A nonlinear refractive index measurement system and method based on two-beam interference

Abstract

The application provides a nonlinear refractive index measurement system and method based on double optical path interference, and relates to the technical field of nonlinear optical measurement. The system comprises a first laser emitting unit, a first light splitting device, a second light splitting device, an interference device, a sample to be measured, a second laser emitting unit and a measurement unit. The first laser emitting unit generates a Gaussian beam. The first light splitting device splits the Gaussian beam into sample beams and reference beams which are parallel to each other. The second light splitting device splits the sample beams and the reference beams which are parallel to each other into two paths. The first sample beams and the reference beams are incident on the sample to be measured. The second laser emitting unit generates a pump beam, and the pump beam is incident on the sample to be measured along the optical path of the first sample beams. The interference device generates interference between the two sample beams and interference between the two reference beams. The measurement unit collects interference images, analyzes the interference images, and calculates the nonlinear refractive index of the sample to be measured. The system is not easily affected by the environment.

Description

Technical Field

[0001] This invention relates to the field of nonlinear optical measurement technology, and in particular to a nonlinear refractive index measurement system and method based on dual-path interference. Background Technology

[0002] Commonly used methods for measuring nonlinear refractive index using dual-path interferometry mainly include:

[0003] Z-scan method: A Gaussian laser beam is focused onto the sample, which is moved along the optical axis (Z-axis). 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, the presence of the nonlinear refractive index causes self-focusing or self-defocusing of the beam, resulting in changes in the far-field light intensity distribution and thus altering the transmittance. The Z-scan method requires precise control of the sample movement; otherwise, significant errors will be introduced. Moreover, this method can only measure the overall nonlinear refractive index of the sample and cannot obtain information on the spatial distribution of the refractive index.

[0004] The Mach-Zehnder interferometry uses a beam splitter to split a beam of light into two beams, which then pass through different optical paths before being recombined to generate interference. One beam passes through the sample under test, while the other serves as a reference beam. When the sample has a nonlinear refractive index, the phase of the light passing through the sample changes, causing the interference fringes to shift. The nonlinear refractive index of the sample is calculated by measuring the shift of the interference fringes. The Mach-Zehnder interferometry requires extremely high stability of the optical path; even minute vibrations or air disturbances can affect the stability of the interference fringes, thus affecting the measurement results. Adjusting the optical path is complex, requiring precise control of the optical path difference and polarization state of the two beams.

[0005] 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 then reflected by mirrors and recombined for interference. One beam passes through the optical path containing the sample to be measured. Changes in the sample's nonlinear refractive index cause a change in the optical path, resulting in a shift in the interference fringes. The nonlinear refractive index of the sample is determined by measuring the amount of this shift. The Michelson interferometry has relatively low measurement sensitivity and may struggle to accurately measure weak changes in nonlinear refractive index. Furthermore, the accuracy and stability of the mirrors significantly affect the measurement results, requiring regular calibration and maintenance.

[0006] Environmental factors have a significant impact on measurement results. Current interferometric methods for measuring nonlinear refractive index have poor stability and accuracy and are greatly affected by the environment. Z-scan emission cannot measure the nonlinear refractive index of thick samples. Summary of the Invention

[0007] The purpose of this invention is to address the problems of poor stability and susceptibility to environmental influences in existing nonlinear refractive index detection technologies by proposing a nonlinear refractive index measurement system based on dual-optical-path interference. The system includes: a first laser emitting unit, a first beam splitting device, a second beam splitting device, an interference device, a sample to be tested, a second laser emitting unit, and a measurement unit.

[0008] The first laser emitting unit is used to generate a Gaussian beam and incident the Gaussian beam onto the beam splitter.

[0009] 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.

[0010] The second beam splitter is used to split the parallel sample beam and reference beam into a first parallel sample beam and a first parallel reference beam, and a second parallel sample beam and a second parallel reference beam; the first sample beam and the first reference beam are incident on the sample to be tested to obtain the target sample beam and the target reference beam.

[0011] The second laser emitting unit is used to generate a pump beam and incident the pump beam onto the sample to be tested along the optical path of the first sample beam;

[0012] The interference device is used to generate interference between the target sample beam and the second sample beam, as well as interference between the target reference beam and the second reference beam.

[0013] The measurement unit is used to acquire interference images, analyze the target interference images, and calculate the nonlinear refractive index of the sample under test.

