X. Q. Jin

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.

Experimental Demonstrations and Extensive Comparisons of End-to-End Real-Time Optical OFDM Transceivers With Adaptive Bit and/or Power Loading

Experimental demonstrations are reported for end-to-end real-time optical orthogonal frequency division multiplexing (OOFDM) transceivers incorporating three widely adopted adaptive loading techniques, namely, power loading (PL), bit loading (BL), and bit-and-power loading (BPL). In directly modulated distributed-feedback (DFB) laser-based, intensity-modulation, and direct-detection (IMDD) transmission systems consisting of up to 35-km single-mode fibers (SMFs), extensive experimental comparisons between these adaptive loading techniques are made in terms of maximum achievable signal bit rate, optical power budget, and digital signal processing (DSP) resource usage. It is shown that BPL is capable of supporting end-to-end real-time OOFDM transmission of 11.75 Gb/s over 25-km SMFs in the aforementioned systems at sampling speeds as low as 4 GS/s. In addition, experimental measurements also show that BPL (PL) offers the highest (lowest) signal bit rate, and their optical power budgets are similar. The observed signal bit rate difference between BPL and PL is almost independent of sampling speed and transmission distance. All the aforementioned key features agree very well with numerical simulations. On the other hand, BPL-consumed DSP resources are approximately three times higher than those required by PL. The results indicate that PL is a preferred choice for cost-effective OOFDM transceiver design.

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 Real-Time Optical OFDM Transmission at 7.5 Gb/s Over 25-km SSMF Using a 1-GHz RSOA

The 7.5-Gb/s real-time end-to-end optical orthogonal frequency-division-multiplexing (OOFDM) transceivers incorporating variable power loading on each individual subcarrier are demonstrated experimentally using a live-optimized reflective semiconductor optical amplifier intensity modulator having a modulation bandwidth as narrow as 1 GHz. Real-time OOFDM signal transmission at 7.5 Gb/s over 25-km standard single-mode fiber is achieved across the C-band in simple intensity modulation and direct detection systems without in-line optical amplification and dispersion compensation.

Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers

The feasibility of implementing 128-QAM in off-the-shelf component-based real-time optical OFDM (OOFDM) transceivers incorporating advanced channel estimation, on-line performance monitoring and live parameter optimisation, is experimentally investigated, for the first time, in intensity-modulation and direct-detection (IMDD) single-mode fibre (SMF) and multi-mode fibre (MMF) transmission systems involving directly modulated DFB lasers. The highest ever spectral efficiency of 5.25bit/s/Hz is demonstrated successfully in the aforementioned simple systems. Experimental investigations show that, it is feasible to transmit 5.25 Gb/s 128-QAM-encoded OOFDM real-time signals over 25 km MetroCor(TM) SMFs and 500 m 62.5/125 microm OM1 MMFs. The impact of key parameters on the transmission performance of the real-time OOFDM transceivers with 128-QAM encoding are explored, based on which optimum signal clipping ratios are identified.

Optimization of adaptively modulated optical OFDM modems for multimode fiber-based local area networks [Invited]

Focus Issue on Orthogonal-Frequency-Division Multiplexed Communications Systems and Networks The impact of a wide range of different parameters of various components involved in optical modems using adaptively modulated optical orthogonal-frequency-division multiplexing (AMOOFDM) on the transmission performance of AMOOFDM signals is explored thoroughly, in single-channel, unamplified, multimode-fiber (MMF)-based, intensity modulation and direct detection (IMDD) transmission links. Practically available optimum component parameters are identified, based on which the AMOOFDM modems are optimized. It is shown that the optimized AMOOFDM modems enable a >70% increase in the capacity versus reach performance without compromising link loss margins, in comparison with that achieved without modem optimization. In addition, the validity of the identified optimum parameters and the feasibility of the optimized AMOOFDM modems are also statistically verified for implementation in the vast majority of installed MMF links. Statistical investigations show that the optimized AMOOFDM modems can support >50Gbits/s signal transmission over 300m in 99.5% of already installed MMF links with loss margins of >7dB. Furthermore, it is also confirmed statistically that the optimized AMOOFDM modems have excellent performance flexibility and great robustness to various fiber and/or system implementation-related impairments.

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

Improved transmission performance of adaptively modulated optical OFDM signals over directly modulated DFB laser-based IMDD links using adaptive cyclic prefix

The impact of Adaptive Cyclic Prefix (ACP) on the transmission performance of Adaptively Modulated Optical OFDM (AMOOFDM) is explored thoroughly in directly modulated DFB laser-based, IMDD links involving Multimode Fibres (MMFs)/Single-Mode Fibres (SMFs). Three ACP mechanisms are identified, each of which can, depending upon the link properties, affect significantly the AMOOFDM transmission performance. In comparison with AMOOFDM having a fixed cyclic prefix duration of 25%, AMOOFDM with ACP can not only improve the transmission capacity by a factor of >2 (>1.3) for >1000 m MMFs (<80 km SMFs) with 1 dB link loss margin enhancement, but also relax considerably the requirement on the DFB bandwidth.

Optical OFDM Synchronization With Symbol Timing Offset and Sampling Clock Offset Compensation in Real-Time IMDD Systems

End-to-end real-time Optical Orthogonal Frequency Division Multiplexing (OOFDM) signal synchronization that is capable of simultaneously compensating for both Symbol Timing Offset (STO) and Sampling Clock Offset (SCO) is experimentally demonstrated, for the first time, over 64-quadrature amplitude modulated (QAM)-encoded, 11.25-Gb/s, 25-km, Intensity-Modulation and Direct-Detection (IMDD) single mode fiber (SMF) systems employing directly modulated distributed feedback lasers. In comparison with manual synchronization, almost-perfect compensations of arbitrary STOs are achieved. The OOFDM synchronization technique can also compensate for the SCO effect with an accuracy of <; 1 ppm for initial SCOs as large as 4000 ppm. The technique has a number of salient advantages including low complexity, fast tracking speed, high accuracy, and suitability for high-speed optical transmission systems.

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.