Microwave photonics, which brings together the worlds of radiofrequency engineering and optoelectronics, has attracted great interest from both the research community and the commercial sector over the past 30 years and is set to have a bright future. The technology makes it possible to have functions in microwave systems that are complex or even not directly possible in the radiofrequency domain and also creates new opportunities for telecommunication networks. Here we introduce the technology to the photonics community and summarize recent research and important applications.

Microwave photonics combines two worlds

Microwave photonics (MWP) is an emerging field in which radio frequency (RF) signals are generated, distributed, processed and analyzed using the strength of photonic techniques. It is a technology that enables various functionalities which are not feasible to achieve only in the microwave domain. A particular aspect that recently gains significant interests is the use of photonic integrated circuit (PIC) technology in the MWP field for enhanced functionalities and robustness as well as the reduction of size, weight, cost and power consumption. This article reviews the recent advances in this emerging field which is dubbed as integrated microwave photonics. Key integrated MWP technologies are reviewed and the prospective of the field is discussed.

Integrated microwave photonics

Recent advances in photonic integration have propelled microwave photonic technologies to new heights. The ability to interface hybrid material platforms to enhance light–matter interactions has led to the development of ultra-small and high-bandwidth electro-optic modulators, low-noise frequency synthesizers and chip signal processors with orders-of-magnitude enhanced spectral resolution. On the other hand, the maturity of high-volume semiconductor processing has finally enabled the complete integration of light sources, modulators and detectors in a single microwave photonic processor chip and has ushered the creation of a complex signal processor with multifunctionality and reconfigurability similar to electronic devices. Here, we review these recent advances and discuss the impact of these new frontiers for short- and long-term applications in communications and information processing. We also take a look at the future perspectives at the intersection of integrated microwave photonics and other fields including quantum and neuromorphic photonics. This Review discusses recent advances of microwave photonic technologies and their applications in communications and information processing, as well as their potential implementations in quantum and neuromorphic photonics.

Optical signal processing brings together various fields of optics and signal processing - namely, nonlinear devices and processes, analog and digital signals, and advanced data modulation formats - to achieve high-speed signal processing functions that can potentially operate at the line rate of fiber optic communications. Information can be encoded in amplitude, phase, wavelength, polarization and spatial features of an optical wave to achieve high-capacity transmission. We revisit advances in the key enabling technologies that led to recent research in optical signal processing for digital signals that are encoded in one or more of these dimensions. Various optical nonlinearities and chromatic dispersion have been shown to enable key sub-system applications such as wavelength conversion, multicasting, multiplexing, demultiplexing, and tunable optical delays. We review recent advances in high-speed optical signal processing applications in the areas of equalization, regeneration, flexible signal generation, and optical control information (optical logic and correlation).

All-Optical Signal Processing

This paper reviews the future directions of next generation passive optical networks. A discussion on standardized 10 Gb/s passive optical network (PON) systems is first presented. Next, new technologies that facilitate multiple access beyond 10 Gb/s time division multiple access (TDMA)-PONs will be reviewed, with particular focus on the motivation, key technologies, and deployment challenges. The wavelength division multiplexed (WDM) PON will be discussed and in combination with TDMA, the hybrid WDM/TDMA PON will be reviewed in the context of improving system reach, capacity, and user count. Next, discussions on complementary high-speed technologies that provide improved tolerance to system impairments, capacity, and spectral efficiency will be presented. These technologies include digital coherent detection, orthogonal frequency division multiple access (OFDMA), and optical code division multiple access (OCDMA).

