Greg Taft

Measurement of 10-fs laser pulses

We report full characterization of the intensity and phase of /spl sim/10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks.

Direct diode-pumped Kerr-lens mode-locked Ti:sapphire laser

We describe a Ti:sapphire laser pumped directly with a pair of 1.2W 445nm laser diodes. With over 30mW average power at 800 nm and a measured pulsewidth of 15fs, Kerr-lens-modelocked pulses are available with dramatically decreased pump cost. We propose a simple model to explain the observed highly stable Kerr-lens modelocking in spite of the fact that both the mode-locked and continuous-wave modes are smaller than the pump mode in the crystal.

ULTRASHORT OPTICAL WAVEFORM MEASUREMENTS USING FREQUENCY-RESOLVED OPTICAL GATING

We measure the intensity and phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser using the recently developed technique of frequency-resolved optical gating. These results represent to our knowledge the shortest complete optical waveform characterization measurements performed to date. We also verify recent theoretical calculations that predict that the main limitation on the pulse duration from these lasers is the presence of uncompensated higher-order dispersion.

Multi-microjoule, MHz repetition rate Ti:sapphire ultrafast regenerative amplifier system

We demonstrate a cryogenically cooled Ti:sapphire ultrafast regenerative amplifier laser system producing >20 μJ energies at 50 kHz, >12 μJ at 200 kHz and >3.5 μJ at 1MHz with repetition rates continuously tunable from 50 kHz up to 1.7 MHz in a footprint of only 60x180 cm². This laser uses down-chirped pulse amplification employing a grism stretcher and a glass-block compressor, achieving sub-60-fs pulse duration. This laser represents a several-times improvement in repetition-rate and average power over past Ti:sapphire-based ultrafast lasers in this class. We discuss the unique challenges and solutions for this laser system. This laser system has wide applications especially in ultrafast photoemission, nonlinear imaging and spectroscopy, as well as for micro/nano-machining and ultrafast laser therapy and surgery.

Measurement of the Intensity and Phase of Ultrashort Pulses Using Frequency-Resolved Optical Gating

Recently, we developed a technique for measuring the full time-dependent intensity, I(t), and phase, ϕ(t), of the complex electric field, E(t), of an ultrashort laser pulse. This technique, Frequency-Resolved Optical Gating (FROG),1–5 has been demonstrated for pulses in the ultraviolet, visible, near-infrared, and mid-infrared. It can measure pulses from millijoules to a few picojoules (the latter using the second-harmonic-generation version). It is also routinely used to measure a single ultrashort laser pulse.

Direct diode pumped Kerr lens modelocked Ti:Sapphire laser oscillator

We describe a Ti:sapphire laser pumped directly with 445nm laser diodes. With 44 mW average power at 800 nm and bandwidth for <;50 fs pulses, Kerr-lens-modelocked pulses are available with dramatically decreased pump cost.

Practical Issues, Marginals, Error Checks, and Error Correction

The usefulness of a new scientific technique is determined by the details of its implementation. Many clever techniques that are otherwise rigorous and sound fail to become useful workhorse laboratory techniques because of difficulties in experimental implementation, noise sensitivity, cumbersomeness, or other more fundamental limitations. Often these details go unreported (who wants to write a paper explaining how he failed to be able to do something?), resulting in a loss of valuable time and resources for research groups that attempt to use the technique. It’s therefore incumbent upon the developers of an experimental technique to determine whether practical limitations will render the technique less than ideal.

Systematic error and its elimination in the measurement of 10-femtosecond laser pulses

focused together into a fused silica substrate in a beam geometry resulting in a third-order nonlinear interaction that is phase matched, independent of wavelength. The spectrum of the mixing signal is recorded as one pulse is delayed, yielding a spectrogram that is then deconvolved to yield the complex field profile. The third order of the nonlinearity allows unambiguous interpretation of the direction of time, as well as the sign of any residual chirp. Background-free measurements such as this are essential in determining the true baseline of the wave form and in measuring contrast ratio. In early results from our lab presented here, we have compressed 160 pJ, 25 fs input pulses to 11 fs (see Fig. la), as well as shorter pulses that have a higher pedestal. This pulse has a clean rising edge that is especially critical for high-field experiments. For this measurement, the grating compressor was used; note that the residual phase (Fig. Ib) is dominated by thirdorder dispersion that should be eliminated with the use of both the prisms and gratings. The present results are the shortest high-power wave forms characterized with use of the FROG technique.