Laser diode module with distributed feedback, single-mode fibre waveguide, and optical isolator

The design of a module of a distributed-feedback laser diode is proposed. The diode emits at the wavelength 1.55 μm and is efficiently coupled by a two-lens configuration to a single-mode fibre waveguide. The diode characteristics are stabilised by placing an optical isolator between the matching lenses. The isolator consists of two birefringent prisms and an yttrium iron garnet plate surrounded by an Nd–Fe–B ring magnet. The efficiency of coupling the diode radiation into the single-mode fibre exceeds 30% even after the isolator losses are taken into account. The measured degree of isolation produced by this device is at least 35 dB. An experimental dependence of the relative intensity of noise I_RIN on the degree of backreflection was determined for a distributed-feedback laser diode matched to a single-mode fibre. The intensity I_RIN of the investigated module was –140 dB Hz^-1.