Real-time experimental demonstration of DSP-enabled soft-ROADMs with multi-level flexible add/drop functions for cloud access networks.

Making use of digital filtering, drop RF signal-driven intensity modulation and passive optical coupling, DSP-enabled flexible ROADMs, termed soft-ROADMs, are experimentally demonstrated in real-time, which are free from both optical filters and O-E-O conversions and are inherently transparent to major network design characteristics. In a 4-channel IMDD optical network node incorporating FPGA-based orthogonal digital filter multiplexing, fully real-time soft-ROADM dynamic add and drop operations at both sub-wavelength and spectrally overlapped orthogonal sub-band levels are extensively, experimentally explored, along with their performance robustness against condition variations of practical networks associated with low-cost optical/electrical components. It is shown that the soft-ROADMs introduce optical power penalties as low as 1.4dB for add operation and 2dB for drop operation. For received optical powers fixed at -10dBm, the add operation can tolerate a differential optical input dynamic range of 6.5dB (1.5dB) for sub-wavelength (sub-band) add operation. On the other hand, robust drop operation performances are obtainable over a ~5dB (16°) drop RF signal amplitude (phase) variation range. This work is a significant milestone in demonstrating the technical feasibility of utilising soft-ROADMs to create a programmable networking environment capable of addressing elastic 5G slicing and the SDN paradigm.

Experimental demonstrations of 30Gb/s/λ digital orthogonal filtering-multiplexed multiple channel transmissions over IMDD PON systems utilizing 10G-class optical devices.

By utilizing digital orthogonal filtering (DOF) in the digital domain, we report, for the first time, experimental demonstrations of aggregated 30.078Gb/s/λ transmissions of DOF-multiplexed spectrally-overlapped and/or frequency gapless six channels over IMDD PON systems incorporating off-the-shelf and low-cost 10G-class optical devices. Experimental results show that simple adaptive channel power loading implemented in the digital domain enables very similar transmission performances of individual channels regardless of their locations in the digital filter space. As a direct result of the interplay between the transmission system-associated negative chromatic dispersion and the intensity modulation-induced frequency chirp, negative power penalties of >0.2dB are experimentally observed for all the involved channels under various transmission system configurations. In addition, excellent performance robustness of the demonstrated systems is also obtainable for various transmission distances up to 45km.

DSP-enabled reconfigurable and transparent spectral converters for converging optical and mobile fronthaul/backhaul networks.

Dynamically reconfigurable and transparent signal spectral conversion is expected to play a vital role in seamlessly integrating traditional metropolitan optical networks and mobile fronthaul/backhaul networks. In this paper, a simple digital signal processing (DSP)-enabled spectral converter is proposed and extensively investigated, for the first time, which just utilizes a single standard dual-parallel Mach-Zehnder modulator (DP-MZM) driven by SDN-controllable RF signals and DC bias currents. As an important thrust of the paper, optimum operating conditions of the proposed converter are analytically identified, statistically examined and experimentally verified. Optimum operating condition-supported spectral converter performances in IMDD-based network nodes are explored both theoretically and experimentally in terms of frequency detuning range-dependent conversion efficiency, spectral conversion-induced OSNR/power penalty and transparency to input signal characteristics. The proposed spectral converter has unique advantages including low configuration complexity, strict transparency, SDN-controllable performance reconfigurability and flexibility, as well as negligible spectral conversion-induced latency.

Experimental demonstration of a DSP-based cross-channel interference cancellation technique for application in digital filter multiple access PONs.

A DSP-based cross-channel interference cancellation (CCIC) technique with initial condition-free, fast convergence and signal modulation format independence, is experimentally demonstrated in a two-channel point-to-point digital filter multiple access (DFMA) PON system based on intensity-modulation and direct-detection (IMDD). The CCIC-induced transmission performance improvements under various system conditions are fully investigated for the first time. It is shown that with one iteration only the CCIC technique can achieve a reduction in individual OFDM subcarrier BERs of more than 1000 times, an increase in transmission capacity by as much as 19 times and an increase in optical power budget by as much as 3.5dB. The CCIC technique thus has the potential to drastically improve the transmission performance of DFMA PONs.

