JP5357210B2 - Optical deflector and optical deflector control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problems that, when a light deflector is controlled by providing a trap filling operation and a deflection operation as control voltage applying to a KTN, reproducibility of a deflection angle is bad due to the occurrence of variation in the deflection angle according to a length of the deflection operation or the repeating number of deflection operation in which control voltage is actually applied, and that the dispersion of the deflection angle occurs due to the occurrence of temporal or spatial dispersion or variation in crystal in the quantity of electrons trapped in the crystal. <P>SOLUTION: In a light deflector using an electro-optic crystal such as a KTN, a period for erasing a voltage history to release remaining trapped electrons is provided after a deflection period for performing a deflection operation and before a trap filling period for filling the trap with electrons. During the period for erasing a voltage history, the electrons in the crystal are released from the trap by raising the temperature of the electro-optic crystal. An old history of trap filling voltage is erased and the state of the trapped electrons in the crystal is turned to the initial state for every control cycle. <P>COPYRIGHT: (C)2013,JPO&amp;INPIT

Description

  The present invention relates to a method for controlling an optical deflector. More specifically, the present invention relates to an optical deflector using KTN and a control method thereof.

  Currently, there is an increasing demand for light control elements for deflecting laser light in projectors and other video equipment, laser printers, high-resolution confocal microscopes, barcode readers, and the like. Conventionally, a technique for rotating a polygon mirror, a technique for controlling the deflection direction of light using a galvanometer mirror, a light diffraction technique using an acoustooptic effect, and a micromachine technique called MEMS have been proposed.

In recent years, an optical deflector that deflects light as a result of realizing a space charge control state by injecting charges into an electro-optic crystal to generate an electric field gradient and an electro-optic effect to cause a refractive index gradient has been proposed. (Patent Document 1). Unlike a galvano mirror, a polygon mirror, a MEMS mirror, or the like, this optical deflector using an electro-optic crystal does not have a movable part, so that high-speed light deflection is possible. As the above-mentioned electro-optic crystal, potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1): hereinafter referred to as KTN) crystal, or a material having the same effect as KTN, lithium K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1) and the like are known. Hereinafter, an optical deflector using KTN will be described as an example of the electro-optic crystal as described above.

  FIG. 1 is a diagram showing a configuration of an optical deflector using KTN. Electrodes 102 and 103 are formed on the upper and lower surfaces of the KTN crystal 101. A control voltage is applied from the control voltage source 104 between the two electrodes. Incident light 105 travels in the z direction from the left end face of the KTN crystal in the drawing, and is deflected in the KTN crystal 101 to obtain outgoing light 106 whose traveling direction is changed in the x-axis direction. Although details will be described later, a deflection angle corresponding to the applied voltage can be obtained.

  A control signal corresponding to the application of the optical deflector is given from the control voltage source 104. For example, a sine wave or sawtooth control signal is applied according to the application of the optical deflector. In order to obtain an appropriate maximum deflection angle, a drive voltage of about several hundred volts is applied to KTN. However, when the optical deflector is controlled only by the driving voltage for causing the deflection, there has been a problem associated with an increase in the driving speed. That is, there is a problem that the ideal space charge control state is not realized because the moving distance of the electrons injected from the electrodes by the driving voltage is shorter than the distance between the electrodes, and the deflection angle is reduced.

  To solve this problem, by applying a burst voltage to KTN before applying a drive voltage for causing deflection, electrons are injected into the crystal during the burst voltage application, and electrons are trapped in the trap. A control method that captures the water has been proposed. In other words, by filling the traps in the crystal with electrons, it is possible to generate an electric field distribution or inclination in the crystal, thereby realizing light deflection.

