Optical waveguides – Integrated optical circuit
Reexamination Certificate
2001-07-26
2004-09-21
Nasri, Javaid H. (Department: 2839)
Optical waveguides
Integrated optical circuit
Reexamination Certificate
active
06795600
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the art of optical integrated circuits and more particularly to apparatus and methods for mitigating polarization dependence in optical integrated circuits.
BACKGROUND OF THE INVENTION
Optical integrated circuits (OICs) come in many forms such as 1×N optical splitters, optical switches, wavelength division multiplexers (WDMs), demultiplexers, optical add/drop multiplexers (OADMs), and the like. Such OICs are employed in constructing optical networks in which light signals are transmitted between optical devices for carrying data and other information. For instance, traditional signal exchanges within telecommunications networks and data communications networks using transmission of electrical signals via electrically conductive lines are being replaced with optical fibers and circuits through which optical (e.g., light) signals are transmitted. Such optical signals may carry data or other information through modulation techniques, for transmission of such information through an optical network. Optical circuits allow branching, coupling, switching, separating, multiplexing and demultiplexing of optical signals without intermediate transformation between optical and electrical media.
Such optical circuits include planar lightwave circuits (PLCs) having optical waveguides on flat substrates, which can be used for routing optical signals from one of a number of input optical fibers to any one of a number of output optical fibers or optical circuitry. PLCs make it possible to achieve higher densities, greater production volume and more diverse functions than are available with fiber components through employment of manufacturing techniques typically associated with the semiconductor industry. For instance, PLCs typically comprise optical paths known as waveguides formed on a silicon wafer substrate using lithographic processing, wherein the waveguides are made from transmissive media including lithium niobate (LiNbO
3
) or other inorganic crystals, silica, glass, thermo-optic polymers, electro-optic polymers, and semiconductors such as indium phosphide (InP), which have a higher index of refraction than the chip substrate or the outlying cladding layers in order to guide light along the optical path. By using advanced photolithographic and other processes, PLCs are fashioned to integrate multiple components and functionalities into a single optical chip.
One important application of PLCs and OICs generally involves wavelength-division multiplexing (WDM) including dense wavelength-division multiplexing (DWDM). DWDM allows optical signals of different wavelengths, each carrying separate information, to be transmitted via a single optical channel or fiber in an optical network. For example, early systems provided four different wavelengths separated by 400 GHz, wherein each wavelength transferred data at 2.5 Gbits per second. Current multiplexed optical systems employ as many as 80 wavelengths, and systems are contemplated having more than 160 wavelength channels with 50 GHz spacing, carrying data at 10 Gbits per second in each channel.
In order to provide advanced multiplexing and demultiplexing (e.g., DWDM) and other functions in such networks, arrayed-waveguide gratings (AWGs) have been developed in the form of PLCs. Existing AWGs typically provide multiplexing or demultiplexing of 40 channels or wavelengths spaced at 100 GHz, and AWGs are contemplated to accommodate 128 wavelengths spaced at 25 GHz. As illustrated in
FIG. 1
, a conventional demultiplexing AWG
2
includes a base
4
, such as a silicon substrate, with a single input port
6
, and multiple output ports
8
. Multiple wavelength light is received at the input port
6
(e.g., from an optical fiber in a network, not shown) and provided to an input lens
10
via an input optical path or waveguide
12
in the substrate base
4
.
The input lens
10
spreads the multiple wavelength light into an array of waveguides
14
, sometimes referred to as arrayed-waveguide grating arms. Each of the waveguides or arms
14
has a different optical path length from the input lens
10
to an output lens
16
, resulting in a different phase tilt at the input to the lens
16
depending on wavelength. This phase tilt, in turn, affects how the light recombines in the output lens
16
through constructive interference. The lens
16
thus provides different wavelengths at the output ports
8
via individual output waveguides
18
, whereby the AWG
2
can be employed in demultiplexing light signals entering the input port
6
into two or more demultiplexed signals at the output port
8
. The AWG
2
can alternatively be used to multiplex light signals from the ports
8
into a multiplexed signal having two or more wavelength components at the port
6
.
A problem with the AWG
2
of
FIG. 1
is polarization dependence of the waveguides
14
, caused by waveguide birefringence. Waveguide birefringence is experienced in varying degrees with waveguides fabricated from the above-mentioned materials. For example, where the waveguides
14
are formed by depositing a glass film on a silicon substrate, the difference in thermal expansion coefficient between the glass film and the silicon substrate base
4
causes stress applied on the waveguides
14
in a direction parallel to the surface to be different from that in a perpendicular direction.
Waveguide birefringence results, wherein the refractive index of the waveguides
14
in the direction parallel to the substrate surface becomes different from that in the perpendicular direction. The birefringence, in turn, causes polarization dependence in the waveguides
14
, where the optical path length difference (e.g., between adjacent waveguides
14
) changes depending on the polarizing direction of light. In this situation, shifts occur between the transverse electric (TE) and transverse magnetic (TM) mode peaks, where the shift changes according to polarization. Consequently, the device characteristics change in accordance with the polarized state of the light provided to the device
2
. For instance, the peak coupling in a particular channel or waveguide
14
can vary according to the polarities of the various wavelength components, causing polarization dependent wavelength (PDW) shift.
Referring to
FIG. 2
, this polarization sensitivity or dependence in AWGs and other dispersive components has been heretofore addressed by bisecting the waveguides
14
and placing a waveplate, such as a half-waveplate
20
, in a slot
21
between waveguide portions
22
and
24
. Thus located, the waveplate
20
reduces or eliminates polarization dependence in the AWG
2
. In particular, it has been found that the waveplate
20
causes polarization swapping partway along the optical paths of the bisected waveguides
14
, such that any input polarization samples each propagation constant equally and provides essentially no shift in peak-wavelength with changes in input polarization. Thus, the spectrum for the TE and TM modes coincide through the use of the waveplate
20
. Conventionally, the waveplate
20
is placed at the precise center of the grating arms or waveguides
14
to eliminate the wavelength shift resulting from birefringence.
However, this waveplate location causes the interface between the waveplate
20
and the waveguide ends of portions
22
and
24
to be perpendicular or near perpendicular. As a result, a small fraction of the light propagating through the waveguide portions
22
may be reflected back toward the input lens
10
, leading to unacceptably high back-reflection and directivity measurements Thus, although the convention use of the waveplate
20
reduces or eliminates the polarity sensitivity problems associated with waveguide birefringence, back-reflection is increased. Consequently, there remains a need for better solutions to polarity dependence in optical integrated circuits such as AWGs, which avoid or mitigate the back-reflection problems associated with the convention employment of waveplates in such devices.
SUMMARY OF THE INVENT
Amin & Turocy LLP
Lightwave Microsystems Corporation
Nasri Javaid H.
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