Salim Faci

Microwave On/Off Ratio Enhancement of GaAs Photoconductive Switches at Nanometer Scale

This paper reports on performances enhancement of photoconductive switches in term of On/Off ratio and insertion losses. The optimization parameters on which the research has been focused are gap dimensions reduction to nanometer scale. The device characterization results up to a microwave frequency of 40 GHz and under CW illumination at a wavelength of 800 nm are presented. On/Off ratio reveals a value of 13 dB at 20 GHz under 100 mW optical power.

Electrical Modeling of Semiconductor Laser Diode for Heterodyne RoF System Simulation

The increasing use of optoelectronic devices in high data rate communication systems drives the need of precise electrical circuit modeling that allows the study of important parameters on link performances such as nonlinearity and noise level. In 60-GHz band radio over fiber system, the millimeter-wave signal generation offers simple configuration for the base station. Various techniques have been proposed such as optical heterodyning where the frequency difference between two optical carriers mixed in a photodetector generates the desired electrical carrier. Phase noise and linewidth of the optical sources determine the purity of the generated signal. In this paper, the optical phase noise is integrated into an electrical equivalent model of the laser diode to simulate radio over fiber systems in an electrical simulator. The laser output is represented here in the optical field with both intensity and phase noises. The influence of optoelectronic devices on the modulated analog or complex digital signals can be also analyzed. Two uncorrelated laser diodes are used to generate a millimeter-wave signal. Physical parameters of these lasers are determined from static response and relative intensity noise measurements. Phase noise contribution of individual lasers to the millimeter-wave signal is performed and compared with theoretical expectations.

Simulation of heterodyne RoF systems based on 2 DFB lasers: application to an optical phase-locked loop design

This paper presents a simulation approach of optical heterodyne systems by using the equivalent circuit representation of a distributed feedback laser (DFB) in the electrical domain. Since the electrical representation of the DFB laser is developed from the rate equations, its characteristics such as non-linearity, relative intensity noise (RIN), and phase noise can be predicted precisely for various biasing conditions. The model is integrated in a heterodyne radio over fiber (RoF) system where two DFB lasers are used to generate a millimeter-wave (mm-wave) signal. An optical phase-locked loop is also introduced to reduce the phase noise on the mm-wave signal. The optical phase noise contribution of individual lasers to the mm-wave signal is evaluated and compared with theoretical results. It is shown that the phase noise of the mm-wave is reduced considerably depending on the loop bandwidth and propagation delay. With the circuit simulation approach proposed, optical and mm-wave phase noises can be studied together with other circuit environments such as parasitic effects and driver circuits.

Optically-controlled microwave phase shifting and sampling by efficient photoconductive switching on LT-GaAs substrate integrated technology

Microwave photonics contributes through ultrafast devices to the processing of high data rates. In this area, microwave photoconductive switches (MPCSs) in integrated technology have proved their performances to control the transmission of high frequency signals in complex systems. Their ability to switch microwave signal phase and magnitude is fully defined by a complex frequency-dependant ON/OFF ratio RON/OFF determined from S-Parameters measurements in microwave frequency domain. This paper reports on a new design of MPCSs to be used, after realization and evaluation, as a basic block in optically controlled MMIC devices for application in high frequency samplers or phase shifters.

Direct and external modulation of IF over fiber systems for 60 GHz wireless applications

Wireless domestic applications involving high data rates are required to work on millimeter wave band. Signal propagation at this frequency range is affected by walls and oxygen absorption which limits communication distances to few meters in one room. Radio coverage can be extended to other rooms by optical links. Performances of such photonic systems are dependent on optoelectronic devices, electrical driving, and receiver circuits. In this paper, radio-over-fiber (RoF) links based on the intensity modulation and direct detection technique are investigated for transmission of a broadband OFDM signal. Direct and external modulations are exploited to analyze system performances according to the ultra wideband (UWB) millimeter-band standard. To avoid component tolerances at high frequencies, an intermediate frequency modulation of the optical transducers is chosen. Optoelectronic and optical components of RoF links are modeled by equivalent electrical circuits with consideration of noise and nonlinearities. These models are validated in system simulation by error vector magnitude evaluation with a measurement setup according to the UWB centimeter-band standard.

Photonic components advanced modelling for RoF link design

In this paper, we present the main results of the developed electrical modelling of photonic components allowing simulation at both circuit and system levels of a photonic-microwave link. Distortions on RF analog signal and complex modulated digital signal can be evaluated respectively for defence and telecommunication applications.

Nano photoconductive switches for microwave applications

This paper addresses the interaction between light wave technologies and semiconductors devices at the nanoscale. Research works aiming at the development of emerging 1D and 2D nano materials such as nanodots, nanowires, nanotubes and nanoribbons open the way to overcome the performances bottleneck of conventional microwave photoconductive switches. Such new materials offer new opportunities for the confinement of light/matter interaction and exhibit interesting energy band diagram in an optical wavelength spectrum covering visible to NIR. Strong material interests stays for the generation of very high local density of carriers in contrast with a high dark resistivity, in association with a high carrier mobility. These challenges can be reached today thanks to nanotechnology processes with a high compatibility constraint with submicrometer light coupling solutions and microwave devices and circuits technologies. Modeling and design tools dedicated to photoconductive effect description at nanometer scale, for its implementation in passive and active components must be set up in order to exalt this effect for microwave signal processing functionalities such as switching, generation, amplification and emission over a large frequency bandwidth. This paper will report on latest demonstrations of high performance photoconductive switches for high frequency applications at 0.8μm and 1.5μm based on LT-GaAs, GaAs nanowires and GaInAsSb semiconductor materials.

Simulation approach of wireless OFDM and FBMC signals transmission over fiber based on equivalent electrical models

In this paper, we present the impact of a Radio over fiber (RoF) link on the transmission of a signal dedicated to wireless link. Comparison is given between OFDM and FBMC based wireless signal by simulation with the electrical modelling approach of O/E and E/O devices and optical components. The considered link concerns the Intensity Modulation - Direct Detection (IM-DD) architecture where the wireless signal is transmitted at the intermediate frequency. The simulation system includes the noise sources and nonlinearity of each component making thus the easy study of the impact of each one separately. It is then possible to quantify the degradation of the transmitted signal in term of error vector magnitude (EVM) or bit error rate (BER). Thus, the system architecture can be optimized for various applications and furthermore depending on the number of users.

How electrical modelling of semiconductor lasers can help to analyse complex photonic systems

In this paper, we present main results of electrical modelling of various semiconductor lasers specifying their advantages and drawbacks in terms of gain, noise, and non-linearity. As these components are considered as an electro-optical transducer in photonic-microwave links, their required characteristics and performances that cannot be unique and are strongly dependent on dedicated applications. Some sources must have a small linewidth for the generation of millimetre wave signals and others need high optical power and linearity. A focus on the modelled transducers is presented.

The effect of laser characteristics on millimetre wave optical generation techniques

In this paper two millimetre-wave (mm-wave) generation techniques involving optical heterodyne detection and the Mach-Zehnder Modulator (MZM) based optical comb technique are investigated. The heterodyne detection system is followed by a second order electrical feedback loop to reduce the phase noise. Simulations are carried out to study the effects of the laser parameters (e.g. RIN, Henry coefficient) on the generated mm-wave signal. The optical comb is made by a single driven MZM. The optimum biasing point is investigated and two lasers with different characteristics are used successively to generate optical comb. The two mm-wave generation methods are compared based on their carrier to noise ratio (CNR) performance.