Filter topologies for optical add-drop multiplexers

Details Filter topologies for optical add-drop multiplexers Filter topologies for optical add-drop multiplexers

1999-09-08

2001-02-13

Spyrou, Cassandra (Department: 2874)

Optical waveguides

With optical coupler

Plural

C359S199200, C359S199200

Reexamination Certificate

active

06188816

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to filter designs and more specifically to both apparatus and network level filter topographies for Optical Add-Drop Multiplexers (OADMs).
BACKGROUND OF THE INVENTION
The amount of information communicated over an optical fiber communication system is increased with the use of optical wavelength division multiplexing. Wavelength Division Multiplexed (WDM) systems employ WDM signals consisting of a number of optical signals at different wavelengths, hereinafter referred to as channels or information carrier signals, to transmit information on optical fiber cables. Each channel is modulated by one or more information signal, resulting in the capability to transmit a significant number of information signals over a single optical fiber cable. It is recognized that although a WDM signal comprises a plurality of wavelengths capable of carrying channels, not all the wavelength must contain a channel.
To facilitate the subtraction and/or addition of particular channels to and/or from the WDM signal at different points within a network, OADMs are employed that consist of a plurality of optical filters. These OADMs are used to selectively extract Channels, hereinafter referred to as drop channels, from a WDM signal while the remaining channels, hereinafter referred to as through channels, travel through. The OADMs can also be used to add channels, hereinafter referred to as add channels, to a WDM signal, using wavelengths that have been vacated as a result of channels being dropped at the OADM in question or at an OADM earlier in the transmission path. Since more than one channel usually needs to be accessed at a network node, multi-channel OADMs are used such that a plurality of channels can be dropped and/or added from and/or to a received WDM signal.
There are a number of different implementations for a multi-channel OADM. One key factor that must be considered when considering different implementations is the cost of the filters that are utilized. Filters increase in cost as their Figure of Merit (FOM) increases, their FOM being a measure of the complexity of the filter. One skilled in the art would understand that the FOM increases as the tolerance of the filter that is used increases. One factor that can cause an increase in the required tolerance is an increase in the ratio of the passband to dead band, described herein below. Hence, when considering the cost of any particular OADM, one must consider the number of filters required and the overall tolerance of those filters.
With reference to
FIGS. 1 and 2
, well-known implementations for OADMs are now described. Firstly,
FIG. 1
depicts an OADM that comprises a separate channel demultiplexer
102
and channel multiplexer
104
. For this OADM implementation, the demultiplexer
102
extracts channels from a WDM signal while the multiplexer
104
inserts channels within the WDM signal output from the demultiplexer
102
.
The channel demultiplexer
102
, in this case with four channels to be dropped, comprises a first alignment block
106
, first and second columnating lenses (CL)
108
,
110
, first, second, third and fourth drop filters (DF)
112
a
,
112
b
,
112
c,
112
d
and first, second, third and fourth clean-up filters (CF)
114
a
,
114
b
,
114
c
,
114
d
. The alignment block
106
is utilized to ensure that beams of light being transmitted between the filters and columnating lenses are aligned properly for optimal performance. The first columnating lens
108
receives an input WDM signal S
IN
(t) in a form capable of being transmitted on a fiber optic cable, transforms the signal into an extended beam signal, and transmits the extended beam WDM signal in the direction of the first drop filter
112
a
. In the example being shown in
FIG. 1
, the drop filter
112
a
receives the extended beam WDM signal, filters out a channel at wavelength &lgr;1 with the use of a single wavelength filter, and forwards the remainder of the extended beam WDM signal onto the next drop filter
112
b
. The isolation of the channel at wavelength &lgr;1 is not perfect and so an additional filter may be required to ensure that only the required channel is sent on for further processing, in this case this is done with the first clean-up filter
114
a
. Additional pairings of drop and clean-up filters proceed within the channel demultiplexer
102
of the OADM, each operating similar to that of the first drop and clean-up filters
112
a
,
114
a
but for different wavelengths (&lgr;2,&lgr;3,&lgr;4). In the case depicted in
FIG. 1
, after four drop filters the resulting extended beam WDM signal is received by the second columnating lens
110
which converts the signal to a form transmittable over fiber optic cable. The signal output from the second columnating lens
110
, although not carrying the channels that were dropped within the demultiplexer
102
of the OADM, still may contain channels at other wavelengths.
The channel multiplexer
104
of the OADM of
FIG. 1
comprises a second alignment block
116
, third and fourth columnating lenses
118
,
120
and first, second, third and fourth add filters
122
a
,
122
b
,
122
c
,
122
d
. The second alignment block
116
operates in a similar manner to the first alignment block
106
, as do the third and fourth columnating lenses
118
,
120
with respect to the first and second columnating lenses
108
,
110
respectively. Each of the add filters
122
a
,
122
b
,
122
c
,
122
d
are single wavelength filters that insert channels at the wavelengths &lgr;1, &lgr;2, &lgr;3, and &lgr;4 respectively. An output WDM signal S
OUT
(t) that is similar to the input WDM signal S
IN
(t) but with different channels at wavelengths &lgr;1 through &lgr;4 is transmitted from the fourth columnating lens
120
.
With the separate channel demultiplexer and multiplexer
102
,
104
, a large number of filters are required. For each of the drop channels, two single wavelength filters are required, while for each of the add channels, one single wavelength filter is needed. For the case shown in
FIG. 1
with four channels being dropped and then subsequently added, twelve single wavelength filters are used. In general, it can be seen that for N channels being dropped and added, 3N single wavelength filters are required. In addition, if higher through isolation is required, an additional single wavelength filter per wavelength would be required. This would bring the total to 4N single wavelength filters required for this design.
There are other well-known implementations for the OADM depicted within
FIG. 1
that would result in a similar number of filters being required. One such implementation does not utilize the alignment blocks
106
,
116
, but rather has columnating lenses on either side of each filter element. Instead of the WDM signal being transported from filter to filter within extended beam format, fiber optic cable is used.
Another well-known implementation for an OADM is a band OADM as depicted within FIG.
2
. In this implementation, an input WDM signal S
IN
(t) is received at a first band filter
202
which extracts a band of wavelengths, in this case wavelengths &lgr;1 through &lgr;4, and passes a WDM signal comprising the remaining channels to an isolation filter
204
. The isolation filter
204
ensures that no channels are at wavelengths &lgr;1 through &lgr;4 without extracting or inserting any channels at other wavelengths. Subsequently, the isolation filter
204
outputs the resulting WDM signal to a second band filter
206
at which point a band of wavelengths, in this case &lgr;1 through &lgr;4, are inserted, generating an output WDM signal S
OUT
(t)
The channels that are extracted at the first band filter
202
are separated by a series of single wavelength drop filters
208
a
,
208
b
,
208
c
,
208
d
. These individual channels are then further filtered with respective clean-up filters
212
a
,
212
b
,
212
c
,
212
d
and output for further processing. As depicted in
FIG. 2
, the channels inserted at the second band filter
206
are combined prior to the i

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