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Added noise reduces chaos

10 Aug 2007

European researchers have discovered that introducing small amounts of noise into a laser system can make chaotic behaviour more controllable.

It may seem unlikely that adding noise to a system can make it more predictable, but a European research team has achieved just that by introducing small amounts of noise into the electrical input of a vertical cavity surface-emitting lasers (VCSELs). These lasers – which have been used for more than a decade in optical communication systems – produce highly controllable light emission, but the polarization of the laser light shifts erratically between two states.

In their experiments, the researchers observed a correlation between the amount of electrical noise introduced into the laser system and the resultant polarization shifts. And by precisely controlling the quantity of noise, they were able to predict the polarization state in real time. Such a development could allow designers to exploit the polarization mode to encode data transmissions.

“We found that noise finally makes the system more predictable, in the sense that the VCSEL polarization fluctuates regularly in time instead of fluctuating randomly,” said Marc Sciamanna, head of photonic and communications systems at SUPELEC a laboratory in Metz, France, that is run by the CNRS, the French national research agency. The project also involved scientists at the Free University of Brussels and the University of Navarra in Spain.

In today’s communications networks, data is transmitted by rapidly changing the frequency and phase of the light. Using the polarization state as well could provide an additional “channel” that could significantly boost data rates.

The experimental setup comprises of a VCSEL that is subject to optical feedback from two cavities. An extremely short external cavity is used to tune the polarization-switching current, while a longer cavity is used to introduce a time delay into the system. Noise is added along with the input DC injection current to the VCSEL.

Sciamanna’s team investigated the effect of adding different amounts of noise to the system. They found that the polarization flips that had previously been chaotic began to follow a specific pattern. This effect became more pronounced as the noise level was increased to a certain point, but then any additional noise brought the system back to its chaotic state.

The idea that noise can be used to dynamically synchronize a system has already been observed in several biological processes, and within the optical sphere has become known as coherence resonance. “This is the first experimental proof, to the best of our knowledge, of the coherence resonance phenomenon in a system that combines bistability, noise and delay,” Sciamanna told optics.org.

Sciamanna says that the technique could be used to control laser dynamics and VCSEL polarization. But the research is still in its early stages, and issues such as thermal stability, durability, and synchronization need to be addressed before the method can be used in an actual device. “In our system, it is very difficult to precisely align the optical feedback signal. The ideal solution would be to have the feedback channel integrated into the laser device   in an all-integrated photonic chip.” adds Sciamanna.

The team published their results in the journal Physical Review Letters.

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