JPS6232455B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a device used for extracting or coupling light from an optical data bus in, for example, optical fiber transmission.

[Prior art]

A conventional device of this type is shown in FIG. In the figure, 1, 2, 3 are optical fibers,
4, 5, and 6 are gradient index lenses; 7 is a wedge-shaped reflecting mirror with two perpendicular reflecting film surfaces; 7a;
is the reflective film surface.

A data bus is a system in which a large number of terminal devices share a single transmission path for data transmission, and there are two methods: transmitting only one direction of signals and using a separate transmission path for each direction, and the other method using a single transmission path. There is a method in which signals are shared and transmitted in both directions.

When considering a system for bidirectionally transmitting signals as an optical data bus, an optical T-type branch/coupler is required. Each terminal device is connected to a trunk optical fiber (optical bus) via an optical T-type branching coupler.

Next, the operation of the conventional optical T-type branching coupler will be explained. Opposing optical fibers 2 and 3 are placed on the same optical axis, and their tips are provided with gradient index lenses 5 and 6 for collimation (hereinafter referred to as rod lenses).
is attached so that the ridgeline of the wedge-shaped reflecting mirror 7 is on this optical axis, and the reflective film surface 7a is arranged at an angle of 45 degrees with respect to the optical axis. There is an optical fiber 1 having a collimating rod lens 4 at its tip whose optical axis is on an extension line of has been done. Rod lenses have a distribution in which the refractive index decreases radially outward from the central axis of the lens in proportion to the square of the radius, and like normal optical lenses, they have an image forming effect, and the period length of the lens decreases in proportion to the square of the radius. If a lens with a length of 1/4 is used, the light from the point light source at the end face will be converted into parallel rays. The part of the optical fiber through which light propagates is called the core, which has a number of tens of parts.
Since it is as small as μm, it is considered to be almost a point light source, and when brought to the end face of this lens and the end face of an optical fiber, it can be made into almost parallel light rays. In the conventional example above,
Taking advantage of this fact, a rod lens with a quarter period length is used.

Therefore, the light emitted from the optical fiber 1 is converted into a nearly parallel beam by the rod lens 4, and then divided into two by the two reflecting surfaces 7-a of the right-angle reflecting mirror 7, each splitting at 90 degrees. One side is focused by the rod lens 5 and sent to the optical fiber 2.
The other beam is condensed by the rod lens 6 and coupled to the optical fiber 3 for propagation.

Similarly, the light emitted from the optical fiber 2 is made into a nearly parallel beam by the rod lens 5, half of it is reflected by the reflecting mirror 7, condensed by the rod lens 4, and directed to the optical fiber 1, and the other half is not reflected and is reflected by the rod lens. The light is focused by 6 and coupled and propagated to optical fiber 3. Furthermore, the light emitted from the optical fiber 3 is similarly divided into two parts and coupled and propagated to the optical fibers 1 and 2. As described above, it functions as a three-terminal optical T-type branching coupler.

Since the conventional optical T-type branching coupler is constructed as described above, the ridge line formed by the two reflecting surfaces must be polished with great precision to prevent chipping or sagging. Dropping or sagging is unavoidable and increases losses.

Also, when changing the branching ratio, use optical fibers 2 and 3.
It is necessary to shift the ridgeline of the wedge-shaped reflector from the optical axis of the optical fiber, but this will cause a change in the mode distribution propagating through the optical fiber, and depending on the optical fiber length, etc., the amount of branching in other optical T-branch couplers may be reduced. There was a problem with change.

[Summary of the invention]

This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and its purpose is to provide an optical T-type branching coupler that does not cause a change in the mode distribution of the optical fiber even if the branching ratio is changed. It is said that

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 2, reference numerals 11, 12, and 13 are optical fibers, 14, 15, and 16 are lenses, 17 is a tabular crystal body having birefringence, and 7c is oblique with its crystal optical axis. 18 is a 1/2 wavelength plate, and 19 is a total reflection prism.

