JPH03196115A - Optical isolator - Google Patents

Abstract

PURPOSE:To eliminate the polarized wave dependency by connecting two pieces of polarized wave separator/coupler to a main optical path, forming two pieces of branch optical paths between each polarized wave separator/coupler, and connecting a Faraday rotator, a polarizer and a Faraday rotator to each branch optical path in this order. CONSTITUTION:Wiring of an optical waveguide consisting of main optical paths 1a, 1b and branch optical paths 2a, 2b is executed on an isolator substrate 1. The main optical path 1a is connected to a first polarized wave separator/ coupler 3a, and the optical path is divided into the branch optical paths 2a, 2b therefrom. Also, a polarized wave separator/coupler 3b is connected to the upstream of the main optical path 1b, as well, and the branch optical paths 2a, 2b are focused thereto. In such a state, in the branch optical path 2a, a Faraday rotator 4a, a polarizer 5a and a Faraday rotator 6a are placed in this order, and also, in the branch optical path 2b, a Faraday rotator 4b, a polarizer 5b and a Faraday rotator 6b are placed in this order. In such a way, the optical isolator is provided with a characteristic which does not depend on a polarized wave and can be inserted between optical fibers of a non- polarized wave holding type and used for a regular optical communication.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical isolator, and more particularly to an optical isolator without polarization dependence.

(Prior art) In optical communication and optical measurement systems, the purpose is to remove reflected light from optical components in the optical path of output light from a light source in order to perform highly reliable optical communication and high-precision optical measurement. An optical isolator is placed in the optical path.

Generally, this optical isolator includes a Faraday rotator and
It consists of two polarizers arranged before and after the polarizers with their planes of polarization tilted at 45° with respect to each other, and a magnet for saturating the Faraday rotator.

The output light traveling in the forward direction passes through the first polarizer and then
The plane of polarization is rotated by 45 degrees due to the Faraday effect (magneto-optical effect) of the Faraday rotator, and the light passes through the second polarizer as it is.

On the other hand, when the return light due to reflection, that is, the reflected light traveling in the opposite direction, passes through the second polarizer and enters the Faraday rotator, its plane of polarization is further rotated by 45 degrees, so that it is relative to the first polarizer. Since the plane of polarization is 90°,
It will no longer be able to pass through this first polarizer. That is, reflected light is removed.

In the case of the optical isolator having the above structure, a polarizer is arranged on the input side. Therefore, the optical isolator performs ideal operation only when the polarization direction angle of the input light matches the angle of the polarization plane of the polarizer.

Conversely, if the polarization direction angle of the input light is perpendicular to that of the polarizer, the output from the optical isolator will be zero, and the so-called insertion loss will become infinite.

For this reason, in the optical isolator having the above structure, it is necessary that the polarization direction angle of the input light is maintained at a constant angle commensurate with the angle of the polarization plane of the polarizer.

However, the above conditions are usually not satisfied in optical isolators placed between optical fibers.

The reason is that ordinary optical fibers used for optical communication are
This is because it does not have polarization maintaining properties, and its polarization state changes over time depending on environmental conditions such as temperature, wind, pressure, and vibration.

The present invention aims to solve the above-mentioned problems in conventional optical isolators and to provide an optical isolator without polarization dependence.

(Means and effects for solving the problem) In order to achieve the above object, in the present invention, two polarization splitters/combiners are connected to the main optical path, and each of the polarization splitters/combiners is connected to the main optical path. There is provided an optical isolator characterized in that two branched optical paths are formed therebetween, and each branched optical path is connected to a Faraday rotator, a polarizer, and a Faraday rotator in this order.

The optical isolator of the present invention will be explained based on the basic configuration diagram shown in FIG.

In the figure, on an isolator substrate l such as a Si substrate, there are main optical paths 1a, lb and a branch optical path 2a.

An optical waveguide consisting of 2b is wired. These two main optical path branch optical paths are channel waveguides formed on the substrate l or optical fibers fixed on the substrate l. and,
The tips of the main optical paths 1a and lb are each connected to an optical fiber (not shown).

The main optical path 1a is connected to a first polarization splitter/combiner 3a,
From here, the optical path is divided into branch optical paths 2a and 2b.

