Complete AM and residual PM noise measurements of an external cavity semiconductor laser hybridly modelocked at 10 GHz

Abstract

Modelocked semiconductor and fiber lasers have been shown to operate with very low noise levels making them suitable as sources of sampling pulses for applications in analog-to-digital converter (ADC) technologies. In spite of the need for long cavities in fiber lasers, obtaining multigigahertz sampling rates has been possible by harmonic modelocking at high integer multiples of the relatively low fundamental cavity frequencies. Semiconductor lasers, on the other hand, have the capability of operating in both regimes (cavities can be either long or short). For harmonically modelocked lasers, noise spurs will arise at harmonics of the cavity frequency, while fundamental modelocking has the ability to eliminate these spurs. At multigigahertz sampling rates, every doubling in the sampling frequency causes the ADC resolution to fall by /spl sim/1 bit due to phase modulation (PM) noise (timing jitter) in the sampler. Accurate evaluation of pulse train noise then becomes an important issue. Commonly, reported measurements of PM noise extend out to limited offset frequencies (several tens of megahertz or lower). To consider the noise characterization complete, however, sideband measurements should extend to the highest offset frequencies possible (i.e. the Nyquist frequency). For harmonically modelocked lasers, this offset range will include noise spurs arising from harmonic beating mechanisms. We report measurements of AM and residual PM noise out to the Nyquist offset frequency (5 GHz) of a hybridly modelocked external cavity semiconductor laser for fundamental cavity frequency modelocking as well as harmonic modelocking.