JPH04140709A - Optical isolator - Google Patents

Abstract

PURPOSE:To reduce the size and cost of the optical isolator by constituting the optical isolator of the nonreciprocal spherical lens alone. CONSTITUTION:The nonreciprocal spherical lens 2 is inserted and fixed to the center of a sleeve 20. Input fiber cords 24, 28 inserted and fixed with optical fibers 24a, 28a into ferrules 26, 30 are inserted into the sleeve from both sides. Both end faces of the fibers 24a, 28a are set as the focal positions of the nonreciprocal spherical lens 2. The end faces of the input optical fiber 24a and the output optical fiber 28a are subjected to diagonal polishing and the nonreflection treatment by non-reflection vapor deposited films. A permanent magnet 22 is provided in the central part of the sleeve 20 to impress a satd. magnetic field at which the YIG crystal constituting the nonreciprocal spherical lens 2 optically rotates the transmitted light by 45 deg.. The optical isolator is constituted of the nonreciprocal spherical lens 2 alone in such a manner by which the size and cost of the optical isolator are reduced.

Description

[Detailed Description of the Invention] Summary Regarding an optical isolator that does not have polarization dependence, the purpose of this invention is to provide an optical isolator that is composed only of a lens system that has a non-reciprocal effect and is downsized and cost-reduced. In an optical isolator consisting of first and second birefringent crystals and an optical rotator, one surface of the first and second birefringent crystals is processed into a spherical surface and the other surface is processed into a flat surface, and the plane of polarization is changed by applying a magnetic field as an optical rotator. A parallel flat magneto-optic crystal plate that rotates by 45° is employed, and the first and second birefringent crystals are oriented on both sides of the magneto-optic crystal plate with an angular difference of 45° between their optical axes. The flat sides of the first and second birefringent crystals are fixed such that the center of each spherical surface coincides with the central axis of the forward transmission optical path to form a non-reciprocal lens, and the plane of the first and second birefringent crystals is the center of the forward transmission optical path. The non-reciprocal lens is arranged and configured so as to be slightly inclined from the direction perpendicular to the axis.

INDUSTRIAL APPLICATION FIELD OF THE INVENTION The present invention relates to optical isolators without polarization dependence.

For example, in an optical transmission system that uses an optical fiber as a transmission path, a portion of light that enters the optical fiber from a light source may be reflected at a joint between the optical fibers and return to the light source. When such reflected feedback light occurs, especially when the light source is a single conductor laser operating at high speed, its operation becomes unstable and noise is generated in the optical signal. In the second case, an optical isolator is required to remove reflected feedback light. The general requirements for this type of optical isolator are (1) high extinction ratio, that is, low loss for forward direction light and low loss for reverse direction light; (2) no polarization dependence; and (2) no polarization dependence.

Conventional technology A typical optical isolator consists of a polarizer that transmits polarized light in a specific direction, a 45° Faraday rotator that always performs Faraday rotation in the same direction using a permanent magnet and magneto-optic crystal, and a polarizer that transmits polarized light in a specific direction. and an analyzer arranged with its transmission polarization plane rotated by 45 degrees with respect to the analyzer. With this configuration, forward light with a polarization state in a specific direction is transmitted, but light in the opposite direction is treated as linearly polarized light by the analyzer.
Since this polarized light is rotated by a 45° FARade rotator in a direction opposite to the light in the forward direction with respect to the propagation direction, it cannot be transmitted through the polarizer, and the function of an optical isolator is achieved.

However, the optical isolator with this configuration has polarization dependence even for light in the forward direction. That is, even in the forward direction, there is a problem in that only polarized light having a specific plane of polarization is transmitted, while light in other polarized states is rejected by the polarizer, and thus is not used effectively.

Therefore, as an optical isolator with improved polarization dependence,
JP 61-58809, JP 61-58811, JP 55-22729, JP 55-1130
The technique described in No. 20 has been proposed. According to the first two optical isolators described in Tokkosho, there are two birefringent tapered plates that act as a polarizer and an analyzer, a 45° Faraday rotator, and a coupling lens that couples a light beam to an optical fiber. An optical isolake is constructed from these.

