Fiber-optic communication | EPFL Graph Search

Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared or visible light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is preferred over electrical cabling when high bandwidth, long distance, or immunity to electromagnetic interference is required. This type of communication can transmit voice, video, and telemetry through local area networks or across long distances. Optical fiber is used by many telecommunications companies to transmit telephone signals, internet communication, and cable television signals. Researchers at Bell Labs have reached a record bandwidth–distance product of over 100 petabit × kilometers per second using fiber-optic communication. Background First developed in the 1970s, fiber-optics have revolutionized the telecommunications industry and have played a major role in the advent of the Inform

Gigahertz free-space electro-optic modulators based on Mie resonances

Ileana-Cristina Benea-Chelmus

Electro-optic modulators are essential for sensing, metrology and telecommunications. Most target fiber applications. Instead, metasurface-based architectures that modulate free-space light at gigahertz (GHz) speeds can boost flat optics technology by microwave electronics for active optics, diffractive computing or optoelectronic control. Current realizations are bulky or have low modulation efficiencies. Here, we demonstrate a hybrid silicon-organic metasurface platform that leverages Mie resonances for efficient electro-optic modulation at GHz speeds. We exploit quasi bound states in the continuum (BIC) that provide narrow linewidth (Q = 550 at lambda(res) = 1594 nm), light confinement to the non-linear material, tunability by design and voltage and GHz-speed electrodes. Key to the achieved modulation of Delta T/T-max = 67% are molecules with r(33) = 100 pm/V and optical field optimization for low-loss. We demonstrate DC tuning of the resonant frequency of quasi-BIC by Delta lambda(res) = 11 nm, surpassing its linewidth, and modulation up to 5 GHz (f(EO,- 3dB) = 3 GHz). Guided mode resonances tune by Delta lambda(res) = 20 nm. Our hybrid platform may incorporate free-space nanostructures of any geometry or material, by application of the active layer post-fabrication.

NATURE PORTFOLIO2022

Dynamics of quantum dot lasers

Pablo Moreno

The exceptional performance of self-assembled Quantum Dot (QD) materials renders them extremely appealing for their use as optical communications devices. As lasers, they feature reduced and temperature independent threshold current and proper emission wavelength at the fiber telecommunication windows. These characteristics, together with the low linewidth enhancement factor and broad spectrum, make QD materials extremely attractive for application as light emitters or amplifiers. There exist, nevertheless, several unclear issues which prevent QDs from conquering the new generation of optoelectronic devices. Their differential efficiency is lower than expected. The output power of QD lasers is lower than that of their quantum well counterpart. Still, it is their dynamics which has incited the majority of studies. The modulation bandwidth of these devices seems to be limited by the relaxation of carriers from the upper energetic layers to the low levels within the dot. Besides, the electron-hole interaction is widely unknown, the extent of the electron-hole Coulombic attraction is not yet established. Throughout this thesis I present a theoretical and experimental study of the gain and phase dynamics of quantum dot lasers. I explain the appearance of different decay times observed in pump and probe experiments in QD amplifiers as a result of the different electron and hole relaxation times, by means of an electron-hole rate-equation model. The ultrafast hole relaxation first leads to an ultrafast recovery of the gain, which is then followed by electron relaxation and, on the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. Our results corroborate with spectral measurements of the dephasing and gain in QD amplifiers. Additionally, our work is compared with existing pump and probe results. Exploiting the capacity of QD lasers to emit at two different wavelengths corresponding to the ground state (GS) and excited state (ES), I present a theoretical study of the QD dynamics, based on a linearization of the QD rate-equations. The results predict the existence of single oscillation frequency of GS and ES, meaning that both states are highly coupled. In order to verify our theory, we perform two kinds of experiments. By modulating these lasers at high frequency, we measure separately the dynamics of GS and ES. However, in contradiction to our theory, two different modulation frequencies are found. Additional temporally-resolved measurements of the laser dynamics reveal a surprising effect. By injecting a sub-bandgap pump in an InAs/InGaAs QD laser, the emitted photons are depleted. Through additional transmission and photocurrent measurements, we relate this observation with carrier photoexcitation, which was so far only theoretically addressed. The role of carrier photoexcitation in our experimental laser dynamics is further supported by a rate-equation model. Impelled by this finding, we study the effect of carrier photoexcitation in the static and dynamic characteristics. We find that carrier photoexcitation reduces the efficiency of QD lasers, which is one of the major QD handicaps, and depletes the GS lasing after the ES threshold, as observed experimentally. Moreover, by adding carrier photoexcitation to our linearization of the rate-equations, we find that the theory predicts the appearance of two lasing resonance frequencies, in agreement with our previous experimental results. Additionally, we deal with the improvement of carrier relaxation. In tunnel injection devices, carriers are given an additional path towards the ground state of the dot by growing a quantum well layer close to the QD active plane. Through the quantum-mechanical tunneling effect, carriers relax from the nearby quantum well layer to the QDs, which speeds up relaxation. We aim at the increase of the modulation bandwidth while keeping the good performances quantum dot lasers have exhibited, such as low and temperature insensitive threshold current and proper emission wavelength. In the final part of this work, we present dynamical measurements of 1.5 µm InAs/InP tunnel injection and non-tunnel injection QD lasers, which display remarkable static characteristics. After proving with static measurements that tunnel injection is actually taking place in these structures, we show several dynamic measurements. Pump and probe measurements on QD devices show that the tunnel injection samples exhibit a slightly faster relaxation time than the non-tunnel injection samples used as reference, meaning that relaxation time is improved with tunnel injection. However, by probing the device with an ultrafast pump no improvement of the dynamic characteristics is observed. These results confirm that the laser dynamic properties of InP QD lasers, both standard and tunnel-injection designs, are actually not limited by relaxation of carriers. We point towards the size distribution of these quantum dash-like structures as the limiting factor of the modulation frequency.

