JPH0451013A - Optical circulator - Google Patents

Abstract

PURPOSE:To obtain an optical circulator having small loss, high isolation and no dependency on polarized light by fitting a Faraday element and optical rotatory elements to each terminal of an optical circulator and separating a polarized light beam splitter under specified conditions. CONSTITUTION:A Faraday element 11-i (i is 1-4 in the figure), optical rotatory elements 9-i, 10-i and polarized light separating elements 13-i, 14-i for ensuring no dependency on polarized light are fitted to each terminal 15-i of an optical circulator and a polarized light beam splitter 12 for setting the output direction of light beams is further fitted. The polarized light separating films of the splitter 12 are continuously arranged along an optical path with leakage so as to remove leaking light in succession. The effect of accuracy in the rotational angle of each of the oscillating surfaces of the elements 11-i, 9-i, 10-i and the effect of the extinction ratio of the splitter 12 on the isolation of the resulting optical circulator are reduced and small loss and high isolation can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Usage Fees> The present invention relates to a polarization-independent optical circulator with low loss and high isolation.

<Prior Art> FIG. 5 shows a typical configuration of a conventional polarization-independent optical circulator.

The one in Fig. 5 is a four-terminal optical circulator, with four optical fibers 1-1 to 1-4 and four lenses 2-1 to 1-4.
2-4, one each of Faraday element 3 and optical rotation element 4, two each of polarizing beam splitters 5-1.5-2 and right-angle prisms 6-1.6-2.

In the conventional optical circulator shown in FIG. 5, unpolarized light is transmitted from one of the optical fibers 1-i (i=1 to 4) through the lens 2-i to one polarizing beam splitter 5-1 or 5-.
2, where the oscillation plane of the electric field is separated into two straight sight beams that are orthogonal to each other. These two straight a-light beams pass through the Faraday element 3 and the optical rotation element 4 to rotate their vibration planes by 0 degrees or 90 degrees, respectively, and then pass through the other polarizing beam splitter 5-2 or 5-1.
and is multiplexed into a predetermined optical fiber 1-j (j=1 to 4, ]
≠i).

By the way, in the above-mentioned conventional polarization-independent optical circulator, there is an optical path in which the input light from the optical fiber 1-1 is coupled to a predetermined optical fiber 1-j, and another optical fiber 1-k.
(k=1 to 4, k≠11 to ≠J) overlaps with the optical path in which input light is coupled to a predetermined optical fiber 1-1.

Therefore, if the rotational angle accuracy of the vibration plane in the Faraday element 3 or the optical rotation element 4 is not sufficient, or if the extinction ratio of the polarizing beam splitter 5-1.5-2 is not sufficient, a predetermined isolation can be obtained. The problem was that I couldn't do it.

In particular, the polarizing beam splitters 5-1 and 5-2 are arranged so that unnecessary polarization components generated in each polarizing beam splitter are added to the polarizing beam splitters 5-1 and 5-2.
Isolation ratios greater than 1 or 5-2 cannot be achieved.

In order to improve this problem, in the configuration shown in FIG.
1.2-3 and the polarizing beam splitter 5-1, and between the lens 2-2.2-4 and the polarizing beam splitter 5-2, an optical circulator with a birefringent crystal plate (not shown) is provided. It has been proposed (Japanese Patent Application No. 1-156018 ``Polarization-independent optical circuit'').

In this optical circulator, an optical fiber 1-i (i
= 1 to 4) is pre-separated by a birefringent crystal plate with a large extinction ratio into two linear sight beams whose electric field vibration planes are orthogonal to each other, and then sent to a polarizing beam splitter 5-1 or 5-2. incident on . Therefore, the polarizing beam splitter 5
The optical path of the unnecessary polarized beam generated according to the extinction ratio of −1 or 5-2 is connected to another optical fiber 1-k (k=1 to 4, k,
4i) and is coupled to the optical fiber 1-i. As a result, in the optical circulator with the refracted crystal plate added, its isolation constraint does not depend much on the extinction ratio of the polarizing beam splitter 5-1 or 5-2.

