Philippe Gravey

Transparent optical packet switching: network architecture and demonstrators in the KEOPS project

This paper reviews the work carried out in the ACTS KEOPS (Keys to Optical Packet Switching) project, describing the results obtained to date. The main objective of the project is the definition, development, and assessment of optical packet switching and routing networks, capable of providing transparency to the payload bit rate, using optical packets of fixed duration and low bit rate headers in order to enable easier processing at the network/node interfaces. The feasibility of the KEOPS concept is assessed by modeling, laboratory experiments, and testbed implementation of optical packet switching nodes and network/node interfacing blocks, including a fully equipped demonstrator. The demonstration relies on advanced optoelectronic components, developed within the project, which are described.

Theory of two‐wave mixing gain enhancement in photorefractive InP:Fe: A new mechanism of resonance

We present a new model (including both temperature and electron‐hole effects) of two‐beam coupling in photorefractive semiconductors under an external dc field E0. This model predicts that the exponential gain Γ can exhibit an intensity‐dependent resonant behavior, yielding a π/2 phase shift of the space‐charge field with respect to the incident fringe pattern. This optimum intensity strongly depends on crystal temperature but it is practically independent of the grating period. As an illustration this model is applied to InP:Fe. In this case the resonance occurs when the hole photoionization rate and the electron thermal emission rate are equal. Values of Γ as high as 20 cm−1 at 1.06 μm, for a fringe spacing of 15 μm and E0 =10 kV/cm, are predicted. The comparison between theory and experimental data requires taking into account the variation of the pump intensity throughout the sample (due to optical absorption), which reduces the volume where the resonance condition is fulfilled; in this way a satisfac...

Multi-Band OFDM Transmission at 100 Gbps With Sub-Band Optical Switching

In this paper, we present an original work on sub-wavelength optical switching performed over a coherent multi-band orthogonal frequency-division multiplexing (MB-OFDM) super-channel operating at 100 Gbps. After having demonstrated that dual-polarization MB-OFDM (DP-MB-OFDM) is as efficient as single-carrier dual-polarization quaternary phase shift keying (DP-QPSK) technology to transport 100 Gbps data-rate over a 10 × 100-km G.652 fiber-based transmission line, we show that optical add-drop of OFDM sub-bands as narrow as 8 GHz inside a 100 Gbps DP-MB-OFDM signal constituted of four sub-bands is feasible in the middle of this 1000-km transmission line. The flexible optical add-drop multiplexer (FOADM) implemented here is constituted by the association of an ultra-narrow pass-band and stop-band optical filter. The design and realization of such ultra-selective optical filters is presented, while the impact of their physical features over the quality of transmission is discussed. To prove that several add-drop multiplexers can be cascaded, our FOADM is introduced into a G.652 fiber-based recirculating loop and the impact of the cumulated filtering transfer function as well as the crosstalk inside the OADM are investigated. A typical use case for the introduction of such FOADM into long-haul transport networks is given, and the capital expenditure (CAPEX) cost advantage for the multi-layer transport networks is highlighted. By the proof of concept delivered here, combination of super-channel and sub-wavelength optical switching pushes network flexibility far away of what is today proposed by system vendors, opening new horizons for an optimized use of multi-layer transport networks.

Multiservice optical network: main concepts and first achievements of the ROM program

With the tremendous introduction of internet protocol (IP) applications, the quality-of-service (QoS) becomes more and more an emergent issue. Concrete solutions can be adopted (IP/ATM/SONET/WDM) opening the way to new types of applications (interactive applications through the exploitation of voice and video) in a short-term approach. However, all the telecommunication community tries to provide new solutions offering capacity and flexibility in a simpler manner. In this paper, we present the concepts of a multiservice optical network studied in the framework of a French Research Program. The QoS could be offered through the combined exploitation of electronic memories in the edges and optical resources in the core of the optical network and through the coexistence of different types of connections. In particular, the traffic shaping in the edges is highlighted through simulation and demonstrates the real impact of this function to maintain the logical performance at its highest level. To propose concrete solutions for its implementation, two network scenarios are proposed. The first one, for the backbone, exhibits a novel optical packet switching architecture taking benefit of the massive presence of wavelengths to solve the contention. The second one, for the metro, shows a second optical packet switching architecture really adapted to the cost constraints (upgradability, compactness, granularity).

