JP2989983B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator using a birefringent crystal plate, and more particularly to a polarization-independent optical isolator that operates without considering the plane of polarization of incident light.

[0002]

2. Description of the Related Art In an optical transmission system using an optical fiber, when signal light is transmitted over an optical fiber over a long distance, the plane of polarization of the signal light is not preserved. Therefore, in an optical transmission system using an optical fiber, a polarization-independent optical isolator that functions as an optical isolator without depending on the polarization direction of light is used.

Recently, an optical fiber amplifier using an Er-doped fiber has been attracting attention. Even in this case, the optical fiber amplifier oscillates in the Er-doped fiber due to return light reflected from each optical component or a connection point, which increases noise. Cause
An optical isolator for removing reflected return light is required. When the signal light is transmitted through the Er-doped fiber, the polarization plane of the signal light is not preserved. Therefore, a polarization-independent optical isolator that does not depend on the polarization direction of the signal light is used for the optical fiber amplifier. Various types of polarization-independent optical isolators have been proposed, such as JP-A-1-287528 and JP-A-4-75311.

Here, an example of a conventional example will be described with reference to FIGS. 4 and 5. FIG. 4 (b) show the configuration of the conventional example, and FIGS. 4 (a) and 4 (c) show the polarization state when light passes through each component when viewed from the optical fiber F1 side. Is shown.
(A) is a forward direction (+ Z direction), (c) is a reverse direction (-
Z direction). In the forward direction, at the time of incidence, the polarization component orthogonal to the plane including the extraordinary wave vector of the refracted light and the optical axis of the birefringent crystal plate (hereinafter referred to as the main cross section) is P1, and the parallel polarization component is P1. P2, a black circle at the center of each polarized light component, and an open circle at one end of the polarized light component P1 for the orthogonal polarized light component P1 to distinguish each polarized light component. Further, the central axis in which the incident light travels is indicated by black circles. Note that the polarization components P1, P at the time of forward incidence
2 is also used in the reverse direction. Further, a polarized light component orthogonal to the main cross section of each birefringent crystal plate is defined as ordinary light, and a polarized light component parallel thereto is defined as extraordinary light.

[0005] In FIG. 4, from FIG. 4 (b), a converging lens (omitted), a birefringent crystal plate 7, a Faraday rotator 11 for rotating the polarization plane of transmitted light by 45 ° from the incident direction between the optical fibers F 1 and F 2, Birefringent crystal plate 8, birefringent crystal plate 9, and birefringent crystal plate 10 are arranged in this order. The optical axes of the birefringent crystal plates 7, 8, 9, and 10 have an inclination angle of 45 ° with respect to the optical axis, and the thickness ratio is √2: 1: (1 + √2 / 2): √2.
/ 2. The main cross sections of the birefringent crystal plates 9 and 10 are coincident, but are orthogonal to the main cross section of the birefringent crystal plate 8. Further, the main cross sections of the birefringent crystal plates 9 and 10 and the main cross section of the birefringent crystal plate 8 are respectively rotated by 45 ° with respect to the main cross section of the birefringent crystal plate 7.
3A, the light incident on the birefringent crystal plate 7 is separated into ordinary light P1 and extraordinary light P2 with respect to the main cross section. The ordinary light P1 proceeds straight as it is, and the extraordinary light P2 moves by √2 L horizontally and is emitted from the birefringent crystal plate 7 to the Faraday rotator 11. The polarization plane is rotated by 45 ° by the Faraday rotator 11, and P
1, P2 enters the birefringent crystal plate 8. Birefringent crystal plate 8
P1 is ordinary light, and P2 is extraordinary light.
Goes straight as it is, and P2 further moves horizontally by L and exits from the birefringent crystal plate 8. In this case, the moving distance of P1 and P2 is (1 + √2) L for P1 with respect to 0. Orthogonal polarization component P1, which is incident on the birefringent crystal plate 9,
Since P2 is extraordinary light and P2 is ordinary light with respect to the birefringent crystal plate 9, P2 goes straight and P1 becomes (1 + √2 /
2) The light is emitted from the birefringent crystal plate 9 after moving horizontally L. At this time, since the moving distances of P1 and P2 are both (1 + √2) L, the optical path lengths of the two orthogonal polarization components are equal.
The incoming and outgoing positions of the light are not on the same straight line.

