JPH026425Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a temperature-compensated optical fiber phase modulator.

(Conventional technology and its problems) In recent years, research has been progressing on coherent optical communication aimed at increasing the functionality of communication, optical fiber gyroscopes aimed at miniaturizing equipment, and increasing reliability. A light phase control means is also required.

In addition, a polarization compensator that converts arbitrary polarized light into linearly polarized light in a desired direction is required for connecting waveguide optical devices and single mode fibers and for coherent optical communications. A system in which phase modulators are connected in series was proposed in patent application 116342/1986.
It is stated in

As a phase modulator used for the above purpose,
This method changes the phase difference between two orthogonal polarization components of light propagating through an optical fiber by applying pressure or stretching to the optical fiber and changing the magnitude and direction of birefringence within the fiber. It is expected that there will be no need to emit light and there will be no loss.

As an optical fiber type phase modulator that applies pressure or stretch to an optical fiber as described above, one having the configuration shown in FIG. 1 has been proposed. In the configuration shown in FIG. 1, a polarization-maintaining optical fiber is wound around a cylindrical piezoelectric element, and a voltage applied to the cylindrical piezoelectric element causes the polarization-maintaining optical fiber to stretch. Due to the elongation that occurred at this time, the two orthogonal directions of the polarization-maintaining optical fiber
By changing the magnitude of birefringence in the directions of the two optical axes, the propagation constant difference changes and the phase difference between two orthogonal polarization components can be changed. In the case of a polarization-maintaining optical fiber, since the difference in propagation constant between the two optical axes is originally made large, the change in the difference in propagation constant that occurs when elongation is applied is also large. However, since the original difference in propagation constants is large, the temperature dependence of the difference in propagation constants is also large, and there is a drawback that the difference in propagation constants changes greatly due to a slight change in temperature. Therefore, even if the same voltage is applied to the cylindrical piezoelectric element, the amount of change in the phase difference between the two orthogonal polarization components will vary greatly depending on the temperature. These drawbacks also apply to phase modulators that apply pressure to an optical fiber using a plate-shaped piezoelectric element or the like. This is a big problem in the polarization compensation device that connects two phase modulators in cascade as described above, and at some point an appropriate voltage is applied to the two phase modulators to perform polarization compensation. However, if the temperature changes, the voltage value to be applied will change, and in the worst case, even if you try to control the voltage applied to the phase modulator by monitoring the polarization state of the output light from the polarization compensator, it will not be possible to control the voltage applied to the phase modulator. A situation arises.

(Objective of the invention) An object of the invention is to eliminate the above-mentioned drawbacks and provide a temperature-compensated optical fiber phase modulator.

(Structure of the invention) The present invention comprises a plurality of optical fibers having refractive index anisotropy in a direction perpendicular to the light transmission direction, and a means for stretching or applying pressure to the optical fibers, and a plurality of optical fibers are provided. The fibers are connected in series in the light transmission direction so that the main axis of the refractive index anisotropy is oriented in a certain direction and the fiber is inclined at 90 degrees with respect to this certain direction, and the main axis is in a certain direction. The configuration is such that the sum of the lengths of the optical fibers facing in this direction is equal to the sum of the lengths of the optical fibers whose principal axes are inclined at 90 degrees with respect to the certain direction.

(Operation and principle of the present invention) Assuming that the optical fibers constituting the phase modulator as described above are an even number of optical fibers connected with their principal axes of refractive index anisotropy inclined at 90 degrees to each other,
The change in the propagation constant difference between two mutually orthogonal polarization components due to temperature fluctuation repeats positive, negative, positive, negative, etc. an even number of times in an even number of optical fibers, and each optical fiber is long. Since the magnitudes are made equal, their absolute values are equal. Therefore, changes in the propagation constant difference due to temperature fluctuations in two adjacent optical fibers cancel each other out. Since the phase modulator is composed of an even number of optical fibers, the change in the propagation constant difference due to temperature fluctuation in the phase modulator as a whole becomes zero, which means that temperature compensation has been performed.

In the above explanation, in order to make it easier to understand, the case where there are an even number of optical fibers with the same length has been explained as an example, but it also applies when there is an odd number of optical fibers, or when the lengths are not all the same. That is,
The number and length of the optical fibers need only be determined so that the positive and negative changes in the propagation constant difference due to temperature fluctuations are the same.

