Nonlinear refractive index measurement system and method based on dual-optical-path interference

Abstract

The invention provides a nonlinear refractive index measurement system and method based on dual-optical-path interference, and relates to the technical field of nonlinear optical measurement, the system comprises a first laser emission unit, a first light splitting device, a second light splitting device, an interference device, a to-be-measured sample, a second laser emission unit and a measurement unit; the first laser emission unit generates a Gaussian beam; the first light splitting device splits the Gaussian beam into a sample beam and a reference beam which are parallel to each other; the second light splitting device splits the sample light beam and the reference light beam which are parallel to each other into two paths; the first sample light beam and the reference light beam enter a to-be-tested sample; the second laser emission unit generates a pumping light beam, and the pumping light beam is emitted to the sample to be measured along the light path of the first sample light beam; the interference device generates interference between the two sample light beams and interference between the two reference light beams; the measuring unit is used for collecting interference images, analyzing the interference images and calculating the nonlinear refractive index of the to-be-measured sample. The system is not easily influenced by the environment.

Description

Technical Field

[0001] The present invention relates to the technical field of nonlinear optical measurement, and in particular to a nonlinear refractive index measurement system and method based on double-path interference. Background Art

[0002] Common methods for measuring nonlinear refractive index by double-path interference mainly include: Z-scan method: A Gaussian laser beam is focused on a sample, and the sample moves along the optical axis (Z-axis) direction. The nonlinear refractive index information of the sample is obtained by measuring the change in far-field transmittance. When the laser beam passes through the sample, due to the existence of the nonlinear refractive index, self-focusing or self-defocusing of the beam will occur, resulting in a change in the far-field light intensity distribution, and thus the transmittance changes. In the measurement process of the Z-scan method, it is necessary to accurately control the movement accuracy of the sample, otherwise large errors will be introduced. Moreover, this method can only measure the overall nonlinear refractive index of the sample and cannot obtain the spatial distribution information of the refractive index.

[0003] Mach-Zehnder interferometry: A beam of light is split into two beams by a beam splitter, and then recombined to generate interference after passing through different optical paths. One beam of light passes through the sample to be measured, and the other beam is used as a reference light. When the sample has a nonlinear refractive index, the phase of the light passing through the sample will change, resulting in the movement of the interference fringes. The nonlinear refractive index of the sample is calculated by measuring the movement of the interference fringes. Mach-Zehnder interferometry has extremely high requirements for the stability of the optical path. Tiny vibrations or air disturbances may affect the stability of the interference fringes, thus affecting the measurement results. The optical path adjustment is relatively complex, and it is necessary to accurately control the optical path difference and polarization state of the two beams of light.

[0004] Michelson interferometry: Similar to the Mach-Zehnder interferometry, it is also based on the principle of light interference. The Michelson interferometer splits a beam of light into two beams, which are reflected by reflectors and then recombined to interfere. One beam of light passes through the optical path where the sample to be measured is placed. The change in the nonlinear refractive index of the sample will cause a change in the optical path, and thus the movement of the interference fringes. The nonlinear refractive index of the sample is determined by measuring the amount of movement of the interference fringes. The measurement sensitivity of Michelson interferometry is relatively low, and it may be difficult to accurately measure weak changes in the nonlinear refractive index. Moreover, the accuracy and stability of the reflectors have a great impact on the measurement results, and regular calibration and maintenance are required.

