Contents I. Introduction
100Gbit Ethernet (100GbE) is being strongly promoted as a way of dealing with the recent rapid increase in the data rate of data communication systems. The 100GbE specifications stipulate the use of 4-channel 25-Gbit/s signals over four wavelengths in the band, and we have developed a monolithically integrated light source array (MILSA) that consists of four DFB lasers, four electroabsorption modulators (EAMs) operating at 25 Gbit/s and one 4‒1 multi-mode interference coupler (MMI) as a wavelength multiplexer (MUX) [1]–[3]. The wavelength interval of each DFB laser is 800 GHz (approximately 4.5 nm in the band) in accordance with the 100GbE specifications. Our MILSA for DFB lasers integrated with EAMs (EADFB laser array) shows good 40-km-transmission characteristics thanks to the clear EAM waveforms. However, an issue with the EADFB laser array chip is that it has an on-chip loss of about 7 dB due to the MUX, including the principal 6-dB loss of the 4‒1 MMI. Since such MUX-induced loss is common in MILSAs, MUXs are key devices in various kinds of MILSAs, including both our 100GbE light source and other wavelength division multiplexing light sources such as tunable lasers. A transversal filter (TF) based on two MMIs is a possible candidate as a MUX device. An MMI-based TF has already been demonstrated as a demultiplexer (DEMUX) for an optical orthogonal frequency division multiplexing receiver on a silica planar lightwave circuit [4]. It has the advantage of a compact size compared with arrayed waveguide grating filters (AWGs) when the number of channels is limited to several. But, we should make it smaller not to increase the chip size of our conventional MILSA using a 4‒1 MMI as a MUX.
