JP2000039590A - Reflection type circulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To show a high isolation characteristic, to reduce both of length and size and to reduce a cost by miniaturizing optical parts to be used and reducing the number of parts. SOLUTION: This reflection type circular is constituted so that two pieces of double reflection elements 12, 14, a 45 deg. Faraday rotator 16 and a 1/2 wavelength plate 18 inserted between them are arranged in a line, further, an incident/emission part 20 is provided on the one end of the line and a polarization rotary element (1/4 wavelength plate 32) reciprocally rotating a polarization plane at 90 deg. and a reflection body (mirror 34) are arranged on the other end of the line. The incident/emission part consists of the array of a multi-core ferrule (three cores ferrule 24) having three pieces or more of fibers 22, a common fiber connecting lens 26 and an optical path correction element 28 making an emission beam from the fiber the beam in parallel to an optical axis and making the beam in parallel to the optical axis being a return beam connecting to the fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circulator for separating light passing through a transmission line in fields such as optical communication and optical measurement. More specifically, the present invention relates to a multi-core ferrule at one end of an optical circulator body. A light input / output unit using the same is provided, and a mirror or the like is provided at the other end, and light is reflected by the mirror to reciprocate the optical circulator main body portion and couple to a fiber of a multi-core ferrule, so that a small-sized The present invention relates to a reflection-type optical circulator with improved performance and higher performance.

[0002]

2. Description of the Related Art An optical circulator emits light from one port to a specific other port only, such as emitting incident light from a port to a port and emitting incident light from a port to the port. This is an optical device having a separating function. This type of optical circulator uses a 45-degree Faraday rotator configured to apply a fixed magnetic field by a permanent magnet to the Faraday element, and realizes non-reciprocity of the light beam by rotating the polarization plane by 45 degrees.

Conventionally, optical circulators of various configurations have been developed. As an example, three birefringent elements are arranged in a line at intervals, a pair of a Faraday element and a half-wave plate is inserted between the birefringent elements, and input / output sections are provided at both ends. There is a configuration (Japanese Unexamined Patent Publication No.
No. 61001). A fixed magnetic field is applied to the Faraday element by a permanent magnet to form a 45-degree Faraday rotator whose polarization plane rotates 45 degrees. This forms an optical circulator that couples the light emitted from one port to the opposite port, the light emitted from the port to the opposite port, and if necessary, the light emitted from the port to the opposite port. it can. Here, each port is composed of a combination of a fiber and a lens.

[0004] Most of the optical circulators currently in practical use are of the three-port type. The reason is that in a general bidirectional transmission system or reflected light measurement system, if there are three ports: a port for connecting a light emitting element, a port for connecting a transmission fiber or a sample to be measured, and a port for connecting a light receiving element This is because it is often sufficient. Therefore, it is sufficient if the operation of port → port and port → port can be realized.

[0005]

As described above, in the conventional optical circulator having a plurality of ports, each port has one port.
Lenses are combined in correspondence with the respective fibers. That is, collimated coupling is achieved by combining individual lenses for each fiber. Therefore, a distance commensurate with the lens diameter is required as the distance between the fibers, and each optical element on the optical circulator body side must be inevitably enlarged, and the optical circulator is reduced in size, diameter, and cost. Had become a major obstacle.

[0006] Further, rutile single crystals of the same shape are used as the birefringent elements at both ends, and they are arranged to face each other, thereby providing the functions of polarization separation and synthesis. However, since one element is used for each function, the overall length becomes longer,
This is also an obstacle to miniaturization and cost reduction of the optical circulator.

An object of the present invention is to provide an optical circulator which exhibits high isolation characteristics, can be reduced in both length and diameter, and can be manufactured at a low cost by downsizing optical elements and reducing the number of parts. It is.

