Contents 1. Introduction
To deal with the increase of communication traffic especially in data center, 100GbE has been standardized [1]. Among them, 100GBASE-LR4 and -ER4, 400GBASE-LR8 use single-mode fibers (SMFs) as a transmission medium. Since, in these standards, LAN-WDM system with 800-GHz spacing around 1.3 µm are used, a four-wavelength multiplexer (MUX) is essential in a transmitter (for 400GBASE-LR8, two MUX is necessary). Various O-band MUXs have been demonstrated such as MMI couplers [2] and two-stage Mach-Zehnder interferometers (MZIs) [3]–[5]. Although 1by4 MMIs are simple, there is a 6-dB insertion loss. In two-stage MZIs, MZIs with free spectral range (FSR) of 3200 and 1600 GHz (approximately 18 and 9 nm at wavelength region) are cascaded in order to realize 800-GHz spaced multiplexing. Especially Si-based MUX [4], [5] is a promising candidate, considering the recent development of hybrid integration techniques on Si platform [6]. One of the problems in two-stage MZI is the control of peak wavelength position. Slight deviations in the waveguide width of MZI result in the peak wavelength fluctuation. Especially, the peak wavelength of 1600-GHz filter is very difficult to control. Since the maximum power consumption of the transmitter is severely limited, it is desirable to minimize the use of wavelength turning element (such as heater) as much as possible. Therefore, fabrication tolerant MUX is highly desired for 100/400GbE transmitter.
