Harmonic RF carrier generation and broadband data upconversion using stimulated Brillouin scattering

We propose and experimentally demonstrate a harmonic radio frequency (RF) carrier generation and broadband data upconversion technique with single mode and single sideband (− SSB) modulation for radio-over-fiber (RoF) systems using stimulated Brillouin scattering (SBS). The optical carrier-to-sideband ratio (CSR) of the single mode and SSB modulated signal can be easily adjusted to achieve the best received sensitivity performance of the RoF system. By using this method, we successfully demonstrate generation of third-harmonic RF carrier at 32.625 GHz with f_LO of 10.875 GHz and upconversion of 1.25-Gb/s data to the RF carrier band. In addition, the data bandwidth is independent of the Brillouin gain profile. Finally, the transmission performance of the RoF downlink system is examined.     

A full-duplex radio-over-fiber link with 12-tupling mm-wave generation and wavelength reuse for upstream signal

A full-duplex radio-over-fiber (RoF) link with a novel scheme to generate 60 GHz mm-waves from a 5 GHz RF signal source is investigated. In the RoF downlink, the required frequency of the RF oscillator is reduced greatly. Since the optical carrier is not modulated by downstream data, part of it is reused to carry upstream data and the upstream data is transmitted to the central station using optical single-sideband modulation. In this way, a single wavelength is used for both downstream and upstream transmissions. Based on this scheme, a full-duplex RoF link is built and its transmission performance is analyzed. Theoretical analysis and numerical simulation show that the downstream signal cannot only eliminate code form distortion caused by time shift of the code edges, but also reduce the influence of the fading effect as the 60 GHz DSB optical mm-wave signal is transmitted along the fiber, and the upstream signal is immune to both fading effect and time shift of the code edges.     

Impact of interferometer delay time on the performance of ODSB-SC RoF system with wavelength interleaving

In this paper, we investigate the impact of interferometer delay time in a 5 Gb/s optical double sideband-suppressed carrier (ODSB-SC) RoF system transmitting two wavelength interleaved radio frequency (RF) signals at 10 and 15 GHz over an optical fiber. Here, an optical Mach–Zehnder modulator is used for both optical carrier suppression and signal modulation. At the receiver, delay interferometer is used for the separation of RF frequency signals. We analyze the performance of the RoF system by varying the value of delay time of interferometer from 0.02 to 0.14 ns. The result shows that the RoF system performance is optimum for the time delay of 0.1 ns. Further, the optical spectrums, RF spectrums and eye diagrams of two interleaved RF signals have been compared.     

Performance improvement and wavelength reuse in millimeter-wave radio-over-fiber links incorporating all-fiber optical interleaver

Simultaneous downlink performance improvement and uplink wavelength reuse in a full-duplex millimeter-wave (MMW) radio-over-fiber (RoF) system by using a simple and cost-effective all-fiber optical interleaver are proposed and demonstrated. The MMW RoF downlink performance improvement is based on suppressing optical carrier-to-sideband ratio (OCSR), with which the mechanism is confirmed by theoretic analysis and derived experimental results. Measured results show that, by suppressing OCSR using a fabricated all-fiber optical interleaver, the downlink optical receiver sensitivity is improved about 2.1 dB. The downlink data rate is 1.25 Gbit/s and the carrier frequency is 58.1 GHz; the link consists of 6 km optical single-mode fiber and 1 m wireless connection. On the other hand, with the interleaver suppressing downlink OCSR, simultaneously an optical carrier is recovered from the RoF downlink and is reused for RoF uplink transmission. The uplink is operated at 62.9 GHz and the data rate is the same 1.25 Gbit/s. With the recovered optical carrier, a laser-free remote access point is achieved. The principle, structure, and fabrication of an all-fiber optical interleaver are also presented in this paper.     

Carrier generation and IF signal up-conversion using optical injection locking and stimulated Brillouin scattering

We theoretically investigate the intermediate frequency (IF) signal up-conversion with dual modes modulation (DMM) and single-mode modulation (SMM) of two single sideband (SSB) modulation modes for radio-over-fiber (RoF) systems. The simulation results indicate that the SMM can overcome the dispersion-induced power penalties of the generated carrier and up-converted IF signals. Then, we experimentally demonstrate an approach for RF carrier generation with SSB modulation and IF signal up-conversion with SMM using a combination of direct modulation in an injection-locked distributed feedback (DFB) laser and stimulated Brillouin scattering (SBS). The experimental results are compared with the predictions of the numerical simulation in SMM condition, and both are in good agreement with each other. The advantages and limits of the proposed method are discussed.     

