JPH04204712A - Optical isolator - Google Patents

Abstract

PURPOSE:To eliminate the dependency on polarized light by setting at least either of the aperture diameter on a forward direction side and on a backward direction side at the size of >=3 times the beam sepn. width at the time when the ordinary ray and extraordinary ray separated by double refraction polarizing plates transmit a Faraday rotor. CONSTITUTION:At least either one of the aperture diameter on the forward direction side and on the backward direction side is set at the size of >=3 times the beam sepn. width at the time when the ordinary ray and extraordinary ray separated by the double refraction polarizing plates 3, 3' transmit the Faraday rotor 4. The two rays reflected by the aperture in the forward direction pass a polarization separating plate 3, the Faraday rotor 4, a lambda/2 plate 6, and a polarization separating plate 3'. These rays are emitted to the positions apart from an optical fiber 1' by the beam sepn. width at the time when the rays respectively pass the Faraday rotor. The rays are then released from an optical isolator. The high isolation is, therefore, maintained. The optical isolator having no dependency on polarized light is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical isolator used, for example, in optical communication or optical transmission, and particularly relates to an optical isolator without polarization dependence used between optical fibers.

[Prior Art] Conventionally, there have been examples of optical isolators used in optical communication and optical transmission as shown in FIGS. 8 and 9.

8 and 9, 1 and 1' are optical fibers,
2 and 2' are lenses, 3, 3', and 3# are birefringent polarization separation plates, 4 is a Faraday rotator, and 6 is a % wavelength plate (λ/2 plate).

In FIG. 8, the birefringent polarization separation plates 3 and 3' have the same separation width between the ordinary ray and the extraordinary ray.

First, light that is emitted from the optical fiber 1 and collimated by the lens 2 passes through the birefringent polarization separation plate 3, where it is separated into an extraordinary ray polarized horizontally to the plane of the paper and an ordinary ray polarized vertically. These rays are connected to the Faraday rotator 4
Each plane of polarization rotates by 145 degrees. Here, the rotation sign of the polarization plane is defined as 0 in the direction in which the hour hand moves toward the direction in which the light travels. Next, each plane of polarization is further rotated by 145 degrees using a λ/2 plate 6. In the end, before and after passing through the Faraday rotator and the λ/2 plate, a ray polarized in the horizontal direction on the paper becomes a ray polarized in the vertical direction, and a ray polarized vertically becomes a ray polarized in the horizontal direction. . If the optical axes of the birefringence separation plates 3 and 3' are made to coincide, the horizontally polarized light ray and the vertically polarized light ray will become extraordinary rays and ordinary rays, respectively, with respect to the birefringence polarization separation plate 3'. Therefore, two rays can be combined. Therefore, lens 2
' can be coupled to the optical fiber 1', and light traveling in this direction can pass through with almost no loss regardless of its polarization direction (FIG. 8(a)).

On the other hand, with this component arrangement, the light emitted from the optical fiber 1' and collimated by the lens 2' is transmitted to the birefringent polarization separation plate 3'.
The beam is separated into an extraordinary ray polarized horizontally to the plane of the paper and an ordinary ray polarized vertically.

Each light beam passing through the λ/2 plate 6 rotates the plane of polarization by 145 degrees. Next, when it passes through the Faraday rotator 4, it is rotated by +45 degrees. After all, the polarization directions of the two light beams do not change before and after passing through the λ/2 plate and the Faraday rotator.

Therefore, the two light beams are not combined by the birefringent polarization separation plate 3, but are emitted to a position separated from the optical fiber 1 by the beam separation width when passing through the Faraday rotator. Since the two lenses do not couple into one optical fiber, light cannot pass in this direction (Fig. 8(b)).

In FIG. 9, the birefringent polarization separation plates 3, 3' and 3'' have a separation width of ordinary rays and extraordinary rays of ff: 1:
It was from 1.

