JPS5928894B2 - electro-optic light modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an optical modulator that temporally modulates the amplitude of a laser beam;
In particular, it relates to optical modulators that utilize the electro-optic effect of lithium tantalate crystals.

Optical modulators that modulate the intensity of laser light at high speed according to electrical signals are used in devices that transmit and process high-speed, large-capacity information such as in optical communication and optical information processing, and in high-speed devices such as laser displays and laser recording devices. It is an essential element for devices that display and record speed.

Electro-optic light modulators use the Pockels effect, an effect in which the refractive index of a crystal for light changes in proportion to the magnitude of the electric field applied to the crystal from the outside, and can be used for other effects such as mechanical vibration or acousto-optic. It has a wider modulation bandwidth than optical modulators that use effects, and can modulate light at a much faster speed. For this reason, it is becoming widely used for the above-mentioned purposes. The principle of conventional optical modulators using the electro-optic effect, for example, the principle of the transverse electric field type using ferroelectric crystal LiTaO3, which is currently widely used, is widely understood as described below.

An electric field applied in the z-axis direction of the crystal causes a phase difference between the optical electric field components in the two principal axes of the index ellipsoid of a light wave propagating through the crystal. Therefore, linearly polarized light incident with an oscillating electric field component at an arbitrary angle with respect to the main axis of the crystal is
When the light exits the crystal, it generally becomes elliptically polarized light. The intensity of the light is modulated by transmitting this light through an analyzer that transmits only a component in one direction of the crystal principal axis. The phase difference between the optical electric field components in the above two principal axis directions is 2π φ=-(nz-nY)1 λ where nY■no-''2nδγ31E ゛ 1 nZ■nE-dn6γ33E, and λ is , 1 is the length of the crystal in the direction of light transmission, n0; nE is the refractive index of the crystal for ordinary and extraordinary rays, respectively, γ31 and γ33 are the electro-optic coefficients, and E is the electric field applied to the crystal. It is strength.

The modulation band of this type of optical modulator is approximately determined by the electrical capacity of the element. The capacity is 10 to 20 in a normal design.
It can be about pF. Therefore, as mentioned above, the modulation bandwidth is wide. However, conventional optical modulators have had several drawbacks. For example, one of them is that in the phase difference φ in the previous equation, there is a term that does not depend on the strength of the applied electric field.
A so-called natural birefringence term (2π/λ)(NEnO)l is included. Therefore, the modulation operation becomes unstable due to changes in ambient temperature. To compensate for this, as is well known, a λ/2
Attempts have been made to insert a plate or to make the electric field application axes of two crystals of equal length, such as the z-axes in the above example, orthogonal to each other. These methods can compensate for the above effects if the geometric arrangement of the two crystals is ideal, but there are imperfections in the lengths of the two crystals and the orthogonality of the crystal axes. Therefore, it is not possible to provide complete compensation. This increases the complexity of crystal processing and element assembly during actual manufacturing. Furthermore, the presence of the natural birefringence term in the above-mentioned phase difference requires a crystal with extremely good homogeneity. This is because if there is a non-uniform portion of natural birefringence (NE-NO) inside the crystal used, the degree of extinction will be lowered. As described above, with conventional electro-optic light modulators, it is extremely difficult to obtain an element that is less affected by ambient temperature and has a high modulation degree. The above-mentioned difficulty with conventional optical modulators, namely the deterioration of the extinction degree due to temperature changes in natural birefringence, is 2.
This occurs because the phase difference between the two oscillating electric field components is sensitive to temperature changes.

