Recent developments

Recent developments

Recent developments Laser developments Acousto-optically at RR E Q-switched laser The Royal Radar Establishment have produced a self-contained flas...

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Recent developments Laser developments Acousto-optically

at RR E

Q-switched laser

The Royal Radar Establishment have produced a self-contained flashlamppumped Nd-glass laser which can produce pulses with peak powers in the megawatt region using an acoustooptic switch. The switch is a development of an earlier model which was designed at the RRE to control a continuously pumped Nd-YAG laser. Previously the use of acousto-optic switches with flash pumped lasers had not been thought practicable because it was considered firstly that the hold off capability would not be sufficiently great, and secondly that the opening time of the switch would be too slow. Indeed, these objections are probably valid when considering their use with really high gain materials such as Nd-YAG, but this is not the case with Nd-glass which has a low specific gain and a long build up time for Q-switched pulses. The laser is capable of generating single pulses that have an energy of 80 mJ and a pulse width of 40 ns, corresponding to a peak power of 2 MW.

The complete device is about the size of a small gas laser and shows the

extreme simplicity of a laser using this type of Q-switch. An Nd doped ED2 glass laser rod, 4 mm in diameter and 5 cm long, is pumped in a compact elliptical cavity by a high pressure xenon flash tube. The resonator consists of two plane parallel mirrors, the output mirror having a reflectivity of 85%. The flash lamp is powered from an NiCd battery, which charges a compact paper capacitor to about 1.5 kV via a transistor dc converter. The switch consists of a block of dense glass, in which longitudinal sound waves are generated at around 20 MHz by a PZT transducer fed from a transistor oscillator and power amplifier. The sound is absorbed in the opposite surface, which is corrugated and loaded with an acoustic absorbing material.

insertion loss. Also, the switch can be made from material having a higher damage threshold than LiNbO,, which is one of the most widely used materials for electro-optic switches. The spinning mirror Q-switch needs a motor which is often noisy and heavy, and the speed requires optimization for each different input level, whereas the acousto-optic switch is light, noiseless, and needs no optimization. Miniature pulsed Nd- YA G laser

A miniature self-contained solid state laser has also been developed. This is believed to be the most compact 0.5 J device so far produced: it weighs

The designers claim that the acoustooptic switch is superior to other forms of Q-switch for this application since it combines the versatility of the electro-optic switch with the low insertion loss of the rotating prism. Whereas the electro-optic switch requires voltages in the kilovolt region and is used in conjunction with a polarizer often having an insertion loss of up to 1O%, the acousto-optic switch can be driven from transistor circuitry using a few tens of volts, and has almost negligible Miniature

Acousto-optically

148

Q-switched

laser

pulsed Nd-YAG

laser

460 g; its dimensions are only 77 mm x 70 mm x 53 mm, and it fits snugly in the palm of the hand. Prime power is provided by a 4 V, 0.5 Ah NiCd battery, which is capable of delivering 50 shots before recharging is required. This feeds a transistor invertor circuit which develops several hundred volts to charge an electrolytic capacitor. When this energy is discharged into a xenon flash lamp, !aser oscillations at 1.06 pm are produced from a 3 mm diameter by 30 mm long Nd-YAG rod. The laser operates in the normal, long pulse mode, with oscillations lasting for about 1 ms. Despite the small overall size the device is capable of delivering an output energy of up to 600 mJ at an efficiency of 1.4%. This energy is sufficient to produce a

OPTICS AND LASER TECHNOLOGY.

