PhD Defense by Andrea Pertoldi

Thulium fibre lasers for quantum applications

 Precise optical control of atomic energy levels is a cornerstone of quantumtechnologies based on ultra-cold atoms. As more atomic species gain relevance for applications from optical clocks to quantum computing, the demand for laser systems tailored to specific transitions is increasing. These systems must provide high stability, low noise, and compact form factors compatible with existing experimental platforms. Fiber lasers are well suited due to their robustness, low noise, and power scalability. However, the narrow emission bandwidths of conventional erbium- and ytterbium-doped fiber lasers limit the range of possible wavelengths. This gap is addressed by introducing thulium as a dopant, whose broad emission spectrum extends the accessible wavelength range, and this thesis establishes the foundation for modular, low-noise laser systems based on Tm-doped fibers. In particular, distributed-feedback (DFB) fiber lasers with free-running linewidths in the in the kilohertz range were developed at 1762 nm, 1970 nm, 2052 nm, and 2094 nm, along with Tm-doped amplifiers delivering Watt-level output powers. These lasers are used to either directly address barium transitions or act as pump sources in nonlinear waveguide-based conversion schemes that produce visible light at 689 nm and 698 nm for second-stage cooling and clock transitions in strontium. For this, two frequency conversion approaches are explored: a cascaded scheme based on third-harmonic generation and a more direct method using sum-frequency generation in combination with a commercial Yb-doped fiber laser. The latter system produced over 80 mW of 689 nm with linewidths as low as 12 kHz and was successfully integrated into a strontium lattice clock experiment for second-stage cooling to microkelvin temperatures. Beyond laser development, this work investigates fundamental mechanisms that influence laser stability. In particular, the impact of resonant acoustic waves on the frequency noise of DFB fiber lasers is examined in detail, alongside a complete study of coherent instabilities in thulium-doped amplifiers caused by seed laser frequency modulation. These findings inform engineering guidelines to improve the performance and robustness of both seed lasers and amplifiers. Altogether, this work demonstrates the feasibility of laser platforms based on Tm-doped fibers for quantum applications. The systems developed here form the basis for compact, scalable, and low-noise solutions for quantum technologies, and have already translated into commercial laser products.  

Online link: 

https://ucph-ku.zoom.us/j/68442325698?pwd=e71MJIlqZi9RPqhqea4blXsu35lFbS.1

Meeting ID: 684 4232 5698

Passcode: 162102

 Committee:

Dr Steve Lecomte, CSEM Neuchâtel (CH)
Prof. Karsten Rottwitt, DTU (DK)
Assoc. Prof. Leonardo Midolo, NBI