Gigabit-converged wired and wireless networks for simultaneous multi-services transmission

A novel architecture of converged radio-over-fiber(RoF) and wavelength division multiplexed passive optical network(WDM-PON) system,namely RoF-WDM-PON,is demonstrated.20-GHz 1-Gb/s radio frequency(RF) signals and 1-Gb/s baseband(BB) signals are simultaneously generated and transmitted using optical carrier suppression(OCS) modulation techniques.The proposed scheme is compatible with the conventional RoF and PON system.25-km single-mode fiber(SMF) transmission is successfully achieved.     

Single-sideband modulated radio-over-fiber system based on long period fiber grating

We present a prototype for optical single-sideband (SSB) modulated radio-over-fiber (RoF) system by employing a long period fiber grating (LPFG). A LPFG with 13.78 nm base width of transmission spectrum and 0?23.2 dB of transmission depth was designed by using commercial software. Then it is used in RoF SSB modulation scheme. In the scheme, a Mach?Zehnder modulator modulates the light wave with millimeter-wave driving signals to realize optical double-sideband (ODSB) modulation, the generated ODSB modulation signals pass through a LPFG. Due to the negative slope in transmission spectrum, the lower sideband experiences higher attenuation than the upper sideband. Thus the conversion from ODSB to optical single sideband with carrier (OSSB + C) can be easily achieved by using only one LPFG. Also, the carrier to sideband ratio (CSR) can be reduced by using a LPFG, results show the CSR can be decreased from 12.49 dB to 1.1 dB.     

Effects of Direct Current Bias-Drifting on Radio on Fiber Link

The effects of direct current (dc) bias-drifting on radio on fiber (RoF) links are quantitative investigated and simulated. Single sideband (SSB) modulation and carrier suppressed double sideband (CS-DSB) modulation RoF links are considered, power variation of targeted signal and harmonic suppression on account for bias-drifting are calculated and evaluated. Our results suggest that power variation can be maintained within 1.1 dB with dc bias drift ε less than 5% in the SSB modulation RoF link and the degradation of harmonic suppression is evident with ε exceed 2% in the CS-DSB modulation RoF link.     

Stable RF transfer over a fiber-optic ring with DSBCS modulation and a DSB RF signal

We propose a radio frequency(RF)transfer technique with passive phase noise compensation over a fiber-optic ring.By adopting different frequencies and same wavelength transmission and double sideband(DSB)with carrier suppression(DSBCS)modulation,the impact of backscattering can be effectively suppressed.A stable RF signal can be obtained via frequency mixing at an arbitrary access site along the fiber-optic ring.As the two directional transmissions adopt the same fiber and same wavelength from the same laser,the bidirectional propagation symmetry can be maximally guaranteed.We experimentally demonstrate 2 GHz RF signal transfer along a 100 km standard single-mode fiber-optic ring.     

Simulative investigation of nonlinear distortion in single- and two-tone RoF systems using direct- and external-modulation techniques

In this paper, we investigated a Radio-over-Fiber (RoF) system consisting of two different system set-ups using direct- and external-laser modulation techniques to study frequency response. Further, second- and third-harmonic generations of single- and two-tone RoF systems have been studied. In this work, we also measured the electric Rf power of two receiving channels and BER at received optical power at different modulating Rf frequencies up to 20 GHz using EDFA or SOA amplifiers. The results have been compared for the electric Rf power of receiving channels obtained using a sin² Mach Zehnder modulator and a linear modulator, and an improvement in the received Rf power with linear modulator in comparison with the sin² modulator is observed.     

Employing inverse return-to-zero and Manchester formats for uplink wavelength reuse in RoF systems

The inverse return-zero (IRZ) modulation format and Manchester format were investigated as wavelength reusing schemes in the RoF downlink systems. Since both of the modulation formats featured power remaining in the bit slot regardless of the bit value, the downlink optical carrier can be reused as the uplink light source. The performances of two formats were analyzed in the symmetrical and asymmetrical duplex RoF system. It was found that IRZ is more suitable for symmetrical system, and the duty cycle of IRZ pulse is the critical factor on the asymmetrical RoF system performance. However the Manchester format demonstrated the large system tolerance on asymmetrical duplex RoF system with the receiver sensitivity degradation within 1 dB.     

