FR2549661A1 - Device for coupling a terminal by optical fibre to a link channel in closed loop and method of transmitting information within a system using such devices. - Google Patents

Abstract

The coupling device according to the invention for terminals connected to a serial bus with optical fibres fn-1' fn,n+1 in closed loop comprises two electrooptical switches C1, C2 provided with first waveguides G11, G21 coupled to the bus and coupled between themselves f, and second waveguides G12, G22 coupled respectively to a detector PD and to a laser diode LD. Each coupling device DCn operates, depending on the condition of the switches C1, C2, as detector (partial extraction), as regenerator (total extraction and re-emission) or as optical "short-circuit" (passive mode). The method of transmitting within a system employing such coupling devices, comprises steps for setting the energy extraction by the terminals permitting optimisation of the transmissions.

Description

 The invention relates to a device for coupling a terminal to a closed-loop optical fiber link channel and more particularly a serial bus type channel. It also relates to an optimized method of transmitting information between terminals of a transmission system connected to this serial bus.

 Such an architecture raises problems which are specific to it and among which, by way of non-exhaustive examples, those which will be discussed below.

 A first problem to be solved is that raised by the failures of the terminals connected to the serial bus. Indeed, a partial or total failure of one or more terminals should not affect the overall operation of the system, which would have the effect of greatly degrade the reliability rate.

 In some applications, access to the failed terminal is difficult or impossible.

 It is customary to provide the terminals connected to the link channel coupling devices for disconnecting faulty terminals, preferably automatic, while maintaining continuity of the link at these terminals.

 In older systems, it has been used either passive optical coupling elements partial sampling of the light energy carried by the connecting channel, or electromechanical switches.

 The passive coupler elements very quickly limit the total number of terminals that can be connected in series due to the fixed losses provided by each coupler. Electromechanical switches limit the speed of transmission because of their inertia.

 It has been proposed more recently switches having a structure called integrated optics.

 Many examples of achievements have been experimented. These switches, consisting of single-mode waveguides and made on a suitable substrate, generally electro-optical material, for switching the light from one waveguide to another with great efficiency, the crosstalk being better that - 30 dB, with low control voltages, of the order of volt, and high bandwidths of several gigahertz. There can be mentioned, for example, switches made on substrates made of lithium niobate.

Thus, among the various components developed in recent years, in the field of integrated optics, shots are known using two parallel portions of optical waveguide and whose coupling is controlled using a set of electrodes under the action of a variable voltage applied to these electrodes. Such couplers have been described for example in the French patent applications published under the numbers FR
A-2,299,662 and FR-A-2,309,890.

 These switches therefore allow, in particular, a higher transmission rate.

 However, the coupling devices of the known Art as proposed do not make it possible to make the best use of the potential possibilities of the currently available optical elements and, among others, those of optical fibers mono-link modems and electro-optical switches.

 A first limitation is the lack of flexibility of the system architecture. Two types of serial transmission can basically be conceived: the transmission in "bus" mode, in which all the terminals constantly receive the messages conveyed by the link channel, this reception being carried out physically by a partial sampling of energy, the circulation of messages not being interrupted; and transmission in "cascade" mode known by the English name of "Daisy chain", mode for which each message from an upstream terminal is systematically intercepted by each terminal before being re-transmitted to the next terminal or terminal downstream. Once the choice is made, it is usually difficult or impossible to switch from one mode to another.

 A second limitation is encountered in the mode of operation of each terminal. Three cases of operation can be distinguished: an active mode in which the terminal receives the messages by taking a fraction of the light energy conveyed by the connecting channel; a regenerative mode in which the terminal intercepts each message and regenerates it before transmission; and a passive mode in which the terminal behaves as an optical short circuit or "by pass" according to the commonly used English expression, for example following a failure of this terminal which requires the disconnection of the terminal .

 It is also difficult, if not impossible, to obtain these three modes with certain devices of the Known Art.

