Andrew J. Phillips

WDM FSO network with turbulence-accentuated interchannel crosstalk

A wavelength division multiplexing (WDM) access network using high-speed free-space optical (FSO) communication for the distribution link is proposed. Combining FSO communication with optical fiber can reduce the system cost and provide high-bandwidth access in regions where optical fiber installation is problematic. The WDM channels suffer from interchannel crosstalk, while the FSO communication performance in a clear atmosphere is limited by atmospherically induced scintillation. These impairments, plus the amplified spontaneous emission noise from optical amplification, combine in a potentially problematic way, particularly in the upstream direction, which is investigated here. This turbulence-accentuated crosstalk effect is considered for the cases of 1) signal turbulent but crosstalk not and 2) crosstalk turbulent but signal not. Error floors are obtained in each case. The FSO link length that can be supported in the general case of the hybrid network is investigated.

Nonlinear Evolution of Gaussian ASE Noise in ZMNL Fiber

This paper investigates the evolution of kurtosis of the input Gaussian amplified spontaneous emission (ASE) noise in a nonlinear fiber with negligible dispersion. The nonlinear Schrodinger equation (NLSE) describing propagation in optical fibers is simplified such that the fiber represents a zero memory nonlinear (ZMNL) system, and this approximation allows the development of analytical formulas for the statistical moments of the output noise. It is possible to calculate moments of all integer orders and the explicit expressions for the first four moments are given. The investigations show that the ASE noise does not preserve its Gaussian character when Kerr nonlinearity is significant. This observation proves that the common assumption of the Gaussian output ASE is not necessarily valid. Numerical simulations are provided to support the derivation. Kurtosis deviating significantly from the value typical for Gaussian noise is also an indicator that BER calculation in the coherent systems based on the assumption that ASE is Gaussian is likely to be inaccurate.

DPPM FSO communication systems impaired by turbulence, pointing error and ASE noise

Performance analyses, using a modified Chernoff bound on the bit error rate, are given for an optically preamplified digital pulse position modulation (DPPM) free-space optical (FSO) communication system impaired by pointing error and atmospheric turbulence combined, in addition to the amplified spontaneous emission noise arising from the optical amplification. An improvement of about 7-9 dB (depending on the turbulence level) can be potentially achieved using the DPPM scheme over an equivalent on off keyed nonreturn-to-zero based FSO system.

Performance evaluation of optically preamplified dicode pulse position modulation receivers

An analysis for optically preamplified dicode pulse position modulation (DiPPM) receivers is presented for the first time. The performance at two different bit rates (622u2009Mbit/s and 2.5u2009Gbit/s) is analysed and compared with equivalent optically preamplified pulse position modulation (PPM) and on-off keyed non-return-to-zero (OOK NRZ) systems. Additionally a new derivation for the average power of the DiPPM pulse stream is provided and the sensitivity performance is compared with that obtainable from a previous formula, with results that are somewhat more conservative. For optically preamplified systems it is shown that PPM significantly outperforms DiPPM and OOK NRZ, when using the DiPPM average power formula derived here, but DiPPM could still show a sensitivity benefit over OOK NRZ of 4.2u2009dB. Copyright

Optimising performance of a confocal fluorescence microscope with a differential pinhole

The signal-to-noise ratio (SNR)-resolution trade-off is of great importance to bio-imaging applications where the aim is to image the sample using as little light as possible without significantly sacrificing image quality. In this paper the inherent SNR-resolution tradeoff in Confocal Fluorescence Microscopy (CFM) systems is presented by means of an effective tradeoff curve. A CFM system that employs a differential pinhole detection scheme has recently been shown to offer increased resolution, but at the expense of SNR. An optimum profile for the differential pinhole is identified in this paper that offers improved performance over a conventional (circular pinhole) system. The performance enhancement is illustrated through computer simulation.

