GB2131245A - Optical data link - Google Patents

Abstract

Each optical head of a free space optical data link includes a lens (21,22) at whose focus an end of an optical fibre (23,24) is arranged. The opto-electronic transducers (25, 28), for converting electronic signals to be transmitted to optical form and converting received optical signals to electronic form, are arranged at the other ends of the optical fibres so that the transducer may be arranged remotely from the optical heads, for example in the buildings on the roofs of which the optical heads are arranged. Adjustment means are provided for use in aligning the optical heads in attitude and azimuth, focusing, and adjusting the optical fibre to lie on the axis of the lens. A method is described of achieving maximum transmitted signal amplitude by modulation of an audio signal at the receiver in accordance with the received intensity and feeding back the modulated signal via 32 to the transmitter end to allow correct alignment of the transmitter. <IMAGE>

Description

SPECIFICATION Optical data link This invention relates to an optical data link and, in particular, to a free-space optical data link.

According to one aspectofthe present invention there is provided a free-space optical data link system comprising a pair of aligned optical heads separated by a free-space data transmission path whereby data can betransmitted in optical form at least in a first direction along the path between the heads, wherein each optical head is provided with at least one respective optical fibre and includes a lens and support structure means for mounting one end ofthe optical fibre atthefocus of the lens, and wherein in use of the system a data signal to be transmitted along the path is supplied in optical form to one optical head via its respective optical fibre and a corresponding data signal is available in optical form at the other end of the other optical head's respective optical fibre.

According to a further aspect of the present invention there is provided an optical head arrangement, for use in a free-space optical data link system, comprising an optical fibre, a main optical head body with a boretherethrough,asinglelenselement mounted at one end ofthe bore and supportstructure means for the optical fibre removably secured atthe other end of the bore and such that in the secured position one end ofthe optical fibre is arranged at the focus of the lens.

According to another aspect of the present inven- tin there is provided a method of adjusting optoelectronictransducers of a free-space optical data link system formaximum transmitted signal amplitude, which system includes a pair of aligned optical heads separated byafree-space data transmission path over which data can be transmitted in optical form at least inafirstdirection, each optical head including a lens and means for arranging one end of a respective optical fibre atthe focus ofthe lens, wherein the data to be transmitted is converted to optical form by a transmit opto-electronictransducer applied at the other end of one optical fibre and wherein the received optical signal at the one end ofthe other optical fibre is converted to electronic form by a receive optoelectronic transducer at the other end of the other optical fibre, which method includes the steps of obtaining an outputfrom the receive transducer which is proportional to the received signal amplitude, applying said output to a variable frequency oscillator whereby to obtain a corresponding speech frequency variable tone, transmitting the variable tone to the transmittransducerovera speech frequency link and adjusting thetransmittransducerfor maximum tone pitch which corresponds to maximum signal amplitude.

Embodiments of the invention will now be de scribed with reference to accompanying drawings in which: Figure 1 shows, partially sectioned and somewhat schematically, an optical head (single optical path) for use in an optical data link according to the present invention; Figure 2 shows a schematic of an optical data link system according to the present invention, and Figure 3 shows, in section, one path of an optical head (twin optical paths) for use in an optical data link according to the present invention.

Afree-space optical data link may be employed in situations where the installation of a wire or optical fibre is prohibited or inexpedient. Typical examples might be the bridging of rivers, estuaries or major road complexes, the rapid interconnection of sites separated by an established built-up area; orion major civil engineering projects where the location of the link terminals might require change atfairlyfrequent intervals and where the intervening terrain is subject to continuous excavation.

The linkterminals of conventional free-space optical links include electronics for converting an electrical signal to be transmitted to an optical signal, electronicsforconverting a received optical signal to a corresponding electrical signal, and optics systems for launching the optical signal to be transmitted from one linkterminal to another linkterminal with which it is in "line-of-sight" and for receiving the optical signal transmitted from the other linkterminal tothe one terminal. Generally the link terminals which are arranged,forexample, on the roofs of buildings have included both the optics and the electronics systems, including launching lasers and photodetectors, and are thus highly susceptible to temperature variations.