However, even in an optical circulator with a refracted crystal plate added, since the configuration is basically as shown in FIG.
and the optically active element 4, and while the isolation greatly depends on the rotation error of the vibration plane of the Faraday element 3 and the optically active element 4, I[ has not been improved.

<Problems to be Solved by the Invention> The purpose of the present invention is to provide a structure of an optical circulator that has a small influence on the rotational angle precision of the vibration plane of a Faraday element or an optical rotation element, and on the isolation ratio of the extinction ratio of a polarizing beam splitter. The object of this invention is to realize a polarization-independent optical circulator with low loss and high isolation.

Means for Solving the Problems> The configuration of the polarization-independent optical circulator according to the present invention is such that the number of optical circulator terminals is 2 (N+
1) In each optical circulator, there is a polarization beam splitter in which N+1 polarization separation films intersect at equal angles, an optical fiber, a lens coupled to this optical fiber, a polarization separation element coupled to this lens, and a straight line. A first optical rotation element that rotates the direction of polarized light by +45 degrees, and a second optical rotation element that is arranged in parallel with the first optical rotation element and rotates the direction of linearly polarized light by 145 degrees.
and a Faraday element which rotates the direction of linearly polarized light by +45 degrees and is coupled to a polarization separation element by sandwiching the first and second optical rotation elements or by sandwiching the first and second optical rotation elements. The unpolarized light input from the optical fiber is output to the polarizing beam splitter as two linearly polarized beams in the pli state of the polarizing beam splitter, and the unpolarized light input from the polarizing beam splitter is outputted as two linearly polarized beams in the S polarized state. N+1 optical circulator terminals of group A that combine and output polarized beams, an optical fiber, a lens coupled to this optical fiber, a polarization separation element coupled to this lens, and a direction of direct & II polarized light of +45 a first optical rotation element that rotates by 145 degrees; a second optical rotation element that is arranged in parallel with the first optical rotation element and rotates the orientation of linearly polarized light by 145 degrees;
The polarization separation element has a first and second optical rotation element sandwiched therebetween, or a Faraday element coupled between the first and second optical rotation elements, and converts the unpolarized light input from the optical fiber into the polarized beam. Two states of S polarization of the splitter! !
N+1 B beams are output as 1111 light beams to a polarizing beam splitter, and two linearly polarized beams in the p-polarized state inputted from the polarizing beam splitter are combined and output.
and optical circulator terminals of the group A and B, and the arrangement relationship between the optical circulator terminals of the groups A and B and the polarization beam splitter is such that each optical circulator terminal of the group A is connected to a separate optical circulator terminal of the group B. The light from one separate optical circulator in group A is reflected and combined by the two polarization separation films of the polarization beam splitter, and the light from one separate optical circulator in group A is connected to the two polarization separation films of the polarization beam splitter at each optical circulator terminal in group B. It is characterized by its ability to bind through membranes.

Function> In the conventional polarization-independent optical circulator, as described above, all the signal light beams emitted from each terminal pass through the same Faraday element and optical rotation element. Moreover, the polarizing beam splitter is used to make the optical circulator polarization independent and to determine the output direction of the optical beam.
shared by two functions.

In contrast, in the optical circulator of the present invention, each terminal has a Faraday element, an optical rotation element, and a polarization separation element for making polarization independent, and the output direction of the light beam is determined separately from each polarization separation element. There is a polarizing beam splitter.

Then, the polarization separation films of the polarization beam splitter are arranged in series in the optical path where the leakage occurs, and the leakage light is removed one after another.

Therefore, the influence of the rotation angle accuracy of the vibration plane of the Faraday element or the optical rotation element and the extinction ratio of the polarizing beam splitter on the isolation of the optical circulator is reduced, and low loss and high isolation are achieved.

Embodiment> A four-terminal polarization-independent optical circulator, which is an embodiment of the present invention, will be described with reference to FIG.

