GB2127643A

GB2127643A - Optical data link

Info

Publication number: GB2127643A
Application number: GB8320161A
Authority: GB
Inventors: Arthur Edward Brewster; William Hugh Powell
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-09-24
Filing date: 1983-07-27
Publication date: 1984-04-11
Also published as: GB2127643B

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 for use in aligning the optical heads is described with reference to Figs. 1 and 3 and a method of achieving maximum transmitted signal amplitude is described with reference to Fig. 2. Instead of lenses, reflective or catadioptric systems may be employed, for example a parabolic mirror. The optical fibre may be mounted by means of a spider arrangement (Fig. 4) with respect to a mirror. <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 aspect of the present invention there is provided a free-space optical data link system comprising a pair of aligned optical head separated by a 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 an optical focussing arrangement and support structure means for mounting one end of the optical fibre at the focus of the optical focussing arrangement, 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 bore therethrough, an optical focussing arrangement including an optical element mounted at one end of the bore and support structure means for the optical fibre such that one end of the optical fibre is arranged at the focus of the optical element.

According to another aspect of the present invention there is provided a method of adjusting opto-electronic transducers 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 an optical focussing arrangement and means for arranging one end of a respective optical fibre at the focus of the optical focussing arrangement, wherein the data to be transmitted is converted to optical form by a transmit opto-electronic transducer applied at the other end of one optical fibre and wherein the received optical signal at the one end of the other optical fibre is converted to electronic form by a receive opto-electronic transducer at the other end of the other optical fibre, which method includes the steps of obtaining an output from 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 transmit transducer over a speech frequency link and adjusting the transmit transducer for maximum tone pitch which corresponds to maximum signal amplitude.

Embodiments of the invention will now be described 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; 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, and Figure 4 illustrates schematically the mounting of an optical fibre at the focus of a parabolic mirror in an alternative version of an optical head.

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; or in major civil engineering projects where the location of the link terminals might require change at fairly frequent intervals and where the intervening terrain is subject to continuous excavation.

The link terminals of conventional free-space optical links include electronics for converting an electrical signal to be transmitted to an optical signal electronics for converting a received optical signal to a corresponding electrical signal, and optics systems for launching the optical signal to be transmitted from one link terminal to another link terminal with which it is in "line-of-sight" and for receiving the optical signal transmitted from the other link terminal to the one terminal.

Generally the link terminals which are arranged, for example, 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 link of the present invention, however, each link terminal 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 or transmitting optical signals, or it may have a single bore which is optically divided for transmitting and receiving optical signals, or it may have twin optical paths in physically separate parallel bores (binocular) one path being used for transmitting and the other for receiving.The electronics systems for converting electric signals to optical signals, and vice versa, may be arranged remotely from the optical heads, for example inside the buildings to be linked, the optical heads being connected to the electronics systems by the optical 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 requirements for the lens are that it should have sufficient aperture to collect most of the cone of rays emerging from the fibre and that is 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 alignment of two optical heads each is adjustably mounted on a support and each may include a telescopic viewfinder whose aim is fixed in relation to the axis of the head body, or alternatively particularly in the case of separate transmit and receive optics arrangement either thereof 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 of the accompanying drawings which also illustrates somewhat schematically means for adjusting the position of the optical path in two directions at right angle to one another relative to a fixed axis for critical or fine 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 as 3, is held in a fixed 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 1 5. The connector 5 is screwed into a bore at one end of a fibre focussing socket 6 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 socket 6. The outer circumference of socket 6 is provided with a fine screwthread along its length, which thread cooperates with a matching thread provided at one end of an optical fibre adaptor and filter mount 12. A filter may be mounted as at 1 3 in the member 12. 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 at 13 would 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. 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 prevent the ingress of dirt and moisture into the bore 2 and onto the innermost face of lens 3.

