Compact Hybrid Integrated 100-Gb/s Transmitter Optical Sub-Assembly Using Optical Butt-Coupling Between EADFB Lasers and Silica-Based AWG Multiplexer

A compact hybrid-integrated 100 Gb/s (4 lane × 25.78125 Gb/s) transmitter optical sub-assembly (TOSA) has been developed for a 100 Gb/s transceiver for 40-km transmission over a single-mode fiber. The TOSA has a simple configuration in which four electro-absorption modulator-integrated distributed feedback (EADFB) lasers are directly attached to the input waveguide end-face of a silica-based arrayed waveguide grating (AWG) multiplexer without bulk lenses. To achieve a high optical butt coupling efficiency between the EADFB lasers and the AWG multiplexer, we integrated a laterally tapered spot-size converter (SSC) for the EADFB laser and employed a waveguide with a high refractive index difference of 2.0% for the AWG multiplexer. By optimizing the laterally tapered SSC structure, we achieved a butt-coupling loss of less than 3 dB, which is an improvement of around 2 dB compared with a laser without an SSC structure. We also developed an ultracompact AWG multiplexer, which was 6.7 mm × 3.5 mm in size with an insertion loss of less than 1.9 dB. We achieved this by using a Mach–Zehnder interferometer-synchronized configuration to obtain a low loss and wide flat-top transmission filter spectra. The TOSA body size was 19.9 mm (L) × 6.0 mm (W) × 5.8 mm (H). Error-free operation was demonstrated for a 40-km transmission when all the lanes were driven simultaneously with a low EA modulator driving voltage of 1.5 V at an operating temperature of 55 °C.

I. Introduction

More compact and cost-effective optical transceivers are needed for 100 Gb/s Ethernet (100 GbE) to deal with the exponential growth in data traffic generated in telecommunication networks and data center networks. 100 GbE, standardized in IEEE 802.3ba [1], has a long reach specification for 10 km or 40 km transmission over single-mode fiber (SMF), namely 100 GBASE-LR4 or 100 GBASE-ER4. The specification defines a multi-lane configuration that has four 25.78125 Gb/s lanes with local area network wavelength division multiplexing (LAN-WDM), which is assigned a wavelength channel spacing of 800 GHz in the 1.3 μm wavelength band. A transceiver for 100 GbE is also defined in the multi-source agreement (MSA). To further reduce both the body size and the power consumption of the 100 GbE transceiver, a CFP4 transceiver [2] defined in the centum form-factor pluggable MSA or a QSFP28 transceiver [3] defined in the quad small form-factor pluggable MSA has been enacted. However, with a multi-lane configuration it is complicated to fabricate such a compact transceiver because we have to further miniaturize the optical components installed in the transceiver such as the transmitter optical sub-assembly (TOSA) and the receiver optical sub-assembly (ROSA). Nevertheless, the multi-lane configuration must be a promising approach even for the next generation 400-Gb/s Ethernet (400 GbE), which is currently being discussed by an IEEE task force [4]. Therefore, an integration technique that satisfies the increasing lane number is an important technical issue for a future TOSA and ROSA.