CN105352915B - A kind of dynamic measurement method of refractive index Two dimensional Distribution - Google Patents

Abstract

The invention relates to a dynamic measurement method for two-dimensional distribution of refractive index, which combines digital holographic interferometry and total internal reflection. The phase shift of the reflected light in the process of total internal reflection is related to the incident angle of the incident light and the refractive index of the media on both sides of the interface of the total internal reflection prism. The second exposure digital holographic interferometry is used to dynamically measure the hypotenuse surface of the total internal reflection prism. The phase shift difference of the reflected light before and after the sample is measured, and according to the relationship between the phase shift difference of the reflected light and the refractive index of air and the sample, the two-dimensional refractive index dynamic distribution of the sample can be directly calculated. This method only needs the refractive index of the sample to ensure that the total internal reflection of the incident light occurs at the interface of the total internal reflection prism, so a wide range of refractive index distribution measurement can be realized.

Description

A Dynamic Measuring Method of Two-Dimensional Distribution of Refractive Index

technical field

The invention relates to a dynamic measurement method for two-dimensional distribution of refractive index, in particular to a method for dynamically measuring two-dimensional distribution of refractive index by using the phase shift characteristic of reflected light during total internal reflection combined with digital holographic interferometry.

Background technique

Refractive index is an important optical parameter, and its accurate measurement is very important in the fields of material analysis, biochemical sensing, and optical component parameter design. At present, the methods used to measure the refractive index include the natural collimation method, the minimum deflection angle method, etc., and these methods are all based on the laws of refraction and reflection. The traditional instrument for measuring the refractive index is the refractometer, but it needs to be calibrated in advance. In recent years, a variety of refractive index sensors based on special optical fiber devices have been widely used due to their high sensitivity and high measurement accuracy, but their measurement range is small and complex manufacturing processes are required. In addition, both refractometers and fiber optic refractive index sensors can only measure the refractive index of homogeneous materials. However, the measurement object in the actual situation is often non-uniform, and the existing methods are difficult to measure its refractive index distribution, especially for the dynamic refractive index distribution of some measurement objects in chemical and physical processes. Someone also proposed a method that can realize the dynamic measurement of the two-dimensional distribution of the refractive index in a large range (Y.Chu, et al. "Full-field refractive index measurement with simultaneous phase-shift interferometry," Optik 125(13), 3307-3310( 2014).), but this method requires a complex optical path structure and cumbersome data processing methods, which brings great inconvenience to practical applications.

Contents of the invention

technical problem to be solved

In order to avoid the deficiencies of the prior art, the present invention proposes a dynamic measurement method for the two-dimensional distribution of the refractive index, which uses the phase shift characteristics of the reflected light during total internal reflection and combines digital holographic interferometry to dynamically measure the two-dimensional distribution of the refractive index. Dimensional distribution, the involved optical path structure is simple, and the subsequent data processing method is simple.

Technical solutions

A dynamic measurement method for two-dimensional distribution of refractive index, characterized in that the steps are as follows:

Step 1: Divide horizontally polarized or vertically polarized parallel light into two beams, one of which is incident from a right-angled side of a right-angle prism, and then undergoes total internal reflection at the interface between the hypotenuse of the prism and the air at an incident angle θ, Then emerge from the other right-angled side as the object light wave and reach the target surface of the image acquisition device;

Another beam of parallel light as a reference light wave meets and interferes with the object light wave on the target surface of the image acquisition device, and is recorded by the image acquisition device to obtain a reference digital hologram H ₀ ;

Step 2: Place the sample to be tested in the reflection area of the object light wave on the surface of the hypotenuse of the rectangular prism, and continue recording to obtain N digital holograms H _i , i=1, 2, 3...N;

Step 3: According to Kirchhoff's diffraction theory, use the computer to simulate the diffraction propagation process of light waves numerically, and reconstruct the holograms H ₀ and H _i respectively to obtain the complex amplitude distribution of the original light wave, and further reconstruct the holograms H 0 and H _i The phase distribution of the object light wave is different from the phase distribution of the object light wave reconstructed from H ₀ , and the phase difference distribution of the object light wave after placing the sample is calculated Δφ _o (x, y);

Step 4: Due to the difference in the refractive index distribution of the sample on the hypotenuse surface of the prism, the reflected light wave will produce an additional phase shift difference, and the phase difference distribution of the object light wave obtained in step 3 is equal to the additional phase shift difference distribution of the reflected light. According to the additional phase shift difference distribution of the reflected light The relationship between the shift distribution and the refractive index distribution of the sample is obtained to obtain the two-dimensional refractive index distribution of the sample:

When horizontally polarized parallel light is used, the additional phase shift difference generated by the reflected light wave is:

When vertically polarized parallel light is used, the additional phase shift difference generated by the reflected light wave is:

Where: n ₁ is the refractive index of the prism, and n ₂ (x, y) is the refractive index distribution of the sample to be measured.

