Over the last 20 years there has been considerable research and development of infrared laser sources. This interest stems from the presence of two low attenuation windows in the atmosphere between 3-5μ,m and 8-14μ.m, on the basis of which, a wide range of military, industrial and medical applications have been proposed. In particular the CO2 laser with its lasing transitions between 9-11μm, has and continues to be, the focus of much attention. Although the CO2 laser was first demonstrated in 1964 by Patel', it is only in more recent years, with the application of improvements in the understanding of laser physics in conjunction with advances in relevant technologies, that high power devices which are also compact, efficient, reliable and long lived, have made practical applications feasible.
This paper describes a laser source for the 4.6 to 4.8 micron band. The laser is based on a line tunable TEA CO2 laser operating between 9.2 and 9.6 microns. A frequency doubling crystal is then used to convert the radiation into the shorter wavelength band. Pulse powers > 100kW have been measured for the doubled wavelength when using AgGaS2 as the doubling crystal.
Some of the many different device types which are available for the detection of thermal infrared radiation are reviewed, with emphasis on the ongoing trends towards higher operating temperature and increasing array size.
Model for HgCdTe photoconductor sensitivity was constructed by solving a three dimensional (3-D) transport equation. The model takes into account recombination velocities at crystal side walls and surfaces, device temperature rise due to bias current Joule heat and temperature dependence of ambipolar mobility. Parameters, i.e., recombination velocity at crystal side wall, minority carrier bulk lifetime and majority carrier concentration were fixed by comparing the measured responsivity and carrier lifetime with the calculated responsivity and carrier lifetime. Using parameters thus fixed, noise was calculated and was compared with the measured noise. Good agreement between the calculated and the measured noises was obtained.
Linear pyroelectric arrays of up to 64 elements have been available for some time. Individual array elements are defined by photolithographic electrode patterning on the pyroelectric ceramic, and absorption of radiation is maximised across the wavelength range of interest by electrodeposition of 'platinum black on to the active element electrodes. Direct thermal conduction paths between adjacent elements in a monolithic array produce significant amounts of thermal crosstalk, decreasing the image resolution. Reticulation techniques have therefore been developed to isolate the elements thermally and hence decrease the crosstalk. Detector elements are individually compensated by large non-blacked areas connected in series opposition with the active element. This provides cancellation of common mode signals such as those produced by temperature drift and vibration. Discrete JFET chips have traditionally been used as buffer amplifiers, but MOSFET arrays have now been developed, giving a significant simplification in assembly. In its simplest form the MOSFET array consists of 32 transistors integrated onto a single chip; two of these chips are used to provide the 64 source followers for a 64 element linear pyroelectric array. A major advantage of MOSFET arrays is the lower gate leakage current than that obtainable with JFETs. This results in a lower current noise, which is manifested as an improvement in D* at low frequencies. The advantage of the MOSFET buffers is even more marked at elevated temperatures, because the leakage current of a JFET is strongly temperature dependent. As the element count of linear arrays increases so do the problems of interfacing the array to the following electronics; packages with more than 68 leads are very bulky. One solution to this problem is to incorporate multiplexers into the package, thus reducing the number of connections needed. To this end, multiplexed MOSFET arrays have been developed. The disadvantage of multiplexing immediately after the source follower is that no noise bandwidth limiting can be incorporated before the multiplexer, and the high frequency noise is aliased into the base band. This leads to a deterioration in the achievable signal to noise ratio. In order to improve on this, an integrated circuit has been developed which consists of 16 rising gain amplifiers and band limiting filters followed by a 16 way multiplexer. Eight of these integrated circuits have been incorporated into a single hybrid with a 128 element pyroelectric array.
This paper is concerned with the use of Joule-Thomson refrigerators in infrared detection systems such as thermal imagers, missile seekers, and guidance systems. Although the principle behind the minicooler is simple there exists a range of designs, either simple or complex, which cover many requirements from one-shot fast cool systems for missile guidance to systems requiring the minimum of logistic support and reliability for remote thermal imaging applications. The first step in examining the use of J-T minicoolers is to look at the applications and how each relates to the cryogenic system used.
