JPS649607B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fiber optical phase modulation method for coupling signals to an optical transmission path of a fiber telemetry signal transmission device, which is one type of signal transmission device using optical fibers.

Signal transmission systems using optical fibers have features such as resistance to electromagnetic induction noise, good insulation, light weight, and wide transmission band, and are useful not only for public communications but also for industrial plants, power generation substations, and ships. ,
It is also expected to be used in data transmission equipment such as in aircraft. In many such signal transmission systems, data is exchanged between a central monitoring device processing unit and a large number of terminals. Therefore, in order to reduce the amount of wiring between them, a loop-shaped trunk signal line is often installed that originates from the central monitoring equipment processing unit and returns again, and branches connecting each terminal from various points in the loop. Provide a signal line for
A so-called loop data highway or data bus system is used. There are several ways to construct such a data highway system using fiber optics.

There is a method for signal transmission by inputting a signal into an optical fiber from an arbitrary point without cutting the optical fiber and inserting a coupler. A well-known example uses two single-mode fibers,
The light emitted from one light source is divided into two by an acousto-optic device, into light whose wavelength is increased by the frequency of the sound wave in the acousto-optic device, and light with the original wavelength without increasing the wavelength. Injected into a single mode fiber, the light with increased wavelength is transmitted from the outside of the fiber, giving phase modulation to the light in the fiber, and making it different from the light emitted from this fiber without changing the wavelength described above. The light transmitted through the single mode fiber is multiplexed at the fiber output end, and the light is received by a light receiver with square-law characteristics.
In so-called heterodyne detection, the electrical output is obtained as an amplitude modulation signal using the frequency of the sound wave in the acousto-optic element as a carrier, the amplitude of which changes depending on the amount of phase change applied from the outside of the fiber. By using different frequencies for phase modulation applied from the outside of the fiber at each input point, a large number of signals can be inserted from arbitrary points and these can be discriminated at the receiving end.

A conventionally known method of applying a phase change to light waves traveling through a fiber is to sandwich an optical fiber from above covered with a piezoelectric plate, apply an electric field to both sides of the piezoelectric plate, and generate compressive vibrations in the piezoelectric plate. wake up,
This vibration is transmitted to the fiber through a coating made of chemical resin, changing the refractive index in the fiber and imparting a phase change to the transmitted light. This method is inefficient or has low bandwidth. In other words, the mechanical impedance of the piezoelectric body is 30 to 40×
While it is around 10 ⁶ Kg/s・m ² , it is much smaller at 3×10 ⁶ Kg/s・m ² for resin. Therefore, almost no elastic energy permeates into the resin.
In addition, piezoelectric ceramics are close to free vibration,
The bandwidth is narrow due to the high resonance sharpness.

Another method is to wrap a fiber around a piezoelectric ceramic circle and utilize changes in the fiber's elongation as the circle expands in the forward direction. It is sensitive to changes in phase due to changes in fiber elongation, and because it utilizes resonance in the piezoelectric body's radial direction, it has the advantage of being able to obtain a large phase change with a small applied electric field. This method uses only the radially axially symmetric spreading resonance vibration of the piezoelectric circle, and the bandwidth is also very narrow due to the high sharpness of the resonance.

All of these conventional methods of imparting phase changes to light waves in optical fibers have difficulty achieving both efficiency and bandwidth.

SUMMARY OF THE INVENTION An object of the present invention is to provide a fiber optical phase modulation method that eliminates the above-mentioned difficulties and has a simple configuration.

According to the present invention, by winding and fixing an optical fiber around a piezoelectric body that vibrates in multiple modes, it is possible to obtain a highly efficient fiber optical phase modulation method with a wide bandwidth.

The details of the present invention will be further explained based on examples and with reference to the drawings. Figure 1 shows a general configuration of a method of winding an optical fiber around a piezoelectric ceramic cylinder to impart phase modulation to the light transmitted through the fiber.
1 is an optical fiber wire, 2 is a coating that protects this fiber wire, and 3 is a cylindrical piezoelectric ceramic. Conventionally, the method of driving this element is to provide electrodes uniformly on the upper and lower surfaces of the cylinder and apply an alternating electric field between them, or to apply electrodes to the entire surface of the outer and inner cylindrical surfaces, respectively, and to apply an alternating electric field between them. It is used by applying an electric field. With this method of applying an electric field, the only radial vibration excited in the piezoelectric ceramic cylinder is a symmetrical spreading vibration in which the outside of the cylinder swells uniformly. Since the sharpness of this vibration is extremely high, the element can only be driven at almost a single frequency.

