Interaction between light and sound waves in a photonic chip

24 June 2014

Thomas Büttner and Dr Irina Kabakova from the University School of Physics Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS)
Thomas Büttner and Dr Irina Kabakova from the University School of Physics Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS)

Thomas Büttner and Dr Irina Kabakova from CUDOS, the ARC Centre of Excellence at the University of Sydney's School of Physics, have exploited the interaction between light and sound waves in a photonic chip to produce a high repetition rate train of short optical pulses.

Traditional pulsed sources such as those based on mode-locked fibres and solid-state lasers provide repetition rates that are typically limited to tens of megahertz. This limited repetition rate is an artefact of either the length of the fibre cavity (in metres) or the maximum speed of the electronics used to drive the experiment, or indeed both. The physical limitations of traditional methods frustrate researchers who demand increasingly compact, integrated pulse sources with much higher gigahertz repetition rates. The work of the CUDOS team, with main contributors Thomas Büttner and Dr Irina Kabakova, promises a way to break through the existing gigahertz repetition rate ceiling by providing a compact and high frequency pulse source.

"This breakthrough represents really interesting and new physics - we show for the first time how the interaction between light waves and sound waves can stimulate the generation of stable, short optical pulses on a chip. The physics of this interaction is quite beautiful and the applications are really exciting. This is the first demonstration of an optical-phononic chip; harnessing the interaction between photons (optical waves) and phonons (sound waves)." - Professor Ben Eggleton, Director, CUDOS.

"Our technique enables the formation of optical pulses from a continuous wave in just a few centimetres-long cavity. The pulses are synchronized by sound waves and the fiber nonlinearity; the gigahertz frequency of the sound waves is what determines the repetition rate of optical pulses," said Thomas Büttner, who is a PhD candidate at the School of Physics.

The researchers chose chalcogenide glass fibres and waveguides on a chip to demonstrate their frequency combs due to, amongst other things, their highly nonlinear nature. When a light wave enters the chalcogenide waveguide, it can modulate the glass density and excite acoustic phonons, which is another name for moving density fluctuations.

These phonons scatter the light in a direction which is opposite to the travelling optical wave. The backscattered light reinforces acoustic vibrations and amplifies the scattering in a process called stimulated Brillouin scattering (SBS). Each backscattered wave has a small frequency shift as a result of a Doppler shift taking place when light interacts with the acoustic phonon, which is itself traveling at the speed of sound. If the medium is placed in between two mirrors, the optical feedback strongly amplifies the SBS, leading to the generation of multiple waves, known as a 'frequency comb'. Each of the waves in this frequency comb is separated by the same frequency shift, proportional to the speed of sound.

Gigahertz frequency combs are particularly important for telecommunication applications which operate a gigahertz rates. "Our findings will help to develop a compact and high-frequency pulse source for applications in metrology, microwave photonics and communications," said Dr Kabakova.

"Another important application of gigahertz frequency combs can be seen as a precise ruler, which can provide a frequency reference or act as a clock for signal synchronization in communication networks," said Dr Kabakova.

In addition, these results will be useful in microwave photonics for generation of low-noise microwave signals. This can be achieved by combining two or more comb lines which produces a radio-frequency signal in the gigahertz range.

The Centre for Ultrahigh bandwidth Devices for Optical Systems, CUDOS is an ARC Centre of Excellence.

Read the paper online here

Contact: Tom Gordon

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