Online self-calibration measurement method of liquid crystal polarization phase-shifting Fizeau interferometer

Abstract

The invention discloses an online self-calibration measuring method of a liquid crystal polarization phase-shifting Fizeau interferometer, which comprises the following steps: firstly, controlling software to adjust the voltage of a liquid crystal variable retarder and synchronously acquiring a series of phase-shifting interferograms at a CCD (Charge Coupled Device) end; all the interferograms are preprocessed according to light intensity changes of the series of phase-shifting interferograms so as to remove background information and normalize modulation degrees; and finally, Hilbert transformation is performed on a result obtained after column averaging of all the interferograms, and the phase shifting amount of each interferogram relative to the first interferogram is calculated according to the obtained phase. According to the method, online in-situ self-calibration can be quickly realized in the Fizeau interference device, the precision of phase shift and interference measurement is improved, and the method can be used for interferograms with few fringes.

Description

Technical Field

[0001] The present invention belongs to the field of optical interference detection, and in particular relates to an online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer. Background Art

[0002] Liquid crystal is a state of matter between crystals and liquids, combining the ordered arrangement of crystal molecules with the fluidity of liquids. Liquid crystal molecules are arranged in an orderly pattern in space, and their physical properties exhibit anisotropy in certain directions. The relatively weak interactions between liquid crystal molecules allow them to rapidly adjust their molecular arrangement in response to changes in the external physical environment, thereby altering their optical properties. The most notable of these changes is birefringence, a property particularly pronounced in nematic liquid crystals. Under the influence of an electric field, the molecular arrangement of nematic liquid crystals changes. When light passes through the liquid crystal, it is split into o-light and e-light. These two types of light propagate at different speeds within the liquid crystal, resulting in a phase difference. By varying the electric field to manipulate the alignment of the liquid crystal molecules, the refractive index along the z-axis can be effectively controlled, thereby modulating the lightwave. This phase-modulating property of nematic liquid crystals allows the creation of liquid crystal variable retarders. By varying the applied voltage, the alignment of the liquid crystal molecules can be manipulated, thereby varying the phase difference between o-light and e-light, enabling precise control of the phase delay of the lightwave. Compared with traditional electro-optical crystal devices, liquid crystal variable retarders also have the advantages of high transmittance, low loss and fast response, which makes them have broad application scenarios in polarization modulation, interference phase shifting, optical communication and other fields.

[0003] In optical interferometry, accurately calibrating the electrically controlled phase delay of liquid crystal variable retarders (LCVRTs) is essential for achieving high-precision optical measurements. Numerous methods exist for calibrating the phase delay of LCVRTs, including the light intensity method, the Stokes vector method, the Wollaston prism calibration method, and the Sagnac interferometry method. However, none of these methods are feasible in Fizeau interferometry. Therefore, a method for in-situ online calibration is urgently needed. Summary of the Invention

[0004] The present invention proposes an online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer, which can quickly realize online in-situ self-calibration in a Fizeau interferometer measurement device and improve the accuracy of phase shift and interference measurement.

[0005] The technical solution for achieving the purpose of the present invention is: an online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer, comprising the following steps:

[0006] Step 1: Adjust the voltage of the liquid crystal variable retarder by sending voltage control instructions, and synchronously collect n consecutive original phase-shifting interference patterns I at the CCD end. n (x,y).

[0007] Step 2: Based on n consecutive original phase-shifting interference patterns I n (x, y) light intensity changes, pre-process each original phase-shift interferogram, remove background information and normalize the modulation index to obtain the pre-processed phase-shift interferogram

[0008] Step 3: Preprocess the phase-shifted interferogram The columns are added and averaged to obtain a one-dimensional interference signal right Perform Hilbert transform to obtain the one-dimensional interference signal Hilbert transform result I H (x).

[0009] Step 4: Calculate the Hilbert transform result I of the one-dimensional interference signal H (x), and unpack to obtain the phase distribution φ represented by the nth original phase-shifting interferogram n (x), to obtain the phase shift calibration result δ n ′.

[0010] Step 5: The phase shift of the liquid crystal polarization phase-shifting Fizeau interferometer is calibrated multiple times and averaged to obtain an accurate online self-calibration phase shift-voltage curve.