In addition, in all cases, the presence of a terminal connected to the bus results in energy losses that fluctuate depending on the operating mode. It is then necessary, for energy considerations, operating safety margins and allowable error rate, to impose either a fixed number of terminals or use rigid communication protocols. In any case, a fixed number of terminals does not entirely solve the problems related to fluctuations in energy losses related to the operating mode of each terminal.
Finally, an important consideration concerns the transit time of the signals on the bus, which also varies according to the operating mode of the terminals. Indeed, during a regeneration of the signals, an additional delay is introduced by the electronic circuits of the bus. regeneration.

 All these effects should be taken into account for the adoption of a transmission protocol, which leads to a reduction in the flexibility of the transmission systems.

 The object of the invention is to overcome the difficulties which have just been recalled by allowing at the same time an optimization of the transmissions and an increase in the overall flexibility of the system.

The invention therefore relates to a coupling device for connecting an information processing terminal to an optical fiber link channel for unidirectional serial-type information transmissions, in a closed loop, characterized in that qutil ~ comprises first and second switches in integrated optics comprising a substrate of electro-optical material in which have been formed two light waveguides comprising adjacent sections and provided with voltage-controlled electrodes creating an interacting electric field on both sections of adjacent waveguides so as to cause a controllable transfer, in whole or in part, of light energy guided in puy guides to the other guide, each switch being optically coupled by a first end of a first guide to the optical fiber of the connecting channel, respectively upstream and downstream, and the two switches being coupled mutually r a second end of this guide; optoelectronic means for detecting light energy and for converting this energy into electrical signals, coupled to one end of the second guide of the first switch, optoelectronic means for emitting controllable optical light radiation coupled to one end of the second guide of the second switch, electronic control and signal processing circuits receiving electrical signals from the conversion, generating control signals of the optoelectronic means for emitting light radiation and dialoguing with said terminal;
and in that the electronic circuits further generate voltage control signals applied to the electrodes of the first and second switches so as to selectively realize three distinct modes of connection from said terminal to the link channel: a passive mode, in the absence of control signals, for which all the energy conveyed by the light waves is transmitted from upstream to downstream of the coupling device, an active mode associated with a first state of the control signals for which a first fraction of the energy conveyed by the incident light waves is taken from the first guide of the first switch, coupled to the connecting channel, and transferred to the second guide and detected, the remaining action being transmitted, and a mode of regeneration for a second state of the control signals for which all the incident light-wave energy is transmitted from the first guide of the first switch to the second g uide, regenerated by the optoelectronic emission means and reinjected into the connecting channel- by transfer of the second guide of the second switch in the first guide.

 The invention also relates to an optimized ~ -transmission information process in a system using such a device.

The invention will be better understood and other features and advantages will appear on reading the description which follows with reference to the appended figures and among which:
- Figures 1 to 3 illustrate an example of an electro-optical switch used by the invention;
FIGS. 4 and 5 are two alternative embodiments of a coupling device according to the invention;
FIG. 6 schematically illustrates a network of terminals connected by a closed loop serial bus;
- Figure 7 schematically illustrates a particular step of the method of the invention.

 The device of the invention using, as switching elements, integrated electro-optical switches, it is first necessary to briefly recall the main characteristics and their operation.

 FIG. 1 schematically represents a directional coupler in integrated optics, the operation of which is recalled as an electro-optical switch. This exemplary embodiment illustrated with reference to FIGS. 1 to 3 is given by way of non-limiting example. Two optical waveguides 1 and 2 are obtained in a substrate 4 by creating zones in which the refractive index is greater than that of the substrate. Among others, a conventional method of realization is the diffusion of titanium on the surface of a lithium niobate substrate. The guides 1 and 2, made of the same electro-optical material, have the same width and same thickness. They are.

parallel, at least on a straight portion of length L and are then distant from a value d whose order of magnitude is a few times the wavelength of the optical radiation to be propagated in the guides.

 Metal electrodes E1 and E2 are deposited on the face of the substrate carrying the guides. They completely cover the guides 1 and 2 on the length L. They are connected to a voltage source 3.

