Pulse-shaper-assisted spatio-temporal ultrashort pulse characterisation

Pulse-shaper-assisted spatio-temporal ultrashort pulse characterisation Seth L. Cousin1 , Juan M. Bueno2 , Nicolas Forget3 , Dane R. Austin1 , and J. Biegert1,4 1. ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain ´ 2. Laboratorio de Optica, Universidad de Murcia, Campus de Espinardo, 30071 Murcia, Spain 3. FASTLITE, Centre Scientifique d’Orsay—Bˆat. 503—BP 45, 91401 Orsay, France 4. ICREA-Instituci´o Catalana de Recerca i Estudis Avanc¸ats, 08010 Barcelona, Spain

Ultrashort pulse characterization is sufficiently mature that temporal amplitude and phase profiles are now routinely acquired. However, most implementations either ignore or average over spatial variations. Nontrivial spatiotemporal couplings play a significant role in many ultrafast sources, such as chirped-pulse and optical parametric amplifiers, and can strongly affect many applications, such as high-harmonic generation and materials processing. This motivates the development of methods for full spatio-temporal characterisation [1, 2]. Most current designs involve augmenting an existing temporal characterisation method with additional components such as a spatially resolved interferometer [3]. Such implementations usually complicate usage and it is therefore important to devise a simple setup with a minimum number of components and straightforward usage. Here, we present a robust and straightforward implementation of spatio-temporal measurement by combining a Shack-Hartmann wavefront sensor with the Dazzler acousto-optic programmable dispersive filter [4] from Fastlite. The Dazzler selects spectral slices of the pulse. The spatial phase of each slice is measured using a Shack-Hartmann wavefront sensor. Then, the Dazzler is operated as a bFROG [5] to retrieve the spectral phase and amplitude at a single spatial position. The wavefronts at each frequency may then be related by this spectral phase, providing a three-dimensional measurement of the phase. The experiment is depicted in Fig. 1. The test pulses were produced

τ Dazzler

Pinhole t

χ(2)

Beamsplitter

ω

Spectrometer

Shack-Hartmann wavefront sensor

Figure 1: Schematic of the experiment. The green rectangle indicates the components used for the bFROG measurement.

Figure 2: (a) Spectrum at the center of the pulse. (b), (c) Wavefront at 720 nm and 790 nm respectively, as indicated by the dashed lines in subfigure (a). (d) Reconstructed volume of half peak spatio-temporal intensity. (e) Reconstructed arrival time vs. position.

using a gas-filled fiber compression system, and had 14 fs FWHM duration, 100 µJ energy and 750 nm center wavelength. To measure wavefronts for all frequencies in the pulse, we programmed the Dazzler to operate as a bandpass filter, and selected 5 nm-wide slices at 27 wavelengths uniformly spaced across the spectrum, which is shown in Fig. 2(a). We measured the spatial phase of these slices using a Shack-Hartmann wavefront sensor. Two examples are given in Fig. 2(b) and (c). We obtained the spectral phase and amplitude of a central spatial region of the pulse which we selected using a pinhole by programming the Dazzler to produce time-delayed replicas and directing its output onto a collinear bFROG setup consisting of an 80 mm lens, a 20 µm BBO crystal, a color filter, and a spectrometer. We used the measured spectral phase to relate the absolute phase of each wavefront slice, allowing a threedimensional phase φ(x, y, ω) to be reconstructed up to the usual absolute phase and arrival time ambiguities. Combined with the intensity, we were then able to reconstruct the pulse in any combination of time or frequency and position or wavenumber domains. Figure 2(d) illustrates this by showing the 50% of peak intensity volume in the spatio-temporal domain. The volume is a simple ellipsoid, showing that our pulse is close to a Gaussian profile in time and space. Another useful measure of the pulse quality is the arrival time as a function of position, shown in Fig. 2(e). In conclusion, this technique provides robust and compact spatio-temporal pulse measurement by exploiting mature and commercially available technologies. References [1] C. P. Hauri, J. Biegert, U. Keller, B. Schaefer, K. Mann, and G. Marowski, “Validity of wave-front reconstruction and propagation of ultrabroadband pulses measured with a Hartmann-Shack sensor,” Opt. Lett. 30, 1563–1565 (2005). [2] E. Rubino, D. Faccio, L. Tartara, P. K. Bates, O. Chalus, M. Clerici, F. Bonaretti, J. Biegert, and P. D. Trapani, “Spatiotemporal amplitude and phase retrieval of space-time coupled ultrashort pulses using the Shackled-FROG technique,” Opt. Lett. 34, 3854–3856 (2009). [3] C. Dorrer, E. M. Kosik, and I. A. Walmsley, “Direct space-time characterization of the electric fields of ultrashort optical pulses,” Opt. Lett. 27, 280 (2002). [4] F. Verluise, V. Laude, Z. Cheng, C. Spielmann, and P. Tournois, “Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter: pulse compression and shaping,” Opt. Lett. 25, 575 (2000). [5] N. Forget, V. Crozatier, and T. Oksenhendler, “Pulse-measurement techniques using a single amplitude and phase spectral shaper,” J. Opt. Soc. Am. B 27, 742–756 (2010).