In FIG. 1, the polarization beam splitter 12 has two polarization separation films that intersect on a straight line at equal angles (90 degrees), and four polarization separation films separated by this straight line. Four optical circulator terminals 15-i (i=1 to 4) are arranged so as to face each of the pieces.

Each optical circulator terminal 15-1 connects the optical fiber 7-
i, a lens 8-i, a first optical rotation element 9-i,
Second optical rotation element 10-i and Faraday element 11-1
, polarizing beam splitter 13-1, and right angle prism 1
4-1. However, the combination of the polarization beam splitter 13-1 and the right-angle prism 14-1 forms a polarization-independent polarization separation element.

Each of the first optical rotation elements 9-i passes through it! Each of the second optical rotation elements 10-1, which rotates the direction of the cap light (vibration plane of the electric field) by +45 degrees, rotates the direction of the linearly polarized light passing through it by +45 degrees.
It shall be rotated by 5 degrees, and the first and second
Both optical rotation elements 9-i and 10-i are arranged in parallel.

Also, is the Faraday element 11-1 an optical fiber? -i
For the linearly polarized light traveling from the polarizing beam splitter 12 to the polarizing beam splitter 12, it is rotated by 145 degrees and
The linearly polarized light traveling from 2 to the optical fiber 7-i is rotated by +45 degrees.

Note that the rotation of the polarization direction is considered based on the direction in which the light travels, and the sign of the rotation angle is meaningful only if the rotation is in the opposite direction.

At each optical circulator terminal 15-1, the optical fiber 7-
1, a lens 8-i, a polarization separation element (polarization beam splitter 13-1 and a right-angle prism 14-i), and first and second optical rotation elements 9-i and 10-i arranged in parallel,
Faraday elements 11-i are arranged in this order.

(The order of the optical rotation element and the Faraday element may be reversed.) Also, among the four optical circulator terminals, two terminals 15-1, 15-3 and the other two terminals 15-2, 15-
4 is the first. The arrangement of the second optical rotation element is different, with the former two terminals 15-1.15-3 having the first optical rotation element 9-1°9- on the polarization beam splitter 13-1.13-3 side.
3, the second optical rotation element 10-1.10-3 is arranged on the right angle prism 14-1.14-3 side, and the latter two terminals 1
5-2, 15-4, polarizing beam splitter 13-2.
The second optical rotation element 10-2, 10-4 is placed on the 13-4 side, and the first optical rotation element 9- is placed on the right angle prism 14-2, 14-43.
2.9-4 is placed.

This allows two optical circulator terminals 15-1.1
5-3, the polarization beam splitter 12 has a polarization state of 2
It outputs two linearly polarized light beams, and also combines two linearly polarized light beams in the 81M optical state inputted from the polarization beam splitter 12. In addition, the other two optical circulator terminals 15-2 and 15-4 conversely output a linearly polarized beam in the s-polarized state to the polarizing beam splitter 12,
Also, two linearly polarized beams in the plH optical state inputted from the polarizing beam splitter 12 are combined.

Furthermore, two optical circulator terminals 15-1 and 15-2
sandwich the polarizing beam splitter 12 and face it directly, and the other two
The two optical circulator terminals 15-3 and 15-4 are also placed directly opposite each other with the polarizing beam splitter 12 in between, and two optical circulator terminals 15-1 and 15-4 of another combination are placed on the same side of the polarizing beam splitter 12. The other two optical circulator units 5-2 and 15-3 are also arranged on the other side of the polarizing beam splitter 12.

With this arrangement, four optical circulator terminals 15-1
~15-4, the light from the terminal 15-4 is reflected and combined by the two polarization separation films of the polarizing beam splitter 12 to the terminal 15-J, and the light from the terminal 15-1 is transmitted to the terminal 15-2. The light passes through the two polarization separation films of the polarizing beam splitter 12 and is combined, and the light from the terminal 15-2 is reflected by the polarizing beam splitter 12 and combined into the terminal 15-3, and is transmitted to the terminal 5-4. The light from the terminal 15-3 passes through the polarizing beam splitter 12 and is combined.

