JP6487835B2 - Optical deflector and control method thereof - Google Patents

Description

  The present invention relates to an optical deflector and a control method thereof, and more particularly to an optical deflector using a KTN crystal having an electro-optic effect and a control method thereof.

For the optical deflector that changes the traveling direction of light, a technology that rotates a polygon mirror, a technology that controls the deflection direction of light using a galvanometer mirror, a light diffraction technology that uses the acousto-optic effect, a micromachine technology called MEMS, etc. are applied. ing. In recent years, an electro-optic crystal having an electro-optic effect, specifically, potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1)) crystal (KTN crystal) or K ₁₋ added with lithium _{y L y Ta 1-x Nb} x O 3 (0 <x <1,0 <y <1) crystal (KLTN crystal) optical deflector using the have been proposed (e.g., see Patent Document 1). Unlike the optical deflector described above, the optical deflector utilizing the electro-optic effect is a solid element having no movable part, and can deflect light at high speed. Hereinafter, for simplicity, two types of KTN crystal and KLTN crystal are collectively referred to as KTN crystal unless explicitly distinguished.

  A KTN crystal is known as a substance having a large electro-optic effect, the refractive index of which changes greatly when a relatively low voltage is applied. Furthermore, when Ti or Cr is used as the application electrode, charges can be injected into the KTN crystal. By utilizing the internal electric field generated by the charge, a high-speed and wide-angle optical deflector can be realized. Therefore, it is possible to replace very common optical components such as lenses, prisms, and mirrors with an optical deflector (KTN optical deflector) using a KTN crystal in applications where they need to move at high speed (for example, patents). Reference 2).

  In recent years, attention has been focused on a medical optical tomographic imaging system using a high-speed wavelength swept light source in which a KTN optical deflector is incorporated in an external resonator by utilizing the high-speed property of refractive index control in the KTN crystal. The medical optical tomographic imaging system can acquire a clearer image or an image with a wide field angle if the optical deflector can be driven at a high frequency in MHz. For this reason, further high frequency driving is expected for the KTN deflector, which is a key device for realizing high speed.

  FIG. 1 shows a configuration of an optical deflector using a conventional KTN crystal. 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 is incident on the left side surface of the KTN crystal 101 and is deflected in the KTN crystal 101 while traveling in the z-axis (optical axis) direction. The light is emitted from the right side surface of the KTN crystal 101 as outgoing light 106 while changing the traveling direction in the x-axis direction. At this time, a deflection angle θ corresponding to the applied voltage is 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 the KTN crystal 101.

International Publication No. 2006/137408 JP 2012-074597 A JP2012-242612A

  However, when a conventional optical deflector using an electro-optic crystal such as a KTN crystal is driven at high speed by modulating an applied voltage, a desired deflection angle is reduced due to heat generation based on dielectric loss or mechanical loss of the crystal. It was not obtained. In particular, at a frequency where the control signal exceeds MHz, the deflection angle is greatly deteriorated due to a decrease in the dielectric constant due to heat generation.

  Further, since the KTN crystal has a large dielectric constant during driving, the capacitance between the electrodes is increased, and the control voltage source is large and expensive. There is also an application example in which the control signal is a signal including a high frequency component such as a triangular wave instead of a sine wave and applied to the KTN crystal. In this case, there is a problem that the output of the control signal is limited depending on the specification of the control voltage source as well as the above-described problem of heat generation.

  Furthermore, if the resolution point of the deflection angle of the KTN optical deflector is to be improved, it is necessary to increase the crystal thickness, which has been a major obstacle to improving the deflector characteristics.

  An object of the present invention is to provide an optical deflector in which when a KTN crystal is driven by a high-speed control signal, there is no decrease in dielectric constant due to heat generation, and a desired deflection angle can be obtained.

  In order to achieve such an object, the present invention, in an optical deflector using an electro-optic crystal such as a KTN crystal, injects a charge into the electro-optic crystal by applying a DC voltage to the electro-optic crystal, The light deflection of the incident light is controlled by modulating the intensity of the irradiation light having a wavelength shorter than the wavelength of the incident light and irradiating the electro-optic crystal.

  As described above, according to the present invention, the light incident on the electro-optic crystal can be deflected only by modulating the intensity of the irradiation light, so that it is not necessary to apply a high voltage AC voltage. The problem of heat generation of the electro-optic crystal can be avoided.

  In addition, since the driving power source for the irradiation light has a low modulation voltage, a small and inexpensive power source may be used, so that an economical optical deflector can be realized.

It is a figure which shows the structure of the optical deflector using the conventional KTN crystal | crystallization. It is a figure for demonstrating the method of calculating | requiring a trap electron density from refractive index distribution. It is a figure which shows the structure of the optical deflector concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the optical deflector concerning the 2nd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  When the electron density filled in the trap of the KTN crystal is Ntrap, the deflection angle obtained when incident light passes through the KTN crystal is expressed by the following equation (Patent Document 3).

