B integral and nonlinear refractive index measurement method based on near-field regulation and control technology

Abstract

A B integral and nonlinear refractive index measuring method based on a near-field regulation and control technology comprises the following steps: adjusting a loading pattern of an amplitude type liquid crystal spatial light modulator by using the amplitude type spatial light modulator according to the light intensity distribution of an incident laser beam and the spatial resolution of a wavefront sensor, and shaping the shape of the incident laser beam; light intensity distribution with gradient intensity is formed and is emitted to a to-be-detected sample; and receiving a light beam emitted from the to-be-measured sample by using a wavefront sensor, selecting a spatial light beam shape containing a high-intensity region in which B integral is accumulated and a low-intensity region in which B integral is not accumulated, extracting total nonlinear phase shift by using the low-intensity region as a reference point, and calculating to obtain B integral and nonlinear refractive index. The non-linear refractive index of a single material can be measured, B integral accumulated in a section of light path can also be measured, a sample cannot be damaged in the measurement process, and measurement is accurate.

Description

Technical Field

[0001] The present invention relates to a measurement method applied in the field of high-power laser nonlinear optics. More specifically, the present invention relates to a method for measuring the nonlinear refractive index of optical elements and the B-integral in a high-power laser system in the application scenario of high-power laser nonlinear optics. Background Art

[0002] ICF and other high-energy high-density scientific researches have put extremely high requirements on the output energy and power of high-power laser devices. As a nonlinear phase shift during the transmission of the optical field, the B-integral is an important parameter for evaluating the safe operation of the device. With the increase of output energy and power, the B-integral will continuously increase. When the B-integral increases to a certain extent, irreversible damage will occur to the optical materials in the system. Therefore, monitoring the B-integral of the laser system and clarifying the maximum output capacity under the safe operation of the device are of great significance for the design, safe operation, and performance improvement of high-power laser devices. At the same time, optical materials for high-power laser systems should be selected with lower nonlinear refractive indices, which can not only protect optical elements from being damaged, but also reduce the distortion of the output beam.

[0003] Existing methods for measuring the nonlinear refractive index usually adopt the Z-scan technique. In this technique, under a tightly focused configuration, a Gaussian beam is incident, the position of the sample on the optical axis (Z-axis) is changed, and the transmittance through a finite aperture located on the optical axis is measured in the far field to obtain the Z-scan curve, and the nonlinear refractive index of the material is obtained by calculation. In this method, there is a real focus of the laser system in the sample, which is prone to thermal distortion and has a large measurement error. At the same time, this method can only measure the nonlinear refractive index of a single-piece sample, and has great limitations on the shape of the incident laser, the thickness of the sample, and the number of samples. Currently, for the B-integral test method based on the entire system, the method of measuring the spectral broadening amount is usually adopted, but the application of this technique to high-power laser systems requires an additional high-precision spectral test unit. Therefore, it is necessary to design a high-precision B-integral and nonlinear refractive index measurement system with a wider application range and a simpler and easier-to-operate structure. Summary of the Invention

[0004] Aiming at the deficiencies existing in the above problems, the present invention provides a method for measuring the B-integral and nonlinear refractive index based on the near-field modulation technique, which is used to solve the strict limitations of the traditional Z-scan method on the beam shape, sample thickness, and sample position. It can measure the B-integral accumulated in a section of the optical path, and estimate the maximum power density borne by the optical path components by monitoring the B-integral to prevent damage to the optical path components.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] A method for measuring B-integral and nonlinear refractive index based on near-field modulation technology, characterized in that it includes:

[0007] Using an amplitude-type spatial light modulator to adjust the loading pattern of the amplitude-type liquid crystal spatial light modulator according to the light intensity distribution of the incident laser beam and the spatial resolution of the wavefront sensor. After shaping the shape of the incident laser beam, a light intensity distribution with a gradually changing intensity gradient is formed and directed towards the sample to be measured;

[0008] Using a wavefront sensor to receive the light beam emerging from the sample to be measured, and selecting a spatial light beam shape that includes both a high-intensity region where B-integral accumulates and a low-intensity region where B-integral does not accumulate. Using the low-intensity region as a reference point to extract the total nonlinear phase shift, and calculating the B-integral and the nonlinear refractive index.