[0014] Optionally, the first laser emitting unit includes: a laser emitter and a polarizer;

[0015] The laser emitter is used to generate a fundamental Gaussian beam and emit the fundamental Gaussian beam onto a polarizer;

[0016] The polarizer is used to modulate the fundamental mode Gaussian beam into a horizontal polarization direction and transmit it to the first beam splitter.

[0017] Optionally, the first beam splitting device includes: beam splitting prism BS1, reflector M1, reflector M2, and beam splitting prism BS2;

[0018] The beam splitter BS1 is used to split the Gaussian beam emitted from the first laser emitting unit into a reflected beam L1 and a transmitted beam L2, and to transmit the reflected beam L1 and the transmitted beam L2 to the reflector M2 and the reflector M1, respectively. The light transmitted from the reflected beam L1 to the reflector M2 and then through the beam splitter BS2 is the sample beam; the light transmitted from the transmitted beam L2 to the reflector M1 and then through the beam splitter BS2 is the reference beam; and the sample beam and the reference beam are transmitted to the second beam splitter.

[0019] Optionally, the second beam splitting device includes: beam splitting prism BS3, reflector M6, and beam splitting prism BS5;

[0020] The sample beam and the reference beam pass through the beam splitter 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 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 reflector M6 to obtain the second sample beam and the second reference beam.

[0021] Optionally, the second laser emitting unit includes: a laser emitter, a beam shaper, a polarizer, a reflector M3, and a reflector M4;

[0022] The laser emitter is used to generate pump light and emit the pump light to a beam shaper;

[0023] The beam shaper is used to expand the pump light and then transmit it to the polarizer;

[0024] The polarizer is used to modulate the expanded pump light into a horizontal polarization direction and transmit it to the reflectors M3 and M4.

[0025] The reflectors M3 and M4 are used to reflect the horizontally polarized pump light to the beam splitter BS5, and to make the reflected horizontally polarized pump light have the same optical path as the first sample beam.

[0026] Optionally, the interference device includes: a filter and a beam splitter BS4;

[0027] The filter is placed between the sample to be tested and BS4 to filter the pump beam.

[0028] The beam splitter 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.

[0029] Optionally, the mirror orientation of the M6 ​​is adjustable.

[0030] Optionally, the position of the reflector M1 can be adjusted so that the reference beam obtained after the reflected beam is reflected by the beam splitter BS2 is parallel to the sample beam, and the distance between the reference beam and the sample beam is 5mm.

[0031] This invention also proposes a nonlinear refractive index measurement method based on dual-path interference, which, based on the above-described system implementation, includes:

[0032] Construct the measurement optical path;

[0033] The magnitude of the pump light is changed and its intensity is recorded. At the same time, the interference patterns between the target sample beam and the second sample beam, as well as the interference patterns between the target reference beam and the second reference beam, are collected in the measurement optical path.

[0034] Analyze the interferograms to obtain the relative displacement distance between the two sets of interferograms;

[0035] The nonlinear refractive index of the sample under test is calculated based on the interferogram.

[0036] The beneficial effects of the technical solution provided by this invention are:

[0037] This invention uses a first laser emitting unit to incident a Gaussian beam onto a beam splitter, which splits the Gaussian beam into two parallel sample beams and a reference beam. One sample beam and the reference beam are simultaneously incident onto the sample under test. A second laser emitting unit sends a pump beam along the optical path of the sample beam onto the sample under test, changing the intensity of the pump beam to alter the refractive index of the sample. The relative movement of the two sets of interference fringes—the target sample beam and the other sample beam, and the target reference beam and the other reference beam—characterizes the magnitude of the refractive index change. The measurement unit analyzes the two sets of interference images of the sample beam and the reference beam to calculate the nonlinear refractive index, thereby effectively avoiding the poor stability of single-path interferometry devices and enabling the measurement of the nonlinear refractive index of thick samples. Attached Figure Description

[0038] Figure 1 This is a structural diagram of a nonlinear refractive index measurement system based on dual-optical-path interference according to an embodiment of the present invention;

[0039] Figure 2 This is a schematic diagram of the light intensity distribution after interference of two sets of sample beams and a reference beam in an embodiment of the present invention;

[0040] Figure 3 This is a transverse distribution diagram of the longitudinal grayscale value integral of the interference pattern in an embodiment of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0042] A structural diagram of a nonlinear refractive index measurement system based on dual-path interferometry according to an embodiment of the present invention is shown below. Figure 1 It includes: a first laser emitting unit, a first beam splitting device, a second beam splitting device, an interferometer, a sample to be tested, a second laser emitting unit, and a measurement unit.

