Accurate pulse-to-pulse stability measurement for femtosecond lasers

Short-term energy stability metrology of pulsed lasers is the source of widespread misleading comparisons. The most used value for this characterization in commercial products and laboratory applications is the rms pulse-to-pulse stability (P-to-P). For frequency-specific analysis, the relative intensity noise (RIN) power spectral density is measured and the integrated RIN over a certain bandwidth is given. In principle, integrated RIN and P-to-P correspond to the same value. In practice, the different experimental setups often cause results to differ significantly. In this paper, we detail the specifics of measurement setups for short-term stability characterization and focus on measurements with stability better than 1% rms. The sensitivity of the P-to-P approach with respect to the oscilloscope specifications is shown by measuring a commercial femtosecond amplifier using three different oscilloscopes. A simple modified setup, including a low-pass filter at the laser repetition rate to turn the photodetected pulse train into a quasi-sinusoidal response, is proposed. This approach allows to overcome the oscilloscope limitation in standard P-to-P setups, allowing to obtain reliable measurements lower than 1% rms with a resolution of the order of 0.1%.