Robust and tunable 16.375Gb/s dual-band optical OFDM transmissions over directly modulated VCSEL-based 200m OM2 MMFs.

Utilizing low-cost, 2.2GHz modulation bandwidth, uncooled and standalone directly modulated VCSEL (DM-VCSEL)-based real-time dual-band optical OFDM (OOFDM) transmitters, aggregated 16.375Gb/s transmissions of OOFDM signals having bandwidths approximately 3.8 times higher than the VCSEL manufacturer-specified modulation bandwidths, are experimentally demonstrated, for the first time, over 200m OM2 MMF links based on intensity modulation and direct detection. The aggregated signal transmission capacities of the aforementioned links vary by just 8% for various OM2 MMFs ranging from 100m to 500m, and by just 10% over a 1GHz passband carrier frequency detuning range. Such dual-band OOFDM adaptability-induced excellent performance robustness and large passband frequency tunability can significantly relax the requirements on VCSEL modulation bandwidth for achieving specific transmission performances for cost-sensitive application scenarios such as data centers.

Real-time experimental demonstrations of software reconfigurable optical OFDM transceivers utilizing DSP-based digital orthogonal filters for SDN PONs.

Real-time optical OFDM (OOFDM) transceivers with on-line software-controllable channel reconfigurability and transmission performance adaptability are experimentally demonstrated, for the first time, utilizing Hilbert-pair-based 32-tap digital orthogonal filters implemented in FPGAs. By making use of an 8-bit DAC/ADC operating at 2GS/s, an oversampling factor of 2 and an EML intensity modulator, the demonstrated RF conversion-free transceiver supports end-to-end real-time simultaneous adaptive transmissions, within a 1GHz signal spectrum region, of a 2.03Gb/s in-phase OOFDM channel and a 1.41Gb/s quadrature-phase OOFDM channel over a 25km SSMF IMDD system. In addition, detailed experimental explorations are also undertaken of key physical mechanisms limiting the maximum achievable transmission performance, impacts of transceiver's channel multiplexing/demultiplexing operations on the system BER performance, and the feasibility of utilizing adaptive modulation to combat impairments associated with low-complexity digital filter designs. Furthermore, experimental results indicate that the transceiver incorporating a fixed digital orthogonal filter DSP architecture can be made transparent to various signal modulation formats up to 64-QAM.

Self-seeding-based 10Gb/s over 25km optical OFDM transmissions utilizing face-to-face dual-RSOAs at gain saturation.

Self-seeded passive optical networks (PONs) are currently attracting extensive research interest. In this paper, a novel self-seeded PON transmitter is, for the first time, proposed and experimentally demonstrated, which incorporates two face-to-face-positioned reflective semiconductor optical amplifiers (RSOAs) operating at their gain saturation regions: one RSOA directly driven by an upstream electrical signal and the other RSOA biased at a fixed current. Detailed experimental explorations are undertaken of the dynamic performance characteristics of the proposed transmitter. It is shown that, in comparison with previously reported self-seeded transmitters each employing a reflective mirror and a single electrical signal-driven RSOA, the proposed transmitter has a number of salient advantages including, considerably narrowed optical signal spectra, up to 16dB reduction in RINs of intensity-modulated optical signals, and residual intensity modulation crosstalk suppression as high as 10.7dB. The aforementioned features enable experimental demonstrations of real-time self-seeded 10Gb/s optical OFDM (OOFDM) transmitters. In particular, by making use of two low-cost RSOAs having their 3-dB modulation bandwidths as small as 1.125GHz, 10Gb/s over 25km adaptive OOFDM transmissions with power penalties of 0.6dB are experimentally achieved in the simple self-seeded IMDD PON systems.

Record-high and robust 17.125 Gb/s gross-rate over 25 km SSMF transmissions of real-time dual-band optical OFDM signals directly modulated by 1 GHz RSOAs.

