CN101776488B - Method for measuring optical phase by using synchronous phase-shifting interference method and implementing device - Google Patents

Abstract

A method and device for obtaining optical phase by synchronous phase-shifting interference method and realizing optical path, relating to a method and device for dynamically detecting wave surface information. In the present invention, the semi-transparent and semi-reflective prism and the polarization beam-splitter prism are used to split the light source into six beams, and at the same time, combined with the method of polarization interference, six synchronous phase-shifting interference fringe images are collected in the air domain at one time, and the six interference fringes are respectively imaged on six CCDs. on target. The shape of the test wavefront can be obtained by analyzing the six interference fringe patterns. Since the influence of vibration is consistent with the influence of the synchronous phase-shifting interference fringe pattern, the influence of vibration can be eliminated by subtraction and division in the phase-shifting algorithm, thus realizing high-precision wave surface measurement suitable for more working environments. And there is no need to perform PZT linearity correction before each measurement. Moreover, the method of the present invention is simple, the principle is clear, each component in the optical path is easy to obtain, and the whole device is compact and easy to use.

Description

Method and device for measuring optical phase by synchronous phase-shifting interference method

technical field

The invention relates to a method and a device for dynamically detecting wavefront information, in particular to a method for obtaining an optical phase by using a synchronous phase-shifting interference method and realizing an optical path.

Background technique

In optical wavefront interferometry, the laser is usually used as a coherent light source, and two wavefronts are generated by dividing the amplitude. One wavefront is projected to the standard reference mirror and returned to form a reference wavefront, and the other wavefront is projected to the device under test and returned to form a reference wavefront. The test wavefront, the reference wavefront and the test wavefront meet in space to form an interference field, and the digital image acquisition device records the interference field in the form of an interference fringe pattern, and the shape of the test wavefront can be obtained by analyzing the interference fringe pattern. At present, the method of optical phase shift measurement is widely used in domestic and foreign commercialized laser wavefront interferometers. In this method, the piezoelectric ceramic transducer (PZT) is used as an actuating device for optical phase shifting. The standard reference mirror is connected to the PZT device through an elastic structure. When a step voltage is applied to the PZT, the PzT produces a step displacement. This displacement is transmitted to the standard reference mirror through the elastic structure, so that the phase of the reference wavefront can be changed in steps, that is, the optical phase shift can be realized (usually the phase shift step is 90°), and some amplitude shifts can be obtained by collecting the interference field during the phase shift process. Interference fringe pattern. The accuracy of wavefront measurement can be further improved by analyzing multiple phase-shifted interference fringe patterns. However, this PZT optical phase shift is a time-domain phase shift. When there is vibration in the test environment, the phase-shifted interferograms collected at different times will introduce phase errors. This error is usually random and difficult to eliminate. It It is the main source of error in time-domain phase-shifting interferometry. Even if the influence of environmental vibration is not considered, the interferometer must perform linear correction on the voltage displacement characteristic of PZT before each measurement to ensure the accuracy of phase shift during the measurement process. In view of the fact that time-domain PZT optical phase shifting is not suitable for on-site interferometry with vibration, the most direct idea is to generate several phase-shifting interference fields in the air domain at the same time, and record these interference fields synchronously to obtain several synchronous phase-shifting fields. Interference fringe plot. Since the influence of vibration is consistent with the influence of the synchronous phase-shifting interference fringe pattern, the influence of vibration can be eliminated by subtraction and division in the phase-shifting algorithm, thus realizing high-precision wave surface measurement suitable for more working environments and PZT linearity correction is not required before each measurement. According to the research of JamesE.Millerd and NealJ.Brock et al. in 2001, the holographic grating was used to split the beam into four beams, and combined with the polarization interference method, four interference fringe patterns with a phase shift of 90 degrees were simultaneously obtained in space. This method obtained better measurement results. However, due to the use of holographic gratings for light splitting, in order not to appear unnecessary diffracted light and not to change the wavefront of incident light, the requirements for the grating manufacturing process are relatively high, so the overall cost of this type of interferometer is relatively high.

Contents of the invention

In order to overcome the disadvantages of the time-domain measurement principle of the conventional phase-shifting laser plane interferometer, the present invention provides an optical path device that can measure the optical phase by using the synchronous phase-shifting interferometry method. The method of six split beams combined with polarization interference collects six synchronous phase-shifting interference fringe patterns at one time in the airspace, and these six interference fringes are respectively imaged on six CCD target surfaces.