The method shown in Figure 9 has some differences from Figure 8 in its configuration, polarization state of the light beam, and light propagation path, but in the forward direction, it combines two separated polarized lights that are perpendicular to each other. They are coupled to the other optical fiber by a lens, and in the opposite direction, the two separated polarized lights remain separated and are not coupled to the optical fiber, so they are basically the same.

In this way, an optical isolator without polarization dependence is possible.

[Problem to be solved by the invention]

Normally, it is necessary to attach an aperture to ensure the strength of the optical isolator and to specify the beam incidence position and usable beam diameter, but there is a problem that attaching an aperture significantly deteriorates the isolation.

[Means to solve the problem]

An object of the present invention is to provide an optical isolator with high isolation and no polarization dependence.

In order to achieve this objective, in the optical isolator of the present invention, one of the aperture diameters in the forward direction and the aperture diameter in the reverse direction is fixed by one or more birefringent polarization splitting plates. 3 of the beam separation width when the separated ordinary and extraordinary rays pass through the Faraday rotator
It is characterized by more than double the amount.

Hereinafter, the present invention will be explained in detail using Examples.

In Figures 1 and 2, 1 and 1' are single mode fibers, 2 and 2' are lenses, and 3°3' and 3''
4 is a Faraday rotator, 5 is a magnet, 6 is a λ/2 plate, and 7 and 7' are apertures.

Optical fibers 1 and 1' and lenses 2 and 2' are optically adjusted in advance so that parallel beams can be obtained.

The beam diameter was approximately 300 pm.

As the Faraday rotator 4, bismuth-substituted magnetic garnet was used and fixed to the samarium cobalt magnet 5. λ
/2 As the plate 6, crystal was used. Two sets of optically adjusted optical fibers and lenses were optically coupled by separating them by the length of an optical isolator inserted between them. In between, fix a holder whose components can be adjusted, polarization separation plate 3,
The Faraday rotator 4, the λ/2 plate 6, and the polarization separation plate 3' are arranged in this order (in the configuration shown in FIG. 2, the polarization separation plate 3, the Faraday rotator 4, the polarization separation plate 3', and the polarization separation plate 3''). 3.6 while monitoring the transmitted light II (insertion loss and isolation) using a laser light source and optical power meter.
and 3' adjustment around the optical axis (3.3' in the configuration shown in Figure 2)
and 3#) and the optical axes of the fiber 1' and lens 2' were adjusted. The beam portion NWA at the Faraday rotator of this optical isolator was 580 t1m, and the isolation was 43 dB. I attached an aperture to this.

The optical isolator of Example 1 had the structure shown in FIG. 1, and the aperture opening diameter on the forward direction side was 0.7 m, and the aperture diameter on the reverse direction side was 1.8 m. Isolation was 43 dB.

The optical isolator of Example 2 had the structure shown in FIG. 2, and the aperture diameter on the forward direction side was 0.7 m, and the aperture diameter on the reverse direction was 1.8 m. Isolation was 43 dB.

3 and 4 are diagrams showing propagation paths of light incident from opposite directions in the optical isolators of Example 1 and Example 2, respectively. In Fig. 3, the two light beams reflected by the forward aperture are the polarization splitting plate 3, Faraday rotator 4, λ/2 plate 6, polarization splitting plate 3' (in Fig. 4, the polarization splitting plate 3, Faraday rotator 4, , polarization separation plate 3', and polarization separation plate 3#), and are emitted from the optical isolator at a position separated from the optical fiber 1' by the beam separation width when passing through the Faraday rotator.

Therefore, high isolation is maintained.

As in Examples 1 and 2, the aperture opening diameter in the opposite direction is three times or more the separation width when the ordinary ray and extraordinary ray separated by the polarizing AM plate quickly pass through the Faraday rotator. An optical isolator with 1.7 is preferable because the incident position of the beam and the usable beam diameter can be arbitrarily determined depending on the type of aperture on the forward direction side.

