Fiber-optical data-communication system using carriers of different wavelengths

A plurality of data-transmitting stations communicate with respective data-receiving stations via channels using optical carriers of different wavelengths, the several channels sharing a common light guide between an optical multiplexer at an outgoing terminal and an optical demultiplexer at an incoming terminal. To minimize cross-talk between channels, the data streams modulating their carriers are so synchronized that the signal peaks appearing beyond the demultiplexer on any pair of spectrally adjacent channels are relatively offset by half a signal period of their data streams or of the faster one of these two data streams. The synchronization can be carried out through electronic control of the channel modulators at the transmitting stations or with the aid of optical delay lines inserted in every other channel upstream of the multiplexer. A similar phase relationship is maintained between incoming and outgoing data streams at a subscriber station having a transmitter and a receiver communicating via an optical duplexer with a single light guide.

FIELD OF THE INVENTION 
My present invention relates to a multichannel data-communication system of 
the fiber-optical type operating by wavelength-division multiplexing. 
BACKGROUND OF THE INVENTION 
In such a communication system, messages sent by several transmitters to 
respective receivers travel over paths sharing a common light guide 
between an optical multiplexer at an outgoing terminal and an optical 
demultiplexer at an incoming terminal. Each transmitter is linked with its 
associated receiver by an individual channel defined by an optical carrier 
of a particular wavelength differing from those of all the other carriers. 
These channels, accordingly, occupy predetermined relative positions in 
the spectrum of wavelengths which are not necessarily related to the 
geographic locations of the stations. In the ensuing description, the term 
"adjacent channels" refers to those channels whose carriers lie next to 
each other in that wavelength spectrum. 
The number of channels that can be accommodated by a common light guide is 
limited by interference phenomena giving rise to cross-talk between 
adjacent channels. Thus, the carriers must be sufficiently spaced apart to 
minimize such interference. 
Various solutions have already been proposed to overcome the problem of 
cross-talk. In an article (Paper C7.3) presented by H. Ishio and T. Miki 
at the IOOC '77 conference held in Tokyo, there has been described a 
comparison system in which the output signal of a photodetector receiving 
incoming signals from a channel of a wavelength-division-multiplex system 
is modified by a corrective electrical signal derived by suitable 
attenuation and polarity inversion from the optical inter-channel 
interference. 
The output voltage or current of a photodetector oscillates in random 
fashion about a mean value constituting the useful signal. These 
oscillations account for a noise component of an amplitude proportional to 
the actual signal level. Since the noise accompanying an interfering 
signal is obviously uncorrelated with the noise accompanying the affected 
signal, the above-described compensation technique cannot reduce noise 
interference but is in fact likely to intensify same. This problem, 
accordingly, is specific to optical communication systems using 
photoelectric signal conversion. 
Moreover, the level of the optically interfering signal must be exactly 
known to enable an effective reduction of cross-talk without further 
signal degradation. The electrical components needed for signal 
attenuation and polarity inversion also tend to introduce additional noise 
due in part to drift and instability phenomena. 
OBJECT OF THE INVENTION 
The object of my present invention, therefore, is to provide improved means 
for the suppression of cross-talk in a fiber-optical communication system 
of the type referred to with avoidance of the aforestated drawbacks. 
SUMMARY OF THE INVENTION 
I have found that, for a given level of interfering optical signals, the 
performance degradation in a multichannel optical system largely depends 
on the relative phasing of the mutually independent data streams 
modulating the carriers of adjacent channels. In fact, the phase 
dependence of cross-talk increases with the signal level. 
Thus, I have determined that maximum degradation occurs when the data 
streams of adjacent channels (whose cadences are based on a common clock 
frequency) are in phase, with their signal peaks coinciding. Conversely, 
cross-talk is at a minimum when these data streams are relatively offset 
by half a signal period so that a peak on one channel coincides with a 
trough on the other channel. This assumes, of course, that the two signal 
periods are the same; if they are different but harmonically related, i.e. 
if the bit rate or cadence of one data stream is a multiple of the other, 
the offset should be by half the signal period of the data stream of 
higher cadence. 
In accordance with my present invention, therefore, I provide synchronizing 
means connected to the channel modulator of at least one transmitter for 
staggering the signal period of its outgoing data stream with reference to 
that of the data stream of at least one other channel as observed at one 
of the terminals of the common light guide, more particularly at the 
incoming or receiving-side terminal in the case of unidirectional 
signaling. With two-way communication, as where a transmitter and a 
receiver of a subscriber station are coupled to the light guide through a 
duplexer at the proximal terminal of that guide, the offset should exist 
at this proximal terminal. 
The synchronizing means according to my invention may be either an 
electronic phase shifter in the electrical path or an optical delay device 
in the light-guide path of a channel. 
