Measurements and active control of the temporal characteristics of femtosecond laser pulses

Abstract

Measurements and active control of the temporal characteristics of femtosecond laser pulses are investigated. The effect of quadratic, cubic and quartic phase terms on the temporal profile of frequency-chirped laser pulses is analyzed, and a new technique is developed for the characterization of femtosecond laser pulses by applying an evolutionary algorithm to an interferometric autocorrelation measurement. The absolute phase term is discussed as a parameter of temporal characteristics in the case of few-cycle pulses. Chirped femtosecond laser pulses, generated by adjusting the grating separation of a pulse compressor in a chirped-pulse amplification laser, are investigated in time, frequency, and time-frequency domains. The quadratic phase terms and the pulse duration are analyzed with respect to grating detuning, and the effect of the high-order phase terms on the pulse duration is also examined. A time-frequency analysis of chirped femtosecond pulses is performed using the Wigner distribution (WD) function. The chirp structure is graphically interpreted in the WD using a peak detection method, which resolves the multicomponent frequencies that cannot be seen in the standard Fourier analysis. A new technique to recover the temporal phase of a femtosecond laser pulse from its interferometric autocorrelation trace and laser spectrum is presented. The phase retrieval algorithm, called `Evolutionary Phase Retrieval from Interferometric Auto-Correlation (EPRIAC),` is optimized for the characterization of few-cycle optical pulses and is tested with numerically generated femtosecond pulses. In actual applications, a 29-fs negatively chirped pulse and a sub-10-fs pulse containing high-order phase distortion are successfully characterized. Adaptive compression of femtosecond laser pulses is demonstrated using a low-loss pulse shaper that employs a micro-machined deformable mirror and a Brewster-cut prism. Much enhanced transmission efficiency (more than 85 %) is achie...