Bidirectional transmission system, especially one using optical fiber, employing a single carrier for both transmission directions

This bidirectional transmission system, especially one using optical fiber, between a source terminal and a user terminal employing a single carrier for both transmission directions obtained from a source in the source terminal characterized in that it comprises for each transmission direction means for modulating one parameter of said carrier, this parameter being intensity for the two directions and subject in all cases to the condition that the modulation applied first for transmission in the downward direction from the source terminal to the user terminal the depth of modulation is sufficiently high to leave sufficient power for application of the second modulation to which that applied second can be applied for transmission in the upward direction from the user terminal to the source terminal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a bidirectional transmission system, 
especially one using optical fiber, employing a single carrier for both 
transmission directions. 
The present invention cad be used among other things to provide 
bidirectional transmission on a link, especially an optical fiber link, 
between two equipments of the link (called terminals) when one of these 
terminal equipments (called a user terminal) must be as simple as possible 
and in particular must not incorporate any carrier source, especially any 
optical carrier source. 
A user terminal of this kind may be a subscriber terminal of a 
telecommunication network such as the telephone network, for example, 
especially in the context of connecting subscribers to the network using 
optical fiber. This application will be taken by way of specific example 
in what follows. 
2. Description of the Related Art 
Various techniques have been considered for providing bidirectional 
transmission via optical fiber between subscriber terminals and their 
local telephone network central office. 
For each of the techniques mentioned below, only one connection need be 
considered (between a subscriber and the central office), even if a set of 
subscribers can be connected to the central office by the same link, in 
particular by multiplexing. 
FIG. 1 summarizes a first technique. 
In this technique the central office CL and the subscriber terminal TA are 
connected by two optical fibers and information is transmitted from the 
central office to the subscriber (this is called the downward direction) 
on a fiber f.sub.1 separate from the fiber f.sub.2 on which information is 
transmitted from the subscriber to the central office (this is called the 
upward direction). 
The drawbacks of this first technique are the duplication of all the 
transmission equipments (two optical senders S'.sub.1, S'.sub.2 each 
comprising an optical source, two optical fibers f.sub.1, f.sub.2 and two 
optical receivers D'.sub.1, D'.sub.2) and the presence of an optical 
source at the subscriber terminal. 
A second technique summarized in FIG. 2 and described in the document IEEE 
Global Telecommunications Conference, H. Kobrinski, L. S. Smoot, and T. J. 
Robe, "A passive photonic loop architecture employing wavelength-division 
multiplexing" uses the great bandwidth of optical fiber to provide 
bidirectional transmission on a single fiber f between the central office 
CL and the subscriber terminal TA. In addition to the optical senders 
S'.sub.1, S'.sub.2 and the optical receivers D'.sub.1, D'.sub.2, optical 
couplers C'.sub.1, C'.sub.2 are then required to distinguish between the 
two transmission directions, respectively that to the central office and 
that to the subscriber terminal. Two wavelengths may be assigned to a 
call, a wavelength .lambda..sub.d for the downward direction and a 
wavelength .lambda..sub.m for the upward direction. Because of the nature 
of light, the same wavelength may be used for both transmission 
directions, however, as described in the document Electronic Letters, Vol. 
20, No. 18, pp. 722-723, 1984, A. P. McDonna, D. J. McCartney, and D. B. 
Mortimore, "1.3 .mu.m bidirectional optical transmission over 31 km of 
single-mode fibre using optical couplers". The major disadvantage of this 
type of technique is again the presence of an optical source in the 
subscriber terminal. 
A so-called "ping-pong" variant of the second technique reserves some 
timeslots for transmission in the upward direction and other timeslots for 
transmission in the downward direction. Various drawbacks are then 
incurred over and above the one mentioned above: 
simultaneous transmission in both directions is not possible; 
synchronization of transmission times allowing for the propagation times 
between the central office and the subscriber is required. 
As previously mentioned, a system which uses an optical carrier generated 
at the user terminal is unattractive, especially for a subscriber 
connection system, for two reasons: 
The wavelength assigned to transmission in the upward direction must be 
controlled to minimize interference with other calls, whether or not it is 
different from that used for transmission in the downward direction. 