In order to achieve the above object, the present invention adopts the following method and realizes the optical path of obtaining six synchronous phase-shifting interference fringe patterns:

Using the method of synchronous phase-shifting interferometry to measure the optical phase, two beams of wavefronts are generated by the method of dividing the amplitude, one beam of wavefronts is projected to the standard reference mirror and returns to form a reference wavefront, and the other beam of wavefronts is projected to the DUT and returned to form a test wavefront , the reference wavefront and the test wavefront meet in space to form an interference field, and the digital image acquisition device records the interference field in the form of an interference fringe pattern, and the shape of the test wavefront can be obtained by analyzing the interference fringe pattern. The specific method is: using a linearly polarized plane light source as the light source, the light source is divided into two beams of transmitted light I ₁ and reflected light I ₂ perpendicular to each other by the first semi-transparent prism; wherein the transmitted light I ₁ passes through the second semi-transparent The half-reflective prism is then divided into two beams of transmitted light I ₁₁ and reflected light I ₁₂ perpendicular to each other; the transmitted light I ₁₁ is reflected back by the measured mirror, and then divided into two beams of transmitted light perpendicular to each other by the second half-reflective prism I ₁₁₁ and reflected light I ₁₁₂ ; wherein the transmitted light I ₁₁₁ light returns to the first half-reflective prism, and is divided into two beams of mutually perpendicular transmitted light and reflected light I ₁₁₁₂ by the first half-reflective prism; wherein The reflected light I ₁₁₁₂ is reflected by the standard reference mirror through the one-eighth wave plate and returns to be divided into two beams of mutually perpendicular transmitted light I ₁₁₁₂₁ and reflected light I ₁₁₁₂₂ by the first half-transparent prism again; wherein the transmitted light I ₁₁₁₂₁ is polarized by The beam-splitting prism divides two beams of transmitted light I ₁₁₁₂₁₁ and reflected light I _{111212 whose} propagation directions and polarization states are perpendicular to each other, wherein the reflected light I ₁₁₁₂₁₂ and the reflected light I ₂₂ divided by the polarization beam-splitting prism superimpose and interfere, and the first A graph of interference fringes. The transmitted light I ₁₁₁₂₁₁ and the reflected light I ₂ are superimposed and interfered with the transmitted light I ₂₁ split by the polarizing beam splitter, and the second interference fringe pattern is obtained. The reflected light I ₁₁₁₂₂ is divided into two beams of transmitted light I ₁₁₁₂₂₁ and reflected light I ₁₁₁₂₂₂ which are perpendicular to each other through the second half-reflective prism again; the transmitted light I ₁₁₁₂₂₁ is reflected back by the measured mirror again, and passes through the second semi-reflective prism again. The transflective prism is divided into two beams of mutually perpendicular transmitted light I _1112211 and reflected light I _1112212 ; wherein the reflected light I _1112212 is divided into two beams of transmitted light I 11122121 and reflected light I _11122122 whose propagation direction and polarization state are all perpendicular to each other by the polarization _beam splitter , where _the reflected light I _11122122 overlaps and interferes with the reflected light I ₁₁₂₂ split by the polarizing beam splitter prism 11 to obtain the third interference fringe pattern. The transmitted light I _11122121 overlaps and interferes with the transmitted light I ₁₁₂₁ split by the reflected light I ₁₁₂ through the polarization beam splitter prism, and the fourth interference fringe pattern is obtained. The reflected light I ₁₁₁₂₂₂ is divided into two beams of transmitted light I _1112221 and reflected light I _1112222 whose propagation direction and polarization state are perpendicular to each other by the polarization beam splitter prism, wherein the reflected light I _1112222 and the reflected light I ₁₂ are divided into the reflected light I by the polarization beam splitter 11 ₁₂₂ superimposed interference occurs, and the fifth interference fringe pattern is obtained. The transmitted light I _1112221 and the reflected light I ₁₂ are superimposed and interfered with the transmitted light I ₁₂₁ split by the polarization beam splitter prism, and the sixth interference fringe pattern is obtained. The above-mentioned first half-reflective prism, second half-reflective prism, polarizing beam splitter, polarizing beam splitting prism, and polarizing beam splitting prism can all divide a beam of light into two beams of mutually perpendicular light transmitted light and reflected light; And there is a half-wave loss in the reflection of the first half-reflective prism and the second half-reflective prism to the light reflected by the measured mirror; The polarization states of light are perpendicular to each other. The above six interference fringe patterns are collected by six CCD target surfaces at one time in the airspace, and the optical phase is obtained by calculating the interference light intensity value of each interference fringe pattern by using a phase shifting algorithm.

