Walid Mathlouthi

Suppression of Turbulence-Induced Scintillation in Free-Space Optical Communication Systems Using Saturated Optical Amplifiers

A laboratory-simulated free-space optical link under various turbulence levels is implemented to propose and experimentally demonstrate the use of saturated optical amplifiers as a simple and efficient approach for suppression of scintillation due to atmospheric turbulence. The use of erbium-doped fiber amplifier (EDFA) or semiconductor optical amplifier (SOA) requires the received signal be coupled into a fiber. The system performance of receiver structures employing a saturated EDFA and a SOA (in saturation and conversion modes) are measured and compared to that of fiberless direct detection (DD). It is shown that in higher turbulence levels, where no data transmission can be achieved by DD, remarkable eye opening results when using saturated amplifiers

Fast and Efficient Dynamic WDM Semiconductor Optical Amplifier Model

A novel state-variable model for semiconductor optical amplifiers (SOAs) that is amenable to block diagram implementation of wavelength division multiplexed (WDM) signals and fast execution times is presented. The novel model is called the reservoir model, in analogy with similar block-oriented models for Raman and erbium-doped fiber amplifiers (EDFAs). A procedure is proposed to extract the needed reservoir model parameters from the parameters of a detailed and accurate space-resolved SOA model due to Connelly, which was extended to cope with the time-resolved gain transient analysis. Several variations of the reservoir model are considered with increasing complexity, which allow the accurate inclusion of scattering losses and gain saturation induced by amplified spontaneous emission. It is shown that at comparable accuracy, the reservoir model can be 20 times faster than the Connelly model in single-channel operation; much more significant time savings are expected for WDM operation. The model neglects intraband SOA phenomena and is thus limited to modulation rates per channel not exceeding 10 Gb/s. The SOA reservoir model provides a unique tool with reasonably short computation times for a reliable analysis of gain transients in WDM optical networks with complex topologies

Dense SS-WDM Over Legacy PONs: Smooth Upgrade of Existing FTTH Networks

We propose a hybrid passive optical network (PON) architecture supporting time-division multiplexing (TDM) and dense spectrum-sliced wavelength-division multiplexing (SS-WDM) over the legacy PON infrastructure. We use a fiber Bragg grating (FBG)-based self-seeded reflective semiconductor optical amplifier (RSOA) transmitter in conjunction with a recently proposed balanced receiver (BR); identical transceiver pairs are placed at the central office and customer side. Self-seeded RSOAs obviate the need for centralized sources, providing a high power, directly modulated source. Intensity noise mitigation of this thermal source is investigated by operating the RSOA in saturation and employing the recently proposed BR. We study the optimal reflectivity for seeding that balances signal power and noise cleaning to achieve the best bit error rate (BER) possible; channel widths are comparable with dense WDM when using coherent sources.

An Innovative Receiver for Incoherent SAC-OCDMA Enabling SOA-Based Noise Cleaning: Experimental Validation

We propose a new low complexity receiver for spectral amplitude optical coded division multiple access (SAC OCDMA) that enables intensity noise reduction using semiconductor optical amplifiers (SOAs). Compared to the standard receiver requiring two optical filters at the receiver side, our receiver requires only one optical filter. While a 1.4-dB power penalty in incurred, network capacity is unchanged, i.e., BER floors due to intensity noise have the same level. The primary motivation for the low complexity receiver is not reduced component count, but rather modifying the receiver so that promising SOA noise mitigation techniques might be employed to increase system capacity. SOA noise cleaning suffers from a major limitation: filtering after the SOA can negate most of the signal enhancement, the so-called post SOA filtering issue. The only solution to date for the post-SOA filtering effect in SAC-OCDMA is prohibitively complex McCoy , J. Lightw. Technol., vol. 25, no. 1, pp. 394-401, Jan. 2007, i.e., requiring multiple SOAs per client. We demonstrate that our proposed receiver drastically limits the client side filtering, thus maintaining noise suppression and overcoming the post-SOA filtering effect. We compare BER at up to 10 Gb/s with and without noise cleaning. When a noise cleaning module is used, BER improvement of several orders-of-magnitude is observed when only a few users are active in the network. Examination of the noise properties, however, leads us to conclude that highly populated networks will have diminished improvement.