  FIG. 2 is a diagram for explaining a method of applying a control voltage in which a trap filling time is provided in the KTN crystal. The horizontal axis represents time, and the vertical axis represents control voltage. The time for applying the control voltage is divided into a time 202 for applying the trap filling voltage and a time 204 for applying the deflection voltage for causing the deflection. In this example, an example of a driving method in which a positive constant voltage 201 is applied as a burst voltage for trap filling is shown. The deflection voltage 203 is a sine wave signal, but it may be a sawtooth signal or a signal of any other waveform. In addition, although the example which applies the positive constant voltage 201 was shown in FIG. 2, it is good also as what applies a negative constant voltage. A triangular wave or a sawtooth wave can also be applied as the burst voltage.

  As shown in FIG. 2, by providing the trap filling time 202, even if there is no electron injection during the deflection voltage application operation 204, the electric field gradient is caused by the electrons trapped in the trap during the trap filling operation 202. Occur. As a result, the refractive index is tilted by the electro-optic effect, so that high-speed and wide-angle light deflection can be realized. As long as trap filling is sufficiently performed and a necessary deflection angle can be maintained, there is no limitation on the length of the trap filling time and the deflection time described above. Further, as shown in FIG. 2, it is not always necessary to repeat the trap filling time and the deflection time. What is necessary is just to provide the trap filling time before the single deflection time without repeating.

International Publication No. 2006/137408 Pamphlet

Koichiro Nakamura et al., “Space-charge-controlled electro-optic effect: Optical beam deflection by electro-optic effect and space-charge-controlled electrical conduction”, JOURNAL OF APPLIED PHYSICS 104, 013105, 2008 Jun Miyazu et al., `` 400 kHz Beam Scanning Using KTa1-xNbxO3 Crystals '', Conference on Laser and Electro-Optics, Quantum Electronics and Laser Science conference Proceedings (CLEO / QELS 2010)

  However, as shown in FIG. 2, when control is performed by providing a trap filling time and a deflection time as the control voltage applied to the KTN, the number of repetitions of the deflection operation in which the control voltage is actually applied depending on the length of the deflection time. As a result, the deflection angle fluctuates and the reproducibility of the deflection angle is poor. This is considered to be because electrons remain in the trap even if the application of the deflection voltage is stopped after the end of the deflection operation. Since the amount of electrons trapped in the crystal varies temporally or spatially in the crystal, the deflection angle varies. Hereinafter, this problem will be described more specifically.

When the electron density filled in the trap of the KTN crystal is N _trap , the deflection angle obtained when the optical signal passes through the KTN crystal is expressed by the following equation.

In the above equation, the deflection angle θ _pp is the maximum deflection angle deflection width in the x-axis direction when a sine wave is applied as the deflection voltage, with reference to FIG. n is the refractive index of KTN, and L is the length of the KTN crystal in the z-axis direction in FIG. g ₁₁ is an electro-optic constant, e is an elementary electric quantity, and ε is a dielectric constant. V is the maximum amplitude voltage of the deflection voltage, and d is the thickness of the KTN crystal in the z direction in FIG.

As can be seen from Equation (1), the deflection angle θ _pp is proportional to the electron density N _tarp charged in the trap.

FIG. 3 is a diagram illustrating the temporal variation of the trap electron density. The horizontal axis in FIG. 3 is time, and the vertical axis is the electron density N _trap filled in the trap in the KTN crystal. FIG. 3 shows a situation in which the trap filling operation (T _trap ) and the deflection operation (T _scan ) are alternately repeated three times (i = 1, 2, 3). At this time, the trap electron density N _trap changes as indicated by the curves 304, 301, 305, 302, 306, and 303.

During the initial trap filling operation (T _trap ), the trap electron density increases from 0 to N ₁ by applying the trap filling voltage. Thereafter, during the first deflection operation (T _scan ), for example, when a sine wave control voltage is applied for deflection, the trap electron density gradually decreases. Next, during the second trap filling operation (T _trap ), the trap electron density increases from the last value of the curve 301 to N ₂ by applying the trap filling voltage. Thereafter, during the second deflection operation (T _scan ), when the control voltage is applied for deflection, the trap electron density gradually decreases. During the third trap filling operation (T _trap ), the trap electron density increases from the last value of curve 302 to N ₃ by applying the trap filling voltage.