In the optical data linkofthe present invention, however, each linkterminal basically comprises an optical head containing no active components, input and/or output optical signals being conveyed to and from the optical heads through conventional optical fibres. An optical head may have a single optical path and thus be capable of only receiving ortransmitting optical signals, or it may have a single bore which is optically divided fortransmitting and receiving optical signals, or it may have twin optical paths in physically separate parallel bores (binocular) one path being used fortransmitting and the otherfor receiving.The electronicssystemsforconverting electricsignalsto optical signals, and vice versa, may be arranged remotely from the optical heads,forexample inside the buildings to be linked, the optical heads being connected tothe electronics systems bytheoptical fibres.

To launch an optical data signal into free space from one optical head to another, the output end of an optical fibre may simply be placed at the focus of a simple positive lens, comprising the transmitting optics of the optical head, so that its image is projected on the receiving optics of a distant optical head. The basic design requirementsforthe lens are that it should have sufficient apertureto collect most of the cone of rays emerging from the fibre and that its focal length should be such as to project a remote image of a suitable size.Typically for optical fibres having a 50 micron core diameter and NA=2, a lens with a 50 mm diameter and 120 mm focal length may be employed To facilitate initial alignmentoftwo optical heads each isadjustably mounted on a support and each may include a telescopioviewfinderwhose aim is fixed in relation to the axis of the head body, or alternatively particularly in the case of separate transmit and receive optics arrangementeitherthere- of may be converted to a telescope by the replacement of an optical fibre adaptor, by means of which the end of the optical fibre is arranged at the focus of the simple positive lens, with an eyepiece. Critical alignment may be achieved by method and means described in detail hereinafter.

As indicated above, in one possible optical arrangement an optical head includes a single optical path.

Such an arrangement is shown in Figure 1 ofthe accompanying drawingswhich also illustrates somewhat schematically means for adjusting the position ofthe optical path in two directions at right angle to one another relative to a fixed axis for critical orfine alignment purposes.

In a main body 1 of an optical head of, for example, duraluminium a bore 2 is provided. The bore 2 may be for either the receiving or transmitting optical path; alternatively it may be optically divided as is described in more detail hereinafter. At a front end (left hand end in the drawing) of each bore a respective objective lens, such as3, is held in afixed mounting.

An optical fibre 4 with a polished end face is mounted in a spring-loaded fibre connector 5 including elements 7 and 8 and spring 15. The connector 5 is screwed into a bore at one end of a fibre focussing socket6 until the optical fibre 4 engages the end face of an apertured pot-like member 10, comprising a fibre end locator, which is screwed into threaded bore 9 at the other end of fibre focussing socket6. The outer circumference ofthe socket 6 is provided with a fine screwthread along its length, which thread co-operates with a matching thread provided at one end of an optical fibre adaptor and filter mount 12. Afilter may be mounted as at 13 in the member 12. The purpose of such a filter is to minimise the possibility of interferencefrom spuriouswideband light sources, such as the sun.Afilteremployed at 13 would be a narrow bandpass optical filtertunedto thetransmitter wavelength and arranged in the receiving optics light path. There is no need to similarly arrange a filter in the transmitting optics light path. The member 12 is screwthreaded for engagement with a corresponding thread on the main body 1. A ring seal 12' may be arranged in a groove of the filter mount in order to preventthe ingress of dirt and moisture into the bore 2 and onto the innermost face of lens 3.

The position ofthe polished end face of the fibre 4 is adjusted during manufacture relative to the lens 3 such that it is focussed to infinity, that is parallel light incident on the left-hand side of lens 3 is focussed on the polished end face of the fibre 4; adjustmentforthis purpose being provided by means of the screwthreads on socket 6 and member 12. Once the adjustment has been correctly achieved socket 6 is locked in position by means of lock ring 14.Sincefora given pair of optical heads for a data linkthe lens employed will bethesame, and the lens to fibre distances will bethe same, adjustment may be carried out by putting two lenses together, thus obviating the necessity for supplying a parallel beam lightsourceor means for determining when the beam emergent from the left-hand side of lens 3 is parallel.