Next, the operation of this invention will be explained using FIG. In FIG. 3a, the light emitted from the optical fiber 11 is
The birefringent crystal 17 separates the light into ordinary rays (.) and extraordinary rays (〓), which are linearly polarized lights perpendicular to each other.
Ordinary rays travel straight through the birefringent crystal 17, and extraordinary rays travel obliquely. The extraordinary ray exits the birefringent crystal 17 and travels parallel to the incident ray and is coupled and propagated into the optical fiber 12. On the other hand, the ordinary ray is a 1/2 wavelength plate 18
(the optical axis of the 1/2 wavelength plate makes an angle of 45 degrees with the polarization direction of the ordinary ray), the polarization direction is rotated by 90 degrees, the direction is changed by the two total reflection prisms 19, and the polarization direction is changed again by the two total reflection prisms 19. Although it is incident on the birefringent crystal 17,
This time, the polarization direction is rotated 90 degrees, so
This corresponds to an extraordinary ray, which travels obliquely within the crystal 17, exits the birefringent crystal 17, and is coupled and propagated to the optical fiber 13 as light parallel to the incident ray.

In addition, in FIG. 3b, the light emitted from the optical fiber 13 is separated into the ordinary ray and the extraordinary ray, as described above, and the ordinary ray travels straight, and the light emitted from the optical fiber 12
is jointly propagated to After the extraordinary ray travels diagonally through the crystal 17 circles, its direction is changed by two total reflection prisms 19, and after its polarization direction is rotated by 90 degrees by a 1/2 wavelength plate 18, it is redirected. The light enters the birefringent crystal 17, but the polarization direction of the light corresponds to ordinary light, and after passing straight through the crystal 17, it is coupled and propagated into the optical fiber 11.

Similarly, the light emitted from the optical fiber 12 is separated by the birefringent crystal 17 into an ordinary ray and an extraordinary ray, and the light is coupled and propagated to the optical fibers 11 and 13, respectively.

Therefore, the light from the optical fiber 11
2, 13, the light from the optical fiber 12 is the optical fiber 1
1, 13, the light of optical fiber 13 is optical fiber 1
1 and 12, and function as an optical T-type branching coupler. If the light from the optical fiber is unpolarized, the power ratio between the ordinary light and the extraordinary light within the birefringent crystal 17 will be 1:1, resulting in a 3 dB optical T-type branching coupler.

FIG. 4 shows another embodiment of this invention.
This is an example of the configuration of an optical T-type branching coupler having a branching ratio other than 3dB branching. Optical fiber 1 of birefringent crystal 17
A beam splitter 21 and a quarter wavelength plate 20 are added to the above embodiment on the first side.

The light emitted from the optical fiber 12 is separated into ordinary light and extraordinary light, and the extraordinary light passes through the birefringent crystal 17.
The light incident on the 1/4 wavelength plate 20 and the beam splitter 21 and transmitted through the beam splitter 21 is coupled to the optical fiber 11. On the other hand, the light reflected by the beam splitter 21 passes through the 1/4 wavelength plate 20 twice, which is equivalent to passing through a 1/2 wavelength plate, and the polarization direction is rotated by 90 degrees, causing birefringence. crystal 1
7, the polarization direction is rotated by 90 degrees through a 1/2 wavelength plate 18 and two total reflection prisms 19, and the polarization direction is changed and enters the birefringent crystal 17 again.
The light is transmitted as extraordinary light and is coupled and propagated to the optical fiber 13. Ordinary light from the optical fiber 12 passes straight through the birefringent crystal 17, merges with the light reflected by the beam splitter, and is coupled and propagated to the optical fiber 13.
The same applies to the light emitted from the optical fiber 13. Therefore, if the reflectance of the beam splitter 21 is r, the distance between the optical fibers 12 and 13 is (1+r)/2,
Between optical fibers 11 and 12, optical fibers 11 and 13
The coupling ratio is (1-r)/2, and by changing the transmission/reflection ratio of the beam splitter 21,
It is possible to construct an optical T-branch coupler with a branching ratio other than 3 dB.

Examples of birefringent crystals include calcite (CaCO ₃ ). Although not described in the above embodiments, lenses 14, 15,
16 is used.

