Crystal crunch

Experts at Southampton University are investigating the properties of a nonlinear crystal to develop an all-optical signal-processing device to cope with increasingly high-speed telecommunications.

The crystals, periodically poled lithium niobate (PPLN) waveguides, will be used to demonstrate relatively unexplored optical effects known as quadratic nonlinearities. By creating compact, quadratic processing modules, the researchers hope to achieve signal processing at an entirely optical level.

'We will need to accommodate much larger amounts of data much faster than before,' said Dr Periklis Petropoulos, one of the project investigators.

'In order to increase the capability of the networks that we already use, based on optical fibres, it will be increasingly important to be able to process data signals while they are in the optical form, without converting them into electronics first.

'The crystals we will be investigating provide the technology that will allow us to do this.'

Existing alternatives to the new waveguides would be nonlinear optical fibres or semiconductor optical amplifiers and at the moment, the only all-optical processing of signals installed in transmission systems is the optical amplifier, such as the erbium doped fibre amplifier (EDFA).

Apart from that, any processing that is required is done by electronics, which means signals are converted from an optical signal to an electronic signal, processed in an electronic form, then converted back into optical.

The waveguides have so far only been used in research laboratories to demonstrate basic telecoms devices, for instance using PPLN to convert wavelengths or for demultiplexing of signals.

Petropoulos's team wants to target more advanced applications, such as the regeneration of telecommunication signals (suppressing the noise in signals) and the characterisation of complex wavelengths (using the device to more accurately detect ultrafast features of an optical wave form).

Efficiency and speed are two features of the PPLN crystals that have attracted the interest of the Southampton team.

'The main feature is that processing can be performed at ultra-fast time scales. This means response times of the order of a femtosecond (10^-15 of a second) or even less,' said Petropoulos.

'The second attraction is that these waveguides, which have wide conversion bandwidths, are among the most efficient nonlinear optical devices that are available today, meaning that we will need less optical power to operate them.'

Using the crystals, the scientists aim to create devices that can process multiple signals in parallel at speeds of 40Gbit/s, with the help of quadratic nonlinearities.

'Nonlinear devices allow different signals to interact with each other, so they allow us much greater flexibility on the level of processing that we can apply,' said Petropoulos.

He said although in some contexts nonlinearities are considered detrimental, the Southampton team has an alternative point of view.

'There are two ways of seeing nonlinearities: if you are looking at transmission systems, when you are trying to transmit your signal from point A to point B, you want to minimise those nonlinearities because you do not want the shape of your signal to change as a function of the power that you are feeding to your transmission line. You want it to be as direct as possible.

'In this case, however, we are trying to make lumped devices that will be at specific points in our optical network, and we want to enhance the nonlinearities in order to manipulate the properties of our signals.'

Nonlinear effects enable new frequencies to be generated and, in particular, Petropoulos said quadratic nonlinearities make it possible to see optical processes such as second harmonic generation (SHG; also known as frequency doubling) or difference frequency generation.

'An obvious advantage is that you can generate new wavelengths. In our case, we want to cascade different nonlinearities within the nonlinear crystal in order to make our applications possible,' he said.

In SHG, photons interacting with a nonlinear material effectively 'combine' to form new photons with twice the energy. These have twice the frequency but half the wavelength of the initial photons.

'We will be trying to combine different types of nonlinearities for our processing applications, so we will be using this SHG together with other types of nonlinearities in the same crystal,' Petropoulos added.

By exploiting quadratic nonlinearities in PPLN, the new device would be able to process several wavelengths simultaneously with minimal cross-talk between them, and while maintaining the phase information of the signals.

This means that a variety of data formats can be accommodated, such as phase-modulated signals.

The Southampton team will design the crystals and the devices will be fabricated by PPLN specialists Taiwan-based HC Photonics.