Have you wondered why simulations done in the lab never quite produce accurate results when determining the capacity of optical fibre?


Dr Lidia Galdino of the department of Electronic and Electrical Engineering of University College London discovered during the course of her research that transceivers that transmit and receive optical signals have a much greater effect on the system performance than one had previously considered.


Dr Galdino had been working for the UNLOC research project. She had been performing a series of experiments that involved maximising the capacity of optical fibre systems. This meant testing new techniques for studying the nonlinearities and their impact on the capacity of data transmission through a typical optical fibre.


Dr Galdino and her team were particularly focused on a technique called as digital back-propagation (DBP). This technique detects the amplitude and phase of the light and digitally reverses their journey to in order to negate the distortions.


This research has been carried out on labs around the world and it has been found that the theoretical results are not the same as those of the practical experiments, and are in fact exaggerated. Dr Galdino was determined to find out why such discrepancies were caused in the first place and where the other experiments were going wrong.


Until now it was thought that the polarisation mode dispersion (PMD), which is a term used to describe the random rotation of light polarization in fibre cannot be explained by DBP alone. This was the reason, experts said, the lab results did not match up. Even so, even after compensating for the nonlinearity because of PMD, it was hard to explain the discrepancy between the theoretical and practical results.


Dr Galdino tried to make sense of this. First she entered certain parameters for the transceiver into the simulation. To her surprise, once the transceiver parameters were entered into the simulation, the theoretical results matched the practical results.


Dr Galdino explains, “This is hugely significant for the design of fibre infrastructure. It is now possible to identify all sources of error in system performance and, therefore, to find techniques to mitigate them.”


Following the experiment, Dr Galdino wrote in a paper published in the Optics Express that the fact that the transceiver noise interfered nonlinearly with the signal meant that the noise did not increase in the same proportion as the rate at which the signal power was increased. Until now the impact of the transceiver noise was underestimated by researchers, as it was thought that the interference caused by the noise increased at a steady rate in tune with the increased in the signal power.


Dr. Galdino wrote, “The noise produced by the transceiver comes from digital-to-analogue and analogue-to-digital converters in the transmitter and receiver respectively. This fundamental noise is present even in our state-of-the-art equipment, but in time manufacturers will be able to improve on this. Our research will help systems designers to correctly predict and design next-generation, high-performance optical transmission systems.”


“We should consider transceiver noise as a more fundamental limit to DBP performance than PMD. Importantly we can now precisely estimate the achievable gains by applying DBP in realistic optical systems. We’ve been doing all the right things and DBP is still one of the most powerful ways to maximise capacity or increase transmission distances. Our data shows that applying DBP to real-world systems can double the achievable data rate to 1Tb/s over 1000km – longer than the length of the UK,” Dr Galdino said.


As Daniel Semrau, a PhD student and co-author of Dr Galdino’s study explains, “We’ve proposed a new approach that correctly accounts for the nonlinear interference between the transceiver noise and signal in an optical fibre link. For the first time, every system designer can easily predict their transmission system performance in seconds.”


This goes to show that the noise that the transceiver produces comes from analogue-to-digital and digital-to-analogue converters in the in the transmitter and receiver. Even our top-of-the-line, world class equipment have this noise, but over time we will be able to improve on this. The research from UNLOC will certainly allow systems designers such as us to design highly accurate, high-performance, next-generation optical transmission systems.




Dr Galdino demonstrates for the first time that transceiver noise interferes nonlinearly with the signal


Testing Reliability as Part of an Integrated System