Next, the operation as an optical circulator will be explained.

(1) When signal light enters the lens 15-1 from the outside: When the optical fiber t< 7-1, the light emitted from the lens 8-1
1 into a parallel light beam, and a polarizing beam splitter 13-
1, the vibration plane of the electric field is separated into linearly polarized light parallel to the plane of FIG. 1 (referred to as linearly polarized beam A) and linearly polarized light perpendicular to the plane of FIG. 1 (referred to as linear dimming beam B).

First, the linearly polarized beam A is polarized by the polarizing beam splitter 13-
1, it passes through the optical rotation element 9-1 (rotated by +45 degrees) and the Faraday element 1l-1 (rotated by -45 degrees) and enters the polarization beam splitter 12, so there is no rotation of the polarization direction after all. .

On the other hand, the linearly polarized beam B is polarized by the polarizing beam splitter 13-
1 and the right angle prism 14-1, the optical rotation element 1O
-1 (-45 degree rotation) and Faraday element 1l-1 (-4
5 degree rotation), the polarization direction is rotated by 90 degrees and the light beam enters the polarization beam splitter 12.

That is, at the terminal 15-1, both the linearly polarized beam B and the linearly polarized beam B enter the polarizing beam splitter 12 as p-polarized light, so they pass through the polarizing beam splitter 12 and enter the Faraday element 11 at the adjacent terminal 15-2. -2 is reached.

Then, the linearly polarized beam A is transmitted between the Faraday element 1l-2 (rotated by +45 degrees) at the terminal 15-2 and the optical rotation element 1O-2 (
-45 degree rotation), there is no rotation of the polarization direction, and p
The light enters the polarization beam splitter 13-2 in a polarized state.

On the other hand, the linearly polarized beam B is transmitted to the Faraday element 1l-2 (rotated by +45 degrees) and the optical rotation element 9-2 (
+45 degrees rotation), the polarization direction is rotated 90 degrees,
The light is then reflected by the right-angle prism 14-2 and enters the polarizing beam splitter 13-2 in an S-polarized state.

Therefore, the straight IIi light beam A and the straight IIi polarized beam B are combined by the polarizing beam splitter 13-2, focused by the lens 8-2, and coupled to the optical fiber 7-2.

(2) When signal light enters the terminal 15-2 from the outside: The light emitted from the optical fiber 7-2 is made into a parallel light beam by the lens 8-2, and the electric field is oscillated by the polarizing beam splitter 13-2. The light is separated into linearly polarized light whose surface is parallel to the paper (referred to as linearly polarized beam C) and linearly polarized light whose surface is perpendicular (referred to as linearly polarized beam D).

First, the linearly polarized beam C is polarized by the polarizing beam splitter 13-
2, passes through the optical rotation element 1O-2 (rotated by -45 degrees) and the Faraday element 1l-2 (rotated by -45 degrees),
Eventually, the polarization direction is rotated by 90 degrees and the light beam enters the polarization beam splitter 12.

On the other hand, the linearly polarized beam D is transmitted through the polarizing beam splitter 13-
2 and the right angle prism 14-2, the optical rotation element 9-
2f+45 degree rotation) and Faraday element (-45 degree rotation)
Since the light passes through and enters the polarization beam splitter 12, there is no rotation of the polarization direction after all.

That is, at the terminal 15-2, the M-ray scratch light beam CX[v! Both the i-polarized beam D is S-polarized and the polarized beam splitter 12
The light is reflected by the polarizing beam splitter 12 and reaches the Faraday element 11-3 at the adjacent terminal 15-3.

Then, the linearly polarized beam C is connected to the Faraday element 1l-3 (+45 degree rotation) at the terminal 15-3 and the optical rotation element 9-3 (+45 degree rotation).
45 degrees rotation), the polarization direction is rotated 90 degrees, and pl
! The light enters the polarizing beam splitter 13-3 in the state of light.

