JP2017203847A - Light deflector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light deflector capable of keeping a KTN crystal at a constant temperature by a single temperature controller.SOLUTION: A light deflector includes an electro-optic crystal and an electrode pair formed on opposing faces of the electro-optic crystal and comprising a positive electrode and a negative electrode to generate an electric field in the electro-optic crystal, in which an optical axis of incident light is set to be orthogonal to the direction of the electric field and a voltage is applied between the pair of electrodes to deflect the incident light. The light deflector includes: a first conductor holding part having a first temperature detector and in contact with the positive electrode; and a second conductor holding part having a second temperature detector in contact with the negative electrode. The electro-optic crystal is kept at a constant temperature by temperature control means based on the temperature detected by the first and second temperature detectors connected in series.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an optical deflector, and more particularly, to an optical deflector structure capable of reducing the temperature control mechanism required for stable operation of the optical deflector.

Optical deflectors are widely used for printers, displays, optical instruments for inspection such as OCT, laser microscopes for living body observation, and the like. Conventionally, an optical deflector using a polygon mirror, a galvanometer mirror, a MEMS mirror, or the like has been proposed. In recent years, an optical deflector using a potassium tantalate niobate (KTN: KTa _1-x Nb _x O ₃ , 0 <x <1) crystal, which is an electro-optic crystal, has been reported as a high-speed optical deflector. The KTN optical deflector is a so-called transmission type optical deflector. An electric field is generated inside the KTN crystal by electron injection and voltage application to the KTN crystal, and a refractive index distribution is generated inside the KTN crystal. The optical axis of the incident light is set so as to be orthogonal to the direction of the electric field, and the light transmitted through the crystal is deflected by the application of the electric field. A mechanism that does not have a driving portion such as a galvanometer mirror realizes light deflection that is faster than conventional light deflection.

  The deflection angle θ in the KTN optical deflector is expressed by the following equation (for example, see Non-Patent Document 1).

n is the refractive index of KTN, g ₁₁ is the electro-optic constant, e is the elementary charge, N is the number of electrons, L is the crystal length, ε is the dielectric constant, V is the applied voltage, and d is the distance between the electrodes. Since KTN has a larger dielectric constant than other electro-optic crystals, wide-angle deflection is possible. In addition, the dielectric constant of KTN is temperature-dependent and becomes a maximum near the phase transition temperature of tetragonal and cubic crystals. Therefore, for wide angle and stable operation, the temperature of KTN crystal is around the phase transition temperature. It is necessary to keep it constant. KTN generates a large amount of heat due to its large dielectric constant, and in order to keep the temperature of the KTN crystal constant, a temperature control mechanism that takes into account the heat generation from the KTN crystal is required.

JP 2013-195916 A

J. Miyazu, T. Imai, S. Toyoda, M Sasaura, S. Yagi, K. Kato, Y. Sasaki, and K. Fujiura, “New Beam Scanning Model for High-Speed Operation Using KTa1-xNbxO3 Crystals”, Appl Phys. Express, vol. 4, no. 11, 111501, November 2011.

  As a temperature control mechanism for keeping the temperature of the KTN crystal constant, a mechanism using a thermistor as a temperature detector and a Peltier element as a cooling heating means is known (for example, see Patent Document 1).

  FIG. 1 shows a first example of a temperature control mechanism of a conventional optical deflector. The upper and lower surfaces of the KTN crystal 1 are sandwiched between graphite sheets 2a and 2b and are held by two metal blocks 3a and 3b. Electrodes for applying a control voltage are formed on the upper and lower surfaces of the KTN crystal 1, and are electrically connected to a control voltage source via metal blocks 3a and 3b. The graphite sheets 2a and 2b are inserted in order to prevent the KTN crystal from being broken by vibration when a high frequency control voltage is applied to the KTN crystal 1. On both sides of the KTN crystal 1, aluminum nitride (AlN) 4a and 4b are inserted as heat transfer materials for positioning the KTN crystal 1 and keeping the temperatures of the two metal blocks uniform.

  A Peltier element 6 is disposed between the metal block 3a and the support plate 5, and a thermistor 7 is embedded in the metal block 3a. The temperature control device 8 detects the temperature by the thermistor 7 and heats the metal block 3 a by the Peltier element 6 to maintain the KTN crystal 1 at an appropriate set temperature.

