GB2427282A - Acousto-optic deflection - Google Patents

Abstract

Apparatus for acoustically deflecting optical radiation to form a directed optical beam including a deflector means 1 for receiving optical radiation 8 and deflecting the optical radiation with acoustic wave(s) 6 wherein the deflector means is arranged to modulate the acoustic wave(s) such that acoustic waves of differing frequency simultaneously deflect received optical radiation by differing respective amounts thereby to form a directed optical beam, and to control the distribution of light intensity across the directed optical beam by controlling said modulation. A corresponding method is also disclosed. The apparatus may be used to control a laser display projector in an aircraft simulator machine to simulate a diffuse light source.

Description

Acousto-optic Deflection The present invention relates to apparatus and

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

It is well know that deflection of a light beam may be based on the deflection 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 photo-elastic 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 the acoustic diffraction 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-optic devices known as "Bragg cells". The angle through which incident optical radiation is deflected by interaction with the acoustically generated diffraction grating is dependent upon the ratio between the wavelength of the optical radiation and the wavelength of the acoustic wave. Thus, by suitably controlling the wavelength of the acoustic wave one may control the form of the phase grating and thereby the angle of optical deflection imparted by that grating.

Optical display projectors, such as laser projectors, sometimes employ acoustic-optic deflectors as the means for controllably deflecting a laser beam to cause the laser beam to scan across a projector display surface thereby to "draw" a projected image thereupon. By suitably controlling the extent to which a laser beam is deflected by an acousto- optic deflector one may control the position at which the deflected laser beam impinges upon a display surface. That is to say, the position of the laser beam "footprint" upon the display surface may be controlled by suitably controlling the degree of opticaJ deflection imparted by the acoustooptic deflector of the laser projector.

Conversely, the visual appearance of the laser "footprint" - such as its shape, or intensity - are typically controlled separately of the acoustooptic deflector by means of suitable optical elements such as lenses or mirrors with which one may control the appearance of the beam footprint.

However, the degree of control of the appearance of the beam "footprint" which may be implemented using such separate optical elements is often limited. The very provision of such optical elements is itself expensive, as is the expertise typically required to install or prepare such separate optical elements. This is particularly problematic when rapid variations in the appearance of a deflected beam footprint are required. A correspondingly rapid variation/control of the separate optical elements is essential in this case if success is to be found.

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

The present invention is concerned with, among other things, the appearance of a projected light beam "footprint" in terms of the shape, intensity, and/or the distribution of brightness across the "footprint" (e.g. spatial intensity profile). The invention relates to acousto-optic deflectors (such as Bragg cells) arranged to be driven by a drive signal (e.g. an electrical signal) and being responsive to the drive signal to produce an acoustic wave for use in deflecting light.

At its most general, the present invention proposes modulating the acoustic waves generated in an acousto-optic deflector with modulation signals chosen to shape in a desired way the distribution of light intensity within light (e.g. a light beam) deflected by the deflector. For example, the present invention may involve modulating the drive signal used to drive the acousto-optic deflector with the modulation signals thereby to cause it to generate the modulated acoustic waves.

In this way, the present invention may obviate the need of separate optical elements for controlling the appearance of e.g. a projected light beam "footprint". By implementing such control acoustically one may implement changes in "footprint" appearance as rapidly as the acoustooptic deflector is able to respond to appropriate changes in the drive signal thereof. Thus, simple changes in the modulation of the drive signal may implement corresponding desired changes in the distribution of light intensity within the light deflected by the driven deflector, thereby very rapidly changing the appearance of the footprint of projected light deflected by the acoustic-optical deflector.

In a first of its aspects, the present invention may provide an apparatus for acoustically deflecting optical radiation to form a directed optical beam, including a deflector means for receiving optical radiation and deflecting the optical radiation with acoustic wave(s) wherein the deflector means is arranged to modulate (e.g. with a continuous or a noncontinuous modulation spectrum) the acoustic wave(s) such that acoustic waves of differing frequency simultaneously deflect received optical radiation (preferably a light beam) by differing respective amounts thereby to form a directed optical beam, and to control the (e.g. continuous or non-continuous) distribution of light intensity within the directed optical beam by controlling said modulation. The apparatus may include control means for controlling the modulation applied to the acoustic waves and for controlling the distribution of light intensity accordingly Since the frequency of acoustic waves within the deflector means determines the degree of deflection imparted to optical radiation, by simultaneously providing a plurality of acoustic waves of differing frequency, the present invention produces a corresponding plurality of simultaneous optical deflections of a plurality of respective differing amounts. Of course, it will be understood that references to deflecting optical radiation with acoustic waves may refer to the deflection of the former by interaction with the wavefronts of the latter within the acoustic transmission medium of the deflector means. Also, such deflection may be expressed as a collision or interactions between photons (light quanta) and phonons (sound quanta).

By controlling the spectral distribution of the simultaneous different acoustic waves, both in terms of the range of different frequencies employed and in terms of the relative strengths of each acoustic wave of a given frequency, the present invention may control the distribution - in terms of both position and relative intensity - of deflected optical radiation.

The deflector means may be arranged to generate said acoustic waves as a carrier wave (e.g. a continuous wave of fixed frequency) modulated by a modulation signal having a signal frequency spectrum, the deflector means being arranged (e.g. using the control means) to selectively shape said spectrum according to a selected distribution of light intensity to be produced within the directed optical beam. The deflector means may be arranged to generate an electrical drive signal having a predetermined waveform as described herein and may drive an acousto-optic means (e.g. a Bragg cell or the like) using the electrical drive signal to produce the acoustic waveform. The electrical drive signal waveform may be substantially the same, or similar, to the resulting acoustic waveform. The modulation signal frequency spectrum may be substantially centred upon the carrier wave frequency, or may be centred away from the modulation signal frequency. For example, in practice, the modulation signal spectrum of the electrical drive signal may be centred about 0Hz when it is being generated, but when it is applied to the carrier wave, it is centred about the carrier wave. The acoustic waveform is typically generated directly from this electrical waveform.