On the other hand, the linearly polarized beam D is transmitted through the Faraday element 1l-3 (
+45 degree rotation) and optical rotation element 1O-3 (-45 degree rotation), so there is no rotation of polarization direction (, right angle prism 14
After being reflected at -3, the light enters the polarizing beam splitter 13-3 in the state of 541i light.

Therefore, the linearly polarized beam C and the orthogonal S-polarized beam D are combined by the polarizing beam splitter 13-3, focused by the lens 8-3, and coupled to the optical fiber 7-3.

(3) When signal light enters the terminal 15-3 from the outside: For the light emitted from the optical fiber 7-3, see the previous section (
Considering the same way as the light incident from the optical fiber 7-1 of the terminal 15-1 explained in 1), it is coupled to the optical fiber 7-4 of the terminal 15-4.

(4) When signal light enters the terminal 15-4 from the outside: For the light that enters the optical fiber 7-4, see the previous section (
Similar to the light incident from the optical fiber 7-2 of the terminal 15-2 described in 2), by considering the same, the light is coupled to the optical fiber 7-1 of the terminal 15-1.

Therefore, between the four optical circulator terminals, the terminal 1s
-1 → terminal 15-2, terminal 15-2 → terminal 15-3, terminal 15-3 → terminal 15-4, terminal 15-4 → terminal 15-
An operation of an optical circulator is performed in which light is coupled to the light source.

Next, achieving high isolation will be explained. Here, a qualitative explanation will be given using the leakage of light from the terminal 15-1 to the terminal 15-4 as an example. In addition, for ease of understanding, optical leakage will be divided into two parts.

That is, the light incident on the polarization beam splitter 12 is not only p-polarized, but also depends on the extinction ratio of the polarization beam splitter 13-1 and the polarization directions of the optical rotation element 9-1, the optical rotation element 10-1, and the Faraday element 11-1. An amount of S-polarization is included that is related to the rotation angle error.

First, among the pli light with a large amount of light, the Faraday element 1
1-4 is the leakage of the polarization beam splitter 12, and since the configuration of the terminal 15-4 is the same as the configuration of the terminal 15-2, the leaked p-polarized light is transmitted to the optical fiber 7-4.
join to.

However, the Faraday element 11-1 of the terminal 15-1
The leakage of p-polarized light from the polarization beam splitter 12 to the Faraday element 11-4 at the terminal 15-4 is attenuated in the same way as a polarization filter consisting of two stages of ordinary polarization beam splitters connected in cascade. Compared to the conventional optical circulator shown in FIG. 5, deterioration of isolation due to leakage of p-polarized light is significantly improved.

On the other hand, most of the S-polarized light that enters the polarizing beam splitter 12 from the Faraday element 11-1 is reflected by the polarizing beam splitter 12 and enters the Faraday element 11-4.

Of this S-polarized light, the light that has passed through the Faraday element 1l-4 (rotated by +45 degrees) and the optical rotation element 1O-4 (rotated by -45 degrees) has its polarization direction not rotated, so most of it is sent to the polarizing beam splitter 13-4. is reflected. Furthermore, Faraday element 1l-4 (rotated by -+-45 degrees) and optical rotation element 9-4 (+
Since the polarization direction is rotated by 90 degrees, most of the light passes through the polarizing beam splitter 13-4.

Therefore, regarding the leakage caused by the S-polarized light incident on the polarization beam splitter 12 from the Faraday element 11-1,
The amount of light incident on the polarizing beam splitter 12 is as small as the leakage amount of a conventional optical circulator, and the amount of light coupled to the optical fiber 7-4 is filtered by the polarizing beam splitter 13-4, so the amount of light is very small. Since the amount of S-polarized light is even smaller, deterioration in isolation due to leakage of S-polarized light is also significantly improved compared to conventional optical circulators.

