JPH02100016A - Polarization non-dependence type optical circuit and device thereof - Google Patents

Abstract

PURPOSE:To execute single core type bidirectional optical communication at a low loss by separating and synthesizing an incident light beam to ordinary light and extraordinary light by a double refractive element and a non-phase inversion type element, separating an incident light from an opposite direction to ordinary light and extraordinary light, reflecting the light by a total reflecting mirror provided with a slit, reflecting the light by a total reflecting mirror, separating the light to the other optical system, and synthesizing the light to the ordinary light and the extraordinary light. CONSTITUTION:The beam from P1 transmits the total reflecting mirror (a) with the slit and is made incident on a planar double refractive crystal (b). The ordinary light and the extraordinary light are separated therein. The ordinary light and the extraordinary light are rotated 45 deg. by a Faraday rotator (c) and transmit the planar double refractive crystal (d). The ordinary light and the extraordinary light are thereafter rotated 45 deg. by a Faraday rotor (e) and are synthesized by the planar double refractive crystal (f). The incident light from P2 is reflected to the position different from the beam position in a forward direction in the total reflecting mirror part (a) with the slit. The ordinary light and extraordinary light made incident to the mirror of the total reflecting mirror (a) and reflected sideways are synthesized by the planar double refractive crystals (g), (h). The single core type bidirectional optical communication to the longer distance, the higher density of transmission wires, etc., are enabled in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical device for optical fiber communication, optical measurement, etc., and relates to a non-reciprocal and polarization-independent optical circuit and its device.

[Prior Art] Recently, optical communication systems and optical application equipment that use semiconductor lasers as light sources have come into widespread use. Optical isolators are used to eliminate Particularly recently, the amount of light returned to the semiconductor laser, that is, the reflection attenuation ω, is required to be approximately 60 dB.

A directional coupler as shown in FIG. 2 is used to measure this return loss.

[Problem that the invention sought to solve] However, as shown in Figure 2, the distance between the incident lights is at least 50% (=
3dB) is always a loss. Moreover, the polarization state of the returned light is not necessarily uniform, and the ratio between the reflected light value and the amount of transmitted light is not constant due to the polarization dependence of the half mirror. Therefore high precision.

It becomes difficult to measure high dynamic ranges.

Similarly, if the directional coupler is used in core type bidirectional optical communication, the loss will increase, making highly reliable communication impossible.

In view of this point, the present invention allows light of any polarization state to be transmitted with low loss in the forward direction.

In the opposite direction, light of any polarization state has low loss,
The purpose of this study is to provide a polarization-independent optical circuit for transmission to boats other than forward-incidence boats.

[Means for Solving the Problems] Fig. 1 shows a diagram of the principle configuration of the present invention, and shows the states of ordinary light and extraordinary light in the case of forward direction light (a) and backward direction light (b), respectively. Illustrated at the top of the page. In one embodiment of the present invention, a indicates a total reflection mirror with a slit, b,
d, f, Q.

h indicates a tabular birefringent crystal, and c and e are Faraday rotators magnetized in the same direction by permanent magnets. b, d
, f, g, and h, it is preferable to use calcite, which can utilize open planes, from the viewpoint of material availability, price, and performance. Of course, the present invention also encompasses the use of other birefringent materials such as rutile.

Also, if the thickness of the tabular birefringent crystal is 1, the ratio of the thicknesses of the birefringent crystal is b, d, f, Cl, h=1:
The relationship is J2:1:2:2. The optical axis of the crystal is inclined at 41 to 49 degrees from the surface in a plane that includes the surface and its normal, and at angles of 41 degrees or less and 49 degrees or more, the thickness of the crystal is adjusted to obtain the separation width of ordinary and extraordinary light. I have to make it longer.

The angles from the surface of each birefringent crystal were the same.

Further, the plane of polarization of the Faraday rotator rotates by 45° in a magnetically saturated state. b, d, f The optical axis of the birefringent crystal is y
The relationship between the crystal optical axis projection lines when projected onto the −z plane is a position rotated by 45° from the crystal optical axis projection line of the pre-birefringent crystal in the direction of rotation of the plane of polarization by the Faraday rotator in the forward direction.

9.h When the crystal optical axis of a birefringent crystal is projected onto the X-Z plane, the relationship between the crystal optical axis projection lines is 90°.

