Composite liquid crystal achromatic wave plate

Abstract

A composite achromatic wave plate relates to the technical field of optics, and can realize the function of a 0-1 achromatic wave plate in the wave band of 450-700nm by controlling the synchronous change of the phase delay of an odd number of liquid crystal phase adjustable retarders (LCVRs), namely changing the external voltage of wave plates on two sides and the fast axis direction of a central wave plate. Parameters required for realizing 1 / 2 and 1 / 4 achromatic wave plate functions in a 400-750 nm wave band and parameters required for realizing switching between two working states in a 500-600 nm wave band are respectively obtained by using numerical simulation. Results of the embodiment show that the designed composite wave plate can realize 1 / 2 and 1 / 4 achromatic wave plate functions at 450-700nm, and the maximum deviations from target phase delay are 2.4% and 2.56% respectively; when an angle of 64 degrees is formed between the fast axis direction of the zero-level wave plate and the reference axis, the wave band of 520-620 nm can be converted between the working states of 1 / 2 and 1 / 4 achromatic wave plates by changing the voltage. The composite achromatic wave plate provided by the invention is convenient to operate, can be adjusted in real time, expands the achromatic range of a device, and is suitable for a self-adaptive optical system.

Description

Technical Field

[0001] The invention relates to the field of optical technology, and in particular to a composite liquid crystal achromatic wave plate. Background Art

[0002] A wave plate is a device that can adjust the polarization state of a light beam. Common wave plates include half-wave plates and quarter-wave plates, which are mostly made of uniaxial crystals such as quartz. However, due to its strong wavelength dependence, that is, incident light of different wavelengths will produce different phase differences (i.e., chromatic aberration), which in turn leads to a decrease in imaging quality. In the field of geometric optics, the essence of chromatic aberration is the aberration caused by the different refractive indices of optical materials for different wavelengths. In order to eliminate chromatic aberration, a composite lens composed of two different materials of glass is usually used. However, this method has some problems, such as poor temperature stability of phase retardation and large influence of the incident angle. In order to improve the optical properties of the wave plate so that it is not affected by factors such as wavelength, researchers have developed achromatic wave plates with a composite structure. These wave plates are usually composed of two or more wave plates with the same birefringence. However, traditional achromatic wave plates are usually designed only for a specific wavelength range, and their efficiency may not be high for multi-wavelength or wide-wavelength light sources. In addition, they cannot be adjusted in real time, are not suitable for adaptive optical systems, and require multiple wave plates with different wavelength ranges, which increases the complexity of the design. Summary of the invention

[0003] In view of this, the purpose of the present invention is to provide a composite liquid crystal achromatic wave plate, which is used to solve the problems that the existing achromatic wave plates can only be designed for a specific wavelength range, are inefficient, cannot be adjusted in real time, are not suitable for adaptive optical systems, and require multiple wave plates with different wavelength ranges.

[0004] The technical solution adopted by the present invention is:

[0005] The present invention provides a composite liquid crystal achromatic wave plate, which is composed of an odd number of liquid crystal phase adjustable retarders, including a liquid crystal phase adjustable retarder located in the center and a plurality of liquid crystal phase adjustable retarders arranged symmetrically on both sides. The composite liquid crystal achromatic wave plate is a composite structure, which is composed of a plurality of birefringence wave plates cascaded.

[0006] Preferably, the composite liquid crystal achromatic wave plate has an odd number of achromatic wave plates F0, F ±1 、F ±2 …F ±n (where n is a positive integer) are δ0, δ ±1 ,δ ±2 …δ ±n (where n is a positive integer), the angles between the fast axis of the wave plate and the vertical x-axis are θ0 and θ±1 ,θ ±2 …θ ±n .

[0007] Preferably, the composite liquid crystal achromatic wave plate comprises wave plates F arranged in sequence along the optical path. -n 、F -n+1 、F -n+2 ...F0, F1, F2...F n (where n is a positive integer).

[0008] Preferably, the composite liquid crystal achromatic wave plate is characterized in that the phase delay of the wave plate is electrically controlled and the bilaterally symmetrical wave plate F -n With F n (where n is a positive integer and n≠0) have the same fast axis direction and the same phase delay at the central wavelength.

[0009] Preferably, the composite liquid crystal achromatic wave plate changes the F -1 The external voltage of F0 and F1 wave plates and the fast axis direction of F0 wave plate can realize the function of 0-1 achromatic wave plate in the band of 450-700nm.