As described above, from the optical fiber 7-1 to the optical fiber 7-4
The isolation to the Faraday elements 11-1 and 1
1-4, optical rotation elements 9-1 and 9-4, or optical rotation element 1
Even if the rotational angle accuracy of the vibration plane at 0-1 and 10-4 is on the same level as the conventional one, the polarizing beam splitter 12.1
Even if the extinction ratios of 3-1 and 13-4 are at the same level as the conventional ones, they are significantly improved.

Furthermore, Faraday element 1.1-2, optical rotation elements 9-2 and 1
0-2, the right angle prism 14-2, the polarizing beam splitter 13-2, etc., the light emitted from the optical fiber 7-1 can be transmitted to the optical fiber 7-3. It is clear that there is no progress in this direction.

Similarly, the light emitted from the optical fiber 7-2 is not coupled to the optical fibers 7-1 and 7-4, and the light emitted from the optical fiber 7-2 is not coupled to the optical fibers 7-1 and 7-4.
The fact that the light emitted from -3 is not coupled to the optical fibers 7-1 and 7-2, and the light emitted from the optical fiber 7-4 is not coupled to the optical fibers 7-2 and 7-3, Easy to understand. Therefore, the optical circulator of the present invention can achieve high isolation.

Note that if the positions of the optical rotation element 9-i and the optical rotation element 10-1 in each optical circulator terminal 15-1 are reversed from the embodiment shown in FIG. becomes. In addition, polarizing beam splitter 1
3-1 and the right-angle prism 14-1, the same function can be obtained even if other polarization separation elements such as a Savart plate are used.

2 and 3 are a top view and a side view of another embodiment of the present invention, and the reference numerals in the figures indicate the same elements as in the embodiment shown in FIG. 1. As can be seen from a comparison between FIG. 1, FIG. 2, and FIG. 3, in this embodiment, the optical circulator terminal 15
-i is rotated 90 degrees. By this, the optical circulator terminal 15-1 and the optical circulator terminal 15-
4 or between the optical circulator terminal 15-2 and the optical circulator terminal 15-3, and the sides of the polarizing beam splitter 12 can be shortened, the distance between the optical fiber 7-1 and the optical fiber 7-1 can be reduced. -j (i≠J) can be shortened.

In the embodiment shown in FIGS. 2 and 3, the polarizing beam splitter 13-1 is rotated 90 degrees with respect to the embodiment shown in FIG. 1, so the operation is the same as in the embodiment shown in FIG. 15
-1→terminal 15-2, terminal 15-2→terminal 15-3, terminal 15-3 terminal) 5-4, terminal 15-4→terminal 15-
In order to obtain the operation of the optical circulator 1, the optical rotation element 9-
1 and the optical rotation element 10-1 are interchanged with those in the embodiment shown in FIG.

If the positions of the optical rotation element 9-i and the optical rotation element 10-1 are made the same as in the embodiment shown in FIG. 1, it is clear that the optical coupling becomes an optical circulator in the opposite direction to that explained in FIG. be.

FIG. 4 shows a six-terminal light-independent optical circulator as yet another embodiment of the present invention.

In FIG. 4, the polarization beam splitter 16 has three polarization separation films that intersect on one straight line at equal angles (60 degrees), and the polarization beam splitter 16 of the six films separated by this straight line ! Six optical circulator terminals 15-i (i = 1 to 6) are arranged facing each membrane piece.

Each optical circulator terminal 15-1 has an optical fiber 7-i, a lens 8-1, a first optical rotation element 9-i, and the like in each of the embodiments shown in FIGS. 1, 2, and 3. , a second optical rotation element 10-i, a Faraday element 11-1, a polarizing beam splitter 13-i, and a right-angle prism 14-1.