  FIG. 2 shows changes in the capacitance of the KTN crystal with respect to the environmental temperature in the first example. It is the result of measuring the capacitance between the electrodes provided on the upper and lower surfaces of the KTN crystal 1 according to the environmental temperature. The capacitance of KTN is expressed by the following equation.

S is an electrode area provided in the KTN crystal 1. As shown in the equation (2), since the capacitance is proportional to the dielectric constant, the change in the dielectric constant can be detected by looking at the variation of the capacitance. When the variation in permittivity required for a system using a KTN optical deflector is replaced with a capacitance, the allowable range is ± 0.02 nF, but according to FIG. 2, a variation of ± 0.2 nF is observed.

  FIG. 3 shows a second example of a temperature control mechanism of a conventional optical deflector. In order to suppress fluctuations in the dielectric constant, two Peltier elements 6a and 6b are disposed between the metal block 3a and the support plate 5a, and between the metal block 3b and the support plate 5b, respectively. Two thermistors 7a and 7b are also arranged in each of the metal blocks 3a and 3b, and the temperature is controlled from above and below the KTN crystal 1 by the two temperature control devices 8a and 8b. With such a configuration, the temperature of the KTN is kept constant and fluctuations in the dielectric constant are suppressed.

  However, the KTN optical deflector of the second example requires two temperature control devices for driving the Peltier elements, which increases the size of the KTN optical deflector and increases the manufacturing cost. It was.

  An object of the present invention is to provide an optical deflector capable of keeping the temperature of a KTN crystal constant with a single temperature control device.

  In order to achieve the above object, according to one embodiment of the present invention, there is provided an electro-optic crystal, and a positive electrode and a negative electrode that are formed on opposing surfaces of the electro-optic crystal and generate an electric field inside the electro-optic crystal. An optical deflector for deflecting the incident light by applying a voltage between the electrode pair, the optical axis of the incident light being set so as to be orthogonal to the direction of the electric field, A first conductor holding portion that contacts the positive electrode and includes a first temperature detector, and a second conductor holding portion that contacts the negative electrode and includes a second temperature detector, and is connected in series. The electro-optic crystal is held at a constant temperature by temperature control means based on the temperatures detected by the first and second temperature detectors.

  According to the present invention, since the electro-optic crystal is controlled to be held at a constant temperature based on the temperatures detected by the first and second temperature detectors connected in series, one temperature control means is provided. Thus, the temperature of the KTN crystal can be kept constant.

It is a figure which shows the 1st example of the temperature control mechanism of the conventional optical deflector. It is a figure which shows the change of the capacitance of the KTN crystal with respect to the environmental temperature in a 1st example. It is a figure which shows the 2nd example of the temperature control mechanism of the conventional optical deflector. It is a figure which shows the optical deflector concerning Example 1 of this invention. It is a figure which shows the change of the capacitance of a KTN crystal with respect to environmental temperature in Example 1. FIG. It is a figure which shows the optical deflector concerning Example 2 of this invention. It is a figure which shows the optical deflector concerning Example 3 of this invention.

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

  FIG. 4 shows an optical deflector according to the first embodiment of the present invention. FIG. 4 shows a KTN optical deflector viewed from the incident direction of light. The upper and lower surfaces of the KTN crystal 11 are sandwiched between graphite sheets 12a and 12b, and are held by two metal blocks (conductor holding portions) 13a and 13b. On the upper and lower surfaces of the KTN crystal 11 facing each other, an electrode pair consisting of a positive electrode and a negative electrode for applying a control voltage is formed, and electrically connected to the control voltage source via the metal blocks 13a and 13b that are in contact with the electrode pairs. It is connected. By applying a voltage from the control voltage source and injecting electrons into the KTN crystal 11, an electric field is generated inside the KTN crystal 11, and a refractive index distribution is generated inside the KTN crystal 11. The optical axis of the incident light is set so as to be orthogonal to the direction of the electric field, and a voltage is applied between the electrode pair to deflect the incident light.

  The graphite sheets 12a and 12b are inserted in order to prevent the KTN crystal from being destroyed by vibration when a high frequency control voltage is applied to the KTN crystal 11. On both sides of the KTN crystal 11, aluminum nitride (AlN) 14a, 14b is inserted as a heat transfer material for positioning the KTN crystal 11 and keeping the temperatures of the two metal blocks uniform.

