An online self-calibration measurement method of liquid crystal polarization phase shifting type fizeau interferometer

Abstract

The application discloses an online self-calibration measurement method of a liquid crystal polarization phase-shifting Fizeau interferometer, which comprises the following steps: firstly, a software controls the voltage of a liquid crystal variable retarder and synchronously collects a series of phase-shifting interferograms at a CCD end; secondly, all the interferograms are preprocessed according to the light intensity variation of the series of phase-shifting interferograms to remove background information and normalize modulation degree; and finally, the results after column average of all the interferograms are subjected to Hilbert transform, and the phase-shifting amount of each interferogram relative to the first interferogram is calculated according to the obtained phase. The method can realize online in-situ self-calibration in the Fizeau interferometer quickly, improve the precision of phase-shifting and interference measurement, and can be used for the interferogram with few fringes.

Description

Technical Field

[0001] This invention belongs to the field of optical interferometric detection, specifically an online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer. Background Technology

[0002] Liquid crystals are a state of matter intermediate between crystals and liquids, possessing both the ordered molecular arrangement of crystals and the fluidity of liquids. Liquid crystal molecules are arranged in an ordered spatial order, exhibiting anisotropy in certain directions. The relatively weak intermolecular forces allow liquid crystals to rapidly adjust their molecular arrangement in response to changes in the external physical environment, thereby altering their optical properties. The most significant of these is the birefringence effect, 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-rays (light emitted by the electron beam) and e-rays (light emitted by the electron beam). These two types of light travel at different speeds within the liquid crystal, resulting in a phase difference. By changing the electric field to control the alignment of the liquid crystal molecules, the refractive index along the z-axis can be effectively controlled, thus modulating the light wave. Based on the tunable phase characteristic of nematic liquid crystals, variable retarders can be fabricated. By changing the applied voltage, the alignment of the liquid crystal molecules can be adjusted, thereby altering the phase difference between the o-ray and e-ray, achieving precise control over the amount of phase delay. Compared with traditional electro-optic crystal devices, liquid crystal variable delay devices also have advantages such as high transmittance, low loss and fast response, which makes them suitable for a wide range of applications in polarization modulation, interference phase shifting, optical communication and other fields.

[0003] In optical interferometry apparatuses, accurately calibrating the electrically controlled phase delay of a liquid crystal variable retarder is fundamental to achieving high-precision optical measurements. There are many methods for calibrating the phase delay of a liquid crystal variable retarder, including the light intensity method, the Stokes vector method, the Wollaston prism calibration method, and the Sagnac interferometry. However, none of these methods are feasible in the Fizeau interferometer apparatus; therefore, an online, in-situ calibration method is urgently needed. Summary of the Invention

[0004] This invention proposes an online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer, which can quickly achieve online in-situ self-calibration in the Fizeau interferometer, thereby improving the accuracy of phase shifting and interferometric measurements.

[0005] The technical solution to achieve the objective of this invention is as follows: 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 delay circuit by sending a voltage control command, and simultaneously acquire n consecutive original phase-shifting interferograms I at the CCD end. n (x,y).

[0007] Step 2: Based on n consecutive original phase-shifting interferograms I n The intensity variation in (x,y) is used to preprocess each original phase-shifted interferogram, removing background information and normalizing the modulation index to obtain the preprocessed phase-shifted interferogram.

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

[0009] Step 4: Calculate the Hilbert transform result I of the one-dimensional interference signal. H The phase angle of (x) is obtained, and the phase distribution φ represented by the nth original phase-shifted interferogram is obtained by unwrapping. 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-calibrated phase shift-voltage curve.

[0011] Compared with the prior art, the significant advantages of this invention are:

[0012] (1) This invention does not require rotation or the addition of polarization devices, and can realize online in-situ calibration of the phase shift of a liquid crystal polarization phase-shifting Fizeau interferometer. It has the characteristics of simple operation and high measurement accuracy.

[0013] (2) The present invention does not involve Fourier transform operations during implementation, so there are no special requirements for the carrier frequency in the phase-shifted interference diagram. It is applicable to cases with few fringes and has a wider range of applications.

[0014] (3) The related technologies of this invention can be further promoted and applied to other interferometric measurement devices involving liquid crystal polarization phase shifting, such as Michelson interferometers, Mach-Zehnder interferometers, etc., and have broad application prospects in the field of interferometric measurement. Attached Figure Description

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

[0016] Figure 2 A schematic diagram of the phase shift-voltage curve measured online by this invention. Detailed Implementation

[0017] The present invention will now be described in further detail with reference to the accompanying drawings.

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

[0019] Step 1: Adjust the voltage of the liquid crystal variable delay circuit and simultaneously acquire n consecutive original phase-shifting interferograms I. n (x,y):

[0020]

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

[0022] Step 2: Based on the light intensity changes of the above n original phase-shifting interferograms, preprocess each original phase-shifting interferogram to obtain the preprocessed phase-shifting interferogram.

