JPH02272419A - Small-sized two-stage optical isolator - Google Patents

Abstract

PURPOSE:To prevent reflection on an interface and to stabilize characteristics by arranging two Faraday rotators so that when light is made incident on the forward direction of the two-stage optical isolator, the light strikes on the surface of an LPE film first and then strikes on a substrate. CONSTITUTION:The Faraday rotators 1''a and 1''b and center polarizers 2b and 2c are fixed to a united center holder 5 and arranged so that the film surface of the LPE film is on the incidence side of the optical isolator. By this arrangement, reflected light from the interface is returned again through the Faraday rotators to enter the incidence-side polarizer, but the plane of polarization is rotated by 90 deg. through the Faraday rotators and almost all rays of the light is separated by the polarizers and does not return to the incidence side. Therefore, the reflection on the interface between the substrate and LPE film can be removed extremely efficiently.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to basic research on high-speed, large-capacity optical communications and compact two-stage optical isolators applied to such optical communication systems, and particularly relates to compact two-stage optical isolators. This invention relates to the structure of a compact two-stage optical isolator for improving the performance of.

[Background Art] In recent years, with the amazing progress of optical fibers, research and development and practical application of communication devices using optical fibers to increase speed and capacity are being seriously considered. Semiconductor lasers with narrow spectral line widths are often used as light emitting sources in these devices.

In particular, the oscillation characteristics are sensitively affected by the feedback of reflected light (return light), which is applied to high-speed, large-capacity optical communication systems, etc., and the noise increases. A two-stage optical isolator has become essential.

Fig. 2 is a diagram for explaining the structure of a conventional small two-stage optical isolator, in which Fig. 1a and lb are Faraday rotators,
2a, 2b, 2c, 2d are polarizers, 3a, 3b are hollow permanent magnets, 4a, 4b are rotary holders 5 for fixing the polarizers 2a, 2d, Faraday rotators and central polarizers 2b, 2c. 6 is an outer case for fixing the entire two-stage optical isolator.

The two permanent magnets 3a and 3b in FIG. 2 are magnetized in opposite directions. Therefore, when light enters from the right side of the drawing, that is, when light passes through the optical isolator in the forward direction, the optical polarization plane is rotated by 45 degrees in the right optical isolator, and rotated by 145 degrees in the left optical isolator. . As a result, no rotation of the plane of polarization is observed between the incident light (A) and the output light (B).

Another possibility is to connect two permanent magnets so that they are attracted to each other. In this case, the rotation angle of the plane of polarization between the incident light and the output light is 906.

The problem here is that, as shown in Figure 2, in the structure of the conventional two-stage optical isolator, two Faraday rotators 1
The end faces of a, lb and the four polarizers 2a, 2b, 2c, and 2d are almost perpendicular to the optical axis, and there is a risk that the reflected light from the end faces of each Faraday rotator and polarizer will return to the semiconductor laser as is. There is. To avoid this, a high-precision anti-reflection coating is applied to the end face of each component.

In recent years, in order to miniaturize two-stage optical isolators, P
Bi-substituted garnet produced by the E method has come into use. In this case, as shown in FIG. 3, essentially the same structure as in FIG. 2 is used. In the figure,
1'' a, lo b are Faraday rotators made of a Bi-substituted garnet film with a substrate made by the LPE method. Figure 4 shows an enlarged view of this Faraday rotator. indicates a garnet film. In such an optical isolator, the substrate side is on the light incident side.

In particular, recently, small two-stage optical isolators with high reverse loss have come into use, and slight reflections that were not noticeable with conventional single-stage optical isolators pose a problem in the configuration of new optical communication systems such as coherent optical communication. It started to become.

In order to avoid this, it has been proposed to tilt the Faraday rotator with respect to the optical axis as shown in FIG. By adopting this configuration, the effect of deterioration of semiconductor laser characteristics due to reflection at the entrance surface of the Faraday rotator is significantly reduced.

However, when LPE membrane is used with the aim of miniaturization.

A new problem arises. That is, since the LPE film is always grown on the substrate, when it is actually used as a Faraday rotator, it is always used together with the substrate. At this time, there is a problem that reflection from the interface between the substrate and the LPE film cannot always be avoided. For this reason, even if a highly accurate antireflection film is produced on the end face of the Faraday rotator, it is not possible to expect much effect in reducing the overall reflectance.

On the other hand, high-precision optical communication systems such as the coherent optical communication mentioned above have high reverse loss and L
It is desired to develop a compact two-stage optical isolator that suppresses internal reflection from the interface between the PE film and the substrate as much as possible.

However, to date, the prior art optical isolator structures of FIGS. 3 and 5 have not received much attention in this regard.

[Problem to be solved by the invention] In the conventional small two-stage optical isolator, no attention has been paid to solving the reflection from the interface between the substrate and the LPE film as in the structure. However, there was a drawback that it was not possible to stably realize a high-performance compact two-stage optical isolator.

That is, a high-performance two-stage optical isolator cannot be realized with the configuration of the prior art.