[0005] The influence of environmental factors on the measurement results is relatively large. At present, the stability and accuracy of measuring the nonlinear refractive index by interferometry are poor, and it is greatly affected by the environment. The Z-scan method cannot measure the nonlinear refractive index of thick samples. Summary of the Invention

[0006] The object of the present invention is to propose a non - linear refractive index measurement system based on double - path interference to solve the problems of poor stability and susceptibility to environmental influence in existing non - linear refractive index detection technologies. The system includes: a first laser emission unit, a first beam - splitting device, a second beam - splitting device, an interference device, a sample to be measured, a second laser emission unit, and a measurement unit; The first laser emission unit is used to generate a Gaussian beam and incident the Gaussian beam on the beam - splitting device; The first beam - splitting device is used to split the Gaussian beam into a sample beam and a reference beam that are parallel to each other; The second beam - splitting device is used to split the parallel sample beam and reference beam into a first sample beam and a first reference beam that are parallel to each other, and a second sample beam and a second reference beam that are parallel to each other; The first sample beam and the first reference beam are incident on the sample to be measured to obtain a target sample beam and a target reference beam; The second laser emission unit is used to generate a pump beam and incident the pump beam along the optical path of the first sample beam on the sample to be measured; The interference device is used to generate interference between the target sample beam and the second sample beam, and interference between the target reference beam and the second reference beam; The measurement unit is used to collect an interference image and analyze the target interference image to calculate the non - linear refractive index of the sample to be measured.

[0007] Optionally, the first laser emission unit includes: a laser emitter and a polarizer; The laser emitter is used to generate a fundamental - mode Gaussian beam and emit the fundamental - mode Gaussian beam to the polarizer; The polarizer is used to modulate the fundamental - mode Gaussian beam to a horizontal polarization direction and transmit it to the first beam - splitting device.

[0008] Optionally, the first beam - splitting device includes: a beam - splitting prism BS1, a mirror M1, a mirror M2, and a beam - splitting prism BS2; The beam - splitting prism BS1 is used to split the Gaussian beam emitted from the first laser emission unit into a reflected beam L1 and a transmitted beam L2, and transmit the reflected beam L1 and the transmitted beam L2 to the mirror M2 and the mirror M1 respectively. The light that the reflected beam L1 is transmitted through the prism BS2 after being transmitted to the mirror M2 is the sample beam; The light that the transmitted beam L2 is reflected by the prism BS2 after being transmitted to the mirror M1 is the reference beam; And the sample beam and the reference beam are transmitted to the second beam - splitting device.

[0009] Optionally, the second beam - splitting device includes: a beam - splitting prism BS3, a mirror M6, and a beam - splitting prism BS5; The sample beam and the reference beam pass through the beam splitter prism BS3 to obtain the reflected beam and the transmitted beam of the sample beam, and the reflected beam and the transmitted beam of the reference beam. The transmitted beam of the sample beam and the transmitted beam of the reference beam are reflected by the beam splitter prism BS5 to obtain the first sample beam and the first reference beam; the reflected beam of the sample beam and the reflected beam of the reference beam are reflected by the mirror M6 to obtain the second sample beam and the second reference beam.

[0010] Optionally, the second laser emission unit includes: a laser emitter, a beam shaper, a polarizer, a mirror M3 and a mirror M4; The laser emitter is used to generate pump light and emit the pump light to the beam shaper; The beam shaper is used to expand the pump light and then transfer it to the polarizer; The polarizer is used to modulate the expanded pump light to a horizontal polarization direction and transfer it to the mirrors M3 and M4; The mirrors M3 and M4 are used to reflect the pump light with a horizontal polarization direction to the beam splitter prism BS5 and make the reflected pump light with a horizontal polarization direction have the same optical path as the first sample beam.

[0011] Optionally, the interference device includes: a filter, a beam splitter prism BS4; The filter is arranged between the sample to be measured and BS4 and is used to filter the pump beam; The beam splitter prism BS4 is used to transmit the second sample beam and the second reference beam, reflect the target sample beam and the target reference beam, and transfer the target sample beam, the target reference beam, the second sample beam and the second reference beam to the measurement unit.

[0012] Optionally, the mirror surface direction of the mirror M6 is adjustable.

[0013] Optionally, adjust the position of the mirror M1 so that the reference beam obtained after the reflected beam is reflected by the beam splitter prism BS2 is parallel to the sample beam, and the distance between the reference beam and the sample beam is 5 mm.