[0010] Preferably, for the composite liquid crystal achromatic wave plate, when the phase delay δ of any wavelength light in a band deviates from the target phase delay δ′0 within 3% of 2π, the wave plate can be considered to have achromatic characteristics in this band. This condition can be expressed as:

[0011]

[0012] Select the max|δ-δ'0| / 2π in a band, i.e. the infinite norm, as the criterion for judging the achromatic characteristics, and use the optimization algorithm to optimize it. The optimization can obtain the optimal θ0 and δ0, δ0 that meet the minimum corresponding max|δ-δ'0| / 2π in the band range. ±1 ,δ ±2 …δ ±n (where n is a positive integer). θ0 is the angle between the fast axis of the central wave plate and the vertical x-axis, δ0 is the phase delay of the central wave plate, δ ±1 ,δ ±2 …δ ±n is the phase delay of the wave plates on both sides.

[0013] Preferably, the composite liquid crystal achromatic wave plate can obtain the optimal central wavelength phase delay δ0, δ100, δ200, δ300, δ400, δ500 for realizing the function of a 0-1 composite achromatic wave plate at 400nm-750nm after optimization algorithm simulation. ±1 ,δ ±2 …δ±n (where n is a positive integer) and the corresponding angle θ0 between the fast axis and the reference axis of the central wave plate. At the same time, the simulation also obtained that θ0 can be kept unchanged by changing δ0, δ ±1 , δ ±2 …δ ±n (where n is a positive integer) The corresponding parameters for realizing the 0-1 achromatic wave plate function simultaneously within 500nm-600nm.

[0014] Preferably, the composite liquid crystal achromatic wave plate uses a driving device to apply voltage, and then measures the voltage amplitude required to work in the 0-1 wave plate state. These voltage values ​​correspond to different phase delay values ​​δ0, δ ±1 , δ ±2 …δ ±n (where n is a positive integer). BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0016] Figure 1 Schematic diagram of the composite achromatic wave plate structure;

[0017] Figure 2 The phase delay deviation of 400-750nm corresponding to the optimal parameters when the target is a 1 / 4 wave plate;

[0018] Figure 3 This is the principle diagram of the optical path for measuring voltage amplitude using the light intensity method;

[0019] Figure 4 is the equivalent phase delay δ at different wavelengths of 400-750nm when working in the 1 / 4 wave plate state;

[0020] Figure 5 The phase delay deviation of 400-750nm corresponding to the optimal parameters when the target is a 1 / 2 wave plate;

[0021] Figure 6 is the equivalent phase delay δ at different wavelengths of 400-750nm when working in the 1 / 2 wave plate state;

[0022] Figure 7 is the equivalent phase delay δ at different wavelengths of 400-750nm when working in the adjustable 1 / 4 wave plate state;

[0023] Figure 8is the equivalent phase delay δ at different wavelengths of 400-750nm when working in the adjustable 1 / 2 wave plate state. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution in the embodiment of the present invention will be clearly and completely described in conjunction with the drawings in the embodiment of the present invention. It should be noted that, in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In the description of the present invention, it should be understood that the orientation or position relationship indicated by the terms "center", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc. is based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. Moreover, the term "include", "comprise" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such a process, method, article or device. In the absence of further restrictions, the elements defined by the phrase "comprising..." do not exclude the existence of other identical elements in the process, method, article or device comprising the elements. If there is no conflict, the embodiments of the present invention and the various features in the embodiments can be combined with each other, all within the protection scope of the present invention.

[0025] Embodiment 1:

[0026] like Figure 1 As shown, this embodiment provides a composite liquid crystal achromatic wave plate, including a wave plate F -1 , F0, F1, change the F -1 The external voltage of F0, F1 wave plates and the fast axis direction of F0 wave plate realize the function of 1 / 4 composite achromatic wave plate at 400nm-750nm.

[0027] The specific implementation includes the following steps:

[0028] S10: First, numerical simulation is performed on the 1 / 4 achromatic wave plate to obtain the parameters required to realize the 1 / 4 achromatic wave plate function in the 400-750nm band. The achromatic characteristics of the composite wave plate are studied by formula (1):

[0029] For a certain central wavelength λ0, an odd number of wave plates F-1 The phase delays of F0 and F1 are δ -1 The angles between the fast axis direction of the three wave plates δ0, δ1 and the vertical x-axis are θ -1 , θ0, θ1, keep F -1 The fast axis direction and phase delay of F1 at the central wavelength λ0 are the same, and the fast axis angle θ0 and phase delay δ0 of wave plate F0 are variable.

[0030] Assuming that the birefringence difference Δn of the liquid crystal material used does not change with wavelength, for any wavelength λ other than the central wavelength λ0 within a band, δ=2πΔnd / λ, the corresponding δ and φ of the composite wave plate are obtained as follows:

[0031]

[0032] Equations (2) and (3) represent the relationship between the phase delay of the composite wave plate, the equivalent fast axis and the wavelength, from which the working characteristics of the composite wave plate in a wide band can be inferred.

[0033] Select the max|δ-δ0′| / 2π in a band as the infinite norm as the criterion for judging the achromatic characteristics, and find the θ0 and δ0 that minimize them. ±1 Parameters, where θ0 is the angle between the fast axis of the central wave plate and the vertical x-axis, δ0 is the phase delay of the central wave plate, δ ±1 is the phase delay of the wave plates on both sides.

[0034] This is an optimization problem, and the objective function is max|δ-δ′0| / 2π in the band. We choose to use the optimization algorithm for optimization. The optimization can obtain the optimal θ0 and δ0,δ that meet the minimum corresponding max|δ-δ0′| / 2π in the band. ±1 Parameters.

[0035] The designed central wavelength λ0 is 532nm, the band is 400nm-750nm, and the 1 / 4 wave plate is used as the target, that is, δ0′=90°. The optimization algorithm is used to obtain the minimum max|δ-δ0′| / 2π of each wavelength, which is 0.62%, corresponding to the δ -1 =δ1=145°, δ0=218°, θ0=72°, and the obtained max|δ-δ′0| / 2π varies with wavelength as shown in Figure 2 As shown in the figure, max|δ-δ0′| / 2π is 0.62% at this time, which is much smaller than 3%. The composite wave plate can theoretically realize the function of a quarter achromatic wave plate.

[0036] S20: Then the achromatic characteristics of the composite wave plate are tested. The testing method is as follows:

[0037] For the designed composite tunable achromatic wave plate, it is necessary to test the precise value of its phase delay within a certain spectrum. The azimuth angle special value method commonly used in the light intensity method to measure the phase delay is selected to measure the designed device. Its measurement steps and debugging methods are relatively convenient. The phase delay of the device to be tested can be obtained by analyzing the system incident light and transmitted light intensity values. Use a fiber optic spectrometer as the reference light and receiving end of the test optical path. The principle of measuring the optical path is as follows: Figure 3 As shown, by rotating the two polarizers and the sample to be tested in the system at different angles, the phase delay of the sample is derived.

[0038] The above method is used to verify the achromatic characteristics of the composite wave plate. It is necessary to measure the phase delay of the three LCVRs under different voltages and find the phase delay that satisfies the previously simulated 1 / 4 wave plate state at the central wavelength of 532nm. -1 , δ0, δ1 voltages.

[0039] The composite achromatic wave plate used in the experiment is a composite structure composed of multiple birefringent wave plates, which is composed of three multi-stage wave plates of birefringent materials cascaded. The LCVR used is made of liquid crystal material SLC103024-000, with a cell thickness of 9mm. The voltage is applied using a driving device, and then the voltage amplitude required to work in the 1 / 4 and 1 / 2 wave plate states is measured using the light intensity method.

[0040] The steps for measuring voltage amplitude using the light intensity method are as follows:

[0041] S21: First, keep the fast axis directions of the three-stage LCVR consistent;

[0042] S22: Rotate the fast axis of the 0th-stage LCVR clockwise by an angle of θ0.

[0043] In order to keep the polarization direction of the polarized light reaching the receiving end of the fiber spectrometer unchanged, three polarizers are used for measurement. The last two polarizers are the polarizer and the analyzer, and the polarization directions are 45° to the polarization direction of the first polarizer. The polarizer can be rotated to achieve a state of being orthogonal and parallel to the polarization direction of the analyzer. The equivalent optical axis of the composite wave plate at the central wavelength of 532nm is kept at a 45° angle to the analyzer direction for measurement, and the spectra of I1 and I2 in equations (4) and (5) can be obtained, and the equivalent phase delay δ of the corresponding band can be calculated by equation (6).

[0044]

[0045]

[0046] The composite wave plate working in the 1 / 4 wave plate state is measured, and the spectral data is processed to obtain Figure 4The target phase delay δ0′=90°, which satisfies |δ-δ'0| / 2π≤3%, is between 79.2°~100.8°, that is, Figure 4 The middle dotted line is between 430.899nm and 750.312nm. Therefore, the composite wave plate actually working in the 1 / 4 wave plate state has achromatic characteristics between 450-700nm, and the maximum deviation from the target phase delay is 2.56%.

