1 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark
In this report all-optical 2R-regeneration in optical communication systems is investigated. A simple regenerator device based on concatenated semiconductor optical amplifiers (SOAs) and electro absorbers (EAs) is introduced and examined. Experiments show that the monolithic SOA-EA 2R-regenerator has a sharp step-like transfer function with a threshold that is easily adjusted by simply changing the bias voltage over the electro absorbers. Measurements on a modulated signal show that the device can improve the extinction ratio of a degraded signal with more than 5 dB, and improve the receiver sensitivity with more than 8 dB compared to the back-to-back case, using a degraded signal. The noise properties and cascadability of the proposed device are examined through modeling. Furthermore the influence of the saturation properties of the EA on the regeneration performance is investigated. Calculations show that it is possible to increase the nonlinearity of the transfer function and improve the regenerating properties by lowering the saturation power of the EA and concatenating several SOA-EA pairs, although this also adds more noise to the signal. In order to analyze the influence of the regenerator properties on the bit error rate in a cascade of regenerators, a general model for a 2R-regenerator, neglecting timing jitter, is developed. The model conceptually divides the regenerator into a linear amplifying part and a nonlinear reshaping part. The amplifier adds noise in the form of amplified spontaneous emission and the nonlinear transfer function redistributes the noise and improve the extinction ratio of the signal. The model shows that the best choice of decision threshold is made by considering the properties of the cascaded regenerators rather than the signal going into the cascade. Furthermore the interplay between different regenerator properties like noise figure, nonlinearity and extinction ratio is examined. The results show that an increase in nonlinearity can compensate for a higher amplifier noise figure or increase the reach of a transmission link. These kinds of investigations make it possible to compare different kinds of regenerators and guide the optimization of regenerators. Theoretical modeling and direct measurement of the probability density function (PDF) of a continuous wave (cw) signal after an SOA show the nonlinear noise redistribution due to gain saturation and carrier dynamics. The redistribution gives both a reduction in the width and change in the overall shape of the PDF, compared to a linear amplification. The redistribution of noise, as described by the changes in the PDF, is central to the understanding of regeneration. The changes of the noise in an SOA is examined closer by use of a small-signal model, including first and second order noise terms, and a large-signal time-domain simulation. The calculations show the noise spectra and the bandwidth dependence of the noise redistribution in the device. Both pass-through amplification and wavelength conversion through cross-gain modulation is investigated and compared.