Mode-locking of thulium-doped and Erbium-doped fiber lasers

This thesis reports work on passive mode-locking of thulium-doped and erbium-doped fiber ring lasers using the technique of polarization additive pulse mode-locking (PAPM). A self-starting, mode-locked Tm+3-doped fiber laser was demonstrated with 360 to 500 fsec pulses tunable from 1.8 to 1.9 pm, the largest tuning range demonstrated from a rare-earth doped fiber. This laser operated in the soliton regime due to the large negative group-velocity dispersion (GVD) of the fiber at 1.8 jim. A possible application to optical coherence tomography on biomedical tissue was explored. A stretched-pulse Er+3-doped laser at 1.55 m was optimized for the application of frequency-doubling to 775 nm where the pulses can be used as a seed for a Ti:Sapphire regenerative amplifier. This laser incorporated segments of fiber with positive and negative GVD to avoid operation in the soliton regime. Compressed fundamental pulses of 100 fsec and 2.7 nJ were obtained, and three nonlinear crystals, d-BaB 20 4 (BBO). KNbO3 (potassium niobate), and LiB305 (LBO), were evaluated for frequency doubling. Near transform-limited pulses at 771 nm with average powers of 8.7 mW were obtained with a 1-cm BBO crystal, corresponding to conversion efficiencies of up to 10%. Frequency resolved optical gating (FROG) measurements were performed on both the fundamental and doubled pulses to better characterize the laser. The effect of linear birefringence on P-APM was explored through numerical simulations for the case of standard fibers, where the two are of the same order. Although reduced by the birefringence, pulse shaping still occurred and there was no inherent periodicity due to the fiber beat-length. Measurements of birefringence and temperature sensitivity of both standard and polarization maintaining (PM) fibers were also performed. Experimental work toward an environmentally stable Er+3-doped fiber laser included two different schemes. The first design was comprised of only PM-fiber, but stable mode-locking was prevented by the temperature dependence of the PM-fiber birefringence. The second scheme, the sigma laser, was a traveling-wave cavity which used both non-PM and PM-fiber. Environmental stability was achieved by canceling changes in linear phase bias in the non-PM fiber and using linear polarization along one of the axes in the PM fiber. Stretched-pulse operation of the sigma laser was achieved with sub-110 fsec, > 1 nJ pulses generated directly from the cavity. Thesis Supervisor: Hermann A. Haus Title: Institute Professor Thesis Supervisor: Erich P. Ippen Title: Elihu Thompson Professor of Electrical Engineering