Abstract

Asynchronous optical sampling (ASOPS) was first demonstrated in 1987 [1]. It employs two mode-locked lasers operating at slightly different repetition rates f and f+Δf which serve as pump and probe lasers. Thus the relative time-delay of the pulses from the pump laser compared to the pulses from the probe laser is repetitively ramped from zero to 1/(f+Δf). ASOPS therefore permits time-domain spectroscopy without mechanical delay stage. By employing 1 GHz repetition rates instead of 82 MHz in a high-speed ASOPS implementation, the time resolution could be improved from 1,5 ps [1] to ~159 fs [2]. Higher repetition rates have the additional benefit of permitting higher scan rates Δf. Today high-speed ASOPS is a technique to acquire time-resolved pump-probe data and perform THz spectroscopy with unprecedented signal-to-noise ratio (typically 10⁻⁷) in very short data acquisition times (typically a few seconds) [2]. Reduction of timing jitter between the asynchronously linked laser repetition rates is of major concern for high-speed ASOPS and is key to a further improvement of the time resolution to values well below 100 fs and eventually approach the limit given by the laser pulse duration. In our experimental setup (Fig. 1(a)) the 1 GHz repetition rate Ti:sapphire lasers deliver ~700 mW of output power at center wavelengths independently tunable between 750 and 850 nm and a pulse duration of ~35 fs. The lasers repetition rate offset stabilization is performed in a master-slave configuration using the detected 11. harmonics of the repetition rates, a harmonic frequency shifter and a phase-locked loop (PLL). Using higher harmonics H of the repetition rates has two advantages. First the error signal for the PLL becomes more sensitive. Second the maximum bandwidth which can be used for the PLL without modulating sidebands from the frequency shifter into the error signal must be well below HxΔf. To exploit the possible higher feedback bandwidth a weight reduced cavity mirror of the slave laser was mounted on a high bandwidth piezoelectric transducer which enables ~25 kHz of PLL-bandwidth. In comparison to an electronic trigger the used optical trigger enables an enhanced long-term stability and preserves the obtained time resolution of the high-speed ASOPS by avoiding slow drifts between trigger and detected signals. The current optimal operating value of the repetition rate difference Δf for high-speed ASOPS is 5 kHz and permits a time resolution of ~60 fs over the full 1 ns measurement window with a relative error of the time-delay axis calibration below 10⁻⁴. We demonstrate the enhanced capability of the new high-speed ASOPS setup in a high-bandwidth (> 4 THz) THz-setup and by a measurement of coherent optical phonons in GaAs (at 8,8 THz), see Figs. 1(b) and 1(c).

© 2009 IEEE