Jose A. P. Morgado

Directly modulated laser parameters optimization for metropolitan area networks utilizing negative dispersion fibers

The optimization of an uncooled directly modulated laser operated at 10 Gbit/s for metropolitan area networks utilizing negative dispersion fibers is presented. The laser optimization is performed in order to accomplish two goals: maximizing the back-to-back sensitivity and the dispersion tolerance of negative dispersion single-mode fiber. Extensive numerical simulations reveal that a significant improvement of system performance can be achieved by optimizing simultaneously four laser intrinsic parameters, namely linewidth enhancement factor, photon lifetime, gain parameter, and gain compression factor. The optimal laser parameters have been obtained at a temperature of 25/spl deg/C. The optimized laser shows a weak dependence of back-to-back sensitivity and dispersion tolerance on variations of laser intrinsic parameters as well as on laser temperature up to 85/spl deg/C. Nevertheless, the gain compression factor is the most stringent intrinsic parameter, because it controls the right balance between the transient and adiabatic chirps. The results show also that the impact of laser parasitics on system performance is only slightly altered with the temperature increase up to 85/spl deg/C. Back-to-back sensitivities of about -27.8 dBm and -29.1 dBm and 1 dB dispersion tolerances of about 1550 ps/nm and 1580 ps/nm have been achieved at 25/spl deg/C and 85/spl deg/C, respectively. The dispersion tolerance doubles the value of 750 ps/nm reported in a practical experiment.

Rigorous assessment of small-signal analysis for linear and dispersive optical communication systems operating near zero-dispersion wavelength

This paper presents a rigorous small-signal theory for linear single-mode fibers taking into account the first- and second-order fiber dispersion. From this theory, exact small signal intensity modulation (IM) and frequency modulation (FM) fiber transfer functions are presented. Exact expressions of the intensity and frequency noise spectra at fiber output due to laser noise taking into account all Langevin noise terms are also derived. Accurate numerical simulations of the IM and FM fiber transfer functions, and intensity and frequency noise spectra after linear transmission along single-mode fiber are compared with theoretical predictions and very good agreement is achieved. In addition, the theoretical predictions are compared with other authors results and the discrepancies are thoroughly explained. A new expression for the transmission distance which can lead to further significant reduction of intensity noise spectrum in systems using single-mode lasers with reduced linewidth enhancement factor is presented. The theoretical and simulation results show that the magnitude of the small-signal IM and FM fiber transfer functions, and the intensity and frequency noise spectra after linear single-mode fiber transmission are not affected by second-order fiber dispersion. The theory indicates that second-order fiber dispersion solely introduces delay distortion in the IM and PM fiber transfer functions. So, with linear transmission second-order dispersion effects on the intensity and frequency noise have proved to be irrelevant even for very long broad-band fiber systems.

Experimental study of coexistence of multi-band OFDM-UWB and OFDM-baseband signals in long-reach PONs using directly modulated lasers

Transmission of coexisting Orthogonal Frequency Division Multiplexing (OFDM)-baseband (BB) and multi-band OFDM-ultra-wideband (UWB) signals along long-reach passive optical networks using directly modulated lasers (DML) is experimentally demonstrated.When optimized modulation indexes are used, bit error ratios not exceeding 5 × 10⁻⁴ can be achieved by all (OFDM-BB and three OFDM-UWB sub-bands) signals for a reach of 100 km of standard single-mode fiber (SSMF) and optical signal-to-noise ratios not lower than 25dB@0.1 nm. It is experimentally shown that, for the SSMF reach of 100km, the optimized performance of coexisting OFDM-BB and OFDM-UWB signals is mainly imposed by the combination of two effects: the SSMF dispersion-induced nonlinear distortion of the OFDM-UWB signals caused by the OFDM-BB and OFDM-UWB signals, and the further degradation of the OFDM-UWB signals with higher frequency, due to the reduced DML bandwidth.

A perspective on optical-wireless converged NG-FTTH networks using directly modulated lasers

OFDM-based next generation fiber to the home (NG-FTTH) networks using directly modulated lasers (DMLs) have been proposed as a solution to deliver quintuple play services addressing cost-effective optical-wireless convergence. In this article, we highlight the principles and discuss the main technical challenges of the optical infrastructure of these networks. A first experimental assessment of transmission of coexisting baseband and ultra-wideband signals along distances typical of NG-FTTH networks using DMLs is presented.

