GB2195508A - Optical fibre transmission system - Google Patents

Abstract

In a duplex optical mobile transmission system in which the cable link is likely to be connected and disconnected frequently, a safety interlock system is provided which will automatically prevent optical transmission in the event that the link is disconnected or otherwise broken, so that operating personnel are automatically protected from the dangers of eye damage from the optical source in the system. Each terminal (T1, T2) has a "received signal fail" detector (3) which disables (4) the optical transmitter (2) if the received optical signal is absent. An initializing arrangement allows override of the disabling for a short period. <IMAGE>

Description

SPECIFICATION Optical fibre transmission system This invention relates to an optical fibre transmission system, particularly a mobile system or other non-permanent installation in which connection and disconnection of the system is likely to occur.

In optical fibre transmission systems, particularly on a duplex fibre optic system which is likely to be frequently connected and disconnected, there is a possibility of danger to operating personnel. As an example this can occur in a tactical military system.

In such systems where expanded beam connectors are normally used, the expanded beam connectors concentrate the optical energy in to a narrow beam, and this is shown schematically in Fig. 1. Here two connector parts A and B associated respectively with receive and transmit fibres and have focussing and collimating lenses which expand the beams for coupling together via the parallel light beam.

Such a beam is largely unattenuated with distance and the infra-red radiation emitted by such a system can be a safety hazard to operating personnel, especially when a laser is used as the electro-optic transmissive element, or when a light emitting diode is used at low temperatures when the power increases dramatically.

In a system such as shown in Fig. 2, the operating personnel are being continually exposed to this risk when setting up and dismantling the system. The system comprises a vehicle containing optical line transceiver equipment with an expanded beam connector like the parts A and B of Fig. 1 but duplex. A fibre optic cable link having at each end a part of the connector AB, and a fibre optic cable connects with another transceiver equipment (not shown) but similar to that in the vehicle.

It could itself be in another vehicle.

The only known method presently used of providing protection to the operating personnel, are mechanical shutters on the optical connector arrangements that snap shut when the connectors are separated. A similar example in the purely electrical field is common in the domestic household where 13amp ring main plugs and sockets are similarly protected when the plug is removed from the socket. However such arrangements, particularly in more hostile environments than are normally encountered within the home, make them complex and prone to jamming by dirt and grit. Secondly of course such a mechanical system will not protect operating personnel in the event that the transmission cable is damaged.

It is an object of the present invention to overcome this problem and provide an alternative solution.

According to the present invention there is provided a transceiver in or for an optical transmission system, said transceiver comprising an optical transmission element for transmitting information to the system and an optical receiving element for receiving information from the system, and an automatic feedback arrangement sensitive to the presence of a received optical signal, said feedback arrangement being responsive to a change in level of the signal indicative of a break or disconnection, to prevent transmission from the transmission element.

In order that the invention can be clearly understood reference will now be made to the remainder of the accompanying drawings, in which: Figure 3 shows a block diagram of a duplex fibre optic system according to an embodiment of the present invention.

Referring to Fig. 3 of the drawings there is shown two terminals T1 and T2 which are substantially identical, each having an optical receiver 1 and transmitter 2. The optical receiver 1 is coupled to a "received signal fail" detector 3 which will raise an output "flag" when an optical signal is not being received.

This will provide an alarm condition to tell the operator that the system has a fault. This detector can comprise a photodetector, amplifier and an electromechanical relay or an electronic switch, although some transmitters have in any event a "disable" connection. A control signal is also provided on the optical transmitter 2 to disable the optical output therefrom when this input "flag" is raised. If the "received signal fail" representing the "flag" is connected to the "transmit disable" input 4, the optical output will be cut off as soon as the transmission path is broken.

During operation the most likely causes of the transmission path being broken are: 1. disconnection by an operator 2. mechanical damage causing breakage of the fibre of the cable.

This system will protect the operator from being exposed from any of the radiation in both the above cases.

With the embodiment described, as with most optical systems, a signal i.e. a modulation product (binary ones or zeros) is present even when no data is being sent, and because of this fact there is no danger of the feedback system not working during periods when the system is in an idle state. When the system is initially connected up however, the interlock or feedback system will need to be overridden for a fraction of a second otherwise the transmitter will be permanently inhibited. This will be sufficient time for the system to settle down. This override can be provided by several means, as follows. Firstly an external switch as shown, termed "initialise", and a timer limits the active time of this switch for safety reasons.Secondly the override can be disabled for a short period during "power up". This will require the equipment to be switched on and off to initialise the system.

Thirdly the interlock can be periodically overridden, automatically for a fraction of a second. With this arrangement, when the link is broken, the interlock will be automatically initialised when the link is remade. Because the override is only active for a fraction of a second, there is no danger to the operator.

The present invention is particular suitable for solving the problem of eye safety particularly but not exclusively when using expanded beam connectors. It is also especially applicable to systems using high power lasers.

It is also applicable to duplex systems operating over a single optical path or physically separate "go" and "return" paths, provided the paths are coupled with the same connector.

Claims (4)

1. A transceiver in or for an optical transmission system, said transceiver comprising an optical transmission element for transmitting information to the system and an optical receiving element for receiving informa- tion from the system, and an automatic feedback arrangement sensitive to the presence of a received optical signal, said feedback arrangement being responsive to a change in level of the signal indicative of a break or disconnection, to prevent transmission from the transmission element.

2. A transceiver or system as claimed in claim 1, wherein the transceiver includes an - initialising arrangement for overriding the feedback system.

3. A transceiver or system as claimed in claim 2, wherein the initalising arrangement comprises a timing device for limiting the initialising period to a fraction of a second.

4. A transceiver or system substantially as hereinbefore described with reference to and as illustrated in Fig. 3 of the accompanying drawings.