Asynchronous optical sampling (ASOPS) is a method for time domain spectroscopy on ultrashort time scales which does not rely on a mechanical delay stage and thus enables scan rates in the kilohertz regime. Laser Quantum’s ASOPS technology has been developed and improved over 15 years for applications in spectroscopy using femtosecond lasers. The first commercial ASOPS system has been released to the market in 2005. Since 2010, the 3rd generation stabilisation unit based on the patented DDS technology (direct-digital-synthesis, DDS) has emerged, ensuring a time resolution better than 50 fs over a full scan range of 1 ns when used in combination with two 1 GHz Ti:Sapphire laser systems. When employing ASOPS there is a small difference in the repetition rate of the pump and probe lasers respectively, which leads to a difference in the pulse-to-pulse separation between pump and probe laser beams which increases linearly with Δt= ΔfR/fR2. The ASOPS Engine consists of two femtosecond laser systems with almost identical repetition rates, the patented TL-1000 ASOPS stabilisation unit and a fast acquisition card for high-speed measurements. The ASOPS Engine has been available so far with only the 1 GHz taccor laser range, and is now also available with the venteon and gecco laser series which run at a repetition rate of 84 MHz.
The two femtosecond lasers are working in a master slave configuration in which the repetition rate of the slave laser is controlled to be equal to the master laser repetition rate up to a small offset using a phase locked loop. The difference between the repetition rates depends on the lasers employed and typically is 100 Hz (for 84 MHz) up to 10 kHz (for 1 GHz systems). For specific purposes, the TL-1000 ASOPS unit supports values even beyond those between 4 µHz and 20 kHz. The difference frequency can be easily controlled using a touch screen. The lock procedure between the lasers employed is achieved using a push button. Laser Quantum’s patented stabilization electronics enables a very high time resolution over the complete measurement window and enables the highest time resolution values on the market today.