Femtosecond laser is a laser, which emits optical pulses with the pulse duration of few femtoseconds to hundreds of femtoseconds (1 fs=10^-15 s). The generation of femtosecond pulses achieved through the mode locking process in which the laser resonator contains either active element in the form of an optical modulator or saturable absorber a non-liner passive element that results the formation of femtosecond pulses circulating in the laser resonator. The important parameters of femtosecond lasers are pulse duration, pulse repetition rate and pulse energy that determines the femtosecond lasers applications. The field of ultrashort pulse generation has had roughly three decades to develop and can thus be considered relatively mature. The most common types of femtosecond lasers are based on bulk Titanium-Sapphire, ytterbium-doped and chromium-doped and lasers. Whereas Titanium-Sapphire based femtosecond laser can generate optical pulses up to 5 fs with averaged power hundreds of milliwatts operating at 80 MHz. The non-linear processes in atomic, molecular, plasma and solid-state physics including short and long-range dynamics of the atom and the evaluation of the extremely excited systems can be investigated now by employing the femtosecond lasers.
The transformation of laser light into white light with substantial spectral bandwidth is called white-light continuum generation. In the white-light continuum generation, spectral broadening is achieved by the propagation of laser pulses in a highly nonlinear optical material, such as transparent solid materials, liquids and gasses. White-light continuum spectrum spans from visible to infrared region. In general, It is accepted theoretically and experimentally that the self-focusing is the primary process responsible for the phenomena of white-light continuum generation, in this process, the pulse is compressed in space and as a result, the peak intensity increases correspondingly. The other major processes responsible for starting the mechanism of self-broadening are a self-steepening and self-phase modulation. This interesting phenomenon of the white-light continuum is an ideal broadband light source for some interesting applications like seed pulse for optical parametric chirped pulse amplification (OPCPA), femtosecond time-resolved spectroscopy, short pulse generation, a tunable laser source, optical coherence tomography, material characterization, spectral interferometry, and pump-probe experiments.
Different diagnostics techniques employ to characterize the femtosecond laser systems such as auto-correlation, frequency resolved optical gating (FROG), and cross-correlation frequency resolved optical gating (XFROG). Other complex techniques like spectral phase interferometry for direct electric-field reconstruction (SPIDER) and dispersing a pair of light electric fields (TADPOLE) which is the combination of FROG and SPIDER. The autocorrelation technique fails to provide information about the phase of the pulse. Therefore, the shape of temporal profile guessed before to make the experimental measurements. On the other hand, SPIDER technique can provide spectral and temporal information, but the experimental setup is quite complicated and challenging in the alignment. The FROG technique, which can be described as a spectrally resolved auto-correlation measurement, simple in setup and efficient to characterize the spectral and temporal evolution of the femtosecond pulses. There are different versions of FROG diagnostic techniques; the most sensitive version of FROG is second harmonic generation (SHG) FROG. Further, it can be a categorized into multi-shot FROG and single-shot FROG. The FROG techniques involve the gating of pulse with itself and measured the spectrum versus the delay between the two pulses, while in the XFROG technique a well-known pulse is gated with the unknown measured pulse. The XFROG and FROG techniques gives the phase and pulse intensity versus time or frequency domain.