The transmission performance of the single sideband optical millimeter-wave with BPSK signal in the duplex radio-over-fiber link

This paper has investigated the transmission performance of the single sideband (SSB) optical millimeter (mm)-wave with signal carried by the sideband in BPSK format in duplex radio-over-fiber (RoF) system theoretically and numerically. The SSB optical mm-wave signal is generated by a LiNbO₃ Mach–Zehnder modulator and there exists an optimal modulation index to generate the SSB optical mm-wave with a maximal RF photocurrent. The SSB optical mm-wave is much suitable for the duplex ROF link with the uplink lightwave recovered from the downlink because the optical carrier carries no signal. In such a duplex RoF link, although there are the spurs on the optical carrier, they have little influence on the downlink and the uplink signal even if the modulation index is large.     

A subcarrier demultiplexing scheme based on a coefficient-switchable microwave photonic filter in radio-over-fiber systems

A flexible method for subcarrier demultiplexing in radio-over-fiber (RoF) systems is proposed and experimentally demonstrated. The subcarrier demultiplexer is realized by a coefficient-switchable microwave photonic filter, which is based on biasing Mach–Zehnder modulator (MZM) at different transmission slopes. The 2.4 GHz and 2.7 GHz subcarrier signals both modulated with 100 Mbps on-off keying (OOK) data are transmitted and demultiplexed in the RoF system. The experimental result shows that each of the two subcarriers can be individually selected out with the other one well depressed. The proposed subcarrier demultiplexing scheme is simple, feasible, cost-effective and has good potential applications in the multiplexed RoF systems.     

Photonic generation of 60 GHz-band vector signals for RoF systems based on optical vector signal down-conversion

A novel photonic method of 60 GHz-band vector signal generation for RoF systems based on optical vector signal down-conversion is proposed and experimentally demonstrated in this paper. In the proposed method, the target vector signals are first generated in the optical domain with the help of mature commercial optical devices and then directly distributed to the base stations (BSs) through the fiber link. The generated vector signals can be automatically down-converted to the 60 GHz band in the BSs after O/E conversion, and then directly transmitted to the users without any further processing. With the proposed method, higher spectrum efficiency and system capacity will be obtained compared with the traditional OOK RoF systems while almost no extra system complexity and cost is brought in. According to the characteristics of different types of vector signals, two particular modulation schemes are provided, which are then verified by corresponding simulations and experiments. In the experimental 60 GHz RoF system, the 622 MSym/s 60 GHz-band 8-QAM and 4-QAM signals generated with two different schemes respectively are successfully transmitted over 50 km SMF and 5 m wireless channel without any compensation, and the power penalty are both about 1.7 dB at the BER of 10^− 9.     

A full-duplex radio-over-fiber system using direct modulation laser to generate optical millimeter-wave and wavelength reuse for uplink connection

In this paper, we present a full-duplex radio-over-fiber system incorporating both optical millimeter-wave (mm-wave) generation and wavelength reuse for uplink connection. The optical double sidebands (DSB) signal is generated by using only one inexpensive broadband direct modulation laser (DML), to which a mixing RF signal is applied. An optical interleaver is then used to separate the first-order optical sidebands from the optical carrier of optical DSB signal. The separated first-order optical sidebands are beat to generate mm-wave signal that has double the frequency of the RF drive signal, while the separated optical carrier is reused as light source to remodulate uplink signal. Both detailed theoretical analysis and experiments to demonstrate the feasibility of the proposed system are presented. Experiment result shows that the bidirectional 2.5 Gb/s data can be successfully transmitted over 40 km standard single-mode fiber (SSMF) with less than 2 dB power penalty.     