The optical isolators described in the latter two publications include two parallel plate-shaped birefringent plates that act as a polarizer and an analyzer, a 45° Farade single rotator, and a 1/2 It consists of a wave plate and a coupling lens that couples a light beam to an optical fiber.

Problems to be Solved by the Invention However, the optical isolators described in the above-mentioned gazettes and publications do not require coupling with optical fibers or light-emitting elements in addition to the non-reciprocal part consisting of a polarizer, optical rotator, and analyzer. A lens system is required, and if it is constructed as an optical isolator that can be interfaced with an optical fiber transmission line, it will be quite large and costly.

The present invention has been made in view of these points, and its purpose is to provide an optical isolator that is composed of only a lens system having a non-reciprocal effect and is downsized and cost-reduced. It is.

Means for Solving the Problems In the optical isolator of the present invention, one surface of the first and second birefringent crystals is processed into a spherical surface and the other surface is processed into a flat surface, and the plane of polarization is rotated by 45 degrees by applying a magnetic field as an optical rotator. A parallel flat magneto-optic crystal plate is used. Then, the flat sides of the first and second birefringent crystals, which are oriented on both sides of the magneto-optic crystal plate with an angular difference of 45° between their optical axes, are
The centers of the respective spherical surfaces are fixed so as to coincide with the central axis of the forward transmitted optical path to form a non-reciprocal lens, and the planes of the first and second birefringent crystals are perpendicular to the central axis of the forward transmitted optical path. This non-reciprocal lens is placed so that it is slightly tilted from the center.

The present invention allows the lens itself to function as an isolator by imparting a non-reciprocal effect to the lens, making it easy to downsize and integrate with other optical devices.The principle is shown in Figure 1. This will be further explained using . 2
is a non-reciprocal spherical lens, which employs a magneto-optic crystal plate 4 in the shape of a parallel plate cut to a thickness that rotates the plane of polarization by 45 degrees by applying a magnetic field with a polarization rotator. The magneto-optic crystal plate 4 is made of YIG with a refractive index n=2.2, for example.

6 is a first birefringent crystal that acts as a polarizer, and 8 is a second birefringent crystal that acts as an analyzer.

The first and second birefringent crystals 6.8 are made of rutile, for example. One surface of the first and second birefringent crystals 6 and 8 is polished into a spherical surface and the other surface is polished into a flat surface, and the centers of the spherical surfaces of the first and second birefringent crystals thus polished are in the forward direction. The flat sides of the first and second birefringent crystals 6 and 8 are placed on both sides of the magneto-optic crystal plate 4 so that they coincide with the central axis of the transmitted optical path and have an angular difference of 45° between their optical axes. They are glued together with an optical adhesive to form a ball lens 2 having a non-reciprocal effect as shown.

The non-reciprocal spherical lens 2 is arranged such that the planes of the first and second birefringent crystals 6.8 are inclined by θ, for example 1 to 4 degrees, from the direction perpendicular to the central axis of the forward transmission optical path. .

2 is a spherical lens made of glass such as BK-7, and the positional relationship is such that the non-reciprocal spherical lens 2 focuses on the input optical fiber 12 and the spherical lens 10 focuses on the output optical fiber 14. It is fixed at

Operation First, the forward direction operation will be explained with reference to FIG. 1(A). While the light emitted from the input optical fiber 12 enters the first birefringent crystal 6 of the non-reciprocal spherical lens 2 and passes through it, it becomes an ordinary ray 0 shown by a solid line and an extraordinary ray e shown by a broken line. separated. The polarization planes of the ordinary light and the extraordinary light are rotated, for example, by 45° in the clockwise direction by the magneto-optic crystal plate 4.
The light is incident on the second birefringent crystal 8. Since the optical axis of the second birefringent crystal 8 is rotated by 45° clockwise in the direction of the light beam with respect to the optical axis of the first birefringent crystal 6, the ordinary and extraordinary rays of the first birefringent crystal 6 are 2 correspond to the ordinary light and extraordinary light of the birefringent crystal 8, respectively. Therefore, the second
The ordinary light and the extraordinary light transmitted through the birefringent crystal 8 are emitted in parallel to each other, and are sent to an output optical fiber 14 by a ball lens 10.
is combined with

