JPS6257971B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an optical isolator, and more specifically, the present invention relates to an improvement in an optical isolator, and more specifically, by providing a magneto-optic effect element itself with a function as a polarizer, there is no need to arrange an external polarizer. This article relates to an optical isolator that has been devised so that it can save time.

As is well known, an optical isolator is a two-terminal element with irreversibility that transmits light only in one direction and not in the opposite direction.For example, in an optical communication system, when the transmitting device This is used when preventing interference caused by reflected light from the receiving side.

In particular, when a semiconductor laser is used as an optical oscillator, the oscillation state is disturbed when laser light reflected from the outside and returned enters the oscillation region.
As a result, the oscillation waveform of the semiconductor laser is distorted, the wavelength and output become unstable, and noise increases. Therefore, in an optical communication system using a semiconductor laser, an optical isolator is installed between the semiconductor laser and the optical fiber.

Conventionally used optical isolators use Faraday rotation elements, as shown in Figure 1A,
As shown in B, it is a combination of a first polarizer 1, a Faraday rotation element 2, and a second polarizer 3, which are sequentially arranged along the optical axis X. Now, considering the case where the light from the semiconductor laser 4 heads toward the optical fiber 5 (this is called the forward direction), as shown in FIG. 4A, the light L1 from the semiconductor laser 4 passes through the first polarizer 1 Then, due to the action of the external magnetic field H in the Faraday rotation element 2, the incident linearly polarized light L2 becomes linearly polarized light L3 whose plane of polarization is oriented in the traveling direction and rotated by 45 degrees clockwise, for example. ,
A second polarizer 3 is arranged to be rotated by 45 degrees about the optical axis so that the linearly polarized light L3 passes through as is.
The light enters the optical fiber 5 through the On the other hand, in the case of the opposite direction as shown in FIG.
passes through the second polarizer 3 as it is, but is now rotated counterclockwise by 45 by the Faraday rotation element 2.
The linearly polarized light L5 obtained due to the degree of rotation has a polarization plane when compared with the linearly polarized light L2 at the time of incidence described above.
The light is rotated by 90 degrees, and therefore, the first polarizer 1 prevents the light from passing through and does not return to the semiconductor laser 4. Thus, with the above configuration, it is possible to effectively function as an optical isolator.

However, in the optical isolator having the above configuration, it is necessary to arrange polarizers before and after the Faraday rotation element, which has the disadvantage that the optical isolator becomes large. In particular, optical isolators used in optical communication systems using semiconductor lasers are required to have extremely high performance, and the polarizers incorporated therein are generally made in the shape of a prism using natural calcite crystals. For this reason, it is extremely expensive, for example, several hundred thousand yen per piece, making an optical isolator that requires two polarizers much more expensive than a semiconductor laser, making it difficult to use for optical communication. This was a very big problem in widely developing the system in various fields.

The present invention was made in view of the above-mentioned state of the prior art, and its purpose is to provide a magneto-optical material,
By combining it with an isotropic transparent body having a refractive index that is almost the same as that, it exhibits a good optical isolator function, making it unnecessary to install a polarizer externally at all, thereby significantly reducing the cost of the device. The objective is to provide an optical isolator that can

The present invention, devised to achieve the above object,
By skillfully applying the surface function of the magneto-optical material, the magneto-optic material itself also has the function of a polarizer, and is devised so that the incident optical axis and the outgoing optical axis are parallel. An isotropic transparent body having a refractive index equal to that of the two magneto-optical materials is placed between two magneto-optical materials having an inclined plane cut at a complementary angle to the Brewster angle, and the inclined plane produces a polarizing effect. They are constructed so that they are closely integrated with each other. However, what should be noted here is that although both end faces of the magneto-optical material are cut at a complementary angle to the Brilleuster angle, the Brilleuster angles of both end faces are not the same. This is because one side is usually in contact with air, while the other side is in contact with an isotropic transparent body having the same refractive index.

Therefore, in the present invention, regarding the inclined end faces of both magneto-optical materials, since the light does not move backward in the optical axis direction due to reflection there, there is no need to apply a non-reflective coating to the inclined faces, making it easier to manufacture. However, it is possible to prevent deterioration of isolation characteristics due to light traveling backward in the optical axis direction.