EPFL2008

Object-oriented method and architecture for virtual private network service management

One of the major trends in the evolution of current corporate networking is an increasing need for high performance long distance communications. Provided as an alternative to leased lines networks, virtual private networks (VPNs) are gaining increasing acceptance among customers and network providers by providing corporate networking between geographically dispersed customer premises based on a shared public switched network infrastructure. As a contribution towards this evolution, the present thesis proposes a generic object-oriented architecture for open distributed VPN services integrating service and management issues all along the modeling process. The aim of this VPN architecture is not only to provide a descriptive model of VPNs but also to illustrate possible ways of operating on that model to solve VPN related service or management issues. The architecture proposed is generic in the sense that, firstly, it is network technology independent and, secondly, that it captures the fundamental characteristics common to all VPNs. This latter aspect is very important as, due to the lack of standardization in this area, current VPNs are more marketable products than well-defined telecommunications services. The notion of distribution is also very important as telecommunications systems in general, and VPNs in particular, are intrinsically distributed. Indeed, a VPN can be seen as a distributed application running on the multiple nodes of telecommunications networks. To effectively deal with this de facto distribution and to exploit at best the benefits it can provide the VPN service and management services have been conceived from the very beginning as distributed applications. For this purpose, and due to the lack of satisfactory existing solutions in this area, an object-oriented method for the specification and design of open distributed telecommunications and management services has been developed. The proposed method combines the ODP (Open Distributed Processing) concepts and stucturing rules with the systematic development process advocated by an object-oriented software engineering method called Fusion. This method provides an architectural framework and a seamless thread from problem definition to the realization of the distributed telecommunications system, based on hierarchically related abstraction levels. Mapping rules, guidelines and notations are proposed on how and when enterprise, information and computational models should be built, in particular based on models performed at a higher abstraction level. The method has then been applied to the specification of the generic VPN architecture itself. In the enterprise viewpoint the scope of the VPN service has been refined and limited to the provision, within the public domain boundary, of internetworking services between customer premises networks. The main actors involved in using, providing and managing the VPN service have been described as well as interactions between them. Based on their requirements, fundamental VPN characteristics have been identified, among which the notions of virtual resource and of closed user group are the most relevant. In the information viewpoint, a generic object-oriented network information model has been developed based on standard network resources. This model has been recursively applied to specify the object models pertaining to the different layers of the VPN architecture. Based on this architecture, a management service allowing customers to dynamically configure the logical connectivity of their VPN has been specified down to the network elements level. The realization of this functionality in terms of application components suitable for distribution and interactions between them has been the purpose of the computational viewpoint. Finally, the generic VPN architecture has been refined and applied to the particular context of ATM-based VPNs. In addition to describing how VPNs can be built on ATM networks this case study addresses a very important issue in such an environment, narnely bandwidth management. In this respect, an open distributed VPN bandwidth management service has been specified that allows VPN customers to dynamically modify the bandwidth allocated to the virtual path connections connecting the different sites of their organization. The final outcome of this specification process is the definition of a computational architecture for VPN bandwidth management extending from VPN level down to the physical network elements. This work has been carried out in the frame of the RACE (Research and Development in Advanced Communications Technologies in Europe) project R2041 PRISM (Pan-European Reference Configuration for IBC Services Management).

EPFL1996

Object-oriented method and architecture for virtual private network service management

EPFL1996

Dynamics of quantum dot lasers

Pablo Moreno

EPFL2008