JPS63200117A - Multistage optical isolator - Google Patents

Abstract

PURPOSE:To realize a small-sized highly functional multistage optical isolator where the plane of polarization is not rotated, by magnetizing N-number of magnets in the axial direction and arranging magnets so that magnetic poles having the same polarity of adjacent magnets face each other. CONSTITUTION:Cylindrical magnets 3a and 3b' having a thickness L have opposite magnetization directions and are arranged a length (x) apart from each other so that their S poles face each other. Beam spliters P1 and P3' in the incidence side and the exit side are directed to the same direction, namely, the direction of 0 deg., and an analyzer P2 interposed between two Farady rotating elements FR-1 and FR-2' is directed to the direction of 45 deg.. Consequently, the light incident on a point (a) passes the Farady rotating element FR-1 to have the plane of polarization rotated at 45 deg. and passes the analyzer P2 inclined at 45 deg. and is made incident on the second Farady rotating element FR-2'. Since the element FR-2' is magnetized in the opposite direction, the plane of polarization of the light passing the element FR-2' is rotated at 45 deg. in the opposite direction. Thus, the small-sized multistage optical isolator having a large opposite loss is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an isolator that has the function of controlling light propagating in the opposite direction along the same optical path when light propagates in one direction.

[Conventional technology]

For example, when attempting to propagate light generated by a semiconductor laser as a light source through an optical fiber, reflected light may occur due to dispersion of light within the optical fiber, local non-uniformity of refractive index, presence of connections, etc. It may propagate in the opposite direction along the optical path and be coupled back into the light source semiconductor laser. In such a state, the oscillation of the semiconductor laser becomes unstable and the noise of the generated light increases, which is undesirable. Therefore, in such a case, it is effective to use an optical isolator to suppress the reflected light.

FIG. 1 shows an example of the configuration of a conventional optical isolator. In the figure, light incident from point a is converted into parallel light by lens 1, and then enters polarizer P1. Polarizer P1
selectively passes only polarized light in a certain direction, such as vertically polarized light, from the incident light. The light emitted from the polarizer PI is Y I G
(YlLOl, an oxide single crystal whose main component is Fetus), etc., enters the Faraday rotator element 3, which produces an output light whose polarization direction is rotated by 45 degrees. As shown in the figure, PR is placed at the center of the cylindrical magnet 3 and is magnetized in a direction almost parallel to the optical path.The emitted light from the Faraday rotary element PR is sent to the analyzer P.
However, the polarization direction of the analyzer P2 is inclined by 45 degrees from the vertical direction. For this reason, the Faraday rotating element PR
The incident light passes through the analyzer P2 and exits, passes through the lens 2, and produces an output converged at point b. Therefore, for example, by placing the end of the optical fiber at point b, it is possible to couple the light incident even at point a to the optical fiber.

On the other hand, as mentioned above, the reflected light generated in the optical fiber, etc. enters the polarizer P2 (acted as an analyzer in the previous example) from point b through the lens 2, and the component light that matches the polarization direction of the polarizer P2 passes through the lens 2. The light passes through P2 and enters the Faraday rotary element 3. As is well known, in the Faraday rotation element PR, the direction of rotation of the plane of polarization changes depending on the relationship between the incident direction of light and the magnetization direction of the Faraday rotation element material. In this case, the coordinate system of the arrangement is rotated by 45 degrees in the same direction as the incident light, so (the polarization direction of the output light of the Faraday rotator PR is the propagation direction of the analyzer Pi (which acted as a polarizer last time). Therefore, the Faraday rotating element PR
The incident light from is blocked by the analyzer Pi and does not propagate to the side of a. Therefore, the reversely incident light that couples to the semiconductor laser placed at point a is blocked, and S/N deterioration in the semiconductor laser is prevented. .

In the optical isolator shown in FIG. 1, a Rochon prism or a Glan-Thompson prism has conventionally been used for the polarizer and analyzers Pi and P2. It is said that the extinction ratio of these prisms is limited to approximately 50 dB. In addition, the above-mentioned YIG and CB used as Faraday rotation elements
I C (GdzOs, Rig's.

The extinction ratio of the material (oxide single crystal mainly composed of YtOi+PezQs) is said to be about 40 dB. The reverse loss of an optical isolator constructed by combining several such components cannot be increased very much because the effects of each component are superimposed on each other; a typical single optical isolator has a reverse loss of about 30 dB or more. The specifications are as follows.

Now, as the optical density and speed of optical communication increases year by year, research on coherent optical communication has become active in various places. It is said that a reverse loss of 30 dB of an optical isolator is not sufficient for such coherent optical communication, and that a reverse loss of 60 dB or more is required. Naturally, the configuration of only one stage of optical isolators as shown in FIG. 1 is insufficient, and it is conceivable to connect two stages of conventional optical isolators as shown in FIG. 2. As a result, in principle, 60 dB or more can be achieved. However, if this is used as is, the distance between the laser diode a side and the output side to the optical fiber will be too large, and the coupling efficiency between the two will be significantly reduced.

