JPH0341809B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber switch for switching the optical path of a light beam transmitted through a fiber, and more particularly to an electronic optical switch having no mechanically movable parts.

In optical fiber communication systems, optical switches are widely used for switching between regular and backup lines, switching optical parts, and for measurement in order to improve line reliability and facilitate maintenance. Optical switches are also used not only for communication systems between distant places, but also for data transmission systems that connect information processing equipment within a limited area, such as connection nodes between highways and terminals in so-called optical data highways. There is. That is, when it is necessary to avoid a specific terminal, for example, when the terminal is out of order, undergoing maintenance or inspection, or when not in use, the optical switch is used to bypass the route.

One type of optical switch that has been developed for this purpose is one that electromagnetically displaces a prism or mirror. That is, the light emitted from an optical fiber is converted into a parallel beam, the optical path is spatially changed by inserting a prism or a mirror into this optical path, and the optical fiber on which the light is focused again is selected. This type of optical switch has the characteristics of high crosstalk and low optical insertion loss, and is easy to use, but because it has a mechanical movable mechanism, it is difficult to obtain a high switching speed, and the number of switching cycles is limited. There are concerns about longevity and reliability for extremely long intervals of operation. for example,
There is instability in that a certain state is held almost fixedly for a long period of time, and when a switch operation is suddenly performed, the device does not operate correctly. Therefore, it is difficult to use it in optical submarine repeaters, etc., where it is difficult to easily replace or inspect elements.

The following devices are also known as devices that electronically switch fiber light without having mechanically movable parts. A polarizing element that transmits incident light to a polarization conversion element that has the function of rotating the polarization direction of transmitted light by 90 degrees by applying a voltage or current, and then changes the direction of the optical path depending on the polarization direction of the light. The light is directed to different optical fibers by transmitting the light through the optical fiber. Electro-optic materials such as electro-optic crystals and liquid crystals can be used as polarization conversion elements.
Magneto-optical materials such as iron garnet crystals and highly concentrated lead glass have been used. Conventional optical switches of this type have several drawbacks. One of these is that the constituent polarizing elements are expensive or have insufficient characteristics. As a polarizing element that changes the optical path according to the polarization direction of the light mentioned above,
Polarizing prisms made of calcite, which has long been known as a material with high birefringence, rutile crystal, which is also a material with high birefringence, formed into a prism shape and polished, and the reflective surface of a total reflection prism made of glass. There are some that have a three-layer dielectric film formed thereon to serve as a polarizing element. Calcite is a natural stone and is expensive; the crystal material itself for rutile prisms is expensive; and because it has a high refractive index, it is necessary to form a good anti-reflection film on the prism entrance surface. Polarizing elements made of a dielectric multilayer film have drawbacks such as deterioration of polarization properties when light of a wavelength deviates from the design wavelength is incident because the wavelength characteristics of the multilayer film are sensitive to incident light.

The second drawback of conventional optical switches is that they require a large light transmission section of the polarization conversion element or a cylindrical lens, which means that the applied external field is large or the cylindrical The problem is that light transmission loss cannot be avoided due to lens aberration.

That is, the arrangement of the optical system is such that a collimating lens that collimates light onto an input fiber and a focusing lens that focuses this collimated light onto an output fiber are provided, and the aforementioned polarization conversion element or polarizing element is provided in the optical path between these lenses. be. Since the collimated light has a cross section of about 1 mmφ, the cross section of the polarization conversion element needs to be several mm square or more than several mmφ. For this reason, when a magneto-optical material is used for the polarization conversion element, the demagnetizing field coefficient becomes large, the external magnetic field required to illegally sum the magnetization becomes large, and the current value for generating this magnetic field also becomes large. When an electro-optic material is used, a vertical element with a small electro-optic effect is used, and the applied voltage is as high as several hundred volts. If a plate-shaped polarization conversion element is used, the demagnetizing field coefficient can be lowered and a horizontal electro-optical element can be used.
For this reason, cylindrical lenses that reduce the beam width of the collimated light in one dimension are provided before and after the polarization conversion element. It is difficult to obtain a high-performance cylindrical lens with a focal length of several millimeters, and its large absorption deteriorates the optical insertion loss of the element.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fiber optic switch which eliminates the above-mentioned disadvantages.