AUGUST

1973

small weld, or punch a hole in metal foil, Royal Radar Establishment, St Andrew’s Road, Malvern, Worcester-shire,UK Laser alignment technique New procedures and equipment developed at the National Physical Laboratory allow accurate alignment over ranges up to 1 km. The technique has been adapted to provide static, quasi-static, or dynamic measurements of absolute alignment or displacement, and has already been applied successfully to a number of conventional engineering alignment problems. In the basic system, light from a low power helium-neon laser is diverged by a suitable lens and illuminates a circular Fresnel zone plate. The zone plate forms an image on a translucent screen bearing a rectangular grid. From optical considerations, the centre of the image, the centre of the zone plate, and the centre of curvature of the wavefronts diverging from the laser will lie on a straight line. Many alignment and deflection measurements can be interpreted as a need to check the constant or changing misalignment of three points. In this case, by relating the optical components of the system to these points by mechanical means, their misalignment in both vertical and horizontal planes may be observed by noting the position of the image relative to the screen grid. Alternatively, the zone plate or screen may be moved by a known amount to give a fiducial setting of the image on the screen centre. Important advantages of this technique in comparison with many conventional alignment instruments are the insensitivity of the reference axis to tilt of the optical units, and the simplicity and cheapness of the equipment. For some short range applications, the zone plate is replaced by a simple lens, and a tungsten-halogen lamp may suffice as source. It is sometimes helpful to define the source position by means of a pinhole. For dynamic measurements, or measurements over an extended period of time, the screen is replaced by a single or dual axis position-sensing photo-diode with associated display and recording equipment. Over long ranges, the sight line may be bent significantly by transverse gradients of refractive index in the atmosphere. However, it has

OPTICS AND LASER TECHNOLOGY.

Components

of a laser alignment

set

been found in a series of field experiments that an adequate correction can often be obtained by sampling the temperature gradient with an array of thermistors. Accuracy is better than *O. 1 mm at short ranges and about +l mm at 1 km, under prescribed conditions. Alignment sets embodying the technique have been produced and applied to such varied problems as the straightness of long guide rails, bridge movements under dynamic loads, supertanker flexure, dam wall deflections, settlement of concrete piles, and the calibration of large metrological instruments. Division of Mechanical and Optical Metrology, National Physical Laboratory, Teddington, Middlesex TWIl OLW, UK

average pulse duration. Calculations of fundamental pulse duration from streak records of second harmonic pulses have had to assume a Gaussian intensity versus time waveform. However, as the waveforms are asymmetrical 4 and the second harmonic pulses are shorter than the fundamental (approaching the time resolution limitation of the streak camera), accurate measurement of the fundamental pulse duration has been difficult. The Imacon 600 streak camera used in obtaining the direct pulse measurements was fitted with a mesh grid Sl image tube; the photo-cathode was specifically processed for good sensitivity at 1.06 p. A linear cavity

High speed camera with SI photo-cathode John Hadland have produced an ultra high speed camera fitted with an Sl photo-cathode which has allowed direct measurement of the fundamental pulse durations from a mode locked Nd-glass laser. Measurements of pulse durations have previously been made either using the ‘two photon fluorescence’ (TPF) technique L2 or by recording the second harmonic pulses with a streak camera.3 It has been shown that the individual pulse duration increases with time throughout the train,3 thus TPF measurements only give an

AUGUST

1973

Direct duration measurement from a mode-locked Nd-glass the lmacon 600 camera. The a pulse taken from the centre split into two pulses separated

of pulses laser using record shows of the train by 57 ps

149

Nd-glass laser was used to generate a train of pulses with a separation of 7 ns. The laser incorporated a flowing dye cell 1 mm wide using Kodak 9860 dye. The pulse train was reflected off an optical flat 5.74 mm thick which produced two pulses from each incident pulse, separated by 57 ps. The light transmitted through the glass was attenuated and triggered an S 1 vacuum photo-diode unit with transistorized output. This supplied the trigger pulse for the Imacon 600. The part of the train to be recorded was selected by adjusting a filter in front of the photodiode. The laser pulses were focused to a narrow line on the cathode by using a slit and relay optics. The four stage magnetically focused intensifier was optically coupled to the Imacon screen using a 1 : 1 80 mm fl relay lens. The intensifier was operated at 30 kV and the screen recorded by a 1: 1 lens. Kodak 2485 3.5 mm film was used and processed to a gamma of 1.5 in Kodak 857 developer. The record shown is of a pulse taken from the centre of the train split into two pulses separated by 57 ps. The writing speed was 29 mm ns-l. Microdensitometer traces show each pulse to be 9 ps in duration. This is in close agreement with TPF measurements. Giordmaine, J. A., Rentzepis, P. M., Shapiro, S. L., Wecht, K. W. ApplP/~ys Lerters I1 (1967) 216 Shapiro, S. L. App[Phys Letters 13 (1968) 19 Dewhurst, R. J., Obaidi, H. Al., Ramsden, S. A., Hadland, R., Harris, R. Opt Commun 6 (1972) 4 Von Der Linde, D., Laubereau, A. Opf Cornnzun 3 (1971) 279