Assuming that the forward direction is (P1→P2), in FIG. 1(a), the beam from Pl passes through a total reflection mirror a with a slit and enters a flat birefringent crystal. Here, the ordinary light and the extraordinary light are separated and enter the Faraday rotator C. The ordinary light and the extraordinary light are rotated by 45 degrees due to Faraday rotation and are transmitted through the tabular birefringent crystal d. The state of the light here is such that the plane of polarization of the ordinary light is rotated 45 degrees to the right from the y-Z plane, and the plane of polarization of the extraordinary light is orthogonal to the plane of polarization of the ordinary light. Tsuneko.

The position of the extraordinary light is within the Z-X plane. Thereafter, it is rotated by 45 degrees by a Faraday rotator e, and the ordinary light and extraordinary light are matched by the tabular birefringent crystal f, and are coupled to the subsequent optical system with low loss. Therefore, the light can be transmitted without depending on the plane of polarization.

When the opposite direction is set to (P2→P3), in Figure 1(b), the non-reciprocity caused by the Faraday rotator and the separation phenomenon of ordinary and extraordinary light caused by the tabular birefringent crystal are used.
The light incident from P2 is at a position different from the beam position in the forward direction on the part a of the total reflection mirror with a slit.

Therefore, since there is an isolation effect here, the ordinary light does not go out to Pl, but enters the mirror of the total reflection mirror a with a slit and is reflected laterally.

The extraordinary light is combined by the tabular birefringent crystals Q and h and projected onto P3 with low loss.

When used in single-core bidirectional optical communication, etc., as shown in Figure 3, the signal light from the LD (laser diode) from Pl passes through the polarization-independent optical circuit with low loss, and the P2 emitted to. Thereafter, the light is transmitted into the fiber, from P4 to P6, and is received by a PD (photodiode). At this time P
There is very little crosstalk to P3 and P5. Similarly, the signal from P5 becomes a transmission path of P4→P2→P3, and there is very little crosstalk to P1 and P6. Normally, near-end reflected light increases crosstalk, but as in the above principle, P3. By arranging the circular slit i in the P6 section, the amount of crosstalk could be attenuated.

The purpose of the circular slit is to prevent forward direction signal light from being combined with reverse direction signal light as reflected light from a connector or the like in the optical fiber transmission line, thereby preventing an increase in the amount of crosstalk. That is, near-end reflected light with high reflected light intensity in an optical fiber transmission line has many higher-order mode components, so the light intensity is high at the core circumferential portion of the optical fiber cross section. However, since the backward signal light is transmitted over a certain distance, the higher-order modes are attenuated and the lower-order mode components are higher.

Therefore, the beam diameter of the near-end reflected light is large and the beam diameter of the backward signal light is small. Utilizing this, it becomes possible to attenuate the near-end reflected light with the circular slit.

As mentioned above, optical fiber 1 tree and polarization independent optical circuit 2
Two-way optical communication can be realized by using two-way optical communication.

When used in a return loss measuring device, as shown in Figure 4, the light from the LD light source is collimated (parallelized) by a lens, enters a polarization-independent optical circuit, and is transmitted with low loss. transmitted over optical fiber. The light emitted from the fiber is collimated by a lens and enters the object to be measured j. The reflected light from the object to be measured returns within the fiber,
The light is emitted to P3 of the polarization-independent optical circuit.

Measure this light intensity with a power meter. The measurement method is to first remove the object to be measured and measure the light intensity of P4 with a power meter. Next, connect power meter k to P3, and
4, and measure the amount of reflected light. The return loss is defined by the following equation.

Return loss=−10·log(P4/P3) When measured as above, a high dynamic range with a return loss of 1fi60 dB can be obtained.

[Example] An example of the present invention is shown in FIG.
The width of the slit was set to allow the incident light from to pass through. A total reflection film is applied to the inclined surface. The tabular birefringent crystals b, d, f, g, and h are made of calcite with an optically polished surface.
Ratio of calcite plate thickness, d, f, Q, h=1 :J
The ratio is 2:1:2:2. A Bi-substituted rare earth magnetic garnet crystal was used as the Faraday rotator, and the crystal was magnetically saturated with a permanent magnet to have a thickness such that one rotation angle of 77 rad of the plane of polarization was 45°.

[Operation] In the forward direction (P1→P2), the incident beam passes through the slit portion of the slitted total reflection mirror a and enters the calcite plate, where it becomes ordinary light.