Aggregated 17.125 Gb/s real-time end-to-end dual-band optical OFDM (OOFDM) transmissions over 25 km SSMF IMDD systems with 7 dB receiver sensitivity improvements are experimentally demonstrated, for the first time, by utilizing low-cost transceiver components such as directly modulated 1GHz RSOAs and DACs/ADCs operating at sampling speeds as low as 4GS/s. The demonstrated OOFDM transceivers have both strong adaptability and sufficiently large passband carrier frequency tunability, which enable full use of highly dynamic spectral characteristics of the transmission systems. This results in the achievements of not only excellent performance robustness to variations in system operating conditions but also significantly relaxed requirements on RSOA small-signal modulation bandwidth. It is shown that the aforementioned transmission capacity only varies by <23% over a RSOA-injected optical power variation range as large as 20dB, and that the 1 GHz RSOAs can support successful transmissions of adaptively modulated OOFDM signals having bandwidths of 8.5 GHz. By taking into account the adopted 25% cyclic prefix and a typical 7.3% FEC overhead, the demonstrated real-time OOFDM transmission systems are capable of conveying 11.6 Gb/s user data.

Robust real-time 15.125 Gb/s adaptive optical OFDM transmissions over 100 m OM2 MMFs utilizing directly modulated VCSELs subject to optical injection locking.

Optical injection locking (OIL) is an effective approach for significantly enhancing the modulation bandwidths of VCSELs. The frequency responses of OIL-VCSELs are, however, very sensitive to the applied OIL conditions. This brings about strong difficulties in practically utilizing the OIL-enhanced modulation bandwidths to achieve highly robust transmission performances of directly modulated OIL-VCSEL-based multi-mode fibre (MMF) links for cost-sensitive application scenarios such as data-centers. In this paper, directly modulated OIL-VCSEL-based real-time dual-band optical OFDM (OOFDM) transceivers with tunability in both the electrical and optical domains are experimentally demonstrated, for the first time, utilizing DACs/ADCs at sampling speeds as low as 4GS/s. The transceivers can support 15.125 Gb/s adaptive OOFDM transmissions over 100 m OM2 MMF links based on intensity modulation and direct detection. More importantly, the adaptability and tunability of the demonstrated transceivers enable the achievement of excellent robustness of the aggregated OOFDM transmission capacity to OIL condition variations. It is shown that, over a large diversity of OIL conditions that give rise to significantly different system frequency responses, the aggregated OOFDM transmission capacity only vary by <11% in the aforementioned transmission link.

Experimental demonstrations of record high REAM intensity modulator-enabled 19.25Gb/s real-time end-to-end dual-band optical OFDM colorless transmissions over 25km SSMF IMDD systems.

Record-high 19.25Gb/s real-time end-to-end dual-band optical OFDM (OOFDM) colorless transmissions across the entire C-band are experimentally demonstrated, for the first time, in reflective electro-absorption modulator (REAM)-based 25km standard SMF systems using intensity modulation and direct detection. Adaptively modulated baseband (0-2GHz) and passband (6.125 ± 2GHz) OFDM RF sub-bands, supporting signal line rates of 9.75Gb/s and 9.5Gb/s respectively, are independently generated and detected with FPGA-based DSP clocked at only 100MHz as well as DACs/ADCs operating at sampling speeds as low as 4GS/s. The two OFDM sub-bands are electrically multiplexed for intensity modulation of a single optical carrier by an 8GHz REAM. The REAM colorlessness is experimentally characterized, based on which optimum REAM operating conditions are identified. To maximize and balance the signal transmission performance of each sub-band, on-line adaptive transceiver optimization functions and live performance monitoring are fully exploited to optimize key OOFDM transceiver and system parameters. For different wavelengths within the C-band, corresponding minimum received optical powers at the FEC limit vary in a range of <0.5dB and bit error rate performances for both baseband and passband signals are almost identical. Furthermore, detailed investigations are also undertaken of the maximum aggregated signal line rate sensitivity to electrical sub-band power variation. It is shown that the aforementioned system has approximately 3dB tolerance to RF sub-band power variation.

