Sagnac wide-band linear polarization imaging structure

Abstract

The application belongs to the technical field of polarization imaging, and relates to a Sagnac wide-band linear polarization imaging structure, which comprises a second plane mirror, a first polarization beam splitter, a first polarizer, a first imaging lens and a first focal plane array arranged in sequence and at intervals on a first optical axis, and a compensation plate for compensating for a shearing amount is arranged on one side of the first polarization beam splitter, and the compensation plate is arranged close to the second plane mirror; the first plane mirror is arranged on a second optical axis perpendicular to the first optical axis and is arranged at an interval from the first polarization beam splitter and is located on a side away from the compensation plate; a first blazed grating and a second blazed grating are reversely arranged at intervals and are located on an optical path between the first plane mirror and the second plane mirror, and the first blazed grating is close to the first plane mirror. The system has small system adjustment workload, high accuracy, sufficient shearing amount compensation, good wide-band polarization imaging image precision and good overall stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polarization imaging, and particularly relates to a Sagnac wide-band linear polarization imaging structure. BACKGROUND

[0002] Polarization imaging technology can be divided into time sequence type polarization imaging technology and snapshot type polarization imaging technology according to different imaging detection methods and technical schemes. The time sequence type polarization imaging technology refers to measuring the polarization state of a target in multiple directions multiple times by using a single detector, and finally obtaining the polarization information of the target through calculation and fusion, which is a time-sharing type polarization imaging technology, and is mainly divided into mechanical polarization light modulation type and phase delay modulation type. Generally speaking, the time sequence type polarization imaging scheme has simple structure and low cost, but generally can only image the polarization of a static target or a static target, and contains mechanical or electric control components, which is greatly affected by vibration or environmental factors. The snapshot type polarization imaging technology can detect moving targets and has a wide range of applications. At present, the snapshot type polarization imaging technology is divided into amplitude division type, aperture division type, focal plane division type and channel modulation type, wherein the amplitude division type needs to use multiple detectors, has complex structure and high cost, and has the advantage of high resolution. The aperture division type adopts a coaxial optical path, has a small field of view angle and loses spatial resolution, and has high system stability after debugging and is easy to integrate. The focal plane division type needs to use micro-nano processing technology, and can only detect linear polarization at present. Different types of polarization imaging schemes also have the trend of mutual fusion, such as the simultaneous polarization imaging technology combining amplitude division and aperture division, which comprehensively utilizes the advantages of high resolution of amplitude division and small volume of aperture division. The channel modulation type is a new generation of simultaneous polarization imaging technology, which has high resolution, compact structure, no moving parts and can obtain polarization information in real time compared with traditional polarization imaging technology, and is a research hotspot in recent years.

[0003] The channel modulation type polarization imaging technology is different from the traditional polarization filter imaging method, and can simultaneously detect the two-dimensional spatial distribution information of the scene polarization information through modulation and demodulation of the Stokes parameter information of the target. The core of the technology is a light splitting modulation module, which is generally made of a birefringent crystal. According to the difference of the light splitting element or configuration, the channel modulation type polarization imaging system has a wedge prism type, a Savart type, a Wollaston prism type, a polarization grating type and a Sagnac interferometer type.

[0004] In order to obtain wide-band polarization imaging and improve the detection capability of targets in low-light environments, in 2009, Kudenov MW et al. from the University of Arizona used a Sagnac interferometer structure to replace the Savoy plate for light splitting based on the snapshot-type Savoy plate polarization interferometry imaging system. At the same time, the third blazed grating 12 and the fourth blazed grating 15 were used to eliminate dispersion, thus obtaining a dispersion-compensated polarization Sagnac interferometer. The structure of the dispersion-compensated polarization Sagnac interferometer is shown in the figure. Figure 1 As shown, the system includes a second polarization beam splitter 11, a third blazed grating 12, a fourth blazed grating 15, a third plane mirror 17, a fourth plane mirror 18, a second analyzer 13, a second imaging lens 14 and a second focal plane array 16, wherein the second polarization beam splitter 11, the third plane mirror 17 and the fourth plane mirror 18 are all tilted, with a horizontal line passing through the center of the second polarization beam splitter 11 as the third optical axis, and a vertical line passing through the center of the second polarization beam splitter 11 as the fourth optical axis, the second polarization beam splitter 11, the third blazed grating 12 and the third plane mirror 17 are arranged in sequence and spaced apart on the third optical axis, the fourth plane mirror 18, the fourth blazed grating 15, the second polarization beam splitter 11, the second analyzer 13, the second imaging lens 14 and the second focal plane array 16 are arranged in sequence and spaced apart on the fourth optical axis, there is an angle between the third plane mirror 17 and the third optical axis, and there is an angle between the fourth plane mirror 18 and the fourth optical axis.

[0005] After the system is set up, but before it is actually used, since the third blazed grating 12 and the fourth blazed grating 15 are respectively used to compensate for the dispersion of the carrier frequency in each optical path, the positions of the third blazed grating 12 and the fourth blazed grating 15 need to be adjusted during the adjustment process. However, since the spacing and level of all components on the third optical axis and the fourth optical axis are adjusted separately, the system installation and adjustment is not convenient, the adjustment workload is large, and the accuracy is poor. In addition, the shear compensation of the system is insufficient, which makes it difficult to perform polarization imaging in a wide band, affecting the image accuracy and overall stability of the system. Summary of the Invention

[0006] In view of this, the present invention provides a Sagnac wide-band linear polarization imaging structure to solve the technical problems of large system adjustment workload and poor accuracy, and insufficient shear compensation, which lead to poor image accuracy and overall stability of wide-band polarization imaging.

