman fiber laser system for improving the high-order Raman threshold comprises a backward multimode pumping signal beam combiner, a Raman low reflection grating, a Raman high reflection grating and a Raman gain fiber, wherein a first end of the backward multimode pumping signal beam combiner is provided with a plurality of pumping arms and a signal arm, each pumping arm is correspondingly connected with a pumping source respectively, the signal arm is used as an output fiber of signal light, a second end of the backward multimode pumping signal beam combiner is connected with the Raman low reflection grating, the Raman low reflection grating is connected with one end of the Raman gain fiber, and the other end of the Raman gain fiber is connected with the Raman high reflection grating;
The output optical fiber of the signal light and the pumping arm are integrated together based on the backward multimode pumping signal beam combiner, so that the signal light can be rapidly amplified and output at the pumping light injection end, the acting distance of the high-order Raman light in the optical fiber is shortened, the gain accumulation of the signal light on the high-order Raman is reduced, and the threshold value of the high-order Raman is effectively improved.
Further, a backward return filter is connected between the pump source and the pump arm of the backward multimode pump signal beam combiner.
Further, the output optical fiber of the signal light is connected with a first output end cap for outputting the signal light, and an antireflection film covering the wavelength range of the signal light is plated on the first output end cap.
Further, the raman high reflection grating is connected with a second output end cap for outputting unconverted residual pump light, and an antireflection film covering the wavelength range of the residual pump light is plated on the second output end cap.
Further, the Raman low reflection grating comprises a first-order Raman low reflection grating, the Raman high reflection grating comprises a first-order Raman high reflection grating, the second end of the backward multimode pump signal beam combiner is connected with the first-order Raman low reflection grating, the first-order Raman low reflection grating is connected with one end of a Raman gain optical fiber, and the other end of the Raman gain optical fiber is connected with the first-order Raman high reflection grating.
Further, the center wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near a Raman gain peak of the pump light in the Raman gain fiber.
Further, the Raman low reflection grating comprises a first-order Raman low reflection grating and a second-order Raman low reflection grating, the Raman high reflection grating comprises a first-order Raman high reflection grating and a second-order Raman high reflection grating, the second end of the backward multimode pump signal beam combiner is connected with the second-order Raman low reflection grating, the second-order Raman low reflection grating is connected with the first-order Raman low reflection grating, the first-order Raman low reflection grating is connected with one end of the Raman gain optical fiber, the other end of the Raman gain optical fiber is connected with the first-order Raman high reflection grating, and the first-order Raman high reflection grating is connected with the second-order Raman high reflection grating.
Further, the center wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain optical fiber, the center wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are located near the Raman gain peak of the first-order Raman grating in the Raman gain optical fiber, and the output signal light is at the center wavelength corresponding to the second-order Raman grating.
Further, the raman low reflection grating comprises a first-order raman low reflection grating, a second-order raman low reflection grating and an n-order raman low reflection grating, the raman high reflection grating comprises a first-order raman high reflection grating, a second-order raman high reflection grating and an n-order raman high reflection grating, the second end of the backward multimode pump signal combiner is sequentially connected with the n-order raman low reflection grating and the n-order raman high reflection grating, the second-order raman low reflection grating and the first-order raman low reflection grating, the first-order raman low reflection grating is connected with one end of the raman gain fiber, and the other end of the raman gain fiber is sequentially connected with the first-order raman high reflection grating, the second-order raman high reflection grating and the n-order raman high reflection grating, and the n-order is any order before the zero dispersion wavelength of the fiber.
Further, the raman gain fiber is silicate fiber, tellurate fiber or fluorite fiber, etc., the raman gain peak of silicate fiber is 440cm -1, the raman gain peak of tellurate fiber is 750cm -1, and the raman gain peak of fluorite fiber is 580 cm -1.
Further, the pump source is a semiconductor laser, an ASE source or a fiber laser.