The position of the 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; adjustment for this purpose being provided by means of the screwthreads on socket 6 and member 12. Once the adjustment has been correctly achieved socket 6 is iocked in position by means of lock ring 14. Since for a given pair of optical heads for a data link the lens employed will be the same, and the lens to fibre distances will be the same. adjustment may be carried out by putting two lenses together, thus obviating the necessity for supplying a parallel beam light source or 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 1 6 which is mounted to a tripod or other stand arrangement (not shown) via a pan/tilt block 1 6a permitting coarse adjustment of the aim of the optical head for which the optical fibre adaptor and filter mount 12 is unscrewed as a unit from the body and replaced by an eyepiece in order to form a telescope. The body 1 is mounted to the plate 16 via a universal joint 1 7, which may be comprised by an elevation hinge 1 7a and an azimuth hinge 1 7b. Fine elevation and azimuth adjustment is achieved by means of a finely threaded 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 1 9 which can be screwed along a worm shaft 1 9a.Thus by screwing the pin 1 8 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 9 the azimuth of the optical head may be adjusted. In use once set the correct position is maintained by spring-loaded locking studs 20 between plate 1 6 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 at the heads are then adjusted for maximum signal amplitude as will now be described with reference to Figure 2 of the 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 opto-electric transducer, comprising a pulse frequency modulation transmitter 25 including an optical beam source for example a semiconductor laser from an electronic input signal 26 applied thereto and converted to an electronic output signal 27 by an opto-electronic transducer comprising pulse frequency modulation receiver 28 including a photodetector. It is desirable that the electronics associated with the optical heads are adjusted for optimum 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.For this purpose it is necessary to have some means of measuring the signal received at each optical head and then to adjust both the transmit and receive electronics for maximum signal amplitude. At the receiver this may be achieved by watching a meter and simply adjusting the receiver aim for maximum signal amplitude, however, it is necessary to know at the transmitter exactly what is happening at the receiver when making adjustments to the transmitter. This may be achieved by taking an output 29 from the pulse frequency modulation receiver 28 from a point such as at the receiver photodetector such that the output is related (proportional) to the signal amplitude. This output is applied to a variable frequency (pitch) oscillator 30 which converts it to a variable tone output at speaker 31.This variable tone output may then be transmitted to the transmitter end of the optical link over speech frequency link 32, such as a telephone line or walkie-talkie radio, for example. Thus a person at the transmit end of the optical link can "hear" the effect of adjustment of the alignment of the transmitting optical head with the receiving optical head and the alignment of the transmitting optical head can be readily adjusted for maximum signal amplitude at the receiver. The variable frequency (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 or voltage changes, changes its amplitude with signal level so that the pitch level varies.

Preferably the optical link transmits over the video 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 transmittabie as up to eight megabits in binary format. The bandwidth that can be transmitted is limited only by the bandwidth of the electronics. Fundamentaliy at 1 micron carrier wavelength the bandwidth could be 20x1012 Hz.

In the case of a binocular arrangement of transmitting and receiving optical paths, to facilitate initial alignment of the optical heads each head may incorporate a telescopic viewfinder whose aim is fixed 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 of the arrangement illustrated in Fig. 3. One such alignment and focussing arrangement is employed for each of the transmitting 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 of transmitting optic path, similar bore is provided for the 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 rear end of an adjustable focussing tube 45, which is urged towards the objective lens 42 by a compression spring 46 engaged between a shoulder 47 of the tube 45 and a rear end plate 48 secured to the main body 40. A front end plate 49 is secured to the opposite end of the body 40. A focussing nut 50, which engages a fine thread on the end of the focussing tube 45 serves to axially move the spring-loaded tube 45 and hence the end of the optical fibre 43 to the correct focal position, as determined by putting the two lenses together, as described above. A filter 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 of the tube 45 which provides a shoulder 47 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 movement.

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 of the externally-tapered 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 element 57, which also has an external taper. The internal and external tapers of the adjuster element 57 also have their axles 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 of the adjuster elements can be achieved without seizing. Typically the angle of taper is 30 . To facilitate critical adjustment the focussing nut 50 and the adjuster elements 56 and 57 may be provided with peripheral tommybar holes (not shown).

Thus, if the adjuster elements 56 and 57 are so orientated that their 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, the adjuster elements 56 and 57 are rotated independently, the optical fibre may be displaced in any direction at will, up to a maximum of 1 mm, in order to compensate for any alignment errors.

In the use of an optical data link system it will also be necessary to accurately align the two terminals (optical heads) and for this purpose a neo-concentric eccentric taper 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 of flexure to allow the rear of the 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 of the phosphor-bronze 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 mainpulation for adjustment purposes.The pan/tilt block 59 will generally be mounted to a tripod or other stand arrangement (not shown) via means permitting coarse adjustment of the aim of the optical head, and the adjusters 61 and 62 will be employed only for the critical final adjustment of the 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 easier to perform with the independent fine elevation and azimuth adjusters described with reference to Fig. 1 , than the neoconcentric adjusters 61 and 62, due to difficulty in interpreting which adjuster to 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 siop 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 power wedge 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 receives areas resulting in a corresponding optical loss in comparison with a binocular arrangement of two optical paths with the lenses each of the same size as the one lens of a single optical path, but this may be compensated for by using a correspondingly larger lens for a partitioned lens arrangement. Another possible optical arrangement would be a central round lens for the transmit path surrounded by an annular lens for the 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 ability to achieve a very narrow beam angle, of the 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 receiver field of view reduces the risk of noise and interference from spurious light sources such as street lighting or the sun.