In the reverse direction, as shown in FIG. 3C, the reflected return light incident on the birefringent crystal plate 10 is separated into two orthogonally polarized components, ie, extraordinary light P1 and ordinary light P2 by the main cross section of the birefringent crystal plate 10. You. The ordinary light P2 goes straight as it is, and the extraordinary light P1 moves horizontally (√2 / 2) L and exits from the birefringent crystal plate 10 to the birefringent crystal plate 9. The two orthogonal polarization components P1 and P2 incident on the birefringent crystal plate 9 are such that P1 is extraordinary light and P2 is ordinary light with respect to the birefringent crystal plate 9, so that P2 goes straight and P1 is (1 + √2 / 2). (2) The light moves horizontally to return to the central axis where the incident light travels, and exits to the birefringent crystal plate 7. As for the two orthogonal polarization components P1 and P2 incident on the birefringent crystal plate 7, since P1 is ordinary light and P2 is extraordinary light with respect to the birefringent crystal plate 7, P1 goes straight as it is and P2 moves L horizontally. The light is emitted to the Faraday rotator 11. Faraday rotator 1
The two orthogonal polarization components P1 and P2 that have passed through 1 are each rotated by 45 ° in the polarization plane. At this time, since the Faraday rotator is non-reciprocal, the polarization direction differs from the polarization component in the forward direction by 90 °. The two orthogonal polarization components P1 and P2 that have passed through the Faraday rotator 11 enter the birefringent crystal plate 6.
At this time, P1 becomes extraordinary light and P2 becomes ordinary light with respect to the birefringent crystal plate 6, and P1 moves horizontally by 2L and the birefringent crystal plate 6
And both polarized components are emitted from a position different from the position of the optical fiber F1, so that the return light can be blocked without returning to the optical fiber F1.

In FIG. 5, (b) shows that the birefringent crystal plate 12, the Faraday rotator 16 for rotating the polarization plane of the transmitted light by 45 °, and the birefringent crystal plate 13 between the optical fibers F1 and F2 from the incident direction. 4 in the opposite direction to the Faraday rotator 16
Faraday rotator 17 rotating by 5 °, birefringent crystal plate 1
4. The Faraday rotator 18 that rotates the polarization plane by 45 ° in the direction opposite to the polarization plane rotation direction of the Faraday rotator 16 and the birefringent crystal plate 15 are arranged in this order. The thickness ratio of the birefringent crystal plates 12, 13, 14, 15 is 1: 、, respectively.
2: 1: √2. The birefringent crystal plates 12, 1
4 are the same, but the birefringent crystal plate 13,
The 15 main sections are orthogonal. The main cross sections of the birefringent crystal plates 13 and 15 are rotated 45 ° with respect to the main cross sections of the birefringent crystal plates 12 and 14, respectively.