Compared with the prior art, the invention has the technical effects that:
According to the Raman fiber laser system for improving the high-order Raman threshold, based on the experimental structure of the backward multimode pump signal beam combiner, the output end of the signal light and the injection end of the pump light can be integrated together, so that the effect of rapidly amplifying the signal light at the injection end of the pump light is achieved, meanwhile, the effective acting distance (equivalent to the reduction of the transmission distance of the high-order Raman in the optical fiber) between the signal light and the high-order Raman is reduced, the effective improvement of the high-order Raman threshold is achieved, and the output of the signal light with higher power is achieved.
In addition, because the directions of the signal light output and the pump light injection are opposite, the residual pump light and the generated signal light can be effectively separated, the use of a light splitting device is reduced, and the experimental operation is simplified.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to the structures shown in these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic diagram of an optical path structure of a raman fiber laser system for raising a higher order raman threshold in embodiment 1 of the present invention;
fig. 2 is a schematic diagram of the optical path structure of a raman fiber laser system for raising the higher-order raman threshold in embodiment 2 of the present invention;
FIG. 3 is a graph showing the power and spectral evolution of comparative example 1, wherein (a) is a graph showing the power evolution of comparative example 1, and (b) is a graph showing the spectral evolution of comparative example 1;
FIG. 4 is a view showing the construction of the optical path of the Raman fiber laser system employed in comparative example 2;
FIG. 5 is a graph showing the power and spectrum evolution obtained in comparative example 2, wherein (a) is a graph showing the power evolution obtained in comparative example 2, and (b) is a graph showing the spectrum evolution obtained in comparative example 2;
fig. 6 is a schematic diagram of the optical path structure of a raman fiber laser system for raising the higher-order raman threshold in embodiment 3 of the present invention;
The drawings are marked:
101. The device comprises a first output end cap, a second output end cap, a 2-way backward multimode pumping signal beam combiner, a 3-way first-order Raman low reflection grating, a 4-way first-order Raman high reflection grating, a 5-way Raman gain fiber, a 6-way backward return filter, a 7-way second-order Raman low reflection grating, a 8-way second-order Raman high reflection grating, and pumping light.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Because the raman gain is different from the energy level transition of the rare earth ion doped optical fiber, the raman fiber laser has obvious threshold characteristic, and raman laser can be generated only when the pump light power reaches a certain level, in order to reduce the light emitting threshold of the raman signal light, the length of the raman gain optical fiber needs to be increased, however, the overlong raman gain optical fiber can reduce the threshold power of high-order raman, which limits the further improvement of the power of the raman fiber laser.
The invention is based on the backward multimode pump signal beam combiner 2 to integrate the output end of the signal light with the incident end of the pump light 9, thereby realizing rapid amplification of the signal light at one end of pump injection, reducing the acting distance of the high-order Raman in the optical fiber and further improving the threshold value of the high-order Raman.
One embodiment provides a Raman fiber laser system for improving a high-order Raman threshold, which comprises a backward multimode pump signal beam combiner 2, a Raman low reflection grating, a Raman high reflection grating and a Raman gain fiber 5;
The center wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near a Raman gain peak of the pump light in the Raman gain fiber.
The types of the Raman gain optical fiber comprise, but are not limited to, graded index optical fiber and step index optical fiber, the cladding forms of the Raman gain optical fiber comprise, but are not limited to, single cladding optical fiber and multi-cladding optical fiber, and the matrix types of the Raman gain optical fiber comprise, but are not limited to, silicate optical fiber, tellurate optical fiber and fluorite optical fiber. The raman gain peak of silicate fiber is located at 440cm -1, the raman gain peak of tellurate fiber is located near 750cm -1, and the raman gain peak of fluorite fiber is located at 580 cm -1.
The writing modes of the Raman low-reflection grating and the Raman high-reflection grating comprise but are not limited to ultraviolet writing and femtosecond writing, and the types of the Raman low-reflection grating and the Raman high-reflection grating comprise but are not limited to fiber gratings based on graded-index fibers and fiber gratings based on step-index fibers.