[0008]

According to the present invention, two birefringent elements, a 45-degree Faraday rotator and a half-wave plate inserted between them are arranged in a line, and one end of the line is further arranged. Is a reflection type optical circulator in which a polarization rotating element for rotating the polarization plane by 90 degrees reciprocally and a reflector are arranged at the other end of the row. Here, the input / output section is 3
Multi-core ferrules with more than two fibers, a fiber coupling lens common to them, and a beam parallel to the optical axis for the light emitted from the fiber and a beam parallel to the optical axis for return light coupled to the fiber And an optical path correction element.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION A polarization rotating element for rotating a polarization plane 90 degrees reciprocally may be a quarter-wave plate or a 45-degree Faraday rotator. In the case where a three-core ferrule is used, for example, the optical path correction element may be a prism having a shape in which two parallel ridges of a rectangular parallelepiped are obliquely cut off, or a trapezoidal prism, or a prism having a central gap. A combination of two symmetrically arranged right-angled triangles or wedge-shaped prisms may be used. Of the three fibers arranged side by side, the light emitted from the outer fiber and the light incident thereon are refracted by the slope of the prism and follow an appropriate optical path,
The light emitted from the central fiber and the light incident on the central fiber follow an appropriate optical path by traveling straight in the central parallel plane portion of the prism or in the gap between the prisms.

When the birefringent element near the entrance / exit portion is divided into two and a half-wave plate is inserted between the birefringent element pieces,
Although the number of components is slightly increased, it is possible to eliminate the polarization dispersion generated on the forward path and the return path.

[0011]

FIG. 1 is a configuration diagram showing one embodiment of a reflection type optical circulator according to the present invention, and is an example of a three-port configuration. The optical circulator body 10 includes a first birefringent element 12 and a second birefringent element 1 that are spaced apart from each other.
4 and 45-degree Faraday rotator 1 inserted between them
This is a configuration in which 6 and 1/2 wavelength plates 18 are arranged in a line.
Here, both birefringent elements 12 and 14 are made of rutile, both end faces are parallel to each other, and their optical axes are set at an angle of 45 degrees with the optical axis. The optical axes of the first birefringent element 12 and the second birefringent element 14 are perpendicular to each other in the optical axis direction (within the plane of FIGS. 2 and 3 described later). The optical axis of the half-wave plate 18 is set to ± 22.5 degrees from the horizontal axis when viewed in the optical axis direction (in the paper plane in FIGS. 2 and 3 described later). Faraday rotator 1
Reference numeral 6 denotes a configuration in which an external magnetic field is applied to the Faraday element by a permanent magnet, and the polarization plane is set to be 45 degrees counterclockwise when viewed from the port on the left hand side of the drawing. In order to simplify the drawing, only a Faraday element is illustrated as a Faraday rotator, and illustration of a permanent magnet is omitted.

The optical circulator body 10 is designed on the assumption that light reciprocates using reflection. That is, the input / output section 20 is provided at one end (here, the end near the first birefringent element 12) of the row constituting the optical circulator body portion 10, and the other end (here, the second birefringent element 12).
(The end near the birefringent element 14). Here, the first birefringent element 12 has a polarization separating / combining function, and the second birefringent element 14 has an optical path shifting function.

The input / output section 20 comprises an arrangement of a three-core ferrule 24 having three fibers 22, a fiber coupling lens 26 common to them, and an optical path correction element 28. The optical path correction element 28 is connected to the lens 26
The outgoing light passing therethrough is converted into a beam parallel to the optical axis, and the beam parallel to the optical axis, which is return light, is incident on the fiber 22 through the lens 26 and is coupled.

The reflector 30 comprises a polarization rotator 32 for rotating the polarization plane 90 degrees back and forth and a mirror 34. For example, a quarter-wave plate can be used as the polarization rotation element. The optical axis of the quarter-wave plate is set at 45 degrees from the horizontal axis when viewed in the optical axis direction (within the plane of the paper in FIGS. 2 and 3).

2 and 3 show specific states of the polarization plane and the directions of the optical axes, and the operation of the optical circulator will be described with reference to FIGS. In addition, each rectangular part in the drawing represents the emission surface of each optical element.

