fs-laser source

Starting point is the so called "seed" laser - a passive mode locked Ti:sapphire laser. The self-mode-locking process is based on the nonlinear Kerr effect in the active material Ti:Al₂O₃. It is one of the simplest cavity arrangements and was introduced by Sibbet et al. at the beginning of the 1990's. In our case it is optically pumped by a frequency doubled Nd:YVO₄ laser with a cw-output of 3W at a wavelength of 532nm. This leads to 40fs pulses of 1nJ at a frequency of 785nm.

The second step is the amplification of the output of the seed-laser. Therefore, a commercial regenerative Ti:sapphire amplifier ("Spitfire" by Spectra-Physics) is used, converting the low-power output of the seed-laser at 80 MHz repetition rate to high-power output at 1kHz. For running the Spitfire a second pump laser is needed. We use a frequency-doubled Nd:YLF laser running at 1kHz repetition rate. It can be seen as the "(cardiac) pacemaker". Henceforth, we end up with 80fs pulses of approximately 1mJ at a wavelength of 800nm. This means an increase by a factor of 10⁶. Throughout the following this is going to be our source, which will be spectrally broadened, amplified, temporally compressed, and so on, to get suitable ultrashort light pulses for various applications.

NOPA

For spectroscopic applications, it is desireable to have a tunable broadband ultrashort laser source. Our non-collinear optical parametric amplifier (NOPA) is such a handy tool, being pumped by the 80fs output of the amplifier system at 800nm. A small fraction of the input is used to generate a high-quality white-light continuum with spectral components between 450nm and 830nm. This is accomplished in a sapphire-plate of 1mm thickness. The remaining fraction of the input is frequency-doubled to 400nm and amplifies the white-light seed through parametric interaction in a 1mm thick BBO crystal. Through the non-collinear arrangement of the seed and pump-pulses a lot of spectral components can effectively be phase-matched simultaneously. Therefore, no temporal broadening of the pulse occurs. The NOPA generates amplified tunable pulses between 450 and 700nm which are compressed in a subsequent prism compressor to a pulse duration of approximately 25fs (FWHM).

Pulse shaper

Control over the dependecy of the phase of the different spectral components traveling in the pulse (i.e., the "chirp") is crucial for ultrafast spectroscopy. Our setup is equipped with a programmable acousto-optical dispersive filter (the so-called "Dazzler" by Fastlite) which offers arbitrary control over spectral amplitude and phase simultaneously. This provides the ability to compensate for the different orders of the chirp separately. The active medium of the device is a birefringent crystal. By interaction with a supersonic wave launched in this crystal, the different (optical) spectral components can be diffracted selectively from the ordinary to the extraordinary axis. The "Dazzler" can be controlled by a software running on a standard PC.