JP2002318356A - Variable optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a variable optical attenuator which has small polarization dependency, wavelength dependency, temperature dependency, and power consumption. SOLUTION: The variable optical attenuator is equipped with an optical fiber 32 which is laid on the top surface of a piezoelectric single-crystal plate having a polarization reversal layer in piezoelectric bimorph structure and has an end surface where spatial light is made incident and emitted, an optical fiber which is put opposite the optical fiber 32 at a specific distance along the optical path and fixed at a spatial position and has an end surface where the spatial light is made incident and emitted, and a means which applies a voltage to the piezoelectric single-crystal plate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical component for variably attenuating light intensity in an optical communication system.
The present invention relates to a variable optical attenuator for variably attenuating light intensity by an electric signal in a wavelength multiplexing optical communication device.

[0002]

2. Description of the Related Art Conventionally, as a variable optical attenuator used in an optical repeater of an optical communication system, there is an optical attenuator using a variable Faraday rotator. FIG. 4 is an explanatory diagram of a conventional variable optical attenuator. As shown in FIG. 4, the conventional variable optical attenuator has an optical configuration.

The operation of the variable optical attenuator shown in FIG. 4 is as follows. The light emitted from the optical fiber 41a is
2a is converted into a parallel beam by a wedge-shaped birefringent crystal 4.
3a. When passing through this crystal, the ordinary light component and the extraordinary light component of light are refracted according to their respective refractive indexes.

Next, each light component is rotated by a variable Faraday rotator 44 on the plane of polarization, then passes through a wedge-shaped birefringent crystal 43b and is converted into a convergent light beam by a lens 42b. The light of the portion is coupled to the optical fiber 41b.

Here, wedge-shaped birefringent crystals 43a and 43
The direction of the c-axis (optical axis) of b is substantially orthogonal. Therefore, when the Faraday rotation angle by the variable Faraday rotator 44 is approximately 90 °, light that has passed through the wedge-shaped birefringent crystal 43a as ordinary light also passes through the wedge-shaped birefringent crystal 43b as ordinary light.

As a result, the wedge-shaped birefringent crystals 43a and 43a
The change in the traveling direction of the light received by 3b cancels out, and all ordinary light components are coupled to the optical fiber 41b. Also,
The situation is the same for light passing through the wedge-shaped birefringent crystal 43a as extraordinary light.

On the other hand, when the Faraday rotation angle is 0 °, light passing through one of the wedge-shaped birefringent crystals as ordinary light passes through the two wedge-shaped birefringent crystals because the other passes as extraordinary light. The change in the direction of travel does not cancel out. As a result, the light is transmitted to the optical fiber 41b.
Does not combine with

Accordingly, in this example, when the Faraday rotation angle is 90 °, the attenuation is zero, and the attenuation increases as the Faraday rotation angle approaches 0 °.

As the Faraday rotator 44 used here, there is a method of changing the Faraday rotation angle by changing the angle between the saturation magnetization vector and the traveling direction of light while the magnetization of the Faraday rotation crystal is saturated. And practical. As a method for changing the direction of the saturation magnetization vector, a method using a combined magnetic field by a permanent magnet and an electromagnet is practical.

Conventionally, such a variable optical attenuator has been often used in an optical communication apparatus.

[0011]

In the conventional variable optical attenuator as described above, the incident light is divided into two polarization components, guided to a variable Faraday rotator having an electromagnet, and then combined. Thus, a method of coupling to an optical fiber has been adopted. As a result, it has not been easy to reduce the polarization dependence of the optical attenuation. In addition, because of the wavelength dependence and temperature dependence of the Faraday rotation crystal, reducing the wavelength dependence and the temperature dependence of the optical attenuation,
It was difficult. Further, it has not been easy to reduce the power for driving the electromagnet.

An object of the present invention is to provide a variable optical attenuator having small polarization dependence, wavelength dependence, temperature dependence and power consumption.

