Coherent crosstalk attenuation apparatus

A optical device is provided for reducing coherent crosstalk associated with a particular channel within a communications network. An attenuator is coupled between an emitter associated with a first node and a receiving element associated with a second node. The attenuator adjusts the signal power level generated by the emitter such that the signal power is within the dynamic range of the receiving element.

FIELD OF INVENTION 
The present invention relates generally to optical communication systems 
and more particularly to a system and apparatus for matching the output of 
optical transmitters with the dynamic range of optical receivers within a 
communications network, as well as substantially reducing crosstalk 
associated with particular optical channels. 
BACKGROUND OF INVENTION 
Wavelength Division Multiplexing (WDM) is a technique used to transmit a 
plurality of optical channels via an optical waveguide medium where each 
channel carries information signals within a network or system. Each 
channel within the WDM signal is associated with a particular wavelength, 
thereby increasing the information capacity of fiber optic systems. WDM 
optical networks have traditionally been used for long haul point-to-point 
networks. However, with the increasing demands on communication systems, 
WDM optical networks can also be used in smaller system configurations, 
such as local telephone or data networks. In these systems, communication 
signals are usually transmitted over a limited geographic area to various 
nodes within a network, thereby avoiding the need for amplifiers. A 
particular node can be configured to drop one or more information bearing 
or payload channels from the WDM signal, process the information contained 
in the dropped channels and add channels containing new information to the 
WDM signal for transmission to other nodes in the network. An optical 
add/drop multiplexer present at each node may be used to drop the 
particular channel from the WDM signal and subsequently add the channel 
back to the WDM signal prior to transmission to another network node while 
allowing the remaining channels to passthrough. 
Nodes within these types of networks can be separated by optical paths of 
differing lengths. In order for nodes to successfully receive WDM signals 
within the network, the power associated with the WDM signal transmitted 
between nodes varies depending upon the length of the optical path between 
the origination and destination nodes. For example, when a channel is 
added back to the WDM signal at a particular node within a network, the 
associated transmitter provides sufficient power to allow the signal to 
travel to the destination node and its associated receiver. However, if 
the destination node of a particular channel is in close proximity to the 
transmitting or source node, the transmitting node may provide too much 
power to the signal. This causes a problem because optical receivers have 
a corresponding "dynamic range" whereby an increase in the input optical 
power and the associated output electrical current have a linear or 
substantially linear relationship. If input power levels increase 
excessively, the receiver output current can saturate and no longer 
increase with corresponding increases in input optical power. As a result, 
the optical signal input to the receiver cannot be accurately detected. 
Moreover, when a channel is dropped by an optical add/drop multiplexer, a 
portion of that channel signal may leak into the pass-through channels. 
When the dropped channel is subsequently added back to the pass-through 
channels, coherent crosstalk may occur between the added channel signal 
and the leaked portion of that channel. This crosstalk may be sufficient 
to cause signal recognition problems. 
Thus, there is a need to control/fix the output power of node transmitters 
to correspond to the dynamic range of node receivers within a network. In 
addition, there is a need to reduce coherent crosstalk associated with 
particular channel transmissions. 
SUMMARY OF INVENTION 
The present invention meets the above-referenced needs by providing an 
optical device including a first transfer element coupled along a closed 
optical path which carries a plurality of information bearing channels, 
each at a respective wavelength. The first transfer element is configured 
to add a first of the plurality of optical channels where the first 
optical channel has an associated signal power. A second transfer element 
is coupled to the first transfer element along the optical path. The 
second transfer element is configured to select the first optical channel 
from the plurality of optical channels. An attenuator is coupled to the 
first transfer element and is configured to adjust the power associated 
with the first optical channel received at the second transfer element.

DETAILED DESCRIPTION 
Turning to the drawings in which like reference characters indicate the 
same or similar elements, FIG. 1 schematically illustrates a portion of an 
optical network 10 having optical nodes 20.sub.1, 20.sub.2 . . . 20.sub.N 
and corresponding optical paths 40.sub.1, 40.sub.2, 40.sub.2 . . . 