The main body 1 is mounted to support plate 16 which is mounted to a tripod or otherstand arrangement (not shown) via a pan/tilt block 16a permitting coarse adjustmentoftheaim oftheoptical headfor which the optical fibre adaptor and filter mount 12is unscrewed as a unit from the body and replaced by an eyepiece in orderto form a telescope.The body 5 is mounted to the plate 16 via a universal joint 17, which may be comprised by an elevation hinge 17a and an azimuth hinge 17h Fine elevation and azimuth adjustment is achieved by means of a finelythreaded pin 18 engageable with a correspondingly finely threaded bore in the body 1, the tip of pin 18 being engaged with a circumferential groove of a worm gear 19 which can be screwed along a worm shaft 19a. Thus by screwing the pin 18 in or out of the bore in order to change its effective length the elevation of optical head may be adjusted, and by rotation of the worm gear 1 9the azimuth ofthe optical head may be adjusted. In use once set the correct position is maintained by spring-loaded locking studs 20 between plate 16 and head 1.When fine adjustment has thus been made the pre-focussed respective optical fibre adaptor and filter mount are secured in position in the bodies 1 of the heads and the electronics atthe heads are then adjusted for maximum signal amplitude as will now be described with reference to Figure 2 ofthe accompanying drawings.

Figure 2 illustrates schematically lens 21 and 22 and optical fibres 23 and 24 of one optical path of a data link. The optical signal to be transmitted between the lens 21 and 22 is generated by means of an optoelectrictransducer, comprising a pulse frequency modulation transmitter 25 including an optical beam sourceforexamplea semiconductorlaserfrom an electronic input signal 26 applied thereto and converted to an electronic output signal 27 byan opto-electronictransducercomprising pulsefrequency modulation receiver 28 including a photodetector.

It is desirable that the electronics associated with the optical heads are adjustedforoptimum operation under good atmospheric and other operating conditions so that when bad operating conditions, for example, mist are pertaining the signal transmitted can still be successfully received. Forthis purpose it is necessaryto have some means of measu ring the signal received at each optical head and then to adjust both the transmit and receive electronics for maximum signal amplitude. Atthe receiverthis may be achieved bywatching a meterandsimplyadjusting the receiver aim for maximum signal amplitude, however it is necessary to know atthe transmitter exactlywhatis happening atthe receiver when making adjustments to the transmitter. This may be achieved bytaking an output 29 from the pulse frequency modulation receiver 28 from a pointsuch as at the receiver photodetector such thatthe output is related (proportionalito the signal amplitude. This output is applied to a variable frequency (pitch) oscillator30which convertsitto avariabletone output at speaker 31. This variable tone output may then be transmitted tothetransmitter end of the optical link over speech frequency link32, such as a telephone line or a walkie-talkie radio, for example.Thus a person at the transmit end of the optical link can "hear"the effect of adjustment of the electronics for the transmitting optical head on the electronics associated with the receiving optical head and the electronics forthe transmitting optical head can be readily adjusted for maximum signal amplitude at the receiver. The variablefrequency (pitch) oscillator may be comprised by any oscillator which responds to change in voltage or current output from the receiver and which, as the current orvoltage changes, changes its amplitude with signal level so that the pitch level varies.

Preferablytheoptical linktransmits overthevideo band part of the spectrum which is useful for many possible applications and voice hifi or data may also be transmitted thereover, the data being transmittable as upto eight megabits in binary format. The bandwidth that can be transmitted is limited only by the bandwidth of the electronics. Fundamentally at 1 micron carrier wavelength the bandwidth could be 20 x 1012 Hz.

In the case of a binocular arrangement oftransmit- ting and receiving optical paths, to facilitate initial alignment of the optical heads each head may incorporate a telescopic viewfinder whose aim isfixed in relation to the axis of the head body. In order to accommodate mechanical tolerances and centralisation errors in the several lenses employed, provision must be made to adjust for strict parallelism between the viewfinder and the transmit and receive optical paths. It is also necessary to provide a critical focussing adjustment for each optical fibre relative to its objective lens.

