JPH0248635A - Optical isolator - Google Patents

Abstract

PURPOSE:To improve isolation characteristic by setting the hole diameter of a light passing window of a polarizer side prism holder to a specific value. CONSTITUTION:A light passing window 30 of a prism holder 10 for containing a prism 12 of a polarizer 14 is made smaller to the hole diameter being roughly equal to an effective beam diameter of an isolator, and as for an incident light for passing through this window 30, the plane of polarization is rotated by 45 degrees by a Faraday rotor 26 constituted of a magneto-optical crystal 24 and a permanent magnet 22, and the light is outputted through an analyzer 20. By controlling this beam light diameter, a reflected beam is isolated satisfactorily and does not return to a light source, and an isolation characteristic is enhanced. In this regard, when the peripheral part of the window 30 of the holder 10 is formed to a tapered shape, it is prevented that an unnecessary incident light returns directly to the light source.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical isolator in which a polarizer, a Faraday rotator, and an analyzer are arranged in this order.
This invention relates to an optical isolator with improved isolation characteristics by reducing the area of the light passing window of the polarizer-side prism holder.

[Prior Art] An optical isolator is an irreversible optical device that allows light to pass in one direction but blocks light to pass in the opposite direction, and is used, for example, in optical communication systems that use a semiconductor laser as a light source. It is used to prevent laser light from returning to the light source side due to reflection.

Optical isolator 7 (Typically, a Faraday rotator with a magneto-optic crystal mounted in a permanent magnet is placed between a polarizer and an analyzer, both of which have prisms mounted in a prism holder, and are coupled together. A Faraday rotator rotates the plane of polarization of incident light by 45 degrees, and a polarizer and an analyzer each rotate the plane of polarization of incident light by 45 degrees.
They are combined in five different directions.

[Problems to be Solved by the Invention] Generally, in this type of optical isolator, high isolation can be obtained near the central axis, but the isolation characteristics worsen as the distance from the central axis increases.

In conventional optical isolators, the prism holder that holds the prism has the same external shape on both the polarizer and analyzer sides, and the light passing window has a sufficiently large area on both the entrance and exit sides, allowing all light to pass through. It is designed to let you do so. Therefore, even though the light from the light source spreads, almost all of it enters the optical isolator, and the component of the reflected return light from the output side that travels backward through the optical isolator passes through the light passing window of the polarizer-side prism holder. This resulted in a decrease in isolation.

The purpose of the present invention is to eliminate the drawbacks of the prior art as described above, and to improve isolation characteristics by preventing components of the reflected return light that are thick (deviating from the central axis) from reaching the light source side. Our purpose is to provide optical isolators.

Means for Solving Caa] The present invention provides an optical isolator structure in which a Faraday rotator, in which a magneto-optic crystal is mounted in a permanent magnet, is arranged between a polarizer and an analyzer, both of which have prisms mounted in a prism holder. It is assumed that In order to achieve the above object, the present invention is characterized in that the light passing window of the polarizer-side prism holder is reduced to a hole diameter approximately equal to the effective beam diameter of the isolator.

Further, as a preferred embodiment, there is a structure in which the peripheral part of the window on the light incident side of the polarizer side prism holder is made into a Taber shape or a black body.

[Function] In the present invention, the light passing window of the polarizer side prism holder has a small hole diameter that is almost equal to the effective beam diameter of the isolator, so the light passing through it is limited to the vicinity of the central axis, and the isolation is extremely good. Further, the reflected return light from the emission side is blocked by the small light passing window in the expanded portion, and the amount returning to the light source side is minimized.

If the periphery of the light passing window of the polarizer side prism holder is made into a tapered shape or a black body, the incident light will spread out and the light that hits the periphery of the light passing window will be diffused or absorbed and will not return directly to the light source. It disappears. Therefore, when a semiconductor laser is used as a light source, the stability of its operation is further improved.

[Embodiment] FIG. 1 is an exploded perspective view showing an embodiment of an optical isolator according to the present invention. This optical isolator has a cylindrical permanent magnet 22 between a polarizer 14 having a prism 12 mounted inside a prism holder 10 and an analyzer 2° having a prism 18 mounted inside a prism holder 16. It has a structure in which a Faraday rotator 26 equipped with a magneto-optic crystal 24 is arranged and integrated.

The Faraday rotator 26 applies a magnetic field to the magneto-optic crystal 24 using a permanent magnet 22, and changes the plane of polarization of the light passing through it to about 4
It is rotated 5 degrees.

Correspondingly, the prism 12 of the polarizer 14 and the analyzer 20
The prisms 18 and 18 are combined in such a way that the planes of polarization through which the light passes are 45 degrees different from each other.