FIG. 2 shows a state where light is transmitted from the outer port to the center port. The light from the port is separated into ordinary light and extraordinary light by the first birefringent element 12 and passes through the Faraday rotator 16 and the half-wave plate 18. At this time, the extraordinary light has its polarization plane rotated by 90 degrees, but the ordinary light does not rotate as a result because the polarization plane returns to the original state.
Since the extraordinary light is generated for the second birefringent element 14, the light travels with an optical path shift. The quarter-wave plate 32 converts the linearly polarized wave into a circularly polarized wave, and the return light reflected by the mirror 34 is
The light is again converted into linearly polarized light by the 波長 wavelength plate 32. After all,
In the quarter wave plate 32, the plane of polarization is 90
The second birefringent element 14 becomes ordinary light. Each of the light beams has a polarization plane rotated by 90 degrees by the half-wave plate 18 and the Faraday rotator 16.
The other does not rotate. Then, the ordinary light and the extraordinary light are combined by the first birefringent element 12 to form one light beam, which is emitted to the central port. In this way, the light emitted from the port couples to the port, does not return to the port, and does not couple to the port.

FIG. 3 shows a state in which light is transmitted from a center port to an outer port. In this case, the state is exactly the same as the state where light is transmitted from one port to another. The light from the port is separated into ordinary light and extraordinary light by the first birefringent element 12, and the Faraday rotators 16 and 1
/ 2 wavelength plate 18. At this time, the extraordinary light has its polarization plane rotated by 90 degrees, but the ordinary light does not rotate as a result because the polarization plane returns to the original state. Since the extraordinary light is generated for the second birefringent element 14, the light travels with an optical path shift. The quarter-wave plate 32 converts the linearly polarized wave into a circularly polarized wave,
The return light reflected by the is again converted into linearly polarized light by the 波長 wavelength plate 32. As a result, in the quarter-wave plate 32, the polarization plane rotates 90 degrees while the light reciprocates, and the second birefringent element 14
Becomes ordinary light. Each light beam is ２
One of the polarization planes of the wave plate 18 and the Faraday rotator 16 is rotated by 90 degrees, but the other is not. Then, the ordinary light and the extraordinary light are combined by the first birefringent element 12 to form one light beam, which is emitted to the outer port. In this way, the light emitted from the port couples to the port, does not return to the port, and does not couple to the port.

Even if light is emitted from the port, it can pass through the optical circulator main body and the like, be reflected by the mirror, and return again along the optical path of passing through the optical circulator main body and the like again. No port exists. Therefore, it is not coupled to the port.

In the embodiment shown in FIG.
Although a quarter-wave plate is used as the polarization rotation element for rotating 0 degrees, a 45-degree Faraday rotator can be used instead. The polarization plane can be rotated by 90 degrees while the light reciprocates from the Faraday rotator to the mirror to the Faraday rotator.

The details of the input / output section 20 will be described with reference to FIG. As described above, the input / output unit 20 is a three-core ferrule 24 in which three fibers 22 are accommodated in one ferrule.
And an arrangement of one fiber coupling lens 26 common to the three fibers 22 and an optical path correction element 28. The optical path correction element 28 converts the light emitted from the outer fiber into an oblique parallel beam by the lens 26 into a beam parallel to the optical axis and offsets the return light (away from the optical axis). Function) to obliquely enter a beam parallel to the optical axis into the lens 26 and couple it to the outer fiber. The optical path correction element 28 converts the outgoing light from the central fiber toward the center of the lens 26 into a parallel beam along the optical axis without any change in the angle, and returns light along the optical axis toward the center of the lens 26 out of the return light. The parallel beam also has a function of being incident on the central fiber without any change in the angle and coupling. As the optical path correction element 28, a prism that transmits an inclined plane for parallel beams on both sides and transmits a vertical plane for a central parallel beam is used. Here, a prism having an integral structure in which two parallel and adjacent two ridges of a rectangular parallelepiped (square plate) are cut obliquely to form a slope is used, but a prism having a trapezoidal cross section may be used.

