GB2145836A

GB2145836A - Laser recording system

Info

Publication number: GB2145836A
Application number: GB08323317A
Authority: GB
Inventors: D P Fernie; R M Pettigrew
Original assignee: PA Consulting Services Ltd
Current assignee: PA Consulting Services Ltd
Priority date: 1983-08-31
Filing date: 1983-08-31
Publication date: 1985-04-03
Also published as: GB2145836B; GB8323317D0

Abstract

An acousto-optic cell (12) of eg fluoro-carbon liquid or TeO2 can accommodate e.g. a 52 mu s T.V. line in about 32 mm length, and is illuminated by a pulsed (22) laser light source (10), each pulse is of sufficiently short duration to freeze the information in the cell and project it onto recording means (20). Information to be recorded is carried on an electrical input signal which has a bandwidth not exceeding 30 MHz and a carrier frequency less than 50 MHz and means is provided for launching into the cell sound waves which are derived from the input signal and which have an amplitude representative of the information to be recorded, wherein the laser light pulses pass through the cell and are spatially modulated in accordance with the information to be recorded. The spatially modulated laser light is projected onto a photo-responsive medium to form a record of the information. <IMAGE>

Description

SPECIFICATION Laser recording system Field of the invention This invention relates to a laser recording sytem for recording information, such as carried on a video input signal or analog/digital data stream onto a photo-responsive medium.

Background to the invention US Patents Nos. 4,005,275 and 4,107,701 disclose systems using pulsed lasers and acousto-optic devices to record or display an image representative of an input signal which energises the acousto-optic device. The input signal generates an acoustic wave in the acousto-optic device, and the laser light passing through the acousto-optic device is diffracted in accordance with the acoustic wave. The modulated laser light thus not only carries the information to be displayed, but also illuminates the recording or display medium.

In order to record or display a line of information, the laser produces a burst or pulse each time the acoustic wave fills the acousto-optic device. The latter must therefore have a length which holds a complete line of information so that this length can be illuminated by a pulse of laser light in order to "freeze" the information in the line. Hitherto, such cells have been made of materials having speeds of sound which require the cells to have a substantial length in order to accommodate lines having lengths in time comparable to those in video or TV systems. Such cells are difficult and expensive to fabricate.

An object of one aspect of the invention is to provide a laser recording system having an acousto-optic cell made of a material having a low speed of sound therein, so that the cell can be made of compact dimensions, making it less expensive and easier to illuminate the whole length of the cell by the pulse of laser light.

An object of a further aspect of the invention is to provide a laser recording system with a laser which is "naturally" pulsed in synchronism with the input signal, in contrast to the prior art which uses continuously operating lasers whose optical output remains constant in time before pulsing.

Such lasers can be made to operate at pulse repitition frequencies comparable to video line frequencies without loss of average power or the complexity associated with eg cavity-damped, mode-locked Argon ion lasers.

The latter class of lasers can only be pulsed at frequencies well beyond those associated with video line rates.

The above prior art referred to high bandwith information-recording systems with bandwidths in excess of 500 Hz which precludes the use of acousto-optic materials with other than negligible acoustic attenuation but the present invention is concerned with information recording systems with much lower bandwidths and carrier frequencies.

Summary of the invention According to one aspect of the invention a laser recording system for recording information comprises a laser pulsed to produce a sequence of laser light pulses, an acoustooptic cell made of material having a velocity of sound therein less than 1,500 metres per second, said information being carried on an electrical input signal which has a bandwidth not exceeding 30 MHz and a carrier frequency less than 50 MHz, means for launching into the cell sound waves which are derived from the input signal and which have an amplitude representative of the information to be recorded, the laser light pulses passing through the cell and being spatially modulated in accordance with the information to be recorded, and recording means including a photo-responsive medium onto which the spatially modulated laser light is projected to form a record of the information.

The laser recording system may be used to provide a permanent record of any suitable information, but it is envisaged that the invention will have particular use in automatic labelling, printed circuit board manufacture, lithography, and light valve addressing.

