JPH02201416A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain the small-sized optical isolator which is tolerant to the proximity effect of a ferromagnetic body by specify the relation among the external diameter of magnetic yokes, the diameter of passing light, the external diameter and internal diameter of a cylinder type magnet, and the minimum size of the section of a Faraday rotator perpendicular to the optical axis. CONSTITUTION:The magnetic yokes 5'a and 5'b are discoid having the external diameter Dyo and also has a light passing hole with the diameter Dyi in the center, and then 0.9Dmi+0.1Dmo <= Dyo <= 0.8Dmo+0.2Dmi and Dyi <= 1.2Df, where Dmo is the external diameter and internal diameter of the cylinder type permanent magnet 3 are Dmo and Dmi and the minimum size of the section of the Faraday rotator 1 perpendicular to the optical axis is Df. Consequently, the optical isolator which is hardly affected even when the ferromagnetic body come close from outside is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to an optical isolator applied to high-speed, large-capacity optical communication systems, etc., and particularly relates to the structure of an optical isolator for improving performance.

"Prior Art" In recent years, with the amazing progress of optical fibers and semiconductor lasers, 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. #S High-performance semiconductor lasers used in high-speed, large-capacity optical communication systems, etc. are sensitively affected by the feedback of reflected light from optical fibers, etc.
There are phenomena in which the spectral line width broadens and the oscillation characteristics of the semiconductor laser itself become unstable. Therefore, in order to prevent this returned light from reaching the semiconductor laser, an optical isolator with high reverse direction loss has become essential.

FIG. 6 is a cross-sectional view for explaining the structure of a conventional optical isolator, in which 1 is a Faraday rotator, 2a and 2b are polarizers, 3 is a hollow permanent magnet, and 4a.

4b is a rotating holder for fixing the polarizers 2a and 2b, 7 is a fixed holder for fixing the Faraday rotation P, and 6 is an optical isolator for providing a rotation center axis to the rotating holders 4a and 4b. This is an outer case to reinforce the overall strength.

The optical isolator with the conventional structure shown in Figure 6 has an open magnetic circuit, and when another magnetic material approaches from the outside, the magnetic field applied to the Faraday rotator changes. , the characteristics of the optical isolator may deteriorate.

In particular, in recent semiconductor laser modules, the entire module is often hermetically sealed by a method such as laser welding in order to ensure reliability. Therefore, an optical isolator placed near a semiconductor laser is sometimes housed in a cylindrical housing made of a ferromagnetic material such as an iron plate with excellent weldability.

At this time, if the initial setting magnetic field applied to the Faraday rotator 1 is not sufficient, the characteristics of the optical isolator of the conventional structure will be significantly deteriorated due to the proximity of the housing.

In order to avoid this, it is conceivable to use a large magnet to make the applied magnetic field sufficiently large and to design it with a margin. However, this is inconsistent with the overall miniaturization of the optical isolator. Therefore, in the prior art, it was necessary to compromise on appropriate dimensions while taking performance into consideration.

As described above, with the structure of the prior art, there is a limit to the miniaturization of an optical isolator that is less affected by the proximity of a ferromagnetic material from the outside. This is the big problem.

In particular, recently, with the progress of optical communications, there has been an increasing demand for miniaturization of laser modules equipped with optical isolators, and there is a strong desire for the emergence of compact optical isolators that are resistant to the proximity effect of ferromagnetic materials.

"Problems to be Solved by the Present Invention" A proposal to make the magnetic circuit of an optical isolator into a closed magnetic circuit using magnetic yokes 5a and 15b has already been published in patent publication 1983-1989.
462. In this case, as shown in FIG. 7, the outer diameter DLIIO of the single cylindrical permanent magnet and the outer diameter Dyo of the magnetic yoke are the same. The curves in the figure indicate the flow of magnetic flux. In this case, as shown in FIG. 8, when the ferromagnetic cylindrical housing 8 is in close contact with the outside of the permanent magnet 3, the magnetic yokes 5a and 5b are attached to the permanent magnet 3 through the ferromagnetic housing 8 on the outside. This results in the formation of another closed magnetic path including the magnetic flux, and a portion of the magnetic flux flowing in the direction of the Faraday rotor is absorbed by the housing 8. The magnetic field applied to the Faraday rotator is therefore significantly reduced, similar to the prior art embodiment of FIG.

Therefore, even with this method, the proximity effect of the ferromagnetic material cannot be reduced.

As described above, the conventional optical isolator structure has the problem that when actually assembling a laser module, it is not possible to realize a compact optical isolator that is resistant to the proximity effect of ferromagnetic materials.

An object of the present invention is to solve the above problems and provide a compact optical isolator that is resistant to the proximity effect of ferromagnetic materials.

"Means for Solving the Problems" The optical isolator of the present invention includes a Faraday rotator disposed at the center of a cylindrical permanent magnet 3, two magnetic yokes disposed on both sides of the Faraday rotator, and both of them. In an optical isolator whose main components are two polarizers disposed on the outside, the magnetic yoke is disk-shaped with an outer diameter Dyo and has a light passage hole with a diameter Dyi in the center, and the cylindrical permanent The outer diameter of the magnet is Dmo and the inner diameter is DIa.
i, when the minimum dimension of the cross section perpendicular to the optical axis of the Faraday rotator 1 is Df, each dimension is 0.9Dmi+0.1Dmo≦Dyo≦0.8Dmo+
It is characterized by a relationship of 0.2DmiDyL≦1.2Df.

