CN112269262B - Method for verifying stimulated Brillouin scattering ultrasonic grating structure in water - Google Patents
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- CN112269262B CN112269262B CN202011012773.3A CN202011012773A CN112269262B CN 112269262 B CN112269262 B CN 112269262B CN 202011012773 A CN202011012773 A CN 202011012773A CN 112269262 B CN112269262 B CN 112269262B
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Abstract
The invention discloses a method for verifying a structure of an underwater stimulated Brillouin scattering ultrasonic grating, which is a model of the underwater stimulated Brillouin scattering ultrasonic grating structure established on the basis of a stimulated Brillouin scattering coupling wave theory and is used for obtaining a non-uniform distribution model of refractive index in water. By considering the nonlinear effect generated when strong laser is incident into water to excite stimulated Brillouin scattering, a transmission expression of the pump light and the scattered light in the water is established, light intensity distribution of each space position in the transverse transmission process is obtained, and further a refractive index distribution function in the water is established. Through the research on the grating theory in the medium, a plurality of grating models with the spectral distribution most similar to the stimulated Brillouin scattering spectral distribution are selected, and the refractive index function established by the method is modulated according to the refractive index modulation rule. And obtaining the refractive index distribution relation when the stimulated Brillouin scattering occurs in water through the comparison of the reflection spectrums.
Description
Technical Field
The invention relates to the technical field of stimulated Brillouin scattering ultrasonic gratings, in particular to a method for verifying a structure of a stimulated Brillouin scattering ultrasonic grating in water.
Background
Stimulated brillouin scattering is an important method for obtaining phase conjugate light in nonlinear optics, and is widely researched and applied to the aspects of lasers, pulse shaping, optical fiber communication, optical fiber sensing, optical information storage, all-optical signal processing, ocean detection and the like due to the characteristics of threshold value, pulse width compression, high gain, high precision and the like. The research on the marine laser radar system based on the stimulated brillouin scattering phase conjugation property is also a major issue in the field of marine remote sensing. Compared with the traditional laser radar technology, the stimulated Brillouin scattering laser radar system has the advantages of high resolution, high signal-to-noise ratio, no contact and real-time initiative, but in the practical application process, the system faces the limit of detection depth and the improvement of the performance of the stimulated Brillouin scattering laser radar system guided by an imperfect theoretical model. At present, the research on stimulated brillouin scattering in water is mostly focused on the experimental aspect, theoretical research is relatively lagged behind, and particularly reports of analyzing stimulated brillouin scattering from the microscopic mechanism are rare. The stimulated Brillouin scattering ultrasonic grating structure is an effective means for researching acousto-optic interaction and understanding a coupling mechanism of an optical wave field and an acoustic wave field from the distribution of medium refractive indexes, and the forming mechanism is that when strong laser is incident into water, a strong elastic acoustic wave field is generated due to electrostrictive effect excitation, so that the density of the water is periodically changed, namely the refractive index periodically fluctuates along with time and space, the stimulated Brillouin scattering ultrasonic grating structure is equivalent to a moving grating structure, certain characteristics of stimulated Brillouin scattering are influenced, and the research work is mainly concentrated in an optical fiber medium. On the basis of the stimulated Brillouin scattering coupling wave theory, the invention combines the nonlinear effect generated when the laser is transmitted in water to establish a model of light intensity distribution and refractive index distribution in water, uses MATLAB simulation software to simulate, compares reflection spectrum images under various different medium grating refractive index modulation rules, analyzes the stimulated Brillouin scattering characteristics, finally obtains the refractive index modulation distribution which best meets the experimental result, and further verifies the structure of the stimulated Brillouin scattering ultrasonic grating in water.
Disclosure of Invention
The invention aims to solve the problems that: method for verifying stimulated Brillouin scattering ultrasonic grating structure in water
The technical scheme provided by the invention for solving the problems is as follows: a method of validating a stimulated Brillouin scattering ultrasonic grating structure in water, the method comprising the steps of,
the method comprises the following steps: according to the stimulated Brillouin scattering theory, constructing a stimulated Brillouin scattering coupling wave equation in water, and establishing a transverse one-dimensional coupling wave model;
step two: obtaining the light intensity distribution of each spatial position in water through coherent superposition of incident light and scattered light complex amplitude, and establishing a refractive index distribution function relation in water;
step three: selecting several grating models which are closest to the distribution of the stimulated Brillouin scattering spectrum in water in the medium grating theory, modulating the refractive index distribution function obtained in the second step according to the refractive index modulation rule, respectively drawing the reflection spectra of the several different refractive index modulation gratings through MATLAB simulation software, and comparing and verifying the reflection spectra with the experimental result.