[0011] Compared with the prior art, the present invention has the following significant advantages:

[0012] (1) The present invention does not require rotating or adding polarization devices, and can realize the online in-situ calibration of the phase shift amount of the liquid crystal polarization phase-shifting Fizeau interferometer, which has the characteristics of simple operation and high measurement accuracy.

[0013] (2) The implementation of the present invention does not involve Fourier transform operations, so there is no special requirement for the carrier frequency in the phase-shift interferogram. It can be applied to the case of few fringes and has a wider range of applications.

[0014] (3) The relevant technology of the present invention can be further promoted and applied to other interference measurement devices involving liquid crystal polarization phase shift, such as Michelson interferometer, Mach-Zehnder interferometer, etc., and has broad application prospects in the field of interference measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Flowchart of the online self-calibration measurement method for the liquid crystal polarization phase-shifting Fizeau interferometer.

[0016] Figure 2 Schematic diagram of the phase shift-voltage curve measured by online self-calibration of the present invention. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below with reference to the accompanying drawings.

[0018] Combine Figure 1 The online self-calibration measurement method of a liquid crystal polarization phase-shifting Fizeau interferometer described in the present invention comprises the following steps:

[0019] Step 1: Adjust the voltage of the liquid crystal variable retarder and synchronously collect n consecutive original phase-shifting interferograms I n (x,y):

[0020]

[0021] Where n represents the interference pattern index value, a(x,y) represents the interference pattern background, and b(x,y) represents the interference pattern modulation degree. Represents the relative phase distribution to be measured, δ n is the phase shift of the nth interference pattern.

[0022] Step 2: Based on the light intensity changes of the n original phase-shifting interferograms, pre-process each original phase-shifting interferogram to obtain the pre-processed phase-shifting interferogram

[0023]

[0024] in, is the maximum light intensity of the 1st to nth original phase-shifting interference patterns at point (x, y), is the minimum light intensity of the 1st to nth original phase-shifting interference patterns at point (x, y).

[0025] Step 3: The above pre-processed phase-shifted interferogram The columns are added and averaged to obtain a one-dimensional interference signal right Perform Hilbert transform to obtain the one-dimensional interference signal Hilbert transform result I H (x):

[0026]

[0027] Where j is the imaginary factor, for The result after a 90° phase shift.

[0028] Step 4: Calculate the Hilbert transform result I of the one-dimensional interference signal H (x), and unpack to obtain the phase distribution φ represented by the nth original phase-shifting interferogram n (x), to obtain the phase shift calibration result δ n ′:

[0029]

[0030] Among them, φ1(x) is the phase distribution represented by the first original phase-shifting interferogram.

[0031] Step 5: The phase shift of the liquid crystal polarization phase-shifting Fizeau interferometer is calibrated multiple times and averaged to obtain an accurate online self-calibration phase shift-voltage curve.

[0032] It should be noted that in step 1, the voltage sampling is performed at equal intervals by controlling the liquid crystal variable retarder, and the sampling interval is 0.01V / 0.02V, which is adjusted according to the accuracy requirements; and the pitch and tilt of the Fizeau interferometer standard flat crystal are adjusted to make the interference fringes vertically distributed.

[0033] It should be noted that the premise for implementing step 2 is to have a large number of phase-shifted interferograms as data samples.

[0034] It should be noted that in step 4, the phase shift calibration result obtained is a wrapped phase wrapped between (-π, π], so an additional unwrapping operation is required to obtain the final phase shift-voltage curve.

[0035] It should be noted that this method does not require the extraction of phase information through Fourier transform, so there is no requirement for the number of fringes, and phase shift calibration can be achieved in the case of few fringe interferences.

