GB2256337A - Thermally stabilised optical devices - Google Patents

Abstract

An optical data communications system, wherein optical apparatus in the form of optical transceivers 14 are thermally stabilised by mounting the same on a thermally conductive wall 20 of a plenum chamber wherethrough a temperature regulated air flow may be passed. The transceivers are part of an optical communications system comprising a fly-by-fibre control for an aircraft. The transceivers 14 connect to processing circuitry 26 via coaxial inner conductors 34 and connectors 24. <IMAGE>

Description

Thermally Stabilised Optical Devices This invention concerns optical devices used as sources in optical data transmission. Optical data transmission has considerable advantages over conventional electrical data transmission systems particularly in harsh environmental conditions which exist in aircraft or similar environments.

Optical transmitters of such systems have, heretofore, generally been in the form of Light Emitting Diodes (LED's) but such transmitters are unable to operate at the very high speed data rates (e.g. > 5OOMbits/sec) towards which data transmission requirements are moving. Further, the requirement is for larger numbers of optical ports for inter-module communication. These requirements dictate the use of lasers as optical transmitters.

Lasers, whilst providing greatly enhanced optical performance, suffer from adverse temperature performance.

The problems associated with a rise in operating temperature include 1) An exponential rise in threshold level; 2) A decrease in lasing slope efficiency; 3) A shift in centre optical wavelength; and 4) A dramatic decrease in device lifetime.

These adverse effects can be controlled, beneficially, if the operating temperature of the laser is also controlled. For optimum efficiency, the laser, the optical transceiver unit, or other optical device should itself be temperature controlled independently of the ambient temperature and of any heat generated by the device.

A known form of temperature stabilisation is the use of a Peltier device controlled by a feedback loop to heat or cool an associated device. For example, a laser may be mounted in direct thermal contact with a controlled surface of a Peltier device. In such circumstances, control of the laser operating temperature can be effected over a wide range (e.g. -40 to +700C). In order to effect such control, a Peltier device may need a current of 1 Amp. In the particular case, a military aircraft may have a large number of laser transmitters (e.g. > 100) serving the optical data bus. It is then not only extremely expensive but also impractical to provide Peltier devices to cool each such laser transmitter. The maximum power consumption is prohibitive.Additionally, problems associated with avionics such as mechanical stresses, wide temperature variations and constant switching on and off of equipment for servicing and repair, arise. Further, the reliability of the optical devices themselves is reduced in dependence upon the extra circuitry associated with the Peltier devices.

It is an object of the present invention to provide thermally stabilised optical devices in a manner wherein the aforesaid disadvantages are overcome.

According to the present invention, there is provided an optical data transmission system comprising a plurality of optical devices, a plenum chamber wherethrough a temperature stabilised fluid may be passed, the plenum chamber having one wall of a thermally conductive material whereto are mounted said optical devices in thermal contact with said wall.

The plenum chamber may be defined by a hollow aluminium plate to one surface of which the optical devices are mounted.

Preferably, the other surface of the aluminium plate comprises a back plane having connectors whereto line replacement modules may be attached. The backplane may be in the form of a printed circuit board providing a digital bus and power distribution lines.

Miniature co-axial connectors preferably extend through the plenum chamber for electrically connecting the optical devices to the replaceable line modules.

One or more optical data transmission fibres may extend along that surface of the plenum chamber whereto the optical devices are mounted. Each optical device may connect to the optical fibres. Where more than one optical device is connected to a single fibre, wavelength division multiplexing means may be provided for combining or splitting of data from or to the respective optical devices.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a diagrammatic representation of an aircraft wherein thermally stabilised optical devices, according to the present invention, are deployed; Figure 2 is a diagrammatic underside view of a plurality of thermally stabilised optical devices according to the invention; Figure 3 is a diagrammatic mid-section of the arrangement shown in Figure 2; Figure 4 is a fragmentary diagrammatic perspective view of part of the arrangement shown in Figure 2; and Figure 5 is a graphical representation of the power output of a laser diode of an optical device according to the present invention exemplifying the advantageous thermal stability achieved.

Referring firstly to Figure 1, there is shown a diagrammatic representation of a military aircraft 50. The aircraft is adapted to "fly-by-wire" and control means 52, each comprising at least a receiver and an actuator, when instructed, operate control surfaces of the aircraft 50 such as ailerons 54, rudder 56, elevators 58 etc. Connectors 60 are provided for feeding the instructions to the control means 52 from a master control 62 situated in an avionics bay 64 of the aircraft 50. The connections 60 are, in the case of the present invention, optical fibres and the aircraft may then be described as a "fly-by-fibre" aircraft.

