GaAs-AlxGa1-xAs strip buried heterostructure lasers

A new laser structure, the strip buried heterostructure (SBH) laser, using an active strip on a low-loss waveguide is described, theoretically analyzed, and experimentally characterized under both pulsed and CW operations. The waveguiding properties and laser threshold conditions are analyzed using the effective refractive index approximation. The calculated results agree fairly well with measured results. Pulsed output powers as high as 400, 230, and 130 mW/mirror without AR coating have been obtained at "catastrophic" mirror failure for SBH lasers with 10, 5, and 3 μm strips, respectively. Throughout this entire current injection range, stable transverse mode operation and excellent linearity and symmetry in light-current characteristic from each mirror were obtained. Intermodulation products were measured for 5 μm strip SBH lasers. At an average power of 4 mW and modulation depth of 80 percent, the third-order component was about 48 dB below the fundamental. Excess noise above threshold was not present. For SBH lasers with efficient injection current confinement and active strip area of
380 \times 10 \mu
m, pulsed and CW current thresholds were about 74 and 85 mA, respectively. The average external quantum efficiency was about 45 percent. CW operation has been achieved at a heat-sink temperature as high as 115°C. The current threshold has an exponential variation with temperature exp (
T/T_{0}
) where
T_{0} = 110
K. For SBH lasers with strip widths
\sim5 \mu
m, stable fundamental mode oscillation in both transverse directions was observed up to 9 times threshold. Far-field beam divergences of
8-10\deg
and
26-30\deg
were obtained in directions parallel and perpendicular to the junction plane, respectively. Single-longitudinal-mode operation up to twice threshold was obtained for SBH lasers with strip widths
\lsim5 \mu
m under both pulsed and CW operations. Relaxation oscillation and self-pulsation were not observed for SBH lasers with clean transverse modes in pulsed response up to current injection levels as high as
4 \times I_{th}
. For these particularly good diodes, self-pulsation was not observed before aging under CW operation in preliminary experiments. Aging studies have not yet been performed. Densely packed monolithic linear arrays of SBH lasers with center-to-center spacing of 15 μm were also studied. They exhibited similar laser characteristics as in single SBH lasers, but with increased maximum pulsed output power/mirror, approximately (
310 \times N
) and (
150 \times N
) mW for arrays with ∼10 and
\sim4 \mu
m strips, respectively.
N
is the number of lasers in the array. Spectral behavior of these arrays was also studied as a function of injection current and temperature.