40.sub.N. Each node may, for example, represent separate geographical 
locations within network 10 linked by a segment of optical fiber. 
Moreover, network 10 may be configured as a ring, star, or other network 
architecture. 
Each node 20.sub.1 . . . 20.sub.N is configured to receive a WDM signal 
having a plurality of wavelengths .lambda..sub.1 . . . .lambda..sub.j 
carried via optical paths 40.sub.1 . . . 40.sub.N. The wavelengths 
included in the WDM signal are typically, but not necessarily, within the 
1500 nm range which corresponds to the minimum signal attenuation 
associated with silica-based fibers. Each optical channel carries 
information at an associated data rate within network 10. Each node 
20.sub.1 . . . 20.sub.N is configured to drop and add one or more payload 
channels at a respective one of the wavelengths .lambda..sub.1 . . . 
.lambda..sub.j through the use of transfer elements, for example, optical 
add/drop multiplexers (OADMs). An example of a type of add/drop 
multiplexer which may be employed in accordance with the present invention 
is described in copending U.S. patent application entitled "Optical 
Add/Drop Multiplexer" filed on Oct. 23, 1997 having unofficial Ser. No. 
08/956,807 and assigned to the assignee of the present invention 
(hereinafter referred to as "the Optical Add/Drop Multiplexer 
Application") which is incorporated herein by reference. Each node 
includes at least one OADM configured to drop one or more payload channels 
having wavelengths .lambda..sub.1 . . . .lambda..sub.j. Each OADM may also 
be configured to drop the same or different channels from the WDM signal 
depending on the desired network configuration. Each node 20.sub.1, 
20.sub.2 . . . 20.sub.N includes at least one transceiver 60.sub.1, 
60.sub.2 . . . 60.sub.N associated with OADMs 50.sub.1 . . . 50.sub.N, 
respectively. 
Although the following description refers to node 20.sub.1, it is 
understood that this description can be applied to nodes 20.sub.1, 
20.sub.2 . . . 20.sub.N within network 10 with a difference being that 
each node may drop/add the same or different WDM channels. In addition, 
OADM 50.sub.1 is being described as an example of an OADM capable of 
dropping and adding at least one payload channel consistent with the OADM 
disclosed in the Optical Add/Drop Multiplexer Application. However, it 
should be understood that other OADM configurations may be employed which 
are capable of performing the same or similar function, for example, the 
combination of optical circulators and one or more fiber gratings as 
described in co-pending U.S. Patent Application Ser. No. 08/846,086 
entitled "Optical Add-Drop Multiplexers Compatible With Very Dense WDM 
Optical Communication Systems" filed on Apr. 25, 1997. 
OADM 50.sub.1 includes a filtering element 55 and a combining element 60. 
Filtering element 55, for example an interference filter, is configured to 
drop at least a payload channel having a particular wavelength, for 
example .lambda..sub.1. The remaining WDM channels at wavelengths 
.lambda..sub.2 . . . .lambda..sub.j pass through OADM 50 to combining 
element 60 via line 56. The payload channel having wavelength 
.lambda..sub.1 is supplied to terminal equipment where the information 
carried on the payload channel is processed. The payload channel is added 
back to the pass-through WDM channels by way of combining element 60, for 
example an interference filter, and supplied to either the next OADM 
within node 20.sub.1 or to the next node, e.g. node 20.sub.2, by way of 
optical path 40.sub.2. 
Transceiver 60.sub.1 associated with OADM 50.sub.1 includes a receiving 
element 65, such as a photodiode, which receives the payload channel 
having wavelength .lambda..sub.1 via line 70 and generates electrical 
signals in response thereto. These signals are used to modulate, either 
directly or externally, a second light source 75 included in transceiver 
60.sub.1 which is coupled to line 78. In this manner, light source 75 is 
used to transmit the information received via the payload channel having 
wavelength .lambda..sub.1 in the 1.5 .mu.m range to a channel having the 
same or a different wavelength, e.g. 1.3 .mu.m so that it can be 
recognized by customer receiving equipment (e.g. SONET equipment) coupled 
to transceiver 60.sub.1 via line 78. 