All optical microwave photonic filters with a round-trip configuration

We propose novel photonic microwave multi-tap filters using a section of Hi-Bi fiber in a round-trip configuration, together with their application in a 20 km single mode fiber (SMF) radio-over-fiber (RoF) link. We demonstrate for the first time that the effect of chromatic dispersion in these multi-tap filters can be reduced in an RoF link when a linearly chirped fiber Bragg grating (FBG) is adopted. This result can be used to combat radio frequency power fading in an RoF link. The configurations are free from optical coherent interference and phase noise. Measured results show 3-tap filters with notch rejections greater than 35 dB and suppression of the level of secondary sidelobes by adjusting a polarization controller. PACS 42.81.-I; 42.81.Gs; 07.50.Qx     

Generation of optical carrier suppression millimeter-wave signal using one dual-parallel MZM to overcome chromatic dispersion

In this paper, we have proposed a novel approach to generate optical carrier suppression (OCS) mm-wave with signal only carried by one first-order sideband using a dual-parallel MZM in radio-over-fiber (RoF) system, and the transmission performance was also investigated. As the optical mm-wave signal is transmitted along the fiber, there is no time shift of the data symbols resulting from the group velocity dispersion in the fiber because the signal is only modulated on one sideband. The simulation results showed that the eye diagram keeps open and clear even when the optical mm-wave signals are transmitted over 96-km and the power penalty is about 1-dB after fiber transmission distance of 60-km, which is quite consistent with our theoretical analysis. Furthermore, the proposed OCS optical mm-wave generation approach is also used in a full-duplex RoF link based on wavelength reuse at the base station for the uplink due to another pure sideband without carrying data. The bidirectional 2.5-Gbit/s data is successfully transmitted over 40-km standard single mode fiber with less than 0.5-dB power penalty in the simulation. Both the unidirectional RoF downlink and the full-duplex RoF system have good performance.     

Spatially inhomogeneous carrier concentration dependence of the reflectivity of semiconductors

The excite-and-probe technique was used to study the optically induced charge carrier concentration dependence of the reflectivity of semiconductors, both experimentally and theoretically, in the case of an exponential carrier distribution. The second harmonic of an Nd : YAG laser (pulse duration 25 ps) was used as the exciting beam; the fundamental wave served as the probe beam. At a carrier concentration of about 2.5×10²¹ cm^–3 a minimum reflectivity was obtained if the angle of incidence was greater than 40°. For increasing absorption constants of the semiconductor at the exciting frequency, this minimum value of the reflectivity was shown to increase. The dependence of the reflectivity on the exponentially distributed carrier concentration was studied experimentally for thin amorphous silicon films produced by the glow-discharge technique. For an absorption constant of 6×10⁵ cm^–1 at 532 nm, good agreement was found between the numerical calculations and the experimental results.     

应用马赫-曾德尔调制器的光微波链路建模

在考虑马赫-曾德尔调制器(MZM)的消光比、双臂驱动不平衡度等非理想特性参量对其进行细致的理论建模的基础上,建立了包括光源、双臂驱动马赫-曾德尔调制器、光传输模块、光电探测器的基本光微波链路模型,并得到双边带调制(DSB)和单边带调制(SSB)链路的全阶解析响应结果,着重分析不同调制方式下马赫-曾德尔调制器对光微波链路幅度和相位(延时)性能的影响。理论计算和数值仿真结果表明,对双边带调制链路,消光比和双臂驱动不平衡度影响链路的幅频特性而不影响相位特性,对单边带调制链路,二者同时影响链路的幅频和相频特性。理论模型和数值结果可为使用马赫-曾德尔调制器的光微波系统如光纤无线电(RoF)和光控微波波束形成网络等提供定性和定量性能分析的基础。     

Simulative investigation on the impact of laser-spectral width in single-tone radio-over-fiber transmission system using optical single side-band technique

In this paper, we analyzed the impact of laser-spectral width incorporating dual-electrode Mach–Zehnder modulator (DEMZM) in single-tone radio-over-fiber (RoF) transmission system by simulation setup. It is shown that an improvement in the measurement of received radio frequency (RF) power is achieved by reducing the laser line width from 100 MHz to 100 kHz, which further improves the BER rate and optical link by transmitting the information with low power. The results are calculated for 20 and 50 km optical single sideband (OSSB)–RoF transmission system by varying the chirp from 0 to −3 as it requires less bandwidth than optical dual sideband (ODSB)–RoF system and is tolerable for power degradation due to a chromatic fiber-dispersion, through a standard single-mode fiber (SSMF) carried by a continuous wave (CW) laser at 1550 nm of laser-spectral width varying from 100 MHz to 100 kHz with CW power of 10 mW that modulates a single RF channel of 20 GHz. Further, deployment of such lasers with OSSB scheme helps the telecom industry to reduce the designing cost of RoF communication systems.