According to the present invention, there is provided a crystal plate having a high refractive index and optically polished on both sides, and an optical axis of a light beam that is incident on the crystal plate at the Brillister angle and transmitted through the crystal plate, and a reflected light beam. a lens that parallelizes the optical axis of the lens, and a polarization conversion element that transmits the two light beams that have been parallelized by the lens and rotates the polarization of the light beam by 90 degrees due to applied trauma. ,
A lens that causes the optical axes of the light beam that has passed through the polarization conversion element to intersect on the focal plane, and a lens that is arranged so that the light beam that has passed through the lens is incident at a Brewster angle at a position where the optical axes intersect. By using the same crystal plate as the above crystal plate, an inexpensive and high-performance optical switch can be obtained.

Polarizing filters have been known since ancient times, which improve the degree of linear polarization of transmitted light by making light incident on a crystal plate with a high refractive index, such as a silicon crystal, at the Brewster's angle. However, the light reflected by the polarizing filter is not used here. A simple configuration in which transmitted light and reflected light from a high refractive index crystal plate are optically converged without difficulty by a lens having appropriate parameters, and this optical system is combined with a polarization conversion element. The present inventor is the first to construct an industrially useful optical switch.

The details of the present invention will be further explained using examples.
FIG. 1 shows a first embodiment of the present invention, in which 1 is an incident optical fiber, 2 is a collimating lens, 3 is a polarizing element on the incident side, 4 and 6 are condensing lenses, 5 is a polarization conversion element,
7 is a polarizing element on the output side, 8 and 9 are focusing lenses, 1
0 and 11 are output optical fibers. The light emitted from the input optical fiber 1 is converted into a parallel light beam 12 by the collimating lens 2, and is incident on the polarizer 3. The principle of the polarizer 3 will be described later, but its function is to
2 into P wave 14 and S wave 13. lens 4
is provided so that its focal position is the reflection position on the polarizer 3. The lens is assumed to have a high NA value, for example, about 0.5. Since the angle formed by the P wave 14 and the S wave 13 emitted from the polarizer 3 is as narrow as about 32 degrees, as will be described later, their optical axes are Collimated P polarized light 16, S polarized light 1
It becomes 5. The beam diameter at the rear focal position of the lens 4 is focused to a size obtained by multiplying the beam diameter at the output end face of the optical fiber 1 by the magnification ratio of the focal length ratio of the lenses 2 and 4. For example, core diameter 50μm
If an optical fiber 1 is used and the focal lengths of lenses 2 and 4 are selected to be equal, the diameter of the light beam at the rear focal position of lens 4 will be about 50 μm since the image is formed at a same magnification. Therefore, the polarization converter 5, which has the function of transmitting this beam and rotating the polarized light by 90 degrees, may have a small width in the direction perpendicular to the paper plane in FIG. 1, that is, it may have a thin plate shape. The two linearly polarized lights 15 and 16 incident on the polarization converter 5 may be in the same polarization state or polarized light perpendicular thereto, depending on the state of the external field such as voltage or current applied to the polarization converter. The polarization state is determined as follows. Now, the polarization is changed to 90℃ by the polarization converter 5.
If it is rotated, the incident S-polarized light 15 is polarized into P-polarized light, and the P-polarized light 16 is polarized into S-polarized light, and exits from the polarization conversion element 5. The lens 6 and the output polarizer 7 through which these light waves pass are the same as the lens 4 and the input polarizer 3 described above, and are arranged so as to be symmetrical with the focal position of the lens 4 in between. That is, one of the focal positions of the lens 6 is the focal position of the lens 4, and the other is the light reflecting position of the polarizer 7. The optical axes of the two polarized lights transmitted through the lens 6 are converged toward the focal position of the lens 6, and the respective luminous fluxes become parallel fluxes. Since the lens 6 has the same characteristics as the lens 4 as described above, the two polarized lights 17,
The convergence angle of the optical axis 18 is the same as the angle formed by the two polarized lights 13 and 14 that exit the incident polarizer 3 and head towards the lens 4. Since the light beam 15, which is S-polarized light, has undergone polarization conversion as described above, the light beam 17 is P-polarized light. The light beam 17 which is P-polarized light incident on the output polarizer 7 passes through the polarizer 7 and becomes a light beam 2.
0 and is coupled to the exit fiber 11 via the lens 9. When the P-polarized light beam 16 undergoes polarization conversion, it becomes S-polarized light 18. This light beam is reflected by the polarizer 7, merges with the previous light beam 20, and is coupled to the output fiber 11. That is, the light beam emitted from the optical fiber 1 is polarized by the polarizer 3.
The polarized light is separated by the polarization converter 5, polarized by the polarization converter 5, multiplexed by the output polarizer, and coupled to the output fiber 11. On the other hand, if the state of the external field such as the voltage and current applied to the polarization converter 5 is set so that rotation of polarization does not occur, the aforementioned light beam 17 is S-polarized light, which is the same polarization state as the light beam 13. This light beam is reflected by the polarizer 7, becomes a light beam 19, and passes through the lens 8 to the light beam 11.
is combined into a separate light beam 10. Furthermore, since the light beam 18 is P-polarized light, it passes through the polarizer 7,
It merges with the previous light beam 19 and also forms optical fiber 1.
Combined with 0. In this case, the light exiting the optical fiber 1 is coupled into the exit optical fiber 10.
That is, by switching the state of the polarization converter, the output light of the optical fiber 1 can be coupled into either of the two output optical fibers 10 or 11.
The operation of a so-called 1×2 optical switch is realized.

