The present invention relates to apparatus and methods of deflecting an optical beam by acoustical methods and particularly, though not exclusively, relates to acousto-optical beam deflection for optical display projection.

It is well know that deflection of a light beam may be based on the diffraction of light produced by an acoustic wave travelling through a transparent medium. The acoustic wave produces a periodic variation in density (i.e. mechanical strain) along its path which, in turn, gives rise to corresponding changes in refractive index within the medium due to the photoelastic effect. Therefore, a moving optical phase- diffraction grating is produced in the medium. Any light beam passing through the medium and crossing the path of the acoustic wave is diffracted by its phase grating.

Many existing acousto-optic modulators or deflectors operate according to this principle, which is generally known as the "Bragg regime" and the corresponding acousto-optical devices known as "Bragg cells". The angle through which incident optical radiation is deflected by interaction with the acoustic phase grating is dependent upon the ratio between the wavelengths of the optical radiation and the acoustic radiation. Thus, by suitably controlling the wavelength of the acoustic radiation one may control the form of the phase grating and thereby the angle of optical deflection imparted by that grating.

This property is currently used in optical display projectors of various types, but particularly those which generate a projected image by the suitable deflection of a laser beam (or multiple laser beams). The complexity of the image such a projector is able to project is sensitively dependent upon the speed with which it is able to

"draw" the image on a display surface using a deflected laser beam(s). This, in turn, is dependent upon the speed with which the laser beam can be deflected, within a given image frame, in the process of "drawing" a projected image.

When highly detailed or complex projected images are required, including many thousands of separately defined image objects within an overall image scene (e.g. representing images of many separate light sources within the same overall scene), it is typically necessary to allow the laser beam of the projector to settle or "dwell" at a particular fixed position upon the display surface upon which the image is being projected in order than an image point of suitably high brightness can be produced which is clearly distinguishable, within the overall image, as a bright source of light. This is particularly important when the overall projected image is, for example, the image of a runway including many bright runway lights as might be produced when simulating the view scene by a pilot in an aircraft simulator machine.

The requirement to have a projector laser beam settle or dwell at a fixed point for a relatively extended period of time (e.g. several microseconds) necessarily limits the number of such light points which might be drawn within a typical image "frame" period. Often, several thousand such light points are required to be drawn within each frame of a projected image typically employed when simulating the view scene from an aircraft simulator machine. Unfortunately, to increase the number of available light points per image frame currently requires one to increase the duration of each frame (thereby enabling the projector more time to draw the additional individual light points within that frame) but this results in a drastic reduction in the quality of the perceived projected image as the eye is able to perceive the jump between successive image frames as the image frame duration grows. The alternative is to reduce the dwell time accorded to each light point, but this results in

a reduction in the perceived brightness of the projected light point and is generally found to be unacceptable.

The present invention aims to overcome at least some of the aforementioned problems.

At its most general, the present invention proposes making temporally sequential changes to the acoustic wave used to deflect optical radiation so that multiple different optical deflections occur simultaneously as a result of both the original acoustic wave and the changed acoustic wave which follows the original wave. For example, the acoustic wave in a first form may produce a first deflection(s) while the acoustic wave when changed to a second form produces a different deflection(s). By sequentially changing from the first waveform to the second waveform one may produce, in sequence, the first deflection(s) resulting from the action of only the first waveform, then simultaneously both the first and the second deflection(s) resulting from the action of both the first and second waveforms as the latter begins to replace the former in this role, followed by only the second deflection(s) resulting from the action of only the second waveform after it has wholly replaced the first waveform. In this way, one may continue illuminating with optical radiation in a first direction of deflection even while one begins to illuminate in a second direction of deflection. This temporally "parallel" illumination method has a two-fold benefit. Either one may illuminate in the first direction of deflection for a time period longer than would otherwise be the case, or one may illuminate in a greater number of successive (sequential) directions of deflection within a given period of time without reducing the amount of time each direction is illuminated.

Accordingly, in a first of its aspects, the present invention may provide apparatus for acoustically deflecting optical radiation, including a deflector means for receiving

optical radiation and deflecting the optical radiation with acoustic radiation wherein the deflector means is arranged to change the acoustic radiation such that sequential acoustic waves of differing waveform simultaneously deflect the optical radiation by differing respective amounts.

