Fast and high capacity optical switching techniques have the potential to enable low latency and high throughput optical data center networks (DCNs) to afford the rapid increasing traffic boosted by multiple applications. Flexibility of the DCN is of key importance to provide adaptive and dynamic bandwidth and capacity to handle the variable traffic patterns of heterogeneous applications. Aiming at improving the network performance and the system flexibility of optical DCNs, we propose and investigate a novel optical DCN architecture named ROTOS based on reconfigurable optical top of rack (ToR) and fast optical switches. In the proposed DCN architecture, the novel optical flexible ToRs employing multiple transceivers (TRXs) and a wavelength selective switch (WSS) are reconfigured by the software-defined networking (SDN) control plane. The bandwidth can be dynamically allocated to the dedicated optical links on-demand according to the desired oversubscription (OV) and intra-/inter-cluster traffic matrix. Numerical investigations of the novel architecture under realistic traffic model indicate that dynamically allocating the TRXs and elastically controlling the WSS, a packet loss below 1E-5 and a server-to-server latency lower than 3 μs can be guaranteed for different traffic patterns at load of 0.4. With respect to the DCN with static interconnections, the average packet loss of ROTOS decreases two orders of magnitude and the average server-to-server latency performance improves by 21.5%. Scalability investigation to a large number of servers shows limited (11%) performance degradation as the network scale from 2560 to 40960 servers. Additionally, the dynamic bandwidth allocation of the DCN is experimentally validated. Network performance results show a packet loss of 0.05 and 5.85 μs end-to-end latency at the load of 0.8. Finally, investigations on the cost and power consumption confirm that the ROTOS DCN architecture has 28.4% lower cost and 35.0% better improvement for power efficiency with respect to the electrical switch based DCNs.

Low polarization-dependent semiconductor optical amplifiers (SOA) co-integrated with passive circuits via an easy fabrication process are highly demanded in optical communication systems. However, despite several SOAs have been reported, co-integration of polarization-independent bulk SOA with passive waveguides in the InP platform to generate complex systems remains an issue. In this work, we design, simulate, fabricate and characterize a low polarization-dependent SOA based on a bulk active layer that is co-integrated with passive waveguides and exhibits its potential application to realize photonic integrated circuits. Simulation results are in good agreement with the experimental results. Based on the simulation results, the designed SOAs with the length of 300 μm, 750 μm, and 1500 μm cover broadband with 3-dB bandwidth of around 65 nm, 40 nm, and 37 nm, respectively. Characterization of SOAs with different lengths between 300 μm and 1500 μm co-integrated with passive waveguide has been performed. Broadband gain and polarization-dependent gain (PDG) less than 2 dB is measured for 300 μm SOA. Also, at 1550 nm, output saturation power of 12.8 dBm is achieved at the current density of 11.6 kA/cm². For the longest SOA with 1500 μm length, the higher gain of 16 dB with PDG lower than 2.5 dB and saturation power of 7.4 dBm at the wavelength of 1588 nm and current density of 4.1 kA/cm² are achievable. Thus, one could design SOA based photonic circuits with higher gain and saturation power and low PDG by exploiting different SOA lengths and design at the wavelength of interest, opening to potential application in photonic integrated circuits like low-loss and polarization-insensitive switches.

We assess the performance of a modular photonic integrated circuit (PIC) for space and time optical switching as an optical add-drop node for metro-access network applications. The PIC based on semiconductor optical amplifier (SOA) technology enables the implementation of compact, low cost and lossless add-dropping through fast optical switching. Data integrity measurements demonstrate error-free switching operation with less than 1 dB and 2.5 dB power penalty for 10 Gb/s and 40 Gb/s NRZ OOK optical signal, respectively. The dynamic control of the SOA based optical gates by the switch controller permits the power equalization of the switched channels. The experimental results also demonstrate dynamic forwarding, dropping and multicasting operation for a packetized 10 Gb/s NRZ OOK optical signal. The transparency of the SOA technology towards the optical signal’s data rate allows to avoid costly and power consuming dedicated interfaces at the add-drop node and open a path towards efficient statistical multiplexing.

The rapid increasing traffic in data centers (DCs) puts tremendous pressure to the present multi-tier network architectures and electrical switching techniques. Switching traffic in the optical domain featuring ultra-high bandwidth, therefore, has been intensively investigated to build the high capacity data center networks (DCNs). To handle the variable traffic pattern in DCs, the network reconfigurability with adaptable optical bandwidth allocation is of key importance to flexibly assign the optical bandwidth. To this end, we propose and experimentally evaluate a software-defined networking enabled reconfigurable optical DCN with dynamic bandwidth allocation in this work, based on novel optical top of racks exploiting a wavelength selective switch. Experimental assessments show that the proposed solution can automatically reallocate the optical bandwidth in real-time to adapt the dynamic traffic pattern. Compared with the conventional optical DCN with static bandwidth provision, the end-to-end latency performance of the reconfigurable scheme with adaptable bandwidth allocation improves of 58.3% and the average packet loss decreases one order of magnitude. Moreover, the reconfigurable optical DCN features deterministic latency performance, with much lower time variations of packets delivery completion. Based on the experimental parameters, the simulation platform is also built to validate the good scalability of the proposed reconfigurable DCN. Numerical results illustrate the negligible performance degradation (11%) as the network scales from 2560 to 40 960 servers.

Due to the lack of optical buffer, high packer loss caused by packet contention is one of the main challenges for the optical switching data centers (DCs). Flow control (FC) protocol employing electrical buffers at the top of racks (ToRs) and exploiting packet retransmission mechanism in case of contention has been extensively investigated to decrease the packet loss in optical DCs. However, the packet retransmission and the head-of-line (HOL) blocking in electrical buffers at substantial load traffic introduce extra latency and buffer-overflow that still cause packet loss. To overcome these issues, a novel contention resolution technique based on a software-defined networking (SDN) enabled optical polling flow control is proposed and experimentally assessed in this paper. Experimental assessments show that the proposed contention resolution scheme achieves zero packet loss and 7.4 μs latency performance at the load of 0.4. In addition, we numerically modelled and investigated the scalability of the proposed contention resolution technique in a large scale DCN based on the experimental parameters. Results prove the excellent scalability performance of OPFC scheme, in which the packet loss increases from 2.1E-3 to 9.02E-3 and the average latency increases 5.17 μs at the load of 0.5 as the OPFC based network scales from 4 to 40960 servers.

The system performance of a photonic integrated WDM wavelength selective switch is assessed with 10, 20 and 40Gb/s NRZ-OOK. Results show lossless operation, limited penalty for 8 WDM channels and > 8dB power dynamic range.