Subcarrier-multiplexed Signals Research Articles

Investigation of laser phase noise tolerance in simplified coherent PON with subcarrier multiplexing

In this paper, we experimentally investigate the laser phase noise tolerance for a 100 Gb/s/λ coherent passive optical network (PON) with simplified optical network units (ONUs) transceivers when distributed feedback (DFB) laser is used as the local oscillator (LO). In the proposed scheme, the digitally subcarrier-multiplexed (DSCM) signals are generated in the coherent transmitter at the optical line terminal (OLT) side based on Alamouti encoding in two polarizations and received by a single-polarization heterodyne receiver at the ONU side. The experimental results show that the power budget can achieve 34.2 dB and 33.9 dB after a 25 km standard single-mode fiber (SSMF) link when external cavity laser (ECL) and DFB lasers are used as the LOs at the ONU side. Polarization and carrier phase tracking schemes for the DSCM systems based on frequency domain pilot tones were used in digital signal process (DSP). At the same time, the frequency offset estimation and phase noise compensation algorithms are conducted using residual carriers. By choosing an appropriate width for the low-pass filter, the optimal performance for the phase noise compensation can be achieved. The pre-equalization operation at the OLT side is also verified for the subcarrier-multiplexed signals to provide better and flatter performances.

Tolerance of SCM Nyquist and OFDM signals for heterogeneous fiber-optic and millimeter-wave mobile backhaul links under the effect of power amplifier saturation induced clipping

Extremely high frequency carrier communications such as millimeter-wave (mmW) is considered a promising solution for the future mobile networks. To compensate for the large path loss of the millimeter-wave links, saturated power amplifiers are inevitably used during amplify-and-forward relaying, which potentially causes system degradation due to power saturation induced clipping. In this paper, the tolerance of OFDM and Nyquist subcarrier multiplexing (SCM) signals against power amplifier saturation induced clipping is studied in the context of fiber-optic and millimeter-wave integrated communications for heterogeneous mobile backhaul networks. In the system, 10 Gbaud OFDM and Nyquist SCM signals are converted to an intermediate frequency passband before converted to the optical domain for a radio-over-fiber transmission to a remote antenna unit (RAU). At RAU, the optical radio signals are converted back to the electrical domain at millimeter-wave range. Those are then amplified by an power amplifier prior to a millimeter-wave communication to a remote radio head (RRH) for detection. Since Nyquist SCM signal exhibits a level of lower peak-to-average power ratio (PAPR) than that of OFDM, Nyquist SCM signals are more tolerant against the clipping caused by the power amplifier saturation. For 16 QAM, at clipping ratio (CR) of 4.5 dB, one-band Nyquist SCM signal shows a signal-to-noise ratio (SNR) performance over 10 times higher than that of OFDM. While the increase in the number of Nyquist SCM bands reduces the tolerance, Nyquist SCM signal maintains a better tolerance than OFDM one with respect to 16 QAM, 32 QAM and 64 QAM formats, which suffer severer impact with larger SNR penalty for higher modulation levels.

Toward Practical Kramers-Kronig Receiver: Resampling, Performance, and Implementation

The Kramers-Kronig (KK) receiver has recently drawn a great deal of attention due to its capability of retrieving the phase information from the directly-detected intensity waveforms. However, a technical challenge associated with the implementation of this attractive receiver is the high sampling rate of digital signal processing (DSP). Since nonlinear operations (such as logarithm and exponential functions) included in the conventional KK algorithm broaden the signal spectrum significantly, digital upsampling is required at the beginning of DSP. To solve this problem, we have recently proposed and demonstrated a new KK algorithm operable without the digital upsampling. In this upsampling-free algorithm, we removed the exponential function by expressing the complex signal in the Cartesian form (i.e., real plus imaginary) and replaced the logarithm function with its mathematical approximation. In this paper, we present the detailed comparison between the conventional and upsampling-free KK algorithms through numerical simulations. We investigate the performance of the KK receivers when we employ orthogonal frequency-division multiplexing and Nyquist subcarrier multiplexing signals formatted in 16-quadrature amplitude modulation (16-QAM) and 64-QAM. Also investigated in this paper is the performance limitation of the upsampling-free KK algorithm. Finally, we propose the schematic diagrams of hardware implementation for the two KK algorithms, and study their complexity as well as their power consumption. The results show that the upsampling-free KK algorithm reduces the complexity and power consumption by a factor of 7 to 10, in comparison with the conventional algorithm at the expense of small sensitivity penalties. For example, the sensitivity difference between the two algorithms is shown to be < 1 dB for the 16-QAM formatted signals.

