Full-duplex radio-over-fiber system based on a modified single-side band modulation

A full-duplex radio-over-fiber system based on a modified single-sideband using external modulator is proposed and demonstrated. At the central station, a CW lightwave is intensity-modulated by a RF signal to generate a DSB signal. After the central carrier and the two first-order sidebands are separated by a FBG, the central carrier is modulated with a baseband data at 2.5 Gbit/s. Then, it is recombined with the un-modulated first-order sidebands to generate optical millimeter-wave by an optical coupler with a certain coupling coefficient and transmitted to the base station over single-mode fiber. The central carrier and one of the first-order sidebands are beaten to generate the mm-wave when they are detected by an optical receiver. Another first sideband is reused as carrier for uplink connection. The dispersion performance of the generated mm-wave is theoretically analyzed; one can see that the effect of dispersion and requirement of the optical power are reduced. The PIN-PD can avoid working in a high-DC saturation range which may distort the RF components and depress the responsibility of the detector. The stimulant results show that the system can reduce the effect of dispersion effectively, and immune the fading effect and the walking-off signals. It is suitable for a long distance transmission.     

10 Gb/s optical carrier distributed network with W-band (0.1 THz) short-reach wireless communication system

Millimeter-wave (mm-wave) operated in W-band (75 GHz–0.11 THz) is of particular interests, since this frequency band can carry signals at much higher data rates. We demonstrate a 10 Gb/s optical carrier-distributed network with the wireless communication system. The mm-wave signal at carrier frequency of 0.1 THz is generated by a high speed near-ballistic uni-traveling carrier photodiode (NBUTC-PD) based transmitter (Tx), which is optically excited by optical short pulses. The optical pulse source is produced from a self-developed photonic mm-wave waveform generator (PMWG), which allows spectral line-by-line pulse shaping. Hence these optical pulses have high tolerance to fiber chromatic dispersion. The W-band 10 Gb/s wireless data is transmitted and received via a pair of horn antennas. The received 10 Gb/s data is envelope-detected and then used to drive an optical modulator at the remote antenna unit (RAU) to produce the upstream signal sending back to the central office (CO). 20 km single mode fiber (SMF) error free transmission is achieved. Analysis about the optimum repetition rate of the optical pulse source and the transmission performance of the upstream signal are also performed and discussed.     

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.     

Influence of the modulation index of Mach-Zehnder modulator on RoF link with ASK millimeter-wave signal

In this paper, the influence of the modulation index of LiNbO₃ Mach-Zehnder modulator on the radio over fiber (RoF) link based on single sideband (SSB) optical millimeter (mm)-wave with ASK signal is theoretically and numerically investigated. Our investigation shows that there exists an optimal modulation index to generate the SSB optical mm-wave with a maximal RF photocurrent. Although the fiber dispersion distorts the code form and degrades the performance of the RF signal demodulated from the SSB optical mm-wave after fiber transmission, it does not cause the closure of the eye diagram. However, the influence of the fiber dispersion becomes more obvious as the modulation index increases. For the duplex RoF link with the optical carrier of the uplink recovered from the downlink, a larger modulation index of the downlink causes a worse crosstalk from the downlink to the uplink.     

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.     

Frequency sextupling technique using two cascaded dual-electrode Mach–Zehnder modulators interleaved with Gaussian optical band-pass filter

A frequency sextupling technique for the generation of millimeter-wave (mm-wave) is proposed and investigated. The proposed technique is comprised of two cascaded dual-electrode Mach–Zehnder modulators (MZMs), which are interleaved with Gaussian optical band-pass filter (GOBF). The first MZM, biased at minimum transmission, is only used for optical carrier suppression modulation, and the second MZM, biased at maximum transmission, is used for both even-order optical harmonic generation and data signal modulation. The GOBF between two MZMs is used to suppress the high-order optical harmonics beyond the first-order optical harmonics. On the basis of theoretical analysis and simulated demonstrations, it is concluded that with the use of an RF signal at 10 GHz, which carries the data signal and drives the MZMs, an mm-wave signal at 60 GHz can be obtained. The simulation results show that the proposed sextuple leads to a 7 dB improvement in receiver sensitivity in comparison with the modulation technique, i.e. using two cascaded dual-electrode MZMs without GOBF. Furthermore, the eye diagrams show that the quality of generated mm-wave signal is satisfactory. The proposed technique is verified by experiments.     