[0014] The present invention also proposes a non-linear refractive index measurement method based on double-path interference, which is realized based on the above system and includes: Construct a measurement optical path; Change the magnitude of the pump light, record the magnitude of the light intensity of the pump light, and simultaneously collect the interference patterns between the target sample beam and the second sample beam, and the interference patterns between the target reference beam and the second reference beam in the measurement optical path; Analyze the interference patterns to obtain the magnitude of the relative moving distance of the two groups of interference patterns; Calculate the non-linear refractive index of the sample to be measured based on the interference patterns.

[0015] The beneficial effects brought by the technical solution provided by the present invention are as follows: In the present invention, a Gaussian beam is incident on a beam splitting device through a first laser emitting unit. The beam splitting device splits the Gaussian beam into two mutually parallel sample beams and reference beams. One of the sample beams and the reference beam are simultaneously incident on a sample to be measured. A second laser emitting unit injects a pump beam along the optical path of the sample beam into the sample to be measured, and changes the refractive index of the sample to be measured by changing the intensity of the pump beam. The relative movement of two sets of interference fringes of the target sample beam and the other sample beam, and the target reference beam and the other reference beam emitted from the sample characterizes the magnitude of the refractive index change. The measuring unit analyzes two sets of interference images of the sample beam and the reference beam to calculate the nonlinear refractive index, thereby effectively avoiding the disadvantage of poor stability of a single optical path interference device and being able to measure the nonlinear refractive index of a thick sample. Description of the Drawings

[0016] Figure 1 is a structural diagram of a nonlinear refractive index measurement system based on double - path interference according to an embodiment of the present invention; Figure 2 is a schematic diagram of the light intensity distribution after the interference of two sets of sample beams and reference beams respectively in an embodiment of the present invention; Figure 3 is a longitudinal grayscale value integral transverse distribution diagram of the interference pattern in an embodiment of the present invention. Detailed Embodiments

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

[0018] A structural diagram of a nonlinear refractive index measurement system based on double - path interference according to an embodiment of the present invention is as shown in Figure 1 , and includes: a first laser emitting unit, a first beam splitting device, a second beam splitting device, an interference device, a sample to be measured, a second laser emitting unit, and a measuring unit.

[0019] The first laser emitting unit is used to generate a Gaussian beam and inject the Gaussian beam into the beam splitting device. The first laser emitting unit includes: a laser emitter and a polarizer. The laser emitter is used to generate a fundamental mode Gaussian beam and emit the fundamental mode Gaussian beam to the polarizer. The polarizer is used to modulate the fundamental mode Gaussian beam to a horizontal polarization direction and transmit it to the first beam splitting device. In a further embodiment, the above - mentioned laser emitter may be a helium - neon laser.

[0020] The first beam splitting device is used to split the fundamental mode Gaussian beam with a horizontal polarization direction into a sample beam and a reference beam that are parallel to each other. The first beam splitting device includes: a beam splitting prism BS1, a mirror M1, a mirror M2, and a beam splitting prism BS2. The beam splitting prism BS1 is used to split the fundamental mode Gaussian beam with a horizontal polarization direction emitted from the first laser emitting unit into a reflected beam L1 and a transmitted beam L2, and transmit the reflected beam L1 and the transmitted beam L2 to the mirror M2 and the mirror M1 respectively. After the reflected beam L1 is reflected by the mirror M2, it is then transmitted through the optical prism BS2 to obtain the sample beam. After the transmitted beam L2 is reflected by the mirror M1, it is then reflected by the optical prism BS2 to obtain the reference beam. Adjust the position of the mirror M1 so that the reference beam obtained after the reflected beam is reflected by the beam splitting prism BS2 is parallel to the sample beam, and the distance between the reference beam and the sample beam is 5 mm. Then transmit the sample beam and the reference beam to the second beam splitting device.

