ABSOLUTE MEASUREMENT METHOD FOR THE PHASE OF THE COMPLEX COHERENCE COEFFICIENT

Abstract

A method for absolute phase measurement for complex coherence coefficient of an object comprises: firstly, processing fringe data measured by interferometry on any aperture pairs of an equivalent pupil plane of an optical imaging system to obtain an interference term and envelope and extreme values thereof; secondly, selecting extreme value A of light intensity from an interval of relatively stable extreme values change on one side of a curve envelope with zero optical path difference, and selecting two extreme values B and C of light intensity closest to extreme value A on other side of the curve envelope with zero optical path difference, where B>AC or C>AB; thirdly, counting number of fringe periods between A and C or A and B; and fourthly, according to the number of fringe periods, selecting a formula to calculate absolute phase of complex coherence coefficient of the object.

Claims

1. A method for an absolute measurement of a phase of a complex coherence coefficient, comprising providing an optical system comprising an equivalent pupil plane with a plurality of baselines, wherein each of the baselines comprises a pair of apertures at two end points P.sub.1 and P.sub.2 on the equivalent pupil plane, providing a light source that emits optical signals through optical paths to the equivalent pupil plane of the optical system, obtaining interference fringes through interference coupling for the optical signals received through the pair of apertures of the two end points P.sub.1 and P.sub.2 of the baseline on the equivalent pupil plane of the optical system, controlling an optical fiber retarder to sweep through a position of a zero optical path difference of two optical paths through a computer, performing DC removal and denoising on an interference curve of the interference fringes to obtain an interference term and envelope and extreme values thereof, and obtaining the absolute phase of the complex coherence coefficient based on selection and comparison of the extreme values on the interference curve, wherein the absolute phase is the absolute phase of the complex coherence coefficient of any of the baselines on the equivalent pupil plane of the optical system.

2. The method of claim 1, wherein the selection and comparison of the extreme values on the interference curve are based on maximum values, and the method further comprises selecting a relative maximum value A of a light intensity from an interval of relatively stable relative maximum values change in a main lobe on one side of the zero optical path of the interference curve, selecting two extreme values B and C of light intensity that are closest to the relative maximum value A from an interval of relatively stable extreme value change in the main lobe on the other side of the zero optical path, where B>AC, and counting a number of fringe periods between A and C, and when the number of fringe periods is an even number, calculating and obtaining the absolute phase $=\frac{A-C}{B-C},$ and when the number of fringe periods is an odd number, calculating and obtaining the absolute phase $=\left(1+\frac{A-C}{B-C}\right).$

3. The method of claim 1, wherein the selection and comparison of the extreme values on the interference curve are based on minimum values, and the method further comprises selecting a relative minimum value A of light intensity from an interval of relatively stable relative minimum values change in the main lobe on one side of the zero optical path of the interference curve, selecting two extreme values B and C of light intensity that are closest to the relative minimum value A from an interval of relatively stable extreme value change in the main lobe on the other side of the zero optical path, where C>AB, and counting the number of fringe periods between A and C, and when the number of fringe periods is an odd number, calculating and obtaining the absolute phase $=.Math.\frac{A-C}{B-C}.Math.,$ and when the number of fringe periods is an even number, calculating and obtaining the absolute phase $=\left(1+.Math.\frac{A-C}{B-C}.Math.\right).$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a flow chart showing the method for absolute phase measurement of an object complex coherence coefficient in the present invention.

[0035] FIG. 2 shows an optical path layout diagram of a target phase to be measured in a sub-aperture interference test in the present invention.

[0036] FIG. 3 shows the pattern of a target to be measured in the present invention.

[0037] FIG. 4 is an imitated diagram of an interference fringe without an effect of dispersion in the present invention, where the vertical axis represents DC-removed and normalized light intensity.

[0038] FIG. 5 shows the variation of the phase calculation value of a target to be tested with the position of an extreme value when there is no dispersion effect in the present invention, where the vertical axis represents phase/.

