General Information

In the past, we investigated both free-space laser links and fiber-based systems.

Much of our research towards intersatellite and ground-to-satellite laser links was done in cooperation with the European Space Agency (ESA). It included

optical homodyne and heterodyne reception,

Doppler lidar systems,

investigation of the impact of atmosphere on laser beam propagation,

optical communication links to high-altitude platforms (HAPs), and even

quantum key distribution using entangled photons.

In classical fiber communications we concentrated on wavelength division multiplexing systems (WDM). Our research activities aimed at the increase of attainable system distance and capacity. Together with Bell Laboratories (USA) we demonstrated record coarse WDM (CWDM)) transmission capacities based on potentially low-cost vertical-cavity surface-emitting lasers (VCSELs) in combination with forward error correction (FEC), but without the need of dispersion compensation.

More recently we apply our knowledge of optical waveguiding to optical data transmission on printed circuit boards. This research project also stimulated the investigation of the properties of multi-mode VCSELs when feeding multicore waveguides

Optical data transmission on printed circuit boards

In cooperation with AT&S (Austria Technologie & Systemtechnik AG) we develop opto-electronic circuit boards which incorporate short-distance optical interconnects. Optical lines have the advantage that they do not suffer from radio frequency interference. They provide extremely high bandwidths and also exhibit negligible crosstalk even when packed very densely. We investigate the properties of suitable waveguides and optoelectronic components. Light is propagated in a polymer layer into which a core of slightly higher refractive index has been inscribed by two-photon absorption. Multi-mode VCSELs (vertical cavity surface emitting lasers) operating at the wavelength of 850 nm enable, in combination with GaAs photodiodes, to transmit data rates of up to 8 Gbit/s with a bit error ratio of BER = 10⁻⁹. Electronic chips placed immediately before the VCSEL and after the photodiode allow for low-power driving and transimpedance amplification of the photocurrent, respectively. Another task consists of designing and manufacturing electronic equipment as needed for the transmission of high-definition video signals at 1.5 Gbit/s over the opto-electronic boards.

: Photograph of experimental opto-electronic board equipped with SMA connector interfaces, VCSEL, optical waveguide, and photodiode

Coupling of multi-mode VCSELs into multi-core waveguides

To increase the effective diameter of a waveguide manufactured by two-photon absorption, several parallel cores may be inscribed, thus forming a multi-core waveguide. To better understand the coupling between a multi-mode VCSEL and the multi-core waveguide we

a) developed a software simulating the light propagation in arbitrary waveguide structures using the finite-difference method, and

b) performed extensive experiments concerning the influence of even slight back reflections on the emission of the VCSEL.

: Colour coded intensity distribution in the direction of light propagation (z) when coupling an LP₀₂ mode at z = 0 into a multi-core waveguide consisting of seven core areas with Gaussian-shaped refractive index distribution. (The dashed white lines indicate core-cladding transitions).