As is clear from FIG. 1, the feature of the present invention is that the light passing window 3o of the polarizer side prism holder IO is made small to a hole diameter approximately equal to the effective beam diameter of the isolator. On the other hand, the light passing window 32 of the prism holder 16 on the analyzer side has a hole diameter comparable to that of the conventional structure.

It is desirable that the window area of the light passing window 30 of the polarizer side prism holder 10 be as small as possible without increasing the insertion loss of light. For example, the beam diameter of a semiconductor laser is about 10-10 mm in diameter. Therefore, the diameter of the light passing window 30 of the polarizer-side prism holder 10 is also set to approximately match the diameter.

2A and 2B are exaggerated depictions of the operation of this optical isolator. Figure 2A shows the case where the light travels in the forward direction, and Figure 2B shows the case where the light travels in the reverse direction.

As shown in FIG. 2A, light from the light source enters the optical isolator through a small light passage window 30 of the polarizer-side prism holder IO. Because of the light passing window 30, only light substantially along the central axis passes through, resulting in extremely good isolation. The basic operation as an optical isolator is the same as that of the prior art. The linearly polarized light that has passed through the polarizer prism 12 is rotated by 45 degrees in its plane of polarization by a Faraday rotator 26, and then outputted by an analyzer 20.

The reflected return light reflected from the end face of the optical fiber, etc., travels through the optical isolator in the opposite direction as shown in FIG. 2B. Analyzer 20
The plane of polarization of the light that has passed is rotated by 45 degrees by the Faraday rotator 26 and enters the polarizer 14. At this time, the polarization plane of the reflected return light is deviated by 90 degrees from the direction of the polarization plane that is allowed to pass through the prism I2, so it should be blocked from passing, but due to various factors, a small amount of the light passes through the prism 12. 9 In the present invention, since the light passing window 30 is set to have a small opening area, the portion indicated by the dashed diagonal line is the light passing window 3.
0 and does not return to the light source side. What may return to the light source side is the component that travels backwards near the center axis of the optical isolator, but as mentioned above, light that passes near the center axis of the optical isolator originally has extremely good isolation characteristics. , these effects together make it possible to significantly reduce the amount of light that actually returns to the light source side.

Now, FIGS. 3 and 4 are perspective views and sectional views showing other embodiments of the present invention. Since the basic configuration is the same as that shown in FIG. 1, corresponding parts are denoted by the same reference numerals and a description thereof will be omitted. This embodiment is significantly different from the previous embodiments in the shape of the entrance surface side of the prism holder 10 on the polarizer side. In other words, the peripheral part of the light passing window 30 of the polarizer side prism holder 10 is the tapered surface 34.
It has become. If the tapered surface 34 is made like this, the fourth
As shown in the figure, when the light from the light source enters the optical isolator, the component that spreads from the vicinity of the central axis is transmitted to the tapered surface 34.
The reflected light can be prevented from returning directly to the light source because it is largely reflected outward. Therefore, when a semiconductor laser is used as a light source, it is possible to further reduce reflected return light that interferes with stable oscillation operation.

Furthermore, if the polarizer-side prism holder is made of a black body, unnecessary light is absorbed thereby, so that reflected light toward the light source side can be further reduced.

[Effects of the Invention] As described above, the present invention provides the light i1 of the polarizer side prism holder.
? Since the 1ilA window is made as small as the effective beam diameter of the isolator, only the light near the center axis of the optical isolator passes through, and the light that spreads from the center axis of the reflected return light is blocked, which reduces the amount of reflected return light to the light source. decreases and improves isolation. Therefore, according to the present invention, it is possible to manufacture an optical isolator with good isolation even if optical components equivalent to those of the prior art are used.

In particular, by making the periphery of the light passing window of the polarizer side prism holder tapered or making it a black body, it is possible to reduce the amount of light reflected directly to the light source.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view showing one embodiment of the optical isolator according to the present invention, FIGS. 2A and B are sectional views showing its operation, and FIG. 3 is a perspective view showing another embodiment of the present invention. FIG. 4 is a sectional view showing the operation. 10... Prism holder, 12... Prism, 14
... Polarizer, 16... Prism holder, 18...
Prism, 20... Analyzer, 22... Permanent magnet, 2
4... Magneto-optic crystal, 26... Faraday rotator,
30.32 River light passing window, 34... Tapered surface. Patent applicant: Fuji Electrochemical Co., Ltd. Agent: Shigeru Mi

Claims (1)

[Claims] 1. In an optical isolator in which a Faraday rotator, in which a magneto-optic crystal is mounted in a permanent magnet, is arranged between a polarizer and an analyzer, both of which have prisms mounted in a prism holder, the polarizer side An optical isolator characterized in that the light passing window of the prism holder is reduced to a hole diameter approximately equal to the effective beam diameter of the isolator. 2. The optical isolator according to claim 1, wherein the peripheral portion of the light passing window of the polarizer-side prism holder is tapered.