Noise and stability of actively modelocked fiber lasers

Abstract

The timing jitter of a modelocked laser is fundamentally limited by the amplified spontaneous emission in the laser cavity. While one cannot, even in principle, remove this source of noise, one does have control over the pulse timing by using filtering and modulation. In this thesis, we report on the advances made in developing the understanding of timing jitter and stability in actively modelocked soliton fiber lasers. The main achievements reported here are: the development of a theory for quantum-limited timing jitter for the cases of amplitude and phase modulation (AM and PM, respectively); identification of a set of characteristic coefficients governing the physics of pulse retiming that depend on the laser parameters; construction of an apparatus-including the development of harmonically modelocked soliton fiber lasers in both a ring and a sigma configuration-to measure the predicted coefficients; and residual phase-noise measurements of the quantum-limited timing jitter using homodyne detection. The measurements of the characteristic coefficients and the timing jitter were found to be in good agreement with the theory. In addition, a theory for the case of harmonic modelocking was developed, and it is shown that the supermodes reveal pulse-to-pulse correlation statistics and must be included in measurements and calculations of the timing jitter. For the case of uncorrelated timing jitter between different pulses in the laser cavity, the supermodes are predicted to have the same timing jitter spectrum as the baseband mode, and this is confirmed by measurements.

(cont.) A scheme for reducing the timing jitter of a pulse train outside of the laser cavity using group-velocity dispersion and phase modulation is described, and it is shown theoretically that a reduction in the timing jitter is possible, but only at the expense of the carrier-frequency fluctuations. It is also shown that two-photon absorption in a semiconductor mirror structure prevented pulse dropouts in a short harmonically modelocked soliton fiber laser producing picosecond pulses at 2 GHz.