Studies of optomechanics can revolutionize telecommunications

Researchers at the University of Campinas are conducting experiments using optical microcavities to try to control nonlinear effects

By Karina Toledo, in Munich

Agência FAPESP – In the field of fiber optic communications, one of the factors that limits the amount of information that can potentially be transmitted over a particular channel is a nonlinear effect known as Brillouin scattering.

The phenomenon occurs as the luminous power increases and the electric field of the incident light creates a type of acoustic wave that spreads in the material and scatters the light. This effect is particularly harmful in long-distance (approximately 20 kilometers) communication links where a few milliwatts (mW) of power are enough to cause the photons to begin returning to the source of emission instead of proceeding to the receiver.

Through FAPESP funded experiments (Optomechanics in photonic and phononic crystals and Nanophotonics in Group IV and III-V semiconductors) carried out at the Gleb Wataghin Physics Institute (IFGW) of the University of Campinas (Unicamp), Brazil, a group made up of researchers Thiago Alegre and Gustavo Wiederhecker is trying to understand just how this and other optomechanics occur as a result of light’s interaction with mechanical movements in optical microcavities, with the goal of controlling them in the future.

Alegre discussed the topic Thursday (10/16/14) during FAPESP Week Munich.

“Among the practical implications of the optomechanical systems are the creation of tiny devices such as light modulators, keys, optical memories and Brillouin lasers that can be used together in telecommunications systems. It would be a revolution much like what occurred in electronics a few decades ago. By reducing the size of transistors, which are the basic devices, it was possible to increase and add functionalities to a single chip,” said Alegre in an interview with Agência FAPESP.

The group from Unicamp, which also includes researcher Newton Frateschi, has focused on the study of optomechanics, such as Brillouin scattering, on microcavities made of silicon discs measuring approximately 10 microns (μ) in diameter – equivalent to one tenth the thickness of a strand of hair. The discs are supported by a pedestal measuring only 1μ in diameter.

“We designed and fabricated optical cavities in which it is possible to observe Brillouin scattering for frequencies between 10 and 20 gigahertz, values close to the frequency modulation in telecommunications systems. The optical cavities resemble a Frisbee. We connected the light in this system using an optical fiber measuring approximately 2µm, and thanks to the reflection of light on the rim, it turns hundreds of times on the disc for a few nanoseconds before dissipating. This creates what are known as whispering gallery modes. Thanks to this effect, the light remains much longer in the cavity, interacts more times with the material and enhances the optomechanics,” explained the researcher.

According to Alegre, Brillouin scattering is a particularly interesting example of optomechanics because it allows generating laser copies and changing their color. “You shine the light at a certain frequency and a similar color in the light spectrum is scattered in an adjacent frequency. It is possible to change the color of the laser in a passive and transparent cavity through merely the interaction of mechanical movement and light,” Alegre explained.

The researcher went on to explain that by placing one cavity next to another, it is possible to strengthen or diminish the scattering effect. This occurs because the incident light and the scattered light are both in whispering gallery mode, which further increases the amount of time the light remains in the system and thus, the optomechanical interaction.

The final goal, added the researcher, is to be able to select a priori the frequency at which the scattering will occur. “In fiber optics, this frequency is practically fixed and depends only on the material (glass, in the case of fiber optics). The idea is that, based on the design of the cavity, you would already know or be able to determine what the resulting frequency would be,” Alegre explained.

Applications

Fiber optic communication currently uses wavelength-division multiplexing (WDM) technology. The protocol allows for a network to use optical signals at different frequencies, directly related to the wavelength, on the same physical channel.

“When different conversations, or exchanges of information, occur on these systems simultaneously, each is transmitted at a different frequency (color). But there are times when the telecommunications centers need to change the frequency of one of the conversations because, on a given portion of the physical channel, that frequency is already occupied,” Alegre said.

To change the frequency now, the optical pulse has to be converted to electrical form and generate an optical pulse in another frequency. Optomechanics, such as Brillouin scattering, if well-controlled, would allow this system to be completely optical. That way, it would be possible to reduce the size as well as the expenditures on routers and devices that handle the change in the wavelength.

According to the researcher, control of Brillouin scattering will enable important advances from the standpoint of fundamental physics.

It could be used to cool mechanical modes (which have nothing to do with temperature, but rather reduce the vibration of certain mechanical movements of an object) to their lowest possible energy state – the so-called zero point –, at which they behave according to the laws of quantum mechanics.

“This would allow us to study what happens to the mechanical movement when you cool to close to zero degrees on the Kelvin scale. In this situation, the cavities practically stop vibrating and behave like a quantum system. This causes new effects that go beyond what we see in classical physics,” the researcher said.

Also on the panel dedicated to the discussion of nanotechnology and photonics was researcher Sven Höfling, from the School of Physics and Astronomy of the University of St. Andrews in the UK, who presented data from a study to develop a new type of laser formed by excitons (material state in which a positive particle and a negative particle are attracted to each other).