JPH01142525A - Optical isolator - Google Patents

Abstract

PURPOSE:To enable the shortening of an optical path length, the easy adjustment of an optical axis and the miniaturization of an optical system including a coupling lens by constituting an optical isolator in such a manner that magnetic fields can be impressed simultaneously to two pieces of Faraday rotors by a piece of magnet magnetized radially in a radial direction. CONSTITUTION:The magnetic field in a backward direction is impressed along the optical axis direction to the Faraday rotor 4 and the magnetic field in a forward direction is impressed along the optical axis direction to the Faraday rotor 5 by the magnet 6. The magnetic fields to be impressed to the Faraday rotors 4 and 5 are preferably respectively the max. magnetic fields of the magnetic field in the forward and backward directions along the optical axis direction. The impression of such magnetic fields is executed by a piece of the cylindrical magnet or the cylindrical magnet formed by combining the plural magnets and the max. magnetic fields formed by the magnet exist preferably at two points in the axial direction. The small-sized high-performance optical isolator which is extremely small in size, is simple in the optical axis adjustment and has the performance equal to the performance obtainable by using two pieces of the conventional optical isolators connected in series is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical isolator used in optical communications, optical application products, and the like. Specifically, miniaturization and high extinction can be achieved by simultaneously applying a magnetic field to two Faraday rotators using a cylindrical magnet magnetized in the radial direction or a magnet with an equivalent magnetic field distribution. You can get the ratio. The present invention relates to a two-stage integrated optical isolator that is easy to handle.

[Conventional technology]

FIG. q is a schematic explanatory diagram showing a conventional two-stage integrated optical isolator. In the figure, 11 and 3 are polarizers, Da and S are 7 Alade rotators, 6 is a magnet, 7 is the traveling direction of light (optical axis direction), and 3 is the applied magnetic field (magnetic force lines) of the magnet.

A conventional optical isolator has a configuration in which a Faraday rotator with a Faraday rotation angle of 173 degrees is placed between a polarizer and an analyzer adjusted so that the polarization direction of light is aS degree with respect to the polarizer. It has become. The characteristics of such optical isolators often depend on the polarization and Faraday rotator used; the forward insertion loss is /dB or less, the extinction ratio is about 3O-IIOdB, and the extinction ratio of IIOdB or more is 1 It was extremely difficult to obtain a single optical isolator. Therefore, it has been considered to use two optical isolators connected in series (for example, JP-A-Sho t/-/
1I72.111).

[Problem that the invention seeks to solve]

However, if two optical isolators are connected in series, the optical path length will of course be doubled, making it difficult to efficiently couple the semiconductor laser light to an optical fiber, etc. The optical coupling circuit is not large,
The disadvantage is that the polarization directions of the optical isolators in the front and rear stages must also be matched. In addition, the conventional two-stage integrated optical isolator basically consists of
In other words, it is composed of two magnets, two Faraday rotators, and three or two polarizers, and the optical coupling circuit is large in size, just like using a number of optical isolators connected in series. However, the problem of low coupling efficiency to optical fibers and the like could not be solved.

In addition, the conventional method has the disadvantage that the Faraday rotator must be machined to be smaller than the inner diameter of the magnet to be incorporated, and that the effective light beam diameter is small because the Faraday rotator is fixed using mounting parts. Unfortunately, in order to solve this problem, the magnet's inner and outer diameters were increased, which led to the increase in the size of the optical isolator.

[Means for solving problems]

As a result of intensive studies, the inventors of the present invention found that although it has a structure in which - optical isolators are integrated, it is very small, the optical axis adjustment is easy, and the conventional optical isolator P is connected in series. The present invention was achieved by discovering a compact, high-performance optical isolator that has performance equivalent to that used in the conventional optical isolator.

The present invention will be explained in detail below.

FIG. 1 is a schematic explanatory diagram showing an example of the single-stage integrated optical isolator of the present invention. In the figure, 11 and 3 are polarizers, and the polarization directions between the polarizers 1 and 3 and 1 and 3 are adjusted to be 3 degrees. 5 and 5 are Faraday rotators, which respectively rotate the plane of polarization of light by lIs degrees. Reference numeral 6 indicates a cylindrical magnet magnetized in the radial direction, ri indicates the optical axis direction (direction in which light travels), and za indicates the applied magnetic field (lines of magnetic force).

A magnetic field in the opposite direction along the optical axis direction is applied to the Faraday rotator DA, and a magnetic field in the forward direction along the optical axis direction is applied to the Faraday rotator j by the magnet 6. The magnetic fields applied to the Faraday rotators 5 and 5 are preferably the maximum magnetic fields in the head direction and the opposite direction along the optical axis direction, respectively.

Application of such a magnetic field can be realized by using a single cylindrical magnet or a cylindrical magnet that is a combination of a plurality of magnets, and it is preferable that the maximum magnetic field formed by the magnet is located at several places in the axial direction. As such a magnet, a rare earth sintered magnet or a plastic magnet may be used, and the magnet may be magnetized line-symmetrically about the optical axis. For example, a magnet as shown in FIG. 3 can be used. In addition, by increasing the maximum magnetic field of the magnet, Faraday rotator materials with a large saturation magnetic field can be used, making it possible to widen the selection range of Faraday rotator materials, but without increasing the magnet dimensions. A ferromagnetic material such as iron can also be placed outside these magnets to increase the maximum magnetic field.

