Dynamic optical add-drop multiplexers and wavelength-routing networks with improved survivability and minimized spectral filtering

A method and apparatus for constructing an optical wavelength-routing network in which each network node is a dynamic add-drop multiplexer (OADM) with minimized spectral filtering effect on pass-through channels and survivability upon power failure. By using cascaded tunable reflection filters as the building blocks, strictly add-drop non-blocking OADMs for single input/output fibers, double input/output fibers, and 3 input/output fibers can be constructed for application to unidirectional and bidirectional ring networks and mesh networks of arbitrary degree. Methods and apparatus for minimizing various types of out-of-band and in-band crosstalk occurring within the dynamic OADMs are also described.

FIELD OF THE INVENTION 
The present invention relates generally to a fiber-optic communications 
system, and particularly to a network which uses wavelength-division 
multiplexing. 
BACKGROUND 
In wavelength-division multiplexed (WDM) fiber-optic systems, multiple 
wavelengths are carried on a common fiber such that information on the 
wavelengths can maintain their separate integrity and be separated only 
before conversion from optical to electronic format. Currently, backbone 
point-to-point links have been implemented using multiple wavelengths to 
increase the fiber information-carrying capacity. For a WDM 
wavelength-routing network, which has a topology beyond a simple 
point-to-point link, it is desirable that per-wavelength routing be done 
at the nodes of the network (P. E. Green, "Optical Network Update," IEEE 
Journal on Selected Areas in Communications, vol. 14, pp. 764-779, June 
1996). This has the benefits of reducing the equipment required to detect, 
process and re-transmit information which was intended to be passed on to 
the next node. To minimize handling, it is desirable to keep the 
information on its original wavelength or optical carrier, and to route it 
using optical components which distinguish based on wavelength. It is 
further desirable that the routing be dynamically reconfigurable such that 
any one wavelength could be re-routed without interruption to any other 
wavelength which may be already carrying information. 
The simplest form of purely optical wavelength-routing node is an optical 
add-drop multiplexer (OADM) with single input and output fibers, in which 
incoming data may either be passed through the node or dropped to a local 
receiver. In purely optical wavelength routing, all the data on a 
particular wavelength per fiber is considered an inseparable data stream. 
If data from a particular wavelength is dropped, this wavelength is now 
available on the outbound direction, and hence new data can be added from 
a local transmitter. More complex nodes may include multiple input and 
output fibers that data streams need to be routed between. 
There have been various proposed implementations for a wavelength-routing 
node, particularly an OADM. Implementations related to the present 
invention are described in U.S. Pat. No. 5,448,660, issued Sep. 5, 1995, 
entitled "Wavelength Selective Optical Switch", and U.S. Pat. No. 
5,479,082, issued Dec. 12, 1995, entitled "Device for Extraction and 
Re-insertion of an Optical Carrier in Optical Communications Networks" 
both by Calvani et al. Calvani discloses a tunable optical bandpass filter 
and 1.times.1 add-drop multiplexing network. Here, there are at least two 
circulators per wavelength. Thus, amplifiers may be needed to compensate 
for losses as the number of wavelengths increases. 
The principle of using reflection filters in combination with a circulator 
to separate WDM signals is described by L. Quetel, et al. in "Programmable 
Fiber Grating Based Wavelength Demultiplexer," presented at OFC, Feb. 28, 
1996, paper WF6. 
Many OADM implementations operate by demultiplexing all the wavelengths and 
then performing space-switching on a per-wavelength basis. Then, the 
wavelengths to be passed (rather than dropped) are re-multiplexed onto the 
output fiber. There are various well-known wavelength-selective devices 
which can implement a de/multiplexer. The de/multiplexer can be 
implemented by low-loss wavelength-selective devices such as gratings. The 
multiplexer also can be implemented by simple broad-band wavelength 
combiners such as a star coupler, at the expense of higher loss. For 
example, the architecture reported by P. A. Perrier, et al. ("4-Channel 
10-Gbit/s Capacity Self-healing WDM Ring Network with Wavelength Add/drop 
Multiplexers," presented at OFC, Feb. 29, 1996, paper ThD-3) uses a star 
coupler and tunable transmission filters to separate the wavelengths prior 
to the space switch, and then another star coupler to re-multiplex. Other 
architectures use integrated waveguide grating devices to demultiplex and 
multiplex with minimal loss. See, for example, H. Toba, et al., "An 
Optical FDM-based Self-healing Ring Network Employing Array Waveguide 
Grating Filters and EDFA's with Level Equalizer," IEEE Journal of Selected 
Areas in Communications, vol. 14, pp. 800-813, June 1996. See also U.S. 
Pat. No. 548,850, issued Jan. 30, 1996, entitled "Tunable Add Drop Optical 
Filtering Method and Apparatus", by B. Glance. 
There is particular interest in implementing dual-fiber ring topologies 
with wavelength routing network nodes to provide self-healing capability 
as described by both Toba and Perrier. In this case, it is essential that 
the signal can traverse the entire ring at least once and possibly wrap 
around almost two times. A SONET supports up to 16 nodes in a ring. 
