CN114136465B - Instantaneous two-step phase-shift transverse shearing interferometry system and method - Google Patents

Abstract

The invention relates to an instantaneous two-step phase-shift transverse shear interferometry system and method. The problems of complex light path, high debugging difficulty, high cost, complex structure, poor stability and extremely easy introduction of extra errors in the prior art are solved. The invention is characterized in that a laser emergent light source is reflected by a microscope objective and a lens through a reflecting mirror, and then reaches a tested element through standard mirroring of a beam splitting prism, and then is vertically incident into a first crystal polarization beam splitter after passing through a polarizer, two light waves are respectively split into four linearly polarized light beams in the vertical direction, so that the two-to-two shearing of four wave surfaces is realized, each pixel-level micro polarization unit on the phase mask is in one-to-one matching correspondence with a pixel point on the CCD camera target surface through a phase mask arranged in front of the CCD camera target surface, and each two pixel-level micro polarization units are spliced and combined into a super pixel, and the light transmission axes of two adjacent micro polarization units are respectively along the x-axis direction and the y-axis direction.

Description

An instantaneous two-step phase-shift transverse shear interferometry system and method

Technical areas:

The invention relates to the technical field of optical measurement, and specifically relates to an instantaneous two-step phase shift transverse shear interferometry system and method.

Background technique:

Transverse shear interference technology uses a certain optical-mechanical system to divide the wavefront to be measured into two identical wavefronts to be measured with a certain lateral displacement in space. The application of this technology avoids the systematic errors introduced when using standard wavefronts in optical interference. It can also simplify the structure of the system to a certain extent and reduce the impact of the test system on the accuracy of optical components, thereby improving measurement accuracy.

In the existing technology, there are still problems such as inaccurate calculation of the shear amount caused by inaccurate reset of the shear elements, and low surface shape reconstruction accuracy caused by too few sampling points of the shear interference pattern. There are currently many methods to achieve lateral shear interference, such as the common-path and compact wavefront diagnosis system based on cross grating lateral shearing interferometer (Tong Ling, ect, "Common-path and compact wavefront diagnosis system based on cross grating lateral shearing interferometer". Applied OpticsVol .53, Issue 30, pp.7144-7152, 2014) proposed a common path compact diagnostic system for continuous and transient wavefront measurement based on a cross grating transverse shear interferometer. This system uses a cross grating as a shear The element uses a mask to allow only ±1 order diffracted light to pass through, achieving lateral shearing in both the x and y directions. The system has a simple structure and is easy to operate, but it has very high requirements for grating production accuracy and low lateral sampling resolution, resulting in great limitations in its image resolution.

For example, a single-shot freeform surface profiler (YONG BUM SEO,ect, "Single-shot freeformsurface profiler" Vol. 28, No. 3/3 February 2020, Optics Express, 3401-3409) proposed a method based on spatial phase shifting transverse A new method for measuring free-form surfaces using shear interferometry. Birefringent crystals are used to achieve shear interference, but the optical path construction is relatively complex. It is a non-common optical path system, which is difficult to debug and prone to additional errors. For example, in the patent "Phase-shifting transverse shear interferometer (200710045147.2)", "Polarization phase-shifting A polarization phase-shifting double shear interference wavefront measuring instrument and its detection method are described in "Double Shear Interference Wavefront Measuring Instrument and its Detection Method (200710047254.9)" and "Polarization Phase-Shifting Double Shear Interference Wavefront Measuring Instrument (200720075604.8)" , which uses a fixed wave plate and a rotating analyzer to form a phase-shifting system, and achieves shearing through two parallel flat plates. This instrument requires very high positioning accuracy for parallel plates, and achieves phase shift through a certain mechanical motion mechanism. Therefore, the acquisition of interference patterns is sensitive to changes in the system environment.

