JP3261391B2 - Polarization-independent optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator having a low polarization dependency, using a birefringent plate between end faces of a pair of optical fibers to be optically coupled.

[0002]

2. Description of the Related Art A polarization independent optical isolator using a birefringent plate functions by separating or combining optical paths of polarization components whose polarization directions are orthogonal to each other (Japanese Patent Publication No. 60-29297). No., JP-A-4-146
411 etc.). FIG. 1 shows a conventional polarization-independent optical isolator, in which a lens 3 and a birefringent element 4 are provided between an end face of an optical fiber 1 and an end face of an optical fiber 2. (Which means an element having a forward coupling action and a reverse isolation action).
The two polarizations pass through the lens 3 and the birefringent element 4
After being separated into two, the light is collected again, recombined, and enters the optical fiber 2. On the other hand, the orthogonal polarization of the return light in the opposite direction is split into two by the birefringent element 4, but does not return to the optical fiber 1 as off-axis light when passing through the lens 1.

When a polarization-independent optical isolator is inserted between optical fibers, the insertion loss of the optical isolator increases unless the light once separated in the forward direction is accurately recombined. Cause the nature. The smaller the light spot diameter, the greater the insertion loss unless the optical axis is aligned with higher precision. Recombination of the two separated polarizations into alignment with the core of the optical fiber is generally difficult, and the loss of the two polarizations is different. Therefore, it has been difficult to reduce the polarization dependence of the conventional polarization independent optical isolator. Conventionally, in order to solve this problem, one or more lenses have been added in addition to the lens 3 of FIG. 1 to reduce the insertion loss. This is because it can be made larger. However, this increases the number of parts, increases the number of assembling steps, increases the size, and increases the cost.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to improve the polarization independence of a polarization independent optical isolator by a simpler means than in the prior art.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a polarization independent optical isolator in which an optical isolator element including a birefringent plate and one lens are arranged between the light input / output end faces of two optical fibers. Contact stomach, the core diameter of both the optical fiber end face neighborhood
It expands in a tapered shape toward the end face, at least in the forward direction.
Provided is an optical isolator, wherein the mode field diameter MFD of the core end face of the optical fiber on the light emitting surface side is MFD <0.66d when the separation distance between two orthogonal polarization components is d.

[0006]

[Act] To illustrate this point more specifically, the refractive index n _o for ordinary light, when the refractive index n _e and thickness birefringent parallel plate perpendicularly light t incident for extraordinary light, the ordinary light at the exit point The extraordinary light and the separation distance d are expressed by the following equation. ^{_{d = (n e 2 -n o}} 2) t / (n e 2 + n o 2) On the other hand, the core of the optical fiber the position of the incident light to the optical isolator using a birefringent plate from the opposite direction by its birefringence Isolation by deviating from the position (indicates the degree to which backward light is separated from the core of the optical fiber)
, But the isolation is a function of the mode field diameter MFD. Isolation = 17.36 × d ² / (MFD) ² Therefore, in order to ensure the isolation of 40 dB or more, if the MFD is set to the maximum value within the range satisfying the following condition, the MFD <0.66d, the reverse is 40 dB or more. Isolation, as well as increasing the diameter of the light beam in the forward direction, reducing loss and increasing the intensity of the recombined light, and increasing the diameter of the light to increase the directional dependence of polarization. Is suppressed, and the performance of the polarization independent optical isolator can be improved. In general, the positions of the ordinary light and the extraordinary light after passing through the element slightly shift, but if the core is enlarged, the loss due to the minute shift can be reduced, and as a result, the polarization dependence can be suppressed.

A technique for enlarging the core diameter near the end face of an optical fiber in a tapered shape toward the end face is disclosed in JP-A-63-33706.
Therefore, the same method can be adopted in the present invention to form an optical fiber end. The technique disclosed in the publication does not suppress the polarization dependence of the polarization independent optical isolator as in the present invention, but enlarges the diameter of the light beam transmitted in the fiber end and adjusts the degree of polarization. Thus, the technique merely reduces the coupling loss.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 2, a lens 13 is provided between a pair of optical fibers 11 and 12 having waveguide cores 16 and 18.
The two birefringent plates 14 and 14 were joined by using two rutile plates having a thickness of t = 0.5 mm and joining them with a Faraday rotator 15 made of a garnet film therebetween. The birefringent plate is n _o = 2.44, a n _e = 2.69, the separation distance listed above ^{_{d = (n e 2 -n o}} 2) t /
^{_{(N e 2 + n o 2}} ) was ² = ^0.0486mm. Therefore, MFD <0.66 × d = 32.08 μm.
That is, the mode field diameter MFD is 32.08 μm
If less than 40, isolation of 40 dB or less can be ensured with respect to the return light beam. Therefore, the end face diameter of the tapered core enlarged portion 17 (and the enlarged portion 19 if there is incident light in the opposite direction) in this range. You just have to select On the other hand, since the diameter of the light beam in the forward direction is increased, not only the recombination of the separated polarization is increased, but also the dependence on the optical polarization is reduced.

[0009]

According to the present invention, the diameter of a forward light beam can be increased without sacrificing isolation, so that the polarization dependence of a polarization independent optical isolator is reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the operation of a conventional polarization-independent optical isolator.

FIG. 2 is a diagram showing a polarization independent optical isolator according to an embodiment of the present invention.

[Explanation of symbols]

 11, 12 Optical fiber 13 Lens 14 Birefringent plate 15 Garnet plate 16, 18 Core 17, 19 Core enlarged part

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-287528 (JP, A) JP-A-2-116815 (JP, A) JP-A-60-213903 (JP, A) JP-A-4- 371911 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 27/28

Claims (1)

(57) [Claims] 1. A polarization-independent optical isolator in which an optical isolator element including a birefringent plate and one lens are arranged between light input / output end faces of two optical fibers, in the vicinity of both optical fiber end faces. The core diameter is increased in a tapered shape toward the end face, and at least the mode field diameter MFD of the core end face of the optical fiber on the light emitting face side in the forward direction is set so that the separation distance between two orthogonal polarization components is increased. MFD <0.66d, where d is an optical isolator.