[0021] The second beam splitting device is used to split the parallel sample beam and reference beam into a parallel first sample beam and first reference beam, and a parallel second sample beam and second reference beam. The first sample beam and the first reference beam are incident on the sample to be measured to obtain a target sample beam and a target reference beam. The second beam splitting device includes: a beam splitting prism BS3, a mirror M6, and a beam splitting prism BS5. The sample beam and the reference beam pass through the beam splitting prism BS3 to obtain the reflected beam and transmitted beam of the sample beam, and the reflected beam and transmitted beam of the reference beam. The transmitted beams of the sample beam and the reference beam are reflected by the beam splitting prism BS5 to obtain the first sample beam and the first reference beam. The reflected beams of the sample beam and the reference beam are reflected by the mirror M6 to obtain the second sample beam and the second reference beam. The mirror surface direction of the mirror M6 is adjustable.

[0022] The second laser emitting unit is used to generate a pump beam and incident the pump beam along the optical path of the first sample beam on the sample to be measured. The second laser emitting unit includes: a laser emitter, a beam shaper, a polarizer, a mirror M3, and a mirror M4. The laser emitter is used to generate a pump light and emit the pump light to the beam shaper. The beam shaper is used to expand the pump light and transmit it to the polarizer. The polarizer is used to modulate the expanded pump light into a horizontal polarization direction and transmit it to the mirrors M3 and M4. The mirrors M3 and M4 are used to reflect the pump light with a horizontal polarization direction to the beam splitting prism BS5 and make the reflected pump light with a horizontal polarization direction have the same optical path as the first sample beam.

[0023] When the sample to be measured is irradiated by a strong laser, a nonlinear response occurs, the refractive index of the sample changes, and the interference fringes of the sample beam shift.

[0024] An interference device is used to generate interference between a target sample beam and a second sample beam, and between a target reference beam and a second reference beam. The interference device includes: a filter and a beam splitting prism BS4. The filter is disposed between the sample to be measured and BS4 and is used to filter the pump beam to prevent the pump beam from entering the measurement unit. The beam splitting prism BS4 is used to transmit the second sample beam and the second reference beam, reflect the target sample beam and the target reference beam, and transmit the target sample beam, the target reference beam, the second sample beam and the second reference beam to the measurement unit.

[0025] The measurement unit is used to collect an interference image and analyze the target interference image to calculate the nonlinear refractive index of the sample to be measured. In a further embodiment, the above measurement unit may include a collection device and an analysis device. In some embodiments, the above collection device may be a CCD camera. The above analysis device may be a measurement device for measuring the nonlinear refractive index based on double-path interference with data processing, network communication, and program running functions, such as a computer, or other devices or equipment that can achieve the same or similar functions.

[0026] The present invention also provides a method for measuring the nonlinear refractive index based on double-path interference, which is implemented based on the above system and includes: (1) Construct a measurement optical path according to the above system, and the optical path to be measured is referred to Figure 1 , and a peripheral device with data processing, network communication, and program running functions is also required.

[0027] (2) Conduct multiple groups of experiments at room temperature, change the magnitude of the pump light. For example, in one group of experiments, adjust the wavelength of the first laser emission unit to 532 nm and the wavelength of the second laser emission unit to 780 nm, record the intensity of the pump light, and simultaneously collect the interference patterns between the target sample beam and the second sample beam, and between the target reference beam and the second reference beam in the measurement optical path.

[0028] It can be understood that the second laser emission unit injects the pump beam into the sample to be measured along the optical path of the sample beam, changes the intensity of the pump beam to change the refractive index of the sample to be measured. The movement of the interference pattern of the first sample beam is due to the refractive index change and environmental vibration factors, and the movement of the first reference beam is only due to environmental vibration factors. The relative movement of the two sets of fringes accurately characterizes the refractive index change caused by the incidence of the pump light.