For example, the patent "A prism-based transverse shear interference spectrum imager and imaging method (202011643062.6)" describes a transverse shear interference spectrum imager and imaging method based on a right-angle reflection prism, which uses a tetrahedral prism to generate a The reference light and test light are displaced laterally, and one of the right-angle reflection prisms is installed on a one-dimensional displacer. The one-dimensional displacer can be used to control the right-angle reflection prism to move along its hypotenuse or right-angle side to change the shear amount. Therefore, certain motion mechanism support is required, so changes in the environment will affect the measurement results. The realization of shearing in the system needs to ensure that the four right-angle reflection prisms are in a certain spatial attitude. The reflection surface shape error and spatial positioning error of the right-angle reflection prism will reduce the measurement accuracy of the system.

To sum up, the problems of the existing technology are: the optical path of the interference method to achieve multi-directional lateral shearing is complex, and two or more optical paths need to be built to achieve multi-directional shearing, which is difficult to debug, high cost, complex structure, and stable Poor performance and easily introduce additional errors.

Contents of the invention:

The purpose of the present invention is to provide an instantaneous two-step phase shift transverse shear interferometry system and method to overcome the existing technology's complex optical paths, difficulty in debugging, high cost, complex structure, poor stability and easy introduction of additional errors. The problem.

In order to achieve the above purpose, the technical solution of the present invention is as follows: an instantaneous two-step phase shift transverse shear interferometry system, including a laser light source 1, a lens 2, an objective lens 3 and a plane mirror 4 arranged sequentially on the optical axis, and also It includes coaxially arranged optical element 5 under test, standard mirror 6, beam splitting prism 7, polarizer 8, first crystal polarizing beam splitter 9, first λ/4 wave plate 10, and second crystal polarizing beam splitter. 11. The second λ/4 wave plate 12, the pixel-level phase mask 13 and the CCD camera 14. The CCD camera 14 is connected to the computer; the light transmission axis of the polarizer 8 is 45° relative to the x-axis direction. , the angle between the fast axis direction of the first λ/4 wave plate and the positive direction of the x-axis is 45°, and the angle between the optical axis direction and the positive direction of the x-axis of the first crystal polarizing beam splitter 9 is 45° when placed horizontally. The two crystal polarizing beam splitters 11 are placed orthogonally thereto, and the angle between the fast axis direction of the second λ/4 wave plate and the positive direction of the x-axis is 90°; the phase mask 13 is installed in front of the target surface of the CCD camera 14 And consistent with the size of the target surface, each pixel-level micro-polarization unit on the phase mask 13 matches one-to-one with the pixel points on the target surface of the CCD camera 14, and every two pixel-level micro-polarization units are spliced and combined into one. For super pixels, the transmission axes of two adjacent micro-polarization units are along the x-axis direction and the y-axis direction respectively; the above two crystal polarization beam splitters are birefringent crystals.

The measurement method using the above-mentioned instantaneous two-step phase shift transverse shear interference device includes the following steps: Set the measured object 5 on the exit side of the standard mirror 6 on the main optical axis

① The incident light reflected by the object under test 5 passes through the standard mirror 6 and is imaged on the beam splitting prism 7, passes through the polarizer 8 and then enters the first crystal polarizing beam splitter 9;

②The two horizontal light waves emitted from the first crystal polarization beam splitter 9 pass through the first λ/4 wave plate 10 and become two beams of circularly polarized light with opposite rotation directions. The left and right circularly polarized lights pass through the second crystal polarization splitter again. The beam device 11 splits the beam vertically to form four beams of linearly polarized light;

③The linearly polarized light emitted in step ② passes through the second λ/4 wave plate 12, and the four emitted linearly polarized lights become two groups (two left-handed, two right-handed) of circularly polarized light with opposite rotation directions;

④The light waves emitted in step ③ interfere after passing through the phase mask 13 in front of the target of the CCD camera 14. Each pixel-level micro-polarization unit on the phase mask 13 interacts with each pixel on the imaging panel of the CCD camera 14. One-to-one matching, and every two pixel-level micro-polarizing units are spliced and combined into a super pixel. The angles of the light transmission axes of two adjacent micro-polarizing units are along the x-axis and y-axis directions respectively. The light waves pass through the phase mask. After version 13, phase shift interference occurs, and a transversely sheared polarization interference original image is obtained by the CCD camera 14.

