Chalcogenide Glasses Based on Germanium Disulfide for Second Harmonic Generation

High second‐order susceptibilities are created by thermal poling in bulk germanium disulfide based chalcogenide glasses. Experimental conditions of the poling treatment (temperature, voltage, time) were optimized for each glass composition. The second‐order nonlinear signals were recorded by using the Maker fringes experiment and a second‐order coefficient χ⁽²⁾ up to 8 pm V^–1 was measured in the Ge₂₅Sb₁₀S₆₅ glass. This value is obtained using a simulation based on accurate knowledge of the thickness of the nonlinear layer. Two mechanisms are proposed to explain the creation of a nonlinear layer under the anode: the formation and the migration of charged defects towards the anode may mainly occur in Ge₂₀Ga₅Sb₁₀S₆₅ and Ge₂₅Ga₅S₇₀ glasses, whereas the migration of Na⁺ ions towards the cathode may be responsible for the accumulation of negative charges under the anode in Ge₃₃S₆₇ and Ge₂₅Sb₁₀S₆₅ glasses. Different electronic conductivity behaviors seem to be at the origin of the phenomenon. In parallel, the potential effect of the poling treatment on the structural and electronic properties is studied using Raman spectroscopy and secondary ion mass spectroscopy measurements.     

Effect of circuit switched services on the capacity of HSDPA

This paper focuses on the effect of circuit switched (CS) services on high speed downlink packet access (HSDPA) packet services in the context of the UMTS FDD system. An objective of the analysis is to provide guidelines for the UMTS planning process to prioritize services when resources are dynamically shared between circuit and packet services. An analytical model, taking into account some of the features introduced in HSDPA (namely adaptive modulation and coding (AMC), hybrid automatic repeat request (HARQ) and fast cell selection) and the interaction between CS services and HSDPA, is proposed to approximate the HSDPA cell capacity. Simulation results to assess the efficiency of the analytical model are also reported.     

Dynamics of a two-mode semiconductor laser

The authors analyze the properties of a semiconductor laser in a ring cavity when two modes can oscillate. With this model it is shown that phase-sensitive mode-mode interactions can influence the laser dynamics through their dependence on the spatial Fourier components of the carrier density. In particular, these interactions provide a mechanism by which single-mode solutions are destabilized while periodic and quasi-periodic two-mode solutions can occur     

Optical and structural properties of new chalcohalide glasses

New class of chalcohalide glasses has been prepared in the GeS₂–In₂S₃–CsI system with regard to their potential non-linear properties. The study of glass-forming region was undertaken to select glassy compositions, which present high non-linear (NL) optical properties with a low two-photon absorption. Thermal analyses, structural examination by Raman spectroscopy, non-linear optical measurements were investigated as a function of CsI contents. Introduction of CsI has shifted the band-gap edge towards the blue region of the absorption optical spectrum and therefore has limited the two-photon absorption. Their NL refractive index n₂ are 60 times higher than silica glasses without any NL absorption. Moreover, second harmonic signal was observed in thermally poled samples similar to silica glass. However, this second order non-linearity is not temporally stable.     

Cross-Layer design in HSDPA system to reduce the TCP effect

This paper focuses on the interaction between the transport control protocol (TCP) layer and the radio interface in the high-speed downlink packet access (HSDPA) wireless system. In the literature, studies of the interaction between TCP and wireless networks are focused on the evaluation of user bit rate in the case of dedicated channels. In this paper, the interaction between TCP, hybrid automatic repeat request (HARQ), and scheduling techniques (especially, proportional fair scheduling) is conducted. Analytical models to evaluate HSDPA cell capacity, user bit rate, and interaction with TCP layer are presented. Even if as expected the bit rate per flow decreases strongly with the congestion frequency in the wired network, it is shown that the overall capacity achieved by HSDPA is not as affected by the TCP layer. Using this result, a method to reduce the effect of TCP on wireless network without losing much cell capacity is proposed. This method has the advantage of modifying the scheduling algorithm only and of not requiring any change to the TCP protocol.     

VCB: An Efficient Resource Sharing Scheme for Cellular Mobile Systems

A new algorithm called Virtual Channel Borrowing (VCB) used to improve the resource sharing performance of a Fixed Channel Assignment (FCA) scheme is proposed and analyzed. The underlying idea is to exploit regions of cell overlap to achieve a balanced distribution of traffic load in non uniform traffic conditions while reducing call blocks and handoff failures. Simulation results show a performance improvement at the expense of additional signaling. Another advantage of VCB is that it can be integrated easily with small additional cost.     

Stability of a detuned single mode homogeneously broadened ring laser

We study analytically the influence of detuning on the properties of a homogeneously broadened single mode ring laser. In the good cavity domain the stationary output is stable. In the bad cavity domain the stationary output becomes unstable via a Hopf bifurcation. Near line center this Hopf bifurcation is subcritical, leading to unstable small amplitude oscillations. Far from line center this Hopf bifurcation is supercritical and leads to stable small amplitude output.     

Method to improve thermal poling efficiency in silica glasses

For the first time, thermal polings are performed on Suprasil I samples (Heraeus) under square alternating voltages at variable frequency. Experimental results exhibiting a large increase (×13 compared to a continuous voltage poling) of the second-harmonic signal generated within a sample poled at a frequency of 1 mHz are presented     

TCP Performance over UMTS-HSDPA System

The performance of TCP based applications in a UMTS-HSDPA system is evaluated, using an analytical model. HSDPA (High Speed Downlink Packet Access) is an evolution of the UMTS standard over the air interface to achieve higher bit rates through the introduction of adaptive modulation and coding, Hybrid ARQ, fast scheduling, fast cell selection and MIMO. The proposed model is used to evaluate the effect of the TCP protocol on the bit rate of various data services (at 32, 64 and 128 kbps). The results show that the bit rate per flow decreases strongly if congestion in the wired network increases. The interaction between cell capacity and the TCP layer is also studied. A semi-analytical model to evaluate the cell capacity is also presented in this paper, and the correspondence between cell capacity and guaranteed TCP bit rate is reported in this paper for a Round Robin scheduler. The findings are supported by simulation results.