A waveform may be defined as a curve showing a given precise wave shape at a given time. The waveform of an acoustic wave may be defined as the waveform associated with the pressure variations within or along the acoustic wave.

Accordingly, two or more separate deflections may be achieved simultaneously using different non-overlapping acoustic radiation waveforms, each acoustic waveform being dedicated to a predetermined optical deflection. The differences in sequential waveforms may be differences in the monochromatic frequency between successive monochromatic waves, or differences in frequency components/weightings as between polychromatic acoustic waves (e.g. comprising a plurality of simultaneous superposed monochromatic acoustic waves e.g. forming a wave packet), or simply a change from a monochromatic wave to a polychromatic wave, or vice versa. Preferably, the deflector means is arranged to change the frequency of the acoustic radiation such that sequential acoustic waves of differing frequencies simultaneously deflect the optical radiation by differing respective amounts.

Preferably, the deflector means is arranged to sequentially change the frequency of the acoustic radiation so that a succeeding acoustic frequencies are generated only after generation of immediately preceding acoustic frequencies has ended. Thus may ensure that a succeeding acoustic wave does not spatially overlap the preceding acoustic wave it replaces thereby enabling a distinct spatial separation as between preceding and immediately succeeding acoustic waves and their resulting optical deflections.

The deflector means is preferably arranged to direct the acoustic radiation so as to propagate in a direction overlapping (e.g. crossing) the path of the optical radiation therewith to deflect the optical radiation at the overlap region. The deflector means is preferably arranged to change the waveform (e.g. frequency) of the acoustic radiation sufficiently rapidly that a preceding acoustic waveform (e.g. preceding frequencies) is still present within the overlap region as an immediately succeeding (following) acoustic waveform (e.g. succeeding frequencies) enters the overlap region. The deflector means may be arranged to change the waveform (e.g. frequency) of the acoustic radiation sufficiently rapidly that a preceding acoustic waveform (e.g. frequencies) is present only in parts of overlap region where the succeeding (following) acoustic waveform (e.g. frequencies) is substantially not present. For example, the succeeding waveform may follow the preceding waveform into the overlap region with very little delay, or practically no delay, relative to the time it takes either wave to traverse the overlap region. The result is the presence of a wave train as "seen" by the overlapping optical radiation, in which the leading part of the train has a waveform different from that of the trailing part, with little or substantially no gap between the leading and trailing parts. The optical radiation which "sees" the leading part of the wave train is deflected according to the waveform of that part, while the optical radiation which "sees" the trailing part is deflected differently according to the different waveform of the trailing part. As the wave train traverses the overlap region, the trailing part increasingly replaces the leading part, but until such time as complete replacement is achieved, a simultaneous multiple deflection of optical radiation is produced.

Preferably, the deflection apparatus includes a transmission medium for receiving said optical and acoustic radiation through which the optical and acoustic radiation are transmissible and within which said deflection occurs, wherein the deflector

means is arranged to sequentially input into the transmission medium said sequential acoustic waves of differing form (e.g. acoustic frequency). For example a Bragg cell may be employed in which the transmission medium is a crystal cell (e.g. TeO ₂ crystal), but other transmission media may be used such as would be readily apparent to the skilled person. The deflection means may include an acoustic transducer means attached to the transmission medium (e.g. at a surface thereof) and arranged to generate a predetermined acoustic wave at an interface with the transmission medium such that the acoustic wave propagates into the transmission medium towards the region of the transmission medium where overlap with the received optical radiation will occur.

Preferably, the surface of the transmission medium towards which the acoustic radiation propagates, from the transducer means, is shaped or cut (e.g. bias cut) to substantially avoid or prevent acoustic radiation reflecting back therefrom towards the transducer means or the overlap region. Acoustically absorbing material may be arranged upon that surface to suppress acoustic reflection therefrom. The transmission medium is also preferably arranged to receive and admit optical radiation into the medium, to permit propagation of the admitted optical radiation through the overlap region within the transmission medium, and to subsequently exit the transmission medium. Alternatively, the deflection means may be arranged to generate acoustic waves which propagate across a surface of the transmission medium, and the transmission medium is arranged to receive optical radiation at that surface such that the overlap region is formed thereat and received optical radiation may be deflected by reflection from an acoustic wave with which it has interacted at the surface. The deflection means may be arranged to generate the acoustic radiation so as to propagate in the longitudinal or shear propagation mode.