As is conventional, the aircraft has an engine 66 from whence an air bleed 68 supplies air to an environmental conditioning system 70 which supplies conditioned air, for example, to the cockpit space of the aircraft.

Referring now to Figures 2, 3 and 4, there is shown a diagrammatic representation of thermally stabilised optical devices, according to the invention, located in the avionics equipment bay 64 of, for example, the military aircraft 50.

In this aircraft, the remote equipment is controlled and the status thereof is monitored by remotely located control means 52 and sensors each including an optical transceiver.

Control and status signal, in the form of serial data bits are transmitted at high data bit rates along one or more optical fibres, the connections 60, located in conduits originating at (or terminating at) an optical conduit 10 in an avionics equipment bay 64 of the aircraft 50.

As is well known, each fibre connection 60 may carry data signals from and/or to a plurality of remote equipments, the data signals being multiplexed onto and split from the fibre using wavelength division or time division multiplexing, appropriate multiplexors 72, combiners and/or splitters 74 being provided for each remote location and/or, as necessary, for each of a plurality of optical transceivers 14 (see Figure 4) in the avionics equipment bay 64. Each optical transceiver 14 may be connected into the system by a single fibre 12, as shown, or by a plurality of fibres represented at 12a.

Each transceiver 14 comprises an optical transmitter 76 having an electro-optical source such as a laser diode, and an optical receiver 78 having a detector such as a PIN photodetector.

It will be appreciated that there may be in excess of one hundred of these optical transceivers 14 each monitoring or controlling a respective remote equipment. The transceivers 14 are thus mounted, at a relatively high density, on a plate 16. If all, or a majority of the optical transmitters of the transceivers 14 include a laser diode, it is essential that their operating temperature be stabilised and that heat generated thereby, and by the elements associated therewith, be removed.

The plate 16 is of a material of high thermal conductivity such as aluminium. The optical transceivers 14 or, where separate, the optical transmitters 76 thereof, are mounted to the plate 16 using a thermally conductive paste 80 to ensure good thermal contact therewith so that the plate 16 acts as a heat sink.

In accordance with the present invention, the plate 16 constitutes one surface of a plenum chamber 18 wherethrough a cooling fluid such as air from the environmental conditioning system 70 may be passed. The plate 16 preferably constitutes one surface of a hollow rectangular-sectioned aluminium extrusion. Cooling ribs or fins 82 may be provided on the inner surface of the plate 16 projecting into the plenum chamber 18.

The opposite surface 20 of the plenum chamber 18 supports a printed circuit board 22 providing power and other low frequency digital signal circuits, arranged as a backplane whereto connectors 24 for line replacement modules 26 of processing and control circuitry, may be mounted.

Signal connections between the connectors 24 and the circuit board 22, on the one hand, and the optical transceivers 14 on the other hand, may be made by conventional co-axial connecting links 28 extending through the chamber 18. Such connectors are known and comprise male inners 30, for example, projecting from the rear of each optical transceiver 14 which mate with and project through female outers 32 mounted on and projecting from the rear surface of the backplate PC board 22. Conventionally, the female outers are grounded and the male inners carry data signals and are soldered or otherwise connected at their projecting ends to the circuit board 22 or the connectors 24 as appropriate. Such co-axial connectors may have a diameter of the order of 2mm or less and, in consequence, do not obstruct the flow of the coolant through the plenum chamber 18.

Additionally, insulated power connectors 34 are provided between the optical transceivers 14 and power busses on the circuit board 22. As will be seen in Figure 2, power connectors 34 have been shown only for the first pair of transceivers 14 and co-axial signal connectors 30, 32 have been shown only for the next three transceivers 14 but it will be appreciated (and is shown in Figure 2) that such connections are provided, as necessary, for each of the transceivers 14.

As described above, many modern military (and civil) aircraft have the ability to provide a constant supply of relatively clean air at constant pressure and at a preselected temperature via an air-feed from the engines.

Such supply may easily be tapped to provide the flow of cooling fluid through the plenum chamber 18 to remove heat from the plate 16 and hence from the vicinity of the optical transceivers 14.

Figure 5 is graphically illustrative of the advantage of maintaining temperature stability of optical devices.

