JP2534703B2 - Polarization control device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization control device for polarization control used in an optical communication system, an optical fiber sensor or the like, and more particularly to a waveguide type polarization control device. is there.

(Prior Art) A coherent optical communication system, an optical fiber sensor, or the like requires a polarization controller for converting light having an arbitrary polarization state into constant linearly polarized light. Of the devices used for this type of polarization controller, the waveguide type is superior in the following points. That is, a waveguide type device is promising as a device used for polarization control because it is small and lightweight, mass producibility, and integration with other elements are possible.

Conventionally, as such a waveguide type polarization control device,
Some of them were presented by Shimada et al. In 1986 (1986), Proc. This is a combination of a phase shifter and a mode converter for TE or TM for any incident polarization.
It is to convert to mode.

(Problems to be Solved by the Invention) However, this has a limitation in the polarization control range. That is, in the conventional waveguide polarization control device described above, depending on the polarization state of incident light, for example, when the polarization angle continues to rotate in a certain direction, the drive voltage of the phase shifter or the mode converter continues to rise, and finally the limit voltage is reached. And the polarization control becomes impossible. As described above, in the conventional waveguide type polarization control device, the polarization control range is limited,
There is a problem that it is not possible to control arbitrary fluctuations of incident polarized light.

It is an object of the present invention to provide a polarization control device which eliminates the limitation of the polarization control range and can perform polarization control with respect to arbitrary fluctuation of incident polarization.

(Means for Solving the Problems) A polarization control device of the present invention comprises a channel optical waveguide formed on a surface portion of a substrate having an electro-optical effect, and an in-plane surface perpendicular to the light propagation direction of the channel optical waveguide. For producing an electric field of any magnitude in any direction in the channel optical waveguide portion of, in order to produce birefringence of any magnitude in any direction in the same plane and in the same portion where the electric field occurs. Forming first and second elements in series on the channel optical waveguide and on both sides of the channel optical waveguide and stripe electrodes provided on the same plane as the surface portion on which the channel optical waveguide is formed. Composed.

(Operation) In the two elements connected in series which constitute the polarization control device according to the present invention, the birefringence of any size is channeled in any direction in the plane perpendicular to the light propagation direction of the channel optical waveguide. It can be generated in the optical waveguide portion. By keeping the retardation in each element constant at π / 2 and π radians and rotating the direction of birefringence, each element functions similarly to a normal λ / 4 plate and a λ / 2 plate, respectively.
By setting the birefringence direction of each element that operates as a wave plate to an appropriate direction, any incident polarized light can be converted into any linearly polarized light as in the case of a normal wave plate. Here, the voltage required to rotate the direction of birefringence changes periodically, and does not continue to rise. That is, the polarization control is performed without the problem that the drive voltage continues to rise even if the incident polarization changes.

(Example) Next, this invention is demonstrated with reference to drawings.

FIG. 1 is a perspective view of a polarization control device showing an embodiment of the present invention.

This polarization control device forms a channel optical waveguide for propagating light, and along the waveguide, each channel optical waveguide portion is arbitrarily oriented in any direction within a plane perpendicular to the above-described light propagation direction. Lithium niobate (LiNbO ₃ ) substrate 100 as a substrate having an electro-optical effect for forming first and second elements that cause birefringence of the magnitude of
Is used.

On the surface of the lithium niobate substrate 100, the first
As shown in the figure, a channel optical waveguide 101 is formed in the central portion along the longitudinal direction. The channel optical waveguide 101 is formed in this embodiment by selectively diffusing Ti on a flat substrate. The propagation direction of light in the channel optical waveguide 101 is the C-axis direction of the lithium niobate substrate 100. Therefore, when using lithium niobate, the channel optical waveguide 101 is set in consideration of this point.

At the one end side and the other end side of the lithium niobate substrate 100, the first
Element 102 and a second element 102 'are provided. These operate in the same manner as the wave plate, and are capable of producing birefringence of any size in the channel optical waveguide 101 portion in any direction in the plane perpendicular to the light propagation direction described above. An electrode is provided for generating an electric field of an arbitrary magnitude in the channel optical waveguide 101 portion in an arbitrary direction within the vertical plane.