The deflection means most preferably includes a deflection control means arranged to generate deflection control signals with which to control the waveform of the acoustic wave(s) generated by the acoustic transducer means. The deflection control means most preferably includes an electrical or electronic signal generator arranged to generate electrical drive signals with which to drive the acoustic transducer means to generate an acoustic wave having a waveform determined by the drive signals. A drive signal may possess a signal waveform which is predetermined to cause the transducer means to generate a predetermined acoustic waveform in response to receiving the drive signal waveform. The deflection control means may include a computer or processor for controlling the generation, content and timing of the drive signals.

Most preferably, the deflection control means is arranged to generate drive signal according to a method of Direct Digital Synthesis (DDS) in which a drive signal waveform is generated directly using digital deflection control signals. For example, the deflection control means may be arranged to store data corresponding to sequential points along a reference waveform in digital format, and to generate a drive signal waveform by retrieving reference waveform data points from storage in sequence and corresponding to sequential points (which may omit intermediate points) on the reference waveform separated by an increment determined by the frequency or phase of the drive signal to be constructed from the retrieved data. In this way, the DDS method constructs a modified version of the reference waveform in which each point on the constructed waveform is located at a phase within that waveform which corresponds with a point on the reference waveform having the same phase within the reference waveform. However, the modification is determined according to the aforementioned increment of data retrieved and, therefore, the "speed" with which the waveform being constructed acquires a given phase and,

thus, the overall frequency of the constructed waveform e.g. if the waveform is monochromatic.

Preferably, this DDS method includes converting the digital constructed waveform into an analogue format before passing the drive signal waveform to a transducer means for driving the latter.

The deflection means may be arranged to change the form (e.g. frequency) of said acoustic wave to change from a preceding substantially monochromatic first frequency to a succeeding substantially monochromatic second frequency differing from the first frequency. Alternatively, the preceding and/or succeeding sequential acoustic waves may be polychromatic or formed by a differing superposition of a respective plurality of simultaneous acoustic frequencies. The deflection means may be arranged to complete said change of the waveform (e.g. frequency change) of the acoustic radiation in a period of time less than about one hundred nanoseconds, more preferably less than about 50 nanoseconds, and yet more preferably less than about 1 nanosecond.

An advantage of employing a DDS method as described above is the speed with which the deflection means may change the waveform (e.g. frequency) of the drive signal and, ultimately, the acoustic waveform. For example, a change in drive signal frequency may be achieved simply by instantaneously changing the increment employed in sequential data retrieval of the data points from the aforementioned reference waveform. The result will be a correspondingly substantially instantaneous change in the frequency of the constructed waveform generated from the retrieved data points and in the drive signal waveform and, ultimately, in the acoustic waveform generated according to the drive signal waveform. Preferably, the change in the acoustic waveform is substantially instantaneous.

A further advantage of the DDS method is that a change in drive signal waveform may be generated so as to appear continuously and smoothly from that part of the preceding waveform at which the change occurred. Accordingly there is no need to provide a "settling time", to allow "ringing" to subside, as would be the case with many other waveform generation methods since there are effectively no discontinuous changes in waveform which might result in ringing within the waveform. A practical consequence of this is that very rapid desired waveform changes can be made with practically instant effect.

The deflection means is preferably arranged to generate said acoustic radiation so as to propagate in a longitudinal propagation mode. This has the advantage of higher propagation speeds within the medium through which it propagates. Consequently, a relatively large number of differing acoustic waveforms may be caused to propagate through the aforementioned overlap region in a relatively short period of time resulting in a large number of differing optical deflections occurring rapidly in that short period. The deflector means is most preferably arranged to generate an acoustic waveform having a frequency (or having frequency components if not monochromatic) within the range of about 100 MHz to about 600 MHz, and more preferably within the range of about 300 MHz to about 600 MHz. This also applies to the drive signal waveform generated by the deflection means. Most preferably, the deflection means is arranged to generate any specified frequency component of the acoustic/drive signal waveform (or the waveform itself if monochromatic) to within an accuracy of about 1 KHz resolution (in frequency) thereby to enable accurate fine- tuning of resulting optical deflections.