In FIG. 1A, light incident on the birefringent crystal plate 12 is separated into an ordinary light P1 and an extraordinary light P2 with respect to the main section. The ordinary light P1 proceeds straight as it is, and the extraordinary light P2 moves horizontally by L and is emitted from the birefringent crystal plate 1 to the Faraday rotator 16. The polarization plane is rotated by 45 ° by the Faraday rotator 16, and P 1 and P 2 enter the birefringent crystal plate 13. Also for the birefringent crystal plate 13, P1 is ordinary light and P2 is extraordinary light, so that P1 goes straight on and P2 is further 2L.
The light moves horizontally and exits from the birefringent crystal plate 13. In this case, the moving distances of P1 and P2 are as follows: P1 is 0, and P2 is (1
+ √2) L. The Faraday rotator 17 rotates the plane of polarization by 45 ° in the direction opposite to that of the Faraday rotator 16, and P 1 and P 2 enter the birefringent crystal plate 14. The orthogonal polarization components P1 and P2 are
Since 1 is ordinary light and P2 is extraordinary light, P1 goes straight ahead and P
2 is horizontally moved by L and emitted to the Faraday rotator 18. The polarization plane is rotated by 45 ° in the opposite direction to the Faraday rotator 16 by the Faraday rotator 18, and P 1 and P 2 enter the birefringent crystal plate 15. The orthogonal polarization components P1 and P2 incident on the birefringent crystal plate 15 also travel straight with respect to the birefringent crystal plate 15 because P1 is ordinary light and P2 is extraordinary light. P2 moves horizontally, is orthogonally combined with P1, and is emitted to the optical fiber. At this time, the positions of the optical fiber F1 and the optical fiber F2 are on the same straight line, but the moving distance of P1 and P2 is P1 = 0; P2 = (2 + √2) L. The lengths are different.

In the opposite direction, as shown in FIG. 3C, the reflected return light incident on the birefringent crystal plate 15 is 2 L horizontally to two orthogonally polarized components, ordinary light P1 and extraordinary light P2, due to the main cross section of the birefringent crystal plate 15. It moves and exits from the birefringent crystal plate 15 to the Faraday rotator 18. The two orthogonal polarization components P1 and P2 that have passed through the Faraday rotator 18 have their polarization planes rotated by 45 °, and are emitted to the birefringent crystal plate 14. At this time, since the Faraday rotator is non-reciprocal, the polarization direction differs from the polarization component in the forward direction by 90 °. Birefringent crystal plate 14
Of the two orthogonally polarized light components P1 and P2 incident on the birefringent crystal plate 14, P1 becomes extraordinary light and P2 becomes ordinary light, so that P2 goes straight, and P1 moves L horizontally and exits to the Faraday rotator 17. I do. The two orthogonally polarized light beams P1 and P2 that have passed through the Faraday rotator 17 and entered the birefringent crystal plate 13 are such that P1 is ordinary light and P2 is extraordinary light with respect to the birefringent crystal plate 13; P2 is moved to the Faraday rotator 16 by # 2L horizontal movement. The two orthogonal polarization components P1 and P2 that have passed through the Faraday rotator 16 enter the birefringent crystal plate 12. At this time, the birefringent crystal plate 12
P1 becomes extraordinary light, P2 becomes ordinary light, and P1 moves L horizontally and is emitted from the birefringent crystal plate 12, and both polarized components are emitted from a position different from the position of the optical fiber F1. The return light can be blocked without returning.

In the conventional example described above, the optical isolator function is performed regardless of the polarization direction of the incident light.

[0011]

A long-distance optical amplification repeater system using an optical fiber amplifier (EDFA) using Er-doped fiber has a high transmission speed and several Gbits.
/ S or more, polarization dispersion becomes one of the causes of deterioration of transmission characteristics. It is the optical isolator that causes polarization dispersion among the optical components constituting the EDFA. In order to reduce the polarization dispersion, two orthogonal polarization components until the signal light is separated and combined into two orthogonal polarization components in the forward direction.
It is necessary to make the optical path lengths of the polarization components equal.

In FIG. 4, the optical path lengths of the orthogonal two-polarized light components until the signal light is separated and combined into the orthogonal two-polarized light components in the forward direction are equal, but the incoming and outgoing light are not on the same straight line. Optical adjustment at the time of assembly becomes complicated,
Further, since the thicknesses of the polarizing element materials are not the same, several types of polarizing elements must be used, which leads to an increase in the cost of the optical isolator itself.