Hereinafter, the present invention will be explained in detail based on the drawings.
FIG. 2 is an explanatory diagram schematically showing an embodiment of the present invention. As shown in the figure, an optical isolator 10 according to the present invention includes two materials (for example, yttrium-iron-garnet single crystal) 12a and 12b that exhibit a magneto-optic effect, and a material that is interposed between them. It consists of a transparent transparent body 14. The first magneto-optical material 12a has one end face at a complementary angle (i.e., 90 degrees - φ) of the Brewster angle φ, which depends on the relative refractive index of the material (the medium in contact with it is air) ), and the other end face is also the inclined surface B, which is cut so that it is complementary to the Brewster angle, which depends on the relative refractive index with the medium that is in contact with the axial direction. When the inclined surface A and the inclined surface B are rotated relative to each other by 45 degrees in the Faraday rotation direction with respect to the optical axis, the surface orientations correspond to each other, and the polarization plane of the incident light is changed relative to the optical axis by the action of the external magnetic field Ha. This is an element with an optical path length LP that can be rotated by 45 degrees. What should be noted here is that one end surface A is normally in contact with air, while the other end surface B is in contact with the transparent body 14 (strictly speaking, adhesive). Therefore, even if the complementary angle of Brillester's angle is called, the angles at both end surfaces A and B are different.

Further, the second magneto-optical material 12b also
It may have exactly the same shape as the magneto-optical material 12a. Here, the inclined surfaces at both ends of the second magneto-optical material 12b are represented by C and D, respectively. First magneto-optic material 12a and second magneto-optic material 12b
This means that the inclined surfaces B and C are parallel to each other and are arranged in series so as to face each other (so that their optical axes substantially coincide). An isotropic transparent body 14 is interposed between them. Therefore, both end surfaces of the isotropic transparent body 14 have inclined surfaces parallel to each other and cut at complementary angles to the Brewster angle, and are in close contact with the inclined surfaces B and C, respectively. The directions of the external magnetic fields are opposite to each other in the first magneto-optic material 12a and the second magneto-optic material 12b, and this can be easily achieved by arranging the magnets. For example, by locating the S pole just near the transparent body 14 and locating the N poles at both ends thereof, good external magnetic fields Ha and Hb can be applied to both magneto-optical materials. Of course,
A solenoid coil may be arranged around the outer periphery of both magneto-optical materials 12a and 12b to form a predetermined magnetic field depending on the direction of the current.

Note that these three members are joined together with a transparent adhesive. Both end faces of the integrated optical isolator 10 must be polished, but unlike conventional optical isolators, there is no need to apply an anti-reflection coating.

For example, when the magneto-optical material is YIG (yttrium-iron-garnet single crystal), its refractive index n is approximately
2.2, titanium oxide (n=
2.25), strontium titanate (n=2.21), or a mixture of Tl-Cl and Tl-Br (n=2.19)
etc.

The operation of the optical isolator having such a configuration is as follows. In the case of FIG. 2, the inclined surface A of the first magneto-optic material 12a of the optical isolator 10 is arranged to face the semiconductor laser 4. First of all,
As shown in the figure, the light L10 emitted from the semiconductor laser 4 is incident on the inclined surface A of the first magneto-optic material 12a of the optical isolator 10 at a Brewster angle φ. At this time, all the components polarized within the incident plane (the plane that includes the propagation direction of the incident ray and the normal to the inclined plane A set at the point of incidence) are refracted to form an angle complementary to the Brewster angle φ. , most of the components polarized perpendicular to the plane of incidence will be reflected. In other words, the inclined surface A cut at a complementary angle to the Brewster angle φ functions as a polarizer, and guides only the component polarized within the incident plane into the optical isolator 10. The light that is refracted by the inclined surface A and introduced into the optical isolator 10 has its polarization plane rotated, for example, clockwise about the optical axis X by the action of the external magnetic field Ha.
At that time, as described above, the first magneto-optical material 12a
The length from the point of incidence on slope A to slope B is set to a length that rotates the polarization plane of the incident light by 45 degrees, so the light that reaches the exit point on slope B is
It becomes linearly polarized light with a plane of polarization rotated 45 degrees clockwise with respect to the direction of travel. As mentioned above, the inclined surface B is set at the complementary angle of the Brillester angle corresponding to the surface orientation obtained by rotating the surface orientation of the inclined surface A by 45 degrees in the Faraday rotation direction with respect to the optical axis
Since the isotropic transparent body 14 having approximately the same refractive index is located in contact with the inclined surface B of the magneto-optical material 12a, the light that reaches the exit point of the second inclined surface B goes straight as it is. and enter the transparent body 14.

Thereafter, the light travels straight through the transparent body 14 and enters the second magneto-optical material 12b, where it is further polarized to remove undesirable polarization components, and by the action of the external magnetic field Hb in the opposite direction. The plane of polarization rotates counterclockwise about the optical axis X. At that time, as described above, the inclined surface C of the second magneto-optical material 12b
The length from the incident point to the inclined surface D is set to a length that rotates the polarization plane of the incident light by 45 degrees, so the light that reaches the exit point of the inclined surface D will be It becomes linearly polarized light with the plane of polarization rotated 45 degrees counterclockwise. Since the inclined surface D is set at the complementary angle of the Brillester angle, which corresponds to the surface orientation obtained by rotating the surface orientation of the inclined surface C in the direction of Faraday rotation by 45 degrees with respect to the optical axis The light passes through as it is and reaches the optical fiber 5.

