JPH02238433A - light modulator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light modulation device that uses an electro-optic crystal to control the phase of light.

[Conventional technology]

In optical modulators using conventional electro-optic crystals, in order to compensate for the temperature characteristics of the crystals, the method described in O plus E February 1987 issue (Na87) How to Use Optical Crystals (Yutaka Ohno and Atsushi Nakayama), etc. As shown in FIG. do.

In FIG. 3, 6 is an electro-optic crystal, 12 is an electro-optic crystal whose crystal axis is rotated by 90 degrees with respect to 6, and 11 is a linearly polarized laser beam. By applying a voltage, an electro-optic crystal can be moved in two crystal axis directions: one direction parallel to the applied voltage (Z-axis) out of two directions perpendicular to the optical axis (Y-axis), and the other direction perpendicular to the applied voltage (Z-axis). It is a crystal that can change the refractive index (X-axis) at different rates. If the refractive index in the vertical and horizontal directions is different, and the refractive index in the vertical direction is np and the refractive index in the horizontal direction is ns, the optical path length Qp in the crystal for P-polarized light and the optical path length Qs in the crystal for S-polarized light are determined by the physics of the crystal. Letting the target length be a, it becomes as follows. Q p ” n p Q ・
...(1) Qs=nsQ −
(2) As is clear from this, when the optical path lengths of the laser beams are different, a phase difference occurs between the S-polarized light and the P-polarized light.

In other words, the electro-optic crystal 6 is an optical element that can control the phase difference between P and S polarized light by applying a voltage. At this time, the phase difference φ generated by the applied voltage V is expressed as (3). Here, the first term in equation (3) must be removed because it degrades the extinction ratio due to natural refraction and causes variations in the output depending on temperature. To compensate for this, by passing the laser beam through the electro-optic crystal 12 whose crystal axis has been rotated by 90 degrees, the phase difference φ cancels out the phase due to natural birefringence and becomes as follows. λ λ d 2π a s Qλ
d In this way, when a voltage is applied to an electro-optic crystal, the phase difference between two laser beams whose polarization planes are orthogonal to each other can be changed in proportion to the magnitude of the voltage. For example, when a sinusoidal voltage is applied, Then, the phase difference of light can be modulated according to its amplitude and frequency.6 The sensitivity of an electro-optic crystal is generally determined by the voltage Vπ required to generate a phase difference π (this is called a half-wave voltage). In the configuration shown in FIG. 3, Vπ is expressed as 2(neraa-norzg)Q. As is clear from equation (5), Vπ depends on the shape of the crystal, that is, the length Q in the optical axis direction and the thickness d in the applied voltage direction. In other words, the selection of the optical axis and voltage application axis also differs greatly. The configuration shown in Figure 3 is the most sensitive usage example out of several. [Problems to be Solved by the Invention] In the above-mentioned conventional technology, it has been shown that the phase difference between two orthogonal linearly polarized lights can be changed in proportion to the applied voltage. Even in the example in the figure, π
In general, a high voltage is required to generate a phase difference of a certain degree; for example, even in terms of crystal size, ■π can reach several hundred volts, which is necessary for high-voltage crystal drive amplifiers.

An object of the present invention is to provide a highly sensitive electro-optic crystal that enables phase modulation with an amplitude of about π to 2π at the voltage level of an oscillator or power supply without using an amplifier or the like. [Means for solving the problem] In order to achieve the above object, we created a stacked structure in which an electro-optic crystal is cut thinly in the direction of the voltage application axis, the axial direction is alternately reversed, and electrodes are arranged alternately between positive and negative. This is what I did.

[Effect]

The sensitivity of an electro-optic crystal, ie the half-wave voltage Vπ, is determined by the electric field strength and effective length within the crystal.

Therefore, the longer the crystal length Q in the optical axis direction and the thinner the thickness d in the voltage application direction, the higher the sensitivity.