The operation of the optical circulator and the principle of the high isolation ring are the same as in the embodiment shown in FIG. 1, and in this embodiment as well,
The two linearly polarized beams respectively emitted from the optical circulator terminals 15-1.15-3.15-5 are in the p-polarized state, and the optical circulator terminals 15-2.15-4,
Assuming that the two linearly polarized beams respectively emitted from l5-6 are in the S polarization state, terminal 15-1→terminal 15
-2, terminal 15-2 → terminal 15-3, terminal 15-3 → terminal 15-4, terminal 15-4 → terminal 15-5, terminal 15-
5→terminal 15-6 and terminal 15-6→terminal 15-1 as an optical circulator. It is also clear that if the linearly polarized beam emitted from the optical circulator terminal 15-1 is in a polarization state opposite to that described above, the optical circulator becomes an optical circulator in which the coupling of light is polarized in the opposite manner to that described above.

If the present invention is applied as in the embodiment shown in FIG. 4, an optical circulator with four or more terminals can be realized with low loss and high isolation, unlike the conventional example.

In short, in the case of a polarization-independent optical circulator in which the number of terminals through which light is input and output is 2 (N + 1), where N is a natural number, (al (N + 1) polarization separation films are arranged at equal angles). Polarizing beam splitter (1
2, 16), (bl Optical fiber (7), lens (8), polarization separation element (13, 14), optical rotation element (9) that rotates the direction of linearly polarized light by ±45 degrees, arranged in parallel with this It consists of an optical rotation element (10) that rotates the direction of linearly polarized light by 145 degrees, and a Faraday element (11) that rotates the direction of linearly polarized light by ±45 degrees, and converts the unpolarized light input from the optical fiber (7) into the polarized beam. The splitter (12° 16) outputs two linearly polarized beams in the p-polarized state, or the two linearly polarized beams in the s-polarized state inputted from the polarizing beam splitter (12, 16) are sent to the optical fiber (7). ) of (N+1) optical circulator terminals (15-1°15-3.15-5 ・15-2N+
1) and (C1 optical fiber (7) lens (8),
Ii optical separation element (13, 14), the direction of linearly polarized light is ±
An optical rotation element (9) that rotates by 45 degrees, and an optical rotation element (10) that rotates the direction of linearly polarized light by 145 degrees, which is arranged in parallel with this element.
, a Faraday element (
11), which inputs unpolarized light from the optical fiber (7) to the polarizing beam splitter (12°16).
output as two linearly polarized beams in the state of IliB light, or use the polarizing beam splitter (12, 16
) of group B which combines two linearly polarized beams in the p-polarized state input from the optical fiber (7) and outputs it.
) optical circuit, L, -ta terminals (15-2, 15-4,
15-6...15-2 (N+1)) constitute an optical circulator.

Then, polarizing beam splitters (12, 16) and optical circulator terminals (15-1, 15-3) of group A.

15-5...) and group B optical circulator terminal (1
5-2, 15-4, 15-6...), the positional relationship between them is (dl 1st optical circulator terminal in B group (1
5-2) is the first optical circulator terminal in group A (
The light from the polarizing beam splitter (12, 1)
6) at the position where the two polarization separation films are transmitted and combined, and (ej) the second optical circulator terminal in group A (1
5-3) is the first optical circulator terminal in group B (
The light from the polarizing beam splitter (12, 1
6) at the position where it is reflected and combined by the two polarization separation films, and the second optical circulator terminal (1) in the ifl B group.
5-4) is the second optical circulator terminal in group A (
The light from the polarizing beam splitter (12, 1
The light from the optical circulator terminal in group A passes through the polarizing beam splitter and is coupled to the optical circulator terminal in group B in the following order: The arrangement is such that the light from the optical circulator terminal in group B is reflected by the two polarization separation films of the polarizing beam splitter (12, 16) and coupled to the optical circulator terminal in group A. In this case, the first optical circulator terminal (15-1) in group A converts the light from the (N-1-1)th optical circulator terminal (15-2 (N+1)) in group B into a polarized beam. The two polarization separation films of the splitter (12, 16) are arranged at positions where the light is reflected and combined.