The variation of optical power output against ambient temperature, normalised to 1 milliwatt output at 200C, is shown. It will be seen from the dotted line that the optical output power may vary by +258 approximately over the temperature range of -200C to 600C. In comparison, optical devices, temperature stabilised by the simple means provided by present invention, have a variation of less than +58 over the same temperature range as can be seen by the solid line graph. Such stabilisation reduces the mechanical and thermal stresses to which the optical devices are subject leading to greater reliability and longevity of the electro-optical source, such as a laser diode.

In effect, the invention entails the replacement of a conventional structural support for the optical transceivers and for the PC backplane and line replacement module connectors, for one in the form of a hollow plate of a material of good thermal conductivity wherethrough a cooling fluid such as air may be fed.

The exit temperature of the cooling fluid may be sensed, or the optical power output of one or more of the transceivers may be sensed to provide a feedback loop exercising control of the temperature or rate of flow of the input cooling fluid.

Air has been- suggested as the cooling fluid. As stated above, many aircraft (particularly military aircraft) already have the Environmental Control System (ECS) in which at least one specific location within the aircraft e.g. the avionic equipment bay, is provided with a temperature regulated airflow as part of the equipment thermal management procedures. This airflow is usually bled off from the engine or engines of the aircraft and suitably conditioned before use. It is, in such aircraft, an ideal source of'coolant for feeding through the plenum chamber 18 of the stabilised system of the present invention, to provide location specific temperature stabilisation of the optical devices in the manner described herein.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto.

For instance, the plate 16 may be of a high thermal conductivity metal other than aluminium and need not define one surface of an integral box. It may form a separate plate acting as a lid to a plenum chamber 18, the other three sides of which are of a different material. In its preferable form as a rectangular sectioned aluminium extrusion, the extrusion may include integral internal cooling fins and/or deflections for ensuring optimum excess heat removal from the regions of the transceivers. Aligned holes are punched and/or drilled in opposite surfaces of the box to enable the connections 30, 32 and 34 to be made.

Additionally, further holes, threaded or through, may be provided to facilitate fixing of the PC backplate 22 and the connectors 24 to the surface 20 of the chamber 18.

Other variations are possible within the scope of the present invention as defined in the appended claims.

Claims (13)

1. Thermally stabilised optical apparatus for an optical data transmission system, the apparatus comprising a plenum chamber wherethrough a temperature stabilised fluid may be passed, a first wall of the plenum chamber being formed of a thermally conductive material and having mounted, on its outer surface, a plurality of optical devices in thermal contact with the first wall.

2. An apparatus as claimed in claim 1 wherein the plenum chamber is formed of an extrusion of rectangular cross-section of a material having a high coefficient of thermal conductivity.

3. 'An apparatus as claimed in claim 2 wherein the material is aluminium.

4. An apparatus as claimed in claim 2 or 3 wherein the interior surface of the first wall has cooling fins thereon.

5. An apparatus as claimed in any preceding claim wherein that surface of the plenum chamber, opposite the first wall, serves as a support for a printed circuit board for providing power and/or signal connections to the optical devices.

6. An apparatus as claimed in any preceding claim wherein that surface of the plenum chamber, opposite the first wall, has connectors for line replacement modules secured thereto.

7. An apparatus as claimed in any preceding claim wherein the first wall and the opposite wall of the plenum chamber, have aligned holes for receiving power and/or signal connections to the optical devices.

8. An optical data communication system, for an aircraft, comprising a plenum chamber having at least a first wall of a material of high thermal conductivity, a plurality of optical transceivers mounted on an outer surface of the first wall, optical fibres extending from the transceivers to remote equipment for controlling and/or monitoring the status of said remote equipment, a printed circuit board secured to the outer surface of a second wall of the plenum chamber, opposite the first wall, and power and/or signal connectors extending through the plenum chamber between the optical transceivers and the printed circuit board.

9. A system as claimed in claim 8 wherein the printed circuit board acts as a back plane and a plurality of connectors for line replacement modulus are secured thereto and/or to the second wall.

10. A system as claimed in claim 8 or 9 wherein each optical transceiver includes an optical transmitter having an electro-optical transducer and wherein at least the electro-optical transducer is secured to the first wall with a thermally conducting paste therebetween.

11. A system as claimed in claim 10 wherein a plurality of the electro-optical transducers are laser diodes.

12. An apparatus or a system as claimed in any preceding claim including means for passing temperature regulated air through the plenum chamber.

13. A thermally stabilised optical apparatus, or an optical data communications system incorporating such apparatus, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.