Infrared achromatic phase retarder

Abstract

The invention belongs to the field of polarization optics and instruments, and discloses an infrared phase aberration eliminating phase retarder. The phase retarder is formed by attaching two rhombic prisms made of infrared optical materials, and one or more reflecting surfaces of the phase retarder are coated with metal reflecting films. Incident light beams are reflected for four times in the prism and then are emitted, and a specific amount of phase difference is introduced between a p component and an s component of light waves through total internal reflection and metal film reflection. The phase delayer can be designed to be used for near-infrared, medium-wave and long-wave infrared bands, has a certain phase aberration eliminating capability in the designed bands, can realize phase delay of 90 degrees, 180 degrees or other specific values, is parallel to an emergent light beam and an incident light beam, has no transverse movement of the light beam under the normal incidence condition, and is also easy to process and manufacture. The achromatic phase retarder can be used in various infrared polarization optical systems and is used for realizing conversion of infrared light wave polarization states.

Description

Technical Field

[0001] This invention belongs to the field of polarization optics and instruments, and relates to an infrared achromatic phase delay device that utilizes total internal reflection and metal film reflection to generate phase delay. This invention can be widely applied in various infrared polarization optical systems and measurement and analysis instruments to achieve the transformation of the polarization state of infrared light waves. Background Technology

[0002] This invention relates to polarization optical devices, and more particularly to an achromatic infrared phase delayer capable of generating a phase delay between two orthogonal polarization components of an infrared light wave within a relatively large infrared band. The achromatic effect described in this invention refers to the elimination of phase chromatic aberration.

[0003] A phase retarder is a polarization optical element that introduces a phase difference between two orthogonal polarization components of a light wave without changing its amplitude. Common phase retarders include waveplates, compensators, Fresnel prisms, and liquid crystal phase retarders. Phase retarders are widely used in various polarization-dependent optical systems and instruments to change or test the polarization state of light waves, making them a very widely applied fundamental polarization optical element.

[0004] Waveplates are typically made of materials such as quartz crystals, MgF2 crystals, mica, and organic polymers, and their application range is generally in the 200nm–2000nm wavelength band. Beyond this band, the transmittance of the material is very low, or the birefringence of the material is unknown, making these materials unsuitable for waveplate fabrication. Furthermore, crystalline materials exhibit significant birefringence dispersion, limiting waveplates made from crystalline materials to monochromatic light or narrow-band spectra. While achromatic waveplates can be fabricated within these bands using two or more different crystalline materials in combination, such designs are demanding, complex, and costly, and their actual achromatic performance is not very strong due to design and manufacturing errors. Waveplates made from high-molecular-weight organic polymer materials, such as polyester film and polyvinyl alcohol, generally have lower precision and are also unsuitable for mid-wave and long-wave infrared bands. Compensators, such as the common Barbinette-Soler compensator, are also made from crystalline materials like quartz and suffer from the same problems. Liquid crystal phase retarders are adjustable phase retarders designed using the electrically controlled birefringence effect of liquid crystals. They allow for continuous adjustment of the birefringence phase retardation and are a relatively new type of device. However, liquid crystal phase retarders suffer from poor temperature stability, slow response time, and susceptibility to voltage stability issues. Furthermore, liquid crystal materials exhibit dispersion characteristics and are unsuitable for mid- and long-wave infrared bands. Fresnel prisms are a common type of prism-type phase retarder. They utilize total internal reflection within the prism to introduce a specific phase difference between the p and s components, offering advantages such as simple and stable structure, good achromatic performance, and excellent temperature stability. Theoretically, Fresnel prisms can be fabricated using infrared optical materials, but suitable materials for producing a 90-degree phase retardation are difficult to find in the mid- and long-wave infrared bands. Additionally, while the incident and emitted beams of a Fresnel prism are parallel, there is a significant lateral offset, which is inconvenient in practical applications, especially when rotating the phase retarder is required.

