Mehdi Alem

Optical sinc-shaped Nyquist pulses of exceptional quality

Sinc-shaped Nyquist pulses possess a rectangular spectrum, enabling data to be encoded in a minimum spectral bandwidth and satisfying by essence the Nyquist criterion of zero inter-symbol interference (ISI). This property makes them very attractive for communication systems since data transmission rates can be maximized while the bandwidth usage is minimized. However, most of the pulse-shaping methods reported so far have remained rather complex and none has led to ideal sinc pulses. Here a method to produce sinc-shaped Nyquist pulses of very high quality is proposed based on the direct synthesis of a rectangular-shaped and phase-locked frequency comb. The method is highly flexible and can be easily integrated in communication systems, potentially offering a substantial increase in data transmission rates. Further, the high quality and wide tunability of the reported sinc-shaped pulses can also bring benefits to many other fields, such as microwave photonics, light storage and all-optical sampling.

Analytical model and experimental verification of the critical power for modulation instability in optical fibers.

Modulation instability is thoroughly investigated and a simple analytical model for its power critically modifying the wave properties in terms of system parameters is derived and experimentally validated. The differences on the modulation instability gain spectrum in lossless and lossy optical fibers are analyzed based on theoretical models and numerical simulations. In particular the impact of background noise on the behavior of modulation instability is studied analytically and verified by measurements and simulations. The proposed analytical model is experimentally validated by monitoring the wave propagation along an optical fiber using a Brillouin optical time-domain analyzer. This way, the evolution of the optical signal traveling through optical fibers, especially, the pump depletion and the recurrence phenomenon are investigated.

Generation of nyquist sinc pulses using intensity modulators

Optical sinc-shaped Nyquist pulses are produced based on the generation of an ideal frequency comb using cascaded intensity modulators. Nyquist pulses with 9.8-ps temporal width, 82-fs jitter and more than 40 dB SNR are achieved.

Mitigating modulation instability in Brillouin distributed fibre sensors

A thorough study of the generation of modulation instability (MI) in distributed Brillouin fibre sensors is presented. ASE noise co-propagating with the pump pulses within the MI gain spectrum has been identified to be an important factor seeding modulation instability and reducing its power threshold. The paper describes how optical narrowband filtering reduces the pump depletion resulting from MI, allowing pump pulses to propagate through longer distances in standard optical single-mode fibres.

Modelling the depletion length induced by modulation instability in distributed optical fibre sensors

An analytical model for the depletion length of modulation instability in single-mode optical fibres is proposed. The model gives the possibility to determine the maximum sensing distance that distributed optical fibre sensors can reach before being limited by the pump depletion induced by modulation instability. The important role of the noise level in the evolution of both modulation instability and the respective power depletion is clarified. The model gives a closedform expression helpful for a predictive design and is validated comparing the analytical results obtained by the model with measurements in a 25 km long Brillouin fibre sensor.

Analytical expression and experimental validation of the Brillouin gain spectral broadening at any sensing spatial resolution

A novel and simple analytical expression to describe the Brillouin gain spectral broadening as a function of the spatial resolution in time-domain Brillouin distributed fiber sensors is deduced. The proposed model is experimentally validated using a pump-probe Brillouin sensing setup and also compared with numerical and approximate results. In addition, a compact mathematical form is presented for the peak gain reduction resulting from incomplete acoustic-wave activation in Brillouin sensors with short spatial resolution. Both mathematical expressions can be used together to quantitatively predict the impact of the spatial resolution on the signal-to-noise ratio and frequency uncertainty of the sensor.

Mitigation of modulation instability in Brillouin distributed fiber sensors by using orthogonal polarization pulses

A technique based on the use of orthogonally-polarized pulses is proposed to mitigate the detrimental impact of modulation instability on Brillouin distributed fiber sensors. While the theoretical underpinnings of the method are described by introducing a detailed model for the vector modulation instability, the technique is experimentally validated in a 25-km sensing link. Numerical and experimental results demonstrate that the use of orthogonally-polarized pulses not only mitigates the impact of modulation instability, but also the four-wave mixing occurring in systems using pumps with parallel polarization; thus, providing an important sensing range enhancement with a reduced pump depletion.

Highly tunable method to generate sinc-shaped Nyquist pulses from a rectangular frequency comb

A method to produce highly-stable optical sinc-shaped pulses is proposed based on the generation of a rectangular-spectrum frequency comb. Nyquist pulses with <1% power distortion, 82-fs jitter and more than 40 dB SNR are achieved.

Performance limit of two-pump brillouin fiber sensors obtained by Manakov Modulation Instability

The Manakov modulation instability is shown to determine the performance limit of long-distance Brillouin fiber sensors utilizing orthogonally-polarized pumps. Numerical simulations of the Manakov equation are supported by the experimental measurement of a 25-km Brillouin fiber sensor to verify the model. The study reveals that the use of orthogonally-polarized pulses decreases the pump depletion due to modulation instability leading to an enhancement of the sensing distance.

Towards highest spectral efficiency: Optical sinc-shaped Nyquist pulses generation from rectangular frequency comb

In this paper, we review a method to produce optical sinc-shaped Nyquist pulses with unprecedented quality. The method is based on the synthesis of a rectangular shaped and phase-locked frequency comb from a combination of intensity modulators. The result is a highly flexible pulse generator that can easily be integrated in already installed communication systems. All-optical pulse shaping methods for highest spectral efficiencies are attractive since high-bitrate spectrally efficient channels can be combined into multiple Tbit/s superchannels, enabling ultrahigh data transmission rates and a simultaneous ultrahigh spectral efficiency. Ideal sinc-shaped Nyquist pulses thus unlock the path towards minimal inter-symbol interference and no-guard band wavelength multiplexing. Here we will report on the generation, modulation and multiplexing of the sinc-shaped pulses obtained while using off-the-shelf components.