Abdullah S. Karar

Linewidth-Tolerant and Low-Complexity Two-Stage Carrier Phase Estimation for Dual-Polarization 16-QAM Coherent Optical Fiber Communications

Two novel linewidth-tolerant, low-complexity feedforward carrier phase estimation algorithms are described for dual-polarization 16-ary quadrature-amplitude-modulation with coherent detection. For both algorithms, the carrier phase is estimated in two stages. The first stage employs either a simplified quadrature-phase-shift-keying (QPSK) partitioning algorithm or the blind phase search (BPS) algorithm. The second stage employs a novel QPSK constellation transformation algorithm. The performance and linewidth tolerance of both algorithms are evaluated using experimental and simulation data, and the hardware complexity is assessed. For both proposed two-stage algorithms, the linewidth symbol duration product is 1.3 × 10-4 for a 1 dB penalty in optical signal-to-noise ratio at a bit error ratio of 10-3. This performance is comparable to a single-stage BPS algorithm with a large number of test phases, but with a reduction of the hardware complexity by factors of about 2.5-11.

Reducing the complexity of perturbation based nonlinearity pre-compensation using symmetric EDC and pulse shaping

Perturbation based nonlinearity pre-compensation has been performed for a 128 Gbit/s single-carrier dual-polarization 16-ary quadrature-amplitude-modulation (DP 16-QAM) signal. Without any performance degradation, a complexity reduction factor of 6.8 has been demonstrated for a transmission distance of 3600 km by combining symmetric electronic dispersion compensation and root-raised-cosine pulse shaping with a roll-off factor of 0.1. Transmission over 4200 km of standard single-mode fiber with EDFA amplification was achieved for the 128 Gbit/s DP 16-QAM signals with a forward error correction (FEC) threshold of 2 × 10(-2).

100 Gb/s Intensity Modulation and Direct Detection

Recent advances in short reach 100 Gb/s intensity modulation and directed detection systems are briefly reviewed. As an illustrative example of using digital signal processing enabled transmitters and receivers to allow relatively modest symbol rates, the generation and detection of 112 Gb/s 16-QAM half-cycle Nyquist subcarrier modulation are considered using dual-polarization, single-carrier and single-polarization, dual-carrier implementations. High bandwidth directly modulated passive feedback lasers are used to generate the optical signals and pre-amplified receivers are used to detect the received signals in a back-to-back system and after transmission over 4 km of single-mode fiber.

10 Gbps silicon waveguide-integrated infrared avalanche photodiode

We have fabricated monolithic silicon avalanche photodiodes capable of 10 Gbps operation at a wavelength of 1550 nm. The photodiodes are entirely CMOS process compatible and comprise a p-i-n junction integrated with a silicon-on-insulator (SOI) rib waveguide. Photo-generation is initiated via the presence of deep levels in the silicon bandgap, introduced by ion implantation and modified by subsequent annealing. The devices show a small signal 3 dB bandwidth of 2.0 GHz as well as an open eye pattern at 10 Gbps. A responsivity of 4.7 ± 0.5 A/W is measured for a 600 µm device at a reverse bias of 40 V.

Generation and Detection of a 56 Gb/s Signal Using a DML and Half-Cycle 16-QAM Nyquist-SCM

A 16-QAM signal is generated using subcarrier modulation with a subcarrier frequency of half the symbol rate, Nyquist pulse shaping, and a directly modulated passive feedback laser at bit rates of 14, 28, and 56 Gb/s. Using polarization multiplexing emulation, a pre-amplified direct detection receiver and digital signal processing, loss margins of 12.6 and 8 dB are achieved for a 112 Gb/s dual polarization signal at back-to-back and after 4 km transmission, respectively.

Pulse shaping for 112 Gbit/s polarization multiplexed 16-QAM signals using a 21 GSa/s DAC

The implications of increasing the symbol rate for a given digital-to-analog converter (DAC) sampling rate are investigated by considering the generation of 112 Gbit/s PM 16-QAM signals (14 Gsym/s) using a 21 GSa/s DAC with 6-bit resolution.

Electronic Pre-Compensation for a 10.7-Gb/s System Employing a Directly Modulated Laser

We investigate electronic pre-compensation for a 10.7-Gb/s system employing a directly modulated laser (DML). Initially, the nonlinear distortion resulting from the direct modulation of the laser is mitigated by pre-compensation based on reversal of the large signal rate equations and the use of digital signal processing to generate an appropriate modulating current. This concept is then utilized in a novel look-up table optimization scheme for electronic dispersion pre-compensation. Simulation results show pre-compensation of up to 350 km of single mode fiber at a required optical signal to noise ratio <; 16 dB for a bit error rate of 10-3 . It is shown that a 42.8 GSa/s digital to analog converter is sufficient for exploiting the full potential for electronic pre-compensation using a DML.

Electronic Post-Compensation of Dispersion for DML Systems Using SCM and Direct Detection

A novel electronic dispersion post-compensation algorithm is described and experimentally demonstrated for short reach optical links employing a directly modulated laser (DML) as a transmitter with subcarrier modulation (SCM) and digital signal processing enabled direct detection receiver. The proposed algorithm utilizes the functional relationship between the DML output optical power and chirp in deducing the received optical phase, which is normally lost under direct detection. Subsequently, standard frequency domain equalization is performed to mitigate the link dispersion. The proposed algorithm is applied to a single polarization 56 Gb/s and a dual polarization 112 Gb/s half-cycle Nyquist SCM signal transmitted over 4 km of single mode fiber. A 3 dB and 5 dB improvement in received optical power relative to the uncompensated system performance is achieved for the 56 Gb/s and 112 Gb/s signals at a bit error ratio of 1.0×10-3, respectively.

Electronic precompensation of the nonlinear distortion in a 10 Gb/s 4-ary ASK directly modulated laser

The nonlinear distortion resulting from the direct modulation of a semiconductor laser using 4-ary amplitude shift keying at 10 Gb/s is mitigated by precompensation based on the use of digital signal processing to generate an appropriate modulating current.

Monolithic silicon waveguide photodiode utilizing surface-state absorption and operating at 10 Gb/s

We have fabricated a waveguide integrated monolithic silicon infrared detector. The photodiode consists of a p-i-n junction across a silicon-on-insulator (SOI) rib waveguide. Absorption is due to surface-states at the silicon/air interface of the waveguide. A 2 mm long detector shows a response of 0.045 A/W (calculated as a function of coupled light) and is capable of operation at 10 Gb/s at a reverse bias voltage of 2 V.