The light emitted from the three-core ferrule 24 containing the three fibers 22 (the distance between the two cores on both sides is defined as x) is emitted with their optical axes being parallel to each other and spreading. After passing through the lens 26, an angle α corresponding to the offset from the center is generated. Let the focal length of the lens 26 be f
Then, this angle α is α = α with respect to the central axis of the lens.
tan ^-1 (x / f). The prism functions to convert the oblique light into a beam parallel to the optical axis. Assuming that the angle A of the inclined surface of the prism is n, the refractive index is n, and A = sin ⁻¹ (n · sin (A−sin ⁻¹ (sin α / n)). For example, a material having a refractive index of 1.5 (for example, BK7) is used for the prism.
Material, wavelength 1.55 μm), the angle A ≒ 2
・ It becomes parallel to the optical axis when α. If the distance between the lens 26 and the optical path correction element 28 is set so that the beam interval d is 6 times lower than the beam waist diameter (currently, usually about 50 μm), the light leaking to the adjacent port is −. It can be expected to be less than 50 dB.

When such a combination of the three-core ferrule 24, the common single lens 26, and the optical path correcting element 28 is applied to the optical circulator main body 10 to form a configuration as shown in FIG. Can be adjusted,
Each optical element only needs to have an effective diameter and a thickness corresponding to a distance of about six times the beam diameter, so that remarkable miniaturization is possible. For example, if rutile is used as the birefringent element as described above, and its optical axis is inclined at 45 degrees from the optical axis, the optical path is about 1/10 of the thickness (length in the optical axis direction) of the birefringent element. The shift amount is obtained. The optical path shift amount may be about six times the beam waist diameter.
If the thickness is 0 μm, the birefringent element has a thickness of about 3 mm.
It suffices if there is an effective diameter of about 5 mm square. Further, in the present invention, since the reciprocation of light is used in the reflection type, the optical circulator main body is also shortened. From these facts, it is possible to achieve a significant size reduction (shortening and diameter reduction) as compared with the related art.

In the above embodiment, a prism having an integral structure is used as an optical path correction element. However, as shown in FIG. 5, a combination of two prisms may be used. This optical path correction element is a combination of two wedge-shaped prisms 29 symmetrically arranged with a center gap therebetween. A right-angled triangular prism may be used instead of the wedge-shaped prism 29. The light emitted from the outer fiber passes through the lens 26 to become a parallel beam, but is emitted obliquely. When this obliquely emitted beam passes through the wedge-shaped prism 29, it is refracted by its slope and becomes a beam parallel to the optical axis. The light emitted from the central fiber passes through the center of the lens 26 and becomes a parallel beam and exits. This emitted beam passes through the gap between the two wedge-shaped prisms 29 as it is. The return light (incident light to the fiber) is also
The beam parallel to the offset optical axis is refracted by the prism and coupled to the outer fiber, and the non-offset beam passes through the gap between the prisms and couples to the central fiber.

The configuration of the optical path correction element has a large number of components, but does not significantly increase the cost because prisms having the same shape may be combined. In addition, since the central beam does not enter the prism, there is an advantage that the beam is hardly affected by stray light or the like.

FIG. 6 is an explanatory diagram showing another embodiment of the present invention. The birefringent element near the entrance / exit portion in FIG. 1 is divided into two equal parts, and a half
The wave plate 46 is inserted. The portion indicated by reference numeral 40 is the portion that was the birefringent element 12 in FIG. Other configurations may be the same as those in FIG. 1, and the same components are denoted by the same reference numerals, and
Description is omitted. Birefringent element piece 42, 波長 wavelength plate 4
6 and the birefringent element piece 44 function as one birefringent element, whereby the optical path lengths are equal on both the forward path and the return path, and polarization dispersion does not occur. In other words, the polarization is exchanged between the two birefringent element pieces by the half-wave plate 46 to make the optical path length difference zero.

