Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-05-03
2004-02-24
Bruce, David V. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
C385S043000, C385S050000
Reexamination Certificate
active
06697552
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to planar lightwave circuits for use in optical signal routing applications, in particular, planar lightwave circuits having arrayed waveguide gratings.
BACKGROUND
The increase in Internet traffic, the number of telephones, fax machines, computers with modems, and other telecommunications services and equipment over the past several years has caused researchers to explore new ways to increase fiber optic network capacity by carrying multiple data signals concurrently through telecommunications lines. To expand fiber network capacity, fairly complex optical components have already been developed for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM).
In a WDM system, multiple optical data signals of different wavelengths are added together in a device called a multiplexer and the resulting data signal is transmitted over a fiber optic cable. The wavelength division multiplexed signal comprises a plurality of optical signals having a predetermined nominal wavelength difference from each other. A demultiplexer separates the multiple optical data signals of different wavelength. Any WDM system must include at least one component to perform the function of optical multiplexing (namely, the multiplexer) and at least one component to perform the function of optical demultiplexing (namely, the demultiplexer). The optical multiplexer and the optical demultiplexer are each examples of optical wavelength routers.
In general, an optical wavelength router has at least one input optical port and at least on output optical port. In an optical router, light may be transmitted from a specific input port to a specific output port only if the light has an appropriate wavelength. Complex WDM systems may require optical wavelength router components that are more complex than a multiplexer or a demultiplexer. For example, an arrayed waveguide grating (AWG) or an integrated reflection grating may be used in a multiplexer, a demultiplexer, or a more general optical router.
Planar lightwave circuit technology is one technology that may be used to implement an optical wavelength router. A planar lightwave circuit (PLC) is an application of integrated optics. In a PLC, light is restricted to propagate in a region that is thin (typically between approximately 1 &mgr;m and 30 &mgr;m) in one dimension, referred to herein as the lateral dimension, and extended (typically between 1 mm and 100 mm) in the other two dimensions. A plane that is perpendicular to the lateral dimension of the PLC is defined as the plane of the PLC. The longitudinal direction is defined as the direction of propagation of light at any point on the PLC. The lateral direction is defined to be perpendicular to the plane of the PLC. The transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
In a typical example of a PLC, a slab waveguide comprises three layers of silica glass are used 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. In this example, each layer is 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. In this example, adjacent layers 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. Another 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.
A PLC optical router comprises an optical waveguide for each input optical port and an optical waveguide for each output port. Each input and output optical waveguide confines the light in both the lateral and the transverse direction. A PLC optical router also comprises at least one region comprising a slab waveguide, which confines the light in the lateral direction but not in the transverse direction. A PLC optical router further comprises at least one optical dispersive region, which may be either an arrayed waveguide grating (AWG) region or an integrated reflection grating.
FIG. 1
depicts an AWG optical router that acts as a demultiplexer
10
. A plurality of optical signals incident on one input optical port propagates through the device in the following sequence: the signals propagate through an input waveguide
12
, which is a input waveguide associated with the input port; through an input slab waveguide
14
, which has the function of expanding the optical field in the transverse direction by diffraction; through the dispersive region
16
(namely, the array waveguide region) comprising an array of AWG waveguides
18
for modifying the direction of propagation for each wavelength constituent according to the wavelength of the constituent of the plurality of signals; through an output slab waveguide
20
for focusing the signals of different wavelength coupled from the dispersive region
16
into a plurality of predetermined positions in accordance with the predetermined wavelength difference; through a plurality of output waveguides
22
each associated with one output port.
The dispersive property of the arrayed waveguide grating (AWG) region is attributable to the construction of the plurality of waveguides within the waveguide grating region such that adjacent waveguides have a predetermined length difference in accordance to the required dispersive properties of the dispersive region
16
, so that each signal at different wavelength coupled to and traveling over each channel waveguide is provided with a phase difference from each other in accordance with the predetermined length difference. Each of the output waveguides
22
includes an input end
24
, which is arranged at a predetermined position, so that each separated signal at each wavelength is coupled to each output waveguide
22
and emerges from an output end
26
thereof.
In operation, the wavelength division multiplexed signals coupled into the input channel waveguide
12
expand into the input slab waveguide
14
by diffraction. Then, the expanded signals are distributed to the channel waveguides
18
of the arrayed-waveguide grating
16
. Because each channel waveguide
18
of the arrayed-waveguide grating
16
has a predetermined waveguide length difference, each signal, after traveling over each channel waveguide
18
to the output slab waveguide
20
, has a predetermined phase difference according to its waveguide length difference. Since the phase difference depends on the wavelength of the signal, each signal at different wavelength is focused on a different position along the arc boundary
28
of the output slab waveguide
20
. As a result, separated signals, each having a different wavelength, are received by the plurality of output channel waveguides
22
and emerge therefrom, respectively.
The general principles and performance of an AWG multiplexer are similar to the AWG demultiplexer, except that the direction of propagation of light is reversed, the ports that act as inputs for the demultiplexer act as output ports for the multiplexer, and the
Lam Jane
McGreer Kenneth
Xu Hao
Zhao Liang
Barber Therese
Bruce David V.
Lightwave Microsystems Corporation
Morrison & Foerster / LLP
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