JPS5828716A - Optical isolator - Google Patents

Abstract

PURPOSE:To constitute an optical isolator even with a double refractive crystal, by arranging two sheets of double refractive crystal so as to eliminate the effect of the double refraction. CONSTITUTION:Linear polarized wave light is made incident to the axis (a) which is vertical to the axis (c) of a magnetooptic crystal having large thickness. Thus the output light is turned into an oval polarized wave due to the double refraction. In this case, a Faraday rotational angle theta equals to (180Ft/piB)sin(piB/ 180), where B=360(no-ne)t/lambda, F: Faraday rotational coefficient, lambda: wavelength of light, no: refractive index of normal light, ne: refractive index of abnormal light and B: phase delay angle due to the double refraction. Then the two crystals having the same thickness along the direction 6 of easy magnetization of the crystal are provided as shown in the figure so that the axes (c) of the crystals are orthogonal to each other. Thus the delay angle B equals to 360/lambda{(no- ne)t+(ne-no)t}=0, and theta=2Ft is obtained. This is identical with an equation of an isotropic magnetooptical material showing no double refraction.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-performance optical isolator that prevents the oscillation of a laser light source from becoming unstable due to its own reflected light.

The configuration of the photon isolator is shown in Figure 1. In the optical isolator, the linearly polarized light propagating in the magnetization direction of the magneto-optic material is rotated p and t0 by Faraday rotation and passes through the polarizer 3 on the output side, but the reflected light is further rotated +t0 by the magneto-optic material IC. The polarization planes of the polarizers on the incident side are orthogonal,
It has the function of blocking the passage of light. Here, when linearly polarized light undergoes Faraday rotation, linear polarization must be maintained.

That is, there is no problem if the magneto-optical material l is glass or cubic system. However, if the magneto-optical material l is a birefringent material, such as orthoferrite or hexagonal ferrite, if the axis of easy magnetization does not coincide with the optical axis, the birefringence will affect the Faraday rotation, resulting in linear polarization. is no longer held and becomes elliptically polarized. This leads to a deterioration of the extinction ratio of the optical isolator, and the optical isolator loses its function. Therefore, even if there is a material with a high figure of merit or a material with extremely low loss at the new wavelength, the birefringence In the case of Faraday rotation with It is characterized by

Figures 2 and 2 are diagrams of the configuration of a magneto-optic crystal between two polarizers or polarizing plates, where l and j are magneto-optic crystals with the same thickness along the crystal's easy magnetization direction (arrow) B, respectively. The optical axes of the two are perpendicular to each other. The configuration of the present invention is F6
This will be explained by taking as an example a rhombohedral crystal represented by F3 and FeBO3. In this crystal system, the optical axis coincides with the C axis, which is a three-fold symmetry axis, and the easy axis of magnetization is perpendicular to C1III. When linearly polarized light is incident on the a-axis perpendicular to the C-axis of the crystal, which has a thickness of ζt, the emitted light becomes elliptically polarized due to the birefringence.

At this time, the linearly polarized wave plane at the time of incidence corresponds to the long axis of the elliptical polarized wave plane, and the rotation angle θ due to Faraday rotation at this time is given by the following equation.

60 ■= -T-tno-n,) t (2
) where F is the Faraday rotation system (dey / (yo
), λ is the wavelength of light, no is the h1 refractive index of the ordinary ray, ne is the refractive index of the extraordinary ray, and B is the phase delay angle due to birefringence.

As can be seen from equation (1), even if l- is large, θ is extremely small due to Bne and si+IJ'i compared to the case where it is assumed that there is no birefringence.

Therefore, as shown in FIG. 2, the beams 1qj of the two birefringent magneto-optic crystals are arranged such that their C axes are orthogonal to each other. Here, i1=:12:==i, so 13
-o.

Therefore, if B→0 in equation (1), θ=
2 't (4) is obtained.

Equation (4) is exactly the same as the equation for Faraday rotation in an isotropic magneto-optical material such as cubic crystal or glass that does not exhibit normal birefringence.

When using the present invention as an optical isolator, θ=4
After polishing the crystal substrates so that the thickness is 0 and the thickness is t, as shown in Figure 1, the C axes, that is, the optical axes are perpendicular to each other, and then they are bonded together so that the polarization planes are ≠ j
It may be inserted between two polarizers. For example, FeF is transparent in the visible light range.

The Faraday rotation coefficient of is λ = 0.7 μm and p =
io.

Since dey/cm, t = It, jmm.

As explained above, the optical isolator of the present invention can be constructed using birefringent crystals, so there are no restrictions on the selection of crystals. Even crystals like FeF,
It becomes possible to apply it as an optical isolator, and since the apparent reduction in the Faraday effect due to birefringence is eliminated, the crystal thickness can be reduced, which has the advantage of reducing the loss of the optical isolator.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the basic configuration of an optical isolator, and FIG. 2 is a diagram showing the configuration of a magneto-optic crystal according to the present invention. , 5... magneto-optic crystal exhibiting birefringence, 6...
・Arrow indicating easy magnetization direction. Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. In magneto-optic crystals that have a birefringence and whose easy axis of magnetization does not coincide with the optical axis, two magneto-optic crystals with the same thickness along the easy axis of magnetization are placed so that their optical axes are orthogonal to each other. The total thickness of the plate and outer plate of the two magneto-optic crystals is such that the plane of polarization of light of the desired wavelength rotates by +t0 along the axis of easy magnetization. A polarizer in which the magneto-optic crystals have mutually polarized planes of μjO! Optical isolator 0 characterized by a structure set between f and bamboo polarizing plates.