  A Peltier element 16 is disposed between the metal block 13a and the support plate 15, and thermistors (temperature detectors) 17a and 17b are embedded in the metal blocks 13a and 13b, respectively. The temperature control device 18 detects the temperature by the thermistors 17a and 17b, and heats or cools the metal block 13a by the Peltier element 16, thereby maintaining the KTN crystal 11 at an appropriate set temperature (constant temperature).

  Here, the temperature control device 18 detects the temperature by measuring the resistance values of the thermistor 17a and the thermistor 17b connected in series. That is, the temperature rise of both the metal blocks 13a and 13b due to the heat generation of the KTN crystal 11 can be fed back. Further, not only the heat generation of the KTN crystal 11 but also the temperature change of the metal blocks 13a and 13b due to the change of the environmental temperature can be dealt with, and the dielectric constant of the KTN crystal 11 can be kept constant.

  Further, such a configuration enables temperature control by using only one set of the temperature control device 18 and the Peltier element 16. Accordingly, it is possible to prevent the KTN optical deflector from becoming large and to reduce the manufacturing cost.

  FIG. 5 shows changes in the capacitance of the KTN crystal with respect to the environmental temperature in Example 1. An applied voltage having a frequency of 20 kHz and an amplitude of 600 Vpp is applied to the KTN crystal 11. The capacitance of the KTN crystal of the metal blocks 13a and 13b was measured while changing the ambient temperature between 20-40 ° C. Compared with FIG. 2 of the conventional example, according to Example 1, the change in the capacitance of the KTN crystal is within ± 0.02 nF of the allowable range, and even if the environmental temperature changes by 20 ° C., it is almost affected. You can see that it is not.

  The KTN crystal is electrostricted when a voltage is applied, and the dielectric constant changes depending on the pressure applied to the KTN crystal. For this reason, in order to absorb the electrostriction of the KTN crystal and keep the pressure on the KTN crystal constant, a holding mechanism of the KTN crystal using a spring or the like is effective (for example, see Patent Document 1). For example, pressure applying means for applying a predetermined pressure to the KTN crystal in a direction perpendicular to the surface on which the positive electrode is formed and the surface on which the negative electrode is formed by two metal blocks (conductor holding portions) 13a and 13b. Can be provided. The presence / absence and shape / configuration of such a holding mechanism may be arbitrary. Further, the upper and lower heat transfer may have any shape as long as the insulation between the upper and lower electrodes is maintained. The structure of the thermistor also may be in any configuration, including setting locations, number, etc. If detecting the temperature of the upper and lower electrodes.

  FIG. 6 shows an optical deflector according to Embodiment 2 of the present invention. The optical deflector according to the first embodiment performs uniaxial light deflection in the y-axis direction, whereas the optical deflector according to the second embodiment performs biaxial optical deflection of the x axis and the y axis. The configuration of the optical deflection element in the previous stage that performs optical deflection in the y-axis direction is the same as that of the optical deflector of the first embodiment. The upper and lower surfaces of the KTN crystal 21 are sandwiched between graphite sheets 22a and 22b, and are held by two metal blocks (conductor holding portions) 23a and 23b. On the upper and lower surfaces of the KTN crystal 21 facing each other, an electrode pair consisting of a positive electrode and a negative electrode for applying a control voltage is formed, and electrically connected to the control voltage source via the metal blocks 23a and 23b that are in contact with the electrode pairs. It is connected. On both sides of the KTN crystal 21, aluminum nitride (AlN) 24a and 24b are inserted as heat transfer materials for positioning the KTN crystal 21 and keeping the temperatures of the two metal blocks uniform. The metal block 23 a is placed on the upper surface of the heat transfer plate 25, and the Peltier element 26 is disposed on the lower surface of the heat transfer plate 25. The thermistors 27a and 27b are embedded in the metal blocks 23a and 23b, respectively.

  A half-wave plate 29 is inserted between the former stage optical deflection element and the latter stage optical deflection element that performs the optical deflection in the x-axis direction, and is disposed on the same optical axis. The latter optical deflection element is installed so as to form an angle of 90 degrees with the optical axis as the central axis. The outgoing light from the front stage light deflection element passes through the half-wave plate 29 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. The left and right surfaces of the KTN crystal 31 are sandwiched between graphite sheets 32a and 32b, and are held by two metal blocks (conductor holding portions) 33a and 33b. On the opposite left and right surfaces of the KTN crystal 31, an electrode pair consisting of a positive electrode and a negative electrode for applying a control voltage is formed, and electrically connected to the control voltage source via metal blocks 33a and 33b that are in contact with the electrode pair. It is connected. Above and below the KTN crystal 31, aluminum nitride (AlN) 34a and 34b are inserted as heat transfer materials for positioning the KTN crystal 31 and keeping the temperatures of the two metal blocks uniform. One side surface of the metal blocks 33 a and 33 b is placed on the upper surface of the heat transfer plate 25. The thermistors 37a and 37b are embedded in the metal blocks 33a and 33b, respectively.