The coupler is shown in section in FIG.
specify the pace of electric field lines created when the source 3
delivers a DC voltage V. It is known that these -champ lines are
curves, traverse the guides and the substrate - from an electrode to
the other, and are, at the surface z where the electrodes are arranged,
perpendicular to this surface. Thus, we can consider that the fields
electric actuators acting on the guides 1 and 2 are substantially perpendicular to the surface X, equal in absolute value and opposite signs. The
tangential component is very small, and moreover, of the same meaning for both guides.

In the case of lithium niobate, it is known that the electro effects
Optics are maximal when the electric field is parallel to the c axis of the crystal. Thus, in FIGS. 1 and 2, the axis c has been represented
normal surface E. On the other hand, when the axis c is perpendicular to
electric fields, the electro-optical effects are very small. This would be the
case if the axis c was parallel to the surface i :. In this case, we obtain by
against the maximum effects with an electrode arrangement different from
that shown in Figures 1 and 2. Indeed, if one has. the electrodes E1 and E2 on either side of the guides I and 2, contiguous thereto
but not covering them, the field lines cross the guides in
a direction substantially parallel to the surface.

By repeating the configurations of FIGS. 1 and 2, the operation
of the switch is illustrated by the curves in Figure 3. It is assumed that
light energy is introduced into the guide l only, which guide
is coupled in this case to the connecting channel. On the diagram of Figure 3
are represented the variations of the ratio sl / eI, el being the coupled energy
in the guide 1 and sl being the energy present in this same guide 1 as it spreads, as a function of the distance along the z axis
traveled by the light waves, the origin being taken at the end of the
zone of length L corresponding to the exttemity by which the wave is
injected into the guide.

The curve C1 is obtained when the voltage V delivered by the source 3
is zero. Due to the coupling between the two guides, the energy passes
progressively from guide 1 to guide 2 according to a sinusoidal law, although
that the ratio s1 / e1 varies according to the same law. I1 goes through zero at the end of
length 10 called coupling length at rest where energy transfer
is total. The transfer becomes zero after 2 lo, and so on.

When the voltage V applied to the electrodes is not zero, it
causes by electro-optical effect variations in propagation speed
waves in the guides 1 and 2 substantially of the same absolute value and
contrary signs. A difference is thus obtained between the "phase constant" parameters associated with the two guides, much lower than the value of these "phase constants" in the case where the voltage V is zero.

This difference has two effects: a decrease in the effective coupling length and a decrease in the coupling ratio: The curve C2 is obtained for a low value of V. If this value is increased, it reaches the Sas of the curve C3, case for which the coupling length is equal to 2. In this case, for z - lo, where the transfer is total with
2 1o being
V = O, the transfer is zero. By choosing the length of the substrate rl or
o an odd multiple of lo, one collects, when the voltage V is zero, all the energy in the guide 2 and there is a value of the difference of the phase constants for which on the contrary all energy is collected in the guide 1. We have a switch.

 This type of switch can therefore operate indifferently as an "all-or-nothing" switch, or as an optical element taking a fraction of the energy, depending on the amplitude of the variations of the voltage applied between the electrodes, a property which is exploited in the scope of the invention.

 In addition, the necessary control voltages are small amplitudes, of the order of a volt, and the switching times are also small: times less than 50 ps have been obtained with certain switches.

 Finally, the minimal propagation losses of such waveguides, typically less than 0.5 dB cm, make it possible to envisage the integration of several devices of this type on the same substrate to lead to the configurations of the devices according to the invention which will be detailed in the following.

 FIG. 4 illustrates a first example of a coupling device according to the invention.

 In the example illustrated, the coupling device DCn constitutes the interface between a terminal T n and a single optical fiber link.

The index "n" is an arbitrary number. The total number of terminals will be referenced p. The terminal T n is therefore connected to the optical fiber link via its coupling device DCn between the terminals T 1 and Tn. The connecting fibers fn 1 and fn, n + 1 provide the transmissions, on the one hand, between the terminals Tn-1 and Tn and, on the other hand, between the terminals Tn and
Tn + 1. The flow of information is assumed, in the illustrated example, to flow in the terminal direction Tn-1 to terminal Tn + 1.

