Publication
The invention of the laser in the 1960s heralded a new era in photonics. Owing to both, academic and industrial research, photonics technologies have long become part of our everyday life. This PhD thesis explores two subfields of laser technologies: Frequency combs and fiber lasers.
An optical frequency comb is an optical spectrum consisting of many equidistant lines. A dual frequency comb, comprising two such combs, is a powerful tool for measuring optical frequencies and distances with unprecedented precision. This thesis focuses on frequency comb technology at 2 micron wavelength, since this spectral region overlaps with the fingerprint of atmospheric molecules, such as CO2, NH3 and NO2, but also contains an atmospheric transmission window for potential ranging applications. All-fiber setups were investigated in order to provide portable sources that are resilient against ambient disturbances. A key component in this work is thulium-doped active fiber, which delivers significant broadband gain in the 2 micron band.
The first part of the thesis involves generating a free-running dual comb using a thulium-doped fiber laser (TDFL). The goal was to create two frequency combs in one oscillator by simultaneously mode-locking at orthogonal polarization. This approach eliminates the need for stabilization in spectroscopic and ranging applications. Several fiber laser cavity architectures have been investigated to identify the optimal design for this purpose (linear, circular, figure-of-9), as well as different mode-locking techniques (material saturable absorbers, nonlinear amplifying loop mirror, hybrid), and the incorporation of chirped-fiber Bragg gratings (CFBGs) for wavelength-stabilization.
During the investigation, a mode-locked TDFL with a broad tunability from 2022.1 nm to 2042.2 nm was implemented. This was achieved by applying mechanical tension or compression to a CFBG. Furthermore, a figure-of-9 all-fiber TDFL was demonstrated, generating 560 fs long pulses at 1948 nm wavelength. Self-starting passive-mode-locking was achieved utilizing an in-fiber Faraday rotator.
Concerning the free-running dual comb, a hybrid cavity layout combining a nonlinear amplifying loop mirror with a saturable absorber mirror turned out to be a promising approach. Pulsed emission occurred at two repetition rates and a stable beating signal was observed. Yet, the two pulse trains could not be separated. Nevertheless, this demonstration underscores the potential feasibility of the free-running dual comb at 2 microns.
In the second part of the thesis, active fiber resonators incorporating thulium-doped fiber, while operating below the lasing threshold were investigated. These resonators have the potential to generate cavity solitons serving as frequency combs. While active fiber resonators have been demonstrated at telecom wavelength, they have now, for the first time, been investigated at 2 micron wavelengths. Given the increased fiber propagation losses a