The first output end cap 101 is coated with an antireflection film covering the wavelength range of the signal light, and the second output end cap 102 is coated with an antireflection film covering the wavelength range of the residual pump light. The anti-reflection film can reduce end face feedback of the signal light and the residual pump light and avoid end face damage of the optical fiber.
The backward multimode pump signal combiner 2 is an (n+1) ×1 backward pump signal combiner, where the value of N is not limited, and includes 2, 6 and other commercial models.
The pump source includes, but is not limited to, a semiconductor laser, an ASE source, or a fiber laser.
Specifically, in embodiment 1, a raman fiber laser system for raising a higher order raman threshold is provided, as shown in fig. 1, a first end of the backward multimode pump signal combiner 2 has a plurality of pump arms and a signal arm, where each pump arm is correspondingly connected to a pump source, the signal arm is connected to an output tail fiber of a first output end cap 101, an output end of the first output end cap 101 outputs signal light, a second end of the backward multimode pump signal combiner 2 is connected to an input end of a first order raman low reflection grating 3, an output end of the first order raman low reflection grating 3 is connected to an input end of a raman gain fiber 5, an output end of the raman gain fiber 5 is connected to an input end of a first order raman high reflection grating 4, and an output end of the first order raman high reflection grating 4 is connected to a second output end cap 102 outputting remaining pump light.
After the pump light 9 is injected from the pump arm of the backward multimode pump signal combiner 2 by the pump source, the raman laser with the wavelength corresponding to the signal light is selected under the mode selection effect of the raman grating, and finally the raman laser is coupled into the signal arm of the backward multimode pump signal combiner 2, and is output through the first output end cap 101, and the unconverted residual pump light is output through the second output end cap 102. The experimental structure can effectively separate the generated signal light and the residual pump light.
Embodiment 2 provides a raman fiber laser system for improving a higher order raman threshold, as shown in fig. 2, comprising a backward multimode pump signal combiner 2, a raman low reflection grating, a raman high reflection grating, a raman gain fiber 5 and a backward return filter 6;
The first end of the backward multimode pump signal beam combiner 2 is provided with a plurality of pump arms and a signal arm, wherein each pump arm is correspondingly connected with a pump source, a backward return filter 6 is connected between the pump source and the pump arm of the backward multimode pump signal beam combiner 2, the signal arm is connected with an output tail fiber of a first output end cap 101, the output end of the first output end cap 101 outputs signal light, the second end of the backward multimode pump signal beam combiner 2 is connected with the input end of a first-order Raman low reflection grating 3, the output end of the first-order Raman low reflection grating 3 is connected with the input end of a Raman gain optical fiber 5, the output end of the Raman gain optical fiber 5 is connected with the input end of a first-order Raman high reflection grating 4, and the output end of the first-order Raman high reflection grating 4 is connected with a second output end cap 102 outputting residual pump light.
The backward light filter 6 is used for filtering the backward transmitted signal light so as to prevent the backward transmitted signal light from being transmitted back into the pump source to damage the pump source. Types of the backward pass filter 6 include, but are not limited to, a tilted grating, a commercial cladding light filter, an optical fiber device prepared based on a special optical fiber.
After the pump light 9 is injected from the pump arm of the backward multimode pump signal combiner 2 by the pump source, the raman laser with the wavelength corresponding to the signal light is selected under the mode selection effect of the raman grating, and finally the raman laser is coupled into the signal arm of the backward multimode pump signal combiner 2, and is output through the first output end cap 101, and the unconverted residual pump light is output through the second output end cap 102. The experimental structure can effectively separate the generated signal light and the residual pump light.
