JP4696283B2 - High-precision control method of liquid crystal light beam deflector considering wavefront curvature - Google Patents

Description

  The present invention can be applied to an optical system of an optical antenna when performing spatial optical communication such as optical space communication and optical wireless communication, for example, and is applied to the optical system of an optical antenna to control the deflection of a light beam. In particular, the liquid crystal light beam deflector takes into account the wavefront curvature, and in particular, even if there is a phase distortion that varies with the deflection angle due to nonlinearity between the liquid crystal voltage and the transmitted light phase, the deflection angle The present invention relates to a control method that suppresses phase distortion due to the non-linearity of the liquid crystal and enables beam deflection with high efficiency.

  FIG. 1 shows a state of spatial optical communication such as (a) optical space communication between a satellite and a ground station and (b) optical wireless communication between buildings. It is already well known that when a transmitting station or a receiving station moves by such a communication method, it is necessary to control the optical antenna in accordance with the movement. In some cases, the liquid crystal light beam deflector is used for the control for this purpose. Such a liquid crystal light beam deflector is used as an optical device for spatial light communication shown in FIG. 1 and also as a more general active optical component such as laser optics and its application, not limited to communication applications. .

  Conventionally, as described in Non-Patent Document 1 or 2, many of the optical deflecting devices using liquid crystal that have been proposed so far have binary refraction and reflection at the boundary surface due to the change in the refractive index of the liquid crystal. Many of them are intended for switching, not for continuous deflection. In those assuming binary deflection, the phase wrapping of the refractive index gradient distribution is fixed, and dynamic phase blazing (connection) is not taken into account, so it is extended to continuous deflection. Was impossible. Here, the phase brazing means that the wavefront phase is smoothly connected to the wavefront phase deflected by the voltage applied to the adjacent electrode.

  Patent Document 1 “Optical Deflector, Optical Scanning Device, and Information Reading Device” discloses a device that applies parallel voltage between transparent electrodes to cause parallel stripes to appear in liquid crystal. In this method, the parallel stripes are used as a diffraction grating and deflected by diffraction. As will be described later, this is different in operating principle from the deflector according to the present invention.

  Patent Document 2 “Gradient Index Optical Deflector and Optical Deflection Method” discloses a method of controlling a refractive index gradient using a potential gradient caused by an applied voltage. However, this is characterized in that fine adjustment of the wavefront curvature in consideration of non-linearity, that is, focus adjustment cannot be performed, and this is different from the present invention.

  Among the resistive electrode type liquid crystal light beam deflectors which are one of the liquid crystal light beam deflectors, FIG. 2 shows a (a) front view and (b) side view of a liquid crystal light beam deflector with a ladder electrode. In this type of liquid crystal deflector, as shown in FIG. 3A, the electrode surface formed in a stripe shape on the liquid crystal panel is made of a material having resistivity, and the drive voltage applied to both ends of the electrode cell is applied to the cell surface. A potential gradient is generated, and a voltage with a potential gradient is applied to a homogeneously aligned liquid crystal layer in accordance with the potential gradient to generate a refractive index gradient. Here, the homogeneous alignment is an alignment in which the major axis direction of the liquid crystal molecule group sandwiched between two substrates is parallel to the substrate surface. This is also called parallel orientation. As a result, as shown in FIG. 3B, a refractive index distribution is formed on the liquid crystal panel, and the incident laser light is deflected according to the refractive index distribution.

  In that case, the nonlinearity between the voltage applied to the liquid crystal and the transmitted light wavefront phase causes phase wavefront curvature distortion that varies with the deflection angle. In order to enable the deflection of the light beam with high efficiency, it is necessary to suppress the phase wavefront curvature distortion caused by the nonlinearity of the liquid crystal regardless of the magnitude of the deflection angle.

  Conventionally, in a liquid crystal device, continuous deflection is possible, but in order to create a change in refractive index with a constant thickness of the liquid crystal layer, the accuracy of the transmitted wavefront is equal to the voltage applied to the liquid crystal that the liquid crystal has. It will be influenced by nonlinearity between the phase shift of light passing through the liquid crystal.

