Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-04-25
2003-05-13
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S037000, C385S024000, C385S046000, C385S028000, C385S039000, C385S015000, C385S014000
Reexamination Certificate
active
06563988
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed to an apparatus and method which is useful in wavelength division multiplexing systems. More particularly, this invention is directed to an optical apparatus having a predetermined group velocity dispersion of signals.
BACKGROUND
Wavelength division multiplexing (WDM) is a technique for increasing transmission capacity in fiber optic communication systems. To this end, a number of optical devices have been researched and developed including optical routers.
A typical optical router has at least one input port and at least one output port. Light associated with an optical signal is coupled from an input port to an output port according to the carrier wavelength of the optical signal. Examples of optical routers include multiplexers, demultiplexers, and N×N optical routers.
A number of problems, however, must be overcome whenever a WDM system is used for high data rate transmission. These problems include group-velocity dispersion (GVD) and differential group delay (DGD).
GVD is one problem that arises when data rates are increased in a WDM system. If the magnitude of the group-velocity dispersion of the system is sufficiently large, the optical pulses that are transmitted as adjacent pulses will be received as pulses that overlap to a significant extent. The overlapping of adjacent pulses increases the bit error rate of the system and consequently degrades the performance of the fiber optic system. In order to prevent this performance degradation, typically all components of the system are required to have a value of group-velocity dispersion within a certain tolerance. The tolerance limits become smaller as the data rate increases for a particular application. For high bit rate applications the optical router is commonly required to have a value of group-velocity dispersion that is low or sufficiently close to zero throughout the band associated with each wavelength channel. The optical router must have a low value of the absolute value of the group-velocity dispersion within a substantial portion of the passband of each channel.
Differential group delay (DGD) is another problem that must be overcome when data rates are increased in a WDM system. DGD is the group delay for the polarization state that provides that largest group delay minus the group delay for the polarization state that provides the lowest group delay. In order to prevent a fiber optical transmission system from being degraded by polarization mode dispersion, each of the components must have sufficiently low DGD. In general, DGD is positively correlated with GVD. Therefore, ensuring that GVD is low typically ensures that DGD is low.
One technique for fabricating an optical wavelength router is planar lightwave circuit (PLC) technology. A typical PLC comprises planar waveguides and/or channel waveguides. Examples of planar and channel waveguides are shown in H. Kogelnik,
Theory of Optical Waveguides,
Guided-Wave Optoelectonics T. Tamir ed., Springer-Verlag, Berlin, 1988, and also by H. Nishihara, M. Haruna, and T. Suhara,
Optical Integrated Circuits,
McGraw Hill, New York, 1987.
In a planar (or slab) waveguide, light is generally restricted to propagate in a region that is thin (typically between 3 &mgr;m and 30 &mgr;m) in one dimension, referred to herein as the lateral dimension or height, and extended (typically between 1 mm and 100 mm) in the other two dimensions. Herein, we refer to the plane that is perpendicular to the lateral dimension of the PLC as the plane of the PLC. The longitudinal direction is defined to be the direction of propagation of light at any point on the PLC. Further, the lateral direction is defined to be perpendicular to the plane of the PLC and the transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
In a channel waveguide, light has an optical field that is substantially confined in both the lateral direction and the transverse direction. In a typical channel waveguide, the field is substantially confined within a region that extends between 3 &mgr;m and 30 &mgr;m in the lateral direction, herein referred to as the height, and extends between 3 &mgr;m and 100 &mgr;m in the transverse direction, herein referred to as the width.
Typically, the optical field of light that propagates in a channel waveguide comprises a linear combination of normal modes. The normal modes may be denoted as E
x
pq
and E
y
pq
, where p and q may be any non-negative integer and x and y are used to denote the polarization of the mode, x referring to the lateral direction and y referring to the transverse direction. See H. Nishihara, M. Haruna, and T. Suhara,
Optical Integrated Circuits,
McGraw Hill, New York, 1987, p. 29. Herein &phgr;
i
refers to either E
x
0,i−1
, or E
y
0,i−1
or a linear combination of E
x
0,i−1
and E
y
0,i−1
as the case may be. That is, &phgr;
1
refers to a mode that has no nodes in either the lateral or the transverse directions and &phgr;
3
refers to a mode that has no nodes in the lateral direction and two nodes in the transverse direction. Herein the &phgr;
1
mode may be referred to as the fundamental mode or, alternatively to the first mode. Herein the &phgr;
3
mode may be referred to as the third mode.
There are various approaches to building a PLC. In a typical example of a PLC, a slab waveguide comprises three layers of silica glass with the core layer lying between the top cladding layer and the bottom cladding layer. Channel waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. Further, each layer may be doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer. As a second example, slab waveguides and channel waveguides comprise three or more layers of InGaAsP and adjacent layers can have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the slab waveguide and/or channel waveguide may comprise an optically transparent polymer. A fourth example of a slab waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
In addition to the channel and slab waveguides described above, various PLCs may comprise at least one optical dispersive region such as, for example, an arrayed waveguide. An arrayed-waveguide grating router (AWGR) is a planar lightwave circuit and comprises at least one input channel waveguide, an input slab waveguide, an arrayed-waveguide grating (AWG), an output slab waveguide, and at least one output channel waveguide. The edge of the input slab waveguide to which the input waveguides are attached is referred to herein as the input focal curve. The edge of the output slab waveguide to which the output waveguides are attached is referred to herein as the output focal curve.
The arrayed-waveguide grating comprises an array of waveguides. The length of the i
th
waveguide in the AWG is denoted as L
i
. The angular dispersion that is provided by the AWG is determined in part by the difference in length between adjacent waveguides, L
i+1
−L
i
. The details of construction and operation of the AWGR are described in M. K. Smit and C. Van Dam,
PHASAR
-
Based WDM
-
Devices: Principles, Design, and Application,
IEEE Journal of S
Knauss Scott A
Lightwave Microsystems Corporation
Morrison & Foerster / LLP
Sanghavi Hemang
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