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A method is described for producing two controllable pulses from a Q-switched solid state laser, the time separation of the pulses being variable from about 30 nsec to a few microseconds. The technique involves Q-switching the system with a time variant loss which has two steps, the application of the first step only partially depleting the population inversion. The method has been demonstrated with an electro-optically Q-switched Nd:CaWO, laser.

IntroductionMinimum Resolvable Temperature Difference (MRTD) is now the most widely used parameter for describing both the temperature sensitivity, and spatial resolution of thermal imaging systems.It can be measured in the laboratory, using fairly simple equipment, it can be calculated from component parameters, and a good correlation has been established between MRTD and the field performance of systems, eg detection and recognition ranges of targets. However, both the strength, and the weakness of MRTD lies in the fact that it is a subjective parameter. It is measured by an observer viewing standard bar targets, and so it combines the spatial resolution and noise characteristics (thermal resolution) in the correct way.It also takes account of the performance degradations due for example to cosmetic defects.On the other hand, being a subjective measurement, there is bound to be some variations in measured values, particularly from one laboratory to another. This causes problems, if only from an administrative point of view, when testing a system against a given specification.The Royal Aircraft Establishment has been making measurements on thermal imaging systems for the last 62 years and considerable experience has now been built up in the use of MRTD. This paper discusses the techniques of measurement, and describes methods which are being developed to try to standardize MRTD values between laboratories. It also describes longer term work to move to completely objective measurement methods. MRTD measurementsMRTD is a measurement of the temperature difference at which a 4 -bar target can just be resolved, as a function of the bar spatial frequency.The form of the standard target, and a typical MRTD curve are shown in Figure 1. Measurements are made with an observer looking at the display, so that the parameter includes his spatial and temporal integration of the noise, together with his eye resolution.During a measurement the temperature difference between the bars and the background is gradually increased until the pattern can just be resolved, using a predetermined resolution criterion. During this process it is permissible for the observer to adjust the controls of the system, and to move the target position by small amounts within the field of view. He is also allowed to move his head position, although for certain applications a fixed viewing distance may be necessary. At low spatial frequencies, the observer is limited by noise on the display, and any cosmetic defects that may be present.As the temperature increases, the four bars suddenly appear as separate objects. At higher frequencies, the system's spatial resolution is a more important factor, and the observer sees a square target long before he can resolve the individual bars. Measurements are usually made with the target bars perpendicular and parallel to the scan direction, in the centre and at the edge of the field of view. In addition other bar orientations (eg 450) and other positions may be included for particular purposes.A diagram of the arr...

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The output characteristics of a slowly Q-switched Nd in CaWO4 laser have been studied by varying the switching time of an ADP electro-optic Q-switch from 20 to 1200 ns. The results are in broad agreement with the behaviour predicted by theoretical treatments of the subject.