  The parameters will be described with reference to FIG. 1. The deflection angle θp-p is the deflection width of the maximum deflection angle in the x-axis direction of the emitted light 106 when a sine wave is applied as a control signal. n is the refractive index of the KTN crystal 101, and L is the length of the KTN crystal 101 in the z-axis direction. g11 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 control signal, and d is the thickness of the KTN crystal in the z-axis direction. As can be seen from the equation (1), the deflection angle θp-p is proportional to the electron density Ntarp filled in the trap inside the KTN crystal. Thus, the deflection angle is determined by the product of the amount of charge injected into the trap and the applied voltage to be driven.

FIG. 2 is a conceptual diagram for explaining a method of obtaining the trap electron density from the refractive index distribution. It is the result of measuring the retardation distribution of the electro-optic crystal (KTN crystal) before and after irradiation of the LED by the method described in Patent Document 3, and converting it into a refractive index distribution according to the following formula.
Δn = Retardation / L (2)

  In the above equation, Δn is the amount of change in refractive index, and L is the length of the electrode in the optical axis direction. The length of the electrode in the electro-optic crystal used in the measurement of the trap electron density described in FIG. 2 is 3.4 mm. The refractive index distribution can be obtained from the retardation measurement value using the relationship of the above equation (2). From the quadratic function curve shown in the graph of FIG. 2, assuming that the traps in the crystal are uniformly filled with electrons, the trap electron density can be obtained from the following equation (Patent Document 3).

  In the residual refractive index distribution shown in FIG. 2, a refractive index distribution curve having a substantially quadratic function is obtained before the KTN crystal is irradiated with ultraviolet light from the LED light source (before LED irradiation in the figure). This is a refractive index distribution when trapped electrons are uniformly present in the crystal. Next, after irradiating with ultraviolet light (after LED irradiation in the figure), the refractive index distribution as described above disappears, the trapped electrons are released, and there are no electrons in the crystal. Show.

  That is, it was confirmed that the amount of charge injected into the crystal can be varied by irradiating the KTN crystal with ultraviolet light from the LED light source. In this measurement, the charge injection amount was measured by applying the DC voltage to the KTN crystal for a certain time and then measuring the residual charge when the DC voltage was turned off. When the KTN crystal is irradiated with ultraviolet light, the DC voltage is turned off.

(First embodiment)
The amount of charge injected into the electro-optic crystal (KTN crystal) is determined by the magnitude of the applied DC voltage. On the other hand, as described above, the amount of injected charge is determined by the wavelength and intensity of light irradiated from the LED light source to the KTN crystal. Here, when the LED light source is irradiated with ultraviolet light while applying a DC voltage to the KTN crystal, the amount of charge determined by the balance between the charge entering and exiting the KTN crystal remains in the crystal as residual charge. Become.

  Furthermore, the amount of charge that should remain in the KTN crystal can be varied at high speed by controlling the light irradiation intensity by the LED light source by modulating the current. Since the deflection amount of the optical deflector is determined by the DC voltage and the residual charge amount, the deflection angle is controlled by modulating the light irradiation intensity of the LED light source while only the DC voltage is applied. The principle optical deflector can be realized.

  Conventionally, when an optical deflector using an electro-optic crystal such as KTN is driven at a high speed, the dielectric constant due to the heat is generated at a frequency where the control signal exceeds MHz because of heat generation based on the dielectric loss or mechanical loss of the crystal. The deflection angle was deteriorated due to the decrease in. According to this embodiment, since it is not necessary to apply a high AC voltage, the heat generation of the electro-optic crystal hardly poses a problem.

  Further, like KTN crystal, since it has a large dielectric constant at the time of driving, the capacitance between the electrodes is increased, and a large and expensive control voltage source is required. According to the present embodiment, the power source for driving the LED may be a small and inexpensive power source because the modulation voltage is low. Even if the control signal is a signal including not only a sine wave but also a high frequency component such as a triangular wave, it can be easily applied to an LED driving power source.

  As described above, when driving at a high speed, a desired deflection angle with respect to the applied voltage becomes small due to heat generation. That is, the number of resolution points also decreases. According to this embodiment, since the problem of heat generation can be avoided by modulating the light irradiation intensity by the LED, a desired deflection angle with respect to the applied voltage can be increased, so that the number of resolutions can be improved without increasing the crystal thickness. can do.