[0039] FIG. 6 shows an imitated diagram of an interference fringe with an effect of dispersion in the present invention, where the vertical axis represents DC-removed and normalized light intensity.

[0040] FIG. 7 shows the variation of the phase calculation value of a target to be tested with the position of an extreme value when there is an effect of dispersion, where the vertical axis represents phase/.

DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS

[0041] The following example shows a phase of a target to be measured in a sub-aperture interference test in the present invention.

[0042] An optical path layout diagram of simulated measurement is shown in FIG. 2. An extended light source enters into a collimator after passing through a target and is output after being collimated. Then, the light source is collected by aperture pairs with a specific spacing, then passes into a 21 coupler for interference through optical fiber and an optical fiber retarder, and finally is recorded by an optical power meter. During measurement, the interference signal is collected and recorded while controlling and adjusting the optical fiber retarder through a computer. The target is a black and white periodic fringe pattern with a period of 50 microns, and as shown in FIG. 3, the focal length of the collimator is 2260 mm, and the spacing between centers of the two apertures is 75 mm. Before testing, a position of the target is adjusted to make the apertures symmetrical to a central axis of the pattern in a field of view, and in this case, it is recorded as a zero position of the target. The light source of the target is turned on, and the optical fiber retarder is adjusted, so that it sweeps through the position of the zero optical path difference of two optical paths, and interference data is recorded after measurement. After that, the target is translated every 2.5 microns, and the optical fiber retarder is repeatedly adjusted, so that it sweeps through the position of the zero optical path difference of two optical paths, and interference data is recorded after measurement. The acquired raw data is filtered to remove DC for calculating and obtaining an interference term, and an intensity of the normalized interference term is further obtained.

[0043] Assuming that the optical path is not affected by dispersion, an intensity curve of the interference term DC-removed and normalized in a simulation test is shown in FIG. 4. According to a method based on measuring an extreme value of an interference fringe envelope disclosed above, phase values of the target to be measured at positions of 2.5 microns, 7.5 microns, 12.5 microns, 17.5 microns and 22.5 microns are calculated and obtained. Theoretical phase values should be 0.1, 0.3, 0.5, 0.7 and 0.9 respectively. Simulation results show that when the relative maximum value A is located at different positions of the optical path difference, the calculated phase values of the target to be measured are slightly different, as shown in FIG. 5.

[0044] Assuming that the optical path is affected by dispersion, a spectral response function is added in the simulation test. The resulting intensity curve of the interference term, DC-removed and normalized, is depicted in FIG. 6. According to the method based on measuring an extreme value of an interference fringe envelope disclosed above, phase values of the target to be measured at positions of 2.5 microns, 7.5 microns, 12.5 microns, 17.5 microns and 22.5 microns are calculated and obtained. Theoretical phase values should be 0.1, 0.3, 0.5, 0.7 and 0.9 respectively. Simulation results show that due to the influence of optical path dispersion, the calculated accuracy of the phase value of the target to be measured is related to the position of the relative maximum value A away from the zero optical path difference, that is, it is affected by relative stability of the extreme value change. The phase test accuracy in this case is slightly inferior to the measurement result without dispersion, as shown in FIG. 7. This problem can be solved by calibration before measurement. That is, before measuring a phase of the target to be measured, the phase of a standard target can be measured to determine an extreme value in an optical path difference interval with a small measurement error, thereby guiding the subsequent measurement and data processing of the phase of the target to be measured.

[0045] The foregoing is merely a preferred embodiment of the present disclosure and is not intended to limit the patent scope of the present disclosure. Any equivalent method or process transformation derived from the description of the present disclosure and the contents of the accompanying drawings, or directly or indirectly used in other related technical fields are similarly encompassed within the scope of patent protection of the present disclosure.

[0046] The method is simple in principle and is not affected by an external environment of an instrument and is widely applied.