A polarization splitter/combiner 3b is also connected upstream of the main optical path 1b, and a branch optical path 2a is connected thereto.

2b is focused. In the figure, if the propagation direction from left to right is the forward direction, the polarization splitter/combiner 3a operates as a polarization separator, and the polarization splitter/combiner 3b operates as a polarization coupler.

Here, as the polarization splitter/combiner, a dielectric multilayer film type or a birefringence grating type can be used.

The former type separates two orthogonal polarized components by transmission and reflection, so when the allowable bending radius of the waveguide is R, the branched optical paths 2a and 2 after separation are
The spacing SL between b is S.

=R. For example, when the branched optical paths 2a and 2b have cores with a relative refractive index difference (Δ) of 0.75% and a cross section of 4 μm, R is approximately 5 .mu.m, which is quite large.

On the other hand, the latter type separates polarized components by transmission and diffraction, and the diffraction angle can be as small as a few degrees, so the split optical path 2a + 2b after separation is
The interval S1 can be made extremely small, s<<1 m. Therefore, if this type of polarization splitter/combiner 3a, 3b is used, the branched optical paths 2a, 2b can be brought close to each other, so that the Faraday rotator can be used in common to the two branched optical paths 2a, 2b. It can be placed as follows.

In the branched optical path 2a, a Faraday rotator 4a that rotates polarization by 45 degrees by a magnet (not shown), a polarizer 5a, and a Faraday rotator 6a that rotates polarization by 45 degrees by a magnet (not shown) are arranged in this order. Further, in the branch optical path 2b, a Faraday rotator 4b which produces a polarization rotation of 45 degrees, a polarizer 5b, and a Faraday rotator 6b which also produces a polarization rotation of 45 degrees are arranged in this order. In other words, polarization-dependent optical isolators of the conventional structure described above are formed in each of the branched optical paths 2a and 2b.

In this optical isolator, the incident light incident on the main optical path 1a is separated into two orthogonal polarization components by the polarization splitter/combiner 3a, and the light is separated into two orthogonal polarization components into the respective branch optical paths 2a and 2b.
propagate.

The polarized wave components traveling through each of the branched optical paths 2a and 2b undergo polarization rotation of 45'' by Faraday rotators 4g and 4b, and
, 5b, and reach Faraday rotators 6a, 6b. Faraday rotator 6a.

6b, each polarization component undergoes a further polarization rotation of 45°,
The light has a polarization direction angle orthogonal to the light separated by the polarization splitter/combiner 3a, is combined in the polarization splitter/combiner 3b, and is emitted from the main optical path 1b.

On the other hand, the reflected light incident on the main optical path 1b is separated into two orthogonal polarization components by the polarization splitter/combiner 3b, and propagates in the opposite direction through the branched optical paths 2a and 2b.

The polarized wave components traveling in opposite directions on each of the branched optical paths 2a and 2b are given polarization rotation of 45° by Faraday rotators 6a and 6b. This rotation direction is the polarizer 5a.

Since the direction is perpendicular to the polarization plane of the polarizer 5b, each polarized wave component does not pass through the polarizers 5a and 5b. That is, reflected light is removed.

Even if a very small amount of leaked light occurs, this leaked light is again given a 45° polarization rotation by the Faraday rotators 4a and 4b and is outputted from the polarization splitter/combiner 3a in the direction of the arrow p in the figure. Therefore, the light does not return to the input direction of the main optical path 1a.

In this way, the optical isolator of the present invention can achieve isolation equivalent to when two stages of conventional optical isolators are used, and after once separating the incident light into two polarized components, each branched optical Since the isolator operation is performed in this manner, the operation can be performed without depending on the polarization state of the incident light.

(Example) An optical isolator as shown in the plan view of FIG. 2 was manufactured. That is, first, a quartz thin film 12 was formed on a Si substrate 11 by a flame deposition method.

A core was embedded in this quartz thin film 12 to create main optical paths 13a, 13b and branch optical paths 14a, 14b as shown in the figure.

The core embedded in the quartz thin film 12 has a high refractive index ratio (△=
0.75%), but as shown in FIG. 3, the substrate l! The part connected to the optical fiber 19 at the end of is
In order to reduce the coupling loss between the two, the spot size was expanded. That is, the main optical paths 13a, 13b
The end has a tapered structure 11a, and the cross section near the tip is 5 μm, and Δ=0.21%.

