Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-04-03
2004-03-30
Snow, Walter E. (Department: 2862)
Optical waveguides
With optical coupler
Plural
C398S079000
Reexamination Certificate
active
06714702
ABSTRACT:
This application relates to an optical demultiplexing system and method such as may be employed in the system and method described in U.S. provisional application relating to a high performance optical multiplexer and demultiplexer being filed on the same day as the present application by David Boertjes and Kim Roberts, to be assigned to Nortel Networks, under the Nortel Networks reference of 13587RO.
FIELD OF THE INVENTION
The invention relates to an optical demultiplexing system and method, and particularly to a system and method for splitting an optical signal carrying a number of information channels at different frequencies.
BACKGROUND OF THE INVENTION
Optical communications systems increasingly use wavelength division multiplexing (WDM) in which a number of distinct optical signals are transmitted at different wavelengths, generally down an optical fiber. For example, optical communication in the so-called “C” band may allow the transmission of 40 channels, or frequencies, at regular intervals, each carring 10 Gb/s of data. One optical signal can be transmitted at each frequency down a single optical fiber. Other bands and/or other numbers of channels may be used, for example, 56 channels in the “L” band, each carrying 10 Gb/s.
A key component in WDM systems is a demultiplexer for splitting apart optical signals at a plurality of wavelengths into the individual channels at individual wavelengths. This may be done using a splitter and a number of different filters tuned to the individual frequencies, by components that demultiplex the light directly, or a combination of these approaches.
One approach to filtering and demultiplexing is to use an arrayed waveguide grating (AWG), also known as a phased-array device. The operation and design of AWGs is described, for example, in “PHASAR-Based WDM-Devices: Principles, Design and Applications”, Meint K. Smit, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 2, June 1996.
FIGS. 1
to
3
illustrate an AWG device.
FIG. 1
shows a top view of the AWG,
FIG. 2
a side section through a waveguide of the AWG and
FIG. 3
is a detailed top view of part of the AWG device. A plurality of optical waveguides
3
are devil on a substrate
1
in a known way. For example, to define the waveguides a buffer layer
5
may be deposited on the substrate, a core
7
deposited along part of the buffer layer to define the waveguide
3
and a cladding layer
9
provided to cover the core and buffer layers. The refractive indices of the buffer
5
, core
7
and cladding
9
layers are selected so that light is guided along the waveguide in the region of the core.
The arrayed waveguide device includes an array
11
of waveguides
3
arranged side by side on the substrate and extending between an input star coupler
13
and an output star coupler
15
. The input and output star couplers
13
,
15
are defined by a wide core region in which light can travel freely in the two-dimensional plane of the substrate. This region is known as the free propagation region. Input
17
and output
19
optical waveguides are provided to feed input light into the array
11
of waveguides and to output light from the array respectively. There may be a plurality of input waveguides
17
or output waveguides
19
.
As an example
FIG. 3
illustrates the output star coupler of a system with a singe input waveguide and a plurality of output waveguides. The ends
21
of the array of waveguides
11
are usually on a geometric circle
23
of radius r whose centre is at the centre
25
of an image plane
27
. The output waveguides
19
are arranged on the image plane, which also constitutes a circle. Note that the centres of the circles are not coincident, and may not have equal radii.
The length of the individual waveguides
3
of the array
11
differ (see
FIG. 1
) and the shapes of the star couplers
13
,
15
are chosen so that light input to the input optical waveguide
17
passes through the array
11
of waveguides and creates a diffraction pattern on the output waveguide or waveguides, such that light of a predetermined cent wavelength creates a central interference peak at the centre
25
of the image plane. Light with frequencies slightly higher or lower than the predetermined central frequency is imaged with a central interference peak slightly above or below the centre of the image plane.
In order to achieve this result the optical path length difference between adjacent waveguides of the array is chosen so that it is an integral multiple of the central wavelength. Accordingly, light at the central wavelength which enters the array of waveguides in phase will also leave in phase and thus will create the central diffraction spot at the centre of the image plane. Light with a slightly different frequency will arrive at the output star coupler with slight phase differences across the array, which will cause the light to be imaged to a spot on the image plane a little away from the central spot.
Accordingly, the plurality of output waveguides arranged on the output plane receive light of slightly different frequencies. Equally spaced output waveguides correspond to equally spaced frequencies, at least to a first order of approximation.
FIG. 3
shows the effect of one or more output waveguides connected to the output star coupler
15
. It is alternatively or additionally possible to arrange a plurality of input waveguides on the input star coupler with the some effect.
An AWG has a number of properties. One important property is that the distance of the image spot along the image plane as a function of wavelength is substantially linear in wavelength, for wavelengths around the central wavelength. Accordingly, it is possible to separate a number of signals with regular channel separations by positioning output waveguides at substantially regular intervals along the output plane.
A second important property is that the AWG has a repeat frequency. In other words, the interference properties as a function of frequency repeat with a period in the frequency domain. This period is known as the free spectral range (FSR). The free spectral range is a function of the difference in optical length between adjacent waveguides; a large optical length difference results in a small FSR and vice versa.
Although an AWG can carry out demultiplexing, it is not generally practical to demultiplex a large number of channels using a single AWG. For example, it can be impracticable to demultiplex each of the 40 channels in the C-band using a single AWG, for four reasons. Firstly, the C-band covers some 4 000 GHz so the AWG would need an FSR of at least this much. This would result in small path length differences between each waveguide of the array of the AWG and hence a physically large AWG device. Secondly, it would be necessary to provide 40 output waveguides, which would also lead to a large device. Thirdly, the accumulated cross-talk into one channel from the other thirty-nine channels may be excessive. Finally, in some applications it is desired to process a group of channels, e.g. for dispersion compensation, so a multi-stage process might be preferred.
However, all alternative system with separate AWGs in each frequency range would greatly increase the parts count of an optical system and would likewise be inconvenient and difficult to manufacture.
Accordingly, there is a need for an improved optical demultiplexer capable of accurately dividing an optical signal having a moderate or large number of optical channels into individual channels.
Furthermore, in some cases there is a need to apply some processing on optical signals in broad frequency bands as well as to divide the optical signal into marrow frequency bands or individual channels.
Further, the manufacturing costs of optical components can be considerable and it would be beneficial to reduce these costs.
SUMMARY OF INVENTION
In a first aspect of the invention there is provided an optical system, comprising; an optical splitter for splitting an input optical signal between optical outputs; and a plurality of
Collar Andrew J
Day Stephen
Whiteaway James E
Barnes & Thornburg
Nortel Networks Limited
Snow Walter E.
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