Error-free transmission for incoherent broad-band optical communications systems using incoherent-to-coherent wavelength conversion

A new method is presented to exploit the cross-gain modulation mechanism in semiconductor optical amplifiers to reduce the intensity noise of spectrum-sliced communications systems via conversion of the modulated incoherent optical signal into a coherent signal before the detection process. Wavelength-division-multiplexed networks using broad-band sources have performance limited by intensity noise; conversion from an incoherent to a coherent signal reduces the intensity noise and enables the elimination of the bit-error-rate floor.

SOA gain recovery wavelength dependence: simulation and measurement using a single-color pump-probe technique.

Measurements to date of the wavelength dependency of gain recovery time in semiconductor optical amplifiers (SOAs) have mostly used pump-probe techniques with a pump and probe operated on distinct wavelengths. Choice of pump wavelength, and its relative proximity to the probe wavelength, could influence measurements and impede unambiguous observation of wavelength dependence on recovery dynamics. We use a single-color pump-probe measurement technique to directly access the wavelength dependence of the gain recovery time in bulk InGaAsP SOAs. We used ultrashort pulses from a single mode locked laser to measure unambiguously the spectral dependency and temporal behavior of SOAs. Simulation results using a model that takes into account intra-band and inter-band contributions to SOA saturation, as well as experimental results for the SOA tested, show recovery rate dependency similar to gain spectrum.

High-Bit-Rate Dense SS-WDM PON Using SOA-Based Noise Reduction With a Novel Balanced Detection

The major drawback of incoherent broadband sources (BBSs) is their inherent intensity noise. Semiconductor optical amplifiers (SOAs) can be exploited at the transmitter to mitigate this noise. Optical filtering at the receiver, however, leads to the return of most of suppressed noise. Wider filtering at the receiver is the best known strategy to maintain performance gains, at the price of reduced spectral efficiency due to the tradeoff between noise cleaning and adjacent channel crosstalk. We introduce a novel balanced receiver for wavelength division multiplexing (WDM) systems that maintains greater noise cleaning and leaves spectral efficiency unchanged. Unlike standard receivers, our balanced scheme does not filter the desired signal. In this paper, we first demonstrate that the newly proposed receiver is equivalent to standard WDM receivers when no SOA for noise cleaning is present at the transmitter. Although a 2.9-dB power penalty is incurred, network capacity is unchanged, i.e., bit error rate (BER) floors due to intensity noise are the same. When SOAs are employed to mitigate severe intensity noise, we show that our receiver outperforms the wide filtering strategy by two orders of magnitude. Dense WDM capacity is demonstrated up to 10 Gb/s using a thermal source, a saturated SOA, and the balanced detection scheme. A BER of 10-6 is achieved at 10 Gb/s; further improvement is possible using low overhead forward error correction or a better SOA design. This demonstrates the ability of spectrum-sliced wavelength division multiplexing (SS-WDM) passive optical networks (PONs) to operate at 10 Gb/s at good spectral efficiency. Error performance better than 10-9 is achieved up to 8 Gb/s with 30-GHz optical channel bandwidth and 100-GHz spacing.

BER performance of an optical fast frequency-hopping CDMA system with multiple simultaneous users

We report the bit error rate (BER) performance of an optical fast frequency-hopping CDMA system for up to 16 simultaneous users. Previous predictions of BER are shown to be overly pessimistic (beat noise limited) or optimistic (interference limited).

Beat noise effects on spectrum-sliced WDM

In this paper, we review first, the different noise terms involved in amplified and filtered spontaneous emission detection using the standard beating theory (or the semi-classical field expansion theory) of the noise in optical amplifiers. We derive BER calculations using the following approaches: 1) non-ideal electrical and optical filters combined with a Gaussian approximation for the resultant noise process and an ideal integrate and dump electrical filter and an ASE source combined with non-Gaussian Erlang noise statistics.

Electrical-to-optical conversion of OFDM 802.11g/a signals by direct current modulation of semiconductor optical amplifiers

We report experimental results showing direct current modulation of a commercial semiconductor optical amplifier (SOA) for electrical to optical conversion of a 64-QAM OFDM analog signal at 54 Mbit/s on 2.5 and 5 GHz carriers. This is believed to be the highest frequency yet experimentally reported for RF signal transmission using a directly modulated semiconductor optical amplifier