The cavity length-dependent characteristics of compressively strained InGaAsP/InGaAsP multiple quantum well planar buried heterostructure lasers operating at λ ∼ 1.3 µm were investigated under continuous-wave mode. The uncoated 600 µm long laser exhibits Pmax = 33.6 mW and Ith = 12.9 mA at 25 °C with dλ/dT = 0.35 nm/K and dλ/dPe = 0.044 nm/mW, leading to stable beam characteristics of 19.7° (parallel) × 24.1° (perpendicular). From the inverse slope efficiency versus cavity length plot, the loss parameters of internal differential efficiency (ηi) and internal optical loss (αi) were extracted, i.e., ηi = 78% and αi = 10.6 cm−1. The transparent current density of Jtr = 0.12 kA/cm2 and modal gain of G = 49.5 cm−1 were also estimated from cavity length-dependent threshold current density measurements. Metal–dielectric Au/Ti/SiO2 layers for high reflectivity (HR) coating were analyzed using theoretical calculations and experimental results. For the HR-coated 600 µm long laser with Au (150 nm)/Ti (5 nm)/SiO2 (250 nm), Pmax increased up to 61 mW at 25 °C with a reduced Ith of 10.6 mA compared to the uncoated laser, providing an HR of about 94%.