It is difficult to guarantee wavelength stability because of the distance 
between subscribers and between each subscriber and the central office. 
The use of specific wavelengths for transmission in the upward direction 
requires each subscriber to have a different send equipment (including an 
optical source) compatible with the receive equipment at the central 
office. 
A technique for dispensing with an optical source at the subscriber 
terminal described in the document Electronic Letters, Vol. 23, No. 18, 
pp. 943-944, 1987, H. Kobrinski and S. S. Cheng, "Laser power sharing in 
the subscriber loop" is summarized in FIG. 3. 
As in the previous techniques, data d'.sub.1 from the central office is 
transmitted to the subscriber by modulating an optical carrier of 
wavelength .lambda..sub.d from an optical source in a sender S'.sub.1. For 
transmission in the upward direction another optical source S".sub.1 in 
the central office sends to the subscriber on the same fiber f' a carrier 
at a wavelength .lambda..sub.m different than .lambda..sub.d. Using a 
modulator M'.sub.2 the subscriber modulates this optical carrier with the 
data d'.sub.2 to be sent. The modulated optical carrier, which is still at 
the wavelength .lambda..sub.m although this wavelength is now denoted 
.lambda..sup.r.sub.m (r=relayed) is relayed, in this example over the same 
fiber, to the central office. 
In a system based on this technique the absence of any optical source at 
the subscriber terminal eliminates the need for wavelength control between 
all network subscribers. Control of the light sources at the central 
office is still required, however, but this is easier to implement because 
the sources are all located in the central office. However, a system of 
this kind has the drawback of requiring two optical sources rather than 
one source at the central office. 
A variant of this technique using a single optical carrier is described in 
the document Electronics Letters, Vol. 22, No. 10, pp. 528-529, 1986, T. 
H. Wood, E. C. Carr, B. L. Kasper, R. A. Linke, C. A. Burus and K. L. 
Walker, "Bidirectional fibre-optical transmission using a 
multiple-quantum-well (MQW) modulator/detector". This variant uses a 
common component for modulation and detection at the subscriber terminal. 
This component cannot function simultaneously as a modulator and as a 
detector, however, so that transmission in the upward direction cannot 
take place at the same time as transmission in the downward direction. 
A technique enabling simultaneous bidirectional transmission of optical 
signals between a subscriber and a central office using a single optical 
carrier described in the document Conference on Optical Fiber 
Communication (Atlanta, Ga., U.S.A., 24-26 February 1986), Technical 
Digest, paper MH4, pp. 14-15, P. J. Duthie, M. J. Wale, J. Hankey, M. J. 
Goodwin, W. J. Stewart, I. Bennion and A. C. Carter, "Simultaneous 
bidirectional fiber-optic transmission using a single source" is 
summarized in FIG. 4. 
A single optical source included in a sender S'.sub.1 at the central office 
is used for simultaneous bidirectional transmission between the central 
office and a subscriber terminal. For transmission in the downward 
direction the carrier from the optical source is amplitude modulated, the 
resulting light signal is transmitted over a fiber f' and some of the 
light, sampled by a coupler C'.sub.2, is detected at the subscriber 
terminal by the receiver D'.sub.2. For transmission in the upward 
direction the subscriber uses a directional coupler type modulator 
M'.sub.2 to superimpose his signal d'.sub.2 on the non-detected part of 
the light used for transmission in the downward direction from the coupler 
C'.sub.2. The resulting optical carrier is reflected by a mirror for 
transmission to the central office at the same wavelength .lambda.' now 
denoted .lambda.'.sup.r (r=reflected) via the same fiber f'. To enable the 
central office to reconstitute the signal d'.sub.2 sent by the subscriber, 
the transmission bit rate in the upward direction must be lower than that 
in the downward direction, however, so this technique cannot be used to 
transmit data at the same bit rate in both directions. Another drawback of 
this technique is that, in the case of data transmitted in digital code, 
the information conveyed in the downward direction must be coded to avoid 
long sequences of zero values during which the subscriber could not send 
(because he would not be receiving any light). 
