Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-05-15
2004-02-10
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S129000
Reexamination Certificate
active
06690863
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical waveguide devices, and more particularly to passive optical waveguide devices, as well as integrated optical circuits including passive optical waveguide devices.
BACKGROUND OF THE INVENTION
In the integrated circuit industry, there is a continuing effort to increase device speed and increase device densities. Optical systems and technologies promise to deliver increasing speed and circuit packing density in the future. Optical waveguides typically include optical waveguide devices to provide optical functionality. Such optical waveguide devices can perform a variety of optical functions in integrated optical waveguide circuits such as optical signal transmission and attenuation.
In one aspect, optical waveguide devices include a variety of passive optical waveguide devices and/or a plurality of active optical waveguide devices. For example, certain gratings, lenses, filters, photonic crystals, and the like can be fabricated as passive optical waveguide devices. Similarly, active optical waveguide devices may function as filters, gratings, lenses, deflectors, switches, transmitters, receivers, and the like. Availability of a variety of passive and active optical waveguide devices and/or electronic devices provides a desired range of functionality. The availability of these devices is useful in making optical waveguide circuits simpler to design and fabricate.
A passive optical device does not change its function over a period of time excluding device degradations. A large variety of passive optical devices that include, e.g., optical fibers, slab optical waveguides, or thin film optical waveguides, may provide many optical functions. As such, the output or optical functionality of passive optical waveguide devices cannot be tuned or controlled. Additionally, passive active devices cannot be actuated (i.e., or turned on and off) depending on the present use of a region of an optical waveguide.
Many active optical waveguide devices such as modulators, filters, certain lenses, and certain gratings are precisely tunable. Tunability of certain active optical waveguide devices is important in making them more functional and competitive with present electronic circuits and devices.
Silicon-on-Insulator (SOI) and CMOS represent two technologies that have undergone a considerable amount of research and development relating to electronic devices and circuits. SOI technology can also integrate optical devices and circuits. It would be desirable to provide active optical waveguide device functionality and/or passive optical waveguide device functionality based largely on the CMOS devices and technology as well as manufacturing methods that allow for simultaneous fabrication of optically active and passive waveguide elements.
One embodiment of prior-art optical waveguide device is an arrayed waveguide grating (AWG) as shown in FIG.
2
. The AWG
400
includes an input coupler
402
, a plurality of arrayed waveguides
404
, and an output coupler
406
. The AWG
400
can be configured as a wavelength-division demultiplexer (if light signals travel from the left to the right in
FIG. 2
) or a wavelength-division multiplexer (if light signals travel from the right to the left in FIG.
2
). In the AWG
400
, each arrayed waveguide
404
has a different length between the input coupler
402
and the output coupler
406
. The difference in length between each one of the different arrayed waveguides
404
corresponds to an optical phase shift of m2&pgr;, where m is an integer for the central design wavelength of the light that is applied to the AWG
400
. Since each arrayed waveguide
404
has a different length, the light passing through the longer arrayed waveguides arrives at the output coupler
406
later than the light passing through the shorter arrayed waveguides.
AWGs
400
, however, are difficult and expensive to produce. Each arrayed waveguide
404
is measured and formed separately. The operation of the AWG
400
requires that the different arrayed waveguides
404
differ in length by a distance equal to an m2&pgr; optical phase shift for the central design wavelength that the AWG is designed to multiplex/demultiplex. The cross-sectional area and the material of each arrayed waveguide
404
of the AWG
400
is constant to maintain the effective mode index (or the propagation constant &bgr;) of the different arrayed waveguides
404
, and therefore provide a uniform velocity of light traveling through the different arrayed waveguides. As such, in present designs, each arrayed waveguide
404
of the AWG
400
: a) has precisely calculated and measured lengths; b) has the same precisely produced and measured cross-sectional areas; c) has different lengths, such that the difference between the successive lengths, &Dgr;l is such that &bgr; &Dgr;l=m2&pgr;; and d) is smoothly-curved through a gradual radius of curvature to reduce bending losses of light flowing through the arrayed waveguide
404
. Due to these requirements, the AWG
400
is challenging to design and fabricate since it is difficult to ensure the precise relative lengths of each one of the arrayed waveguides
404
. Both the precision requirements and fabrication tolerances place extreme requirements on the manufacturing process. These waveguides traditionally use different indices of glass to make the core and the cladding. Silicon is used in the fabrication process but does not participate in the optical function. A 6″ Si wafer may be able to accommodate 5-50 AWGs
400
depending on the design requirements and the available index contrast between the core and the cladding, which is generally of the order of a few percent. The waveguides in AWGs are designed to be polarization independent so that both the polarizations of the input light are more or less treated equally. Considerable time and human effort is therefore necessary to produce precise AWGs
400
.
It would therefore be desirable to fabricate passive optical waveguide devices (such as AWGS) using standard CMOS fabrication techniques which, when combined with active optical functions such as a modulator on the same substrate, could form the basis of a WDM system on a chip. It would also be desirable to fabricate such passive optical waveguide devices as AWGs and interferometers in a manner that the lengths and shapes of the arrayed waveguides are simple to accurately calculate, measure, and produce. Furthermore, it would be desired to apply active optical waveguide devices as tuning devices associated with optical circuits including passive optical waveguide devices, wherein much of the fabrication errors inherent in passive optical waveguide devices or device degradation over time can be dynamically tuned out by tuning the associated active optical waveguide devices.
SUMMARY OF THE INVENTION
One aspect of the invention relates to a passive optical waveguide device deposited on a wafer. The wafer includes an insulator layer and an upper semiconductor layer formed at least in part from silicon. The upper silicon layer forms at least part of an optical waveguide, such as a slab waveguide. The passive optical waveguide device includes an optical waveguide, a gate oxide, and a polysilicon layer (i.e., a layer formed at least in part from polysilicon) In some embodiments, the optical waveguide is formed within the upper semiconductor layer, a gate oxide layer that is deposited above the upper semiconductor layer, and a polysilicon layer that is deposited above the gate oxide layer. The polysilicon layer projects a region of static effective mode index within the optical waveguide. The region of static effective mode index has a different effective mode index than the optical waveguide outside of the region of static effective mode index. The region of static effective mode index has a depth extending within the optical waveguide. A value and a position of the effective mode index within the region of static effective mode index remains substantially unchanged over time. The region of static effective mode i
Si Optical, Inc.
Ullah Akm Enayet
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