Photonic RF vector signal generation with enhanced spectral efficiency using precoded double single-sideband modulation.

In this study, a novel photonic vector signal at frequency (RF) bands generation scheme based on the beating of double single sidebands (SSBs) is proposed and experimentally demonstrated. The double SSBs carry separate constant- or multi-amplitude quadrature-amplitude-modulation vector signals are generated from a single I/Q modulator. By adopting phase and amplitude precoding, different constellations can be generated, such as 3-ary phase-shift keying (PSK), 4-PSK, 7-PSK, 8-PSK, and so on. In this work, 10-Gbaud 7-PSK vector signal generation at 20 GHz enabled by two precoded 4-PSK SSB signals via a single I/Q modulator is theoretically and experimentally investigated. Compared to a single-drive Mach-Zehnder modulator or conventional I/Q modulator-based photonic vector signal generation scheme, the spectrum efficiency can be doubled. Differential coding is also implemented at the transmitter side for accurate demodulation of 7-PSK into two 4-PSK signals. The bit-error ratio for 10-Gbaud 7-PSK vector signals can be under hard-decision forward-error-correction threshold of 3.8×10-3 after 10 km standard single-mode fiber transmission.

Transmission of single-carrier 400G signals (515.2-Gb/s) based on 128.8-GBaud PDM QPSK over 10,130- and 6,078 km terrestrial fiber links.

We experimentally demonstrate the coherent transmission system with the highest ETDM-based symbol rate of 128.8-GBaud over record breaking distances. We successfully transmitted single-carrier 515.2-Gb/s PDM-QPSK/9-QAM signals over 10,130km/6,078-km, respectively, over 100km spans of TeraWave SLA + fiber. To the best of our knowledge, it is the highest ETDM-based symbol rate reported so far, and the longest WDM transmission distance with single-carrier 400G signals. For the first time, the 515.2-Gb/s single-carrier PDM-QPSK signals in 200-GHz-grid are successfully transmitted over distance above 10,000km in terrestrial transmission environment. We have also demonstrated the transmission of single carrier 128.8-GBaud filtered QPSK signals in 100-GHz-grid over 6,078-km, which has the line spectral efficiency (SE) of 5.152 (b/s/Hz).

Experimental demonstration of iterative post-equalization algorithm for 37.5-Gbaud PM-16QAM quad-carrier Terabit superchannel.

We experimentally demonstrate a quad-carrier 1-Tb/s solution with 37.5-GBaud PM-16QAM signal over 37.5-GHz optical grid at 6.7 b/s/Hz net spectral efficiency. Digital Nyquist pulse shaping at the transmitter and post-equalization at the receiver are employed to mitigate the impairments of joint inter-symbol-interference (ISI) and inter-channel-interference (ICI) symbol degradation. The post-equalization algorithms consist of one sample/symbol based decision-directed least mean square (DD-LMS) adaptive filter, digital post filter and maximum likelihood sequence estimation (MLSE), and a positive iterative process among them. By combining these algorithms, the improvement as much as 4-dB OSNR (0.1nm) at SD-FEC limit (Q(2) = 6.25 corresponding to BER = 2.0e-2) is obtained when compared to no such post-equalization process, and transmission over 820-km EDFA-only standard single-mode fiber (SSMF) link is achieved for two 1.2-Tb/s signals with the averaged Q(2) factor larger than 6.5 dB for all sub-channels. Additionally, 50-GBaud 16QAM operating at 1.28 samples/symbol in a DAC is also investigated and successful transmission over 410-km SSMF link is achieved at 62.5-GHz optical grid.

Time-domain digital pre-equalization for band-limited signals based on receiver-side adaptive equalizers.