The British Aerospace Dynamics Division has been active in the infrared field since the early 1950's. Originally infrared sensors were, and in many cases still are, cooled with Joule-Thomson coolers associated with rechargeable gas-bottles. Today, there is also a need for zero maintenance systems. To meet this need British Aerospace has productionised a range of viable Stirling Cycle cryogenic coolers. This paper summarises the intensive development programme which has taken place over the last seven years on such applications as the British Aerospace infrared linescan system for the Tornado and their suitability for further applications; both infrared and other sensors where signal-to-noise improvements at low temperatures are beneficial.
Philips Usfa BV makes a range of small cryogenic coolers for cooling the sensors of thermal imaging systems. These coolers operate around 80 K and have cooling capacities of 0.25 W, 0.5 W and 1.0 W. The diameters of the "cold fingers" are respectively 5, 7 and 10mm. The standard compressor used at present has a diameter of 71mm and a length of 145mm, and we are currently developing two new compressors : one for the 0.25 W and 0.5 W cold fingers and one for the 1.0 W cold finger. The 1.0 W compressor has a diameter of 60mm and a length of 122mm. The compressor for the 0.25 W and 0.5 W cold fingers is 44mm in diameter and 142mm long. During the last few years a lot of experience has been gained with various cooler-Dewar combinations, one of the most widely used being the 0.5 W type UA 7041 cooler combined with the R185 Mullard Dewar, a TICM class II Dewar containing Sprite detectors. This paper mainly reports on the measured performance of the UA 7041 cooler with this Dewar. It includes :
- basic technical principles of the cooler.
- typical cooling capacity of the UA 7041.
- typical cooling demand of the R185 Dewar.
- performance of the UA 7041
- R185 combination.
- performance of the UA 7043
- R175 combination.
- results of lifetime testing.
The Rutherford Appleton Laboratory (RAL) is involved with two space projects that require cooled infrared detectors operating at about 80 K; the Along Track Scanning Radiometer (ATSR), and the Improved Stratospheric and Mesospheric Sounder (ISAMS). In addition, a two stage Stirling-cycle cooler is being developed for space use. This will act as a precooler for a 4 K Joule-Thomson expansion stage. This paper describes the development and performance of these refrigerators.
New cold shields for infrared detectors are described, with which the signal-to-noise ratio was considerably improved. The function of two of these cold shields is based upon the use of intermediate images which make it possible to shield off housing radiation at very small incident angles to the optical axis. With these cold shields, the uniformity of the pictures was improved and the formation of undesirable background patterns was successfully eliminated.
This review discusses the intrinsic and extrinsic loss mechanisms and compares the physical properties of infrared optical materials. Some recent developments in bulk materials and infrared fibres are discussed together with an indication of present information gaps and material gaps.
This paper reviews some of the advances that have been made in antireflection coatings for the 8-14 μm infrared waveband. These include abrasion resistant chemically durable coatings on lenses and windows for thermal imaging systems, broadband coatings covering the 8-14 μm and 3-5 μm spectral regions and multispectral windows and coatings combining the infrared and microwave bands. Various deposition methods are discussed including the use of MBE for the production of laser resistant coatings.
We summarise a previously described method of economically manufacturing high precision aspherics by vapour deposition on infra-red materials. Then, on a specific lens example, we show the influence of two parameters (time of deposit and centering) on the final modulation transfer function. Finally, we introduce a set of orthonormal functions best suited to simulate aspherics for high aperture lenses.
The improvements obtained on cooling atmospheric remote-sensing instruments for space flight applications has promoted research in the characterization of optical filters necessary for spectral selection. By modelling the effects of temperature on the dispersive spectrum of some constituent thin film materials, the cooled performance can be simulated and compared. Two actual filters are discussed for the 7 pm region, one a composite cut-on/cut-off design of 13% HBW and the other an integral narrowband design of 4% HBW.