Each coupling device DCn comprises a pair of switches C1 and C2 of a type similar to those recalled with reference to FIGS. 1 to 3, an optoelectronic detection unit PD, for example a photodiode, coupled to one of the guides. of the wave G12, the first switch C1, an optoelectronic device for emitting radiant energy LD, for example a semiconductor laser diode, coupled to one of the waveguides
G22, the second switch C2 and various electronic circuits which will be detailed in what follows.

The link fiber fnl I between the upstream terminal and the terminal T is
n coupled to one end of the other waveguide, G11, of the switch
C1 and the optical fiber connecting fn, n + 1 between the downstream terminal and the terminal T n is coupled to one end of the other waveguide G21 of the switch C2.

The second ends of these waveguides G11 and G21 of the switches C1 and C2 are coupled together by an optical fiber f ensuring. the continuity, within each coupling device, of the closed loop optical fiber link. Each switch has a pair of control electrodes, respectively-Ell-E12 and E21
E22.

 In a further variant not illustrated, the two switches C1 and C2 are integrated on a common substrate which comprises a single link waveguide replacing the waveguides G11 and G21 as well as the optical fiber f, as well as two additional waveguides analogous to the guides G12 and G22 and adjacent over specific distances to the single guide for example, each in end regions of the common substrate.

 Finally, the optical coupling between the waveguides G12 and G22 and the opto-electronic organs PD and LD can be effected, as illustrated in FIG. 4, using intermediate optical fibers, respectively D and or directly by the slice of the substrate.

The electronic circuits of each coupling device according to the invention may comprise an amplifier A receiving and amplifying electrical signals delivered by the optoelectronic detection device
PD, signals resulting from the conversion of the detected optical signals; a storage device R, for example a shift register or a circulating memory receiving on an input ER the amplified signals and a control circuit D1 of the optoelectronic transmission unit LD receiving on a serial output SR of the memory R control signals provided by it.

 These signals may be the signals received at the input or their copying, or modified signals which will be re-transmitted by the coupling circuit via the switch C2 on the link channel.

 Finally, each coupling device comprises control circuits CC and signal processing. These DC circuits are electrically coupled by bidirectional transmission links, for example of parallel type, with, on the one hand, the memory R and on the other hand, with the terminal T n by 1T connections. Furthermore, by two links 11 and 12, the DC circuits transmitting control signals of the state of the switches C1 and C2 applied across the electrodes E11 - E12 and E2l - E22.

 A guided wave conveying the optical power information Pd, received by the coupling device DCn, can be processed in three different modes, as has been recalled, depending on the amplitude of the control signals transmitted to the switches C1 and C2 by the connections 11 and 12 and applied to the electrodes Ell E E12 and E21 - E22.

The first mode or passive mode is the mode due to a power failure of the device or a failure. In this case, the two switches C1 and
C2 are in an "on" state for the optical signals - transmitted by the optical fiber fnl 1, n so that these signals pass through without being intercepted by the coupling device and are thus retransmitted via the waveguides G11 and G21 and the optical coupling fiber f of these guides, to the downstream optical fiber fn, nt 1
The second mode is the regenerator mode. In this case, all the energy of the guided wave is taken by the switch C1 and transmitted, via the optical fiber fD, where the optoelectronic detection element PD, converted into signals amplified by the amplifier A and from these signals is extracted the information transmitted by the optical fiber link channel for local operation, if necessary. Finally, these signals are fed back by the modulation circuit D1, the optoelectronic emission element LD, the optical fiber fL and the switch C1 in the downstream optical fiber n, n + 1.

 Finally, in the third mode or active mode, the energy conveyed by the transmitted optical signals is partially taken up by the switch C1. Another fraction of this energy is thus transmitted to the downstream optical fiber fn, n + 1 via the transmission guide. wave G11, the optical fiber f and the waveguide G2l. The fraction of energy taken by the switch Clest as previously, detected and converted into electrical signals for local exploitation thereof.

 In a second variant embodiment, illustrated in FIG. 5, the modulation of the signal retransmitted to the downstream optical fiber f n, n + 1 is directly carried out using the control electrodes E 21 - E 22 of the second switch.