Experimental demonstration of record high 19.125 Gb/s real-time end-to-end dual-band optical OFDM transmission over 25 km SMF in a simple EML-based IMDD system.

Record high 19.125 Gb/s real-time end-to-end dual-band optical OFDM (OOFDM) transmission is experimentally demonstrated, for the first time, in a simple electro-absorption modulated laser (EML)-based 25 km standard SMF system using intensity modulation and direct detection (IMDD). Adaptively modulated baseband (0-2GHz) and passband (6.125 ± 2GHz) OFDM RF sub-bands, supporting line rates of 10 Gb/s and 9.125 Gb/s respectively, are independently generated and detected with FPGA-based DSP clocked at only 100 MHz and DACs/ADCs operating at sampling speeds as low as 4GS/s. The two OFDM sub-bands are electrically frequency-division-multiplexed (FDM) for intensity modulation of a single optical carrier by an EML. To maximize and balance the signal transmission performance of each sub-band, on-line adaptive features and on-line performance monitoring is fully exploited to optimize key OOFDM transceiver and system parameters, which includes subcarrier characteristics within each individual OFDM sub-band, total and relative sub-band power as well as EML operating conditions. The achieved 19.125 Gb/s over 25 km SMF OOFDM transmission system has an optical power budget of 13.5 dB, and shows almost identical bit error rate (BER) performances for both the baseband and passband signals. In addition, experimental investigations also indicate that the maximum achievable transmission capacity of the present system is mainly determined by the EML frequency chirp-enhanced chromatic dispersion effect, and the passband BER performance is not affected by the two sub-band-induced intermixing effect, which, however, gives a 1.2dB optical power penalty to the baseband signal transmission.

REAM intensity modulator-enabled 10Gb/s colorless upstream transmission of real-time optical OFDM signals in a single-fiber-based bidirectional PON architecture.

Reflective electro-absorption modulation-intensity modulators (REAM-IMs) are utilized, for the first time, to experimentally demonstrate colorless ONUs in single-fiber-based, bidirectional, intensity-modulation and direct-detection (IMDD), optical OFDM PONs (OOFDM-PONs) incorporating 25 km SSMFs and OLT-side-seeded CW optical signals. The colorlessness of the REAM-IMs is characterized, based on which optimum REAM-IM operating conditions are identified. In the aforementioned PON architecture, 10Gb/s colorless upstream transmissions of end-to-end real-time OOFDM signals are successfully achieved for various wavelengths within the entire C-band. Over such a wavelength window, corresponding minimum received optical powers at the FEC limit vary in a range as small as <0.5 dB. In addition, experimental measurements also indicate that Rayleigh backscattering imposes a 2.8 dB optical power penalty on the 10 Gb/s over 25 km upstream OOFDM signal transmission. Furthermore, making use of on-line adaptive bit and power loading, a linear trade-off between aggregated signal line rate and optical power budget is observed, which shows that, for the present PON system, a 10% reduction in signal line rate can improve the optical power budget by 2.6 dB.

First real-time experimental demonstrations of 11.25Gb/s optical OFDMA PONs with adaptive dynamic bandwidth allocation.

End-to-end real-time experimental demonstrations are reported, for the first time, of aggregated 11.25Gb/s over 26.4km standard SMF, optical orthogonal frequency division multiple access (OOFDMA) PONs with adaptive dynamic bandwidth allocation (DBA). The demonstrated intensity-modulation and direct-detection (IMDD) OOFDMA PON system consists of two optical network units (ONUs), each of which employs a DFB-based directly modulated laser (DML) or a VCSEL-based DML for modulating upstream signals. Extensive experimental explorations of dynamic OOFDMA PON system properties are undertaken utilizing identified optimum DML operating conditions. It is shown that, for simultaneously achieving acceptable BERs for all upstream signals, the OOFDMA PON system has a >3dB dynamic ONU launch power variation range, and the BER performance of the system is insusceptible to any upstream symbol offsets slightly smaller than the adopted cyclic prefix. In addition, experimental results also indicate that, in addition to maximizing the aggregated system transmission capacity, adaptive DBA can also effectively reduce imperfections in transmission channel properties without affecting signal bit rates offered to individual ONUs.