However, if the length d is increased for this purpose, there is a limit because the modulator itself becomes too large and becomes inconvenient to use, and there is also a limit to the extent to which the laser beam can pass through the thickness d. . Therefore, according to the present invention, a highly sensitive electro-optic crystal can be realized without deteriorating the effective area by stacking a plurality of apparently tingling crystals with large electric field strength.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1 is a configuration diagram showing an overview of the present invention, and FIG. 2 is an exploded view for explaining how to assemble the device shown in FIG. The components in both figures are exactly the same and are shown below. 1 is a crystal driving power supply for applying an electric field (voltage) to the crystal. 2
, 3, 4.5 are electro-optic crystals, 6, 7, 8, 9.10 are electrodes for voltage application, and 11 is a laser beam for modulation.

First, with reference to Figure 2, we will explain how to assemble the device to realize the present invention. In the case of this embodiment, as shown in the figure, light is passed through the crystal in the Y-axis direction and a voltage is applied in the Z-axis direction, which is a high-sensitivity usage example. In this example, an electro-optic crystal is sliced into four parts in the direction of voltage application, and each crystal is divided into 2 to 5 parts. The individual crystals are then stacked and assembled in the same way as before, but at this time,
Alternately overlap each other with the voltage application axis direction reversed. That is, the crystals 2 and 4 are arranged so that the positive direction of the Z axis points upward in the figure, whereas the crystals 3 and 5 are arranged so that the positive direction of the Z axis points downward. Further, electrodes are alternately arranged between positive and negative electrodes between each crystal and at both ends thereof. Electrodes 6, 8, 10 are positive electrodes, and electrodes 7, 9 are negative electrodes. In this example, a lead wire is attached to a thin plate as an electrode and used by sandwiching it between the crystals, but instead of this configuration, for example, a metal such as copper or gold is deposited on both sides of the crystal. It may also be configured with a lead wire attached to the end. In any case, with this configuration, it is possible to apply an equal voltage to all (four) crystals from the positive to the negative direction of the Z axis, and by processing all the crystals to have the same thickness. An equal electric field can be generated in the positive to negative direction of the Z-axis of all crystals.

According to this embodiment, a uniform strong electric field is generated within the crystal, and for example, when the laser beam 11 is passed through, the laser beam separates and travels through four crystals in its cross section, but the laser beam does not pass through all the crystals at all. Since it receives the same amount of phase modulation, the light passing through this crystal can be regarded as one light passing through a uniform crystal. However, in this case, an electric field strength four times (times the number of crystal divisions) as in the conventional example can be obtained for a given applied voltage, so the electro-optic crystal with the configuration of the present invention has an electric field strength four times as large as that in the conventional example (the number of crystal divisions). This has the effect of obtaining a sensitivity of (multiple the number of divisions).

〔Effect of the invention〕

According to the present invention, by dividing the dimension of a conventional electro-optic crystal in the voltage application direction into n parts (n: an integer), the electric field strength inside the crystal can be increased by n times. This has the effect of increasing the sensitivity of the instrument by n times. In other words, a voltage of 1/n is required to generate a certain phase scene.

[Brief explanation of drawings]

1 and 2 are block diagrams showing an outline of an embodiment of the present invention, and FIG. 3 is a block diagram showing a conventional example. 61...Crystal drive power supply, 2, 3, 4, 5. ...Electro-optic crystal, 6
, 7, 8, 9. 10... Electrode, 11... Laser beam.

Claims (1)

[Claims] 1. In an optical modulator consisting of an electro-optic crystal having an electro-optic effect, electrodes and a power source for applying a voltage to this crystal, applying a voltage to the electro-optic crystal, dividing the dimension into equal parts in the direction, and applying the voltage. The orientation of the eigenaxis of the crystal in the direction (
The sensitivity of the crystal is doubled by the number of divisions, by arranging the positive and negative electrodes so that they all face each other, and placing electrodes with alternating polarity between each crystal, and bringing them into close contact. optical modulator.