## 1 Introduction  
Currently deployed optical networks are able to dynamically allocate network resources with a granularity of a wavelength channel in the so-called _fixed grid optical networks_[1], or, more recently, with some variability on the spectral width of each wavelength channel and adapting the modulation format in the _flexible/elastic optical networks_[2]. All these networks follow the connection oriented paradigm [3] and hence may collectively be referred to as optical circuit switching (OCS) networks [4]. Compared to static optical networks, where no optical switching is done, the flexibility in the wavelength dimension enables a better utilization of the network resources, a reduction of the energy consumption and lowering the costs [5]. In spite of this, it is expected that the increased demand of network capacity, which is likely to grow steadily over the following years [6], will push the requirements of optical networks even further. In order to meet these challenges, further increasing of the network granularity may be the key point to enhance the network utilization, reduce the energy consumption and lower the costs. In the past, the need to increase the network granularity motivated the emergence of optical packet switching (OPS) and optical burst switching (OBS) networks [7]. However, both OPS and OBS networks never abandoned the academic circles due to its great implementation complexity and the ability of electronical switching to effectively compete with the benefits promised by OPS/OBS. Recently, research attention has been focused on hybrid optical networks [4]. These hybrid optical networks combine OCS _flexible/elastic lightpaths_ for network services that require high availability with OPS _datagrams_[3] that deliver optical packets on a \"best effort\" approach. Hybrid OCS/OPS networks may make use of standard single mode fibers (SMFs) [4] or multicore fibers [8]. Moreover, OPS subsystems such as the burst-mode receiver with non-coherent on-off keying (OOK) modulation format are currently used in the upstream of optical access networks such as passive optical networks (PONs) [9] and upgraded coherent burst-mode receivers may play an important role in future datacenter networks [10; 11]. Whether for hybrid OCS/OPS networks, PONs or datacenter networks, fast optical switching technologies and digital coherent burst-mode receivers are going to be essential technologies to leverage the development of optical networking over the next years. Previously, extensive research has been carried out in our group on OPS-related subsystems. Our OPS node architecture has an all-optical data plane that is made of optical switches and fiber delay lines (FDLs). Owing to the optical switch transparency, optical packets with different data rates and high-order modulation formats can be handled without changing the OPS node architecture. Figure 1 shows an evolution of the OPS node capacity per port over time. Employing differential quaternary shift keying (DQPSK) modulated, fixed length, colored optical packets over 64 wavelength channels and a (Pb,La)(Zr,La)O\({}_{3}\) (PLZT) [12] optical switch with packet buffering, a switching capacity of 1.28 Tb/s/port was demonstrated [13]. Using polarization division multiplexing (PDM)-DQPSK modulation format, we demonstrated a switching capacity of 1.28 Tb/s/port with 32 wavelength channels [14] and later 2.56 Tbit/s/port with 64 wavelength channels [15; 16]. In this paper, we will focus on the state-of-the-art optical switching subsystems that make use of wavelength division multiplexing (WDM) optical packets, advanced coherent modulation formats such as PDM-QPSK and PDM 16 quadrature amplitude modulation (QAM) and PLZT optical switches capable of switching multi-level QAM optical signals. On Section 2, we investigate the requirements of digital coherent burst-mode receivers (DCBMRXs) and describe a novel digital coherent burst-mode receiver that makes use of a state of polarization (SOP) estimator based on the Stokes parameters for rapid packet demodulation [17].