Recent Developments in Frequency-Resolved Optical Gating: Measurement of Ultraweak Ultrashort Pulses

Frequency-Resolved Optical Gating (FROG) [1] has been used to fully characterize ultrashort laser pulses over a wide range of wavelengths, pulse lengths, pulse energies, time-bandwidth products, and pulse repetition rates. There are, however, many situations in which the sensitivity of FROG is inadequate. For example, ultrafast spectroscopy experiments that generate signal pulses of sub-picojoule energies offer much additional information about the medium if the weak signal pulse could be better characterized. [2] In this work, we combine FROG with Spectral Interferometry (SI) [3] to yield a simple method for measuring nearly arbitrarily weak pulses. Here, FROG is used to characterize a reference pulse, and SI sensitively measures the phase difference between the reference pulse and an unknown pulse, thus yielding the full intensity and phase of the unknown, potentially ultraweak, pulse. We demonstrate this method on a pulse train containing 40 zeptojoules (zJ) of energy, or about 1/5 of a photon per pulse (See Figs. 1-3). This method is quite general because ultraweak ultrashort pulses do not exist by themselves; an optical nonlinearity is required to make ultrashort pulses, which are then strong enough to be measured with FROG and hence act as reference pulses. Because this method uses FROG, and because ST involves simply measuring the spectrum of a pair of pulses, we call it Temporal Analysis by Dispersing a Pair Of Light E-fields (TADPOLE).