JP2516463B2 - Polarization-independent optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is for bidirectional non-polarized light, which does not affect the polarization direction, for blocking the reflected return light of an optical system in optical communication and optical measurement using a semiconductor laser. Dependent optical isolator.

[Prior Art and Problems] As a polarization-independent optical isolator used for bidirectional optical communication, optical measurement, etc., there is an optical circulator shown in FIG. 2, which includes two polarization beam splitters 15, 16 and two total reflections. It is composed of mirrors 17, 18, Faraday rotator 19, and optical rotator 20, and has four ports of 11, 12, 13, and 14 as light beam entrances, and the incident light from 11 port is emitted to 12 port, The incident light from the port does not return to the 11th port because it is emitted to the 13th port. Similarly, when incident from 14 ports, 11
It is emitted to the port, and when it is incident from the 13th port, it is emitted to the 14th port, and this relation is not related to the polarization plane of light. When this is used as a bidirectional polarization-independent optical isolator,
Although it will be used as a port of 11 → 12 and 13 → 14,
In order to connect the optical fiber cable as a whole, the light input / output ports are rotated by 90 °, which complicates fiber connection in a large system. In addition, a high-performance dielectric multilayer film such as a polarization beam splitter and a total reflection mirror is required, which makes the entire system expensive. In terms of performance, since the Faraday rotator and the optical rotator are integrated together, the elliptical component reverts as it is when there is a temperature change or wavelength change, which causes deterioration of the extinction characteristic.

In view of this point, an object of the present invention is to provide a polarization-independent optical isolator which has a technically simple structure and which can be used bidirectionally with high productivity.

[Means for Solving the Problems] In the present invention, two birefringent crystal plates whose crystal optical axes are inclined with respect to the optical axis, two Faraday rotators for rotating the polarization plane by 45 °, polarizing glass, etc. Polarization direction is 90 with adjacent polarizing plate
° Divided into three different parts, and composed of a three-part polarizing plate and two permanent magnets for magnetizing the Faraday rotator, which are arranged on the same plane perpendicular to the ray axis. If this is inserted between two optical fibers arranged in parallel and optically coupled by a lens or the like, one of the optical fibers has a forward optical path and the other optical fiber has a reverse optical path. The reflected return light generated in the optical path is blocked as a polarization independent optical isolator having a high extinction characteristic. By making the magnetization directions of the two Faraday rotators face each other, the temperature between the two Faraday rotators is
Since the wavelength characteristics are mutually compensated, an optical isolator having a wide band temperature and a wide band wavelength can be realized.

FIG. 1 shows an example of the configuration of the present invention, and there is a three-divided polarizing plate 5 as shown in FIG. 3, which is arranged so that the polarizing direction differs from that of the adjacent polarizing plate by 90 °. The material is preferably thin such as polarizing glass, and it is necessary to shorten the distance between the lenses as much as possible in consideration of the coupling between the optical fibers in order to reduce the coupling loss. Also PB
In a structure such as S or Rochon prism that separates different kinds of polarized light, the light after separation becomes a problem, and therefore an absorption type polarizing plate is preferable. Recently, there are polarizing glasses, polarizing films, polarizing chips having a laminated structure of metal dielectrics, and the like, which can be selected according to the characteristics.

The birefringent crystal plates 6 and 7 include rutile and calcite in consideration of price and stability, and in recent years, rutile (TiO ₂ ) single crystal, which is a synthetic body and has sufficient crystal characteristics, is desirable.
When the crystal optical axis is tilted by about 43 ° with respect to the ray direction, the maximum separation effect is obtained, but if the optical path length is l, then l / 10
The separation width is about the same.

Faraday rotators 8 and 9 are created by the FZ method Y
IG single crystal, Bi-substituted rare earth iron garnet prepared by LPE method is used. LP considering system miniaturization
EIG is superior to Y garnet, but YIG is preferable for those with small temperature dependence. A permanent magnet for magnetizing the Faraday rotator requires a magnetic field of about 2KG, and an SmCo magnet with little temperature change is preferable.

Next, regarding the behavior of light rays, when unpolarized light enters from the port 1 in FIG. 1 (a), it is separated into ordinary rays and extraordinary rays by the first birefringent crystal plate 6 and the first Faraday rotation. The plane of polarization is rotated by 45 ° by the child 8, and both the ordinary ray and the extraordinary ray are distributed to the three-divided polarizing plates 5-1 and 5-2 in FIG. At this time, the direction of the three-division polarizing plate is adjusted so that the maximum transmission is obtained in the state of being rotated through 45 ° through the port 1. After transmission, it enters the second Faraday rotator 9 with the opposite magnetization direction, returns 45 °, and becomes the original polarization state.
Enter the second birefringent crystal plate 7. When the second birefringent crystal plate 7 is arranged in mirror symmetry with respect to the first birefringent crystal plate 6, both the ordinary ray and the extraordinary ray converge on the port 2 and go from the port 1 to the port 2 regardless of polarization. Be combined. On the other hand, for the return light from the port 2 to the port 1, the light travels in the same manner up to the second Faraday rotator as shown in (b), but in the second Faraday rotator, the second direction is the same as the forward direction. The ordinary rays and extraordinary rays separated by the birefringent crystal plate of are rotated by 45 ° respectively, so that the ordinary rays are
At 5-1, the extraordinary ray is absorbed and extinguished at 5-2 of the three-division polarizing plate.