[0007] The technical solution of the present invention is:

[0008] A Sagnac wide-band linear polarization imaging structure, comprising:

[0009] A second plane reflector, a first polarization beam splitter, a first analyzer, a first imaging lens, and a first focal plane array are sequentially arranged on the first optical axis, wherein a compensation plate for compensating for shear is provided on one side of the first polarization beam splitter, and the compensation plate is provided close to the second plane reflector;

[0010] The first plane reflecting mirror and the first polarization beam splitter are spaced apart and arranged on a second optical axis perpendicular to the first optical axis and located on a side away from the compensation plate;

[0011] A first blazed grating and a second blazed grating are arranged in opposite directions and spaced apart and located on an optical path between the first plane reflector and the second plane reflector, with the first blazed grating being close to the first plane reflector;

[0012] The incident light is sequentially divided into p-polarized light and s-polarized light by the compensation plate and the first polarization beam splitter. The p-polarized light is reflected by the first plane mirror and diffracted to the first order by the first blazed grating. The diffraction angle is then eliminated by the second blazed grating. After being reflected by the second plane mirror, the light passes through the compensation plate, the first polarization beam splitter, the first analyzer and the first imaging lens in sequence and is focused on the first focal plane array. The s-polarized light passes through the second plane mirror, the second blazed grating, the first blazed grating and the first plane mirror in sequence, and is reflected by the first polarization beam splitter. It then passes through the first analyzer and the first imaging lens in sequence and interferes with the light transmitted by the p-polarized light on the first focal plane array to form interference fringes.

[0013] Furthermore, the first blazed grating and the second blazed grating have the same structure and are placed in parallel.

[0014] Furthermore, the distances between the first polarization beam splitter and the first plane reflector and the second plane reflector are the same.

[0015] Furthermore, the first polarization beam splitter includes a flat beam splitter and a narrowband beam splitter film arranged in sequence, the compensation plate is arranged on one side of the narrowband beam splitter film, and the material and thickness of the compensation plate are the same as those of the flat beam splitter.

[0016] Furthermore, the compensation plate is a glass substrate.

[0017] Furthermore, the first polarization beam splitter, the first plane reflector, and the second plane reflector are all tilted, and they respectively conform to the following relationship with the first optical axis and the second optical axis:

[0018] ɑ=β=90° / 2,

[0019] γ=φ=ɑ+δ=β+δ,

[0020] Wherein, ɑ is the angle between the first polarization beam splitter and the first optical axis, in degrees; β is the angle between the first polarization beam splitter and the second optical axis, in degrees; γ is the angle between the first plane reflector and the second optical axis, in degrees; φ is the angle between the second plane reflector and the first optical axis, in degrees; and δ is the grating blaze angle of the first blazed grating and the second blazed grating, in degrees.

[0021] Furthermore, the δ is 20°~25°.

[0022] Furthermore, the δ is 22.5°.

[0023] Compared with the prior art, the present invention provides a Sagnac wide-band linear polarization imaging structure, in which the incident light is divided into p-polarized light and s-polarized light by the first polarization beam splitter. The p-polarized light is reflected by the first plane mirror and diffracted to the first order by the first blazed grating, and then the diffraction angle is eliminated by the second blazed grating. After being reflected by the second plane mirror, it passes through the first polarization beam splitter, the first analyzer and the first imaging lens in sequence and is focused on the first focal plane array. The s-polarized light passes through the second plane mirror, the second blazed grating, the first blazed grating and the first plane mirror in sequence, and is reflected by the first polarization beam splitter. It passes through the first analyzer and the first imaging lens in sequence and interferes with the light transmitted by the p-polarized light on the first focal plane array to form interference fringes. The present invention utilizes first and second blazed gratings, spaced oppositely and positioned on the optical path between the first and second plane reflectors, to enhance system stability and image accuracy in collaboration with a first polarization beam splitter and a compensation plate. Because the first and second blazed gratings are arranged on a single optical path within the entire system, adjustment of their spacing and level is facilitated, thereby enhancing system adjustability and accuracy. Furthermore, the use of a compensation plate made from the same glass substrate material as the first polarization beam splitter effectively compensates for excess shear. By comprehensively considering the optical path, wavelength, and polarization characteristics, the present invention enhances system stability, improves imaging quality, and enhances the applicability of the technology. Its practicality is highly worthy of promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the structural diagram of DCPSI.

[0025] Figure 2 This is the schematic diagram of MDCPSI.

[0026] Figure 3 This is the expanded diagram of the MDCPSI optical path.

[0027] Figure 4 Schematic diagram of light propagation and structure of the first polarization beam splitter.

[0028] Figure 5The glass substrate of the first polarization beam splitter causes the outgoing s-polarized light to undergo a lateral shift.

[0029] Figure 6 is the lateral displacement of s-polarized light.

[0030] Figure 7 is the relationship between the original shear amount and the additional shear amount and the wavelength.

[0031] Figure 8 Set for the incident light Stokes vector.

[0032] Figure 9 DCPSI interferogram (a) and MDCPSI interferogram (b).

[0033] Figure 10 Interference diagrams of DCPSI (a) and MDCPSI (b) x Axis light intensity distribution.

[0034] Figure 11 Interference fringe intensity diagram for different splitting ratios.

[0035] Figure 12 is the intensity value at coordinate x=200 when the splitting ratio of the first polarization beam splitter is different.

[0036] Figure 13 Interference fringe intensity diagram for different polarization diffraction efficiency ratios.

[0037] Figure 14 The intensity distribution of interference fringes when the first blazed grating and the second blazed grating have different diffraction efficiencies of other orders.

[0038] Figure 15 The first plane mirror is offset to the right by β, resulting in a difference in the lengths of the two arms.

