Today pulsed lasers have reached pulse durations lower than ~100fs. Modern measurement devices and electronics only have bandwidths that allow resolution of pulses in the nanosecond range and above. Thus, in order to measure pulse lengths and shapes below this, certain tricks have to be employed. A common way to do this is to use, what is known as an optical autocorrelator. In an autocorrelator, the input pulse is split into two parts; one part is delayed relative to the other, and the temporal overlap of the two sub-pulses is derived through nonlinear interaction followed by a measurement which can be performed with a low bandwidth detector system. Nonlinear frequency conversion is a process, where light at one wavelength is converted to light at a different wavelength. The efficiency of this process is depending on the intensity of the incident light; meaning that a reduction of 50% of the incoming light will reduce the output by 75%. The output power is then measured as function of the relative delay of the two sub-pulses, if the delay is shorter than the pulse duration the pulses will partly overlap and the nonlinear generated power will be higher, resulting in an increased average power. If the conversion efficiency is measured as function of delay an envelope of the autocorrelation function of the pulse can be measured.

The project is mainly experimental but also contains some theory. You will work on understanding the basic physics behind the nonlinear effects and you will get to work in the laboratory. In the laboratory we have a mode-locked laser giving pulses of a few ps. You will get to build an optical autocorrelator and characterize laser pulses.