[0009] Specifically, the method for realizing the measurement of the nonlinear refractive index is characterized in that it includes:

[0010] A laser, a first half-wave plate, a polarization beam splitter prism, and a second half-wave plate are sequentially arranged in the direction of the incident laser beam of the amplitude-type spatial light modulator;

[0011] A beam reducing system, a sample to be measured, a beam expanding system, and an attenuator are sequentially arranged between the amplitude-type spatial light modulator and the wavefront sensor along the laser beam transmission direction;

[0012] Rotate the second half-wave plate to adjust the laser output by the laser to the required polarization state, and then rotate the first half-wave plate to adjust the light intensity of the laser output by the laser to the weakest light intensity that can be detected. Observe with a wavefront sensor and record the current wavefront as the reference wavefront Δφ(x,y) L ;

[0013] Load a shielding pattern on the amplitude-type spatial light modulator to form a light intensity distribution with a gradually changing intensity gradient, and rotate the first half-wave plate to increase the light intensity of the laser output by the laser. Observe with a wavefront sensor and record the current wavefront as the total wavefront Δφ(x,y) tot ;

[0014] Calculate the nonlinear refractive index n 2 , and the formula is as follows:

[0015]

[0016] where λ is the laser wavelength output by the laser, I 0 is the central light intensity, and L is the propagation distance, that is, the thickness of the sample to be measured.

[0017] Specifically, the method for realizing the measurement of the B-integral is characterized in that it includes:

[0018] A laser, a first beam expander / contractor system, a first half-wave plate, and a polarization beam splitter prism are sequentially arranged in the incident laser beam direction of the amplitude-type spatial light modulator;

[0019] A second beam expander / contractor system, a path to be measured, a third beam expander / contractor system, and an attenuator are sequentially arranged between the amplitude-type spatial light modulator and the wavefront sensor along the laser beam transmission direction;

[0020] Rotate the first half-wave plate to adjust the laser light intensity output by the laser to the weakest detectable light intensity, observe with the wavefront sensor, and record the current wavefront as the reference wavefront Δφ(x,y) L ;

[0021] Load a shielding pattern on the amplitude-type spatial light modulator to form a light intensity distribution with a gradually changing intensity gradient, and rotate the first half-wave plate to make the laser light intensity output by the laser fully incident on the path to be measured, and record the data measured by the wavefront sensor as the total wavefront Δφ(x,y) tot , and obtain that the magnitude of the B integral generated by the path to be measured is Δφ(x,y) tot -Δφ(x,y) L .

[0022] Furthermore, the first beam expander / contractor system is used to adjust the beam aperture to match the light-transmitting aperture of the amplitude-type spatial light modulator, the second beam expander / contractor system is used to adjust the beam aperture to match the path to be measured, and the third beam expander / contractor system adjusts the beam aperture to match the wavefront sensor.

[0023] Furthermore, the laser output by the laser is a square flat-top beam, a circular flat-top beam, a square Gaussian beam, or a circular Gaussian beam.

[0024] Furthermore, a CCD detector is further included, which is located before the wavefront sensor and is used to detect the light field distribution and determine the laser power density.

[0025] Through the above technical solutions, the advantages and characteristics of a method for measuring the B integral and the nonlinear refractive index based on the near-field control technology provided by the present invention are:

[0026] The present invention overcomes the strict limitations of existing measurement methods on the beam shape, sample thickness, and position, and can achieve non-linear phase shift measurement of a single sample or a section of the optical path under any beam shape. The present invention uses an amplitude-type liquid crystal spatial light modulator to shape the beam shape, flexibly adjusts the pattern loaded on the liquid crystal according to the light intensity distribution of the incident laser beam and the spatial resolution of the wavefront sensor, forms a light intensity distribution with a gradually changing intensity gradient, and selects a spatial beam shape that includes both a high-intensity region where the B integral accumulates and a low-intensity region where the B integral does not accumulate, so that the low-intensity region can be used as a reference point to directly extract the total non-linear phase shift and improve the measurement accuracy. The experimental device of the present invention is simple and convenient to measure. By simply rotating the half-wave plate, the non-linear phase shift generated by the non-linear medium under different light intensities can be measured, and the non-linear refractive index of the sample can be accurately deduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art.

