Demands for large-capacity high-density transmission in parallel optical interconnection networks, such as computer networks, data-link networks, and switching systems, are growing rapidly. Optical transmission technologies for parallel optical-data-link applications have therefore attracted a great deal of attention. The factors that affect optical technologies differ considerably from those affecting optical-fiber transmission systems. The key requirements are small-skew operation, low power consumption, integration technology, high reliability, and low cost. Optical device technologies may have to be developed specifically for this area of application to satisfy such requirements. Semiconductor laser arrays, used as light sources in parallel optical interconnection, are a key component in this field. In this paper, the key issues concerning our 1.3-μm InGaAsP/InP n-type modulation-doped strained-MQW monolithic laser arrays on a p-InP substrate used in high-density parallel optical interconnection are reviewed to aim at meeting these needs. The most recently reported results and future prospects for laser performance and technologies are examined with an emphasis on careful optimization of a strained-MQW active layer, the design of the laser cavity, the modulation-doped effect, narrow beam divergence, and uniform characteristics.