High speed ultra short pulse fiber ring laser using photonic crystal fiber nonlinear optical loop mirror

A scheme to generate high speed optical pulse train with ultra short pulse width is proposed and experimentally studied. Two-step compression is used in the scheme: 20 GHz and 40 GHz pulse trains generated from a rational harmonic actively mode-locked fiber ring laser is compressed to a full width at half-maximum (FWHM) of ~ 1.5 ps using adiabatic soliton compression with dispersion shifted fibers (DSF). The pulse trains then undergo a pedestal removal process by transmission through a cascaded two photonic crystal fiber (PCF)-nonlinear optical loop mirrors (NOLM) realized using a double-ring structure. The shortest output pulse width obtained was ~ 610 fs for 20 GHz pulse train and ~ 570 fs for 40 GHz pulse train. The signal to noise ratio of the RF spectrum of the output pulse train is larger than 30 dB. Theoretical simulation of the NOLM transmission is conducted using split-step Fourier method. The results show that two cascaded NOLMs can improve the compression result compared to that for a single NOLM transmission.     

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.     

Simultaneous demodulation and dispersion compensation of WDM DPSK channels using optical ring resonator

We have introduced and comprehensively analyzed a novel scheme of simultaneous demodulation and dispersion compensation of wavelength division multiplexed (WDM) non-return-to zero (NRZ) differential phase shift keying (DPSK) optical link using an optical ring resonator (ORR) based filter. Using extensive numerical simulation we have demonstrated the transmission of 10.7 Gb/s WDM DPSK channels having 50 GHz and 100 GHz spacing over 400 km of unrepeatered reach at 20 dB optical-signal-to-noise-ratio (OSNR) to achieve a bit error rate (BER) of 10^? 3.     

Low capacitance ESD protection circuits for GaAs RF ICs

We present a novel electrostatic discharge (ESD) protection circuit for GaAs radio frequency (RF) integrated circuits (ICs), which are targeted for 10 Gb/s fiber-optic communication applications. The robustness, parasitic impedance, and loading effect of the new ESD protection circuit are studied and compared with the conventional diode-based ESD protection technique. Two versions of this type of ESD protection circuit were fabricated with a 60-GHz InGaP heterojunction bipolar transistor (HBT) technology. These two circuits can withstand, respectively, 2700 and 5000 V human body model (HBM) ESD stress and provide a similar level of ESD protection to RF ICs. The corresponding impedances of the off state are represented by an equivalent shunt capacitance and shunt resistance of 0.22 pF and 500 Ω, and 0.5 pF and 250 Ω, at 10 GHz. This ESD protection circuit can protect the 10 Gb/s RF ICs against much higher level ESD stress than conventional diode-based ESD protection circuits even with smaller size.     

Sub-THz continuous wave generation scheme using high-order harmonics modulated lightwave

We propose a sub-THz continuous wave (CW) generation scheme using a high-order harmonics modulated lightwave (HML) to reduce an electronic dependency of a conventional double sideband suppressed carrier (DSB-SC) scheme. The electronic dependency should be overcome to increase frequency tunability of the conventional DSB-SC scheme. This is because the frequency of a local oscillator (LO), f_LO, should be one-half frequency of the frequency of a desired sub-THz CW in the conventional DSB-SC scheme. The proposed scheme is formed by adding an optical feedback loop to the conventional DSB-SC scheme. In order to verify our proposed scheme, a 120 GHz CW is generated using the LO with f_LO = 20 GHz. Based on our experimental results, we have found that the frequency of the LO can be reduced by our proposed scheme up to one-sixth (20 GHz) of 120 GHz. The 120 GHz CW generated by the proposed scheme has 52 dB higher photomixed output power with narrow spectral linewidth than that of the 120 GHz CW generated by the conventional DSB-SC scheme using the LO with f_LO = 20 GHz. Consequently, our proposed scheme can be helpful to reduce the electronic dependency of the conventional DSB-SC scheme.     