The elements common to the device of FIG. 4 bear the same references and will not be redescribed. The essential difference relates to the fact that modulation member, reference D2 in FIG. 5, transmits, by a link 12, control signals applied to the terminals of the electrodes.
E21 - E22. In this case, the transmission element LD, for example a semiconductor laser diode, is controlled by a direct current generator GI.

The emission is therefore also continuous, the optical signal being modulated at the C2 switch.

 For data transmission starts of less than 1 Gb s-1, the configuration illustrated in FIG. 4 is preferably selected with direct modulation of the laser diode. For higher data rates, the configuration just described in relation to FIG. 5 is preferably selected.

 FIG. 6 schematically represents a closed-loop network using coupler devices according to the invention.

In FIG. 6, p devices have been represented: DC1, DC2,.
... DCp, coupling as many terminals T1 to Tp to the serial link channel at
optical fibers: fl, 2 to fp, each terminal T by its associated coupler device DC n
tion, in each moment, according to one of the modes just described, which allows, at the level of the transmission system shown in FIG. 6, to combine, as has been recalled, two modes of connection of the :: terminals
- the connection in bus mode in which all the terminals are irrigated by the messages,
- and the connection in "cascade" or "Daisy chain" mode in which each message is systematically intercepted by each terminal and relayed thereafter to the terminal arranged in avaL
This is an additional advantage offered by the transmission device according to the invention which, due to the provisions adopted, allows greater operating flexibility.

 To this end, and according to two important aspects, the invention proposes a particular method of transferring data in the form of "packets" of fixed length. To transmit a message, the terminal stores the incoming packet while transmitting its message, it thus positions itself in the regenerative mode and operates in this mode until the return of the transmitted message. At this point, it can either go back to passive mode or send another message if necessary.

This process allows:
to effectively use the serial transmission bus, since several terminals can transmit at the same time,
- to have a control of the good propagation of the message by waiting for its return,
- to allow, according to one of the most important characteristics, the adjustment of the levies of active terminals and the reconfiguration of the system.

 The adjustment procedure according to the invention will now be described in detail in connection with FIG. 7.

 This figure shows two terminals communicating with each other Tx and T connected via their coupling devices DCx and DCy, to the bus alone and separated by (n - 1) passive terminals.

The purpose of the adjustment is to have as few regenerative terminals as possible to speed up the transmission time. However, the number of passive determinals is variable and unpredictable. It evolves constantly over time. Each terminal must act in such a way that the optical power at its output is sufficient for the next terminal given the number of passive terminals to be crossed at the considered location. The decision to regenerate the signal or to only partially pick it up is determined according to the following parameters:
n - 1 = number of passive terminals between two active terminals, Pd = minimum power required for a signal
optical can be detected,
P
e = power transmitted by a terminal,
Pr = power effectively received by a terminator that each terminal must receive at least a power equal to
Given the number of passive terminals, the received signal is attenuated by
an An coefficient, linked to all the connections.Therefore, to obtain
a good functioning, that the power emitted is greater than the value
An.Pd. Under these conditions, a terminal T must regenerate the signal if the
received power-P ry minus the Pdy high power required for its
operation is less than Anpd, ie:
P <AnPd + P
ru dry
For proper operation, each terminal must be
know the value n. Adjustment procedures for this value will be -
-working regularly and after detecting a received power
insufficient on a terminal.

One embodiment of the fit is undo passing through the bus
series of particular messages for which the transmitted power is known.

By measurement of the received power, it is possible to evaluate the value n according to
the following operations:
1) The terminal T memorizes in its control circuits (FIGS.
x
or 6: cc) the number of passive terminals and therefore the value n at a
instant t arbitrary;
2) at a later time f + A t, the number of passive terminals
increases with m, with m> (n-1). Under these conditions, the power received by the terminal T is no longer sufficient;
3) the terminal T sends a particular message to send to the
Tx terminal to transmit at maximum calibrated power.
of this message, the terminal T by measuring the power received,
there
the value of the number of passive terminals between
terminals Tx and T and transmits this value to the terminal Tx y
The time required to perform an adjustment procedure is the time required for the guided light wave to perform two loop turns.

