Aparna P. Shreenath

Frequency-resolved optical gating and single-shot spectral measurements reveal fine structure in microstructure-fiber continuum

Cross-correlation frequency-resolved optical gating with an angle-dithered nonlinear-optical crystal permits measurement of the intensity and the phase of the ultrabroadband (as much as 1200 nm wide) continuum generated from microstructure optical fiber. Retrieval revealed fine-scale structure in the continuum spectrum. Simulations and single-shot spectrum measurements confirmed that the fine structure does exist on a single-shot basis but washes out when many shots are averaged.

Crossed-beam spectral interferometry: a simple, high-spectral-resolution method for completely characterizing complex ultrashort pulses in real time

We present a high-spectral-resolution and experimentally simple version of spectral interferometry using optical fibers and crossed beams, which we call SEA TADPOLE. Rather than using collinear unknown and reference pulses separated in time to yield spectral fringes-and reduced spectral resolution-as in current versions, we use time-coincident pulses crossed at a small angle to generate spatial fringes. This allows the extraction of the spectral phase with the full spectrometer resolution, which allows the measurement of much longer and more complex pulses. In fact, SEA TADPOLE achieves spectral super-resolution, yielding the pulse spectrum with even better resolution. Avoiding collinear beams and using fiber coupling also vastly simplify alignment. We demonstrate SEA TADPOLE by measuring a chirped pulse, a double pulse separated by 14 ps, and a complex pulse comprising two trains of pulses with a time-bandwidth product of ~400.

Experimental Studies of the Coherence of Microstructure-Fiber Supercontinuum

The phase coherence of supercontinuum generation in microstructure fiber is quantified by performing a Youngs type interference experiment between independently generated supercontinua from two separate fiber segments. Analysis of the resulting interferogram yields the wavelength dependence of the magnitude of the mutual degree of coherence, and a comparison of experimental results with numerical simulations suggests that the primary source of coherence degradation is the technical noise-induced fluctuations in the injected peak power.

Measurement of the intensity and phase of attojoule femtosecond light pulses using Optical-Parametric-Amplification Cross-Correlation Frequency-Resolved Optical Gating

We use the combination of ultrafast gating and high parametric gain available with Difference-Frequency Generation (DFG) and Optical Parametric Amplification (OPA) to achieve the complete measurement of ultraweak ultrashort light pulses. Specifically, spectrally resolving such an amplified gated pulse vs. relative delay yields the complete pulse intensity and phase vs. time. This technique is a variation of Cross-correlation Frequency-Resolved Optical Gating (XFROG), and using it, we measure the intensity and phase of a train of attenuated white light continuum containing only a few attojoules per pulse. Unlike interferometric methods, this method can measure pulses with poor spatial coherence and random absolute phase, such as fluorescence.

The Measurement of Ultrashort Light Pulses—Simple Devices, Complex Pulses

We review the state of the art of ultrashort-light-pulse measurement using frequency-resolved-optical-gating (FROG). Recent developments have extended the state of the art considerably. FROG devices for measuring the intensity and phase of ultrashort laser pulses have become so simple that almost no alignment is required. In addition, such devices not only operate single shot, but they also yield the two most important spatio-temporal distortions, spatial chirp and pulse-front tilt. With other FROG variations, it is now possible to measure more general ultrashort light pulses (i.e., pulses much more complex than common laser pulses), with time-bandwidth products as large as several thousand and as weak as a few hundred photons, and despite other difficulties such as random absolute phase and poor spatial coherence.

Numerical simulations of optical parametric amplification cross-correlation frequency-resolved optical gating

We perform numerical simulations of cross-correlation frequency-resolved optical gating with the nonlinearities, optical parametric amplification, and difference-frequency generation for measuring broadband pulses. We show that use of a noncollinear beam geometry that matches the group velocities of the pump, signal, and idler pulses permits use of relatively thick crystals for high gain without significant distortion in the measured trace, yielding bandwidths of ~100 nm.

Spatially Resolved Spectral Interferometry

We present a simplified, alignment-free spectral interferometer using optical fibers. Using spatial fringes the spectral resolution is improved and time-domain filtering is unnecessary. To demonstrate this technique, we measure temporal chirp and a 14-ps double-pulse.

Frequency-resolved optical gating: the state of the art

Summary form only given. Frequency-Resolved Optical Gating (FROG) can completely measure ultrashort laser pulses in almost every situation. We have recently introduced a remarkably simple SHG FROG device that overcomes essentially all of the alignment difficulties of pulse-measurement devices. First, we replace the usual beam splitter, delay line, and beam combining optics with a single element, a Fresnel biprism. Second, we use a thick SHG crystal, which not only gives considerably more signal, but also simultaneously replaces the spectrometer. The resulting device, which we call GRating-Eliminated No-nonsense Observation of Ultrafast Incident Laser Light E-fields (GRENOUILLE), has zero sensitive alignment degrees of freedom and hence is extremely simple to align.

An introduction to the characterization of ultrashort laser pulses

We review the state of the art of ultrashort laser pulse characterization techniques. Two main methods will be mentioned: frequency-resolved optical gating (FROG) and spectral-phase interferometry for direct electric-field reconstruction (SPIDER). Basics of the techniques are introduced, and a comparison will be made on the pros and cons of both methods. We will then present some recent developments in the field of pulse characterization, including the development of an ultracompact and robust pulse characterization device-GRENOUILLE, the extension of pulse-measurement techniques into both time and space, and the measurement of extremely complex and extremely weak pulses.

Spatially resolved spectral interferometry: A simple method for measuring the spectral phase with high resolution

We present a simplified, alignment-free version of spectral interferometry using optical fibers. Spectral resolution is significantly improved using spatial fringes, avoiding time-domain filtering. We demonstrate this technique by measuring temporal chirp and a 12-ps double-pulse.