Radio-over-fiber Transport Systems Research Articles

Self-Start Multi-Wavelength Laser Source with Tunable Delay-Line Interferometer and Optical Fiber Reflector for Wireless Communication System

The radio-over-fiber (RoF) technique has gained a lot of interest recently, as the millimeter-wave signals can be generated and delivered in the optical domain with the advantages of low attenuation, high capacity, and being free from electromagnetic noise interference (EMI). In this paper, we propose and experimentally prove a self-start multi-wavelength laser source based on a distributed feedback laser diode (DFB-LD) for the RoF transport system. The self-start multi-wavelength laser source generates stable laser power with less than 0.18 dB power fluctuation and exhibits good stability. In order to estimate the transmission performance, data is externally modulated onto the multi-wavelength by a reflective semiconductor optical amplifier (RSOA) and transmitted through single-mode fiber (SMF). The experimental result proves that the proposed RoF transport system achieves error-free transmission and clear eye diagrams.

Fiber Link Health Detection and Self-Healing Algorithm for Two-Ring-Based RoF Transport Systems.

A two-ring-based radio over fiber (RoF) transport system with a two-step fiber link failure detection and self-healing algorithm is proposed to ensure quality of service (QoS) by automatically monitoring the health of each fiber link in the transport system and by resourcefully detecting, locating, and bypassing the blocked fiber links. With the assistance of the fiber Bragg grating remote sensing technique, preinstalled optical switches, and novel single-line bidirectional optical add/drop multiplexers, the optical routing pathways in the RoF transport system can be dynamically adjusted by the proposed algorithm when some fiber links are broken. Simulation results show that except in some extreme situations, the proposed algorithm can find the blocked fiber links in the RoF transport system and animatedly adjust the status of preinstalled optical switches to restore all blocked network connections, thereby ensuring QoS in the proposed RoF transport system.

Full-duplex hybrid FTTH and RoF transport system based on polarization modulators

This work demonstrates a full-duplex hybrid fiber-to-the-home (FTTH) and radio-over-fiber (RoF) transport system to transmit one wired signal and two wireless signals based on polarization modulation techniques. Here, the generated odd-order and even-order sidebands of polarization modulated signals can be colorlessly separated with the help of polarization beam splitters. This system has less interference among the wired signal, microwave and millimeter-wave signals. The uplink design is done by reusing the optical carrier and transmitting it through a single mode fiber (SMF) to the central office. The system is simulated using Orthogonal Frequency Division Multiplexing (OFDM) with 4-Quadrature Amplitude Modulation (QAM) as input. Frequency doubling and quadrupling is also used, such that optical microwave at 7.5 GHz and millimeter-wave at 15 GHz are generated successfully using polarization modulators by directly applying 3.75 GHz signal. This system is proved to have error-free transmissions, open eye diagrams, and clear constellation diagrams.

Upgradable radio-over-fiber transport system

An upgradable radio-over-fiber (RoF) transport system is proposed based on a vertical-cavity surface-emitting laser (VCSEL)-composed wavelength selector. The passband window of the wavelength selector can be electrically adjusted to select a specific lightwave from multiple injected optical carriers and attenuate the others. By adding the VCSEL-composed wavelength selector installed inside each base station (BS), the proposed transport system can remain as the original tree topology, but its overall network capacity can be flexibly extended (reduced) by adding (removing) an additional optical carrier into (out of) the optical network. Each BS can dynamically receive data from one of the multiple injected optical carriers. This method is useful because the bandwidth requirement of each BS, which is serviced by an RoF transport system, may significantly change with the visitor flow rate. Experimental results prove that the BSs in the proposed upgradable RoF transport system can be reallocated to multiple logical RoF links and can be dynamically switched to receive data from any one of the logical RoF signals without modifying the original network structure. The transmission performances of these optical carriers are ensured by low bit error rate, clear eye, and optical/electrical spectra diagrams.

