JPS5937802B2 - electro-optic light modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical modulator that temporally modulates the phase, polarization, intensity, etc. of laser light, and particularly to an optical modulator that utilizes the electro-optic effect of crystals.

Electro-optic modulators are used in many systems that use laser light, such as optical communication systems, optical memory systems, laser display systems, and laser recording devices, which transmit, read, display, and record large amounts of information at high speed. is an essential element.

Electro-optic light modulators use the so-called Bockels effect, an effect in which the refractive index of a crystal for light changes in proportion to the magnitude of an electric field applied from outside the crystal. It has a wider bandwidth than a modulator that uses the acousto-optic effect, and can modulate light at a faster speed. For this reason, it is becoming widely used for the purposes mentioned above. Even among optical modulators that use the electro-optic effect of crystals, optical modulators that use the Bockels coefficient, which creates the effect of rotating the index ellipsoid by applying an electric field, are better than those that use the Bockels coefficient to deform the index ellipsoid. When no external electric field is applied, the amount of light transmitted is smaller than that of an optical modulator that uses this effect, that is, it has a superior extinction ratio.

Lithium tantalate is well known as a material with a large Bockels coefficient that contributes to the rotation of the index ellipsoid. This material has optical anisotropy. Therefore, in order to increase the modulation efficiency, the period Λ is set to 2π/Λ■2πIne
-nolλ (Create the period of the application electrode so that

Here, no and ne are the refractive indexes of lightning and extraordinary light, and λ is the wavelength of light in direct air. The structure and operation of such an electro-optic light modulator are detailed in Japanese Patent Application No. 51-82095. However, an electro-optic light modulator having such a configuration is affected by ambient humidity. That is, when the circumferential polarization changes, the magnitude of birefringence Ine-no1 of the crystal changes. This breaks down the condition for phase matching between orthogonal polarized light components, ie, lightning and extraordinary light, that is, the equational relationship in the above equation, and reduces the efficiency of conversion between lightning and extraordinary light. In other words, the insertion loss of the optical modulator for light increases. As described above, conventional electro-optic light modulators have insufficient characteristics. SUMMARY OF THE INVENTION An object of the present invention is to provide a high performance and stable electro-optic light modulator that does not have the drawbacks of conventional electro-optic light modulators as described above.

According to the present invention, among the electro-optic coefficient tensor components of a crystal, a component that contributes to the effect of causing rotation of an optical index ellipsoid when an electric field is applied to the crystal is used, and In an electro-optic optical modulator that controls the magnitude of the amplitude between components orthogonal to each other in the polarization direction by applying an electric field using electrodes provided in the By providing a structure in which the temperature increases or decreases, an electro-optic light modulator that is stable against changes in ambient temperature can be obtained.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is one of the conventional examples shown for explaining the principle of an electro-optic light modulator. In the figure, 1 is a crystal that is cut parallel to the Z-axis, shaped and polished to transmit light in the Y-axis direction, has low optical anisotropy, and has a large Bockels coefficient that contributes to the rotation of the index ellipsoid. , for example, the concentration of lithium oxide is 10% higher than the concentration of tantalum oxide in terms of molar ratio.
~30% larger lithium tantalate crystal (LiTa)
O3) and lead niobate crystal (゛PbO, xNb2O5; 1
．． It is an X-cut plate of electro-optic crystal such as 5〈x〈2.1).

Electrodes 6, 7, 8, and 9 are provided on the opposing X planes of this crystal plate 1 so that an electric field is applied in the X-axis direction, and the direction of the electric field is reversed in the light transmission direction. connects an oscillator 10 that generates a modulation signal. When the applied electric field is zero, linearly polarized incident light 3 having its polarization direction in the C-axis direction of the crystal 1 exits the crystal 1 while maintaining its polarization plane. The analyzer 2 is arranged so as to transmit only the optical electric field component vibrating in a direction perpendicular to the plane of polarization, and when the applied electric field is zero, the intensity of the modulated light 4 is zero. When an electric current is applied to the crystal, as the incident light 3 passes through the crystal 1, its energy is converted into a polarization component orthogonal to the incident light. In the above-mentioned crystal, among the electro-optic coefficient tensor components, V5l is involved. For this conversion to be accomplished efficiently, a period of electric power equal to the difference between the two propagating wave transmissions is required. In general, coupling between two modes occurs efficiently in two states where their phase velocities are approximately the same. When there is a difference in phase velocity, the reception of energy between the two modes causes ripples in the light transmission direction. If the sign of the coupling coefficient is inverted in accordance with the phase of the undulation, the conversion will always be additive. In the electro-optic light modulator described above, the electrode period A
If is selected so as to satisfy equation (1), the efficiency of conversion will increase. However, as the ambient temperature changes, the refractive index of the crystal changes. The magnitude of birefringence, which is the difference between the ordinary refractive index and the extraordinary refractive index, also changes. For this reason, even if the electrode period A is determined to achieve matching at a specific temperature, matching will no longer be achieved if the ambient temperature changes. Efficiency is therefore significantly degraded. FIG. 2 shows the modulation characteristics when an optical modulator is constructed as shown in FIG. 1 using a lithium tantalate crystal having a value of Ne--NO=3.times.10@4. Actual alignment is a characteristic when alignment is achieved correctly. The broken line is Ne-NO 2×
This is the characteristic when the ratio changes by 10-5, and the efficiency is significantly degraded. As described above, in a single-period electrode structure, the characteristics deteriorate due to changes in ambient temperature. FIG. 3 is a plan view showing a single-sided electrode structure of an embodiment of the present invention in which the electrode period is changed in the light transmission direction.

That is, the electrode period near the center of the crystal in the light transmission direction is selected so as to satisfy equation (1) at room temperature, and thereby the front period is wide and the rear period is narrow. Or vice versa. With this configuration, when the ambient temperature changes, the location in the crystal where proper matching can be achieved changes, but the modulation characteristics do not deteriorate. Figure 4 uses the aforementioned lithium tank oxide crystal and has an electrode period of 2.4 m1!
The number of electrodes is 30 from L to 1.6 mm, and the crystal length is 31 m.
711 is the modulation characteristic when the crystal thickness is 0.3711 m.

Although the applied voltage is as high as about 70 V, the change in modulation characteristics is small even if the crystal birefringence changes from 2 to 2.6×10 −4 . Therefore, the modulation characteristics are automatically compensated for changes in ambient vorticity. As is clear from the above description, according to the present invention, a stable electro-optic light modulator can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating the principle of an electro-optic light modulator, where 1 is an electro-optic crystal, 2 is an analyzer, 3 is incident light, 4 is modulated light, and 6-9 is an itch-applying electrode.

Claims (1)

[Claims] 1 Among the electro-optic coefficient tensor components of a crystal, when an electric field is applied to the crystal, the tensor component that contributes to the effect of causing rotation of the optical index ellipsoid is used to In an electro-optic light modulator that controls the magnitude of the amplitude between electric field components of light whose polarization directions are orthogonal to each other by applying an electric field using arranged electrodes,
An electro-optic light modulator, characterized in that the electrodes are arranged so that the period thereof monotonically increases or decreases.