JPH08171075A - Optical switch - Google Patents

Abstract

PURPOSE: To suppress the polarization dependency of polarized wave dispersion and an insertion loss and to improve the insertion loss and the cut-off characteristic. CONSTITUTION: An input fiber 10, a first lens 12, a polarizer 14, a 45 deg. Faraday rotator 16, an analyzer 18, a second lens 20 and an output fiber 22 are provided on the optical axis and an electromagnet 24 bi-directionally magnetizing the Faraday rotator 16 is provided. The polarizer 14 and the analyzer 18 are composed of the same wedge type birefringence plates, the tapered directions are mutually different by 180 deg., the opposed surfaces are parallel, the respective optical axes are parallel and made to be 45 deg. viewing from the optical axis. A compensation plate 30 composed of birefringence parallel plates is inserted on the optical axis between the first and the second lenses. The optical axis of the compensation plate 30 and the optical axis of the adjacent polarizer or the analyzer make 90 deg. viewing from the optical axis, the compensator is inclined to the optical axis so as to make the shifting amount of beams match each other and cancel the delay and the optical axis of the compensation plate 30 is designed so as to be oblique to the plane.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical switch in which a wedge-shaped birefringent plate and a Faraday rotator are combined, and the transmission and blocking of light is controlled by reversing the direction of a magnetic field applied to the Faraday rotator by an electromagnet. It is a thing. More specifically, according to the present invention, by inserting a compensating plate made of a birefringent parallel flat plate, the beam shift amounts of the ordinary light and the extraordinary light generated in the wedge-shaped birefringent plate are made equal to each other, and the delay due to the difference in propagation velocity is offset. The present invention relates to a polarization-independent optical switch. This optical switch is, for example, a long distance,
It is useful in fields such as high-speed, large-capacity optical communication.

[0002]

2. Description of the Related Art In an optical communication system and an optical measuring device, an optical switch for spatially switching the traveling direction of light is required. An example of a polarization-independent 1 × 1 type optical switch is disclosed in Japanese Examined Patent Publication No. 4-2934.

An optical switch of this type has a structure as shown in FIG. 6, for example. That is, the input fiber 10 and the first beam for collimating the light emitted from the input fiber 10
Lens 12 and a polarizer 14 composed of a wedge-shaped birefringent plate,
The Faraday rotator 15 that performs 90-degree polarization rotation, the analyzer 18 that is a wedge-shaped birefringent plate, the second lens 20 that focuses the parallel beam on the output fiber 22, and the output fiber 22 are arranged in this order in the order of the optical axes. It is installed on top. An electromagnet 23 that magnetizes the Faraday rotator 15 in a predetermined direction is arranged near the Faraday rotator 15. The polarizer 14
And the analyzer 18 are made of the same birefringent material, have the same taper angle and different taper directions by 180 degrees, the surfaces facing each other are parallel to each other, and the respective optical axes are parallel and viewed from the optical axis. There is a relationship that makes sense.

The ordinary light o and the extraordinary light e are separated by the polarizer 14, but in the non-biased state (see FIG. 6A), the ordinary light o and the extraordinary light e are parallel because the analyzer 18 maintains the same polarization state. And is focused on the output fiber 22 by the second lens 20. This is the ON state. In a state in which a current is supplied to the electromagnet and a magnetic field H is applied (see B in FIG. 6), the Faraday rotator 15 rotates the polarization plane by 90 degrees, so that the ordinary light o and the extraordinary light e are reversed in the analyzer 18.
Since the ordinary light o and the extraordinary light e spread further, they cannot be condensed on the output fiber 22 even with the second lens 20. This is the off state. In this way, the electromagnet 23 can perform switching control of light transmission / blocking.

[0005]

By the way, the birefringent plate is
Since there is a difference in refractive index between the polarized light having a vibration direction parallel to the optical axis and the polarized light having a vibration direction perpendicular to the optical axis, the ordinary light o and the extraordinary light e have different refraction angles. Inputting causes polarization dispersion and beam shift. In the case of a negative uniaxial crystal, since the refractive index of extraordinary light is larger than that of ordinary light, the shift amount is large with respect to the input light and the propagation speed is slow, as shown in FIG. Therefore, the beam shift distance s and the delay δ of the two light rays are large, and the polarization dependency occurs in the insertion loss when performing the fiber collimate coupling in the lens system.