Transceiver 60.sub.1 includes a photodiode 80 which receives information 
signals from customer transmission equipment at a particular wavelength 
via line 79 and generates electrical signals in response thereto. These 
signals are used to modulate, either directly or externally, light source 
85 which transmits the optical channel having wavelength .lambda..sub.1 to 
OADM 50.sub.1 by way of combining line 90. An optical attenuator 95 is 
disposed between light source 85 and OADM 50.sub.1 along line 90. 
Attenuator 95 reduces the intensity of the light signal produced at source 
85. The degree of attenuation can depend on the configuration of network 
10 and the power level balancing needed for signal transmission between 
nodes 20.sub.1 . . . 20.sub.N. Attenuator 95 may be a variable or fixed 
attenuator. In the case of a variable attenuator, the amount of 
attenuation may be controlled remotely, for example, through the use of a 
service channel carried within network 10. Alternatively, the service 
channel may also be used to control the power associated with light source 
85. Attenuator 95 may be included in a separate module along line 90 or 
may be included within transceiver 60.sub.1. After attenuation, the 
optical channel having wavelength .lambda..sub.2, is added to the WDM 
pass-through channels by way of combining element 60 and supplied to OADM 
50.sub.2 via line 40.sub.2. 
A small portion of the channel having wavelength .lambda..sub.1 dropped by 
OADM 50.sub.1 may be passed-through with the remaining WDM channels 
supplied to combining element 60 by way of line 56. This signal can cause 
crosstalk problems when the dropped channel is added back to the WDM 
signal at combining element 160. However, the attenuation of the dropped 
channel having wavelength .lambda..sub.1 at the transmitting node within 
network 10 is such that the strength of the signal leaked into the 
pass-through channels of OADM 50.sub.1 is sufficiently low to not 
substantially interfere with the added channel. The placement of 
attenuator 95 in close proximity to light source 85 reduces the power 
associated with the added signal so that cross talk levels associated with 
the added channel are alleviated at destination nodes within network 10. 
OADM 50.sub.2 receives the WDM signal from OADM 50.sub.1 and drops a 
payload channel having a particular wavelength, for example 
.lambda..sub.2, by way of filtering element 155. The remaining WDM 
channels .lambda..sub.1, .lambda..sub.3 . . . .lambda..sub.j pass through 
OADM 50.sub.2 to combining element 160 via line 156. Transceiver 60.sub.2 
is coupled to OADM 50.sub.2 by way of line 170 and includes a receiving 
element 165, such as a photodiode, which receives the payload channel 
having wavelength .lambda..sub.2 and generates electrical signals in 
response thereto. 
Receiving element 165 has a corresponding dynamic range where the input 
power has a linear or substantially linear relationship with the output 
electrical current. If the optical signal generated at source 85 has as 
its intended destination node 20.sub.2, for example, and node 20.sub.2 is 
in close proximity to node 20.sub.1, the power associated with this 
optical signal must be within the dynamic range of receiving element 165 
in order to be accurately detected. Accordingly, attenuator 95 adjusts the 
power associated with the optical signal generated at node 20.sub.1 by 
source 85 such that when the signal is received by node 20.sub.2, the 
associated signal power is within the dynamic range of receiving element 
165. Attenuator 95 may also be configured so that the total link loss 
approaches the maximum loss budget of network 10. 
Alternatively, the power associated with light source 85 may be altered by 
reducing the drive current associated with light source 85 if directly 
modulated or by adjusting the modulating element (not shown) drive if 
externally modulated. In either case, the signal power associated with 
source 85 is within the dynamic range of receiving element 165. 
