JP2796315B2 - Polarization-independent optical circuit and device therefor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device in optical fiber communication, optical measurement, and the like, and relates to a non-reciprocal and polarization-independent optical circuit and an apparatus therefor.

[Prior art] Recently, optical communication systems and optical equipment using a semiconductor laser as a light source have been widely used, and the return light of the semiconductor laser has been developed for the purpose of improving the accuracy and reliability of those systems and equipment. An optical isolator has been used to eliminate the noise. Particularly, recently, the amount of return light to the semiconductor laser, that is, the return loss, is required to be about -60 dB. A directional coupler as shown in FIG. 2 is used for measuring the return loss.

[Problems to be Solved by the Invention] However, as shown in FIG. 2, the reflectance of the forward light is 50%.
(= 3dB) plus 50% (= 3dB) of reverse reflectance, 3dB + 3
dB = 6 dB always results in loss. In addition, the polarization state of the return light is not always uniform, and the ratio between the amount of reflected light and the amount of transmitted light is not constant due to the polarization dependence of the half mirror. Therefore, it is difficult to measure with high accuracy and high dynamic range. Similarly, in the single-core bidirectional optical communication, if the directional coupler is used, the loss increases, so that highly reliable communication cannot be performed.

In view of this point, the present invention transmits light of any polarization state in the forward direction with low loss, and transmits light of any polarization state in the reverse direction with low loss, and transmits polarized light to ports other than the forward incidence port. An object of the present invention is to provide a wave-independent optical circuit.

[Means for Solving the Problems] FIG. 1 is a diagram illustrating the principle of the present invention, wherein the states of ordinary light and extraordinary light are forward light (a) and reverse light (b).
Each is shown above the optical system. In one embodiment of the present invention, a indicates a slit total reflection mirror, b,
d, f, g, and h are plate-like birefringent crystals, and c and e are Faraday rotators magnetized in the same direction by permanent magnets. b, d,
It is preferable to use calcite, which can utilize a cleavage plane, from the viewpoint of material supply power, price, and performance for the plate-like birefringent crystal of f, g, h. Of course, the use of other birefringent materials, such as rutile, is also included in the present invention.

If the thickness of the plate-like birefringent crystal b is 1, the ratio of the thickness of the birefringent crystal is It becomes the relationship. The crystal optical axis is inclined by 41 to 49 ° from the surface in a plane including the surface and its normal, and is 41 ° or less and 49 °.
At angles above 角度, the thickness of the crystal must be increased in order to obtain a separation width between ordinary light and extraordinary light.

The angle from the surface of each birefringent crystal is the same. The polarization plane of the Faraday rotator rotates 45 ° in the magnetically saturated state. The relationship between each crystal optical axis projection line when projecting the crystal optical axis of the b, d, f birefringent crystal on the yz plane is as follows. The position is 45 ° rotated from the crystal optical axis projection line. The relationship between the crystal optical axis projection lines when the crystal optical axes of the g, h birefringent crystals are projected on the xz plane is 90 °.

Assuming that the forward direction is (P1 → P2), in FIG.
The beam from P1 is transmitted through a total reflection mirror a with a slit and is incident on a flat birefringent crystal b. 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 ° by the Faraday rotation, and the flat birefringent crystal d
Through. The state of light here is such that the polarization plane of ordinary light is y-
Rotated 45 ° to the right from the x-plane, the polarization plane of the extraordinary light is orthogonal to the polarization plane of the ordinary light. The positions of the ordinary light and the extraordinary light are in the zx plane. Thereafter, the light is rotated by 45 ° by the Faraday rotator e, and the ordinary light and the extraordinary light coincide with each other by the flat birefringent crystal f, and are coupled to the subsequent optical system with low loss. Therefore, the light can be transmitted independently of the plane of polarization.

