Optical frequency marking method and different frequency channel communication network using it

In an optical frequency marking method and a frequency channel communication network using this method, an optical reference frequency is scanned across a scanning band to produce frequency coincidences with a monitored frequency which constitutes the frequency to be marked. During each scanning half-cycle it effects a go path and a return path according to a known law to produce a go frequency coincidence and a return frequency coincidence. A marker interval is measured which is the time elapsed between said two coincidences. This interval marks the monitored frequency and enables said frequency to be locked by comparison with a set point interval. The invention finds a particular application in the marking and stabilization of carrier frequencies of a closely-spaced different frequency channel network.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The present invention concerns a method of marking optical frequencies. 
This method finds a particular application in a communication network 
using optical fibers to guide carriers produced by semiconductor laser 
sources. The carriers are modulated with data to be transmitted and occupy 
a succession of different frequency channels constituting transmission 
channels. Some known networks of this kind, known as dispersed source 
networks, comprise the following components: 
A succession of terminals each comprising optical send and receive means 
for sending and receiving data conveyed by a succession of modulated waves 
occupying a succession of different frequency channels in a predetermined 
communication band and resulting from the modulation of a succession of 
carrier waves of said channels, respectively. 
A reference block sending an optical reference wave at a reference 
frequency. 
Optical fibers linking said terminals to each other and to said reference 
block to transmit said modulated waves and said reference waves. 
The send means of each terminal comprise: 
a monitored source comprising a semiconductor laser emitting a monitored 
wave to constitute the carrier wave of one channel, said wave having a 
monitored frequency constituting a carrier frequency of said channel, 
frequency control means for controlling said monitored frequency, 
modulation means for modulating said monitored wave and constituting a 
modulated wave, and 
frequency comparator means comparing said reference and monitored 
frequencies and operating on said frequency control means to lock said 
monitored frequency relative to said reference frequency. 
Other networks of this kind are called grouped source networks and comprise 
a plurality of sources in the same terminal. 
The availability of indium phosphide semiconductor laser sources which are 
strongly coherent and whose wavelength can be tuned has made it possible 
to adopt dense spaced multiplexing techniques in implementing these 
various networks. These techniques are directed to enabling these networks 
to operate with as small as possible a spacing between adjacent channels, 
to obtain the maximum benefit from the transmission window of the fibers 
around the frequency defined by the wavelength 1 550 nm. 
The resulting drawback, which constitutes a major obstacle to the 
development of this type of technique, resides in the need to stabilize 
the frequency at which these sources emit and to define them with great 
accuracy. The problem is prevent unwanted progressive drift of the carrier 
frequency of one channel creating problems of crosstalk with an adjacent 
channel such as to compromise transmission. Also, the carrier frequency 
provides the means of identifying the channel and must therefore be 
defined absolutely. This is why this frequency must be monitored, that is 
to say measured or at least marked, the source and the carrier wave being 
therefore also monitored. 
The problem is particularly difficult in the case of dispersed source 
networks because the sources cannot be grouped together at the same 
geographical site; this is the case with an interactive communication 
network in particular. 
Various solutions have been proposed for this problem: the reference 
frequency may be a single frequency. It may then be distributed through 
the same fibers as the modulated waves if this frequency is in the margin 
of the communication band occupied by the transmission channels. The 
monitored sending source within a terminal is then stabilized relative to 
this reference frequency using a fixed or scanning Fabry-Perot 
interferometer. On this point reference may usefully be had to the 
article: 
Journal of lightwave technology, vol. 6 No 0 11 November 1988, pages 
1770-1781, DENSELY SED FDM COHERENT STAR NETWORK WITH OPTICAL SIGNALS 
CONFINED TO EQUALLY SED FREQUENCIES--B.S. GLANCE, J. STONE, K. J. 
POLLOCK, P. J. FITZGERALD, C. A. BURRUS, JR., B. L. KASPER, and L. W. 
STULZ. 