Clearly, extensions to as many nodes as possible is highly desirable. 
One key limitation in optical wavelength routing as described above is the 
number of nodes that can be traversed without unacceptable corruption of 
the information. Consider a given wavelength as it passes through 
successive nodes. By necessity, the demultiplexing component has a limited 
optical bandwidth. If used, a wavelength-selective multiplexer also has a 
limited bandwidth. Passing through multiple components successively 
narrows the effective optical passband experienced by that wavelength. 
Eventually, the passband may become so narrow that distortion to the 
information results. In the best case, the distortion occurs when the 
passband is comparable to the information bandwidth, but in practice it 
may happen much sooner. Inaccuracies in the center frequency or width of 
each component passband, carrier wavelength fluctuation and tolerance are 
factors. See, for example, N. N. Khrais, et al., "Effect of Cascaded 
Misaligned Optical (De)multiplexers on Multiwavelength Optical Network 
Performance," presented at OFC, Feb. 29, 1996, paper ThD-4. It is thus 
highly desirable to minimize spectral filtering effects in a 
wavelength-routing OADM network node. The present invention addresses such 
a need. 
Another potential problem of the OADM implemented by conventional methods 
is the unpredictable results on wavelength routing upon node power 
failure. In some cases, the input signals can be totally lost due to, for 
example, the walk-away of the tunable filter in the configuration 
described by Perrier. The present invention addresses such a need. 
SUMMARY 
In accordance with the aforementioned needs, the present invention is 
directed to an optical add-drop multiplexer and network which can 
dynamically route on a per-wavelength basis with minimized spectral 
filtering of the pass-through wavelengths. This feature allows a 
wavelength to pass-through a large number of routing nodes without 
distortion to the information. Additionally, the dynamic OADMs constructed 
according to this invention have the advantage of network survivability 
upon node power failure. 
Dropped wavelengths are assumed to be demultiplexed so they can each be 
separately detected with minimal crosstalk or interference from other 
information streams. Yet another feature of the present invention provides 
a state of operation for which the only narrowband wavelength selectivity 
experienced by the signal is at the final demultiplexing stage just prior 
to detection at the final node. 
Still other features of the present invention advantageously reduce losses 
for pass-through signals. Yet other features of the present invention 
provide reduced add-to-add, drop-to-drop, add-to-drop, and drop-to-add 
crosstalk. 
Accordingly, an embodiment of a 1.times.1 dynamic optical add/drop 
multiplexer (OADM) having an OADM input fiber and an OADM output fiber in 
accordance with the present invention includes: a demultiplexer having an 
output connected to a receiver; a multiplexer having an input connected to 
a transmitter; an output 3-port coupling mechanism; and a 1.times.2 
tunable spectral switch (TSS). The TSS includes: an input 3-port coupling 
mechanism; a plurality of serially connected tunable reflection filters 
(TRFs) having a TRF input coupled to a second port of the input coupling 
mechanism; a first TSS output coupling a TRF output to a first input of 
the output coupling mechanism; the TSS having a TSS input coupling a first 
port of the input coupling mechanism to the OADM input fiber; a second 
input of the output coupling mechanism coupled to a multiplexer output; an 
output of the output coupling mechanism connected to the OADM output 
fiber; and, a second TSS output coupling a third port of the input 
coupling mechanism to a demultiplexer input. 
The output coupling mechanism preferably is a 2.times.1 coupler for 
reducing spectral filtering on an add signal. Under normal operating 
conditions, a pass-through signal is not reflected by any tunable 
reflection filter. If power is interrupted to the OADM, all incoming 
signals are passed-through the tunable reflection filters and routed to 
the output fiber. 
