Dispersive Fourier Transformation for Versatile Microwave Photonics Applications

Abstract: Dispersive Fourier transformation (DFT) maps the broadband spectrum of an ultrashort optical pulse into a time stretched waveform with its intensity profile mirroring the spectrum using chromatic dispersion. Owing to its capability of continuous pulse-by-pulse spectroscopic measurement and manipulation, DFT has become an emerging technique for ultrafast signal generation and processing, and high-throughput real-time measurements, where the speed of traditional optical instruments falls short. In this paper, the principle and implementation methods of DFT are first introduced and the recent development in employing DFT technique for widespread microwave photonics applications are presented, with emphasis on real-time spectroscopy, microwave arbitrary waveform generation, and microwave spectrum sensing. Finally, possible future research directions for DFT-based microwave photonics techniques are discussed as well

Efficient table-top dual-wavelength beamline for ultrafast transient absorption spectroscopy in the soft X-ray region.

We present a table-top beamline providing a soft X-ray supercontinuum extending up to 370 eV from high-order harmonic generation with sub-13 fs 1300 nm driving pulses and simultaneous production of sub-5 fs pulses centered at 800 nm. Optimization of high harmonic generation in a long and dense gas medium yields a photon flux of  ~ 1.4 × 106 photons/s/1% bandwidth at 300 eV. The temporal resolution of X-ray transient absorption experiments with this beamline is measured to be 11 fs for 800 nm excitation. This dual-wavelength approach, combined with high flux and high spectral and temporal resolution soft X-ray absorption spectroscopy, is a new route to the study of ultrafast electronic dynamics in carbon-containing molecules and materials at the carbon K-edge

Distinguishing non-resonant four-wave-mixing noise in coherent stokes and anti-stokes Raman scattering

A method of examining a sample comprises exposing the sample to a pump pulse of electromagnetic radiation for a first period of time, exposing the sample to a stimulant pulse of electromagnetic radiation for a second period of time which overlaps in time with at least a portion of the first exposing, to produce a signal pulse of electromagnetic radiation for a third period of time, and interfering the signal pulse with a reference pulse of electromagnetic radiation, to determine which portions of the signal pulse were produced during the exposing of the sample to the stimulant pulse. The first and third periods of time are each greater than the second period of time

Advanced signal processing methods for plane-wave color Doppler ultrasound imaging

Conventional medical ultrasound imaging uses focused beams to scan the imaging scene line-by-line, but recently however, plane-wave imaging, in which plane-waves are used to illuminate the entire imaging scene, has been gaining popularity due its ability to achieve high frame rates, thus allowing the capture of fast dynamic events and producing continuous Doppler data. In most implementations, multiple low-resolution images from different plane wave tilt angles are coherently averaged (compounded) to form a single high-resolution image, albeit with the undesirable side effect of reducing the frame rate, and attenuating signals with high Doppler shifts. This thesis introduces a spread-spectrum color Doppler imaging method that produces high-resolution images without the use of frame compounding, thereby eliminating the tradeoff between beam quality, frame rate and the unaliased Doppler frequency limit. The method uses a Doppler ensemble formed of a long random sequence of transmit tilt angles that randomize the phase of out-of-cell (clutter) echoes, thereby spreading the clutter power in the Doppler spectrum without compounding, while keeping the spectrum of in-cell echoes intact. The spread-spectrum method adequately suppresses out-of-cell blood echoes to achieve high spatial resolution, but spread-spectrum suppression is not adequate for wall clutter which may be 60 dB above blood echoes. We thus implemented a clutter filter that re-arranges the ensemble samples such that they follow a linear tilt angle order, thereby compacting the clutter spectrum and spreading that of the blood Doppler signal, and allowing clutter suppression with frequency domain filters. We later improved this filter with a redesign of the random sweep plan such that each tilt angle is repeated multiple times, allowing, after ensemble re-arrangement, the use of comb filters for improved clutter suppression. Experiments performed using a carotid artery phantom with constant flow demonstrate that the spread-spectrum method more accurately measures the parabolic flow profile of the vessel and outperforms conventional plane-wave Doppler in both contrast resolution and estimation of high flow velocities. To improve velocity estimation in pulsatile flow, we developed a method that uses the chirped Fourier transform to reduce stationarity broadening during the high acceleration phase of pulsatile flow waveforms. Experimental results showed lower standard deviations compared to conventional intensity-weighted-moving-average methods. The methods in this thesis are expected to be valuable for Doppler applications that require measurement of high velocities at high frame rates, with high spatial resolution

Nonlinear interferometric vibrational imaging

A method of examining a sample, which includes: exposing a reference to a first set of electromagnetic radiation, to form a second set of electromagnetic radiation scattered from the reference; exposing a sample to a third set of electromagnetic radiation to form a fourth set of electromagnetic radiation scattered from the sample; and interfering the second set of electromagnetic radiation and the fourth set of electromagnetic radiation. The first set and the third set of electromagnetic radiation are generated from a source; at least a portion of the second set of electromagnetic radiation is of a frequency different from that of the first set of electromagnetic radiation; and at least a portion of the fourth set of electromagnetic radiation is of a frequency different from that of the third set of electromagnetic radiation