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

FIG. 2 shows an embodiment of the optical fiber phase modulator according to the present invention. In FIG. 2, elliptically clad polarization-maintaining optical fibers 21 and 22 are sandwiched between a piezoelectric element 23, and the birefringence axes of the polarization-maintaining optical fibers 21 and 22 make an angle of 90° to each other at a point 24. In other words, the polarization-maintaining optical fibers 21 and 22 are connected such that the long axes of the elliptical clads form an angle of 90° with each other. The connection point 24 is a bisecting point of the length of the polarization maintaining optical fiber which is a component of this phase modulator. In addition, in Figure 2, 2 is used for convenience of explanation.
Although the polarization-maintaining optical fibers 21 and 22 are written as being separated in the book, they are actually closely connected. As described above, the optical fiber type phase modulator according to the present invention is constructed.

The light propagating through the polarization maintaining optical fiber 21 is transmitted through the piezoelectric element 2.
3, pressure is applied, and this pressure changes the propagation constant difference between the two orthogonal polarization components, and the phase difference between the two polarization components changes with respect to when no voltage is applied to the piezoelectric element. In Fig. 2, the pressure is applied in the y direction, so the direction of pressure application, that is, the polarization component in the y direction is perpendicular to x.
It is assumed that the phase advance is larger than the polarization component in the direction. At this time, even if the two polarization-maintaining optical fibers 21 and 22 are connected with the birefringence axis tilted by 90 degrees at the connection point 24, the pressure in the polarization-maintaining optical fiber 22 is still in the same direction, y-direction. Since this is applied, the phase of the polarized light component in the y direction of the light propagating through the polarization maintaining optical fiber 22 also advances greatly. On the other hand, if the change in the phase difference that occurs in the polarization-maintaining optical fiber 21 up to the connection point 24 due to temperature fluctuation is Δβ, then the temperature of the phase difference that occurs in the polarization-maintaining optical fiber 22 after the connection point 24 The amount of change is also equal to Δβ. This is because the connection point 24 is a point that bisects the length of the polarization-maintaining optical fiber constituting this phase modulator. However, at the connection point 24, the polarization maintaining optical fiber 2
1 and 22 are connected with their birefringence axes tilted at 90 degrees to each other, so polarization maintaining optical fibers 21 and 22
Although the magnitude of the temperature change in the phase difference at is the same, the signs are different. In other words, if the temperature variation in the phase advance of the x-direction polarized light component with respect to the y-direction polarization component is Δφ in the polarization-maintaining optical fiber 21, it becomes -Δφ in the polarization-maintaining optical fiber 22, and propagation due to temperature fluctuations in the entire phase modulator. The amount of change in the constant difference becomes zero.

(Effect of the invention) The above is the operating principle of this optical fiber type phase modulator. In this way, the polarization maintaining optical fiber is By connecting the birefringent axis at an angle of 90 degrees, changes in the propagation constant difference due to unnecessary temperature fluctuations can be removed without impairing the magnitude of the phase modulation effect, making it possible to eliminate changes in the propagation constant difference due to unnecessary temperature fluctuations. Optical fiber phase modulation with temperature compensation. equipment can be obtained easily.

The present invention is not limited to the above embodiments. For example, another example is an optical fiber type phase shifter in which two polarization-maintaining optical fibers of equal length are connected with their birefringent axes tilted at 90 degrees, and only one of them is wound around a cylindrical piezoelectric element. A configuration in which pressure is applied to a modulator or a single mode fiber using a piezoelectric element, and the birefringence axis is tilted at 90 degrees at the point bisecting the length of the single mode fiber, or a polarization maintaining optical fiber is used. Increase the number of pieces, for example, the length is l, 2l,
Even in a configuration in which the optical fibers 3l, 1, and 3l are connected so that the change in the propagation constant due to temperature fluctuation is positive, negative, negative, positive, or positive, the change in the propagation constant is zero as in the above example. can do.

[Brief explanation of the drawing]

Figure 1 shows a known example of an optical fiber type phase modulator in which a single polarization maintaining optical fiber is wound around a cylindrical piezoelectric element, and Figure 2 shows an optical fiber type phase modulator with temperature compensation according to the present invention. FIG. 3 is a diagram showing details of a modulator. 11, 21, 22: polarization maintaining optical fiber, 1
2: Cylindrical piezoelectric element, 23: Plate piezoelectric element.

Claims (1)

[Scope of utility model registration request] The plurality of optical fibers have an anisotropy of refractive index in a direction perpendicular to the light transmission direction, and a means for stretching or applying pressure to the optical fibers, the plurality of optical fibers having anisotropy of refractive index are connected in series in the light transmission direction, including those whose principal axes are oriented in a certain direction and those whose principal axes are oriented at 90 degrees with respect to this certain direction, and whose principal axes of refractive index anisotropy are oriented in a certain direction. A light characterized in that the sum of the lengths of the optical fibers is equal to the sum of the lengths of each optical fiber whose principal axis of refractive index anisotropy is inclined at 90 degrees with respect to the certain direction. Fiber phase modulator.