Next, the operation in the reverse direction will be explained with reference to FIG. 1(B). The reflected return light emitted from the output optical fiber 14 is collimated by the ball lens 10 and then enters the non-reciprocal ball lens 2. When this reflected feedback light passes through the second birefringent crystal 8, it is divided into ordinary light and extraordinary light and refracted in different directions.The plane of polarization is rotated by 45 degrees counterclockwise by the magneto-optic crystal plate 4, and the light is emitted. be done. The ordinary light of the second birefringent crystal 8 whose polarization plane has been rotated by 45 degrees is relative to the optical axis of the first birefringent crystal 6, which has an optical axis shifted by 145 degrees with respect to the optical axis of the second birefringent crystal 8. The polarized light is rotated by 90 degrees and is therefore refracted by the first birefringent crystal 6 as extraordinary light. On the other hand, the extraordinary light of the second birefringent crystal 8 whose plane of polarization has been rotated by 45 degrees is similarly refracted by the first birefringent crystal 6 as ordinary light.

That is, the ordinary light and the extraordinary light of the second birefringent crystal 8 are transformed by the magneto-optic crystal plate 4 into the extraordinary light and the extraordinary light of the first OI refractive crystal 6, respectively.
After being converted into ordinary light, reflected feedback light is emitted from the first birefringent crystal 6 in a direction different from that of the incident light. Therefore, even if this emitted light is narrowed down by the non-reciprocal spherical lens 2, it will not be coupled to the core of the input optical fiber 12, and reflected feedback light will be effectively prevented from entering the input optical fiber 12.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a cross-sectional view of an embodiment of an optical isolator that can be interfaced with an optical fiber transmission line constituted by the non-reciprocal spherical lens 2 described in FIG. 1. A non-reciprocal ball lens 2 is inserted and fixed in the center of the sleeve 20, and a fiber wire 24a is inserted into the ferrule 2 from one side of the sleeve 20.
The human-powered fiber cord 24 inserted and fixed into the ferrule 30 is inserted from the opposite side, and the output fiber cord 28 whose fiber wire 24a is also inserted and fixed into the ferrule 30 is inserted, and one side of the fiber wires 24a and 28a is inserted. is fixed at the focal position of the non-reciprocal spherical lens 2 where the optical coupling efficiency is maximum. The end face of the human-powered fiber wire 24a and the end face of the output fiber wire 28a are subjected to oblique polishing and anti-reflection treatment using a non-reflection vapor deposited film.

A permanent magnet 22 is provided in the center of the sleeve 20, and a YIG crystal forming the non-reciprocal spherical lens 2 applies a saturated magnetic field to rotate the transmitted light by 45°. In this way, according to this embodiment, the optical isolator, which was conventionally composed of a lens system and a non-reciprocal part, can be composed only of the non-reciprocal spherical lens 2, thereby reducing the size and cost of the optical isolator. can be achieved.

Next, referring to FIG. 3, another embodiment of the non-reciprocal spherical lens will be described. This non-reciprocal spherical lens 32 is YIC,
a magneto-optical crystal plate 34 such as, first and second birefringent crystals 36 and 38 polished so that one surface is spherical and the other surface is flat;
1/2 wavelength plate 40. First birefringent crystal 3
6 is bonded to one surface of the magneto-optic crystal plate 34 with an optical adhesive on its flat side so that the center of its spherical surface coincides with the central axis of the forward transmission optical path, and the other surface of the magneto-optic crystal plate 34 is bonded with an optical adhesive. A half-wave plate 40 is also bonded with an optical adhesive. A second birefringent crystal 38 whose optical axis coincides with the direction of the optical axis of the first birefringent crystal 36 is placed on the half-wave plate 40, and the center of its spherical surface coincides with the central axis of the forward transmission optical path. The planar side is bonded with an optical adhesive in this manner.

