Dispersion dependence of linewidth in actively mode-locked ring lasers.

We studied the spectral linewidth narrowing of wavelength-swept actively mode-locked ring lasers (AMLLs). The numerical calculations for the static-state AMLL led us to predict that anomalous dispersion would narrows the linewidth. We examined the effect experimentally using AMLL setups for normal and anomalous dispersive cavities via the normal or inverse use of a linearly chirped fiber Bragg grating. The experiment indicated that the cavity with anomalous dispersion always generates narrower linewidth lasers than its normal dispersion equivalent. The cavity with anomalous dispersion also achieved a 0.08 nm linewidth. Using the anomalous dispersion setup, we observed instantaneous linewidth broadening during wavelength sweeping. Although the coherence of AMLL decreased drastically when the sweep rate became very rapid beyond a single roundtrip, narrow-linewidth lasing was observed within a single roundtrip. In summary, we demonstrated the use of 150 kHz wavelength sweeping with a 40-nm range and a 2.7-m short-length anomalous dispersion cavity.