We theoretically and experimentally investigate a time-domain digital pre-equalization (DPEQ) scheme for bandwidth-limited optical coherent communication systems, which is based on feedback of channel characteristics from the receiver-side blind and adaptive equalizers, such as least-mean-squares (LMS) algorithm and constant or multi- modulus algorithms (CMA, MMA). Based on the proposed DPEQ scheme, we theoretically and experimentally study its performance in terms of various channel conditions as well as resolutions for channel estimation, such as filtering bandwidth, taps length, and OSNR. Using a high speed 64-GSa/s DAC in cooperation with the proposed DPEQ technique, we successfully synthesized band-limited 40-Gbaud signals in modulation formats of polarization-diversion multiplexed (PDM) quadrature phase shift keying (QPSK), 8-quadrature amplitude modulation (QAM) and 16-QAM, and significant improvement in both back-to-back and transmission BER performances are also demonstrated.

Performance comparison of spectrum-narrowing equalizations with maximum likelihood sequence estimation and soft-decision output.

Maximum likelihood sequence estimation (MLSE) offers effective equalizations for bandwidth-limited optical signal on mitigation towards inter-symbol-interference (ISI) impairment. In this paper, we provide the first comprehensive comparisons and analysis of three post-compensation algorithms on the same modeling platform for high spectral-efficiency (SE) optical systems employing the spectral prefiltering. Those algorithms include 1-tap constant modulus algorithm (CMA) and 3-tap MLSE, regular CMA and digital filter with 2-tap MLSE, and constant multi-modulus algorithm (CMMA) with 2-tap MLSE. Furthermore, a novel and effective approach is proposed and verified for the generation of both hard value and soft value at the output of MLSE in order to be compatible with the implementation of soft-decision forward error correction (SD-FEC) decoding process.

Optical independent-sideband modulation for bandwidth-economic coherent transmission.

We proposed and implemented a bandwidth-economic coherent optical transmission technology using optical independent-sideband (O-ISB) modulation. Generation of two software-defined, 30-GBd O-ISB channels per wavelength sharing one set of transmitter hardware was demonstrated, which was made possible by digital single-sided up-conversion, channel pre-equalization and optical IQ modulation with precise amplitude and delay matching between driving signals. With 120-Gb/s PM-QPSK per O-ISB channel, we successfully delivered 16 O-ISB channels in 8 wavelengths at 3.86-bits/s/Hz over 2100 km SSMF. In addition, the system performance was evaluated by using regular and enhanced Rx DSP, respectively. For metro and regional network applications where small footprint, low power consumption and low cost are required features, the proposed two-channels-per-wavelength O-ISB modulation may be considered an economic option.

Transmission of 8 × 480-Gb/s super-Nyquist-filtering 9-QAM-like signal at 100 GHz-grid over 5000-km SMF-28 and twenty-five 100 GHz-grid ROADMs.

We experimentally demonstrate a highly filtering-tolerant multi-modulus equalization (MMEQ) process for very aggressively spectrum-shaped 9-ary quadrature-amplitude-modulation (9-QAM)-like polarization division multiplexing quadrature phase shift keying (PDM-QPSK) signal to achieve 400-Gb/s wavelength-division-multiplexing (WDM) channels on the 100-GHz grid for ultra-long-haul reach and high tolerance of the filter narrowing effect caused by reconfigurable optical add-drop multiplexers (ROADMs). We successfully transmitted 8 channels 480-Gb/s super-Nyquist (channel occupancy much less than signal baud rate) WDM signals at 100-GHz grid over 25 × 200 km conventional single-mode fiber-28 (SMF-28) with post Raman amplification and 25 ROADMs at a net spectral efficiency (SE) of 4b/s/Hz, after excluding the 20% soft-decision forward-error-correction (FEC) overhead. The system performance is significantly enhanced by the MMEQ based on 9-QAM-like constellations compared to the conventional 4 point QPSK constellation. A record transmission distance over conventional SMF-28 with a large number of ROADMs is firstly reported on the 400-Gb/s channels at 100-GHz grid.

WDM transmission of 108.4-Gbaud PDM-QPSK signals (40 × 433.6-Gb/s) over 2800-km SMF-28 with EDFA-only.