This may be achieved by means ofthe arrangement illustrated in Fig. 3. One such alignment and focussing arrangement is employed for each ofthetransmitting and receiving optic paths although only one is shown in the drawing. In a main body 40 of, for example, duralumin three parallel bores are provided, only one of which 41 is shown. The bore 41 may be for either the receiving ortransmitting optic path, a similar bore is provided forthe remaining optic path whereas the third bore is for the viewfinder. At a front end (left hand end in the drawing) of each bore a respective objective lens, such as 42, is held in a fixed mounting.

An optical fibre 43 with a polished end face is plugged into an adaptor 44 fitted to the rearend of an adjustable focussing tube 45, which is urged towards the objective lens 42 by a compression spring 46 engaged between a shoulder 47 ofthetube 45 and a rear end plate 48 secured to the main body 40. Afront end plate 49 is secured to the opposite end of the body 40. Afocussing nut 50, which engages a finethread on the end ofthe focussing tube 45 serves to axially move the spring-loaded tube45 and hence the end ofthe optical fibre 43 to the correctfocal position, as determined by putting the two lenses together, as described above. Afilter may be mounted in a filter mount 51 in the focussing tube.The purpose of such a filter is to minimise the possibility of interference from spurious wideband light sources, such as the sun. A filter employed in mount 51 will be a narrow bandpass optical filter tuned to the transmitter wavelength and arranged in the receiving optics light path. There is no need to similarly arrange a filter in the transmitting optics light path.

In an extension portion 52 ofthetube 45 which provides a shoulder47 for the spring support, there is a guide slot 53. The slot 53 is engaged by a pin 54, fixed to the main body 40, which prevents rotation of the focussing tube, whilst allowing fore and aft move- ment.

The focussing tube 45 is a sliding fit in a cylindrical bore 55 of an externally-tapered adjuster element 56.

The axis of bore 55 is offset in relation to the axis ofthe externallytapered surface of element 56 by, for example, 0.5 mm in a presently preferred binocular optical head. The external taper of element 56 is lapped into and thus rests with the internal taper of an adjuster ejement 57, which also has an external taper.

The internal and external tapers ofthe adjuster element 57 also have their axes offset, by 0.5 mm in the example quoted. The external taper of adjuster element is lapped into and thus rests with a tapered bore 58 in the rear plate 48. The bore 58 is bored to lie nominally concentric with the optical axis of the objective lens 42. The various tapers, and the nut 50 and element 56, are held in close engagement due to the spring 46. The taper angles are such that, with the inclusion of a high-viscosity lubricant, smooth precision rotation ofthe adjuster elements can be achieved without seizing. Typically the angle oftaper is 30 . To facilitate critical adjustmentthefocussing nut 50 and the adjuster elements 56 and 57 may be provided with peripheral tommy-bar holes (not shown).

Thus, ifthe adjuster elements 56 and 57 are so orientated thattheir axis offsets (0.5 mm) exactly cancel, the optical fibre end will be brought to lie on an axis close to the optical axis of the objective lens 42. If, however,theadjusterelements56and 57 are rotated independently, the optical fibre may be displaced in any direction atwill, upto a maximum of 1 mm, in orderto compensate for any alignment errors.

In the use of an optical data link system itwill also be necessary to accurately align the two terminals (optical heads) and for this purpose a neo-concentric eccentrictaper or cone system as described above may be employed. The optical head, comprising the main body 40 and end plates 48 and 49, is mounted to a pan/tilt block 59. The front end plate 49 is bolted to the block 59 via a flat, phosphor-bronze spring 60 which provides a high degree of rigidity whilst being capable offlexureto allowthe rearofthe head to be deflected a small distance in any direction. Between the rear plate 48 and the block 59 is a pair of neo-concentric tapered adjusters 61 and 62, which operate as described above for elements 56 and 57, the movement thereof being communicated to the block 59 via, for example, a steel ball 63 seated between the inner eccentric adjuster 61 and a socket 64 provided therefor on the block. Pressure on the ball is maintained by suitable preloading ofthe phosphorbronze spring 60. If the taper axes of the adjusters 61 and 62 are offset by 1 mm, a total possible movement of 2 mm in any direction is obtained. The adjusters 61 and 62 may be provided with tommy-bar holes (not shown) for ease of manipulation for adjustment purposes.The pan/tilt block 59 will generally be mounted to a tripod or other stand arrangement (not shown)via means permitting coarseadjustmentof the aim ofthe optical head, and the adjusters 61 and 62 will be employed onlyforthe critical final adjustment ofthe aim.

