## 1 Introduction  
Driven by the ever-growing field of big data applications and cloud computing paradigms, the growth of data-center traffic is increasing at a very steep rate. Up to 75% of the emerging traffic load within data centers is transferred between servers and between racks [1, 2]. To cope with the unprecedented traffic explosions and unbalanced traffic distributions, data center network (DCN) architecture design has become a major research priority, since DCs have hundreds of thousands of servers for data storage and processing [3]. The design of DCN architecture needs to satisfy several requirements, such as high bandwidth, flexibility, fast reconfiguration, scalability, dynamical adaption, and low cabling complexity [4, 5]. The conventional wired DCN architecture [6, 7] has so far been the dominant architecture in research. However, this wired architecture, constructed by millions of meters of copper and optical fibers, cannot efficiently support the aforementioned requirements due to the fundamental limitation of wired connections, which leads to issues such as wire ducting, heat dissipation, space utilization, and energy efficiency [8]. Furthermore, the scaling and upgrading of the DCN to support increasing services becomes extremely complicated, which incurs extra maintenance costs in current DC infrastructures. At the same time, the traditional wired hierarchical tree-based DCN architecture has either been oversubscribed, failing to adapt to the dynamic and unpredictable traffic outbursts, or overprovisioned, which is extremely costly and inefficient [9]. Therefore, introducing optical wireless technology into the DCN is a promising solution to address the inherent restrictions of wired DCNs [10; 11]. Firstly, if the cable complexity problems are removed, more flexible architectures can be explored for better space utilization and reduced power consumption. Secondly, the OW-DCN architecture allows simple implementation and scalability, easy relocation, and fast reconfiguration by adding or redirecting plug-and-play wireless modules on top of each rack. Moreover, benefitting from the inherent advantages of optical wireless technology, ultra-high data rates, low latency, and high capacity can be gained with low transmission power, since it exhibits negligible waveguide dispersion and almost zero attenuation in a wide unlicensed spectrum range [12; 13; 14]. Additionally, wireless on-demand links can be implemented to dynamically adapt to the burst and changing traffic outbursts. So far, only a small number of studies on OW-DCN have been conducted. Most existing approaches either focus on theoretical model simulations without sufficient experimental support [15; 16; 17; 18], or only show some preliminary experimental demonstrations with low data rates (10 Gb/s), without considering the full network architecture deployment [19; 20; 21]. The optical wireless technologies, including photonic integrated circuits [16], MEMS [10; 20], digital micro-mirror devices [21], switchable mirrors [22], and pedestal mounted transceiver modules with height and rotation control [18], have been studied for the development of OW-DCNs. However, these approaches have drawbacks, such as slow speed (milliseconds reconfiguration time), complexity control methodology, and small steering angles, which result in limited throughput and scalability. These deficiencies necessitate the innovation of OW-DCN in terms of full architecture interconnection, switching system design, and experimental verification. In this paper, a practical OW-DCN architecture based on a distributed arrayed waveguide grating router (AWGR), a semiconductor optical amplifier (SOA)-based wavelength selector (SWS), and a field-programmable gate array (FPGA)-based fast switch scheduler is proposed. The full architecture design and the optical packet switching system are experimentally demonstrated.