[0028] Figure 1 It is a flowchart of the method for measuring the B integral and the non-linear refractive index based on the near-field regulation technology of the present invention.

[0029] Figure 2 It is a schematic structural diagram of the method for measuring the non-linear refractive index by using an amplitude-type liquid crystal spatial light modulator for near-field regulation provided in Embodiment 1 of the present invention.

[0030] Figure 3 It is a schematic structural diagram of the method for measuring the B integral by using an amplitude-type liquid crystal spatial light modulator for near-field regulation provided in Embodiment 2 of the present invention.

[0031] Figure 4 It is the gamma curve of the amplitude-type liquid crystal spatial light modulator provided in Embodiment 2 of the present invention.

[0032] Figure 5 It is the pattern loaded on the amplitude-type liquid crystal spatial light modulator provided in Embodiment 2 of the present invention for the laser.

[0033] Figure 6 It is the light intensity distribution of the laser beam before and after being modulated by the amplitude-type liquid crystal spatial light modulator provided in Embodiment 2 of the present invention.

[0034] In the figure, 1 - laser, 2 - amplitude-type spatial light modulator, 3 - sample to be measured or optical path, 4 - wavefront sensor, 5 - half-wave plate, 6 - polarization beam splitter prism, 7 - half-wave plate, 8 - beam reducing system, 9 - beam expanding system, 10 - attenuation sheet, 11 - CCD detector, 12 - first beam expanding or reducing system, 13 - second beam expanding or reducing system, 14 - third beam expanding or reducing system, 21 - polarizer, 22 - amplitude-type liquid crystal spatial light modulator, 23 - computer, 24 - analyzer. Detailed implementation mode

[0035] The present invention will be further described below in conjunction with the accompanying drawings and specific examples, but it is not intended to limit the present invention.

[0036] Please refer to Figure 1 , Figure 1 , which is a flowchart of the B-integral and nonlinear refractive index measurement method based on near-field regulation technology of the present invention. As shown in the figure, a B-integral and nonlinear refractive index measurement method based on near-field regulation technology includes:

[0037] Using an amplitude-type spatial light modulator to adjust the loading pattern of the amplitude-type liquid crystal spatial light modulator according to the light intensity distribution of the incident laser beam and the spatial resolution of the wavefront sensor. After shaping the shape of the incident laser beam, a light intensity distribution with a gradually changing intensity gradient is formed and projected onto the sample to be measured;

[0038] Using a wavefront sensor to receive the light beam exiting from the sample to be measured, and selecting a spatial light beam shape that includes both a high-intensity region where the B-integral accumulates and a low-intensity region where the B-integral does not accumulate. Using the low-intensity region as a reference point to extract the total nonlinear phase shift, and calculating the B-integral and the nonlinear refractive index.

[0039] Example 1: As shown in the appendix Figure 2 , which is a schematic structural diagram of the nonlinear refractive index measurement method using an amplitude-type liquid crystal spatial light modulator for near-field regulation provided by the present invention, including a Gaussian distribution laser 1 with a wavelength of 1030 nm, a pulse width of 220 fs, and a spot diameter of 20 mm, a first half-wave plate 5, a polarization beam splitter prism 6, a second half-wave plate 7, a polarizer 21, an amplitude-type liquid crystal spatial light modulator 22 with a clear aperture of 22 mm × 22 mm and a resolution of 30.6 μm, a computer 23, an analyzer 24, a beam reducing system 8 with a beam reduction ratio of 10, a BBO sample 3 with a thickness of 5 mm, a beam expanding system 9 with a beam expansion ratio of 2, an attenuation sheet 10, a CCD detector 11, and a wavefront sensor 4 with a resolution of 30 μm and an accuracy of 10 nm RMS.

[0040] Since the intensity gradient distribution of the Gaussian beam itself is within the resolution range of the wavefront sensor 4, the pattern loaded on the amplitude-type liquid crystal spatial light modulator 22 is a full-white pattern. The beam is reduced in size to increase the optical power entering the BBO sample 3. The laser beam transmitted through the sample 3 is expanded and then attenuated by the attenuator 10 and received by the wavefront sensor 4 to detect the distribution of the optical field. The beam expander system 9 can effectively reduce the optical power while making the size of the output beam better match the resolution of the wavefront sensor 4.

