Ultralow-power second-harmonic generation frequency-resolved optical gating using aperiodically poled lithium niobate waveguides

We discuss ultralow-power second-harmonic generation (SHG) frequency-resolved optical gating (FROG) in the telecommunication C-band using aperiodically poled lithium niobate (A-PPLN) waveguides as the nonlinear medium. A key theme of this work is that the phase-matching curve of the nonlinear medium is engineered to obtain an optical bandwidth adequate for measurement of subpicosecond pulses while retaining the optimum nonlinear efficiency consistent with this constraint. Our experiments demonstrate measurement sensitivity (defined as the minimum product of the peak and average pulse powers at which a reliable nonlinear signal can be detected) of 2.0 x 10−6 mW^2 in a collinear SHG FROG geometry, approximately 5 orders of magnitude better than previously reported for any FROG measurement modality. We also discuss asymmetric Y-junction A-PPLN waveguides that permit background-free SHG FROG and a polarization-insensitive SHG FROG technique that eliminates the impairment that frequency-independent random polarization fluctuations induce in the FROG measurement. Finally, we applied these SHG FROG techniques in chromatic dispersion and polarization mode dispersion compensation experiments. In these experiments the FROG data enabled complete correction of distortions incurred by subpicosecond pulses passing through optical fibers; these results also demonstrate the ability to retrieve extremely complex pulses with high accuracy.