Image processing provides low-frequency jitter correction for synchroscan streak camera temporal resolution enhancement

Abstract

The temporal resolution of a synchroscan streak camera is mainly limited by the intrinsic tube resolution, the laser pulse width and the synchronization jitter between the camera and the laser source. Studies show that laser phase noise is localised principally at low frequency. A previous system was designed in order to eliminate the very low frequency jitter. The system allows the streak camera to accumulate a signal during over a long period (several hours) without significant temporal resolution degradation. In order to work properly, this system use a laser reference directly coupled to the streak camera on the top of the photocathode. The localisation of this laser reference spot is locked at a predefined position and then, the temporal axis of the streak camera image is locked. To allow this control, the software changes the phase between the deflection plate voltage and the synchroscan signal. The resolution obtained was about 2 ps Full Width at Half Maximum (FWHM) which is the best resolution available in the accumulation mode and this can be achieved whatever the accumulation time. In this paper, we describe an upgrade of this system which uses the laser reference information to accumulate the different frames after a retiming. It calculates the centre of gravity (COG) of the laser reference, shifts the image on the temporal axis with a sub-pixel resolution to place this COG to a predefined position. Then the frames are accumulated. By this way, the inter frame jitter is reduced. This system benefits from the very high temporal resolution of the streak camera to make to correction so that it can be very efficient. In photon counting, the temporal resolution with this system is improved to a value of 1,5 ps FWHM. With a signal to noise ratio of about 1000 the acquisition time is 35 minutes.