Practical multisegment composite-cavity semiconductor lasers

Abstract

To provide good light sources of narrow linewidth for coherent optical communication systems we have proposed a new type of external cavity semiconductor laser with multisegment cavity structure.1 Its advantages are: (1) stable single longitudinal mode operation with high side-mode suppression ratio is easily obtained; (2) two-branch output beams standing in direction can be used. In this paper we report a practical device with multisegment cavity configuration, which basically consists of a 1.5-μm antireflection coated laser diode (LD) coupled by a graded-index (GRIN) lens to a thin Fabry-Perot etalon. The etalon is fixed on a piezoelectric transducer (PZT) and its angle of inclination can be adjusted by three fine screws for laser wavelength tuning and for optimization of alignment. The facet reflectivities of the LD, GRIN lens, and F.P. etalon are matched for achieving good single longitudinal mode oscillation. Analyses and experiments indicate that the frequency instability originates mainly from the fluctuation of the environmental temperature and mechanical disturbances in external cavity lasers.2 To reduce these effects all components are stuck on a microcrystal glass plate with low expansion coefficient and their interthermal expansions are compensated, i.e., 2αi,Li ≈ 0, where αi, and Li, are the thermal expansion coefficient and lengths of each component, respectively. The LD temperature is controlled within 0.1°C using a small Peltier cooler. All the components are put in a copper box. The device size is 32 × 26 × 65 mm3 and the actual cavity length is 2-4 cm.