###### Abstract  
Data centers are crucial to the growth of cloud computing. Next-generation data center networks (DCNs) will rely heavily on optical technology. Here, we have investigated a bidirectional wavelength-division-multiplexed (WDM) free space optical communication (FSO) system for deployment in optical wireless DCNs. The system was evaluated for symmetric 10 Gbps 16\(-\)quadrature amplitude modulation (16-QAM) intensity-modulated orthogonal frequency-division multiplexing (OFDM) downstream signals and 10 Gbps on-off keying (OOK) upstream signals, respectively. The transmission of optical signals over an FSO link is demonstrated using a gamma-gamma channel model. According to the bit error rate (BER) results obtained for each WDM signal, the bidirectional WDM-FSO transmission could achieve 320 Gbps over 1000 m free space transmission length. The results show that the proposed FSO topology offers an excellent alternative to fiber-based optical interconnects in DCNs, allowing for high data rate bidirectional transmission. free space optics; wavelength division multiplexed; wireless data center networks 20222229703  
(WDM) technology that provides a high fan-out (i.e., a significant number of channels per FSO connection) and elastic topologies [9]. Indoor FSO links can achieve superior performance at longer distances due to the customary conditions of DCNs. They are less subject to optical channel impairments such scintillation, alignment problems, and atmospheric turbulence [5]. Recently, FSO-based systems have been investigated for deployment as access networks, both for fronthaul and backhaul [10]. Such systems have several advantages, including high bandwidth, spectrum utilization without a license, reduced power consumption, smart hardware, as well as immunity against electromagnetic interference (EMI). Moreover, FSO systems are less expensive to deploy than optical fiber-based communication systems. There are significant set-up and maintenance costs and time associated with optical access networks, especially passive optical networks (PONs) and radio over fiber (RoF) systems. Therefore, FSO communication systems are expected to significantly reduce both deployment cost and time [4; 5]. FSO is mainly impacted by atmospheric attenuation and random variation of signal intensity and phase caused by atmospheric turbulence, known as intensity scintillation [11]. This phenomenon is comparable to multipath fading in radio frequency (RF) wireless communication and is caused by random fluctuations in the refractive index of the atmosphere resulting from variations in temperature and pressure [12]. Various FSO statistical channel models are used for turbulence estimation, such as the log-normal model, K model, negative exponential model, gamma-gamma model, and log-normal Rician model [13]. The gamma-gamma FSO model is commonly used to simulate atmospheric turbulence and attenuation, with characteristics ranging from mild to strong [4]. FSO systems have been extensively investigated in recent years. The performance of an FSO link based on on-off keying (OOK) coding has been investigated and improved by employing a coherent detection and dynamic decision threshold in [14]. An efficient blind detection of OOK signals utilizing an instantaneous channel fading coefficient is proposed in [15]. A visible light FSO link employing a hybrid intensity modulation and direct detection (IM/DD) system is proposed in [16]. The spectral efficiency of the system has been enhanced by integrating the asymmetrically clipped optical orthogonal frequency division multiplexing (OFDM) and OOK modulation formats. An FSO link employing hybrid modulation formats such as quadrature amplitude and multi-pulse position modulation techniques has been studied with and without intensity turbulence [17]. [PERSON] and [PERSON] [18] investigated the performance of an FSO link based on a differential phase shift keying (DPSK) modulation format with and without incorporating Manchester coding.