Sound system

A new type of light modulator that uses acoustic waves rather than electrical currents promises to be a boost to the telecommunications industry. Siobhan Wagner reports.

A group of scientists from a German research institute have developed a new type of light modulator that is driven by surface acoustic waves. It is 300 times smaller and can transport more data than comparable modulators.

Light modulators convert electrical signals into light so they can be transferred into fibre-optic networks. The new devices could one day be used in lasers by the telecommunications industry to improve signal modulation.

The modulator, which has been developed at the Paul Drude Institute for Solid State Electronics, Berlin, consists of a device called a Mach-Zehnder interferometer (MZI). This divides and recombines light waves, using interference to change the amplitude of the wave.

An interferometer works on the principle that two waves coinciding in the same phase will add to each other, while two waves with opposite phases will cancel each other out, assuming both have the same amplitude.

The modulator divides an incoming light signal into two light beams. The refractive index of the material through which one of the beams passes can be altered and, as a result, the light passing through the material is slowed down. This puts the beam in a different phase to the other and, when the two beams are recombined, the reunited rays interfere with each other. Thus, the transmitted light intensity is altered.

Until now MZIs have mainly been composed of electrical insulators such as lithium niobate and the refractive index change is driven by electrical currents. The new MZI is made of a semiconducting compound, gallium arsenide, and uses acoustic surface waves to modulate incoming light signals. Its interaction region is only 15 micrometres across.

For this modulator, researchers built a miniature acoustic source that converts electrical current into acoustic waves. These place a strain on the device and induce changes in the refractive index of the beams. The interference encodes the signal from the original electric current — usually electronic digital data — into a modulated light wave.

The use of sound waves, rather than electrical currents, is one of the main reasons why the researchers were able to reduce the size of the modulator, said Paul Santos, the leading scientist behind the development. ‘The difference is that the change in refractive index you achieve using acoustics is normally larger than what you achieve using electric fields,’ he said.

Santos said he and the other scientists want to develop a more efficient process to generate surface waves. They would like to reduce the number of transducers used to convert electrical current into acoustic waves but the compound they are using, gallium arsenide, is a weak electrical material and requires large amounts of electrical excitement.

Santos said the principle of using acoustic waves to drive light modulation could be applied to materials other than gallium arsenide. ‘Gallium arsenide is important but in this context, if we are able to make these kind of modulators on silicon or indium phosphite, it would be more interesting from an application point of view because we would cover the data communication wavelength,’ he said. ‘The telecommunications industry is mostly based on indium phosphite materials and on silicon insulators.’

However, gallium arsenide has benefits over other materials. Unlike silicon, it is an efficient light-source and a modulator made of the compound could be manufactured with an integrated light-source.

Also, while the acoustic-optic modulator is smaller than electro-optic modulators, Santos said there are drawbacks. ‘The acoustic waves have a velocity that is not very high so if you want to make on and off switches the operation will not be very fast. This is not a device for every application.’