The above-mentioned method of measuring optical phase by synchronous phase-shifting interferometry can be realized with the following optical path: a linear polarization-stabilized laser is used as the light source, and the output laser first passes through a rotatable half-wave plate, and then enters a beam expander system to obtain a linear polarization Polarized flat light source. Place the first half-reflective prism at -45° to the projected light in the projection direction of the linearly polarized plane light source, and place the second half-reflective prism at +45° to the transmitted light in the direction of the transmitted light. And place the mirror under test at a 90° angle to the transmitted light in the direction of the transmitted light of the second half-reflective prism. Place a polarizing beamsplitter prism at -45° to the reflected light in the direction of the reflected light of the first half-reflective prism, and place an eighth-wave plate and a standard reference at 90° to the reflected light in the opposite direction mirror. In the reflected light direction of the second half-reflective prism 4, a polarization beam splitter prism with a +45° angle to the reflected light beam is placed, and a -45° polarization beam splitter prism is placed in the opposite direction to the reflected light beam. Among them, the first half-reflective prism is set so that there is a half-wave loss when the light is reflected by the first half-reflective prism and projected to the one-eighth wave plate and the standard reference mirror and projected in the opposite direction; The half-reflective prism is set so that there is a half-wave loss when the light is reflected by the second half-reflective prism and projected to the polarizing beam splitter and projected in the opposite direction; A half-reflective prism in which the polarization states of light are perpendicular to each other; the fast axis of an eighth-wave plate is perpendicular to the direction of light projection.

The beam expander system consists of two convex lenses.

The method of the invention is simple, the principle is clear, each component in the optical path is easy to obtain, and the whole device is compact and easy to use. The method and device introduced by the invention can realize high-precision dynamic measurement of high-precision optical interference.

Description of drawings

Fig. 1 is a schematic diagram of the optical path and structure of the spectroscopic, imaging and synchronous phase shifting device for measuring the optical phase by using the synchronous phase shifting interference method proposed by the present invention.

In the figure: 1. Frequency-stabilized laser whose output is linearly polarized laser, 2. Half-wave plate, 3. Beam expander system, 4. Semi-transparent and half-reflective prism, 5. Polarizing beam splitter prism, 6. Standard reference mirror, 7, 8 One-third wave plate, 8, polarization beam splitter prism, 9, semi-transparent and half mirror prism, 10, measured mirror, 11, polarization beam splitter prism, among the figure (1), (2), (3), (4), (5) and (6) are the light intensity values of the six interference fringe patterns with mutual phase shift of 90 degrees measured by the same type of CCD detector.

Detailed ways

The method and device for measuring light splitting, imaging and synchronous phase shifting in an optical phase device using the synchronous phase shifting interference method proposed by the present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Embodiment: As shown in Figure 1, the frequency-stabilized laser 1 outputs linearly polarized laser light, and the half-wave plate 2 is used to adjust its vibration transmission direction to the horizontal direction, and then the laser beam is expanded to the linearly polarized plane light source through the beam expander system 3 , leading in a polarization interference system composed of half mirror prisms 4, 9 and polarization beam splitter prisms 5, 8, 11. Each component is placed on the optical platform through an adjustable optical bench, and the optical path of each component is adjusted to the same horizontal plane. Wherein, the beam expander system 3 is made up of two convex lenses whose focal lengths are respectively 5cm and 8cm. Two rectangular prisms of anti-prism 4,9 adopt different mediums, so that there is a half-wave loss in the reflected light incident from the upper surface of semi-transparent half-reflective prism 4, and the incident reflected light from the lower surface of semi-transparent half-reflective prism 9 exists Half-wave loss; polarization splitter prisms 5, 8, 11 make the polarization states of transmitted light and reflected light vertical.

