S. Betti

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.

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. >

Analysis of distortion effects in subcarrier-multiplexed (SCM) externally modulated lightwave systems: a generalized approach

A generalized analysis is presented for the evaluation of intermodulation distortion effects in case that external intensity modulators are used for the transmission of analog/digital (AM/QAM) channels in an SCM based optical access network. The proposed model can provide useful information to select external modulator and system parameters for performance optimization.

Effect of clipping impulsive noise in a hybrid optical‐fiber/coaxial network for transmission of subcarrier‐multiplexing (SCM) analog and digital channels

In Italy, Telecom Italia has started a plan to develop a broadband network platform that will offer broadcast video, interactive multimedia, and telecommunication services: The SOCRATE project is based on analog and digital channel transmission, the subcarrier multiplexing technique, and hybrid fiber / coaxial architecture. In this article, the effect of clipping impulsive noise is considered in the evaluation of the performance of the analog / digital multichannel transmission system.

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>>

Effect of the turn-on delay of a semiconductor laser on clipping impulsive noise

The performance of hybrid multichannel AM/QAM fiber-optic transmission systems is constrained by clipping impulsive noise, which must be suitably taken into account to select system parameters. In this letter, the influence of semiconductor laser turn-on delay is considered in the evaluation of the effects due to clipping impulsive noise.

Performance of a 16-QAM channel in a hybrid AM-VSB/QAM fiber-optic transmission system based on a subcarrier-multiplexing (SCM) technique

The performance of a hybrid multichannel optical-fiber system for the transmission of analog (AM) and digital (QAM) channels by a subcarrier-multiplexing (SCM) technique is presented. The experimental results are compared with a theoretical model recently proposed that takes into account the clipping effect in semiconductor lasers, which gives rise to an impulsive noise affecting system performance.