FTTH and Two-Band RoF Transport Systems Based on an Optical Carrier and Colorless Wavelength Separators

A hybrid fiber-to-the-home (FTTH) and radio-over-fiber (RoF) transport system is proposed to transmit one wired signal and two wireless signals simultaneously through an optical lightwave. By properly utilizing the polarization characteristic of a phase-remodulated and polarization-remodulated optical lightwave, the first- and second-order sidebands of the lightwave can be colorlessly separated for optical double- and quadruple-frequency RoF transmissions. Experimental results prove that the independent FTTH and two-band RoF signals can be simultaneously generated and separated by relatively low-frequency radio-frequency devices, optical modulators, and polarization beam splitters. The transmission performances are proven by proper bit error rate (BER) performance and clear eye and constellation diagrams. The interference among the transmitted wired signal and double- and quadruple-frequency RoF signals is acceptable. In this case, the wavelength of the optical carrier can be flexibly assigned, and the frequency of the transmitted wireless signals can be easily adjusted without affecting the functionality of the developed optical fiber transport system. The presented architecture can be flexibly employed to support various types of FTTH and microwave and millimeter-wave over fiber transmissions.

A Distribute Feedback Laser Diode Composed Microwave Photonic Bandpass Filter for SCM-Based Optical Transport Systems

In this paper, a distribute feedback laser diode (DFB LD) composed microwave photonic bandpass filter is proposed and experimentally demonstrated for subcarrier multiplexing (SCM)-based radio over fiber (RoF) transport systems. To realize the photonic filter, a multiwavelength laser, composed by directly modulating single-tone RF signal with a DFB LD, is developed to simultaneously generate six identical optical carriers. When these optical carriers are phase-modulated with SCM signals and communicated through optical fiber to optical network units (ONUs), a bandpass filter function is accomplished automatically due to the interaction of the accumulated chromatic dispersion and the optical interference among those transmitted optical carriers. Experimental results proof that the central frequency of the passband window of the photonic filter is not fixed, instead, it can be shifted accurately by alerting the optical channel spacing of the generated six optical carriers. By applying the proposed photonic filter for SCM-based RoF transport systems, each connected ONU will receive dedicated SCM signals only and the other SCM signals will be rejected, so no more electrical microwave filter is required to be deployed in the ONU. With the proposed scheme, the SCM-based RoF transport systems can be greatly simplified and the system transmission efficiency can be promoted and controlled centrally.

Radio-over-fiber transport systems with an innovative phase modulation to intensity modulation converter

Robust optical phase modulation (PM) schemes have been widely employed in the development of radio-over-fiber (RoF) transport systems. However, the signals produced by these PM schemes require a delay line interferometer (DI), a fiber Bragg grating (FBG), an optical band-pass filter (OBPF), or dedicated dispersive devices to convert them back into the intensity modulation (IM) signal format prior to detection by a photodetector (PD). Inserting a costly DI into a transport link can significantly increase overall cost, and the use of a fixed working window OBPF, FBG, or dedicated dispersive devices to achieve the conversion is inflexible. To overcome these problems in PM-based RoF transport systems, a long-reach RoF link with an innovative PM-to-IM converter is proposed and experimentally demonstrated. Compared with the published PM schemes, the proposed architecture can utilize a vertical-cavity surface-emitting laser (VCSEL) to achieve the same performance of the published PM-to-IM converter at a lower cost. In addition, by adjusting the VCSEL driving current, a tunable range exceeding 350 GHz is experimentally obtained for the converter working window. The proposed long-reach RoF transport system is the first to achieve such PM-to-IM conversion by using a VCSEL and can contribute to the development of RoF transport systems.