A novel full-duplex radio-over-fiber system based on dual octupling-frequency for 82 GHz W-band radio frequency and wavelength reuse for uplink connection

A novel full-duplex radio-over-fiber system is proposed and demonstrated, in which an external modulator and an optical interleaver are used to generate dual octupling-frequency optical millimeter waves for two base stations and wavelength reuse for uplink connection. This scheme is simplified and low-cost because no additional laser is utilized for uplink connection at two base stations and one laser at central office works for two base stations simultaneously. The frequency of local oscillator signal is reduced largely due to frequency octupling. The theorem about how to generate optical millimeter waves is analyzed. The simulation results show that the bidirectional 5 Gb/s system has a perfect performance because the power penalty for the downlink and uplink signals of two base stations are less than 0.6 dB after successful transmission over 60 km standard single mode fiber.     

Study of all-fiber asymmetric interleaver based on two-stage cascaded Mach–Zehnder Interferometer

In order to improve the transmission efficiency of optical-fiber communication system with 10 Gb/s + 40 Gb/s, an all-fiber interleaver with unequal passband is proposed and discussed, which is based on a two-stage cascaded Mach-Zehnder interferometer (MZI). The optimum value of structural parameters, such as splitting ratios of the couplers and the physical length differences of the interferometer arms, were chosen. One set of optimized data is validated in the experimental result. The experimental results and the theoretic analysis indicate that an all-fiber optical interleaver with 3 dB passband width in odd channels and even channels could be obtained, which having more than 60 GHz passband and 30 GHz passband, for transmission speed of 40 Gb/s and 10 Gb/s, respectively. By assigning different portions of spectrum to the 10 Gb/s and the 40 Gb/s channels, the bandwidth efficiency requirement of the 40 Gb/s channel is relieved, and therefore longer transmission distance can be achieved.     

Novel optical packet switching scheme using OFDM label with one core router and two edge routers

In this paper, we propose and experimentally demonstrate a novel optical packet switching scheme with one core router and two edge routers, in which an orthogonal frequency division multiplexing (OFDM) signal is generated as a label. In this experiment there are two transmission spans, each span consists of 50-km SMF-28 and an erbium-doped optical fiber amplifier (EDFA) without dispersion management. A 10 Gb/s on-off keying (OOK) optical payload and a 2.5 Gb/s OFDM optical label are generated, encapsulated, and transmitted in the first span. And then old label is replaced by new label, the generated new optical packet after transmission over the second span is detached and detected. The transmission performance of the optical label and payload is experimentally investigated at the ingress router, core router and egress router.     

High-repetitive frequency millimeter-wave signal generation using multi-cascaded external modulators based on carrier suppression technique

We have numerically and experimentally investigated how to generate high-repetitive frequency millimeter (mm)-wave using multi-cascaded intensity modulators based on optical carrier suppression (OCS) scheme. We have found the rule how to generate the high-repetitive frequency mm-wave by adjusting the repetitive frequency of the radio-frequency (RF) signals and the phase relation between the RF signals on the intensity modulators. Based on this rule, we have experimentally demonstrated to generate over 80 GHz optical mm-wave.     

A filterless optical millimeter-wave generation based on frequency octupling

A novel method of a filterless optical millimeter-wave (MMW) signal generation with frequency octupling using four nested Mach–Zehnder modulators (MZMs) is proposed for Radio-over-fiber systems. By symmetrically biasing the MZMs and using two RF driving signals with 90 deg phase delay, a cost-effective, high-quality and filterless optical millimeter-wave at 80 GHz with an optical harmonic distortion suppression ratio exceeding 40 dB is obtained. The proposed system is insensitive to the MZM bias drift, which demonstrates a relatively higher stability. So it is a viable solution for the future ultra-high frequency MMW applications.     