  The temperature control device 28 detects the temperature of the preceding optical deflection element by the thermistors 27a and 27b, and detects the temperature of the subsequent optical deflection element by the thermistors 37a and 37b. The four thermistors 27a, 27b, 37a, and 37b are connected in series, and the temperature control device 18 can feed back the temperature rise of both optical deflection elements by measuring these series resistance values. The temperature control device 28 heats the metal blocks 23a, 33a, 33b by the Peltier element 26, and maintains the KTN crystals 21, 31 at an appropriate set temperature.

  According to the second embodiment, the entire optical deflector including the optical deflection elements at the front and rear stages can be controlled by one temperature control device. In addition, about the structure of a heat exchanger plate, what kind of shape may be sufficient if it can contact | connect each metal block and can couple | bond a Peltier element thermally.

  The setting of the temperature control device when a plurality of thermistors are connected in series will be described. The temperature detected by the thermistor is calculated from the following equation.

Ta is an absolute temperature at a ° C., Ra is a zero load resistance value at a ° C., T ₂₅ is an absolute temperature at 25 ° C., R ₂₅ is a resistance value of the thermistor at 25 ° C., and B is a B constant. When two thermistors are connected in series as in Example 1, when the respective resistances are R ₁ and R ₂ , the detected temperatures are:

Is approximated by the following equation.

Compared with the equation (3), by multiplying the resistance value R _{25 of the} thermistor by two, _Ra and R ₁ + R ₂ can be treated equally. That is, the Peltier element can be driven even when a plurality of thermistors are connected in series as in the case where a conventional single thermistor is connected, by simply changing the resistance value R _{25 of the} thermistor twice. Can do.

  FIG. 7 shows an optical deflector according to Embodiment 3 of the present invention. The upper and lower surfaces of the KTN crystal 41 are sandwiched between graphite sheets 42a and 42b, and are held by two metal blocks (conductor holding portions) 43a and 43b. On the upper and lower surfaces of the KTN crystal 41 facing each other, an electrode pair consisting of a positive electrode and a negative electrode for applying a control voltage is formed, and electrically connected to the control voltage source via the metal blocks 43a and 43b in contact with each. It is connected. On both sides of the KTN crystal 41, aluminum nitride (AlN) 44a and 44b are inserted as heat transfer materials for positioning the KTN crystal 41 and keeping the temperatures of the two metal blocks uniform. A Peltier element 46 is disposed between the metal block 43a and the support plate 45. A plurality of thermistors (a plurality of temperature detection elements) 47a and 47c are provided inside the metal block 43a, and a plurality of thermistors (a plurality of temperature detection elements) are provided inside the metal block 43b. Thermistors (a plurality of temperature detecting elements) 47b and 47d are embedded.

The thermistor 47a (resistance value R ₁ ) and the thermistor 47b (resistance value R ₂ ) are connected in series, and the thermistor 47c (resistance value R ₁ ) and the thermistor 47d (resistance value R ₂ ) are connected in series. The thermistor set and the latter thermistor set are connected in parallel and connected to the temperature controller 48. In this case, the four combined resistors are

It becomes.

  The average temperature at the four locations is

It becomes.

  At this time, the temperature detected by the temperature control device 48 is the same as that in Expression (3). Therefore, the temperature control device 48 can be applied as it is without changing the setting of the temperature control device 38 of the second embodiment.

(Electro-optic material)
In the present embodiment, the KTN crystal has been described as an example of the electro-optic crystal, but a single crystal material having a perovskite crystal structure having inversion symmetry can also be used. As a single crystal material related to KTN, the main component of the crystal is composed of groups Ia and Va in the periodic table, group Ia is potassium, and group Va includes at least one of niobium and tantalum. Can be used. Moreover, 1 or multiple types of periodic table group Ia except potassium as an additional impurity, for example, lithium, or IIa group can also be included. For example, a cubic phase KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ , 0 <x <1, 0 <y <1) crystal may be used.