In one alternative solution a plurality of reference frequencies can 
constitute what is known as a "comb". The distribution of a comb of 
optical frequencies is supported by an ancillary fiber network so as not 
to disturb the use of the transmission channels. The frequency comb may be 
produced within the reference block by angular modulation of a coherent 
reference source. In this case the optical spectrum has a series of 
equi-distant lateral lines whose spacing is equal to the modulation 
frequency. Each monitored source can then be locked in frequency or in 
phase to one of the lateral lines of the reference source after heterodyne 
or homodyne detection. Reference may usefully be had on this topic to the 
article: 
FREQUENCY STABILIZATION TECHNIQUES FOR COHERENT LIGHTWAVE SYSTEMS--M. W. 
MAEDA, SPIE vol. 1175 Coherent Lightwave Communications (1989)--p. 4-11. 
and the article: EXPERIMENTAL RELATIVE FREQUENCY STABILIZATION OF A SET OF 
LASERS USING OPTICAL PHASE-LOCKED LOOPS--LEONID G. KASOVSKY and BENITE 
JENSEN--IEEE PHOTONICS TECHNOLOGY LETTERS. VOL. 2 No 7 JULY 1990 p. 
516-518. 
In the simpler case where the sources are grouped together in the same 
terminal, which is the case with a distribution network, other techniques 
have been proposed. One is the "heterodyne spectroscopy" technique 
described in the article: COHERENT OPTICAL-FIBER SUBSCRIBER LINE--E. J. 
BACCHUS--R. P. BRAUN--W. EUTIN--E. GROBMANN--H. FOISEL--K. HEIMES--B. 
STREBEL.--ELECTRONICS LETTERS Dec. 5, 1985 Vol. 21 No 25/26--P. 1203-1205. 
Another is the so-called "reference impulse" method described in the 
article: SHIMOSAKA--THG3 Frequency Locking of FDM optical sources using 
widely tunable DBR LDs--N. SHIMOSAKA--K. KAEDE, S. MURATA--OFC.88/THURSDAY 
MORNING/168. A common feature of these methods is the use of a so-called 
"master laser" reference source the optical frequency of which is 
periodically scanned across the band occupied by the network. The 
reference wave that it emits is mixed with all the channels and by 
heterodyne detection this produces a succession of electrical impulses at 
coincident frequencies. 
In the so-called "heterodyne spectroscopy" technique the current value of 
the master laser control electrical signal at the time of the pulse is 
compared with its nominal value--value representing the frequency reserved 
to this channel. Any difference with respect to this nominal value is 
directly related to the frequency offset and can be used to correct the 
sending frequency of the monitored source. 
The so-called "reference pulse" technique uses a stream of reference pulses 
produced by detecting the lightwave emitted by the master laser and 
transmitted by a calibration Fabry-Perot cavity whose free band gap is 
precisely equal to the frequency spacing to be maintained between the 
carrier frequencies of adjacent channels. Any time offset between a 
reference pulse and a respective coincidence pulse indicates drift of the 
respective monitored frequency and constitutes an error signal that can be 
used to lock that frequency. 
A particular object of the present invention is to enable simple 
implementation of a communication network with different frequency 
channels operating securely despite close spacing between adjacent 
channels. A more general object of the present invention is to define an 
optical frequency marking method which is both simple and accurate so that 
using it in a communication network with different frequency channels 
makes it possible to achieve the previously indicated particular object of 
the invention, even if the sources to be monitored are divided between 
geographical locations. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention consists in an optical frequency 
marker method whereby an optical reference frequency scans a scanning band 
to cause coincidence between two frequencies one related to said reference 
frequency and the other related to a monitored frequency to be marked, 
said frequency coincidences being used to mark said monitored frequency 
relative to values assumed by said reference frequency, in which method 
the reference frequency is scanned through a half-cycle comprising a go 
path and a return path effected according to a known law to produce a go 
frequency coincidence and a return frequency coincidence during said go 
path and said return path, respectively, and a marker interval is measured 
consisting of the time interval which elapses between said two 
coincidences, this interval constituting a marker of said monitored 
frequency. 
