Improved fiber-chip coupling using low-loss tapered fibers

Abstract

A key issue in hybrid integration of optical waveguidc devices such as plU)todiOdes, modulators or semiconductor optical amplifiers is efficient coupling of light from a fiber into (lie waveguide device. Typically, a coupling loss of 7dB occurs when a cleaved single mode fiber (SME) is used for direct coupling toa waveguide device. This large value arises from the fact that the circular modal spot (diameter about I Op iii) of the SME I Op iii is poorly matched to the small (I —3pm) elliptical mode profile of a semiconductor wavcguide structure Major improvements can he achieved by introduction of mode profile transformers such as integrated spot size converter (SSC) Ij, the employment of micro lenses 121, or tapered fibers 31. Among these approaches, the most straightforward solution to increase the coupling efficiency is to employ tapered or lcnsed SMEs is spot size reducer. 'Ibis technique neither requires sophisticated fabrication techniques nor additional lcnses. In this paper. we report on the fabrication of low—loss tapered SMI"s will] mode field diameters( MFDs) as small as 0.8 pm. Improved coupling efficiency up to 80 (3 has been achieved when coupling 1 .55piii light from tapered fibers into I .55 pm InGaAsP/InP waveguide modulators and photodetectors