JPS59224819A - Faraday rotator - Google Patents

Abstract

PURPOSE:To obtain a light guide layer shape which has a high extinction ratio by removing the difference of propagation constants between TE mode and TM made originating from the structure of the light guide layer by determining refractive index differences between an intermediate and a gap layer contacting the light guide layer, and the light guide layer below specific values. CONSTITUTION:The light guide layer 2 made of, for example, substitution type YIG of magnetic garnet is laminated on a non-magnetic garnet substrate 4 of GGG, etc., with the interposing intermediate layer 3 of substitution type YIG, and the gap layer 1 made of substitution type YIG is stacked on the light guide layer. The refractive indexes of the respective layers and the thickness of the light guide layer 2 satisfy single-mode condition. The refractive indexes of the intermediate layer and gap layer need not be equal completely at any time, and when the refractive index difference from the light guide layer is <=0.3%, a >=20dB quenching ratio is obtained by a Faraday rotator with the light guide layer which is 1-5mum thick.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a Faraday rotator in the shape of an optical waveguide layer for thin-film optical isolators and optical circulators.

[Background of the invention]

The development of optical waveguide layers having the Faraday effect is being researched at many research institutions. For example, A.

X GGG (Gd3G
@5012) substitution type Y I G (Ys
A full substitutional YIG layer is formed via an intermediate layer consisting of F-Q Oat (abbreviation for F-Q Oat), and a mismatch is created in the lattice constants of this intermediate layer and the optical waveguide layer to intentionally create refractive index anisotropy in the crystal itself. The TE mode is generated from the structure of the optical waveguide layer.

A method is used to compensate for the difference in the propagation constant of the 1M mode (Appl, Opt, vot201981.
P2444).

This method achieves a power transfer rate of up to 96% in TE mode and 1M mode, but this method does not meet the minimum extinction ratio of 20, which is the minimum extinction ratio required for optical isolators in optical communications.
It is not possible to achieve more than dB. As the reason why the extinction ratio does not increase, a method is used to predict the difference in propagation constant between the TE mode and 1M mode that occurs from above the structure of the optical waveguide layer, and adjust the mismatch between the lattice constants between the optical waveguide layer and the intermediate layer. ing. In this method, since the optical waveguide layer is in contact with air, the difference in propagation constant between the TE mode and the 1M mode becomes very large, resulting in an extinction ratio of 20 dB for the lattice constant of the optical waveguide layer and the intermediate layer. The disadvantage is that it cannot be adjusted with sufficient precision. In addition, since the optical waveguide layer is in direct contact with air, which has a large refractive index difference, the difference in propagation constant between the TE mode and the 1M mode has a large dependence on the optical waveguide layer thickness, resulting in a lack of reproducibility and The drawback is that it cannot be controlled.

[Purpose of the invention]

An object of the present invention is to provide a Faraday rotator having an optical waveguide layer shape that eliminates the difference in propagation constant between the TE mode and the 1M mode resulting from the structure of the optical waveguide layer, and exhibits a high extinction ratio.

[Summary of the invention]

There are two modes of light that propagate throughout the optical waveguide layer: TE mode and TR+ mode. When the optical waveguide layer and the dielectric layer sandwiching it are isotropic, the propagation constants of the TE mode and TM mode (
The propagation speed) is different, as if the TI mode and the 'IN
The mode moves as if it were entirely propagating through a medium with a different refractive index. The apparent refractive index felt by each mode is the upper refractive index and the effective refractive index, but this difference in effective refractive index is caused by the Faraday effect, when the power transmission of the TE mode and ':TM mode is performed as follows: This is the factor that causes the maximum power transfer reduction as shown in Eq.

Here, F is the maximum power transfer, θF is the Faraday rotation power, and Δβ is the effective refractive index difference between the TE mode and the 1M mode multiplied by the wave number (2π/λ.).

As a result, linearly polarized light becomes circularly polarized light, causing a decrease in extinction ratio. FIG. 1 shows the relationship between the effective refractive index difference and the extinction ratio when using light with a wavelength of 1.3 μm and setting the Faraday rotation power to 200 degrees/Crn. In order to obtain an extinction ratio of 20 dB or more, the effective refractive index difference between TE mode and 1M mode must be
It is shown that it is necessary to keep the value below 1.5×10.

