JPS589B2 - optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention generally relates to optical isolators, and more particularly to improving the wavelength characteristics and temperature characteristics of optical isolators using the Faraday effect.

Isolators are generally used in communications equipment to absorb reflected waves and unify the transmission direction.In optical fiber transmission systems, optical isolators are also used to stabilize the light source, connect points of fibers, input/output ends, etc. This is necessary to maintain transmission quality, such as by removing backward waves reflected from

As shown in FIG. 1, the conventionally proposed optical isolator consists of a magneto-optic material 1 having a Faraday effect, a pair of polarizers 2 and 3, and a magnet 4 for applying a magnetic field to the magneto-optic material 1. It is configured.

For example, when constructing an optical isolator for a wavelength of 1.15μ, the magneto-optical material 1 is yttrium iron garnet (abbreviated as Y3Fe5O12-YIG), which is a ferromagnetic material.
, a Glan-Thompson prism is used as a polarizer.

The best isolation property is obtained when the rotation angle (optical power) of the plane of polarization due to the Faraday effect of YIG is 45° and the angle between polarizer 2 and polarizer 3 is 45°.

However, in general, the Faraday effect (the Verdet constant of paramagnetic materials, the Faraday optical rotation power of ferromagnetic materials) has wavelength dependence and temperature dependence, and the optical rotation power changes by 45 degrees due to changes in wavelength or temperature.
deviation, leading to deterioration of isolation characteristics.

For example, FIG. 2 shows the wavelength characteristics of an optical isolator fabricated using YIG according to a construction method that has been proposed in the past, where curve a shows the reverse loss and curve a shows the forward loss.

As is clear from the figure, the wavelength dependence of the reverse loss is large, and in order to achieve isolation over a wide band, it is necessary to insert multiple stages of isolators in series, which increases the forward loss.

Furthermore, conventional optical isolators have the disadvantage that their characteristics change significantly when the temperature changes.

Therefore, the present invention aims to improve the above-mentioned drawbacks of conventional optical isolators, and its object is to provide an optical isolator with improved wavelength characteristics and temperature characteristics.

The optical isolator according to the present invention is characterized by a structure in which a pair of polarizers orthogonal to both ends of a magneto-optical material and an optically active material are combined, and will be explained in detail below with reference to the drawings.

FIG. 3 shows an example of the structure of an optical isolator according to the present invention, in which reference number 1 is a magneto-optical material having a Faraday effect, 2 and 3
are polarizers placed at right angles to each other, and 4 is a magneto-optical material 1.
A magnet 5 for applying an external magnetic field to is an optically active material having natural optical rotation.

Possible magneto-optical materials include paramagnetic materials in which the Faraday effect is induced by an external magnetic field, or ferromagnetic and ferrimagnetic materials in which the Faraday effect is induced by spontaneous magnetization; Since the absorption is large, it cannot be used in the recalibration area.

On the other hand, the optically active material may be an inorganic or organic crystal (for example, quartz, tellurium oxide, glucose, etc.), or a liquid or an aqueous solution of an organic material.

Both the Faraday effect and natural optical rotation rotate the plane of polarization of light (
optical rotary power)
However, in the Faraday effect, the sign of the optical rotation power is reversed depending on the propagation direction of the light, whereas in the natural optical rotation power, the sign of the optical rotation power is unchanged regardless of the propagation direction of the light.

In FIG. 3, the thickness of the magneto-optic material 1 and the optically active material 5 along the optical axis is such that the plane of polarization is rotated by 45 degrees.

Therefore, the light propagating in the direction of arrow a is rotated by 45 degrees in the magneto-optic material 1 and by 45 degrees in the optically active material 5, and the plane of polarization is rotated by 90 degrees. Since the propagating light is rotated by 45° in the optically active material 5 and by 145° in the magneto-optic material 1, the rotation of the plane of polarization is 0 after all.

Here, since the polarizers 3 and 4 are perpendicular to each other, the light propagates in the direction a, but not in the direction b.

Next, the characteristics of the optical isolator shown in FIG. 3 will be explained with reference to FIGS. 4 and 5.