In order to avoid this, when the two isolators, that is, the two permanent magnets 3a and 3b are brought close together, the magnetic forces of the opposing magnetic pole surfaces attract each other, and the important Faraday rotary element PR-1,
The magnetic field applied to PR-2 becomes weaker. Depending on how they are approached, PR-1 and PR-2 may become unsaturated and each may be unable to maintain its individual performance as an optical isolator.

Furthermore, the polarization planes on the incident side and the output side are rotated by 90 degrees, and it becomes necessary to arrange the polarization directions of the optical systems of the laser diode a and the fiber b to be perpendicular to each other.

[Problem that the invention seeks to solve]

As described above, conventional multi-stage optical isolators have the disadvantages of large dimensions and 90 degree rotation of the plane of polarization. The purpose is to provide an isolator.

[Means for solving problems]

That is, the multi-stage optical isolator of the present invention includes N Faraday rotary elements, N cylindrical magnets containing each of these elements, and N+1 beam splitters that act as polarizers and analyzers. In a multi-stage optical isolator in which Faraday rotators are arranged alternately, the N
Each magnet is magnetized in the axial direction, and adjacent magnets are characterized in that the polarities of adjacent magnets are arranged so that the same polarity faces each other.

[For production]

By using the optical isolator structure of the present invention, it is possible to realize a compact, high-performance multi-stage optical isolator that does not rotate the plane of polarization.

〔Example〕

FIG. 3 shows the configuration of an embodiment of the two-stage optical isolator of the present invention. That is, a cylindrical magnet 3a with a thickness L,
3b' have opposite magnetization directions, and are separated by a distance X so that their south poles face each other. Also, the beam splitter PI, which acts as a polarizer and analyzer,
P2. P3° is arranged as shown. The input and output side PIs, P3', face the same direction, that is, the 0° direction. Two Faraday rotating elements PR-1, PR-2
The analyzer P2 sandwiched between `` is facing in the direction of 45°.

With this configuration, the light incident from point a passes through the Faraday rotation element PR-1, and the plane of polarization changes to 4.
The second
into the Faraday rotating element FR-2°. Here, PR
-2" is magnetized in the opposite direction, the polarization plane of the light passing through it is rotated 45 degrees in the opposite direction. That is, the polarization plane of the light is a
It returns to the direction of the polarization of the first light incident from the point, and is approximately 0°
It passes through an analyzer P3° arranged in the direction.

In the embodiment of the present invention in which this direction is the forward direction of the optical isolator, when light passes in the forward direction, it can be passed with extremely low insertion loss without changing the plane of polarization. This is extremely convenient since it is not necessary to design in advance an optical system with a polarization plane inclined at 45° or 90° in consideration of the presence of an optical isolator, as in the conventional case. Furthermore, by bringing the south poles closer to each other, there is an effect that the magnetic field applied to the Faraday rotary element becomes stronger than in the case of a single-stage optical isolator.

On the other hand, the light incident from point b is turned 45 degrees in the opposite direction by PR-2', and is orthogonal to the direction of passage of analyzer P2.

First, a reverse loss of 30 dB is ensured here. Next, the light leaking from analyzer P2 is rotated 45 degrees in the forward direction, perpendicular to analyzer P1, and further attenuated by 30 dB. Total 60
This results in an attenuation of dB. In this way, there is no change in the way the backward loss is added compared to the prior art.

FIG. 4 shows another embodiment of the present invention that utilizes the effect of strengthening the magnetic field. That is, the distance X between the two magnets is O, and they are in close contact. However, in order to make this configuration possible, the central analyzer P2 needs to be as thin as possible, for example, in the form of a film. Of course, it is possible to make them come into close contact with each other in the case of the conventional structure, but the magnetic charges in the close contact portion cancel each other out and the appearance disappears. Therefore, this is equivalent to simply using long magnets, and the magnetic field applied to the Faraday rotation elements PR-1 and PR-2 becomes extremely weak.

Figure 5 shows the results of numerical calculations to clarify this relationship. The vertical axis of the graph is the magnetic field strength at the center point of the two magnets at 0.0°, and the horizontal axis is the magnetic field at the 0 and O゛ points, where the distance between the two magnets is normalized by the magnet's inner diameter and is expressed as x/Di. The strength is the same because the magnet arrangement is symmetrical. When the S and N poles face each other as in the prior art, the directions are the same, but when the same poles face each other as in the present invention, the directions are opposite. The calculation conditions are: Do
/Di=2. L/Di=1. The straight line C is the magnetic field strength at the center when two magnets are sufficiently far apart, that is, the magnetic field limit in the case of a single magnet. Curve A is the case of the prior art, and it can be seen that the magnetic field strength gradually becomes weaker as the two magnets approach.On the other hand, in the case of the embodiment of the present invention, the two magnets are The magnetic field strength gradually becomes stronger as you get closer.The effect is particularly noticeable at X
When compared at =0, there appears to be a difference in magnetic field strength of approximately twice that between the a and the mouth. Therefore, if the magnetic field strength of a single magnet is designed to be on the edge of the saturation magnetic field, it is clear that the two-stage optical isolator of the prior art will not be able to maintain its performance as a result. On the other hand, the magnetic field strength will become stronger in this case, so we will be able to push the limits of design further in this respect.

