JP2979434B2 - Optical isolator using magneto-optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a short wavelength (0.6-0.
Magneto-optical element used in the Faraday rotator of 8μm band) optical isolator to an optical isolator using.

[0002]

2. Description of the Related Art Conventionally, in optical communications, optical measurement, magneto-optical disk systems, etc., a short wavelength (0.6 to 0.8 .mu.m band) semiconductor laser or the like is used as a light source to remove noise due to return light to the light source. The use of optical isolators has been proposed. This type of optical isolator has a Faraday rotator. Use of the semi-magnetic semiconductor _{Cd 1-x Mn x Te (} 0 <× ≦ 1) has been studied as a Faraday rotator material.

[0003] Generally _{Cd 1-x Mn x Te (} 0 <× ≦ 0.
7) Crystals are produced by solidification from the liquid phase by the Bridgman method, but twin defects are known to occur.
When a crystal having such twin defects is used as a Faraday rotator, a problem arises in that sufficient quenching characteristics cannot be obtained. The generation of the twin defects is attributed to the phase transformation of the substance, and it is described that the twin defects are formed during a phase transition from a high-temperature phase of hexagonal to a low-temperature phase of cubic.

In order to avoid the twins mentioned above, the THM method (Trav
elling Heater Method) has been proposed. The THM method is a method of growing a crystal at a temperature lower than the phase transformation temperature by lowering the melting point using Te as a solvent.

[0005]

However, the above-mentioned THM method has a growth rate of 1/10 or less as compared with the Bridgman method, and is inferior in mass productivity due to the difficulty in obtaining a large single crystal, resulting in high cost. There was a disadvantage of becoming.

[0006] Therefore, the technical problem of the present invention, Cd _1-x Mn _x T manufactured by excellent Bridgman method in productivity
to provide an optical isolator using the magneto-optical element having a practically sufficient quenching properties e crystals as full <br/> Arade rotating element.

[0007]

Means for Solving the Problems] Verdet constant of Cd _1-x Mn _x Te crystal since it does not depend on the orientation of the crystal plane through which light enters, conventionally constituted the Faraday rotator in any plane. On the other hand, the twin defect generated in this crystal has a regular structure in which twin planes are stacked vertically in the [111] direction. The present inventors have experimentally found that the extinction characteristic of a crystal depends on the crystal orientation.

According to the present invention, {111} plane is an optical surface Cd _1-x Mn x Te (where, 0.1 ≦ x ≦ 0.7)
The magneto-optical element ing from a single crystal as a Faraday rotator
Used, a polarizer and an analyzer through this Faraday rotator
An optical isolator in which the polarizers are opposed to each other.
And the polarization direction of the analyzer is the maximum extinction in the {111} plane.
It is characterized by being inclined 22.5 degrees with respect to the optical direction.
Optical isolator is obtained.

[0009]

[0010] That is, in the present _{invention, Cd 1-x Mn x Te} (0.1 ≦ × ≦ 0.7) was grown by the Bridgeman method as a Faraday rotator material for an optical isolator according to the twin planes of the crystal {111 ｝ The surface is an optical surface, and the laser beam is made incident on this surface perpendicularly. The extinction axis (same as the main axis direction of the refractive index) is set at 2 ° from the polarization direction of the analyzer.
Tilt 2.5 degrees to return the polarization plane of the return light 22.5 from the extinction axis.
By placing the element within the range of 22.5 to 45 degrees from the extinction axis at which the extinction ratio becomes low, the return light does not pass through the polarization plane. The extinction characteristics can be ensured.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a method for measuring the extinction ratio of a magneto-optical element according to the present invention. In FIG. 1, linearly polarized light is expressed as Cd _1-x M
n _x Te is incident perpendicularly incident light 2 on the crystal plane of the crystal 1,
The extinction ratio of the outgoing light 3 was measured in a state where the direction (polarization azimuth) θ of the polarization plane of the incident light was changed and no magnetic field was applied. still,
Arrow 4 is the polarization direction. The extinction ratio I [dB: decibel] is defined as I = −10 log ₁₀ (intensity of transmitted light at the time of extinction / intensity of incident light). The measurement results are shown in FIG.