Experimental demonstration and optimisation of a synchronous clock recovery technique for real-time end-to-end optical OFDM transmission at 11.25Gb/s over 25km SSMF.

A simple, digital signal processing-free, low-cost and robust synchronous clocking technique is proposed and experimentally demonstrated, for the first time, in a 64-QAM-encoded, 11.25Gb/s over 25km SSMF, real-time end-to-end optical OFDM (OOFDM) system using directly modulated DFB laser-based intensity-modulation and direct-detection (IMDD). Detailed experimental investigations show that, in comparison with the common clock approach utilised in previous experimental demonstrations, the proposed clocking technique can be implemented to achieve no system BER performance degradation or optical power budget penalty and more importantly to improve system stability. As a viable synchronous clocking solution for real-time OOFDM transmission, this work is a vital step towards the realisation of practical OOFDM transmission systems and has particular significance for synchronisation of OOFDM multiple access-based passive optical networks where highly accurate synchronisation of all network elements is essential.

Real-time experimental demonstration of low-cost VCSEL intensity-modulated 11.25 Gb/s optical OFDM signal transmission over 25 km PON systems.

The feasibility of utilising low-cost, un-cooled vertical cavity surface-emitting lasers (VCSELs) as intensity modulators in real-time optical OFDM (OOFDM) transceivers is experimentally explored, for the first time, in terms of achievable signal bit rates, physical mechanisms limiting the transceiver performance and performance robustness. End-to-end real-time transmission of 11.25 Gb/s 64-QAM-encoded OOFDM signals over simple intensity modulation and direct detection, 25 km SSMF PON systems is experimentally demonstrated with a power penalty of 0.5 dB. The low extinction ratio of the VCSEL intensity-modulated OOFDM signal is identified to be the dominant factor determining the maximum obtainable transmission performance. Experimental investigations indicate that, in addition to the enhanced transceiver performance, adaptive power loading can also significantly improve the system performance robustness to variations in VCSEL operating conditions. As a direct result, the aforementioned capacity versus reach performance is still retained over a wide VCSEL bias (driving) current (voltage) range of 4.5 mA to 9 mA (275 mVpp to 320 mVpp). This work is of great value as it demonstrates the possibility of future mass production of cost-effective OOFDM transceivers for PON applications.

Significant improvements in optical power budgets of real-time optical OFDM PON systems.

Based on a comprehensive theoretical optical orthogonal frequency division multiplexing (OOFDM) system model rigorously verified by comparing numerical results with end-to-end real-time experimental measurements at 11.25Gb/s, detailed explorations are undertaken, for the first time, of the impacts of various physical factors on the OOFDM system performance over directly modulated DFB laser (DML)-based, intensity modulation and direct detection (IMDD), single-mode fibre (SMF) systems without in-line optical amplification and chromatic dispersion compensation. It is shown that the low extinction ratio (ER) of the DML modulated OOFDM signal is the predominant factor limiting the maximum achievable optical power budget, and the subcarrier intermixing effect associated with square-law photon detection in the receiver reduces the optical power budget by at least 1dB. Results also indicate that, immediately after the DML in the transmitter, the insertion of a 0.02nm bandwidth optical Gaussian bandpass filter with a 0.01nm wavelength offset with respect to the optical carrier wavelength can enhance the OOFDM signal ER by approximately 1.24dB, thus resulting in a 7dB optical power budget improvement at a total channel BER of 1 × 10(-3).

Real-time experimental demonstration of optical OFDM symbol synchronization in directly modulated DFB laser-based 25km SMF IMDD systems.