We experimentally demonstrated the transmission of 40 × 433.6-Gb/s Nyquist wavelength-division-multiplexing (N-WDM) optical time-division-multiplexing (OTDM) over 2800-km single-mode fiber (SMF)-28 with Erbium-doped fiber amplifier (EDFA)-only amplification, adopting polarization-division-multiplexing carrier-suppressed return-to-zero quadrature-phase-shift-keying (PDM-CSRZ-QPSK) modulation as well as post filter and 1-bit maximum likelihood sequence estimation (MLSE). Each channel occupies 100 GHz, yielding a spectral efficiency of 4.05 b/s/Hz. The bit-error ratio (BER) of all channels is less than the pre-forward-error-correction (pre-FEC) limit of 3.8 × 10(-3) after 2800-km SMF-28 transmission.

Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection.

We introduce an "ultra-dense" concept into next-generation WDM-PON systems, which transmits a Nyquist-WDM uplink with centralized uplink optical carriers and digital coherent detection for the future access network requiring both high capacity and high spectral efficiency. 80-km standard single mode fiber (SSMF) transmission of Nyquist-WDM signal with 13 coherent 25-GHz spaced wavelength shaped optical carriers individually carrying 100-Gbit/s polarization-multiplexing quadrature phase-shift keying (PM-QPSK) upstream data has been experimentally demonstrated with negligible transmission penalty. The 13 frequency-locked wavelengths with a uniform optical power level of -10 dBm and OSNR of more than 50 dB are generated from a single lightwave via a multi-carrier generator consists of an optical phase modulator (PM), a Mach-Zehnder modulator (MZM), and a WSS. Following spacing the carriers at the baud rate, sub-carriers are individually spectral shaped to form Nyquist-WDM. The Nyquist-WDM channels have less than 1-dB crosstalk penalty of optical signal-to-noise ratio (OSNR) at 2 × 10(-3) bit-error rate (BER). Performance of a traditional coherent optical OFDM scheme and its restrictions on symbol synchronization and power difference are also experimentally compared and studied.

60 GHz millimeter-wave gigabit wireless services over long-reach passive optical network using remote signal regeneration and upconversion.

This study proposed and experimentally demonstrated a cost-efficient scheme that can deliver 60 GHz millimeter-wave (mm-wave) multi-gigabit wireless services over 125 km long-reach passive optical networks (PONs) without any dispersion compensation. By introducing a remote local exchange (LE) stage with robust signal regeneration and all-optical upconversion functionalities, the proposed long-reach optical-wireless access network can easily accommodate over 128 users with 2.5 Gb/s shared bandwidth as well as shifting the capital expenditure of multiple hybrid optical network units (ONUs) toward single LE headend. Experimental verification shows that the power penalties for wireless and wired services are 1.8 dB and 0.4 dB at 10(-9) BER after 125 km optical fiber transmission.

Compact polymeric four-wavelength multiplexers based on cascaded step-size MMI for 1G/10G hybrid TDM-PON applications.

A novel polymeric four-wavelength multiplexer based on a new design concept of cascaded step-size multimode interferometer (CSS-MMI) is first presented. It can be fabricated by using planar lightwave circuit (PLC) technology and is highly compatible with both current 1-Gbps and future 10Gbps optical transceivers for applications in next generation 1-Gbps and 10-Gbps coexisting time-division-multiplexed passive optical network (TDM PON). By combining two types of proposed CSS-MMI 1??2 wavelength splitters, a compact 1.5cm-long four wavelength multiplexer is realized without requiring complicated design and tedious calculation. In addition, the upstream wavelength channel at 1310 nm has 0.18 dB low insertion loss and three downstream wavelength channels at 1490 nm, 1550 nm and 1590 nm exhibit insertion losses of 1.21 dB, 1.34 dB and 1.02 dB, respectively, with their extinction ratios ranging from 11.71 dB to 32.07 dB.