[0023]

[0024] in, This represents the maximum light intensity at point (x,y) for the first to nth original phase-shifted interferograms. This represents the minimum light intensity at point (x,y) for the first to nth original phase-shifted interferograms.

[0025] Step 3: The preprocessed phase-shifting interferogram is then... The columns are added together and averaged to obtain a one-dimensional interference signal. right Perform a Hilbert transform to obtain the Hilbert transform result I of the one-dimensional interference signal. 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 The phase angle of (x) is obtained, and the phase distribution φ represented by the nth original phase-shifted interferogram is obtained by unwrapping. n (x) to obtain the phase shift calibration result δ n ′:

[0029]

[0030] Wherein, 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-calibrated phase shift-voltage curve.

[0032] It should be noted that in step 1, the voltage sampling of the liquid crystal variable delay device is controlled at equal intervals, with a sampling interval of 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 step 2 requires a large number of phase-shifting interferograms as data samples.

[0034] It should be noted that the phase shift calibration result obtained in step 4 is a wrapped phase 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 under the condition of few-fringe interference.

[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 starts from 0V and increases by 0.01V per second at sampling intervals. Simultaneously, 1001 consecutive raw phase-shifting interferograms are acquired at the CCD camera until the driving power supply voltage reaches the set maximum value of 10V. The 1001 raw phase-shifting interferograms acquired by the CCD camera are preprocessed to remove background information and normalize the modulation index, resulting in preprocessed phase-shifting interferograms. The columns of the preprocessed interferograms are summed and averaged to obtain a one-dimensional interference signal. A Hilbert transform is performed on this signal to obtain the Hilbert transform result. The phase angle of the Hilbert transform result is calculated, and the phase distribution represented by all raw phase-shifting interferograms is obtained by unwrapping the signal, yielding the phase shift calibration result. The above steps are repeated to obtain 10 sets of phase shift calibration results within the same voltage range, which are then averaged. Figure 2 The online self-calibration measurement phase shift-voltage curve was obtained, and the calibration results corresponding to typical voltage values ​​are shown in Table 1.

[0038] Table 1. Phase shift calibration results

[0039]

[0040] In summary, this invention proposes an online self-calibration method for liquid crystal polarization phase-shifting Fizeau interferometers. This method integrates image preprocessing for interferometric detection, Hilbert transform phase extraction, and unwrapping techniques. By synchronously controlling the voltage of the liquid crystal variable retarder and acquiring interferograms, it achieves high-precision in-situ calibration of the phase shift. The innovation of this method lies in its elimination of the need for Fourier transform and complex optical path adjustment. It eliminates background noise through normalized modulation and combines one-dimensional interference signal mean processing and phase unwrapping algorithms to simplify the operation process and improve the calibration stability of the liquid crystal variable retarder phase shift in scenarios with few fringe interference.

[0041] The technical features of the embodiments described above can be combined arbitrarily. For the sake of brevity, not all combinations of the technical features in the above embodiments are described. However, as long as the combinations of these technical features do not contradict each other, they should be considered within the scope of this specification.

[0042] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should 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, The steps are as follows: Step 1: Adjust the liquid crystal variable retarder voltage and synchronously acquire n consecutive raw phase-shifted interferograms I n (x,y): Where n represents the interferogram index value, a(x,y) represents the interferogram background, and b(x,y) represents the interferogram modulation degree. δ represents the relative phase distribution to be measured. n Let n be the phase shift of the nth interferogram; Step 2: Based on the light intensity changes of the above n original phase-shifting interferograms, preprocess each original phase-shifting interferogram to obtain the preprocessed phase-shifting interferogram. in, This represents the maximum light intensity at point (x,y) for the first to nth original phase-shifted interferograms. The minimum light intensity at point (x,y) for the first to nth original phase-shifted interferograms; Step 3: The preprocessed phase-shifting interferogram is then... The columns are added together and averaged to obtain a one-dimensional interference signal. right Perform a Hilbert transform to obtain the Hilbert transform result I of the one-dimensional interference signal. H (x): Where j is the imaginary factor, for The result after a 90° phase shift; Step 4: Calculate the one-dimensional Hilbert transform of the interference signal I H (x) and unwrap to obtain the phase distribution φ n (x) to obtain the phase shift calibration result δ n ′: Wherein, φ1(x) represents the phase distribution characterized 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-calibrated phase shift-voltage curve.

2. The online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that, In step 1, the voltage sampling is controlled by the liquid crystal variable delay device at equal intervals, with a sampling interval of 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.

3. The online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that, In step 4, the obtained phase shift calibration result is a wrapped phase 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 for a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that, Since Fourier transform is not required to extract phase information, there is no requirement for the number of fringes, and phase shift calibration can be achieved under the condition of few-fringe interference.

5. The online self-calibration measurement method for a liquid crystal polarization phase-shifting Fizeau interferometer according to claim 1, characterized in that, No additional conditions are required to achieve in-situ online detection.

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