SUMMARY OF THE INVENTION 
A particular object of the present invention is to provide a bidirectional 
transmission system using a single carrier for both transmission 
directions enabling simultaneous transmission in both directions and which 
is free of the above mentioned drawbacks. 
The present invention consists in a bidirectional transmission system, 
especially one using optical fiber, between a source terminal and a user 
terminal employing a single carrier for both transmission directions 
obtained from a source in the source terminal characterized in that it 
comprises for each transmission direction means for modulating one 
parameter of said carrier, this parameter being intensity for both of the 
two directions subject in all cases to the condition that for the 
modulation applied first for transmission in the downward direction from 
the source terminal to the user terminal, the depth of modulation is 
sufficiently small to always leave sufficient power for application of the 
second modulation for transmission in the upward direction from the user 
terminal to the source terminal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a first embodiment of the invention simultaneous bidirectional 
transmission using a single optical carrier between a source terminal 
including an optical source providing said carrier and a user terminal is 
obtained by modulating the intensity of said carrier for transmission in 
both directions, namely the downward direction (from the source terminal 
to the user terminal) and the upward direction (from the user terminal to 
the source terminal), the modulation applied for transmission in the 
downward direction being effected between a low level Pb relatively far 
from the zero level and a high level Ph and the modulation applied for 
transmission in the upward direction being effected between a level Po 
near the zero level and the level Pb, in other words on the light emitted 
by the optical source which was not modulated for transmission in the 
downward direction. 
The modulation for transmission in the downward direction is direct 
modulation of the intensity of said carrier, for example. 
FIG. 6 shows how an optical signal of this kind is generated by direct 
modulation. The diagram shows the general shape of the output optical 
power characteristic of an optical source such as a laser diode as a 
function of its control current "i" and the timing diagram of the output 
optical power, in this example for a digital control current comprising 
the sequence 1001010100111010, for example, in this instance with 
modulation applied between the levels Pb and Ph as defined above. 
As shown in FIG. 5, in this first embodiment the source terminal equipment 
comprises: 
an optical sender S.sub.1 including an optical source such as a laser diode 
emitting at a wavelength .lambda. and provided with a control input for 
application of an intensity modulation signal d.sub.1, in this instance a 
direct modulation signal, to be transmitted in the downward direction, 
an optical receiver D.sub.1 for detecting a modulation signal d.sub.2 
transmitted in the upward direction, 
a passive coupler C.sub.1 enabling a single fiber f to be used for both 
transmission directions and three ports of which are respectively 
connected to the output of the sender S.sub.1, to the input of the 
receiver D.sub.1 and to one end of the fiber f, 
an optical isolator I.sub.1 which protects the output of the optical sender 
S.sub.1 from the light received by the source terminal. 
As also shown in FIG. 5, in this first embodiment the user terminal 
equipment comprises: 
a first passive coupler C.sub.2 enabling a single fiber to be used for both 
transmission directions, 
a second passive coupler C".sub.2 enabling part of the received optical 
carrier to be sampled so that the message d.sub.1 that it conveys in the 
downward direction can be detected by an optical receiver D.sub.2 of this 
terminal, the other part being intended to be modulated by the message 
d.sub.2 to be sent by the user terminal and returned to the source 
terminal, still at the wavelength .lambda. but now denoted .lambda..sup.r 
(r=relayed), 
modulation means M.sub.2, for example an electro-optical component of the 
intensity modulator or semiconductor optical amplifier type which receives 
said part intended to be modulated from the coupler C".sub.2 and an 
intensity modulation control signal embodying the data d.sub.2 to be sent 
by the user terminal, 
an optical isolator I.sub.2 which protects the output of the modulator 
M.sub.2 from the light received by the user terminal, 
the optical receiver D.sub.2 for detecting the signal sent in the downward 
direction, restoring the data d.sub.1 sent by the source terminal. 
The coupler C".sub.2 has a first port connected to a second port of the 
coupler C.sub.2, a second port connected to the optical receiver D.sub.2 
and a third port connected to an input of the modulation means M.sub.2, as 
mentioned above. 
The isolator I.sub.2 has a port connected to the output of the modulation 
means M.sub.2 and a port connected to a third port of the coupler C.sub.2. 