Next, regarding ports 4 to 3, the polarization directions of the polarizing plates are reversed from those of ports 1 to 2, so that ports 4 to 3 are in the forward direction and ports 3 to 4 are in the reverse direction, and the light is extinguished. It As a result, a bidirectional polarization-independent optical isolator is formed.

Example A bidirectional polarization-independent optical isolator according to the present invention was formed according to the material specifications shown in Table 1. The third split polarizing plate is the third
Figure 5-1; 1 x 1 mm, 5-2; 2 x 1 mm, 5-3; 1 x 1 mm processed into one. If the polarization direction is arranged as shown in the figure, for the ordinary ray and the extraordinary ray rotated by 45 ° by the first Faraday rotator, the forward ray is transmitted and the backward ray is absorbed and the isolation effect is obtained. can get. A rod lens and a single-mode optical fiber are placed in each port.

The rod lens is adjusted at 0.25 pitch so that it propagates almost as collimated light between the ports. The beam diameter is about 0.4 mmφ in the middle of the port, and the coupling loss is about 0.3 dB at 30 mm intervals without optical components. The birefringent crystal plate is cut out at about 43 ° to the plane perpendicular to the optical axis to obtain a rectangular parallelepiped, and further processed by inclining it at about 7 ° to the plane perpendicular to the optical axis, and the near end to the optical fiber on the incident side. High extinction characteristics can be obtained by tilting the incident surface of the birefringent crystal plate in consideration of reflection. Although rutile single crystal is used in this embodiment, any stable substance having birefringence may be used. In the case of rutile, the separation width between ordinary and extraordinary rays is given by about 1/10 of the crystal length in the ray direction. Therefore, a separation of 1.2 mm (1200 μm) can be obtained. When the ray incident from the port 1 passes through the first birefringent crystal plate, the extraordinary ray is separated to the upper side. If the beam diameter is about 400 μm, the separation between the ordinary ray and the extraordinary ray is 800 μm, and they are sufficiently separated even considering the spread of the Gaussian beam, and the leaked light can be ignored. The first Faraday rotator rotates 45 ° in the direction according to the magnetization direction, transmits the different polarization planes of the split polarizing plate, and converges again on the port 2. Considering the processing accuracy and non-uniformity of the anti-reflection film, the boundary of the polarizing plate is approximately
Since a 0.4 mm width cannot be expected to have a sufficient polarization effect, it is necessary to adjust the optical path length of rutile to make the separation width wider. In this case, the distance between the lenses is 30 mm, but the effective optical path length is rutile length lR, Faraday rotator thickness tF, polarizing plate thickness tP, and air gap gap g (about 4 mm in this embodiment), Let the respective refractive indices be nR, nF, nP, ng, However, since nR = about 2.6, nF = about 2.3, nP = about 1.5, ng = 1, the effective optical path length is about 14 mm and the spread of the Gaussian beam emitted from the lens inserts an optical isolator component. It is half that when not using it, and it is possible to utilize a beam with a sufficiently small luminous flux, and the structure is such that there is little loss. As a measured value, the optical loss at both ends of the optical fiber was 1.3 dB, and the extinction characteristic was 40 dB or more. Moreover, in the case of the magnetic field facing structure, extinction characteristics of 35 dB or more were obtained over a wide range with respect to temperature and wavelength fluctuations.

[Advantages of the Invention] The present invention is a polarization-independent optical isolator that can be used bidirectionally at the same time, and not only has the economic advantage of reducing the cost of optical components, but also integrates two optical isolators. Therefore, it can contribute to the miniaturization of the system.

[Brief description of drawings]

 FIG. 1 is a block diagram showing an embodiment of the present invention. FIG. 2 is a block diagram of a conventional optical circulator. FIG. 3 is a configuration diagram of the three-division polarizing plate in the present invention. 1,2,3,4: Port, 5: Three-division polarizing plate 6,7: Birefringent crystal plate, 8,9: Faraday rotator

Claims (4)

(57) [Claims] 1. A first birefringent crystal plate whose crystal optical axis is tilted with respect to the optical axis, a first Faraday rotator for rotating a plane of polarization by 45 °, and a polarizing plate whose polarization direction is adjacent to 90 °. Divided into three different parts and arranged in the same plane perpendicular to the ray axis, the three-part polarizing plate, the second Faraday rotator, and the first birefringent crystal plate, and the crystal optical axis is mirror-symmetric. A bidirectional polarization-independent optical isolator comprising a second birefringent crystal plate and first and second permanent magnets for magnetizing the first and second Faraday rotators. 2. The bidirectional polarization-independent optical isolator according to claim 1, wherein a flat polarizing glass is used as the three-division polarizing plate. 3. The bidirectional polarization-independent optical isolator according to claim 1, wherein the magnetization directions of the first and second permanent magnets are opposite to each other. 4. A bidirectional polarization-independent optical isolator according to claim 1, wherein the entrance and exit surfaces of the ray axes of the first and second birefringent crystal plates are inclined.