When conditions are such that fine visual alignment is not practicable, the alignment may be carried out using the adjusters 61 and 62 with the aid of a variable pitch oscillator, as described with reference to Fig. 2.

However, such "blind" adjustment is easierto perform with the independent fine elevation and azimuth adjusters described with reference to Fig. 1, than the neo-concentric adjusters 61 and62,dueto difficulty in interpreting which adjusterto adjust and which direction to adjust it in. Thus embodiments can be envisaged in which the head mounting shown in Fig. 3 is replaced by the head mounting shown in Fig. 1.

The adjuster elements 56 and 57 and the nut 50 would normally be adjusted for correct settings during manufacture of the optical head, such that during field use only adjustment of pan/tilt arrangement would be necessary. The use of conical bearing surfaces, provided by the various tapers, serves to eliminate slop and backlash, although the surfaces need not be conical.

Whereas the single optical path arrangements described above each employ a generally round lens other alternatives are possible. For example a single lens may be partitioned into two halves by positioning a low powerwedge prism over one half, the receive half. The prism results in separated receive and transmit images, the separation being extremely stable as it is fixed by the angle of the prism. In this arrangement both the transmit and receive paths share the same "telescope" barrel, that is the same bore in the main optical head body, which greatly simplifies the problems of alignment, adjustment and stability.Partitioning the lens in this way however halves the transmit and receive areas resulting in a corresponding optical loss in comparison with a binocular arrangement of two optical paths with the lenses each ofthe same size as the one lens of a single optical path, butthis may be compensated for by using a correspondingly larger lens for a partitioned lens arrangement. Another possible optical arrangementwould be a central round lensforthetransmit path surrounded byan annularlensforthe receive path.

In comparison with radio links and conventional free-space optical link designs, the system of the present invention has a significant advantage in its abilitytoachieveaverynarrowbeam angle,ofthe order of 0.05 , without the necessity to resort to very large and expensive optical systems. The narrow transmitted beam has the effect of minimising path losses and much reducing the possibility of unauthorised interception, while the equivalent narrow receiverfield of view reduces the risk of noise and interference from spurious light sources such as street lighting orthe sun.

The optical heads contain no active components, both input and output signals being conveyed in the form of lightthrough optical fibres. Thus the free- space system will remain compatible with current or future developments in optical fibre transmitters or receivers. The limiting range ofthe system will depend on the power ofthe optical transmitter and the sensitivity of the optical receiver with which it is used.

Rangesofthe kilometre orderare achievable with current equipment.

Simple single-element lens may be used in the projection system ratherthan the more expensive types having correction for the usual lens aberrations.

This is because the monochromatic nature ofthe light emitted by conventional semiconductor light sources makes it unnecessary to correctfor chromatic aberration and becausethevery small diameter of the fibre end enables itto be treated as an on-axis point source to which off-axis errors, such as distortion, astigmatism and field curvature, will not apply. Spherical aberration may be minimised by suitable choice of the relative curvatures ofthe two lens surfaces.

Claims (24)

1. Afree-space optical data link system compris- ing a pair ofaligned optical heads separated bya free-space data transmission path whereby data can be transmitted in optical form at least in a first direction along the path between the heads, wherein each optical head is provided with at least one respective optical fibre and includes a lens and support structure means for mounting one end ofthe optical fibre atthe focus ofthe lens, and wherein in use of the system a data signal to be transmitted along the path is supplied in optical form to one optical head via its respective optical fibre and a corresponding data signal is available in optical form atthe other end of the other optical head's respective optical fibre.

