JP2840711B2 - Optical isolator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used for removing return light to a light emitting source such as a laser.

[Conventional technology]

In a device for optical communication or the like, when laser light emitted from a light emitting element is coupled to an optical fiber, a part of the emitted light is reflected at the fiber end face or scattered inside the fiber. Then, the reflected light and the scattered light return to the light emitting element, and make the laser oscillation unstable.

Therefore, an optical isolator is usually arranged in front of the light emitting element in order to prevent return light from returning to the light emitting element.

FIG. 5 is a view showing a conventional optical isolator, wherein FIG. 5 (a) is a cross-sectional view on the optical axis center line, and FIG. 5 (b) is a side view.

As shown in the figure, this optical isolator is constructed by inserting and fixing a Faraday rotator 84 sandwiched between two polarizing beam splitters 83 and 85 inside a cylindrical magnet 81. . Note that the polarization beam splitter 83 and the polarization beam splitter 85 have their polarization planes relatively 45 around the optical axis.
さ れ It is fixed in a shifted state.

[Problems to be solved by the invention]

By the way, since the polarizing beam splitter 83, the Faraday rotator 84, and the polarizing beam splitter 85 are formed in a quadrangular shape, as is clear from FIG. Small diameter, usually 1/3 to 1/5 of outer diameter of entire optical isolator including magnet 81
The opening diameter was only about the same.

For this reason, if the aperture diameter is increased, the outer diameter of the magnet 81 becomes 3 to 5 times larger than that, and there is a problem that the outer diameter of the entire optical isolator becomes considerably large.

Further, since both surfaces of the polarization beam splitters 83 and 85 and the Faraday rotator 84 are perpendicular to the optical axis, return light is generated by both surfaces.

Also, this type of bulk type polarizing beam splitter 83,85
Is expensive, and its use has made it difficult to reduce the cost of the optical isolator.

The present invention has been made in view of the above-described points, and aims to reduce the size and cost of an optical isolator by reducing the number of components, and to increase the opening diameter with respect to the outer diameter of the optical isolator. It is another object of the present invention to provide an optical isolator that can generate return light due to both surfaces of each optical component.

[Means for solving the problem]

In order to solve the above problems, an optical isolator according to the present invention comprises, on a surface of a substrate, a Faraday rotator that rotates a direction of a polarization plane by a predetermined angle around an optical axis, and a polarization beam splitter surface, and the substrate With two,
Each is tilted at a predetermined angle with respect to the optical axis, and is installed in a cylindrical magnetic field applying magnet such that the Faraday rotators are on the inside, and the outer peripheral surface of each substrate is the inner wall surface of the magnetic field applying magnet. To make contact.

[Action]

As described above, since the Faraday rotator and the polarizing beam splitter are formed integrally on the substrate, the number of parts is reduced,
The optical isolator can be reduced in size and cost.

Further, since the outer peripheral end surface of the substrate on which the Faraday rotator and the polarizing beam splitter are formed is in close contact with the inner cylindrical surface of the magnet, the aperture diameter increases.

Further, since the substrate on which the Faraday rotator and the polarization beam splitter are formed is inclined at a predetermined angle with respect to the optical axis, light reflected on both surfaces of each optical component is not reflected in the optical axis direction of the incident light. The performance as an isolator is improved.

〔Example〕

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a view showing one embodiment of an optical isolator used in the present invention. FIG. 1 (a) is a cross-sectional view taken along the optical axis center line.
FIG. 2B is a right side view.

As shown in the figure, this optical isolator is a plate-shaped GG.
G. A Faraday rotator 3 made of a magnetic garnet film is attached on one surface of the substrate 1 and a polarizing beam splitter surface 5 is formed on the other surface.
Is tilted by a predetermined angle θ with respect to the optical axis, and inserted and fixed in the cylindrical magnet 7.

 Hereinafter, each component will be described.

The magnet 7 is formed in a cylindrical shape, and the Faraday rotator 3
To provide a magnetic field in the direction of the optical axis.

