A seamless hybrid fiber‐wireless (HFW) architecture represents an alternative access technology that is especially beneficial for customers living in rural and underdeployed regions. As for the optical infrastructure, gigabit passive optical network (GPON) technology is widely used nowadays in Europe and around the globe. GPON is a leading technology for passive optical networks. This chapter describes the key components required to build coherent radio‐over‐fiber radio access units (RAUs) for extending the reach of a GPON network. The target of the field trial was to verify that the developed RAUs work in a real life telecommunication environment. The chapter discusses the development of RAUs for transparently bridging a GPON network using an E‐band wireless point‐to‐point link. Some novel key photonic and electronic components required for seamlessly converting the optical baseband signals to E‐band wireless signals and vice versa are developed. The low‐cost goal of the wireless GPON extension system mandates deep integration of the active components.

The ever-increasing demands in traffic fueled by bandwidth hungry applications are pushing data centers to their limits challenging the capacity and scalability of currently established transceiver and switching technologies in data center interconnection (DCI) networks. Coherent optics emerged as a promising solution for inter-DCIs offering unprecedented capacities closer to data centers and relaxing the power budget restrictions of the link. QAMeleon, an EU funded R and D project, is developing a new generation of faster and greener sliceable bandwidth-variable electro-optical transceivers and WSS switches able to handle up to 128 Gbaud optical signals carrying flexible M-QAM constellations and novel modulation techniques. A summary of the progress on the QAMeleon transponder and Reconfigurable Optical Add/Drop Multiplexer (ROADM) concepts is presented in this paper.

Novel compact fully-hermetic E-band (71-86 GHz) coherent photonic mixer (CPX), featuring a rectangular waveguide WR12 output and providing an RF output power up to +15 dBm, is reported in this work. According to our knowledge, this is the highest reported output power level radiated directly from a photonic mixer module in the E-band frequency range. The fabricated WR12-CPX allows direct optical-to-wireless conversion of optical baseband or IF-band signals, e.g. for radio-over-fiber (RoF) fronthauling in mobile communications. The module comprises an InP-based balanced-PD (BPD) chip, a GaAs HEMT MMIC medium power amplifier, and a laminate-based grounded coplanar waveguide to rectangular waveguide (GCPW-WR12) transition. The transition couples the optically generated, e.g. via heterodyning, millimeter-wave signal from the output of the BPD chip to the WR12. It is based on a double-slot antenna structure and developed on a Rogers RT/duroid 5880 laminate with a thickness of 127 µm. The presented GCPW-WR12 transition enables the development of fully-hermetic photonic packages, which is required to improve the durability of the BPD chip. The transition design is optimized by utilizing a 3D electromagnetic simulation software for achieving a wide operational bandwidth with an average insertion loss (IL) of about 1.6 dB and a return loss (RL) higher than 15 dB in the frequency range of 71-86 GHz. Finally, the RF responsivity of the WR12-CPX module and the hermeticity of the transition are experimentally characterized.

Compact planar laminate-based transitions for integrating high-frequency photodiodes (PDs) with rectangular waveguides (WRs) are presented for the WR15 to WR1 standard waveguide bands, i.e., from 0.05 THz up to 1.1 THz. The transitions couple the optically generated (e.g., via heterodyning) millimeter-wave or terahertz signals from the grounded coplanar waveguide (GCPW) output of the PD chip to the WR. To our knowledge, this is the first scalable integration concept that enables hermetic photodiode packaging up to the terahertz frequency range. For the WR15-WR6 bands, all transitions are designed on ultrathin microfiber reinforced PTFE composites (127-μm Rogers RT/duroid 5880 laminate). For the WR5-WR2.2 and the WR1.5-WR1 bands, liquid crystalline polymer ULTRALAM 3850 laminates are used with a thickness of 50 and 25 μm, respectively. The proposed GCPW-WR transition design is based on a double-slot antenna structure and enables the development of fully hermetic photonic packages, which is required to improve the durability of the PD chip. The transition designs are optimized by systematic EM-wave propagation modeling for achieving a wide operational bandwidth of up to 30% of the respective center frequency for each WR band. The optimized transitions exhibit an average insertion loss of about 1.5 dB and a return loss of about 10 dB for all waveguide bands (WR15-WR1). Based upon the systematic numerical modeling, generic guidelines are developed that allow designing a specific transition for any given waveguide band to 1.1 THz. In addition to the numerical analysis, GCPW-WR12 transitions for E-band (60-90 GHz) operation are fabricated and integrated with InP-based balanced-PD chips and GaAs HEMT MMIC medium power amplifiers in a fully hermetic package. Furthermore, hermetic WR12 coherent photonic mixer (CPX) modules are developed. The packaged WR12-type CPX allows direct optical-to-wireless conversion of optical baseband or IF-band signals, e.g., for radio-over-fiber fronthauling in mobile communications. The WR12-CPX module provides RF output power of up to +15 dB·m at 77.5 GHz.

