High-performance 1.55-/spl mu/m wavelength GaInAsP-InP strongly index-coupled and gain-matched distributed-feedback (DFB) lasers with periodic wirelike active regions mere fabricated by electron beam lithography, CH/sub 4//H/sub 2/-reactive ion etching, and organometallic vapor-phase epitaxial regrowth, whose index-coupling coefficient was more than 300 cm/sup -1/. In order to design lasers for low threshold current operation, threshold current dependences on the number of quantum wells and the wire width mere investigated both theoretically and experimentally. A record low threshold current density of 94 A/cm/sup 2/ among 1.55-/spl mu/m DFB lasers was successfully obtained for a stripe width of 19.5 /spl mu/m and a cavity length of 600 /spl mu/m. Moreover, a record low threshold current of 0.7 mA was also realized at room temperature under CW condition for a 2.3-/spl mu/m-wide buried heterostructure with a 200-/spl mu/m-long cavity. Finally, we confirmed stable single-mode operation due to a gain-matching effect between the standing-wave profile and the wirelike active region.