Abstract

Frequency-resolved optical gating (FROG) is a versatile technique for the measurement of ultrashort pulses across a wide range of wavelengths, and common approaches rely on using a detector array to record the spectrum of the second-harmonic autocorrelation signal of a pulse. Implementing an analogous method to measure mid-infrared pulses is inconvenient because the sensitivity of readily available silicon CCD devices is poor at the second-harmonic frequency of pulses with wavelengths longer than 2.1 μm. In this work we demonstrate a technique which generates a cross-FROG (XFROG) trace by sum-frequency mixing a mid-infrared pulse with a time-delayed near-infrared pulse. By using an adapted version of the principal components generalised projections retrieval algorithm [1] we have been able to characterise both pulses in amplitude and phase. A self-modelocked Ti:sapphire oscillator was used to provide a near-infrared pulse at a wavelength of 835 nm and also served as the pump source for a periodically-poled lithium niobate optical parametric oscillator (OPO) operating with an idler output at 2 35 μm. Approximately 15 mW of 835 nm light was coupled out of the pump beam before the OPO and was incident on a stepper-motor-based delay line (step resolution 10 5 fs) before being combined collinearly with the OPO idler output using a dichroic beamsplitter. The two beams were focused into a 2-mm thick KTA sum-frequency-mixing crystal and the spectrally-resolved 616 nm mixing signal was recorded using a 1200 lines/ mm monochromator and a 512 element linear NMOS-CCD array.

© 2000 IEEE