However, the arrangement of each terminal within the optical circulator terminal (15-i) is as follows: +il optical fiber (7-i), lens (8-i)
i), polarization separation element (13-i, 14-i), parallel optical rotation element (9i, 1O-1), Faraday element (1
1-λ), (jl optical fiber (7-i), lens (8-i
) Polarization separation element (U3-j, 14-i), Faraday element (11-i), parallel optical rotation element (9-i, 10i)
It may be in this order.

<Effects of the Invention> As explained above, in the present invention, each terminal of the optical circulator has a Faraday element and an optical rotation element, and also has a polarization beam splitter and an optical beam output for polarization independence. By separating the polarizing beam splitter and the polarizing beam splitter for determining the direction, it is possible to reduce the influence of the rotation angle accuracy of the vibration plane of the Faraday element or optical rotation element and the extinction ratio of the polarizing beam splitter on the isolation calculation of the conventional optical circulator. This can be reduced in some cases. As a result, a polarization-independent optical circulator with low loss and high isolation can be realized. Another advantage is that an optical circulator with four or more terminals can be realized.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an embodiment of the polarization-independent optical circulator of the present invention, Fig. 2 is a top view showing another embodiment of the polarization-independent optical circulator of the invention, and Fig. 3 is a side view of Fig. 2. 4 are diagrams showing an example of the configuration of the optical circulator of the present invention. is an optical rotation element, 10-i is an optical rotation element, 11-1 is a Faraday element, 12 is a polarizing beam splitter, 13-1 is a polarizing beam splitter, 14-i is a right angle prism, 15-i
is an optical circulator terminal, and 16 is a polarizing beam splitter. Figure 2 Example (top view) Figure 3 Example (side view) Figure 4 Example

Claims (1)

[Claims] The number of optical circulator terminals is 2 (N+
1) In each optical circulator, there is a polarization beam splitter in which N+1 polarization separation films intersect at equal angles, an optical fiber, a lens coupled to this optical fiber, a polarization separation element coupled to this lens, and a straight line. A first optical rotation element that rotates the direction of polarized light by +45 degrees, and a second optical rotation element that is arranged in parallel with the first optical rotation element and rotates the direction of linearly polarized light by -45 degrees.
and a Faraday element which rotates the direction of linearly polarized light by ±45 degrees and is coupled to a polarization separation element by sandwiching the first and second optical rotation elements or by sandwiching the first and second optical rotation elements. The unpolarized light input from the optical fiber is output to the polarizing beam splitter as two linearly polarized beams in the p-polarized state of the polarizing beam splitter, and the two linearly polarized beams in the s-polarized state input from the polarizing beam splitter are A group of N+1 optical circulator terminals that combine and output linearly polarized beams, an optical fiber, a lens coupled to this optical fiber, a polarization separation element coupled to this lens, and a direction of linearly polarized light set to +45. a first optical rotation element that rotates by 45 degrees, and a second optical rotation element that rotates the direction of linearly polarized light by -45 degrees, which is arranged in parallel to the first optical rotation element.
and a Faraday element which rotates the direction of linearly polarized light by ±45 degrees and is coupled to a polarization separation element by sandwiching the first and second optical rotation elements or by sandwiching the first and second optical rotation elements. The unpolarized light input from the optical fiber is outputted to the polarizing beam splitter as two linearly polarized beams in the s-polarized state of the polarizing beam splitter, and the unpolarized light input from the polarizing beam splitter is outputted as two linearly polarized beams in the p-polarized state. It is equipped with N+1 optical circulator terminals of group B that combine and output linearly polarized beams, and the arrangement relationship between the optical circulator terminals of groups A and B and the polarization beam splitter is different from that of each optical circulator in group A. The light from one separate optical circulator terminal of group B is reflected and combined by the two polarization separation films of the polarizing beam splitter, and the light from one separate optical circulator terminal of group B is connected to each optical circulator terminal of group B. An optical circulator characterized in that light from the optical circulator is transmitted through two polarization separation films of a polarization beam splitter and combined.