As shown in FIG. 7, the first birefringent element piece 4
2, the light is separated into ordinary light o and extraordinary light e, and
The wave plate 46 rotates the plane of polarization by 90 degrees. In the divided second birefringent element piece 44, the relationship between the extraordinary light e and the ordinary light o is inverted, and the light is further separated. Two birefringent element pieces 42, 4
4 (the length in the optical axis direction) is set to be the same (just half the thickness of the birefringent element 12 shown in FIG. 1) and the optical axis is rotated by 180 degrees about the optical axis. The optical path length of the light beam on the outward path and the optical path on the return path are the same, and a reflection type optical circulator having no polarization dispersion (dispersion compensation method) can be configured. Although the number of parts is certainly increased, the divided birefringent element pieces 42 and 44 need only be half the size of the birefringent element 12 in FIG. Does not increase much and does not increase the cost significantly.

In each of the above embodiments, a three-core ferrule having three fibers is used. However, if necessary, a multi-port ferrule having four or more fibers arranged side by side may be used. An optical circulator can also be configured.

[0028]

As described above, according to the present invention, a polarization rotator and a reflector are incorporated at one end of the optical circulator main body to form a reflection type, thereby providing a substantially two-stage high isolation. While reducing the number of parts,
There is an effect that the length dimension can be reduced. In addition, since the multi-port ferrule is used to make the multi-port, a common lens is combined with the multi-port ferrule, and the optical path is appropriately corrected by the optical path correction element. Since assembly can be performed, the number of assembly steps can be reduced. Further, since the beam interval can be made very narrow, the thickness (length in the optical axis direction) of the optical element used and the cross-sectional area perpendicular to the optical axis can be small, and the optical circulator can be remarkably miniaturized (length dimension). (Shortening and diameter reduction) are possible.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of a reflection type optical circulator according to the present invention.

FIG. 2 is an explanatory diagram of the operation from the port to the port.

FIG. 3 is an explanatory diagram of the operation from the port to the port.

FIG. 4 is an explanatory view showing an example of an input / output unit.

FIG. 5 is an explanatory view showing another example of the input / output unit.

FIG. 6 is an overall configuration diagram showing an embodiment of a dispersion-compensating reflective optical circulator according to the present invention.

FIG. 7 is an explanatory diagram of the main part.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical circulator main body part 12, 14 Birefringent element 16 Faraday rotator 18 1/2 wavelength plate 20 Input / output part 22 Fiber 24 Three-core ferrule 26 Lens 28 Optical path correction element 30 Reflection part 32 Polarization rotation element 34 Mirror

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiro Masuda 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Inventor Tsuguo Tokumasu 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. F-term (reference) 2H099 BA06 CA06 CA07 CA08

Claims (6)

[Claims] 1. A birefringent element, a 45-degree Faraday rotator and a half-wave plate inserted between them are arranged in a row, and an input / output section is provided at one end of the row. At the other end of the row, a polarization rotator for rotating the polarization plane by 90 degrees in a reciprocating manner and a reflector are arranged, and the input / output section is a multi-core ferrule having three or more fibers, and is shared by them. And an optical path correction element that couples a beam parallel to the optical axis as return light into a beam parallel to the optical axis and a beam parallel to the optical axis as output light from the fiber. Reflective optical circulator. 2. The reflection type optical circulator according to claim 1, wherein the polarization rotating element for rotating the polarization plane by 90 degrees in a reciprocating manner is a quarter-wave plate. 3. The reflection type optical circulator according to claim 1, wherein the polarization rotator for rotating the polarization plane 90 degrees reciprocally is a 45-degree Faraday rotator. 4. A three-core ferrule as a multi-core ferrule, and the optical path correction element is a prism or a trapezoidal prism in which two parallel ridges of a rectangular parallelepiped are obliquely cut off. 3. The reflective optical circulator according to any one of 3. 5. A three-core ferrule as a multi-core ferrule, wherein the optical path correction element comprises a combination of two right-angled triangular or wedge-shaped prisms symmetrically arranged with a center gap therebetween. The reflective optical circulator according to any one of the above. 6. A birefringent element near an input / output unit is divided into two parts, and a half-wave plate is inserted between the two birefringent element pieces to eliminate polarization dispersion. The reflective optical circulator according to any one of the above.