According to the non-reciprocal spherical lens 32 configured in this way, forward light traveling rightward in the figure is directed to the first birefringent crystal 36.
The plane of polarization is rotated by 45 degrees by the magneto-optic crystal plate 34, and then the 1/2 wavelength plate 4 separates the light into ordinary light and extraordinary light.
0, its polarization plane is rotated by 45°, and the light is incident on the second birefringent crystal 38. The ordinary light and extraordinary light of the first birefringent crystal 36 whose polarization planes have been rotated by a total of 90 degrees become extraordinary light and ordinary light with respect to the second birefringent crystal 38, and are refracted accordingly. The light is focused at the focal point of the non-reciprocal spherical lens 32 and coupled to an output side optical fiber (not shown).

On the other hand, the reflected feedback light from the output side optical fiber is separated into ordinary light and extraordinary light by the second birefringent crystal 38, and its polarization plane is rotated by 145 degrees by the 1/2 wavelength plate 40, but the magneto-optic crystal Since the plate 34 has directionality, the plane of polarization is +45°
After being rotated in the opposite direction, the ordinary light and the extraordinary light of the second birefringent crystal 38 whose plane of polarization is not rotated enter the first birefringent crystal 36 as they are. Therefore, when the ordinary rays and extraordinary rays pass through the first birefringent crystal 36, they are further refracted as ordinary rays and extraordinary rays of the first birefringent crystal, and are not focused on the focal position of the non-reciprocal spherical lens 32. It functions as an optical isolator without being coupled to an incident-side optical fiber (not shown) placed at this focal position.

FIG. 4 shows a non-reciprocal drum lens 42 constructed by cutting both ends of the non-reciprocal spherical lens 32 shown in FIG. 3 in a direction perpendicular to the parallel plane of a flat magneto-optic crystal plate 34.
This non-reciprocal drum lens 42 is composed of a flat magneto-optic crystal plate 34', first and second birefringent crystals 36' and 38', and a half-wave plate 40'. The operation of this non-reciprocal drum lens 42 is similar to the operation of the non-reciprocal ball lens 32 shown in FIG. 3, so a description thereof will be omitted.

Next, with reference to FIG. 5, a non-reciprocal rod lens according to another embodiment of the present invention will be described. To form this non-reciprocal rod lens 44, first and second birefringent crystals 48, 50 are processed into a tapered shape at a predetermined taper angle. Then, a converging rod lens 46 made of quartz glass and having a length capable of rotating the plane of polarization cut off at the taper angle by 45 degrees so that both ends thereof are parallel to each other is used as an optical rotator. The first and second tapered birefringent crystals 48.50 are bonded to both end faces of the crystal with an optical adhesive, with the top and bottom parts reversed and their respective optical axes having an angular difference of 45° from each other. . The operation of the non-reciprocal rod lens 44 configured in this way is similar to the operation of the non-reciprocal ball lens 2 explained in FIG. The optical fiber transmission lines on the incident side and the output side can be coupled using only the non-reciprocal rod lens 44 without providing any.

Next, with reference to FIG. 6, an optical coupler with a built-in two-projection isolator using a non-reciprocal lens according to the present invention will be described. Reference numeral 52 designates an input port, into which a non-reciprocal drum lens 56 is inserted and fixed from one end of the sleeve 54, and a human-powered fiber cord 60 having an optical fiber strand 60a inserted and fixed into a ferrule 62 is inserted and fixed from the other end. A permanent magnet 58 that applies a saturation magnetic field to the magneto-optic crystal plate of the non-reciprocal drum lens 56 is fitted onto the sleeve 54.

In the output port 4, a non-reciprocal drum lens 68 is inserted and fixed from one end of the sleeve 66, and an output fiber cord 72, in which an optical fiber element 72a is inserted and fixed in a ferrule 74, is inserted and fixed from the other end. Reciprocal drum lens 6
A permanent magnet 70 that applies a saturation magnetic field to the magneto-optic crystal plate 8
is constructed by being fitted onto the sleeve 66. The non-reciprocal drum lens 68 is 90
It is inserted and fixed into the sleeve 66 at the rotated position.

A coupler film 7 is provided between the input port 52 and the output port 4.
The monitor boat 80 has a drum lens 84 inserted and fixed from one end of the sleeve 82, and an optical fiber 86a is inserted into the ferrule 88 from the other end. A monitor fiber cord 86 is inserted and fixed.