[0039] Figure 16 is the interference pattern for different arm length differences.

[0040] Figure 17 is the light intensity distribution on the x-axis of the interference pattern. DETAILED DESCRIPTION

[0041] The present invention provides a Sagnac wide-band linear polarization imaging structure to solve the above-mentioned problems. In order to enable those skilled in the art to better understand and implement the technical solution of the present invention, the technical solution of the present invention will be clearly and comprehensively described below with reference to the accompanying drawings.

[0042] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0043] In addition, it should be further explained that in the description of the embodiments of the present invention, unless otherwise specified, " / " means or, for example, A / B can mean A or B: "and / or" in the text is merely a description of the association relationship of associated objects, indicating that three relationships may exist, for example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, in the description of the embodiments of the present invention, "multiple" refers to two or more than two.

[0044] The terms "first," "second," "third," and "fourth" below are used for descriptive purposes only and should not be understood to imply or suggest relative importance or implicitly indicate the number of the technical features indicated. Therefore, features defined as "first," "second," "third," and "fourth" may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, "plurality" means at least two, for example, two, three, etc., unless otherwise clearly and specifically limited.

[0045] In the present invention, unless otherwise specified or limited, the terms "installed," "connected," "connect," "fixed," etc. should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two components or interaction between two components, unless otherwise specified. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0046] In the present invention, unless otherwise expressly specified or limited, when a first feature is "above" or "below" a second feature, it may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediary. Furthermore, when a first feature is "above," "above," or "above" a second feature, it may mean that the first feature is directly above or diagonally above the second feature, or simply means that the first feature is at a higher level than the second feature. When a first feature is "below," "below," or "below" a second feature, it may mean that the first feature is directly below or diagonally below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0047] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it may be directly on the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in the present invention are for illustrative purposes only and do not represent the only implementation methods.

[0048] Example 1

[0049] Polarization Sagnac interference structure, the full name of which is Polarized Sagnac Interferometer, referred to as PSI system, has a carrier frequency inversely proportional to the wavelength and is only suitable for monochromatic light polarization imaging. In order to increase the amount of light entering the system and realize the detection of targets in low-light environments, we need to eliminate the carrier frequency dispersion caused by wavelength and obtain a wide-band polarization imaging structure. In order to obtain wide-band polarization imaging and improve the detection capability of targets in low-light environments, in 2009, Kudenov MW et al. from the University of Arizona used a Sagnac interferometer structure to replace the Savoy plate for light splitting based on the snapshot Savoy plate polarization interference imaging system, and used the third blazed grating 12 and the fourth blazed grating 15 to eliminate dispersion, thus obtaining a dispersion-compensated polarization Sagnac interferometer, the full name of which is Dispersion Compensation Polarization Sagnac Interferometer, abbreviated as DCPSI. The structure of the dispersion-compensated polarization Sagnac interferometer is shown as follows: Figure 1 As shown, it includes a second polarization beam splitter 11, a third blazed grating 12, a fourth blazed grating 15, a third plane mirror 17, a fourth plane mirror 18, a second analyzer 13, a second imaging lens 14 and a second focal plane array 16, wherein the second polarization beam splitter 11, the third plane mirror 17 and the fourth plane mirror 18 are all tilted, with the horizontal line passing through the center of the second polarization beam splitter 11 as the third optical axis, and the vertical line passing through the center of the second polarization beam splitter 11 as the fourth optical axis, the second polarization beam splitter 11, the third blazed grating 12 and the third plane mirror 17 are arranged in sequence and spaced apart on the third optical axis, the fourth plane mirror 18, the fourth blazed grating 15, the second polarization beam splitter 11, the second analyzer 13, the second imaging lens 14 and the second focal plane array 16 are arranged in sequence and spaced apart on the fourth optical axis, there is an angle between the third plane mirror 17 and the third optical axis, and there is an angle between the fourth plane mirror 18 and the fourth optical axis.

[0050] After the system is set up, but before it is actually used, since the third blazed grating 12 and the fourth blazed grating 15 are respectively used to compensate for the dispersion of the carrier frequency in each optical path, it is necessary to focus on adjusting the positions of the third blazed grating 12 and the fourth blazed grating 15 during the adjustment process, because the spacing and level of all components on the third optical axis and the fourth optical axis need to be adjusted. However, in its structure, the third blazed grating 12 and the fourth blazed grating 15 are located on the two arms of the PSI, which makes installation and adjustment inconvenient, the adjustment workload is large, and the accuracy is poor. In addition, the shear compensation of the system is insufficient, which makes it difficult to perform polarization imaging in a wide band, affecting the image accuracy and overall stability of the system.

[0051] Based on the above reasons, the present invention proposes a Sagnac broadband linear polarization imaging structure, also known as modified dispersion compensation polarization Sagnac interferometer, the full name of which is Modified dispersion compensation Polarization Sagnac interferometer, abbreviated as MDCPSI. Figure 2 As shown, compared with the structure of the dispersion-compensating polarization Sagnac interferometer, the first blazed grating 3 and the second blazed grating 4 are moved from the right-angled sides of the two arms to the oblique sides, which facilitates alignment and leveling and precise adjustment of the spacing between the first blazed grating 3 and the second blazed grating 4, and a glass substrate is added to one side of the first polarization beam splitter 1 as a compensation plate 9, which is abbreviated as CP and is used to perform shear compensation in the system.