New Trends of Radio over Fiber Communication Systems for Ultra High Transmission Capacity

Radio-over-fiber (RoF) systems have been widely investigated due to such advantages of optical fiber as low loss, large bandwidth, and transparent characteristics for radio signal transmission. By utilizing RoF systems, various radio frequency (RF) signals including cellular services and wireless local area network (WLAN) signals can be efficiently distributed to densely populated areas or outdoor ranges. This paper investigates RoF transport systems that have the potential to offer large transmission capacity, significant mobility and flexibility, as well as economic advantage due to its broad bandwidth and low attenuation characteristics. The high performance of RoF communication systems are investigated against traditional optical communication systems using different coding formats over wide range of the affecting operating parameters. Moreover we have analyzed the transmission bit rates and products per channel based standard single mode fiber made of both silica-doped and plastic materials with using modified Shannon technique in addition to use different coding format such as non-return-to-zero (NRZ) code for ultra long haul transmission applications. We have taken into account the bit error rate (BER) for RoF systems with comparing it with traditional optical fiber communication systems as a proof for improvement of signal to noise ratio.

RoF transport systems with OSNR enhanced multi-band optical carrier generator

A combination of an economic multi-band optical carrier generator and a novel optical signal to noise ratio (OSNR) enhancement circuit is proposed and demonstrated for radio over fiber (RoF) transport systems. Different from normal RoF transport systems which a central station (CS) needs multiple dedicate wavelength laser diodes (LDs) to support various base stations (BSs), the proposed scheme can employ a single LD to provide multiple optical carriers for various BSs. To verify this scheme, 8 coherent optical carriers are firstly generated using a single LD and a local oscillator (LO). Subsequently, their OSNR values are optimized by the developed OSNR enhancement circuit. An up to 15 dB OSNR enlargement in those optical carriers is experimentally achieved. To demonstrate the practice of the proposal, a pair of those optical carriers is employed to experimentally achieve frequency up-conversion process in a RoF transport system. Clear eye diagram and error free transmission reveal that with a proper carrier selector the proposed scheme can be employed to support multiple RoF transmissions. Furthermore, this proposal also presents a high possibility to achieve 60 GHz RoF transmission using a 10 GHz LO, a LD and a low frequency external modulator.

Simplified radio-over-fiber transport systems with a low-cost multiband light source

In this Letter, low-cost radio-over-fiber (ROF) transport systems are proposed and experimentally demonstrated. By utilizing a laser diode (LD) and a local oscillator (LO) to generate coherent multiband optical carriers, as well as a self-composed wavelength selector to separate every two carriers for different ROF transport systems, no any other dedicated LD or electrical frequency upconverting circuit/process is needed in the central station (CS). Compared with current ROF systems, the required numbers of LDs, LOs, and mixers in a CS are significantly reduced. Reducing the number of components not only can simplify the network structure but can also reduce the volume and complexity of the relative logistics. To demonstrate the practice of the proposed ROF transport systems, clear eye diagrams and error-free transmission performance are experimentally presented.

Full-duplex ROF transport systems based on a broadband ASE light source and nonlinear distortion suppression scheme

A full-duplex radio-over-fiber (ROF) transport system employing a broadband amplified spontaneous emission (ASE) light source and nonlinear distortion suppression scheme is proposed and demonstrated. A 10 GHz/70 Mbps data rate (compatible WiMAX format) signal is externally modulated and transmitted over a 120 km single-mode fiber (SMF) transmission both for down/up-link. Good bit error rate (BER) performance and a clear eye diagram were achieved in our proposed systems. Such a proposed full-duplex ROF transport system is suitable for long-haul microwave optical links.

TO EMPLOY SOA-BASED OPTICAL SSB MODULATION TECHNIQUE IN FULL-DUPLEX ROF TRANSPORT SYSTEMS

A full-duplex radio-over-fiber (ROF) transport system employing semiconductor optical amplifier (SOA)-based optical single sideband (SSB) modulation technique is proposed and demonstrated. For our proposed approach, it is relatively simple to implement as it requires only one SOA to generate optical SSB signal. Over an80 km single-mode fiber (SMF) transmission, low bit error rate (BER), clear eye diagram, and low third-order intermodulation distortion to carrier ratio (IMD3/C) were achieved. Our proposed full-duplex ROF transport systems are suitable for the long-haul microwave optical links. Corresponding author: H.-H. Lu (hhlu@ntut.edu.tw).