JPS62145207A - Waveguide type optical demultiplexer - Google Patents

Abstract

PURPOSE:To obtain an integrated demultiplexer with good characteristics which is effective as a device for ultra wavelength multiplex transmission by employing a method in which variation in refractive index as a diffraction grating is caused by forming projections and recessed on the flank of a waveguide. CONSTITUTION:The diffraction grating 4 is formed on the side wall of the waveguide 3 by employing a two-layer masking method which utilizes sandwiching characteristics. An input signal 5 is made incident on one end surface of the waveguide 3 and propagated in the waveguide 3 and part of the input signal which has wavelength satisfying the Bragg condition is diffracted by the diffraction grating 4 and propagated in a waveguide 3'. A photodetecting element or external waveguide is connected to the tip of the waveguide 3' and the demultiplexed output signal 6 is detected, thus constituting the waveguide type optical demultiplexer.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a waveguide type optical demultiplexer used as a transmission component for optical communications or an optical integrated circuit element.

Conventional technology In recent years, with the progress of optoelectronics technology, light emitting elements such as semiconductor lasers and P
Light receiving elements such as IN photodiodes, and functional elements such as switches and optical metering devices are being actively developed. Furthermore, research on ultra-wavelength multiplexing communication is steadily progressing in tandem with optical integration technology as a means of effectively utilizing the optical system. The hybrid type, in which the various elements that make up an optical integrated circuit are individually integrated on a substrate, and the monolithic type, in which the various elements making up an optical integrated circuit are integrated on the same substrate using the same materials, are being developed in parallel. However, recently, improvements in shrimp technology and process technology have demonstrated the possibility of practical devices in the latter, and the mainstream of development has shifted to monolithic types.

Various elements on such an optical integrated circuit are interconnected by optical waveguides formed on the substrate. By completely forming a diffraction grating on an optical waveguide, a wavelength selection function can be provided, and a demultiplexer can be constructed. Many waveguide-type optical demultiplexers have been proposed to date, and some have been prototyped. Figure 2 shows a complete perspective configuration diagram of a typical demultiplexer circuit (
Special Publication No. 54-38143). In the figure, 11 is a substrate, 12 is an optical waveguide layer, and 13 is a diffraction grating formed on the optical waveguide layer and having a splitting function. Input source 14 incident on optical waveguide 12
is subjected to a diffraction effect by a diffraction grating in the waveguide, and only wavelengths that satisfy the bladder condition are separated and extracted.

Problems to be Solved by the Invention However, the above-mentioned method had the following drawbacks. The modes propagating through the waveguide include a TE mode and a TM mode whose polarization planes are orthogonal to each other. The TM mode, which has a plane of polarization perpendicular to the substrate surface, has a large loss due to the influence of the metal film on the waveguide surface, and there are problems with consistency due to the fact that the plane of polarization of other light emitting elements, such as semiconductor lasers, is usually the TE mode. The TE mode is more commonly used as a waveguide mode because it has many disadvantages in terms of the waveguide mode.

By the way, as shown in FIG. 3, for light having an incident angle of θ and a wavelength of λ in the medium, the diffraction grating 13 having a grating constant given by Δ=λ/2 sin θ satisfies the Bragg condition. The incident light 16 is diffracted against the grating surface in the direction of the reflection angle θ, and acts as a filter. I
A C-type optical filter may have a configuration in which a demultiplexer section with a diffraction grating in the waveguide and a light receiving element are integrated, but for ultra-wavelength multiplex transmission, it is possible to combine multiple demultiplexers and a light receiving element section. A light receiving element must be integrated on the substrate.

At this time, the ideal angle of incidence θ with respect to the diffraction grating is 46° from the viewpoint of multiplicity and low crosstalk characteristics. However, in the case of the duplexer with the above configuration, Brag
It is known that g diffraction efficiency is significantly affected. First, when the incident light in TE mode enters the diffraction grating at 45°, no diffraction occurs. Regarding this phenomenon, see (Bell System Technical Journal) B
e1l 5yst.

Tech, Journal Vol, 41 No+9
．． H. Kogelnik's paper “Coupled
Wave Theory for ThickHolo
gram gratings”.

For the above-mentioned reasons, it has been difficult with conventional methods to obtain characteristics that satisfy the conditions required for an original IC device having a demultiplexing function used in ultra-wavelength multiplexing transmission.

SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a complete method for manufacturing a diffraction grating that can eliminate the above-mentioned drawbacks, ie, has the characteristics required for an integrated duplexer.

Means for Solving the Problems In order to solve the above problems, according to the present invention, first, a first optical waveguide formed on a substrate surface is arranged at an intersection angle of approximately 90' with respect to the waveguide. It has at least one coupled second optical waveguide, and a diffraction grating having a wavelength selective reflection function is formed at the coupling portion of @1 and the second optical waveguide. At this time, the entire waveguide type optical demultiplexer can be constructed by forming the diffraction grating on the side wall of the first optical waveguide at the coupling portion of the first and second optical waveguides.

As the working waveguide structure, a slab type waveguide or a lip type waveguide which performs the original confinement three-dimensionally is often used because of ease of fabrication or good propagation characteristics. The present invention uses the latter method, and forms an optical waveguide by three-dimensionally processing the waveguide layer. In the conventional example, a diffraction grating with periodic irregularities formed on the surface of the waveguide is used, but in the present invention, a diffraction grating with periodic irregularities formed on a part of the side wall of the lip type waveguide is preferably used. In other words, in order to function as a diffraction grating, a slight perturbation change in the shape or refractive index is required, but in the conventional example, the diffraction grating has a refractive index change in the direction perpendicular to the substrate surface. In this case, the change in refractive index in the direction parallel to the substrate surface is utilized.

As mentioned above, for traveling waves in TE mode, there is a problem that the diffraction efficiency decreases significantly under the 46° incident condition, but on the other hand, there is no problem for traveling waves in TM mode even under the same conditions. It has been known.

In the present invention, by creating unevenness on the side surface of the waveguide to provide a change in the refractive index as a diffraction grating, the TE
The aforementioned effects on the mode can be removed. Therefore, it becomes easy to adopt the structure required for IC implementation, and it becomes possible to construct a high-performance optical demultiplexer.

EXAMPLES Next, the present invention will be described with reference to the drawings. FIG. 1 shows all the embodiments of the present invention, in which 1 is an InP substrate, 2 is an I
An optical waveguide layer made of nGaAs P, 3 and 3' are optical waveguides formed in a rib shape, 4 is a diffraction grating formed on the side wall of the optical waveguide 3, 5 is an input signal, and θ is an output signal. The widths of the waveguides 3 and 3' may be set so as to satisfy all single mode conditions. The S/R ratio can be improved by etching the regions of the optical waveguide layer 2 other than the optical waveguides 3 and 3' to be thinner than the cutoff film thickness for the guided light in order to eliminate surface propagation of stray light components. . Further, the diffraction grating 4 can be formed on the side wall of the waveguide 3 by using a two-layer mask method utilizing side etching characteristics. In this configuration, of the input signal S that is incident from one end surface of the waveguide 3 and propagates inside the waveguide 3, only the wavelength that satisfies the plug condition in the diffraction grating 4 is diffracted and propagated through the waveguide 3'. At the tip of 3' is a light receiving element b, which is connected to an external guide path and detects the separated output signals to form the entire waveguide type optical demultiplexer. Since the waveguide type optical demultiplexer with the diffraction grating structure configured as described above has a periodic change in refractive index parallel to the substrate surface, its incidence angle dependence is also low for TE mode optical signals. This means that a structure that is advantageous for integration can be adopted.

Note that in this example, InClaAs on an InP substrate
Although we have described a waveguide type optical demultiplexer using a P layer as an optical waveguide layer, other materials such as GaAs-based semiconductor materials or ferroelectric materials such as L I N b 03 may also be used. Needless to say, it can also be applied.

Effects of the Invention As described above, according to the present invention, it is possible to realize an integrated demultiplexer with very good characteristics that is effective as a device for ultra-wavelength multiplexing transmission, and its practical effects are great. .

[Brief explanation of drawings]

Fig. 1 is a perspective view showing the structure of a waveguide type optical demultiplexer according to an embodiment of the present invention, Fig. 2 is a perspective view of the structure of a conventional demultiplexer circuit, and Fig. 3 is an explanatory diagram of Bragg diffraction conditions. be. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Optical waveguide layer, 3 and 3'...0.・Optical waveguide, 4 =--diffraction grating, 5・
...Input signal, 6...Output signal. Name of agent: Patent attorney Toshi Nakao
2 Figure/, Jl! l14/+ Kakucho

Claims (1)

[Scope of Claims] A first thin-film optical waveguide formed on a substrate surface; at least one or more second optical waveguides coupled to the first optical waveguide and arranged at an intersection angle of approximately 90°; In the waveguide type optical demultiplexer, in which a region having a wavelength selective reflection function is provided at a coupling portion between the first optical waveguide and the second optical waveguide, a side wall of the first waveguide in the coupling portion has a periodicity. 1. A waveguide type optical demultiplexer, characterized in that a diffraction grating path having irregularities is formed.