[0005] Infrared polarization optical systems and instruments frequently require infrared phase retarders with achromatic capabilities, but existing technologies lack suitable phase retarders with good achromatic characteristics for the mid- and long-wave infrared bands. The closest to practical infrared achromatic phase retarder described in the literature is the device in the patent "Achromatic Infrared Retarder" (DH Goldstein, US 4917461, 1990). This device employs a three-part structure, consisting of two infrared prisms and a metal mirror, providing a 90° phase retardation in the 3–14 μm band with a chromatic aberration within approximately 1.6°, and ensuring the outgoing beam is parallel to the incident beam. However, this phase retarder suffers from drawbacks: the three-part structure is difficult to assemble, prone to structural misalignment, and the collimated beam undergoes a mirror flip when passing through the phase retarder. The main objective of this invention is to solve these two problems, while also achieving good achromatic capability and a large incident aperture angle. Summary of the Invention

[0006] The purpose of this invention is to overcome the instability and misalignment issues of existing infrared achromatic phase retarders with a three-segment structure, as well as the problem of beam mirror flipping, while still providing good infrared achromatic capability. To this end, based on the physical mechanism of light wave reflection, an infrared achromatic phase retarder is designed, characterized by:

[0007] (a) Composed of two identical rhombic prisms made of preferred infrared optical material, wherein the second end face of the first rhombic prism and

[0008] The first end faces of the second rhombic prism are attached face-to-face to form a mirror-symmetric structure, and the principal sections of the two rhombic prisms are kept parallel to each other; a metal reflective film layer can be deposited on the reflective surface of each rhombic prism; the optical axis of the phase retarder is perpendicular to the front and rear end faces of the prism.

[0009] (b) In the case of normal incidence, the incident light ray is perpendicular to the first end face of the first rhomboid prism and passes through the two sides of the first rhomboid prism.

[0010] After two internal reflections from the parallel inclined planes, the light enters the second oblique prism. After further two internal reflections from the two parallel inclined planes of the second oblique prism, it exits from the second end face of the second oblique prism. The outgoing ray is parallel to the incident ray with no lateral deviation, and...

[0011] They are located on the same straight line; if the incident light is a collimated beam, then the outgoing light is a collimated beam parallel to the incident light; in the four reflections, there may be one or more reflections by the metal film, and the rest are total internal reflections.

[0012] (c) The infrared optical materials, refractive index, and prism apex angle should be selected according to the working band and the required phase delay; the infrared optical materials used should be isotropic, stress-free, and have high transmittance.

[0013] (d) The total phase delay generated can be 90°, 180° or other values, depending on the specific design.

[0014] The sharp apex of the rhomboid prism can be appropriately cut off according to tooling and volume requirements. The infrared optical materials include, but are not limited to, calcium fluoride, zinc selenide, arsenic sulfide, zinc sulfide, silicon, germanium, and gallium arsenide. The metal reflective film can be a high-reflectivity film such as gold, silver, or aluminum; the type of metal film is selected based on its optical properties and adhesion to the substrate material. The film generally needs to have a thickness of 100 nm or more. The internal reflection occurs a total of four times within the phase retarder, from the incident end face along the beam propagation path to the exit end face, with each rhomboid prism accounting for two of these reflections.

[0015] Compared with the prior art, the phase delayer designed in this invention has the following advantages: simple structure, consisting of only two identical rhombic prisms, easy to assemble and adjust, and the structure is not easily misaligned after assembly and adjustment; the outgoing beam is parallel to the incident beam, with no lateral offset and no vertical mirror flip; it has a certain degree of achromaticity; it is suitable for near-infrared, mid-wave and long-wave infrared bands; it has good temperature stability; and the reflection angles of the four internal reflections are complementary, which can effectively increase the incident aperture angle. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure and optical path of the infrared achromatic phase delayer of the present invention (Example 1);

[0017] Figure 2 A schematic diagram of the shape of one of the rhombic prisms that constitute an infrared achromatic phase delayer;

[0018] Figure 3 This is a schematic diagram of the internal reflection angle and optical path when light is incident directly into an infrared achromatic phase delayer.

[0019] Figure 4 This is a schematic diagram of the incident angle, internal reflection angle, exit angle, and light path when light is incident at an oblique angle.

[0020] Figure 5 The curve showing the relationship between the incident aperture angle and the total phase delay in Example 1;

[0021] Figure 6 The curve showing the relationship between the incident aperture angle and the total phase delay in Example 2;

[0022] Figure 7This is a schematic diagram of the infrared achromatic phase delayer described in Example 3;

[0023] Figure 8 The curve showing the relationship between the incident aperture angle and the total phase delay in Example 3;

[0024] Figure 9 This is a schematic diagram of the infrared achromatic phase delayer described in Example 4;

[0025] Figure 10 The curve showing the relationship between the incident aperture angle and the total phase delay in Example 4 is shown.

