In large active ring lasers, the free spectral range is about 100-times smaller than the bandwidth. As a consequence, the laser runs multi-mode on various longitudinal modes: a well-known nuisance that has interfered with precision measurements for decades. We have developed a method based on injection locking, in which we use an external laser to steer the mode index of the active ring laser. The method is remarkable robust and can now be found on the arXiv: https://arxiv.org/abs/2601.19676. 

Quantum frequency conversion is well established between VIS and NIR wavelength, but more challenging if large gaps in wavelength are to be bridged, or if wavelengths in the blue and the UV are involved. Many years ago, we had presented octave-spanning frequency conversion of single photons from the NIR to the UV. This early work could only accept one polarization state of the incoming photon. In an upgraded version of the experiment, we now present polarization-insensitive frequency conversion based on a different type of crystal. Congratulation to Katrin for this nice result! (https://doi.org/10.1364/AO.581771) 

Pound-Drever-Hall locking is the most common approach to lock a laser to an optical cavity, but so-called residual amplitude modulation is a common threat that leads to frequency drift. Ring resonators are sensitive to polarization by design, so it might be straightforward to consider a Hänsch-Couillaud scheme here as well. Combining a Hänsch-Couillaud scheme with lock-in detection allowed us to improve the stability of passive ring laser gyroscopes, now reaching the same performance level as in active operation mode. This work, with Jannik as lead author, has been published with Optics Letters: https://doi.org/10.1364/OL.581271

We have one PhD student less in our group: Thorsten defended his PhD thesis with flying colours! After many years in our group and intensive work on the mercury project, isotope shift spectroscopy became the central piece of his PhD. Congratulations, Dr. Groh!