Compared with the prior art, the beneficial effects of the present invention are:

(1) Easy to operate, the present invention includes a measurement system that does not require any mechanical movement, and can simultaneously obtain an instantaneous two-step phase shift transverse shear interference pattern with a fixed phase shift amount through a single exposure.

(2) In the process of obtaining the shear interference pattern, the shear interference and synchronous phase shift are realized through the common optical path optical system, and are suitable for wavefront measurement of low-coherence light. There is no need to build two or more optical paths and system debugging. Simple.

(3) The present invention uses a pixel phase mask to achieve the simultaneous acquisition of two lateral shear interference patterns with a fixed phase shift amount, without using mechanical phase shift mechanisms such as expensive piezoelectric transducers and linear converters. The transient interference measurement of the wave surface has strong anti-interference ability, improves the measurement speed and reduces the cost of measurement.

(4) A pixel-level micro-polarization unit array is used to achieve synchronous phase shift, which is simple, reliable, and highly computationally efficient.

(5) The shear amount of the polarizing crystal beam splitter is fixed, which avoids the shear amount calculation problem and reduces calculation errors.

Picture description:

Figure 1 is a schematic diagram of a system for achieving instantaneous two-step phase shift transverse shear interferometry.

Figure 2 is a schematic diagram of two crystal polarization beam splitters realizing instantaneous two-step phase shift transverse shear interference.

Figure 3 is a schematic diagram of the pixel phase mask corresponding to the pixel points on the CCD detector and the direction of its pixel-level micro-polarization unit array.

Figure 4 is the original polarization interference image collected by the CCD camera after passing through the mask.

Figure 5 is two transverse shear interference fringe patterns with a fixed phase shift obtained after mask mapping processing.

The reference symbols are explained as follows:

Laser light source 1, lens 2, objective lens 3, plane mirror 4, optical element under test 5, standard mirror 6, beam splitter prism 7, polarizer 8, first crystal polarizing beam splitter 9, first λ/4 wave plate 10, a second crystal polarizing beam splitter 11, a second λ/4 wave plate 12, a pixel phase mask 13, and a CCD camera 14.

Detailed ways:

The present invention will be described in detail below with reference to the drawings and examples.

The core idea of the present invention is to realize instantaneous two-step phase shift transverse shearing by combining two crystal polarizing beam splitters with birefringence effect.

Referring to Figure 1, the present invention provides an instantaneous two-step phase-shift transverse shear interferometry system, which includes a laser light source 1, a lens 2, an objective lens 3 and a plane mirror 4 arranged on the optical axis, and the main optical axis is sequentially coaxial. It is provided with the measured optical element 5, standard mirror 6, beam splitting prism 7, polarizer 8, first crystal polarizing beam splitter 9, first λ/4 wave plate 10, second crystal polarizing beam splitter 11, and Two λ/4 wave plates 12, a pixel-level phase mask 13 and a CCD camera 14. The CCD camera 14 is connected to the computer; the angle of the polarizer 8 is 45° relative to the x-axis direction, and the first λ/4 wave The angle between the fast axis direction of the film and the positive direction of the x-axis is 45°. When the first crystal polarizing beam splitter 9 is placed horizontally, the angle between the light axis direction and the positive direction of the x-axis is 45°. The second crystal polarizing beam splitter 11 Placed orthogonally to it, the angle between the fast axis direction of the second λ/4 wave plate and the positive direction of the x-axis is 90°; the phase mask 13 is installed in front of the target surface of the CCD camera 14 and is consistent in size with the target surface, Each pixel-level micro-polarization unit on the phase mask 13 is matched one-to-one with the pixels on the target surface of the CCD camera 14, and every two pixel-level micro-polarization units are spliced and combined to form a super pixel. The light transmission axes of the micro-polarizing units are along the x-axis direction and the y-axis direction respectively.

