GB2331882A

GB2331882A - Modular infrared data transmission system

Info

Publication number: GB2331882A
Application number: GB9719147A
Authority: GB
Inventors: Stephen Patrick
Original assignee: CABLE FREE SOLUTIONS Ltd
Current assignee: CABLE FREE SOLUTIONS Ltd
Priority date: 1997-09-09
Filing date: 1997-09-09
Publication date: 1999-06-02
Also published as: GB9719147D0; GB2331882A9

Abstract

A versatile infrared transmission system is described using aligned transmitter/receiver units which send and receive signals along a line of sight using a modulated beam of infrared radiation. By adopting a modular approach to the design of the units, they can be adapted, by the use of exchangeable interface cards, to operate over short or long range in a flexible manner to transmit either digital or analogue signals to various standards without changing any optical or other system parameters. Additionally, the divergence angle of the optical transmitter and the field of view of the receiver are chosen to be similar, so as to minimise beam alignment problems.

Description

2331882 VERSATILE INFRARED DATA TRANSMISSION SYSTEM This invention relates

to a versatile infrared data transmission system. It is based on a novel combination of technologies or application rather than uniqueness of individual concepts.

According to the present invention we provide a system for communicating data by modulated beams of infrared radiation using aligned transmitter and receiver units where divergent transmitted light beams and receivers of similar field-of-view are employed to solve the inherent problems of beams alignment and achieve reliable communication. The divergence of the beams and receiver field-of-view can be dynamically adjusted by an onboard microprocessor. A modular approach to design allows scaleability of specific embodiment by variation of optical component specification for long or short range systems, and the option for a flexible manner in which the emitted light is modulated permits further applications. Furthermore, the use of interchangeable data interface cards gives rise to a flexible communications system which can thus be configured to support binary data at a wide range of data rates and standards or arbitrary analogue signals by use of an FM modulator. Alignment aids make the system quick to install and align. An optional in-built Bit-error- rate tester allows measurement of link performance. Diagnostic tools aid service and performance monitoring, and in-built thermal management gives protection against variations in weather.

By way of example, an embodiment of the invention is illustrated in the accompanying drawings in which Figure 1 is a diagramatic illustration of a transmitter reciever unit., and Figure 2 is a detailed functional diagram of the same.

As shown each unit comprises an optical transmitter, optical receiver, processing electronics, data interfaces, control electronics, power supply and mechanical housing. Two units are required to make a complete link which can operate as a bi-directional communications system.

Controlled Beam Divergence. The transmitted light beam is emitted from the unit at a controlled divergence which can be adjusted dynamically by a microprocessor. In the preferred embodiment this may be between 0.5 and 5 degrees of beam divergence, which is 1 effected by moving the source of radiated light away from the focal point of a lens. The receiver field-of-view can similarly be adjusted by the same controller. In the preferred embodiment this may be between 0.5 and 5 degrees of acceptance angle which similarly is effected by moving an optical detector away from the focal point of a lens. The combined benefit of these systems is that: (i) the requirement for pointing accuracy of the units to achieve communication is relaxed; (ii) reliability of communication is increased by negating the 'wandering' of the beam through atmospheric thermal effects by allowing a relativelylarge beam footprint; (iii) the system can rapidly be configured for different applications or operating ranges; (iv) automatic adjustment of the transmit and receive beam parameters could be made to optimise link performance dynamically.

Scaleability. To allow for a family of products, or specific embodiments of different performance characteristics, the optical and electro-optical design can be scaled. Most obviously, in a high-performance system, largeaperture lenses for transmit and receive are used, and for lowerperformance, smaller ones. In the preferred embodiment for a highperformance system, a number or laser diodes are used, with the modulated light combined using fibre optic cables to a single emitting point. The preferred embodiment for a lowerperformance system uses a single laser diode, still with a coupling fibre to ensure optical eye safety of the overall system. The means for modulating the laser diode is an electronic circuit, which a number of identical circuits are used for a multiplelaser system, and a single 20 circuit for a single-laser system. In both preferred embodiments, the optical detector (an avalanche photo-diode) and associated electronic circuits used in the receiver are common irrespective of size of receiver lens.

Flexibility of light modulation. In a specific embodiment of a multipielaser system, provision is made for allowing modulation of each laser separately with a different electrical signal (such as separate analogue RF carriers such as those used in radio systems) with the addition of a device for optical mixing of the resultant independent optical channels. This technique overcomes inherent difficulties known to those skilled in the art in overcoming distortion when modulating either a single high-powered laser diode (due to device non linearity) or multiple smaller diodes (due to non-perfect matching between devices) with a single wide-bandwidth analogue signal. In a specific embodiment of a low- power single-laser system, such separate RF carriers can be electrically combined and subsequently applied to the laser diode in the conventional manner.

