Q. W. Zhang

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.

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.

Robust real-time 15.125Gb/s adaptive optical OFDM transmissions over 100m 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.

Adaptability-enabled record-high and robust capacity-versus-reach performance of real-time dual-band optical OFDM signals over various OM1/OM2 MMF systems [invited]

Real-time dual-band optical orthogonal frequency-division multiplexing (OOFDM) transceivers incorporating digital-to-analog and analog-to-digital converters operating at sampling speeds as low as 4 GS/s and field-programmable gate-array-based digital signal processing clocked at only 100 MHz are employed to explore the maximum achievable transmission performance of electro-absorption modulated laser-based multimode fiber (MMF) systems subject to conventional optical launching. Making use of adaptive subcarrier bit and subcarrier/subband power loading, record-high 20.125 Gb/s transmission over 800 m OM2 MMF of real-time dual-band OOFDM signals is experimentally demonstrated, for the first time, with an optical power penalty as low as 1 dB. In addition, extensive experimental explorations of dual-band OOFDM capacity versus reach performance are also undertaken over the aforementioned systems consisting of different MMF types and lengths. It is shown that 20.125 Gb/s over 100 m and 19.625 Gb/s over 1000 m OOFDM transmissions are obtainable in OM2-only MMF systems and that 20 Gb/s over 100 m and 19.375 Gb/s over 1000 m OOFDM transmissions are achievable in OM1-only MMF systems. Furthermore, in various OM1 and OM2 MMF systems, minimum optical power penalties of <;1 dB are observed for MMF transmission distances ranging from 300 to 800 m. Experimental results indicate that >19 Gb/s over 1000 m transmission of dual-band OOFDM signals are practically feasible in any legacy MMF systems.

25.25-Gb/s Real-Time Multi-Band Optical OFDM Transmission Over 300-m MMFs With IQ Modulated Passband

Record-high 25.25-Gb/s real-time end-to-end optical orthogonal frequency-division multiplexing transmissions over an intensity-modulation and direct-detection-based 300-m OM2 multi-mode fiber links are achieved with a tri-sub-band transceiver architecture. A 9.75-Gb/s baseband signal is RF multiplexed with two 7.75-Gb/s sub-bands, which are IQ modulated onto a 6.125-Gdirect-detectionHz RF carrier. The adaptive bit/power-loaded, independently power-optimized sub-bands sampled at 4 GS/s are multiplexed to intensity modulate an EML. Similar BERs and optical penalties are observed for all sub-bands.

Stage-Dependent DSP Operation Range Clipping-Induced Bit Resolution Reductions of Full Parallel 64-Point FFTs Incorporated in FPGA-Based Optical OFDM Receivers

The fast Fourier transform (FFT) is the fundamental algorithm at the heart of an optical OFDM (OOFDM) transceiver. The high digital signal processing (DSP) complexity has become one of the most significant obstacles to experimentally demonstrating real-time high-capacity OOFDM transceivers. In this paper, stage-dependent clipping of FFT DSP operation dynamic range is proposed and extensively explored, for the first time, based on which an improved stage-dependent minimum bit resolution map is numerically identified by taking into account the DSP operation dynamic range-clipping and precision of FFT DSP operations. The validity and high accuracy of the identified minimum bit resolution map is experimentally verified in 25 km SSMF OOFDM transmission systems based on intensity modulation and direct detection. Experimental results show that, compared to a previously reported FFT stage-dependent bit resolution map including the FFT DSP operation precision only, the improved minimum bit resolution map offers significant bit resolution reductions of up to 3-bits for full-parallel pipelined 64-point FFT architectures. As a direct result, an approximately 30% reduction in FPGA logic resource usage is achievable compared to the spiral FPGA design.

RSOA Intensity Modulator Frequency Chirp-Enhanced Optical OFDM PON Performance

A reflective semiconductor optical amplifier intensity modulator (RSOA-IM) frequency chirp considerably increases the system frequency response of an intensity-modulation and direct-detection (IMDD) PON system over the passband signal spectral region. Adaptive bit and power loading of optical OFDM (OOFDM) enables full use of the RSOA-IM frequency chirp-enhanced passband system frequency response characteristics. As a direct result, without requiring extra electrical and optical equalization schemes, an RSOA-IM with a 3-dB small-signal modulation bandwidth as low as 1 GHz can support 40 Gb/s OOFDM signal transmissions over 25 km SSMF IMDD PON systems free from optical amplification and chromatic dispersion compensation. This paper suggests that significantly relaxed requirements on both RSOA-IMs 3-dB small-signal modulation bandwidth and frequency chirp are feasible for achieving desired OOFDM transmission performances for next-generation PONs.

RSOA intensity modulator frequency chirp-enabled 40Gb/s over 25km IMDD PON systems

Adaptive bit and power loading of optical OFDM (OOFDM) enables full utilizations of RSOA intensity modulator (RSOA-IM) frequency chirps to significantly enhance system performances. 1GHz-RSOA-IMs can support 40Gb/s@25km OOFDM transmissions in simple IMDD PON systems.