JP2549334B2 - Optical isolator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a wavelength of 0.6 to 0.8 [μ
m] for an optical isolator used in the band.

[0002]

2. Description of the Related Art Conventionally, as a light source of an optical device (system) such as optical communication, optical measurement, magneto-optical disk, etc., a semiconductor laser or a gas laser having a wavelength of 0.6 to 0.8 [μm] described above is used. Are often used. By the way, a part of the light emitted from such a laser is returned to the laser itself to generate wavelength fluctuation and noise. Therefore, an optical isolator for removing the noise of the returned light to such a light source has been put into practical use.

The material of this Faraday rotator has the general formula C
It has been proposed to use a semi-magnetic semiconductor represented by d _1-x Mn _x Te (where 0 <x ≦ 1).

In general, the semi-magnetic semiconductor Cd _1-x Mn _x Te has a cadmium-manganese content within the range of 0 <x ≦ 0.7.
Tellurium crystals are obtained by solidifying from a liquid phase by a manufacturing method called Bridgman method. At this time, twin defects are generated due to the phase transition from the hexagonal wurtzite structure to the cubic zinc blende structure, and the twin defects are caused by twin planes ((11
1) A uniaxial optical anisotropy is generated in the lattice row <111> in the direction in which the planes) overlap. Therefore, when this crystal is used as a Faraday rotator for an optical isolator, (1
11) It is advantageous to make the light incident perpendicular to the plane.

[0005]

However, in the case of producing a cadmium-manganese-tellurium crystal by the Bridgman method, in reality, the yield decreases due to the composition fluctuation due to the fluctuation of the melt interface during crystal growth. There's a problem. Since the cadmium-manganese-tellurium crystal having a varied composition has a residual stress, even if the (111) plane is used, a practically sufficient extinction ratio often cannot be obtained.

Therefore, the technical problem of the present invention is to improve the yield of the Faraday rotator by securing the extinction ratio of the Faraday rotator manufactured by the Bridgman method, and to provide a high-performance optical isolator at a low cost. To do.

[0007]

According to the present invention, cadmium manganese tellurium produced by the Bridgman method and represented by the general formula Cd _1-x Mn _x Te (where 0 <x ≦ 0.7) is used. A magneto-optical element having a twin plane of the crystal (the same as the (111) plane) as an optical surface is used as a Faraday rotator with a Faraday rotation angle θ _F [degree], and the polarizer and the analyzer face each other through the Faraday rotator. In the arranged optical isolator, the polarization directions of the polarizer and the analyzer are θ with respect to either one of two orthogonal eigenvectors existing in the (111) plane of the dielectric constant tensor. _F / 2
[Degrees] inclined, and the polarization direction of the analyzer is (90−θ _F /
2) It is tilted by [degrees], and the Faraday rotation of the return light is -θ _F / 2 to θ _F / 2 across the eigenvector.
It is possible to obtain an optical isolator that occurs in the region of.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical isolator according to the present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 shows the general formula Cd _1-x Mn _x Te produced by the Bridgman method.
(However, 0 <x ≦ 0.7), the orientation of the ingot 1 of the cadmium-manganese-tellurium crystal is shown. The ingot 1 is provided as a magneto-optical element (Faraday rotator) of an optical isolator, and its crystal growth direction is approximately given by the following equation (Equation 1).

[0009]

[Equation 1]

The ingot 1 has a dimension in the <1-11> direction of about 60 mm and has a (111) plane (twin plane S).
Is taken out by vertically cutting the ingot 1.

FIG. 2 shows incident light having an extinction ratio I obtained by measuring the extinction ratio I without applying a magnetic field by vertically injecting light into the (111) plane of the ingot 1 to rotate the oscillation plane of the electric field vector in the plane. 3 shows the direction dependence of the polarization vector of. Here, the extinction ratio I is experimentally shown by the following equation (Equation 2).

[0012]

[Equation 2]

The extinction ratio I is defined by I = (light transmittance in orthogonal Nicols) / (light transmittance in parallel Nicols). The extinction axis of θ = π / 2n (n is an integer) that minimizes the extinction ratio I in Equation 2 is <1-10>, <-1-12>.
Of these, <1-10> coincides with the crystal growth orientation. As a result, it is considered that birefringence is generated in the <111> axis direction instead of being optically isotropic. The dielectric constant tensor here is shown by the following equation (Equation 3).

[0014]

(Equation 3)

The eigenvector in this case is <111.
>, <1-10>, <-1-12>. Such a result is a fact experimentally found by the present invention.

Then, when a magnetic field is applied in parallel to any of the eigenvectors of the dielectric constant tensor of a crystal having birefringence and linearly polarized light is made incident in the Faraday arrangement, the extinction ratio I is It becomes as shown in Formula (Formula 4).

[0017]

[Equation 4]

However, here, ε = (n ₀ Δn) / ε ₁ is defined, where n ₀ is the refractive index, Δn is the birefringence, and ε ₁ is the non-diagonal component of the dielectric constant tensor proportional to the applied magnetic field. .
Further, θ _F represents the Faraday rotation angle, and θ represents the angle between the polarization vector of the incident light and the eigenvector perpendicular to the magnetic field. The equation (4) is obtained by expanding the extinction ratio I with the reciprocal H ⁻¹ of the magnetic field H to find the lowest order term, and is found by the present invention. According to the equation 4, θ = −θ _F / 2 +
Since I = I _{0 at} 90 ° n (n is an integer), it can be seen that the influence of birefringence does not appear.