As can be seen from FIG. 3, when the trap electron density in the KTN is initially zero, the trap electron density N _trap becomes N ₁ , N ₂ every time one control cycle of the trap filling operation and the deflection operation is performed. , N ₃ and gradually increase. This is because electrons remain in the trap even after one deflection operation (T _scan ) is completed. Therefore, when trap trapping is continued in the state in which trap electrons remain in the previous control cycle, trap electron density obtained as a result of filling during the new filling operation is 1 This is different from the trap electron density in the previous control cycle.

FIG. 4 is a diagram conceptually showing changes in the trap electron density when the control cycle is repeated. The horizontal axis represents the number i of iterations of the control cycle, the vertical axis represents the trap electron density N _i in the last point in the trap filling operation of each control cycle. In Figure 4, the first cycle start time, shows a case where a certain amount of trapped electron density is already present, the N _i repeats the cycle number that has reached the equilibrium value. In the case of FIG. 4, the trap electron density is not stable unless a predetermined number of control cycles are passed. Therefore, the deflection angle obtained as shown in the equation (1) also varies.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an optical deflector using an electro-optic crystal having excellent deflection angle stability and an optical deflector control method. To do.

In order to achieve the above object, the present invention is characterized in that a control voltage is applied to at least two electrodes formed on opposite surfaces of an electro-optic crystal, and the electro-optic effect is applied to the inside of the crystal. In the method of controlling an optical deflector that deflects incident light that is incident substantially perpendicular to the electric field formed by the control voltage by generating a gradient of the refractive index distribution of the electro-optic crystal, a DC voltage or a positive polarity is applied to the electro-optic crystal. And a first driving step for applying a trap filling voltage composed of an alternating voltage including a negative DC voltage, and a second voltage for applying a deflection voltage for deflecting incident light as the control voltage following the first driving step. and second driving step, subsequent to said second driving step, a step of increasing the temperature of the electrooptic crystal, which is captured by the trap in the crystal A method of controlling an optical deflector, characterized in that it comprises the steps of removing a child.

  The invention according to claim 2 is the method according to claim 1, wherein in the second driving step, the amplitude of the deflection voltage is changed so as to cancel the time variation of the electron density in the electro-optic crystal. The maximum deflection angle of the emitted light obtained by the optical deflector is kept constant.

  According to a third aspect of the present invention, in the method of the first aspect, the first driving step, the second driving step, and the step of raising the temperature are repeated.

K The invention according to claim 4, in the method of claim 1, wherein the electro-optical crystal, potassium tantalate niobate _{(KTa 1-x Nb x O} 3 where (0 <x <1)) was added crystals or lithium _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1).

  The invention according to claim 5 is an electro-optic crystal in which at least two electrodes are formed on opposing surfaces, and a control voltage is applied to the electrodes to generate a gradient of an internal refractive index distribution by an electro-optic effect. An electro-optic crystal that deflects incident light that is incident substantially perpendicular to the electric field formed by the control voltage, and a control voltage power source that generates the control voltage to be applied to the electrode, the electro-optic crystal A trap filling voltage composed of a DC voltage or an alternating voltage including a positive polarity and a negative polarity DC voltage, and a deflection voltage for deflecting incident light are generated as the control voltage applied after the trap filling voltage is applied. Subsequently to the application of the control voltage power supply and the deflection voltage, the temperature of the electro-optic crystal is raised and the electric power captured by the trap in the electro-optic crystal is increased. An optical deflector, characterized in that a temperature varying means for removing.

  According to a sixth aspect of the present invention, in the optical deflector according to the fifth aspect, the control voltage power supply changes the amplitude of the deflection voltage so as to cancel out the time fluctuation of the electron density in the electro-optic crystal, and outputs it. The maximum deflection angle given to the incident light is kept constant.