A 2.times.2 dynamic optical add/drop multiplexer (OADM) having two OADM 
input fibers and two OADM output fibers, in accordance with the present 
invention includes: two 1.times.2 tunable spectral switches (TSS); two 
2.times.1 TSSs; two drop fibers; and two add fibers. A first 1.times.2 TSS 
has an input connected to a first OADM input, and a first output of the 
first 1.times.2 TSS is connected to a first drop fiber. A second 1.times.2 
TSS has an input connected to a second OADM input, and a first output of 
the second 1.times.2 TSS connected to a second drop fiber. A first 
2.times.1 TSS has a first input connected to a second output of the first 
1.times.2 TSS. A second input of the first 2.times.1 TSS is connected to a 
first add fiber; and an output of the first 2.times.1 TSS is connected to 
a second OADM output. A second 2.times.1 TSS has a first input connected 
to a second output of the second 1.times.2 TSS. A second input of the 
second 2.times.1 TSS is connected to a second add fiber. An output of the 
second 2.times.1 TSS connected to a first OADM output. The 2.times.2 OADM 
also includes at least three 2.times.2 tunable spectral switches (TSS) 
which may be interposed according to one of the groups (a, or b, or c) 
consisting of: 
(a) a first 2.times.2 TSS interposed between one of: (i) the two 1.times.2 
TSSs and the OADM input fibers, or (ii) the first outputs of the 1.times.2 
TSSs and the drop fibers; a second 2.times.2 TSS interposed between 
either: (i) the two 2.times.1 TSSs and the OADM output fibers, or (ii) the 
second inputs of the 2.times.1 TSSs and the add fibers; a third 2.times.2 
TSS interposed between the 1.times.2 TSSs and the 2.times.1 TSSs; or 
(b) a first 2.times.2 TSS interposed between the two 1.times.2 TSSs and the 
OADM input fibers; a second 2.times.2 TSS interposed between the first 
outputs of the 1.times.2 TSSs and the drop fibers; a third 2.times.2 TSS 
interposed between either: (i) the two 2.times.1 TSSs and the OADM output 
fibers, or (ii) the second inputs of the 2.times.1 TSSs and the add 
fibers; or 
(c) a first 2.times.2 TSS interposed between either: (i) the two 1.times.2 
TSSs and the OADM input fibers, or (ii) the first outputs of the 1.times.2 
TSSs and the drop fibers; a second 2.times.2 TSS interposed between the 
two 2.times.1 TSSs and the OADM output fibers; and a third 2.times.2 TSS 
interposed between the second inputs of the 2.times.1 TSSs and the add 
fibers. 
Preferably, each 2.times.1 TSS includes: an isolator; a plurality of 
serially connected tunable reflection filters (TRFs); and a 3-port 
circulator; wherein a first input of the 2.times.1 TSS is connected to an 
input of the isolator; a second input of the 2.times.1 TSS is connected to 
a first port of the circulator; wherein the TRFs have a first end 
connected to an output of the isolator, and a second end of the TRFs are 
connected to a second port of the circulator. A third port of the 
circulator is connected to the output port of the 2.times.1 TSS. 
The 2.times.2 TSS preferably includes a plurality of tunable reflection 
filters; and two circulators; the first input of the 2.times.2 TSS 
connected to the first port of the input circulator; the second input of 
the 2.times.2 TSS connected to the first port of the output circulator; 
and a plurality of serially connected tunable reflection filters having a 
first end connected to the second port of the input circulator and a 
second end connected to the second port of the output circulator, and the 
third port of the input circulator connected to the first output port of 
the 2.times.2 TSS, and the third port of the output circulator connected 
to the second output port of the 2.times.2 TSS.

DETAILED DESCRIPTION 
FIG. 1 illustrates a block diagram of a multi-wavelength dynamic optical 
add-drop multiplexer (OADM) 101 having features of the present invention. 
As depicted, the OADM 101 includes up to F inputs (I.sub.1 to I.sub.F) and 
F outputs (O.sub.1 to O.sub.F) Each input I.sub.m carries up to N 
wavelength multiplexed channels (I.sub.m1, I.sub.m2, . . . , I.sub.mN) 
with I.sub.mk at the wavelength .lambda..sub.k. The OADM 101 includes a 
routing module 102 in accordance with the present invention. For maximum 
add-drop capability, the OADM 101 may include up to F transmitter arrays 
103-105 and F receiver arrays 106-108. Each array contains N transmitters 
or receivers. The OADM 101, referred to as an F.times.F N-channel dynamic 
OADM, is capable of either dynamically dropping any input channel I.sub.mk 
(1 m F, 1 k N) to one of the receivers or passing it to one of the 
outputs. To avoid collisions, any two input channels at the same 
wavelength should not be passed to the same output. Simultaneously, when 
any output O.sub.m has an empty wavelength slot due to channel dropping, a 
new channel at that wavelength could be added from the transmitter and 
directed to O.sub.m. The OADM is dynamic if the add-drop condition for any 
channel is reconfigurable at a later time. 
In order to achieve per-wavelength add-drop control, the routing module 102 
must include wavelength-selective filter devices. As will be described 
with reference to FIG. 4, the routing module 102 according to the present 
invention has features which minimize spectral filtering effects from each 
node and thus enhances network transparency and enables signal channels to 
be routed through as many nodes as possible. 
FIG. 2 illustrates a system diagram of a conventional method for 
implementing the routing module 102 (FIG. 1) for an F.times.F N-channel 
dynamic OADM by using F demultiplexers (DEMUX) 201-203, N 2F.times.2F 
optical space switches (OSS) 211-213, and F multiplexers (MUX) 204-206 as 
the routing module 102. Channels from input I.sub.m are separated by 
DEMUX-m with channel I.sub.mk sent to OSS-k. Similarly channel A.sub.mk, 
defined as the add channel at the wavelength .lambda..sub.k from TX-m, is 
also sent to OSS-k. Depending on the state of OSS-k, channel I.sub.mk can 
either go to one of the drop ports (D.sub.1k, D.sub.2k, . . . , D.sub.Fk) 
or exit the OADM by first going through one of the multiplexers 204-206. 
