A switchable optical filter

Abstract

The application discloses a switchable optical filter, which adopts a three-stage polarization grating cascaded beam splitting and combining structure, a partition type electrically controlled optical switch is arranged between the first and second polarization gratings, each partition corresponds to a dispersion spot one by one, and each wavelength beam is independently controlled to be turned on or turned off; wide-band uniform light splitting is realized by polarization selective diffraction, wavelength gating is completed by polarization, partition type electrically controlled polarization adjustment and orthogonal detection polarization, the last two gratings complete beam combining and collimation, and a full solid-state transmission light path is constructed. The application is a full solid-state electrically controlled optical filter, has fast response speed, no mechanical wear, balanced light splitting efficiency in a wide spectrum, supports multi-wavelength parallel output, has a simplified structure and is flexible in regulation and control.

Description

Technical Field

[0001] This invention relates to optical filtering devices, and more specifically to a transmissive, electrically controlled, switchable filter. Background Technology

[0002] Spectral filters are core components in optical systems used to filter beams of target wavelengths. Based on their wavelength adjustment capability, they can be divided into two categories: fixed filters and tunable filters. Among them, grating-type tunable filters achieve wavelength separation based on the principle of diffraction scattering, featuring good splitting properties and a wide applicable spectral range, making them the most widely used filtering solution in the industry.

[0003] Currently, mainstream grating-type tunable filters generally adopt a structure of a single diffraction grating combined with a mechanical slit: multi-wavelength mixed light is collimated by an incident collimating lens and then enters the grating, forming a dispersive light spot arranged in wavelengths in space through diffraction; then the slit is translated by a motor to align the light passage with the light spot of the target wavelength to achieve wavelength selection; the filtered monochromatic light is finally output after being focused and corrected by the collimating lens group.

[0004] This type of solution has a mature principle and clear spectral dispersion, but it has inherent defects that are difficult to overcome: First, wavelength switching relies entirely on mechanical moving parts such as motors and transmission mechanisms, resulting in slow response speeds. A single switch typically takes hundreds of milliseconds, and long-term reciprocating motion can easily cause mechanical wear, backlash errors, and temperature drift, leading to insufficient long-term stability and limited lifespan of the device. Secondly, after beam splitting by a single grating, there is natural angular divergence in the beam. An optical system consisting of multiple sets of focusing lenses and correction lenses must be configured to achieve coaxial output. The number of optical path components is large, the overall structure is complex, the assembly and adjustment are difficult, and the size of the device is difficult to compress.

[0005] Besides mechanical grating filtering solutions, existing electrically controlled tunable filters, such as Lyot-type liquid crystal tunable filters and acousto-optic tunable filters, can achieve rapid wavelength adjustment without mechanical movement, but they are not based on the spatial dispersion beam splitting mechanism and do not have the ability to independently control the on / off state of multiple discrete wavelengths. At the same time, these solutions generally suffer from problems such as high-order stray light, poor coaxiality of the output beam, and low laser damage threshold, making it difficult to meet the needs of application scenarios such as multi-wavelength parallel output and high-power optical paths. Summary of the Invention

[0006] Purpose of the invention: The purpose of this invention is to provide a switchable filter to solve the problems of slow response, poor stability, short lifespan, and complex optical path of existing technologies, which cannot independently select multiple paths.

[0007] Technical Solution: The switchable filter of this invention includes an incident collimating lens, a diffraction grating, and a partitioned electro-optical switch arranged sequentially along the incident optical axis. The diffraction grating consists of three polarization gratings with the same vector direction but different periods, arranged sequentially along the optical path as a first polarization grating, a second polarization grating, and a third polarization grating, with the period of the second polarization grating being smaller than that of the first polarization grating. The partitioned electro-optical switch is located between the first and second polarization gratings, with each partition corresponding one-to-one with the spatial position of each wavelength beam dispersed by the first polarization grating, used to independently control the on / off state of each wavelength beam. The second polarization grating is used to deflect and combine the selected wavelength beams, and the third polarization grating is used to correct the combined beam to a coaxial output parallel to the incident optical axis.

[0008] This invention employs a beam splitting and combining architecture with a three-stage cascaded polarization grating. Spatial dispersion beam splitting is achieved through the first polarization grating, wavelength selection is achieved through the intermediate partitioned electro-optical switch, and beam combining and collimation correction are achieved through diffraction angle compensation in the latter two stages, thus constructing a fully transmissive coaxial optical path.

[0009] Furthermore, the polarization grating is a liquid crystal polymer polarization grating based on the geometric phase principle. This type of polarization grating forms a pure phase diffraction structure through the periodic orientation arrangement of liquid crystal molecules, and achieves phase dispersion by relying on polarization-selective diffraction. It has the characteristics of high diffraction efficiency, thin and light device, and easy large-area fabrication. Its outgoing light polarization state is uniform and controllable, and can form a full-link polarization adaptation with the subsequent polarization-type electrically controlled gating mechanism.

[0010] Furthermore, the liquid crystal polymer polarization grating is a broadband polarization grating. It can be fabricated using a double-twisted structure with two layers of liquid crystal polymer thin films stacked in a mirror image. Through the phase compensation effect of the reverse twisted structure, the phase evolution deviation of different wavelengths is offset, maintaining a stable high-order diffraction efficiency in a wide spectral range from visible light to near-infrared.

[0011] Furthermore, the partitioned electro-optical switch comprises a first linear polarizer, a partitioned TN-type liquid crystal cell, and a second linear polarizer arranged sequentially along the optical path, with the transmission axes of the two linear polarizers being orthogonal to each other. The entire optical switch utilizes the electro-optic effect of the TN-type liquid crystal cell to complete polarization state switching, achieving a response speed in the millisecond range.

[0012] Furthermore, a strip-shaped ITO transparent electrode is fabricated on one side of the substrate of the partitioned TN-type liquid crystal cell. Each strip electrode is arranged along the grating dispersion direction and can be driven independently. The extension direction of the strip electrode is perpendicular to the grating vector direction. This arrangement precisely matches the elongated light spot shape after dispersion by the first polarization grating, realizing a one-to-one spatial correspondence design of a single electrode corresponding to a single monochromatic wavelength beam, ensuring the accuracy of independent single-wavelength control and the on / off extinction ratio.

[0013] Furthermore, the exit surface of the third polarization grating directly serves as the output end of the device, and the combined beam is directly emitted from the third polarization grating. Self-beam combining and self-collimation are achieved through the diffraction angle compensation effect of the three-level grating, eliminating the need for additional output collimating lenses and beam correction lens groups.

[0014] Furthermore, the broadband polarization grating employs a double-twisted structure, consisting of two mirror-symmetrically stacked liquid crystal polymer films. The liquid crystal molecules within the two films are twisted in opposite directions with equal twist angles. When incident light passes sequentially through the two oppositely twisted liquid crystal layers, the polarization state evolution deviation introduced by the first layer can be compensated and corrected by the mirror structure of the second layer, thus offsetting the phase evolution deviation at different wavelengths. This maintains a stable higher-order diffraction efficiency across a wide spectral range from visible to near-infrared.

[0015] Furthermore, the partitioned TN-type liquid crystal cell is filled with nematic liquid crystal, and the liquid crystal molecules are aligned to form a 90° helical twisted structure. When no driving voltage is applied, the incident linearly polarized light can rotate its polarization direction synchronously with the helical arrangement of the liquid crystal molecules, and the on / off control of the light beam can be achieved in conjunction with the orthogonally arranged linear polarizers on both sides.

[0016] Furthermore, the width of each strip electrode along the grating dispersion direction matches the width of the corresponding monochromatic beam spot dispersed by the first polarization grating, with each strip electrode corresponding to the spatial position of a monochromatic beam. This size-space matching design ensures that a single wavelength beam only falls into the control region of the corresponding electrode, avoiding optical crosstalk between adjacent wavelength channels.

[0017] Furthermore, the switchable filter is an all-solid-state transmissive electronically controlled device that relies on a partitioned electronically controlled optical switch to complete wavelength switching, and there are no mechanical moving parts in the optical path for wavelength adjustment.

[0018] The complete working process of the switchable filter described above is as follows: A multi-wavelength mixed incident beam is collimated into a parallel beam by an incident collimating lens and then incident on a first polarization grating. The first polarization grating, through transmission-type angular dispersion, splits the beams of different wavelengths into multiple independent monochromatic sub-beams with different diffraction angles along the dispersion direction. Each spatially discrete monochromatic sub-beam corresponds to an independent zone of an incident zone-type electro-optical switch. By independently applying electrical signals to each zone, the on / off state of the corresponding wavelength beam is controlled, allowing the target wavelength beam to be transmitted and the non-target wavelength beam to be cut off. The selected monochromatic sub-beams continue to propagate to a second polarization grating, where they are deflected and combined, causing the wavelength beams to converge towards the central optical axis of the system. The combined beam undergoes angle correction by a third polarization grating, ultimately forming a coaxial combined beam output parallel to the incident optical axis.

[0019] By adjusting the driving electrical signals of each zone of the electronically controlled optical switch, flexible switching of single or multiple discrete target wavelengths can be achieved. The wavelength combination can be dynamically adjusted simply by modifying the electrical signal allocation rules, supporting multiple working modes such as single-wavelength output and multi-wavelength parallel output.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The present invention is electrically controlled, and the wavelength switching response is rapid, which can avoid the performance degradation caused by mechanical wear and position drift, and improve long-term working stability and service life; the splitting efficiency is balanced in a wide spectral range, supports multi-wavelength parallel output, the power consistency of each output beam is good, and the wavelength adjustment is flexible and convenient; the overall structure is simple and compact, the device size is small, the assembly and adjustment difficulty is low, and it can be adapted to a variety of application scenarios. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of the filter; Figure 2 This is a side view of a wideband polarization grating based on a double-twisted structure. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0023] The wideband switchable filter provided by this invention is an all-solid-state transmissive coaxial optical device that can be applied to scenarios such as spectral detection, multi-wavelength beam gating, and optical imaging in the visible light band. It can achieve millisecond-level electronically controlled switching of multi-channel discrete wavelengths within the 400-700nm working band and supports single-wavelength or multi-wavelength parallel output.

[0024] like Figure 1 As shown, the filter comprises, coaxially arranged along the propagation direction of the incident principal optical axis, an incident collimating lens 1, a first broadband liquid crystal polymer polarizing grating 2, a first linear polarizer 3, a partitioned TN-type liquid crystal cell 4, a second linear polarizer 5, a second broadband liquid crystal polymer polarizing grating 6, and a third broadband liquid crystal polymer polarizing grating 7. The grating vector directions of the first polarizing grating 2, the second polarizing grating 6, and the third polarizing grating 7 are consistent, but their grating periods are different. The grating period of the second polarizing grating 6 is less than that of the first polarizing grating 2. The three gratings sequentially perform the third-order diffraction functions of beam divergence, beam convergence and deflection, and beam collimation and combining along the optical path.

[0025] The first linear polarizer 3, the partitioned TN-type liquid crystal cell 4, and the second linear polarizer 5 together constitute a partitioned electro-optical switch, positioned between the first polarizing grating 2 and the second polarizing grating 6. The striped partitions of the partitioned TN-type liquid crystal cell 4 are arranged along the dispersion direction of the grating, corresponding one-to-one with the spatial positions of each monochromatic beam dispersed by the first polarizing grating 2, and are used to independently control the on / off state of each wavelength beam. The exit surface of the third polarizing grating 7 directly serves as the output end of the device, and the combined beam is emitted directly without the need for an additional output collimating lens.

[0026] In this embodiment, the effective focal length of the incident collimating lens 1 is about 50mm. It is used to shape the incident multi-wavelength mixed beam (such as an optical fiber output beam or a free space source beam) into a highly parallel collimated parallel beam with a parallelism of ≤0.1°, so that it enters the first polarization grating 2 with a vertical incident attitude, providing stable incident conditions for subsequent grating dispersion beam splitting, and ensuring the consistency and stability of the diffraction angle of each wavelength.

[0027] All three polarization gratings are broadband liquid crystal polymer polarization gratings based on the Pancharatnam-Berry geometric phase principle, and their cross-sectional structures are as follows: Figure 2 As shown, the grating is composed of a glass substrate and two mirror-symmetric stacked liquid crystal polymer functional films. The liquid crystal molecules inside the two films are twisted in opposite directions and have complementary twist angles, forming a double-twist compensation structure. When incident light passes through the two reverse-twisted liquid crystal layers in sequence, the polarization state evolution deviation introduced by the first layer can be compensated and corrected by the mirror structure of the second layer. This breaks the limitation of single-layer polarization gratings that only have narrow-band high-efficiency diffraction, and maintains a stable higher-order diffraction efficiency in the working wavelength range of 400-700nm, with the difference in diffraction efficiency between wavelengths ≤15%.

[0028] The first polarizing grating 2 has a grating period of 2.5 μm and performs the function of diffracting beams. According to the grating equation, it performs transmission diffraction on the incident parallel multi-wavelength beams, causing different wavelength beams to produce different diffraction deflection angles, and spatially splitting them into mutually separate monochromatic sub-beams. The second polarizing grating 6 has a grating period of 1.0 μm, which is smaller than the period of the first polarizing grating. It performs the function of converging deflection. According to the grating equation, a smaller grating period corresponds to a larger diffraction deflection angle for the same wavelength. Through the period matching design, each wavelength tilted beam obtains an additional deflection pointing towards the optical axis of the system center, and gradually moves closer to the optical axis along the propagation path. The third polarizing grating 7 has a grating period of 1.5 μm and performs the function of collimation correction. It applies the final angle correction to each beam after it is converged by the second polarizing grating, so that the output direction of all selected wavelength beams is restored to be strictly parallel to the incident principal optical axis.

[0029] The partitioned electro-optical switch is composed of a first linear polarizer 3, a partitioned TN-type liquid crystal cell 4, and a second linear polarizer 5 arranged sequentially along the optical path, with the transmission axes of the two linear polarizers orthogonal to each other. The first linear polarizer 3 acts as a polarizer, converting the circularly polarized monochromatic light diffracted by the first polarization grating into linearly polarized light; the second linear polarizer 5 acts as an analyzer, controlling the on / off state of the light beam through polarization state matching.