The apparatus may comprise a first deflector means for receiving optical radiation and deflecting the optical radiation with acoustic radiation as discussed above, and a

second deflector means for receiving optical radiation deflected by the first deflector means and for deflecting that received optical radiation with acoustic radiation as discussed above. The second deflector means is most preferably arranged to deflect received optical radiation in a direction which is transverse to the direction in which the first deflector means deflected the optical radiation subsequently received by the second deflector means. For example, the direction of deflection imparted by the second deflector means may be substantially perpendicular to the direction of deflection imparted by the first deflector means. Thus, the present invention may provide apparatus comprising two deflector means arranged in succession and forming an optical train in which two successive transverse optical deflections are imparted to optical radiation (e.g. a laser beam) thereby providing deflection in two dimensions.

In a further of its aspects, the present invention may provide a laser display projector including apparatus for acoustically deflecting optical radiation according to the invention in its first aspect. The laser projector may be a laser projector arranged to form a visible display upon a display surface by projection of a suitably deflected laser beam onto the display surface. Accordingly, the display projector may be arranged to simultaneously project light from the same laser beam at different regions of a surface in use.

In yet another of its aspects, the present invention may provide a vehicle simulator machine (e.g. an aircraft simulator) including a laser display projector according to the invention in its further aspect.

It is to be understood that the invention in any of the above aspects implements a method acoustically deflecting optical radiation, and this method is a further aspect of the invention.

In a second of its aspects, the present invention may provide a method of acoustically deflecting optical radiation, including receiving optical radiation, deflecting the optical radiation with acoustic radiation, and changing the acoustic radiation such that sequential acoustic waves of differing waveform simultaneously deflect the optical radiation by differing respective amounts.

The method preferably includes changing the frequency of the acoustic radiation such that sequential acoustic waves of differing frequencies simultaneously deflect the optical radiation by differing respective amounts. The method may include sequentially changing the frequency of the acoustic radiation so that succeeding acoustic frequencies are generated only after generation of immediately preceding acoustic frequencies has ended.

The method may include directing the acoustic radiation so as to propagate in a direction overlapping the path of received optical radiation therewith to deflect the optical radiation at the overlap region, and changing the form (e.g. frequency) of the acoustic radiation sufficiently rapidly that preceding acoustic wave (e.g. preceding frequencies) are still present within the overlap region as immediately succeeding acoustic wave (e.g. succeeding frequencies) enter the overlap region.

Preferably, the method includes directing the acoustic radiation so as to propagate in a direction overlapping the path of received optical radiation therewith to deflect the optical radiation at the overlap region, and changing the form (e.g. frequency) of the acoustic radiation sufficiently rapidly that a preceding acoustic waveform (e.g. frequency) are present only in parts of overlap region where a succeeding acoustic waveform (e.g. frequency) is not present.

The method may include providing a transmission medium for receiving said optical and acoustic radiation through which the optical and acoustic radiation are transmissible and within which said deflection occurs, and sequentially inputting into the transmission medium said sequential acoustic waves of differing form (e.g. acoustic frequency).

The method may include storing data corresponding to sequential points along a reference waveform in digital format, and generating a drive signal waveform by retrieving the reference waveform data points from storage in sequence and corresponding to sequential points on the reference waveform separated by an increment determined by the frequency or phase of the drive signal to be constructed from the retrieved data, then generating the acoustic waveform according to the drive signal.

Preferably, the method includes completing said change of the waveform (e.g. frequency change) of the acoustic radiation in a period of time less than one nanosecond. Preferably, the change is substantially instantaneous.

The method preferably includes generating said acoustic radiation so as to propagate in a longitudinal or shear propagation mode. Preferably, the method includes changing the form (e.g. frequency) of said acoustic wave to change from a preceding substantially monochromatic first frequency to a succeeding substantially monochromatic second frequency differing from the first frequency.

In another of its aspects, the present invention may provide a method of laser display projection including a method for acoustically deflecting optical radiation according to the invention in its second aspect. In yet a further of its aspects, the present

invention may provide method of vehicle simulation including a method of laser display projection described above.