  Therefore, the optimal design of the pretilt angle of the liquid crystal molecules left the problem that the linear operation region must be expanded at the rising edge of the liquid crystal molecules. Here, the pretilt angle is an angle at which liquid crystal molecules are pre-aligned before applying a voltage to the liquid crystal. In the vertical liquid crystal, the angle is relative to a straight line perpendicular to the liquid crystal layer, and in the horizontal liquid crystal, the angle is relative to the liquid crystal layer surface. .

Furthermore, it is said that the effect on the wavefront accuracy cannot be improved unless the control method is devised for processing the nonlinear error and saturation curve that cannot be ignored as an optical component even though they remain. Ended up leaving problems. For example, the convergence of the light beam by the corrugated lens and the deflection of the light beam using the ladder-type electrode exist as separate problems.
JP 7-261203 A JP 7-64123 A W. Klaus, at al., “Efficient liquid crystal wavefront modulators” Proc. SPIE, vol3015, pp.84-92 (1997) W. Klaus, et al., “Adaptive LC lens array and its application,” Proc. SPIE, 3635, pp. 60-73 (1999)

  FIG. 12 shows the state (retardation) of the wavefront phase at the liquid crystal aperture with respect to the voltage applied to the liquid crystal panel. From this figure, it can be seen that the nonlinearity of the liquid crystal, that is, the applied voltage and the transmitted light wavefront phase have a nonlinear relationship.

  In general, it is known that if the refractive index is changed so that the thickness of the liquid crystal layer is constant and continuous deflection is possible, the accuracy of the transmitted wavefront is affected by the nonlinearity of the liquid crystal. For this reason, normally, the linear operation region is expanded at the rising edge of the liquid crystal molecules by the optimum design of the pretilt angle of the liquid crystal molecules. However, minute nonlinear errors and saturation curve portions remain, and how to deal with them is a problem.

  In the present invention, with respect to how to process the remaining non-linear error and saturation curve portion, the influence on the wavefront accuracy is improved by devising the control method of the refractive index. This improvement realizes a liquid crystal wavefront modulator that integrates wavefront convergence and deflection, and enables continuous operation regardless of the change angle and the wavefront convergence rate.

  The present invention has been made in view of such problems, and in a method for controlling the operation of a liquid crystal light beam deflector, the wavefront distortion of the transmitted light of the wave crystal light beam deflector is reduced, and an arbitrary deflection angle is obtained. Another object of the present invention is to provide a highly accurate control method of a liquid crystal light beam deflector that enables highly efficient beam deflection.

  By suppressing the phase wavefront curvature distortion, the beam control accuracy at the time of deflection can be improved, and at the same time, the electro-optical characteristics of the liquid crystal can be used up to the nonlinear region. Also, as a result of the above suppression, it becomes possible to correct the wavefront curvature with a single light beam deflector, that is, to also serve as a focusing device. An optical system can be provided.

  In order to suppress the phase wavefront curvature distortion, in the present invention, the amount of wavefront curvature distortion caused by the nonlinearity of the liquid crystal can be calculated in real time in advance by deriving the second derivative of the phase wavefront curvature. This makes it the best to minimize wavefront distortion even when phase wrapping in 2 pi cycles is required, such as when the tilt is steep with respect to the modulation range of the liquid crystal when the wavefront tilt is given for the purpose of deflection. The return method can be calculated. Further, an algorithm for sequentially calculating the phase for each electrode is constructed using this as an evaluation function. That is, the best value is preferentially given to the center of the aperture with high light intensity to be the initial value, and the phase plane is extended to the peripheral part while performing brazing (connection) under optimum conditions so as to obtain the wavefront curvature. At this time, by giving a bias value to the evaluation of the wavefront curvature, the wavefront curvature can be finely adjusted.