According to a seventh aspect of the invention, the optical deflector according to claim 5, wherein the electro-optical crystal, potassium tantalate niobate _{(KTa 1-x Nb x O} 3 (0 <x <1)) doped crystal, or lithium K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1) .

  As described above, according to the present invention, an optical deflector with excellent reproducibility of the deflection angle can be realized.

FIG. 1 is a diagram showing a configuration of an optical deflector using KTN. FIG. 2 is a diagram for explaining a method of applying a control voltage in which a trap filling time is provided in the KTN crystal. FIG. 3 is a diagram illustrating the temporal variation of the trap electron density. FIG. 4 is a diagram conceptually showing changes in the trap electron density when the control cycle is repeated. FIG. 5 is a conceptual diagram showing the configuration of the optical deflector of the present invention. FIG. 6 is a graph showing the time variation of the trap electron density with the crystal temperature as a parameter. FIG. 7 is a conceptual diagram illustrating a method for obtaining the trap electron density from the refractive index distribution. FIG. 8 is a conceptual diagram showing temporal variation of the trap electron density when a control voltage including a history erasing operation is used in the present invention. FIG. 9 is a diagram conceptually showing changes in the trap electron density when the control cycle is repeated in the present invention. FIG. 10 is a conceptual diagram showing a control voltage control method in which the deflection voltage amplitude applied during the deflection operation (T _scan ) is corrected. FIG. 11 is a diagram for explaining the actual control timing of the optical deflector including the voltage history erasing operation for raising the temperature of the electro-optic crystal of the present invention.

According to the present invention, in an optical deflector using an electro-optic crystal such as KTN, the electro-optic crystal is heated and remains after a certain period of deflection operation and before filling the trap with electrons. It is characterized in that residual electrons are eliminated as much as possible by thermally exciting the trapped electrons to release them from the bound state from the traps. As a result, the voltage history of the deflection operation is deleted. The electro-optic crystal to which the present invention can be applied includes potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1): hereinafter referred to as KTN) crystal, or lithium as a material having the same effect as KTN. _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1) and the like added.

  During the above-described voltage history erasing operation, the electrons remaining in the trap remaining after the deflection operation are released from the trap, thereby erasing the history of the remaining electrons so far, and in each control cycle, trap electrons in the crystal. Return the state to the initial state. By providing this history erasing operation, the reproducibility of the deflection angle in the optical deflector is improved. Hereinafter, the configuration and operation of the present invention will be described in detail.

  FIG. 5 is a conceptual diagram showing the configuration of the optical deflector of the present invention. The optical deflector 600 of the present invention includes an electro-optic crystal 601 and electrodes 602 a and 602 b formed on the upper and lower surfaces of the crystal 601. In FIG. 5, the lower electrode 602b is not displayed. Specifically, the optical deflector 600 of the present invention is an optical deflector using a rectangular KTN crystal chip. A control voltage power source 604 that supplies a control voltage to be applied to KTN is connected between the two electrodes. The control voltage power source 604 can output various control voltage signals as described with reference to FIG. In addition, it has a driving capability capable of applying a voltage of about several hundred volts that can produce a practical deflection angle with respect to KTN. Incident light 605 is incident from one surface of the electro-optic crystal 601, is deflected, and exits from the opposite end surface.

  The optical deflector 600 of the present invention further includes a temperature control stage 603. The temperature control stage can set the temperature of the electro-optic crystal 601 to an arbitrary temperature, and can control the temperature of the electro-optic crystal 601 at a predetermined temperature change rate. In FIG. 5, the electro-optic crystal 601 is described as physically held, but any configuration may be used as long as the temperature of the electro-optic crystal 601 can be controlled. Therefore, the electro-optic crystal 601 may not be in contact. In FIG. 5, the temperature control stage 603 is shown as a mechanism that holds the electro-optic crystal 601, but there is naturally an accompanying control circuit for controlling the temperature of the electro-optic crystal. Next, a method for controlling the control voltage in the optical deflector of the present invention will be described in detail.

