JPS6310403B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) (Technical Field of the Invention) The present invention relates to improvements in so-called polarization-maintaining optical fibers that transmit data while maintaining the plane of polarization in one direction.

(b) Background of the technology The progress in the development of semiconductor lasers, semiconductor photodetectors, and optical fibers as transmission lines has led to significant advances in so-called optical communication technology, which has many advantages over conventional telecommunications, and has already reached the point where it can be put into practical use. Significant progress is being made.

The optical fibers used in this case are both clad type made of quartz material, and utilize total internal reflection based on the difference in refractive index between the central core and the outer cladding to emit laser light as a medium. Transmit with minimal loss.

This optical fiber uniformly transmits the polarization plane of laser light in any direction.

On the other hand, unlike the conventional optical fibers mentioned above, a single mode laser beam with a polarization plane in a specific direction propagates while maintaining that polarization plane, but there are also so-called polarization preserving fibers that attenuate laser beams with polarization planes in other directions as much as possible. It is being developed for special purposes.

If such a polarization-maintaining fiber is used, for example, two laser beams with slightly different wavelengths can be input into one end of the fiber with their polarization planes matched, and an output corresponding to the phase difference between the two can be obtained from the other end. , it is possible to provide a heterodyne detection function in the optical state without electrical conversion.

Furthermore, an angular velocity sensor (gyro) required for indicating the movement direction and controlling the attitude of a moving object can be manufactured with high sensitivity using this polarization preserving fiber. That is, the rotational angular velocity can be measured with high precision by utilizing the phenomenon (Sagnatsk effect) in which the phase shift (θ) of a light wave propagating in a coil made of the fiber shifts in proportion to the rotational angular velocity (Ω) of the coil. Ru.

Figure 1 shows a diagram of its principle. Laser light with a wavelength λ from a semiconductor laser 1 is reflected by a mirror 2 and exits from one end face of a polarization-maintaining fiber coil 3 which is wound N times into a coil shape with a radius R. The laser beam is input as shown by the solid line arrow, and the laser beam that has passed through one mirror 2 is inputted from the other end face as shown by the dotted line arrow, and the outputs of both are reflected or passed through the mirror 2 and combined and sent to the photodetector 4. It is then converted into electrical output,
The signal is recorded on a recorder 6 via an amplifier 5.

The optical phase difference θ that occurs when the coil 3 rotates at a rotational angular velocity Ω is expressed by the so-called Sagnac equation θ=8π ² R ² N/λC (C is the speed of light), and the electrical output proportional to this is expressed by the recorder 6. obtained from.

As described above, polarization preserving fibers have high expectations for various special uses in the future, so it is desirable that they be produced at low cost, have good characteristics, and be easy to handle.

(c) Prior art and problems Two examples of this type of polarization preserving fiber conventionally produced are shown in cross section in Figure 2 A and B. In A, the cladding 1 surrounding the core 11 is shown.
2 is configured in an elliptical shape, and this is the jacket 1.
3 has an enclosing structure, and due to thermal strain during manufacturing, the refractive index changes with the maximum and minimum in the major and minor axis directions, so that the laser beam with polarization planes in both axis directions is mainly In B, cavities 16 and 16' are provided in the cladding 15 surrounding the core 14 symmetrically with respect to the axis, and similar changes in the refractive index due to thermal strain during manufacturing are caused to produce a similar effect. It has special characteristics.

As might be inferred at first glance, it is not only quite difficult to produce a fiber with such a structure, but also extremely difficult to achieve uniform properties throughout its entire length. Furthermore, when connecting fibers, it is necessary to match the cross-sectional shapes in addition to the conventional center alignment, and such cross-sectional shape matching is not easy for fibers whose core strain is less than 10 microns.

Furthermore, if it is to be made into a coil shape and used as an angular velocity sensor as described above, the relative position of the cross section with respect to the winding surface can be kept constant if consideration is given to fluctuations in the refractive index due to internal stress generated during winding. Not only is it extremely difficult to handle, but the specific polarization plane output is relatively large compared to others, that is, in order to increase the S/N ratio, a length of several to several tens of lengths is required. For miniaturization, it is necessary to make it into a coil shape.

Figure 2 (c) shows the diameter of the drum (coil) on the horizontal axis and the preservation characteristic of the plane of polarization on the vertical axis. As shown by the dotted line waveform, the polarization preservation characteristic cannot be maintained unless the diameter of the drum around which the fiber is wound is increased to a certain extent, making it unsuitable for miniaturization.

(d) Purpose of the invention The present invention eliminates the drawbacks of conventional fibers of this type, can be produced in the same process as ordinary fibers,
The purpose of this study is to obtain a new polarization-maintaining optical fiber that can connect fibers by simply aligning their centers.