The present invention having the above features is a liquid crystal panel in which a plurality of ladder-type electrodes having a stripe pattern are arranged opposite to other electrodes through which light can pass through a homogeneously oriented liquid crystal layer.
A voltage is applied between each of the ladder-type electrodes and the electrodes through which light can pass, and between the stripes in order to create a potential difference gradient in the transverse direction of the stripe of the ladder-type electrode and in the liquid crystal layer. By applying a voltage, a refractive index gradient is generated in the liquid crystal layer and along the crossing direction of the stripe of the ladder electrode.
Deriving the second derivative of the wavefront phase of the light beam transmitted through the liquid crystal layer having the refractive index gradient with respect to the displacement of each ladder electrode in the stripe transverse direction, and calculating the amount of wavefront curvature distortion in real time,
Regarding the phase difference between sequentially adjacent wavefront phases in the ladder-type electrode, the wavefront phase of the ladder-type electrode that reduces the calculated wavefront curvature distortion amount is changed from the electrode near the center of the opening where the light intensity is strong to the peripheral portion. Blazing is performed by sequentially selecting and smoothing the wavefront phase as a whole .

  Obviously, if the wavefront tilt is slow, it is best to use a continuous wavefront, but the effect of the non-linearity of the liquid crystal increases as the wavefront tilt increases. On the other hand, the wavefront phase to be adjusted may not be within the phase modulation range of the liquid crystal. In this case, the phase is turned back with a period of 2 pi using the periodicity of the optical vibration. Even in the above case, it is possible to perform the optimal phase folding while taking into account the nonlinearity of the liquid crystal according to the target wavefront inclination.

Further, as described above, when calculating the phase shift, (1) a wavefront phase boundary condition is preferentially given to the strong part of the light beam of the liquid crystal panel to obtain an initial value, and (2) the phase shift is calculated. The wavefront phase is extended to the weak part of the light beam of the liquid crystal panel while performing the above brazing so that the equiphase surface by the above becomes the curvature of the wavefront.

Further, when performing brazing as described above , the wavefront curvature is finely adjusted by giving a bias value to the calculated value of the voltage applied to the ladder-type electrode as appropriate. This treatment makes it possible to perform focus adjustment with the same device alone.

  Further, the two liquid crystal panels as described above, the first and second liquid crystal panels are arranged so as to be incident with the light beams being chained, and the stripe direction of the first liquid crystal panel is the stripe of the second liquid crystal panel. In the case where the liquid crystal panels are arranged so as to be orthogonal or oblique to the direction, the above-described problem can be solved by controlling one of the first liquid crystal panel and the second liquid crystal panel by the above method.

  In the present invention, the residual wavefront error due to the nonlinearity of the liquid crystal is improved by devising the control method for the resistive electrode type beam deflector of the liquid crystal device. This improvement is due to the fact that the wavefront curvature is controlled by adding an optimal process to the phase brazing process, and the accuracy close to the theoretical value is achieved. In addition to the high accuracy that is the original function of the beam deflector, the focus adjustment can be realized by the same device alone by controlling the wavefront curvature.

  In general, when beam deflection is performed continuously using liquid crystal, the nonlinear distortion of the liquid crystal changes accordingly, and correction with a simple phase correction plate may be difficult. When the present invention is applied, it is effective even when parameters such as the deflection direction are continuously changed.

As a specific example, FIGS. 4 to 6 show a state in which the aforementioned phase is blazed. FIG. 4 shows a case where the wavefront phase converges symmetrically so that the center of the deflector is concave. FIG. 5 shows a case where the wavefront phase is deflected with a gradient to one side as a whole deflector. FIG. 6 shows a case where the wavefront phase is deflected to one side while forming a concave surface as the entire deflector. In both cases, the wavefront phase is blazed, so that a phase shift occurs . It can also be seen that all phase errors are within 0.1 radians.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  FIG. 7 shows an optical system from laser light oscillation to detection which is the best mode for carrying out the present invention. FIG. 7A is a block diagram in the case of beam deflection to infinity, and FIG. 7B is a block diagram in the case of beam deflection when observing the liquid crystal aperture.

  7A shows a liquid crystal panel 2 that controls the deflection in the X-axis direction and a liquid crystal panel that controls the deflection in the Y-axis direction, having the same structure as that shown in FIG. 4 are arranged in order in the direction of the optical axis, and this is taken as one unit. In FIG. 7A, the polarizer 1, the unit, and the lens 5 having a polarization angle of 90 ° from the incident direction are arranged in this order. FIG. 2B also shows an optical system in which the polarizer 7 having a polarization angle of 45 °, the unit, the analyzer 8 having a polarization angle of 45 °, and the lens 5 are arranged in this order.