FIG. 5 shows still another embodiment of the present invention, in which rod lenses 22 to 24 are used as lenses,
A 1/2 wavelength plate 18 is pasted between total reflection prisms 19 to create a structure that does not emit light into the space and is not affected by humidity or dust in the air.

In the above embodiment, the case where the light emitting element is coupled to an optical fiber has been described, but it may be coupled directly to a light emitting element or a light receiving element.

Further, in the above embodiment, a beam splitter plate is used as the optical beam splitter, but a beam splitter film may be provided on one end surface of the quarter wavelength plate.

Furthermore, although a total reflection prism was used in the above embodiment, the same effect can be expected even if a normal reflection mirror is used in combination.

〔Effect of the invention〕

As described above, according to the present invention, the configuration in which a birefringent crystal and a wavelength plate are combined has the advantage that the branching ratio can be arbitrarily changed regardless of the propagation mode of the optical fiber.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view showing a conventional optical T-type branching coupler, FIG. 2 is a plan view showing an optical T-type branching coupler according to an embodiment of the present invention, and FIG. 3 is an embodiment of the present invention. FIG. 4 is a plan view showing another embodiment of the invention, and FIG. 5 is a plan view showing still another embodiment of the invention. 11, 12, 13 are optical fibers, 14, 15,
16 is a lens, 17 is a birefringent crystal, 18 is 1/2
19 is a total reflection prism, 20 is a quarter wavelength plate, and 21 is a beam splitter plate. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. A birefringent crystal plate having a planar shape having an entrance surface portion and an exit surface portion, and whose optical axis of the crystal has an inclination angle with respect to the flat plate, and a birefringent crystal plate having an incident surface portion and an exit surface portion side a deflection means provided in the crystal plate and the deflection means for returning the light emitted from the output surface in a direction opposite to the emission direction parallel to the output light and re-entering the crystal plate; It is composed of a 1/2 wavelength plate that rotates the polarization direction by 90 degrees and a plurality of light input/output ends provided on both sides of the crystal plate, and of the light emitted from the first input/output end, the above Light that is incident perpendicularly to the birefringent crystal plate and becomes an ordinary ray in the crystal plate is transmitted through the crystal plate, passes through the 1/2 wavelength plate, and is again directed in parallel to the crystal plate by the deflection means. Among the light that is incident on the crystal plate, passes through the crystal plate as an extraordinary ray, is coupled to the third input/output end, and is emitted from the first input/output end, the inclination angle is determined as an extraordinary ray in the crystal plate. It is arranged so that the light that passes through the input terminal parallel to the incident position is coupled to the second input/output end, and the light that exits from the second input/output end goes to the first and third input/output ends. , an optical branching coupler in which light emitted from the third input/output end is coupled to the first and second input/output ends. 2. A birefringent crystal plate having a planar shape having an entrance surface portion and an exit surface portion, and whose crystal optical axis has an inclination angle with respect to the flat plate surface, and a birefringent crystal plate provided on the exit surface side and from the exit surface. a deflection means for returning the emitted light parallel to the emitted light in a direction opposite to the emission direction and re-entering the crystal plate; and a deflection means provided in the crystal plate and the deflection means to rotate the polarization direction of the light by 90 degrees. a 1/2 wavelength plate, a plurality of light input/output ends provided on both sides of the crystal plate, and a first input/output end of the plurality of light input/output ends and a birefringent crystal plate. It consists of a 1/4 wavelength plate in between and a light beam splitter that reflects some of the light and transmits the other part, and of the light emitted from the first input/output end, it is perpendicular to the birefringent crystal plate. The light that is incident on the crystal plate and becomes an ordinary ray passes through the crystal plate, and the above 1/2
After passing through the wave plate, the deflection means causes the crystal plate to enter the crystal plate again in parallel, and after passing through the crystal plate as an extraordinary ray, it is coupled to the third input/output end, and is coupled to the first input/output end. Among the light emitted from the crystal plate, the light is transmitted through the crystal plate at an inclined angle as an extraordinary ray, and the light emitted parallel to the incident position is coupled to the second input/output end. An optical branching coupler in which light emitted from an input/output end is coupled to first and third input/output ends, and light emitted from the third input/output end is coupled to first and second input/output ends.