John Hadland (Photographic Instrumentation) Limited, Newhouse Laboratories, Bovingdon, Hertfordshire, UK

Ion laser for the vacuum ultraviolet Scientists at the Naval Research Laboratory in Washington have reported the measurement of laser emission at 154.82 nm and 155.08 nm from triply ionized carbon ions. They hope that the travelling wave low pressure discharge technique used to produce these wavelengths may also be applied to obtain emissions down to the x-ray region. The construction cf the transverse discharge system allows the length to be varied

150

154.82 nm

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4.0-

Y .c 2 3,00 5 oz 2.0

-

Laser emission from CIV ions: measurement

without disturbing the inversion density, and thus the measurement small signal gains.

of

Workers at NRL previously obtained laser emission at 116.1 nm from hydrogen using a travelling wave excitation. However, they believe that molecules cannot generate laser emission at wavelengths much shorter than this because sufficiently widely spaced energy levels do not exist. Such levels are present in ions and there is even a possibility of x-ray emission. Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington DC 203 75, USA CO2 laser rangefinder Ranging experiments using a frequency modulated CO, laser and optical heterodyne detection are being carried out at the Royal Radar Establishment. These show some success: in clear conditions, diffuse targets at ranges up to 10 km have been detected. Such a system could have some advantages over present laser rangefinders: the method of detection gives increased sensitivity and only a)ow laser power is necessary. It is also preferable from the point of view of safety, since damage caused by radiation of 10 pm wavelength is by surface burning and the threshold power density for this is much higher than the threshold for retinal damage with visible lasers. The laser used is a commercial singlemode single-transition, high-stability model. A triangular frequency modulation of 1.5 MHz amplitude and

of gain for the two lines observed

2 kHz repetition frequency is imposed on the beam by modulating the cavity length with a piezoelectrically driven laser mirror or an intercavity electrooptic modulator. The output is transmitted by a telescope of 15 cm aperture giving a diffraction limited diverging beam, and the same telescope collects the radiation backscattered from the target. A cooled PbSnTe photo-diode is used for detection. Output is either observed directly on an oscilloscope or is processed by a spectrum analyser. At present, the maximum range is approximately equal to the visible when this is less than 5 km, although one experiment in thin fog was able to detect a 1 km target when it could not be seen visually. It is hoped that with improved signal processing and modulation, significantly better performance will be obtained. Industrial Applications Unit, Royal Radar Establishment, St A ndrew ‘s Road, Malvern, Worcester, UK

‘Green light’ for liquid crystal displays RCA are now to go ahead with full scale commercial production of liquid crystal displays. The first efforts of the new Liquid Crystal Products organization will concentrate on displays designed for watches, clocks, calculators, and laboratory instruments. RCA first demonstrated liquid crystal displays in 1968, and in 1971 four developmental types were introduced by the Solid State Division. RCA are

OPTICS AND LASER TECHNOLOGY.