The acousto-optic device is preferably crystalline tellurium dioxide (Te02) which exhibits a particularly low speed of sound along one axis, this being referred to as the "slow shear" mode. Tellurium dioxide operating in the slow shear mode has a speed of sound of only 617 metres per second. This means that if the information to be recorded is carried on a video signal typically having (in the U.K.) a line duration of 64 micro seconds of which about 52 micro seconds carries useful information, the cell can be as small as about 32 mm. Other suitable materials for the acoustooptic device are fluoro-carbon liquids.

According to another aspect of the invention a laser recording system for recording information comprises a laser operated naturally in a pulsed mode to produce a sequence of laser light pulses, an acousto-optic cell, said information being carried on an electrical input signal which has a bandwidth not exceeding 30 MHz and a carrier frequency less than 50 MHz, means for launching into the cell sound waves which are derived from the input signal and which have an amplitude representative of the information to be recorded, the laser light pulses passing through the cell and being spatially modulated in accordance with the information to be recorded, and a photo-responsive medium onto which the spatially modulated laser light is projected to form a record of the information.

The laser may be operated at any wave length corresponding to the photoresponsive material used. For example, rare gas halide excimer lasers can be used to expose materials sensitive in the near ultra-violet whilst semiconductor diode lasers or solid state lasers based on Alexandrite or Nd:YAG can be used to address or expose sensors or materials responsive in the near infra red region. Laser based on vapours of Gold, Copper, Manganese, Barium and Strontium are suited to video rate image production at wavelengths from the near ultra-violet through the visible to the mid infra red.

In both aspects of the invention the input signal is preferably at standard video line frequency which is 1 5.625 kHz in the U.K. or at a line frequency determined by the pulse repetition rate of the illuminating laser. The laser is preferably synchronised directly from the input signal, and the duration of the laser pulses is preferably small so that no appreciable smearing of information occurs. For example, for a video bandwidth of 5.5. MHz which is common in the U.K., the number of resolution elements along the video line is approximately 300. In addition the active length of the line in time is approximately 50 micro seconds. The time associated with a given resolution element is therefore about 200 nano seconds.In order, therefore, to freeze the video line in the acousto-optic cell the laser pulse duration should be well below this figure of 200 nano seconds to avoid appreciable coalition of adjacent picture elements. A pulse duration below 50 nano seconds has been found to be suitable.

After the laser light has been spatially modulated it is projected onto the photoresponsive medium using appropriate optical elements. To form a two-dimensional record each line is scanned vertically using a frame scanner which may consist of a rotating mirror whose rotation is synchronised by means of the video input signal. Alternatively, scanning may be accomplished by translating the photoresponsive medium in a direction perpendicular to the projected line. In order to generate the spatially modulated record or image, the laser light is focussed through one of two small orifices which serve to pass either zero order (undiffracted) light or first order diffracted light. The image under first order illumination relative to zero order illumination is in "inverse video", i.e. of complementary brightness.

A laser recording system according to the invention will now be described, by way of example, with reference to the accompanying drawing, which shows the system in diagrammatic form.

Detailed description of the drawing.

The system comprises a pulsed laser which operates naturally in a pulsed mode with an output matched to the spectral response of the photoresponsive medium. This can lie between the near ultra violet to the mid infra red region of the spectrum. The pulses of laser light are fed to an acousto-optic modulator cell 1 2, (sometimes called a Bragg cell), fabricated out of crystalline tellurium dioxide.

The cell 1 2 is energised. though a piezoelectric transducer 1 3. by an electrical signal carrying video information at the standard video line frequency or at a frequency determined by the pulse repetition rate of the illuminating laser. The cell 1 2 is arranged so that the acoustic waves derived from the input signal are launched into the tellurium dioxide in a direction along which the speed of sound is particularly low, this mode of operation being called the slow shear mode.The cell 1 2 need only be about 32mm long to accommodate a line duration of 52 micro seconds (corresponding to a standard video line in the U.K.) The pulses produced by the laser 10 have a pulse duration of less than 50 nano seconds, and each pulse is produced just as the information in one video line fills up the cell 12.

The pulse illuminates the whole cell and the light is spatially modulated along the length of the cell in dependence upon the video information in the line. This technique therefore, in effect, "freezes" the information in each video line and allows the laser pulse leaving the cell 1 2 to convey the information in the video line.