The frequencies whose coincidences are used may be the reference and 
monitored frequencies themselves. Alternatively, frequency coincidence may 
be the condition achieved by the difference between the reference and 
monitored frequencies passing through a predetermined value, for example 
the frequency specific to an electric filter. 
In a second aspect, the present invention consists in a different frequency 
channel communication network comprising: 
a succession of terminals each comprising optical send means and optical 
receive means for sending and receiving data conveyed by a succession of 
modulated waves occupying a succession of different frequency channels and 
resulting from the modulation of a succession of carrier frequencies of 
said channels, respectively, the set of said channels occupying a 
predetermined communication band, 
a reference unit sending a reference wave having a reference frequency, and 
optical fibers linking said terminals to each other and to said reference 
unit to transmit said modulated waves and said reference wave, 
the send means of each of said terminals comprising: 
a monitored source consisting of a semiconductor laser emitting a monitored 
wave to constitute the carrier wave of a channel, said wave having a 
monitored frequency constituting a carrier frequency of said channel, 
frequency control means for controlling said monitored frequency, 
modulator means for modulating said monitored wave and constituting a 
modulated wave, and 
frequency comparator means comparing said reference and monitored 
frequencies and operating on said frequency control means to lock said 
monitored frequency relative to said reference frequency, 
in which network said reference frequency effects at least one scanning 
half-cycle according to a known law as a function of time and during which 
said frequency progressively moves from a first end to a second end of a 
scanning band on a go path and then returns from said second to said first 
end on a return path, said scanning band being chosen so that said 
scanning half-cycle produces a go frequency coincidence during said go 
path and then a return frequency coincidence during said return path, said 
frequency comparator means of each terminal measuring a marker interval 
consisting of the time interval between the go and return frequency 
coincidences of the same scanning half-cycle, said frequency comparator 
means producing an error signal representing a difference between the 
marker interval and a set point interval, said error signal being supplied 
to said frequency control means to lock said monitored frequency to a set 
point frequency defined by said set point interval. 
How the present invention may be put into effect will now be described in 
more detail by way of non-limiting example with reference to the appended 
diagrammatic drawings. If the same component is shown in more than one 
figure it is always denoted by the same reference symbol. It must be 
understood that the components mentioned may be replaced by other 
components implementing the same technical function.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, an optical communication network comprises the 
following components whose functions are known from the MAEDA article: 
A succession of terminals T1 through T8. Each terminal, for example 
terminal T1, comprises optical send means E and optical receive means R 
for sending and receiving data. In the network as a whole data is carried 
by a succession of modulated waves occupying a succession of different 
frequency channels C1, C2. These waves are obtained by modulating carrier 
waves of these channels. The set of the latter occupies a predetermined 
communication band DC. 
A reference unit BR sending a reference wave at a reference frequency FR. 
Communication optical fibers GC etc which handle the exchange of messages 
between the various terminals through the intermediary of a star coupler 
CC. 
Finally, reference optical fibers GR which transmit the reference wave from 
the reference unit BR to all of the terminals through a unidirectional 
coupler CR. 
Referring to FIG. 5, the send means of each of the terminals T1 etc 
comprise: 
a monitored source 2 comprising a semiconductor laser emitting a monitored 
wave to constitute the carrier wave of one channel, said wave having a 
monitored frequency F1 constituting a carrier frequency of said channel, 
frequency control means 4 for controlling said monitored frequency, 
modulator means 6 for modulating said monitored frequency to constitute a 
said modulated wave, and 
frequency comparator means for comparing said reference and monitored 
frequencies and operating on said frequency control means to lock said 
monitored frequency to said reference frequency. 
The frequency comparator means use an optical frequency marking method in 
accordance with the present invention and will be described at the same 
time as this method. 
This method comprises the following operations known from the BACCHUS 
article in the case where the sources to be monitored are grouped together 
in the same terminal and can therefore be controlled without difficulty 
over electrical connections: 
Frequency scanning effected by the reference frequency FR. This scanning 
constitutes marker scanning causing this frequency to assume a succession 
of values. The scanning rate is greater than the rate of any variation of 
the monitored frequency F1. 