With such a structure, the optical isolator becomes less susceptible to influence even when a ferromagnetic material from the outside comes close to the optical isolator.

"Example" Examples will be described in detail below.

FIG. 1 is a cross-sectional view for explaining one embodiment of the present invention.
Magnetic yoke 5° with outer diameter Dyo slightly smaller than mo
It has a structure in which the magnets a and 5'b are brought into close contact with the side planes of the cylindrical permanent magnet 3. Also, there is a polarizer 2 on the outside of both of them.
Rotary holders 4a and 4b, on which holders a and 2b are mounted, are arranged.

Here, in order to realize a compact optical isolator that is strong against the proximity effect of ferromagnetic materials, what is important in design is the optimal relationship between the dimensions of the magnetic yokes 5°a and 5'b and the dimensions of the cylindrical permanent magnet 3. is to find out.

FIG. 2 shows an enlarged view of the magnetic yokes 5'a, 5°b, cylindrical permanent magnet 3, and Faraday rotator 1 shown in FIG. a, 5
°b used the same size.

In such a structure, if the length of the Faraday rotator 1 and the length of the cylindrical permanent magnet 3 are equal, the magnetic yokes 5°a, 5
The Faraday rotator l contacts ``b'', forming a closed magnetic path.The direction in which the magnetic flux flows is shown by the arrow in FIG. However, most of the magnetic flux lines at the outer periphery leak into free space.

FIG. 3 shows a case where a ferromagnetic cylindrical housing 8 is closely attached to the optical isolator of this embodiment shown in FIGS. 1 and 2 from the outside. At this time, in the part of the ferromagnetic cylindrical housing 8 near the end of the cylindrical permanent magnet 3,
Magnetic charges with opposite signs to N and S of the permanent magnets are induced, and the magnetic field applied to the Faraday rotator 1 becomes somewhat weaker. However, in the embodiment of the present invention, the outer diameter Dmo of the magnetic yoke is made small to make it difficult for the magnetic flux lines at the outer circumference of the single cylindrical permanent magnet 3 to enter the Faraday rotor 1 in advance. Therefore, even when the ferromagnetic housing approaches, there is little change in the magnetic flux flowing through the Faraday rotator 1, and as a result, the decrease in the magnetic field in the area where the Faraday rotator 1 is placed is not so large. .

FIG. 4 shows the results of actually verifying the effects of the present invention using several examples in which the dimensions of the magnetic yoke were varied.
Here, the characteristics of the conventional optical isolator shown in FIGS. 6 and 8 are also shown.

The horizontal axis in Fig. 4 is the outer diameter Dy of the magnetic yokes 5'a and 5'b.
o is normalized by the outer diameter Dmo of the cylindrical permanent magnet 3,
The vertical axis shows the measurement results of the reverse direction loss and insertion loss of the experimental optical isolator. However, at this time, the dimensions of the cylindrical magnet are Dmo==6.0mmφ, Dmi=3.0II1mφ
In this embodiment, the inner diameter of the magnetic yoke was DyC1, 6 mmφ, and the Faraday rotator was a prism with a side of 2.0 wrlI. Also, the length of the cylindrical permanent magnet 3 is 3
■The length of the Faraday rotator was 2, f3 mm.

Since I used a YIG single crystal as a Faraday rotator,
If the wavelength used is 1.55 μs, the Faraday rotator l
The inner diameter Dmi of the 0-yen n-type permanent magnet 3 is determined in advance as the minimum diameter that can accommodate the Faraday rotator 1 and the fixed holder 7.

The rightmost point in Fig. 4 shows an example of the prior art, with the outer diameter Dyo of the magnetic yokes 5°a and 5°b and the outer diameter D of the cylindrical permanent magnet.
This is the case where it is set to be the same as mo. As can be seen from this result, in the conventional structure, when the optical isolator is inserted closely into the housing 8 made of a ferromagnetic material, both the reverse direction loss and the insertion loss of the optical isolator deteriorate.

Furthermore, the point on the left end of FIG. 4 shows another example of the prior art. Even in the case without a magnetic yoke, if a ferromagnetic housing 8 is attached as in the previous embodiment, the characteristics of the optical isolator are improved. to degrade.

On the other hand, as seen in the embodiment of the present invention, 0.55
In the range of ≦Dyo/DIIlo≦0.90, even when inserted into the housing 8, the deterioration of the reverse direction loss and insertion loss is not so large.

In particular, at the point of Dyo/Dmo=0.8, there is almost no deterioration in the characteristics of the optical isolator before and after it is housed in the housing 8. This is because the magnetic yokes 5''a and 5''b are cylindrical. This shows that the area in contact with the side plane of the molded permanent magnet 3 is an important parameter. That is, Dy.