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1973

now optimistic about the future of the total liquid crystal market, quoting an estimated figure of $6 million for 1973 and greater than $100 million for 1977. RCA, 30 Rockefeller Plaza, New York 10020. USA Ferranti multi-fold

laser in the US

A new laser system announced by American Optical uses the Ferranti high-power multi-fold CO, laser head, which is designed to occupy the minimum space by having zig-zag folds in the resonator tube. The overall length of discharge is 10 m and this is housed in a box measuring 1.5 m by 0.5 m square, which also contains the necessary vacuum manifold and cooling water circuits. The total weight is only 120 lb. Each fold utilizes a single mirror. The mirrors and discharge tubes are mounted on an independently suspended structure within the box which is sufficiently stable for external alignment trimmers to be dispensed with. Both mirrors and discharge tubes may be replaced without need for re-alignment. The laser oscillates

in the TEMOO mode and produces a typical power output of 450 W. The MF 400 laser was first planned and developed by Ferranti in 1970, and by the end of 1971 was being used in the British metal-working industry. American Optical now have all marketing rights to it in North America. A demonstration model of the new AO-65 system is at present being used in a co-ordinate tracing system at their Southbridge plant. Ferranti Limited, Dunsinane Avenue, Dundee DO2 3PN, UK American Optical Corporation, Southbridge, Massachusetts OI550, USA

2 dB km“

limit to fibre losses

Experiments at Corning on the low loss fibre-optic waveguides recently produced indicate that their ultimate lower limit of attenuation at wavelengths between 700-l 100 nm should be around 2 dB km-l. The present fibres have measured attenuation levels down to 4 dB km-’ (as reported in the December 1972 issue of Optics and Laser Technology) and these have enabled a much fuller interpretation of the absorption spectrum. Studies showed that between 700-l 100 nm all signal losses due to absorption can be accounted for to within 20.7 dB km-’ by the waveguide’s OH content. Reduction of the water content of the waveguide material through tighter process controls would thus leave scattering as the major loss mechanism and permit total attenuations of about 2-3 dB km-’ The researchers at Corning estimated that by eliminating the scattering losses due to waveguide imperfections, the figure can be reduced by 30% to reach that of the intrinsic glass. Thus they concluded that a total attenuation of about 2 dB km-’ appears possible. Corning Glass Works, Coming, New York 14830, USA

Sorting by laser

The Ferranti high-power multi-fold laser which is now being used in a new system from American Optical

OPTICS AND LASER TECHNOLOGY.

Two of the new laser code reader systems developed by Bendix have been installed to automatically identify and sort cartons in TRW’s large parts replacement division in Cleveland. Each system is installed next to a

AUGUST

1973

conveyor line. A laser beam scans the codes on the carton. These consist of 0.125 in and 0.0625 in black bars that can be arranged in 16 different combinations, and placement on the carton is restricted only by a +25” tilt limitation. Carton alignment can be off by +40” without causing a loss in reading accuracy. A mechanical memory unit controls sortation. Bendix Recognition Systems, 3.526404 Southfield, Michigan 48076, USA

Measurement light

of the speed of

The value 299 792.4562 ?r 0.001 km set-l for the speed of light that was obtained recently by scientists at the National Bureau of Standards, is around 100 times more accurate than that obtained by Froome in England during 1957. It was the combined result of better frequency and wavelength measurements on the 3.39 pm emission from a stabilized He-Ne laser; these were published by two separate groups at the NBS’s Boulder laboratories at the end of last year. One important factor in the experiments was the stabilization of the laser: a fixed reproducible frequency was obtained by absorption of the beam in methane gas. The frequency is measured in steps, comparing the standard caesium frequency (9.19 GHz) to an infra-red HCn laser, comparing harmonics of that laser to a water vapour laser of higher frequency, and its harmonics to a CO, laser. The difference between two different CO, laser lines is then measured and the second compared to the He-Ne laser, which is locked to the methane absorption line 88.38 THz. Two laser frequencies are compared by mixing them on a catwhisker diode - a high speed detector developed by the NBS and the Massachusetts Institute of Technology. Known frequencies from a klystron oscillator are added and a heterodyne effect allows measure ment to be made. The wavelength determination consists of an interferometric comparison of the stabilized He-Ne laser and the krypton standard lamp. The lamp provides an asymmetric line much wider than that of the laser emission, limiting the accuracy of the wavelength measurement to about ten