Interchangeable Data Interface cards. In all preferred embodiments of the system, a versatile optical communications system is configured for any of a number of digital data interfaces or arbitrary analogue signals by insertion of an appropriate interface card, without change to any optical or other system parameters. (i) Digital data streams may be transmitted directly if suited to the native mode of the system, the specific embodiment of which offering transparent communication of balanced binary data streams from 80kbps to 2 greater than 50OMbps. Physical connection to twisted pair or coaxial copper cables or fibre optic links are provided on various interfaces. (ii) An additional 'regenerator sub-card which can be used for certain data formats allows data received from the optical receiver to be 're generated' in that timing degradation known as'jitter' is removed from the signal. In the specific embodiment this 'regenerator can automatically detect and operate at one of four pre-determined rates of data transmission. (iii) Additional processing can be performed on a data interface card to enable transmission of multiple data signals or data formats unsuitable for direct use. A specific embodiment of a certain interface card allows multiple video images with audio channels to be separately digitised and transmitted across the communications system as a single high-speed digital data stream. (iv) The use of a square-wave FM modulator to convert an incident signal to a form that can be transmitted, and a corresponding demodulator to reconstruct the original signal permits communication of arbitrary analogue waveforms, such as video signals. Auxiliary information (such as low speed data or audio channels) can be added on RF sub-carriers to an analogue signal (in manner well-known to those skilled in such art) prior to FM modulation, and likewise be recovered after demodulation. The above schemes allow preferred embodiments of the optical communications system to be configured to carry analogue or digital data signals or combination of signals simply by insertion of an appropriate interface card.

Link Alignment Aids. Like any free-space communications system of directional nature, optical systems require alignment before they will operate. In all preferred embodiments of the system there are five elements to achieve this: (i) A mechanical alignment sight is fitted which permits coarse alignment by sighting the remote end of the communications link visually. In the preferred embodiment this is a tube running the length of the unit, mounted just above the transmit lens, which offers a 0.5 degree field-of-view. (ii) An FM radio circuit is incorporated which when used with microphone and headphones allows the two operators to talk across the communications link, and listen to tones sent from the remote unit indicating received signal strength at the opposite end. This circuit is far more sensitive than the high bandwidth digital services which the link will carry, and therefore allows users to optimise a partially-aligned link without external radios or telephones. (iii)A10segmentbar-graph meter indicates received optical signal strength. (iv) The microprocessor can offer a more accurate reading of the received signal strength, measured in the industry-standard dBm (decibels referenced to one milliwatt), and can offer display and storage of peak strength encountered during alignment for optimisation purposes. (v) As mentioned above, the transmitted beam divergence and receiver field-of-view can be widened, which speeds initial alignment.

Optional Bit-Error-Rate tester. All preferred embodiments of the system allow insertion of an additional card to perform bit-error-rate measurement of an installed communications link. This card is selected instead of the regular data traffic and the microprocessor can display a 3 count of the number of errors encountered during a given period. The facility to continuously or periodically monitor signal quality and detect periods of poor performance (for example due to change in weather) and such process can be automated by the processor. The system also allows remote monitoring of the link performance. The flexible data card format allows bit-error-rate to be measured across a link even if the link traffic is non-digital, for example if an FM video encoder 1 decoder is being used.

Diagnostic tools. The preferred embodiments contain several features which aid problem diagnosis and maintenance: (i) An external RS232 link allows the microprocessor to remotely display operational data and allow changes to all settings such as beam divergence, bit-error-test or signal loop-back mode. (ii) On-board analog ue-to-d ig ital converters allow the microprocessor to measure and report out-of- tolerance conditions such as laser temperature, system temperature and power supply voltage. (iii) A signal loop-back mode enables an incoming optical signal to be received and returned along the transmit path without processing, which aids verification of link operation. (iv) A real-time-clock with battery back-up allows the microprocessor to keep a log of system operating hours and laser operating life for servicing purposes.

Thermal Management. A built-in heater with electronic sensor and thermostatic control is installed behind the optical window, designed to de-mist the window and thaw ice build-up if necessary. A Peltier-effect heat-pump with calibrated sensor and controller removes heat from the lasers to ensure long operating life especially in hot environments.

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Claims

1. A system for communicating data by modulated beams of infrared radiation using aligned transmitter and receiver units where divergent transmitted light beams and receivers of similar field-of-view are employed to solve the inherent problems of beams alignment and achieve reliable communication, and wherein each unit is constructed with means to receive at least one of a plurality of interchangeable data interface cards to enable configuration of the system to transmit and receive digital data or arbitrary analogue signals without change to the optical or other system parameters.

2. A system according to claim 1 in which the transmitter and recever units can be dynamically adjusted by a microprocessor so as to adjust the divergence of the beam and the field of view respectively.

A system according to claim 1 or 2 in which the transmitter and receiver units are independent modular units to allow scaleability of specific embodiment by variation of optical component specification between the modular units for long or short range systems.

4. A system according to any one of claims 1 to 3 in which the modulation of the emitted light may be varied.

5. A system according to any one of claims I to 4 in which the interchangeable data interface cards are adapted to configure the system to support binary data transfer over a wide range of data rates and standards.

6. A system according to any one of claims I to 4 in which the interchangeable data interface cards include an FM modulator or demodulator to enable the transmission and reception of video signals.

7. A system for communicating data substantially as hereinbefore described with reference to the accompanying drawings.

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