Frequency-shifted Feedback in solid-state lasers
Project description from October 1992 to July 1999
Dipl. Phys. G. Bonnet, Dipl. Phys. O. Barz, Dipl. Phys. M. Stellpflug and Prof. Dr. K. Bergmann
Frequency shifted feedback (FSFB) introduced basically in  can be achieved by inserting an acousto-optic modulator (AOM) into the laser-cavity and closing the resonator via first diffraction order of the AOM (see figure 1).
In a linear cavity the AOM causes a frequency shift of the light which is two times the AOM-frequency per cavity roundtrip time. An optical filter (here: a combination of an etalon and a birefringent filter) then limits the bandwidth of the opticalamplifier and reshapes the spectrum. The complete cycle of non-linear ampiflication and filtering is displayed in figure 2. At the first view it is not clear that such a system will still show laser activity.
Indeed the properties of the emitted light are quite different from those of lasers with simple resonators. The slope of the output-power against the pump-power is non-linear and shows hysteresis. The output is broadband and no stationary mode structure is observed [2,3,4,7,9] if the product of rountriptime and AOM-shift is closed to or bigger than unity.
Therefore it has been used for optical pumping of Rb  and cooling of an atomic beam . The broad amplification bandwidth of this kind of laser was demonstrated by generating a multi-frequency emission via seeding a FSFB laser . Because of the low phase correlation for different frequencies it was discussed as a chaotic light source . Recently  it has been shown with a titanium sapphire (TiSa) system in this laboratory that FSFB is a robust experimental method to achieve a self starting short pulse emission. The concept has been successfully applied also to fiber lasers where pulses down to 2 ps were generated or cw-emission in a FSFB wavelength-swept fiber laser was optimized recently for certain applications .
The thesis of M. Stellpflug was dealing about investigations based on a rate-equation model for the TiSa FSFB laser in . Maybe a paper on that will be published soon.
In there the operation-regimes are classified from the Feigenbaum-chart in fig. 4 and the time-evolution of the spectrum displayed in figure 5 is discussed.
Oliver Barz was setting up a new TiSa-FSFB-Laser in 98 and observed again the short pulses. With that system more detailed investigation could and still can be made. Gerd Bonnet will finish his PhD on this system doing more experiments on the questions:
- Does this laser really show no mode structure ?
- Is the frequency-shift stepwise or continuous ?
- How can you use this light-source for encryption ?
- Does it work on other laser materials ?
The frequency-shifted feedback will also be applied to a Nd:YVO4 system in the end of 99 or beginning of 00. We intend to design a compact setup for a fsfb laser. The project is supplied by the Stiftung Innovation-Rheinland-Pfalz.