[0052] Specifically, the Sagnac broadband linear polarization imaging structure includes:

[0053] A second plane mirror 5, a first polarization beam splitter 1, a first analyzer 6, a first imaging lens 7, and a first focal plane array 8 are sequentially arranged on the first optical axis. A compensation plate 9 is provided on one side of the first polarization beam splitter 1 and is positioned close to the second plane mirror 5. The first plane mirror 2 and the first polarization beam splitter 1 are spaced apart on a second optical axis perpendicular to the first optical axis and are located away from the compensation plate 9. A first blazed grating 3 and a second blazed grating 4 are spaced apart in opposite directions and are located on the optical path between the first plane mirror 2 and the second plane mirror 5, with the first blazed grating 3 being close to the first plane mirror 2.

[0054] Specifically, the first blazed grating 3 and the second blazed grating 4 are arranged in opposite directions, which means that the grooves of the first blazed grating 3 and the second blazed grating 4 are in opposite directions, that is, the blazing directions are in opposite directions.

[0055] The first blazed grating 3 and the second blazed grating 4 are completely identical in structure and are arranged in parallel, and the distance between the two is L. The distance between the first polarization beam splitter 1 and the first plane mirror 2 and the second plane mirror 5 is the same.

[0056] Specifically, in actual work, after the light is incident on the first polarization beam splitter 1, it is divided into two beams of light perpendicular to each other in the vibration direction: the transmitted p-polarized light and the reflected s-polarized light. The p-polarized light is reflected by the first plane mirror 2 and then diffracted to the first order by the first blazed grating 3, and then the diffraction angle is eliminated after passing through the second blazed grating 4 arranged in reverse parallel, at this time the light is parallel to the transmission direction before being incident on the first blazed grating 3, and then the light is transmitted through the first polarization beam splitter 1 after being reflected by the second plane mirror 5, and the exit point deviates from the center by a certain distance, and the deviation size is , wherein is a constant related to the system structure parameters. Similarly, the s-polarized light passes through the second plane mirror 5, the second blazed grating 4, the first blazed grating 3 and the first plane mirror 2 in turn, and is finally reflected by the first polarization beam splitter 1, and the distance of the exit light deviating from the center is , and the distance between the two beams of exit light is the shear quantity, which is proportional to the wavelength. The present application precisely controls the diffraction and compensates for the optical path difference, so that the offset quantity shear quantity of the two beams of polarized light is proportional to the wavelength, and the compensation plate compensates for the optical path difference caused by the difference in film thickness, thereby ensuring the accuracy of the system in high-precision optical measurement.

[0057] It should be noted that the Sagnac wide-band linear polarization imaging structure requires the p-polarized light transmitted by the first polarization beam splitter 1 and the s-polarized light reflected by the first polarization beam splitter 1 to have the same optical path difference, that is, the distance between the first polarization beam splitter 1 and the first plane mirror 2 and the second plane mirror 5 is equal. The first polarization beam splitter 1 includes a flat beam splitter and a narrow-band beam splitter film arranged in turn, but since the manufacturing process of the first polarization beam splitter 1 is to coat a narrow-band beam splitter film on one side of the flat beam splitter, the transmitted light will travel an extra distance compared to the reflected light, and this distance is a nonlinear relationship with the wavelength, which cannot be adjusted by adjusting the length of the two arms to compensate. We can place a compensation plate 9 with the same material and thickness as the flat beam splitter on the other side of the first polarization beam splitter 1 to compensate, forming a laminated structure of the flat beam splitter, the narrow-band beam splitter film and the compensation plate 9, so that the optical path difference of the two beams of light after beam splitting remains the same. Specifically, the compensation plate 9 is a glass substrate.

[0058] The first polarization beam splitter 1, the first plane mirror 2 and the second plane mirror 5 are all arranged obliquely, and they respectively satisfy the following relationship with the first optical axis and the second optical axis:

[0059] ɑ=β=90° / 2 (1)

[0060] γ=φ=ɑ+δ=β+δ (2)

[0061] Wherein, ɑ is the angle between the first polarization beam splitter 1 and the first optical axis, and the unit is °; β is the angle between the first polarization beam splitter 1 and the second optical axis, and the unit is °; γ is the angle between the first plane reflector 2 and the second optical axis, and the unit is °; φ is the angle between the second plane reflector 5 and the first optical axis, and the unit is °; δ is the grating blaze angle of the first blazed grating 3 and the second blazed grating 4, and the unit is °.

[0062] More specifically, δ is 20° to 25°, preferably δ is 22.5°, to ensure that the light is vertically incident on the first blazed grating 3 and the second blazed grating 4 .

[0063] The shearing amount generated by the Sagnac broadband linear polarization imaging structure is:

[0064] (3)

[0065] in, is the shearing amount produced by the Sagnac broadband linear polarization imaging structure, is the wavelength of the incident light, is a constant, The value of is related to the designed Sagnac wide-band linear polarization imaging structure parameters.

[0066] The present invention can compensate for the dispersion of the carrier frequency in the PSI structure by introducing the first blazed grating 3 and the second blazed grating 4. Figure 2 The MDCPSI shown is expanded as follows Figure 3 , p-polarized light propagates from left to right, and s-polarized light propagates from right to left. After passing through the first blazed grating 3, the light is diffracted to the first order. Then, passing through the second blazed grating 4 with the same structure, the diffraction angle is eliminated, and the light is now parallel to its original transmission direction.

[0067] When the light is vertically incident on the first blazed grating 3 and the second blazed grating 4, the diffraction angle can be calculated by the grating equation:

[0068] (4)

[0069] in, is the diffraction angle of the light, m is the diffraction order, d is the grating period, is the wavelength of the incident light.