To demonstrate the technical effects of the present invention, two comparative examples were designed, wherein comparative example 1 is to construct an optical path based on the optical path structure of a raman fiber laser system for improving the higher-order raman threshold provided by the embodiment shown in fig. 2. The pump source in comparative example 1 is 1080nm ytterbium-doped fiber laser, the output tail fiber is 20/400um, and a section of inclined grating inscribed on the same 20/400um fiber is welded after the output tail fiber of the pump source to be used as a backward return filter 6 for filtering backward transmitted signal light, thereby achieving the purpose of protecting 1080nm pump source. The specification of the backward multimode pump signal combiner 2 is (6+1) x 1, wherein the specification of the pump arm optical fiber of the backward multimode pump signal combiner 2 is 20/130um, and the specification of the signal arm optical fiber is 50/400um. The tail fiber of the first output end cap 101 adopts an optical fiber with the same specification as the signal arm optical fiber 50/400um of the backward multimode pump signal combiner 2. The specification of the optical fiber output end of the backward multimode pump signal beam combiner 2 is 150/210um, the specification of the first-order Raman low reflection grating 3, the specification of the first-order Raman high reflection grating 4, the specification of the Raman gain optical fiber 5 and the second output end cap 102 is 150/210um, and the purpose of matching with the specification of the optical fiber output end of the backward multimode pump signal beam combiner 2 is achieved, so that the purpose of minimizing welding loss is achieved. The raman gain fiber 5 used in comparative example 1 had a length of 170 meters. Referring to fig. 3, there is shown a graph of the evolution of the power and spectrum of comparative example 1, wherein (a) is a graph of the evolution of the power of comparative example 1, and (b) is a graph of the evolution of the spectrum of comparative example 1. As shown in fig. 3 (a), when the pump light power is 1890W, the first output end cap 101 outputs 833W of the signal light. Fig. 3 (b) shows the output spectrum at 833W, where the difference in intensity between the signal light and the higher-order raman light is 55dB. It is noted that, as can be seen in fig. 3 (b), the residual pump light at 1080nm has a high signal to noise ratio, and can well separate the signal light from the residual pump light.
For comparison with comparative example 1, a comparative example 2 is provided, referring to fig. 4, which is a light path structure diagram of the raman fiber laser system adopted in the comparative example 2, the comparative example 2 builds a light path based on the light path structure of the raman fiber laser system shown in fig. 4, and the selection type of devices in the light path is identical to that of comparative example 1, except that the positions of the first-order raman low reflection grating 3 and the first-order raman high reflection grating 4 in fig. 4 are exchanged with respect to the positions of the first-order raman low reflection grating 3 and the first-order raman high reflection grating 4 in fig. 2, that is, the comparative example 2 adopting the light path structure shown in fig. 4 realizes pumping of the forward structure, and other conditions are identical to those of comparative example 1.
Referring to fig. 5, fig. 5 is a graph of the evolution of power and spectrum obtained in comparative example 2, where (a) is a graph of the evolution of power obtained in comparative example 2, and (b) is a graph of the evolution of spectrum obtained in comparative example 2. As shown in fig. 5 (a), when the pump light power is 1304W, 584W of signal light is obtained. The output spectrum at 584W in fig. 5 (b) also maintains the intensity difference between the signal light and the higher-order raman light at the 55dB level. It is noted that the residual pump light of comparative example 2 using the optical path structure shown in fig. 4 requires a spectroscope for spectroscopic, so that the residual pump light spectrum at 1080nm has a low signal to noise ratio.
In conclusion, according to the comparison example, the optical path structure based on the backward multimode pump signal beam combiner can effectively inhibit high-order Raman and further improve the level of signal light output power, meanwhile, the optical path structure based on the backward multimode pump signal beam combiner can effectively separate signal light and residual pump light, the use of a light splitting device is reduced, and the obtained spectrum signal to noise ratio is better.
At a physical length ofIn the raman resonator, after the pump laser reaches a certain power level, the gain of the first-order stokes light (i.e. the signal light) is larger than the loss, the power of the first-order stokes light is continuously increased, the corresponding pump light is continuously consumed, and the power is continuously reduced. Similarly, after the power of the first-order stokes light reaches a certain power level, the energy is continuously converted into high-order raman light and amplified. In a backward pumped raman oscillator, the expression of the raman gain is as follows:
;
is the raman gain;
the Raman gain coefficient is related to not only the material of the gain medium, but also the wavelength difference between the pump and the signal light;
Is the pump light power;
is the effective length of the fiber, given by: ;
Is the loss coefficient of the signal light;
Is the physical length of the fiber;
is the effective mode field area for the interaction of the signal light and the pump light.