The add-drop at each channel wavelength is controlled by the space 
switches 211-213 in the wavelength-separated space between the DEMUX and 
MUX. Consequently any input channel that is routed through the OADM has to 
pass the narrowband filters inherent in the DEMUX and the MUX. Although 
the spectral filtering effects of the MUX could be eliminated by using a 
broad-band coupler (at the expense of increased loss), the filtering 
effect of the DEMUX cannot be eliminated, regardless of the type of 
wavelength-selective device utilized. 
FIG. 3 represents an equivalent system diagram of the dynamic OADM of FIG. 
1 where the routing module 102 is implemented by the conventional method 
of FIG. 2. As depicted, the transmitters TX-1-TX-F add signals are assumed 
to be first multiplexed by the additional multiplexers 311-313 to form the 
multi-channel signals A.sub.1 to A.sub.F, which are immediately 
demultiplexed by the demultiplexers 321-323. Signal A.sub.m is 
demultiplexed into A.sub.m1 to A.sub.mN. Similarly, the drop signals are 
assumed to be first multiplexed by multiplexers 314-316 to form the 
multi-channel signals D.sub.1 to D.sub.F which reach the receiver arrays 
RX-1-RX-F after passing through demultiplexers 324-326. Within the routing 
module 301, the interconnections from the outputs of demultiplexers 
201-203 and 321-323 to the inputs of all OSS-k and from the outputs of all 
OSS-k to inputs of multiplexers 204-206 and 314-316, as illustrated by the 
crossed lines in FIG. 2, are represented in FIG. 3 by the input 
interconnect fabric 302 and the output interconnect fabric 303. Each of 
the 2F multi-channel signals I.sub.1 to I.sub.F and A.sub.1 to A.sub.F 
passes through the cascade of a DEMUX, an OSS, and a MUX. The module 301 
is thus called a DEMUX/OSS/MUX routing module. Note that all the 
2F.times.2F optical space switches 211-213 can always be constructed using 
basic 2.times.2 space switches, as depicted therein by switch-like symbols 
enclosed by brackets 331-333. In general there are multiple configurations 
which may implement a 2F.times.2F space switch using 2.times.2 switches. 
To properly achieve the add-drop and routing functions for the F.times.F 
N-channel dynamic OADM, the 2F.times.2F space switches must satisfy the 
following conditions: (1) any two switches are independently controllable 
for per-wavelength routing; (2) when a signal I.sub.mk is dropped, it can 
be dropped to any drop port D.sub.nk (1 n F); (3) when a signal I.sub.mk 
is passed through, it can be passed to any output port O.sub.nk (1 n F); 
(4) when a signal A.sub.mk is added, it can be added to any output port 
O.sub.nk (1 n F); (5) at most one channel is routed to any output; (6) at 
most one channel is routed to any drop port; and (7) establishing a new 
connection from input/add to output/drop has no effect on existing 
connections. These conditions are referred to in the following as strictly 
add-drop non-blocking (SADNB). It is assumed that routing the output of a 
local transmitter to a local receiver is not useful since this information 
could have been transferred within the electrical domain. 
FIG. 4 depicts a system diagram of an F.times.F N-channel dynamic OADM 
having features of the present invention in which the conventional 
DEMUX/OSS/MUX routing module 301 is transformed into a new 
tunable-reflection-filter (TRF) routing module 401 constructed from 
multiple TRF devices TRF-1 . . . TRF-N. The TRF is an optical filter 
device with a narrow reflection bandwidth whose center wavelength can be 
tuned by mechanical, thermal, or electronic means. The principle of using 
reflection filters in combination with a circulator to separate WDM 
signals is described by L. Quetel, et al. in "Programmable Fiber Grating 
Based Wavelength Demultiplexer," presented at OFC, Feb. 28, 1996, paper 
WF6, which is hereby incorporated by reference in its entirety. The TRF is 
preferably a low-loss tunable reflection filter, such as an in-fiber 
UV-written Bragg grating, which may exhibit lower insertion loss for 
pass-through channels. 
