High-resolution Interferometric Diagnostics for Ultrashort Pulses

2.1 Metrology, ultrashort pulses, and ultrafast scienceproved understanding.The highest temporal resolution of a streak camera is approximately 2 ps [18]. The introductionof passively mode-locked dye lasers led to the first subpicosecond pulses [19]. Measurementof such pulses required the use of nonlinear autocorrelation methods, with the general feature thatthe detected signal is the result of the pulse nonlinearly interacting with a delayed replica of itself.By scanning the delay, an estimate of the pulse’s properties is obtained. The intrinsic temporalresolution of such methods is the response time of optical nonlinearities, which in many casesis below one femtosecond. Nonlinear autocorrelations had previously been used in addition tostreak cameras for longer pulse durations [20, 21], but became an essential technique in verifyingand guiding improvements in laser technology below one picosecond. Besides estimating thepulse duration, intensity autocorrelations suggested a hyperbolic-secant pulse shape, consistentwith theoretical models [22], and subtle dynamical effects [23] analogous to those observed inoptical fibres.Whilst nonlinear methods enabled subpicosecond time resolution, linear spectral measurementscontinued to provide useful information. Since the spectrum conveys the presence or absenceof particular wavelengths, but not their arrival times, knowledge of the spectrum can onlyprovide a lower bound on the pulse duration via the uncertainty property of the Fourier transform.Treacy [24, 25] observed that the duration of Nd:glass laser pulses, as inferred from nonlinear autocorrelations,was significantly longer than the minimum duration inferred from the spectrum— the pulses were not Fourier transform-limited. The pulses could be compressed to their transformlimited duration by applying wavelength-dependent delay, and were thus diagnosed to possesschirp — different arrival times for different wavelengths. This was caused by dispersion —the wavelength-dependence of the group velocity of light — in the elements of the laser cavity[26, 27]. The shorter a pulse, the broader its spectrum, and the more sensitive to dispersion it becomes.Therefore, since its diagnosis in the 1960s, measurement of pulse chirp has been crucialin the quest for shorter pulse durations. For example, similar methods to Treacy [24] were used tomeasure and compensate for the chirp of passively mode-locked dye lasers [28]. In the collidingpulse technique [29, 30], which accessed the sub-100 fs regime for the first time, chirp measure-9