Transmission of multi-band OFDM-UWB signals along NG-FTTH networks using directly modulated lasers

Transmission of multi-band OFDM-UWB signals along FTTH networks using directly modulated lasers is experimentally evaluated. Reach of 100 km of standard SMF is achieved with a BER below 10−4 and an OSNR of 25 dB.

Assessment of laser noise influence on direct-detection transmission system performance

A new semi-analytical method of estimating the influence of laser noise on directly modulated direct-detection transmission system performance is proposed. Its accuracy is assessed for different propagation situations and receiver noise type dominance using rigorous mixed semi-analytical simulation. Two different relative intensity noise (RIN) powers at receiver input associated with the two laser currents of the binary levels are considered, and not just a single one corresponding to the laser average current as in other published works. It is shown that, to perform rigorous simulation, suitable methods of solving stochastic laser rate equations should be used, and a number of samples high enough in order to stabilize the sensitivity estimator should be considered. It has been found that, for power penalties of about 1 dB, more than 150 samples may be necessary. For a power penalty due to laser noise of about 4.5 dB, discrepancies not exceeding 0.4 dB are observed between the results obtained using the new semi-analytical approach and using the rigorous simulation. It is shown that the new semi-analytical approach gives more precise results than other approach currently used. For a power penalty due to laser noise not exceeding 2 dB, the discrepancies between the two approaches can reach roughly 1 dB. It is shown that these discrepancies can increase with the laser linewidth. Explicit expressions for system power penalty due to the RIN at the fiber output are presented, for both signal-independent and -dependent noise dominance. It is shown that these expressions give good upper bounds of the real power penalties.

Linearity Improvement of Directly Modulated PONs by Digital Pre-Distortion of Coexisting OFDM-based Signals

Digital pre-distortion of five OFDM-based wired-wireless signals for compensation of directly-modulated PONs nonlinearity is demonstrated experimentally. This technique leads to EVM-compliant levels in all signals and to EVM improvements that reach 5.7dB in UWB signals.

Enhancement of laser phase noise to intensity noise conversion by fiber nonlinearity and its impact on intensity modulation system performance

Laser phase noise conversion to intensity noise due to fiber dispersion may impose severe degradation in intensity modulation and direct detection (IM-DD) system performance. In this paper, we investigate theoretically and numerically the influence of fiber nonlinearity on the conversion of phase noise to intensity noise at fiber output. Excellent agreement of noise characteristics between simulations results obtained from an accurate solution of the noise driven laser rate equations and theoretical predictions has been achieved. Results reveal that due to the fiber nonlinearity a significant enhancement of magnitude of the relative intensity noise (RIN) spectrum as well as its broadening is observed in the anomalous dispersion regime while a magnitude reduction and narrowing of the spectrum is observed in the normal regime. The system penalty due to RIN at the fiber output in IM-DD transmission systems using external modulator is assessed. The maximum transmission distance imposed by RIN for 1 dB of power penalty is presented for various optical power at fiber input in gigabit-per-second transmission systems. It is shown that in the anomalous dispersion regime the RIN enhancement due to the fiber nonlinearity can impose a significant impairment in IM-DD transmission systems through the phase modulation to IM conversion enhancement.

Aging of solar PV plants and mitigation of their consequences

Photovoltaic solar fields are planned to operate during many years, so the monitoring of the plant parameters and performances undoubtedly represents an adequate way to ensure a safe and long term operation for the overall system. Values around 10%/year of photovoltaic system degradation in 1991 fall to 12% in 25 years in PV panels using modules manufactured after 2000, mainly due the recent technology advances seen in the field of renewable. Obviously, these results require that system operation should follow an adequate maintenance. In this work, the main failures in the most critical components of conventional PV solar plants are described and ways to mitigate them are recommended, in order to achieve a reduction of unexpected overthrow occurrences. The photos presented were taken in solar PV plants located in Portugal.

Influence of spectrum location of Gigabit-Ethernet in coexistence with multi-band OFDM-UWB wireless signals on the long-reach PON performance using directly modulated lasers

The influence of spectrum location of OFDM-based Gigabit-Ethernet (GbE) in coexistence with multi-band OFDM-UWB wireless signals on the performance of long-reach PON using directly modulated lasers is experimentally evaluated. It is shown that the central frequency of the GbE signal of 1.5GHz is a good compromise in terms of transmission performance of the coexisting OFDM-(GbE+UWB) signals. With that central frequency of the GbE signal, BERs not exceeding 1.3×10−4 are achieved by all signals for a reach of 100km of standard single-mode fiber without dispersion compensation and an optical signal to noise ratio (OSNR) of 30dB@0.1nm, when optimized modulation indexes are used.