Hi-Fi Optic Analysis Shown

Chalmers University of Technology in Gothenburg, Sweden, in collaboration with spinoff company PicoSolve Inc., have demonstrated a measurement method to analyze optical signals with information encoded in intensity and in the optical carrier phase. They said the method sets a record in the measurement of optical high-speed signals.

The high-fidelity analysis is made possible by using optical instead of electronic sampling, providing extremely high temporal resolution.

"The resolution in existing systems for acquiring high-speed optical signals is limited by the bandwidth in the electronics after detection of the optical signal," PicoSolve said in a statement. "At present, this is around 15 GHz, if what is known as real-time sampling is used -- a method that captures the true signal even if it only occurs as a single event.

"A further limitation that could arise is Nyquist's sampling theorem,* which postulates that the highest signal frequency is half the sampling rate, e.g., 10 GHz at a sampling frequency of 20 GSamples per second. However, this limitation has not been of practical interest for high-speed signals, until now." [*also called the Nyquist criterion: In image acquisition, the postulate that the pickup sampling frequency must be a minimum of twice as high as the rate of brightness change of any detail to be resolved. Harry Nyquist, who was born in 1889 in Sweden and emigrated to the USA in 1907, published his results at the Bell Telephone Laboratories in 1928.]

The researchers at Chalmers said they have solved the problem by sampling the high-speed signals into two parts: The first part involves sampling the signal using an optical technique with a very high bandwidth and then parallelizing the results into four outputs. In the second part, these four outputs are detected and sampled using a commercially available sampling system (4 x 25 GS/s). They said this results in a sampling system of 100 GSa/s and with a "Nyquist-limited" bandwidth of 50 GHz -- three times greater than what is currently available. They said the system also has a memory depth that is the same as for conventional systems, making the concept of significant practical use in the future.

"Installed optical communication systems are, with a few exceptions, implemented by utilizing the optical intensity as a carrier of digital information and operate at bit rates of up to 10 Gbit/s," PicoSolve said. "Commercial systems at 40 Gbit/s are now available and have been installed to increase system capacity."

Increased interest in using the optical carrier phase as an information carrier will result in more efficient us of available bandwidth of optical fibers, the company said. "In this context, it is important to develop tools in order to characterize optical signals in both intensity and phase."

Although optical phase-sensitive measurement solutions have been demonstrated in research laboratories around the world, the temporal resolution of these solutions is limited by the electronics to approximately 20 picoseconds, which is not enough to accurately characterize 40 Gbaud signals, PicoSolve said.

The researchers used an optical sampling technique that can achieve extremely high temporal resolution, demonstrated in experiments with a resolution of three picoseconds. This technique enables complete characterization (intensity and phase) of the optical signals at very high symbol rates, they said.

The team demonstrated measurement of a phase-modulated signal at 40 Gbaud at 2008 OFC/NFOEC last month in San Diego, "although the temporal resolution in principle enables characterization of signals at 100 Gbaud or more," the reserchers said. "The demonstrated technique is also very practical owing to the symbiosis between hardware and software-based algorithms."

The results were also presented at the European Conference on Optical Communication, held in September in Berlin.

For more information, see: M. Westlund, M. Sköld and Peter A. Andrekson, "All-optical, phase-sensitive waveform sampling at 40 GSymbol/s," OFC/NFOEC proceedings, post-deadline paper PDP12, March 2008, or visit: mc2.chalmers.se