[0070] because d It is usually large ≥ 30 μm, so the small angle approximation can be used to simplify:

[0071] (5)

[0072] based on Figure 3 , the shear capacity of the MDCPSI system can be calculated using the following formula:

[0073] (6)

[0074] in, is the shearing amount produced by the Sagnac broadband linear polarization imaging structure, is the wavelength of the incident light, is a constant, The value of is related to the structural parameters of the designed Sagnac broadband linear polarization imaging structure. represents the distance between the first blazed grating 3 and the second blazed grating 4, m is the diffraction order, d is the grating period.

[0075] It can be seen from Equation (6) that the shearing amount of the MDCPSI system is generated by the first blazed grating 3 and the second blazed grating 4 and is proportional to the wavelength, so the carrier frequency is The wavelength term in the denominator is canceled out, and the frequency of the interference fringes is independent of the wavelength, which can achieve wide-band imaging. This is the principle of dispersion compensation of the dispersion-compensated polarization Sagnac interferometer system. It should be noted that in the above formula, is the carrier frequency, is the system shear, is the wavelength of the incident light, is the focal length of the imaging lens.

[0076] The cumulative phase of the transmitted p-polarized light and reflected s-polarized light emitted by the MDCPSI system can be expressed as:

[0077] (7)

[0078] in, The accumulated phase of the transmitted p-polarized light, The accumulated phase of the reflected s-polarized light, represents the imaging focal length, m is the diffraction order, d is the grating period, represents the distance between the two blazed gratings, x i are the image plane coordinates.

[0079] Considering only the diffraction of the first blazed grating 3 and the second blazed grating 4 to the first order, the light intensity formula on the focal plane is calculated based on the phase factor:

[0080] (8)

[0081] in, 、 and is the Stokes parameter, represents the imaging focal length, d is the grating period, represents the distance between the two blazed gratings, x i are the image plane coordinates.

[0082] If a piece with x Axis The light intensity formula at this time can be expressed as:

[0083] (9)

[0084] in, 、 and is the Stokes parameter, represents the imaging focal length, d is the grating period, represents the distance between the two blazed gratings, are the image plane coordinates.

[0085] It can be seen from formula (9) that the linear polarization information of the target, that is, the three Stokes parameters 、 and They are modulated onto different carrier frequencies.

[0086] In the MDCPSI system, after the first blazed grating 3 and the second blazed grating 4 are inserted, the direction of the light remains unchanged after two diffractions, but the position is shifted. When the diffraction angle is small, the shear amount of the system is proportional to the wavelength, thereby eliminating dispersion. However, the interference of the two beams can only modulate three Stokes parameters, so MDCPSI is a wide-band linear polarization imaging system.

[0087] The shearing amount of the Sagnac broadband linear polarization imaging structure is generated by the first blazed grating 3 and the second blazed grating 4. It is required that the lengths of the two arms are equal, and the p-polarized light transmitted by the first polarization beam splitter 1 and the reflected s-polarized light have the same optical path difference. However, in the actual production process of the first polarization beam splitter 1, the metal wire grid is plated on a single-sided glass substrate, such as Figure 4 As shown, the glass substrate has a certain thickness, and the transmitted p-polarized light will be refracted when passing through the glass substrate.

[0088] The single-sided grid makes the optical path of the two beams in the DCPSI structure differ by the thickness of the glass substrate, thus generating additional shear. Figure 5 As shown, the p-polarized light transmitted by the first polarization beam splitter 1 is refracted twice by the glass substrate and then exits at the position remains unchanged, while the reflected s-polarized light is refraction-free, and the exit position is changed from Offset to Location.

[0089] In order to accurately calculate the additional lateral shear caused by refraction, the propagation path of the light in the first polarization beam splitter 1 is locally amplified and passed through Figure 6 The ray trace shown, s-polarized light produces a lateral displacement yes:

[0090] (10)

[0091] in, is the lateral displacement produced by s-polarized light, t is the thickness of the glass substrate of the first polarization beam splitter 1, is the refraction angle of the glass substrate.

[0092] According to the law of refraction:

[0093] (11)

[0094] Combining equations (10) and (11), the lateral displacement produced by s-polarized light is It can be expressed as:

[0095] (12)

[0096] in, is the lateral displacement produced by s-polarized light, is the refractive index of light in the glass substrate of the first polarization beam splitter 1, n is the refractive index in air, t is the thickness of the glass substrate of the first polarization beam splitter 1, is the angle of incidence of the light hitting the glass substrate, is the refraction angle of the glass substrate.

[0097] The total shear amount of the system is:

[0098] (13)

[0099] in, is the shearing amount produced by the Sagnac broadband linear polarization imaging structure, is the lateral displacement produced by s-polarized light, is the wavelength of the incident light, represents the distance between the first blazed grating 3 and the second blazed grating 4,d is the grating period, t is the thickness of the glass substrate of the first polarization beam splitter 1, is the refractive index of light in the glass substrate of the first polarization beam splitter 1 .

[0100] It can be seen from formula (13) that the presence of the glass substrate causes the s-polarized light to produce a lateral displacement of It has a nonlinear relationship with the wavelength, resulting in a decrease in the dispersion compensation effect of the first blazed grating 3 and the second blazed grating 4.

[0101] Taking the first polarization beam splitter 1 of the Sorebo wire grid as an example, it uses fused silica glass as the substrate with a thickness of , the first blazed grating 3 and the second blazed grating 4 periods , and Spacing between , the refractive index of fused silica As shown in Table 1.