  FIG. 6 is a graph of the time variation of the trap electron density with the crystal temperature as a parameter. First, after trap filling is performed for a predetermined time (period A in FIG. 6), the electro-optic crystal is set to a predetermined temperature for a predetermined time (period B in FIG. 6), and then the trap electron density time is reached. The change was measured (period C in FIG. 6). The horizontal axis indicates the elapsed time in seconds, and the vertical axis indicates the trap amount (trap electron density).

  The measurement conditions will be described in more detail as follows. First, in the period A in FIG. 6, a trap filling DC voltage of 300 V was applied for 60 seconds to set the KTN crystal to the initial trap filling state. The trap filling voltage was stopped when the horizontal axis of the graph of FIG. 6 was 0 second, and the initial trap electron density was measured 6 seconds later. Thereafter, in the period B in FIG. 6, KTN was set to a predetermined temperature within 60 seconds. After setting the KTN crystal to a predetermined temperature, the trap electron density was repeatedly measured every 60 seconds in the period C of FIG.

The KTN used in the measurement of FIG. 6 has a chip size of width W 3.20 × length L (light traveling direction) 4.00 × thickness T (voltage application direction) 1.00 mm ³ , and the electrode size is W3.15 × L3.4 mm ² , the operating temperature of a normal KTN is 51.2 ° C. (εr = 17500).

In FIG. 6, a square plot is obtained by measuring the trap electron density while maintaining the KTN at the normal operating temperature of 51.2 ° C. The circle plot and the triangle plot are obtained by measuring the trap electron density by setting the temperature of KTN to 60 ° C. and 70 ° C., respectively. As is apparent from FIG. 6, by increasing the temperature of KTN higher, the trap electron density decreases more quickly and reaches the saturation lower limit. The trap electron density is saturated at a lower limit of about 1 × 10 ²⁰ (m ⁻³ ), which is due to the limit of the trap electron density measurement system.

  FIG. 7 is a conceptual diagram illustrating a method for obtaining the trap electron density from the refractive index distribution. The trap electron density can be determined by measuring the retardation distribution. (For the measurement method of retardation distribution, see Non-Patent Document 1). There is a relationship represented by the following formula between the retardation distribution and the refractive index change amount (Δn) distribution.

  In the above equation, Δn is the amount of change in refractive index, n is the refractive index (2.265 at the retardation measurement wavelength (633 nm)), L is the electrode length, and in the sample used in the measurement shown in FIG. 4 mm. The refractive index distribution can be obtained from the retardation measurement value using the relationship of the above equation (2). In the graph of residual refractive index distribution shown in FIG. 7, a refractive index distribution curve of a quadratic function is obtained before the temperature rises. From this quadratic function curve, it is possible to obtain the trap electron density on the assumption that the trap is uniformly filled with electrons in the crystal. Details are as follows.

Assume that there are uniformly trapped electrons in the crystal. When this trap amount is N _trap , the electric field distribution in the crystal is expressed by the following equation from Gauss's law.

  In the above equation (3), the following equation is obtained by integrating from 0 to x with the cathode of the two electrodes as the origin 0. Here, e is the electron elementary quantity, and ε is the dielectric constant of the electro-optic crystal.

  Since the voltage is obtained by integrating the electric field at the position, when the driving voltage is V and the distance between the two electrodes is d, the following equation is further obtained for the state when the driving voltage is 0.

  From the above equation (5), E (0) is expressed by the following equation.

Substituting the above equation (6) into equation (5), the following equation is obtained.

  When the electro-optic crystal has a secondary electro-optic effect (Kerr effect), the refractive index change Δn (x) is expressed by the following equation.