In this way, the choice of the shape of the frequency spectrum of a modulation signal directly imposes a corresponding shape on the distribution of light intensity produced within the deflected optical radiation. For example, the signal frequency spectrum of the modulation signal may be a substantially continuous spectrum (e.g. no gaps) having a predetermined shape (e.g. amplitude distribution of spectral components).

Applying this modulation signal to the carrier wave has been found to produce a deflected light beam "footprint" having a correspondingly continuous distribution of light (e.g. no gaps) having an intensity distribution mirroring the amplitude distribution of the spectral components of the modulation signal. Accordingly, one may shape the signal frequency spectrum of the modulation signal in any way one chooses to implement a corresponding shape to the light intensity distribution of a deflected optical beam "footprint".

The modulation signal frequency spectrum may be either symmetrical thereby to produce a correspondingly symmetrically distributed light beam "footprint" - or maybe asymmetrical so as to produce a deflected light beam footprint having a correspondingly asymmetrical spreading of light intensity. For example, the modulation signal frequency spectrum may be shaped such that the amplitude distribution of the spectral components of the modulation signal are Gaussian in form. This produces a corresponding spreading of the "footprint" of a deflected light beam such that the distribution of light intensity at any point within the "footprint" of the beam follows a corresponding Gaussian form (i.e. brightest in the centre, becoming dimmer as one moves from the centre of the "footprint" and being symmetrical about the centre of the "footprint"). The Gaussian form of modulation signal frequency spectrum is particularly well suited to simulating the appearance of bright lights in fog as are often required to be produced by display projectors employed in aircraft simulators. These optical projectors are typically driven to generate a simulated view from the cockpit of an aircraft, often requiring the simulation of light runway and/or vehicle/building lights seen by the pilot of an aircraft in flight.

Thus, the carrier wave frequency may determine the position/deflection of the centre of the beam "footprint", while the modulation signal determines the shape and appearance of the "footprint".

The modulation may be a modulation of any one or more of: the amplitude; the frequency; the phase of the acoustic wave(s). The appropriate choice of modulation may be made according to the degree of control of the modulation signal frequency spectrum required, and thereby the degree of control of the distribution of light intensity within the deflected optical radiation. By simultaneously modulating a plurality of different properties of the drive signal (e.g. both amplitude and frequency) one may exercise a greater degree of control over the modulation signal frequency spectrum. For example, an asymmetrical shaping of the modulation signal frequency spectrum may be implemented in this way according to the present invention.

The modulation of the acoustic wave(s) may be a modulation simultaneously of: both the amplitude and the frequency; or, both the amplitude and the phase thereof. The acoustic wave(s) may include a carrier wave having two components in phase quadrature with each other, and the deflector means may be arranged to separately modulate each component to implement a vector modulation. The deflector means may be arranged to implement any of the following types of signal modulation by the modulation signal: Phase, Shift, Keying (PSK), In-Phase and Quadrature Modulation (I/a), Quadrature Amplitude Modulation (QAM), Frequency Shift Keying (FSK) or any other suitable modulation methods such as would be readily apparent to the skilled person. The deflector means is preferably operable to selectively adjust the modulation to render the frequency spectrum of the modulated acoustic wave(s) asymmetrical in shape thereby to render asymmetrical the distribution of light intensity within the directed optical beam Vector modulation is especially well suited to this and the deflector means preferably employs vector modulation to generate such asymmetrical spectra.

The modulation signal frequency distribution may be a continuous distribution having no spectral gaps (or no substantial or effective gaps) . The modulation signal frequency spectrum may be bounded within a predetermined spectral bandwidth outside of which there are substantially no modulation signal spectral components.

The modulation signal frequency spectral components may be provided in any suitable manner such as would be readily apparent to the skilled person and, where a continuous spectral distribution is required, spectral filling techniques may be employed to ensure an effectively continuous spectrum. The deflector means is preferably arranged to modulate the acoustic wave(s) with a random or pseudo- random modulation signal which varies a selected property of the acoustic waves in a desired random or pseudo-random manner. Pseudo-random variation of the selected property may comprise the selection of a pseudo- random value for a selected property of the acoustic wave(s) from within a predetermined range of search values. For example, pseudo-random modulation of the amplitude of the acoustic wave(s) may comprise pseudo- random selection of an amplitude value from within a band range of amplitude values (which may be a continuous range and may contain both positive and negative values), with the subsequent adjustment of the acoustic wave amplitude to match the pseudo-randomly selected amplitude value.

Similar such pseudo-random modulation methodologies may be applied to modulation of the frequency and/or phase of acoustic wave(s). The frequency spectral distribution of the modulation signal may have a variable width of between 0MHz and 15 MHz, and may have a width of between 0Hz to 10MHz, or between 0Hz to 5MHz. The frequency spectral distribution of the modulation signal may be Gaussian. This has the effect of producing a smooth and symmetrical spreading or "diffusion" of a projected beam "footprint" intensity distribution simulating the view of a light in fog.

Preferably, the deflector means includes an acousto-optic means arranged to deflect said received optical radiation with said acoustical wave(s) generated thereby according to a drive signal, and drive means arranged to generating the drive signal and to modulate the acoustic wave by modulating the drive signal.

The deflector apparatus may have an acousto-optic deflector means for deflecting received optical radiation with acoustic waves generated thereby according to a drive signal, a drive means for generating said drive signal the signal comprising a carrier signal modulated with a modulation signal controlled such that the distribution of light energy within a cross-section of a light beam deflected by the acoustic wave(s) is controlled according to the modulation signal thereby to control the distribution of light within the projected "footprint" of the beam in use.