FIG. 2 shows the θ dependence of the extinction ratio measured by the method of FIG. The 0 degree position of θ is an angle indicating the maximum extinction ratio in the {110} plane and the arbitrary figures, and is an angle indicating the minimum extinction ratio in the {111} plane. $ 11
The maximum value of the extinction ratio of the {1} plane is equal to the maximum value of the {110} plane, and the amplitude of fluctuation of the extinction ratio with respect to θ indicates that the {111} plane is smaller than the {110} plane. In addition, it shows that the amplitude of the fluctuation is smaller than the other two in the arbitrary plane, but the absolute value of the extinction ratio is also low.

That is, in FIG. 2, the extinction ratio alternately alternates between a maximum value and a minimum value at every 45 degrees of θ, and the {111} plane which is a twin plane and the {110} plane which is perpendicular thereto.
An azimuth θ _max indicating the maximum extinction ratio exists on the surface. In any other surface, the θ dependence of the extinction ratio is small, but the absolute value of the extinction ratio I is also small. This experimental result indicates that the crystal has birefringence, and its principal axis direction is the direction (extinction direction) showing the maximum extinction ratio. That is, {111}
There are three in total, two in the plane and the [111] axis perpendicular to it. The birefringence in the {111} plane is smaller than that in the plane including the [111] axis. On the other hand, for comparison,
The extinction ratio was measured in the same manner as in the measurement method of the example, even for a crystal having no twin defects produced by the HM method.
The extinction ratio was constant. Therefore, it is considered that the optical anisotropy of the crystal prepared by the Bridgman method was induced by twin defects.

A magneto-optical element according to an embodiment of the present invention and an optical isolator using the same are manufactured as follows. Composition by Bridgman method _{Cd 1-x Mn x Te (} x
= 0.2, 0.5). These crystals had a structure in which twin planes overlapped at intervals of about 10 μm in the [111] direction. A {111} plane parallel to the twin plane, a {110} plane perpendicular to the {111} plane, and a plane having an arbitrary orientation are cut out, and optical polishing and an anti-reflection film are deposited.
A Faraday rotator having a Faraday rotation angle of 45 degrees at 0 Gauss was constructed. The composition of x = 0.5 has a wavelength of 63
The one with 3 nm and x = 0.2 was a 780 nm Faraday rotator. Tables 1 and 2 show the measurement results of the extinction ratio of the Faraday rotator described above.

In Tables 1 and 2, {110}
The extinction ratio of the surface and any surface was measured by setting the polarization direction of the analyzer so as to obtain the maximum extinction ratio. In the case of the {111} plane, the polarization direction of the analyzer is set to 22.5 degrees from the extinction axis, and
7.5 degrees, an arbitrary angle. In the case of 22.5 degrees, the plane of polarization for 45-degree Faraday rotation is within 22.5 degrees from the main axis, and in the case of 67.5 degrees, the plane of polarization is 22.5 to 45 degrees from the main axis. Was set as follows. For comparison, the extinction ratio of a crystal having the same composition without twin defects and produced by the THM method is also shown. From Table 1 and Table 2, {111}
The extinction ratio when the polarization direction of the analyzer is tilted by 22.5 degrees from the extinction axis on the surface is closest to the value of the crystal produced by the THM method, and satisfies the practical level of 40 dB.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

As described above, according to the present invention, according to the present invention, it is possible to provide with good quenching characteristics, the optical isolator using a light isolator Faraday rotator materials at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a method for measuring an extinction ratio of a magneto-optical element according to an embodiment of the present invention.

FIG. 2 is a diagram showing the θ dependence of the extinction ratio of the magneto-optical element according to the embodiment of the present invention measured by the method of FIG.

[Explanation of symbols]

Reference Signs List 1 Cd _1-x Mn _X Te crystal 2 incident light 3 outgoing light 4 polarization direction

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁶ , DB name) G02F 1/09 501 G02B 27/28 JICST file (JOIS)

Claims (1)

(57) [Claims] _1. A Cd _1-x M in which a {111} plane is an optical surface
n _x Te (where, 0.1 ≦ x ≦ 0.7) that Do monocrystalline
Using the magneto-optical element as a Faraday rotator,
A polarizer and an analyzer are arranged facing each other via a Ladder rotator.
An optical isolator, wherein the polarizer and the analyzer are polarized.
The light direction is relative to the maximum extinction direction in the {111} plane.
An optical isolator characterized by being inclined at 22.5 degrees.
Ta .