Long-wavelength InGaAsP/InP multiquantum well distributed feedback and distributed Bragg reflector lasers grown by chemical beam epitaxy

We demonstrated the successful operation of long-wavelength InGaAsP low threshold-current index-coupled and gain-coupled DFB lasers grown by chemical beam epitaxy (CBE). For index-coupled DFB lasers, buried-heterostructure six-QW DFB lasers (250 μm long and as-cleaved) operated at 1.55 μm with CW threshold currents 10-15 mA and slope efficiencies up to 0.35 mW/mA (both facets). A side-mode suppression ratio (SMSR) as high as 49 dB was obtained. The lasers operated in the same range even at high temperatures (70°C checked). For gain-coupled DFB lasers, gain-coupling is accomplished by using a InGaAsP quaternary grating or quantum-well grating that absorbs the DFB emission. The use of a quantum-well grating, in particular, greatly facilitates the reproducible regrowth (defect-free) over grating and the control of the coupling coefficient. CW threshold currents mere in the range of 10-15 mA for 250-μm and 13-18 mA for 250-μm and 500-μm cavities, respectively. Slope efficiencies were high, ~0.4 mW/mA (both facets). SMSR was as high as 52 dB and remained in the same DFB mode with SMSR staying ~50 dB throughout the entire current range. Linewidth×power products of 1.9-4.0 mere measured with minimum linewidths of 1.8-2.2 MHz. No detectable chirp was measured under 2.5 Gb/s modulation. Unlike index-coupled DFB lasers in which mode partition events decrease slowly even when biased above threshold, these lasers have mode partition events shut off sharply as bias approaches threshold (≳0.95 I_th). A very small dispersion penalty of 1.0 dB was measured at 10^-11 BER in transmission experiments using these lasers as sources at 1.7 Gb/s over an amplified fiber system of 230 km. No self-pulsation was observed in these gain-coupled DFB lasers. Gain-switching at 4 GHz with a 100% optical modulation depth and a FWHM pulse width of 23 ps was achieved with these gain-coupled DFB lasers. The peak power was ~72 mW and the FWHM bandwidth was 0.14 nm. We also fabricated InGaAs/InGaAsP multiquantum-well DBR lasers by CBE. Taking advantage of uniform thickness growth and proper design of weak and long gratings, a record high SMSR of 58.5 dB was obtained