Dispersion and noise of 1.3 μm multimode lasers in microwave digital systems

The intensity noise and the performance in a 1.7 Gb/s digital system of 1.3 μm InGaAsP multilongitudinal mode lasers is discussed. The total intensity noise, mode partitioning, and the impact of dispersion on optical noise are measured. It is found that under CW conditions the total simulated emission from unpackaged lasers is inherently quiet, with an integrated optical signal-to-noise ratio (SNR) of 26.8±1.5 dB over a bandwidth of 1.5 GHz and 5 mW/facet. The optical SNR decreased as a function of increasing reflection. Intense mode partitioning decreased the SNR of the main mode by ~20 dB and reduced the effective coherence length to ~2 cm in glass fiber. At 1.7 Gb/s, the power penalities associated with laser bias and fiber dispersion are reported. The best receiver sensitivity is obtained when the laser is biased 1.3 mA below threshold. In general, it is found that as the bit rate increases, the optimum transmitter design becomes progressively more restrictive