This circuit design implements an integrator intended to allow digitization of the energy output of a pulsed laser, or the energy of a received pulse of laser light. It integrates the output of a detector upon which the laser light is incident. The integration is performed constantly, either by means of an active integrator, or by passive components. If an active integrator is used, closed-loop dc biasing is added with a time constant much longer than the laser pulse width. The output of the integrator goes to a track-and-hold amplifier (THA), using a zero-potential switch topology. The track/hold control is derived from timing information obtained either by a threshold comparator on the detector, or a peak detector. Laser pulses of varying widths can be accommodated by adjusting the characteristics of the timing control circuitry. The output of the THA is available for digitization at a later time. Bandwidth limiting can be used in the signal path as necessary, depending on the noise characteristics of the signal.

Prior integration techniques utilize threshold comparators to “start” and “stop” the integration. Some implementations require reset circuitry, which can create offset at the output. Starting and stopping the integration usually involves clipping off the beginning and/or end of the signal; this introduces greater errors as the signal amplitude decreases. Also, as the signal speed increases, the comparator speed must increase, and thus its power consumption rises. As the signal (pulse) gets narrower, the comparator delay time may cut off significant portions of the signal unless electronic delay is introduced. This adds complexity, mass, and timing uncertainty.

The advantage of this integration technique is that it does not depend on exact threshold adjustment to start or stop the integrator, nor does it require the use of switches to discharge the integration capacitor. Further, there is no pedestal introduced in the integrator.

This circuit is intended to implement an integrator that does not require time gating, delay, or reset circuitry, in order to avoid the limitations that these elements impose. The integrator is part of a pulsed laser energy monitor. This implementation of a continuous integrator is designed to be used in a laser transmit energy monitor.

The final design had the ability to integrate pulses down to 50 mV at 4.5 ns, and 250 mV at 3 ns, with 0.8-percent electrical accuracy.

This work was done by Jeremy Karsh of Goddard Space Flight Center. GSC-15843-1

Photonics Tech Briefs Magazine

This article first appeared in the April, 2011 issue of Photonics Tech Briefs Magazine.

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