title: Wavelength shifting of intra-cavity photons: Adiabatic wavelength tuning in rapidly wavelength-swept lasers.
abstract: We analyze the physics behind the newest generation of rapidly wavelength tunable sources for optical coherence tomography (OCT), retaining a single longitudinal cavity mode during operation without repeated build up of lasing. In this context, we theoretically investigate the currently existing concepts of rapidly wavelength-swept lasers based on tuning of the cavity length or refractive index, leading to an altered optical path length inside the resonator. Specifically, we consider vertical-cavity surface-emitting lasers (VCSELs) with microelectromechanical system (MEMS) mirrors as well as Fourier domain mode-locked (FDML) and Vernier-tuned distributed Bragg reflector (VT-DBR) lasers. Based on heuristic arguments and exact analytical solutions of Maxwell's equations for a fundamental laser resonator model, we show that adiabatic wavelength tuning is achieved, i.e., hopping between cavity modes associated with a repeated build up of lasing is avoided, and the photon number is conserved. As a consequence, no fundamental limit exists for the wavelength tuning speed, in principle enabling wide-range wavelength sweeps at arbitrary tuning speeds with narrow instantaneous linewidth.
authors.affiliation: Institute for Nanoelectronics, Technische Universität München, Arcisstr. 21, D-80333 Munich, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Jirauschek
authors.affiliation: Institut für Biomedizinische Optik, Universität zu Lübeck, Peter-Monnik-Weg 4, D-23562 Lübeck, Germany ; Lehrstuhl für BioMolekulare Optik, Fakultät für Physik, Ludwig-Maximilians-Universität München, Oettingenstr. 67, D-80538 Munich, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Huber
keywords: (140.3430) Laser theory; (140.3600) Lasers, tunable; (140.7260) Vertical cavity surface emitting lasers; (170.4500) Optical coherence tomography