The problem is not the absolute value of /Dtao, but the difference between the inner diameter Dmi and the outer diameter Dmo. Considering this point, the above relationship is 0.1 (Dmo-Dmi)+Dmi≦Dy.

It can be transformed as ≦0.8(Dmo-Dmi)+Dmi. That is, it can be seen that if the dimensions of the magnetic yokes 5'a and 5°b are within this range, the proximity effect of the ferromagnetic material can be significantly reduced.

Next, the range of the inner diameter Dyi of the magnetic yokes 5'a and 5'b was examined. It is preferable that the inner diameter Dyi is smaller because the magnetic circuit is closer to a closed magnetic path, but if it is too small, the effective area of transmitted light will be reduced, and there is a possibility that a sufficient amount of light cannot be transmitted from the semiconductor laser to the optical fiber or the like. Therefore,
I want to make the inner diameter Dyi as large as possible. However, if the size is too large, the distance between the Faraday rotator and the magnetic yoke will increase, and the magnetic resistance will increase, making it impossible to make it strong against the proximity effect of the ferromagnetic material, which is the original purpose.

For the purpose of knowing this limit value, the Dyo/
At the point Dmo=0.75, Dyi is 1.6+nm, 1.8
mm, 2.0mm, 2.2mm, 2.4mm, 2.6m
An experiment was conducted by changing only m.

As a result, Dyi is up to 2.4m+e, that is, the length of one side of the prismatic Faraday rotator is 2.4m+e. It was found that the characteristic deterioration due to housing modification was not so severe up to 1.2 times the axis m, and the present invention was effective.

In the present embodiment, a prismatic Faraday rotator 1 is used, but the effect of the present invention is the same even if the Faraday rotator 1 has another shape, for example, a cylindrical shape. In this case, the length of one side of the prism is considered to correspond to the diameter of the cylinder. Further, as another shape, a rectangle can be considered, but in this case, the length of the short side is considered to correspond to the rectangle. That is, to summarize the above and express the scope of the present invention, the inner diameter Dyi of the magnetic yokes 5°a and 5°b is equal to 1.0 mm of the minimum lateral dimension of the cross section perpendicular to the optical axis of the Faraday rotation p. If it is twice or less, it is appropriate, and this is the upper limit of the inner diameter Dyi.

FIG. 5 is a structural diagram of an optical isolator showing another embodiment of the present invention. That is, the magnetic yoke 5''a of the present invention,
5''b is provided with slight protrusions 9a and 9b at the center, which rotate so as to slide on the inner circumferential surface of the cylindrical magnet 3.

If the holders 4'a and 4'b, to which polarizers are attached in advance, are fixed to the magnetic yokes 5''a and 5''b with adhesive, the magnetic yokes 5''a and 5''b become cylindrical magnets. 3
Since the adhesive is suctioned, the entire shape can be maintained even before the adhesive hardens. Therefore, even after applying the adhesive, the optical isolator can be adjusted by rotating the polarizer until the adhesive is cured. The heights of the protrusions 9a, 9b are preferably as small as possible within a range that ensures the center of rotation of the polarizers 2a, 2b.

"Effects of the Invention" As explained in detail using the examples above, it can be seen that the optical isolator using the magnetic yoke of the present invention is extremely stable against the proximity effect of a ferromagnetic material from the outside. .

3; Cylindrical permanent magnet, 4 a, 4 b, 4' a, 4'b; Rotating holder 5
a, 5 b, 5' a, 5' b, 5"a, 5"b; Magnetic yoke, 6; Outer case, 7; Fixed holder 8; Ferromagnetic housing, 9 a r 9 b i protrusion

[Brief explanation of the drawing]

1, 2, 3, and 5 are diagrams showing embodiments of the present invention; FIG. 4 is a diagram showing characteristics of the optical isolator of the present invention; FIGS. 6, 7, FIG. 8 is a diagram showing a conventional example. l; Faraday rotator, 2 a + 2 b i polarizer, Figure 2 Figure 3 Figure 4 Figure Dyo/Dm. Figure 8

Claims (2)

[Claims] (1) Faraday rotator placed in the center of a cylindrical magnet,
In an optical isolator whose main components include two magnetic yokes disposed on both sides of the Faraday rotator and two polarizers disposed on both sides of the magnetic yokes, the magnetic yokes are disc-shaped with an outer diameter Dyo. The cylindrical magnet has a light passage hole in the center with a diameter of Dyi, and the outer diameter of the cylindrical magnet is D.
When mo and the inner diameter are Dmi, and the minimum dimension of the cross section perpendicular to the optical axis of the Faraday rotator is Df, each dimension is 0.9Dmi + 0.1Dmo≦Dyo≦0.8Dmo
An optical isolator characterized by having a relationship of +0.2DmiDyi≦1.2Df. (2) In the optical isolator according to claim 1, the disk-shaped magnetic yoke has a circular protrusion at the center, and this protrusion enters a hollow part of a cylindrical permanent magnet, and the magnetic yoke is arranged in a cylindrical permanent magnet. An optical isolator characterized in that it is rotatable so as to slide on an inner circumferential surface and a side plane of the optical isolator.