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Statistical Analysis of Non-linear Optical Devices In Ultrashort Light Pulse Detection in Lightwave Communication Systems

Matinfar, Mehdi | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41809 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Salehi, Jawad
  7. Abstract:
  8. In this dissertation, the statistical analysis of ultrashort light pulse detectors, in presence of different noise sources, is presented. By investigating performance degradation of ultrashort light pulses detection based on linear conventional detectors, the necessity of all optical preprocessors prior to the low speed photodetectors is approved. By using these preprocessors, we achieve high discrimination between noise and ultrashort light pulses at the photodetector input, for simplicity in detection of light pulses by conventional low speed photodetectors. Among various types of preprocessors, in this dissertation, we focus on optical crystals with dominant imposed nonlinear second harmonic generation (SHG) effect in them. In these crystals, by applying phase-matching conditions, SHG nonlinear effect will present as the most dominant nonlinear effect. Performance of fiber optic communications receivers with optical SHG crystals is evaluated. By classifying optical crystals based on the optical path in them, two different models, Thin and Thick crystals, are obtained. A unique method for statistically analysis each of these models is presented. Thick crystals are statistically modeled by using the well known Papoulis theorem in narrowband systems. Then by using Turin lemma the moment generating functions of decision variables in Thick crystal receiver structure are obtained. Finally, performance of the receiver is evaluated by using Saddle Point approximation. Analytical evaluated and Gassian approximated performances are compared with the precise performance obtained using Monte Carlo simulation. High precision of the presented analytical approach for performance evaluation is approved. It is approved that the Gaussian approximation is useless for performance evaluation of these receiver types, especially in high received power conditions. The statistical analysis of the receivers with Thin crystal is presented by evaluating first, second and third order moments used in Gram-Charlier expansion of the probability density function (pdf) of decision variables. By integrating the pdf of decision variables, the error probability of the receiver in detection of bit "0" and "1" is evaluated. Spectrally phase encoded optical CDMA (SPE-OCDMA) systems are considered as one of the optical communication systems which use ultrashort light pulses for data transmission. Performance of the receivers with optical SHG crystals in these systems in presence of multi access interference noise, optical amplifier amplified spontaneous emission (ASE) noise, photodetector shot noise and electronic thermal noise is evaluated. The effect of optical path through the crystal and other system parameters are completely investigated and discussed. Finally, erformances of the receivers with Thin and Thick crystals are compared. In the environments with dominant thermal and shot noise compared to ASE noise, receivers with Thick crystals outperform the receivers with Thin crystals and vice versa. Receivers with Thin crystals are more sensitive to the optical path through the crystal, than the receivers with Thick crystals. In SPE-OCDMA systems, when the summation of multi access interference noise and optical amplifier ASE noise is the dominant source of noise compared to the thermal and shot noises, performance of the receivers with Thin crystals outperforms the Thick one
  9. Keywords:
  10. Optical Communication ; Photodetector ; All Opticall Detectors ; Nonlinear Pre Processors ; Ultrashort Light Pulses ; Ultrafast Optical Communication System

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