Referring again to FIG. 4, assume maximum add-drop capability such that N 
TRFs are connected in series with TRF-k controlling the add-drop of 
channel .lambda..sub.k. For a given WDM channel wavelength spacing 
.DELTA..lambda.=.lambda..sub.k+1 -.lambda..sub.k, the TRF bandwidth should 
be small enough to reflect only .lambda..sub.k and pass all the other N-1 
channels in the ON state while passing all N channels in the OFF state. As 
depicted, the TRF module 402 includes N cascaded TRFs, wherein each filter 
TRF-k controls the reflection (when the TRF center wavelength is tuned at 
.lambda..sub.k, the ON state) or transmission (when the TRF center 
wavelength is detuned to that is away from .lambda..sub.k, the OFF state) 
of the channel at .lambda..sub.k. Therefore, along with the two 
circulators 404 and 405, .lambda..sub.k will travel from in-1 to out-1 or 
from in-2 to out-2 when TRF-k is ON (corresponding to a BAR state in a 
2.times.2 space switch) and travel from in-1 to out-2 or in-2 to out-1 
when TRF-k is OFF (corresponding to a CROSS state of a 2.times.2 space 
switch). In practice can be constructed such that a TRF-k in the OFF state 
has a flat response for all N channels. The module 403 in the bracket thus 
functions as a 2.times.2 N-channel tunable spectral switch (TSS) with the 
transmission or reflection of all channels being independently 
controllable. Because the module 403 achieves independent wavelength 
switching with all channels multiplexed, it is concluded that the 
DEMUX/OSS/MUX module 301 of FIG. 3 can be transformed into the 2F.times.2F 
N-channel TRF module 401 of FIG. 4 with internal 2.times.2 TSS modules 
connected exactly in the same fashion that the 2.times.2 space switches 
are connected in any OSS-k of FIG. 3. Outside the TRF routing module 401, 
the arrangement of array transmitters 411-413, transmitter multiplexers 
421-423, array receivers 431-433, and receiver demultiplexers 441-443 are 
exactly the same as outside the module 301 in FIG. 3. 
FIG. 5 illustrates a system diagram of the simplest 1.times.1 N-channel 
dynamic OADM, a node for a single fiber unidirectional optical ring or bus 
network, constructed according to this invention, by setting F=1 in FIG. 
4. Any channel I.sub.1k at a wavelength .lambda..sub.k can be dropped to 
port D.sub.1 when TRF-k is ON, and conversely, passed to the output 
O.sub.1 when TRF-k is OFF. Dropped channel I.sub.1k goes to the k-th 
receiver in the receiver array 502 by first passing through the DEMUX 512. 
Simultaneously, the k-th transmitter in the transmitter array 503 can add 
a channel at the wavelength .lambda..sub.k which passes through the MUX 
513, reflected by TRF-k and exits at the circulator 405 output O.sub.1. 
Since any channel that is routed from I.sub.1 to O.sub.1 experiences the 
out-of-band flat response of TRF-1 to TRF-N, the spectral filtering effect 
is minimized, thus reducing system penalties when the signal is routed 
through a large number of nodes in a ring or bus network. The filtering 
effects of the DEMUX and the MUX is less of a problem because the signal 
experiences such effects only once, i.e. when it is dropped or added. A 
second concern in implementing the OADM of FIG. 5 is the in-band and 
out-of band crosstalk that may contaminate the signal channel due to 
imperfect isolation of practical TRFs and/or circulators. Consider, as an 
example of out-of-band crosstalk, that TRF-1 is ON and TRF-2 is OFF so 
that ideally I.sub.11 is dropped to port D.sub.1 and I.sub.12 passes 
through. In practice I.sub.12 can be slightly reflected by TRF-2 even in 
the OFF state. The DEMUX 512, however, provides a desirable second 
isolation stage. The in-band crosstalk on the dropped channel I.sub.11 can 
result from the leakage of A.sub.11 (when transmitting) through TRF-1 in 
the ON state. This type of crosstalk is referred to as add-to-drop 
crosstalk. Similarly, I.sub.11 in reality can leak through TRF-1 and 
result in drop-to-add crosstalk on A.sub.11. 
FIG. 6 shows a system diagram of a dynamic OADM according to the present 
invention (which has been modified from FIG. 5) to remove add-to-drop 
crosstalk and also to eliminate spectral filtering effects on add signals. 
The add-to-drop crosstalk is removed by replacing the output circulator 
405 in FIG. 5 by a 2.times.1 directional coupler 601. Here, the input 
circulator 604 and the TRF module 603 actually function as a 1.times.2 
N-channel TSS module 605. To further eliminate spectral filtering on the 
add signals, an N.times.1 star coupler 602 can be used in place of the MUX 
513. The reduction of the crosstalk and spectral filtering is, however, 
achieved at the expense of additional power loss. 
As will be discussed with reference to FIG. 15b, two independent 1.times.1 
OADMs such as those illustrated by FIG. 5 or FIG. 6 could be arranged in 
parallel to form a WDM ring network with bidirectional (clockwise and 
counter-clockwise) signal transmission capability. Each 1.times.1 OADM 
independently handles the channel add-drops for one direction. In this 
arrangement there is no interaction of signals between the two OADMs. 
Moreover, circulators can be employed at the inputs and outputs of the two 
OADMs to convert the two inputs or two outputs to a single fiber, which 
results in the single-fiber bi-directional transmission between the ring 
nodes. 
Consider now a more common 2.times.2 N-channel dynamic OADM with inputs 
I.sub.1 and I.sub.2, outputs O.sub.1 and O.sub.2, add ports A.sub.1 and 
A.sub.2, and drop ports D.sub.1 and D.sub.2 obtained from FIG. 3 for F=2. 
