Patrick O’Shea

Highly simplified device for ultrashort-pulse measurement.

We show that a frequency-resolved optical gating device using (1) a thick nonlinear crystal to replace the usual thin crystal and spectrometer and (2) a Fresnel biprism to replace the beam splitter and delay line yields a remarkably simple single-shot ultrashort-pulse intensity-and-phase measurement device with no sensitive alignment parameters and significantly greater sensitivity.

Relative-phase ambiguities in measurements of ultrashort pulses with well-separated multiple frequency components

Ultrashort-pulse characterization techniques, such as the numerous variants of frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction, fail to fully determine the relative phases of well-separated frequency components. If well-separated frequency components are also well separated in time, the cross-correlation variants (e.g., XFROG) succeed, but only if short, well-characterized gate pulses are used.

Increased-bandwidth in ultrashort-pulse measurement using an angle-dithered nonlinear-optical crystal.

We show that the usual phase-matching-bandwidth constraint in ultrashort-laser-pulse measurement techniques is overly restrictive. Specifically, the phase-matching bandwidth need not exceed the pulse bandwidth on every pulse. Instead, only the phase-matching bandwidth integrated over the measurement period need exceed the pulse bandwidth. We show that angle-dithering a second-harmonic-generation crystal that is otherwise too narrowband (that is, too thick) can yield sufficient phase-matching bandwidth and an accurate pulse measurement. We apply this technique to frequency-resolved optical gating (FROG) and show that accurate pulse measurements can be made using a comparatively very thick and hence narrowband crystal. An additional advantage of using a thick crystal is increased signal strength.

Ultrashort-laser-pulse measurement using swept beams

We demonstrate a frequency-resolved optical gating (FROG) device that uses a sweepshot geometry that combines the advantages of multishot and single-shot pulse-measurement devices, has only one moving part, a galvanometer, and requires no computer control. Like a multishot device, it focuses the beam to a small spot (rather than a line focus) and has a high intensity in the nonlinear medium. Like single-shot devices, it makes measurements quickly, generating an entire FROG trace on a single camera screen (rather than requiring many camera downloads).

Measuring Spatial Chirp and Pulse-Front Tilt in Ultrashort Pulses Using Single-Shot FROG

Because their generation involves considerable spatio-temporal manipulations, ultrashort laser pulses commonly suffer from spatio-temporal distortions. The most common such distortions are spatial chirp and pulse-front tilt. Devices that use dispersive elements such as prisms and diffraction gratings introduce spatial chirp and pulse-front tilt (fig.1), only-in principle-to remove it afterwards. Unfortunately, even slight misalignments leave residual distortions in the output pulse. Since ultrashort pulses are very broadband, these distortions are usually noticeable and problematic.

Origin of supercontinuum generation in microstructured fibers

We investigate the propagation of femtosecond pulses in microstructured fibers under conditions in which a supercontinuum is generated. We find that higher-order dispersion primarily determines the spectral envelope and that the spectrum contains a highly complicated underlying sub-structure which is highly sensitive to input fluctuations.

Highly SimplifiedUltrashort Pulse Measurement

A thick nonlinear crystal can replace the usual thin crystal and spectrometer, and a Fresnel biprism can replace the beam splitter and delay line in a FROGdevice, yielding a remarkably simple single-shot ultrashort-pulse intensity-a nd-phase measurement device with no sensitive alignment parameters and significantly greater sensitivity.

Very Simple FROG Apparatus: GRENOUILLE

The first goal in developing a measurement technique is to make it work. Once that’s done, the next step is to make it simple.