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

In a second of its aspects, the present invention may provide a display projector apparatus including apparatus for acoustically deflecting optical radiation according to the invention in its first aspect. The optical radiation is preferably a laser beam.

The projector may be arranged to project calligraphic light points upon a display surface, the light points may simulate bright lights within a simulated scene.

In a third of its aspects, the present invention may provide a vehicle simulator apparatus including a display projector apparatus according to the invention in its second aspect: for example, an aircraft simulator, the display projector being arranged to project a simulated scene to be viewed from the cockpit of the simulator.

It is understood that the invention is described above as an implementation of a method of acoustically deflecting optical radiation, and that each aspect, and preferred or optional feature thereof, embodies a corresponding method encompassed by the invention, as discussed below for example.

In a fourth of its aspects, the present invention may provide a method for acoustically deflecting optical radiation to form a directed optical beam, including deflecting optical radiation with acoustic wave(s) modulated such that acoustic waves of differing frequency simultaneously deflect optical radiation by differing respective amounts thereby to form a directed optical beam, and controlling the distribution of light intensity within the directed optical beam by controlling said modulation.

Thus, the present invention may provide the modulating of a drive frequency for an acousto-optic deflector with a modulation signal having a plurality of frequency values chosen to cause a spreading of the distribution of optical intensity across a radiation beam deflected by the acousto-optic deflector.

The method preferably includes generating said acoustic wave(s) as a carrier wave modulated by a modulation signal having a signal frequency spectrum, and selectively shaping said spectrum according to a selected distribution of light intensity to be produced within the directed optical beam.

The modulation may be a modulation of any one or more of: the amplitude; the frequency; the phase of the acoustic wave(s) The modulation of the acoustic wave(s) may be a modulation simultaneously of: both the amplitude and the frequency; or, both the amplitude and the phase thereof. The acoustic wave(s) may include a carrier wave having two components in phase quadrature with each other, and the method may include separately modulating each component to implement a vector modulation. The method preferably includes selectively adjusting..the modulation to render the frequency spectrum of the modulated acoustic wave(s) asymmetrical in shape thereby to render asymmetrical the distribution of light intensity within the deflected optical radiation.

The method may include modulating the acoustic radiation with a random or pseudo- random modulation signal which varies a selected property of the acoustic radiation in a desired random or pseudo-random manner. The frequency spectral distribution of the modulation signal may be Gaussian.

The method preferably includes providing an acousto-optic means arranged to deflect optical radiation with said acoustical wave(s) generated thereby according to a drive signal, and generating the drive signal and modulating the acoustic wave(s) by modulating the drive signal.

The method may comprise imposing a first deflection upon received optical radiation with acoustic wave(s) as discussed above, and imposing a subsequent second deflection upon the first-deflected optical radiation using acoustic wave(s) as discussed above. The second deflection is most preferably a deflection that is in a direction substantially transverse to the direction in which the first deflection of optical radiation occurs. For example, the second deflection imparted upon the optical radiation may be substantially perpendicular to the direction of deflection imparted by the first deflection of that radiation. Thus, the present invention may provide a method of two successive transverse optical deflections of optical radiation (e.g. a laser beam) thereby providing deflection in two dimensions.

In a fifth 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 fourth aspect.

In a sixth of its aspects, the present invention may provide a method of vehicle simulation including a method of laser display projection according to the invention in its fifth aspect.

Examples of the invention follow with reference to the accompanying drawings, in which: Figure 1A illustrates an acousto-optic 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 1 B schematically illustrates the drive signal spectrum incorporating both the suppressed carrier signal spectral component and the modulation signal spectral components; Figure 1C schematically illustrates the light intensity distribution across the cross-section of a laser beam deflected by the acousto-optic deflection apparatus of Figure 1A driven according to a drive signal having a drive signal spectrum according to Figure 1B; Figure 2 illustrates a schematic representation of a drive signal generator operable to generate a drive signal according to amplitude modulation; Figure 3A illustrates a schematic view of a drive signal generator operable to generate a drive signal according to vector modulation; Figure 3B schematically illustrates an asymmetrical drive signal spectrum generated according to vector modulation employed in the drive signal generator of Figure 3A, including the suppressed carrier signal spectral component and the modulation signal spectral components; Figure 3C schematically illustrates the light intensity distribution across the cross-section of a laser beam deflected by an acousto-optic deflector driven according to the drive signal spectrum illustrated in Figure 3B; Figure 4A schematically illustrates a two-stage acousto-optic deflector employing the first deflector for deflecting an incoming light beam in the X- direction, followed by a subsequent acousto-optic deflector for deflecting the optical output of the X-deflector in a Y-direction (perpendicular to the previous deflection); Figure 4B schematically illustrates the three-dimensional light intensity distribution across the cross-section of the deflected light beam output from the Y- deflector of Figure 4A: Figure 4C illustrates the appearance or "footprint" of a projected light beam possessing the intensity distribution illustrated schematically in Figure 4B; Figure 5 schematically illustrates a laser display projector for a vehicle simulator machine and projector laser beam deflections; Figure 1 schematically illustrates an acousto-optic deflector apparatus (1) including a Bragg cell comprising a transmission medium (2) in the form of a Te02 crystal (other crystals, such as quartz or PbMO4, or the like, may be used), 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 that are transmitted to the acoustic transducer via the signal transmission line (5). These drive signals possess waveforms having a specified frequency distribution of spectrum which is predetermined to cause the transducer (3) to generate an acoustic waveform having a corresponding frequency spectrum in response to receiving the electrical drive signal waveform.

