Lidar-signal compression by photomultiplier gain modulation: influence of detector nonlinearity.

The application of photomultiplier gain modulation to the compression of wide-dynamic-range lidar signals is investigated in relation to the effect of the gain level on anode-signal linearity. Gain reduction is achieved by the coupling of modulation signals through either multidynode or focus-grid gating networks. This technique facilitates signal recovery and prevents detector nonlinearity and dynode damage caused by high near-field lidar signals. The measurements were performed in the current mode primarily on a 50-mm-diameter, 12-stage photomultiplier (EMI 9214) with a bialkali photocathode. With 3- or 4-dynode-based modulation made at a photomultiplier voltage of 1300 V and a gain of 1 x 10(7), signals of ~6 mA can be maintained at the 1% linearity limit from 100% to 0.2% modulation, corresponding to a 500-fold reduction in the lidar-signal dynamic range. A significant advantage to dynode modulation is that it preserves the shot-signal-to-noise ratio of the incoming signal, which is not true for focus-grid modulation or external predetection schemes such as controlled obscuration or Pockels-cell modulation that attenuate the as-yet unamplified signal.