In the backward pumping structure, the pump light and the signal light are in backward propagation, and due to loss in the optical fiber, the pump light firstly enters the optical fiber and starts to undergo attenuation, so that the pump light power interacted with the signal light is reduced, and meanwhile, the effective acting distance (equivalent to the effective transmission distance of the high-order Raman in the optical fiber) between the signal light and the high-order Raman is reduced, so that the occurrence probability of the high-order Raman effect is reduced.
Embodiment 3 provides a raman fiber laser system for improving a higher order raman threshold, as shown in fig. 6, comprising a backward multimode pump signal combiner 2, a raman low reflection grating, a raman high reflection grating, a raman gain fiber 5 and a backward return filter 6;
The first end of the backward multimode pump signal beam combiner 2 is provided with a plurality of pump arms and a signal arm, wherein each pump arm is correspondingly connected with a pump source, a backward return filter 6 is connected between the pump source and the pump arm of the backward multimode pump signal beam combiner 2, the signal arm is connected with an output tail fiber of a first output end cap 101, the output end of the first output end cap 101 outputs signal light, the second end of the backward multimode pump signal beam combiner 2 is connected with the input end of a second-order Raman low reflection grating 7, the output end of the second-order Raman low reflection grating 7 is connected with the input end of a first-order Raman low reflection grating 3, the output end of the first-order Raman low reflection grating 3 is connected with the input end of a Raman gain optical fiber 5, the output end of the Raman gain optical fiber 5 is connected with the input end of a first-order Raman high reflection grating 4, the output end of the first-order Raman high reflection grating 4 is connected with the input end of a second-order Raman high reflection grating 8, and the output end of the second-order high reflection grating 8 is connected with a second output end cap 102 outputting residual pump light.
The center wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are positioned near the Raman gain peak of the pump light in the Raman gain optical fiber, the center wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are positioned near the Raman gain peak of the first-order Raman grating in the Raman gain optical fiber, and the output signal light is at the center wavelength corresponding to the second-order Raman grating.
After the pump light 9 is injected from the pump arm of the backward multimode pump signal combiner 2 by the pump source, the raman laser with the wavelength corresponding to the signal light is selected under the mode selection effect of the raman grating, and finally the raman laser is coupled into the signal arm of the backward multimode pump signal combiner 2, and is output through the first output end cap 101, and the unconverted residual pump light is output through the second output end cap 102. The experimental structure can effectively separate the generated signal light and the residual pump light.
In addition, the raman low reflection grating and the high reflection grating in the invention can be any order before the zero dispersion wavelength of the optical fiber, specifically, the raman low reflection grating comprises a first-order raman low reflection grating, a second-order raman low reflection grating and an n-order raman low reflection grating, the raman high reflection grating comprises a first-order raman high reflection grating, a second-order raman high reflection grating, an n-order raman high reflection grating, the second end of the backward multimode pump signal combiner is sequentially connected with the n-order raman low reflection grating, the first-order raman low reflection grating is connected with one end of a raman gain optical fiber, the other end of the raman gain optical fiber is sequentially connected with the first-order raman high reflection grating, the second-order raman high reflection grating, the n-order raman high reflection grating, and the second-order optical fiber is any order before the zero dispersion wavelength. Conventional communication fibers have zero dispersion wavelengths around 1.3um and 1.5 um. The center wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are positioned near the Raman gain peak of the pump light in the Raman gain optical fiber, the center wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are positioned near the Raman gain peak of the first-order Raman grating in the Raman gain optical fiber, and the like, and the output signal light is at the center wavelength corresponding to the n-order Raman grating.
The technical features of the above embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
The above examples merely represent a few embodiments of the present application, which are described in more detail and are not to be construed as limiting the scope of the application. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the application, which are all within the scope of the application. Accordingly, the scope of the application should be assessed as that of the appended claims.
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