JPS5824762B2 - Shyuuseki Hikari Mode Bunpa Souchi - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical mode demultiplexing device in an integrated optical circuit.

In the fields of optical information processing and optical communications, vigorous research and development is being carried out to realize integrated optical circuits for the purpose of stabilizing and downsizing optical devices.

An integrated optical circuit is an optical waveguide provided on or near the surface of a small substrate, which is equipped with a microscopic light source optical amplifier, optical modulator,
Active elements such as photodetectors, optical resonators, optical directional couplers,
Refers to an optical device in which passive elements such as optical demultiplexers are combined and integrated.

Optical waveguides, which are the most basic of the elements that make up integrated optical circuits, are often made of optically transparent materials with a slightly higher refractive index on a substrate made of materials such as dielectrics or semiconductors. It can be obtained by forming a thin film consisting of the following by using means such as vapor deposition, sputtering, or epitaxial growth.

Generally, when a light wave propagates in an optical waveguide, there are several allowed propagation modes (hereinafter simply referred to as modes), and the phase propagation constant and energy distribution are determined by the refractive index distribution of the optical waveguide.

In the case of the above-mentioned thin film optical waveguide, since the refractive index is approximately constant in the thin film, its propagation constant and energy distribution are determined by the thickness of the thin film and the refractive index of the thin film and the substrate.

Therefore, if the thickness of the thin film changes, the propagation constant for each mode light wave will change.

Integrated optical circuit elements such as optical reflectors, optical filters, and prisms that essentially utilize this fact are conventionally known.

FIG. 1 is a structural schematic diagram of an embodiment of a known integrated optical prism.

An optically transparent thin film 12 is formed as an optical waveguide on a substrate 11 by sputtering.

The waveguide 12 consists of two regions 14 and 15 having different thicknesses with a boundary line 13 as a boundary.

As explained above, the propagation constant of the light wave 16 propagating through this waveguide is different in the regions 14 and 15.
It is refracted by crossing the boundary 13.

Moreover, this holds true for all mode light waves that are allowed in -carry, regardless of whether they are low-order or high-order mode light waves.

An integrated optical prism having such a structure can function as an optical mode demultiplexer by utilizing the fact that the propagation direction of each mode light wave changes for the reason described above.

However, in integrated optical circuits, in certain applications of optical mode demultiplexers, for example, the propagation mode of the lower-order mode group does not change from among the multi-mode light wave group, and only the higher-order mode group It may be required to be used as a monitor for each mode group or a propagation path switch for each mode group to change the propagation direction.

For such applications, the above-mentioned known integrated optical prisms have a disadvantage in that they do not satisfy the above-mentioned reason, ie, the requirement that the propagation mode of the lower-order mode group must be kept unchanged.

A known example of an optical mode demultiplexer that changes the propagation mode of only a group of higher-order modes to be demultiplexed is JP-A-48-1.
Patent application No. 0265 known in 1984-3634
I can list one invention.

In such a known example, the configuration of the optical mode demultiplexer includes a thin film light guide having a uniform refractive index distribution and a film thickness of the light guide.
It consists of stripes that decrease with an incline, and the optical mode splitting effect occurs in the part where the film thickness is sloped.

This method utilizes total internal reflection in order from higher-order modes to lower-order modes.

Therefore, when this known example is applied to the above-mentioned monitor in mode group units or to an integrated optical mode demultiplexer as a propagation path switch, a serious drawback can be pointed out.

The first problem is the difficulty in manufacturing a large number of the above-described tapered film thickness strip structures by integrating them at desired locations in an integrated optical circuit.

That is, the thickness and inclination angle of the film thickness gradient stripes are usually within several microns and several degrees, and therefore, even if such a structure is manufactured by means such as etching, ion milling, selective deposition, etc. Today, it is easy to point out the difficulty in uniformly integrating and manufacturing a large number of devices.

The second drawback of the known example is that the known example utilizes mode demultiplexing in the tapered film thickness stripe, so the propagation mode of each mode essentially changes, and therefore a specific mode cannot be detected. group,
In other words, it is not suitable for the above-mentioned optical mode demultiplexer where it is required to keep the propagation mode of a group of low-order modes unchanged.

