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]

The present invention describes systems and methods of providing optical information transmission systems. An exemplary embodiment of the present invention includes a precoder configured to differentially encode a binary data signal, a duobinary encoder configured to encode the differentially encoded binary data signal as a three-level duobinary signal, an electrical-to-optical conversion unit configured to convert the three-level duobinary signal into a two-level optical signal, and an optical upconversion unit configured to modulate the two-level optical signal onto a higher frequency optical carrier signal and transmit the modulated higher frequency optical carrier signal onto an optical transmission medium.

Systems and methods for providing an optical information transmission system

Successful joint experiments with Deutsche Telecom (DT) on long-haul transmission of 100 G and beyond are demonstrated over standard single-mode fiber (SSMF) and inline erbium-doped fiber amplifier-only amplification. The transmission link consists of eight nodes and 950-km installed SSMF in DT's optical infrastructure with the addition of lab SSMF for extended optical reach. The first field transmission of 8 × 216.8-Gb/s Nyquist-WDM signals is reported over 1750-km distance with 21.6-dB average loss per span. Each channel modulated by a 54.2-Gbaud PDM-CSRZ-QPSK signal is on 50-GHz grid, achieving a net spectral efficiency (SE) of 4 bit/s/Hz. We also demonstrate mixed data-rate transmission coexisting with 1 T, 400 G, and 100 G channels. The 400 G uses four independent subcarriers modulated by 28-Gbaud PDM-QPSK signals, yielding the net SE of 4 bit/s/Hz while 13 optically generated subcarriers from single optical source are employed in 1 T channel with 25-Gbaud PDM-QPSK modulation. The 100 G signal uses real-time coherent PDM-QPSK transponder with 15% overhead of soft-decision forward-error correction. The digital postfilter and 1-bit maximum likelihood sequence estimation are introduced at the receiver DSP to suppress noise, linear crosstalk, and filtering effects. Our results show the future 400 G and 1 T channels utilizing the Nyquist wavelength division multiplexing technique can transmit long-haul distance with higher SE using the same QPSK format.

Field Transmission of 100 G and Beyond: Multiple Baud Rates and Mixed Line Rates Using Nyquist-WDM Technology

We present two key system technologies on the two ends of RF spectrum to support optical-wireless access using radio-over-fiber (RoF) based distributed antenna system (DAS) for in-building environments The proposed technologies can enable protocol-independent, multi-carrier broadband connectivity to the end users' devices deployed in the in-door facilities by supporting existing and emerging wireless services with lower radio frequency carrier (from 500 MHz up to 3 GHz) as well as future-proof very high throughput (VHT) gigabit wireless services using 60 GHz mm-wave band For low frequency wireless services, a broad spectrum band-shifting technique is experimentally demonstrated to simultaneously transmit multi-service multiple input multiple output (MIMO) radio signals over in-building active DAS This approach facilitates easy upgrading of existing non-MIMO services to MIMO services or adding future MIMO services in a simple and cost-effective way For wireless-VHT service, for the first time to our knowledge, we demonstrate simultaneous all-optical up-conversion of multiple, separate gigabit wireless services at 60 GHz and 64 GHz mm-wave band using one single lightwave transmitter This scheme can be used to achieve seamless convergence of very high throughput wireless personal area network (VHT-WPAN) with more conventional low frequency distributed antenna based optical-wireless access system

Multi-Band Transport Technologies for In-Building Host-Neutral Wireless Over Fiber Access Systems

We introduce an "ultra-dense" concept into next-generation WDM-PON systems, which transmits a Nyquist-WDM uplink with centralized uplink optical carriers and digital coherent detection for the future access network requiring both high capacity and high spectral efficiency. 80-km standard single mode fiber (SSMF) transmission of Nyquist-WDM signal with 13 coherent 25-GHz spaced wavelength shaped optical carriers individually carrying 100-Gbit/s polarization-multiplexing quadrature phase-shift keying (PM-QPSK) upstream data has been experimentally demonstrated with negligible transmission penalty. The 13 frequency-locked wavelengths with a uniform optical power level of -10 dBm and OSNR of more than 50 dB are generated from a single lightwave via a multi-carrier generator consists of an optical phase modulator (PM), a Mach-Zehnder modulator (MZM), and a WSS. Following spacing the carriers at the baud rate, sub-carriers are individually spectral shaped to form Nyquist-WDM. The Nyquist-WDM channels have less than 1-dB crosstalk penalty of optical signal-to-noise ratio (OSNR) at 2 × 10(-3) bit-error rate (BER). Performance of a traditional coherent optical OFDM scheme and its restrictions on symbol synchronization and power difference are also experimentally compared and studied.

Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection

A new optical millimeter-wave generation scheme to double the beating frequency without suppressing the carrier by taking advantages of the out-of-phase property between sidebands of a phase-modulated optical carrier is proposed for the first time. Theoretical analysis shows that the generated 60 GHz optical millimeter-wave (mm-wave) can tolerant ±0.016 nm wavelength drifting with filter bandwidth ranging from 70 to 100 GHz to sustain first to second harmonic suppression ratio of 18 dB. The doubled frequency is continuously tunable from 60 to 90 GHz within 100 GHz filter bandwidth with RF power variation of less than 2 dB. In addition, simultaneously generating and transmitting multi-band signal: millimeter-wave band, microwave band, and baseband leveraging the same concept is also proposed. Error-free transmission of 2.5 Gb/s wireless baseband signals carried by the generated 60 GHz mm-wave is successfully demonstrated in both single- and multi-band network environments over a combined optical fiber and wireless distance with a proper equivalent isotropically radiated power of about 20 dBm for in-building access. Moreover, dispersion effect on the generated frequency-doubled optical mm-wave is analyzed by experimentally comparing the link performance of both single mode fiber (SMF-28) and dispersion-shifted fiber cases. It is concluded that for single-band service delivery, the proposed scheme is immune to the interference from the dispersion-induced, redundant 1st harmonics; however, to deliver multi-band services, launching lightwave at zero-dispersion wavelength over SMF-28 is highly recommended to mitigate inter-band interference.

Hung-Chang Chien

Papers

Systems and methods for providing an optical information transmission system

Field Transmission of 100 G and Beyond: Multiple Baud Rates and Mixed Line Rates Using Nyquist-WDM Technology

Multi-Band Transport Technologies for In-Building Host-Neutral Wireless Over Fiber Access Systems

Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection

Optical Millimeter-Wave Generation and Transmission Without Carrier Suppression for Single- and Multi-Band Wireless Over Fiber Applications