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

Simple single-element lens may be used in the projection system rather than the more expensive types having correction for the usual lens aberrations. This is because the monochromatic nature of the light emitted by conventional semiconductor light sources makes it unnecessary to correct for chromatic aberration and because the very small diameter of the fibre end enables it to be treated as an on-axis point source to which offaxis errors, such as distortion, astigmatism and field curvature, will not apply. Spherical aberration may be minimised by suitable choice of the relative curvatures of the two lens surfaces.

Whereas the embodiments described above employ simple positive lens other optical focussing arrangements may alternatively be employed, which focussing arrangements may be refracting, reflecting or catadioptric. A reflecting system may be comprised by a first-surfacereflecting parabolic mirror or by a Mangin mirror.

Both surfaces of a Mangin mirror are spherical, with the refiection occurring at the second surface. Spherical aberration is corrected as the light enters via the first surface and the body of the glass, achieving the equivalent of a parabolic mirror. The particular advantage of employing an optical fibre with a reflecting system is that whilst the source or detector provided by the fibre end must lie in the field of view of the reflecting system, the small fibre diameter will minimise obscuration of the reflected light by the source or detector.

The optical fibre may be mounted with its end at the mirror focus by a spider arrangement such as is used to support the diagonal mirror of a Newtonian telescope. Aiternatively, since it may be desirable to provide weather protection by including a front window on the optical head, the optical fibre end may be attached to the centre of such a front window, thereby eliminating residual obscuration due to the legs of the spider.

Although it is practicable to merely plug the fibre into a central hole of the window, it is preferable to attach the fibre to the inside of the window, entering radially from edge to centre, and bending it adjacent the centre at a right angle to face the mirror, the radius of curvature of the right angle bend being sufficiently large so as not to impair the transmission properties of the fibre. Any associated supporting structure at the bend should be designed to keep the obscuration to a minimum. For a simple front window the glass may be optically flat and not contribute to the focussing parameters of the system. However, it is also possible to support the fibre at the centre of a correcting plate of a Schmidt optical system, or other systems of the same family, all of which can be considered to "parabolise" a spherical primary mirror.However, since correcting plates do not generally lie at the focus of the mirror, it is necessary to extend the fibre forward to the correct focal position such as by means of a supporting rod coinciding with the optical axis of the system. In the case of a very long extension with the possibility of mechanical instability it would be preferable to locate the fibre end by means of a spider rather than attach it to the correcting plate. Alternatively, the support may be a single rod extending radially inward from the wall of the assembly, the fibre passing up the rod and having a right-angled bend adjacent its end in order to position the end at the focus, in a similar manner to that described above. The extreme lightness of the fibre enables the supporting structure to be correspondingly light with minimal optical obscuration.

A possible spider for locating a fibre end at the focus of a mirror is shown in Fig. 4. A mirror 70, which may be parabolic is mounted at the end of a housing tube 71. In the case of a receiver, parallel input light is collected by the mirror 70 and focussed onto the end 72 of an optical fibre 73 via which it is transmitted to a photodetector (not shown). Two sets of three fine, tensioned steel wires 74 are attached to the fibre, and each wire of a set is attached to the housing tube with a 1200 interval with respect to the other wires of the set. The two planes of attachment of the wires on the tube (A1, A2) have a greater separation than the two points of attachment (F1, F2) in the fibre 73, so that the two spider elements oppose each other axially to centralise the fibre and maintain alignment with the optical axis.

It is additionally possible to use Cassegrainian, Gregorian or Maksutov optical systems, however, the advantage of the small diameter of the fibre would then be lost since the fibre would not lie in the field of view, the obscuration in these systems would be due to the presence of a secondary mirror which must always cover an appreciable area.

Claims

1. A free-space optical data link system comprising a pair of algined optical heads separated by a 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 an optical focussing arrangement and support structure means for mounting one end of the optical fibre at the focus of the optical focussing arrangement, 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.

2. A free-space optical data link system as claimed in claim 1, including a respective optoelectronic 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 back to electronic form for subsequent transmission from the system.

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

4. A free-space optical data link system as claimed in claim 3 wherein the optical heads include a respective lens as the optical focussing arrangement for each optical fibre, which two lenses of each head are arranged in a binocular configuration.

5. A free-space optical data link as claimed in claim 1, wherein each optical head includes a single lens as the optical focussing arrangement 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. A free-space optical data link as claimed in claim 4 or claim 5, wherein the support structure means includes an optical fibre adaptor and filter mount associated with each fibre, which adaptor and filter mounts are adapted to be removably secured in main bodies of the optical head such that the adaptor and filter mount may be replaced by an eyepiece, whereby to convert the optical head to a telescope for optical head alignment purposes, and that the said one optical fibre ends are automatically located at the focus of the respective lens when the adaptor and fibre mounts are resecured in the main bodies.