As a Faraday rotator material, it is desirable to have a material with low light absorption and a large Faraday rotation angle (the angle at which the plane of polarization rotates due to the 7 Alladay effect). Examples of such materials include magnetic garnet with multiple substitutions of bismuth (the molecular formula is R3).
-xFes-yMy012; R is a rare earth ion containing a single or multiple Y, or AI or Ga) is suitable, and is more compact than the conventional single-stage integrated optical isolator using YIG (molecular formula: Y3Fe5012). becomes possible. When a magnetic field is applied to the Faraday rotator, the polarization direction of the light passing through the Faraday rotator is 4tS.
Rotate degrees. The direction of rotation of polarized light is the Faraday rotator
This changes depending on whether the Faraday rotation coefficient is positive or negative.If a material with a positive Faraday rotation coefficient is used as a 7Arad rotator, when the direction of travel of the light and the direction of the applied magnetic field are the same, the polarization direction of the light will change. rotates in the direction to tighten the right-hand screw. In other words, polarizers 1 and 2 and polarizers 3 and 3, which have been adjusted so that their polarization directions are 73 degrees, are rotated by 11 degrees in the same direction as the polarization direction rotated by the 7-Alade rotator used. Bye.

Furthermore, if a birefringent polarizing plate or the like is used in place of the polarizer shown in FIG. 1, and an optical isolator with no polarization dependence is created on the input side, it becomes possible to further reduce the size of the optical isolator compared to the conventional optical isolator.

〔Example〕

The present invention will be explained in more detail below using examples.
The present invention is not limited to the examples unless it exceeds the gist thereof.

Example 1 It had the shape of the inner diameter Q, j■, the outer diameter 7 m, and the length D4 shown in Fig. 1 (a), and was magnetized in the radial direction with the magnetic field shown in the C-th rotation in the optical axis direction. act3-xB 1xFe5- with a thickness of 200 μm as a Faraday rotator using a magnet.
y Gay 012 and a 3W angle polarizer, the first
The one-stage integrated optical isolator assembled with the configuration shown in the figure has the dimensions of outer diameter twsz length/, 21111, and has a dimension of 21111.
Approximately 1/2 the length compared to gII+1, length 2.3+w)
, the volume ratio was x/g, s, and miniaturization was achieved.

〔Effect of the invention〕

The single-stage optical isolator according to the present invention can simultaneously apply a magnetic field to two Faraday rotators using one magnet magnetized radially in the radial direction. The optical path length can be made shorter than that of the conventional two-stage integrated optical isolator, the optical axis can be easily adjusted, and the optical system including the coupling lens can be made smaller.

In addition, in order to stabilize the oscillation mode of a semiconductor laser and reduce wavelength shift, the temperature of the semiconductor laser and optical isolator may be controlled to a constant level, but it is necessary to reduce the size and weight of the thermostat used for this purpose. Can be done. Furthermore, when assembling the optical isolator of the present invention, it is only necessary to attach the Faraday rotator to the outside of the magnet, making it simpler and easier to manufacture than the conventional method of incorporating the Faraday rotator into the magnet. Become.

Note that by combining two or more stages of optical isolators of the present invention, an optical isolator with an even higher extinction ratio can be constructed.

[Brief explanation of the drawing]

Fig. 1 is a schematic explanatory diagram showing an embodiment of the two-stage integrated optical isolator of the present invention, and Fig. 2 is a perspective view of a cylindrical magnet magnetized in the radial direction used in the optical isolator of Fig. 1. ) and the distance dependence of the magnetic field strength of the magnet in the optical axis direction, FIG. 3 is a perspective view showing an example of a magnet that can be used in the optical isolator of the present invention, and FIG. 9 is a conventional magnet. FIG. 2 is a schematic explanatory diagram showing a C-stage optical isolator. In the figure, /, co, 3 are polarizers, da, 5 are Faraday rotators, 6 are magnets, 7 is the traveling direction of light (optical axis direction), g is the applied magnetic field of the magnet, and 9 is the magnetization direction.

Claims (5)

[Claims] (1) In an optical isolator that includes a cylindrical magnet, a Faraday rotator, and a polarizer, the magnet forms forward and reverse magnetic fields along the optical axis direction, and the magnetic fields are applied to two sets of Faraday rotators. An optical isolator characterized by: (2) The optical isolator according to claim 1, wherein the magnet is a magnet that forms two maximum magnetic fields in the axial direction. (3) The optical isolator according to claim 1, wherein the magnet has a magnetization direction that is line-symmetrical about the optical axis. (4) The optical isolator according to claim 1, wherein the magnet is a magnet magnetized in a radial direction. (5) The optical isolator according to claim 1, wherein the Faraday rotator is a magnetic garnet substituted with bismuth.