[0047] Embodiment 2:

[0048] Based on the method of Example 1, this embodiment provides a composite liquid crystal achromatic wave plate, including a wave plate F -1 , F0, F1, change the F -1 The applied voltage of F0, F1 wave plates and the fast axis direction of F0 wave plate are used to realize the function of 1 / 4 composite achromatic wave plate at 400nm-750nm and test the function of composite achromatic wave plate.

[0049] The test steps are as follows:

[0050] S10: First, numerical simulation is performed on the 1 / 4 achromatic wave plate to obtain the parameters required to realize the function of the 1 / 2 achromatic wave plate in the 400-750nm band. The achromatic characteristics of the composite wave plate are studied by formula (1):

[0051] The designed central wavelength λ0 is 532nm, the band is 400nm-750nm, and the optimization is carried out with the 1 / 2 wave plate as the target. δ0′=180°. The minimum max|δ-δ′0| / 2π of each wavelength obtained by the optimization algorithm is 0.99%, corresponding to δ -1 =δ1=176°, δ0=176°, θ0=123°. |δ-δ'0| / 2π varies with wavelength as Figure 5 As shown in the figure, max|δ-δ0′| / 2π is 0.99% at this time, which is much smaller than 3%. The composite wave plate can theoretically realize the function of a 1 / 2 achromatic wave plate.

[0052] S20: Then test the achromatic characteristics of the composite wave plate:

[0053] The composite wave plate working in the 1 / 2 wave plate state is measured, and the spectral data is processed to obtain the phase delay at different wavelengths. Figure 6 For the target phase delay δ0′=180°, the range of |δ-δ'0| / 2π≤3% is between 169.2°~190.8°, that is Figure 6In the middle dotted line range, it can be seen that this range is between 441nm and 719nm. Therefore, the composite wave plate actually working in the 1 / 2 wave plate state has achromatic characteristics between 450-700nm, and the maximum deviation from the target phase delay is 2.4%.

[0054] Embodiment 3:

[0055] This embodiment provides a composite liquid crystal achromatic wave plate, including a wave plate F -1 , F0, F1, change the F -1 , F0, F1 wave plate external voltage, to achieve the conversion between 1 / 2 and 1 / 4 composite achromatic wave plate. The specific implementation steps are as follows:

[0056] S10: First, the simulation shows that θ0 can be kept unchanged by changing δ -1 ,δ0,δ1 are the corresponding parameters for achieving both 1 / 2 and 1 / 4 achromatic wave plate functions in the range of 500nm-600nm

[0057] If the value of θ0 is fixed, δ can be changed by applying an external voltage. -1 The values ​​of δ0 and δ1 can realize the conversion between 1 / 2 and 1 / 4 compound achromatic wave plates. Then the compound wave plate can be tuned by applying voltage only without changing the fast axis direction of the 0th order wave plate. -1 According to formulas (1) and (7), the deviation between the equivalent phase delay δ and the target phase delay δ0 = 180° / 90° in different cases is obtained, and a θ0 value is found so that the composite wave plate can be -1 , δ0, δ1 simultaneously realize the functions of 1 / 2 and 1 / 4 achromatic wave plates within a certain band.

[0058]

[0059] After the optimization algorithm optimization simulation, it is found that when θ0 = 64°, when δ -1 =δ1=189°, δ0=188°, which can realize the function of a 1 / 2 achromatic wave plate with a max|δ-δ′0|| / 2π of 2.26% at 500nm-600nm; similarly, when δ -1 =δ1=293°, δ0=195°, which can realize the function of a 1 / 4 achromatic wave plate with max|δ-δ′0| / 2π being 2.41%.

[0060] S20: Next, the tunability of the composite achromatic wave plate between the 1 / 2 wave plate and the 1 / 4 wave plate is tested. The test results are as follows:

[0061] S11: Fixed θ0 = 64°, when the central wavelength is 532nm, δ -1 =δ1=293°, δ0=195°, the phase delay of the composite wave plate working in the 1 / 4 wave plate state at different wavelengths is measured as follows Figure 7 As shown in the dashed line range, |δ-δ0′| / 2π≤3% is satisfied, and the maximum deviation is 2.99% between 489.102nm and 626.805nm wavelength.

[0062] S12: When the central wavelength is 532nm, -1 =δ1=189°, δ0=188°, the phase delay of the composite wave plate working in the 1 / 2 wave plate state at different wavelengths is measured as follows Figure 8 In the dotted line range, |δ-δ0| / 2π≤3% is satisfied, and the maximum deviation is 3% between the wavelengths of 518.44nm and 685.147nm.

