Laser oscillation of an organic distributed-feedback laser at the edge of a mini stopband

Abstract The conditions for determining the oscillation wavelength of organic distributed-feedback (DFB) lasers were investigated by employing 5,5″-bis(biphenyl-4-yl)-2,2′:5′,2″-terthiophene (BP3T) single crystals with various thicknesses as an active layer. Analysis using experimentally obtained refractive indexes revealed that the oscillation wavelengths of some BP3T DFB lasers are at the edge of a stopband, whereas the others are at an edge of a mini stopband, which satisfies the phase-matching condition among different transverse modes and has not been considered in DFB lasers so far. The coupling of different transverse modes was verified by finite-element optical-waveguide simulations of the BP3T DFB laser.