Separate into extraordinary light. The separation width is approximately 171 times the thickness of the calcite plate.
It is 0. Faraday rotator C has constant light.

The plane of polarization of both extraordinary light rotates to the left with respect to the direction of travel of the light. If you want to rotate it to the right, just reverse the direction of the magnetic field. In the calcite plate d, 4 points from the surface in the plane including the polarization direction of the extraordinary light.
The optical axis of the crystal is in the 5° direction, and as shown in the figure, the ordinary light and the extraordinary light transmitted through d are at right angles to each other, and the plane containing each other's rays is parallel to the Z-X plane. Following the same principle, a Faraday rotator e and a calcite plate f are provided to combine the ordinary light and extraordinary light separated by b.

On the other hand, in the opposite direction (P2→P3), 1. The light emitted from the calcite plate will be emitted at a position shifted from the incident position in the forward direction. Therefore, as shown in the figure, the light is projected onto the mirror portion of the total reflection mirror a with a slit, is totally reflected by the mirror, and is emitted to the side. In order to match the separated ordinary light and extraordinary light, calcite plates Q and h were provided, and the ordinary light and extraordinary light were made to match. For this reason, P
This becomes a polarization-independent optical circuit having the ability that the light incident from P2 exits from P2 and does not exit from P3, and the light that enters from P2 exits from P3 and does not exit from Pl.

[Effect of the invention] As described above, light of any polarization plane can be transmitted with low loss in the forward direction (P1→P2), and in the reverse direction (P2→P2).
P3) can transmit light of any polarization plane with low loss. Furthermore, there is an isolation effect that does not return to Pl. Therefore, by using the polarization-independent optical circuit described above, the return loss G can be measured with high precision.

The high dynamic range makes it possible to develop high-performance optical devices, and by using this circuit in single-core bidirectional optical communication, it is possible to increase long distances, increase the density of transmission lines, and reduce size and weight. It becomes possible.

[Brief explanation of drawings]

FIG. 1 shows a basic configuration diagram of a polarization-independent optical circuit according to the present invention. (a): Forward light beam path (b); Reverse light beam path FIG. 2 shows a basic configuration diagram of a conventional bulk type directional coupler. FIG. 3 shows a schematic diagram when the polarization-independent optical circuit of the present invention is used for single-core bidirectional optical communication. FIG. 4 shows a schematic diagram when the polarization-independent optical circuit of the present invention is used in a return loss measuring device. a Total reflection mirror with Ni slit b; d; f: g: h: Tabular birefringent crystal Cue: Faraday rotator Bi-circular slit Patent applicant Namiki Precision Jewel Co., Ltd. + ＼ -/ Ha ( ↑ Figure 2 + @1 figure

Claims (4)

[Claims] (1) A light beam incident from a total reflection mirror with a slit is separated into ordinary and extraordinary light by a birefringent element, and the plane of polarization is rotated by 90° with a non-reciprocal element sandwiching the birefringent element. The ordinary light and the extraordinary light are combined and emitted, and then the light beam that enters from the opposite direction passes through the above optical system in the reverse direction and is separated into the ordinary light and the extraordinary light, each of which has a beam position different from that in the forward direction. A polarization-independent optical circuit characterized by a configuration in which the slitted total reflection mirror reaches the slit-attached total reflection mirror and is reflected to be separated into other optical systems, and the ordinary light and the extraordinary light are combined by a birefringent element. (2) From a total reflection mirror with a slit, a first flat birefringent crystal, a first Faraday rotator, a second flat birefringent crystal, a second Faraday rotator, and a third flat birefringent crystal. The optical system is arranged in this order, and the optical axis of the plate-shaped birefringent crystal is inclined with respect to the surface, and the first, second,
The thickness ratio of the third tabular birefringent crystal is 1:
√2:1, and the light beam entering from the slit passes through the optical system and is emitted, while the light beam entering from the opposite direction passes through the optical system in the opposite direction and is reflected by the total reflection mirror. By this, another optical device consisting of a fourth tabular birefringent crystal and a fifth tabular birefringent crystal each having a thickness ratio of 2:2 corresponding to the thickness of the tabular birefringent crystal. A claim (
1) Polarization-independent optical circuit as described above. (3) A single-core bidirectional optical communication device using two polarization-independent optical circuits and one optical fiber according to claim (1). (4) A return loss measuring device using the polarization-independent optical circuit according to claim (1).