Tobias Witting

Broadband astigmatism-free Czerny-Turner imaging spectrometer using spherical mirrors

We describe the elimination of the astigmatism of a Czerny-Turner imaging spectrometer, built using spherical optics and a plane grating, over a broad spectral region. Starting with the principle of divergent illumination of the grating, which removes astigmatism at one chosen wavelength, we obtain design equations for the distance from the grating to the focusing mirror and the detector angle that remove the astigmatism to first order in wavelength. Experimentally, we demonstrate near diffraction-limited performance from 740 to 860 nm and over a 5 mm transverse spatial extent, while ray-tracing calculations show that barring finite-aperture and detector size limitations, this range extends from 640 to 900 nm and over 10 mm transversely. Our technique requires no additional optics and uses standard off-the-shelf components.

Improved ancilla preparation in spectral shearing interferometry for accurate ultrafast pulse characterization

We report a version of spectral phase interferometry for direct electric field reconstruction (SPIDER), in which spectral filters are used to produce the quasi-monochromatic fields required for upconversion. The advantages of this approach include improved calibration accuracy, robustness for strongly chirped input pulses, simplicity, and compactness. We verify the technique experimentally by measuring the spectral chirp of a grating compressor using a spatially encoded arrangement (SEA-)SPIDER.

High precision self-referenced phase retrieval of complex pulses with multiple-shearing spectral interferometry

We show that using multiple shears in spectral shearing interferometry is a powerful technique for improving precision, thus enabling the measurement of more complex pulses and resolving phase ambiguities. We derive an efficient and robust optimal phase reconstruction algorithm for extracting the spectral phase from interferograms taken at an arbitrary number of different shears. We show that if the shear is easily adjustable then a multishear measurement always offers a superior precision, even when considering the loss of precision of the raw data necessitated by multiple acquisitions. We present numerical examples and demonstrate an experimental implementation of the measurement of a double pulse using two shears.

Lateral shearing interferometry of high-harmonic wavefronts

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects.

Ultrashort pulse characterization by spectral shearing interferometry with spatially chirped ancillae

We report a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER), in which two spatially chirped ancilla fields are used to generate a spatially encoded SPIDER interferogram. We dub this new technique Spatially Encoded Arrangement for Chirped ARrangement for SPIDER (SEA-CAR-SPIDER). The single shot interferogram contains multiple shears, the spectral amplitude of the test pulse, and the reference phase, which is accurate for broadband pulses. The technique enables consistency checking through the simultaneous acquisition of multiple shears and offers a simple and precise calibration method. All calibration parameters--the shears, and the upconversionfrequency--can be accurately obtained from a single calibration trace.

Gold-SPIDER: spectral phase interferometry for direct electric field reconstruction utilizing sum-frequency generation from a gold surface

We report on a version of spectral phase interferometry for direct electric field reconstruction (SPIDER) that uses no nonlinear crystals, instead relying on the harmonic signal from a gold mirror. Surface harmonic generation holds the promise of being able to upconvert extremely broad bandwidths over a large tuning range, thus providing access to both extremely short pulses and wavelengths outside of traditional methods. In this proof of principle demonstration, SPIDER traces for chirped and transform-limited 55 fs pulses are presented.

Measuring sub-Planck structural analogues in chronocyclic phase space

Phase space quasi-probability distributions of certain quantum states reveal structure on a scale that is small compared to the Planck area. Using an analog between the wavefunction of a single photon and the electric field of a classical ultrashort optical pulse we show that spectral shearing interferometry enables measurement of such structure directly, thereby extending an idea of Krzysztof Wodkiewicz and others. In particular, we use multiple-shear spectral interferometry to fully characterize a pulse consisting of two sub-pulses which are temporally and spectrally disjoint, without a relative-phase ambiguity. This enables us to compute the Wigner distribution of the pulse. This spectrographic representation of the pulse field features fringes that are tilted with respect to both the time- and frequency axes, showing that in general the shortest sub-Planck distances may not be in the directions of the canonical (and easily experimentally accessible) directions. Further, independent of this orientation, evidence of the sub-Planck scale of the structure may be extracted directly from the measured signal.

Characterising spatio-temporal coupling of extreme ultraviolet ultrashort pulses from high harmonic generation

This work has demonstrated a simple method to measure spatio-temporal coupling in XUV high harmonic generation via the use of spatial shearing interferometry. The method is based on a well established technique used in the visible domain which is robust to noise and can also be measured in a single shot that such a method can prove valuable in further understanding of propagation and phase-matching effects in high harmonic generation, as well as providing a tool for performing reference based time-resolved spectroscopy.

Phase retrieval of complex ultrashort pulses using multiple-shearing spectral interferometry

High time-bandwidth product (TBP) pulses arise frequently in ultrafast optics, such as in coherent control, supercontinuum generation, filamentation, telecommunications, and micromachining. Pulse measurement techniques with sufficient sampling points compared to the Whittaker-Shannon requirement therefore find application in all of these contexts. Theoretically, most techniques, including time-frequency and spectral shearing methods, are capable of handling complex pulses. In practice however, the additional resolution requirements introduce a compromise between signal-to-noise and acquisition time, raising the question of how to optimally use ones detector resources to obtain the most precise measurement.

Characterizing spatio-temporal coupling of extreme ultraviolet ultrashort pulses from high harmonic generation

We demonstrate a tool for performing measurements of space-time coupling of ultrashort, extreme ultraviolet pulses from high harmonic generation which can be used to study propagation and phasematching effects during the generation process.