Abstract

Recent advances in ultrafast laser technology and measurement techniques have pushed optical communication rates beyond a Terabit per second [1] and moved high-speed opto-electronics into the Terahertz domain [2]. Similar advances in high-field laser-matter interactions promise compact table-top sources of tunable short-pulse VUV- and X-radiation with brightness comparable to large electron storage ring facilities [3]. Also of great interest to the high energy and particle accelerator communities, large-amplitude relativistic Langmuir waves excited by super-strong ultrafast laser pulses in plasmas may lead to both “compact” (kilometer) TeV scale accelerator facilities and small laboratory/medical facility table-top electron accelerators with GeV beam energies [4]. Continued advancement in our understanding of the underlying fundamental physics in these fields depends critically on the successful development, refinement, and application of sensitive optical diagnostics with femtosecond temporal resolution. As the dominant interaction between a probe laser pulse E(t) and these materials is a shift in optical phase, E(t) → E(t)e^iϕ(t), where ϕ(t) = (n(t)wz/c) and n(t) is the refractive index, optical diagnostics designed to directly measure the phase change ϕ(t) have shown the most promise. The inter-related techniques of spectral blueshifting [5], longitudinal (i.e. frequency domain) interferometry (LI) [6], and frequency resolved optical gating (FROG) [7] have been successfully used by many researchers to extract from spectral power density measurements sensitive details of ultrafast time-domain phase shifts. The first two of these techniques, being linear optical effects, have been shown to be powerful tools for measuring DC and slowly varying time-domain phase shifts with extremely high sensitivity (one part in 3000 of a fringe) over a temporal span of many pulse-widths about the peak of the pulse. On the other hand, the optically nonlinear FROG, which accurately recovers detailed variations in temporal phase about the intense portions of the pulse but is largely insensitive to DC and slowly varying terms, acts as a complementary diagnostic for highly structured phase shifts, such as those in ionization fronts. We have developed a new marriage of standard FROG with multi-pulse longitudinal interferometry, termed Multi-pulse Interferometric FROG, or MI-FROG, which promises to become a powerful real-time diagnostic of ultrafast dynamics: a femtosecond phase-sensitive oscilloscope if you will.

© 1997 Optical Society of America