[0029] Analyze the interference pattern to obtain the magnitude of the relative movement distance between the two sets of interference patterns.

[0030] Calculate the nonlinear refractive index of the sample to be measured based on the interference pattern.

[0031] The analysis device stores interference intensity diagrams of different sample beams and reference beams under the condition of increasing pump light intensity, as shown in Figure 2 shown below. Figure 2 Figure Figure 2 is a schematic diagram of the light intensity distribution after the interference of two groups of sample beams and reference beams in the embodiment of the present invention. The movement of the interference pattern of the sample beam is due to refractive index changes and environmental vibration factors, and the movement of the reference beam is only due to environmental vibration factors. The relative movement of the two sets of fringes accurately characterizes the refractive index change caused by the incident pump light.

[0032] The measurement principle of the embodiment of the present invention is based on the optical Kerr effect. The optical Kerr effect is a third-order nonlinear optical effect, which means that when strong light passes through certain substances, the refractive index of the substances will change with the change of light intensity. This effect originates from the nonlinear response of the medium, and the electron cloud distribution of the substance is distorted under the action of the strong light field. Specifically, the refractive index of the substance can be expressed as where is the refractive index of the substance under low light intensity, is the nonlinear refractive index, is the light intensity.

[0033] Since the pump light irradiates the sample to be measured, it causes the nonlinear response of the sample, changes the refractive index of the sample, and the resulting optical path difference can be expressed as:

[0034] where represents the change in optical path difference, is the change in the refractive index of the sample to be measured, is the thickness of the sample to be measured.

[0035] The change in optical path difference is characterized by the number of moving interference fringes, which can be expressed as:

[0036] where represents the number of moving interference fringes, is the laser wavelength emitted by the laser emitter in the first laser emission unit.

[0037] The analysis device converts the bright and dark interference pattern into the size distribution of the picture gray value along the horizontal direction. From the horizontal distribution of the gray value of the interference fringes, it can be obtained that:

[0038] where represents the number of moving interference fringes, is the width of the bright fringe, is the relative moving distance of the corresponding peaks of the two sets of interference fringes.

[0039] Eliminate , to obtain:

[0040] The nonlinear refractive index of the sample to be measured is:

[0041] Wherein, represents the nonlinear refractive index of the sample to be measured, and I represents the light intensity.

[0042] The analysis device converts the interference pattern with bright and dark fringes into the size distribution of the picture gray value along the horizontal direction, as Figure 3 shown, Figure 3 is the integral horizontal distribution diagram of the longitudinal gray value of the interference pattern in the embodiment of the present invention. The light and dark degrees of the black and white image are divided according to the gray value of 0-255. The larger the value, the higher the brightness. Figure 3 The abscissa in Figure 2 represents the horizontal pixels in Figure 3 The ordinate represents the total integral of the longitudinal gray value under this pixel. By calibrating the relative moving distance of the two groups of peaks in

[0043] 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. 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 will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A non - linear refractive index measurement system based on double - path interference, characterized in that, The system includes: a first laser emission unit, a first beam splitter, a second beam splitter, an interference device, a sample to be measured, a second laser emission unit, and a measurement unit; The first laser emission unit is used to generate a Gaussian beam and incident the Gaussian beam on the beam splitter; The first beam splitter is used to split the Gaussian beam into a sample beam and a reference beam that are parallel to each other; The second beam splitter is used to split the parallel sample beam and reference beam into a first sample beam and a first reference beam that are parallel to each other, and a second sample beam and a second reference beam that are parallel to each other; The first sample beam and the first reference beam are incident on the sample to be measured to obtain a target sample beam and a target reference beam; The second laser emission unit is used to generate a pump beam and incident the pump beam on the sample to be measured along the optical path of the first sample beam; The interference device is used to generate interference between the target sample beam and the second sample beam, and interference between the target reference beam and the second reference beam; The measurement unit is used to collect an interference image and analyze the target interference image to calculate the nonlinear refractive index of the sample to be measured.