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Next-Generation Broadband Access Networks and Technologies

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

The deployment rate of wavelength division multiplexed passive optical networks (WDM-PONs) is expected to accelerate with the availability of cost-efficient wavelength-specific transmitters. Fueled by this promise, we propose and experimentally demonstrate a novel scheme that facilitates the use of reflective semiconductor optical amplifiers (RSOAs) as colorless upstream transmitters. Central to the scheme is the use of a passive reflective path that is placed at the remote node (RN) to reflect a spectral slice of the broadband amplified spontaneous emission (ASE) light emitted from each RSOA. The reflected spectral slice, termed as a seeding light, establishes a self-seeding of the RSOA with measurements indicating the self-seeded output to be incoherent with a low relative intensity noise. The subsequent direct modulation of the self-seeding RSOA with nonreturn-to-zero data at 1.25 Gb/s for upstream transmission exhibits good transmission and crosstalk performance after traversing 21 km of single-mode fiber. Our proposed scheme eliminates the need for centralized broadband sources, external modulators, and active temperature control within the RN and between the RN and the optical network unit. Aside from the feasibility study of self-seeding RSOAs, we investigate the upstream performance dependence on the characteristics of the seeding light. Our investigations reveal that there exists a noise floor limit of the bit error rate (BER) of the self-seeded upstream signal. The noise floor is shown to vary with an initial optical seeding power that affects the level of ASE noise suppression of the self-seeded upstream signal. None the less, the RSOA self-seeds at a user-defined wavelength with a sufficient suppression of ASE noise to achieve a BER=10-9 with only -30.5 dBm of initial optical seeding power. Our characterization of the frequency response of the RSOA reveals a high-pass filter response that suppresses the modulation on the reflected seeding light, and thus stabilizing the self-seeded output. Collectively, these features highlight the potential of using the self-seeding RSOAs to realize a cost-efficient WDM-PON solution in the near future

Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks

Radio-over-fiber technology provides a simpler pathway for the distribution of wireless signals in broadband wireless networks via optical means. The technology has evolved over the last 30 years, with many challenges already overcome and with many more to address. This paper provides an overview of the technology evolutionary path of RoF with a review of its past, evaluation of its present, and a discussion of its challenges into the future.

Evolution of Radio-Over-Fiber Technology

In this paper, we investigate the reduction of intermodulation distortion (IMD) in fiber-radio systems incorporating a dispersion-tolerant optical single sideband with carrier modulation. We present a systematic analysis and quantification of the third-order IMD generated due to optical components in the nonlinear optical front-end. Our proposed technique to improve the optical front-end linearity is by the removal of the optical components that contribute most to the third-order IMD in the RF domain. We experimentally demonstrated the proposed technique with two- and three-tone tests and showed more than 9-dB improvement in the overall carrier-to-IMD ratio. The proposed technique was also investigated via simulation analysis for a larger number of radio channels and showed an IMD suppression of >10 dB. In addition, the proposed technique is not only able to improve the carrier-to-interference of the radio signals but also to enable simultaneous baseband transmission, thereby facilitating the merging of millimeter-wave fiber-radio systems with other wired-access infrastructure. We present a detailed investigation and characterization of this technique.

Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment

Background: Recent advances in enamel and dentine adhesive technology have resulted in the emergence of many new adhesive systems. Self-etching bonding systems do not require a separate etching step and the newest systems are the “all-in-one” systems which combine etching, priming and bonding into a single application. This study reports laboratory enamel microshear bond strengths of a self-etching priming and three all-in-one systems and also evaluates two different microshear bond test methods. Methods: One hundred and nineteen enamel specimens were bonded (0.8mm diameter) with either Clearfil Protect Bond (Kuraray), Xeno III (Dentsply), G Bond (GC) or One-Up Bond F (Tokuyama) using Palfique Estelite resin composite and stored in 37oC water for seven days. The microshear bond test method used either a blade or wire to apply the shear stress. Results were analysed with one-way ANOVA and post hoc (Tukey) multiple comparison analysis. Results: Clearfil Protect Bond demonstrated higher and more consistent bond strengths than Xeno III, G Bond or One-Up Bond F. The wire method showed much greater reliability in results, with a coefficient of variation half that of the blade method. Conclusions: All-in-one adhesives seem to be less reliable than the two-step self-etching priming adhesive when bonding to enamel. Test method can significantly affect results in the microshear bond test method.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1834-7819.2006.tb00438.x

Ka-Lun Lee

Papers

Fiber-Wireless Networks and Subsystem Technologies

Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks

Evolution of Radio-Over-Fiber Technology

Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment

Comparison of microshear bond strengths of four self-etching bonding systems to enamel using two test methods.