[0013]

A variable optical attenuator according to the present invention comprises a movable optical fiber using bending displacement of a piezoelectric single crystal having an inverted polarization layer having a piezoelectric bimorph structure, and another optical fiber. An optical attenuator is configured using the optical coupling of the optical attenuator.

The state of optical coupling between an optical fiber attached to the surface of a piezoelectric single crystal having an inverted polarization layer having a piezoelectric bimorph structure and another optical fiber is simplified as follows. Can be approximated by the state of light coupling in the optical system.

Actually, the traveling direction of the light emitted from the end face of the optical fiber attached to the surface of the piezoelectric single crystal having the inverted polarization layer having the piezoelectric bimorph structure is inclined with the bending of the piezoelectric single crystal. However, the decrease in the coupling efficiency due to the inclination is negligibly small as compared with the decrease in the overall efficiency due to the axis deviation.

As a simplified optical system, a light coupling state when two optical fibers having light input / output end faces are opposed to each other will be described. First, the Z axis is determined along the center axis of the optical fiber, the X axis and the Y axis are determined in a direction perpendicular to the Z axis, and a coordinate system is set around the Z axis.

In this coordinate system, the central axes of the two opposing optical fibers are substantially parallel, the difference between the Z coordinates of the light entrance / exit surfaces of the two optical fibers is small, and the two axes in the X-axis direction are small. Assuming that the distance (axis shift) between the central axes of the optical fibers is X ₀ , the light coupling efficiency η between the two optical fibers is:
Equation (1) is obtained.

Η = exp {− (X ₀ / W) ² } (1) where W is the mode field diameter of the two optical fibers.

Further, the optical attenuation amount _{a t} expressed in dB
Is as shown in Expression (2).

A _t = −10 · log ₁₀ (η) (2)

As an example, W = 5 μm, X ₀ = 11 μm
When the light attenuation _{a t} becomes 21.0DB.

That is, the variable optical attenuator of the present invention comprises an optical fiber having an end face for emitting and emitting spatial light, provided on the surface of a piezoelectric single crystal plate having a domain-inverted layer having a piezoelectric bimorph structure, A variable optical attenuator comprising: an optical fiber having an end face for inputting / outputting spatial light facing the fiber at a distance in the optical path direction, and a unit for applying a voltage to the piezoelectric single crystal plate. .

Further, according to the present invention, the piezoelectric single crystal plate
This is a variable optical attenuator using iNbO ₃ single crystal.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A variable optical attenuator according to an embodiment of the present invention will be described below.

FIG. 1 is an explanatory diagram of a polarization-inverted bimorph of a variable optical attenuator according to an embodiment of the present invention. In FIG. 1, the variable optical attenuator of the present invention is a polarization-inverted bimorph 1.
1 is provided with a movable optical fiber 12 on the surface thereof.

Here, the domain-inverted bimorph 11
This is a domain-inverted bimorph made of iNbO ₃ single crystal. On its upper and lower surfaces, electrodes 15a and 15b for applying a DC voltage are formed. On the upper surface, an optical fiber 12 having a mode field diameter of 5 μm is formed. Is pasted. One end of the polarization inversion type bimorph 11 is
It is fixed by the fixing block 13, and the DC power supply 14
When a DC voltage is applied, the other end is displaced in the direction indicated by the displacement direction 16.

FIG. 2 is an explanatory diagram of the operation of the domain-inverted bimorph used in the variable optical attenuator according to the embodiment of the present invention. In FIG. 2, reference numeral 21 denotes 14 of LiNbO ₃ single crystal.
It is a 0-degree rotating Y-plate, and its dimensions are 0.5 mm in thickness, 7 mm in width, and 25 mm in length. The width direction is the X direction,
The upper half of the plate is polarized in the direction of the polarization direction 27b, and the lower half is reversed in polarization and polarized in the direction of the polarization direction 27a.