[0026] Figure 11 This is a schematic diagram of the infrared achromatic phase delayer described in Example 5;

[0027] Figure 12 The curve showing the relationship between the incident aperture angle and the total phase delay in Example 5; Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] like Figure 1 As shown, the infrared achromatic phase retarder consists of prisms 1 and 2, which are identical in shape and material. The second end face 4 of prism 1 and the first end face 5 of prism 2 are fitted together face-to-face, maintaining the parallelism of the principal sections of the two prisms. Prisms 1 and 2 are essentially rhomboid prisms with two sharp vertices removed. Figure 2 As shown, taking prism 1 as an example, its first end face 3 and second end face 4 are parallel to each other, its first inclined plane 7 and second inclined plane 8 are parallel to each other, the acute apex angle 16 is α in size, and the apex angles 17 and 18 are cut off, the amount of removal being such that no light rays are cut off, or it may not be removed at all. Figure 1 In the phase retarder, the optical axis 15 is perpendicular to the incident end face 3 and the exit end face 6, and passes through the center of both end faces. Ray 11 is incident parallel to the optical axis 15, undergoes four reflections inside the phase retarder, and exits. The exiting ray 12 is parallel and collinear with the incident ray 11. Similarly, ray 13 is incident parallel to the optical axis 15, undergoes four reflections inside the phase retarder, and exits. The exiting ray 14 is parallel and collinear with the incident ray 13. Figure 1 In the embodiment shown, a metal reflective film is coated on the inclined surface 9 of the prism 2. Therefore, in the four reflections of light inside the phase delayer, the first three are total internal reflections, and the last is an internal reflection of the metal-coated film.

[0030] According to the above description, a feature of the phase delayer of the present invention is that it is composed of only two rhomboid prisms (with sharp apex angles that can be cut off) that are identical in shape and material. The structure is simple and reliable, and easy to process and adjust.

[0031] According to the above description, another feature of the phase delayer of the present invention is that the outgoing beam and the incoming beam are completely parallel. Under normal incidence (i.e., the incident light is parallel to the optical axis), there is no lateral offset between the outgoing beam and the incident beam, and the outgoing light and the incident light are on the same straight line.

[0032] The third feature of the phase delayer described in this invention is that, by selecting different optical materials, prism apex angles, and the number of coating surfaces, various phase delay values, including but not limited to 90° and 180°, can be achieved. The number of surfaces of the metal reflective coating can be one or more.

[0033] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are only for illustrating the present invention in detail and are not intended to limit the invention. In fact, the present invention can be implemented using many other specific embodiments, such as changing materials, wavebands, coatings, or altering prism angles. The scope of protection of the present invention is defined by the appended claims.

[0034] Example 1

[0035] This embodiment describes an infrared achromatic phase delayer suitable for the infrared 3–5 μm band, capable of generating a 90° phase delay. For example... Figure 1 , 2 As shown, the design wavelength is 4μm, the infrared optical material is zinc selenide (ZnSe), the acute apex angle α of prism 1 and prism 2 is 36.64°, and the inclined surface 9 of prism 2 is coated with a gold film with a thickness of not less than 100nm.

[0036] First consider the case where the light is incident normally, such as Figure 3 As shown, the angle of incidence of the light ray inside prism 1 is θ1, and the angle of incidence inside prism 2 is θ2. According to geometric relationships, θ1 = θ2 = α. The light ray undergoes total internal reflection on surfaces 7, 8, and 10. The phase delay produced by a single total internal reflection is determined by the following formula.

[0037]

[0038] In the formula θi is the incident angle for total internal reflection, n1 is the refractive index of the prism material, and n2 is the refractive index of the external medium of the prism. The phase delay δM caused by the reflection of light on the gold-plated surface 9 is determined by the following set of formulas.

[0039]

[0040] δM = arg(rs / rp)

[0041] In the formula θi is the incident angle when reflected from the gold-plated surface 9, θt is the corresponding refraction angle, and n1 is the refractive index of the prism material. Let be the complex refractive index of the gold film, and n and κ be the optical constants of the gold film. The total phase delay produced by the phase retarder is δ = 3δTIR + δM. Table 1 below gives the relevant data for the 3–5 μm band.

[0042] Table 1.

