Measuring Weak Laser Pulses Using Frequency-Resolved Optical Gating

Abstract

Frequency-Resolved Optical Gating [1] (FROG) measurements based on polarization gating [2] (PG FROG), self diffraction [3] (SD FROG), and second-harmonic generation [4] (SHG FROG) have been used successfully to measure ultrashort laser pulses with spatially uniform temporal pulse shape and phase with durations from 9 fs to 2 ps and with wavelengths from the ultraviolet to the near infrared. Temporal Analysis by Dispersing a Pair of Light E-fields [5] (TADPOLE), a combination of FROG and spectral interferometry, has been used to measure pulse trains with average pulse energies of 42 zeptojoules (42 × 10-21 J). None of these techniques, however, has proved capable of measuring the pulses from a Ti:sapphire laser without a temporal ambiguity or an amplified reference pulse. The difficulty lies in the fact that the pulses from such oscillators are too weak for PG or SD FROG, and SHG FROG has a time-reversal ambiguity due to the symmetry of the second-harmonic signal in time. TADPOLE, while capable of measuring extremely weak pulses, uses FROG to measure a reference pulse and thus has the same restrictions Here we use surface third-harmonic generation (THG) to make an unambiguous measurement of the intensity and phase of pulses from a Ti:sapphire oscillator. In addition, we describe a new method for using TADPOLE for measuring ultrashort laser pulse fields that vary arbitrarily in time and in space.