Among the optical phenomena used to control light, methods that use the effects of light diffraction and mode coupling have virtually no deterioration in extinction due to changes in ambient temperature in principle. A well-known example of an element that diffracts and modulates light is the electro-optic crystal C.
A device with interdigital electrodes on the surface, which diffracts the light that passes through the bottom of the electrodes through periodic changes in the refractive index caused by the electric field applied between the electrode fingers within the surface, thereby achieving light intensity modulation. It is. The features of this optical modulator are that, as mentioned above, it uses the effect of diffraction, so it is almost unaffected by temperature changes, and because the diffracted light, that is, the modulated light, can be spatially separated, it is possible to use an analyzer. It is something that is not necessary. However, the electric lines of force generated by the interdigital electrodes are localized near the bottom of the electrode, so in order to increase the diffraction efficiency, the light must be made into a ribbon shape and transmitted near the bottom surface of the electrode. . Therefore, a cylindrical lens is required. Also,
Diffraction occurs due to the electrode itself, and a decrease in extinction cannot be avoided. By applying an external electric field, we can optically modulate a crystal with electro-optic coefficient tensor components that causes coupling between optical electric field components that propagate independently with their oscillating electric field directions perpendicular to each other in the absence of an applied electric field. By using the crystal as a medium and applying an electric field with electrodes placed in appropriate directions, incident light with only one of the above oscillating electric field components will be converted into light with a linearly polarized component as it passes through the crystal. .

Light intensity modulation is achieved by transmitting the emitted light through an analyzer. An optical modulation element that uses this mode coupling effect does not utilize the above-mentioned phase change of light, but uses a change in amplitude, so it is not affected by natural birefringence. In this type of optical modulator, in order for the incident light to be completely converted into components orthogonal to each other in the vibration direction, the propagation constant of each of the two components,
That is, it is necessary that the refractive index be equal or that there be a period of the electro-optical constant (or periodicity of the applied electric field) having a wave number equal to the difference in propagation constants. Many of the known electro-optic crystals have different refractive indices in the directions in which the components of the electro-optic constant tensor contributing to the above operation can be utilized. Therefore, electrodes must be provided for applying a periodic electric field. However, in ordinary crystals, this difference in refractive index is large, so it is necessary to make the periodic spacing of the electrodes fine. Therefore, electric lines of force penetrate only near the surface of the crystal. As in the case of the optical modulator that utilizes the effect of diffraction, the electric field that effectively acts on the light wave weakens, so it is inevitable that the applied voltage and crystal length will increase. As has become clear in the field of so-called integrated optical circuit technology, which is a technology for confining optical energy near the surface of a dielectric crystal substrate, the propagation constants of many light propagation modes propagating near the surface of a substrate are very close to each other. are doing.

If the substrate has an electro-optic effect, the above operation can be achieved with electrodes having relatively coarse periodic intervals. However, as is well known, it is extremely difficult to make adjustments to couple a light beam emitted from a light source such as a laser into the waveguide mode propagating near the surface of the substrate, and to extract it back into the air as a beam. , and the loss of light during input and output is also large. It is difficult to realize an optical modulation element with low optical insertion loss using the above technology. In this way, mode-coupled optical modulation elements are more effective than elements that utilize phase changes or diffraction effects.
Although it is theoretically predicted that the device will have high performance, a construction method for realizing a practical device has not yet been found. SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the conventional electro-optic light modulators and provide a high-performance, stable mode-coupled light modulator.

According to the present invention, a lithium tantalate crystal having a composition in the range of Li/Ta-1.0 to 1.5 is used as a modulation medium, and electrodes for applying an electric field in the X-axis (or Y-axis) direction are opposed to each other. It is installed on two X planes (or Y planes), and the light passing direction is
By oriented in the axial (or X-axis) direction, a high-performance and stable optical modulator can be obtained.

Embodiments of the present invention will be described with reference to the drawings. In Figure 1, 1 is tantalum acid made with an arbitrary molar ratio of Li2O and Ta2O5, cut parallel to the Z axis (C axis) of the crystal, shaped and polished to transmit light in the y axis direction. This is an X-cut plate of lithium solid solution crystal. Electrodes 6 and 7 are provided on the opposing X planes of this crystal so that an electric field is applied in the X-axis direction, and the direction of the electric field is periodically reversed in the light propagation direction, as shown in FIG. Oscillator 5 that generates a modulation signal
The output of is connected in such a way that the electric field described above is generated in the crystal. Linearly polarized incident light 2 with its polarization direction in the C-axis direction is
When the applied electric field is zero, light is emitted from the crystal while maintaining its polarization plane. The analyzer 4 is arranged so as to transmit only the optical electric field component vibrating in the X-axis direction perpendicular to the plane of polarization, and when the applied electric field is zero, the intensity of the modulated light 3 is zero. When an electric field is applied to the crystal, as the incident light 2 passes through the crystal, its energy is converted into a component that vibrates in the X-axis direction. Among the components of the electro-optic constant tensor of the lithium tantalate crystal, R5l is involved in this. The incident light is extraordinary light in the crystal, and its refractive index is NE. The emitted light is NO. A lithium tantalate crystal with a normal stoichiometric composition (Li/Ta-1.0) has an NE-2.
180, NO=2.176. As mentioned above, in order to achieve 100% of the above mode conversion, the periodicity of the electric field is required to be equal to the difference in the propagation constant of each mode. For this purpose, the period A of the electrode 6 as shown in FIG. 1 must be set so as to satisfy the following.

