An T. Nguyen

Wideband wavelength conversion of 16 Gbaud 16-QAM and 5 Gbaud 64-QAM signals in a semiconductor optical amplifier

We demonstrate wavelength conversion based on four-wave mixing in a semiconductor optical amplifier of signals with quadrature amplitude modulation (QAM). We first demonstrate wavelength conversion of 16 Gbaud 16-QAM signals over the entire C-band using two co-polarized pumps with low power penalty at the forward error correction threshold (FEC) for a wide range of input optical-signal-to-noise-ratio (OSNR). We also demonstrate for the first time wavelength conversion of 5 Gbaud 64-QAM signals in a semiconductor optical amplifier with bit-error rate below the FEC threshold over the entire C-band and investigate the dependence of the power penalty on input OSNR with a single pump configuration.

BER Performance of Coherent Optical Communications Systems Employing Monolithic Tunable Lasers With Excess Phase Noise

We analyze the bit error rate (BER) performance of optical coherent communication systems employing multi-section monolithic tunable lasers with large excess phase noise. The BER of the coherent system utilizing a second-order decision-directed digital phase-locked loop (DD-PLL) for phase tracking was computed numerically. Experimental results have also been demonstrated with a 16-quadratic-amplitude modulation coherent system deploying a sampled-grating distributed Bragg reflector laser at 16 Gbaud that confirms the performance of the second-order DD-PLL in tracking the excess phase noise.

Enabling In-Band Bidirectional OFDM-Uplink and OOK-Downlink Transmission in Long-Reach RSOA-Based WDM-PON Systems

We present a simple, straightforward technique to enable bidirectional transmission of analog and digital signals in single-fiber wavelength division multiplexing passive optical network systems employing reflective semiconductor optical amplifiers (RSOA). An On-Off-Keying (OOK-DL) downlink modulation is effectively erased in a saturated RSOA, reflecting a carrier for retransmitting on the same fiber an in-band analog WiFi orthogonal frequency division multiplexing uplink signal (OFDM-UL). We investigate the operating conditions and the optimization procedure to attain required signal quality for both the uplink and downlink. We demonstrate experimentally the maximum distance and the best attainable performances for different lengths of standard single mode fiber (transmission). Bit error rate below the forward error correction threshold is obtained for WiFi OFDM-UL signal over 40 km and up to 80 km of SSMF using 64QAM and QPSK modulation modes, respectively. In all cases, we achieve error-free operation for the OOK-DL signal at 1 Gb/s.

Downstream modulation index tuning to enable full-duplex OOK-DL/OFDM-UL transmission in RSOA-based Radio-over-Fiber system

In this paper, we investigate the possibility of transmitting full-duplex digital and analog signal in a carrier reused, re-modulating Radio-over-Fiber (RoF) system based on reflective semiconductor optical amplifier (RSOA). We study the effect of tuning the modulation index of the On-Off-Keying Downlink (OOK-DL) signal on its reception performance and the amplitude modulation erasing in RSOA. By properly selecting operating conditions, downlink residual modulation on the reflected optical carrier can be minimized to allow the transmission of a WiFi Orthogonal Frequency Division Multiplexing Uplink (OFDM-UL) signal. We experimentally demonstrate bidirectional transmission over 20km of standard single mode fiber (SSMF) and show an operational region where we can achieve error-free for a 1Gb/s OOK-DL signal while we remain below forward error correction (FEC) threshold for WiFi OFDM-UL using QPSK, 16QAM, and 64QAM modulation modes.

Error vector magnitude based parameter estimation for digital filter back-propagation mitigating SOA distortions in 16-QAM.

We investigate the performance of digital filter back-propagation (DFBP) using coarse parameter estimation for mitigating SOA nonlinearity in coherent communication systems. We introduce a simple, low overhead method for parameter estimation for DFBP based on error vector magnitude (EVM) as a figure of merit. The bit error rate (BER) penalty achieved with this method has negligible penalty as compared to DFBP with fine parameter estimation. We examine different bias currents for two commercial SOAs used as booster amplifiers in our experiments to find optimum operating points and experimentally validate our method. The coarse parameter DFBP efficiently compensates SOA-induced nonlinearity for both SOA types in 80 km propagation of 16-QAM signal at 22 Gbaud.