  FIG. 3 shows the configuration of the optical deflector according to the first embodiment of the present invention. Electrodes 202 and 203 are formed on the upper and lower surfaces of the KTN crystal 201, which is an electro-optic crystal, as an electrode pair composed of a positive electrode and a negative electrode for generating an electric field inside the crystal. A control voltage (DC voltage) is applied from the control voltage source 204 between the two electrodes. Incident light 205 (1.3 μm band) is incident on the left side surface (xy plane) of the KTN crystal 201 so as to be orthogonal to the direction of the electric field, and travels in the Z-axis (optical axis) direction to enter the KTN crystal 201. Be biased at. The light is emitted from the right side surface of the KTN crystal 201 as outgoing light 206 by changing the traveling direction in the x-axis direction. Further, an LED light source 207 that irradiates ultraviolet light having a wavelength of 405 nm is disposed at a position 10 mm away from the side surface (xz plane) along the optical axis of the KTN crystal 201.

  The size of the KTN crystal 201 is 4 mm (length in the z-axis direction) × 3 mm (width in the y-axis direction) × 1 mm (thickness in the x-axis direction). A metal film such as copper is formed as an electrode on the upper and lower surfaces of the KTN crystal 201. A Peltier element (not shown) for controlling the temperature of the KTN crystal is thermally coupled to the lower portion of the electrode 203. Here, the phase transition temperature of the KTN crystal is set so that the relative dielectric constant of the KTN crystal is 20000 at 30 degrees.

  With such a configuration, after irradiating ultraviolet light from the LED light source 207, a DC voltage of −240 V is applied from the control voltage source 204. While applying only the DC voltage from the control voltage source 204, the LED light source 207 irradiates a 20 kHz triangular wave intensity-modulated light modulation signal. In the LED light source 207, a function generator is used to drive the LED in a drive current range of 0 to 10 mA so as to obtain a 20 kHz triangular wave. A polarization angle of 100 mrad was realized by the deflection angle according to the DC voltage and the deflection angle according to the light modulation signal. In this way, there is no deterioration of the deflection angle, and high-speed (frequency exceeding MHz) light deflection is possible.

  Further, since the control voltage source 204 only needs to apply a DC voltage, it is not necessary to apply a high AC voltage. Since the LED light source 207 can easily drive the LED by a modulation signal including a high frequency component with a simple configuration using an existing light source, an economical optical deflector can be realized.

(Second Embodiment)
FIG. 4 shows a configuration of an optical deflector according to the second embodiment of the present invention. The optical deflector of the first embodiment performs uniaxial light deflection in the x-axis direction, whereas the second embodiment performs biaxial light deflection of the x-axis and the y-axis. The configuration of the optical deflection element in the previous stage is the same as the configuration of the optical deflector of the first embodiment. Electrodes 302 and 303 are formed on the upper and lower surfaces of the KTN crystal 301. A control voltage (DC voltage) is applied from the control voltage source 304 between the two electrodes. Incident light 305 (1.3 μm band) is incident on the left side surface (xy plane) of the KTN crystal 301 and is deflected in the KTN crystal 301 while traveling in the z-axis (optical axis) direction. An LED light source 307 that irradiates ultraviolet light having a wavelength of 405 nm is provided on a side surface (xz plane) along the optical axis of the KTN crystal 301.

  A half-wave plate 321 is inserted between the front-stage optical deflection element and the rear-stage optical deflection element, and is disposed on the same optical axis. The front-stage optical deflection element and the rear-stage optical deflection element It is installed so as to form an angle of 90 degrees with the axis as the central axis. The outgoing light from the front stage light deflection element passes through the half-wave plate 321 and enters the rear stage light deflection element.

  The configuration of the optical deflection element at the subsequent stage is the same as the configuration of the optical deflector of the first embodiment. Electrodes 312 and 313 are formed on both side surfaces of the KTN crystal 311. A control voltage (DC voltage) is applied from the control voltage source 314 between the two electrodes. Incident light from the preceding optical deflection element changes its traveling direction in the x-axis direction and is emitted from the right side surface of the KTN crystal 311 as outgoing light 316. An LED light source 317 that irradiates ultraviolet light having a wavelength of 405 nm is provided on the upper surface (yz plane) along the optical axis of the KTN crystal 301.

  The size of the two KTN crystals 301 and 311 is 4 mm (length in the z-axis direction) × 3 mm (width in the y-axis direction) × 1 mm (thickness in the x-axis direction). As in the first embodiment, electrodes are formed on the KTN crystals 301 and 311 and Peltier elements (not shown) are thermally coupled. Here, the phase transition temperature of the KTN crystal is set so that the relative dielectric constant of the KTN crystal is 20000 at 30 degrees.