Examples of the invention follow with reference to the accompanying drawings, in which:

Figure 1 illustrates an acousto-optical deflection apparatus through which an acoustic waveform and optical radiation propagate in crossing paths such that the former deflects the latter where they cross;

Figure 2 illustrates deflection apparatus comprising two acousto-optical deflectors of the type illustrated in figure 1 ;

Figure 3 illustrates schematically the stages of a DDS waveform generator employed in the deflectors of figures 1 and 2;

Figure 4 illustrates schematically the process of changing a waveform according to a DDS method;

Figure 5 schematically illustrates a laser display projector for a vehicle simulator machine and the timings of a succession of simultaneous multiple projector laser beam deflections;

Figure 6 illustrates views of the optical train of deflection apparatus illustrated in figure 2.

Figure 1 schematically illustrates an acousto-optical deflector apparatus (1) including a Bragg cell comprising a transmission medium (2) in the form of a TeO ₂ crystal, an acoustic transducer (3) attached to a face of the transmission medium, and a deflection control means (4) operably connected to the acoustic transducer (3) via a drive signal transmission line (5).

The deflection control unit (4) includes electronic signal generator apparatus arranged to generate electrical drive signals which are transmitted to the acoustic

transducer via the signal transmission line (5). These drive signals possess a specified wave form (e.g. specified frequency, or a frequency distribution, and/or amplitude) which is predetermined to cause the transducer (3) to generate a corresponding predetermined acoustic waveform in response to receiving the electrical drive signal waveform.

Schematic representations of two different sequential waveforms (WF1 , WF2) are illustrated as propagating across the crystal transmission medium (2) from the acoustic transducer (3). The acoustic transducer is arranged to generate waveforms (WF1 and WF2) to be acoustic waves propagating in the longitudinal mode of propagation through the transmission medium with an acoustic velocity of about 4mm per microsecond. Each acoustic waveform is substantially monochromatic and has a substantially constant amplitude. The first waveform WF1 has a first frequency F1 , while the second waveform WF2 has a higher frequency F2. The second waveform WF2 follows substantially immediately and sequentially after generation of the first waveform WF1 is terminated. This results in a wave train comprising a first part composed solely of the first waveform and a second part, following the first part, composed solely of the second waveform.

As discussed below, the transition of the first waveform into second waveform occurs substantially instantaneously and without discontinuity in either waveform or the transition between them.

A laser beam (7) is directed into the crystal transmission medium (2) in a direction which crosses the path of the acoustic wave train (6) comprised of the first and second waveform. As is well known in the art, the successive pressure wave fronts of each acoustic wave generate a corresponding periodic variation in the refractive index of the crystal transmission medium (2) which results in an effective diffraction

grating from which the incoming laser beam (7) is deflected where it overlaps with the acoustic wave train (6). Consequently, where the laser beam (7) overlaps with those parts of the wave train comprised only of the first waveform WF1, which generate refractive index variations of a periodicity corresponding to that of the first waveform, the laser beam is deflected by an angle (the Bragg angle θi) corresponding with the periodicity of the first waveform thereby to result in an outgoing laser beam portion (11) propagating in a direction (12) deflected by an angle θi relative to the direction of propagation (8) of the incoming laser beam (7).

Similarly, those parts of the incoming laser beam (7) which overlap with those portions of the wave train comprised only of the second waveform WF2 experience refractive index variations of a periodicity corresponding with the periodicity of the second waveform, and consequently undergo an angular deflection through a Bragg angle θ ₂ associated with the periodicity of the second waveform. Thus, those parts of the incoming laser beam (7) not deflected through the Bragg angle θi associated with WF1, are deflected through a different Bragg angle θ ₂ associated with the second waveform WF2 resulting in an outgoing deflected beam portion (9) propagating in a direction (10) deflected by the second Bragg angle θ ₂ relative to the direction of propagation (8) of the incoming laser beam (7).

In this way, two simultaneous differing laser beam deflections are caused to occur in consequence of the sequential input to the Bragg cell of acoustic waveforms of distinctly separate frequency. It is to be understood that additional sequential acoustic waveforms may be input to the Bragg cell following the second waveform WF2, in which the subsequent waveforms differ from the second waveform and result in another simultaneous laser beam deflection. For example, the further different waveform could differ from both the first and the second waveforms and could be

input to the Bragg cell sufficiently quickly that it enters the region of overlap with the incoming laser beam (7) while both the first and second waveforms are still within that region and causing their associated beam deflections. The result would be three simultaneous different laser beam deflections. Of course, the only limit on the number of simultaneous beam deflections that may be produced in this manner comes from a limit on the time it takes a given waveform to traverse across the region of overlap within the Bragg cell between the acoustic wave and the input laser beam, together with the speed with which subsequent differing waveforms may be sequentially input to the Bragg cell.