Acoustic wavefronts (6) are thus caused to propagate from the acoustic transducer (3) across the body of the transmission medium of the Bragg cell as indicated by arrows (7). A diffraction grating (6) is generated by an acousto-electric effect in which pressure variations along the propagating acoustic waveform generate corresponding variations in the refractive index of the transmission medium (2) of the Bragg cell. The nature of the diffraction grating is dependent upon the frequency spectrum of the propagating acoustic waveform.

The transmission medium of the Bragg cell is arranged to receive a beam (8) of laser light to be deflected, and is arranged to receive the laser beam such that it impinges upon the diffraction grating formed by the acoustic waveform and is deflected by an amount dependent upon the nature of the acoustic waveform, Of course, this is a classical picture of the interaction between sound and light within the transmission medium of the Bragg cell. In a Quantum treatment, the interaction may be considered as being between quanta of light (photons) and quanta of sound (phonons). Optical radiation (8) is incident upon an acoustic wavefront (6) within the Bragg cell and is deflected by an angle to OB (first order diffraction) by the grating of which the acoustic wave front effectively forms a part. The "Bragg Angle" B is given by: = arcsin (2(/2AA) Equation (1) Where is the wavelength of the light forming the light beam within the Bragg cell, and AA is the wavelength of an acoustic wave in the Bragg cell.

Thus, the wavelength of the acoustic wave form 2A determines the degree of deflection of the optical radiation incident upon it. It can be shown that the deflected light intensity I is dependent upon the acoustic power P conveyed by the acoustic wave form (6) within the Bragg cell, where: I = I sin2[r/2jP/Pj Equation (2) Where I is the incident light intensity and P0 is a constant determined by the physical properties of the Bragg cell, and the wavelength of the incident light. The diffraction efficiency is 1/10.

When the orientation (OB) of the incident light beam (8) is such that the exact Bragg condition does not quite occur (i.e. equation (1) is not exactly complied with), then diffraction and beam deflection will still occur, but to a lesser diffraction efficiency.

This may also occur when the acoustic frequency FA changes such that it does not exactly satisfy equation (1). It can be shown that a change AFA in the acoustic frequency results in a change e in the angle of deflection of optical radiation by the acoustic wave in question, where AO AF and n is the refractive index of the fly4 transmission medium (2) while VA is the velocity of sound therein. It can be shown that the diffraction efficiency varies as: -{sinc[ir/2J7P +AI /C]}2 Where FA is the change in acoustic frequency, and C is a constant determined by the properties of the Bragg cell, and the optical radiation and acoustic wave(s).

Thus, small changes FA in the frequency of the acoustic waveform within the Bragg cell result in small changes e in the degree of deflection of the light beam incident upon the Bragg cell with an insignificant change in diffraction efficiency.

Referring to Figure 1 B, the deflection control means (4) is arranged to generate an electrical drive signal comprising a suppressed main carrier wave component (indicated as item 10 for clarity) having a frequency of Al Hz and a modulation component (11) which modulates a selected property of the main carrier wave with a noise/pseudo-random modulation, the frequency spectrum of the modulation being substantially continuous and centred upon the frequency of the carrier wave component, and effectively extending over a range of Bi Hz (below the carrier frequency) to Cl Hz (above the carrier frequency).

The electrical drive signal is conveyed to the acoustic transducer (3) of the Bragg cell via the drive signal transmission line (5) and, uponreceipt by the acoustic transducer, causes the transducer to generate the acoustic waveform (6) within the transmission medium of the Bragg cell having substantially the same (or similar) frequency spectrum as that of the drive signal. Figure 1 B schematically illustrates an example of the frequency spectrum of the drive signal.

The simultaneous presence, simultaneously, of a spectrum of acoustic signal frequencies within the Bragg cell results in a simultaneous deflection of the incident laser beam (8) through a corresponding spectrum of beam deflections. The frequency Al of the carrier wave component of the drive signal determines the degree of overall direction (deflection OB) of the laser beam while the spectral range and shape of the modulation signal component of the drive signal determines the degree of simultaneous deflection of some of the light from the incident laser beam into deflection angles either side of that part of the incident beam deflected by the carrier wave component. The upper and lower frequency parts (e.g. Bi and Cl) of the modulation signal spectral range result in a deflection of parts of the light beam by a continuous range of angles 02 centred upon the angle A2 by which the beam as a whole is deflected/directed by the carrier signal component. Since substantially all frequencies within the range Bi Hz to Cl Hz are present in the acoustic waveform (6) forming the diffraction grating within the Bragg cell, then all laser beam deflection angles within the range B2 degrees to C2 degrees are imposed upon the incident laser beam (8). The result is an effective angular spreading of the light within the deflected laser beam (9) by at least an angle 02 = B2 - C2 centred upon a deflection angle A2. A directed laser beam "footprint", having a cross-sectional intensity profile (12) as illustrated in Figure 1C results. Of course, if the spectral shape of the drive signal is not band limited, e.g. in Gaussian, then upper and lower frequencies Bi and Cl merely represent nominal effective limits but it will be understood that very small amounts of drive signal may exceed these limits in reality.

The deflected light intensity is dependent upon the acoustic power of the spectral component of the acoustic waveform causing a given angular deflection. The deflection control means (4) is arranged to control the shape or the frequency spectrum of the drive signal generated thereby so as to correspondingly control the shape of the frequency spectrum of the acoustic waveform generated by the acoustic transducer in response to the drive signal. This enables control of the distribution of light intensity across the directed light beam (i.e. in cross-section), and therefore, the appearance of the "footprint" of the directed light beam (1) projected upon a display surface. Figure 1 C schematically illustrates the correspondence between the spectral profile of the electrical drive signal generated by the deflection control means (4) and the resulting intensity profile of the deflected (directed laser beam (9)).