The measurement process of this system is as follows: the light source is emitted from the laser 1, passes through the microscope objective 2 and the lens 3 to realize the beam expansion collimation system, is reflected by the reflector 4, passes through the beam splitting prism 7 and the standard mirror 6, and reaches the component under test 5. The test wavefront returned from the surface of the component under test 5 passes through the polarizer 8 and then vertically enters the first crystal polarization beam splitter 9. Since the crystal polarization beam splitter has birefringence characteristics, the test wavefront is horizontally split into a certain beam when it exits. Two beams of transversely displaced linearly polarized light with vibration directions perpendicular to each other, namely o-light and e-light. The two beams of light waves pass through the first λ/4 wave plate 10. Since the angle between the fast axis direction of the λ/4 wave plate 10 and the positive x-axis direction is 45°, the two incident linearly polarized lights respectively become Left-hand and right-hand circularly polarized light. The circularly polarized light is vertically irradiated onto the second crystal polarizing beam splitter 11. After being split by the crystal polarizing beam splitter 11, the two circularly polarized lights are again split in the vertical direction into four linearly polarized lights. Realize pairwise shearing of four wave surfaces. The angle between the fast axis direction of the second λ/4 wave plate and the positive direction of the x-axis is 90°. The split light wave generates phase delay after passing through the second λ/4 wave plate 12. See Figure 3. The phase mask 13 in front of the target of the camera 14. Each pixel-level micro-polarization unit on the phase mask 13 matches one-to-one with the pixels on the target surface of the CCD camera 14, and every two pixel-level micro-polarization units The light transmission axes of two adjacent micro-polarization units are along the x-axis and y-axis directions respectively.

The principle analysis of the present invention is as follows:

Two beams of linearly polarized light with lateral displacement are emitted from the crystal polarizing beam splitter, namely o light and e light; when emitted, the phase difference between o light and e light is:

Among them, λ is the working wavelength of the crystal polarizing beam splitter, n _o and n _e are the refractive indexes of o light and e light respectively from the crystal polarizing beam splitter, L _o and L _e are the refractive indexes of o light and e light respectively. The optical path, L _o and L _e can be calculated based on the crystal thickness.

After the above-mentioned outgoing light passes through the phase mask 13, when it passes through the micro-polarizing unit with a transmission direction of 0°, the Jones vector expressions of the four light waves emitted from the second crystal polarizing beam splitter 11 are:

E ₂ =0(3)

E ₄ =0(5)

When passing through the micro-polarization unit with a transmission polarization direction of 90°, the Jones vector expressions of the four light waves emitted from the second crystal polarization beam splitter 11 are respectively:

E ₁ =0(6)

E ₃ =0(8)

These four light waves undergo phase-shift interference after passing through the pixel-level micro-polarizing unit array. When passing through the micro-polarizing unit with a transmission polarization direction of 0°, interference is generated by the superposition of the two light waves E ₁ and E _3. The interference pattern The light intensity expression can be expressed as:

When passing through a micro-polarizing unit with a transmission direction of 90°, the two light waves E ₂ and E ₄ are superimposed to produce interference. The light intensity expression of the interference pattern can be expressed as:

Among them, in formulas (10) and (11), is the initial phase carried by the surface shape of the component under test, and δ _c is the phase difference between the o-light and the e-light emitted from the crystal polarizing beam splitter, which is specifically given by formula (1).

Using an instantaneous two-step phase shift transverse shear interferometry method provided by the present invention, the measured object 5 is placed on the exit side of the standard mirror 6 on the main optical axis, which includes the following steps:

① The incident light reflected by the object under test 5 passes through the standard mirror 6 and is imaged on the beam splitting prism 7, passes through the polarizer 8 and then enters the first crystal polarizing beam splitter 9;

②The two horizontal light waves emitted from the first crystal polarization beam splitter 9 pass through the first λ/4 wave plate 10 and become two beams of circularly polarized light with opposite rotation directions. The left and right circularly polarized lights pass through the second crystal polarization splitter again. The beam device 11 splits the beam vertically to form four beams of linearly polarized light;

③The linearly polarized light emitted in step ② passes through the second λ/4 wave plate 12, and the four emitted linearly polarized lights become two groups (two left-handed, two right-handed) of circularly polarized light with opposite rotation directions;

④The light waves emitted in step ③ interfere after passing through the phase mask 13 in front of the target of the CCD camera 14. Each pixel-level micro-polarization unit on the phase mask 13 interacts with each pixel on the imaging panel of the CCD camera 14. One-to-one matching, and every two pixel-level micro-polarizing units are spliced and combined to form a super pixel. The light transmission axes of two adjacent micro-polarizing units are along the x-axis direction and the y-axis direction respectively. The light waves undergo phase shift interference after passing through the phase mask 13, and a transversely sheared polarization interference original image is obtained by the CCD camera 14.

