Gokalp Kahraman

Time and frequency response of avalanche photodiodes with arbitrary structure

A method is developed for solving the coupled transport equations that describe the electron and hole currents in a double-carrier multiplication (DCM) avalanche photodiode (APD) of arbitrary structure. This solution makes it possible to determine the time and frequency response of the device. The injection can be localized to one or both ends of the multiplication region, or distributed throughout an extended region where multiplication can occur concurrently. The results are applied to conventional APDs with position-dependent carrier ionization rates (e.g., a separate-absorption-grading-multiplication APD) as well as to superlattice multiquantum-well (MQW) structures where the ionizations are localized to bandgap transition regions. The analysis may also be used to determine the dark current and include the carrier trapping at the heterojunction interfaces. The results indicate that previous time-dependent theories only account for the tail of the time response under high-gain conditions and are inaccurate for high-speed devices. >

A fuzzy model based adaptive PID controller design for nonlinear and uncertain processes.

We develop a novel adaptive tuning method for classical proportional-integral-derivative (PID) controller to control nonlinear processes to adjust PID gains, a problem which is very difficult to overcome in the classical PID controllers. By incorporating classical PID control, which is well-known in industry, to the control of nonlinear processes, we introduce a method which can readily be used by the industry. In this method, controller design does not require a first principal model of the process which is usually very difficult to obtain. Instead, it depends on a fuzzy process model which is constructed from the measured input-output data of the process. A soft limiter is used to impose industrial limits on the control input. The performance of the system is successfully tested on the bioreactor, a highly nonlinear process involving instabilities. Several tests showed the methods success in tracking, robustness to noise, and adaptation properties. We as well compared our systems performance to those of a plant with altered parameters with measurement noise, and obtained less ringing and better tracking. To conclude, we present a novel adaptive control method that is built upon the well-known PID architecture that successfully controls highly nonlinear industrial processes, even under conditions such as strong parameter variations, noise, and instabilities.

Spectral properties of photocurrent fluctuations in avalanche photodiodes

The authors provide a stochastic model that describes the time dynamics of double-carrier multiplication in an avalanche photodiode (APD) and obtain the autocorrelation function and the spectral characteristics of the photoelectric current. The photoelectric pulse generated by an APD as a result of a single injected photoelectron is regarded as a nonstationary random function of time (the impulse response function). A discrete stochastic model for the electron-hole motion and multiplication is defined on a spatiotemporal lattice and used to derive recursive equations for the mean, the variance, and the autocorrelation of the impulse response as functions of time. Correlation properties of the impulse response are studied for a conventional and a multilayer (superlattice) APD. The power spectral density of the photocurrent is evaluated. >

Performance failure analysis of EDFA cascades in optical DWDM packet-switched networks

A simple and comprehensive technique to determine the probability that a cascade of erbium-doped fiber amplifiers (EDFAs) may be driven into unacceptable regimes of bit error rate (BER) and/or gain levels is presented. This technique allows network designers to determine the tolerances by which the signal power levels may deviate from their predesigned average values and still give acceptable gain variances and BERs at the receiver. We show that even in the signal power range well above the receiver sensitivities (-38 dBm/ch) where the gain spread is not significant, the corresponding spread in BER due to random arrival of packets might result in unacceptable performance. We show for typical levels of operation, the BER temporarily (for about 3 /spl mu/s) deviates to below 10/sup -9/ (10/sup -15/) with a probability of 10/sup -3/ (10/sup -2/), for 100 (64) channels. We show that the gain spread for a single EDFA can be negligible for a range of signal and pump powers at a given average gain.

An FBG sensor system for low voltage AC signals

An FBG sensor system was realized to sense AC voltages with low amplitudes and low frequencies. ECG signals was sensed and successfully transmitted over a singlemode optical fiber. The FBG sensor system can be expanded to form a sensor network wherein ECG signals measured off many patients can be multiplexed and monitored on a single computer.

Quantum-statistical properties of pulse amplification in optical fibers with gain saturation

We studied the quantum-statistical properties of pulse amplification in erbium-doped fiber amplifiers (OFAs), including saturation of the atomic population inversion and pump depletion. We use a fully quantum theory to describe the atom-field interaction as wed as the light propagation. The generating function of the output photon number distribution (PND) is determined as a function of time during the course of the pulse, with an arbitrary input PND assumed. For input light with Poisson PND, the output PND is shown to be the Laguerre distribution with parameter 1 at all times smaller than the coherence time /spl tau//sub c/, even in the presence of nonlinear effects. An expression for the photon count moment generating function is found for counting times T/spl Gt//spl tau//sub c/. The mean pulse shape is shown to be altered by the nonlinear amplification. The variance is similarly altered, and the excess amplifier noise is greater at the leading side of the pulse. >

Nonlinear Photon Statistics of Pulse Amplification in Optical Fiber Amplifiers

The bit error rate (BER) in soliton amplification through a cascade of Erbium-doped fiber amplifiers (EDFA) is determined accounting for the signal-induced gain saturation. Steady state analysis under pulsed conditions is used since the level population densities respond to the time-averaged power of the soliton stream. Since shorter soliton duration is required for higher bit rates and the average soliton power increases as the duration decreases, EDFAs are operated under saturation at high bit rates. The EDFA gain and amplified spontaneous emission (ASE) noise change with the time-averaged input power and attain different values when operated under saturation than when not. In this work, the span distance and gain and noise levels are accurately determined accounting for the saturation effect in order to meet a bit error rate criteria in a given soliton communication system using a cascade of EDFAs.