Practical Techniques for the Measurement of Ultrashort Laser Pulses

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

The measurement of ultrashort laser pulses remains an arduous task. Theautocorrelator, the device of choice for decades, is complex, comprising many components. The pulse must be split into two; the two pulses must then be spatially and temporally overlapped in a carefully aligned second-harmonic-generation (SHG) crystal; and the pulse relative delay must be scanned while maintaining alignment. Alignment involves four sensitive degrees of freedom and has great potential for error. Worse, bandwidth considerations require a very thin SHG crystal, which severely limits device sensitivity. Frequency-Resolved Optical Gating (FROG), which yields the full time-dependent pulse intensity and phase, requires adding a spectrometer to an autocorrelator apparatus, further complicating an already complicated apparatus. And FROG is the simplest proven intensity-and-phase measurement device. As a result, we have developed a remarkably simple, full-intensity-and-phase pulse-measurement device that 1) uses no spectrometer, 2) uses no beam-splitter or delay line, 3) avoids the thin crystal and instead uses a more convenient, thick SHG crystal, 4) is considerably more sensitive, 5) involves no sensitive alignment parameters, 6) can measure a single pulse, and 7) comprises only a few simple optical components. It yields traces identical to those of SHG FROG, and hence yields the full time-dependent pulse intensity and phase. This device incorporates two key innovations: 1) a Fresnel biprism (a prism with a 170 apex angle), which replaces the beam splitter and delay line, and 2) a thick nonlinear crystal, whose output light is angularly resolved, yielding spectral resolution (as well as improved sensitivity and the usual time-gating), and which replaces the thin crystal and spectrometer.