JPH08286150A - Optical isolator - Google Patents

Abstract

(57) [Abstract] [Objective] A polarization-independent optical isolator for blocking reflected return light from an optical system in an optical fiber communication using a semiconductor laser, particularly in an optical fiber amplifier. An optical isolator main body 10 in which at least a Faraday rotator 13 is arranged between birefringent crystal plates 11 and 14, and a magnet 16 for magnetizing the Faraday rotator is provided on the outer periphery thereof, and both ends of the optical isolator main body. Is an optical isolator having a collimator formed by fixing the focusing rod lens 2 to the tip of the optical fiber 1. The end surface of the focusing rod lens 2 is θ with respect to the plane A perpendicular to the center axis of the collimator. It is inclined.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical fiber communication using a semiconductor laser, and more particularly to a polarization-independent optical isolator for blocking reflected return light from an optical system in an optical fiber amplifier.

[0002]

2. Description of the Related Art Conventionally, an optical isolator of this type is shown in FIG. In the figure, 20 is an incident light beam, 1
Reference numeral 1 is a birefringent crystal plate, and when a light beam 20 is incident on this crystal plate, it is divided into two light beams having vibration planes perpendicular to each other.
One goes straight (ordinary ray) and the other goes diagonally (extraordinary ray). These rays propagate as two parallel rays after passing through the crystal plate. Reference numeral 12 has a crystal axis that is tilted by 67.5 degrees from the direction in which the crystal optical axes of the birefringent crystal plates 11 and 14 are projected on the surface. 1/2 wave plate that rotates 45 degrees, 13
Is a Faraday element composed of a magneto-optical crystal plate, 14 is a birefringent crystal plate in which crystal axes are arranged in the same direction as that of the birefringent crystal plate 11, and 15 is a substrate holding elements 11 to 14.
6 is a magnet for magnetizing the Faraday element 13,
The optical isolator main body 10 is constituted by these elements. At both ends of such an optical isolator body 10,
As shown in FIG. 7, a collimator 26 formed by introducing a spherical lens 17, a ferrule 24, and an optical fiber 18 into a cylindrical holder is arranged to reduce the number of assembling / adjusting steps and to manufacture at a lower cost. (JP-A-4-246615
Issue).

[0003]

Since this type of optical fiber collimator uses a spherical lens, reflection of the signal light incident on the main body of the optical isolator is reflected almost as it is. Processing such as applying a high-quality low-reflection multilayer film is required. As a result, there has been a problem that the manufacturing is complicated and the processing cost is increased. Therefore, an object of the present invention is to provide an optical isolator that can easily reduce the reflected light and can be assembled compactly.

[0004]

An optical isolator according to the present invention comprises at least a Faraday rotator disposed between birefringent crystal plates, and an optical isolator main body having magnets for magnetizing the Faraday rotator on the outer circumference thereof. An optical isolator having a collimator formed by fixing a focusing rod lens to the tip of an optical fiber at both ends of the optical isolator body, wherein an end surface of the focusing rod lens is perpendicular to a center axis of the collimator. It is characterized by being formed by sloping,

Further, in the collimator, the direction of the light emitted from the end face of the collimator is inclined with respect to the center axis of the collimator, and the emitted light is incident on the birefringent crystal plate of the optical isolator body to refract extraordinary light. It is characterized in that it is arranged so as to be in the opposite direction to the direction.

[0006]

According to the above construction, the end surface of the focusing rod lens according to the present invention is tilted by 4 ° to 8 ° with respect to the surface perpendicular to the central axis of the collimator, so that the reflected light or the end surface generated at the end surface is formed. It is possible to realize an optical isolator in which the reflected light incident on is removed to a value at which there is practically no problem. Since the direction of the light emitted from the end face of the collimator and the direction in which this emitted light enters the birefringent crystal plate and is refracted are arranged, the signal light passes near the central axis. The performance of the optical isolator is stable, and the optical isolator can be formed in a small size.

[0007]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is a diagram showing the overall structure of an optical isolator according to this embodiment. In the figure, at least a Faraday rotator 13 is arranged between the birefringent crystal plates 11 and 14, and an optical isolator main body 10 provided with magnets 16 for magnetizing the Faraday rotator on the outer periphery thereof, and both ends of the optical isolator main body 10. Is an optical isolator having a collimator 4 formed by fixing a focusing rod lens 2 to the tip of an optical fiber 1. The end face of the focusing rod lens 2 is a plane perpendicular to the collimator central axis a. It is formed with an inclination (θ).

In the collimator 4, the optical fiber 1 is a single mode type optical fiber and a focusing type rod lens 2 composed of a graded index type optical fiber of a predetermined length is fusion-bonded, and a ferrule 3 of a transparent glass tube is provided on the outer periphery thereof. It is formed by covering. The end surface of the collimator 4 is formed in this way, and then given a predetermined angle (θ) by optical polishing or the like. The end face is formed by cutting a graded index type optical fiber into a predetermined length, which simplifies the structure. However, this is because reflection occurs when the signal light passes, and this is removed.

FIG. 2 shows measured values showing the relationship between the inclination angle θ of the end face and the return loss of the reflected light returning to the single mode optical fiber 1. As a result, when the inclination angle θ is 4 ° or more, it becomes a value that can be ignored in practical use.