That is, the first element 102 is provided by providing the first, second, and third stripe electrodes 110, 111, 112 directly above and on both sides of the channel optical waveguide 101 on one end side on the surface of the lithium niobate substrate 100. Is configured. Although not shown in FIG. 1, the channel optical waveguide 101
A SiO ₂ film is formed between the first stripe electrode 110 and the first stripe electrode 110 by the CVD method. This is to prevent the attenuation of the waveguide propagation light by the metal film.

FIG. 2 shows the first part of the polarization control device of this embodiment.
3 is a cross-sectional view of the element 102 of FIG. As shown in FIG. 2, the first
The voltages V ₁ and V ₂ are respectively applied to the second stripe electrodes 110 and 111 by the power supply 120, and the third stripe electrode 112 is grounded.

Also, the second and third stripe electrodes 111, 112 are
Each is provided so as to be equidistant from the first stripe electrode 110.

As shown in FIG. 1, the second element 102 ′ on the other end side of the lithium niobate substrate 100 has the same structure as the first element 102, and both are connected in series in the light propagation direction of the channel optical waveguide 101. It is connected to the. As described below, the first and second
By applying an appropriate finite voltage to each stripe electrode of each of the elements 102 and 102 ', the birefringence is generated in an arbitrary direction within a plane perpendicular to the light propagation direction of the channel optical waveguide 101, and this birefringence is generated. The refraction retardation is π or π / 2 radians. Therefore, the first and second elements 102 and 102 'function similarly to a λ / 2 plate or a λ / 4 plate wave plate, respectively.

As described above, this polarization control device includes a channel optical waveguide formed in the surface portion of the substrate 100 made of lithium niobate and an arbitrary direction in a plane perpendicular to the light propagation direction of the channel optical waveguide. The first and second elements 102 and 102 'each including a stripe electrode for generating an electric field having a magnitude of 1 to the channel optical waveguide portion are connected in series.

To explain the operation of this polarization control device, when incident light is applied to the polarization device to control the polarization, the first and second elements 102 and 102 'are respectively set to λ / 4.
The first element 102 by operating as a plate and a λ / 2 plate and appropriately setting the directions of the principal axes of birefringence.
Any polarized light that enters the polarization control device from the side is converted into linearly polarized light of any polarization angle. That is, the first element 102 converts incident polarized light into linearly polarized light, and then the second element 102 'converts it into linearly polarized light having an arbitrary polarization angle.

Here, the drive voltage of the first and second elements 102 and 102 'does not exceed a certain value regardless of any change in the incident polarization. That is, the polarization control range is infinite.
Further, linearly polarized light having an arbitrary polarization angle which is incident on the polarization control device from the second element 102 'side is converted into arbitrary polarization, but in this case as well, the polarization control range is infinite.

 Furthermore, it demonstrates with reference to FIG.

As shown in FIG. 2, the third stripe electrode 112 is grounded and the first and second stripe electrodes 110 and 111 are applied with a voltage.
When V ₁ and V ₂ are added, the channel optical waveguide 101
Of electric fields E _V and E in the directions perpendicular and horizontal to the surface of the lithium niobate substrate 100 in the plane perpendicular to the light propagation direction of
_H is generated, and the electric fields E _V and E _H are dominated by the components generated by the voltages V ₁ and V ₂ , respectively. That is, the electric fields E _V and E _H can be arbitrarily set by V ₁ and V ₂ , respectively, and are approximately represented by the following equation (1).

However, W ₁ is the first stripe electrode 110 and the second stripe electrode 111, and the first stripe electrode 110 and the third stripe electrode 111.
Is the distance between the stripe electrodes 112. W ₂ is the distance between the second stripe electrode 111 and the third stripe electrode 112. Here, when the electric fields E _V and E _H are expressed by the equation (2), the strength of the synthetic electric field generated in the channel optical waveguide 101 portion is E ₀ , and its direction is based on the electric field E _H direction. The direction is rotated by θ.

Therefore, by appropriately selecting E ₀ and θ, an electric field of arbitrary strength can be set in the channel optical waveguide 101 portion in any direction. Due to this electric field, Technical Report of IEICE Technical Report 0QE84-125, 1984 by Miura et al.
As shown on pages 25 to 32 of the year, birefringence of a certain size can be set in an arbitrary direction in the waveguide portion. Here, the magnitude of birefringence and the direction of its principal axis are uniquely determined by E ₀ and θ, respectively. By sizing the electric field E ₀ so that the retardation in this element is π or π / 2 radians, the element operates equivalently to a λ / 2 plate or a λ / 4 plate. The electric field E ₀ required for λ / 2 plate or λ / 4 plate operation is given by the following equation (3).

