Hybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter-wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide untethered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. In this paper, we present an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. We also review the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Fiber-Wireless Networks and Subsystem Technologies

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

Erbium-doped fiber devices have been extraordinarily successful due to their broad optical gain around 1.5–1.6 μm. Er-doped fiber amplifiers enable efficient, stable amplification of high-speed, wavelength-division-multiplexed signals, thus continue to dominate as part of the backbone of longhaul telecommunications networks. At the same time, Er-doped fiber lasers see many applications in telecommunications as well as in biomedical and sensing environments. Over the last 20 years significant efforts have been made to bring these advantages to the chip level. Device integration decreases the overall size and cost and potentially allows for the combination of many functions on a single tiny chip. Besides technological issues connected to the shorter device lengths and correspondingly higher Er concentrations required for high gain, the choice of appropriate host material as well as many design issues come into play in such devices. In this contribution the important developments in the field of Er-doped integrated waveguide amplifiers and lasers are reviewed and current and future potential applications are explored. The vision of integrating such Er-doped gain devices with other, passive materials platforms, such as silicon photonics, is discussed.

Erbium-doped integrated waveguide amplifiers and lasers

Polarization management is very important for photonic integrated circuits (PICs) and their applications. Due to geometrical anisotropy and fabrication inaccuracies, the characteristics of the guided transverse-electrical (TE) and transverse-magnetic (TM) modes are generally different. Polarization-dependent dispersion and polarization-dependent loss are such manifestations in PICs. These issues become more severe in high index contrast structures such as nanophotonic waveguides made of silicon-on-insulator (SOI), which has been regarded as a good platform for optical interconnects because of the compatibility with CMOS processing. Recently, polarization division multiplexing (PDM) with coherent detection using silicon photonics has also attracted much attention. This trend further highlights the importance of polarization management in silicon PICs. The authors review their work on polarization management for silicon PICs using the polarization independence and polarization diversity methods. Polarization issues and solutions in PICs made of SOI nanowires and ridge waveguides are discussed.

Polarization management for silicon photonic integrated circuits

The wavelength-division-multiplexed passive optical network (WDM-PON) is considered to be the next evolutionary solution for a simplified and future-proofed access system that can accommodate exponential traffic growth and bandwidth-hungry new applications. WDM-PON mitigates the complicated time-sharing and power budget issues in time-division-multiplexed PON (TDM-PON) by providing virtual point-to-point optical connectivity to multiple end users through a dedicated pair of wavelengths. There are a few hurdles to overcome before WDM-PON sees widespread deployment. Several key enabling technologies for converged WDM-PON systems are demonstrated, including the techniques for longer reach, higher data rate, and higher spectral efficiency. The cost-efficient architectures are designed for single-source systems and resilient protection for traffic restoration. We also develop the integrated schemes with radio-over-fiber (RoF)-based optical-wireless access systems to serve both fixed and mobile users in the converged optical platform.

Key Technologies of WDM-PON for Future Converged Optical Broadband Access Networks [Invited]

We propose converged optical-wireless access networks for future wireless HD services delivery over optical fiber and air links. Key technologies including long reach, flexible reconfigurability, and real SD/HD uncompressed video implementation are experimentally demonstrated.

Wireless HD services over optical access systems: Transmission, networking, and demonstration

For the first time, a long-reach 400G transmission proposal based on quad-carrier PM-QPSK with a net spectral efficiency of 4 b/s/Hz was experimentally demonstrated. Under the assumption of narrowing100-GHz bandwidth after passing through multiple optical nodes, 512-Gb/s quad-carrier PM-QPSK signals were successfully delivered over 2400-km SMF-28 with EDFA-only amplification, and the worst-case BER received at edge channel was small than the soft-decision pre-FEC limit of 2×10-2.

512-Gb/s quad-carrier PM-QPSK transmission over 2400-km SMF-28 subject to narrowing 100-GHz optical bandwidth

We propose a network of bidirectional, wireless data channels, operating in the frequency band of 55–65 GHz, as an efficient means to transfer data between processors and memory nodes distributed on separate boards within the computer box. We address channel throughput, power dissipation, form factor, and crosstalk abatement. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2969–2973, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24779

Toward a 60-GHz wireless, low-power, high-throughput memory access system

We propose and experimentally demonstrate a novel scheme of 25 GHz spaced DWDM-PON with 10 Gb/s DPSK downstream and remodulated 10 Gb/s duobinary upstream with 1 dB power penalty over 25 km SMF without dispersion compensation.

Centralized, colorless, wavelength reusable 25GHz spaced DWDM-PON with 10 Gb/s DPSK downstream and re-modulated 10Gb/s duobinary upstream for next-generation local access system

500Mb/s Quadrature Phase-Shift Keying (QPSK)-Orthogonal Frequency Division Multiplexing (OFDM) signals on 60-GHz mm-wave are experimentally demonstrated over 100-m graded-index polymer optical fiber with less than 1 dB power penalty at BER =10-4.

Hung-Chang Chien

Papers

Wireless HD services over optical access systems: Transmission, networking, and demonstration

512-Gb/s quad-carrier PM-QPSK transmission over 2400-km SMF-28 subject to narrowing 100-GHz optical bandwidth

Toward a 60-GHz wireless, low-power, high-throughput memory access system

Centralized, colorless, wavelength reusable 25GHz spaced DWDM-PON with 10 Gb/s DPSK downstream and re-modulated 10Gb/s duobinary upstream for next-generation local access system

QPSK-OFDM radio over polymer optical fiber for broadband in-building 60GHz wireless access