The electrical signals generated by receiving element 165 modulate light 
source 175. These optical signals are used to transmit the information 
received via the payload channel having wavelength .lambda..sub.2 in the 
1550 nm range to a channel having the same or a different wavelength, e.g. 
1.3 .mu.m so that it can be recognized by customer receiving equipment 
coupled to line 178. Similar to transceiver 60.sub.1, transceiver 60.sub.2 
includes a photodiode 180 which receives information signals via line 179 
and generates electrical signals in response thereto. These signals are 
used to modulate light source 185 which transmits the optical channel 
having wavelength .lambda..sub.2 to OADM 50.sub.2 by way of line 190. An 
optical attenuator 195 is disposed between light source 185 and OADM 
50.sub.2 along line 190 for controlling the power of the light signal 
produced via source 185 to correspond to the dynamic range of the 
receiving element, for example receiving elements 265 and/or 65. The 
optical channel having wavelength .lambda..sub.2, is added to the WDM 
pass-through channels by way of combining element 160 and supplied to OADM 
50.sub.2 via line 40.sub.2. 
As described above, coherent crosstalk problems may arise when two nodes 
are in close proximity. For example, if OADM 50.sub.1 is in relatively 
close proximity to OADM 50.sub.2, and the channel having wavelength 
.lambda..sub.1 is added at node 20.sub.1 and the same channel is dropped 
and added at node 20.sub.2, coherent crosstalk problems will result when a 
portion of the dropped channel leaks through OADM 50.sub.2 and the same 
channel is added back via combining element 160. Typical coherent 
crosstalk levels, for example, using interference filters as the filtering 
and combining elements in each of the OADMs, is in the range of 
approximately -25 dB. These values may vary depending upon the OADM 
configurations and components employed. However, a desirable crosstalk 
level, for example, for the signal received at line 170 of node 20.sub.2 
can be in the range of approximately -40 dB. Therefore, the added channel 
having wavelength .lambda..sub.1 supplied by source 85 of transceiver 
60.sub.1 must be attenuated by 15 dB. Accordingly, attenuator 95 can be a 
15 dB attenuator thereby providing -40 dB of crosstalk for the channel 
having wavelength .lambda..sub.1 supplied from OADM 50.sub.1 to OADM 
50.sub.2. 
OADM 50.sub.N receives the WDM signal from OADM 50.sub.2 and drops a 
payload channel having a particular wavelength, for example 
.lambda..sub.j, by way of filtering element 255. The remaining WDM 
channels .lambda..sub.1 . . . .lambda..sub.j-1 pass through OADM 50.sub.N 
to combining element 260 via line 256. Transceiver 60.sub.N is coupled to 
OADM 50.sub.2 by way of line 270 and includes a receiving element 265, 
such as a photodiode, which receives the payload channel having wavelength 
.lambda..sub.j and generates electrical signals in response thereto. The 
attenuator associated with the transmitting node, for example attenuators 
95, 195, etc., adjust the power of the signal to be received by node 20N 
such that the signal power corresponds to the dynamic range associated 
with receiving element 265. The electrical signals generated by receiving 
element 265 are used to modulate light source 275 which supplies 
information to customer equipment via line 278. 
Transceiver 60.sub.N also includes a photodiode 280 which receives customer 
information signals via line 279. These signals are used to modulate light 
source 285 which transmits the optical channel having wavelength 
.lambda..sub.j to OADM 50.sub.N by way of line 290. Attenuator 295 is 
disposed between light source 285 and OADM 502 along line 290 for 
controlling the power of the light signal produced via source 285 as 
described above. The optical channel having wavelength .lambda..sub.j is 
added to the WDM pass-through channels by way of combining element 260 and 
supplied to OADM 50.sub.N via line 40.sub.N. 
While the foregoing invention has been described in terms of the 
embodiments discussed above, numerous variations are possible. 
Accordingly, modifications and changes such as those suggested above, but 
not limited thereto, are considered to be within the scope of the present 
invention.