Assume that in a dual-fiber bidirectional ring network discussed below, 
I.sub.1 to O.sub.1 corresponds to a transmission in one direction, while 
I.sub.2 to O.sub.2 corresponds to a transmission in the other. In contrast 
to the two independent 1.times.1 OADMs discussed above, the 2.times.2 OADM 
can dynamically drop any input channel to either D.sub.1 or D.sub.2 and 
route any add channel to either O.sub.1 or O.sub.2. The 2.times.2 
N-channel OADM, when constructed by the conventional configuration of FIG. 
3, will include N 4.times.4 optical space switches. 
FIG. 7a and 7b illustrate two possible configurations of 4.times.4 space 
switches for channels at .lambda..sub.k, constructed using 2.times.2 space 
switches. Note that 1.times.2 and 2.times.1 space switches can be regarded 
as 2.times.2 switches with one unused input or output. It is straight 
forward to verify that both FIG. 7a and 7b satisfy the conditions of 
SADNB. 
FIG. 8 illustrates a 2.times.2 N-channel dynamic OADM 801 that is 
constructed according to this invention. The OADM 801 may be obtained by 
transforming FIG. 3 to FIG. 4, given the 4.times.4 OSS configuration 701 
in FIG. 7a, in the following steps. The N-channel TSS modules 802-808 have 
a one-to-one correspondence to the space switches 702-708 in FIG. 7a. The 
TSS cross-drop module 806 enables I.sub.1 or I.sub.2 to be dropped to 
either D.sub.1 or D.sub.2. The TSS cross-add module 808 enables A.sub.1 or 
A.sub.2 to be added to either O.sub.1 or O.sub.2. Similarly the TSS 
cross-pass module 807 enables I.sub.1 or I.sub.2 to be routed to either 
O.sub.1 or O.sub.2. These three functions are transformed from the 
cross-drop switch 706, cross-add switch 708, and the cross-pass switch 707 
in the 4.times.4 OSS-k 701 of FIG. 7a. The two 1.times.2 switches 702 and 
703 and 1.times.2 TSS modules 802, 803 are functionally similar and are 
called drop-control switches. The 2.times.1 switches 704 and 705 and 
2.times.1 TSS modules 804, 805 are functionally similar and are called 
add-control switches. The dynamic OADM 801 is SADNB for all N wavelengths 
since the 4.times.4 OSS of FIG. 7a is SADNB for .lambda..sub.k. Note that 
the 1.times.2 TSS modules 802 and 803 each need only one circulator due to 
the absence of a second input. The circulators 813 and 814 in the 
2.times.1 TSS modules 804 and 805, however, are preferably kept because 
they can greatly reduce both add-to-drop and drop-to-add types of in-band 
crosstalk. Consider the four signals at .lambda..sub.k, i.e. I.sub.1k, 
I.sub.2k, A.sub.1k, A.sub.2k, and both I.sub.1k and I.sub.2k are dropped 
by setting TRF-k in modules 802 and 803 both to ON state. Channel I.sub.1k 
can leak through TRF-k of module 802, reflected by TRF-k in module 807 
assumed at ON state, and appear at port 831 of the 2.times.1 TSS module 
804. To add a channel to output O.sub.2 from port 832, TRF-k in module 804 
will be set at ON state. Therefore the drop-to-add crosstalk from port 831 
will be reflected to the unused output 833. Simultaneously the add signal 
that leaks through TRF-k in module 804 will also exit at 833 and thus the 
add-to-drop crosstalk is also avoided. The circulator 814 in the TSS 
module 805 for O.sub.1 functions identically in crosstalk reduction. 
Alternatively, the circulators 813 and 814 can each be replaced by an 
isolator. Finally imperfect isolation of the TSS module 808 will cause 
in-band crosstalk between ports A.sub.1 and A.sub.2 (referred to as 
add-to-add crosstalk) in the switchable transmitter arrays 841. Similarly 
there is drop-to-drop crosstalk in the switchable receiver arrays 842. 
Those skilled in the art will appreciate that adjacent circulators can be 
combined. For example, the 3-port circulator 880 coupled to the input port 
I.sub.1 and the adjacent 3-port circulator 890 could be replaced by a 
single 4-port circulator. 
Those skilled in the art will also appreciate that in the dynamic OADM 801 
constructed according to this invention, since input channels passing 
through the OADM ring node (from I.sub.1 to O.sub.1 or from I.sub.2 to 
O.sub.2) go through the out-of-band flat response of all TRF modules in 
the path, spectral filtering effects are therefore advantageously 
minimized. Another desirable feature for a ring network that is built 
based on nodes of the OADM 801 is that the network can survive when node 
power fails: all TRFs in the OADM can be designed to consume power only in 
the ON state such that all input channels will be routed through the node 
during a power failure (all TRFs OFF) with channel add/drop automatically 
suppressed. 
FIG. 9 illustrates a second 2.times.2 N-channel dynamic OADM 901 that is 
constructed by transforming FIG. 3 to FIG. 4 given the 4.times.4 OSS 
configuration 711 as shown in FIG. 7b. The N-channel TSS modules 912-916 
one-to-one correspond to the space switches 712-716 in FIG. 7b. Assuming 
identical TSS device imperfections, the OADM 901 has different crosstalk 
performance compared to that of the OADM of FIG. 8. The OADM 901 is worse 
in terms of both add-to-drop and drop-to-add crosstalks because the add 
and drop channels are isolated by only a single TSS module (913 or 914). 
