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The construction and capabilities of a pulsed time-of-flight laser radar device constructed for the measurement of liquid levels is presented. The system consists of a measurement head, an electronic unit and a fibre optic connection between the two. The measurement head includes the laser transmitter, which is a collimator pen laser diode with a peak output power and repetition rate of 10 mW and 1 MHz, respectively, and single axis optics. The novel construction of the optics together with the highly non-divergent beam of the laser diode (< 0.3 mrad) and the mirror-like reflection of the liquid level enable measurement situation to be achieved in which the amplitude of the received pulse remains constant as a function of the measurement range. This simplifies the construction of the system and increases its accuracy, as the errors entailed in automatic gain control electronics can be avoided. On the other hand, the construction increases the accuracy needed in the set-up of the system, as the received beam has to fall on the receiver lens. Time interval measurement is based on the counting of the pulses of an accurate free-running 100 MHz oscillator within start/stop time intervals. By averaging 106 measurement results a resolution of about 1 mm is achieved. The averaging circuit is constructed in such a way that missing stop pulses, e.g. due to momentary fluctuations in liquid level, do no interrupt the measurement. The resolution and accuracy of the system were measured to be 1 mm (a-value) and 2 mm, respectively. If the liquid level fluctuates, the accuracy deteriorates due to dispersion effects in the receiver fibre, which may have a length of up to a hundred of metres. This error can be reduced, however, by use of a mode scrambler at the input to the receiver fibre.
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