Contents 1. Introduction
Boosted by 5G, IoT, and Big Data applications, data traffic and scale out of data center are steadily increasing. With the aim to satisfy the scalable growth in both network traffic volume and connected endpoints while decreasing the cost and the energy consumption, transparent optical DC networks (DCNs) based on fast optical switches have been investigated which could provide high bandwidth and agile (nanosecond) reconfigurability [1] –[3]. Several optical switch architectures have been presented. An example of a modular optical switch [2] is shown in Fig. 1(a). The switch consists of N 1xN multicast and select switches (MCSS) modules, which broadcast the signal to the N possible outputs and use N optical gates to select at which of the N outputs the signal will be forwarded, and N 1xN power combiners. Practical implementation would require the photonic integration of hundreds or even more of optical passive-active components, such as splitters, optical gates, and optical amplifiers to compensate the broadcast losses. Compact, broadband, polarization-insensitive WDM MCSS and amplification based on SOAs can be realized on the InP platform owing to the passive-active co-integration process. It involves the co-integration of active SOAs with several passive elements such as straight/bend waveguides and multimode interferometers (MMIs) which functions as power splitter/couplers. Polarization-insensitive (PI) operation of MCSS is an important feature as the incoming signals to the different ports of the optical switch have no predefined polarization state, as well as the switch can support dual-polarization modulation formats to increase the capacity of the network. Up to now, few MCSSs based on SOAs have been reported in both monolithic and hybrid platforms [3]–[4], but no polarization-independent operation of the MCSS based on bulk SOAs has been demonstrated. Fabrication of MCSS based on bulk SOA is robust and easier, also the designed bulk SOAs are polarization insensitive in wide range of current and wavelength which makes them superior to their counterparts such as MQW and QD SOAs [5] –[7].
