High-resolution Interferometric Diagnostics for Ultrashort Pulses

2. BACKGROUNDments [31, 32] led to the deliberate introduction of dispersive elements into the cavity leading togreater pulse compression [33, 34], eventually down to 27 fs [35, 36].While ultrashort dye lasers were enjoying their zenith in the 1980s, investigations into solidstatematerials revealed the potential for even greater bandwidths, potentially supporting pulsesas short as 1 fs [37]. From a number of different technologies and pulse generation mechanisms,the Ti:Sapph oscillator emerged as a leading contender. By 1993, pulse durations down to about10 fs [38–41] were being produced. However, in this regime of pulse duration, the enormous bandwidthspossess a chirp which cannot be accurately characterised by a simple linear dependenceof arrival time on wavelength. Instead, several orders of a Taylor polynomial expansion must beconsidered for good characterisation and compensation [42]. The combination of the spectrumwith the simple nonlinear autocorrelations, as used previously, did not determine these higher orderterms. This motivated the development of complete characterisation methods, capable of retrievingall information about the pulse up to a few well understood, and hopefully insignificant,ambiguities. These measurements guided improvements to the designs. More sensitive autocorrelationmethods [43] guided the introduction of specially-designed chirped mirrors to compensatefor intra-cavity dispersion [44] with corresponding pulse durations of 8 fs. However, the first completecharacterisation methods such as frequency-resolved optical gating (FROG) [45] and spectralphase interferometry for direct electric-field reconstruction (SPIDER) [46], returned the chirp toall orders [47, 48, 48–50], assisting with design improvements which pushed pulse durations downto around 5 fs [51, 52].2.1.2.2 Metrology and the manipulation of ultrashort pulsesBesides the drive towards shorter pulses, another development in ultrafast science is the ability toamplify, compress and shape ultrashort pulses. Metrology also plays a crucial role in these areas.Pulse amplification systems are often limited by the peak intensity, which causes nonlineardistortions of the pulse or induces damage to the materials. Chirped pulse amplification [53, 54]circumvents this by stretching the pulse to reduce its peak intensity, then performing the amplification,then compressing the pulse by undoing the chirp. The stretching and compression steps10