This paper presents the design and measurements of a 32-Gb/s differential-input differential-output transimpedance amplifier (TIA) employed in dual polarization integrated coherent receivers for 100-Gb Ethernet. A circuit technique is shown that uses a replica TIA to stabilize the operating point of the two shunt-feedback input stages as well as to cancel the dc part of the two complementary input currents and balances their offset. The TIA can be operated in two modes, an automatic gain control mode to retain a good total harmonic distortion (THD) over a wide dynamic range and a manual gain control mode. Electrical as well as optical-electrical characterization of the TIA are presented. It achieves a maximum differential transimpedance of 74 dB $\Omega $ , 33 GHz of 3-dB bandwidth, 12.2 pA/ $\sqrt {\text {Hz}}$ of average input-referred noise current density with the photodiode, 900 mVpp of maximum differential output swing, less than 1% of THD for 600 mVpp differential output swing, and 500 $\mu \text{A}_{\text {pp}}$ differential input current. The linearity of the TIA is furthermore demonstrated with PAM4 measurements at 25 Gbaud. The dual TIA chip is fabricated in a 0.13- $\mu \text{m}$ SiGe:C BiCMOS technology, dissipates 436 mW of power and occupies 2 mm2 of area.

We report on a coherent RoF architecture for connecting multiple ONUs in a GPON network via a 71–76 GHz E-band wireless link to the OLT. Novel coherent photonic mixer (CPX) modules providing up to +17 dBm RF output power in the E-band are used for direct optic-to-RF conversion.

In this paper, we report on a field trial, demonstrating the seamless wireless extension of a real-world 2.5 Gbit/s Gigabit passive optical network (GPON) using a hybrid fiber wireless (HFW) bridge. Direct optic-to-RF conversion using a novel coherent photonic mixer (CPX) and direct RF-to-optic radio access units (RAU) were developed and employed to extend the reach of the GPON network using a 455 m long-distance 73.5 GHz millimeter-wave point-to-point (PTP) wireless link. The wireless GPON bridge was synchronized to the downlink transmission line rate of 2.5 Gbit/s. In the field trial, as total downlink traffic of about 2.5 Gbit/s was transmitted over SMF and the wireless bridge between the OLT and three ONU. The total uplink traffic from two ONUs to the OLT was about 1.2 Gbit/s.

The papers in this special section were presented at the 42nd European Conference on Optical Communication (ECOC 2016). This conference is the largest conference on optical communication in Europe, and one of the largest and most prestigious events in this field worldwide. Optical Communication is the technological basis of the global internet, which transforms the world into an information and communication society where the information is available everywhere at every time.

We demonstrate error-free 80-km transmission of a 400-Gb/s channel in a colorless coherent C-band WDM system using a high-bandwidth micro-ICR. The WDM channels are colorlessly combined at the transmitter and colorlessly split/detected at the receiver.

A hybrid optical transmitter module comprising a 15-segment InP in-phase/quadrature-phase segmented Mach–Zehnder modulator and two SiGe:C BiCMOS drivers featuring integrated 4-b digital-to-analog converter functionality is described. The drivers, fabricated in the 0.13- $\mu \text{m}$ process of IHP, deliver a differential output voltage of $2.5~V_{\mathrm{ pp}}$ across all the 15 segments while dissipating less than 1 W of power each, at the maximum. Clear electrical eye diagrams up to 40-Gb/s from every output are reported. The module allows a wide range of modulation formats, among which up to eight-pulse amplitude modulation and polarization-division-multiplexed (PDM) 64-quadrature amplitude modulation (QAM) back-to-back error-free electro-optical transmission at a record speed of 32 GBd are demonstrated. The 32-GBd PDM 64-QAM signal was transmitted error-free over 80 km of standard single-mode fiber, featuring 7.8-pJ/b energy consumption. The devised hybrid arrangement proves the suitability of SiGe HBT drivers for achieving higher speeds over their CMOS counterparts, with comparable low power dissipation, for advanced optical transceivers.