Due to such a bimorph structure, when a DC voltage is applied in the vertical direction by the DC power supply 24, for example, the upper half of the 140 ° rotated Y plate of LiNbO ₃ single crystal contracts in the longitudinal direction. The lower half extends in the longitudinal direction. As a result, the movable end is displaced in the direction indicated by the displacement direction 26.

FIG. 3 is an explanatory diagram of a variable optical attenuator according to an embodiment of the present invention. FIG. 3A is a vertical sectional view, and FIG. 3B is an external perspective view. The variable optical attenuator of FIG. 3 uses the polarization inversion type bimorph described above, and is configured by combining an optical fiber displaced with the polarization inversion type bimorph and a fixed optical fiber.

In FIG. 3, reference numeral 31 denotes a polarization inverted bimorph, 32 denotes an optical fiber, 33 denotes a fixed block, 35a and 35b denote lead wires, and 34 denotes a fixed optical fiber.

Reference numerals 38 and 39 denote input / output optical fibers, and reference numerals 37a and 37b denote voltage input terminals for applying a voltage to the polarization inversion type bimorph 31.

The variable optical attenuator of the present invention can obtain an optical attenuation of 20 dB at an input voltage of about 140 V.

Also, in the variable optical attenuator of the present invention,
A LiNbO ₃ single crystal having a Curie point of about 1140 ° is used as the piezoelectric single crystal forming the domain-inverted bimorph, and as a result, in the environmental temperature range of −40 ° C. to 80 ° C., the temperature of the bending displacement is increased. There are few dependencies.

When a piezoelectric single crystal having a domain-inverted layer is used as the piezoelectric bimorph, a precise bending displacement can be obtained without the hysteresis and creep seen in piezoelectric ceramics.

Further, the current flowing through the domain-inverted bimorph is very small, and the heat generated by power consumption is reduced, so that the reliability as an element is improved.

Further, the variable optical attenuator of the present invention does not separate polarization, and thus does not have a principle polarization dependence. In the above embodiment, one is variable and one is fixed. However, even if both are variable, a variable optical attenuator can be configured.

Furthermore, since there is no wavelength dependence and temperature dependence due to the Faraday rotator seen in the conventional example, a variable optical attenuator with little wavelength dependence and temperature dependence can be realized.

[0038]

As described above, according to the present invention, it is possible to provide a variable optical attenuator having small polarization dependence, wavelength dependence, temperature dependence and power consumption.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a polarization-inverted bimorph of a variable optical attenuator according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an operation of a polarization inversion type bimorph used in the variable optical attenuator according to the embodiment of the present invention.

FIG. 3 is an explanatory diagram of a variable optical attenuator according to the embodiment of the present invention. 3A is a vertical sectional view, and FIG. 3B is an external perspective view.

FIG. 4 is an explanatory diagram of a conventional variable optical attenuator.

[Explanation of symbols]

11, 31 Polarization inversion type bimorph 12, 32, 41a, 41b Optical fiber 13, 33 Fixed block 14, 24 DC power supply 15a, 15b Electrode 16, 26 Displacement direction 21 140 ° rotation Y plate of LiNbO ₃ single crystal 27a, 27b Polarization Direction 34 Fixed optical fiber 35a, 35b Lead wire 37a, 37b Voltage input terminal 38, 39 Input / output optical fiber 42a, 42b Lens 43a, 43b Wedge-shaped birefringent crystal 44 Variable Faraday rotator

Claims (2)

[Claims] 1. An optical fiber having an end face for transmitting and receiving spatial light, which is provided on a surface of a piezoelectric single crystal plate having a domain-inverted layer having a piezoelectric bimorph structure, and spaced apart from the optical fiber in an optical path direction. A variable optical attenuator comprising: an optical fiber having an end face for receiving and emitting facing spatial light; and means for applying a voltage to the piezoelectric single crystal plate. 2. The variable optical attenuator according to claim 1, wherein the piezoelectric single crystal plate is a LiNbO ₃ single crystal.