Next, the operation of the polarizer will be explained. FIG. 2 is a diagram showing the principle of a polarizer, and 100 is a thin single crystal plate of silicon, gallium arsenide, indium phosphide, or the like. These crystals are transparent over a light wavelength of about 1 μm to 2 μm. For example, an N-type silicon single crystal with a carrier density of 1.4×10 ¹⁶ cm ^-3 has an optical wavelength of
The absorption coefficient in the region of 1.3 μm to 2 μm is 0.1 cm ⁻¹ or less, showing extremely good light transmission characteristics. Also, the refractive index is
It is very high at about 3.5, and the Brillusta angle φ is as large as 74 degrees. When the light beam 101 is incident at a Brillustat angle of φ = 74 degrees (Θ = 16 degrees), the reflections from the upper and lower surfaces of the thin crystal plate are added together, and the reflected light 102 becomes only the S-polarized component, with a reflectance of 84 %. Most of the transmitted light 103 has a P-polarized component, and its transmittance is very high because the above-mentioned absorption effect is low. The S-polarized component contained in this transmitted light 103 is 13%. When three thin crystal plates shown in FIG. 2 are stacked with a space in between, the S polarization component becomes 0.2%, and a polarizing element having sufficient polarization characteristics can be obtained. Also, as mentioned above, Θ=
Since the light beam will be incident at a very shallow angle of 16 degrees, the optical axes of the P-polarized and S-polarized light can be made parallel by using a lens with a high NA value, as shown in Figure 1. .

As the polarization converter 5 in FIG. 1, a conventionally known type can be used. for example
As mentioned above, when using a polarization converter that uses the electro-optic effect of LiTaQ ₃ crystal or a polarization converter that uses the Farade effect of a magneto-optic crystal such as YIG,
The light beam diameter at the position where the polarization converter is placed is 50μ
m, which is very small, requires only a thin crystal plate, and has the advantage that the applied voltage and applied magnetic field strength can be low.

FIG. 1 shows an embodiment of a so-called 1.times.2 optical switch that couples light from one input fiber to one of two output fibers. It is easy to realize a 2.times.2 optical switch by using two optical fibers on the input side and arranging them in the same way as the output fibers in FIG.

In addition, although we have described the case where the same function is achieved with one lens for parallelizing the optical axis of the S-polarized light and the P-polarized light emitted from the polarizing element, each lens is
A combination of two may be used. The same can be applied to a lens that collimates a light beam whose optical axes are parallel, emitted from a polarization conversion element, and guides it to the output-side polarizing element.

As described above, according to the present invention, an inexpensive and high-performance optical switch can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention.
is the input optical fiber, 3 is the input side polarizing element, 4 and 6
5 is a lens with a high NA value, 5 is a polarization conversion element,
7 is an output side polarizing element, and 10 and 11 are output optical fibers. FIG. 2 is a principle diagram showing the operation of a polarizing element, and 100 is a high refractive index crystal plate.

Claims (1)

[Claims] 1 A crystal plate with a high refractive index and optically polished on both sides, an optical axis of a light beam that enters the crystal plate at Brewster's angle and passes through the crystal plate, and an optical axis of a reflected light beam. a lens that collimates the light beams, and the two light beams collimated by the lens are transmitted and the polarization of the light beams is changed to 90 by an applied external field.
a polarization conversion element that rotates the polarization conversion element; a lens that causes the optical axes of the light beams that have passed through the polarization conversion element to intersect on the focal plane; 1. An optical switch comprising a crystal plate identical to the above crystal plate arranged so as to allow light to enter at a corner.