With this improved system, in which the adjacent channels are effectively 
in optical quadrature at the point of demodulation of at least one of 
their carriers, photodetection follows rather than precedes the 
suppression of interference so that no additional noise is introduced into 
the demodulated data stream ahead of the decision stage recognizing the 
incoming symbols. Since the reduction or elimination of cross-talk 
involves only the relative time position of the signals, no advance 
knowledge of the signal level is required. Finally, the electronic or 
optical phase shifters are inherently stable and thus do not cause any 
significant deterioration of the signal-to-noise ratio.

SPECIFIC DESCRIPTION 
In FIG. 1 I have shown a plurality of transmitting stations T.sub.1, 
T.sub.2, T.sub.3, . . . T.sub.n-1, T.sub.n communicating with respective 
receiving stations R.sub.1, R.sub.2, R.sub.3, . . . R.sub.n-1, R.sub.n via 
transmission paths including individual light guides f.sub.1 -f.sub.n on 
the transmitting side, a common light guide F, and individual light guides 
f'.sub.1 -f'.sub.n on the receiving side. The two sets of individual light 
guides are coupled to the common guide F by an optical multiplexer MX at 
an outgoing terminal and an optical demultiplexer DMX at an incoming 
terminal. Transmitting station T.sub.1, which is representative of all the 
other stations on the same side of light guide F, comprises a digital data 
source SD, a line coder CL, a channel modulator MC and a light source SL. 
The latter, which generates a carrier of wavelength .lambda..sub.1, may be 
a light-emitting diode or a laser, for example. The remaining transmitting 
stations T.sub.2 -T.sub.n have carriers of wavelengths .lambda..sub.2 
-.lambda..sub.n, respectively. 
The circuits MC of all the transmitting stations, which modulate the 
signals of the associated data sources upon the respective optical 
carriers, are controlled in parallel by a common clock CK. In the case of 
stations T.sub.2 -T.sub.n, however, respective phase shifters SF.sub.2 
-SF.sub.n are interposed between this clock and their channel modulators. 
Each receiving station, as particularly illustrated for station R.sub.1, 
comprises a photodetector FR working into an amplifier/equalizer AE whose 
output is fed to a decision stage DE controlled by a sync extractor ES 
which monitors the incoming data stream detected and amplified in circuits 
FR and AE. The symbols recovered in stage DE are fed to a load DU. 
It will be assumed, for the sake of simplicity, that the several 
wavelengths .lambda..sub.1 -.lambda..sub.n vary progressively in the order 
in which they appear on the drawing so that the light waves emitted by 
stations T.sub.1 and T.sub.2, for example, constitute a pair of adjacent 
channels. The phase shifters introduced by devices SF.sub.2 -SF.sub.n are 
so chosen that the bit streams modulating the carriers .lambda..sub.2 
-.lambda..sub.n are relatively offset by half a signal period, as defined 
above, at demultiplexer DMX where the channels are separated from one 
another. If the signals do not undergo a significant relative phase shift 
on their way to terminal DMX, i.e. if local guides f.sub.1 -f.sub.n and 
common guide F are short enough, this phase relationship can be taken into 
account in the design of the phase shifters which in that case need not be 
adjustable. In other situations, however, it may be necessary to determine 
the relative phasing of the arriving bit streams at the incoming terminal 
in order to establish the correct mode of operation for these phase 
shifters. Thus, as illustrated in FIG. 2, a phaseshift detector MSF 
monitors the received signals in the outputs of stations R.sub.1 -R.sub.n 
and transmits this information via an ancillary channel A to a control 
circuit CT adjusting the phase shifters SF.sub.2 -SF.sub.n. The inputs of 
detector MSF could be connected in parallel with those of the sync 
extractors ES (FIG. 1) of the respective receiving stations. 
Ancillary channel A could include a further optical carrier transmitted 
over the common guide F and separated from the other carriers by a 
spectral distance greater than that existing between adjacent message 
channels. Such a spacing makes it unnecessary to control the phase of the 
supervisory signals passing over this ancillary channel. 
As illustrated in FIG. 3, the electronic phase shifters SF.sub.2 -SF.sub.n 
could be replaced by as many optical delay devices RO.sub.2 -RO.sub.n 
inserted in the corresponding local light guides f.sub.2 -f.sub.n. 
If relative phase shifts during transmission are insignificant, the 
simplified arrangement of FIG. 4 can be adopted in which a single phase 
shifter SF is inserted between clock CK and the channel modulators of 
every other transmitting station T.sub.2 . . . T.sub.n (n being assumed to 
be even). The odd-numbered stations T.sub.1, T.sub.3, . . . T.sub.n-1 are 
all connected directly to clock CK. If the bit rates or cadences 
controlled by clock CK are harmonically related but not identical for all 
stations, the phase shift introduced by device SF should correspond to 
half a signal period of the data stream having the highest cadence. 
In an analogous arrangement shown in FIG. 5, phase shifter SF is replaced 
by optical delay devices RO.sub.2 . . . RO.sub.n inserted in every 
even-numbered local light guide f.sub.2 . . . f.sub.n. 
