Optical waveguide device and method of forming the same

Details Optical waveguide device and method of forming the same Optical waveguide device and method of forming the same

2000-08-14

2002-05-28

Healy, Brian (Department: 2874)

Optical waveguides

Planar optical waveguide

C385S131000, C385S132000, C385S141000, C385S142000, C385S144000, C385S014000

Reexamination Certificate

active

06396988

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical wave guide device usable for optical communications and a method of forming the same.
As a great deal of rapid and worldwide propagation of internet, the requirement for commercialization of the optical communication devices has been on the increase. For example, 2.5 Gb/s-systems capable of transmissions with large capacity corresponding to thirty thousands telephone lines have been introduced into many areas As the amount of informations to be transmitted through the optical communication system has been on the increase, a wavelength-multiplexing system has been practiced. In the initial state, a few waves were multiplexed in wavelength. Recently, however, a high density wavelength-multiplexing system could be realized at about 80 wave level. For the high density wavelength-multiplexing system, there are important a multiplexer for multiplexing plural optical signals having different wavelengths to introduce the multiplexed optical signal into a single optical fiber or a single optical waveguide as well as a demultiplexer for demultiplexing the multiplexed optical signal into plural optical signals having different wavelengths to introduce the plural optical signals into plural optical fibers or plural optical wave guides. An array wave-guide grid has been attracted as one example.
FIG. 1
is a diagram illustrative of a conventional array wave-guide grid. The conventional array wave-guide grid has an input waveguide
21
, an array waveguide
23
connected through a first star-coupler
23
to the input waveguide
21
and an, output waveguide
25
connected through a second star-coupler
24
to the array waveguide
23
. The array waveguide
23
comprises plural wave guides which are different in length of optical path by a constant difference and are arranged in array, so that the array waveguide
23
serves as a high-order diffraction-grating to exhibit multiplexing and demultiplexing functions. The array waveguide grid has already been commercialized and used in the existent optical communication system, wherein silica-based optical wave guides are formed over a silicon substrate or a silica substrate.
If the silica-based optical wave guides are formed over the silicon substrate, then a difference in thermal expansion coefficient between the silica-based optical wave guides and the silicon substrate causes a thermal stress which generates a birefringence or birefringence or double refraction in a silica-based layer of the silica-based optical wave guide. The birefringence or birefringence or double refraction causes a difference in propagation characteristics of the silica-based optical wave guide between in TE-mode and TM-mode. Particularly, this problem is serious to the device such as the array waveguide grid having a narrow distance of adjacent channel wavelengths and an abrupt transmission wavelength spectrum because a slight difference in wavelength characteristics between in TE-mode and TM-mode causes a large polarization dependency loss. In case of the array waveguide grid device with a frequency distance of 100 GHz, the polarization dependency loss is approximately proportional to a difference &Dgr;&lgr;(=&lgr;
TM
−&lgr;
TE
) between a first transmission center wavelength &lgr;
TM
in TM-mode and a second transmission center wavelength &lgr;
TE
in TE-mode. A proportional constant may be estimated in the range of about a few dB
m to 10 dB
m unless any specific design technique is taken to planarize a peak portion of the transmission wavelength spectrum. If the silica-based optical waveguide, which have practically been manufactured, is applied to the above described array waveguide grid device, then the polarization dependency loss is extremely large, for example, not less than 1 dB since &Dgr;&lgr; is, normally not less than 0.1 nm. The actually used array wave guide grid device having the silicon substrate and the silica-based optical wave guide is further provided with a half-wavelength plate at a center of the array wave guide for canceling a difference in wavelength characteristic between polarized lights.
FIG. 2
is a diagram illustrative of the conventional array wave guide grid device. The conventional array wave guide grid devices has an input waveguide
21
, an array waveguide
23
connected through a first star-coupler
23
to the input waveguide
21
and an output waveguide
25
connected through a second star-coupler
24
to the array waveguide
23
. The array waveguide
23
comprises plural wave guides which are different in length of optical path by a constant difference and are arranged in array, so that the array waveguide
23
serves as a high-order diffraction-grating to exhibit multiplexing and demultiplexing functions. The array waveguide
23
also has a half-wavelength plate
26
at its center position for canceling a difference in wavelength characteristic between polarized lights. The additional provision of the half-wavelength plate
26
suppresses the polarization dependency loss to not more than 0.2 dB which is not practical problem. It is, however, necessary to realize a highly accurate positioning of the half-wavelength plate
26
through many additional processes. The half-wavelength plate is somewhat expensive and makes it difficult to reduce the manufacturing cost of the conventional array wave guide grid device. In this circumstances, it had been required to reduce the polarization dependency loss without the half-wavelength plate. In order to reduce the polarization dependency loss without the half-wavelength plate, it is necessary to reduce the thermal stress to the silica-based layer of the optical wave guide. In order to reduce the thermal stress, it is effective that a dopant concentration of a dopant such as phosphorus or boron of the silica-based layer is adjusted so that the thermal expansion coefficient of the doped silica-based layer is made close to the thermal expansion coefficient of tie silicon substrate. In Japanese laid-open patent publication No. 8-136754, it is disclosed that in order to reduce the thermal stress, a cladding, layer is used to form the optical wave guide, wherein a dopant concentration of a dopant such as phosphorus or boron of the silica-based cladding layer is adjusted so that the thermal expansion coefficient of the silica-based cladding layer is made close to the thermal expansion coefficient of the Silicon substrate.
FIG. 3
is a diagram illustrative of individual variations in thermal expansion coefficients of silica-based glasses doped with individual dopants, for example P
2
O
5
, GeO
2
, B
2
O3, Al
2
O
3
, F and TiO
2
, over a dopant concentration. It is possible to reduce the difference in thermal expansion coefficient between the silica-based glass layer and the silicon substrate by controlling the dopant concentration.
In ELECTRONICS LETTERS vol. 33, No. 13, pp. 1173-1174, June 1997, it is disclosed that the array wave guide grid device has a reduced stress birefringence or double refraction, wherein the transmission center wavelength difference &Dgr;&lgr; is reduced from 0.19 nanometers to 0.03 nanometers.
In accordance with the conventional method, the silica-based film is formed by use of a high temperature heat treatment at a temperature of not less than 1200° C., for example, FHD method, wherein in order to reduce the stress, dopant concentrations of P and B are somewhat increased from the normal concentrations. The high temperature heat treatment causes separated phases of P
2
O
5
and B
2
O
3
in the silica-based glass layer. A large amount of deposition is formed in the layer or on a surface of the layer. The separated phases and the deposition serve for scattering the light, whereby the optical propagation loss is increased.
In the above circumstances, it had been required to develop an optical waveguide device with a reduced polarization dependency and an optical propagation loss as well as a method of forming the device by use of low temperature processes with an optimization to the dopant

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