It is an object of the present invention to solve the above problems and provide a high-performance compact 2
The present invention provides a stage optical isolator.

[Means for Solving the Problems] The compact two-stage optical isolator of the present invention consists of two Faraday rotators and two permanent magnets for separately magnetizing them, and polarized light is applied to both sides of each Faraday rotator. L P E (Liq
When light is incident in the forward direction of the two-stage optical isolator, the light first enters the film surface of the LPE, and then enters the substrate. It features a Faraday rotator.

Further, in the small-sized two-stage optical isolator, the normal line of the end face of the two Faraday rotation elements is 0.0. They are characterized by being tilted at an angle of 5 degrees or more, and the directions of the tilting are not in the same direction.

[Example] The present invention will be explained in more detail below with reference to Examples.

FIG. 1 is a configuration diagram for explaining one embodiment of the present invention. In Fig. 1, Faraday rotators l″a, l°
'b and the central polarizers 21) and 2c are fixed to the integral central holder 5. FIG. 6 is an enlarged view of the Faraday rotators 1'a and 1''b. That is, unlike the prior art, the film surface of the LPE film is arranged on the incident side of the optical isolator.

Now, compared to the conventional structure, the way of arranging the Faraday rotator is simply reflective, but by using the arrangement method of the present invention, reflection from the interface between the substrate and the LPE film can be extremely efficiently reduced. It can be removed. In other words, with this arrangement, the light reflected from the interface is folded back through the Faraday rotator and reaches the polarizer on the incident side, but at this time, the plane of polarization is rotated by 90 degrees by the Faraday rotator, and most of the light is This is because the light is separated by the polarizer and does not return to the incident side.

In contrast, in the conventional structure shown in Figure 3, the plane of polarization returns to the polarizer on the incident side with almost no change, so it passes through the polarizer and reaches the semiconductor laser, which has a negative effect on the oscillation of the semiconductor laser. It is.

FIG. 7 is another embodiment of the invention.

That is, the Faraday rotators 1''a, l''b are tilted with respect to the optical axis, and are designed to prevent reflections at the incident end face of the Faraday rotator from directly returning to the semiconductor laser side. Also, two Faraday rotators 1” a,
The direction of inclination of l''b is opposite to the optical axis, and they are arranged so that the deviation of the optical axis due to the inclination is mutually canceled out.

Table 1 shows the results of experiments on the broadening of the oscillation spectrum line width of a semiconductor laser due to the feedback of light from a load to the semiconductor laser in an example of the prior art and an example of the present invention. The experimental wavelength is 1.55-.

Table 1 Relationship between the optical isolator structure and the spectral linewidth of the semiconductor laser As can be seen from the results of this experiment, the semiconductor laser using the optical isolator of the present invention operates with an extremely narrow linewidth.
It was also found to be stable.

[Effects of the Invention] As is clear from the above explanation, by using the structure of the present invention, the characteristics of a high-performance compact two-stage optical isolator used in a high-precision optical communication system for coherent optical communication can be stably realized. be able to.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one embodiment of the present invention, FIGS. 2, 3, 4, and 5 are structural diagrams for explaining the prior art, and FIG. 6 is a diagram showing the Faraday structure of FIG. Enlarged view of rotor, No. 7
The figure shows another embodiment of the invention. la, lb, l' a, l' b,
1"a, 1"b: Faraday rotator 2a, 2b, 2c, 2dH polarizer, 3a, 3b; permanent magnet, 4a, 4bH rotating holder, 5; central holder 6; outer case, 8; substrate, 9; LPE Membrane Figure 1'a, l'b Figure 4 Figure 5 Procedure Nef IJ official text (spontaneous) a b 1999 1.10.27 Indication of the case 1999 Patent Application No. 94 134 Name of the invention Small size Relationship with the case involving two-stage optical isolator correction

Claims (5)

[Claims] (1) A two-stage optical isolator consisting of two Faraday rotators and two permanent magnets for separately magnetizing them, and a polarizer is provided on both sides of each Faraday rotator, the above-mentioned LPE (LiquidPhase Epitaxial) in which the Faraday rotator has a substrate
The two Faraday rotators are arranged so that when light is incident in the forward direction of the two-stage optical isolator, the light first enters the film surface of the LPE film and then enters the substrate. A small two-stage optical isolator characterized by the arrangement of (2) In the small two-stage optical isolator according to claim 1, the normals of the end faces of the two Faraday rotators are inclined by 0.5 degrees or more with respect to the optical axis, and the directions of the inclinations are in the same direction. A small two-stage optical isolator that is characterized by: (3) The compact two-stage optical isolator according to any one of claims 1 to 2, characterized in that two polarizers are provided between the two Faraday rotators. Isolator. (4) The compact two-stage optical isolator according to any one of claims 1 to 2, characterized in that three or more polarizers are provided between the two Faraday rotators. optical isolator. (5) The small two-stage optical isolator according to any one of claims 1 to 2, characterized in that one polarizer having an extinction of 60 dB or more is provided between the two Faraday rotators. A small two-stage optical isolator.