A simple optical OFDM (OOFDM) synchronization technique utilizing subtraction and Gaussian windowing at the symbol rate is proposed and implemented in FPGA-based OOFDM receivers. End-to-end real-time symbol synchronization of 128-QAM-encoded OOFDM signals at raw bit rates of 6.56Gb/s is experimentally demonstrated, for the first time, over directly modulated DFB laser-based 25km SMF intensity modulation and direct detection (IMDD) systems. Experimental investigations show that the proposed synchronisation technique offers a number of salient advantages including low complexity, fast tracking speed, high accuracy and suitability for high-capacity optical transmission systems.

Wavelength reused bidirectional transmission of adaptively modulated optical OFDM signals in WDM-PONs incorporating SOA and RSOA intensity modulators.

Detailed numerical investigations are undertaken of wavelength reused bidirectional transmission of adaptively modulated optical OFDM (AMOOFDM) signals over a single SMF in a colorless WDM-PON incorporating a semiconductor optical amplifier (SOA) intensity modulator and a reflective SOA (RSOA) intensity modulator in the optical line termination and optical network unit, respectively. A comprehensive theoretical model describing the performance of such network scenarios is, for the first time, developed, taking into account dynamic optical characteristics of SOA and RSOA intensity modulators as well as the effects of Rayleigh backscattering (RB) and residual downstream signal-induced crosstalk. The developed model is rigorously verified experimentally in RSOA-based real-time end-to-end OOFDM systems at 7.5 Gb/s. It is shown that the RB noise and crosstalk effects are dominant factors limiting the maximum achievable downstream and upstream transmission performance. Under optimum SOA and RSOA operating conditions as well as practical downstream and upstream optical launch powers, 10 Gb/s downstream and 6 Gb/s upstream over 40 km SMF transmissions of conventional double sideband AMOOFDM signals are feasible without utilizing in-line optical amplification and chromatic dispersion compensation. In particular, the aforementioned transmission performance can be improved to 23 Gb/s downstream and 8 Gb/s upstream over 40 km SMFs when single sideband subcarrier modulation is adopted in the downstream systems.

Experimental demonstration of a record high 11.25Gb/s real-time optical OFDM transceiver supporting 25km SMF end-to-end transmission in simple IMDD systems.

The fastest ever 11.25Gb/s real-time FPGA-based optical orthogonal frequency division multiplexing (OOFDM) transceivers utilizing 64-QAM encoding/decoding and significantly improved variable power loading are experimentally demonstrated, for the first time, incorporating advanced functionalities of on-line performance monitoring, live system parameter optimization and channel estimation. Real-time end-to-end transmission of an 11.25Gb/s 64-QAM-encoded OOFDM signal with a high electrical spectral efficiency of 5.625bit/s/Hz over 25km of standard and MetroCor single-mode fibres is successfully achieved with respective power penalties of 0.3dB and -0.2dB at a BER of 1.0 x 10(-3) in a directly modulated DFB laser-based intensity modulation and direct detection system without in-line optical amplification and chromatic dispersion compensation. The impacts of variable power loading as well as electrical and optical components on the transmission performance of the demonstrated transceivers are experimentally explored in detail. In addition, numerical simulations also show that variable power loading is an extremely effective means of escalating system performance to its maximum potential.

First experimental demonstration of 6Gb/s real-time optical OFDM transceivers incorporating channel estimation and variable power loading.

The fastest ever 6Gb/s real-time FPGA-based optical orthogonal frequency division multiplexing (OOFDM) transceivers utilizing channel estimation are experimentally demonstrated, for the first time, with variable power loading being incorporated to effectively compensate for the rapid system frequency response roll-off effect. The implemented transceivers are constructed entirely from off-the-shelf components and incorporate crucial functionalities of on-line performance monitoring and live optimization of key parameters including signal clipping, subcarrier power and operating conditions of directly modulated DFB lasers (DMLs). Real-time end-to-end transmission of a 6Gb/s 16-QAM-encoded OOFDM signal over 300m OM1 multi-mode fiber with a power penalty of 0.5dB is successfully achieved in an intensity-modulation and direct-detection system employing a DML.