The first interference fringe diagram: the laser beam emitted by the beam expander system 3 is evenly divided into two beams by the semi-transparent prism 4, and the transmitted light passes through the semi-transparent prism 9 and then shoots to the measured mirror 10, and then transmits the semi-reflective prism again after returning. The transflective prism 9 is reflected by the transflective prism 4, and the reflected light passes through the one-eighth wave plate 7 and then projects to the standard reference mirror 6, and passes through the one-eighth wave plate 7 after returning, and finally transmits the transflective The first interference fringe pattern (1) is obtained by interfering with the prism 4 and another part of the light reflected by the polarizing beam splitter 5 and reflected by the beam expander system 3 and reflected by the half mirror 4 and reflected by the polarizing beam splitting prism 5 .

The second interference fringe diagram: the laser beam emitted by the beam expander system 3 is evenly divided into two beams by the semi-transparent prism 4, and the transmitted light passes through the semi-transparent prism 9 and then shoots to the measured mirror 10, and then transmits the semi-reflective light again after returning. The transflective prism 9 is reflected by the transflective prism 4, and the reflected light passes through the one-eighth wave plate 7 and then projects to the standard reference mirror 6, and passes through the one-eighth wave plate 7 after returning, and finally transmits the transflective The second interference fringe pattern (2) is obtained by interfering with the prism 4 and another part of the light transmitted by the polarizing beam splitter prism 5 and emitted by the beam expander system 3, reflected by the half mirror 4 and transmitted by the polarizing beam splitter prism 5.

The third interference fringe pattern: the first beam of light that forms this interference fringe pattern is: the laser beam emitted by the beam expander system 3 passes through the semi-transparent and half-reflective prisms 4 and 9, and then projects to the measured mirror 10, and then passes through the semi-transparent and semi-reflective prisms again after returning. Reflecting prism 9 transmits and half-reflecting prism 4 reflects and shoots to one-eighth wave plate, shoots to standard reference mirror 6 after exiting, and passes through one-eighth wave plate 7 and half-reflecting prism 4 reflection again after returning and The half-reflective prism 9 transmits to the mirror 10 under test, and after returning, it is reflected by the half-reflective prism 9 and the polarizing beam splitter prism 11 to obtain the first beam of light; the second beam of light forming an interference fringe pattern is: beam expander system 3. The outgoing laser beam passes through half-reflective prisms 4 and 9 and then projects to the mirror 10 under test. After returning, it is reflected by half-reflective prism 9 and polarized beam splitter 11 to obtain the second beam of light. These two beams of light interfere to obtain the third interference fringe pattern (3).

The fourth interference fringe pattern: the first beam of light forming this interference fringe pattern is: the laser beam emitted by the beam expander system 3 passes through the semi-transparent and semi-reflective prisms 4 and 9, and then projects to the mirror 10 under test, and then passes through the semi-transparent and semi-reflective prisms again after returning. Reflecting prism 9 transmits and half-reflecting prism 4 reflects and shoots to one-eighth wave plate, shoots to standard reference mirror 6 after exiting, and passes through one-eighth wave plate 7 and half-reflecting prism 4 reflection again after returning and The half-reflective prism 9 transmits to the mirror under test 10, and after returning back, it is reflected by the half-reflective prism 9 and transmitted by the polarizing beam splitter prism 11 to obtain the first beam of light; the second beam of light forming an interference fringe pattern is: a beam expander system 3. The emitted laser beam passes through half-reflective prisms 4 and 9 and then projects to the mirror under test 10. After returning, it is reflected by half-reflective prism 9 and transmitted by polarizing beam splitter prism 11 to obtain a second beam of light. These two beams of light interfere to obtain the fourth interference fringe pattern (4).

The fifth interference fringe diagram: the first beam of light that forms this interference fringe diagram is: the laser beam emitted by the beam expander system 3 passes through the semi-transparent and half-reflective prisms 4 and 9, and then projects to the mirror 10 under test, and then passes through the semi-transparent and semi-reflective prisms again after returning. Reflecting prism 9 transmits and half-reflecting prism 4 reflects and shoots to one-eighth wave plate, shoots to standard reference mirror 6 after exiting, and passes through one-eighth wave plate 7 and half-reflecting prism 4 reflection again after returning and The semi-transparent and half-reflective prism 9 is reflected and projected to the polarization beamsplitter prism 8, and the first beam of light is obtained through the polarization beamsplitter prism 8. Anti-prism 4 reflection and semi-transparent half-mirror prism 9 reflect and project to polarizing beam splitting prism 8, obtain the second beam of light through polarizing beam splitting prism 8 light splitting reflection. These two light beams interfere to obtain the fifth interference fringe pattern (5).