(e) Structure of the invention The above-mentioned object of the present invention is to provide a jacket made of quartz and composed of three layers: a core, a cladding, and a jacket.
A fiber with a diameter of 50 to
This is achieved by a single mode polarization preserving fiber according to the invention wound on a 200 mm drum.

That is, since the fiber of the present invention has a core, cladding, and jacket each having a concentric circular cross-section, the production process is the same as that for conventional polarization non-maintaining optical fibers, and the coupling between the fibers can be achieved by considering only the center alignment. good. Note that ±0.05 is a manufacturing error that occurs because the refractive index is smaller than that of ordinary fibers.

This optical fiber does not exhibit polarization preservation characteristics when it is straight, but by winding it around a cylindrical drum, the outside of the optical fiber extends at right angles to the axis of the cylindrical drum, and the drum side extends. Shrink. On the other hand, in the horizontal direction with respect to the axis of the cylindrical drum, since no distortion occurs, the shape changes depending on the direction, and exhibits polarization preservation characteristics for the first time. This can be thought of in the same way as a rectangular-circular waveguide converter often used in microwaves.

Of course, conventional communication optical fibers also exhibit slight directionality in their refractive index when bent, but originally there was no consideration given to polarization preservation characteristics, and the refractive indices of the core and cladding were selected to minimize transmission loss. The ratio of their refractive indexes (n1−n2/2n1×100, where n1 and n2 are the refractive indexes of the core and cladding, respectively)
ranges from 0.2 to 0.5%.

Even if such a fiber is bent, its polarization preservation properties are so small that it cannot be used, for example, for the gyro described above.

In the fiber of the present invention, the difference in refractive index between the core and the cladding is made even smaller, and n1-n2/2n1×100% is set to 0.13±0.05%, so that the fiber can be wound as shown in the solid line in Fig. 2C. The smaller the diameter of the attached drum, the greater the distortion in the direction perpendicular to the drum axis of the fiber, and therefore the better the polarization preservation characteristic, making it suitable for miniaturization. This is because the relative change in the refractive index due to the above-mentioned strain in the two directions when bent increases due to the small difference in the refractive index. In this way, we were able to improve the desired polarization preservation property to a value that can be put to practical use.

(f) Embodiment of the Invention FIG. 3 shows an embodiment of the invention by an enlarged cross-sectional view of a fiber (a), a block diagram of a structure for measuring polarization preservation characteristics (b), and a characteristic curve (c).

It has a triple structure of a circular cross-section core 21, a circular cross-section cladding 22, and a circular cross-section jacket 23, and the ratio of the refractive index of the core 21 and the cladding 22 is
An optical fiber 24 having the dimensions shown in FIG. 3A and selected as 0.13 is wound approximately 500 times around a drum 25 with a diameter of 150 mm as shown in FIG. A semiconductor laser beam with a wavelength of 0.78 microns is applied through the coil to propagate the linearly polarized laser beam inside the coil, and the output light from the other end is passed through the condenser lens 28 and the rotating analyzer 29 to the detector 30. and activates the output meter 31.

The characteristic curve shown in FIG. 3C, in which the horizontal axis is the angle θ formed by rotating the analyzer 29 and the linear polarization plane of the incoming light, and the horizontal axis is the indication (dB) of the output meter 32, is a characteristic curve of the optical fiber of the present invention. It shows good polarization preservation.

(g) Effects of the Invention As explained above, the polarization preserving optical fiber according to the present invention can be produced in the same manner as conventional optical fibers, and the fibers only need to be connected to each other by center alignment, and there is no need for drum winding. This fiber exhibits an excellent effect that eliminates the drawbacks of conventional fibers of this type, such as requiring no special consideration.

[Brief explanation of the drawing]

Figure 1 shows the configuration of an angular velocity sensor (gyro) that uses a polarization-preserving fiber, and Figure 2 (a) and (b) show two examples of this type of fiber that have been used in the past, and (c) shows the drum diameter and polarization-preserving fiber. Characteristics, 3rd
The figure shows one embodiment of the present invention, with a fiber cross section (a), a block diagram of a configuration for measuring polarization preservation characteristics (b), and a preservation characteristic curve thereof (c). In the figure, 11, 14, 21 are fiber cores, 12, 15, 22 are claddings, 13, 23 are jackets, 24 is an optical fiber according to the present invention,
25 is a drum, 26 is a polarizing prism, 29 is an analyzer, 30 is a detector, and 31 is an output meter.

Claims (1)

[Claims] 1. It is made of quartz and has a concentric triple structure of a core, a cladding, and a jacket, and the ratio of the refractive index difference between the core and the cladding (n1-n2/2n1×100, where n1 and n2 are the core and the cladding, respectively). , refractive index of the cladding) is 0.13±
A single-mode polarization-maintaining optical fiber that is made by winding a 0.05% fiber into a drum with a diameter of 50 to 200 mm.