Preferably, the approximation process used by the transverse one-dimensional coupled wave model in the first step is one-dimensional approximation, slow-varying envelope approximation or steady-state approximation.
Preferably, the complex amplitude expressions of the incident light and the scattered light in the second step both take into account the nonlinear absorption of water, and the total light intensity in water is a result of the coherence of the two beams:
preferably, the refractive index distribution function relationship of water when the stimulated brillouin scattering occurs in the second step is converted from the spatial distribution of light intensity, and includes two terms: a hyperbolic cosine change, a cosine change:
preferably, the dielectric grating models in the third step are all selected in a non-uniform mode.
Preferably, when the reflection spectrum is drawn in the third step, the used central wavelength is 532nm, and the adopted calculation method is an ABCD matrix method.
Compared with the prior art, the invention has the advantages that: the invention provides a method for modulating a refractive index distribution function in water by utilizing refractive index modulation rules of various medium gratings, which obtains the refractive index distribution condition of an underwater stimulated Brillouin scattering ultrasonic grating through comparison and further verifies the structure of the underwater stimulated Brillouin scattering ultrasonic grating. The invention is also suitable for the research of the stimulated Brillouin scattering ultrasonic grating in other media, and provides theoretical guidance for the research of acousto-optic interaction.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
FIG. 1 is a Gaussian refractive index modulation profile of the present invention;
FIG. 2 is an index type refractive index modulation profile of the present invention;
FIG. 3 is a sinusoidal refractive index modulation profile of the present invention;
FIG. 4 is a comparison graph of the spectrum collected in the stimulated Brillouin scattering experiment and the simulated spectrum of the invention; in the figure, a reflection spectrum corresponding to ICCD10 is an experimental acquisition spectrum, and a reflection spectrum corresponding to a water pool 05 is a reflection spectrum of an underwater stimulated Brillouin scattering ultrasonic grating structure simulated by using a sine refractive index modulation model.
Detailed Description
The embodiments of the present invention will be described in detail with reference to the accompanying drawings and examples, so that how to implement the technical means for solving the technical problems and achieving the technical effects of the present invention can be fully understood and implemented.
In order to make the object, technical scheme, etc. of the present invention more clear, the following further elaboration takes the refractive index modulation function as gaussian, exponential, sinusoidal as an example:
the method comprises the following steps: according to the stimulated Brillouin scattering coupling wave theory, a transverse one-dimensional stimulated Brillouin scattering coupling wave model in water is constructed through reasonable approximate treatment;
the transverse one-dimensional stimulated Brillouin scattering coupling wave theory in the step one is derived from Maxwell equation, and the approximate processing method selected according to the actual situation in water is as follows: one-dimensional approximation: only the light field distribution in the z direction of the light beam transmission direction is considered; slow-varying envelope approximation: the variation of the second derivative of the light field in space and time is much smaller than that of the first derivative; steady state approximation: the laser pulse width adopted by the invention is 8ns, the phonon life is 200ps, the phonon life is far shorter than the pulse duration, and the generated stimulated Brillouin scattering is considered as a steady state.
Step two: considering the nonlinear absorption effect in water, obtaining the light intensity distribution of each spatial position in water through coherent superposition of incident light and scattered light, converting the light intensity distribution on the space into refractive index distribution, and establishing a refractive index distribution function relation in water;
in the second step, the complex amplitude expressions of the incident light and the scattered light both consider the average absorption term and the nonlinear absorption term of water, and the coherent result is
Converts the spatial distribution of light intensity into refractive index distribution
Two items are included: one term is hyperbolic cosine variation and one term is cosine variation.
Refractive index modulation depth, n, of hyperbolic cosine and cosine terms, respectively 0 Is the initial refractive index of water, L is the grating length, the exponential term represents the nonlinear absorption, and α is the absorption coefficient of water.
Step three: and (3) obtaining a non-uniform grating model in a medium grating theory, selecting a plurality of grating models which are closest to the stimulated Brillouin scattering spectral characteristics, analyzing the refractive index modulation rule, modulating the refractive index distribution function established in the step two by using MATLAB simulation software, drawing a reflection spectrogram of the refraction index distribution function, and comparing the reflection spectrogram with an experimental result.
