JPS62103617A - Optical circulator - Google Patents

Abstract

PURPOSE:To make halfwave plates and mirrors unnecessary to reduce the cost by using fiber type polarizing beam splitter. CONSTITUTION:Lenses 36-a and 36-c and a Farady element 4-a are interposed between ends 33-a and 34-a of the fiber type polarizing beam splitter and principal axes of polarization of these ends are inclined at 45 deg. to each other. Ends 33-b and 34-b are provided similarly. Thus, the function of a halfwave plate is fulfilled by adjustment of principal axes of polarization, and the function as an optical circulator independent of polarization is the same as a conventional circulator.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a fiber-type optical circulator that is small in size, has a large eaves, and does not depend on the polarization direction of incident light.

(Prior Art) FIG. 7 is a block diagram of a conventional optical circulator that is configured between optical fibers and is independent of the direction of incident polarization.

It is shown in FIG. FIG. 7 shows a case where light is incident from the optical fiber 1-a, and FIG. 8 shows a case where light is incident from the optical fiber 1-c. Polarized light that oscillates in the direction perpendicular to the plane of the paper (
), and polarized light that vibrates in parallel directions is represented by (→). In FIG. 7, the light emitted from the optical fiber 1-a is collimated by the lens 2-a and enters the bulk polarization beam splitter 3-aK. The bulk polarization beam splitter separates incident light into polarized light perpendicular to the plane of the paper and polarized light parallel to the plane of the paper. One of the separated lights is directly reflected, and the other is reflected by a mirror and then enters Faraday elements 4-a and 4-b, both of which are rotated clockwise by 45 degrees, and are rotated by 1/ Two-wavelength plate 6-a.

6-b, both are rotated clockwise by 45 degrees. As a result, the two polarized lights are each rotated 90° clockwise with respect to the polarization direction before entering the Faraday element. Therefore, one side is reflected by the mirror 7-a, and the other side is directly incident on the bulk polarization beam splitter 3-b, which is combined and focused into the optical fiber 1-C by the lens 2-c. 1-C. In Figure 8, optical fiber 1-c
The light incident from the bulk polarization beam splitter 3-
b nyop1 separated into two orthogonal polarized waves, each l
After being rotated by 45° clockwise by the /2 wavelength plates 6-a and 6-b, the farade elements 4-a and 4-b are rotated by 45° counterclockwise. Therefore, nine beams are combined by the bulk polarization beam splitter 3-a, and in this case, the beams are output to the optical fiber 1-b side.

(Problem to be solved by the invention) In this way, although it functions as a circulator for light with any polarization component, the bulk polarization beam splitter uses a multilayer film whose thickness is precisely adjusted. Because it is used, it is extremely expensive, 1/! The optical circulator itself, including the wave plate and the like, is extremely expensive. Furthermore, since there are many components other than optical fibers, there are problems in that adjustment is difficult when considering coupling with optical fibers, and stability is lacking even after installation.

The present invention is intended to improve the above-mentioned problems, and its purpose is to reduce the number of parts, improve the problem of stability of coupling with optical fibers, and provide a small, lightweight, and low-cost optical circulator. .

(Means for Solving the Problems) A feature of the present invention is that the cutoff wavelength λ of the higher-order mode is set relative to the used wavelength λ. Two birefringent optical fibers with a relationship of 1.3<λ/2°<2.1 are arranged in parallel so that their principal axes are parallel, and a part of the longitudinal direction is integrated to form an optical coupling section. The coupling part has a coupling length that separates the light incident from one port into polarization components in two orthogonal directions when the light passes therethrough, and the coupling part has first and second fiber-shaped polarization components facing each other. Between the wave beam splitter and the fiber-type polarized beam splitter, there are two sets of means for converting the light emitted from the splitter into parallel light and converging it into the fiber again, and means for imparting Faraday rotation to the parallel light. The main polarization axes of the polarization beam splitter boats facing each other are approximately 4
It is located in an optical circulator arranged at an angle of 5°.

According to the invention, the polarization beam splitter is constituted by optical fibers, and means are provided for imparting Faraday rotation between them.

Additionally, there is no need for a 1/2 wavelength plate or mirror, and the number of parts other than the optical fiber is smaller than in conventional configurations, resulting in an optical circulator with excellent coupling stability with the optical fiber and no need for expensive optical components. .