[0063] In summary, for this composite liquid crystal wave plate, tuning between the 1 / 2 and 1 / 4 achromatic wave plates can be achieved by applying an external voltage when the angle θ0 between the fast axis and the reference axis of the 0th order wave plate is 64° between 520nm and 620nm.

[0064] This verifies the composite wave plate proposed in the present invention. -1 The applied voltage of F0, F1 wave plates and the fast axis direction of F0 wave plate can realize the functions of 1 / 2 and 1 / 4 achromatic wave plates in the band of 450-700nm respectively. And when the fast axis angle of wave plate F0 is fixed to θ0 of 64°, the working state of 1 / 2 and 1 / 4 achromatic wave plates can be switched in the band of 520nm-620nm only by changing the applied voltage.

Claims

1. A composite liquid crystal achromatic wave plate, characterized in that: The composite liquid crystal achromatic wave plate is composed of an odd number of liquid crystal phase adjustable retarders, including a liquid crystal phase adjustable retarder located in the center and a plurality of liquid crystal phase adjustable retarders arranged symmetrically on both sides. The composite liquid crystal achromatic wave plate is a composite structure, which is composed of a plurality of birefringence wave plates cascaded.

2. The composite liquid crystal achromatic wave plate according to claim 1, characterized in that: For a certain central wavelength λ0, an odd number of achromatic wave plates F0, F ±1 、F ±2 …F ±n (where n is a positive integer) are δ0, δ ±1 , δ ±2 …δ ±n (where n is a positive integer), the angles between the fast axis of the wave plate and the vertical x-axis are θ0 and θ ±1 ,θ ±2 …θ ±n .

3. The composite liquid crystal achromatic wave plate according to claim 2, characterized in that: The composite liquid crystal achromatic wave plate comprises wave plates F arranged in sequence along the optical path. -n 、F -n+1 、F -n+2 ...F0, F1, F2...F n (where n is a positive integer).

4. The composite liquid crystal achromatic wave plate according to claim 3, characterized in that: The phase delay of the wave plate is electrically controlled and adjusted. -n With F n (where n is a positive integer and n≠0) have the same fast axis direction and the same phase delay at the central wavelength.

5. The composite liquid crystal achromatic wave plate according to claim 4, characterized in that: The composite liquid crystal achromatic wave plate changes the F -1 The external voltage of F0 and F1 wave plates and the fast axis direction of F0 wave plate can realize the function of 0-1 achromatic wave plate in the band of 450-700nm.

6. A composite liquid crystal achromatic wave plate according to claim 5, characterized in that: If the phase delay δ of any wavelength light in a band deviates from the target phase delay δ′0 within 3% of 2π, then the wave plate can be considered to have achromatic characteristics in this band. This condition can be expressed as: Select the max|δ-δ'0| / 2π in a band, i.e. the infinite norm, as the criterion for judging the achromatic characteristics, and use the optimization algorithm to optimize it. The optimization can obtain the optimal θ0 and δ0, δ0 that meet the minimum corresponding max|δ-δ'0| / 2π in the band range. ±1 , δ ±2 …δ ±n (where n is a positive integer). θ0 is the angle between the fast axis of the central wave plate and the vertical x-axis, δ0 is the phase delay of the central wave plate, δ ±1 , δ ±2 …δ ±n is the phase delay of the wave plates on both sides.

7. The composite liquid crystal achromatic wave plate according to claim 6, characterized in that: After the optimization algorithm simulation, the optimal center wavelength phase delay δ0, δ1 can be obtained to realize the 0-1 composite achromatic wave plate function at 400nm-750nm. ±1 , δ ±2 …δ ±n (where n is a positive integer) and the corresponding angle θ0 between the fast axis and the reference axis of the central wave plate. At the same time, the simulation also obtained that θ0 can be kept unchanged by changing δ0, δ ±1 , δ ±2 …δ ±n (where n is a positive integer) The corresponding parameters for realizing the 0-1 achromatic wave plate function simultaneously within 500nm-600nm.

8. A composite liquid crystal achromatic wave plate according to claims 3, 4 and 5, characterized in that: The drive device applies voltage and then measures the voltage amplitude required to operate in the 0-1 wave plate state. These voltage values ​​correspond to different phase retardation values ​​δ0, δ ±1 , δ ±2 …δ ±n (where n is a positive integer).

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