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Vol. 9     No.3   June  2001       

Threshold Detector Unit

threshold.gif (103268 bytes)A Threshold Detector Unit has been developed for the evaluation of electro-optic  countermeasure (EOCM) class solid-state laser systems in field conditions over long ranges. It is not available commercially in the format desirable for such a diagnostic application. Measurement of energy density requires absolute value of the energy as well as the spot size at the target point. Even if the required parameters are computed, i.e., energy density from the measured values of energy and spot size using conventional equipment, the process will be highly cumbersome as the spot sizes typically range from a few centimeters to few meters in realistic battlefield ranges. The detector unit eliminates the need for such a cumbersome test set-up and gives the desired information directly in a compact single gadget. The detector unit has selectable energy density threshold level; and at a selected energy density level, it is capable of establishing whether the energy density at the desired target point is above or below the preset threshold level. When the energy density at  the target point is   more than the set value, the unit gives both an audio and a visible indication. While the audio buzzer sound has been provided for information of the technical personnel sitting inside the instrumentation vehicle on the outcome of the laser shot fired, the glowing flash-light is for the personnel at the source end. The intensity of light is so high that it can be seen from a distance of more than 2 km even during the day time.

The detector unit can be used with both free running and Q-switched types of pulsed solid-state lasers and can be selected with the help of a switch. The system is capable of handling Q-switched laser pulses from 10 ns to 100 ns pulse duration and free running laser pulses from 100 ms to more than 600 ms. It has a , wide measurement range offering, three possible range selections of 5 ÁJ/cm2 to 50 ÁJ/cm2;   5 mJ/cm2 to 50 mJ/cm2 and 5 J/cm2 to 50 J/cm2,   thus making possible the evaluation of EOCM lasers producing pulse energies from hundreds of millijoules to several kilojoules. The range selection is done by using a different calibrated front-end attenuator assembly. Each range setting has 10 further selectable threshold levels. The detector unit has a provision for auto shutoff after 15 s to enable firing of multiple shots. It is battery-operated and needs no human intervention at the target point. The detector unit has been tested in the field with both types of lasers covering pulse energy of less than a joule to kilojoule levels and covering ranges from tens of meters to 2 km.