G. De Marchis

Statistical treatment of the evolution of the principal states of polarization in single-mode fibers

A simple relationship is found for the evolution of the principal states of polarization (PSPs) and their differential group delay in fiber links. A simple expression is found, using the relationship, for the probability of the differential group delay (DGD), considering the evolution of the PSPs as a Brownian motion. The theory has been verified experimentally on an optical cable composed of 12 single-mode, shifted-dispersion fibers 2.2-km long. The results show that the DGD grows as the square root of the length when the length of the fiber is far larger than the correlation length of the perturbation. The measured value of DGD can vary substantially in two fibers belonging to the same ensemble, and in the same fiber, considering two frequencies differing by more than 5 nm. >

Polarization modulated direct detection optical transmission systems

A novel polarization modulated direct detection (PM-DD) system suitable both for binary and multilevel transmission is presented. At the transmitter the optical field is polarization modulated by a standard modulator. The receiver is based on the estimation of the Stokes parameters of the received optical field by means of a direct-detection optical front end and baseband electrical processing. The Poincare sphere rotation induced by the fiber is compensated by means of a purely electronic algorithm and the decision is performed in the Stokes space. The system performance is evaluated by an analytical model when the only relevant noise source is the receiver thermal noise and when erbium-doped optical amplifiers introduce amplified spontaneous emission (ASE) noise. The system is completely compatible with a direct-detection-based optical network, and it is possible to implement efficient multilevel modulation formats. >

A novel multilevel coherent optical system: 4-quadrature signaling

A novel multilevel coherent optical system is proposed. It is based on the exploitation of the property that the electromagnetic field propagating in a single-mode optical fiber can be represented by a four-dimensional vector whose components are the phase and quadrature terms of the two polarization components of the electrical field. This allows a wider use of the resources of the electromagnetic field for information transmission in order to obtain a spectrally efficient modulation format with a limited end. The net performance gain with respect to multilevel amplitude and phase modulation (N-APK) and N-PSK increases with an increase in the number of levels N. For instance, for N=32 the gain is 1.6 and 7.7 dB with respect to N-APK and N-PSK systems. The effect of laser phase noise on the system performance is evaluated. >

Phase noise and polarization state insensitive optical coherent systems

The authors present three different schemes that allow compensation of phase noise and polarisation state change by sending a reference channel that is suitably frequency shifted by using polarization modulation together with Stokes parameters detection or computing and inverting the Jones matrix that describes the fiber polarization state transformation. As a conclusion, some comparisons are made among different approaches in order to show how different systems can be tailored to different requirements both in point-to-point and in multipoint networks. >

Evolution of the bandwidth of the principal states of polarization in single-mode fibers.

The bandwidth in which the first-order approximation of the principal states of polarization of a single-mode fiber can be assumed valid is examined. The principal states of polarization and their bandwidth are found for a fiber with both constant coupling and birefringence, and the relationship with the fibers eigenmodes is examined. On the basis of these results, a fiber cascade is analyzed, and a Monte Carlo simulation provides theoretical values of the bandwidth that have been experimentally verified on a 2-km-long concatenation of single-mode dispersion-shifted fibers.

Speckle and Modal Noise in Optical Fibres Theory and Experiment

Modal noise has been observed in optical fibres excited by partially coherent sources. In this paper the theory of this noise is presented. Following the basic guidelines of the speckle theory, the statistical properties of the fluctuations of the power transmitted through a circular aperture of arbitrary size are evaluated. It is found that, for fibres having an α -exponent index-profile, the maximum attainable signal-to-noise ratio, when the receiving aperture area is equal to the core area, is given by the square root of the number of the guided modes. The hypotheses on which the theory is based are discussed and an experiment, confirming the theoretical results, is presented.

Homodyne optical coherent systems based on polarization modulation

The first homodyne receiver for polarization modulated optical signals is proposed and analyzed. It is based on the detection at the receiver of the four quadratures that completely characterize the received optical field and on the estimation of the Stokes parameters starting from them. This allows homodyne detection to be carried out without phase locking the local oscillator to the received signal. An accurate performance evaluation is carried out both at the quantum limit and in the presence of phase noise for a binary system based on an antipodal decision in the Stokes space. The system structure is described. The quantum limit performance is evaluated, and the phase-noise induced penalty is evaluated using an accurate analytical model. >

Spatial coherence in optical fibers

Abstract The spatial degree of coherence on the cross-section of a graded-index optical fiber is evaluated under the assumption that all the propagating modes are uncorrelated among themselves. A detailed example for the case of parabolic fibers is given.

Phase-noise and polarization state insensitive coherent optical receivers

Different schemes that make it possible to compensate both phase-noise and polarization fluctuations due to single mode fibers are presented. The first two systems, double-frequency phase shift keying (DFPSK) and Jones matrix inversion PSK (JMPSK) rely on the principle of transmitting a reference carrier to compensate for the phase noise effect at the receiver. The polarization independence is attained in DFPSK by polarization diversity detection and in JMPSK by the electronic feedforward technique. The second pair of systems, antipodal Stokes parameters shift keying (ASPSK) and differential Stokes parameters shift keying (DSPSK), is based on polarization modulation and Stokes parameter detection. Because the Stokes parameters are independent of the phase terms common to both polarization components, the systems are widely tolerant with respect to the phase noise. The polarization fluctuation insensitivity is realized in ASPSK by means of a feedforward electronic control circuit, while in DSPSK the adopted decision criterion allows instantaneous polarization fluctuation compensation. The first class of systems is suitable for long-haul applications, the second class for local and metropolitan area networks.<<ETX>>

Feasibility studies for a Mediterranean neutrino observatory — the NEMO.RD Project

Abstract The NEMO.RD Project is a feasibility study of a km 3 underwater telescope for high energy astrophysical neutrinos to be located in the Mediterranea Sea. At present this study concerns: i) Monte Carlo simulation study of the capabilities of various arrays of phototubes in order to determine the detector geometry that can optimize performance and cost; ii) design of low power consumption electronic cards for data acquisition and transmission to shore; iii) feasibility study of mechanics, deployment, connection and maintenance of such a detector in collaboration with petrol industries having experience of undersea operations; iv) oceanographic exploration of various sites in search for the optimal one. A brief report on the status of points i) and iv) is presented here.