The system of FIG. 4 can also be made adaptive, with adjustment of phase 
shifter SF under the control of a monitoring circuit as shown at MSF and 
CT in FIG. 2, in order to compensate for possible variations in the 
transmission characteristics of the light guides or relative drifts of the 
data sources, provided that these variations affect the channels of all 
the parallel-connected stations in a similar manner. 
In FIG. 6 I have shown two transmitter terminals UT.sub.1 and UT.sub.2 each 
of which may comprise one or more transmitting stations working into a 
respective light guide G.sub.1 and G.sub.2. Guide G.sub.2 merges into 
guide G.sub.1 at a junction AC through the intermediary of a conventional 
optical coupler IN. An associated receiver terminal UR, comprising as many 
receiving stations as there are transmitting stations in terminals 
UT.sub.1 and Ut.sub.2 combined, is connected to the opposite end of guide 
G.sub.1 and has a structure similar to that shown in the right-hand 
portion of FIG. 1, including an optical demultiplexer. An optical 
multiplexer will of course be included in either or both transmitter 
terminals UT.sub.1, UT.sub.2 if such terminal encompasses more than one 
station. 
Terminal UT.sub.1 is controlled by a clock CK and may include one or more 
phase shifters or optical delay devices as shown in the preceding Figures. 
Terminal UT.sub.2 receives the timing signal of clock CK by way of a link 
B which may again be an optical channel. 
Junction AC further includes an optical delay device RO' inserted in guide 
G.sub.2 for the purpose of collectively offsetting the data stream or 
streams from terminal UT.sub.2 with reference to those originating at 
terminal UT.sub.1 to provide the aforedescribed quadrature relationship 
between adjacent channels thereof. This offset is particularly important, 
even if the carriers of the two terminals lie in different wavelength 
ranges well separated from each other, where the distance of terminal 
UT.sub.2 from junction Ac is substantially less than the distance of 
terminal UT.sub.1 from that junction so that the signals arriving over 
guide G.sub.2 are less attenuated and therefore of larger amplitude that 
those coming in on guide G.sub.1. 
Delay device RO' in guide G.sub.2 could be replaced by one or more phase 
shifters in the electrical circuits of terminal UT.sub.2, as described 
above with reference to FIGS. 1, 2 and 4. Such phase shifter or shifters 
may again be made adjustable under the control of a monitoring circuit of 
the type illustrated in FIG. 2. 
According to FIG. 7, the connection B from the clock CK associated with 
terminal UT.sub.1 can be replaced by a sync extractor RS including a 
photodetector to which part of the wave energy traveling on guide G.sub.1 
is fed via an ancillary guide G.sub.3 branched off guide G.sub.1 at a 
point DR just upstream of coupler IN. 
It will be apparent that any number of transmitting terminals may be 
optically interconnected in the manner illustrated in FIG. 6 or 7. These 
terminals need not be controlled by a common clock, as in the system of 
FIG. 6, but could have individual clocks suitably synchronized with one 
another, e.g. in the manner shown in FIG. 7. 
In FIG. 8 I have illustrated a subscriber station AU including a 
transmitter TR and a receiver RC respectively similar to stations T.sub.1 
and R.sub.1 shown in FIG. 1. The sync extractor ES of receiver RC works 
into a synchronizer SY by way of a line 1; this synchronizer, in turn, 
controls the channel modulator MC of transmitter TR via a line 2. A common 
light guide L supplies incoming messages to receiver RC via an optical 
duplexer DX and a local light guide f.sub.r while carrying outgoing 
messages arriving from transmitter TR by way of a local guide f.sub.t and 
the duplexer. Here again, because of the proximity of the transmitter, 
substantial cross-talk would exist in the absence of the synchronizer even 
with widely separated wavelengths of the incoming and outgoing carriers. 
If receiver RC and transmitter TR operate with the same cadence, 
synchronizer SY may have the simple structure shown at SY.sub.a in FIG. 
9a. This structure consists of a delay line LR followed by a timer AB 
generating control pulses for modulator MC, the delay of line LR being 
equal to half a signal period or possibly to an odd number of such 
half-periods. If receiver RC has a cadence which is a multiple of that of 
transmitter TR, the synchronizer should have the structure shown at 
SY.sub.b shown in FIG. 9b which includes a frequency divider DF inserted 
between delay line LR and timer AB; the step-down ratio of this divider 
corresponds to the ratio of the two cadences. Conversely, if the cadence 
of receiver RC is a submultiple of transmitter TR, a frequency multiplier 
MF of corresponding step-up ratio is inserted in a synchronizer SY.sub.c, 
as illustrated in FIG. 9c. 
In FIG. 10 I have shown a set of graphs representing error probability at 
the decision stage DE (FIGS. 1 and 8) plotted against phase shift in terms 
of a signal period T, corresponding for instance to half a clock cycle. 
The several curves represent various relative signal levels X, ranging 
from -3dB to -.infin., for a single interfering channel. It will be seen 
that in all instances the cross-talk is at a minimum for a relative phase 
displacement of 0.5T; the reduction of error probability is particularly 
significant where the level X of the interfering signal as measured at the 
affected channel is of the order of -6dB or higher.