Modeling of burst mode 2R optical regenerator cascades for long-haul optical networks

A complete system-level model for cascades of burst mode 2R optical regenerators (BMOR), each consisting of an optical limiting amplifier (LA) followed by an optical re-shaper, is presented for optical packet/burst-switched optical networks. The major distinction compared to continuous-mode 2R OR is that a BMOR can successfully handle the large power variations because of the LA, and the impact of inclusion of this into the system analysis is presented here. The bit error rate is evaluated along a cascade of identical BMORs, interspersed with heterogeneous inter-regenerator links (i.e., varying amplified spontaneous emission noise and net gains) which further complicate the burst mode case relative to the continuous mode. The dependency of performance of the cascade on the characteristics of the LA is investigated.

Theoretical investigation of the role of optically induced carrier pulsations in wave mixing in semiconductor optical amplifiers

A theoretical investigation of the role of interband effects in wave mixing in semiconductor optical amplifiers (SOA) is reported. Carrier density modulation (CDM) caused by optical wave beating in SOAs is examined along with its dependence on different operating parameters. Unlike most wave mixing theories, in which the existence and form of carrier pulsations are assumed a priori, we model the carrier dynamics and optical propagation in the time domain directly without invoking such an assumption. The dependence of CDM on the bias current, input power, and detuning between the pump and probe waves is investigated. Selected simulation results are verified experimentally. Good qualitative agreement is obtained between simulations and experiments for nearly degenerate wave mixing (restricted to 3 GHz by experimental limitations).

Resolution and signal-to-noise ratio improvement in confocal fluorescence microscopy using array detection and maximum-likelihood processing

Confocal fluorescence microscopy (CFM) is widely used in biological sciences because of its enhanced 3D resolution that allows image sectioning and removal of out-of-focus blur. This is achieved by rejection of the light outside a detection pinhole in a plane confocal with the illuminated object. In this paper, an alternative detection arrangement is examined in which the entire detection/image plane is recorded using an array detector rather than a pinhole detector. Using this recorded data an attempt is then made to recover the object from the whole set of recorded photon array data; in this paper maximum-likelihood estimation has been applied. The recovered object estimates are shown (through computer simulation) to have good resolution, image sectioning and signal-to-noise ratio compared with conventional pinhole CFM images.

Enhanced Confocal Fluorescence Microscope performance using a differential pinhole

While evaluating the performance of a microscope, one must consider not only the resolution but also SNR. The two criteria are each important to bio-imaging and interdependent such that attempts of optimising one generally tends to degrade the other. Here a novel detection scheme using an alternative pinhole geometry, a differential pinhole, is introduced. The scheme offers enhanced lateral as well as axial resolution resulting in an improved SNR-resolution tradeoff in a confocal fluorescence microscope. Up to 20 % resolution gain in either direction has been achieved and the performance enhancement has been demonstrated through computer simulations . The new detection scheme if used in conjunction with annular lens apertures could see further resolution gains.

Performance Analysis of DC-Coupled Peak Detecting Burst Mode Receiver in the Presence of ASE Noise and Interchannel Crosstalk

The bit-error-rate (BER) performance of an optically preamplified peak detecting burst mode (BM) receiver is analyzed. The analysis takes into account amplified spontaneous emission (ASE) noise and, for the first time with this receiver architecture, interchannel crosstalk. Furthermore, the ASE beat noises are more appropriately included through the use of a noncentral chi-square (NCCS) distribution in contrast to earlier attempts, which used a Gaussian approximation (GA), though a GA is also developed here for the interchannel crosstalk case, for comparative purposes. Results are presented in terms of BER and penalty curves. The peak detector-based BM receiver shows larger threshold acquisition (TA) penalty than an integrator-based BM receiver in the presence of ASE. Also ASE beat noise modeling using the NCCS distribution shows greater power penalty for interchannel crosstalk than does the GA. The sum of separate crosstalk and TA penalties is found to be less than the combined TA penalty obtained in the presence of interchannel crosstalk.