  Further, in FIG. 7B, an image formed by the lens is received by a CCD (Charge Coupled Device) of the light receiver 6, and the result is signal-processed by a personal computer (PC) 10 and DAC 9 (digital It shows that the two liquid crystal panels are controlled via an analog converter array. Here, the output of the light reception result by the CCD can be switched and recorded also on the beam profiler 12. The DAC array converts a deflection digital signal from the PC into an analog signal, and applies the analog signal as a voltage to the liquid crystal panel.

  FIG. 8A shows an image formed as a result of the beam deflection in the beam deflection optical system shown in FIG. Further, FIG. 8B shows an image formed as a result of the focus adjustment. FIG. 8C shows the relative intensity of each image.

  FIG. 9A shows the state of imaging with respect to the focus parameter, and FIG. 9B shows the peak intensity with respect to the focus parameter.

  FIG. 10 shows the phase delay of the wave front at the liquid crystal aperture surface in the beam deflection optical system (Phase Retardation). FIG. 10A shows the state of the wavefront phase at the liquid crystal aperture when the beam travels straight. No stripes appear. FIG. 10B shows the state of the wavefront phase at the liquid crystal aperture when the beam converges. Concentric stripes are formed, but the center is not deviated from the beam axis. FIG. 10C shows the state of the wavefront phase at the liquid crystal aperture when the beam is deflected. Stripes like the trajectory of the side of the wave appear. FIG. 10D shows the state of the wavefront phase at the liquid crystal aperture when the beam is deflected and converged. A concentric striped pattern whose center is off the axis of the beam is formed.

  It has been confirmed by mathematical formulas and computer simulations that these can also work for continuous wavefront changes (deflection, convergence, divergence).

Further, in FIG. 7B, when the two liquid crystal panels are controlled via the DAC 9 array based on the signal processing results from the personal computer (PC) 10, as shown in FIG. The phase difference of the mold electrode can be calculated within 2π. For example, as shown in FIG. 11 (a), when the wavefront phase adjacent to the wavefront phase is a phase difference exceeding 2π, as shown in FIG. Is within 2π so that the sum is pseudo-equal to the total phase difference. This can be readily understood from the periodicity of light waves by the process, the applied voltage is lowered and can you to improve residual wavefront errors due to the nonlinearity property of the liquid crystal.

  By using the present invention for optical system correction of an optical antenna in spatial light communication, highly accurate deflection and focus adjustment of beam directivity can be realized with a single small liquid crystal element. Realization of an ultra-small optical antenna with driving power can be expected. In addition, the optical device for space can be used not only as a communication application but also as a more general active optical component such as laser optics or its application.

  Further, the beam deflection accuracy can be improved, and at the same time, the wavefront curvature can be controlled by setting the control parameters so that the electro-optical characteristics of the liquid crystal are used up to the nonlinear region. As a result, it becomes possible to correct the wavefront curvature of the beam deflector alone, that is, to also serve as a focusing device. When combined with the convenience of liquid crystal, the laser deflection optical system can be made smaller and more precise.

  The beam deflection device using a liquid crystal panel enables a structure equivalent to a wristwatch size in the entire system including the power supply. Therefore, if it is a communication system, the structure of the optical antenna that tends to be complicated can be simplified.

