Variable repetition frequency asynchronous optical sampling method without a feedback loop

Abstract

The ultrafast pump–probe measurement represents a key technique to study fs–ps dynamics. The asynchronous optical sampling (ASOPS) method realizes fast and long time-range measurement with high time resolution using different repetition frequency pump–probe light pulses. The frequency difference Δ f is an important parameter, as it dictates the measurement time and time resolution. However, usual ASOPS measurements require a complex and precise stabilizer to control Δ f or it is difficult to change Δ f. In this study, we use two free-running titanium/sapphire pulse lasers to develop a variable repetition frequency ASOPS (VRF-ASOPS) method without a stabilizer or feedback loop, where we can easily alter Δ f by changing the cavity length of the probe light laser. To detect the coincidences of the pump–probe light pulses, we cause the instantaneous reflectivity change in a 100 nm platinum film by irradiating the pump light and observe it by the probe light. We use this signal as the trigger signal to directly determine Δ f, which enables us to average and convert the measured responses without a stabilizer or feedback loop. Using this VRF-ASOPS system, we obtain pulse echo signals and 100 GHz Brillouin oscillations, which are equivalent to those measured by the mechanical delay line method, confirming the validity of our developed method.

Progress and Prospects in Optical Ultrafast Microscopy in the Visible Spectral Region: Transient Absorption and Two-Dimensional Microscopy

“…Other studies have utilized digital delay generators to control the time delay between pulses . New approaches have also applied asynchronous optical sampling techniques, in which two laser sources produce pump and probe pulses with different repetition rates to generate a periodically changing temporal delay . The latter methodology has the advantage that long delay times up to tens of nanoseconds can be accomplished by simply adjusting the laser repetition rates.…”

“… 117 New approaches have also applied asynchronous optical sampling techniques, in which two laser sources produce pump and probe pulses with different repetition rates to generate a periodically changing temporal delay. 118 The latter methodology has the advantage that long delay times up to tens of nanoseconds can be accomplished by simply adjusting the laser repetition rates. However, this approach requires high temporal synchronization and stability of the laser systems.…”

Dispersive coherent Brillouin scattering spectroscopy

“…Consequently, the conventional scanning method of delay time limits the signal acquisition speed or increases the electronic complexity. In recent years, various technological advancements have been underway to solve the above issues, such as the development of laser sources including free-running dual comb laser oscillators [31] , [32] and the simplification of the adjustment method for the repetition offset of two mode-locked pulsed laser sources [33] .…”