The two OADMs have similar add-to-add and drop-to-drop crosstalk. The OADM 
of FIG. 9 possesses the same advantages as FIG. 8 of minimized spectral 
filtering for pass channels and survivability upon power failure. 
In order to remove the add-to-add and drop-to-drop in-band crosstalks in 
the OADM of FIG. 8 while still preserving the features of minimized 
spectral filtering for pass channels and survivability upon power failure, 
FIG. 10 illustrates an alternative configuration 1001 for implementing 
just the switchable transmitter array 841 or receiver array 842 portions 
for F=2. The add channels A.sub.1k and A.sub.2k from TX-1 and TX-2, 
respectively, are space switched by 2.times.2 OSS array 1011-1013 before 
they are multiplexed. Since low-loss high-isolation (&gt;50 dB) 2.times.2 
space switches are commercially available, the in-band add-to-add 
crosstalk is essentially removed if the switchable transmitter array 841 
in the OADM 801 is so modified. The configuration 1001 is readily 
extendible to F transmitters by using N F.times.F space switches as the 
switch assembly 1002. For arbitrary F and N, a similar configuration can 
be constructed and used to replace the switchable receiver arrays 842 in 
FIG. 8. 
The 2.times.2 OADM 801 implemented by FIG. 8 can be understood in the 
following modular way on a per-wavelength basis: (1) dropping of any 
I.sub.m is controlled by a 1.times.2 TSS module, (2) adding of any A.sub.m 
is controlled by a 2.times.1 TSS module, and (3) three additional 
2.times.2 TSS modules make possible the functions of cross-drop, 
cross-add, and cross-pass. These three specifications suggest the generic 
system configuration of FIG. 11, from which eight different SADNB 
2.times.2 N-channel dynamic OADM configurations can be constructed 
according to this invention. 
To construct an SADNB 2.times.2 N-channel OADM, only three out of the five 
2.times.2 N-channel TSS modules 1101-1105 in FIG. 11 are needed. However, 
the cross-drop function is enabled only if at least one of the 2.times.2 
TSS modules 1101 and 1102 is retained. Similarly the cross-add function is 
enabled only if at least one of 2.times.2 TSS modules 1103 and 1104 is 
retained. A third 2.times.2 TSS module provides the cross-pass function. 
Accordingly there are eight possible configurations to construct the SADNB 
2.times.2 OADM from FIG. 11 by always incorporating the 1.times.2 and 
2.times.1 modules 1111-1114 and three 2.times.2 modules of eight possible 
combinations: (1101, 1103, 1105), (1101, 1104, 1105), (1102, 1103, 1105), 
(1102, 1104, 1105), (1101, 1102, 1103), (1101, 1102, 1104), (1101, 1103, 
1104), and (1102, 1103, 1104). When any 2.times.2 TSS module is not 
retained, it has straight-through connections from the first input to the 
first output and from the second input to the second output. Also if TSS 
module 1105 is not used, the cascaded connection of a 1.times.2 module and 
a 2.times.1 module (1111 and 1113, or 1112 and 1114) can be implemented by 
a single 2.times.2 module. The OADM of FIG. 8 utilizes 2.times.2 TSS 
modules (1102, 1103, 1105) while the OADM of FIG. 9 utilizes (1101, 1102, 
1104) with the cascaded 1.times.2 module and 2.times.1 module implemented 
by a single 2.times.2 module (module 913 or 914 in FIG. 9). The 2.times.2 
TSS module 1103, if used, can be replaced by a 2.times.2 space switch 
array 1002 (FIG. 10) in between the transmitters and the multiplexers to 
remove add-to-add crosstalk. A similar arrangement can be applied to 
remove drop-to-drop crosstalk on the receiver side. 
Consider next an F.times.F N-channel dynamic OADM to be constructed 
according to this invention. The SADNB 2.times.2 N-channel dynamic OADMs 
in the generic configuration of FIG. 11 is extended to an F.times.F OADM 
configuration in FIG. 12 using F 1.times.2 drop-control TSS modules 
1211-1213, F 2.times.1 add-control TSS modules 1214-1216, along with three 
F.times.F TSS modules for the functions of cross-drop, cross-add, and 
cross-pass. For the same reasons explained pertaining to FIG. 11, three 
out of the five F.times.F N-channel TSS modules 1201-1205 in FIG. 12 are 
needed of which at least one of modules 1201 and 1202 should be retained 
and at least one of modules 1203 and 1204 should be retained. Accordingly, 
there are eight possible configurations. One can verify that all F.times.F 
N-channel OADMs constructed in this way satisfy the conditions of SADNB if 
the F.times.F TSS modules 1201-1205 satisfy the conditions of strictly 
non-blocking (SNB) on a per-wavelength basis: (1) the F inputs are routed 
to F outputs without collision and (2) establishing a new connections from 
inputs to outputs has no effect on existing connections. 
