An ultralow-threshold 1.3-micrometer InGaAsP/InP ten-element monolithic laser array has been grown on a p-InP substrate entirely by metal-organic vapor phase epitaxy. This was achieved by carefully optimizing the strained-MQW active layer, especially the amount of strain, the well thickness, the barrier thickness, the number of wells, and the active layer width. This array has a record-low threshold current, highly uniform threshold current characteristics (1.28 plus or minus 0.07 mA; slope efficiency of 0.37 plus or minus 0.01 W/A), extremely low operating current (14 mA under 5-mW output power), and long-term reliability. It is thus suitable as a practical light source in high-density parallel optical data-link applications. In addition, best-ever continuous-wave threshold currents of 0.58 mA at 20 degrees Celsius and 1.62 mA at 90 degrees Celsius, for a long-wavelength laser, were obtained by using a short cavity (100 micrometers) with high-reflection coatings. The reduced carrier lifetime and threshold current of an n-type modulation-doped strained-MQW laser were experimentally demonstrated to drastically reduce turn-on delay time.