Figure 2 schematically illustrates an example of a deflection control means (4) of an embodiment of the invention operable to generate a drive signal with a frequency spectrum which is symmetrical about the carrier wave frequency therewith to cause the generation of deflected/directed beam intensity profiles which are symmetrical (e.g. as in Figure 1C).

The control means includes a direct digital synthesiser (DDS) unit (16) for digitally synthesising a carrier wave signal component of the drive signal, and for outputting the results in an analog format. The DDS unit is operable to generate a carrier wave having a frequency from 0 MHz to 500 MHz, or thereabouts, as determined by a synthesis control oscillator (15) which provides a first control input to the DDS unit. A second control input (14) is provided by the DDS unit via which other control functions of the DDS unit may be implemented such as would be readily apparent to the skilled person.

The signal output port via which the DDS outputs the analog carrier signal is connected to a signal input port of a signal filter (20) operable to filter the analog carrier signal received thereby. The DDS unit may typically be arranged to sample at a rate of 1 0Hz (the manner and purpose of such "sampling" shall not be discussed here, and will be readily apparent to the skilled person), and the signal filter (20) may correspondingly most preferably be set to reject alias frequencies typically above 600 MHz. The output of the signal filter unit (20) is connected to the input of a signal amplifier (21), the latter being arranged to amplify filtered carrier wave signals emanating from the signal output of the signal filter thereby to raise the filtered carrier signal power level to a level suitable for driving the modulator unit (22) to which the output of the amplifier unit (21) is connected. In this way, the drive control unit (4) is operable to generate an analog carrier signal component of a desired frequency, to filter and then amplify the carrier wave component, and to input the result to a modulator unit for subsequent modulation.

The modulator unit (22) of the drive control unit is an amplitude modulator having not only a carrier signal input port for receiving the carrier signal as discussed above, but a modulation input port for receiving a modulation signal with which to modulate the carrier signal simultaneously received thereby. In the present embodiment the modulation signal is a noise pseudo-random signal derived from a digital pseudorandom noise signal input to a modulation signal input port (13) of the drive control unit, which is connected to the input of a digital spectrum shaping filter (130) arranged to impose a predetermined spectral shape upon the digital noise signal received thereby. The output of the digital spectrum shaping filter is connected to the input of a digital-to-analog converter (DAC) (17) which converts the received digital filtered modulation signal into an analog format. The analogue output port of the DAC unit is connected to the signal input port of a reconstruction filter (18) arranged to filter the frequency spectrum of modulation signals received thereby so as to stop signals of above about 500MHz and remove aliases from the signal. The output port of the reconstruction filter (18) is connected to the modulation signal input port of the modulator unit (22) via an intermediate signal amplifier (19) arranged to amplify to a level suitable for input to the modulator unit filtered modulation signals output from the signal filter unit.

The modulator unit (22) is operable to impose a pseudo-random noise modulation, using the modulation signal received thereby, to the carrier signal simultaneously received thereby, and to output the result along drive signal transmission line (5), as an analog electrical drive signal for driving the acoustic transducer (3) of the Bragg cell. An amplifier (not shown) may be placed between the output of the modulator and the input of the acoustic transducer for amplifying filtered modulated signals output by the modulator thereby to raise the drive signal power level to a level suitable for driving the acoustic transducer.

The modulation signal is generated digitally as a pseudo-random number selected from a sequence of numbers (e.g. a number selected randomly from the range +1 to -1) such that the spectral frequency distribution of the digital modulation signal is substantially or effectively continuous (at least without significant gaps) across a predetermined spectral bandwidth. In other embodiments the spectrum may contain deliberate gaps. The shape of the frequency spectrum of the analog modulation signal subsequently input to the modulator unit (22) is determined by the spectral response of the digital spectrum shaping filter (130) into which the raw digital noise modulation signal is input prior to input to the modulator unit. In this way, the acoustic power associated with a given spectral component of the ultimate acoustic waveform generated in the Bragg cell may be controlled by appropriately controlling the strength of the corresponding spectral component of the modulation signal within the drive signal used to generate the acoustic waveform. This drive signal spectral control is, in the present embodiment, made by a suitable selection of the spectral response of the digital shaping filler unit (130). In other embodiments, or in a modification of the present embodiment, additional spectral control of the modulation signal may be put into effect by appropriately weighting the spectral components of the digital modulation signal input to the drive control unit (4) via the modulation signal input port (13) thereof.

Thus, in use, a carrier wave is generated by the DDS unit (16) of the drive control unit in analog form, and this analog carrier signal is subsequently filtered and amplified before being input to the modulator unit (22). Simultaneously, a digital pseudo-random noise modulation signal input to the drive control unit is digitally filtered to impose a desired frequency spectral shape thereupon, is converted to analogue format, reconstruction filtered to remove undesired frequency spectral components, amplified to an appropriate level and input as a modulation signal to the BPSK modulator unit for use thereby in modulating the carrier wave simultaneously received by the modulator unit. The modulator unit modulates the amplitude of the carrier wave according to the modulation signal and outputs the result as an amplitude modulated electrical analog drive signal. In the present example the analog modulation signal frequency spectrum is output by the digital shaping filler (130) as a Gaussian spectrum centred upon the frequency Al of the suppressed carrier wave (10), predominantly within the range of frequencies Bl Hz to Cl Hz centred on Al Hz. as illustrated in Figure lB. Thus, modulation signal sidebands, of Gaussian form (11), are produced symmetrically either side of a central carrier spectral component (10). This distribution of frequencies results in an acoustic waveform of a corresponding spectral shape and a resultant spread of simultaneous deflections of incident laser beam light through a spread of angles 02. As a result, when the "footprint" of the deflected beam is viewed (e.g. the beam in cross-section), it is found to be centred upon a position A3 determined according to the carrier frequency Al of the drive signal, and to have a spreading of light intensity being substantially Gaussian in form and spread over a centrai location (predominantly within the range of positions B3 to C3 centred upon position A3) determined according to the modulation signal spectral profile contained predominantly within the frequency range Bi to Cl. Thus, the appearance of the "footprint" of the directed beam may be controlled by suitably controlling not only the frequency of the carrier wave component of the drive signal (10) but also the frequency spectrum of the modulation component (11) of the drive signal.

