Polarization desensitized optical waveguide interferometer

Details Polarization desensitized optical waveguide interferometer Polarization desensitized optical waveguide interferometer

2002-01-29

2004-06-29

Lee, John D. (Department: 2874)

Optical waveguides

With optical coupler

C385S011000, C385S014000, C385S043000

Reexamination Certificate

active

06757454

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical waveguide interferometer composed of planar optical waveguides, and more particularly to a technique that compensates for the polarization sensitivity of the optical interferometer, or on the contrary enhances the polarization dependence, by utilizing the dependence of waveguide birefringence on waveguide core width.
BACKGROUND ART
Today, optical wavelength division multiplexing communication systems (WDM systems) utilizing a plurality of optical wavelengths are being developed intensively to increase communication capacity. In the optical wavelength division multiplexing communication systems, arrayed waveguide grating optical wavelength multi/demultiplexers (abbreviated to AWGs from now on) are widely used as optical wavelength multi/demultiplexers for multiplexing a plurality of optical signals with different wavelengths at a transmitting side, and for demultiplexing a plurality of optical signals passing through an optical fiber to different ports at a receiving side.
FIG. 18
shows a circuit configuration of a conventional AWG. Light launched into an input waveguide is diffracted in parallel with a substrate
3
in a first slab optical waveguide
2
, and coupled to a plurality of arrayed waveguides
4
. Since the adjacent arrayed waveguides
4
have a fixed optical path difference, a plurality of light beams have phase differences depending on the wavelengths when they are coupled to a second slab optical waveguide
5
. As a result, the focal points made by the interference between the plurality of light beams change their positions depending on the wavelength. Thus, disposing a plurality of output waveguides
6
at the points of focus in advance makes it possible for the AWG to function as an optical wavelength multi/demultiplexer for multiplexing or demultiplexing the plurality of optical wavelengths in the block.
In the reported AWGs at present, the plurality of arrayed waveguides
4
are designed to have the same core width. The AWGs are fabricated using waveguides of a variety of materials such as glass, polymer and semiconductors, and their results are reported (M. K. Smit, “New focusing and dispersive planar component based on an optical phased array,” Electronics Letters, vol. 24, no. 7, pp. 385-386, March 1988; Y. Hida, et al., “Polymeric arrayed-waveguide grating multiplexer operating around 1.3 mm,” Electronics Letters, vol. 30, pp. 959-960, 1994; and M. Zirngibl, et al., “Polarization compensated waveguide grating router on InP,” Electronics Letters, vol. 31, no. 19, pp. 1662-1664, 1995).
Generally, an optical waveguide formed on a planar substrate has different effective refractive indices for the TM light with an electric field component vertical to the substrate and for the TE light with an electric field component parallel to the substrate. The difference between the effective refractive indices is called waveguide birefringence, and defined by the following equation (1).
B=n
TM
=n
TE
  (1)
where B is the waveguide birefringence, and n
TM
and n
TE
are the effective refractive indices of the TM light and TE light. The waveguide birefringence is caused by stress-induced birefringence, structural birefringence or the like.
AWG center wavelengths of the TM light and TE light are expressed by the following equations (2) and (3).
λ
TM
=
n
TM
·
Δ
⁢
 
⁢
L
m
(
2
)
λ
TE
=
n
TE
·
Δ
⁢
 
⁢
L
m
(
3
)
where &lgr;
TM
and &lgr;
TE
are the AWG center wavelengths of the TM light and TE light, &Dgr;L is the length difference between adjacent arrayed waveguides, and m is a diffraction order (integer).
As seen from the foregoing equations (1)-(3), when the waveguide birefringence B is present, the AWG center wavelengths &lgr;
TM
and &lgr;
TE
of the TM light and TE light differ from each other. Basically, a silica-based glass waveguide has little dependence of a propagation loss on the polarization. However, since the center wavelengths differ for the TM light and TE light, the AWG has a problem of the polarization sensitivity that its characteristic changes depending on the polarization state of incident light.
(First Example of Conventional Technique)
FIG. 19
shows a method of eliminating the polarization dependence. It inserts into the arrayed waveguide
4
a half-wave plate
7
whose principal axis inclines 45° at the center of the AWG via a groove
8
(Y. Inoue, et al., “Polarization sensitivity elimination in silica-based wavelength-division multiplexer using polyimide half waveplate,” IEEE J. Lightwave Technol., vol. 15, no. 10, pp. 1947-1957, October 1997).
The half-wave plate
7
operates as a polarization mode converter between the TM light and the TE light so that the polarization sensitivity is eliminated by exchanging the TM light and the TE light at the center of the AWG to average the overall characteristic.
(Second Example of Conventional Technique)
Another method of eliminating the polarization sensitivity of the AWG is reported. It reduces the thermal stress in the fabrication process of the AWG by providing the silica-based glass with a thermal expansion coefficient corresponding to the thermal expansion coefficient of the silicon substrate by adding much dopant to the silica-based glass, thereby eliminating the polarization sensitivity (S. Suzuki, et al., “Polarization-insensitive arrayed-waveguide gratings using dopant-rich silica-based glass with thermal expansion adjusted to Si substrate,” IEE Electron. Lett., vol. 33, no. 13, pp. 1173-1174, June 1997).
More specifically, adjusting the stress imposed on the silica-based glass layer from the silicon substrate to a value between −1 Mpa and 1 Mpa enables the absolute value of the waveguide birefringence to be limited equal to or less than 2×10
−5
, where the negative sign designates compressive stress and the positive sign designates tensile stress.
The second method of the conventional technique is a more promising candidate than the first method of the conventional technique because it obviates the additional step involved in inserting the half-wave plate
7
, and prevents excess loss as well. The second method, however, has a problem of readily causing cracks in the silica-based glass layer during the fabrication process of the AWG because the compressive stress of the glass is very weak. In addition, since the silica-based glass layer contains a lot of dopant, it is poor in long term weather resistance, and brings about crystallization in the waveguide which will increase the optical insertion loss of the waveguide. The low reliability is a critical problem with the optical communication component to be solved urgently.
In summary, the two methods of achieving the polarization-independence described in the conventional techniques have the problems to be solved. The first conventional method using the half-wave plate has a problem of requiring the additional step involved in inserting the half-wave plate, and of bringing about the excess loss of light. On the other hand, the second conventional method of eliminating the thermal stress of the glass by increasing the dopant of the silica-based glass has a problem of its reliability.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide a low cost, high reliability, polarization-independent optical waveguide interferometer.
DISCLOSURE OF THE INVENTION
We found that the waveguide birefringence varies depending on the core width. Utilizing this phenomenon, the present invention solves the problem of polarization sensitivity of the AWG without additional job or component. More specifically, the polarization sensitivity of the AWG is eliminated by varying the effective core widths of the waveguides of the arrayed waveguide one by one.
To accomplish the object, according to a first aspect of the present invention, there is provided an optical waveguide interferometer composed of optical waveguides on a substrate,

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