  The refractive index distribution in the case where there are uniformly trapped electrons of density N in the crystal is a quadratic function centered on the center (x = d / 2) in the thickness direction of the crystal as follows. .

If the graph of the residual refractive index distribution as shown in FIG. 7 is obtained, the electron density N _trap trapped from the graph can be estimated using the equation (9).

  As described above, by raising the temperature of the KTN crystal, trap-filled electrons remaining after the deflection operation can be released from the trap. By providing an operation for increasing the temperature of the KTN crystal and erasing the history of trap filling so far, the state of trapped electrons in the crystal can be returned to the initial state every control cycle. Next, a method for controlling the optical deflector to which the history erasing time is added will be described.

FIG. 8 is a conceptual diagram showing temporal variation of the trap electron density when a control voltage including a history erasing operation according to the present invention is used. FIG. 8 will be described in comparison with the time variation of the trap electron density in the case of the control voltage provided with only the conventional trap filling operation shown in FIG. In FIG. 8, the horizontal axis represents time, and the vertical axis represents trap electron density. This shows a situation where the trap filling operation (T _trap ) and the deflection operation (T _scan ) are alternately repeated three times (i = 1, 2, 3). At this time, the trap electron density N _trap changes as indicated by the curves 404, 401, 405, 402, 406, and 403. During the initial trap filling operation (T _trap ), the trap electron density increases from 0 to N ₁ by applying the trap filling voltage. Thereafter, during the first deflection operation (T _scan ), for example, when a sine wave control voltage is applied for deflection, the trap electron density gradually decreases. In the method of controlling an optical deflector according to the present invention, the electron remaining in the trap is raised by raising the temperature of the electro-optic crystal of the optical deflector during the voltage history erasing operation with the end of the first deflection operation (T _scan ). Is once released from the trap. Therefore, the trap electron density quickly returns to 0 after the end of the deflection operation (T _scan ).

Subsequent to the first control cycle, the second control cycle begins with a second trap filling operation (T _trap ), but the trap electron density increases again from 0 to N ₁ by applying the trap filling voltage. . Therefore, the trap electron density at the start of the second deflection operation (T _scan ) is the same N ₁ as in the first control cycle. Similarly, in the third control cycle, the trap electron density at the start of the third deflection operation (T _scan ) is the same as N _{1 in the} first cycle. Therefore, even when the control cycle is repeated, the trap electron density is kept substantially constant.

FIG. 9 is a diagram conceptually showing changes in the trap electron density when the control cycle is repeated in the present invention. The horizontal axis represents the number i of iterations of the control cycle, the vertical axis represents the trap electron density N _i in the last point in the filling operation of each cycle. In FIG. 9, plot 410 shows the prior art case where a certain amount of trapped electron density already existed at the beginning of the first cycle. According to the control voltage control method including the voltage history erasing operation of the present invention, the trap electron density _Ni is maintained at a constant value while maintaining the initial trap electron density regardless of the presence or the number of repetitions of the previous control cycle. Be drunk.

The deflection angle θ (t) at time t during the deflection operation (T _scan ) in FIG. 9 is expressed by the following equation with reference to equation (1).

Here, n is the refractive index of KTN, and L is the KTN crystal length in the z-axis direction in FIG. g ₁₁ is an electro-optic constant, e is an elementary electric quantity, and ε is a dielectric constant. V (t) is the deflection voltage, and d is the KTN crystal thickness in the z direction in FIG.

  Here, when the deflection voltage is a sine wave signal, the obtained deflection angle is also a sine wave.

From the equations (11) and (12), the deflection angle is expressed by the following equation.

In equation (13), N _trap (t) is expressed by the following equation as the trap electron density from when the trap filling is completed and the deflection operation (T _scan ) is started.

