Experimental determination of temporal asymmetry of ultrashort laser pulses using unbalanced modified spectrum auto -interferometric correlation technique A. K. Sharma * , M.Raghuramaiah, P. A. Naik, and P. D. Gupta Laser Plasma Division, Centre for Advanced Technology, Indore 452 013, India ABSTRACT We report experimental implementation of an unbalanced modified spectrum auto-interferometric correlation (UMOSAIC) technique for visual detection and estimation of the laser pulse temporal asymmetry without direction-of- time ambiguity. This is based on modification of the spectral contents of second order unbalanced interferometric autocorrelation (U-IAC) signals. We use a commercial AlGaAs light emitting diode LED as a two-photon detector and a commercial audio speaker to record second order U-IAC signals in real time. A pulse asymmetry of ~30% has been estimated for the ~200fs femtosecond laser pulses from a cw mode-locked oscillator. Keywords: Ultrafast laser pulse measurements, Laser pulse shape, Unbalanced interferometric autocorrelation signals, UMOSAIC technique. 1. INTRODUCTION While simple diagnostic systems such as single shot second autocorrelators [1-5] are widely used for measurements of pulse duration, angular chirp and pulse-front tilt of ultrashort laser beams, they cannot provide information on the pulse shape and any temporal asymmetry present in the laser pulse. Many techniques [6-15] have been developed so far to indirectly determine the pulse shape and phase of ultrashort laser pulses using complicated iterative algorithms. For instance, frequency resolved optical gating (FROG) [6], is one of the most commonly used techniques for detailed pulse characterization even for pulses of few cycles duration. In FROG, the autocorrelation is spectrally resolved for each delay step. The autocorrelation is then sampled on a two dimensional time frequency grid, called FROG trace, which is usually sufficient for unique reconstruction of the amplitude and phase of the ultrashort pulse via some iterative phase retrieval algorithm. Further, a third order intensity autocorrelation or third harmonic generation FROG or its variant is required to remove direction-of-time ambiguity in ultrashort pulse shape measurement. There has been a long standing interest in pulse characterization methods using low cost and simple auto (or cross) correlation techniques [12-18]. The interferometric autocorrelation (IAC) coupled with field autocorrelation (or power spectrum) are known to uniquely determine the pulse shape and magnitude of chirp without direction-of-time information[13]. Recently, based on autocorrelation signals, a new technique namely phase and intensity from correlation and spectrum only (PICASO) has emerged which also either uses second order cross-correlation [15] or an unbalanced third order intensity correlation [15] for full characterization of ultrashort laser pulses without any ambiguity in direction-of- time. Though the above stated techniques can provide detailed pulse shape and pulse chirp, it may not be convenient to use them for visual detection of temporal pulse asymmetry, especially when the latter is of a very small magnitude of a few percent. As stated earlier, cross-correlation or higher order autocorrelation signals are required for temporal asymmetry measurement [19-20]. While the cross correlation experiments may not be possible for pulses of smaller duration, recording of a higher order correlation has two disadvantages viz requirement of a large amount of laser pulse energy, and a more complex experimental set-up. In many situations, it is of main interest to detect only the presence of either any temporal asymmetry in the laser pulse or the occurrence of double pulses, rather than to carry out a detailed temporal characterization. It will be interesting if one could detect and measure the pulse asymmetry in a simpler way