GB2211315A - Laser scanning system - Google Patents

Abstract

A laser scanner for a telecine or a telerecorder is proposed in which the laser beams emitted from red, green and blue lasers 11R, 11G, 11B are deflected through individual tracking deflectors 24 to follow individual chirp lenses 28R, 28G, 28B passing through an acousto-optic chirp cell 10. A train of three chirps is launched into the cell and each tracking deflector tracks a different chirp. The frequency characteristics of each chirp are chosen so that the respective laser beam is focused on the film 18 and the intensity control signals applied to the lasers on lines 44 are relatively delayed so as to achieve registration of the red, green and blue scans. Intensity control may be omitted in the case of a telecine machine. <IMAGE>

Description

LASER SCANNING SYSTEMS This invention relates to laser scanning systems using acoustooptic cells to deflect a laser beam. Such systems are of particular utility in telecine machines, also telerecording machines.

The use of an acousto-optic cell to defect as laser beam is described in BBC Research Report 1974/31. Briefly, an acousto-optic cell is a gaseous, liquid or solid crystalline medium through which bursts of v.h.f. sound energy are passed. The periodic nature of the sound energy, which is in effect periodic variation in pressure in the cell sets up corresponding areas of periodic variations in the refractive index of the medium through which it is passing.

This variation in refractive index causes the incoming laser light to be scattered into a regular pattern similar to that produced in a diffraction grating. The pitch of the grating and hence the degree to which the beam of laser light is deflected in the acoustic cell is established by the periodic spatial variation of refractive index which is determined by the frequency of the applied acoustic wave.

Similarly, the intensity of the diffracted light is a function of the power of the sound wave; undiffracted light passes straight through the cell and formsXan undeflected beam. A known acoustooptic cell is illustrated schetnatically in Figure 1. The material used in this example is lead molybdenate, lithium niobate crystal or Tellurium dioxide and the acoustic wave is generated by an electrical signal applied via a lead 1 to an acoustic transducer 2 fixed at one end of the crystal 3. The wave is symbolised by high pressure areas 7 travelling in the direction 8. An incoming beam 4 is part defected (beam 5) and part transmitted (beam 6).

An extension of the principle outlined above is the so-called 'chirp' cell. In this cell, the frequency of the burst of acoustic energy is varied linearly throughout the burst so that one end of the burst is comparatively low frequency and the other end is high frequency. Thus there is a linear variation in the spatial period of the perturbed refractive index of the crystal. The effect of the chirp is to focus the inputted light beam in one dimension.

Focusing in the orthogonal direction to form a spot can be achieved using a cylindrical lens.

The chirp cell is particularly applicable to scanning systems as the chirp is in effect a travelling lens, and the focused laser beam can scan a spot at the velocity of the acoustic chirp, which depends on the medium used in the chirp cell.

This method of scanning is of particular interest to the construction of telecine machines for high definition television (HDTV). For such an application a transit time for the chirp across the cell of 30 is required which can be achieved without having to resort to exotic, expensive materials.

The travelling lens produced by the chirp cell can operate in one of two modes: flooded mode and tracking mode. These two modes of operation are illustrated in Figures 2 and 3 respectively.

In the flooded mode, a single beam deflecting chirp cell 10 is used. The beam from a laser 11 is directed through an objective lens 12 and through an optional spatial filter 14 arranged at its focal point to an expansion lens 16 which expands the beam so that it 'floods' the chirp cell 10 with light across the whole width of the cell. The lenses 12 and 16 and the filter 14 together form a beam expander unit. A part of the flooded beam is deflected by the chirp as it passes across the cell and is focused on a strip of film 18 at a spot 19. Undeflected light passes straight through the chirp cell and falls on to an absorber 20. The position of the scanning spot 19 moves across the film as the chirp travels through the cell 10.The amplitude of the laser beam may be varied in synchronism with the travel of the chirp so as to control the intensity of exposure of the film by the flying spot. Amplitude variation is required for telerecording or for scanning negative film.

In the tracking mode shown in Figure 3 the laser beam is again shone through a beam expander 22 but a conventional tracking deflector 24 is interposed between the expander and the chirp cell 10 to ensure that only the section of the main scanner containing the chirp is illuminated. The tracking deflector tracks the chirp across the cell. An absorber 26 is included between the deflector 24 and the chirp cell to absorb higher order beams produced by the deflector 24. The presence of higher orders of refraction produce very large variations in deflected beam power with changing acoustic frequency and is thus undesirable. In both modes of operation, an orthogonal focusing lens (not shown) is used to focus the beam into the spot 19, i.e. to effect focusing in the direction normal to the plane of the paper.

Flooded mode operation has the advantage of a very rapid flyback time but is very inefficient as only a small part of the beam is deflected. Tracking mode is more efficient but has a slower flyback time and is more complex.