In addition, PbTi _1-x Zr _x O ₃ (PZT) and PLZT to which La is added are also perovskite crystals, and can have inversion symmetry by selecting the composition and use temperature. These can be used not only as a single crystal but also as a polycrystalline sintered body.

(Electrode material)
When a high voltage is applied to the electro-optic crystal, charges are injected from the electrodes, and space charges can be generated in the crystal. This space charge causes a gradient in the magnitude of the electric field in the direction in which the voltage is applied. Therefore, in order to make the electro-optic material function as the optical deflector of the present embodiment, in order to obtain a desired refractive index distribution, when a voltage is applied to the electro-optic crystal, a high-density space is formed inside the electro-optic crystal. Charges should be formed.

  The amount of space charge depends on the carrier injection efficiency. When the carriers contributing to electrical conduction in the electro-optic crystal are electrons, as the work function of the electrode material decreases, the electrode and the substrate approach an ohmic junction, and the carrier injection efficiency increases. Therefore, the electrode formed on the opposing surface of the KTN crystal of this embodiment is preferably a material that forms an ohmic junction with the KTN crystal. For example, Ti (3.84) or the like can be used as an electrode material having a work function of less than 5.0 eV. The parentheses indicate work functions, and the unit is eV. Since the Ti single-layer electrode is oxidized and becomes high resistance, generally, an electrode in which Ti / Pt / Au is sequentially laminated is used to join the Ti layer and the electro-optic crystal. Furthermore, transparent electrodes such as ITO (Indium Tin Oxide) and ZnO can also be used.

  On the other hand, when the carriers contributing to electrical conduction in the electro-optic crystal are holes, the work function of the electrode material is preferably 5.0 eV or more in order to inject holes efficiently. For example, as an electrode material having a work function of 5.0 eV or more, Co (5.0), Ge (5.0), Au (5.1), Pd (5.12), Ni (5.15), Ir (5.27), Pt (5.65), Se (5.9) can be used.

1,11,21,31,41 KTN crystal 2,12,22,32,42 Graphite sheet 3,13,23,33,43 Metal block 4,14,24,34,44 Aluminum nitride (AlN)
5, 15, 45 Support plate 6, 16, 26, 46 Peltier element 7, 17, 27, 37, 47 Thermistor 8, 18, 28, 48 Temperature controller 25 Heat transfer plate

Claims (7)

Incident light so as to be orthogonal to the direction of the electric field, comprising: an electro-optic crystal; and an electrode pair formed on opposite surfaces of the electro-optic crystal and configured to generate an electric field inside the electro-optic crystal. And an optical deflector for deflecting the incident light by applying a voltage between the electrode pair,
A first conductor holding portion in contact with the positive electrode and provided with a first temperature detector;
A second conductor holding part in contact with the negative electrode and provided with a second temperature detector;
An optical deflector characterized in that the electro-optic crystal is held at a constant temperature by temperature control means based on temperatures detected by the first and second temperature detectors connected in series.   The first conductor holding part and the second conductor holding part apply a predetermined pressure to the electro-optic crystal in directions perpendicular to the surface on which the positive electrode is formed and the surface on which the negative electrode is formed. The optical deflector according to claim 1, further comprising pressure applying means for applying the pressure.   3. The optical deflector according to claim 1, wherein the electro-optic crystal is a perovskite single crystal material. The electro-optical crystal, potassium tantalate niobate _{(KTN: KTa 1-x Nb} x O 3, 0 <x <1) the optical deflector according to claim 3, characterized in that the.   5. The optical deflector according to claim 1, wherein the positive electrode and the negative electrode are made of a material that forms an ohmic junction with the electro-optic crystal.   A plurality of optical deflectors according to any one of claims 1 to 5 are arranged on the same optical axis, all of the first and second temperature detectors are connected in series, and all of them are controlled by the temperature control means. An optical deflector characterized in that the electro-optic crystal is maintained at a constant temperature. The first temperature detector includes a plurality of first temperature detection elements,
The second temperature detector includes the same number of second temperature detection elements as the first temperature detection elements,
A set of a plurality of temperature detection elements, wherein one of the first temperature detection elements and one of the second temperature detection elements are connected in series;
6. The electro-optic crystal is held at a constant temperature by the temperature control means based on a temperature detected by a set of the plurality of temperature detecting elements connected in parallel. An optical deflector according to claim 1.