JP2757095B2 - Magneto-optical material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material for a Faraday rotator for an optical isolator used in a wavelength range of 0.6 to 0.8 .mu.m.

[0002]

2. Description of the Related Art Conventionally, semiconductor lasers or gas lasers having a wavelength band of 0.6 to 0.8 μm have been used as light sources for optical systems such as optical communication, optical measurement, and magneto-optical disks. In order to remove noise, an optical isolator utilizing the Faraday rotation effect is used. As a material for the Faraday rotation element, a semi-magnetic semiconductor Cd _1-x Mn _{x is used.}
The use of Te (0 <X ≦ 0.7) has been proposed. Usually, this crystal is produced by the Bridgman method, and in order to avoid deterioration of the extinction ratio due to birefringence due to twin defects, the {111} plane, which is the twin plane of the above crystal, is used as an optical surface, and laser light is used as {111}. The Faraday rotator is formed by vertically incident on the surface. However, when a magnetic field is applied to the crystal to form a Faraday element having a Faraday rotation angle of 45 degrees, there is a problem that the extinction ratio is deteriorated as compared to before the application of the magnetic field, which makes practical use difficult.

[0003]

According to the present invention, Cd _1-X
It is an object of the present invention to secure a practically sufficient extinction ratio in a Faraday element using a {111} plane of Mn _X Te (0 <X ≦ 0.7).

[0004]

The extinction ratio I of a crystal is defined as I = light transmittance in orthogonal Nicols / light transmittance in parallel Nicols. Conventionally, in a magneto-optical material, a magnetic field H
Has been known to increase the extinction ratio I by applying. However, materials other than Cd _1-x Mn _x Te did not hinder practical use. The present invention is based on the experimental finding that the extinction ratio I increases in proportion to the square of the magnetic field H, and that the coefficient is determined by the width of the frequency spectrum of the laser light source. Hereinafter, the description will be made using mathematical expressions.

When the frequency spectrum of the laser light source has a finite width and the Faraday rotation angle θ _F becomes θ _F + δθ _F , the extinction ratio I is expressed by Equation 1.

[0006]

(Equation 1)

In equation (1), P (δθ _F ) represents the probability that the Faraday rotation angle shifts by δθ _F. According to Equation 1,
The extinction ratio I is 2 times the standard deviation of the distribution of the Faraday rotation angle θ _F.
It is raised to the power, and is expressed as in Equation 2.

[0008]

(Equation 2)

The relationship between the Faraday rotation angle shift δθ _F and the wavelength shift δλ is expressed by equation (3).

[0010]

(Equation 3)

The symbols used in Equation 3 are V: Verdet constant, d: crystal thickness, and H: applied magnetic field. In the case of a Faraday element having a Faraday rotation angle of 45 degrees, when Equation 3 is modified and θ _F = 45 ° = π / 4 radians is substituted, Equation 3 is obtained.
Is expressed as Equation 4.

[0012]

(Equation 4)

Therefore, the extinction ratio I expressed by the equation (2)
Δθ _F is substituted and expressed by Equation 5.

[0014]

(Equation 5)

Here, if Δλ is defined by Equation 6, Δλ
Represents the standard deviation of the spectral distribution of the wavelength of the light source and depends on the particular laser used, so that Mn
The extinction ratio I is 1 × 10 ⁻³ or less (I <1 × 10 ⁻³ ) which is considered to be a practical value by selecting the density X and adjusting the expression 7 in the expression 5.
^-3 ).

[0016]

(Equation 6)

[0017]

(Equation 7)

The inventor has experimentally confirmed that this is actually possible, and has accomplished the present invention. That is, the magneto-optical material is characterized in that the Mn concentration X takes a value larger than X ₀ defined by the _{equation (} 12).

[0019]

(Equation 12)

[0020]

[Action] semimagnetic semiconductor cadmium manganese telluride _{(Cd 1-X Mn X Te} ) single crystal, since it has a large Verdet constant transparent in the visible region, a Faraday for an optical isolator wavelength 0.6~0.8μm band It has been proposed for use as a promising material for rotating elements. However, when a magnetic field is applied to the crystal to form a Faraday rotation element having a Faraday rotation angle of 45 degrees, there is a problem that the extinction ratio I deteriorates. This extinction ratio increases in proportion to the square of the magnetic field as shown in Equations 2 and 3, and as shown in Equation 5, this is due to the width δλ of the frequency spectrum of the laser light source; Correspondingly, the Verdet constant depends on the Mn concentration X,
By selecting the Mn concentration X, the extinction ratio I is 1 × 10 ^−3.
The following practical values were experimentally confirmed.