Assuming that the reverse direction is (P2 → P3), in FIG.
Because of the use of the nonreciprocity of the Faraday rotator and the separation of ordinary and extraordinary light by a flat birefringent crystal, P2
Is incident on the total reflection mirror section a with a slit at a position different from the beam position in the forward direction. Therefore, since there is an isolation effect here, P1
The ordinary light and the extraordinary light which are not emitted to the mirror and incident on the mirror of the slit total reflection mirror a and reflected laterally are synthesized by the plate-shaped birefringent crystals g and h and are projected to P3 with low loss.

When used for single-core bidirectional optical communication or the like, as shown in FIG. 3, signal light from an LD (laser diode) from P1 passes through a polarization-independent optical circuit with low loss, and P2 Out. After that, it is transmitted in the fiber, transmitted from P4 to P6,
(Photodiode). At this time, crosstalk to P3 and P5 is very small. Similarly, the signal from P5 is P4 → P2 → P3
And the crosstalk to P1 and P6 is very small. Normally, near-end reflected light increases crosstalk, but as in the above principle,
By arranging the circular slits i in the portions P3 and P6, the amount of crosstalk could be attenuated. Also, since the transmission loss of the optical system including the non-reciprocal element and the birefringent element is 0.5dB,
In the forward and reverse directions, the transmission loss is about 0.5 dB + 0.5 dB = 1 dB. The circular slit prevents the forward signal light from being combined with the reverse signal light as reflected light from a connector or the like in the optical fiber transmission line, thereby preventing an increase in crosstalk. In other words, near-end reflected light having a high reflected light intensity in the optical fiber transmission path has a high light intensity in the core portion circumferential portion of the optical fiber cross section because of a large number of higher-order mode components. However, since the backward signal light is transmitted over a certain distance, the higher-order mode is attenuated and the lower-order mode component is higher. Therefore, the beam diameter of the near-end reflected light beam is large and the beam diameter of the backward signal light is small. By utilizing this, it is possible to attenuate the near-end reflected light by the circular slit. As described above, bidirectional communication can be realized by using one optical fiber and two polarization-independent optical circuits.

When used in a return loss measuring device, as shown in FIG. 4, light from an LD light source is collimated (parallel) 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 the lens and enters the object k. The reflected light from the DUT returns in the fiber,
The light is output to P3 of the polarization independent optical circuit. This light intensity is measured by a power meter j. In the measurement method, first, an object to be measured is removed, and the light intensity of P4 is measured with a power meter. Next, connect the power meter j to P3, place the device under test k on P4,
Measure the amount of reflected light. The return loss is defined by the following equation.

Return loss = -10 · log (P3 / P4) When measured as described above, a high dynamic range with a return loss of 60 dB can be obtained.

[Embodiment] FIG. 1 shows an embodiment of the present invention. In a total reflection mirror a with a slit, a groove is formed in a right-angle prism, and P1 is formed.
The slit width was such that the incident light from the slit could pass through. The inclined surface is provided with a total reflection film. b, d, f, g, h plate-shaped birefringent crystal, the cleavage plane of calcite is an optically polished surface, and the thickness ratio of the calcite plate is It is. A Bi-substituted rare-earth magnetic garnet crystal was used for the Faraday rotator, and the thickness was adjusted so that the Faraday rotation angle of the polarization plane was 45 ° by magnetic saturation with a permanent magnet.

[Operation] In the forward direction (P1 → P2), the incident beam passes through the slit portion of the slit total reflection mirror a and the calcite plate b
And is separated into ordinary light and extraordinary light. The separation width is about 1/10 of calcite slab thickness. In the Faraday rotator c, both the ordinary light and the extraordinary light rotate the polarization plane to the left with respect to the traveling direction of the light. When rotating clockwise, the direction of the magnetic field may be reversed. In the calcite plate d, the crystal optical axis is located at 45 ° from the surface within the plane including the polarization direction of the extraordinary light, and as shown in the figure, the ordinary light and the extraordinary light transmitted through d are perpendicular to each other, and The plane included is parallel to the zx plane. Hereinafter, a Faraday rotator e and a calcite plate f are provided to combine the ordinary light and the extraordinary light separated by b according to the same principle.