Paying attention to equation (14), when the control method of the control voltage provided only conventional trap filling operation, N _i varies in each control cycle. On the other hand, according to the control method of the control voltage including a voltage history erase operation shown in FIG. 8, so that N _i is maintained at a constant value. Further, in the deflection operation (T _scan ), the electron density gradually decreases gradually with a natural logarithmic curve having a certain time constant τ _i .

However, the deflection angle can be kept constant by correcting the amplitude of the deflection voltage applied to the deflection operation (T _scan ). Next, a method of further improving the stability of the deflection angle in the optical deflector control method using the control voltage including the history erasing operation of the present invention shown in FIG. 9 will be described.

FIG. 10 is a conceptual diagram showing a control voltage control method in which the deflection voltage amplitude applied during the deflection operation (T _scan ) is corrected. FIG. 10 also shows an envelope V ₀ (t) 503 and a deflection angle θ ₀ (t) 504 of the deflection voltage signal in addition to the trap electron density N _i (t) 501 during the deflection operation (T _scan ). Yes. In the improved method shown in FIG. 10, the envelope of the deflection voltage is gradually increased so as to cancel the gradual decrease in the trap electron density shown in Equation (14). For example, the envelope V ₀ of the deflection voltage signal is changed as follows:

By correcting the deflection voltage, the deflection angle during the deflection operation (T _scan ) can be maintained at a constant value.

In the above description, the trap electron density change during the deflection operation (T _scan ) is generally a natural logarithmic decrease curve, but may follow other change patterns for some reason. In such a case, the envelope of the deflection voltage may be corrected so as to cancel the trap electron density change according to this change pattern. Such a corrected control voltage can be easily obtained by generating an envelope signal with an arbitrary waveform generator and modulating a sine wave.

  FIG. 11 is a diagram for explaining the actual control timing of the optical deflector including the voltage history erasing operation for raising the temperature of the electro-optic crystal of the present invention. The optical deflector of the present invention shown in FIG. 5 has a temperature control stage 603. The temperature control stage 603 can set the temperature of the electro-optic crystal 601 to an arbitrary temperature, but generally a certain amount of time is required for the temperature setting. The temperature setting depends on the configuration of the temperature variable means in the temperature control stage 603.

  In FIG. 11, the horizontal time axis is expressed in linear real time considering the temperature setting time of the temperature variable means. In the present invention, the control cycle of the control voltage applied to KTN is divided into the following three periods according to the control contents.

The first period is a time (T _trap ) 507 for performing the trap filling operation. In this period, for example, a positive constant voltage 201 is applied as a burst voltage as shown in FIG. Even if there is no electron injection during the deflection voltage application operation described below, the electric field is tilted by the electrons trapped in the trap during the trap filling period. As a result, the refractive index is tilted by the electro-optic effect, so that high-speed and wide-angle light deflection is realized. The trap filling voltage is not limited to a unipolar DC voltage. For example, you may give the alternating voltage which contains two voltage values of positive polarity and negative polarity for every fixed time. By applying an alternating voltage, electrons can be trapped evenly in the vicinity of the two electrodes, and spatial nonuniformity of trapped electrons can be avoided. Further, an AC signal having a frequency lower than that of the high-speed signal used for the deflection operation may be given.

The second period is a time (T _scan ) 505 for performing a deflection operation for applying a deflection voltage, and a deflection operation is performed by applying a high-frequency deflection voltage to the electro-optic crystal. In this deflection period, as shown by V ₀ (t) in FIG. 11, the deflection voltage amplitude value (envelope) so as to cancel the trap electron density change corresponding to the trap electron density change N _i (t). ) Is preferably corrected by modulation. As a result, the deflection angle θ ₀ (t) can be maintained at a constant value.

The third period is a time (T _clear ) 506 for performing the voltage history erasing operation. The temperature of the electro-optic crystal is raised to release the electrons remaining in the trap so that the trapped electrons are generated in the electro-optic crystal. Return to no initial state. When the control cycle is repeated, the voltage history erasing operation includes an operation of returning the electro-optic crystal to the original normal operation temperature. By arranging the above three periods in this order and controlling the deflection voltage and the electro-optic crystal temperature, an optical deflector with excellent reproducibility of the deflection angle can be realized.