Figure 2 schematically illustrates an acousto-optical deflector apparatus for imparting multiple consecutive deflections to the received laser beam in a manner as described with respect to Figure 1. The deflector apparatus includes a first Bragg cell (2A) controllable to acousto-optically deflect laser radiation (7) received thereby into multiple simultaneously deflected beams (9A, 11A) having different respective deflection angles (θi, θ ₂ ...etc) subtended in a common first plane (e.g. the "x" plane perpendicular to the plane of the page of Figure 2).

A second Bragg cell (2B) is positioned adjacent the optical output region of the first Bragg cell (2A) so as to simultaneously receive, as optical input, each of the deflected beams output by the first Bragg cell. The second Bragg cell is controllable to acousto-optically deflect, into multiple simultaneous deflection beams, each of the multiple deflected beams received thereby from the first Bragg cell. The second Bragg cell is arranged such that the beam deflection it imparts results in an angle of deflection subtended in a plane perpendicular to the plane in which the deflection angles of the first Bragg cell are subtended (e.g. the deflection angles of the Bragg cell subtend in the "y" plane parallel to the page of Figure 2).

The second Bragg cell (2B) is also controllable to impart only a common single deflection angle simultaneously to deflection beams (9A, 11A) simultaneously received from the first Bragg cell, if required.

Each of the first and second Bragg cells is generally arranged and operates as describes above with reference to Figure 1 , and each is driven by a common deflection control unit (4). The deflection control unit includes a control signal generator (13) arranged to generate digital control signals which, ultimately, control the operations of the first and second Bragg cells (2A, 2B). Each of the first and second Bragg cells is connected to a control signal output (100) of the control signal generator via a respective one of a first and second signal paths (101A, 101 B).

Each of the first and second signal paths contains a Direct Digital Synthesis (DDS) unit (14) arranged to receive digital control signals output (100) from the control signal generator, to generate an analogue drive signal according to the digital control signal in a manner described in detail below, and to output the analogue drive signal to a signal filter unit (15) connected thereto for filtering the drive signal. The signal filter unit is a low-pass filter arranged to block the passage therethrough of undesired high frequency signal component (e.g. typically associated with noise and/or signal transients). The signal output of the filter unit is connected to the signal input of an acoustic transducer (3A, 3B) associated with the Bragg cell (2A, 2B) served by the filter. In this way, digital control signals generated by the control signal generator (13) are directed along a given signal path (101A, 101B) and are received by a DDS unit (14) which responds by generating an analogue drive signal according to the control signals, and outputs the analogue drive signal to an associated Bragg cell via a low pass signal filter unit.

The digital control signals are typically electrical signals, as are the subsequent analogue drive signals. The control signal unit (13) includes a digital processor unit (not shown) arranged to generate suitable control signals according to a preprogrammed set of instructions and/or according to contemporaneous input instructions from a user.

Of course, the control signals are generated in such a way as to result in the generation of a desired analogue drive signal waveform which, in turn, results in the generation, by the acoustic transducer (3A, 3B) ultimately in receipt of that drive signal, of a desired acoustic waveform within the associated Bragg cell (2A, 2B).

In this way, each of the first (2A) and second (2B) Bragg cells may be driven in tandem according to dedicated control signals from a common control unit (30). Of course, each Bragg cell may have a separate dedicated control unit in other embodiments, or the control unit of Figure 2 may comprise a separate dedicated digital processor unit (not shown) for each Bragg cell. A controlled deflection of an input laser beam (7) in multiple different directions simultaneously may thereby be achieved in two dimensions of deflection (i.e. in both the "x" and "y" directions).

Figure 6 schematically illustrates in more detail the optical train of the acousto-optical deflector of Figure 2. A plan view and a side view are presented. The optical train begins with a cylinder lens (61) arranged such that received laser radiation (7) is focussed upon the first Bragg cell (2A), which is the second optical element in the optical train. A biconvex lens (62) is positioned beyond the optical output side of the first Bragg cell so as to receive and focus each deflected beam (9A, 11A) output thereby onto the second Bragg cell (2B) arranged subsequently in the optical train. A final cylinder lens (63) ends the optical train and is positioned to collimate multiple

diverging deflected optical beams (9B, 11B) emanating from the second Bragg cell (2B).

