Dec 22 2006
Scientists at the Paul Drude Institute for Solid State Electronics (PDI) in Berlin are presenting a new semiconductor device that modulates light and is driven by surface acoustic waves. The modulator is based on a so-called Mach-Zehnder interferometer (MZI). It is 300 times smaller in size than comparable devices and it uses a new way to modulate optical signals. Such acousto-optic devices are important for signal transfer by light. The smaller and the more efficient they are, the more data can be transported. The scientists headed by Dr. Paulo Santos report on their concept in Applied Physics Letters (No. 89, 121104).
The modulator is extremely tiny. Its active region measures only approximately 15 micrometers and is thus 4 to 6 times shorter than the diameter of a human hair. It is not only the small size that makes the device so special, but also the material and the way it works. Up to now, Mach-Zehnder interferometers are made mainly of dielectric material, for example lithium niobate, and driven by electrical currents. The newly developed MZI from the PDI is made of the semiconductor compound gallium arsenide (GaAs). It uses acoustic surface waves to modulate incoming light signals. Both innovations lead to the increase in efficiency and reduction in size, so that more devices can be manufactured on a chip.
Dr. Paulo Santos, leading scientist at the PDI, explains: „MZI modulate light signals through light interference.” Such a modulator divides an incoming light signal by channelling it through different branches. After a short distance, the two light-rays are re-united. On existing GaAs prototypes, which will become commercially available in a few years, these branches are a few millimeters in length. An electrical current changes the refractive index and, therefore, the speed of light in one of the branches. This leads to different phases of the light waves at the end of the two branches - the rejoined rays interfere destructively, thus extinguishing the transmitted light intensity. „The problem is that the efficiency is small“, says Santos, „because dielectric materials react weakly to an electrical stimulus“.
The scientists at the PDI got around the difficulty by building a miniature acoustic source, which converts electrical signals into acoustic surface waves. These acoustic waves propagate on the device and induce changes in the refractive index of the branches of opposite signs. Thus, the scientists made it possible to miniaturize the MZI and at the same time increase its efficiency. Most importantly, GaAs is a very efficient light-source, diode lasers made of GaAs are commercially available. Thus, a modulator based on the PDI concept can be manufactured monolithically with an integrated light-source. Several modulators can fit on a chip the size of a thumbnail. The monolithic building of small-size low-energy devices lowers manufacturing costs and increases data transmission rates in networks and also in computers.
However, there are several technical challenges to be mastered, for example the development of more efficient processes to generate surface waves. The transfer of the concept to other materials is also desirable. To do so, co-operations with other research groups, for example in the Netherlands and in Denmark, are currently underway.