Referring to Figure 5, using the original transversely sheared polarization interference pattern obtained by the present invention, two transversely sheared interference fringe patterns a and b can be simultaneously obtained through corresponding mask acquisition processing, and the two obtained interference fringes are There is a certain fixed phase shift between the pictures: Among them, δ _c is the phase difference between o light and e light emitted by the crystal polarization beam splitter, which is given by formula (1).

Claims (3)

1. An instantaneous two-step phase-shift transverse shear interferometry system, characterized by: including a laser light source (1), a lens (2), an objective lens (3) and a plane mirror (4) arranged sequentially on the optical axis, It also includes a coaxially arranged optical element under test (5), a standard mirror (6), a beam splitter prism (7), a polarizer (8), a first crystal polarizing beam splitter (9), a first λ/4 Wave plate (10), second crystal polarization beam splitter (11), second λ/4 wave plate (12), pixel-level phase mask (13) and CCD camera (14), the CCD camera ( 14) Connected to a computer; the transmittance axis of the polarizer (8) is 45° relative to the x-axis direction, the angle between the fast axis direction of the first λ/4 wave plate and the positive direction of the x-axis is 45°, and the When the first crystal polarizing beam splitter (9) is placed horizontally, the angle between the light axis direction and the positive x-axis direction is 45°. The second crystal polarizing beam splitter (11) is placed orthogonally to it. The fast wavelength of the second λ/4 wave plate The angle between the axis direction and the positive direction of the x-axis is 90°; the phase mask (13) is installed in front of the target surface of the CCD camera (14) and is consistent in size with the target surface. The pixel-level micro-polarizing units are matched one-to-one with the pixels on the target surface of the CCD camera (14), and every two pixel-level micro-polarizing units are spliced and combined into a super pixel. The transmission of two adjacent micro-polarizing units is The optical axes are along the x-axis direction and the y-axis direction respectively. 2. An instantaneous two-step phase shift transverse shear interferometry system according to claim 1, characterized in that: the first crystal polarization beam splitter (9) and the second crystal polarization beam splitter (11) are double refractive crystals. 3. A measurement method using the instantaneous two-step phase shift transverse shear interferometry system according to claim 1, characterized in that it includes the following steps: setting the measured optical element (5) on a standard on the main optical axis The exit side of the mirror (6); ①The incident light reflected by the optical element under test (5) passes through the standard mirror (6) and is imaged on the beam splitter prism (7), passes through the polarizer (8), and then enters the first crystal polarizing beam splitter (9) ; ②The two horizontal light waves emitted from the first crystal polarizing beam splitter (9) pass through the first λ/4 wave plate (10) and become two beams of circularly polarized light with opposite rotation directions. The left and right circularly polarized light passes through the first λ/4 wave plate (10) again. The two crystal polarizing beam splitters (11) split the beam vertically to form four beams of linearly polarized light; ③The linearly polarized light emitted in step ② passes through the second λ/4 wave plate (12), and the four beams of linearly polarized light emitted are respectively transformed into two sets of circularly polarized light with opposite rotational directions. The two groups of circularly polarized light with opposite rotational directions are Two beams of polarized light are left-handed and the other two are right-handed; ④The light waves emitted in step ③ interfere after passing through the phase mask (13) in front of the target of the CCD camera (14). Each pixel-level micro-polarization unit on the phase mask (13) interacts with the CCD camera (14) Each pixel on the imaging panel matches one by one, and every two pixel-level micro-polarizing units are spliced and combined into a super pixel. The angles of the light transmission axes of two adjacent micro-polarizing units are along the x-axis and y-axis respectively. In the axial direction, the light wave undergoes phase shift interference after passing through the phase mask (13), and a transversely sheared polarization interference original image is obtained by the CCD camera (14).