[0102] Table 1 Refractive index of fused silica

[0103]

[0104] The relationship curve between the total shear amount of the system and the wavelength is obtained by MATLAB calculation. Figure 7 As shown in the figure, the original shear amount generated by DCPSI is proportional to the wavelength, and the additional shear amount is approximately inversely proportional to the wavelength. When the wavelength is 550nm, the original shear amount is , additional shear , the two are of similar magnitude, and the additional shear cannot be ignored. Since the additional shear has a nonlinear relationship with wavelength, the wavelength term in the denominator cannot be offset when calculating the carrier frequency. This error will reduce the dispersion compensation effect and must be eliminated or compensated. The solution is to add a glass substrate of the same material and thickness on the other side of the glass substrate of the first polarization beam splitter 1 for compensation. In this way, the optical path of the transmitted p-polarized light and the reflected s-polarized light becomes the same, theoretically completely eliminating the error caused by the glass substrate of the first polarization beam splitter 1.

[0105] The simulation verifies the shear compensation effect as follows: the incident light intensity is uniformly distributed, and the spatial Stokes vector is as follows Figure 8 As shown, the Stokes vector inside the circle is , the Stokes vector outside the circle is .

[0106] When the wavelength width of the incident light is 50 nm, the interference images on the focal plane of the DCPSI and MDCPSI systems are as follows: Figure 9As shown. In order to see it more clearly, cut x The values ​​on the axis, the interference fringes intensity values ​​produced by the two systems are x The distribution on the axis is as follows Figure 10 As shown. Figure 10 It can be seen that in the original DCPSI structure, due to the presence of the glass substrate of the first polarization beam splitter 1, an additional shear amount that is not positively correlated with the wavelength is generated, resulting in dispersion of the carrier frequency and a decrease in the contrast of the interference fringes. The image quality deteriorates more significantly towards the edge. After adding the glass substrate for compensation, the interference pattern of the MDCPSI system is no longer affected by the wavelength.

[0107] When two light waves with the same frequency and the same vibration direction meet at a certain point in space, the vibrations of the light arriving at that point are:

[0108] (14)

[0109] (15)

[0110] in, and is the amplitude of the two light waves at a certain moment, and is the initial amplitude of the two light waves, is the angular frequency of the light waves, t It's time, and is the phase of the two light waves. According to the superposition principle of light, the composite vibration is Still in simple harmonic oscillation, the intensity of the interference light wave after coherence is:

[0111] (16)

[0112] is the total intensity of the two light waves, and are the intensities of the two light waves, and is the phase of the two light waves.

[0113] Fringe modulation is defined as:

[0114] (17)

[0115] in, and are the intensities of the two light waves respectively.

[0116] It can be seen from formula (17) that when and When they are equal, the modulation degree of the interference fringes is the largest, that is, ideally, the modulation degree of the interference fringes is 1. In the MDCPSI system, the interference fringes are generated by the transmitted p-polarized light and the reflected s-polarized light, which take opposite optical paths after passing through the triangular optical path and converge on the focal plane through the imaging lens. Due to the limitations of the actual processing technology, the splitting ratio of the first polarization beam splitter 1 is not the ideal 50:50. After the incident light is diffracted by the first blazed grating 3 and the second blazed grating 4, in addition to the first-order diffraction, other orders of stray light will also enter the system to participate in the imaging, resulting in a decrease in the contrast of the interference fringes. These factors will affect the modulation degree of the interference fringes. Polarization information reconstruction requires that the modulation degree of the interference fringes must not be less than 0.5. Therefore, it is necessary to consider the splitting ratio of the first polarization beam splitter 1 separately. , the diffraction efficiency of the first blazed grating 3 and the second blazed grating 4 for p-polarization and s-polarization 、 , the diffraction efficiency of other diffraction orders of the first blazed grating 3 and the second blazed grating 4 Effect of imaging fringe modulation.

[0117] The MDCPSI system adds two parallel, transmissive first blazed gratings 3 and second blazed gratings 4 to the hypotenuse of the PSI. The diffraction properties of the gratings are used to offset incident light of different wavelengths, so that the shearing of the two final emitted beams is approximately proportional to the wavelength. This results in the interference fringe period on the focal plane being independent of the wavelength, thereby achieving dispersion compensation. MDCPSI requires that the two arms of the system be of equal length. Otherwise, the shearing of the two emitted beams will no longer be proportional to the wavelength, and the carrier frequency expression will contain a component of the inverse of the wavelength, resulting in a poor dispersion compensation effect. In addition to the glass substrate of the first polarization beam splitter 1, the factors that cause the two arms to be of different lengths may also come from assembly errors, that is, the first plane reflector 2 and the second plane reflector 5 are at different distances from the center of the first polarization beam splitter 1. The following analyzes these factors that may cause system errors.

[0118] The first polarization beam splitter 1 transmits p-polarized light and reflects s-polarized light. The ratio of the p-polarized component to the s-polarized component is . When incident light After passing through the first polarization beam splitter 1, the reflected s-polarization and transmitted p-polarization components are: and , after ray tracing calculation, the formula for the interference fringe intensity on the focal plane can be obtained as:

[0119] (18)

[0120] in, is the ratio of the p-polarization component to the s-polarization component, 、 and is the Stokes parameter, is the carrier frequency, are the image plane coordinates.

[0121] It can be seen from formula (18) that the first polarization beam splitter 1 There is an extra DC component in ,right The demodulation causes errors, and The demodulation results will also be affected.

[0122] Set the Stokes parameters of the incident light , different beam splitting ratios are simulated The interference pattern under Figure 11 In order to see the changes of interference fringes more clearly, x =200 coordinates, the intensity value is as follows Figure 12 shown.

[0123] The demodulation must meet the modulation index greater than 0.5, and the project requires that the demodulation error should not exceed 10%. Figure 11 and Figure 12 Get the splitting ratio of different first polarization beam splitters 1 When the values ​​are 1, 1.1, 1.2, and 1.3 respectively, the modulation index of the interference fringes is 0.86, which meets the modulation index requirement. M >0.5. The demodulation error caused is shown in Table 2. The demodulation error increases as the beam splitting ratio difference increases. If the error requirement needs to be met e <10%, the splitting ratio of the first polarization beam splitter 1 should meet the requirements .