Modeling and compensation of transmitter nonlinearity in coherent optical OFDM

We present a comprehensive study of nonlinear distortions from an optical OFDM transmitter. Nonlinearities are introduced by the combination of effects from the digital-to-analog converter (DAC), electrical power amplifier (PA) and optical modulator in the presence of high peak-to-average power ratio (PAPR). We introduce parameters to quantify the transmitter nonlinearity. High input backoff avoids OFDM signal compression from the PA, but incurs high penalties in power efficiency. At low input backoff, common PAPR reduction techniques are not effective in suppressing the PA nonlinear distortion. A bit error distribution investigation shows a technique combining nonlinear predistortion with PAPR mitigation could achieve good power efficiency by allowing low input backoff. We use training symbols to extract the transmitter nonlinear function. We show that piecewise linear interpolation (PLI) leads to an accurate transmitter nonlinearity characterization. We derive a semi-analytical solution for bit error rate (BER) that validates the PLI approximation accurately captures transmitter nonlinearity. The inverse of the PLI estimate of the nonlinear function is used as a predistorter to suppress transmitter nonlinearity. We investigate performance of the proposed scheme by Monte Carlo simulations. Our simulations show that when DAC resolution is more than 4 bits, BER below forward error correction limit of 3.8 × 10(-3) can be achieved by using predistortion with very low input power backoff for electrical PA and optical modulator.

Experimental validation of digital filter back-propagation to suppress SOA-induced nonlinearities in 16-QAM

We experimentally demonstrate the performance of a computationally efficient digital filter back-propagation scheme for post-compensating SOA nonlinearities in coherent detection of 16-QAM signal at 16 Gbaud, and compare the results with simulations.

Full-duplex WiFi analog signal transmission with digital downlink in a radio-over-fiber system employing RSOA-based WDM-PON architecture

We experimentally demonstrate, for the first time, bidirectional analog WiFi signal transmission in a digital wavelength division multiplexed passive optical network (WDM-PON) system. Our solution is compatible with a single optical feeder fiber and customer premises units employing reflective semiconductor optical amplifiers (RSOAs). The downlink optical carrier is modulated which both baseband 1 Gb/s On-Off-Keying (OOK) data and a WiFi signal in the 2.4 GHz band. The optical signal is split, with part going to reception and part reflected by the RSOA and modulated with an uplink WiFi signal. The uplink WiFi signal is in the 2.4 GHz band (no frequency translation), although the RSOA 3 dB response is limited to 1.2 GHz. At the bit-error-rate (BER) threshold of 2×10-3 before forward error correction (FEC), we achieve the transmission link up to 20 km (64-QAM), 30 km (16-QAM), and 40 km (QPSK). In all cases, the 1 Gb/s digital downlink signal is error-free.

An optimized 16-QAM constellation for mitigating impairments of phase noise and limited transmitter ENOB in optical coherent detection systems

A new 16-QAM constellation is proposed combatting laser phase noise and limited resolution of high speed digital-to-analog converters. Experimental results show small but consistent performance improvement at low and high baud-rates compared with square and other reported constellations.

Overcoming Phase Sensitivity in Real-Time Parallel DSP for Optical Coherent Communications: Optically Filtered Lasers

Implementation of real-time giga-Baud optical coherent systems for single-carrier higher-level modulation format such as 64-quadrature amplitude modulation (QAM) depends heavily on phase tracking. For offline digital signal processing, decision-directed phase recovery is performed at symbol rate with the best performance and the least computational effort compared among other best-known algorithms. However, in real-time systems, hardware parallelization and pipelining delay on feedback path pose stringer requirement on the linewidth, or the frequency noise spectral level of laser sources. This leads to the paucity of experiments demonstrating real-time phase tracking for 64- or higher QAM. In this paper, we experimentally investigate the impact of optically-filtered lasers on parallel and pipelined phase tracking in a single-carrier 5 Gbaud 64-QAM back-to-back coherent system. For parallelization levels higher than 24, the optically-filtered laser shows more than 2 dB improvement in optical signal-to-noise ratio penalty compared to that of the same laser without optical filtering.