[0021]

The present invention will be described below in detail with reference to examples. C having various Mn concentrations X by the Bridgman method
A single crystal of d _1-X Mn _X Te (0 <X ≦ 0.5) is prepared,
The (111) plane (twin plane) was subjected to optical polishing and antireflection coating, and the Verdet constant V (X, λ) and the extinction ratio I were evaluated. The wavelength spectrum of the laser light source was observed with a spectrum analyzer. Examination of the dependence of the Verdet constant V on the wavelength λ and the Mn concentration X confirmed that Equation 8 holds in a long wavelength region without light absorption, which is a practical region of the optical isolator.

[0022]

(Equation 8)

The energy E ₀ corresponding to λ ₀ is given by
As shown in Fig. 7, it was confirmed that the value varied linearly with the Mn concentration X.

[0024]

(Equation 9)

FIG. 1 shows the relationship between the reciprocal of the Verdet constant V ^-1 and the square of the wavelength λ ² for Mn concentrations X = 0.45 and 0.1. Also, λ ₀ ² (X) of Expression 8 is shown. Equation 8 is realized for each in the long wavelength region.

Next, the standard deviation of the wavelength fluctuation Δλ = 2.5
FIG. 2 shows the extinction ratio I of a crystal having a center wavelength λ = 0.678 μm and a thickness X = 0.40 and a thickness d = 0.6 mm measured by changing the magnitude of a magnetic field H using a semiconductor laser having a center wavelength of λ = 0.678 μm. Shown in The extinction ratio I is proportional to the square of the magnetic field H ² , and its coefficient is 5.29 × 10 ⁻⁶ [KOe] ⁻² obtained by substituting numerical values into the expression shown in Expression 10 related to Expression 3. And agree well.

[0027]

(Equation 10)

For a laser with Δλ = 1.0 nm, the coefficient is 0.84 × 10 ⁻⁶ [KOe] ⁻² and the extinction ratio I is H ^2.
It shows that it is proportional to

From Equations 5 and 8, the extinction ratio I of the Faraday rotator of the optical isolator is given by Equation 11. Here, since λ ₀ (X) has a relationship of E ₀ = hc / λ ₀ ,
Equation 9 of the empirical formula indicates the relationship with the composition X. Here, c represents the speed of light, and h represents Planck's constant.

[0030]

[Equation 11]

Center wavelength λ = 0.678 μm, Δλ = 2.5
FIG. 3 shows the composition X dependence of the extinction ratio I measured using a laser light source having a wavelength of λ = 0.830 μm and Δλ = 2.0 nm.
(A) and (b) of FIG. At any wavelength, the measured value and Equation 11 are in good agreement. The extinction ratio I decreases as X increases, reaching a practical level. X ₀ that ^satisfies I = 1 × 10 ⁻³ is obtained from Expression 12 obtained by substituting a numerical value into Expression 11.

[0032]

(Equation 12)

Here, the unit of λ and Δλ is μm. X
> X ₀ , the extinction ratio practically satisfies the level I <1 × 10 ⁻³ . λ = 0.678 μm, Δλ = 0.0025n
By substituting m into Equation 12, X ₀ = 0.346, and FIG.
(A). λ = 0.83, Δλ
By substituting = 0.002 into Equation 12, X ₀ = 0.103, which is consistent with the data in FIG.

[0032]

As described above, according to the present invention, it is possible to supply a Faraday rotator material having practically sufficient extinction characteristics, and furthermore, an optical isolator.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the reciprocal V ⁻¹ of the Verdet constant V and the square λ ² of the wavelength λ when the Mn concentration X is 0.45 and 0.1.

FIG. 2 shows the extinction ratio I and the square of the applied magnetic field H ² using two types of semiconductor lasers having a center wavelength λ = 0.678 μm and a standard deviation Δλ of wavelength fluctuation of 2.5 nm and 1.0 nm. The graph which showed the relationship of.

FIG. 3 is a graph showing the relationship between the extinction ratio I and the Mn concentration X measured using a laser light source having a center wavelength λ and a standard deviation Δλ of wavelength fluctuation. FIG. 3A shows that the center wavelength λ is 0.678 μm and the standard deviation Δλ of the wavelength fluctuation is 2.5 n.
extinction ratio I and Mn concentration X measured using a laser light source of m
The graph which showed the relationship with. FIG. 3B shows the center wavelength λ.
Is 0.830 μm and the standard deviation Δλ of the wavelength fluctuation is 2.0.
4 is a graph showing a relationship between an extinction ratio I and a Mn concentration X measured using a laser light source of nm.

Claims (1)

(57) [Claims] 1. A material of a Faraday rotation element for an optical isolator used in a wavelength range of 0.6 to 0.8 μm, which is a twin plane of a cadmium-manganese-tellurium (Cd ₁ -x Mn _X Te) single crystal. 111 ° plane as an optical surface, and Mn
A magneto-optical material characterized in that the concentration X is larger than X ₀ defined by the equation shown in Equation 12, where λ μm is the laser center wavelength and the standard deviation of the wavelength spectral distribution is Δλ μm. (Equation 12)