[0124] Table 2 Demodulation error caused by the first polarization beam splitter 1 with different splitting ratios

[0125]

[0126] Similarly, the different polarization diffraction efficiencies of the first blazed grating 3 and the second blazed grating 4 will also affect the intensity of the two beams of light emitted by Sagnac. Assume that the p-polarization and s-polarization diffraction efficiencies of the first blazed grating 3 and the second blazed grating 4 are and , the formula for the intensity of the interference fringes on the focal plane after ray tracing is:

[0127] (19)

[0128] in, and are the p-polarization and s-polarization diffraction efficiencies, 、 and is the Stokes parameter, is the carrier frequency, are the image plane coordinates.

[0129] It can be seen from the above formula that the different diffraction efficiencies of the first blazed grating 3 and the second blazed grating 4 for p-polarization and s-polarization will cause errors in the four demodulated Stokes vector values. I There is an additional DC component in the expression of and The amplitude of is affected, and the impact is the same as the splitting ratio of the first polarization beam splitter 1.

[0130] Select the Stokes parameters of the incident light , set the p-polarization diffraction efficiency , s-polarization diffraction efficiency are 1, 0.9, 0.8, and 0.7 respectively. The interference patterns under different conditions are simulated as follows Figure 13 shown.

[0131] based on Figure 13 The modulation degree of interference fringes and demodulation error under different p-polarization and s-polarization diffraction efficiencies are shown in Table 3. The modulation degree of interference fringes meets the modulation degree requirements. M >0.5. To meet the error requirements e <10%, the ratio of p-polarization to s-polarization diffraction efficiency needs to meet >0.83.

[0132] Table 3 Interference fringe modulation and demodulation error under different p-polarization and s-polarization diffraction efficiencies

[0133]

[0134] The ideal transmission type first blazed grating 3 and second blazed grating 4 m The diffraction efficiency of the order can be expressed as:

[0135] (20)

[0136] Where: is the peak-to-peak phase of the first blazed grating 3 and the second blazed grating 4, m is the diffraction order, and a is a constant related to the structure and geometric dimensions of the blazed grating.

[0137] If the peak-to-peak phase variation of the continuous transmission first blazed grating 3 and the second blazed grating 4 is The incident light is completely diffracted to the first order. However, the processing technology of the first blazed grating 3 and the second blazed grating 4 is to form a sawtooth groove cross-section on the grating substrate, and each sawtooth is quantized with steps through multiple exposures. The first-order diffraction efficiency depends on the number of exposures.

[0138] In addition to the different diffraction efficiencies of the first blazed grating 3 and the second blazed grating 4 for p-polarization and s-polarization, which result in different intensities of the two split beams and affect the fringe contrast, the other orders of light diffracted by the first blazed grating 3 and the second blazed grating 4 will also enter the system and cause aliasing of interference fringes. Because only the first-order diffraction efficiency was considered in the previous calculation process, but from the formula of carrier frequency, it can be seen that when considering different orders of diffracted light, the carrier frequency will no longer be a fixed value. The carrier frequency of each order of diffracted light is different. Just like the wavelength will cause dispersion before, the carrier frequency and the diffraction order are closely related. m Proportional to the dispersion compensation result, the interference fringe intensity formula on the focal plane after ray tracing is:

[0139] (twenty one)

[0140] in, The first blazed grating 3 and the second blazed grating 4 m The diffraction efficiency of the order.

[0141] It can be seen from formula (21) that the interference fringe intensity I The middle term is the superposition of diffraction fringes of different orders, which will cause aliasing of fringes. The last term is the superposition of the 0th order light intensity without diffraction on the image, which causes the contrast of fringes to be reduced.

[0142] Set the Stokes parameters of the incident light , The diffraction efficiencies of the first blazed grating 3 and the second blazed grating 40, 1, 2, and 3 orders are selected for simulation. The results are as follows: Figure 14 As shown in the figure, as the first-order diffraction efficiency decreases and the diffraction efficiency of other orders increases, the contrast and frequency of the fringes will change, resulting in a decrease in the quality of the fringes.

[0143] based on Figure 14 The modulation and error results of interference fringes under four conditions are shown in Table 4. M >0.5 and error requirements e <10%, the first-order diffraction efficiency of the first blazed grating 3 and the second blazed grating 4 must meet >79%.

[0144] Table 4 Interference fringe modulation and demodulation error at different diffraction efficiencies of the first blazed grating 3 and the second blazed grating 4

[0145]

[0146] The shear in the Sagnac broadband linear polarization imaging structure is generated by the first and second blazed gratings 3 and 4. The two emitted beams are centrosymmetrical, requiring the two arms of the triangular optical path to be of equal length. Due to the glass substrate of the first polarization beam splitter 1, s-polarized light is refracted within the substrate upon exit, resulting in additional lateral shear. This error can be addressed by adding a compensation plate 9.

[0147] However, in the actual experimental setup and system adjustment, due to human operation accuracy errors, it is impossible to ensure that the lengths of the two right-angled sides are completely equal. Assume that the difference in the distances between the first plane reflector 2, the second plane reflector 5 and the center of the first polarization beam splitter 1 is β , the first plane mirror 2 shifts to the right β The difference in arm length is Figure 15 shown.