Generation and applications of carrier-envelope phase stable mid-infrared femtosecond pulses at high repetition rates

Parametric processes can be used to amplify broadband spectra as well as shift their frequency range. Optical parametric chirped pulse amplification (OPCPA) systems are used in the generation of broadband femtosecond pulses with high intensities over many frequency ranges. In the mid-infrared, a region where classical mode-locked laser systems are not as easily available, they offer the chance to generate broadband carrier-envelope phase-stable pulses. This allows to make use of the advantages of the mid-infrared range for applications from strong-field physics to time-resolved spectroscopy. In this thesis, for both of these applications, OPCPA systems are developed and their usefulness is demonstrated by studying ultrafast carrier dynamics in semiconductors. One demonstrated system generates high average power in the 10 W range with sub-three cycle pulses around 2 µm at a repetition rate of 100 kHz. This enables studies in a previously less accessible frequency range with high intensity and with good statistics over short measurement times. Among other strong-field experiments, it was used to study high harmonic generation from silicon under ambient conditions. Another system was developed to measure the field-resolved changes to the dielectric properties of materials following an ultrashort pump excitation. This expands such experiments to higher, previously unobtainable THz probe frequencies (up to 100 THz). The laser system combines octave spanning field-resolved spectroscopy from 3 to 6 µm with a sub-10 fs temporal resolution, 2 MHz repetition rate, and a high dynamic range. These capabilities are demonstrated on studies on photoexcited carrier dynamics in gallium arsenide and germanium. The field-resolved measurement proved useful to reveal scattering dynamics and the previously often neglected initial thermalization dynamics. The high pump intensity permitted studying these materials under strong excitation conditions which allowed to investigate the saturation of the scattering rate with intensity. The work presented in this thesis demonstrates that mid-infrared OPCPA systems are a flexible tool for many scientific applications and can be used to gain new insides into even extensively investigated materials such as bulk semiconductors.Parametrische Prozesse können benutzt werden, um breitbandige Spektren zu verstärken sowie ihren Frequenzbereich zu verschieben. Optisch-parametrische gechirpte Pulsverstärker (engl. optical parametric chirped pulse amplifier: OPCPA) Systeme werden dazu benutzt, breitbandige Femtosekundenpulse mit hohen Intensitäten in vielerlei Frequenzbereichen zu erzeugen. Im mittleren Infraroten, einer Region wo klassische modengekoppelte Lasersysteme schlechter verfügbar sind, erlauben sie es, breitbandige Pulse mit stabiler Träger-Einhüllendenphase zu erzeugen. Dies ermöglicht, die Vorteile des mittleren Infraroten Frequenzbereiches für Anwendungen von Starkfeldphysik bis zur zeitaufgel¨osten Spektroskopie zu nutzen. In dieser Doktorarbeit wurden für diese beiden Anwendungen OPCPA Systeme entwickelt und ihre Nützlichkeit dadurch demonstriet, dass ultra-schnelle Ladungsträgerdynamiken in Halbleitern untersucht wurden. Eines der entwickelten Lasersystem erzeugt Pulse mit hoher Durchschnittsleistung im 10 W Bereich mit weniger als drei optischen Zyklen um 2 µm mit einer Wiederholrate von 100 kHz. Dies erlaubt es, Experimente in einem vorher wenig zug¨anglichen Frequenzbereich mit hohen Intensitäten und guter Statistik bei kurzer Messzeiten durchzuführen. Das Lasersystem wurde für Starkfeldexperimente eingesetzt, inklusive der Erzeugung hoher Harmonischer in Silizium. Ein weiteres Lasersystem wurde entwickelt, um feldaufgelöst Änderungen der dielektrischen Materialeigenschaften durch einen ultraschnellen Anregungspuls zu messen. Dieses System erweitert solche Experimente zu höheren, vorher unerreichten THz Abtastfrequenzen. Es kombiniert ein oktavenbreites Spektrum von 3 bis 6 µm mit einer zeitlichen Auflösung unter 10 fs sowie einem hohen Dynamikbereich. Diese Vorzüge werden an Messungen von photonenangeregten Ladungsträgern in Germanium und Galliumarsenid demonstiert. Die feldaufgelösten Mesungen waren dabei hilfreich, um Streudynamiken und die oft vernachläsige anfängliche Ladungsträgerthermalisierung zu enthüllen. Die hohe Pumpintensität erlaubte es, Sättigungseffekte der Streurate zu beobachten. In dieser Arbeit wird gezeigt, dass OPCPA Systeme im mittlern Infrarot ein vielseitigesWerkzeug sind, dass in vielen wissenschaftlichen Anwendungsgebieten neue Erkenntnisse liefern kann, selbst in hinreichlich untersuchten Materialien wie in kristallinen Halbleitern