  With such a configuration, after the ultraviolet light is irradiated from the LED light sources 307 and 317 to the KTN crystals 301 and 311 respectively, a DC voltage of −240 V is applied from the control voltage sources 304 and 314. While only the DC voltage was applied from the control voltage sources 304 and 314, the LED light source 307 modulated the LED drive current with a 20 kHz triangular wave, and the LED light source 317 modulated the LED drive current with a 200 kHz triangular wave. In the front and rear optical deflection elements, the deflection angle according to the DC voltage and the deflection angle according to the modulation signal, respectively, do not deteriorate the deflection angle and enable high-speed (frequency exceeding MHz) two-dimensional optical deflection. It becomes.

  In the present embodiment, 1.3 μm band light is used as incident light, and ultraviolet light having a wavelength of 405 nm is used as irradiation light from the LED light source. By using irradiation light having a short wavelength, the effects of the present invention can be achieved.

101, 201, 301, 311 KTN crystal 102, 103, 202, 203, 302, 303, 312, 313 Electrode 104, 204, 304, 314 Control voltage source 105, 205, 305 Incident light 106, 206, 316 Emission light 207 , 307, 317 LED light source 321 half-wave plate

Claims (4)

Potassium tantalate niobate _{(KTa 1-x Nb x O} 3 (0 <x <1)) crystal K _1-y or that the addition of _{lithium, L y Ta 1-x Nb} x O 3 (0 <x <1, 0 <y <1) an electro-optic crystal that is one of the crystals, and at least one electrode pair including a positive electrode and a negative electrode that generate an electric field inside the electro-optic crystal, and orthogonal to the direction of the electric field An optical deflector for deflecting the incident light by setting an optical axis of incident light and applying a voltage between the electrode pair,
A light source for irradiating the electro-optic crystal with irradiation light having a wavelength shorter than the wavelength of the incident light;
An optical deflector characterized in that a DC voltage is applied between the electrode pair, and the incident light is deflected by modulating an irradiation intensity of the irradiation light. Potassium tantalate niobate _{(KTa 1-x Nb x O} 3 (0 <x <1)) crystal K _1-y or that the addition of _{lithium, L y Ta 1-x Nb} x O 3 (0 <x <1, 0 <y <1) an electro-optic crystal that is one of the crystals, and at least one electrode pair including a positive electrode and a negative electrode that generate an electric field inside the electro-optic crystal, and orthogonal to the direction of the electric field An optical deflector control method for deflecting the incident light by setting an optical axis of incident light and applying a voltage between the electrode pair,
Applying a DC voltage between the electrode pair to inject charges into the electro-optic crystal;
Irradiating the electro-optic crystal with irradiation light having a wavelength shorter than the wavelength of the incident light, comprising: modulating the irradiation intensity of the irradiation light to deflect the incident light. Control method of optical deflector. Potassium tantalate niobate _{(KTa 1-x Nb x O} 3 (0 <x <1)) crystal K _1-y or that the addition of _{lithium, L y Ta 1-x Nb} x O 3 (0 <x <1, 0 <y <1) An electro-optic crystal that is one of the crystals, and at least one electrode pair including a positive electrode and a negative electrode that generate an electric field inside the electro-optic crystal, and orthogonal to the direction of the electric field The optical axis of the incident light is set, and a voltage is applied between the electrode pair to deflect the incident light, and the first and second optical deflection elements, which are optical deflection elements, A half-wave plate inserted between the first optical deflection element and the second optical deflection element is an optical deflector disposed on the same optical axis,
Each of the first and second light deflection elements includes a light source for irradiating the electro-optic crystal with irradiation light having a wavelength shorter than the wavelength of the incident light,
Each of the first and second light deflection elements applies a DC voltage between the electrode pair, and modulates the irradiation intensity of the irradiation light to deflect the incident light. Deflector. Potassium tantalate niobate _{(KTa 1-x Nb x O} 3 (0 <x <1)) crystal K _1-y or that the addition of _{lithium, L y Ta 1-x Nb} x O 3 (0 <x <1, 0 <y <1) An electro-optic crystal that is one of the crystals, and at least one electrode pair including a positive electrode and a negative electrode that generate an electric field inside the electro-optic crystal, and orthogonal to the direction of the electric field The optical axis of the incident light is set, and a voltage is applied between the electrode pair to deflect the incident light, and the first and second optical deflection elements, which are optical deflection elements, A half-wave plate inserted between the first optical deflection element and the second optical deflection element is a method of controlling an optical deflector disposed on the same optical axis,
In each of the first and second optical deflection elements, applying a direct current voltage between the electrode pair to inject electric charge into the electro-optic crystal;
In each of the first and second light deflecting elements, the step of irradiating the electro-optic crystal with irradiation light having a wavelength shorter than the wavelength of the incident light, wherein the irradiation intensity of the irradiation light is modulated. A method of controlling an optical deflector, comprising: deflecting the incident light.