Until now, when an isotropic medium is used, a difference occurs in the effective refractive index, so in order to compensate for this difference, for example, a mismatch in lattice constant is intentionally created between the optical waveguide layer and the substrate, and the refractive index difference is created in the medium itself. Efforts were taken to provide directionality. However, both of them have a mourning characteristic, i.e. 1.5 X l (1-B
It was a very difficult technique to counteract the following. We have now solved this problem using the effective method shown below.

FIG. 2 shows the Faraday rotator that takes the shape of the optical waveguide layer invented this time. Non-magnetic garnet substrate like GGG 4
An optical waveguide layer 2 made of magnetic garnet, for example, substitutional YIG, is laminated thereon via an intermediate layer 3 made of substitutional YIG, and further, on this optical waveguide layer, substitutional YIG
It has a structure in which a cap layer 1 consisting of the following is laminated. The refractive index of each layer and the thickness of the optical waveguide layer 2 satisfy single mode conditions. In the structure shown in Figure 2, assuming that the intermediate layer 3 and the cap layer 1 have the same refractive index, the effective refractive index difference between the TE mode and the 1M mode is calculated as the difference in the effective refractive index between the optical waveguide layer 2 and these layers. Figure 3 shows the wave layer thickness as a parameter.

Figure 3 shows the wavelength λ assuming that the intermediate layer and cap layer are sufficiently thick.
= Effective refractive index difference i￥ between TE mode and 1M mode when light of 1.3 μm is incident on an optical waveguide layer with a refractive index of 2.21
The calculation results show that by keeping the refractive index difference between the optical waveguide layer and the intermediate layer (camp layer) below 6X10-s (0.3 factor), a TE that provides an extinction ratio of 20 dEi when the optical waveguide thickness is 3 μm is obtained. Effective refractive index difference between mode and 1M mode 1.5 X l
This shows that a value of O-5 or lower can be achieved. Moreover, when paying attention to the optical waveguide layer thickness, which is a parameter, this refractive index difference shows that they are very close to each other, and that strict control of the optical waveguide layer thickness is unnecessary.

Although FIG. 3 shows the case where the refractive indexes of the intermediate layer and the cap layer completely match, it is not necessary that they match completely, and the refractive index difference between the intermediate layer and the optical waveguide layer is 0. If it is 3% or less, an extinction ratio of 20 dB or more can be obtained with a Faraday rotator having an optical waveguide layer shape with an optical waveguide layer thickness of 1 to 5 μm.

The next problem is the refractive index anisotropy (birefringence) of the optical waveguide layer itself, which occurs due to lattice constant mismatch.
To make this less than 'i1.5X10'-'', the mismatch t
It is sufficient to keep the current to 0.00015A or less.

[Embodiments of the invention]

Hereinafter, one embodiment of the invention will be explained with reference to FIG. 2.

EXAMPLE First, a method for forming a Faraday N trochanter having a single mode leading wave layer shape will be described. G GG substrate 4 ligature Y203 +
G d20s + G a20s + F @20s
(YGa)s (F-G
-1a 012 Thickness 1 that sufficiently attenuates the Keeva cent wave
A layer of about 0 μm was formed. After that, it is immersed in a flux with an increased melting amount of Ga2O3, and the intermediate layer 3 has a 0.3 + substitution amount of 0, (15 mol/f, 11. is 0.00
After the optical waveguide layer 2 of 015A or less was laminated to a thickness of 3 μm, a cap layer 1 was laminated under the same conditions as when the intermediate layer 3 was laminated. The extinction ratio of the Faraday N trochanter having the shape of the optical waveguide layer prepared in this manner was measured using a conventional optical method.
I got a B or higher.

Figure 4 shows a Faraday rotator that takes the shape of this optical waveguide layer.
The configuration of the optical isolator when used is shown. A Faraday N trochanter is constructed by placing 5 permanent magnets for applying the full bias magnetic field over the gap 1!1 having a thickness such that the evanescent waves are sufficiently reduced. Further, a part of the cap layer is etched with hot phosphoric acid to form a thin comb, and a metal film 6 is deposited thereon to form a polarizer.