When a magnetic field can be applied to the Faraday rotator by a permanent magnet like an optical isolator, the permanent magnet can be made small, so that the distance between the polarizer and the analyzer can be narrowed,
The amount of beam shift between ordinary light and extraordinary light is small. In this way, the two light beams having a small shift amount can be condensed on the fiber without loss by using a lens having a small wavefront aberration. However, in the case of the optical switch, it is necessary to use the electromagnet 24 for switching as shown in FIG. 8, and in order to efficiently utilize the magnetomotive force, the gap g is reduced and only the Faraday rotator 15 is inserted. I am trying to do it. Therefore, the polarizer 14 and the analyzer 18 are arranged outside the yoke 25, and the distance L between the polarizer 14 and the analyzer 18 becomes considerably long (specifically, L = about 10 mm). Reference numeral 27 represents a coil. Therefore, the beam shift distance s between the ordinary ray and the extraordinary ray becomes large, and a general-purpose lens cannot focus light on the fiber without loss due to the influence of aberration or the like.
The condensing ratio of ordinary light and extraordinary light appears as polarization dependence.

Further, the Faraday rotator has a problem that the extinction ratio is deteriorated in a state where a magnetic field is not applied (in the above-described example, a non-biased ON state), and the insertion loss or the cutoff characteristic is deteriorated.

An object of the present invention is to provide an optical switch that suppresses polarization dependence of polarization dispersion and insertion loss and improves insertion loss and cutoff characteristics.

[0009]

According to the present invention, an input fiber, a first lens for collimating light emitted from the input fiber into a parallel beam, a polarizer comprising a wedge-shaped birefringent plate, and 4 are provided.
A Faraday rotator that rotates polarization by 5 degrees, an analyzer composed of a wedge-shaped birefringent plate, a second lens that focuses a parallel beam on an output fiber, and an output fiber are installed in this order on the optical axis. An electromagnet for bidirectionally magnetizing the Faraday rotator is disposed in the vicinity of the Faraday rotator, and the polarizer and the analyzer are made of the same birefringent material, and have the same taper angle and taper. Direction is 180
The optical switches are arranged so that the surfaces facing each other are parallel to each other and the optical axes thereof are parallel to each other and are arranged at 45 degrees as viewed from the optical axis. A feature of the present invention is that a compensating plate made of a birefringent parallel flat plate is inserted on the optical axis between the optical components between the first lens and the second lens, and The axis and the optical axis of the polarizer or analyzer adjacent to the compensating plate form 90 degrees from the optical axis, and the beam shift amounts of the ordinary ray and the extraordinary ray generated after passing through the wedge-shaped birefringent plate are made equal and the propagation speed is It is tilted with respect to the optical axis so as to cancel out the delay due to the difference, and the optical axis of the compensator is designed to be oblique with respect to the plane.

[0010]

The operation as an optical switch can be realized in principle without a compensating plate and is the same as the conventional one. In the ON state where a forward bias is applied to the electromagnet, the ordinary light and the extraordinary light are parallel when exiting the analyzer and are emitted in the direction of coupling toward the output fiber by the lens, but in the OFF state where a reverse bias is applied to the electromagnet. The ordinary light and the extraordinary light spread when they leave the analyzer and do not couple to the output fiber. The compensator inserted in the present invention functions as a shift compensator for changing the optical paths of the ordinary ray and the extraordinary ray at the time of forward bias by converting the direction and the inclination of the optical axis to convert into a single ray. At the same time, the function of dispersion correction is also fulfilled.
This improves the polarization dispersion of polarization dispersion and insertion loss.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an overall configuration diagram showing an embodiment of an optical switch according to the present invention. This optical switch includes an input fiber 10, a first lens 12 that makes the light emitted from the input fiber 10 a parallel beam, a polarizer 14 that is a wedge-shaped birefringent plate, and a Faraday rotation that rotates 45 degrees of polarization. Element 16, an analyzer 18 composed of a wedge-shaped birefringent plate, and a second lens 2 for converging a parallel beam on an output fiber 22.
0 and the output fiber 22 are installed on the optical axis in this order, and an electromagnet 24 for bidirectionally magnetizing the Faraday rotator 16 is arranged near the Faraday rotator 16. The polarizer 14 and the analyzer 18 are made of the same birefringent material, have the same taper angle and different taper directions by 180 degrees, the facing surfaces are parallel to each other, and the respective optical axes are parallel to each other. It is arranged at 45 degrees when viewed.