This fact can be seen in Figures 5 and 7 of the above-mentioned patent application No. 47-36341.
It is pointed out from the figure.

That is, the low-order mode light wave 11 in FIG. 5 is the high-order mode group 1.
3, it is transmitted through the demultiplexer 10, but at this time, even for the lowest order (m=o) in FIG. 7, the isolateral refractive index decreases as the guide film thickness decreases. propagation mode,
For example, the propagation direction cannot go straight as shown in FIG. 5, but curves.

Therefore, when constructing the propagation path switch for each mode group by continuously combining the structures shown in FIG. 5, considering the new complexity of design and the difficulty of manufacturing as described above,
The drawbacks of the above-mentioned known examples become clearer.

These drawbacks stem from the structure of the thin film light guide and the tapered film strip, and as the degree of integration of optical circuits increases, such a structure becomes more exposed.

As described above, the known examples of optical mode demultiplexers used for demultiplexing the 4-mode group of higher-order modes have many drawbacks.

In contrast, recently, optical waveguides (hereinafter referred to as inclined distribution optical waveguides) have been known that have a refractive index distribution inclined in the depth direction near the surface of an optically transparent substrate.

This is obtained by forming a layer with a higher refractive index than the surrounding area near the surface of the substrate by utilizing ion exchange phenomena, thermal diffusion phenomena, etc. in the substrate material.

The propagation characteristics of light waves propagating through such an optical waveguide are determined by the distribution of refractive index, as in the case of thin film optical waveguides.

However, unlike in the case of a thin film, in a tilted distributed optical waveguide where the refractive index is attenuated in the depth direction from the substrate surface, the propagation constant and energy distribution of any mode light wave will vary from the surface to the depth specific to the mode. It has the characteristic that it is determined by the refractive index in the waveguide leading to , and does not depend on the refractive index distribution state in a deeper region.

Here, the mode-specific depth is often referred to as a transition point, which moves in the depth direction from the surface of the waveguide as the mode order increases.

By providing a disturbance in the refractive index at an arbitrary depth from the surface of such a tilted distributed optical waveguide, all low-order mode light waves having a transition point in the region from the surface to the desired depth are without perturbing its propagation characteristics.
It is possible to change the propagation characteristics and exert effects such as refraction and reflection only on higher-order mode light waves.

An object of the present invention is to overcome the drawbacks of the prior art by using an optical waveguide having a gradient refractive index profile in the depth direction.

The present invention provides a new integrated optical mode demultiplexing device that eliminates manufacturing difficulties associated with integration and disturbances in the propagation state during demultiplexing in mode group units.

According to the present invention, in a multimode integrated optical waveguide having a refractive index distribution that monotonically decreases in the depth direction from the surface of the substrate, the depth of the optical waveguide is such that it affects only the higher-order mode light waves to be demultiplexed. An integrated optical mode demultiplexer characterized by changing the refractive index of the portion in the propagation direction of light waves is obtained.

The advantages of the present invention are that it has a simple structure and is easy to manufacture for integration, and that it can demultiplex only high-order mode light waves from a multimode light wave group by reflecting, refracting, and diffracting it. It's at the point where it's possible.

Next, the present invention will be explained in detail using the drawings.

FIG. 2 is a structural schematic diagram showing an embodiment of the present invention.

In the figure, a tilted distributed optical waveguide 2 containing more Kurilam ions ('rl) than the surrounding area is formed near the surface of an optically transparent K-7 glass substrate 21 using an ion exchange method.
2 is formed.

The depth and refractive index distribution of the optical waveguide 22 are different on both sides 24 and 25 of the boundary line 23.

FIG. 3 shows the refractive index distribution of such an optical waveguide.

That is, curves 31 and 32 respectively correspond to region 2 in FIG.
4 and 25 represent the distribution of refractive index in the depth direction from the waveguide surface.

Here, the refractive index and depth are indicated by n and x, respectively.

Curve 31 has a slope in the X direction that changes continuously and decreases, whereas curve 32 is represented by two attenuation curves whose slope changes discontinuously at x = h (1). be.

Here it is.

is a defined depth. Further, the difference between the lines 31 and 32 when 0≦X≦ho is negligibly small compared to the difference between them when h□<x.

