Monolithic, Mode-Locked Titanium-Sapphire Lasers with Repetition Rates in the Range of 30 GHz to 300 GHz

• Applicant: Professor Dr. Ulrich Wittrock
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 555580329

Mode-locked solid state lasers produce trains of ultrashort laser pulses. Pulse durations range from a few femtoseconds to picoseconds while pulse repetition rates are typically in the range of hundred MHz to a few GHz. The goal of our research is to extend the pulse repetition rate to the range of 30 GHz to 300 GHz. The ideal laser gain medium to achieve this is Titanium-doped Sapphire (Ti:Sa). To date, the highest reported repetition rate for a mode-locked Ti:Sa laser is 10 GHz. As a result of a previous DFG project, we recently achieved a new record of 17 GHz with a mode-locked Ti:Sa laser. However, we are confident that the new laser concept we developed for this purpose can be scaled to much higher repetition rates. The pulse repetition rate of a fundamentally mode-locked laser is equal to the inverse of the round-trip time of the laser pulse inside the resonator. High repetition rates therefore require very short resonators. For example, a mode-locked laser with a repetition rate of 100 GHz needs 3 mm round-trip optical path length of its resonator. In general, mode-locked lasers need to have a large number of longitudinal modes in order to be useful for applications. The reason is that a large number of longitudinal modes is required in order to produce well-defined pulses with a peak power that is orders of magnitude higher than the time-averaged laser power. Mode-locked lasers with a large number of longitudinal modes have a comb-like spectrum in the frequency domain. Such frequency combs are very useful for metrology. A monolithic architecture, where the laser gain medium constitutes the resonator, is ideal for the millimeter-sized resonators that we need. We now want to scale-up the 17-GHz repetition rate of this laser to 300 GHz. This is challenging because several non-linear optical effects have to be properly balanced in order to maintain mode locking. The non-linear effects have to be calculated with sufficient precision and the chirped coatings on the laser crystal have to be manufactured to exacting specifications. Unfortunately, these coatings are expensive and a new laser crystal has to be coated every time the specifications are changed. This makes research on monolithic lasers not only expensive but also time consuming. While research on monolithic lasers is expensive, they could later be mass-produced at very low. In addition, these lasers are very robust because the monolithic laser resonator can not be misaligned. The fact that the monolithic resonator has no movable components also results in very low phase and amplitude noise of these laser. This makes monolithic mode-locked Ti:Sa lasers perfect tools for frequency metrology, e. g. optical atomic clocks. With the envisioned repetition rates of more than 100 GHz they can also bridge the gap between optics and microwave photonics.

• DFG Programme: Research Grants