In FIG. 5, the incident light and the outgoing light are on the same straight line. However, since the thickness of the polarizing element material is not the same, several types of polarizing elements must be used. Leads to increased costs. Also, this optical isolator is used in a long-distance optical amplification system using an EDFA, because in the forward direction, the optical path length of the orthogonal two-polarization components before the signal light is separated and combined into the orthogonal two-polarization components. In such a case, polarization dispersion may occur, leading to deterioration of transmission characteristics.

The present invention has been made in view of such a technical problem, and since a single polarizing element is easy to assemble, material costs and costs can be reduced in an assembling process. Another object of the present invention is to provide an optical isolator capable of reducing polarization dispersion.

[0015]

The present invention uses a birefringent flat birefringent crystal plate disposed between one or a plurality of pairs of optical waveguides, and uses a first birefringent crystal plate from the light incident side. Refraction crystal plate, second birefringence crystal plate, 45 ° rotation non-reciprocal element, 4
The 5 ° rotation reciprocal element, the third birefringent crystal plate, and the fourth birefringent crystal plate are arranged in this order, and the thicknesses of the four birefringent crystal plates are all equal, and the 45 ° rotation nonreciprocal element and the 45 ° °
This is an optical isolator in which the rotation directions of the rotation reciprocal elements are opposite.

Further, in the present invention, the angle between the polarization separation directions of the first and second birefringent crystal plates and the angle between the polarization separation directions of the third and fourth birefringence crystal plates are 90 °, 1
The angle between the polarization split direction of the second and fourth birefringent crystal plates and the angle between the polarization split directions of the second and fourth birefringent crystal plates is 180 °, and the polarization direction of the second and third birefringent crystal plates. Is 90 °, and in the forward direction, the light incident on the first birefringent crystal plate and the light emitted from the fourth birefringent crystal plate are on the same straight line, and the light is incident on the first birefringent crystal plate. An optical isolator that is separated into two orthogonal polarized lights by a birefringent crystal plate and has the same optical path length between the two polarized lights until orthogonally combined by the fourth birefringent crystal plate.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. Since the optical isolator to which the present invention is applied always requires a permanent magnet, illustration and description thereof are omitted. In FIG. 1, FIG. 1 (b) shows the configuration of one embodiment of the present invention, and FIGS. 1 (a) and 1 (c) show the polarization state of light when passing through each component when viewed from the optical fiber F1 side. . (A) shows the forward direction, and (c) shows the reverse direction. In FIG. 6B, a convergent lens (omitted), a birefringent crystal plate 1, a birefringent crystal plate 2, and a polarization plane of transmitted light as a non-reciprocal element between the optical fibers F1 and F2 are set to 45.
Rotating Faraday rotator 5, Half-wave plate or optical rotator 6 rotating the polarization plane of transmitted light by 45 ° in the opposite direction to Faraday rotator 5 as a reciprocal element, Birefringent crystal plate 3, Birefringent crystal plate 4, Convergence They are arranged in the order of lenses (omitted). The thickness ratio of the birefringent crystal plates 1, 2, 3, 4 is 1:
It is composed of 1: 1: 1. The main sections of the birefringent crystal plate 1, the birefringent crystal plate 2, the birefringent crystal plate 3, and the birefringent crystal plate 4 are orthogonal to each other. The main cross sections of the birefringent crystal plate 1, the birefringent crystal plate 3, the birefringent crystal plate 2, and the birefringent crystal plate 4 are the same. In all the birefringent crystal plates, when the light perpendicularly incident on the plane (boundary surface) perpendicular to the center axis where the incident light travels is separated into two orthogonal polarized lights, the incident light is maximized so as to maximize the separation angle. Are inclined at a predetermined angle with respect to the central axis.