In the third step, several non-uniform grating models are selected through the research of a medium grating theoretical model, wherein a Gaussian model, an exponential model and a sinusoidal model are taken as examples. And (5) researching a refractive index modulation rule of each grating model, and modulating the refractive index distribution function established in the step two. The grating length L here is 2 m, the initial refractive index of water is 1.333, and the absorption coefficient is 0.06. The refractive index modulation functions of Gaussian type, exponential type and sinusoidal type are respectively as follows: gaussian type:
sine type:
and (3) index type:
the reason why the selected dielectric grating model is non-uniform in the third step is that the stimulated brillouin scattering is a three-order non-linear optical phenomenon, when the stimulated brillouin scattering occurs in water, the refractive index and the period of the stimulated brillouin scattering show non-uniform changes due to the non-linear polarization effect of the water, and the modulation depth of the refractive index can not be regarded as a constant.
In the third step, when MATLAB simulation software is used for drawing a reflection spectrum, laser with the incident central wavelength of 532nm is mainly researched, and the ABCD matrix method is adopted for calculation.
The experimental results described in step three were collected from ICCD10, the specific procedure was: laser with wavelength of 532nm emitted by a laser 01 sequentially passes through a polarization beam splitter 02, an 1/4 wave plate 03 and a convex lens I04 to reach a water pool 05 to form an ultrasonic grating structure, and a generated backward stimulated Brillouin scattering signal sequentially passes through the convex lens I04, the 1/4 wave plate 03, the polarization beam splitter 02, a reflector 06, a concave lens 07, a convex lens II 08 and an F-P etalon 09 to reach an ICCD10 to obtain a stimulated Brillouin scattering spectrum signal (a reflection spectrum corresponding to ICCD10 shown in figure 4).
The foregoing is merely illustrative of the preferred embodiments of the present invention and is not to be construed as limiting the claims. The present invention is not limited to the above embodiments, and the specific structure thereof is allowed to vary. All changes which come within the scope of the invention as defined by the independent claims are intended to be embraced therein.
Claims (4)
1. A method for verifying a stimulated Brillouin scattering ultrasonic grating structure in water comprises the following steps:
the method comprises the following steps: according to the stimulated Brillouin scattering theory, constructing a stimulated Brillouin scattering coupling wave equation in water, and establishing a transverse one-dimensional coupling wave model; step two: considering the nonlinear absorption effect in water, the light intensity distribution of each spatial position in water is obtained by coherent superposition of complex amplitudes of incident light and scattered light:
wherein the content of the first and second substances,
is the non-linear absorption coefficient of water,
is the length of the grating, the exponential term
Representing the nonlinear absorption effect in water, the light intensity distribution of each spatial position in water is converted into the refractive index distribution functional relation in water, and the refractive index distribution functional relation comprises two terms: one hyperbolic cosine change, one cosine change:
wherein, the first and the second end of the pipe are connected with each other,
which represents the initial refractive index of water,
、
refractive index modulation depths of a hyperbolic cosine term and a cosine term respectively;
step three: and (3) selecting three medium grating models of Gaussian type, exponential type and sinusoidal type in the medium grating theory, modulating the refractive index distribution function relationship obtained in the second step according to the refractive index modulation rule, respectively drawing the reflection spectra of the different refractive index modulation gratings by using an ABCD matrix method through MATLAB simulation software, and comparing and verifying the reflection spectra with the experimental result.
2. The method according to claim 1, wherein the approximation process used by the transverse one-dimensional coupled wave model in the first step is one-dimensional approximation, slow-varying envelope approximation or steady-state approximation.
3. The method according to claim 1, wherein the medium grating model is selected to be non-uniform in the third step.
4. The method according to claim 1, wherein the reflection spectrum is plotted in step three at a center wavelength of 532 nm.
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| Analysis of refractive index distribution of stimulated Brillouin scattering in water;Liu Yanhuan等;《Proceedings of SPIE》;20200131;全文 * |
| 水中布里渊散射动态光栅的理论分析及相关技术研究;葛媛;《中国优秀博硕士学位论文全文数据库（硕士）基础科学辑》;20150215;第3章、第4章第4.1节，第23-38页 * |
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