Accurate retrieval of ultrashort laser pulses using frequency-resolved optical gating (FROG) is often hindered bylocal convergence in existing algorithms. We introduce the Sigma Check, a novel supervised evaluation step, to ourknowledge, designed to detect and mitigate erroneous convergence to local minima. This method evaluates thedifference between measured and retrieved spectrograms, applying targeted perturbation when significant discrepancies are detected. Numerical simulations and experimental measurements demonstrate that the Sigma Checkenhances retrieval accuracy across multiple algorithms, including line-search FROG, extended ptychographiciterative engine, and principal components generalized projections algorithm, providing a systematic approach toimproving FROG pulse reconstruction.

The most widely used frequency-resolved optical gating (FROG) retrieval algorithm solves the trace inversion problem to retrieve the phase distribution of the ultrashort pulse electric field by using preprocessed measured data in each iterative step to improve subsequent guesses. Such algorithms work very well for measurements with high signal-to-noise ratios but can become less reliable in extracting weaker signals buried in noisy data. We introduce the line-search FROG (LSF) algorithm, which enhances noise robustness by treating measurement data passively, using it solely for error evaluation rather than iterative correction. The gradient-free LSF algorithm requires no preprocessing of the measurement data and thus does not make assumptions about the noise in the measured traces. We show that LSF achieves comparable FROG error metrics to a ptychographic retrieval algorithm and COPRA, while producing higher-quality pulse reconstructions with reduced noise contaminations. It is applicable to all FROG geometries, supports blind FROG retrieval, and can reconstruct pulses from incomplete datasets.

Femtosecond laser pulses are essential tools in modern science and technology, yet generating pulses that are both extremely short and experimentally accessible remains a persistent challenge. Conventional post-compression methods rely on the Kerr effect, a third-order nonlinear effect, but these approaches often require complex setups and a large laboratory footprint. This thesis explores a fundamentally different route to pulse compression by utilising the coherently driven transversal optical phonon-polariton modes in Potassium Titanyl Phosphate (KTP). Rather than relying on Kerr-based spectral broadening and subsequent dispersive compression, the method exploits strong second-order nonlinearities in KTP to generate polaritons by optical rectification so that their electric fields may be used for efficient Stokes sideband generation by electro-optic interaction. The resulting interplay with normal dispersion in the nonlinear crystal results in an order of magnitude shorter self-compressed pulses generated in a simple setup. Such pulses demand advanced characterisation methods. Building on insights from applied mathematics and optimisation theory, a new retrieval algorithm for Frequency-Resolved Optical Gating (FROG) measurements is introduced, which we call the Line-Search FROG (LSF) algorithm, that decouples the measurement data from the reconstruction process. This greatly improves the performance of the pulse retrieval fidelity in the presence of large amounts of noise. The LSF algorithm is highly versatile and applicable to all FROG geometries, including the so-called double-blind FROG, with which we managed to measure the phase of a mode-locked dark pulse for the first time. Other pulse measurement techniques such as dispersion scan could benefit as well as the underlying optimisation problem is similar. To further improve the performance of pulse characterisation techniques, we also present the Sigma Check, which is a general algorithmic step that aims to reduce the likelihood of stagnating at local minima. This is achieved by performing an image-recognition step that correctly identifies and counteracts local minimums.

A chirped 470 fs pulse at 1030 nm was used to generate a phonon-polariton wave in bulk KTP by optical rectification. Phase modulation of the pump by the polariton wave resulted in generation of a burst of 20 fs pulses.

Dark optical solitons are solutions to the nonlinear Schrodinger equation in normal dispersion media with positive Kerr nonlinearity, exhibiting a discrete pi phase jump. These solitons are valuable to applications within telecommunication. Recent advancements have demonstrated the generation of two-colour bright-dark soliton pairs through cross-amplitude modulation in laser cavities, resulting in mode locking. In this study we present for the first time full field characterization of the electric field of a dark pulse. We achieved this by performing Blind Frequency Resolved Optical Gating measurements using the synchronous bright pulse as the gate pulse. The retrieved dark pulse verifies the existence of the expected p phase jump in the phase of the dark pulse, confirming theoretical predictions.

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 9050 was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultra-fast transients at frequencies above 1.5 THz.

We demonstrate self-compression of 173 fs pulses centered at 1030 nm down to 19.5 fs through electro-optic phase modulation by the phonon-polariton waves generated in a phase-matched intra-pulse difference-frequency mixing.

We demonstrate self-compression of 173 fs pulses centered at 1030 nm down to 19.5 fs through electro-optic phase modulation by the phonon-polariton waves generated in a phase-matched intra-pulse difference-frequency mixing.