[0041] The laser pulse shaped by the amplitude-type liquid crystal spatial light modulator experiences linear phase shift and nonlinear phase shift in the sample to be measured, and the total phase change of the beam is expressed as:

[0042] Δφ(x,y) tot =Δφ(x,y) L +Δφ(x,y) NL #(1)

[0043] where, Δφ(x,y) L is the linear phase shift, and Δφ(x,y) NL is the nonlinear phase shift. The B-integral is a measure of the total nonlinear phase shift, and the calculation formula of the B-integral is shown in Equation (2):

[0044]

[0045] where, λ is the laser wavelength, n 2 is the nonlinear refractive index of the nonlinear medium, I(z) is the optical field intensity, and L is the propagation distance (the thickness of the nonlinear medium). The optical field intensity, that is, the optical power density in the sample, is obtained by measuring the spot area and the light intensity distribution with the CCD detector 13.

[0046] From Equation (1) and Equation (2), the calculation formula of the nonlinear refractive index can be obtained:

[0047]

[0048] where, I 0 is the central light intensity. The magnitude of the B-integral is the phase change amount measured in the experiment, and substituting it into Equation (3) can calculate the magnitude of the nonlinear refractive index of the sample to be measured.

[0049] According to the formula calculation, the Rayleigh length of the beam in the sample to be measured is 3.05 m. From the formula it can be seen that the spot sizes at 0.3 m before and after the beam waist are the same, and the beam power density does not change. Therefore, the BBO sample 3 can be placed at any position within 0.3 m before and after the beam waist.

[0050] By rotating the second half-wave plate 7 to adjust the polarization state and rotating the first half-wave plate 5 to adjust the average power, the light intensity I measured at the BBO sample 30 is 0 - 30 GW / cm 2 By measuring the phase change corresponding to different incident light intensities of the ordinary light and extraordinary light, the nonlinear refractive indices of the ordinary light direction and extraordinary light direction of the BBO sample can be obtained.

[0051] Example 2: As shown in the attached Figure 3 figure, it is a schematic structural diagram of the B-integral measurement method for near-field regulation using an amplitude-type liquid crystal spatial light modulator provided by the present invention. It includes a flat-top distributed laser 1 with a wavelength of 1030 nm, a pulse width of 220 fs, and a spot of 40 mm × 40 mm, a first beam reduction system 12 with a beam reduction ratio of 2, a half-wave plate 5, a polarization beam splitter prism 6, a polarizer 21, an amplitude-type liquid crystal spatial light modulator 22 with a clear aperture of 22 mm × 22 mm and a resolution of 30.6 μm, a computer 23, an analyzer 24, a second beam expansion system 13 with a beam expansion ratio of 2, a section of optical path 3, a third beam reduction system 14 with a beam reduction ratio of 10, an attenuation sheet 10, a CCD detector 11, and a wavefront sensor 4 with a resolution of 30 μm and an accuracy of 10 nm RMS.

[0052] The laser beam of the laser 1 is adjusted in beam profile by the amplitude-type liquid crystal spatial light modulator 22. The amplitude-type liquid crystal spatial light modulator 22 is controlled by the computer 23. The gamma curve of the amplitude-type liquid crystal spatial light modulator 22 is as shown in the attached Figure 4 figure, the anti-Gaussian pattern loaded according to the flat-top pulse laser distribution is as shown in the attached Figure 5 figure, and the light field distributions before and after beam adjustment are as shown in the attached Figure 6 figure.

[0053] The Rayleigh length of the beam in the optical path to be measured is calculated according to the formula to be 1.22×10 3 m. From the formula it can be obtained that the spot sizes are the same at 27 m before and after the beam waist, and the beam power density does not change. Therefore, the B-integral accumulated in a section of the optical path can be accurately measured by this device.

[0054] By reading the wavefront data detected by the wavefront sensor 4, the B-integral of the optical path 3 to be measured can be obtained according to Equation (1).