[0043]

[0044] As can be seen from Table 1, the fourth characteristic of this phase delayer is that it has good achromaticity in the mid-infrared band with wavelengths of 3 to 5 μm, with a maximum deviation of about 1.7°.

[0045] The following is combined Figure 4 This embodiment describes the incident aperture angle. The angle between the incident ray 21 and the optical axis 15 is... After entering the first end face 3 of prism 1, the angle between the refracted ray and the optical axis 15 is... According to the law of refraction, we have the following relationship: Based on the geometric relationships in the diagram, the angle of incidence when the light undergoes total internal reflection inside prism 1 can be determined. Angle of incidence when reflected inside prism 2 Therefore, the phase delayer described in this embodiment has complementary internal reflection angles θ1 and θ2. Taking a design wavelength λ = 4μm as an example, with normal incidence... The total phase delay is 90.00°. Figure 5 The incident aperture angle is given. The curve showing the relationship between the incident aperture angle and the total phase delay. The total delay of the time phase delayer is 89.0°, and the incident aperture angle is... The total delay of the time phase delay is 91.0°. When light ray 21 travels at an aperture angle... With oblique incidence, the angle between the outgoing ray 22 and the optical axis 15 remains the same. The outgoing ray 22 is parallel to the incident ray 21. At this time, there will be a lateral offset between the two, but the offset is very small and generally has little impact on actual use.

[0046] The phase retarder described in this embodiment has an aperture of 10 mm at its incident end face 3, and the geometric path length of light propagating inside the phase retarder is 47.76 mm, with a "length / aperture" ratio of approximately 4.8:1. Theoretically, triple total internal reflection does not cause any loss in light intensity. At normal incidence, the p-component and s-component have the same transmittance at the incident end face 3 and the exit end face 6, but the reflectances of the p-component and s-component at the zinc selenide-gold film interface are slightly different. The light intensity reflectance of the p-component is Rp = 0.9823, and the light intensity reflectance of the s-component is Rs = 0.9886. Therefore, the bidirectional attenuation factor of this phase retarder is approximately 0.0032.

[0047] Example 2

[0048] This embodiment describes an infrared achromatic phase delayer capable of generating a 90° phase delay, applicable to the infrared 8–12 μm band. A schematic diagram is shown below. Figures 1-4 As shown, the difference from Example 1 is that the selected design wavelength is 10 μm, and the acute apex angle α of both prisms is 34.12°. The infrared optical material used in this example is zinc selenide (ZnSe), and a gold film is deposited on the inclined surface 9 of prism 2. Table 2 provides relevant data for the 8–12 μm band.

[0049] Table 2.

[0050]

[0051] Figure 6 The incident aperture angle at the designed wavelength in this embodiment is given. As shown in the figure, if the allowable phase retardation deviation is ±1°, the incident aperture angle is approximately -5.4° to +2.80°. If the aperture of the incident end face 3 is 10mm, the geometric path length of the light propagating inside the phase retarder is 51.06mm, and the "length / aperture" ratio is approximately 5.1:1. The light intensity reflectivities of the p-component and s-component of the normally incident light at the zinc selenide-gold film interface are Rp = 0.9830 and Rs = 0.9883, respectively, and the bidirectional attenuation factor of this type of phase retarder is approximately 0.0027.

[0052] Example 3

[0053] This embodiment describes an infrared achromatic phase delayer capable of generating a 180° phase delay in the infrared 3–5 μm band. For example... Figure 7 As shown, the preferred center wavelength is 4 μm, the infrared optical material is zinc selenide (ZnSe), the acute apex angle α of both prisms is 40.22°, and gold films are deposited on the reflective surfaces 7 and 9 of prism 1 and prism 2. Table 3 shows the relevant data for the 3–5 μm band. The slight chromatic aberration in the total phase delay mainly comes from the two reflections of the gold films.

[0054] Table 3.

[0055]

[0056] Figure 8 The relationship curve between the phase delay generated by the phase retarder in this embodiment and the incident angle is given. As can be seen from the figure, if the allowable phase delay decreases to 179°, the incident aperture angle can reach approximately ±9.8°. In this embodiment, due to the complementarity of the internal reflection angles θ1 and θ2, the phase delay generated by the two total internal reflections on surfaces 8 and 10 is complementary, and the phase delay generated by the two internal reflections of the gold film on surfaces 7 and 9 is also complementary, thus resulting in a larger incident aperture angle.