Where n ₀ is the refractive index of LiNbO ₃ with respect to ordinary light, and r ₂₂ is LiN
The electro-optic coefficient of bO ₃ , l is the element length, and λ is the wavelength.

In this way, since the same function as that of the wave plate can be obtained, as described above, the first and second elements 102, 10 are provided.
When 2'is operated as a λ / 4 plate and a λ / 2 plate, respectively, and when light is propagated in the channel optical waveguide 101, by setting the direction of each birefringence, any incident polarized light can be converted into any linearly polarized light. Can be converted. Moreover, in this case, the voltage required to rotate the direction of the birefringence changes periodically, and the drive voltage does not continue to rise unlike the conventional case.

To give specific numerical values, W ₁ = 4 μm, W ₂ = 16 μm,
The maximum values of the applied voltages V ₁ and V ₂ to the electrodes required for λ / 2 plate operation when l = 10 mm and λ = 1.55 μm, that is, W ₁ E ₀ and W ₂ E ₀ are 8.4 V and 33.6, respectively. It was V. Also,
The maximum values of V ₁ and V ₂ required for λ / 4 plate operation are half the values for λ / 2 plate operation.

In this device, a slight anisotropy occurs due to the structural dispersion of the waveguide. Therefore, when compensating for this, it is necessary to apply a constant bias to V ₁ or V ₂ .

In this way, in controlling the polarization, the first and second elements
The drive voltage of 102, 102 'does not exceed a certain value. Therefore, it is possible to prevent a situation in which the drive voltage continues to rise and the polarization cannot be controlled as in the conventional case even if the incident polarization changes, and the polarization control range is not limited.

In the polarization control device described above, the channel optical waveguide 101 is formed by ion diffusion on a flat substrate, but the present invention is not limited to this, and it is based on ion exchange, or a ridge type or rib type channel optical waveguide. But it's okay. In these cases, the electrodes may be those that generate an electric field of an arbitrary magnitude in the channel optical waveguide portion in an arbitrary direction within a plane perpendicular to the light propagation direction of the channel optical waveguide 101.

When forming a channel optical waveguide on a lithium niobate substrate, after diffusion of Ti, MgO may be additionally diffused over the entire surface of the substrate. Thereby, the waveguide becomes more isotropic, and in the case of such a configuration, the bias voltage for compensating the anisotropy described above can be reduced.

Further, the substrate used for the polarization control device is not limited to the one using lithium niobate, and may be an electro-optic crystal having a three-fold symmetry axis, an electro-optic ceramic such as PLZT, or the like. When an electro-optic crystal having a three-fold symmetry axis is used, the propagation direction of light in the channel optical waveguide is the optical axis direction.

(Effects of the Invention) As described above, according to the polarization control device of the present invention, the polarization control range is not limited. Therefore, it is possible to control the polarization with respect to an arbitrary fluctuation of the incident polarization.

[Brief description of drawings]

FIG. 1 is a perspective view showing a polarization control device according to an embodiment of the present invention, and FIG. 2 is a view for explaining a cross-sectional structure of an element constituting the polarization control device and its operation description. 100 ... Lithium niobate substrate 101 ... Channel optical waveguide 102, 102 '... Element 110, 111, 112 ... Stripe electrode 120 ... Power supply

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-60-19109 (JP, A) JP-B-50-25344 (JP, B1) JP-B-55-12563 (JP, B1) W. A. Sharcliff, "Polarized light and its applications" (Kyoritsu Publishing, September 5, 1965, pages 92-98)

Claims (2)

(57) [Claims] 1. A channel optical waveguide formed on a surface portion of a substrate having an electro-optical effect, and an arbitrary direction in an arbitrary direction on the channel optical waveguide portion in a plane perpendicular to a light propagation direction of the channel optical waveguide. On the channel optical waveguide and on both sides thereof and the channel optical waveguide, for producing an electric field of a magnitude, thereby producing a birefringence of an arbitrary magnitude in the same plane and in the same portion where the electric field is generated in an arbitrary direction. A polarization control device comprising: first and second elements formed in series, each including a surface portion on which a waveguide is formed and a stripe electrode provided on the same plane. 2. The polarization according to claim 1, wherein retardations of birefringence generated in the channel optical waveguide portion are π / 2 and π radians in the first and second elements, respectively. Control device.