A novel optical picosecond-duration NRZ-to-RZ format converter with simultaneous wavelength multicasting using a single-stage Mach–Zehnder modulator

We propose a novel and simple scheme to achieve NRZ-to-RZ format conversion and simultaneous wavelength multicasting based on a single-stage dual-arm electro-optic Mach–Zehnder modulator (MZM) and a short single mode fiber (SMF). The format conversion and wavelength multicast process are achieved by chirp compensation under the condition of generation of optical flat frequency comb. 40 Gb/s NRZ-to-RZ conversion with one-to-five multiple-wavelength channel multicasting and transmission of the NRZ and the converted signals over 200 km dispersion-managed fiber-link are successfully demonstrated by numerical simulation. Research results show that 40 Gb/s 2 ps RZ signal with wavelength-preserving can be obtained after format conversion. The converted RZ signal presents good transmission performance and can easily be multiplexed to 160 Gb/s using optical time division multiplexing (OTDM) technology. All the multicast channels can be error free after 50 km transmission. Besides, the conversion operation can also greatly reduce the timing jitter of the degraded NRZ signal due to the retiming function of the proposed scheme.     

A new cross-protection dual-WDM-PON architecture with carrier-reuse colorless ONUs

This paper proposes a new cross-protection colorless dual-WDM-PON architecture. The proposed protection scheme can provide 1 + 1 downstream protection and 1:1 upstream protection against both feeder fiber and distribution fiber failures by using the fiber links and AWGs of the neighboring WDM-PON. Wavelength is reused for the down- and up-stream transmissions in dual-WDM-PONs where gain-saturated reflective semi-conductor optical amplifiers (RSOAs) are employed as colorless transmitters in ONUs. The number of extra protection fibers is minimized and wavelength is much more efficiently utilized compared with other protection schemes. The feasibility and operation of the proposed dual-WDM-PON architecture are experimentally verified with 1.25 Gb/s for upstream and 2.5 Gb/s for downstream over 20 km single mode fiber transmission in both working and protection modes.     

A long-reach WDM passive optical network enabling broadcasting service with centralized light source

We propose a long-reach wavelength-division-multiplexed (WDM) passive optical network (PON) to provide conventional point-to-point (P2P) data and downstream broadcasting service simultaneously by superimposing, for each WDM channel, the differential-phase-shift-keying (DPSK) broadcasting signal with the subcarrier multiplexing (SCM) modulated downstream P2P signal, at the optical line terminal (OLT). In the optical network units (ONUs), by re-modulating part of the downstream signal with a reflective semiconductor optical amplifier (RSOA), we realize color-less ONUs for upstream data transmission. The proposed scheme is numerically verified with a 5 Gb/s downstream P2P signal and broadcasting services, as well as 2.5 Gb/s upstream data through a 60 km bidirectional fiber link. In particular, the influence of the downstream lightwave's optical carrier–subcarrier ratio (OCSR) on the system performance is also investigated.     

An 80 Gbps multi-band access radio over fiber link with single side band optical millimeter-wave dispersion-tolerant transmission without FBG and optical filter

We are presenting a new low-cost Single Sideband (SSB) modulated Radio-over Fiber (RoF) communication system for millimeter (mm)-wave multiband wireless communication at the frequencies of 40 GHz, 80 GHz and 120 GHz. Its principle lies in the Carrier Suppressed modulation through a nested dual electrode Mach–Zehnder Modulator (MZM) and product modulator based baseband signal decomposition. In this novel method, the optical signal is decomposed into different SSB signals using a power splitter and product modulators at the base station. This proposed method uses a different technique for a baseband signal decomposition from the existing method. The proposed signal decomposition technique has reduced the nonlinearities due to the FBGs. The proposed method is compared with the existing method in terms of BER, data rate and OSNR. The simulation results disclose that our proposed scheme outperforms the existing methods at a higher data rate of 80 Gbps with a minimum BER and privileged Q factor.