FIG. 13 depicts a SNB 3.times.3 N-channel TSS module 1301 in accordance 
with the present invention. As depicted, the SNB 3.times.3 N-channel TSS 
module 1301 includes three 2.times.2 N-channel TSS modules 1311-1313. The 
system diagram of a 3.times.3 N-channel OADM obtained from FIG. 12 using 
(1202, 1203, 1205) 2.times.2 TSS modules is illustrated in FIG. 14. As 
also explained earlier, the 2.times.1 modules 1405-1407 each includes an 
equivalent isolator 1421-1423. The add-to-drop and drop-to-add crosstalk 
in the OADM 1401 are essentially removed under the same principle for the 
2.times.2 OADM of FIG. 8. The add-to-add and drop-to-drop crosstalks could 
also be removed by modifying the switchable transmitter arrays 1431 and 
receiver arrays 1432 into an extended version (F=3) of the structure as 
illustrated by FIG. 10 (F=2). 
Those skilled in the art will appreciate that a SADNB F.times.F N-channel 
dynamic OADM for F&gt;3 can be constructed as depicted in FIG. 12 along with 
the rules for selecting three F.times.F N-channel TSS modules. The 
required SNB F.times.F TSS modules could be first constructed in one of 
the standard switch configurations: the cross-bar, the route/select, or 
the Clos networks. See, e.g., J. E. Midwinter, Photonics in Switching, 
vol. II, Systems, chapter 3, Academic Press, 1993. 
When the F.times.F (F 3) dynamic OADM 1201 is incorporated in an optical 
mesh network, the two key advantages of minimized spectral filtering for 
pass channels and survivability upon power failure are preserved for 
pre-assigned pass-through connections. For example, for F=3, with the 
3.times.3 TSS module 1411 assuming the configuration 1301 in FIG. 13, all 
channels will be routed from I.sub.1 to O.sub.1, I.sub.2 to O.sub.2, and 
I.sub.3 to O.sub.3 at node power failure. These three pass-through 
connections are also the paths with minimized spectral filtering and 
therefore can be pre-assigned to most frequent signal traffics. Various 
means for removing all four types of crosstalk apply equally well to the 
F.times.F OADM constructed according to this invention. Compared to the 
conventional implementation, all F.times.F OADMs for arbitrary F 
constructed according to this invention potentially have lower insertion 
loss for pass-through channels by using low-loss tunable reflection 
filters such as in-fiber UV-written Bragg gratings. 
Various types of wavelength-routing networks can be constructed using the 
dynamic OADMs implemented according to this invention. Consider first, the 
simple single-fiber ring network configurations shown in FIG. 15a-15c. 
Each block in FIG. 15a-15c represents a dynamic OADM with adds and drops 
denoted by short arrows pointing to and away from the OADM, respectively. 
The unidirectional ring topology of FIG. 15a uses a 1.times.1 OADM 610 
(FIG. 6) as the network node while the bidirectional ring of FIG. 15b uses 
two 1.times.1 OADMs 610 and two circulators 1511-1512 as the network node. 
Each 1.times.1 OADM in FIG. 15b is responsible for wavelength routing and 
add/drop in one direction while the two OADMs have no optical interaction 
within the node. By using 2.times.2 OADM 1110 (FIG. 11) and two 
circulators 1521-1522 as the node, FIG. 15c allows the functions of 
cross-add, cross-drop, and cross-pass. The ring networks of FIG. 15a-15c 
are reduced to linear bus networks if the fibers 1501-1503 are not 
connected. 
FIG. 16a illustrates a dual-fiber bidirectional ring network comprising 
2.times.2 dynamic OADMs 1110 as the ring node. This is an all-optical 
equivalent to a SONET BLSR-2 in which half the capacity in each direction 
carries working traffic, and the other half carries protection traffic. 
Clearly, at each node, cross-add and cross-drop for either direction are 
possible by using the SADNB 2.times.2 OADMs 1110. It is obvious from FIG. 
16a that, upon power failure, the node acts like straight-through fibers 
due to the feature of survivability of the OADM discussed earlier. 
FIG. 16b illustrates two single-fiber unidirectional rings coupled at a 
cross-connect node 1601 using a 2.times.2 dynamic OADM 1110 so that 
traffic can be routed between rings. When each individual ring is 
dual-fiber bidirectional, the cross-connect node will be the 4.times.4 
OADM 1701 of FIG. 17a (system 1210 for F=4 in FIG. 12). Finally, a 
wavelength-routing mesh network is illustrated in FIG. 17b where the 
network nodes can have different degrees (F=2, 3, or 4). 
Now that the invention has been described by way of a preferred embodiment, 
with alternatives, various modifications and improvements will occur to 
those of skill in the art. Thus, it is understood that the detailed 
description should be construed as an example and not as a limitation. The 
proper scope of the invention is defined by the appended claims.