Michelle Rhodes

Complete characterization of a spatiotemporally complex pulse by an improved single-frame pulse-measurement technique

We further develop a simple device, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH), for measuring the complete spatiotemporal intensity and phase, I(x,y,t) and ϕ(x,y,t), respectively, of an arbitrary ultrashort pulse on a single camera frame (and hence, on a single shot). We have increased the measurable bandwidth, eliminated most aberrations, and improved the uniformity of the multiple holograms in the device. We demonstrate these improvements by making single-camera-frame measurements of spatiotemporally complex subpicosecond crossed and chirped pulses from a Ti:sapphire oscillator. In order to display the massive resulting data files—pairs of four-dimensional intensity-and-phase data—we also develop a method for generating intuitive movies of the measured pulses. With these improvements, this device and its resulting movies should be able to perform and intuitively display true single-shot spatiotemporal measurements of most current ultrashort pulses.

Standards for ultrashort-laser-pulse-measurement techniques and their consideration for self-referenced spectral interferometry.

Issues important for new ultrashort-pulse-measurement techniques include the generation of theoretical example traces for common pulses, validity ranges, ambiguities, coherent artifacts, device calibration sensitivity, iterative retrieval convergence, and feedback regarding measurement accuracy. Unfortunately, in the past, such issues have gone unconsidered, yielding long histories of unsatisfactory measurements. We review these issues here in the hope that future proposers of new techniques will consider them without delay, and, as an example, we address them for a relatively new technique: self-referenced spectral interferometry.

Single-shot measurement of the complete temporal intensity and phase of supercontinuum

We demonstrate the first technique for the complete temporal measurement of a single supercontinuum pulse. We achieve large ranges and high resolutions using polarization gating, a tilted gate pulse, and the cancellation of geometrical smearing.

Numerical simulations of holographic spatiospectral traces of spatiotemporally distorted ultrashort laser pulses.

We simulate traces for a catalog of spatiotemporally complex pulses measured using a single-shot complete spatiotemporal pulse-measurement technique we recently developed, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH). The only such technique ever developed to our knowledge, STRIPED FISH measures the complete spatiotemporal intensity I(x,y,t) and phase ϕ(x,y,t) of an arbitrary laser pulse using an experimentally recorded trace consisting of multiple digital holograms, one for each frequency present in the pulse. To understand the effects of various spatiotemporal distortions on the STRIPED FISH trace, we numerically investigate STRIPED FISH trace features for a catalog of pulses, including the spatially and temporally transform-limited pulse, temporal and spatial double pulses, spherically focusing and diverging pulses, self-phase modulated and self-focusing pulses, spatiotemporally coupled pulses, and pulses with complex structures. As a practical example, we also analyze an experimentally recorded trace of a focusing pulse with spatial chirp. Overall, we find that, from STRIPED FISHs informative trace, significant spatiotemporal characteristics of the unknown pulse can be immediately recognized from the camera frame. This, coupled with its simple pulse-retrieval algorithm, makes STRIPED FISH an excellent technique for measuring and monitoring ultrafast laser sources.

Measurement of the ultrafast lighthouse effect using a complete spatiotemporal pulse-characterization technique

Three of the four spatiotemporal pulse amplitude couplings—spatial chirp, angular dispersion, and pulse-front tilt—are well known for their important roles in optics and, especially, in ultrafast optics. The remaining one, only recently identified, corresponds to the pulse arrival time variation with angle and is known as the “ultrafast lighthouse effect.” This effect has important applications in attosecond science, but its characterization has not yet received much attention. In this work, we generate an ultrafast lighthouse and measure it using a recently developed single-frame complete spatiotemporal pulse-characterization technique called STRIPED FISH. We discuss in great detail the measured couplings in different domains and their roles in generating the ultrafast lighthouse effect. In addition, we display the propagation of the measured ultrafast lighthouse with an intuitive movie plot over space and time. We conclude that STRIPED FISH provides a simple and informative approach for measuring the ultrafast lighthouse effect and also other possible spatiotemporal distortions in the pulse, in both the horizontal and vertical dimensions.

Coherent artifact study of two-dimensional spectral shearing interferometry

We study multi-shot intensity-and-phase measurements of unstable trains of ultrashort pulses using two-dimensional spectral shearing interferometry (2DSI). We find that, like other interferometric ultrashort-laser pulse-measurement methods, it measures only the coherent artifact and so yields effectively only a lower bound to the pulse length. We also attempt to identify warning signs of pulse-shape instability in 2DSI and find that it responds to instability with reduced fringe visibility, although this effect is very small when using the small spectral shears appropriate for large temporal ranges. We conclude that 2DSI should be used with caution and large-shear measurements or alternative techniques should be used to verify the stability of the pulse train.

Visualizing spatiotemporal pulse propagation: first-order spatiotemporal couplings in laser pulses

Even though a general theory of first-order spatiotemporal couplings exists in the literature, it is often difficult to visualize how these distortions affect laser pulses. In particular, it is difficult to show the spatiotemporal phase of pulses in a meaningful way. Here, we propose a general solution to plotting the electric fields of pulses in three-dimensional space that intuitively shows the effects of spatiotemporal phases. The temporal phase information is color-coded using spectrograms and color response functions, and the beam is propagated to show the spatial phase evolution. Using this plotting technique, we generate two- and three-dimensional images and movies that show the effects of spatiotemporal couplings.

Measuring spatiotemporal intensity-and-phase complexity of multimode fiber output pulses

We demonstrate ultrashort pulse spatiotemporal field measurement for multimode optical fibers, using a singleframe characterization technique, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH). We measure STRIPED FISH traces and retrieve the pulse field E(x,y,t) or equivalently E(x,y,ω), to generate movies revealing the field structure induced by propagating modes, due to their differences in field spatial distribution, modal propagation velocity and modal dispersion inside the fiber. We launch femtosecond pulses near 800nm from Ti: Sapphire laser to investigate linearly polarized modes LP01, LP11, LP02 and LP21 in multimode fibers.

Unstable multipulsing can be invisible to some ultrashort pulse measurement techniques

Simulations show that variable relative phase of a satellite pulse causes the satellite to wash out of a SPIDER measurement completely. FROG and autocorrelation measurements see satellite pulses but cannot determine their precise properties.

The Coherent Artifact in Interferometric Pulse-Measurement Techniques

We study multi-shot intensity-and-phase measurements of unstable trains of ultrashort pulses using two-dimensional spectral shearing interferometry (2DSI) and self-referenced spectral interferometry (SRSI) in order to identify warning signs of pulse-shape instability.