Such a refractive index distribution can be obtained by selectively applying ion exchange treatment using Tl+ only to region 25 in FIG.
The entire surface including 4 and 25 was subjected to ion exchange treatment using TA+ at a treatment speed different from that in the first stage.

In such an optical waveguide, the aforementioned transition point is 0 < x
The low-order mode light wave group 26 that exists in the range of ≦ho is
As explained above, since it is not affected by the difference in the refractive index distribution of both layers represented by curves 31 and 32, it propagates straight through the boundary 23 without any perturbation being added to its propagation characteristics.

On the other hand, there is a transition point.

The higher-order mode light wave group 27 that exists in the range of ≦X is refracted under the influence of the difference in the refractive index distribution before and after the boundary 23.

That is, such an optical waveguide has a function of easily distributing a group of low-order mode light waves and a group of high-order mode light waves in a multimode integrated circuit.

FIG. 4 is an explanatory schematic diagram of another embodiment of the present invention.

In the figure, in a tilted distributed optical waveguide created on a BK-7 glass substrate 41 by the same ion exchange method as described above,
Only the lens-shaped region 42 was subjected to a two-step ion exchange treatment similar to that described in the previous example, and the curve 32 in FIG.
It has a refractive index distribution as shown below.

Further, the outside of the region 42 is subjected to one-stage ion exchange treatment similar to the previous embodiment, and has a refractive index distribution represented by the curve 31 in FIG. 3.

For the same reason as mentioned above, the low-order mode light wave group 43 is
Although the higher-order mode light waves 44 pass straight through the lenticular region, they are refracted by the lens action of the region 42 .

FIG. 5 is a schematic diagram illustrating still another embodiment of the present invention.

In exactly the same way as the embodiment shown in FIG. 4, in the inclined distributed optical waveguide fabricated on the substrate 51 by the ion exchange method, only the lattice-shaped region 52 having a constant period is formed by the two-step ion exchange process as shown in FIG. It has a refractive index distribution as represented by a curve 32.

Since the region 52 serves as a phase diffraction grating only for the high-order mode light wave group 54, it reflects only these light wave groups 54 and does not have any influence on the low-order mode light wave group 53.

In the above-mentioned embodiments of the present invention, only the ion exchange method was cited as a means for forming the inclined distributed optical waveguide. It is also possible to obtain a refractive index distribution.

As described above, the present invention essentially requires that the refractive index be disturbed only at a predetermined depth from the surface in an optical waveguide having a refractive index distribution inclined in the depth direction, The effects claimed in the present invention cannot be obtained with a structure in which the thickness of a thin film optical waveguide having a uniform refractive index distribution is changed in the propagation direction of light waves.

As mentioned above, the structure of the present invention is manufactured using a multi-stage ion exchange method or a thermal diffusion method. Alternatively, it can be achieved by forming a mask pattern of the desired shape on the substrate in advance for thermal diffusion, and the desired refractive index distribution can be achieved by precisely controlling the temperature of ion exchange or thermal diffusion for one hour, etc. Realized.

These techniques have already been established as integrated circuit manufacturing techniques.

Therefore, the fabrication of the present invention is extremely easy to integrate optical circuits as compared to conventionally known examples.

[Brief explanation of drawings]

FIG. 1 is a structural schematic diagram of an embodiment of a conventional integrated optical prism, FIG. 2 is a structural mechanical diagram showing an embodiment of the present invention, and FIG.
The figure is an explanatory diagram of the refractive index distribution of the optical waveguide of FIG. 2, FIG. 4 is an explanatory schematic diagram of another embodiment of the present invention, and FIG.
Further, an explanatory schematic diagram of another embodiment is shown. In the figure, 11, 21, 41, 51 are substrates, 12
is a thin film optical waveguide, 22 is an ion exchange optical waveguide, 26, 4
3.53 is a low-order mode light wave group, 27°44.54 is a high-order mode light wave group, and 25 and 42.52 are areas where two-stage ion exchange processing was performed.

Claims (1)

[Claims] 1 In a multimode integrated optical waveguide having a refractive index distribution that monotonically decreases in the depth direction from the surface of the substrate, the refractive index in the deep part of the optical waveguide that affects only the higher-order mode light waves to be demultiplexed is An integrated optical mode demultiplexing device characterized by changing the propagation direction of the optical mode.