The Faraday rotator 3 is configured to have a thickness that rotates the plane of polarization of light incident from the optical axis direction by a predetermined angle. In order to prevent surface reflection on the Faraday rotator 3,
An AR coating surface 9 is formed.

The outer peripheral end surface 11 of the GGG substrate 1 on which the Faraday rotator 3 and the polarization beam splitter surface 5 are formed is formed in such a shape that the entire periphery thereof is substantially in close contact with the circular inner wall surface of the magnet 7.

Therefore, when the GGG substrate 1 is viewed from a direction perpendicular to the light incident surface, the GGG substrate 1 has an elliptical shape.

Next, the operation of the optical isolator will be described with reference to FIG.

FIG. 2 is a diagram showing an embodiment of the present invention in which two optical isolators shown in FIG. 1 are connected and used.

In these two optical isolators, the Faraday rotators 3 provided on both sides are located inside, and the polarization plane of the polarizing beam splitter surface 5 provided on both sides is positioned with respect to the optical axis.
They are connected so that they are shifted by 45 °. At this time, the polarities of the end faces of the Faraday rotator-side magnets are set to be opposite.

Each of the Faraday rotators 3 of the optical isolator used in this embodiment has a thickness that rotates the polarization plane of light by 22.5 ° in the optical axis direction.

First, the incident light I ₀ having a predetermined polarization plane, which has entered from the left opening of the left optical isolator, passes through the polarization beam splitter plane 5 as it is, and enters the Faraday rotator 3.

The light whose polarization plane is rotated by about 22.5 ° by the Faraday rotator 3 enters the Faraday rotator 3 of the right optical isolator.

The Faraday rotator 3 further rotates the plane of polarization by about 22.5 °, and the light that has been rotated by 45 ° in total,
The light enters the polarization beam splitter surface 5 and is transmitted as it is under the condition of parallel Nicols.

In this embodiment, the rotation angle of each Faraday rotator 3 is 22.5 °, but the rotation angles of both Faraday rotators 3 may be different. That is, the total rotation angle of the two Faraday rotators 3 may be 45 °.

Next, of the outgoing light Ii emitted from the right optical isolator, return light I ₀ ′ having a random polarization plane generated by surface reflection, scattering, etc., first enters the polarization beam splitter surface 5 of the right optical isolator. , Pass through only the parallel Nicol component and enter the Faraday rotator 3.

The light having a predetermined plane of polarization incident on the Faraday rotator 3 is further rotated by 22.5 ° in the plane of polarization by the Faraday rotator 3 and is incident on the Faraday rotator 3 on the left side.゜ It is rotated.

This light is almost totally reflected by the left polarizing beam splitter surface 5.

This is because the direction of the polarization plane of the return light is in a crossed Nicols state with respect to the polarization beam splitter plane 5.

Here, the difference between the loss of the incident light, ie, the forward loss, and the loss of the return light, ie, the reverse loss, is called isolation. That is, isolation = reverse loss−forward loss [dB] In this embodiment, forward loss = −10 Log ₁₀ Ii / I ₀ [dB] reverse loss = −10 Log ₁₀ Ii ′ / I ₀ ′ [ dB].

The larger the isolation, the higher the performance as an isolator.

For this purpose, the extinction ratio of the polarizing beam splitter is preferably as high as possible, and the total rotation angle of the Faraday rotator 3 is preferably close to 45 °. Further, as the isolator, the smaller the loss of the outgoing light with respect to the incident light, the better, and the smaller the performance change with respect to the temperature change, the better.

By the way, the magnetic garnet film used for the Faraday rotator 3 needs to apply a saturation magnetic field or more for stable operation, and therefore, it is necessary to install the Faraday rotator 3 in a magnet. Therefore, the size of the optical isolator is determined by the size of the magnet. In general, when a cylindrical magnet is used as in the above-described embodiment, the larger the effective aperture diameter of the incoming / outgoing light is, the better the outer diameter of the magnet is. This is because, as the effective aperture diameter is larger than the outer diameter of the magnet, the condition for installing the optical isolator in the optical device is eased, and the optical adjustment becomes easier.