The optical isolator of Example 3 had the structure shown in FIG. 2, and the aperture opening diameter on the forward direction side was 1.81 mm, and the aperture opening diameter on the reverse direction side was 0.7 mm. Isolation was 43 dB.

FIG. 5 is a diagram showing the propagation path of light incident from the opposite direction in the optical isolator of Example 3.

The two light beams emitted from the polarization separation plate 3 are emitted as they are from the optical isolator. Therefore, high isolation is maintained.

The optical isolator of Comparative Example 1 has the structure shown in FIG. 1, and the aperture diameters on the forward and reverse sides are 0.7.
It was set as m. Isolation was 30dB.

The optical isolator of Comparative Example 2 has the structure shown in FIG.
．． 4ms and 7 seconds. Isolation was 33 dB.

6 and 7 are diagrams showing propagation paths of light incident from opposite directions in optical isolators of Comparative Example 1 and Comparative Example 2, respectively. In Fig. 6, the two light beams reflected by the forward aperture are the polarization splitting plate 3, the Faraday rotator 4, the λ/2 plate 6, and the polarization splitting plate 3' (in Fig. 7, the polarization splitting plate 3, the Faraday rotator 4, polarization splitting plate 3', polarization splitting plate 3''), and is emitted to a position away from the optical fiber 1' by the beam separation width when passing through the Faraday rotator. It is reflected again by the aperture on the opposite side. The two light beams are separated by a polarization separation plate 3', a λ/2 plate 6, a Faraday rotator 4, a polarization separation plate 3 (in comparative example 2, a polarization separation plate 3'', a polarization separation plate 3', a Faraday rotator 4, a polarization separation plate The light beams pass through the optical fiber plate 3) and are combined at the optical fiber 1 position.

Therefore, isolation deteriorates significantly.

As is clear from these facts, the ordinary ray and the extraordinary ray separated by one or more birefringent polarizing plates at least one of the aperture diameter on the forward side and the aperture diameter on the reverse side are generated by a Faraday rotator. By making J at least three times the beam separation width when transmitting the beam, it is possible to realize an optical isolator with high isolation and no polarization dependence.

〔Effect of the invention〕

According to the present invention, it is possible to provide a small optical isolator having high isolation, low insertion loss, and no polarization dependence, and is therefore industrially useful.

[Brief explanation of the drawing]

1 and 2 are schematic configuration diagrams showing an example of the optical isolator of the present invention. 1 and 1' are single mode fibers, 2 and 2' are lenses, 3°3' and 3'' are polarization separation plates that utilize birefringence, 4 is a Faraday rotator, 5 is a magnet, 6 is a λ/2 plate, 7 and 7' indicate the apertures. It is a schematic diagram showing the propagation path of light incident from opposite directions.l and 1' are single mode fibers, 2 and 2' are lenses, 3, 3' and 3# are polarization splitting plates using birefringence,
4 is a Faraday rotator, 5 is a magnet, 6 is a λ/2 plate, and 7 and 7' are apertures. FIGS. 8 and 9 are schematic diagrams showing the configuration of a conventional optical isolator and the polarization state and propagation path of light.
A) and (b) are cases where light is incident from the forward direction and the reverse direction, respectively. 1 and 1' are optical fibers, 2 and 2' are lenses, 3.3' and 3# are birefringent polarization separation plates,
4 is a Faraday rotator, and 6 is a λ/2 plate.

Claims (1)

[Claims] (1) In an optical isolator that includes one or more birefringent polarization splitting plates, a Faraday rotator, and two apertures as components, at least one of the aperture diameter on the forward direction side and the aperture diameter on the reverse side One feature is that the beam separation width is at least three times as large as the beam separation width when the ordinary ray and extraordinary ray separated by one or more birefringent polarization separation plates are transmitted through a Faraday rotator. optical isolator.