Figure 3A illustrates an alternative embodiment of the present invention employing a vector drive signal modulation method with which it is possible to generate asymmetrical light intensity distributions across a directed light beam "footprint". In Figure 3A like articles are assigned like reference numerals as compared to the articles illustrated in Figure 2, and, as between Figure 2 and Figure 3A, such articles are substantially the same or similar.

The drive control unit (4) of Figure 3A comprises two digital modulation signal inputs.

Each such digital modulation signal input is ultimately operably connected to a respective one of two corresponding analog modulation signal input ports of a vector modulator unit (31) via a respective signal processing limb. The vector modulator unit (31) is, in this embodiment, an In-Phase and Quadrature (l/Q) modulator having an In-Phase modulation signal input port, and a second and separate Quadrature modulation signal input port. Each of these two modulation signal input ports is served by a respective signal processing limb.

The In-Phase signal processing limb comprises an in-phase digital modulation signal input port (131) leading to a digital spectral shaping filler (1301) arranged to impose a predetermined frequency spectral shape upon the received modulation signal. The output of the spectral shaping filler (1301) is directed to a digital-to-analog convert unit DAC (171) for converting the shaped In-Phase digital modulation signal into an analog format and for outputting the result to an In-Phase modulation signal reconstruction filter unit (181) operable to remove undesired aliases and frequency spectrum components (above 500MHz) from the frequency spectrum of the analog In-Phase modulation signal received thereby. The result is output to the result to the In-Phase modulation signal input of the vector modulator unit (31) via a signal amplifier (191). The corresponding Quadrature modulation signal processing limb comprises corresponding digital Quadrature modulation signal input port (130), subsequent digital shaping filler (1300), and subsequent digitalto-analog converter unit (17Q), Quadrature modulation signal reconstruction filter unit (180) and subsequent Quadrature modulation signal amplifier (19Q) leading to the Quadrature modulation signal input port of the vector modulation unit. The digital-to-analog converter units (171, 17Q), the digital shaping fillers (1301, 130Q), the modulation signal reconstruction filter units (181, 18Q) and the modulation signal amplifiers (191, 190) are substantially the same or similar, and operate in substantially the same or similar manner to the DAC unit (17), the spectral shaping filler (130), the modulation signal reconstruction filter unit (18) and the modulation signal amplifier unit (19) described above with reference to Figure 2. Each of the digital modulation signals input to the In-Phase modulation input port (131) and the Quadrature modulation signal input port (130) is a digital pseudo-random noise signal generated by selection of a pseudo-random number from a pseudo-random number sequence.

The digital shaping filler units (1301, 1300) for each of the In-Phase modulation signal processing limb and the Quadrature phase modulation signal processing limb are each independently controllable to impose a desired modulation signal frequency spectrum profile upon the respective I or 0 digital modulation signals received thereby. In the present embodiment the I and 0 spectral shaping filler units impose a Gaussian frequency spectral shape upon the frequency spectrum of the respective digital modulation signals.

Inserted between the output of the carrier wave amplifier unit (21) and the vector modulator unit (31) of the drive signal control unit (4) is a coupler unit (30) operable to split the carrier wave signal received thereby from the amplifier unit into 900 offset signals (i.e. in-phase Quadrature) for use in the vector modulator unit. A first coupler output port (31A) is connected to a dedicated first carrier input port of the vector modulator unit while a second output port, carrying the carrier wave signal shifted in phase by 90 is output by a second output port (31 B) of the coupler unit into a dedicated second carrier wave input port of the vector modulator unit.

The vector modulator unit is operable to multiply the input to the first carrier wave input port thereof with the In-Phase modulation signal received thereby concurrently, and to multiply the 90 phase-shifted carrier wave signal input at the second carrier wave input port thereof by the Quadrature modulation signal received concurrently thereby, and to subtract the latter product from the former product and output the result as the drive signal upon the drive signal transmission line (5).

Thus, if the carrier wave generated by the DDS unit (16) of the drive control unit is in the form of a cosine wave, and the In-Phase and Quadrature modulation signals are pseudo-random noise signals N, then the output analogue drive signal S(t) generated by the vector modulator unit (31) has the form: S(t) = N(t)cos(wt) - N0 (t)sin(wt) Equation (4) Where N is the In-Phase noise modulation signal (with a Gaussian frequency spectrum) and N0 is the noise modulation signal of the Quadrature component of the drive signal (with a Guassian frequency spectrum). The quantity w is the angular frequency of the drive signal, and the quantity t is time. Thus, as will be appreciated by the skilled person, the relative magnitudes of N1(t) and N0(t) as determined by the vector modulator unit (31) determines the weighting of the In-Phase and Quadrature components of the drive signal and thereby determines the ultimate shape of the spectral frequency distribution of the drive signal. This control enables the user to impose asymmetrical frequency spectral distributions upon the drive signal such as is illustrated in Figure 3B wherein the frequency spectrum of the drive signal remains centred on the frequency Al of the carrier wave generated by the drive control unit, but is not symmetrical in shape either side of the carrier frequency within the frequency bandwidth of the drive signal. In the illustrated example, greater spectral energy is concentrated at frequencies below the carrier frequency with fewer spectral components being concentrated in the frequency range above the carrier frequency.

