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 mum long and as-cleaved) operated at 1.55 mum 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-degrees-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 were in the range of 10-15 mA for 250-mum and 13-18 mA for 250-mum and 500-mum cavities, respectively. Slope efficiencies were high, approximately 0.4 mW/mA (both facets). SMSR was as high as 52 dB and remained in the same DFB mode with SMSR staying approximately 50 dB throughout the entire current range. Linewidth x power products of 1.9-4.0 were 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 (greater than or similar to 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 approximately 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.