JP2534710B2 - Polarization control device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polarization control device used in an optical communication system, an optical fiber sensor, or the like.

(Prior Art) A coherent optical communication system, an optical fiber sensor, or the like requires a polarization controller for converting an arbitrary incident polarized light into an arbitrary outgoing polarized light. Among the devices used for the polarization controller, the waveguide type is small, lightweight, mass producible,
It is promising in that it can be integrated with other devices.

Such a waveguide-type polarization control device is a phase shifter and mode converter based on the phase shifter and mode converter presented by Shimada et al. , And phase shifter in that order.

(Problems to be Solved by the Invention) In the above-mentioned waveguide polarization control device, when the polarization state of incident light or emitted light, for example, the polarization angle continues to rotate in a certain direction, the drive voltage of the phase shifter or the mode converter rises. Continued, and finally the limit voltage was reached and polarization control became impossible.

As described above, the waveguide-type polarization control device using the conventional technique has a problem that the polarization control range is limited.

(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. A first, a second, and a third element which are arranged 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. Formed and configured.

(Operation) In the first, second, and third elements connected in series that constitute the polarization control device according to the present invention, an arbitrary size is obtained in an arbitrary direction within a plane perpendicular to the light propagation direction of the channel optical waveguide. Birefringence can be generated in the channel optical waveguide portion. The retardation in each element is π / 2,
By keeping the π and π / 2 radians constant and rotating the direction of each birefringence, each element is
It works the same as a wave plate, a half wave plate, and a quarter wave plate. 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 outgoing 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, so polarization control is possible regardless of any change in the polarization state of incident light or emitted light. Is. That is, the polarization control device according to the present invention does not limit the polarization control range.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view of a polarization control device showing an embodiment of the present invention. The channel optical waveguide 101 is formed by selectively diffusing Ti into the surface portion of the lithium niobate substrate 100.
Are formed. The propagation direction of light in the channel optical waveguide 101 is the C-axis direction of the lithium niobate substrate 100. On the surface of the lithium niobate substrate 100,
The first, second, and
The first element 102 is formed by providing the third stripe electrodes 110, 111, 112. Here, the second and third stripe electrodes 111, 112 are equidistant from the first stripe electrode 110, respectively. Although not shown in FIG. 1, a SiO ₂ film is formed between the channel optical waveguide 101 and the first stripe electrode 110 by the CVD method.
This is to prevent attenuation of light propagating in the optical waveguide due to the metal film. The second and third elements 103, 104 correspond to the first element 102.
And a structure similar to that of the first, second and third elements 102, 103,
104 is monolithically formed on the same substrate in the light propagation direction of the channel optical waveguide 101.

Here, a 1/4 wavelength plate, 1/2 wavelength plate, 1 / wave plate, 1 / wave plate, and 1 / wave plate are applied to each stripe electrode of the first, second, and third elements 102, 103, 104 by applying a suitable finite voltage as described later.
Works like a four-wave plate. Any polarized light incident on the polarization control device from the side of the first element 102 is reflected by the first element 102.
Is converted into linearly polarized light by the second element 103, then converted into linearly polarized light having an arbitrary polarization direction, and finally the third element 104 is converted.
It is converted into arbitrary polarized light. Here, even if any change in the polarization state of the incident light and the outgoing light,
The drive voltage of the elements 102, 103, 104 of the above does not exceed a certain fixed value. That is, the polarization control device of this embodiment does not limit the polarization control range. In addition, in this embodiment, each element is formed monolithically on one substrate, but each element may be individually manufactured and connected in series.

FIG. 2 is a cross-sectional view of an element that operates as a wave plate that constitutes the polarization control device of this embodiment.
It corresponds to the first, second and third elements 102, 103 and 104 shown in the figure. First and second stripe electrodes 210,
Voltages V ₁ and V ₂ are applied to 211 by a power supply 200, respectively. The third stripe electrode 212 is grounded. In the plane perpendicular to the light propagation direction of the channel optical waveguide 101,
The electric fields E _V and E _{H in the} directions vertical and horizontal to the surface of the lithium niobate substrate 100 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.

Here, W ₁ is the distance between the first stripe electrode 210 and the second stripe electrode 211, and the distance between the first stripe electrode 210 and the third stripe electrode 212. W ₂ is the distance between the second stripe electrode 211 and the third stripe electrode 212. If the electric fields E _V and E _H are expressed by the equation,
The strength of the combined electric field that causes the channel optical waveguide 101 is E ₀
Then, the direction is a direction rotated by θ with respect to the electric field E _H direction.

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 E ₀ , the technical report of the Institute of Electronics and Communication Engineers OQE84-125,198 by Miura et al.
4 years, as shown on pages 25 to 32, the same can set a certain amount of birefringence in the leading portion in any direction. Here, the magnitude of birefringence and the direction of its principal axis are uniquely determined by E ₀ and θ, respectively. By setting the magnitude of the electric field E ₀ so that the retardation in this element is π or π / 2 radians, this element operates equivalent to a ½ wavelength plate or a ¼ wavelength plate. The electric field E ₀ required for a half-wave plate or a quarter-wave plate is
Each is given by the following equation.

Where n ₀ is the refractive index of lithium niobate for ordinary light,
r ₂₂ is the electro-optic coefficient of lithium niobate, l is the element length,
λ is the wavelength.

When W = 4 μm, W ₂ = 16 μm, l = 10 mm, λ = 1.55 μm
Maximum value of V ₁ and V ₂ required for half-wave plate operation, that is, W ₁ E
₀ and W ₂ E ₀ were 8.4V and 33.6V, respectively. The maximum value of V ₁ and V ₂ required for 1/4 wave plate operation is 1/2
It is half the value in the case of wave plate operation.

Note that the waveguide dispersion of the optical waveguide in the present device, since the slight anisotropy occurs, V ₁ and to compensate for this
It is necessary to apply a constant bias to V ₂ .

In the polarization control device described above, the channel optical waveguide is not limited to one formed by ion diffusion on a flat substrate, but may be one formed by ion exchange, or a ridge type or rib type channel optical waveguide. 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.

When forming a channel optical waveguide on a lithium niobate substrate, after diffusion of Ti, MgO is further spread over the entire surface of the substrate.
May be additionally diffused. This makes the waveguide more isotropic, thus reducing the bias voltage to compensate for the anisotropy.

The substrate used for the polarization control device is not limited to lithium niobate, and an electro-optic crystal having a three-fold symmetry axis, or
An electro-optic ceramic such as PLZT may be used. 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.

(Effect of the Invention) In the polarization control device according to the present invention, the polarization control range is not limited. Therefore, polarization control can be performed with respect to any change in the polarization state of incident light or emitted light.

[Brief description of drawings]

FIG. 1 is a perspective view of a polarization control device showing an embodiment of the present invention, and FIG. 2 is a sectional view of an element which operates as a wave plate constituting the polarization control device showing the present embodiment. In the figure, 100 ... Lithium niobate substrate, 101 ... Channel optical waveguide, 102, 103, 104 ... Element, 110, 111, 112, 210, 211, 212 ...
… Striped electrodes, 200 …… electrodes.

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. Polarization control characterized by being formed by serially forming first, second, and third elements each including a surface portion on which a waveguide is formed and a stripe electrode provided on the same plane. device. 2. The retardation of birefringence generated in the channel optical waveguide portion is π / 2, π, π / 2 radians in the first, second and third elements, respectively. The polarization control device according to the first aspect.