Optical oscilloscope is fit for high-speed studies

11 Nov 2008

An all-optical oscilloscope beats the resolution of conventional devices by more than a factor of five.

Physics in the US have made an oscilloscope that can take snapshots of optical waveforms at a resolution fives times better than current devices. Based on an all-optical rather than an electronic design, the oscilloscope should be able to accurately profile modern telecommunications signals and various ultrafast chemical and physical phenomena (Nature 456 81).

Oscilloscopes are used to trace graphs of signals over time. Conventional models are based on microelectronics and, using photodetectors, can take snapshots of optical signals at as low as 30 ps (30 × 10^–12 s) resolution.

But as telecommunication data transmission gets faster and faster, and as scientists want to probe more high-speed systems, oscilloscopes based on microelectronics are being stretched to the limit. This is because they can only cope with a relatively narrow frequency spread or "bandwidth", which holds back their resolution.

Optical is better
All-optical circuits, on the other hand, can process much wider bandwidths. Although optical techniques already exist – indeed, with resolutions going down to a few femtoseconds – these have only been able to take snapshots of small segments of waveforms, and take a long time to update.

A team led by Alexander Gaeta at Cornell University in New York has found a way to exploit the fine resolution of optical techniques for longer waveforms. The researchers make use of the fact that electromagnetic waves have a space–time duality, in that there is a link between their spatial and temporal wavefunctions. This means that the researchers can use a lens to convert the temporal profile of a dispersed snapshot into a detailed, spectral output via a so-called Fourier transformation.

In the Cornell team's device, an input waveform enters an optical fibre and mixes with a pump laser pulse, which ensures that the waveform matches the focal length of the lens. As the waveform travels through the fibre it stretches out or "disperses". Then, at the end of the fibre the lens – a nanoscale silicon waveguide – converts the waveform into a spectrum that can be measured with a spectrometer.

The device can record an input waveform at a resolution of 220 fs (220 × 10^–15 s) over lengths greater than 100 ps, giving the largest length-to-resolution ratio (more than 450) of any snapshot oscilloscope technique. Moreover, the technique uses components that can easily be integrated on chips.