Here, a (111) YIG plate is used as the magneto-optical material, and a C plate of quartz (S 102 ) is used as the optically active material.

The thickness of both of these materials is adjusted so that the plane of polarization rotates by 45 degrees for light of 1.15 μm, for example.

FIG. 4 is a graph showing the wavelength dependence of optical rotation power, where the horizontal axis shows wavelength (μm) and the vertical axis shows optical rotation power (degrees).

It can be seen that the optical rotation power of YIG (curve a) and quartz crystal (curve 2) also decreases as the wavelength increases.

In conventional optical isolators, the change in the amount of optical rotation due to the wavelength of YIG directly affects the deterioration of the reverse loss, whereas in the optical isolator of the present invention, the introduction of an optically active material has a direct effect on the deterioration of the reverse loss. This is simply the difference in the amount of change in optical rotation power between YIG and quartz.

FIG. 5 is a graph showing the wavelength dependence of the optical isolator according to the present invention, where the horizontal axis is the wavelength, the vertical axis is the loss, the curve a is the reverse loss, and the curve A is the forward loss.

By comparing FIG. 5 with FIG. 2, it is clear that the operating range of the optical isolator according to the invention is expanded.

The same holds true for temperature characteristics, and it has been confirmed that, for example, for a wavelength of 1.153 μm, an optical isolator made of a combination of YIG and tellurium oxide (TeO2) improves temperature characteristics.

The temperature characteristics will be explained in detail below.

The temperature coefficient of natural optical rotation power varies greatly depending on the substance; for example, while it is +0.6 m1n/cm·°C for quartz, it is -2.4 m1n/cm·°C for tellurium oxide.

On the other hand, the temperature coefficient of Faraday optical rotation power by YIG is -7
, 8m1n/cm・℃.

From this, when the optical rotation angle is set to 45°, the temperature coefficient of the amount of change Δθ from 45° for each of these substances is as follows.

YIG; -1,44m1n/℃ Quartz; 0,54m1n/℃ Tellurium oxide; -0,65m1n/℃ Tellurium oxide has a temperature coefficient close to that of YIG, so reverse loss deterioration is greatly improved, whereas crystal In the combination of and YIG, the temperature coefficients of both have opposite signs, so the reverse direction loss deteriorates.

FIG. 6 shows the temperature coefficients of reverse loss and forward loss for each material.

Here, curves a and d are YIG (45°) and Te02 (45°).
(°), the curve e shows only YIG (45°), and the curves c and f show the case of YIG (45°) and crystal (45°).

It can be seen from FIG. 6 that the combination of YIG and TeO2 has particularly excellent reverse loss characteristics (curve a).

The reason for setting the rotation of the plane of polarization to 45° is (a
) Since the polarizers 2 and 3 are generally rectangular, it is easier and more stable to assemble if the polarizers are positioned at 0° or 90°, and (b) the normal laser output light Since the plane of polarization is horizontal or vertical, it is desirable that the plane of polarization after passing through the isolator is also horizontal or vertical.
Two points can be mentioned.

As explained above, according to the present invention, by combining a magneto-optic material and an optically active material between a pair of polarizers placed at orthogonal positions, and constructing an optical isolator,
Improvements in wavelength characteristics and temperature characteristics can be measured.

[Brief explanation of the drawing]

Figure 1 is a structural example of a conventional optical isolator, Figure 2 is a graph showing the characteristics of a conventional optical isolator, Figure 3 is a structural example of an optical isolator according to the present invention, and Figures 4 and 5 are according to the present invention. This is a graph showing the characteristics of an optical isolator. FIG. 6 is a graph showing the temperature characteristics of isolators based on various materials. 1; magneto-optical material; 2, 3; polarizer; 4; magnet; 5; optically active material.

Claims (1)

[Claims] 1 a pair of orthogonal polarizers arranged along the optical axis;
The magneto-optical material is arranged between the polarizers and rotates the plane of polarization by 45 degrees due to the Faraday effect, and the optically active material rotates the plane of polarization by 45 degrees due to natural optical rotation. An optical isolator characterized in that the optically active material is tellurium oxide.