Contents I. Introduction
Multi-Channel transmitter optical sub-assemblies (TOSAs) have attracted considerable attention with the recent increase of bit rates [1]–[4]. In 100 Gbit/s Ethernet (100 GbE) TOSAs, four optical signals generated at different wavelengths in the 1.3 band by individual laser diodes and modulated at 25 Gbit/s [5] are combined and passed into a single-mode output fiber (SMF) [6]. In order to avoid the TOSA becoming unduly large, an integration technique is indispensable. In particular, the hybrid integration of electroabsorption modulator integrated with lasers (EMLs) and a waveguide multiplexer is an attractive solution. In contrast to coupling via a direct butt-joint, coupling via a lens has the advantage of preventing laser instability caused by reflection from the end facet of the waveguide. In a TOSA with a parallel optical configuration, the lenses must be actively aligned after mounting the EMLs and the multiplexer optics, because the multiple input cores of the multiplexer cannot be moved [7]. However, aligning the lenses and fixing them by laser welding or with an adhesive is challenging due to the very tight positional tolerance required of the lens (approaching submicron order), as discussed below. This issue has been avoided by constructing the TOSA using monolithic integration of the EMLs and the multi-mode interference multiplexer [2], [3], but, depending on its manufacturing yield, this may be an expensive solution on account of the high cost of InP. Y. Baek et al. have also adopted a lens-free solution with a butt joint between the EMLs and a planar lightwave circuit (PLC) based arrayed waveguide grating (AWG) [4].