The slight residual component that was not reflected is transparent body 1
4, enters and enters the first magneto-optical material 12a, but is again subjected to polarization action on the inclined surface A,
It will be reflected.

The optical fiber 5 is usually made of quartz glass,
Since it is extremely thin, it is impossible to apply a non-reflective coating to its end face, and it is said that normally about 4% of the light is reflected from the end face.
Therefore, the light reflected by the end face of the optical fiber 5 travels back towards the semiconductor laser 4. This reflected light enters the inclined surface D of the second magneto-optical material 12b of the optical isolator 10, is refracted, and enters the inside of the optical isolator 10. Then, inside the optical isolator 10, an external magnetic field
Under the action of Hb, the plane of polarization rotates clockwise with respect to the optical axis X. The plane of polarization
The linearly polarized light that has rotated 45 degrees and reached the inclined surface C has been rotated by exactly 90 degrees when compared with the case in the forward direction, and is therefore reflected by the first inclined surface C and becomes reflected light. In this way, the return light reflected by the optical fiber 5 is reflected by the inclined surface C of the second magneto-optic material 12b of the optical isolator 10, so that almost no return light returns to the semiconductor laser 4. A small amount of residual component that was not reflected passes through the transparent body 14 and is transferred to the first
The light enters and enters the magneto-optical material 12a, but it is again subjected to the polarizing action on the inclined surface A and is reflected. In this way, two of the specific structures as above
An optical isolator can be constructed by combining magneto-optical materials and transparent bodies.

In Figure 2, the black circles and small arrows schematically indicate the polarization direction. The black circles indicate polarization perpendicular to the plane of incidence, and the small arrows perpendicular to the optical axis indicate polarization within the plane of incidence. are shown respectively.

Incidentally, the Faraday rotation angle (°/cm) in a magneto-optical material is a function of the wavelength of light, and in the present invention, since the end face of the optical isolator element is inclined, it can be adjusted by shifting the incident position of the optical axis. Even if the wavelength of the light changes, the plane of polarization can be rotated by 45 degrees. This means that an optical isolator element with a certain shape can be applied to an optical isolator of a different wavelength, which is extremely effective in terms of widening the band and reducing the number of parts.

In addition, with such a configuration, a portion of the laser light passing through the optical isolator 10 is directed to the inclined surfaces A, B, C,
Even if it is reflected by D, it does not move backward in the optical axis direction and is separated, so deterioration of isolation characteristics can be prevented. Therefore, it is not necessary to apply an anti-reflection coating to each inclined surface.

In the above embodiment, the inclined surfaces B and D of the optical isolator are
This is an example of a structure in which the surface orientation of the inclined surfaces A and C is rotated by 45 degrees in the Faraday rotation direction relative to the optical axis, but as shown in FIG. The structure may be rotated in the Faraday rotation direction. Since the operation of the optical isolator in this case is the same as that shown in FIG. 2, corresponding parts are denoted by the same reference numerals and description thereof will be omitted.

Since the present invention is an optical isolator configured as described above, an expensive polarizer is completely unnecessary.
It can be manufactured at low cost, and the structure is relatively simple, making it suitable for downsizing and weight reduction.Furthermore, since the end face of the magneto-optical material is an inclined surface, no anti-reflection coating is required, making it easy to manufacture. The emitted light can be parallelized within the same plane, so it can be easily incorporated into various devices as an optical device, and the light blocking rate can be reduced to 2.
It can produce many excellent effects, such as being able to double the size.

[Brief explanation of the drawing]

1A and 1B are explanatory diagrams of the prior art, FIG. 2 is an explanatory diagram showing one embodiment of the optical isolator according to the present invention, and FIG. 3 is an explanatory diagram showing another embodiment. DESCRIPTION OF SYMBOLS 1... First polarizer, 2... Faraday rotation element, 3... Second polarizer, 4... Semiconductor laser, 5... Optical fiber, 10... Optical isolator, 12a, 12b... Magneto-optics Materials, 14...
Isotropic transparent body.

Claims (1)

[Claims] 1 Both end surfaces are inclined surfaces cut at complementary angles to the Brieucster angle, but the surface orientations of both inclined surfaces correspond when rotated relative to the optical axis by 45 degrees or 225 degrees in the Faraday rotation direction. Two magneto-optical materials with an optical path length that can rotate the plane of polarization by 45 degrees by the action of an external magnetic field are arranged in series, and an isotropic material with a refractive index approximately equal to that of the two magneto-optic materials is placed between the two magneto-optic materials. An optical isolator characterized by being closely integrated with a transparent material interposed.