In addition to the above-mentioned axially symmetrical mode, non-axisymmetrical vibrational modes also exist in the radial vibration mode that exists in an elastic vibrating body having an axially symmetrical shape. For more information on multimode vibration theory, read the book ``Electro-Mechanical Functional Components'' published by the Institute of Electrical Engineers of Japan. FIG. 2 shows the vibration distribution, and FIG. 2a shows the displacement of the outer contour of the lowest order mode of the axisymmetric mode. The dashed line shows the state when there is displacement, and the solid line shows the state when there is no displacement. FIG. 2b is a diagram showing the displacement distribution for the lowest non-axisymmetric vibration mode. As mentioned above, this vibration mode is not excited by conventional electrodes having uniform upper and lower surfaces or electrodes having uniform inner and outer cylindrical surfaces. These two vibration modes, the lowest-order axisymmetric mode and the non-axisymmetric mode, each belong to an independent system, and generally have considerably different vibration frequencies. By bringing these vibration frequencies close together so that both modes can be driven with a single set of drive electrodes, it is possible to expand the frequency characteristics of the phase modulation applied to the optical fiber and flatten the characteristics within that band. This can be done as follows. The vibration frequencies of the two modes described above match when the ratio of the outer diameter to the inner diameter of the hollow cylinder is set to a specific value. For example, if the Poisson's ratio of piezoelectric ceramic is 0.3, and the inner diameter/outer diameter ratio is set close to 0.3, the two frequencies will match. By changing this value, the resonance frequency interval can be set arbitrarily. Further, if an electrode is provided only in the region where both vibrations are in the same phase and driven, two vibration modes will be excited. FIG. 3 is a diagram showing the shape of the electrodes on one side when electrodes are provided on the upper and lower surfaces of the cylinder, and the shaded portions are the electrodes. As shown in FIG. 2, it is sufficient to provide electrodes only in the upper half of the cylindrical cross section where the phases of vibration in both vibration modes are in the same phase. Although it can be excited even if it is provided over the entire upper half as shown in FIG. 3a, in this case, the excitation efficiency of the two vibration modes is not the same, and the efficiency of the axially symmetric mode is higher. In order to make this the same, it is realized by providing the electrode closer to the outer diameter part as shown in FIG. 3b.

By winding an optical fiber around the vibrator configured and driven in this manner as shown in FIG. 1, phase modulation is imparted to the light guided through the fiber.
Figures 4 and 5 compare the frequency characteristics of phase modulation intensity between the conventional method and the method of this embodiment.
FIG. 4 shows a conventional method, and as mentioned above, the frequency bandwidth is extremely narrow. Compared to this, the fifth
According to the method of this embodiment shown in the figure, a wide bandwidth can be obtained.

In Fig. 3, we have described the case where electrodes are provided on the upper and lower surfaces of the hollow cylinder, but when electrodes are provided on the outer and inner cylinder surfaces and the inner cylinder surface of the cylinder, instead of providing electrodes over the combined 360° of the cylinder surfaces, The inner and outer surfaces may be opposed to each other by 180 degrees or less.

In an elastic vibrating body having a rotationally symmetrical shape, a plurality of orthogonal vibration modes have the same resonance frequency. This is called isomorphic degeneracy. By solving the degeneracy of these vibrational modes and appropriately determining the frequency interval, a fiber optical phase modulation element with a wide bandwidth can be realized. Figure 6a,
b shows the vibration displacement distribution of two orthogonal non-axisymmetric vibrations of the circle with dotted lines. Degeneracy can be achieved by slightly breaking the symmetry of the vibrating body, and electrodes can be provided so that both modes can be driven simultaneously. As shown in FIG. 7, this is achieved by cutting out a part of the side of the circle to break the symmetry and providing an electrode in the part where both modes are in phase. The frequency spacing between the two modes can be adjusted depending on the degree of lateral missing.

In this embodiment as well, the band characteristic of the modulation intensity exhibits a wide band characteristic as in the case of the above-mentioned embodiment shown in FIG.

This method of using the isomorphic degenerate mode can also be used in the case of a hollow cylinder. Further, in addition to these modes, a triple mode using three axially symmetrical modes can also be used.

By winding an optical fiber around a piezoelectric vibrator that vibrates in multiple modes as described above, a highly efficient fiber optical phase modulation method with a wide bandwidth can be obtained.

[Brief explanation of drawings]

FIG. 1 shows a general configuration of a fiber optical phase modulation method, in which numeral 1 is an optical fiber wire, 2 is a fiber coating, and 3 is a cylindrical piezoelectric ceramic. FIG. 2 shows the radial vibration displacement distribution of the cylindrical vibrator.
FIG. 3 is a diagram showing the shape of a piezoelectric ceramic drive electrode. FIG. 4 shows the modulation intensity band characteristics of the conventional method, and FIG. 5 shows the modulation intensity band characteristics of the method of the present invention. FIG. 6 shows the variation distribution of the circular non-axisymmetric vibration mode. FIG. 7 is a diagram showing an example of an electrode structure that excites two non-axisymmetric vibration modes.

Claims (1)

[Claims] 1. In a method of winding an optical fiber around the cylindrical surface of a piezoelectric vibrating body having a cylindrical shape and imparting phase modulation to light transmitted through the fiber, a plurality of vibration modes having vibration displacement mainly in the radial direction of the piezoelectric vibrating body are provided. A fiber optical phase modulation method characterized in that the plurality of vibration modes are excited in the piezoelectric vibrating body by applying a voltage to a portion where the vibration modes are displaced in the same phase.