GB2125647A

GB2125647A - Active optical transponder system

Info

Publication number: GB2125647A
Application number: GB08222968A
Authority: GB
Inventors: Peter Sothcott
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-08-10
Filing date: 1982-08-10
Publication date: 1984-03-07
Also published as: GB2125647B

Abstract

An active optical communication system uses an electromagnetic beam (preferably infra-red) from a mobile interrogation station to set up a communications path to a wanted one of a number of other terminals. The beam is aimed at the wanted terminal and modulated by a coder (1) with that terminal's code. At the wanted station the beam is picked up by infra-red diodes (A,B), and the code decoded and checked by a decoder (3). If it is correct for that station, a drive motor (4) directs the receiving diodes (A and B) towards the interrogator for maximum response. In addition a mirror (5) which directs the return beam from the terminal's transmitter (6) to the interrogation terminal is positioned with the receiving diodes. Thus the communication path is set up. <IMAGE>

Description

SPECIFICATION Active optical transponder system This invention relates to an optical communication system for setting up two-way communication between a mobile terminal and a number of other terminals.

According to the invention, there is provided a communication system, which includes an interrogation terminal and a plurality of other terminals, in which to set up communications between the interrogation terminal and said other terminal, a beam of electro-magnetic radiation is directed from the interrogation terminal to the wanted one of the other terminals, in which the beam is modulated with a code identifying the wanted terminal, in which at the terminal whereat a said beam is directed the code modulated on that beam is checked as to whether it is the code for that terminal, and- in which if the code is the one for that terminal the receiving means and the transmitting means thereat are adjusted so as to be aligned in a direction appropriate to the interrogation terminal, whereafter communication over the modulated beam is set up between the interrogation terminal and the wanted other terminal.

According to the invention, there is also provided an optical communication system, which includes an interrogation terminal and a plurality of otherterminals, in which to set up communications between the interrogation terminal and said other terminal, a light beam is directed from the interrogation terminalto the wanted one of the other terminals, in which the light beam is modulated with a code identifying the wanted terminal, in which at the terminal whereat a said light beam is directed the code modulated on that beam is checked as to whether it is the code for that terminal, and in which if the code is one for that terminal the optical receiving means and the optical transmitting means thereat are adjusted so as to be aligned in a direction appropriate to the interrogation terminal, whereafter communication over the modulated light beam is set up between the interrogation terminal and the wanted other terminal.

Thus the basic principle is that the interrogation, i.e. the mobile terminal, radiates a signal coded to indicate the identity of the 'called' station. This, and only this, station responds by emitting its reply, and the channel thus set up is kept open until told to close down by reception of the appropriate signal from the interrogator. A typical use would be to establish a temporary link to a remote sensor or monitor, and to continue to observe its indications for as long as required. By modifying the interrogation code, the user can select any one of a number of such monitoring channels at the remote site. The interrogating signal may also carry information so that, in effect, a two way communication channel is set up and maintained as long as required.

In the interest of security, it is desirable to ensure that the interrogating signal is transmitted via as narrow a beam as possible, that the reply signal's beam is similarly limited in width and is returned only in the direction of the interrogations, and that the device is as small and inconspicuous as possible.

Such desiderata are attainable by the use of light, particularly infra-red.

In the present system, the interrogating light signal is modulated by a suitable code, e.g. that used in a radio pager system, and is then collimated into a narrow beam by a simple optical system. This is directed at the responder, which receives it in a similar optical system, recovers the modulation, examines the code and, if it is the code for that responder's terminal selects the channel of information to be sent (if this facility is provided) and then activates the transmitting element. In addition to decoding the incident beam, the receiver also measures the angle of arrival of the incident beam, and swings the angle of a similarly narrow angle reply beam until it points in the same direction, i.e.

towards the interrogator. At the interrogator the received signal is "captured" in a suitable optical system, and demodulated to recover the transmitted information. Such sharply directive light beams can be readily formed.

The accompanying drawing shows schematically one way of implementing the above-described principle. The operation of the interrogator's coder and transmitter are self-evident, and a very simple optical system - e.g. a single lens or concave reflector, is adequate to produce a reasonably narrow beam. At the responder the incoming beam is split into two as shown. Each portion of the incoming beam is received by the photodiodes A and B, which are optically, if not physically, capable of rotation by the same motor, and have angular directivities in opposite directions.

