Abstract

Frequency-resolved optical grating (FROG) is a techique for measuring the full time-dependent intensity and phase of an arbitrary ultrashort optical pulse.¹-² Unlike the polarization-gated (PG) method, FROG using second-harmonic generation (SHG) has a time-reversal ambiguity. The envelope of the SHG FROG signal field is given by E_SHG−sig(t, τ) ∝E(t)E(t − τ) and is always symmetric in the time delay τ, therefore the resulting spectrograms are somewhat unintuitive. Nevertheless, because of the simplicity of the SHG FROG, it becomes the most common technique for detail characterization of ultrashort optical pulses of laser oscillators. We extend this FROG technique with the use of third-harmonic generation (THG FROG), where the signal field is now given by E_THG−sig(t, τ) ∝E²(t)E(t − τ) which has no time ambiguity. Therefore the resulting THG FROG trace is as appealing as the PG FROG. Although THG is dipole-allowed, the coherent length in the bulk of a material is short therefore THG in the bulk is often too weak to be measured. But THG is much enhanced at an interface³ giving adequate THG photons for pulse characterization of laser oscillators.

© 1996 Optical Society of America