Coherent-Optical In-Line Add/Drop of PDM Tributaries of Subcarrier Multiplexed Signals

We propose an extended coherent optical add/drop multiplexing scheme, in which an individual polarization tributary in a polarization-division multiplexed orthogonal frequency-division multiplexed signal is erased and added by frequency conversion in fiber, which is sequentially inserted in a transmission line. In the scheme, a polarization tributary can be inserted and replaced in a dual polarization spectrally overlapped ultra-dense multiplexed signal. In a proof-of-concept demonstration, we investigate the performance of the proposed optical add/drop multiplexing scheme. A 25-Gb/s quadrature phase-shift keying tributary within a dual-polarization three subcarrier orthogonal frequency division multiplexed signal is erased and added with negligible distortion of the orthogonal polarization tributary and the adjacent subcarriers.

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.

Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems.

In this work we experimentally investigate the improved intra-channel fiber nonlinearity tolerance of digital subcarrier multiplexed (SCM) signals in a single-channel coherent optical transmission system. The digital signal processing (DSP) for the generation and reception of the SCM signals is described. We show experimentally that the SCM signal with a nearly-optimum number of subcarriers can extend the maximum reach by 23% in a 24 GBaud DP-QPSK transmission with a BER threshold of 3.8 × 10(-3) and by 8% in a 24 GBaud DP-16-QAM transmission with a BER threshold of 2 × 10(-2). Moreover, we show by simulations that the improved performance of SCM signals is observed over a wide range of baud rates, further indicating the merits of SCM signals in baud-rate flexible agile transmissions and future high-speed optical transport systems.

Reconfigurable digital receiver for 8PSK subcarrier multiplexed and 16QAM single carrier phase-modulated radio over fiber links

A reconfigurable digital receiver based on the k-means algorithm is proposed for phase-modulated subcarrier multiplexed (SCM) and quadrature amplitude-modulated single carrier, phase-modulated radio-over-fiber links. We report successful demodulation after 40 km single mode fiber transmission with three 50 Mbaud 8PSK SCM signals and a 312.5 Mbaud 16QAM single carrier. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1015–1018, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25905

Effect of Fiber Dispersion and Self-phase Modulation in Multi-channel Subcarrier Multiplexed Optical Signal Transmission

We investigated the combined effect of fiber chromatic dispersion and self-phase modulation (SPM) in multi-channel subcarrier multiplexed (SCM) optical transmission systems. We theoretically analyzed the transmission characteristics of the SCM signals with the effect of SPM and chromatic dispersion in a single-mode optical fiber by numerical simulations based on the nonlinear Schrodinger equation. The numerical simulation results revealed that the effect of fiber dispersion and SPM could occur independently between subcarrier channels in two-channel SCM systems for small optical modulation index (OMI) and large channel spacing. However, for large OMI, small channel spacing, and large fiber launching power, we found a performance degradation of the two-channel system compared to that of a single-channel system. These parameters are therefore important for the optimization of multi-channel SCM systems applicable to radio over fiber networks.

Colorless WDM-PON based on a Fabry-Pérot laser diode and reflective semiconductor optical amplifiers for simultaneous transmission of bidirectional gigabit baseband signals and broadcasting signal

A novel WDM-PON system delivering bidirectional baseband data and broadcasting data is proposed and demonstrated. A subcarrier multiplexing signal is broadcasted to all users by modulating a broadband optical source based on a Fabry-Pérot laser diode. Reflective semiconductor optical amplifiers are used as colorless modulators for the baseband data at both optical line terminal and remote optical network units. Transmission performance including bit error rate of bidirectional gigabit data and error vector magnitude of broadcasting data of many optical channels is investigated. Additionally, the data rate for the broadcasting signal was improved by using an external modulator.