The sixth interference fringe pattern: the first beam of light forming this interference fringe pattern is: the laser beam emitted by the beam expander system 3 passes through the semi-transparent and half-reflective prisms 4 and 9, and then projects to the mirror 10 under test, and then passes through the semi-transparent and semi-reflective prisms again after returning. Reflecting prism 9 transmits and half-reflecting prism 4 reflects and shoots to one-eighth wave plate, shoots to standard reference mirror 6 after exiting, and passes through one-eighth wave plate 7 and half-reflecting prism 4 reflection again after returning and The semi-transparent and half-reflective prism 9 is reflected and projected to the polarizing beam splitter 8, and the first beam of light is obtained through the polarization beam splitting prism 8; the second beam of light forming an interference fringe pattern is: the laser beam emitted by the beam expander system Reflected by the anti-prism 4 and reflected by the half-reflected prism 9, it is projected to the polarization beam splitter 8, and the second beam of light is obtained through the polarization beam splitter 8 and transmitted. These two beams of light interfere to obtain the sixth interference fringe pattern (6).

The CCD detector is used to collect six interference fringe images simultaneously.

Finally, the optical phase can be obtained by using the phase-shift algorithm from the interference light intensity values of each interference fringe pattern, so as to analyze the situation of the measured mirror surface.

Claims (3)