[0043] A first laser emitting unit is used to generate a Gaussian beam and incident the Gaussian beam onto the beam splitter. The first laser emitting unit includes a laser emitter and a polarizer. The laser emitter generates a fundamental mode Gaussian beam and emits it onto the polarizer. The polarizer modulates the fundamental mode Gaussian beam into a horizontal polarization direction and transmits it to the first beam splitter. In a further embodiment, the laser emitter may be a helium-neon laser.

[0044] The first beam splitter is used to split the horizontally polarized fundamental mode Gaussian beam into a parallel sample beam and a reference beam. The first beam splitter includes: a beam splitter prism BS1, a reflector M1, a reflector M2, and a beam splitter prism BS2. Beam splitter BS1 splits the horizontally polarized fundamental mode Gaussian beam emitted from the first laser emitting unit into a reflected beam L1 and a transmitted beam L2, and transmits the reflected beam L1 and transmitted beam L2 to reflector M2 and reflector M1, respectively. The reflected beam L1 is reflected by reflector M2 and then transmitted through beam splitter BS2 to obtain the sample beam. The transmitted beam L2 is reflected by reflector M1 and then reflected by beam splitter BS2 to obtain the reference beam. The position of reflector M1 is adjusted so that the reference beam obtained after reflection by beam splitter BS2 is parallel to the sample beam, with a distance of 5 mm between the two beams. The sample beam and reference beam are then transmitted to the second beam splitter.

[0045] The second beam splitter is used to split a parallel sample beam and a reference beam into a first parallel sample beam and a first parallel reference beam, and a second parallel sample beam and a second parallel reference beam. The first sample beam and the first reference beam are incident on the sample to be tested, resulting in a target sample beam and a target reference beam. The second beam splitter includes: a beam splitter prism BS3, a reflector M6, and a beam splitter prism BS5. The sample beam and the reference beam pass through the beam splitter prism BS3 to obtain a reflected beam and a transmitted beam of the sample beam, and a reflected beam and a transmitted beam of the reference beam. The transmitted beams of the sample beam and the reference beam are reflected by the beam splitter 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 reflector M6 to obtain the second sample beam and the second reference beam. The mirror orientation of the reflector M6 is adjustable.

[0046] The second laser emitting unit generates a pump beam and directs it along the optical path of the first sample beam onto the sample under test. The second laser emitting unit includes a laser emitter, a beam shaper, a polarizer, and mirrors M3 and M4. The laser emitter generates pump light and emits it to the beam shaper. The beam shaper expands the pump light and transmits it to the polarizer. The polarizer modulates the expanded pump light into a horizontally polarized direction and transmits it to mirrors M3 and M4. Mirrors M3 and M4 reflect the horizontally polarized pump light to the beam splitter BS5, ensuring that the reflected horizontally polarized pump light follows the same optical path as the first sample beam.

[0047] When the sample is subjected to strong laser incident light, it produces a nonlinear response, which changes the refractive index of the sample and causes the interference fringes of the sample beam to shift.

[0048] An interferometer is used to generate interference between the target sample beam and the second sample beam, as well as interference between the target reference beam and the second reference beam. The interferometer includes a filter and a beam splitter BS4. The filter is positioned between the sample to be measured and BS4 to filter the pump beam and prevent it from incident on the measurement unit. The beam splitter BS4 transmits the second sample beam and the second reference beam, reflects the target sample beam and the target reference beam, and transmits the target sample beam, the target reference beam, the second sample beam, and the second reference beam to the measurement unit.

[0049] A measurement unit is used to acquire interferometric images and analyze the target interferometric images to calculate the nonlinear refractive index of the sample under test. In a further embodiment, the measurement unit may include an acquisition device and an analysis device. In some embodiments, the acquisition device may be a CCD camera. The analysis device may be a measurement device based on dual-path interferometry for nonlinear refractive index measurement, which has data processing, network communication, and program execution functions, such as a computer, or other devices or equipment capable of performing the same or similar functions.