(Embodiment) FIG. 1 is a schematic diagram of a first embodiment of the present invention. In the same figure, the same parts as those in FIG. ing. FIG. 4 shows a stressed birefringent optical fiber. 61 is the core, 62 is the cladding.

63 is a stress applying section. The stress applying part 63 is the cladding 6
The core 61 is made of glass having a coefficient of thermal expansion greater than 2. Therefore, tensile stress exists in the core 61, and birefringence is induced in the core 61 due to the photoelastic effect. As a result, the propagation constants of light polarized in the X-axis direction and the y-axis direction are β
Assuming β7, the mode birefringence B is: B=(βX-βy)/k (
1) is given. However, =2π/λ, λ is the wavelength of the light used. Here, if B>I×10, linear polarization is sufficiently maintained.

Fiber type polarization beam splitter 31-a, 31-
b is the cutoff wavelength λ of the higher-order mode with respect to the used wavelength λ, as shown in FIG. Two stressed 4-type birefringent optical fibers 72 and 73 having a ratio of 1, 3 and λ/λc of 2.1 are arranged in parallel, and a portion of the longitudinal direction is fused and stretched. At this time, in the fused and stretched portion 71, the stress applying portions are arranged in parallel as shown in the cross-sectional view 74. This is to maintain polarization also in the fused and stretched portion 71. The fused and stretched portion 71 has a tapered shape, and since the cores are close to each other, optical coupling occurs. The electric field distribution in this region is
It can be expressed as a superposition of the lowest-order even mode, which has no zero point in the electric field distribution, and the odd mode, which has one zero point in the field distribution. The propagation constants of even mode and odd mode of light polarized in the X-axis direction are approximately: n r /'odd + '/
The propagation constants of even mode and odd mode of light polarized in the axial direction are Av□, 1. dd, when a stress-applied birefringent optical fiber is used as shown in Fig. 5, approximately. . 7odd'F/'aven #odd
...(2). It is possible to meet or satisfy this requirement. Here, m and n are integers, the integration is performed in the fusion-stretching region, and Z is the longitudinal direction.

The propagation constant of each varies in the longitudinal direction due to the tapered shape. Therefore, it is expressed as a function of Z. When the condition of equation (3) or equation (4) is satisfied, for example, when light is input through the input end 72 in FIG. 73a
It is possible to separate the polarized waves. At that time, if the cutoff wavelength of the higher-order mode of the birefringent optical fiber used is 1.3<'/'c<2.1 with respect to the wavelength used, the loss is 1.
0 dB or less, and crosstalk, which indicates polarization maintaining ability, is also less than -15 dB, showing good characteristics. The fiber type polarization beam splitter used in this example has a Δ: 0.3% and a λc
= O,S Oμm stressed birefringent optical fiber, excess loss 0.5 dB, crosstalk -23d
It is B. When light is incident from 72 in FIG. 7, the ratio of the light intensity at the two output ends 72a and 73a to the incident polarization angle θ is shown in FIG. 1% from 72a when θ=0, that is, incident polarization in the X-axis direction.

99% is emitted from 73a, and when polarized in the X-axis direction, 7
99% is emitted from 2a and 1% is emitted from 73a. It can be seen that this functions as a polarization beam splitter. In FIG. 1, 32-a, 32-b, 3
3-a, 33-b, 34-a, 34-b, 35
-a, 35-b are the ends of the fiber type polarized beam splitter, 36-a, 36-b, 36-c
, 36-d are L/7's.

Here, the end 33-a of the fiber-type polarization beam splitter
and 34-a, with lenses 36-a, 36-c, and Faraday element 4-a sandwiched between them, and the principal axes of polarization are tilted at 45° with respect to each other, as shown in FIG. The same applies to 33-b and 34-b. As a result, as shown in FIG. 7, the function of rotating the plane of polarization by 45 degrees can be achieved by the conventional 1/2 wavelength plate 6-a, eliminating the need for a 1/2 wavelength plate.

As can be seen from the above, the conventional bulk polarization beam splitter 3-a shown in FIG.

