Bertrand Noharet

Retrocommunication utilizing electroabsorption modulators and nonmechanical beam steering

A novel retrocommunication link utilizing reflective multiple quantum well (MQW) optical modulators and nonmechanical beam steering and tracking is demonstrated. Large aperture reflective MQW modulators using AlGaAs/GaAs are optimized and manufactured. The modulators exhibit a contrast ratio larger than 4:1 and a modulation bandwidth of 10 MHz. Nonmechanical beam steering and tracking are studied using nematic liquid crystal (NLC) spatial light modulators (SLMs). The communication link is comprised of a retromodulating array with four MQW modulators and a transceiver using a NLC SLM for beam steering and tracking. Transfer of audio, real-time image data and pseudorandom bit sequences over 100-m range while tracking the moving retromodulator is shown. The link is capable of transferring data at approximately 8 Mbps.

A novel free-space retrocommunication link

Retrocommunication is a new technique for asymmetric free-space optical communication that has attracted interest during recent years. Novel technologies such as multiple quantum well (MQW) optical modulators and non-mechanical laser beam steering and tracking have been studied for implementation in a retrocommunication link. Large and small aperture reflective AlGaAs/GaAs MQW modulators were optimised and fabricated. The modulators exhibit high contrast ratios (from 5 to 100) and high modulation rates (up to 16 Mbit/s). A retroreceiver consisting of four large aperture MQW modulators, associated optics and drive electronics was fabricated. Nematic liquid crystal spatial light modulators have been evaluated, characterised for beam steering and tracking and implemented in a transceiver. Small area MQW modulators, used in focal plane configurations, were studied for static communication links. Results from a novel retrocommunication link utilising a retroreceiver and non-mechanical laser beam steering and tracking will be presented. Bit rates of 8 Mbit/s were observed during non-mechanical tracking of a moving retroreceiver over 100 m range. The demonstrator system was capable of transferring audio-, real-time images or bit streams. The demonstrated principles show promising features for future low weight free-space communication links. Performance calculations including requirements for a retrocommunication link using MQW modulators and non-mechanical beam steering are discussed.

A high-speed modulated retro-reflector communication link with a transmissive modulator in a cat's eye optics arrangement

An optical communication link with a modulated retro-reflector in a cats eye optics arrangement has been evaluated. A transmissive multiple quantum well modulator with a modulation bandwidth in excess of 10 MHz was placed in a wide field-of-view cats eye optic system. A laser transceiver with one line and one broadband source around 1550 nm was used to interrogate the modulated retro-reflector and detect the reflected and modulated signal. Multi-level digital encoding was evaluated as a way to increase the information transfer rate in conditions with relatively high signal-to-noise ratio. The links were evaluated in indoor as well as outdoor conditions.

Multiple quantum-well spatial light modulators for optical signal processing

We report on the development of different surface-normal multiple quantum well (MQW) modulator devices. Owing to their unique speed advantage, arrays of surface-normal MQW modulators are very well suited for fast and parallel signal processing, and can be developed for both digital or analogue signals. We present the design and fabrication process for single surface-normal MQW modulators, as well as for high fill factor (80-90%) 1-D arrays of such modulators. Design issues and trade-offs in terms of modulator size, speed and contrast ratios are described. Contrast ratios in excess of 100:1 have been demonstrated. 3-dB frequency modulation bandwidths in excess of 1GHz have been obtained for single modulator devices of 125μm in diameter. Performances of 1-D arrays of 64 and 128 modulators (pixel size: 2mmx80μm and 2mmx40μm, respectively) are also presented, with response time for programmable filtering in the order of 10 ns.

Development of the SANDRA antenna for airborne satellite communication

Novel avionics communication systems are required for increasing flight safety and operational integrity, for optimizing economy of operations and for enhancing passenger services. One of the key technologies to be developed is an antenna system that will provide broadband connectivity within aircraft cabins at an affordable price. This paper describes the development of an electronically steered Ku-band phased array antenna with low aerodynamic profile. The antenna front-end consists of at least 1800 antenna elements, of which the beam has to be steered continuously to geostationary satellites. Best performance for the beam steering is expected from a hybrid architecture with small sub-apertures having their local own beamformers (using phase shifters). The beamformer to steer the sub-apertures of the entire antenna uses True Time Delays with an optical ring resonator. 1 2