[0050] This invention also proposes a nonlinear refractive index measurement method based on dual-path interference, which, based on the above-described system implementation, includes:

[0051] (1) Construct the measurement optical path according to the above system, and refer to the optical path to be measured. Figure 1 Furthermore, it also requires peripheral devices with data processing, network communication, and program execution functions.

[0052] (2) Multiple sets of experiments were conducted at room temperature, and the size of the pump light was changed. For example, in one set of experiments, the wavelength of the first laser emitting unit was adjusted to 532nm and the wavelength of the second laser emitting unit was adjusted to 780nm. The intensity of the pump light was recorded, and the interference pattern between the target sample beam and the second sample beam in the measurement optical path, as well as the interference pattern between the target reference beam and the second reference beam, were collected.

[0053] It is understandable that the second laser emitting unit incident the pump beam along the optical path of the sample beam onto the sample to be tested, changing the intensity of the pump beam and thus changing the refractive index of the sample to be tested. The movement of the interference pattern of the first sample beam is due to the change in refractive index and environmental vibration factors, while 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 change in refractive index caused by the incident pump light.

[0054] The interference patterns are analyzed to obtain the relative displacement distance between the two sets of interference patterns.

[0055] The nonlinear refractive index of the sample under test is calculated based on the interferogram.

[0056] The analytical device stores interference intensity diagrams of different sample beams and reference beams as the pump light intensity increases, such as... Figure 2 As shown, Figure 2 This is a schematic diagram of the light intensity distribution after interference of two sets of sample beams and a reference beam in an embodiment of the present invention. The movement of the interference pattern of the sample beam is due to changes in refractive index and environmental vibration factors, while the movement of the reference beam is due to environmental vibration factors only. The relative movement of the two sets of fringes accurately characterizes the change in refractive index caused by the incident pump light.

[0057] The measurement principle of this invention is based on the optical Kerr effect, a third-order nonlinear optical effect that refers to the change in the refractive index of a material as strong light passes through it. This effect originates from the nonlinear response of the medium; the electron cloud distribution of the material is distorted under the influence of a strong light field. Specifically, the refractive index of the material... It can be represented as ,in It is the refractive index of a substance under low light intensity. It is a nonlinear refractive index. This represents light intensity.

[0058] The pump light irradiates the sample, causing a nonlinear response that alters its refractive index. The resulting optical path difference can be expressed as:

[0059]

[0060] in, This represents the change in optical path difference. The change in the refractive index of the sample under test. The thickness of the sample to be tested.

[0061] The change in optical path difference, characterized by the number of shifts in interference fringes, can be expressed as:

[0062]

[0063] in The number of shifts in the interference fringes. The wavelength of the laser emitted by the laser emitter in the first laser emitting unit.

[0064] The analysis equipment converts the contrasting light and dark interference patterns into an image with grayscale values ​​distributed horizontally. From the horizontal distribution of the grayscale values ​​of the interference fringes, we can obtain:

[0065]

[0066] in The number of shifts in the interference fringes. The width of the bright stripe. The relative shift distance between the corresponding peak values ​​of the two sets of interference fringes.

[0067] eliminate ,get:

[0068]

[0069] The nonlinear refractive index of the sample under test is:

[0070]

[0071] in, I represents the nonlinear refractive index of the sample under test, and I represents the light intensity.

[0072] The analysis equipment converts the interferometric pattern of contrasting light and dark areas into a horizontal distribution of grayscale values ​​in the image, such as... Figure 3 As shown, Figure 3 This is the horizontal distribution map of the longitudinal grayscale value integral of the interference pattern in this embodiment of the invention. The brightness of the black and white image is divided into grayscale values ​​from 0 to 255, with larger values ​​indicating higher brightness. Figure 3 The horizontal axis represents Figure 2 The horizontal axis represents the pixel, and the vertical axis represents the total integral of the vertical grayscale values ​​at that pixel, thus defining the... Figure 3 The change in refractive index caused by the incident pump light can be obtained by measuring the relative shift distance between the two sets of peaks.