1. Using the method of synchronous phase-shifting interferometry to measure the optical phase, two beams of wavefronts are generated by the method of dividing the amplitude, one beam of wavefronts is projected to the standard reference mirror and returns to form a reference wavefront, and the other beam of wavefronts is projected to the measured mirror and returned to form The test wavefront, the reference wavefront and the test wavefront meet in space to form an interference field, and the digital image acquisition device records the interference field in the form of an interference fringe pattern, and the shape of the test wavefront can be obtained by analyzing the interference fringe pattern; it is characterized in that : With the linearly polarized plane light source as the light source, the light source is divided into two mutually perpendicular transmitted light I ₁ and reflected light I ₂ by the first semi-transparent and half-reflected prism (4), wherein the transmitted light I ₁ passes through the second semi-transparent and semi-transparent The anti-prism is further divided into two beams of transmitted light I ₁₁ and reflected light I ₁₂ which are perpendicular to each other, wherein the transmitted light I ₁₁ is reflected back by the measured mirror (10), and then divided into two beams by the second half-transparent half-reflective prism (9). Two beams of transmitted light I ₁₁₁ and reflected light I ₁₁₂ perpendicular to each other, wherein the transmitted light I ₁₁₁ returns to the first half-reflective prism (4), and is divided into two by the first half-reflective prism (4). Beams of transmitted light and reflected light I ₁₁₁₂ perpendicular to each other, wherein the reflected light I ₁₁₁₂ is reflected by the standard reference mirror (6) through the one-eighth wave plate and returned to be divided into two by the first semi-transparent and half-reflective prism (4) Beams of transmitted light I ₁₁₁₂₁ and reflected light I ₁₁₁₂₂ perpendicular to each other, _wherein the transmitted light I ₁₁₁₂₁ is divided into two beams of transmitted light I 111211 and reflected light I ₁₁₁₂₁₂ whose propagation direction and polarization state are perpendicular to each other by the first polarization beam splitter prism (5) , wherein the reflected light I ₁₁₁₂₁₂ and the reflected light I ₂ are superimposed and interfered by the reflected light I ₂₂ divided by the first polarizing beam splitter prism (5), and the first interference fringe pattern is obtained; the transmitted light I ₁₁₁₂₁₁ and the reflected light I ₂ are passed through the second The transmitted light I ₂₁ divided by a polarizing beam splitter prism (5) is superimposed and interfered to obtain the second interference fringe pattern; the reflected light I ₁₁₁₂₂ is divided into two mutually perpendicular transmission beams through the second half-transparent half-reflective prism (9) again. Light I ₁₁₁₂₂₁ and reflected light I ₁₁₁₂₂₂ , wherein the transmitted light I ₁₁₁₂₂₁ is reflected back by the measured mirror (10) again, and is divided into two mutually perpendicular transmitted light I by the second semi-transparent half-reflective prism (9) again _1112211 and reflected light I _1112212 , wherein reflected light I _1112212 is divided into two beams of transmitted light I 11122121 and reflected light I 11122122 _whose propagation directions and polarization states are perpendicular to each other by a third polarization beam splitter prism (11), wherein reflected light I _11122122 and reflected light I _11122122 The reflected light I ₁₁₂ superimposed and interfered by the reflected light I ₁₁₂ divided by the third polarization beam splitter prism (11) obtains the third interference fringe pattern; the transmitted light I _11122121 and the reflected light I ₁₁₂ pass through the first polarization beam splitter prism (5 ) into the transmitted light I ₁₁₂₁ superimposed and interfered to obtain the fourth interference fringe pattern; Reflected light I ₁₁₁₂₂₂ is divided into two beams of transmitted light I _1112221 and reflected light I _1112222 whose propagation direction and polarization state are perpendicular to each other by the second polarization beam splitter prism (8), wherein reflected light I _1112222 and reflected light I ₁₂ pass through the third The reflected light I ₁₂₂ divided by the polarization beam splitter prism (11) overlaps and interferes, and the fifth interference fringe pattern is obtained; the transmitted light I _1112221 and the reflected light I ₁₂ overlap and interfere with the transmitted light I ₁₂₁ divided by the polarization beam splitter prism (8), Obtain the 6th interference fringe figure; Above-mentioned first half mirror prism (4), second half mirror prism (9), first polarization beam splitter prism (5), the third polarization beam splitter prism ( 11), the second polarizing beamsplitter prism (8) can divide a beam of light into two beams of mutually perpendicular light transmission light and reflected light, and the first semi-transparent half-reflective prism (4), the second semi-transparent There is a half-wave loss in the reflection of the light reflected by the mirror (10) by the half mirror (9), the first polarization beam splitter (5), the third polarization beam splitter (11), the second polarization beam splitter The polarization states of the transmitted light and reflected light produced by the dichroic prism (8) are perpendicular to each other; the above-mentioned six interference fringe patterns are respectively collected by six CCD targets in the airspace once, and the interference light intensity values of each interference fringe pattern are analyzed by phase-shifting algorithm. Calculate the optical phase. 2. Realize the device utilizing the method for measuring optical phase by synchronous phase-shifting interferometry as claimed in claim 1, characterized in that: a linearly polarized frequency-stabilized laser is used as a light source, and the laser output of it first passes through a rotatable half-wave plate , then enter the beam expander system to obtain a linearly polarized plane light source; place the first half-transparent half-reflective prism (4) on the projection direction of the linearly polarized plane light source to form -45° with the projected light, and place it in the direction of its transmitted light Transmit light into the second half-reflective prism (9) of +45°, and place the measured mirror (10°) with the transmitted light at 90° on the direction of the transmitted light of the second half-reflective prism (9). ); Place the first polarization beam splitter prism (5) that becomes -45 ° with reflected light on the reflected light direction of the first half-transparent half-reflective prism (4), and in the direction opposite to the traveling direction of this reflected light Place the one-eighth wave plate and the standard reference mirror that are 90° with the reflected light in turn; place the third one that is +45° with the reflected light on the reflected light direction of the second half mirror (9). Polarization beamsplitter prism (11), and place the polarization beamsplitter prism (8) that becomes-45 ° with reflected light on the opposite direction with the traveling direction of this reflected light; For there is a half-wave loss when the light passes through the first half-reflective prism (4) reflection and casts into the 1/8 wave plate and the standard reference mirror and reflects into the first polarization beam splitter prism (5), the second Half-reflective prism (9) is set so that when the light is projected into the third polarized beam splitter (11) and reflected into the second polarized beam-splitter prism (8) by the second half-reflected prism (9) Wave loss, the first polarizing beam splitter (5), the third polarizing beam splitting prism (11), and the second polarizing beam splitting prism (8) are all semi-transparent and semi-perpendicular to make the polarization states of the transmitted light and the reflected light perpendicular to each other Inverse prisms, the fast axis of the one-eighth wave plate is perpendicular to the direction of light projection. 3. The device according to claim 2, wherein the beam expander system is composed of two convex lenses.

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