Detection of coincidence between the frequencies FR and F1 or (this 
alternative is not shown) between two other frequencies respectively 
related to the monitored frequency and the reference frequency. 
Finally, use of frequency coincidence to mark the monitored frequency 
relative to the succession of values assumed by the reference frequency. 
According to the present invention the marker scanning effected by the 
reference frequency comprises at least one scanning half-cycle AB carried 
out with precise timing and according to a known law. During this 
half-cycle the reference frequency FR moves progressively from a first end 
E1 to a second end E2 of a scanning band DB along a "go" path PA. It then 
returns from this second end to this first end along a "return" path PB. 
This scanning band is chosen so that this scanning half-cycle causes a go 
frequency coincidence QA1 during the go path and then a return frequency 
coincidence QB1 during the return path. 
The operation which uses these frequency coincidences involves measuring a 
marker time H1 consisting of the time interval between the go and return 
frequency coincidences of the same scanning half-cycle. 
The frequency coincidence detection operation comprises the following 
operations: 
Mixing of the reference and monitored waves to form a mixed wave. Referring 
to FIG. 5, this mixing is done by means of an optical coupler 8 which 
samples from the communication fiber GC a small fraction of the monitored 
wave produced by the source 2 and which mixes it with the reference wave 
reaching the terminal T1 via the reference fiber GR. 
Detection of said mixed wave by an opto-electronic detector comprising 
heterodyne receiver 10 and signal shaper 12 to form an electrical 
coincidence pulse when a component of said mixed wave passes through a 
predetermined detection frequency. This latter frequency is preferably 
less than 1 GHz so that it can be easily processed by an electronic 
circuit within the terminal. Two go and return coincidence pulses VA1 and 
VB1 represent two respective go and return frequency coincidences. The 
detector includes the heterodyne receiver 10. For optimum detection in 
this receiver the reference wave has two orthogonal linear polarizations 
which alternate at a sufficiently fast rate to enable the use of the 
signal provided by this receiver irrespective of the polarization of the 
monitored wave. This receiver is followed by the electronic signal shaper 
circuit 12 to produce the coincidence pulses. 
These pulses are received by a timing device comprising a counter 14 which 
counts clock pulses supplied by a clock 16. This counter is triggered by 
each go coincidence pulse and stopped by each return coincidence pulse and 
then reset to zero after supplying the count result, that is to say the 
marker interval H1, to a comparator 18. It includes logic means 15 for 
distinguishing between the go and return coincidence pulses. To this end 
the marker scanning is preferably in the form of a periodic series of 
scanning half-cycles AB each having a half-cycle AB duration and separated 
from each other by separation intervals EF greater than this half-cycle 
duration. 
The coupler 8, the detector 10, 12, the timing device 14, 16 and the 
comparator 18 constitute the frequency comparator means previously 
mentioned. They supply an error signal representing a difference between 
the marker interval H1 and a set point interval. This error signal is 
supplied to the frequency control means 4 to lock said monitored frequency 
to a set point frequency defined by said set point interval. 
The set point interval is, for example, represented by one of the numbers 
stored in a memory 20, the comparator 18 comparing the number of clock 
pulses counted by the counter 14 and the stored number. 
Of course, the stored numbers read by the various comparators 18 etc of the 
various terminals differ from each other to define the succession of 
carrier frequencies of the network. 
It must also be understood that, without departing from the scope of the 
present invention, the set point interval could be a variable interval 
defined by a network management unit rather than a predetermined interval 
defined by a memory 20. 
As shown in FIG. 2, the known reference frequency variation law is 
preferably a linear law and advantageously a law with two opposite slopes 
for the go and return paths. 
FIG. 4 shows the coincidence pulses VA2 and VB2 formed in the terminal T2 
and representing coincidences QA2 and QB2 between the carrier frequency F2 
of this terminal and the reference frequency FR. 
The various components may be of known types. In the network shown the 
coupler is a hybrid coupler. It comprises a coupler element 8A to sample a 
fraction of the monitored wave, a guide 8B to guide this fraction to the 
reference fiber GR and a second coupler element 8C to mix this fraction 
with the reference wave.