Multiple quantum wells spatial light modulators for correlation based processors

To be competitive with their electronic counterparts, correlation-based optical processors require very fast spatial light modulators (SLMs) that can perform simultaneously phase and amplitude modulation. Owing to their ultra-high speed, multiple quantum well (MQW) SLMs have been early identified as very good candidates. However, the coding domain of MQW SLMs is not widely known. We present here a study of available coding domains of MQW SLMs. We demonstrate that pure amplitude modulation, ternary { -1, 0, + 1} modulation and quaternary {0, + 1, ei2π/3, ei4π/3}, modulation are examples of coding domains that can be achieved by tuning a few parameters in the design of Fabry-Perot MQW modulators. We show that ternary and quaternary filters provide much better results than binary filters for the recognition of objects embedded in highly cluttered noise. Finally, we present a technique, the time-averaged pseudo-random encoding technique, which enables encoding of any complex filter onto a quaternary modulator. Combined with the time-averaged pseudo-random encoding technique, MQW SLMs may pave the way to the development of new optoelectronic correlator systems with improved speed and accuracy performance.

Wide-aperture GaAs/AlGaAs multiple quantum well electro-optic modulators

We present design and fabrication methods for surface normal monolithic amplitude modulators with an aperture up to 14 x 14 mm2, a contrast ratio of 6:1 and for low driving voltages (≤8 V). The modulators consist of undoped GaAs/AlGaAs quantum wells embedded in a Fabry-Perot (FP) resonance cavity grown by MOVPE. To improve the device performance the FP cavity, the period and thickness of the quantum well and doping concentration were optimised. Also, the dimension of the modulator were varied from 0.5 x 0.5 to 14 x 14 mm2. The results show that the yield of the modulators increases significantly when decreasing the size of the modulators. To remedy the low yield issue for wide aperture modulator, a pixelated approach was used to divide the mono pixel in a monolithic modulator into several pixels, for example from 4 to 48. The modulation speed of the modulators with different dimensions was characterised by electro-optic (EO) response measurements. The temporal optical response of the large modulators was satisfactory up to the order of MHz modulation frequency where the RC constant limited the performance. A few of the modulators with wide apertures are to be assembled into an optical link system for free-space communication.

Development of a broadband and squint-free Ku-band phased array antenna system for airborne satellite communications

Novel avionic communication systems are required for various purposes, for example to increase the flight safety and operational integrity as well as to enhance the quality of service to passengers on board. To serve these purposes, a key technology that is essential to be developed is an antenna system that can provide broadband connectivity within aircraft cabins at an affordable price. Currently, in the European Commission (EC) 7th Framework Programme SANDRA project (SANDRA, 2011), a development of such an antenna system is being carried out. The system is an electronically-steered phased-array antenna (PAA) with a low aerodynamic profile. The reception of digital video broadcasting by satellite (DVB-S) signal which is in the frequency range of 10.7-12.75 GHz (Ku-band) is being considered. In order to ensure the quality of service provided to the passengers, the developed antenna should be able to receive the entire DVB-S band at once while complying with the requirements of the DVB-S system (Morello & Mignone, 2006). These requirements, as will be explained later, dictate a broadband antenna system where the beam is squint-free, i.e. no variation of beam pointing direction for all the frequencies in the desired band. Additionally, to track the satellite, the seamless tunability of the beam pointing direction of this antenna is also required. In this work, a concept of optical beamforming (Riza & Thompson, 1997) is implemented to provide a squint-free beam over the entire Ku-band for all the desired pointing directions. The optical beamformer itself consists of continuously tunable optical delay lines that enable seamless tunability of the beam pointing direction.

Multiple quantum well spatial light modulators: design,fabrication,characterization

Multiple quantum well spatial light modulators (MQW SLMs) are promising devices for future high-speed applications. We present results obtained with a single-pixel amplitude modulator. We discuss the status of our work on a 128x128-pixel ternary SLM. This SLM will run at 10 kHz and have one low-reflectance level and two high reflectance levels with a phase difference of pi. We also present a study of the relation between the coding domain and the structural design of modulators.

Photonic integration and components development for a K u -band phased array antenna system

In this paper the development of a phased array antenna system using a photonic beamformer is reported. The paper emphasizes on the photonic integration between two main components of the beamformer, namely the photonic beamformer chip and the electroabsorption modulator array. System level simulation is used to determine the required performance of each component. The measurements on the component characteristics are presented.