It is a conceptual diagram of space optical communication (optical space communication between a satellite and a ground station and optical wireless communication between buildings on the ground). It is (a) front view and (b) side view of a ladder-type electrode-liquid crystal light beam polarizer. It is (a) top view and (b) sectional drawing of a liquid crystal light beam deflector with a ladder type electrode. It is a figure which shows the wave front phase and phase error (convergence) corresponding to an electrode. It is a figure which shows the wave front phase and phase error (deflection) corresponding to an electrode. It is a figure which shows the wave front phase and phase error (deflection and convergence) corresponding to an electrode. FIG. 2 is a schematic cross-sectional view showing a beam deflection optical system from oscillation to detection of laser light, (a) a diagram showing beam deflection to infinity, (b) a diagram showing beam deflection when observing a liquid crystal opening surface, It is. It is a figure which shows the image formation by a beam deflection optical system, (a) An image figure of beam deflection, (b) An image figure of focus adjustment, (c) A figure showing the relative intensity of each image. It is a figure which shows the imaging and intensity | strength by a focus parameter, (a) The imaging figure with respect to a focus parameter, (b) The figure showing the peak intensity with respect to a focus parameter. This figure shows the state of wavefront phase at the liquid crystal aperture in the beam deflection optical system, and shows the cases of (a) going straight, (b) convergence, (c) deflection, (d) deflection and convergence (bottom right). It is. It is a figure for demonstrating making a phase shift so that it may become a phase difference of less than 2pi, respectively, and making it pseudo-equal when a wavefront phase adjacent to this wavefront phase exceeds 2π. It is a figure which shows the retardation in the liquid crystal opening surface with respect to the liquid crystal applied voltage in a beam deflection optical system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Polarizer 2 Liquid crystal panel 3 1/4 wavelength plate 4 Liquid crystal panel 5 Lens 6 Imager 7 Polarizer 8 Analyzer 9 Digital analog computer 10 Personal computer 11 Switch 12 Beam profiler

Claims (4)

In a liquid crystal panel in which a plurality of ladder-type electrodes with stripe patterns are arranged opposite to other electrodes that can transmit light through a homogeneously oriented liquid crystal layer,
A voltage is applied between each of the ladder-type electrodes and the electrodes through which light can pass, and between the stripes in order to create a potential difference gradient in the transverse direction of the stripe of the ladder-type electrode and in the liquid crystal layer. By applying a voltage, a refractive index gradient is generated in the liquid crystal layer and along the crossing direction of the stripe of the ladder electrode.
Deriving the second derivative of the wavefront phase of the light beam transmitted through the liquid crystal layer having the refractive index gradient with respect to the displacement of each ladder electrode in the stripe transverse direction, and calculating the amount of wavefront curvature distortion in real time,
Regarding the phase difference between sequentially adjacent wavefront phases in the ladder-type electrode, the wavefront phase of the ladder-type electrode that reduces the calculated wavefront curvature distortion amount is changed from the electrode near the center of the opening where the light intensity is strong to the peripheral portion. A high-precision control method for a liquid crystal light beam deflector in consideration of wavefront curvature, characterized by performing brazing so that the wavefront phases are smoothed as a whole by selecting them sequentially. In the high-precision control method of the liquid crystal light beam deflector in consideration of the wavefront curvature according to claim 1,
Sequentially, the wavefront phase is calculated for each of the ladder-type electrodes, and the wavefront phase adjacent to the wavefront phase is selected from those exceeding 2π, so that the phase difference is less than 2π in a quasi-two cycle. Phase wrapping and phase shifting,
When performing the phase shift,
(1) A wavefront phase boundary condition is preferentially given to the strong part of the light beam of the liquid crystal panel to an initial value,
(2) A liquid crystal light beam considering the wavefront curvature, characterized by extending the wavefront phase to a weak part of the light beam of the liquid crystal panel while performing the above brazing so that the equiphase surface by the phase shift has the curvature of the wavefront. High-precision control method of deflector. In the high-precision control method of the liquid crystal light beam deflector in consideration of the wavefront curvature according to claim 2,
When performing the brazing, adjust the wavefront curvature by adjusting the bias value to the calculated value of the voltage applied to the ladder electrode,
A high-precision control method for a liquid crystal light beam deflector in consideration of wavefront curvature, characterized in that the focus is adjusted by a single device. The first liquid crystal panel and the second liquid crystal panel are light beams in which a ladder-type electrode having a stripe pattern is disposed opposite to an electrode through which light can pass through a homogeneously aligned liquid crystal layer. Arranged so that the beams are incident in a chain,
When the stripe direction of the first liquid crystal panel is arranged so as to be orthogonal or oblique to the stripe direction of the second liquid crystal panel,
The high-precision control method for a liquid crystal light beam deflector in consideration of the wavefront curvature according to any one of claims 1 to 3 is applied to any one of the first liquid crystal panel and the second liquid crystal panel. A high-precision control method for a liquid crystal light beam deflector in consideration of wavefront curvature, which is applied.

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