This asymmetrical distribution of spectral components within the drive signal spectrum imposes a corresponding asymmetrical shape (36) upon the directed light beam intensity distribution in cross-section (i.e. the "footprint" of the beam) such as is illustrated schematically in Figure 3C. While the directed beam is centred at a position A3 determined by the carrier frequency Al of the drive signal, the distribution of light intensity is concentrated at positions between A3 and position B3 corresponding to the concentration of spectral components within the drive signal between carrier frequency Al and the lower sideband of the drive signal spectrum.

This functionality allows a user to produce projected images having an uneven brightness using simply the distribution of light intensity within the "footprint" of a directed beam, and without requiring the directed beam as a whole to be scanned in order to produce the projected light distribution.

Figure 4A schematically illustrates an acousto-optic deflector apparatus for imparting multiple consecutive deflections to a received laser beam in a manner as described with respect to Figure 1A. The deflector apparatus comprises a first Bragg cell (1A) controllable to acousto- optically deflect laser radiation (8) received thereby into a directed optical beam (9) in the manner described above with reference to Figure 1A.

The deflection imposed by the first acousto-optical Bragg cell (1A) is an angle subtended in a first plane (e.g. the "X" plane perpendicular to the plane of the page of Figure 4A). A second Bragg cell (1 B) is positioned adjacent to the optical output region of the first Bragg cell (1A) so as to receive, as optical input, the directed optical beam output by the first Bragg cell. The second Bragg cell is controllable to acoustooptically deflect the deflected beam 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 angle of the first Bragg cell is subtended (e.g. the deflection angle of the Bragg cell subtended in the "Y" plane parallel to the page of Figure 4A.

Each of the first and second Bragg cells is generally arranged and operates as described above with reference to Figure 1A, and each is driven by a dedicated deflection control unit (4A and 4B respectively) arranged to generate dedicated drive signals according to any of the embodiments discussed above with reference to Figure 2 or Figure 3A via respective drive signal transmission lines 5A and 5B. A control unit (37) is operably connected to each of the deflection control units (4A, 4B) of the first and second Bragg cells (1A, 1B) to apply appropriate carrier wave and modulation control signals to each drive signal control unit so as to impose a desired light intensity distribution within the directed optical beam output thereby, and ultimately output by the deflector apparatus of Figure 4A as a whole. In this way the control unit (37) may control the overall deflection of the directed beam (38) ultimately output from the deflector apparatus in two-dimensions (i.e. the "X" dimension and the "Y" dimension), by a suitable control of the carrier signal frequencies input to the first and second Bragg cells (1A, 1B), and may control in two dimensions the distribution of light intensity within the directed optical beam by controlling the modulation signals input to each of two Bragg cells.

For example, in an embodiment of the invention, each of the first and second drive signal control units (4A and 4B) respectively comprises a drive signal control unit as described above with reference to Figure 2. When the modulation signal is a pseudo-random noise signal having a spectral frequency distribution of a Gaussian shape such as described above with reference to Figure 1A, the resulting distribution of light intensity across the directed optical beam (38) output by the second Bragg cell (1 B) of the deflector apparatus has a distribution such as is schematically illustrated in Figure 4B as a two-dimensional Gaussian profile (39). The visual appearance of the iootprint" of the directed optical beam (38) when projected upon a projection surface appears as a bright light seen through thick fog - namely a diffuse or fuzzy disc of light (40) - as illustrated in Figure 4G.

Asymmetrical two-dimensional intensity distributions across the directed light beam (38) may be imposed by using vector modulation in either or both of the drive control units (4A, 4B) of the type described above with reference to Figure 3A.

The light deflector apparatus illustrated in Figure 4A, or in Figure 1A, is particularly suited to controlling the appearance of the "footprint" of a directed laser beam produced by a laser display projector when projected upon a display surface. Figure schematically illustrates a laser display projector unit (50) comprising apparatus for deflecting optical radiation as described above with reference to Figure 4A. The directed light beam (38) output by the deflector apparatus of the laser projector unit impinges upon a display surface (51) thereby to produce a visible "footprint" of scattered or reflected light possessing the appearance illustrated in Figure 4C. In preferred embodiments this projector apparatus is embodied within an aircraft simulator unit and the laser light projector unit (50) is operable to generate images of bright light sources within a view typically seen by an aircraft pilot whilst flying an aircraft. For example, these light points may be bright runway lights and/or bright lights of buildings or vehicles in a simulated scene or view of an airport and/or runway. The ability of the present invention to effectively spread the distribution of light in one or two dimensions of the beam "footprint" in a controllable manner enables a user to simulate extended bright light sources (e.g. rectangular/square illuminated windows of a simulated building image) or the diffuse spreading of brightness of a runway light (or the like) as seen through mist or fog. This controllability is particularly advantageous in allowing a user to accurately simulate a variety of the seeing conditions (weather conditions typically encountered by a pilot in flight) and to do so using the properties of a directed laser beam without the requirement of scanning the laser beam in order to produce the desired "footprint" appearance.

A key benefit of imposing a Gaussian spectral shape to the noise spectrum employed in the drive signal becomes apparent when utilising the embodiment of the invention illustrated in Figure 4A comprising a first and second acousto-optic deflectors for successfully deflecting a given optical beam in the "X" and "Y" directions successively (or vice versa). This is because the Gaussian noise spectrum imposed by the "X" deflector produces a distribution of light intensity at the beam "footprint" in the "X" dimension having a Gaussian shape given by: J=IQexp(-x) Similarly, the intensity distribution imposed in the "Y" dimension of the "footprint" of a beam deflected by the "Y" deflector has the form: I = I exp(-y2) Thus, when both intensity distributions are imposed upon the same beam, such as will be the case when employing the apparatus illustrated in Figure 4A, the resulting intensity distribution is simply the product of the intensity distributions in the "X" and "Y" dimensions and is given by: I = 1 exp(-R2); where R2=x2+y2 Hence, it can be seen that the intensity distribution imposed by the consecutive "X" and "Y" acoustic deflectors, when each is driven using a Gaussian noise spectrum, is a brightness distribution which is circularly symmetrical as is illustrated in Figure 4C.

