A Fast CMOS Array Imager for Nanosecond Light Pulse Detection In Accumulation Mode

摘要

We designed a camera based on a fast CMOS APS imager for high speed optical detection which produces images similarly as a streak camera. This imager produces the intensity information I as function of one spatial dimension and time (I = f(x,t)) from one frame with two spatial dimensions. The time sweeping is obtained by delaying successively the integration phase for each pixel of the same row. For the first FAMOSI (Fast MOs Imager) prototype the start of integration is given by the camera itself. This signal is injected to a laser trigger. This laser emits a 10 nanoseconds light pulse onto the sensor. The temporal evolution of the light pulse is then resolved by the camera with a resolution of 800 ps. In single shot, the maximum dynamic of the camera is estimated to 64 dB and is limited by the readout noise. We decide to work in accumulation mode in order to increase the signal to noise ratio of the camera. But the high laser trigger (about 20 ns rms) does not allow accumulation of several optical events without a large spreading. The camera has been modified in order to be triggered by an external signal delivered by a trigger unit. In this new configuration the laser emit pulses at a repetition rate of 50 Hz. A photodiode detect a part of the laser pulse and generate the trigger signal for FAMOSI. The laser pulse is delayed with an optical fibre before being directed to the camera. The trigger jitter obtained is then less than 100 ps and allows accumulation without significant loss of the temporal resolution. With accumulation the readout noise is attenuated by a N~(1/2) factor. Then with N = 1000 accumulations, the dynamics approach 93 dB. This allows the camera to work similarly as a synchroscan streak camera and then to observe weak signal.