[0055] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for measuring the B-integral and nonlinear refractive index based on near-field modulation technology, characterized in that, it includes: Using an amplitude-type spatial light modulator to adjust the loading pattern of the amplitude-type liquid crystal spatial light modulator according to the light intensity distribution of the incident laser beam and the spatial resolution of the wavefront sensor. After shaping the shape of the incident laser beam, a light intensity distribution with a gradually changing intensity gradient is formed and directed towards the sample to be measured; Using a wavefront sensor to receive the light beam emerging from the sample to be measured, and selecting a spatial light beam shape that includes both a high-intensity region where the B-integral accumulates and a low-intensity region where the B-integral does not accumulate. Using the low-intensity region as a reference point to extract the total nonlinear phase shift, and calculating the B-integral and nonlinear refractive index.

2. A method for measuring the nonlinear refractive index implemented by the method for measuring the B-integral and nonlinear refractive index based on near-field modulation technology according to claim 1, characterized in that, it includes: A laser (1), a first half-wave plate (5), a polarization beam splitter prism (6) and a second half-wave plate (7) are sequentially arranged in the direction of the incident laser beam of the amplitude-type spatial light modulator; A beam reducing system (8), a sample to be measured (3), a beam expanding system (9) and an attenuation sheet (10) are sequentially arranged in the direction of laser beam transmission between the amplitude-type spatial light modulator and the wavefront sensor; Rotate the second half-wave plate (7) to adjust the polarization state of the laser beam output from the laser (1) to the desired state, and then rotate the first half-wave plate (5) to adjust the light intensity of the laser beam output from the laser (1) to the weakest detectable light intensity. Observe using the wavefront sensor (4) and record the current wavefront as the reference wavefront Δφ(x,y). L ; Load a masking pattern onto the amplitude-type spatial light modulator (2) to form an intensity distribution with a gradually changing intensity gradient, and rotate the first half-wave plate (5) to enhance the laser intensity output by the laser (1). Observe using the wavefront sensor (4) and record the current wavefront as the total wavefront Δφ(x,y). tot ; Calculate the nonlinear refractive index n 2 , and the formula is as follows: where λ is the laser wavelength output by the laser (1), I 0 is the central light intensity, and L is the propagation distance, i.e., the thickness of the sample to be measured (3).

3. A method for measuring the B-integral implemented by the method for measuring the B-integral and nonlinear refractive index based on near-field modulation technology according to claim 1, characterized in that, it includes: A laser (1), a first beam expanding / contracting system (12), a first half-wave plate (5) and a polarization beam splitter prism (6) are sequentially arranged in the direction of the incident laser beam of the amplitude-type spatial light modulator; A second beam expanding / contracting system (13), a measurement optical path (3), a third beam expanding / contracting system (14) and an attenuation sheet (10) are sequentially arranged in the direction of laser beam transmission between the amplitude-type spatial light modulator and the wavefront sensor; Rotate the first half-wave plate (5) to adjust the laser intensity output by the laser (1) to the weakest detectable intensity. Observe using the wavefront sensor (4) and record the current wavefront as the reference wavefront Δφ(x,y). L ; Load a masking pattern on the amplitude-type spatial light modulator (2) to form an optical intensity distribution with a gradually changing intensity gradient, and rotate the first half-wave plate (5) so that the laser light intensity output by the laser (1) is fully incident on the optical path to be measured (3), and record the data measured by the wavefront sensor (4) as the total wavefront Δφ(x, y). tot , and obtain that the magnitude of the B integral generated by the optical path to be measured (3) is Δφ(x, y). tot -Δφ(x, y) L .

4. The method for measuring the B-integral according to claim 3, characterized in that, The first beam expanding / contracting system is for adjusting the beam diameter to match the light passing diameter of the amplitude-type spatial light modulator (2), the second beam expanding / contracting system is for adjusting the beam diameter to match the measurement optical path (3), and the third beam expanding / contracting system adjusts the beam diameter to match the wavefront sensor (4).

5. A method for measuring the B-integral and nonlinear refractive index based on near-field modulation technology according to any one of claims 1-3, characterized in that, The laser (1) outputs a square flat-top beam, a circular flat-top beam, a square Gaussian beam or a circular Gaussian beam.

6. A method for measuring the B-integral and nonlinear refractive index based on near-field modulation technology according to any one of claims 1-3, characterized in that, It further includes a CCD detector, located before the wavefront sensor, for detecting the light field distribution and determining the laser power density.

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