Figure 3 schematically illustrates the stages within, and the operation of, a DDS unit (14) as employed in the present embodiments, and preferably in any embodiment of the present invention.

The DDS unit is arranged to store data points representing successive points on a waveform in digital format and to recall/retrieve selected such data points in sequence as along the waveform (increasing phase of the waveform) by controlled increments and use the recalled data points to reconstruct a waveform. The speed with which the DDS unit advances, sequentially, through the store of data points determines the rate of reconstruction of sequential/successive parts of a waveform and so determines the rate of phase advance (e.g. the frequency) of the waveform.

The DDS unit (14) includes a phase accumulator unit (18) having a phase increment input port for receiving phase increment signals from a processor unit (not shown) of the control signal generator (30), and a clock signal input port for receiving clock signals with which to regulate phase increments. The phase accumulator is arranged to generate a data retrieval signal, according to a phase increment command conveyed by the phase increment signal, which is operable to cause retrieval of a given stored data item associated with a point on the stored waveform which is ahead (on the waveform) of the immediately previously retrieved such data item by an amount corresponding to the phase increment command. In this way the phase accumulator (18) is able to generate data retrieval commands, issued in synchrony with a clock signal, which will result in retrieval of stored waveform date points variably separated (in phase) by the increment conveyed in the increment command.

The DDS unit (14) includes a waveform map unit (19) containing the aforesaid stored waveform data points/items, and is arranged to perform the aforesaid data retrieval in response to receipt of retrieval signals issued thereto by the phase accumulator (18), and to generate, increment-by-increment, a digital waveform from the retrieved data items. It is to be understood that each data item contains information defining its phase position in a waveform common to all such stored data items, and its relative amplitude within the waveform.

The waveform map unit contains a data store (e.g. ROM or PROM) containing the waveform data items, which preferably define a sinusoidal waveform, but may define any other waveform if required. Also, a digital signal generator is included which is arranged to generate a digital waveform, increment-by-increment, according to the successively retrieved waveform data items, and to output the digital waveform to a digital-to-analogue (D/A) converter (20) which converts the digital waveform into an analogue form and outputs the result to the low pass filter unit (15) as the analogue drive signal output of the DDS unit.

Figure 4 schematically illustrates a methodology for substantially instantaneously changing the waveform of the drive signal generated by the DDS unit, and for changing the corresponding acoustic waveform generated by the acoustic transducer (3, 3A, 3B) of a Bragg cell as discussed above.

Consider the phase of a sinusoidal stored waveform (within the waveform map unit 19) as phase advances around a circle (21) illustrated in Figure 4. Each data point (22, 27) on the phase circle (21) represents a retrieved waveform data item, both in terms of its phase position within the stored waveform and its relative "amplitude" or "height" within the waveform (i.e. the projection of the data point onto the vertical axis of the phase circle).

A first phase increment signal ("phase increment #1") corresponds to a small phase step size (e.g. phase 23 to phase 24) such that successively retrieved data items are quite close in phase. Since each phase increment (each data retrieval operation) occurs in synchrony with a regular clock signal, this means that small phase increments result in a slow progress of the phase of the constructed wave as time progresses, and the reconstructed waveform has a low frequency. By increasing the size of the phase increment to a larger second value (e.g. "phase increment #2") corresponding a larger phase step size (e.g. phase 25 to phase 26), successively retrieved data items are more spaced in phase. Since the rate of retrieval of data items remains the same as between the first (#1) and the second (#2) phase increments, this means that the phase of the reconstructed wave is advanced more rapidly with use of the large phase increment size. This results in a reconstructed waveform of correspondingly higher frequency.

In the present example, the second phase increment is twice the size of the first phase increment. The transition from the first to the second phase increment size occurs immediately after the first increments have reached a phase of 180°. This results in the reconstruction of a first half of a sinusoid according to the first increments (#1) and an immediately succeeding half sinusoid, following without discontinuity from the first half, having twice the frequency of the first half sinusoid.

