Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-09-08
2003-03-04
Tweel, John A. (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C385S017000, C385S024000
Reexamination Certificate
active
06529302
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of communications, in particular to wavelength encoded optical communication systems, and more particularly to optical switching means i.e. means for adding, dropping and multiplexing or switching wavelength encoded optical channels.
Optical communications systems are a substantial and fast-growing constituent of communication networks. The expression optical communication system, as used herein, relates to any system that uses optical signals to convey information. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, Prentice Hall, N.Y. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from a plurality of sources, time-division multiplexing (TDM) is frequently employed. In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal from one of the signal sources being reconstructed from the portions of the signals collected from the relevant time slots. While this is a useful technique for carrying information from a plurality of sources on a single channel, its capacity is limited by fibre dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g. by laying more fibre optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) is now used for increasing the capacity of existing fibre optic networks. In a WDM system a plurality of optical signal channels are carried over a single waveguide, each channel being wavelength encoded i.e. assigned a distinct part of the spectrum. Ideally each channel will be allocated to a wavelength band centered upon a single wavelength. In practice, due to the shortcomings of available sources and spectral broadening due to the modulation on the carrier and due to the dispersion and propagation of transmission media, each signal channel will spread across the spectrum to a greater or lesser extent. References herein to a wavelength are to be interpreted accordingly.
Optical fibre networks have been explored to permit the transfer of optical signals carrying WDM channels (WDM signals) bearing analogue or digital data, from one optical fibre in one loop, ring, cell of a mesh or line of a network to a different loop, ring, cell of a mesh or line of the network directly, in optical form, without the need to convert the signals into electrical form at interconnection points of the network. These interconnection points (or nodes) comprise optical add-drop multiplexers OADMs or optical cross connects OXCs.
Several methods to achieve optical add drop multiplexing (or switching) and optical cross connect switching are described in the proceedings of the European Conference Optical Communications, September, 1998, Madrid, Spain and the Optical Fibre Conference, February 1998, USA.
FIG. 1
shows a known optical add-drop arrangement. The arrangement has four external ports identified by numbers inside rectangles. External port
1
is the Input port for an input signal comprising a set S
IN
of input channels. This set S
IN
may be made up from a set S
T
of through channels and a set S
D
of drop channels. Port
2
is the Drop port where a drop signal comprising the set S
D
of drop channels emerges. External port
3
is the Thru′ port for the output of a through signal comprising the set S
T
of through channels and any add channels S
A
and port
4
is the Add port for the introduction of an add signal comprising add channels S
A
. The above channels are wavelength encoded, (e.g. WDM) optical channels.
According to the known arrangement of
FIG. 1
, switching of one or more selected channels from the set S
IN
of input channels is achieved by passing the set S
IN
=S
T
+S
D
into a first port Port
1
of a first optical circulator
16
. These channels will exit first optical circulator
16
at a second port
17
. A series of tunable optical filters represented by rectangles are positioned in optical guide
15
leading from second port
17
of first circulator
16
to a first port
19
on second circulator
18
such that selected ones S
T
of the set S
IN
of input channels are passed by the series of filters to second circulator
18
, while the other ones S
D
of the set S
IN
of input channels are reflected back into second port
17
of first circulator
16
to emerge at a third port thereof, i.e., the Drop port
2
, to form the drop signal.
The through channels S
T
enter second circulator
18
at first port
19
thereof and emerge at Thru′ port
3
forming the through signal. If it is desired to add channels S
A
to the through signal at Thru′ port
3
i.e. to replace those channels S
D
dropped as a result of being reflected by one of the filters, these channels S
A
may be inserted at a third port, i.e. Add Port
4
of second circulator
18
such that they emerge at first port
19
and encounter the series of tunable optical filters positioned between the first and second optical circulators. If the add channels S
A
are assigned to some of the same wavelengths as the drop channels S
D
, they will be reflected back into second circulator
18
at first port
19
thereof and will emerge at Thru′ port
3
together with the through channels S
T
.
In the arrangement of
FIG. 1
a filter is required in waveguide
15
leading from the first circulator to the second circulator for each input channel. Switching is achieved by arranging that the filters may be de-tuned or adjusted by an amount comparable to the spectral width of a channel. Hence, if a filter is arranged to normally reflect a particular channel, de-tuning will cause it to pass that channel. Alternatively, the filter may be arranged to normally pass and on de-tuning to reflect a particular channel. This process allows any sub-set of channels to be selected from the total input set S
IN
.
The performance of the system of FIG.
1
and similar systems will depend on the performance of the filters. Important system performance parameters are insertion loss between ports (i.e. power loss between Ports
1
and
2
,
1
and
3
,
4
and
3
) and the signal rejection (i.e. power coupled into the wrong port such as from Port
1
to Port
3
for channels that have been dropped) and from Port
4
to Port
2
for added channels. The ideal filter response would be zero outside of the desired reflection band and 100% within the band so that when tuned or de-tuned for reflection no power is transmitted through the filter and all is reflected. The rejection and insertion losses would then be limited by imperfections in the optical circulators.
In practice compromises have to be made in the design of a filter. A highly reflective design has high loss outside of the reflection band which increases the insertion loss for other channels passing through it. The more filters used, the greater the insertion loss. A lower reflectivity filter produces less insertion loss for the other channels passing through it but passes more of the optical energy of the channel to be reflected so that the rejection performance becomes poorer. This can cause problems, for example if a new channel is added to the signal passed by the filters. If, as is commonly the case, it is desired to reuse the spectral band occupied by a channel that has been dropped in an OADM (i.e. by reflection by a filter) for adding a new channel, then corruption of the new channel can occur due to optical energy from the dropped channel passed by the lower reflectivity filter.
Thus the design of
FIG. 1
will work acceptably
Kirschstein et al.
Marconi Communications Limited
Tweel John A.
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