2. Afree-space optical data link system as claimed in claim 1, including a respectiveopto-electronic transducer at the other end of each optical fibre whereby a data signal for transmission along the path may be supplied to the system in electronic form, transmitted along the path in optical form and converted backto electronic form forsubsequent transmission from the system.

3. Afree-space optical data link system as claimed in claim 1 or claim 2 and wherein each optical head is provided with the respective optical fibres and data can be transmitted in optical form in both directions along the path between the heads.

4. Afree-spaceoptical data link system as claimed in claim 3 wherein the optical heads include a respective lens for each optical fibre which two lenses of each head are arranged in a binocular configuration.

5. Afree-space optical data link as claimed in claim 1, wherein each optical head includes a single lens and wherein by means of a respective prism arranged adjacent each lens the optical path between the heads is partitioned into receive and transmit portions whereby data can be transmitted in both directions.

6. Afree-space optical data link as claimed in any one ofthe preceding claims wherein the support structure means includes an optical fibre adaptor and filter mount associated with eachfibre,which adaptor and filter mounts are adapted to be removably secured in main bodies of the optical head such that the adaptorand filter mount may be replaced by an eyepiece, whereby to convertthe optical head to a telescope for optical head alignment purposes, and thatthe said one optical fibre ends are automatically located atthefocusofthe respective lens when the adaptor and fibre mounts are resecured in the main bodies.

7. Afree-space optical data link as claimed in any ofle ofthe preceding claims wherein each optical head is provided with adjustable mounting means to facilitate alignment of the two optical heads, which adjustable mounting means includes means for coarse and fine adjustment.

8. Afree-spaceoptical data link as claimed in claim 7, wherein the fine adjustment means includes a support plate to which one end of the optical head main body is mounted via a universal joint and a rotatable worm gear mounted to the support plate adjacent the end of the optical head main body opposite the one end thereof, which worm gear is engaged buy a pin mounted to the optical head main body and whose effective length is adjustable, and wherein rotationofthewormgearservestoadjustthe azimuthal position ofthe optical head and adjustment ofthe length ofthe pin serves to adjust the elevational position of the optical head.

9. Afree-space optical data link system as claimed in claim 2 wherein the transmitting opto-electric transducer comprises a pulse frequency modulation transmitter and wherein the receiving opto-electronic transducer comprises a pulse frequency modulation receiver.

10. Afree-space optical data link system as claimed in claim 9, including a variable frequency oscillator and a speaker, and wherein the pulse frequency modulation receiver provides an output proportional to the amplitude of the received signal to the oscillator whose output is applied to the speaker wherebyto provideavariabletoneoutputfortransmission to the transmitting optical head over a speech frequency linktofacilitate adjustment ofthe transmitterfor maximum transmitted signal pitch.

11. Afree-space optical data link as claimed in any one of claims 1 to 5, wherein each optical head is provided with coarse and fine adjustment means to facilitate alignment of the two optical heads, wherein the fine adjustment means include a tube to one end of which the one end of the optical fibre is mounted, which tube is a slide fit in a cylindrical bore of a first adjuster element, which first adjuster element is nested in a second adjuster element, and which second adjuster element is nested in a correspondingly shaped bore of the main body, the first and second adjuster elements being sleeve-shaped, the first adjuster element including an external surface having its longitudinal axis parallel to but offset from the longitudinai axis of its cylindrical bore, the axis of internal and external surfaces ofthe second adjuster element being parallel to but offset from one another, the position of the tube relative to the main body being adjustable by rotation of either or both of the first and second adjuster elements relative to the bore ofthe main body.

12. Afree-space optical data link as claimed in claim 11, wherein the said internal and external surfaces ofthe first and second adjuster elements are conically shaped, and wherein means are provided to prevent rotation of the tube relative to the main body.

13. Afree-space optical data link as claimed in claim 12, wherein means are provided to adjust the axial position ofthe tube and thus the optical fibre relative to the lensforfocussing purposes.