[0030] The partitioned TN-type liquid crystal cell 4 is encapsulated by two glass substrates with ITO transparent electrodes, and the cell is filled with nematic liquid crystal material; the ITO electrodes on one side of the substrate are made into parallel strip structures, the strip electrodes extend in the vertical direction and are periodically arranged in the horizontal dispersion direction, with a total of 24 independent channels, each channel width of about 0.4 mm. Each strip electrode can be driven by an independent voltage, and the channel position corresponds one-to-one with the monochromatic light spot after dispersion by the first polarization grating.

[0031] When no driving voltage is applied, the liquid crystal molecules in the corresponding partition are arranged in a 90° spiral twist along the alignment direction. The incident linearly polarized light rotates 90° synchronously after passing through the liquid crystal layer, aligning with the transmission axis of the second linear polarizer 5. The light beam is transmitted, and the corresponding wavelength channel is in a pass state. When a saturation driving voltage is applied, the liquid crystal molecules are vertically aligned along the electric field direction, the spiral twist structure disappears, the polarization direction of the linearly polarized light does not change, and it is orthogonal to the transmission axis of the second linear polarizer 5. The light beam is cut off, and the corresponding wavelength channel is in an open circuit state. In this embodiment, the channel contrast ratio (extinction ratio) of the optical switch is greater than 5000:1.

[0032] This embodiment employs an all-solid-state transmission optical path design, eliminating mechanical moving parts. The single wavelength switching time is less than 10ms, thus eliminating the impact of mechanical wear, return hysteresis, temperature drift, and positioning errors on device performance, improving long-term operational stability and lifespan. The overall transmission efficiency of the device is greater than 40%, with minimal differences in splitting efficiency across wavelengths within the 400-700nm operating band. It features 24 independent gating channels with an extinction ratio greater than 5000:1, allowing for flexible configuration of single-wavelength or multi-wavelength combination output modes. Utilizing a three-stage cascaded diffraction compensation structure, the output beam coaxiality error is ≤0.05°.

Claims

1. A switchable filter, comprising an incident collimating lens (1) arranged along the incident optical axis, a diffraction grating, and a zoned electro-optical switch, characterized in that: The diffraction grating consists of three polarization gratings with the same grating vector direction but different periods. Along the optical path, they are arranged in sequence as a first polarization grating (2), a second polarization grating (6), and a third polarization grating (7), with the period of the second polarization grating (6) being smaller than that of the first polarization grating (2). The partitioned electro-optical switch is located between the first polarization grating (2) and the second polarization grating (6), with each partition corresponding to the spatial position of each wavelength beam dispersed by the first polarization grating (2). The second polarization grating (6) is used to deflect and combine the selected wavelength beams, and the third polarization grating (7) is used to correct the combined beam to a coaxial output parallel to the incident optical axis.

2. The switchable filter according to claim 1, characterized in that, The polarization grating is a liquid crystal polymer polarization grating based on the geometric phase principle.

3. The switchable filter according to claim 2, characterized in that, The liquid crystal polymer polarization grating is a wideband polarization grating.

4. The switchable filter according to claim 1, characterized in that, The partitioned electro-optical switch includes a first linear polarizer (3), a partitioned TN-type liquid crystal cell (4), and a second linear polarizer (5) arranged sequentially along the optical path. The transmission axes of the two linear polarizers are orthogonal to each other.

5. The switchable filter according to claim 4, characterized in that, The partitioned TN-type liquid crystal cell (4) has a strip-shaped ITO transparent electrode on one side of its substrate. Each strip electrode is arranged along the grating dispersion direction and can be driven independently.

6. The switchable filter according to claim 5, characterized in that, The extension direction of the strip electrode is perpendicular to the grating vector direction, so that each electrode control area corresponds one-to-one with the long strip-shaped light spot dispersed by the first polarization grating (2).

7. The switchable filter according to claim 3, characterized in that, The wideband polarization grating adopts a double-twisted structure, which is composed of two mirror-symmetrically stacked liquid crystal polymer films. The liquid crystal molecules in the two films are twisted in opposite directions and at equal twist angles.

8. The switchable filter according to claim 4, characterized in that, The partitioned TN-type liquid crystal cell (4) is filled with nematic liquid crystal, and the liquid crystal molecules are aligned to form a 90° spiral twisted structure.

9. The switchable filter according to claim 5, characterized in that, The width of each strip electrode along the grating dispersion direction matches the width of the corresponding monochromatic beam spot dispersed by the first polarization grating (2), and each electrode corresponds to the spatial position of a monochromatic beam.

10. The switchable filter according to claim 1, characterized in that, The switchable filter is an all-solid-state transmissive electronically controlled device, and wavelength switching is achieved through a zoned electronically controlled optical switch.

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