Birefringent diffraction grating type polarization splitters and couplers 15a and 15b having a thickness of 300 μm are connected to the separation and coupling locations of the main optical paths 13a and 13b and the branched optical paths 14a and 14b, respectively.

As mentioned above, by using a birefringent diffraction grating,
The branched optical paths 14a and 14b are placed close to each other, and have a common thickness of 19 mm.
0 μm, and the composition is (YbTbBi)sFe, OI!
Faraday rotators 16 and 17 are connected to provide a polarization rotation of 45°.

And the interval S between the branch optical paths 14a and 14b located between these Faraday rotators 16 and 17.

is widened to 1 m so that two orthogonal lamipoles can be handled, and each branched optical path 14a, 14b has a laminated polarizer lamipole 18a, 1 with a cross section of 1 m and a film thickness of 30 μm.
8b were connected to each other.

Here, a birefringent diffraction grating type polarization splitter/combiner 15
a, 15b, Faraday rotator 16.17, polarizer 18
Both a and 18b are arranged in a waveguide as shown in FIG.

That is, a slit C in which each optical element A is housed by bonding it with an optical adhesive B is micro-wrapped and inserted across a conductive wave path such as a main optical path or a branch optical path. These surfaces are coated with anti-reflection coatings to prevent surface reflections, and are coated with anti-reflection coatings on the waveguides to prevent even the slightest amount of reflected light from recombining into the waveguides. It is arranged at a 5° inclination.

Furthermore, when light passes through these optical elements A, since these elements do not exhibit a waveguide effect, a loss occurs due to the distance between the film thickness needles of each element.

Therefore, the portion of the waveguide that crosses the optical element A is formed into a Taber structure D1 to enlarge the spot size and reduce separation loss. In other words, the waveguide has a cross section of 4 μm, Δ=
0.75%, and the portion that crosses the element of the Taper portion DI is:
Cross section: 9 μm opening, Δ=0.15%. In this way, the loss due to a spacing of 100 μm is reduced to 2 in the waveguide.
．． 5 dB, and 0.1 dB at a 4° Taper section.

In this way, the length is 251 m, the width is 2.5 m, and the thickness is 0.
A 5 m optical isolator 20 was obtained.

As shown in FIG. 5, a cylindrical permanent magnet 21
When I set it inside and performed isolation,
It functioned as an optical isolator without polarization dependence.

(Effects of the Invention) As is clear from the above description, the optical isolator of the present invention has two polarization splitters/combiners connected to the main optical path, and two polarization splitters/combiners between each of the polarization splitters/combiners. A book branch optical path is formed, and each branch optical path is characterized in that a Faraday rotator, a polarizer, and a Faraday rotator are connected in this order, so that it has polarization-independent characteristics. It can be inserted between non-polarization maintaining optical fibers used in normal optical communications, and is useful, for example, as an optical isolator for fiber amplifiers.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of the optical isolator of the present invention, Fig. 2 is a plan view showing an embodiment, Fig. 3 is a plan view showing the connection part of the main optical path optical fiber, and Fig. 4 is a guide of each optical element. FIG. 5 is a plan view showing how the optical isolator of the embodiment is set. 1. 11... Substrate, 12... Quartz thin film, l'a,
lb. 13a, 13b--main optical path, 1la-tapered portion, 14
a. 14b--Branch optical path, 3a, 3b, 15a, 15b-
...Polarization splitter/combiner, 4a, 4b, 16...Faraday rotator, 5a, 5b, 18a, 18b
-...Polarizer, 6a, 6b, 17...Faraday rotator, 19...Optical fiber, A...Optical element, B...
・Optical adhesive, C...Slit, D...Waveguide, D
+...Tapered portion, 20...Optical isolator, 21.
··permanent magnet.

Claims (1)

[Claims] Two polarization splitters/combiners are connected to the main optical path, two branch optical paths are formed between each of the polarization splitters/combiners, and each branch optical path includes a Faraday rotator, a polarizer, etc. , an optical isolator characterized in that Faraday rotators are connected in this order.