Reflective SOA-Based Bidirectional WDM-PON Sharing Optical Source for Up/Downlink Data and Broadcasting Transmission

We proposed and experimentally demonstrated the reflective semiconductor optical amplifier (RSOA)-based wavelength-division-multiplexed passive optical network (WDM-PON) scheme where not only up/downlink data services but also broadcasting service could be provided using a single optical source. In the proposed scheme, the digital signal and subcarrier multiplexing (SCM) signal for downstream were simultaneously modulated by a single distributed feedback laser diode and RSOA in optical network unit remodulated downstream source as an upstream. Without performance deterioration of digital signals both in up- and downstream, the proposed WDM-PON scheme can stably offer the SCM signal for broadcasting service. In the experiment, 1-Gb/s digital signals both for up- and downstream and 20-Mb/s SCM signal at 2.2 GHz for broadcasting were demonstrated in 10-km bidirectional link

In-Band Time-to-Live Signaling System for Combined DPSK/SCM Scheme in OLS

We propose and validate experimentally a time-to-live (TTL) signaling system for an optical label swapping scheme based on 10-Gb/s differential phase-shift-keying (DPSK) packets and with 100-Mb/s subcarrier multiplexing (SCM) label. The proposed scheme allows fast packet discarding by using a 3-GHz subcarrier tone. DPSK payload has only a 2.4-dB power penalty at 10/sup -9/ bit-error rate after superimposing the SCM and TTL labeling signal.

Degree-of-Polarization-Based PMD Monitoring for Subcarrier-Multiplexed Signals Via Equalized Carrier/Sideband Filtering

We demonstrate a method for differential-group-delay (DGD) and polarization-mode-dispersion (PMD) monitoring using the degree-of-polarization (DOP) in subcarrier-multiplexed (SCM) systems. Traditional SCM signal show very little DOP sensitivity to DGD/PMD due to the low modulation depth used for generating SCM signals. We use a narrow-band optical filter to equalize the power of the carrier and one of the sidebands by offsetting the filter from the carrier, enabling PMD and DGD monitoring and more than tripling the DOP sensitivity to DGD/PMD. Our technique is simple, uses only a single optical filter, and can be applied to both single- and double-sideband (SSB and DSB) SCM signals as well as single and multisubcarrier systems. Additionally, we show that our monitoring technique is robust to the chromatic dispersion-induced radio-frequency (RF) power fading effect seen in DSB SCM signals. Using this technique to enhance the DOP sensitivity to DGD/PMD and generate a feedback signal to a PMD compensator (PMDC), we obtain an 11-dB improvement in the 5% RF power tail.

Multifunctional Operation of a Fiber Bragg Grating in a WDM/SCM Radio Over Fiber Distribution System

A radio over fiber distribution system incorporating both sub-carrier multiplexing (SCM) and wavelength division multiplexing (WDM) technologies is presented. The SCM signal contains five 155-Mbit/s data channels, centered around 18.5 GHz with 450 MHz spacing. This signal is directly modulated onto three high-speed lasers with emission frequencies spaced by 50 GHz. Bragg filters are employed at the receiver base-station in order to both demultiplex the required optical channel and ensure that the detected signal is single sideband (in order to overcome dispersion limitations on the link). Our results show negligible degradation in system performance for the demultiplexing of the WDM signal compared with the back-to-back performance curves.

Wavelength conversion of SCM signals using semiconductor optical amplifiers: theory, experiments, and applications

In this paper, we deal with the subject of cross-gain modulation (XGM) and cross-phase modulation (XPM) semiconductor optical amplifier-based wavelength conversion of optical channels carrying subcarrier multiplexing signals in a comprehensive way. The equations and models that describe the conversion process and the resulting harmonic and intermodulation distortions are obtained showing the superior performance of XPM over XGM in terms of secondand third-order distortion and contrast ratio. Experimental results for XGM-based wavelength conversion that confirm the results predicted by our theoretical models are presented, and finally, we consider the specific application of wavelength conversion of optical channels carrying full frequency plans such as that of cable television applications.