 To fix ideas, we will now specify the main elements used as well as typical performance obtained with the device- of the invention.

 The closed loop transmission line was made from single-mode optical fiber sections (f1> 2 to fp, l) optimized for transmissions of wavelengths of wavelength centered on the 1.3 micrometer value. The typical attenuation of a wired and spliced optical fiber (every 2.5 km for example) is 1 dB / km.

The opto-electronic emission devices (LD) may be laser diodes of the "Ga In As P / In P" type emitting on a wavelength centered on the value 1.3 micrometer. Under these conditions, the modulated optical power coupled to the optical fiber connection is powder 1 mW, that is to say
O dBm.

Optoelectronic detection devices (PD) may be photodiodes of the "Ga In As" type associated with preamplifiers. A typical sensitivity is - 35 dBm at a rate of 560 Mb s, for a predictable error rate of 10-9
The switches in integrated optics (C1 and C2) can be made from lithium niobate substrates in which we have implanted
diffusion of titanium in the substrate, waveguides. Their bandwidth
is several gigahertz, their switching time being less than
50 / ps.For a flow rate of 560 Mb sl, we can therefore provide attenuation
35dB between the optical power emitted by one of the
coupling and the optical power required to detect a train
digital flow rate 560 Mb s, for the allowable error rate l0 9.

 These 35dB can be divided into insertions losses, mainly due to the insertion of electro-optical switches and connectors, about 5dB by insertion; and loss of transmissions in the optical fibers, about 1 dB / km.

 The possible optical fiber link distance between two terminals of course depends on the mode of operation and the rate of security of the desired transmissions.

Nevertheless, distances of several kilometers between two terminals are achievable with a maximum of about 30 km, under conditions that
have just been recalled, between two neighboring terminals
regenerator.

Claims (9)