The operating principle is that the light 10-1 from the laser is in TE mode,
The TM modes are of equal power and are incident on the optical waveguide layer 2 so that the plane of polarization is oriented at 45 degrees with respect to the film surface. The light is rotated by 45 degrees with a Faraday rotator, becomes completely in TE mode, and is transmitted through a polarizer, becoming an output light 10-2. On the other hand, for the light 11-1 from the opposite direction, only the TE mode is transmitted through the polarizer, and the entire optical rotation of 45 degrees is carried out by the Faraday rotator, which is around 90 degrees 1 from the polarization plane of the light 10-1 from the laser. It becomes light 11-2. Since the laser has excellent polarization characteristics, this light 1
1-2 does not affect the oscillation state at all.

In this way, by making the entire optical isolator thinner, the optical isolator can be made smaller and more reliable than a bulk type optical isolator in which individual optical components are configured in a contact-N configuration.

〔Effect of the invention〕

By reducing the refractive index difference i between the intermediate layer in contact with the optical waveguide layer and the optical waveguide layer to 0.396 or less, the effective refractive index of the 1゛E mode and TM mode generated from the structure of the optical waveguide layer can be reduced. Since the difference can be made smaller, the extinction ratio of 20 dB can be achieved with good accuracy and reproducibility, which was not possible with this method.
As described above, a Faraday rotator can be constructed that takes the leading wave layer shape (9).

[Brief explanation of the drawing]

Figure 1 shows the optical Kefalada rotation power of 200 at a wavelength of 1.3 μm.
Optical waveguide layer shape that separates degrees/cm? Figure 2 shows the calculated value of the extinction ratio for the effective refractive index difference between the TE mode and the TM mode when the beam enters the Faraday N trochanter. Diagram,
FIG. 3 shows the calculation 11i! of the effective refractive index of TE mode and TM mode with respect to the refractive index difference between the optical waveguide layer and these layers, assuming that the refractive index of the intermediate layer and the cap layer are equal! The fully illustrated diagram, FIG. 4, is a block diagram of an optical isolator that fully utilizes the Faraday rotator of the present invention. DESCRIPTION OF SYMBOLS 1... Cap layer, 2... Optical waveguide j-13... Intermediate layer, 4... Substrate, 5... Magnet, 6... Metal film,
10-1, 2...Light from laser (incident light, emitted light)
, 11-1, 2... Light from the opposite direction (incident light, emitted light). Agent Patent attorney Akio Takahashi (10) Figure 1 TE −f−−1<゛trM mode tora exchange email Esuq
Figure 33 Between optical waveguide 4 and f/i (Character 7A) "Layer cutting season difference (7j?
1. ) Sword 4

Claims (1)

[Claims] 1. An optical waveguide layer made of substitutional YIG is laminated on a non-magnetic garnet substrate via an intermediate layer consisting of magnetic garnet and substitutional YIG (Ys F-5012). Furthermore, a cap layer made of substitutional YIG is laminated on the optical waveguide layer, the refractive index of the waveguide layer is increased in the area of the intermediate layer and the cap layer sandwiching it, and the optical waveguide layer, the intermediate layer, and the optical waveguide layer are The total refractive index difference between the wave layer and the cap layer is 0.
A Faraday rotator characterized by having a ratio of 3% or less. On the 2.0GG substrate, substitution type Y I G I Y,,
i”e5012) through an intermediate layer consisting of substitution type YI
An optical waveguide layer made of G is laminated, and a cap layer made of direct conversion type YIG is further laminated on the optical waveguide layer, so that the total MA refractive index of the optical waveguide layer is larger than that of the intermediate layer and cap layer sandwiching it. In addition, a Faraday rotator characterized in that the refractive index difference between the leading wave layer and the intermediate layer, the optical waveguide layer and the cap layer is 0.3% or less, and the lattice constant mismatch is 0.00015 Å or less. 3. In a Faraday rotator having the structure described in claim 1 or 2, means for determining the strength and length of the applied magnetic field so that the plane of polarization of incident linearly polarized light is rotated by 45 degrees. Faraday rotator with all features.