Full distortion induced by dispersion evaluation and optical bandwidth constraining of fiber Bragg grating demultiplexers over analogue SCM systems

We provide a full analysis of the distortion effects produced by the first and second order in-band dispersion of fiber Bragg grating based optical demultiplexers over analogue SCM (Sub Carrier Multiplexed) signals. Optical bandwidth utilization ranges for Dense WDM network are calculated considering different SCM system cases of frequency extension and modulation conditions.

Statistics of PMD-induced power fading for intensity-modulated double-sideband and single-sideband microwave and millimeter-wave signals

Polarization-mode dispersion (PMD) can severely degrade the performance of millimeter-wave fiber-optic links by inducing a power fading penalty of the received signal that is dependent on the subcarrier frequency and accumulated PMD along the fiber. We experimentally investigate the statistics of PMD-induced power fading as a function of the differential group delay (DGD) for intensity-modulated double- and single-sideband subcarrier-multiplexed signals in the absence of chromatic dispersion. We find a similar susceptibility to PMD-induced power fading for both modulation formats with a subcarrier frequency of 7 GHz using a PMD emulator with a Maxwellian distribution of DGD (average DGD /spl sim/40 ps). A significant improvement in the worst case power fading penalty (/spl sim/20 dB) is achieved by using an electronically controlled polarization controller in combination with a single section of polarization maintaining fiber in a dynamic first-order PMD compensator. Furthermore, the results of numerical Monte Carlo simulations support the measured data and show the scalability of PMD-induced power fading for subcarrier-multiplexed signals in the microwave and millimeter-wave region.

Simple, polarisation-independent, and dispersion-insensitive SCM signal extraction technique for optical switching systems applications

A dispersion- and polarisation-insensitive all-optical method for transmitting and extracting a subcarrier multiplexed (SCM) signal using a fibre Bragg grating filter without a conventional SCM circuitry is demonstrated. Fibre transmission experiments conducted on standard singlemode fibres of varying lengths up to 80 km showed excellent sensitivity with no apparent fading effects.

A novel in-service measurement technique using the same wavelength band as SCM signals

This paper proposes a new technique for measuring in-service optical fibers, that uses an optical time domain reflectometer (OTDR). The feature of the proposed technique is that the OTDR light is in the same wavelength band as the video signal, which is distributed by using the subcarrier multiplexing (SCM) technique. In a 40-channel SCM system operating at a signal wavelength of 1.558 /spl mu/m, we show that the required video quality can be maintained, by using the proposed OTDR operating in the 1.55 /spl mu/m band, even though the measured fiber is in service and the OTDR light enters an optical receiver. Moreover, we clarify the conditions for undertaking measurements, without the need for optical filters designed to prevent OTDR light from degrading the SCM signal quality.

Subcarrier multiplexing signal modulation at 60 GHz using two-mode injection-locked Fabry-Perot laser

The subcarrier multiplexing of three 156 Mbit/s binary phase-shift keyed signals is demonstrated at the 60 GHz millimetre-wave via the two-mode injection-locking and direct modulation of a Fabry-Perot laser. The influence of the third-order distortion caused by the multi-tone modulation of the laser is also investigated.

Full-duplex fiber-optic RF subcarrier transmission using a dual-function modulator/photodetector

An electroabsorption waveguide device is presented as a dual-function modulator/photodetector for application as a cost-effective full-duplex transceiver in radio-frequency (RF) fiber-optic links. The spectral modulation and detection properties of the dual-function transceiver are characterized experimentally. Extinction ratio, insertion loss, and responsivity are 12 dB, 7 dB, and 0.8 A/W, respectively. Modulation and detection bandwidths are both in excess of 17 GHz. By employing a dual-lightwave technique, optimum modulation and detection performance is simultaneously achieved. Furthermore, full-duplex error-free optical transmission of RF subcarrier-multiplexed signals over 10 km nondispersion shifted single-mode fiber is demonstrated and a point-to-multipoint optical ring architecture is proposed.