For these measurements, the sample is excited with a pump laser pulse and generates a perturbation inside the material. Subsequently, a pulse of a second pulsed laser, which is slightly delayed against the first pulse, measures the excitation state of the sample resulting from the induced perturbation. in order to get information about the excitation decay over time, the delay time between the two laser pulses needs to be varied. Often, such setups use a mechanical delay line to change the time delay between the two laser pulses but this results in timing limitations caused by the respective actuators. Different approaches like asynchronous optical sampling (ASOPS) or electronically controlled optical sampling (ECOPS) can avoid such restrictions. In ASOPS, the variation in delay time is realized by using two lasers with a slightly different repetition frequency, which leads to a proportionally increasing delay time. And in ECOPS, the cavity length of one laser is tunable. As it is changed, the repetition rate of the laser varies.
Both ECOPS and ASOPS produce an asynchronous delay of the laser pulses and allow a high-speed sampling of dynamic processes without any mechanical parts. But on the other hand, for these high accuracy sampling approaches, the two lasers need to be precisely synchronized and detuned. So, what can help with these issues?
For a precise synchronization of two lasers, their phase positions need to be compared. Usually, a photodetector is applied for this task. However, high-speed applications like ASOPS and ECOPS direct photodetection cannot provide the required time resolution. Cycle’s Balanced Optical Cross-Correlator (BOC) is a high-precision timing detector that can measure the timing jitter of two lasers with femtosecond resolution. Using non-linear optical effects, the timing jitter of two lasers is reduced to < 10fs RMS and it directly allows precise control of the laser cavity for the detuning. Both features make it highly suitable for sophisticated, high-speed measurements like optical sampling applications.