Abstract
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.
Highlights
Optical orthogonal frequency division multiplexing (OOFDM) [1] has been considered as one of the strongest contenders for practical implementation in next-generation, high-speed passive optical networks (PONs) [2,3]. This is because OOFDM has a large number of inherent and unique advantages including, for example, potential for providing cost-effective technical solutions by fully exploiting the rapid advances in modern digital signal processing (DSP) technology, and considerable reduction in transmission system complexity owing to its great resistance to dispersion impairments and efficient utilization of channel spectral characteristics
Based on the optimum loaded subcarrier power profiles, end-to-end real-time transmission of 11.25Gb/s (9.375Gb/s) 64-QAM- (32-QAM)-encoded OOFDM signals is experimentally demonstrated over 25km standard single-mode fibre (SSMF) intensity modulation and direct detection (IMDD) PON systems involving vertical cavity surface-emitting lasers (VCSELs) intensity modulators
The feasibility of utilising low-cost, un-cooled VCSELs as intensity modulators in previously demonstrated real-time OOFDM transceivers has been extensively explored experimentally, for the first time, in terms of achievable signal bit rates, physical mechanisms limiting the transceiver performance and performance robustness. Making use of such transceivers, endto-end real-time transmission of 11.25Gb/s 64-QAM-encoded OOFDM signals over 25km SSMF IMDD PON systems has been experimentally demonstrated with a power penalty of 0.5dB
Summary
Optical orthogonal frequency division multiplexing (OOFDM) [1] has been considered as one of the strongest contenders for practical implementation in next-generation, high-speed passive optical networks (PONs) [2,3]. The transmission performance of VCSEL intensity modulator-based IMDD OOFDM systems has been reported in [8,9] Both works were undertaken using off-line DSP approaches, which do not consider the limitations imposed by the precision and speed of practical DSP hardware for realizing end-to-end real-time transmission. In the present real-time OOFDM transceivers, on-line performance monitoring of total channel bit error rate (BER), individual subcarrier BER and system frequency response are implemented These on-line monitoring functions provide vital information for live control of digital signal power, signal clipping level, digital amplitude of individual subcarriers (here referred to as adaptive power loading) and VCSEL operating conditions to enable live maximization of system performance via minimizing the limitations set by low modulation bandwidths of cheap optical/electrical components. Adaptive power loading utilizes the limited available power and just re-distributes the power among various subcarriers, it does not affect the overall power consumption of the OOFDM transceiver
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