The factors influencing the approach for a particular grade of material are reviewed both from optical and physical viewpoints. The influence of process control on material properties is illustrated and the chemical vapour deposition reaction sequence is discussed with particular reference to zinc sulphide manufacture. The use of a pilot plant facility in process evaluation is described.
Manufacturing methods for diamond machined optical surfaces, for application at infrared wavelengths, require that a new set of criteria must be recognised for the specification of surface form. Appropriate surface form parameters are discussed with particular reference to an XY cartesian geometry CNC machine. Methods for reducing surface form errors in diamond machining are discussed for certain areas such as tool wear, tool centring, and the fixturing of the workpiece. Examples of achievable surface form accuracy are presented. Traditionally, optical surfaces have been produced by use of random polishing techniques using polishing compounds and lapping tools. For lens manufacture, the simplest surface which could be created corresponded to a sphere. The sphere is a natural outcome of a random grinding and polishing process. The measurement of the surface form accuracy would most commonly be performed using a contact test gauge plate, polished to a sphere of known radius of curvature. QA would simply be achieved using a diffuse monochromatic source and looking for residual deviations between the polished surface and the test plate. The specifications governing the manufacture of surfaces using these techniques would call for the accuracy to which the generated surface should match the test plate as defined by a spherical deviations from the required curvature and a non spherical astigmatic error. Consequently, optical design software has tolerancing routines which specifically allow the designer to assess the influence of spherical error and astigmatic error on the optical performance. The creation of general aspheric surfaces is not so straightforward using conventional polishing techniques since the surface profile is non spherical and a good approximation to a power series. For infra red applications (X = 8-12p,m) numerically controlled single point diamond turning is an alternative manufacturing technology capable of creating aspheric profiles as well as simple spheres. It is important however to realise that a diamond turning process will possess a new set of criteria which limit the accuracy of the surface profile created corresponding to a completely new set of specifications. The most important factors are:- tool centring accuracy, surface waviness, conical form error, and other rotationally symmetric non spherical errors. The fixturing of the workpiece is very different from that of a conventional lap, since in many cases the diamond machine resembles a conventional lathe geometry where the workpiece rotates at a few thousand R.P.M. Substrates must be held rigidly for rotation at such speeds as compared with more delicate mounting methods for conventional laps. Consequently the workpiece may suffer from other forms of deformation which are non-rotationally symmetric due to mounting stresses (static deformation) and stresses induced at the speed of rotation (dynamic deformation). The magnitude of each of these contributions to overall form error will be a function of the type of machine, the material, substrate, and testing design. The following sections describe each of these effects in more detail based on experience obtained on a Pneumo Precision MSG325 XY CNC machine. Certain in-process measurement techniques have been devised to minimise and quantify each contribution.
Early attempts to extend staring-mode sensing into the thermal infrared spectrum failed, because the resulting imagery was dominated by fixed pattern noise. The source of this noise was modulation of the infrared background by sensor response non-uniformities. In 1973, use of internal photoemission from Schottky silicide arrays was proposed as a means of achieving the photoresponse uniformity necessary to obtain useful thermal imaging capability. Since that time, there has been a steady evolution in silicide sensor technology. Current silicide cameras have sensitivity comparable with the best scanning systems. These cameras are based upon the largest infrared arrays now available. This paper will describe recent advances in silicide sensors and project future technology trends.
For the last two decades, ground attack and strike aircraft have possessed an impressive capability to reach and destroy targets by day and in favourable weather conditions. Operations in poor weather and darkness have been denied until recently when developments of thermal imaging combined with Night Vision Goggles (NVG) have opened up new operational concepts. Most aircraft have been severely limited at low level by the pilot's ability to acquire visually targets at a reasonable distance such that a first pass attack is possible. Besides the basic problems of terrain screening the pilot is faced with the effects of daylight haze and mist combined with the smoke/dust of a battlefield scenario.
The two most common spectral bands used in the thermal infra red are the 3-5 μm and 8-12 μm bands. For years, substantial controversy has occurred over which of these spectral bands has the greater merit. A qualitative approach to address this question is presented.