Ultrafast spectroscopy is based on the use of two pulses separated by a variable delay, the second pulse (probe) allowing to measure the modifications induced by the first one (pump) within the studied system, and to reconstruct point-by-point the dynamics of the system. Traditionally, the two pulses come from the same laser but travel different optical paths. The delay between the two pulses is obtained by mechanical displacement of the optics of one of the paths, as shown below. It is thus limited to the sub-nanosecond range: a delay beyond a few nanoseconds corresponds to mechanical displacements of more than one meter, making it difficult to maintain the pointing necessary for the experiment.
A more recent technique, called ASOPS (Asynchronous OPtical Sampling) requires two synchronized lasers, whose relative phase can be varied. We thus have two pulses separated by a variable delay, without mechanical translation, and with temporal excursion only limited by the period of the pump laser. Its major drawback is that it requires the frequency of one laser to be locked to the other, which implies modifications to the cavity of one laser and a sophisticated control system. It is also possible to use two amplified lasers, injected by two synchronised oscillators.
The method we have developed also uses two lasers, but without any condition on their repetition rates. It allows to extend the use of ASOPS to any pair of femtosecond lasers, while keeping the advantages of the technique: no mechanical translation, scanning speed, sub-picosecond temporal resolution, temporal excursion only limited by the pump laser period. This technique consists in measuring the repetition rates of the two lasers as well as the evolution of the relative offset between the two pulse trains. The delay between each pair of pulses can then be calculated a posteriori, and this with great precision thanks to the remarkable frequency stability of femtosecond oscillators. The determination of the relative evolution of the two pulse trains can be done either optically, by detecting with a photodiode the linear interference of two temporally overlapping pulses , or electronically, by detecting each pulse train and then performing a digital processing in an FPGA .
At the LOB, we have realized a prototype with optical coincidence detection that has allowed us to reach a resolution of less than 400 fs over the entire 200 ns time domain corresponding to the period of our pump laser. This prototype was applied to the measurement of charge transfer within the photosynthetic reaction center of Rhodobacter Sphaeroides  and achieved a very good signal-to-noise ratio in only a few minutes. A crucial advantage of our method is to allow the use of a chirped pulse oscillator (CPO) as a pump laser, providing much more energy at a lower repetition rate.
Our method can furthermore be extended to amplified femtosecond lasers, as shown above . The amplified pulses can also be selected to provide delays as close to the target values as possible, allowing both fast and logarithmic delay scans . The figure below shows the electronics for both the sub-picosecond precision delay measurements and the triggering of the amplifiers at the optimum times.