Recent developments

Recent developments

However, with a temperature controlled Fabry Perot etalon accessary this can be further decreased to l/500 cm-l. Application notes and a full specific...

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However, with a temperature controlled Fabry Perot etalon accessary this can be further decreased to l/500 cm-l. Application notes and a full specification of the tunable laser are available on request. Systems price range is from Xl 2 000 to &25 000 depending on the tunable ranges required; delivery 90 days. Rofin Ltd, 3 Windhill, Bishop’s Stortford, Hertfordshire, UK

Mesh filters for the far infra-red Cambridge Physical Sciences now supply low-pass and band-pass mesh interference filters for the far infrared. The design of the filters is based on the frequency dependent reflection and transmission properties of metallic meshes. A complete filter usually consists of two to five meshes which are accurately spaced apart. Its transmission characteristics are functions of mesh type, geometry and spacing: destructive interference occurs between wavelengths outside the pass-band. Mesh filters of this kind are said to have sharper cut-offs and better transmission than filters relying on selective absorption in a medium. A range of standard low-pass filters is available with cut-offs at frequencies 140 cm-l, 125 cm-l, 110 cm-l, 67 cm-l 33 cm-l and 10 cm-l. Non-standard low-pass filters and band-pass filters with edges between 200 cm-* and

10 cm-l can be made specially to order. Unwanted transmission in the visible and near infra-red regions may be reduced to typically 0.2% of maximum transmission by use of an additional blocking filter. The filters have a maximum transmission of 90% unblocked or 60% when blocked from cut-off down to the visible region. The filters have a working aperture of 25 mm diameter and are mounted in stainless steel rings. Mesh filters are used in far infra-red interferometry and spectrometry, for laser line separation and in infrared astronomy and radiometry. They have been successfully cooled to liquid helium temperatures and filters made for the Nimbus E weather satellite have withstood thermal cycling and vibrational testing according to NASA specifications for space flight acceptance. Gzmbridge Physical Sciences, Bar Hill, Gzmbridge CB3 8EZ, UK

Improved quality ceramic for laser rods Polycrystalline ceramic laser rods of improved optical quality have been produced at the General Electric Company USA. The composition of the ceramic material - termed Yttralox by GE(USA) ~ is 89 mole % yttrium oxide, 10% thorium oxide and 1% neodymium oxide. It is produced by conventional cold pressing and sintering techniques. Typical rods studied have been 3 in long and l/4 in thick.

Scientist from GENJSA) examines polycrystalline ceramic laser rods

The neodymium doped rods are said to have a gain intermediate between glass and yttrium aluminium garnet but offer a potentially higher average power output than neodymium doped glass. The lasing slope efficiency of a rod is about 43% that of a comparable glass laser rod; its lasing threshold is 8 J, compared to 9 J for glass. The usefulness of this material has previously been limited by high scattering losses, particular optical defects being pores and a variation in the refractive index. Dr Greskovich of GE(USA) found that the optical quality could be improved through a change in processing of the ceramic. The constituent powders are milled in a rubber-lined ball mill for several hours; this reduces the size of the particles and mixes the components more thoroughly. As a result of reducing the pore density and variation in refractive index, GE(USA) claim that the optical quality of the rods has been improved to the point where it is possible to sight through a rod and resolve an object several miles away; further research could now be directed towards possible submicroscopic scattering defects. General Electric Company USA, Research and Development Centre, Schenectady, New York, USA Optical dipsticks for liquids A series of optical dipsticks and depth gauges has been developed by UMIST University of Manchester Institute of

Mesh filters for the far infra-red

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Science and Technology - for measuring and monitoring the level of liquids and for operating switches at pre-set levels.

engineers on making a prototype and sometimes it may be possible for UMIST to design and make simple prototypes.

Such devices, which are widely used commercially, utilize the change in the critical angle at the surface of a transparent solid when it is placed in contact with a liquid. Light is sent down a transparent rod; it is reflected back up again from a conical or bevelled tip provided the tip is dry; if the tip is wet the light leaves the rod and the return signal is lost. Advantages are that the rods can be made of low conductivity, corrosionresistant materials and light can be conveyed to and from the device by light-guides - applications have included gauges for fuels, acids, liquid nitrogen and oxygen.

