Pump it up

US researchers have demonstrated that cleaning up fibre optic signals can be done on a single photonic microchip.


More and more of our communications – from text messages to high-definition television – travel over optical fibre.


But there’s a problem: Light is dimmed by miles of fibre, and the crisp on-and-off pulses that represent the ones and zeros of a digital signal become misshapen and fuzzy. Every 50 miles or so the signal must be reamplified, cleaned up and relaunched.


Now Cornell University researchers have demonstrated that all this can be done on a single photonic microchip, replacing bulky bundles of fibre or electronic amplifiers.


Previously Alexander Gaeta, professor of applied and engineering physics, and Michal Lipson, associate professor of electrical and computer engineering, had demonstrated a light amplifier on a silicon chip using a process called four-wave mixing, which could amplify an optical signal by “pumping” with another beam of light.


Now, they have shown how that the same process can clean up and sharpen the pulses that travel the length of a fibre-optic cable too. If the pumping beam consists of a series of pulses synchronised with the input signal, the process also cleans up “timing jitter,” in which the pulses are not only deformed but also move slightly forward or back in time.


The Cornell researchers used silicon waveguides only a few hundred nanometres across and 1.8 centimetres long embedded in a single silicon chip. The tight dimensions of the waveguide, smaller than the wavelength of the light travelling through it, forces two entering beams of light – the signal and the “pump” – to exchange energy over a very short distance, amplifying the original signal.


Others come out at a wavelength equal to twice the pump wavelength minus the signal wavelength. That last effect can be used to convert a signal from one wavelength to another.


In a series of experiments all using the same nanoscale wave guides, the researchers found that pumping a pulsed signal with a continuous wave light beam at another frequency amplifies the signal but doesn’t clean up the pulses. However, if the arrangement is changed so that the light carrying the signal acts as the pump, the output is both amplified and sharpened. If the pump is a pulsed beam synchronised with the pulse rate of the input signal, the output is amplified and sharpened, and timing jitter is also reduced.


The researchers say that the four-wave mixing approach could be used in multiplexed fibre-optic systems where several wavelengths are used simultaneously to carry multiple signals.