David N. Fittinghoff

Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating

We summarize the problem of measuring an ultrashort laser pulse and describe in detail a technique that completely characterizes a pulse in time: frequency-resolved optical gating. Emphasis is placed on the choice of experimental beam geometry and the implementation of the iterative phase-retrieval algorithm that together yield an accurate measurement of the pulse time-dependent intensity and phase over a wide range of circumstances. We compare several commonly used beam geometries, displaying sample traces for each and showing where each is appropriate, and we give a detailed description of the pulse-retrieval algorithm for each of these cases.

Measurement of the intensity and phase of ultraweak, ultrashort laser pulses

We show that frequency-resolved optical gating combined with spectral interferometry yields an extremely sensitive and general method for temporal characterization of nearly arbitrarily weak ultrashort pulses even when the reference pulses is not transform limited. We experimentally demonstrate measurement of the full time-dependent intensity and phase of a train of pulses with an average energy of 42 zeptojoules (42 x 10(-21) J), or less than one photon per pulse.

Pulse retrieval in frequency-resolved optical gating based on the method of generalized projections

We use the algorithmic method of generalized projections (GPs) to retrieve the intensity and phase of an ultrashort laser pulse from the experimental trace in frequency-resolved optical gating (FROG). Using simulations, we show that the use of GPs improves significantly the convergence properties of the algorithm over the basic FROG algorithm. In experimental measurements, the GP-based algorithm achieves significantly lower errors than previous algorithms. The use of GPs also permits the inclusion of an arbitrary material response function in the FROG problem.

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.

Practical issues in ultrashort-laser-pulse measurement using frequency-resolved optical gating

We explore several practical experimental issues in measuring ultrashort laser pulses using the technique of frequency-resolved optical gating (FROG). We present a simple method for checking the consistency of experimentally measured FROG data with the independently measured spectrum and autocorrelation of the pulse. This method is a powerful way of discovering systematic errors in FROG experiments. We show how to determine the optimum sampling rate for FROG and show that this satisfies the Nyquist criterion for the laser pulse. We explore the low- and high-power limits to FROG and determine that femtojoule operation should be possible, while the effects of self-phase modulation limit the highest signal efficiency in FROG to 1%. We also show quantitatively that the temporal blurring due to a finite-thickness medium in single-shot geometries does not strongly limit the FROG technique. We explore the limiting time-bandwidth values that can be represented on a FROG trace of a given size. Finally, we report on a new measure of the FROG error that improves convergence in the presence of noise.

Third-harmonic generation microscopy by use of a compact, femtosecond fiber laser source

We demonstrate the first use, to our knowledge, of a compact, diode-pumped, femtosecond fiber laser for third-harmonic generation (THG) microscopy. We discuss the utility of this technique, as well as the technical issues involved in using this compact source, and demonstrate the first use, to our knowledge, of imaging by THG backlighting.

Characterization of the polarization state of weak ultrashort coherent signals by dual-channel spectral interferometry.

We demonstrate that dual-channel spectral interferometry in conjunction with a well-characterized reference pulse can be used to time resolve the polarization state of extremely weak ultrashort coherent signals from linear-and nonlinear-optical experiments by measuring the intensity and the phase of two orthogonal polarization components. In this way the signal is completely characterized.

Transient-grating frequency-resolved optical gating

We introduce a transient-grating beam geometry for frequency-resolved optical-gating measurements of ultrashort laser pulses and show that it offers significant advantages over currently used geometries. Background free and phase matched over a long interaction length, it is the most sensitive third-order pulse-measurement geometry. In addition, for pulses greater than ~300 fs in length and ~1 microJ in energy, the nonlinear medium can be removed and the nonlinearity of air can be used to measure the pulse.

Widefield multiphoton and temporally decorrelated multifocal multiphoton microscopy

We demonstrate a widefield multiphoton microscope and a temporally decorrelated, multifocal, multiphoton microscope that is based on a high-efficiency array of cascaded beamsplitters. Because these microscopes use ultrashort pulse excitation over large areas of the sample, they allow efficient use of the high-average power available from modern ultrashort pulse lasers.

Collinear type II second-harmonic-generation frequency-resolved optical gating for use with high-numerical-aperture objectives

Ultrashort-pulse lasers are now commonly used for multiphoton microscopy, and optimizing the performance of such systems requires careful characterization of the pulses at the tight focus of the microscope objective. We solve this problem by use of a collinear geometry in frequency-resolved optical gating that uses type II second-harmonic generation and that allows the full N.A. of the microscope objective to be used. We then demonstrate the technique by measuring the intensity and the phase of a 22-fs pulse focused by a 20x, 0.4-N.A. air objective.