[0148] After ray tracing, the two emitted beams of light and Each of them shifted outwards β The shear amount of the system is:

[0149] (twenty two)

[0150] in, is the shear capacity of the system, is the shear amount generated by the Sagnac broadband linear polarization imaging structure, for and Each of them is offset to the outside, is the wavelength of the incident light, represents the distance between the first blazed grating 3 and the second blazed grating 4, d is the grating period.

[0151] The light intensity at the focal plane is:

[0152] (twenty three)

[0153] in, 、 and is the Stokes parameter, is the carrier frequency, are the image plane coordinates. for and Each of them is offset to the outside, is the wavelength of the incident light, represents the imaging focal length, represents the distance between the first blazed grating 3 and the second blazed grating 4, d is the grating period.

[0154] When the system is built, the additional shear introduced by the difference in length between the two arms is constant, and the carrier frequency expression contains wavelength-related terms, which leads to different periods of interference fringes on the focal plane, that is, dispersion. When the difference in length between the two arms is large, dispersion will cause interference fringes to alias, reduce contrast, and affect demodulation.

[0155] when β When the distance is 0mm, 1mm and 10mm respectively, the interference fringes on the focal plane are as follows Figure 16 As shown, in order to observe the fringe aliasing more intuitively, the intercepted light intensity value is x The distribution on the axis is as follows Figure 17 As shown by Figure 17 It can be seen that when the difference between the two arm lengths is larger, the aliasing of the interference fringes is more serious, the fringe quality drops sharply, and even cannot be demodulated. The demodulation error results caused by different differences in the length of the two arms are shown in Table 5. It can be seen from the table that when the error requirements are met e <10%, the assembly error of the first plane reflector 2 and the second plane reflector 5 cannot exceed 2.8 mm, so the assembly tolerance requirement is relatively high. Using the Sagnac wide-band linear polarization imaging structure provided by the present invention, since the first blazed grating 3 and the second blazed grating 4 are on the same straight line, it is more convenient to adjust the spacing and level and the assembly tolerance is easier to ensure, and the system has high adjustability and accuracy.

[0156] Table 5 Demodulation error caused by different arm length differences

[0157]

[0158] The above disclosure is only a preferred specific embodiment of the present invention. However, the embodiments of the present invention are not limited thereto. Any changes that can be conceived by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. A Sagnac broadband linear polarization imaging structure, characterized in that: include: A second plane reflector (5), a first polarization beam splitter (1), a first analyzer (6), a first imaging lens (7), and a first focal plane array (8) are sequentially arranged on a first optical axis, wherein a compensation plate (9) for compensating for shear is provided on one side of the first polarization beam splitter (1), and the compensation plate (9) is provided close to the second plane reflector (5); The first plane reflector (2) and the first polarization beam splitter (1) are spaced apart and arranged on a second optical axis perpendicular to the first optical axis, and are located on a side away from the compensation plate (9); A first blazed grating (3) and a second blazed grating (4) are arranged in opposite directions and spaced apart and located on an optical path between the first plane reflector (2) and the second plane reflector (5), with the first blazed grating (3) being close to the first plane reflector (2); The incident light is sequentially divided into p-polarized light and s-polarized light by passing through the compensation plate (9) and the first polarization beam splitter (1). The p-polarized light is reflected by the first plane mirror (2) and diffracted to the first order by the first blazed grating (3). The diffraction angle is then eliminated by the second blazed grating (4). The light is then reflected by the second plane mirror (5) and sequentially passes through the compensation plate (9), the first polarization beam splitter (1), the first analyzer (6) and the first imaging lens (7). The light is focused on the first focal plane array (8). The s-polarized light sequentially passes through the second plane mirror (5), the second blazed grating (4), the first blazed grating (3) and the first plane mirror (2). After being reflected by the first polarization beam splitter (1), the light is sequentially passed through the first analyzer (6) and the first imaging lens (7). After that, the light interferes with the p-polarized light on the first focal plane array (8) to form interference fringes.

2. The Sagnac broadband linear polarization imaging structure according to claim 1, characterized in that: The first blazed grating (3) and the second blazed grating (4) have the same structure and are placed in parallel.

3. The Sagnac broadband linear polarization imaging structure according to claim 1, characterized in that: The distances between the first polarization beam splitter (1) and the first plane reflector (2) and the second plane reflector (5) are the same.

4. The Sagnac broadband linear polarization imaging structure according to claim 1, characterized in that: The first polarization beam splitter (1) comprises a flat beam splitter and a narrowband beam splitter film which are arranged in sequence, the compensation plate (9) is arranged on one side of the narrowband beam splitter film, and the material and thickness of the compensation plate (9) are the same as those of the flat beam splitter.

5. The Sagnac wide-band linear polarization imaging structure according to claim 4, characterized in that: The compensation plate (9) is a glass substrate.

6. The Sagnac wide-band linear polarization imaging structure according to claim 1, characterized in that: The first polarization beam splitter (1), the first plane reflector (2), and the second plane reflector (5) are all arranged tilted, and respectively conform to the following relationships with the first optical axis and the second optical axis: ɑ=β=90° / 2, γ=φ=ɑ+δ=β+δ, wherein ɑ is the angle between the first polarization beam splitter (1) and the first optical axis, in degrees; β is the angle between the first polarization beam splitter (1) and the second optical axis, in degrees; γ is the angle between the first plane reflector (2) and the second optical axis, in degrees; φ is the angle between the second plane reflector (5) and the first optical axis, in degrees; and δ is the grating blaze angle of the first blazed grating (3) and the second blazed grating (4), in degrees.

7. The Sagnac broadband linear polarization imaging structure according to claim 6, characterized in that: The δ is 20°~25°.

8. The Sagnac wide-band linear polarization imaging structure according to claim 7, characterized in that: The δ is 22.5°.

Images