14. Afree-space optical data link as claimed in claim 13, wherein the tube is spring-loaded relative to the main body and carries adjacent the one end an adjustable stop member urged into engagement with the first adjuster element underthe action of the spring, which spring is arranged between a shoulder ofthetube adjacent the other tube end and a face of the main body surrounding the bore therein.

15. An optical head arrangement, for use in a free-space optical data link system, comprising an optical fibre, a main optical head body with a bore therethrough, a single lens element mounted at one end ofthe bore and support structure meansforthe optical fibre removably secured at the other end of the bore and such that in the secured position one end of the optical fibre is arranged at the focus ofthe lens.

16. An optical head arrangement as claimed in claim 15, wherein the support structure means includes support means whereby an optical filter can be arranged in-the optical path between the lens and the one end of the optical fibre.

17. An optical head arrangement as claimed in claim 15 or claim 16, further including adjustable mounting means to facilitate alignment of two such optical head arrangements,which adjustable mounting means include means for coarse and fine adjustment.

18. An optical head arrangement as claimed in claim 17, wherein the main body is mounted to a support plate via fine adjustment means, which support plate is mounted for coarse adjustment relative to a stand forthe optical head, and wherein the fine adjustment means comprises a universal joint, via which one end ofthe main body is mountedtothe support plate, and a worm gear/adjustable length pin via which the other end ofthe main body is mounted to the support plate, which worm gear is rotatably mounted to the support plate and engaged by the pin which is mounted to the main body, and wherein rotation ofthe worm gear serves to adjust the azimuthal position of the optical head and adjustment of the length ofthe pin serves to adjust the elevational position ofthe optical head.

19. An optical head arrangement as claimed in claim 17, wherein the main body is mounted to a support plate via the coarse adjustment means, and wherein the optical fibre is mounted to one end of a tube which is mounted to the main body via the fine adjustment means, and wherein the fine adjustment means include a pair of nested adjuster elements, the tubebeingslidableinacylindrical bore of a first adjuster element, the external surface ofthe first adjuster element, the internal and external surfaces of the second adjuster element being conically-shaped and the external surface of the second adjuster element being nested in a corresponding bore of the main body, the longitudinal axes ofthe first adjuster element cylindrical bore and external surface, and the internal and external surfaces ofthe second adjuster element, being parallel but offset with respectto one another, whereby rotation of the first and second adjuster elements serves to adjust the position ofthe tube relative to the main body.

20. A method of adjusting opto-electronictransducers of a free-space optical data link system for maximum transmitted signal amplitude, which system includes a pair of aligned optical heads separated by a free-space data transmission path over which data can be transmitted in optical form at least in a first direction, each optical head including a lens and meansforarranging one end of a respective optical fibre atthe focus ofthe lens, wherein the data to be transmitted is converted to optical form by a transmit opto-electronictransducer applied atthe other end of one optical fibre and wherein the received optical signal atthe one end ofthe other optical fibre is converted to electronic form by a receive optoelectronic transducer at the other end of the other optical fibre, which method includes the steps of obtaining an output from the receivetransducerwhich is proportional to the received signal amplitude, applying said outputto a variable frequency oscillator wherebyto obtain a corresponding speech frequency variable tone, transmitting the variable tone to the transmit transducer over a speech frequency link and adjusting thetransmittransducerfor maximum tone pitch which corresponds to maximum signal amplitude.

21. A method as claimed in claim 20, wherein the transmit opto-electronic transducer is a pulse frequency modulation transmitter and the receive optoelectronic transducer is a pulse frequency modulation receiver, and wherein said output is taken from a photodiode of the receiver.

22. Afree-space optical data link system substantially as herein described with reference to the accompanying drawings.

23. An optical head arrangement for a free-space optical data link system substantially as herein describedwith reference to and as illustrated in Fig. 1 of the accompanying drawings.

24. A method of adjusting a free-space optical data link system for maximum transmitted signal amplitude substantially as herein described with reference to and as illustrated in Figure 2 ofthe accompanying drawings.