3-b and the fiber-type polarization beam splitter 31 in FIG.
-a and 31-b have similar functions, and by adjusting the polarization principal axis as shown in Figure 3, they can function as the 1h wavelength plate in Figure 7, and can be used as polarization-independent optical circulators. The function is exactly the same as in FIG. In this example, the insertion loss including the connection loss with the optical fiber is 2.
．． 5 dB, when light in the opposite direction, that is, light incident from the optical fiber 1-c, enters the optical fiber 1-a, the ratio of the light intensity PI-a entering the optical fiber 1-a and the light intensity PL-b entering the optical fiber 1-b, that is, the light Isolation IOA? ogPI-b/PI-a was 20dB.

FIG. 6 shows an embodiment in which the Farade elements made of YIG are replaced with ball lenses 37-a and 37-b in order to reduce the number of parts. Compared to the configuration shown in Figure 1, the lens 3
6-a, 36-b, 36-c, and 36-d are omitted. The focal length of a spherical lens is 0.0 at a wavelength of 1°3 μm.
7 mm, and both sides of the spherical lens are coated with anti-reflection coatings. The characteristics of the optical circulator are insertion loss 2
．． 9 dB, and the optical isolation was 20 dB. Components other than optical fiber are YIG ball lens 37
-a and 37-b, reliability with respect to earthquake resistance and temperature resistance is improved.

The diameter of the YIG ball lens is 2.1 mm, and the Y of the incident light is
The beam diameter in the IG sphere was 140 μmφ.

(Effects of the Invention) As explained above, the optical circulator of the present invention uses a fiber-type polarized beam splitter instead of a bulk-type polarized beam splitter, and eliminates the conventional 1/2 wavelength plate and mirror. Made unnecessary. This has the following effects.

(1) Since a 1/2 wavelength plate and mirror are not required, the cost can be reduced.

(2) Since the number of parts is reduced, the optical circulator can be easily assembled and downsized.

(3) Since the number of parts other than the optical fiber is reduced, earthquake resistance and temperature resistance are improved.

[Brief explanation of drawings]

FIGS. 1 and 2 are configuration examples of the optical circulator of the present invention, FIG. 3 is a diagram showing the positional relationship of the polarization principal axes at the ends of a fiber-type polarization beam splitter sandwiching a Faraday element, and FIG.
The figure is a cross-sectional view of a stressed ear-shaped birefringent optical fiber, Figure 5 is a configuration diagram of a fiber-type polarization beam splitter, Figure 6 is a diagram showing the dependence of the polarization beam splitter on the incident polarization angle, and Figure 7 and FIG. 8 are block diagrams of a conventional optical circulator. 1-a, 1-b, 1-c-optical fiber, 2-a. 2-b, 2-c--lens, 3-a, 3-b/' bulk polarization beam splitter, 4-a, 4-b... Farade element, 5... magnet, 5- a, 6-b...1/
Two-wavelength plate, 7-a, 7-b-...Mirror, 31-a, 3
1-b...Fiber type polarizing beam splitter, 32-
a, 32-b, 33-a, 33-b, 34-a, 34-
b, 35-a. 35-b... end of fiber polarization beam splitter,
36-a, 36-b, 36-c, 3
6-d.-lens, 37-a, 37-b...YIG
Ball lens, 61... Core, 62... Clad, 63
... Stress applying part, 71... Fusion stretching part, 72°72
a, 73, 73a... End of the fiber type polarization beam splitter, 74... Cross-sectional view of the fused extension part.

Claims (2)

[Claims] (1) Cutoff wavelength λ of higher-order mode with respect to usage wavelength λ
Two birefringent optical fibers in which c is 1.3<λ/λ_c<2.1 are arranged in parallel so that their principal axes are parallel, and a part of the longitudinal direction is integrated to form an optical coupling section. form,
The coupling section includes first and second fiber-type polarization beam splitters that face each other and have a coupling length that separates the light incident from one port into polarization components in two orthogonal directions when the light passes therethrough. , between the fiber-type polarized beam splitter, there are two sets of means for converting the light emitted from the splitter into parallel light and converging it into a fiber again, and means for imparting Faraday rotation to the parallel light, and the above-mentioned opposing fiber type An optical circulator characterized in that the polarization main axes of the ports of the polarization beam splitter are arranged at approximately 45 degrees from each other. (2) The means for turning the light beam into parallel light and converging it into a fiber again, and the means for imparting Faraday rotation to the parallel light are constituted by a Faraday element having a lens function. The optical circulator according to item 1.