The coupler C.sub.2 has a first port connected to one end of the fiber f. 
Note that the user terminal equipment is relatively simple, comprising an 
optical receiver, a modulator and a limited number of passive optical 
components. 
FIG. 7A shows the timing diagram of the optical signal at the point A in 
FIG. 5, in other words the optical power received by the detector which in 
this instance comprises the receiver D.sub.2 of the user terminal. As 
shown in this figure, the decision threshold sd.sub.2 of this receiver is 
advantageously set to a value substantially equal to half the sum of the 
level values Pb and Ph. 
FIG. 7B shows the timing diagram of the optical signal at the point B in 
FIG. 5, in other words at the output of the send modulator M.sub.2 of the 
user terminal equipment. As shown in this figure, the decision threshold 
sd.sub.1 of the receiver D.sub.1 of the source terminal is advantageously 
set to a value equal to half the sum of the level values Po and Pb. 
In a second embodiment of the invention simultaneous bidirectional 
transmission by means of a single optical carrier is achieved by the use 
of frequency modulation for transmission in the downward direction, in 
this instance CPFSK type modulation (Continuous Phase Frequency Shift 
Keying), and intensity modulation for transmission in the upward 
direction. 
FIG. 8 shows the source terminal equipment and the user terminal equipment 
in this embodiment. They differ from those shown in FIG. 5 only in that 
the control input of the optical sender is a control input for application 
of a frequency modulation signal and in that the receiver D".sub.2 
comprises a filter F preceding a detector D"'.sub.2. An FSK signal is 
usually demodulated by heterodyne detection. However, an optical filter 
having an appropriate bandwidth centered on one of the two peaks of the 
modulated signal power spectrum (the modulating signal being a binary 
signal)--see FIG. 9--enables conversion from FSK modulation to ASK (or 
intensity) modulation and subsequent demodulation of the signal by direct 
detection. 
FIG. 10A shows the timing diagram of the optical signal at the point A in 
FIG. 8, in other words the optical power received by the receiver D".sub.2 
of the user terminal. As shown in this figure, the optical power at this 
point remains constant at a level P. 
The frequency modulation employed is, for example, modulation by a binary 
signal in which case the spectrum of the modulated signal in the case of a 
data bit of value "1" in the modulating signal is merely offset in 
frequency relative to that obtained in the case of a data bit of value 
"0", as diagrammatically shown in FIG. 9. 
In FIG. 10A these various data bits are identified by areas that are shaded 
or unshaded according to the value of the bits. 
The user's terminal equipment in this example applies to the CPFSK 
modulated optical carrier intensity modulation between the optical power 
levels Po and P. This equipment therefore modulates the total optical 
power output by the source terminal optical source. 
FIG. 10B shows the timing diagram of the optical signal at the output of 
the user terminal equipment modulator, in other words at point B in FIG. 
8. 
In this embodiment the thresholds sd.sub.1 and sd.sub.2 of the receivers 
D.sub.1 and D".sub.2 may be identical, as shown in FIG. 10B. 
Note that for the two examples of modulation given for transmission in the 
downward direction long sequences of data bits of value "0" do not prevent 
the subscriber sending to the source terminal. This applies even if there 
is no transmission (when there is no data to be sent from the source 
terminal to the user terminal). 
Note also that the modulation examples described above are merely examples 
and that any parameter of the optical carrier supplied by the source 
terminal (intensity, frequency, phase, polarization, etc) could be 
modulated provided that this parameter is different for the two 
transmission directions (except for the intensity, which can be the same 
in both cases) and subject in all cases to the condition that if the 
modulation applied first, for transmission from the source terminal to the 
user terminal, is intensity modulation, the depth of modulation is 
sufficiently small to always leave sufficient power for application of the 
second modulation for transmission from the user terminal to the source 
terminal. 
Note that irrespective of the type of modulation employed, the modulating 
signals can be either digital or analog signals. 
Note also that in the case of digital modulating signals it is not 
necessary for the bit rate of the modulating signal applied second to be 
less than that of the modulating signal applied first and in particular 
that the bit rates of these two modulating signals can be the same.