The control method as in the present invention is preferably applied to the following fields. That is, a certain long time of about 10 seconds is required for the voltage history erasing period, and it takes time to make the filling trap zero (although it is shortened compared with the case where the temperature is not increased). This is effective when there is a period in which the deflector is not operated for several minutes. That is, one control cycle can be applied to a relatively long time. It is also suitable for applications that do not require repetition in a short period. Since the time of each control cycle i is long and the decrease of the trap electron density N _i (t) is relatively large, the increase width of the deflection voltage becomes large. The absolute value of the deflection angle cannot be kept large, but the stability of the deflection angle is high.

  More specifically, it can be used in a method for evaluating electro-optical characteristics of an electro-optical crystal.

  As described above in detail, according to the present invention, an optical deflector excellent in reproducibility of the deflection angle can be realized.

  The present invention can be used for optical instruments.

100, 600 Optical deflector 101, 601 Electro-optic crystal 102, 103, 602a Electrode 104, 604 Control voltage power supply 105, 605 Incident light 603 Temperature control stage

Claims (7)

By applying a control voltage to at least two electrodes formed on opposite surfaces of the electro-optic crystal and generating a gradient of the refractive index distribution in the crystal by the electro-optic effect, an electric field formed by the control voltage is generated. In a method of controlling an optical deflector that deflects incident light that is incident substantially perpendicularly,
A first driving step of applying a trap filling voltage comprising a DC voltage or an alternating voltage including a positive polarity and a negative polarity DC voltage to the electro-optic crystal;
Subsequent to the first driving step, a second driving step of applying a deflection voltage for deflecting incident light as the control voltage;
The step of raising the temperature of the electro-optic crystal subsequent to the second driving step, and the step of removing electrons trapped in the traps in the crystal. Method.   In the second driving step, the amplitude of the deflection voltage is changed so as to cancel out the time variation of the electron density in the electro-optic crystal, and the maximum deflection angle of the emitted light obtained by the optical deflector is constant. 2. The method of claim 1, wherein the method is maintained.   The method according to claim 1, wherein the first driving step, the second driving step, and the step of increasing the temperature are repeated. The electro-optic crystal is potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1)) crystal or K _1-y Li _y Ta _1-x Nb _x O ₃ (to which lithium is added). The method according to claim 1, wherein 0 <x <1, 0 <y <1). An electro-optic crystal in which at least two electrodes are formed on opposite surfaces, and formed by the control voltage by applying a control voltage to the electrodes and generating a gradient of the internal refractive index distribution by the electro-optic effect. An electro-optic crystal that deflects incident light that is incident substantially perpendicular to the applied electric field;
A control voltage power supply for generating the control voltage to be applied to the electrode,
The electro-optic crystal has a trap filling voltage consisting of a DC voltage or an alternating voltage including a positive polarity and a negative polarity DC voltage;
A control voltage power supply configured to generate a deflection voltage for deflecting incident light as the control voltage applied after applying the trap filling voltage;
An optical deflector comprising: temperature changing means for increasing the temperature of the electro-optic crystal following application of the deflection voltage and removing electrons trapped in the traps in the electro-optic crystal.   The control voltage power supply is characterized in that the maximum deflection angle given to the outgoing light is kept constant by changing the amplitude of the deflection voltage so as to cancel out the time variation of the electron density in the electro-optic crystal. The optical deflector according to claim 5. The electro-optic crystal is potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1)) crystal or K _1-y Li _y Ta _1-x Nb _x O ₃ (to which lithium is added). 6. The optical deflector according to claim 5, wherein any one of 0 <x <1 and 0 <y <1) is satisfied.

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