Here, a feature of the present invention is that a compensating plate 30 made of a birefringent parallel flat plate is inserted on the optical axis between the above-mentioned optical parts between the first lens 12 and the second lens 20. That is the point. This compensator 30 has its optical axis and the optical axis of the polarizer or analyzer adjacent to the compensator 90 degrees from the optical axis, and the beam shift amount between the ordinary ray and the extraordinary ray generated after passing through the analyzer. Are tilted with respect to the optical axis so that
The optical axis of the compensator is designed oblique to its plane. In this embodiment, the compensating plate 30 is inserted after the analyzer 18, that is, between the analyzer 18 and the second lens 20.

The electromagnet 24 is a substantially C-shaped flat yoke 2.
6 and a coil 28 wound around the yoke 26,
The Faraday rotator 16 is inserted into the gap g of the yoke 26 perpendicularly to the gap length direction. The Faraday rotator 16 has a size such that a part of the Faraday rotator 16 protrudes outward from the gap, and light rays pass through the protruding portion. In order to efficiently use the magnetomotive force, the gap g is reduced to reduce the Faraday rotator 1
6 is inserted, and the polarizer 14 and the analyzer 18 are the yoke 2
Place outside 6. As the Faraday rotator 16, L
A bismuth-substituted iron garnet single crystal film by the PE method is preferable. The reason is that the bismuth-substituted iron garnet single crystal has a large Faraday rotation coefficient, so that a relatively thin film structure can be formed, and the LPE method has an advantage of high productivity.

An outline of the operating state is shown in FIG. Electromagnet 24
In the ON state in which a forward bias magnetic field H _ON is applied to FIG.
As indicated by A, the extraordinary ray e has a higher refractive index than the ordinary ray o, and because of the use of the electromagnet, the polarizer 14 and the analyzer 18
Since the distance between and is large, the beam shift distance s between the extraordinary light e and the ordinary light o is large when the light passes through the analyzer 18. Further, since there is a difference in propagation speed between the extraordinary light e and the ordinary light o, the extraordinary light e is delayed with respect to the ordinary light o (delay amount δ). This point is the same as the conventional one. In the present invention, the compensation plate 30 is inserted,
The compensator 30 converts the extraordinary light e into the ordinary light o and the ordinary light o into the extraordinary light e. The extraordinary light e has a large refractive index so that the beam shift amount is large and the propagation speed is slow. At the time of emission, the beam shift amounts of both light beams can be made equal to each other, and the delay can be made zero. Then, a single light beam is emitted from the compensating plate 30. Conversely speaking, the inclination when arranging the compensating plate 30 on the optical axis, the angle of the optical axis, etc. are determined so as to be as described above.

In the OFF state in which a reverse bias magnetic field is applied to the electromagnet 24, the ordinary light o in the polarizer 14 is converted into the extraordinary light e in the analyzer 18 and the ordinary light o in the compensator 30, as shown in FIG. 2B. Then, the extraordinary ray e at the polarizer 14 is converted into the ordinary ray o at the analyzer 18 and the extraordinary ray e at the compensator 30, and after passing through the compensator 30, the two rays of the ordinary ray o and the extraordinary ray e are parallel. Therefore, the second lens cannot focus light on the output fiber.

The compensating plate 30 according to the present invention is arranged so as to be tilted with respect to the optical axis as described above, and the optical path is changed so that the beam shift amounts of the ordinary light o and the extraordinary light e are equalized (beam shift distance is It is adjusted so that the delay caused by the difference in propagation velocity becomes zero. Although there is an example in which a compensator is also provided in an optical isolator, in that case, it is merely used to reduce the delay due to the difference in propagation velocity, that is, to correct the polarization dispersion.

Next, an actual design example will be described. FIG.
And each optical component is arranged as shown in FIG. The orientation of each optical component and the direction of the optical axis are as illustrated. The wedge-shaped birefringent plate used for the polarizer 14 and the analyzer 18 is made of rutile single crystal and has a center thickness of 0.48 mm and a taper of 4 degrees.
The refractive indexes are n _o = 2.543 and n _e = 2.709. The distance L between the polarizer 14 and the analyzer 18 is 10 mm. The Faraday rotator 16 is a bismuth-substituted iron garnet single crystal, has a thickness of 0.536 mm and a refractive index of 2.38. The electromagnet 24 is a C-shaped yoke 2 made of a semi-hard magnetic material.
A coil 28 is wound around 6, and a Faraday rotator 16 is inserted in the center of the gap. The compensator 30 made of a birefringent parallel plate is also a rutile single crystal, and the analyzer 18
The optical axis is tilted by 44.8 degrees from the in-plane. This compensator 3
The thickness of 0 is 1.769 mm.