On the other hand, FIG. 3 shows the relationship between the insertion loss and the angle (φ) when the collimator 4 having such an inclination angle θ is arranged oppositely and the other collimator is twisted. When the tilt angle θ becomes large, the insertion loss rapidly increases even for a slight twist angle, and when the tilt angle θ exceeds 8 ° and becomes 10 °, the twist angle needs to be adjusted to 1 ° or less. From this viewpoint, the inclination angle θ is appropriately 8 ° or less.

Further, the relationship between the collimator 4 and the birefringent crystal plates 11 and 14 of the optical isolator body 10 will be examined with reference to FIG. In FIG. 4A, the signal light propagating along the central axis a of the collimator 4 is refracted by the angle α due to the end face inclination angle θ and enters the birefringent crystal plate 11. In the birefringent crystal plate 11, the ordinary light travels straight, but the extraordinary light is refracted at the angle β and propagates. Here, the directions of the angle α and the angle β are opposite to each other.

On the other hand, in the case of FIG. 4B, when the center axis of the collimator 4 is arranged twisted by 180 °, the directions of the angle α and the angle β have the same relationship. In such an arrangement, the signal light passing through the optical isolator main body 10 is located at a position displaced from the central axis a, which is not preferable from the viewpoint of the performance and size of the optical isolator. When the arrangement is such that the directions of the angle α and the angle β are opposite to each other as shown in FIG. 4A, the deviation with respect to the central axis does not become large, so that the influence of the end face inclination angle can be minimized.

In the structure of FIG. 1, the focusing type rod lens 2 has an outer diameter of 150 μm, a length of 920 μm, and a relative refractive index difference of 0.
9%, inclination angle θ5 °; optical isolator main body 10 has birefringence crystal plates 11 and 14 with a thickness of 0.9 mm, Faraday rotator 13 has a thickness of 0.1 mm, and a half-wave plate has a thickness of 0.4 m.
When the wavelength was 1.55 μm, the return loss was 65 dB, the insertion loss was 0.4 dB, and the isolation was 42 dB, and the deviation of the collimator center axis was 200 μm.
Met.

FIG. 5 is a view showing an optical isolator composed of two optical isolator main bodies 10-1 and 10-2 and two magnets 16-1 and 16-2. In this case, the focusing rod lens 2 has an outer diameter of 135 μm and a length of 880.
μm, relative refractive index difference 1.0%, inclination angle θ4 °; the optical isolator main body 10 has birefringent crystal plates 11 and 14 having thicknesses of 0.4 mm and 0.95 mm, respectively, and Faraday rotator 13 having a thickness of 0. When manufactured with a wavelength of 1.55 μm, the return loss was 62 dB, the insertion loss was 0.7 dB, the isolation was 60 dB, and the collimator center axis shift was 140 μm.

[0016]

As described above, since the end surface of the focusing rod lens according to the present invention is inclined by 4 ° to 8 ° with respect to the plane perpendicular to the central axis of the collimator, the reflected light generated at the end surface or It is possible to realize an optical isolator in which the reflected light incident on the end face is removed to a value that causes substantially no problem. Since the direction of the light emitted from the end face of the collimator and the direction in which this emitted light enters the birefringent crystal plate and is refracted are arranged, the signal light passes near the central axis. The performance of the optical isolator is stable, and the optical isolator can be formed in a small size.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of an optical isolator according to an embodiment.

FIG. 2 is a diagram showing a relationship between a tilt angle θ and a return loss.

FIG. 3 shows the relationship between the twist angle φ and the insertion loss.

FIG. 4 is a partially enlarged view of FIG.

FIG. 5 is a diagram showing an overall configuration of an optical isolator according to another embodiment.

FIG. 6 is a diagram showing a configuration of an optical isolator main body according to a conventional example.

FIG. 7 is a diagram showing a conventional collimator.

[Explanation of symbols]

 1, 18: Optical fiber 2: Lens 3, 24: Ferrule 4, 26: Collimator 10: Optical isolator main body 11, 14: Birefringence crystal plate 12: 1/2 wavelength plate 13: Faraday rotator 16: Magnet 17: Ball lens 20: Ray

Claims (3)

[Claims] 1. An optical isolator main body in which at least a Faraday rotator is arranged between birefringent crystal plates, and a magnet for magnetizing the Faraday rotator is provided on the outer circumference thereof, and optical fiber tips are provided at both ends of the optical isolator main body. An optical isolator having a collimator formed by fixing a focusing rod lens to the optical axis, wherein an end surface of the focusing rod lens is formed to be inclined with respect to a plane perpendicular to the central axis of the collimator. Optical isolator. 2. The optical isolator according to claim 1, wherein the end surface of the focusing rod lens is tilted by 4 ° to 8 ° with respect to a plane perpendicular to the central axis of the collimator. 3. The collimator has a direction in which light emitted from its end face is inclined with respect to the center axis of the collimator, and the emitted light is incident on the birefringent crystal plate of the optical isolator body to refract extraordinary light. The optical isolator according to claim 1, wherein the optical isolator is arranged so as to be opposite to the direction.