[0073] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A nonlinear refractive index measurement system based on two-beam interference, characterized in that, The system comprises a first laser emitting unit, a first light splitting device, a second light splitting device, an interference device, a sample to be measured, a second laser emitting unit and a measuring unit; The first laser emitting unit is configured to generate a Gaussian beam and to input the Gaussian beam into the first light splitting device; The first light splitting device is configured to split the Gaussian beam into a sample beam and a reference beam which are parallel to each other; The second light splitting device is configured to split the sample beam and the reference beam which are parallel to each other into a first sample beam and a first reference beam which are parallel to each other, and a second sample beam and a second reference beam which are parallel to each other; the first sample beam and the first reference beam are input into the sample to be measured to obtain a target sample beam and a target reference beam; The second laser emitting unit is configured to generate a pump beam and to input the pump beam into the sample to be measured along the light path of the first sample beam; The interference device is configured 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 measuring unit is configured to acquire an interference image and analyze the interference image to calculate the nonlinear refractive index of the sample to be measured.

2. The nonlinear refractive index measurement system based on dual optical path interference according to claim 1, characterized in that, The first laser emitting unit comprises a laser emitter and a polarizer; The laser emitter is configured to generate a fundamental mode Gaussian beam and to emit the fundamental mode Gaussian beam into the polarizer; The polarizer is configured to modulate the fundamental mode Gaussian beam into a horizontal polarization direction and to transmit the fundamental mode Gaussian beam to the first light splitting device.

3. The nonlinear refractive index measurement system based on dual optical path interference according to claim 1, characterized in that, The first light splitting device comprises a light splitting prism BS1, a mirror M1, a mirror M2 and a light splitting prism BS2; The light splitting prism BS1 is configured to split the Gaussian beam emitted by the first laser emitting unit into a reflected beam L1 and a transmitted beam L2; the reflected beam L1 is transmitted to the mirror M2, and the light transmitted through the light splitting prism BS2 after passing through the mirror M2 is the sample beam; the transmitted beam L2 is transmitted to the mirror M1, and the light reflected by the light splitting prism BS2 after passing through the mirror M1 is the reference beam; and the sample beam and the reference beam are transmitted to the second light splitting device.

4. The nonlinear refractive index measurement system based on dual optical path interference according to claim 1, characterized in that, The second light splitting device comprises a light splitting prism BS3, a mirror M6 and a light splitting prism BS5; The sample beam and the reference beam pass through the light splitting prism BS3 to obtain a reflected beam and a transmitted beam of the sample beam, and a reflected beam and a 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 light splitting 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.

5. The nonlinear refractive index measurement system based on dual optical path interference according to claim 1, wherein, The second laser emitting unit comprises a laser emitter, a beam shaper, a polarizer, a mirror M3 and a mirror M4; The laser emitter is configured to generate a pump beam and to emit the pump beam into the beam shaper; The beam shaper is configured to expand the pump beam and transmit the expanded pump beam to the polarizer; The polarizer is configured to modulate the expanded pump beam into a horizontal polarization direction and transmit the expanded pump beam to the mirror M3 and the mirror M4; The mirror M3 and the mirror M4 are used for reflecting the horizontal polarization direction pump light to the beam splitter BS5, and making the reflected horizontal polarization direction pump light same with the first sample light beam path.

6. The nonlinear refractive index measurement system based on dual optical path interference according to claim 1, wherein, The interference device comprises a filter and a beam splitter BS4; The filter is arranged between the sample to be measured and the beam splitter BS4, and is used for filtering the pump light beam; The beam splitter BS4 is used for transmitting the second sample light beam and the second reference light beam, reflecting the target sample light beam and the target reference light beam, and transmitting the target sample light beam, the target reference light beam, the second sample light beam and the second reference light beam to the measurement unit.

7. The nonlinear refractive index measurement system based on dual optical path interference according to claim 1, characterized in that, The mirror M6 is adjustable.

8. The nonlinear refractive index measurement system based on dual optical path interference according to claim 3, characterized in that, The position of the mirror M1 is adjusted so that the reference light beam obtained by reflecting the light beam through the beam splitter BS2 is parallel to the sample light beam, and the distance between the reference light beam and the sample light beam is 5mm.

9. A method of nonlinear refractive index measurement based on two-beam interference, characterized in that, The system is implemented based on any one of claims 1-8, comprising: Constructing a measurement light path; Changing the size of the pump light, recording the light intensity of the pump light, and collecting the interference patterns between the target sample light beam and the second sample light beam, and the interference patterns between the target reference light beam and the second reference light beam in the measurement light path; Analyzing the interference patterns to obtain the relative moving distance of the two groups of interference patterns; Calculating the nonlinear refractive index of the sample to be measured based on the interference patterns.

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