Optimal dimensioning of the WDM unidirectional ECOFRAME optical packet ring

To efficiently support the high rate and the high dynamicity of the traffic in metro networks, an optical packet-switched WDM ring, named ECOFRAME, is proposed. The key features of the proposed ring are optical transparency and statistical multiplexing of optical packets on parallel WDM channels. Such features can be exploited by properly allocating wavelengths and receivers. This paper aims to optimally dimension the unidirectional ECOFRAME rings. The dimensioning at minimum cost (i.e., for wavelengths and receivers) is modeled with an mixed-integer linear programming formulation. An heuristic algorithm is also proposed, and its performance is compared against the optimal solutions and bounds. When considering the receiver and wavelength cost, results indicate that trading the wavelengths for receivers allows cost saving of up to 75% with respect to WDM optical packet rings with a single dedicated wavelength per node (i.e., single receiver at each node).

Characterization of photorefractive CdTe:V: high two-wave mixing gain with an optimum low-frequency periodic external electric field

We present two-wave mixing results obtained with CdTe:V photorefractive crystals under an external dc or low-frequency square-shaped periodic electric field. The majority photorefractive carriers are holes at 1.32 and 1.54 μm, whereas electrons dominate slightly at 1.048 μm. At this wavelength an intensity-dependent resonant gain enhancement is observed in the presence of a dc field. The effective trap densities are near 1015 cm−3. The highest gains (to as high as 11 cm−1 for a field amplitude of 15 kV/cm at 1.32 μm) are obtained with a square-shaped periodic field at an optimum frequency F0. The dependence of F0 on the different experimental parameters is presented. A two-level model in which the secondary level has a thermal emission rate much larger than the principal one qualitatively describes this nonstandard frequency behavior.

Analysis of double phase conjugate mirror interaction in absorbing photorefractive crystals: Application to BGO:Cu

Abstract The effect of absorption on double phase conjugate mirror (DPCM) in photorefractive materials is studied. An analytical condition for the threshold is derived; in particular the 2WM gain Γ and the absorption α must satisfy γ>2α. The diffraction efficiency above the threshold is numerically evaluated taking into account the saturation of Γ at large fringe modulation. Theoretical predictions are compared with the results obtained with a BGO:Cu crystal at 632.8 nm.

Double-phase conjugated mirror and double color pumped oscillator in photorefractive InP:Fe

We report on double‐phase conjugation at 1.06 μm wavelength in InP:Fe with applied continuous electric field. Conversion efficiencies up to 74% with a grating period of 9 μm are obtained. The dependence of this efficiency on intensity, applied field, and grating period is reported and the results are compared with numerical simulations taking into account space‐charge field nonlinearities. Double color (1.06 and 1.32 μm) pumped oscillation is also observed with conversion efficiencies higher than 30%.

Model for resonant intensity dependence of photorefractive two-wave mixing in InP:Fe

We present a new model, which incorporates both temperature and electron-hole effects, for two-beam coupling in photorefractive semiconductors under an external dc field E(0). We show that the exponential gain ? exhibits an intensity-dependent resonance. The application of this model to InP:Fe allows us to predict a value of ? near 20 cm(-1) for a thin sample at 1.06 microm with E(0) = 10 kV/cm.

Holographic switching between single mode fibres based on electrically addressed nematic liquid crystal gratings with high deflection accuracy

Abstract Accurate control of the deflection angle needed when connecting single mode fibres can be performed in electrically addressed nematic liquid crystal deflectors. We present a reconfigurable 1×5 switch with 30% global efficiency and predict the characteristics of a 16×16 system.