On the other hand, in the reverse direction (P2 → P3), the light emitted from the calcite plate b is shifted from the position where it was incident in the forward direction due to the non-reciprocity caused by the Faraday rotator and the separation of ordinary light and extraordinary light caused by the calcite plate. It will be. Therefore, as shown in the figure, the light is projected on the mirror portion of the total reflection mirror a with a slit, totally reflected by the mirror, and emitted to the side. Calcite slabs g and h to match the separated ordinary and extraordinary light
And matched the ordinary light and the extraordinary light. For this reason, the light incident from P1 exits from P2 and does not exit from P3, and the light incident from P2 emerges from P3 and does not exit from P1, resulting in a polarization-independent optical circuit.

[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 → P3).
Can transmit light of any polarization plane with low loss. There is also an isolation effect that does not return to P1. Therefore, the use of the above-mentioned polarization-independent optical circuit makes it possible to measure the return loss with a high accuracy and a high dynamic range, thereby enabling the development of a high-performance optical device and a single-core bidirectional optical communication. By using this circuit, it is possible to increase the distance, increase the density of transmission lines, reduce the size and weight, and so on.

[Brief description of the drawings]

FIG. 1 shows a principle configuration diagram of a polarization independent optical circuit of the present invention. (A): Forward light beam path (b): Reverse light beam path FIG. 2 shows a principle configuration diagram of a conventional bulk type directional coupler. FIG. 3 is a schematic diagram when the polarization independent optical circuit of the present invention is used for a single-core bidirectional optical communication or the like. FIG. 4 is a schematic diagram showing a case where the polarization-independent optical circuit of the present invention is used in a return loss measuring device. a: Total reflection mirror with slit b; d; f; g; h: Flat birefringent crystal c; e: Faraday rotator i: Circular slit

Claims (4)

(57) [Claims] 1. A light beam incident from a slit of a total reflection mirror with a slit is separated into ordinary light and extraordinary light by a birefringent element, and after a polarization plane is rotated by 90 ° across the birefringent element by a non-reciprocal element, The birefringent element combines ordinary light and extraordinary light and emits the light beam. Polarization-independent light that reaches the above-mentioned total reflection mirror with slit at a position different from the position, is separated into other optical systems by reflection, and combines ordinary light and extraordinary light with a birefringent element circuit. 2. 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. The optical system is arranged in the order of the crystals, and the flat birefringent crystal has a crystal optical axis inclined with respect to the surface, and
2, The thickness ratio of the third planar birefringent crystal is The light beam incident from the slit is transmitted through the optical system and emitted, while the light beam incident from the opposite direction is transmitted through the optical system in reverse and reflected by the total reflection mirror, It passes through another optical system composed of a fourth flat birefringent crystal and a fifth flat birefringent crystal whose thickness ratio is 2: 2 corresponding to the thickness of the flat birefringent crystal. 2. The polarization-independent optical circuit according to claim 1, wherein the optical circuit is configured to emit the light. 3. A polarization-independent optical circuit 2 according to claim 1.
Each of the optical circuits was placed at two points, an optical fiber was arranged between the external input / output ports of each optical circuit, a light emitter was arranged at the light beam incident port of each optical circuit, and the mirror of each optical circuit was reflected. A single-core bidirectional optical communication device in which a light receiver is arranged at a side port. 4. A polarization independent optical circuit according to claim 1, wherein the quantity of incident light at an external input / output port of the optical circuit is determined, and then an object to be measured is placed at the external input / output port. A reflection attenuation measuring apparatus for measuring the natural logarithm of the ratio of the amount of reflected light to the amount of incident light, using the amount of incident light obtained at the side port reflected by the mirror as the amount of reflected light.