This is a very efficient way of controllably producing circularly symmetrical light beam "footprint" profiles.

It will be appreciated that the above embodiments are exemplary only and that modifications and variations of them, such as would be readily apparent to the skilled person, may be made without departing from the scope of the present invention.

Claims (22)

1. CLAIMS: 1. Apparatus for acoustically deflecting optical radiation to form

   a directed optical beam including a deflector means for receiving optical radiation and deflecting the optical radiation with acoustic wave(s) wherein the deflector means is arranged to modulate the acoustic wave(s) such that acoustic waves of differing frequency simultaneously deflect received optical radiation by differing respective amounts thereby to form a directed optical beam, and to control the distribution of light intensity across the directed optical beam by controlling said modulation.
2. 2. Apparatus according to any preceding claim in which the deflector means is arranged to generate said acoustic wave(s) as a carrier wave modulated by a modulation signal having a signal frequency spectrum, the deflector means being arranged to selectively shape said spectrum according to a selected distribution of light intensity to be produced within the directed optical beam.
3. 3. Apparatus according to any preceding claim in which said modulation is a modulation of any one or more of: the amplitude; the frequency; the phase of the acoustic wave(s).
4. 4. Apparatus according to any preceding claim in which the modulation of the acoustic wave(s) is a modulation simultaneously of: both the amplitude and the frequency; or, both the amplitude and the phase thereof, and the deflector means is operable to selectively adjust the modulation to render the frequency spectrum of the modulated acoustic wave(s) asymmetrical in shape thereby to render asymmetrical the distribution of light intensity within the directed optical beam.
5. 5. Apparatus according to any preceding claim in which the acoustic wave(s) includes a carrier wave having two components in phase quadrature with each other, and the deflector means is arranged to separately modulate each component to implement a vector modulation, and the deflector means is operable to selectively adjust the modulation to render the frequency spectrum of the vector modulated acoustic wave(s) asymmetrical in shape thereby to render asymmetrical the distribution of Hght intensity within the directed optical beam.
6. 6. Apparatus according to any preceding claim in which the deflector means is arranged to modulate the acoustic wave(s) with a random modulation signal which varies a selected property of the acoustic wave(s) in a random manner.
7. 7. Apparatus according to claim 6 in which the frequency spectral distribution of the modulation signal is Gaussian.
8. 8. Apparatus according to any preceding claim in which the deflector means includes an acousto-optic means arranged to deflect said received optical radiation with said acoustical wave(s) generated thereby according to a drive signal, and drive means arranged to generating the drive signal and to modulate the acoustic wave(s) by modulating the drive signal.
9. 9. A display projector apparatus including apparatus for acoustically deflecting optical radiation according to any preceding claim.
10. 1O.A vehicle simulator apparatus including a display projector apparatus according to claim 9.
11. 11. A method for acoustically deflecting optical radiation to form a directed optical beam including deflecting optical radiation with acoustic wave(s) modulated such that acoustic waves of differing frequency simultaneously deflect optical radiation by differing respective amounts thereby to form a directed optical beam to control the distribution of light intensity across the directed optical beam by controlling said modulation.
12. 12. A method according to claim 11 including generating said acoustic wave(s) as a carrier wave modulated by a modulation signal having a signal frequency spectrum, and selectively shaping said spectrum according to a selected distribution of light intensity to be produced within the directed optical beam.
13. 13. A method according to any of preceding claims 11 to 12 in which said modulation is a modulation of any one or more of: the amplitude; the frequency; the phase of the acoustic wave(s).
14. 14. A method according to any of preceding claims 11 to 13 in which the modulation of the acoustic wave(s) is a modulation simultaneously of: both the amplitude and the frequency; or, both the amplitude and the phase thereof, and the method includes selectively adjusting the modulation to render the frequency spectrum of the modulated acoustic wave(s) asymmetrical in shape thereby to render asymmetrical the distribution of light intensity within the directed optical beam.
15. 15. A method according to any of preceding claims 11 to 14 in which the acoustic wave(s) includes a carrier wave having two components in phase quadrature with each other, and the method includes separately modulating each component to implement a vector modulation, and selectively adjusting the modulation to render the frequency spectrum of the vector modulated acoustic wave(s) asymmetrical in shape thereby to render asymmetrical the distribution of light intensity within the directed optical beam.
16. 16. A method according to any of preceding claims 11 to 15 including modulating the acoustic wave(s) with a random modulation signal which varies a selected property of the acoustic wave(s) in a pseudo-random manner.
17. 17. A method according to claim 16 in which the frequency spectral distribution of the modulation signal is Gaussian.
18. 18. A method according to any of preceding claims 11 to 17 including providing an acousto-optic means arranged to deflect optical radiation with said acoustical wave(s) generated thereby according to a drive signal, and the method includes generating the drive signal and modulating the acoustic wave(s) by modulating the drive signal.
19. 19. A method of laser display projection including a method for acoustically deflecting optical radiation according to any of preceding claims 11 to 18.
20. 20. A method of vehicle simulation including a method of laser display projection according to claim 19.
21. 21. Apparatus substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.
22. 22. A method substantially as described in any embodiment hereinbefore with reference to the accompanying drawings.