The diode's outputs are equal when they are pointing towards the incident light, but change in opposite senses if they are displaced from this position. Thus, it is easy to detect the magnitude and sense of the change, and to drive the motor to seek the equi-signal position, and so point the diodes in the direction of the incident light. As this is not a null-seeking device, there is always a substantially constant level of total signal to operate the decoder.

In practice, the diodes themselves need not be moved: if they can be made reasonably omnidirectional, e.g. by using several, pointing in different directions, then the steerable directivity can be obtained by rotating a graded attenuator in front of each. The motor also steers the beam of the responding transmitter into the correct direction: as shown, the beam is a simple, fixed, optical system and the steering is done by a small mirror.

Thus the sequence of events is: (a) the code of the addressed responder is selected by the coder 1 at the interrogator, together with the channel of information required (if this facility is provided); (b) the interrogator is visually aligned by a visual sight 2, similar to a rifle sight, on the responder and switched on; (c) at the responder, the signals from each diode A - B are added and examined by a decoder 3. If the code is that for the responder then the response is initiated; (d) the motor 4 turns the directive patterns of both receiving diodes until the signal from each is the same level.They are now aligned with the incident light, which in the present system is infra-red; (e) the mirror 5 associated with the interrogated terminal's transmitter rotates with the diodes, steering the transmitter's optical system to face the interrogation; (f) the appropriate channel of information already having been selected, the transmitter 6 is switched on, and transmits information until the 'stop transmitting' code is received, or the incident light is switched off.

The nature of the link used for the return signal depends on the type of traffic involved: in this case a free space, narrow beam, optical link is used. It uses a transmission terminal actually designed for optical fibre links, the transmitter's light output being taken by fibre to the focus of the transmitting optical system, a similar arrangement applies at the interrogator. Because the light source and 'sink' formed by the fibre ends are so small, a very precise beam can be formed from a very simple optical system. A link from the interrogator to the transponder can be added if required.

The techniques used by 'direction finder' are given by way of example, and one slight complexity would be to provide the ambiguity resolution necessary to obtain omnidirectional coverage, although the mode of use will often not require a full 360". Furthermore, mechanical rotation is not always desirable. However, other arrangements are available which may not have these limitations. More difficult might be to obtain a sharply directional and steerabie transmitted beam from totally fixed elements. Techniques for electrically steering light beams are available, but, at present, only fairly small angles seem obtainable.

Thus the best substitute might be an array of separate, fairly narrow beam, transmitting elements, that element being chosen which points most nearly in the required direction. Although this has the obvious disadvantage of permitting spill-over into directions other than that of the interrogations, it may be adequate for some applications.

Claims

1. A communication system, which includes an interrogation terminal and a plurality of other terminals, in which to set up communications between the interrogation terminal and said other terminal, a beam of electro-magnetic radiation is directed from the interrogation terminal to the wanted one of the other terminals, in which the beam is modulated with a code identifying the wanted terminal, in which at the terminal whereat a said beam is directed the code modulated on that beam is checked as to whether it is the code for that terminal, and in which if the code is the one for that terminal the receiving means and the transmitting means thereat are adjusted so as to be aligned in a direction appropriate to the interrogation terminal, whereafter communication over the modulated beam is set up between the interrogation terminal and the wanted other terminal.

2. A system as claimed in claim 1, in which the beam is a beam of millimetre wave radio beam.

3. A system as claimed in claim 1, in which the beam is a light beam.

4. A optical communication system, which includes an interrogation terminal and a plurality of other terminals, in which to set up communications between the interrogation terminal and said other terminal, a light beam is directed from the interrogation terminal to the wanted one of the other terminals, in which the beam is modulated with a code identifying the wanted terminal, in which at the terminal whereat a said light beam is directed the code modulated on that beam is checked as to whether it is the code for that terminal, and in which if the code if the one for that terminal the optical receiving means and the optical transmitting means thereat are adjusted so as to be aligned in a direction appropriate to the interrogation terminal, whereafter communication over the modulated light beam is set up between the interrogation terminal and the wanted other terminal.

5. A system as claimed in claim 4, in which the light beam is an infra-red beam.

6. A system as claimed in claim 5, in which the receiving means at an interrogated terminal includes a plurality of infra-red diodes, and in which said adjustment is effected by so setting the arrangement as to maximise and to equalise the diodes' response.

7. A system as claimed in claim 6, in which the transmitting means at a said other terminal includes a light-emitting infra-red diode whose output is directed via an optical fibre and a lens on to a mirror, and in which the setting of the mirror is adjusted to make said adjustment of the setting of the transmitting means.

8. An optical communication system, substantially as described with reference to the accompanying drawing.