2. The non-linear refractive index measurement system based on double-path interference according to claim 1, characterized in that The first laser emission unit includes: a laser emitter and a polarizer; The laser emitter is used to generate a fundamental mode Gaussian beam and emit the fundamental mode Gaussian beam to the polarizer; The polarizer is used to modulate the fundamental mode Gaussian beam to a horizontal polarization direction and transmit it to the first beam splitter.

3. A non-linear refractive index measurement system based on double-path interference according to claim 1, characterized in that, The first beam splitter includes: a beam splitting prism BS1, a mirror M1, a mirror M2, and a beam splitting prism BS2; The beam splitting prism BS1 is used to divide the Gaussian beam emitted from the first laser emission unit into a reflected beam L1 and a transmitted beam L2. The light that the reflected beam L1 is transmitted through the optical prism BS2 after being transmitted to the mirror M2 is the sample beam; The light that the transmitted beam L2 is reflected by the optical prism BS2 after being transmitted to the mirror M1 is the reference beam; And the sample beam and the reference beam are transmitted to the second beam splitter.

4. A non-linear refractive index measurement system based on double-path interference according to claim 1, characterized in that, The second beam splitter includes: a beam splitting prism BS3, a mirror M6, and a beam splitting prism BS5; The sample beam and the reference beam pass through the beam splitting prism BS3 to obtain the reflected beam and the transmitted beam of the sample beam, and the reflected beam and the transmitted beam of the reference beam. The transmitted beams of the sample beam and the reference beam are reflected by the beam splitting prism BS5 to obtain a first sample beam and a first reference beam; The reflected beams of the sample beam and the reference beam are reflected by the mirror M6 to obtain a second sample beam and a second reference beam.

5. A non-linear refractive index measurement system based on double-path interference according to claim 1, characterized in that The second laser emission unit includes: a laser emitter, a beam shaper, a polarizer, a mirror M3, and a mirror M4; The laser emitter is used to generate a pump light and emit the pump light to the beam shaper; The beam shaper is used to expand the pump light and transmit it to the polarizer; The polarizer is used to modulate the expanded pump light to a horizontal polarization direction and transmit it to the mirror M3 and the mirror M4; The mirrors M3 and M4 are used to reflect the pump light in the horizontal polarization direction to the beam splitter prism BS5, and make the reflected pump light in the horizontal polarization direction have the same optical path as the first sample beam.

6. The non-linear refractive index measurement system based on double-path interference according to claim 1, characterized in that, The interference device includes: a filter and a beam splitter prism BS4; The filter is arranged between the sample to be measured and BS4 and is used to filter the pump beam; The beam splitter prism BS4 is used to transmit the second sample beam and the second reference beam, reflect the target sample beam and the target reference beam, and transmit the target sample beam, the target reference beam, the second sample beam and the second reference beam to the measurement unit.

7. A non-linear refractive index measurement system based on double-path interference according to claim 1, characterized in that, The mirror surface direction of the mirror M6 is adjustable.

8. A non-linear refractive index measurement system based on double-path interference according to claim 3, characterized in that, Adjust the position of the mirror M1 so that the reference beam obtained after the reflected beam is reflected by the beam splitter prism BS2 is parallel to the sample beam, and the distance between the reference beam and the sample beam is 5 mm.

9. A method for measuring the nonlinear refractive index based on double - path interference, characterized in that, Implemented based on the system according to any one of claims 1-8, including: Construct a measurement optical path; Change the size of the pump light, record the intensity of the pump light, and simultaneously collect the interference patterns between the target sample beam and the second sample beam, and the interference patterns between the target reference beam and the second reference beam in the measurement optical path; Analyze the interference patterns to obtain the relative moving distance between the two sets of interference patterns; Calculate the nonlinear refractive index of the sample to be measured based on the interference patterns.

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