A colour system requires the deflection of a plurality (almost invariably three) laser beams of different colours. Previous suggestions have assumed the use of three flooded mode chirp cells or three chirp cells operated in tracking mode using three conventional acousto-optic deflectors. Either alternative is expensive and, as will be explained more fully below, optical alignment for achromatic (or better still apochromatic) behaviour is difficult.

The object of the present invention is to provide a simpler, cheaper system which also makes optical alignment much easier.

The system according to the invention is defined in claim 1 below.

The invention requires only one chirp cell which operates in the more efficient tracking mode. Moreover, the start and end frequencies of the chirps corresponding to the different lasers can be individually established, whereby a major part of the optical alignment is achieved by electronic control.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a schematic representation of an acousto-optic cell as already described; Figures 2 and 3 show a chirp cell operating in flooded and tracking modes respectively, as already described; Figure 4 is a schematic diagram of a laser scanning system embodying the invention and suitable for use in a telecine machine; and Figure 5 is a similar diagram to Figure 4 showing a system suitable for use in a telerecording machine.

By way of further background to the invention, some of the problems in the way of achieving an apochromatic system using three separate chirp cells will be explained.

An apochromatic correction may be applied before or after a cell (i.e. before deflection of the beam or inbetween the deflected beam and the film.

If correction were applied after the cell a lens with the opposite dispersion to the chirp lens would have to be used. Such a lens is considered to be practically impossible to manufacture due to the large dispersion, (that is variation of refractive index with the wavelength of light) of the chirp diffraction grating.

Correcting before the cell could be achieved in the flooded mode by diverging the flooding beams for red and green (blue light having the shortest wavelength). Adjustment of the overall focus would then be achieved by adjusting the frequency range of the chirp. Although simple, this system does not overcome the basic inefficiency of operation in the flooded mode.

In the tracking mode, the tracking beams must be caused to diverge. However, as it is necessary to correct the tracking deflectors themselves using prisms (in known manner) to provide for apochromatic operation of the deflectors but the deflector and chirp cell adjustments interact which leads to a complex lining-up operation. In addition the area between the deflectors and the chirp cell would be very complex optically as the tracking deflectors each require a separately adjustable mirror to redirect the beam deflected by their respective prisms, and these prism and mirror arrangements would be in addition to the lens system needed to correct the chirp cell.

It should be noted that all the possibilities described require one chirp cell for each ofthe red, green and blue scans required for television.

The system illustrated in Figure 4 overcomes these problems and combines the efficiency of the tracking mode with a system that does not require optical adjustment of the chirp cell.

Figure 4 shows a system having a single chirp cell 10, into which a train of chirps 28 is repeatedly launched. Three lasers 11 R, G, B produce red, green and blue light respectively, and light from each layer is passed through a respective beam expander 22 to a respective tracking deflector 24. Each of the tracking deflectors is arranged so that its beam tracks an individual chirp 28 of the train across the chirp cell 10. Thus, in any train of chirps there will be one chirp that acts as a focusing lens for one particular laser beam. The focused beams fall on a film 18 arranged at a common focal plane. The frequency characteristics of each chirp, that is the beginning and end frequency and chirp duration are so chosen that each of the chirps focuses the particular beam that it is being tracked by on to the same focal plane.As in the basic acousto-optic cell illustrated in Figure 1, an acoustic transducer 2 is attached to one end of the chirp cell 10. The driving unit 30 for the cell is substantially as described in BBC Research Report 1974/31 referred to before. However the equipment includes a chirp generator 32, sequentially producing red, green and blue chirps, and including means for adjusting the stop and start frequency of each chirp. The chirp waveforms from the generator are applied in cyclic sequence to the acoustic transducer 2 of the cell 10.

The image on the film 18 arranged at the focal plane will transmit R, G and B intensity signals in proportion to the colours exposed on the film. The transmitted light falls on a photodetector and multiplier array 21 arranged behind the film. The array 21 outputs electrical signals on lines 41 which are proportional to the R, G and B light falling on the array. These signals are fed to combination circuits 43 in the driving unit 30 which circuits combine the received signals and operate on them to produce properly weighted colour difference signals suitable for video transmission.

The timing circuits 38 are synchronised to insert correctly timed television synchronising pulses and also perform the following functions: (1) They trigger the chirp generator 32 at the beginning of a line, about one third through the line and about two thirds through the line to produce a train of chirps 28R, 28G and 28B for the red, green and blue laser beams.

(2) They provide staggered deflection waveforms on lines 46 to the three tracking deflectors 24, so timed that each beam tracks its corresponding chirp lens 28R, 28G and 28B.

(3) They control the Y-axis film movement in conventional manner to provide the required film scanning.