Figure 6 shows the above effect in order to make it easier to understand.
x/Di=0.0.4. The results of calculating the magnetic field distribution on the central axis in the case of Do are shown. The figure shows only the results near the center of the left magnet, and the calculation conditions are the same as in Figure 5. Negative magnetic field strength means opposite directions. (a) shows the conventional technique, and (b) shows the case of the present invention.

In the case of the prior art, the magnetic field strength decreases as a whole, but when the two come in close contact, the magnetic field of the closely-contacted portion becomes stronger, albeit in a small portion. Also in the case of the present invention,
On the other hand, when they are brought into close contact, there is a small portion where the magnetic field strength becomes weaker. However, this portion is small when viewed from the whole, and since the FPLAD rotation element is not originally used by extending it to this portion, the effect of this portion is not a problem when discussing the effects of the present invention. Rather, in the case of the present invention, rather than completely adhering,
It can be said that this problem can be avoided by using a distance of about x/Di=0.4.

FIG. 7 is a diagram showing another embodiment of a multi-stage optical isolator of the present invention. This case is a case of a three-stage optical isolator. As you can see from this figure, three cylindrical magnets are lined up, and the two adjacent magnets have S poles,
North-polar comrades and same-polar comrades are facing each other. In this arrangement, of the three Faraday rotary elements, excluding PR-2'< FR-
1, PR-3 are magnetized in the same direction. Analyzer P4
is tilted at 45 degrees like P2. In this case, the effect of preserving the plane of polarization, which is one of the effects of the present invention, cannot be achieved, but
The effect of emphasizing the magnetic field strength at the center can be ensured. Also, 3
By layering, a reverse loss of 90 dB or more can be ensured. It will be clear from the above explanation that in the method of making the magnetic poles of adjacent magnets the same according to the present invention, if a four-stage optical isolator is used, the effect of preserving the polarization plane can be ensured again.

In addition, in the embodiments of the present invention, one polarizer and one analyzer are used as a representative example, but experts in this technology know that the effect of the present invention is the same even if a plurality of these are used to improve the extinction ratio. It's obvious for the house.

Furthermore, it will be apparent that embodiments of the present invention may be extended to realize any number of N stages of optical isolators. Regarding the rotation angle of the polarizer and analyzer used for this, if the first one is 0° when viewed from the direction of light incidence, the even numbered one is 4 degrees.
5° and odd numbers become 0°. The accuracy of this setting angle is not so strict in the case of a multi-stage type, and it is sufficiently practical if it falls within the range of ±5° for each stage.

〔Effect of the invention〕

By using the structure of the present invention, multiple magnets can be gathered in a narrow space without impairing the performance of the optical isolator.
A compact multi-stage optical isolator with high reverse loss, which is essential for coherent optical communications, can be realized.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of optical isolators explaining the prior art, and FIGS. 3, 4, and 7 are cross-sectional views of optical isolators according to embodiments of the present invention, and FIGS. 5 and 61! l is a splitter (polarizer,
Analyzer), PR, PR-1゜PR'42. PR-2
°, PR-3; Faraday rotating element, 3° 3a, 3b
, 3b'+3c; cylindrical magnet. 1st rl! J Figure 2 Figure 3 Figure 4 Figure 5 Figure 7 Figure 6 <a>

Claims (5)

[Claims] (1) The Faraday rotation elements are arranged alternately between N Faraday rotation elements, N cylindrical magnets containing each of these elements, and N+1 beam splitters that act as polarizers and analyzers. The N magnets are magnetized in the axial direction, and adjacent magnets are arranged so that the same polarity faces each other. . (2) In the optical isolator according to claim 1, if the angle of the deflection direction of the first beam splitter counting from the light incident direction among the N+1 beam splitters is approximately 0 degrees, then the even-numbered beam splitter The angle of the deflection direction of the beam splitter is 45 ± 5 degrees, and the odd angles including the first are within the range of 0 ± 5 degrees. (3) The multi-stage optical isolator according to claim 1, wherein the N+1 polarizers and analyzers each include a plurality of polarizers and analyzers. (4) The Faraday rotation element is Y_2O_3, Fe
The multi-stage optical isolator according to claim 1, characterized in that it is made of an oxide single crystal whose main component is _2O_3. (5) The Faraday rotation element is Gd_2O_3, B
The multi-stage optical isolator according to claim 1, characterized in that it is made of an oxide single crystal whose main components are i_2O_3, Y_2O_3, and Fe_2O_3.