Therefore, the ingot 1 (cadmium-manganese-tellurium crystal) is used as a Faraday rotator based on the equation (4), and the polarizer and the analyzer are installed through the Faraday rotator under the following installation conditions. By arranging the two to face each other, an optical isolator in which the extinction ratio of the Faraday rotator is sufficiently secured can be constructed. That is, in such an optical isolator, the dielectric constant tensor (11
1) For one of the two orthogonal eigenvectors existing in the plane, the polarizer is θ = θ _F / 2, and the analyzer is θ =
The backward loss of the returning light can be maximized when the direction is set to (90 ° −θ _F / 2).

FIG. 3 shows the arrangement of the polarizer and the analyzer. However, the sign of the Faraday rotation angle θ _F is positive here. Since θ is set arbitrarily in the conventional optical isolator, the extinction ratio I may be deteriorated due to the influence of plural refractions even in such a setting by the equation (4).

By the way, in the optical isolator of the present invention,
Based on the experimental fact that birefringence exists in the twin plane S and the formula derived from the Muskwell equation, the equation (4) is found and the polarizer and the analyzer are arranged to face each other. Set θ _F = 45 ° like an isolator,
Even if θ _F deviates from 45 ° by a few degrees due to a mistake in thickness, etc., by tilting the polarizer by θ _F / 2, use it based on the general Faraday rotation angle θ _F [degree]. It can be planned.

The optical isolator of the present invention will be specifically described below.

First, the composition Cd _{1-x was} obtained by the Bridgman method.
An extinction ratio I was measured by growing a Mn _x Te (x = 0.05, 0.45) single crystal, optical polishing and antireflection coating on the (111) plane. However, x = 0.05 has a wavelength of 0.83 μm, and x = 0.45 has a wavelength of 0.633 μm.
It was evaluated by m. Also, the vibration direction of the electric field vector is θ in the plane.
Rotate (However, the standard of θ = 0 is <-1-12>
When the extinction ratio I was measured without applying a magnetic field (defined as the direction), the extinction ratio I showed the orientation dependence shown in FIG. 2 as described above.

Next, the following equation (Equation 5) was used.

[0025]

(Equation 5)

Where λ = wavelength, d = crystal thickness,
Δn = birefringence, Δn is obtained from this equation 5, and this Δn is the following equation (Equation 6), which is a geometric transformation equation of the above equation 4.
To obtain a theoretical formula.

[0027]

(Equation 6)

Further, as shown in Table 1, the extinction ratio I (its θ dependence and Δn) when the magnetic field H [kOe] is not applied.
And the extinction ratio I (the experimental value and the theoretical value thereof) when the magnetic field H [kOe] was actually applied as shown in Table 2 were measured. In Table 2 (A), the composition x = 0.05, and in Table 2 (B), the composition x = 0.45.

[0029]

[Table 1]

[0030]

[Table 2]

As a result, both the experimental value and the theoretical value of x = 0.05 and x = 0.45 are in excellent agreement, and θ = (− θ
_The extinction ratio I was minimized at _F / 2 + 90n) [degrees]. In the case of a normal optical isolator (θ _F = 45 °), θ
When the extinction ratio I (θ) is set to an arbitrary value, the practical value 1.
Although it is larger than 0 × 10 ⁻³ (30 dB), it has been found that the extinction ratio I (θ) can be set to I <1.0 × 10 ⁻³ in the case of the Faraday rotator included in the optical isolator of the present invention.

Further, in the optical isolator of the present invention, the polarizer is in the direction of θ = θ _F / 2, and the analyzer is in the direction of θ = (90 ° −θ _F /
Although they were installed in the direction of 2), respectively, it was confirmed that the isolation was 30 dB or more for both compositions. By the way, it was found that when a polarizer and an analyzer were installed so that the return light would be incident at θ = 0 as in an ordinary optical isolator, the isolation would be 28 dB, which was not a practical value.

As described above, the optical isolator of the present invention is
Since the extinction ratio of the Faraday rotator is sufficiently secured, the yield in the process of manufacturing the Faraday rotator is improved,
As a result, a high performance optical isolator can be efficiently manufactured.

[0034]

As described above, according to the present invention, it is a twin plane of a cadmium-manganese-tellurium crystal (11
1) Since the extinction ratio of the Faraday rotator whose surface is the optical surface is improved, it is possible to improve the yield in the manufacturing process of the Faraday rotator. As a result, a high-performance optical isolator using a Faraday rotator can be efficiently manufactured at low cost.

[Brief description of drawings]

FIG. 1 shows an orientation of an ingot of a cadmium-manganese-tellurium crystal (Faraday rotator) which is provided in the optical isolator of the present invention and manufactured by the Bridgman method.

FIG. 2 is a diagram showing a manufacturing process of the optical isolator according to the present invention, which is measured without applying a magnetic field.
It shows the dependence of the extinction ratio in the twin plane in the tellurium crystal on the direction of the polarization vector of the incident light.

FIG. 3 shows an arrangement of a polarizer and an analyzer included in the optical isolator of the present invention.

[Explanation of symbols]

1 Ingot (Faraday rotator of cadmium-manganese-tellurium crystal) S twin plane

Claims (1)

(57) [Claims] 1. A general formula Cd produced by the Bridgman method.
Twin plane of the cadmium-manganese-tellurium crystal represented by _1-x Mn _x Te (where 0 <x≤0.7) ((111)
The same as the optical surface) is used as a Faraday rotator having a Faraday rotation angle θ _F [degree] as an optical surface, and a polarizer and an analyzer are arranged to face each other via the Faraday rotator. And the polarization direction of the analyzer exists in the (111) plane of the dielectric constant tensor.
The polarization direction of the polarizer is inclined by θ _F / 2 [degree] and the polarization direction of the analyzer is inclined by (90-θ _F / 2) [degree] with respect to any one of the two orthogonal eigenvectors. And the Faraday rotation of the return light occurs in the region of −θ _F / 2 to θ _F / 2 across the eigenvector.