The spatially modulated pulses of laser light are fed through a field lens 1 5 and a projection lens 1 6 and thence onto a frame scanning mirror 1 8 which rotates about an axis transverse to the plane of the drawing in synchronism with the video input signal applied to the cell 1 2. Alternative scanning of the image may be effected by translation of the photoresponsive specimen. The spatially modulated laser light pulses are projected onto a photo-responsive medium 20 onto which is recorded an image of the information carried by the video signal. The frame scanning mirror 1 8 rotates or the photoresponsive specimen translates with an indexing movement in order to project each line onto the appropriate part of the medium 20 to form a complete permanent record of the information.

The laser 10 is pulsed in synchronism with the video input signal by means of the line synchronising signal applied to the laser, as shown at 22 in the drawing.

The video bandwidth is typically less than 10 MHz, but it is envisaged that bandwidths up to 30 MHz might in the future be used, subject to international agreements. The carrier frequency of the video signal is typically less than 50 MHz because of the relatively high attenuation of tellurium dioxide in the slow shear mode.

By using a multiplicity of Bragg cells in parallel ie one "line" on top of another the requirement to pulse the laser at the video line rate is reduced by a factor equal to the total number of parallel lines. A limiting case of this is a modulator system consisting of N "line" modulators as described above where N is the number of lines to ba recorded.

Claims

1. A laser recording system for recording information comprising a laser pulsed to produce a sequence of laser light pulses, an acousto-optic cell made of material having a velocity of sound therein less than 1,500 metres per second, said information being carried on an electrical input signal which has a bandwidth not exceeding 30 MHz and a carrier frequency less than 50 MHz, means for launching into the cell sound waves which are derived from the input signal and which have an amplitude representative of the information to be recorded, the laser light pulses passing through the cell and being spatially modulated in accordance with the information to be recorded, and recording means including a photo-responsive medium onto which the spatially modulated laser light is projected to form a record of the information.

2. A laser recording system as claimed in claim 1 wherein the acousto-optic device is crystalline tellurium dioxide (Te02) which exhibits a particularly low speed of sound along one axis.

3. A laser recording system as claimed in claim 1 wherein the acousto-optic device is a fluoro-carbon liquid.

4. A laser recording system for recording information comprising a laser operated naturally in a pulsed mode to produce a sequence of laser light pulses, an acousto-optic cell, said information being carried on an electrical input signal which has a bandwidth not exceeding 30 MHz and a carrier frequency less than 50 MHz, means for launching into the cell sound waves which are derived from the input signal and which have an amplitude representative of the information to be recorded, the laser light pulses passing through the cell and being spatially modulated in accordance with the information to be recorded, and a photo-responsive medium onto which the spatially modulated laser light is projected to form a record of the information.

5. A laser recording system as claimed in any of claims 1 to 4 wherein the laser operates at a wavelength corresponding to the photoresponsive material used.

6. A laser recording system as claimed in claim 5 wherein a rare gas halide excimer laser is used to expose materials sensitive in the near ultra-violet.

7. A laser recording system as claimed in claim 5 wherein semiconductor diode lasers or solid state lasers based on Alexandrite or Nd:YAG are used to address or expose sensors or materials responsive in the near infra red region.

8. A laser recording system as claimed in claim 5 wherein the laser is based on vapours of gold, copper, magnanese, barium and strontium for use with video rate image production at wavelengths from the near ultraviolet through the visible to the mid infra red.

9. A laser recording system as claimed in claim 5 wherein the laser is synchronised directly from the input signal, and the duration of the laser pulses is small so that no smearing of information occurs.

1 0. A laser recording system as claimed in claim 9 wherein the pulse duration is below 50 nano seconds.

11. A laser recording system as claimed in any of the preceding claims wherein after the laser light has been spatially modulated it is projected onto the photo-responsive medium using appropriate optical elements.

1 2. A laser recording system as claimed in claim 11 wherein each line is scanned vertically using a frame scanner which consists of a rotating mirror whose rotation is synchronised by means of the video input signal so as to obtain a two-dimensional record.

1 3. A laser recording system as claimed in claim 11 wherein the photoresponsive medium is scanned in a direction perpendicular to the projected line.

1 4. A laser recording system as claimed in claim 11 or 1 2 wherein the laser light is focussed through one of two small orifices which serve to pass either zero order (undiffracted) light or first order diffracted light to form the spatially modulated record or image.

1 5. A laser recording system constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.