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Thesis (Ph.D)--Physics, Georgia Institute of Technology, 2010. / Committee Chair: Rick Trebino; Committee Member: Ahmet Erbil; Committee Member: John Buck; Committee Member: Stephen Ralph; Committee Member: Zhigang Jiang. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Frequency-resolved optical gating (FROG) is the standard for measuring femtosecond laser pulses. It measures relatively simple pulses on a single-shot and complex pulses using multi-shot scanning and averaging. However, experience from intensity autocorrelation suggests that multi-shot measurements may suffer from a coherent artifact caused by instability in the laser source. In this thesis, the coherent artifacts present in modern pulse measurement techniques are examined and single-shot techniques for measuring complex pulse(s) are proposed and demonstrated. The study of the coherent artifact in this work shows that modern pulse measurement techniques also suffer from coherent artifacts and therefore single-shot measurements should be performed when possible. Here, two single-shot experimental setups are developed for different scenarios. First, an extension of FROG is developed to measure two unknown pulses simultaneously on a single-shot. This setup can measure pulses that have very different center wavelengths, spectral bandwidths, and complexities. Second, pulse-front tilt is incorporated to extend the temporal range of single-shot FROG to tens of picoseconds which traditionally can only be attained by multi-shot scanning. Finally, the pulse-front tilt setup is modified to perform a single-shot measurement of supercontinuum, one of the most difficult pulses to measure due to its long temporal range, broad spectral bandwidth, and low pulse energy.

In the last decade, there has been tremendous progress in the field of ultrashort-pulse measurement. However, this effort has focused mostly on the temporal behavior of 100-fs, 800-nm ultrashort pulse, ignoring other pulse lengths, wavelengths, and the very common space-time couplings or so called spatio-temporal distortions. In this thesis work, I do an extensive study of spatio-temporal distortions and their measurement using Frequency Resolved Optical Gating (FROG) and its relatives. I clarify some ambiguities in the descriptions of these effects in the existing theory and establish a more general description of such distortions in ultrashort pulses. I also extend these measurement techniques to different wavelengths and pulse lengths. Specifically, I develop measurement devices for few-cycle NIR pulses, weak and narrowband fiber laser pulses, long (several-ps) NIR pulses, and visible pulses from NOPAs.

A new pulse-retrieval software for Frequency-Resolved Optical Gating (FROG) technique has been developed. The new software extends the capacity of the original FROG algorithm in two major categories. First is a new method to determine the uncertainty of the retrieved pulse field in FROG technique. I proposed a simple, robust, and general technique?tstrap method?ch places error bars on the intensity and phase of the retrieved pulse field. The bootstrap method was also extended to automatically detect ambiguities in the FROG pulse retrieval. The second improvement deals with the spatiotemporal effect of the input laser beam on the measured GRENOUILLE trace. I developed a new algorithm to retrieve the pulse information, which includes both pulse temporal field and the spatiotemporal parameters, from the spatiotemporal distorted GRENOUILLE trace. It is now possible to have a more complete view of an ultrashort pulse. I also proposed a simple method to remove the spatial profile influence of the input laser beam on the GRENOUILLE trace. The new method extends the capacity of GRENOUILLE technique to measure the beams with irregular spatial profiles.

This thesis contains three components of research: a detailed study of the performance of Frequency-Resolved Optical Gating (FROG) for measuring complex ultrashort laser pulses, a new method for measuring the arbitrary polarization state of an ultrashort laser pulse using Tomographic Ultrafast Retrieval of Transverse Light E-fields (TURTLE) technique, and new approach for measuring two complex pulses simultaneously using PG blind FROG. In this thesis, we compare the performance of three versions of FROG to measure complex ultrashort laser pulses: second-harmonic-generation (SHG) FROG, polarization-gate (PG) FROG, and cross-correlation FROG (XFROG). We found that the XFROG algorithm achieves 100% convergence, while PG FROG and SHG FROG GP algorithm achieve 100% convergence after doing the noise deduction and increasing the sampling range. The second part of this thesis describes a method for measuring the intensity, phase and the complete polarization state of a laser pulse having a time-dependent polarization state (i.e. a polarization shaped pulse). This technique is called tomographic ultrafast retrieval of transverse light E-fields (TURTLE). TURTLE typically involves making three FROG measurements: one of the intensity and phase of the pulse's horizontal polarization component, one of its vertical component, and another of the 45o component. Performing a simple minimization using these three FROG measurements, the time-dependent polarization state of the ultrashort pulse can be determined. The third part of this thesis introduces a method for measuring two complex pulses simultaneously using a single FROG device. This technique is based on Polarization-gate (PG) FROG and it is called PG blind FROG. It involves two measurements: One of them is a PG FROG trace using the intensity of pulse 1 to gate pulse 2 and other one is the PG FROG trace using the intensity of pulse 2 to gate pulse 1. An iterative phase retrieval algorithm based on generalized projection (GP) is used to reconstruct the intensity and phase of these two pulses. This approach is an elegant way to measure complex and/or very spectrally broad pulses such as those due to super continuum.

Many essential processes and interactions on atomic and molecular scales occur at ultrafast timescales. The ability to measure and manipulate ultrashort pulses hold the key to probing and understanding these key processes that physicists, engineers, chemists and biologists study today. Measuring ultrashort pulses means that we measure both the intensity (which is a function of time) and the phase of the pulse in time. Or alternately we might measure spectrum and spectral phase (in the corresponding Fourier domain). In the early 1990's, the invention of FROG opened up the field of ultrashort measurement with it's ability to measure the complete pulse. Since then, there have been a whole host of pulse measurement techniques that have been invented to measure all sorts of ultrashort pulses. However, no variation of FROG nor any other fs pulse measurement technique, for that matter, has yet been able to completely measure arbitrary ultraweak femtosecond light pulses such as those found in nature. In this thesis, we will explore a couple of highly sensitive methods in a quest to measure ultraweak ultrashort pulses. We explore the use of Spectral Interferometry, a known sensitive technique as one possibility. We find that it has certain drawbacks that make it not necessarily suitable to tackle this problem. But in the course of our quest, we find that this technique is highly suitable for measuring 10s of picosecond-long shaped pulses. We discuss a couple of developments which make SI highly practical to use for such shaped pulse-measurements. We also develop a new technique which is a variation of FROG, based on the non-linearity of Difference Frequency Generation and Optical Parametric Amplification, which can amplify pulses as weak as a few hundred attojoules to be able to spectrally resolve them and measure the full intensity and phase of these pulses. This technique offers great potential to measure generalized ultraweak ultrashort pulses.