7. A free-space optical data link as claimed in any one of the 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. A free-space optical 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 by a pin mounted to the optical head main body and whose effective length is adjustable, and wherein rotation of the worm gear serves to adjust the azimuthal position of the optical head and adjustment of the length of the pin serves to adjust the elevational position of the optical head.

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

1 0. A free-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 whereby to provide a variable tone output for transmission to the transmitting optical head over a speech frequency link to facilitate adjustment of the transmitter for maximum transmitted signal pitch.

11. A free-space optical data link as claimed in claim 4 or claim 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 includes 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 longitudinal axis of its cylindrical bore, the axis of internal and external surfaces of the 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 of the main body.

12. A free-space optical data link as claimed in claim 11, wherein the said internal and external surfaces of the 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. A free-space optical data link as claimed in claim 12, wherein means are provided to adjust the axial position of the tube and thus the optical fibre relative to the lens for focussing purposes.

14. A free-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 under the action of the spring, which spring is arranged between a shoulder of the tube adjacent the other tube end and a face of the main body surrounding the bore therein.

1 5. A free-space optical data link system as claimed in any one of claims 1,2 or 3, claim 7 or claim 8, as appendant to claims 1,2, or 3, claim 9 or 10, wherein the optical focussing arrangement is comprised by a reflective or catadioptric optical system.

1 6. A free-space optical data link system as claimed in claim 15, wherein the optical heads include a respective parabolic mirror for each optical fibre.

17. A free-space optical data link system as claimed in claim 1 6, wherein the support structure means comprises a fine, tensioned wire spider arrangement.

18. A free-space optical data link system as claimed in claim 16, wherein the optical heads include a front window and the optical fibres are supported thereby.

19. A free-space optical data link system as claimed in claim 15, wherein the optical heads include a respective optical system of the Schmidt type including a primary spherical mirror and a corrector plate, and wherein the optical fibres are supported by the respective corrector plates.

20. 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, an optical focussing arrangement including an optical element mounted at one end of the bore and support structure means for the optical fibre such that one end of the optical fibre is arranged at the focus of the optical element.

21. An optical head arrangement as claimed in claim 20, wherein the optical focussing arrangement including an optical element is comprised by a single lens and wherein the support structure means is such that the optical fibre is removably secured at the other end of the bore and in the secured position the one end of the optical fibre is arranged at the focus of the lens.

22. An optical head arrangement as claimed in claim 21 , 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.

23. An optical head arrangement as claimed in claim 20, wherein the optical element comprises a parabolic mirror.

24. An optical head arrangement as claimed in claim 23, wherein the support structure means comprises a fine, tensioned wire spider arrangement.

25. An optical head arrangement as claimed in claim 23, wherein a front window is secured at the other end of the bore and comprises the support structure means.

26. An optical head arrangement as claimed in claim 20, wherein the optical focussing arrangement is of the Schmidt type and includes a primary spherical mirror, comprising the optical element, and a corrector plate, comprising the support structure means.

27. An optical head arrangement as claimed in any one of claims 20 to 26, 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.

28. An optical head arrangement as claimed in claim 27, 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 for the optical head, and wherein the fine adjustment means comprises a universal joint, via which one end of the main body is mounted to the support plate, and a worm gear/adjustable length pin via which the other end of the 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 of the worm gear serves to adjust the azimuthal position of the optical head and adjustment of the length of the pin serves to adjust the elevational position of the optical head.

29. An optical head arrangement as claimed in claim 27 as appendant to claim 21, 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 tube being slidable in a cylindrical bore of a first adjuster element, the external surface of the 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 of the first adjuster element cylindrical bore and external surface, and the internal and external surfaces of the second adjuster element, being parallel but offset with respect to one another, whereby rotation of the first and second adjuster elements serves to adjust the position of the tube relative to the main body.

30. A method of adjusting opto-electronic transducers 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 an optical focussing arrangement and means for arranging one end of a respective optical fibre at the focus of the optical focussing arrangement, wherein the data to be transmitted is converted to optical form by a transmit opto-electronic transducer applied at the other end of one optical fibre and wherein the received optical signal at the one end of the other optical fibre is converted to electronic form by a receive opto-electronic transducer at the other end of the other optical fibre, which method includes the steps of obtaining an output from 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 transmit location over a speech frequency link and adjusting the transmit head alignment for maximum tone pitch which corresponds to maximum signal amplitude.

31. A method as claimed in claim 30, wherein the transmit opto-electronic transducer is a pulse frequency modulation transmitter and the receive opto-electronic transducer is a pulse frequency modulation receiver, and wherein said output is taken from a photodiode of the receiver.

32. A free-space optical data link system substantially as herein described with reference to the accompanying drawings.

33. An optical head arrangement for a freespace optical data link system substantially as herein described with reference to and as illustrated in Fig. 1, Fig. 3 or Fig. 4 of the accompanying drawings.

34. 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 of the accompanying drawings.