It is assumed that each spot is scanned in the X-axis direction from a start spot 19S to an end spot 19E on the film 18. The presence of a spot at 19S is taken to be the beginning of each video line and so the outputs of the photomultipliers must be so timed that the R G B exposures are in exact horizontal registry when the three signals are combined. Accordingly the G and B signals are delayed by delay lines 41G and 41B respectively whose delays match exactly the triggering delay of the G and B chirps in the generator 32 and correspondingly the delays of the green and blue deflection waveforms for the green and blue tracking deflectors 24 relative to the red deflection waveform for the red deflector 24.

The apparatus may be modified for use as a telerecorder in which case amplitude control of the individual lasers is required.

Such a system is shown in Figure 5. The arrangement is substantially the same as that of Figure 4.

An incoming video signal may be in any suitable form, e.g. R, G, B composite. In any event, separation circuits 42 are utilised in conventional manner to separate out synchronising information and provide separate R, G and B video signals for controlling the intensity of the lasers 11, to which they are fed on lines 44. The timing circuits 38 are synchronised with the television synchronising pulses and perform the functions.

Also included in the driving unit 30 of Figure 4 or Figure 5, but not shown, is an optical feedback system that looks at the spot made by each focused beam to determine how well the chirp lens is performing. The system is a line-by-line correction system which may be implemented using a CCD line sensor to examine the spot profile, and to calculate with the aid of a microprocessor, the corrections that must be applied to the subsequent chirps.

The corrections may conveniently be applied by an RF attenuator on the output of the chirp oscillator in the drive circuit.

The arrangement of Figure 5 is particularly applicable to telerecording. That is, recording onto film from a video signal.

However, exposure control is necessary in a telecine machine if negative film is being scanned and some sort of exposure control is required. In a practical machine, exposure control is included so that the machine may be used as either a telecine or a telerecorder and can scan either positive or negative film.

The arrangement of Figures 4 and 5 dispenses with the need for chirp lens dispersion correction since each chirp lens can have the same focal length, even though the individual lenses diffract different wavelengths.

The system eliminates all of the fundamental adjustments that have made the previously described tracking mode impractical whilst retaining the advantage of the high optical efficiency of tracking mode which in the long term results in longer laser-tube lifetime.

The main system focus and the diferential focus between individual beams is controlled by varying the beginning and final frequencies of the individual chirps. Colour correction and alignment are no longer a problem as a separate signal is sent to each of the tracking deflectors.

Although the drive equipment has been described as hardware, much of the system control could be implemented in software.

The system has the advantage that all maintenance adjustments of the optics are removed, making the system practical for construction of a telecine machine. The adjustments are replaced by increased complexity in the system controller, for example, to control the frequency of the train of chirps. The system is also comparatively cheap as all previous systems have envisaged an individual chirp cell for each of the three colour signals.

Although three tracking deflectors are required which is an additional requirement to flooded mode operation, tracking deflectors are considerably cheaper than chirp cells.

Claims (12)

CLAIMS:

1. A laser scanning system, comprising a plurality of lasers arranged to emit laser light at different wavelengths, an acoustooptic chirp cell having means for launching a train of acoustic chirps into the cell, each chirp being associated with a respective one of the lasers and having a frequency characteristic such that the beams from each laser can be focused on the same focal plane, and a tracking deflector associated with each laser to cause the respective laser beam to track the associated chirp across the chirp cell.

2. A laser scanning system according to claim 1, comprising means for monitoring the focus of each beam at the focal plane and means for adjusting the start and end frequencies of any of the chirps in the train in response to signals from the monitoring means.

3. A laser scanning system according to claim 1 or 2, comprising means for varying the intensity of the beam emitted from each laser in response to an incoming video signal.

4. A laser scanning system according to claim 3, comprising delay means for delaying the intensity variations to the laser beams to achieve horizontal registration.

5. A telecine machine having a laser scanning system according to any preceding claim.

6. A telerecording machine having a laser scanning system according to any of claims 1 to 4.

7. A method of scanning a film using a plurality of lasers arranged to emit light at different wavelengths, comprising the steps of repeatedly launching a train of acoustic chirps into an acousto-optic chirp cell, each chirp having an individual frequency characteristic, and deflecting each of the laser beams to follow a different one of the chirps of the train, the frequency characteristic of each chirp being such that all the beams are focused by the respective chirps onto a common focal plane at which a film is arranged.

8. A method according to claim 7, comprising the steps of monitoring the focus of each laser beam on the film at the focal plane and adjusting the start and end frequencies of the individual chirps to sharpen the focus.

9. A method according to claim 7 or 8, comprising the step of varying the intensity of each laser in response to an incoming video signal.

10. A method according to claim 9, the intensity variations are relatively delayed so as to achieve horizontal registration.

11. A laser scanning system substantially as herein described with reference to Figure 4 or Figure 5 of the accompanying drawings.

12. A method of scanning a film substantially as herein described with reference to Figure 4 or Figure 5 of the accompanying drawings.