In FIG. 1A, light incident on the birefringent crystal plate 1 is separated into ordinary light P1 and extraordinary light P2 with respect to the main cross section of the birefringent crystal plate 1. The ordinary light P1 goes straight as it is, and the extraordinary light P2 moves horizontally by L and is emitted from the birefringent crystal plate 1 to the birefringent crystal plate 2. The two orthogonal polarized light components P1 and P2 incident on the birefringent crystal plate 2 are extraordinary light and P2 are ordinary light with respect to the birefringent crystal plate 2, so that P2 goes straight and P1 is L horizontal. It moves and exits from the birefringent crystal plate 2. At this time, the moving distance of P1 and P2 is L for both P1 and P2.
It is. The two orthogonal polarization components P1 and P2 emitted from the birefringent crystal plate 2 are 45 ° by the Faraday rotator 5,
The Faraday rotator 5 is turned 45
Due to the rotation, the polarization states of P1 and P2 passing through the half-wave plate 6 are the same as the polarization states passing through the birefringent crystal plate 2. The two orthogonal polarization components P1 and P2 incident on the birefringent crystal plate 3 are such that P1 is ordinary light for the birefringent crystal plate 3,
Since P2 becomes extraordinary light, P1 goes straight, and P2 moves L horizontally and is emitted from the birefringent crystal plate 3. The orthogonal polarized light components P1 and P2 incident on the birefringent crystal plate 4 have an extraordinary light and an ordinary light P2 with respect to the birefringent crystal plate 4.
2 goes straight on, P1 moves horizontally by L, and is orthogonally combined with P1 and emitted to the optical fiber F2. P1, P2 at this time
Are both 2L, the optical path lengths of the two orthogonal polarization components are equal, and the incident and exit positions of the light are on the same straight line.

In the reverse direction, as shown in FIG. 3C, the reflected return light incident on the birefringent crystal plate 4 has two orthogonal polarization components, the extraordinary light P1 and the ordinary light P2, due to the main cross section of the birefringent crystal plate 4.
Is separated into The ordinary light P2 goes straight as it is, and the extraordinary light P1
Moves horizontally by L and exits from the birefringent crystal plate 4 to the birefringent crystal plate 3. The two orthogonal polarization components P1 and P2 incident on the birefringent crystal plate 3 are such that P1 is ordinary light, P
Since P2 becomes extraordinary light, P1 goes straight, and P2 moves L horizontally and exits to the half-wave plate 6. The two orthogonal polarization components P1 and P2 that have passed through the half-wave plate 6 and the Faraday rotator 5 rotate their respective polarization directions by 90 °. Therefore, the polarization direction differs by 90 ° from the polarization direction in the forward direction. Birefringent crystal plate 2
Are incident on the birefringent crystal plate 2, P1 is ordinary light, and P2 is extraordinary light.
1 moves straight, P2 moves horizontally L and exits to the birefringent crystal plate 1. The two orthogonally polarized light components P1 and P2 incident on the birefringent crystal plate 1 are extraordinary light at P1 and ordinary light at P2 with respect to the birefringent crystal plate 1, and P1 moves horizontally L and is emitted from the birefringent crystal plate 1. , Both polarization components are optical fiber F1
Is emitted from a position different from the position of the optical fiber F
The return light can be blocked without returning to 1. In this embodiment, regardless of the polarization direction of the incident light, the optical path length of the two orthogonal polarization components to be separated and combined in the forward direction is equal at the same time as performing the optical isolator function, and the incident and exit positions of the light are on the same straight line. is there.

FIG. 2 shows another embodiment utilizing the present invention. FIG. 2B shows the configuration of the embodiment.
(C) shows the polarization state of light when passing through each component when viewed from the optical fiber. (A) The figure shows the forward direction,
(C) The figure shows the reverse direction. Even if the 45 ° rotating non-reciprocal element and the 45 ° rotating reciprocal element of the optical isolator of the present invention shown in FIG. 1 are switched back and forth, the optical isolator function is performed regardless of the polarization direction of the incident light and the light is separated in the forward direction. The same effect as the optical isolator shown in FIG. 1 can be obtained such that the optical path lengths of the two orthogonally polarized light components to be combined are equal, and the light input / output positions are on the same straight line.