[0036] Example 1

[0037] The driving power supply of the liquid crystal variable retarder in the liquid crystal polarization phase-shifting Fizeau interferometer to be calibrated is set to start from 0V, and the voltage increases every second with a sampling interval of 0.01V, and 1001 continuous original phase-shifted interference patterns are synchronously collected on the CCD camera end until the driving power supply voltage of the liquid crystal variable retarder reaches the set maximum value of 10V; the 1001 original phase-shifted interference patterns collected by the CCD camera are preprocessed to remove background information and normalize the modulation index to obtain the preprocessed phase-shifted interference pattern; the columns of the preprocessed interference pattern are added and averaged to obtain a one-dimensional interference signal, which is Hilbert transformed to obtain the Hilbert transform result of the one-dimensional interference signal; the phase angle of the Hilbert transform result of the one-dimensional interference signal is calculated, and the phase distribution represented by all the original phase-shifted interference patterns is unpacked to obtain the phase shift calibration result; the above steps are repeated to obtain 10 groups of phase shift calibration results in the same voltage range and average them, such as Figure 2 , the online self-calibration measurement phase shift-voltage curve is obtained, where the calibration results corresponding to the typical voltage values are shown in Table 1.

[0038] Table 1 Phase shift calibration results

[0039]

[0040] In summary, the present invention proposes an online self-calibration method for a liquid crystal polarization phase-shifting Fizeau interferometer. This method combines image preprocessing, Hilbert transform phase extraction, and unwrapping techniques for interference detection. By synchronously regulating the voltage of the liquid crystal variable retarder and collecting the interference pattern, high-precision in-situ calibration of the phase shift is achieved. The innovation of this method lies in the fact that it does not rely on Fourier transforms and complex optical path adjustments. It eliminates background noise by normalizing the modulation index, and combines one-dimensional interference signal mean processing with a phase unwrapping algorithm to simplify the operation process and improve the stability of the liquid crystal variable retarder phase shift calibration in low-fringe interference scenarios.

[0041] The technical features of the above-described embodiments may be combined in any manner. For the sake of brevity, not all combinations of the technical features in the above-described embodiments are described. However, as long as there are no contradictions in the combination of these technical features, they should be considered to be within the scope of this specification.

[0042] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. An online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer, characterized in that: Here are the steps: Step 1: Adjust the voltage of the liquid crystal variable retarder and synchronously collect n consecutive original phase-shifting interferograms I n (x,y): Where n represents the interference pattern index value, a(x,y) represents the interference pattern background, and b(x,y) represents the interference pattern modulation degree. Represents the relative phase distribution to be measured, δ n is the phase shift of the nth interference pattern; Step 2: Based on the light intensity changes of the n original phase-shifting interferograms, pre-process each original phase-shifting interferogram to obtain the pre-processed phase-shifting interferogram in, is the maximum light intensity of the 1st to nth original phase-shifting interference patterns at point (x, y), is the minimum light intensity value of the 1st to nth original phase-shifting interference patterns at point (x, y); Step 3: The above pre-processed phase-shifted interferogram The columns are added and averaged to obtain a one-dimensional interference signal right Perform Hilbert transform to obtain the one-dimensional interference signal Hilbert transform result I H (x): Where j is the imaginary factor, for The result after 90° phase shift; Step 4: Calculate the Hilbert transform result I of the one-dimensional interference signal H (x), and unpack to obtain the phase distribution φ represented by the nth original phase-shifting interferogram n (x), to obtain the phase shift calibration result δ n ′: Among them, φ1(x) is the phase distribution represented by the first original phase-shifting interferogram; Step 5: The phase shift of the liquid crystal polarization phase-shifting Fizeau interferometer is calibrated multiple times and averaged to obtain an accurate online self-calibration phase shift-voltage curve.

2. The online self-calibration measurement method of a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that: In step 1, the voltage sampling of the liquid crystal variable retarder is controlled at equal intervals, with the sampling interval being 0.01V / 0.02V and adjusted according to the accuracy requirements; and the pitch and tilt of the Fizeau interferometer standard flat crystal are adjusted to make the interference fringes vertically distributed.

3. The online self-calibration measurement method of a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that: In step 4, the phase shift calibration result obtained is a wrapped phase wrapped between (-π, π], so an additional unwrapping operation is required to obtain the final phase shift-voltage curve.

4. The online self-calibration measurement method of a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that: There is no need for Fourier transform to extract phase information, so there is no requirement for the number of fringes, and phase shift calibration can be achieved with few fringe interferences.

5. The online self-calibration measurement method of a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that: In-situ online detection can be achieved without introducing other conditions.

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