Therefore, the present invention provides a GGG having a Faraday rotator 3 formed thereon.
The direction of the outer peripheral end face 11 of the substrate 1 and the direction of the inner cylinder of the magnet 7 are matched,
The structure is such that the effective opening diameter can be obtained to the maximum by making close contact.

With such a configuration, the aperture diameter becomes about 1.4 times as large as that in the case where the conventional bulk type polarizing beam splitter as shown in FIG. 5 is used (because the conventional polarizing beam splitter has a quadrangular shape). From)).

Also, as shown by the dotted line in FIG. 3, if the outer peripheral end surface of the GGG substrate 1 is formed perpendicular to the surface of the GGG substrate 1, the opening diameter D in the case of the present invention is about (D−2T sin θ). Effective diameter.

If D = 3 mm, θ = 45 °, and T = 1 mm, then
The opening diameter is about 1.6 mm, and only about half the opening diameter D of the present invention can be obtained. Furthermore, it is expected that the difference will be further reduced when the difference in the refractive index between the garnet substrate and air is taken into account.

Next, FIG. 4 is an enlarged view showing a part of the Faraday rotator 3 shown in FIG.

As shown in the figure, the thickness t of the Faraday rotator 3
Is approximately expressed by the following equation (however, the direction of the incident light beam and the direction of the applied magnetic field match).

However, θF: required Faraday rotation angle H: magnetic field strength V: Faraday rotator Verdet constant θ: inclination angle of Faraday rotator (incident angle of light) In the above embodiment, cylindrical magnet and elliptical shape Although the Faraday rotator is used, the inside of the magnet has a rectangular shape, and the Faraday rotator can also be rectangular so that the entire circumference is in close contact with it, in other words, the shape of the inner peripheral surface of the magnet is In addition, any structure may be used as long as the entire periphery of the outer peripheral end face of the Faraday rotator is substantially in close contact with the Faraday rotator.

〔The invention's effect〕

As described in detail above, the optical isolator according to the present invention has the following excellent effects.

(1) Since the Faraday rotator and the polarization beam splitter are integrally formed on the substrate, the number of components is reduced, and the size and cost of the optical isolator can be reduced.

(2) Since the entire outer peripheral end surface of the substrate on which the Faraday rotator and the polarization beam splitter are formed is in close contact with the inner cylindrical surface of the magnet, the aperture diameter can be increased. Therefore, the condition for installing the optical isolator on the optical device is reduced, and the optical adjustment is facilitated.

(3) Since the substrate on which the Faraday rotator and the polarizing beam splitter are formed is inclined at a predetermined angle with respect to the optical axis, the light reflected on the surface of the Faraday rotator and the like does not return to the incident light side, and serves as an isolator. Performance can be improved.

Since the two Faraday rotators are arranged inside using two substrates, the polarizers are present on both sides of the Faraday rotator, so that the return light can be completely removed.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of an optical isolator used in the present invention, FIG. 2 is a diagram showing an embodiment of the present invention using two optical isolators shown in FIG. 1, and FIG. G. forming a Faraday rotator 3 and a polarizing beam splitter surface 5
FIG. 4 is a diagram showing a GG substrate 1, FIG. 4 is a diagram showing a state of light passing through a Faraday rotator 3, and FIG. 5 is a diagram showing a conventional optical isolator. In the figure, 1 ... GGG substrate, 3 ... Faraday rotator, 5 ... Polarization beam splitter surface, 7 ...
... a magnet.

Continuation of front page (72) Inventor Toshimichi Yasuda 2-14-9 Tamagawadai, Setagaya-ku, Tokyo Kyocera Corporation Tokyo Yoga Office (56) References JP-A-62-242915 (JP, A) 2-19113 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B 27/28

Claims (1)

(57) [Claims] An Faraday rotator for rotating a direction of a polarization plane by a predetermined angle around an optical axis and a polarization beam splitter surface are formed on a surface of a substrate, and two substrates are provided, each having an optical axis. The Faraday rotators are installed inside cylindrical magnetic field applying magnets so that the Faraday rotators are on the inside, and the outer peripheral surface of each substrate is brought into contact with the inner wall surface of the magnetic field applying magnet. An optical isolator characterized in that: