Optical waveguide circuit

Details Optical waveguide circuit Optical waveguide circuit

2001-12-21

2003-06-17

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C385S014000, C385S046000, C385S129000

Reexamination Certificate

active

06580862

ABSTRACT:

This application is based on Japanese Patent Application Nos. 2000-391100 filed Dec. 22, 2000, 2001-241368 and 2001-241369 both filed Aug. 8, 2001, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide circuit, and more particularly to an optical waveguide circuit which is an optical component used for an optical communication system, and to an optical waveguide circuit with slab waveguides.
2. Description of the Related Art
As the Internet has spread worldwide, it becomes the urgent necessity to construct communication systems that can transmit large amounts of data simultaneously at high speed. As a system that satisfies such a requirement, an optical communication system utilizing optical wavelength division multiplexing (WDM) receives attention, and is being introduced worldwide with the U.S. taking the lead.
To implement optical WDM technique, an optical multiplexer/demultiplexer capable of multiplexing and demultiplexing multiple different wavelengths is absolutely necessary. As one of its actual forms, there is an optical waveguide circuit that implements an optical circuit using optical waveguides on a substrate.
The optical waveguide circuits are ICs in the optical world, which apply the LSI microfabrication technology to form optical waveguides on a planar substrate integrally. Accordingly, they are superior as integrated circuits suitable for mass production, and capable of implementing high performance circuits with complicated circuit configurations. Recently, as the interest in the optical communication system has grown remarkably, the research and development of various materials such as semiconductor, LiNbO
3
, plastics and silica-based glass has been advanced. Among them, silica-based optical waveguides, which are formed on silicon substrates using silica-based glass, have good matching with silica-based optical fibers constituting transmission lines of optical communications, and can implement stable operation of the optical circuits because of the high stability and long reliability based on the characteristics of the material. In addition, since rectangular cores can be formed at high reproducibility, theory and practice match closely, and high performance circuits with complicated optical circuits can be implemented. With such features, they lead other waveguide materials in practical use.
As basic configurations of the optical multiplexer/demultiplexers using the silica-based optical waveguides with such superior characteristics, there are Mach-Zehnder interferometers and arrayed waveguide gratings. Combining the Mach-Zehnder interferometers with arrayed waveguide gratings can implement various optical multiplexing and demultiplexing characteristics.
The Mach-Zehnder interferometer multiplexes or demultiplexes two different wavelengths, or divides different multiple wavelengths alternately and periodically.
FIG. 1A
shows a configuration of a Mach-Zehnder interferometer
101
. It comprises two optical couplers
102
, two optical waveguides
103
interconnecting the two optical couplers
102
, and an input waveguide
104
and an output waveguide
105
for connecting it with other optical couplers
102
. Here, each optical coupler is composed of a directional coupler consisting of two adjacent optical waveguides. Each optical waveguide is composed of a single-mode waveguide. The spacing between wavelengths to be multiplexed or demultiplexed is set by the waveguide length difference between the two optical waveguides
103
.
The arrayed waveguide grating multiplexes and demultiplexes multiple different wavelengths simultaneously. It can achieve such a function with smaller size than achieving it using multiple Mach-Zehnder interferometers.
FIG. 1B
shows a configuration of an arrayed waveguide grating
111
. It comprises two slab waveguides
112
, a waveguide array
113
interconnecting the two slab waveguides
112
, and an input waveguide
114
and an output waveguide
115
which are connected to different slab waveguides
112
. The waveguide array
113
, input waveguide
114
and output waveguide
115
are each composed of a single-mode waveguide. The waveguide array
113
comprises adjacent optical waveguides with different waveguide lengths, and the waveguide length difference between the adjacent waveguides determines the spacing between the wavelengths to be optically multiplexed and demultiplexed.
FIG. 2
is a cross-sectional view of a silica-based optical waveguide constituting these optical multiplexer/demultiplexers. It has a structure in which a core
203
is coated with a cladding
202
formed on a substrate
201
. The substrate
201
consists of a silicon substrate or a silica substrate, and the cladding
202
and the core
203
are composed of silica-based glass. Using the silicon substrate as the substrate
201
serves as a platform for the hybrid mounting of photo-detectors, light emitting devices and the like. It can also prevent cracks of the cladding
202
and core
203
because of compressive stress imposed on them, thereby increasing reliability.
When using the silicon as the substrate, however, the optical characteristics of the Mach-Zehnder interferometer or the arrayed waveguide grating formed by the silica-based optical waveguides have polarization dependence in which the spectrum in a TM mode with the electric field perpendicular to the substrate shifts to a longer wavelength side as compared with that of a TE mode with the electric field parallel to the substrate.
FIG.
3
and
FIGS. 4A and 4B
are graphs each illustrating transmission spectra of a Mach-Zehnder interferometer or those of an arrayed waveguide grating.
FIG. 3
illustrates transmission spectra of a Mach-Zehnder interferometer with a multiplexing/demultiplexing spacing of 0.8 nm. As illustrated in this figure, the peak wavelengths giving the least loss in the TE mode and TM mode are shifted by about 0.25 nm, and the loss of the other mode is greater about a few dB at each peak wavelength.
FIG. 4A
illustrates transmission spectra of an arrayed waveguide grating with a multiplexing/demultiplexing spacing of 0.8 nm, and
FIG. 4B
illustrates enlarged transmission spectra around 1562 nm of FIG.
4
A. As in the Mach-Zehnder interferometer, the peak wavelengths that give the least loss for the TE mode and TM mode are shifted by about 0.25 nm, and the loss of the other mode is greater about a few dB at each peak wavelength.
Such polarization dependence in the optical multiplexer/demultiplexer, which makes the polarization direction of a signal light transmitted through an optical fiber indefinite and varied with time, will cause the insertion loss or crosstalk to vary with time, thereby degrading the reliability of the signal.
The polarization dependence of the optical multiplexer/demultiplexer is brought about by the waveguide birefringence of the optical waveguides constituting the circuit, where the effective refractive index of the TM mode perceives is greater than the effective refractive index of the TE mode. The polarization dependence occurs because of the waveguide birefringence of the two optical waveguides
103
in the Mach-Zehnder interferometer, and of the waveguide array
113
in the arrayed waveguide grating.
Defining that the waveguide birefringence of the silica-based optical waveguide is the difference obtained by subtracting the effective refractive index of the TE mode from the effective refractive index of the TM mode, it takes a value of about 2×10
−4
−3×10
−4
in the single-mode waveguide. Such a waveguide birefringence results from the compressive stress on the optical waveguide, which is a residual thermal stress caused by the difference between the thermal expansion coefficient of the silicon constituting the substrate and that of the silica-based glass used as the material of the optical waveguide. In addition, since the thermal expansion coefficient varies depending on the type and concentration of the

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