Beneficial effect

The present invention proposes a dynamic measurement method for two-dimensional distribution of refractive index. When a beam of parallel light undergoes total internal reflection on the surface of a prism, the reflected light will generate an additional phase shift (i.e. phase change), and the magnitude of the phase shift value is related to the incident angle θ is related to the refractive index (n ₁ , n ₂ ) of the medium on both sides of the prism interface. When the incident angle θ and the prism refractive index n ₁ are determined, the prism-air and prism-sample interfaces are measured by digital holographic interferometry with double exposure The additional phase shift difference distribution of the reflected light, and the two-dimensional refractive index distribution of the sample can be obtained according to the relationship between the additional phase shift difference distribution and the refractive index of the sample and air.

The present invention proposes a dynamic measurement method for the two-dimensional distribution of the refractive index, which utilizes the advantages of fast, high-precision and full-field dynamic measurement of digital holographic interferometry, introduces a total internal reflection prism into the measurement optical path, and can dynamically measure the two dimensions of the refractive index. dimension distribution. The entire measurement system involved has a simple structure, and a subsequent data processing method is convenient. Since the refractive index of air is known, the refractive index distribution of the measured sample can be directly calculated from the measured reflected light additional phase shift difference distribution, which overcomes the need for the initial refractive index of the measured sample to be measured by digital holographic interferometry alone. Disadvantages of distribution. The refractive index of the sample to be measured only needs to satisfy the total internal reflection of the incident light at the interface of the total internal reflection prism, so this method can realize the measurement of the refractive index in a wide range.

Description of drawings

Figure 1: is the optical path diagram of the present invention to dynamically measure the two-dimensional refractive index distribution;

Fig. 2: is the two-dimensional refractive index distribution figure at 9.2s when measuring liquid mixing process in the embodiment;

In the figure: 1-semiconductor pumped solid-state laser, 2-fiber coupler, 3-first optical fiber, 4-fiber beam splitter, 5-second optical fiber, 6-third optical fiber, 7-first collimating lens, 8-second collimating lens, 9-first half-wave plate, 10-second half-wave plate, 11-right-angle prism, 12-mirror, 13-beam splitting prism, 14-image acquisition device, 15-sample.

Detailed ways

Now in conjunction with embodiment, accompanying drawing, the present invention will be further described:

Embodiment: The optical path of the dynamic measurement method of a kind of refractive index two-dimensional distribution designed by the present invention is as shown in Figure 1, comprises: semiconductor pumped solid-state laser 1, fiber coupler 2, first optical fiber 3, optical fiber beam splitter 4 , the second optical fiber 5, the third optical fiber 6, the first collimating lens 7, the second collimating lens 8, the first half-wave plate 9, the second half-wave plate 10, the rectangular prism 11, the mirror 12, the dichroic prism 13 , image acquisition device 14 , sample 15 .

The workflow of the dynamic measurement method for the two-dimensional distribution of the refractive index is as follows:

Step 1: A beam of horizontally or vertically polarized parallel light is incident from one right-angled side of a right-angled prism, and then undergoes total internal reflection at the interface between the hypotenuse of the prism and the air at an incident angle θ, and then emerges from the other right-angled side as the object The light wave reaches the target surface of the image acquisition device;

Step 2: Another beam of parallel light with the same polarization direction from the same laser is used as a reference light wave to meet and interfere with the above-mentioned object light wave on the target surface of the image acquisition device, and the reference digital hologram _H0 is recorded by the image acquisition device;

Step 3: Place the sample to be tested close to the reflection area of the object light wave on the surface of the hypotenuse of the prism. Due to the difference in the refractive index distribution of the sample on the surface of the hypotenuse of the prism, the reflected light wave will produce an additional phase shift difference distribution, and its magnitude is related to the incident angle θ and the prism The refractive index n ₁ and n ₂ of the medium on both sides of the interface are related:

(1) If the original light wave is polarized along the horizontal direction, then when total internal reflection occurs, the polarization plane is perpendicular to the incident plane, which can be recorded as s-polarization, and the additional phase shift difference distribution at this time is

(2) If the original light wave is polarized along the vertical direction, then when total internal reflection occurs, the polarization plane is parallel to the incident plane, which can be recorded as p polarization, and the additional phase shift difference distribution at this time is

Step 4: The additional phase shift difference distribution generated by the above reflected light wave will cause the object light wave to carry the corresponding phase difference distribution Δφ _o (x, y), which reflects the information of the two-dimensional refractive index distribution of the sample, keeping the reference light wave without Change, continuously shoot N digital holograms H _i (i=1,2,3...N) during the change of sample refractive index;

Step 5: According to Kirchhoff's diffraction theory, use the computer to simulate the diffraction propagation process of light waves numerically, reconstruct the holograms H ₀ and H _i respectively, obtain the complex amplitude distribution of the original light wave, and further reconstruct the holograms H 0 and H _i The phase distribution of the object light wave is different from the phase distribution of the object light wave reconstructed from H ₀ , and the phase difference distribution of the object light wave after placing the sample is calculated Δφ _o (x, y);

Step 6: Since the phase difference distribution of the object light wave is equal to the additional phase shift difference distribution of the reflected light, according to the relationship between the additional phase shift difference distribution of the reflected light and the refractive index distribution of the sample, the two-dimensional refractive index distribution of the sample is finally obtained:

(1) If the original light wave is polarized along the horizontal direction, then the expression of n ₂ is

in,

(2) If the original light wave is polarized along the vertical direction, then the expression of n ₂ is

in,

Specific examples:

The linearly polarized light emitted by the semiconductor pumped solid-state laser 1 (wavelength is 532nm) is coupled into the first optical fiber 3 through the fiber coupler 2, and the optical fiber beam splitter 4 is arranged at the end of the first optical fiber 3, which can split the light beam into the first light beam and the second light beam; wherein the first light beam becomes the parallel light of horizontal polarization after the first collimating lens 7 and the first half-wave plate 9, and it enters the rectangular prism 11 (K9 glass, n ₁ as the object light wave with 45°) =1.5195) and total internal reflection (θ=72.7332°) takes place at the center of its hypotenuse surface, then it is reflected by reflector 12 and reaches dichroic prism 13, and the hypotenuse surface of rectangular prism 11 goes up along the horizontal direction and It is adjusted to a certain height to ensure that the outgoing beam is coaxial with the incident beam; the second beam passes through the second collimator lens 8 and the second half-wave plate 10 and becomes horizontally polarized parallel light as a reference light wave; 13 are combined and interfered on the target surface of the image acquisition device 14 (number of pixels: 1280H×960V, pixel size: 4.4 μm) at a certain angle to form an off-axis digital hologram.

Before placing the sample on the hypotenuse surface of the right-angle prism 11, take a hologram as a reference digital hologram H ₀ ; 112 series of holograms H _i (i=1,2,3...112) reflecting the dynamic changes of the two-dimensional refractive index distribution n ₂ (x,y) of the sample are continuously shot at a frame rate of 7.5fps; using digital holographic numerical reconstruction algorithms and The digital holograms taken by the phase subtraction method are numerically reconstructed respectively, and the phase difference distribution Δφ _oi (x, y) of the object light wave is obtained during the dynamic change of the sample; according to the phase difference distribution of the object light wave and the additional phase shift difference distribution of the reflected light , the relationship between the additional phase shift difference distribution of reflected light and the refractive index distribution of the sample (formula (3)), and finally obtain the dynamic change of the two-dimensional refractive index distribution of the measured sample.

The present method combines digital holographic interferometry with total internal reflection. The phase shift of the reflected light in the process of total internal reflection is related to the incident angle of the incident light and the refractive index of the medium on both sides of the interface of the total internal reflection prism. Using double exposure digital holographic interferometry to dynamically measure the hypotenuse surface of the total internal reflection prism. The phase shift difference of the reflected light before and after the sample is measured, and according to the relationship between the phase shift difference of the reflected light and the refractive index of air and the sample, the dynamic distribution of the two-dimensional refractive index of the sample can be directly calculated. This method only needs the refractive index of the sample to ensure that the total internal reflection of the incident light occurs at the interface of the total internal reflection prism, so a wide range of refractive index distribution measurement can be realized.

Claims (1)

1. A dynamic measurement method of two-dimensional distribution of refractive index, characterized in that the steps are as follows: Step 1: Divide the vertically polarized parallel light into two beams, one of which is incident from one right-angled side of the right-angle prism, and then undergoes total internal reflection at the interface between the hypotenuse of the prism and the air at an incident angle θ, and then passes through the other After exiting from the right-angle side, it reaches the target surface of the image acquisition device as the object light wave; Another beam of parallel light as a reference light wave meets and interferes with the object light wave on the target surface of the image acquisition device, and is recorded by the image acquisition device to obtain a reference digital hologram H ₀ ; Step 2: Place the sample to be tested in the reflection area of the object light wave on the surface of the hypotenuse of the rectangular prism, and continue recording to obtain N digital holograms H _i , i=1, 2, 3...N; Step 3: According to Kirchhoff's diffraction theory, use the computer to simulate the diffraction propagation process of light waves numerically, and reconstruct the holograms H ₀ and H _i respectively to obtain the complex amplitude distribution of the original light wave, and further reconstruct the holograms H 0 and H _i The phase distribution of the object light wave is different from the phase distribution of the object light wave reconstructed from H ₀ , and the phase difference distribution of the object light wave after placing the sample is calculated Δφ _o (x, y); Step 4: Due to the difference in the refractive index distribution of samples on the hypotenuse surface of the prism, the reflected light wave will produce an additional phase shift difference, and the phase difference distribution of the object light wave obtained in step 3 is equal to the additional phase shift difference distribution of the reflected light wave. According to the additional phase shift difference distribution of the reflected light wave The relationship between the shift distribution and the refractive index distribution of the sample is obtained to obtain the two-dimensional refractive index distribution of the sample: When vertically polarized parallel light is used, the additional phase shift difference generated by the reflected light wave is: Replace the Δφ _p (x, y) of the above formula with the calculated phase difference distribution of the object light wave after placing the sample Δφ _o (x, y), and obtain the two-dimensional refractive index distribution of the sample in: Where: n ₁ is the refractive index of the prism, and n ₂ (x, y) is the refractive index distribution of the sample to be measured.