John N. Sweetser

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.

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.

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.

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.

Ultrafast optical switching by use of fully phase-matched cascaded second-order nonlinearities in a polarization-gate geometry.

We show that cascaded second-order nonlinear-optical processes can occur in a convenient polarization-gate beam geometry. Our arrangement uses type II phase matching, and both individual second-order processes (upconversion and downconversion) are simultaneously phase matched. This geometry can be applied to efficient ultrafast optical switching. With a beta-barium borate crystal and lightly focused 250-fs, 7.3-microJ pulses, we achieve a switching efficiency of 15% and an on-off ratio of 3 x 10(4) on a pulse-length-limited time scale.

Direct ultrashort-pulse intensity and phase retrieval by frequency-resolved optical gating and a computational neural network

Ultrashort-laser-pulse retrieval in frequency-resolved optical gating has previously required an iterative algorithm. Here, however, we show that a computational neural network can directly and rapidly recover the intensity and phase of a pulse.

Amplified ultrafast optical switching by cascading cascaded second-order nonlinearities in a polarization-gate geometry

Abstract We demonstrate amplified ultrafast all-optical switching of 250-fs pulses using two different types of cascaded second-order nonlinearities (CSNs) simultaneously in a simple polarization-gate geometry using a type II crystal. The first set of CSN processes has the effect of rotating the input-pulse polarization, yielding polarization gating. This signal pulse is then amplified by a second (different) set of CSN processes involving second-harmonic generation of the gate followed by a stimulated parametric down-conversion process. Using these “cascaded cascaded” second-order processes, we observe amplified switching with 320% efficiency and an on-off ratio of 6 × 10 5 in a 1-mm-thick crystal of BBO.

Ultrashort-pulse retrieval using frequency-resolved optical gating and an artificial neural network

Frequency-resolved optical gating (FROG) is a technique that allows the determination of the intensity and phase of ultrashort laser pulses. In FROG, a spectrogram of the pulse, the so- called FROG trace, is produced, from which the intensity and phase is then retrieved using an iterative algorithm. This algorithm performs well for all types of pulses, but it sometimes requires more than a minute to converge, and more rapid retrieval is important for many applications. It is therefore desirable to have a non-iterative computational method capable of inverting the function that relates the pulse intensity and phase to its FROG trace. In previous work, we showed that a neural network can retrieve simple pulses rapidly and directly. This original approach involved feature extraction by computing the lowest-order integral moments of the FROG trace, making it particularly sensitive to the presence of additive noise. Using parallel-processing hardware, we are now able to use FROG traces of limited size (32 X 32 pixel) without any feature extraction as input for a neural net. In addition, FROG traces of 64 X 64 pixel size, typical for experimental data, can be used in conjunction with a more noise-insensitive feature extraction method.

Absolute pointing and tracking based remote control for interactive user experience

We describe a system designed for simple and intuitive interaction with a large screen from a distance. The approach is based on an optical pointing and tracking system which has the important property that the tracking is absolute in nature. This provides several advantages over traditional methods used for input devices and remote controls, especially when the interaction is performed at a distance from the screen and where the user is not constrained to a surface.