When using a crystal with the above theoretical stoichiometric composition, 6
For light of 33 nm, the thickness of aluminum is 160 μm. In order to effectively achieve the mode conversion function, the applied electric field must be effectively directed in the thickness direction of the crystal plate, that is, in the X-axis direction. Therefore, the thickness of the crystal plate is 40~50μ
It must be about m. In order to transmit the incident light 2 through this thin crystal plate, it is necessary to sufficiently narrow down the light beam, and therefore the length of the crystal in the Y-axis direction, that is, the active length cannot be made very long. Furthermore, the difficulty in processing the crystal plate and in manufacturing the device increases. In a lithium tantalate crystal having a composition with a higher Li concentration than the above-mentioned stoichiometric composition (Li/Ta-1), the above-mentioned refractive index difference NE-NO differs depending on the composition ratio.

An experimental report on this can be found in the Journal off the
American Ceramic Society (Tournal)
Of the Caramic Society) Volume 50,
No. 12, pages 657-659 of the paper by Bollman et al. (hereinafter referred to as Document 1), Applied Physics Letters (Appl. Phys. Lett), Vol. 23, No. 4, No. 19
It is described in Miyazawa's paper on pages 8 to 200 (hereinafter referred to as Document 2). FIG. 2 is a diagram showing the magnitude of birefringence depending on the composition ratio, drawn from the experimental values of Document 2. As Li/Ta increases, the difference in refractive index decreases, and Li/Ta-1
．． Around 25, this difference becomes almost zero. Furthermore, Li/
In a crystal with a large Ta value, the sign of the refractive index difference is reversed, as also reported in Reference 1. Judging from Literature 1 and Literature 2, it is considered that the refractive index difference decreases in proportion to the value of Li/Ta until Li/Ta=1.5. Therefore, the stoichiometric composition (Li/Ta-1
．． 0) higher Li concentration than Li/Ta-1.0 to 1
．． If a mode-coupled optical modulator is constructed using a lithium tantalate crystal in the range of If crystals are used, the electrodes may have a uniform structure.

Therefore, the thickness of the crystal can be increased, and an optical modulator with a long working length, high efficiency, and easy manufacture can be constructed. Note that even with such a composition ratio, the optical homogeneity of the crystal is sufficiently high. Further, in the configuration of the embodiment shown in FIG. 1, the same functions and characteristics can be obtained even if the X-axis and Y-axis of the crystal are exchanged.

[Brief explanation of the drawing]

FIG. 1 is a principle block diagram of an embodiment of the present invention, in which 1 is a modulation crystal, 2 is an incident light beam, 3 is an output light beam, 4 is an analyzer, 5 is a modulation signal generator, 6 and r are electric fields. This is an electrode for application.

Claims (1)

[Claims] 1. In an optical modulator that uses an electro-optic crystal as a modulation medium and controls the magnitude of coupling between two light waves whose polarization planes are perpendicular to each other passing through the crystal, Li/Ta=1.0 as the electro-optic crystal. A lithium tantalate crystal having a composition in the range of ~1.5 is used, and the X axis (or Y
The electrodes that apply an electric field in the direction of
1. An electro-optic light modulator, characterized in that the electro-optic light modulator is provided on the Y-axis (or the Y-axis), and the light transmission direction is the Y-axis (or the X-axis).