[0057] Example 4

[0058] This embodiment describes an infrared achromatic phase retarder capable of generating a -135° phase retardation in the infrared 3–5 μm band. The design wavelength is chosen to be 4 μm, and the infrared optical material is silicon (Si). Figure 9 As shown, the acute apex angles of both prisms are 30.52°, and gold films are deposited on the reflective surfaces 7 and 9 of prism 1 and prism 2. Table 4 provides the relevant data for the 3–5 μm band. The slight chromatic aberration in the total phase delay mainly comes from the two gold film reflections.

[0059] Table 4.

[0060]

[0061] Figure 10 The relationship between the phase delay generated by the phase delayer in this embodiment and the incident angle is shown in the curve. As can be seen from the figure, for the design wavelength, if the allowable phase delay decreases to -136°, the incident aperture angle can reach approximately ±13.5°. The "length / aperture" ratio in this embodiment is approximately 5.7:1.

[0062] Example 5

[0063] This embodiment describes an infrared achromatic phase retarder capable of producing a 180° phase retardation in the 8–14 μm infrared band. The design wavelength is chosen to be 10 μm, and the infrared optical material is germanium (Ge). Figure 11 As shown, the acute apex angles of both prisms are 45.41°, and silver films are coated on the reflective surfaces 7 and 9 of prism 1 and prism 2. Table 5 provides the relevant data for the 8–14 μm band. The slight chromatic aberration in the total phase delay mainly comes from the two silver film reflections.

[0064] Table 5.

[0065]

[0066] Figure 12 The relationship between the phase delay produced by the phase retarder in this embodiment at the designed wavelength and the incident angle is shown in the figure. As can be seen from the figure, a significant feature of this phase retarder is that the incident angle has a very small impact on the phase delay. In addition, the "length / aperture" ratio of this embodiment is approximately 4.1:1.

[0067] The specific embodiments of the present invention have been described above with reference to the accompanying drawings. However, these descriptions should not be construed as limiting the scope of the present invention. The scope of protection of the present invention is defined by the appended claims. Any modifications made based on the claims of the present invention are within the scope of protection of the present invention.

Claims

1. An infrared achromatic phase retarder for use in an infrared polarization sensitive optical system to change the polarization state of an infrared light wave, characterized in that: a) two rhombic prisms made of infrared material are used; b) one or more of the reflecting surfaces of the retarder are coated with a metal film; c) a phase retardation is introduced between the p and s components by total internal reflection and metal film reflection; d) the emergent light is parallel to the incident light without lateral displacement, lying on the same straight line. The infrared waveband can be near infrared, mid infrared or far infrared. The infrared material used to make the prisms is an isotropic homogeneous optical material, including but not limited to calcium fluoride CaF2, zinc selenide ZnSe, arsenic sulfide As2S3, zinc sulfide ZnS, silicon Si, germanium Ge, gallium arsenide GaAs, etc. The two rhombic prisms are identical in shape and material; the size of the top angle of the prisms is determined according to the total phase retardation required by the phase retarder, the refractive index of the material, and the optical parameters of the metal film. The second end face (light exit end face) of the first rhombic prism and the first end face (light entrance end face) of the second rhombic prism are stress-free optically cemented (or glued) together in a face-to-face manner, and the main sections of the two rhombic prisms are parallel to each other, and the reflecting surfaces of the prisms can be coated with a metal film. The metal film coated on the reflecting surface can be a high-purity gold film, silver film, aluminum film, etc. The total phase retardation introduced by the phase retarder is not adjustable, but according to different design forms, the total phase retardation can be 90°, 180° or other values. For use in a collimated light path, the incident light beam and the emergent light beam are both collimated light beams, and the emergent collimated light beam is parallel to the incident collimated light beam. ​ ​ ​ ​ 2. The infrared achromatic phase retarder of claim 1, wherein: ​ 3. The infrared achromatic phase retarder of claim 1, wherein: ​ 4. The infrared achromatic phase retarder of claim 1, wherein: ​ 5. The infrared achromatic phase retarder of claim 1, wherein: ​ 6. The infrared achromatic phase retarder of claim 1, wherein: ​ 7. The infrared achromatic phase retarder of claim 1, wherein: ​ 8. The infrared achromatic phase retarder of claim 1, wherein: ​

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