151

times that of the frequency measurement. The NBS report points out that from these results it would seem desirable to improve the length standard: either by replacing the krypton lamp with a stabilized laser or by defining the value of c. National Bureau of Standards, US Department of Commerce, Washington, DC 20234, USA USSR experiment telephone

with laser

Scientists at Yerevan University and the Armenian Academy of Sciences have been transmitting telephone conversations by laser beam between Yerevan and the astrophysical research centre situated in Biurakan, a distance of some 35 km. This is only the first stage in a series of experiments designed partly to look at the possibility of a laser system for space communications. Novosti Information Service, 3 Rosary Gardens, London SW 7 4N W, UK

Monitoring uv

A laser drilling system designed by IRD being tested before delivery to British Nuclear Fuels. It is to assist in advanced gas-cooled reactor research at Windscale in Cumberland. A pulsed ruby laser supplying up to 70 J is used to drill a closely defined hole of 0.030 f 0.005 in diameter in the end of an irradiated fuel can. Highly radioactive fission products built up within the can during irradiation may then escape and be collected for analysis. The system is based on the IRD Series 5 ruby laser and the design problem was mainly one of focusing and alignment: access for viewing, aiming, and firing the laser is limited to a 3 in diameter aperture in a 4.5 ft thick protecting wall, through which a tube (right) will pass. The optical arrangement (centre foreground) allows the target, which is illuminated by a quartz halogen lamp. to be viewed before firing the pulse; the laser is focused through a lens of 15 cm focal length and a 0.25 in thick cell window, both of stabilized glass. The target cans are of stainless steel 0.030 in thick, and a single shot produces the desired hole International Research & Development NE6 ZYD, UK

Company

L td, Fossway,

Newcastle

Upon Tyne,

exposure to industrial

A prototype indicator for measuring the intensity of ultra-violet radiation at wavelengths most damaging to the human body has been produced by CBS. Field tests are now being carried out. The instrument was developed for the Department of Health, and it is hoped that it will be used to measure the exposure hazard to industrial workers in the US: ultra-violet radiation originates from such equipment as arc lamps and arc welding torches,

and there is considerable concern over the amount of injury it causes. The indicator claims to permit monitoring for periods from eight hours per day down to 0.1 s per day. It is portable, weighing less than 6 lb.

Electrical and Electronics Engineers for his contribution to the field of visible light emitting diodes and lasers.

IEEE award for work on diodes

Professor Holonyak first made a p-n junction in GaAsP in 1960 while working for the General Electric Company, and in 1962 produced a visible GaAsP semi-conductor laser. Work is now being done on increasing the efficiency of GaAsP into the infra-red by nitrogen doping.

Professor N. Holonyak from the University of Illinois will receive the Leibmann award of the Institute of

Engineering Publications, 112 Engineering Hall, University of Illinois, Urbana, Illinois 61801. USA

CBS Laboratories, High Ridge Road, Stamford, Connecticut 06905, USA

Optics at the Physics Exhibition The 1973 Physics Exhibition was held from 9-13 April in London, this year at Earls Court. Although there was no increase in the amount of optics or electro-optics on display, the exhibits created considerable interest and covered a wide range from the latest developments in Nd-YAG and dye lasers to liquid crystals.

152

Electra-optical

the position of two reference marks on a dumbell test specimen.

extensometer

H. W. Wallace were demonstrating an extensometer, now commercially available, designed to measure strain in highly extensible clear, coloured, or opaque elastomers. It uses two ‘autofollowers’, each of which contain a light source, a photo-cell, and a small electric motor, to monitor

OPTICS

AND

The autofollowers are mounted on a track (normally vertical) and one automatically locks on to each reference mark. When the mark moves, the signal from the photo-cell is electronically processed to power the electric motor. This moves the entire

LASER

TECHNOLOGY

. AUGUST

1973