The result of ray tracing of this optical switch is shown in FIG. In the forward bias magnetic field (on state),
As shown in A of FIG. 5, after passing through the analyzer 18, the beam shift distance between the ordinary light o and the extraordinary light e is 179 μm, and the extraordinary light e is delayed by 0.71 ps. However, after passing through the compensation plate 30, both the beam shift distance and the delay become zero. In the reverse bias magnetic field (off state), the analyzer 18
After passing through, the opening angle between the ordinary light o and the extraordinary light e is 2.
It is 05 degrees, and even after passing through the compensation plate 30, it opens at 2.05 degrees as it is, so that even if there is a lens, it does not focus on the output fiber.

In the above embodiment, the compensating plate is arranged in the latter stage of the analyzer. However, the present invention is not limited to this structure, and any optical element can be used as long as it is between the first lens and the second lens. It may be inserted between parts.

[0020]

As described above, according to the present invention, a compensating plate made of a birefringent parallel flat plate is tilted at a predetermined angle with respect to the optical axis between the optical components between the first lens and the second lens. By arranging them and setting the direction and tilt of their optical axes to appropriate values, the optical paths of ordinary and extraordinary rays are changed to be converted into a single ray when in the ON state, and at the same time the delay is offset. be able to. Therefore, the polarization dependence of polarization dispersion and insertion loss can be suppressed. Further, the extinction ratio of the Faraday rotator is deteriorated in the non-magnetized state, and the insertion loss or the cutoff characteristic is deteriorated. However, in the present invention, since the magnetic field is applied in both the on state and the off state, the insertion loss and the cutoff characteristic are improved. To do.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of an optical switch according to the present invention.

FIG. 2 is an explanatory diagram of its operation.

FIG. 3 is a perspective view showing an arrangement of main optical components of a prototype optical switch.

FIG. 4 is a plan view showing an arrangement of main optical components of a prototype optical switch.

FIG. 5 is a ray tracing diagram thereof.

FIG. 6 is an explanatory diagram showing an example of a conventional technique.

FIG. 7 is an explanatory diagram of a beam shift distance and a delay in main optical components.

FIG. 8 is an explanatory diagram of a positional relationship between an electromagnet and main optical components.

[Explanation of symbols]

 10 Input Fiber 12 First Lens 14 Polarizer 15,16 Faraday Rotor 18 Analyzer 20 Second Lens 22 Output Fiber 23,24 Electromagnet 25,26 Yoke 27,28 Coil 30 Compensator

Claims (1)

[Claims] 1. An input fiber, a first lens for collimating light emitted from the input fiber into a parallel beam, a polarizer composed of a wedge-shaped birefringent plate, a Faraday rotator for performing 45-degree polarization rotation, and a wedge-shaped element. An analyzer composed of a birefringent plate, a second lens for focusing a parallel beam on an output fiber, and an output fiber are installed on the optical axis in this order, and the Faraday rotation is provided in the vicinity of the Faraday rotator. An electromagnet that magnetizes the child bidirectionally is arranged, the polarizer and the analyzer are made of the same birefringent material, and the taper angles are the same and the taper directions are different by 180 degrees,
An optical switch in which opposing surfaces are parallel to each other, each optical axis is parallel to each other, and is arranged at an angle of 45 degrees with respect to the optical axis, the optical switch being disposed between the first lens and the second lens. A compensating plate made of a birefringent parallel flat plate is inserted on the optical axis between the optical components of the compensating plate, and the compensating plate sees the optical axis and the optical axis of the polarizer or the analyzer adjacent to the compensating plate from the optical axis. The optical axis of the compensator is 90 ° and is tilted with respect to the optical axis so as to match the beam shift amounts of the ordinary ray and the extraordinary ray generated after passing through the wedge-shaped birefringent plate and cancel the delay due to the difference in propagation velocity. An optical switch characterized by being designed obliquely to the plane.