 l. Coupling device for connecting an information processing terminal (T1 to Tp) to an optical fiber link channel (f1,2 to fp, 1 for unidirectional transmissions - serial type information, in a closed loop , characterized in that it comprises first (C1) and second (C2) switches in integrated optics comprising a substrate of electro-optical material in which have been formed two light waveguides comprising adjacent sections (G11-Gl2 and G21-G22) and provided with voltage controlled electrodes (E11-E12 and E21-E22) creating an interacting electric field on the two adjacent waveguide sections so as to cause a controllable transfer, in whole or in part, guided light energy in one of the guides (G11, G21) to the other guide (gel2, G22), each switch being optically coupled by a first end of a first guide (G11 G21) to the optical fiber the link channel, respe and the two switches being mutually coupled by a second end of this guide; opto-electronic means (PD) for detecting light energy and converting this energy into electrical signals coupled to one of the ends of the second guide (G12) of the first switch (C1), opto-electronic means (LD ) for emitting controllable optical power light radiation coupled to one end of the second guide (G22) of the second switch (C2) of the electronic control and signal processing circuits (CC) receiving the electrical signals derived from the conversion, generating control signals of the light emitting optoelectronic means (LD) and interacting with said terminal (tone) and in that the electronic circuits (CC) further generate voltage control signals applied to the electrodes (Ell-El2, E2l-E22) of the first (C1) and second (C2) switches so as to selectively realize three distinct modes of connection of said terminal (tone) to the connecting channel; passive mode, in the absence of control signals, for which all energy conveyed by the light waves is transmitted from upstream to downstream of the coupling device, an active mode associated with a first state of the control signals for which a first fraction of the energy conveyed by the incident light waves is taken from the first guide (G11) of the first switch (C1, coupled to the connecting channel, and transferred to the second guide (gel2) and detected, the remaining fraction being transmitted, and a regeneration mode for a second state of the control signals, for which all the energy of the incident light waves is transmitted from the first guide (G11) of the first switch (C1) to the second guide (G12), regenerated by the opto-electronic means (LD) transmission and reinjected into the transfer channel transfer of the second guide (G22) of the second switch (C1) in the first guide (G21).  2. Device according to claim 1, characterized in that the opto-electronic detection means (PD) comprise a photodiode and optoelectronic emitting means (LD) cqmprendre a semiconductor laser diode; and in that the link channel is a monomode fiber link channel (f1,2 to fp, 1).  3. Device according to any one of claims 1 or 2, characterized in that it further comprises a modulator member (Dl) receiving said electronic circuits (CC) signals representing the information to be transmitted and generating in response control signals modulated by the signals representing the information to be transmitted acting on the power of the light radiation emitted by the optoolectronic transmission means (LD).  4. Device according to any one of claims 1 or 2, characterized in that it comprises a generator (GV) of continuous control current applied to the electropoptic means (LD) for emitting light radiation so as to generate a wave of continuous optical power and in that it further comprises a modulator member (fl2) receiving from said electronic circuits (CC) signals representing the information to be transmitted and generating in response control signals modulated by the signals representing the information to transmitting and applied to the electrodes (E2l-E22) of the second switch (C1) so as to transmit from the second guide (G22) to the first (G2l) a fraction of the optical power output as a function of the amplitude of the control signals, at rhythm of the modulation of these signals.  5. Device according to any one of claims 1 to 2, characterized in that the first (C1) and second (C2) switches are formed on a common substrate.  6. Device according to claim 1, characterized in that said electronic circuits (CC) comprise a storage device (R) electrical signals from the detection and conversion of optical signals transmitted by the connecting channel.  A system for transmitting unidirectional information over an optical fiber link channel (f 2 to PAL) of the closed-loop serial type comprising terminals (T1 to Tp) for processing this information, characterized in that comprises, for each terminal (Tl to Top), a coupling device (DC1 to DCp) according to any one of claims 1 to 6, and in that said electronic circuits (CC) of the coupling devices comprise means for connect controllably to the link channel, the terminals in one of the following two modes: the bus mode for which all terminals are traversed by the same message and the cascading connection mode for which each terminal (Tn) intercepts a message transmitted by the terminal (Tn, l) arranged upstream on said closed loop and retransmitted to the terminal (Tn + 1) arranged downstream and in that these means act on the state of said first (Cl) and second (C2 ) switches in int optics égrée.  8. Method for transmitting information in a system according to claim 7, transmitted by at least one terminal (Tx) connected by its coupling device (DCx) to said optical fiber link channel, characterized in that it comprises the following steps:  - generating by each transmitting terminal (Tx) a fixed length information message;  positioning each transmitter terminal (Tx) and its associated coupling device (DCx) in the regenerator mode;  recording by these terminals (Tx) and their associated coupling devices, incident messages for the time necessary for transmission of said message to be transmitted;  waiting for a time necessary for propagation of this message over the length of said closed loop of the optical fiber link channel;  - control of the good propagation of the message;  - and positioning to another mode of operation in the absence of new message to be transmitted.  9. Method according to claim 8, characterized in that it further comprises the permanent evaluation of the optical energy withdrawals by the passive mode terminals disposed between an upstream terminal (Tx) and a downstream terminal (Ty) communicating between them and the adjustment of the transmitted optical power by the following operations:  storage by the upstream terminal (Tx) of the instantaneous number (n) of terminals operating in passive mode arranged between the upstream (Tx) and downstream (Ty) terminals ::  emission of -message by a light wave of determined optical power transiting ~ by this number of terminals operating in passive mode;  - evaluation by the downstream terminal (T y) of the received optical power and comparison of this power with a determined minimum threshold;  transmission by the downstream terminal of a particular message -when the detected optical power is below this threshold;  - reception and detection of this message by the upstream terminal (tex)  transmission by the terminal (Tx) of a calibrated power optical signal; ;  - Reception and detection of this signal by the downstream terminal (Ty) and evaluation from the detected optical power of a new number of terminals operating in passive mode arranged between the upstream terminals (Tx) and downstream (T;  - transmission of a message by the downstream terminal (Ty) comprising the information of this number;  reception and detection of this message by the upstream terminal (Tx) and updating of the memory of the number of terminals operating in passive mode;  and transmitting information messages conveyed by an optical power light wave determined by this new number.