FIG. 3 shows another embodiment in which the optical isolator of the present invention shown in FIG. 1 is integrated. The optical axes of all birefringent crystal plates, which are polarizing elements of the optical isolator of the present invention, are inclined at a predetermined angle with respect to the central axis where incident light travels,
Since the thicknesses are equal, the birefringent crystal plate used in the optical isolator of the present invention may be of one type. When the plane perpendicular to the center axis of the incident light on the birefringent crystal plate, which is a polarizing element, is square, the birefringent crystal plate 1 and the birefringent crystal plate 2
In, each optical axis may be arranged at a predetermined position, and may be joined so that the main cross sections are orthogonal to each other,
No need to adjust the rotation around the central axis where the incident light travels,
You only need to align the square surface to the predetermined position,
Polarizer B integrating birefringent crystal plate 1 and birefringent crystal plate 2
By rotating the birefringent crystal plate 3 and the birefringent crystal plate 4 integrally by rotating the birefringent crystal plate 180 up and down by 180 °, the assembly process can be greatly reduced.

[0022]

As described above, according to the structure of the present invention, since the optical path lengths of two orthogonally polarized lights to be separated and combined in the forward direction are equal, long-distance transmission using an EDFA in which polarization dispersion becomes a problem. Can be used for systems. Further, since the light incident and exit positions are on the same straight line, the optical adjustment with the collimator lens is not adjusted or is easy. Further, the same birefringent crystal plate can be used, and the integrated polarizer B1 is rotated 180 ° up and down, so that the integrated polarizer B2 can be used.
When the birefringent crystal plate has a square shape, no rotation adjustment is required in assembling the integrated polarizer, so that the number of assembling steps is reduced, and the assembling is facilitated, leading to cost reduction.

[Brief description of the drawings]

FIG. 1 shows a schematic configuration diagram of an optical isolator according to one embodiment of the present invention and polarization components in a forward direction and a reverse direction.

FIG. 2 shows a schematic configuration diagram of an optical isolator according to another embodiment using the present invention, and shows a state of polarization components in forward and reverse directions.

FIG. 3 shows an integrated schematic diagram of an optical isolator of another embodiment utilizing the present invention.

FIG. 4 shows a schematic configuration diagram of a conventional optical isolator and polarization components in forward and reverse directions.

FIG. 5 shows a schematic configuration diagram of a conventional optical isolator and polarization components in forward and reverse directions.

[Explanation of symbols]

 F1, F2 Optical fiber 1-4 Birefringent crystal plate 5 Faraday rotator 6 Half-wave plate

Claims (3)

(57) [Claims] An optical isolator using a birefringent crystal plate disposed between one or a plurality of pairs of optical waveguides, wherein a first birefringent crystal plate, a second birefringent crystal plate,
A 5 ° rotation non-reciprocal element, a 45 ° rotation reciprocal element, a third birefringent crystal plate, and a fourth birefringent crystal plate are arranged in this order, and the four birefringent crystal plates are all equal in thickness. Optical isolator. 2. An angle between a polarization separation direction of said first and second birefringent crystal plates and an angle between a polarization separation direction of said third and fourth birefringent crystal plates is 90 °, and said first and second birefringent crystal plates are 90 °. The angle formed by the polarization splitting directions of the three birefringent crystal plates and the angle formed by the polarization splitting directions of the second and fourth birefringent crystal plates are 180 °, and the angle formed by the polarization directions of the second and third birefringent crystal plates. Angle 9
2. The optical isolator according to claim 1, wherein the angle is 0 [deg.]. 3. In the forward direction, light incident on the first birefringent crystal plate and light emitted from the fourth birefringent crystal plate are on the same straight line, and the light is reflected by the first birefringent crystal plate. The optical isolator according to claim 1, wherein the optical path lengths of the two polarized lights are equal to each other until they are separated into two orthogonal polarized lights and are orthogonally combined by the fourth birefringent crystal plate.