Since the non-reciprocal drum lenses 56 and 68 are incorporated into the input port 52 and the output port 4 in this way,
The optical coupler of this embodiment can obtain a very high extinction ratio. Furthermore, since there is no need to separately provide a coupling lens for coupling to the input fiber cord 60 and the output fiber cord 72, it is possible to downsize the optical coupler. A normal drum lens 84 is arranged in the monitor boat 80, which does not require such a high extinction ratio, to achieve optical coupling.

Although FIG. 6 describes an optical coupler using a non-reciprocal lens based on the principle of the present invention, the application of the non-reciprocal lens is not limited to this, and is used in lens optical systems such as multiplexers/demultiplexers, optical switches, etc. It is also applicable to optical devices equipped with.

Effects of the Invention As described in detail above, the present invention allows an optical isolator, which was conventionally composed of a lens system and a non-reciprocal part, to be composed only of a non-reciprocal lens. This has the advantage that it is possible to provide an optical isolator that is Also, by incorporating it into other functional devices,
It is possible to significantly simplify its structure.

[Brief explanation of the drawing]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is a sectional view of an embodiment of the invention, Fig. 3 is a sectional view of another embodiment of a non-reciprocal spherical lens, Fig. 4 is a sectional view of a non-reciprocal drum lens, FIG. 5 is a side view of a non-reciprocal rod lens, and FIG. 6 is a schematic diagram of an optical coupler with a built-in two-stage optical isolator using the non-reciprocal lens of the present invention. 2... Non-reciprocal spherical lens, 4... Magneto-optic crystal plate, 6... First birefringent crystal, 8... Second birefringent crystal, lO... Spherical lens, 22.58.70 ...Permanent magnet, 32...Non-reciprocal ball lens, 40...1/2 wavelength plate, 42...Non-reciprocal drum lens, 46...Focusing rod lens.

Claims (1)

[Claims] 1. An optical isolator comprising first and second birefringent crystals and a polarizer, wherein one surface of the first and second birefringent crystals (6, 8) is a spherical surface and the other surface is a flat surface, respectively. At the same time, a magneto-optic crystal plate (4) in the form of a parallel plate, which rotates the plane of polarization by 45 degrees by applying a magnetic field, is used as an optical rotator. °
The plane sides of the first and second birefringent crystals (6, 8), which are oriented with an angular difference of A reciprocal lens (2) is formed, and the first and second birefringent crystals (6
, 8), wherein the non-reciprocal lens (2) is arranged so that the plane of the lens (2) is slightly inclined from the direction perpendicular to the central axis of the forward transmission optical path. 2. In an optical isolator consisting of first and second birefringent crystals and an optical rotator, one surface of the first and second birefringent crystals (36, 38) is processed into a spherical surface and the other surface is processed into a flat surface, and the optical rotator is A parallel plate-shaped magneto-optic crystal plate (34) whose polarization plane is rotated by 45 degrees by applying a magnetic field is used in combination with a 1/2 wavelength plate (40). 2 wavelength plate (40
) is fixed thereto, and the first birefringent crystal (
The flat side of the half-wave plate (36) is fixed so that the center of the spherical surface coincides with the central axis of the forward transmission optical path, and the half-wave plate (
40) On the flat side of the second birefringent crystal (38) so that its optical axis coincides with the optical axis of the first birefringent crystal (36), and the center of the spherical surface is the center of the forward transmission optical path. An optical isolator characterized in that a non-reciprocal lens (32) is formed by being fixed in alignment with an axis. 3. In an optical isolator consisting of first and second birefringent crystals and an optical rotator, the first and second birefringent crystals (48, 50) are processed into a tapered shape at a predetermined taper angle, and A focusing rod lens (46) made of glass having a length that can rotate the polarization plane by 45 degrees and cut off at the taper angle so that both ends are parallel to each other is used, and the taper is attached to both end surfaces of the rod lens (46). A non-reciprocal lens (44) is formed by fixing first and second birefringent crystals (48, 50) such that their top and bottom parts are opposite to each other and their optical axes have an angular difference of 45° from each other. An optical isolator characterized by the following: 4. In an optical device equipped with a lens optical system, the non-reciprocal lens (2, 32,
44) An optical device characterized in that a lens optical system is configured by the following.