For example, the first half sinusoid may represent the first waveform (WF1 ) illustrated in Figure 1 , and the second half (and succeeding) sinusoid may represent the second immediately succeeding acoustic waveform (WF2). In this way one may substantially instantaneously change the acoustic waveform in a Bragg cell from one form to another immediately succeeding form. The transition between waveforms may effectively occur within the time period between successive clock signals, and is most

preferably a time period less than about 100 nanoseconds, and more preferably less than 50 nanoseconds or even more preferably less than 1 nanosecond.

Figure 5 illustrates an application of the present embodiments of the invention as a laser projector device including a laser projector unit (31) and a display screen (32) onto which the laser projector unit is arranged to project images by the suitably controlled deflection of a laser beam (32, 33, 34).

The projector includes an acousto-optical deflector apparatus of any embodiment of this invention. The deflector apparatus is driven to produce two simultaneous laser beam deflections each beam deflection being one of a succession of different beam deflections, each lasting for a "dwell" time period Δt. A first beam deflection (32) occurs as a first acoustic waveform WF 1 alone overlaps with received laser radiation within a Bragg cell (2) of the deflector. This acoustic waveform is immediately succeeded by a second acoustic waveform WF2 which causes a different beam deflection (33) in those parts of the laser beam with which it overlaps. However, the succession of acoustic waveforms from W1 to WF2 occurs with sufficient rapidity that the first acoustic waveform remains in overlap with parts of the received laser beam while the second acoustic waveform enters into such overlap. Two different simultaneous beam deflections (32, 33) result.

Subsequently, a third (34), fourth (35) and fifth (36), and so on, acoustic waveforms respectively succeed the second (33), third (34) and fourth (35), etc waveforms within the Bragg cell (2). The timing of this succession is such that a succeeding acoustic waveform enters into overlap with the received laser beam just as the waveform immediately before the preceding acoustic waveform ceases such overlap - having propagated fully across the overlap region. In this way two simultaneous beam deflections are provided in which the two beams effectively step or "walk" across the

display screen (30). This also doubles the rate with which the laser beam can be progressed through successive deflections since a given beam "dwell" time Δt contains two simultaneous deflections rather than just one, consequently projected images may be generated more quickly or images with more image points (beam deflections) per image "frame" may be produced.

This is particularly useful in permitting rapid and/or complex projected image formations for use in laser projectors in vehicle (e.g. aircraft) simulators required to simulate a "view" for the user (e.g. representing runway lights on a simulated projected image from the cockpit of an aircraft simulator) comprising many image light points. Large numbers of these image light points may, according to embodiments or aspects of the present invention, be generated more rapidly thereby enabling the larger number of such light points per frame of the projected image. Light points with an angular spread of about 5 arc minutes or less may be achieved when the projector is used in a typical existing aircraft simulator machine, and a light point brightness of around 25 ftL.

Preferably a given Bragg cell is driven with drive signals of a frequency within the range 100-600 MHz, and the drive frequency is resolved to within a frequency resolution of at least 1KHz so as to enable the fine tuning of beam deflection/alignment in use. Driving the Bragg cells in longitudinal mode results in acoustical waveforms which have a relatively high acoustic velocity (e.g. 4.2mm per microsecond). The Bragg cells typically provide an overlap region (where propagating acoustic waveforms may overlap with received optical radiation) corresponding to an aperture having a diameter of around 25mm. This means that a wave front on a given longitudinal acoustic wave fully traverses the overlap region within about 6 microseconds. Thus, if each projected laser light point is required to "dwell" for a period of time Δt of the order of a microsecond or less (e.g. a few

hundred nanoseconds) then this means that the high velocity of the longitudinal acoustic wave, of a suitably high frequency, allows at least one succeeding acoustic waveform to enter a substantial part of the overlap region within the aforementioned "dwell" time period. This is a key advantage as compared to acoustic waveforms which propagate in the shear mode and which have a correspondingly much lower acoustic velocity. However, it is to be understood that the present invention encompasses the use of shear mode acoustic wave propagation.

In practice about 5000 or more light points may be projected by the projector per image frame when operating at a frame rate of 60Hz.

It is to be understood that modifications and variations to the embodiments described above, such as would be readily apparent to the skilled person, may be made without departing from the scope of the present invention, e.g. as defined in the claims.