Dr J. E. Geake, Physics Department, UMIST, Manchester M60 1 QD, UK

Several variants of the gauge have been designed and made at UMIST. In one version, the rod or probe is fixed at one or more pre-set levels to operate pumps or alarms. In another, multiule rods indicate a series of increments of liquid depth. A further type, reading remotely, uses a multiple photo-cell output, while a different but related device locates the interface between two liquids. If a firm has particular requirements, UMIST can advise their design

Keeping up to date with quantum electronics A course ‘The physics of quantum electronics’, designed for advanced students and research workers will be held from July 9-20 in Washington. It is a continuation of courses held in 1968, 1969 and 1970 and aims to lead students towards an understanding of the present frontier research areas: for example in the field of high-energy, high-power lasers and propagation of laser beams through the atmosphere. Other topics covered include picosecond light pulses, dye lasers, and integrated optics. Professor S. F. Jacobs, Optical Sciences Center, University of Arizona, Tuscan, Arizona 85 721, USA Laser stylus for VLP player A small He-Ne laser and an optoelectronic tracking mechanism are used for play-back of video long

Schematic representation of the Philips VLP system: 1 - video long-playing record (la shows the pattern of pits); 2 - springsuspended lens with automatic focusing of the light beam; 3 - hinged mirror for following the track; 4 - beam-splitting prism; 5 - photo-diode; 6 - light source

playing (VLP) records being developed by Philips. These can, it is claimed, store colour programmes lasting 30 to 45 minutes and the play-back system can be connected to a TV set. The records are of normal LP size and are made from a similar material. The information is stored on the record along a spiral-shaped track which consists of a string of microscopically small oblong pits. All pits are equal in depth and width, and their variation in length and distance from one another contains all the information required: brightness, colour, synchronization and sound. The VLP is played at a speed of 25 revolutions per second and each turn of the track contains the information needed to reproduce one complete image. In play-back, a spot of light from the He-Ne laser is centred on the track by means of an opto-electronic control system. When the spot of light scans the track, the light reflected by the record is modulated in accordance with the pattern of pits. The modulated beam impinges upon a photo-diode delivering an electrical signal which, after suitable amplification and processing, can be fed directly to the input of a television set.

A recent demonstration of fibreoptical communications was arranged at the UK Institution of Electrical Engineers to coincide with the 50th anniversary of the British Broadcasting Corporation. The picture from a conventional television camera is converted into pulses from a solid state laser and transmitted down 0.8 km of liquid filled optical fibre. The signal is reconverted into an electrical signal by an avalanche photo-diode and fed into a conventional television receiver. Professor W. A. Gambling, UK

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Department

of Electronics,

Southampton

University,

Southampton,

The high intensity of the laser light gives a signal sufficiently free from noise. The opto-electronic tracking mechanism allows a small track pitch and also the possibility of producing stills, slow-motion and reverse-motion pictures from the recording; since the pick-up of the signal occurs without

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mechanical contact, neither the record nor the pick-up system wears out. Philips state that they aim to make the VLP system commercially available within a few years. Philips, Eindhoven, The Netherlands Airborne

laser rangefinder

An airborne laser rangefinder unit, TAV 38, is to be developed and manufactured by Thomson-CSF and the CGE Research Centre. The TAV 38 is designed to supply the aircraft-target range to the fire control computer of an airborne weapon system, in particular the Dassault Fl and Mirage aircraft. It is capable of operation between -40°C and +7O”C. The unit consists of a laser rangetinder and a beam deflector. Developed by the Marcoussis Laboratories of the CGE Research Centre, the rangefinder is said to be accurate to within 5 m irrespective of the range (320 m-10 km). Repetition rate is 1 shot per second with a limit of 12 shots per minute. The 25 x low9 s pulses are supplied by a neodyme-doped glass laser at 1.06 ~1wavelength. The echo from the target is received on an avalanche photo-diode. The time interval between the departure of the pulse and its return is measured by a reference clock. The deflector developed by Thomson-CSF is claimed to widen the rangefinder’s operational field and allow the pilot to obtain accurate range measurements. Laboratoires de Marcoussis, Centre de Recherches de la CGE, 91 Marcoussis, France

a

The RCA built meteorological satellite NOAA-2 is equipped entirely with infra-red instruments which supply cloud cover pictures together with vertical temperature readings. The very high resolution radiometer situated on the satellite can sense objects as small as 0.5 miles across and gives more detailed pictures than those previously obtained. The vertical temperature profile radiometer covers an area of the atmosphere from sea level up to 20 miles altitude RCA International

Lrd, 50 Curzon Street, London

b

c

Holographic interferometry enables scientists at the US Naval Research Laboratory boundaries and other crystal defects on the process of solidification.

WI Y 8EU,

UK

d to observe in detail and measure the influence

of grain

They employ large crystal growth chambers to avoid the constraining influence of the container walls on diffusion, convection, and crystal geometry. To record these crystallization events at unpredictable locations with high resolution, the researchers have developed a holographic system with an off-axis reference wave. They employ an argon ion laser with an acousto-optics switch which allows the recording of a time sequence of holograms at rates up to 4 per second and with individual exposure times as short as 50 ~1s. The reconstructed images permit the detailed study of sample volumes of a few cubic centimeters with a resolution as high as 400 cycles per mm. The photographs show the development grams were taken at 15 s intervals Naval Research Laboratory,

of features near a grain boundary

tri-junction

at a solid-liquid

interface

in camphena.

The four holo-

Washingron DC, USA

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