RAINBOW

Photonics is an SME-driven, export-oriented as well as knowledge-intensive part of the German economy. Increasingly complex optical functions are needed here for applications in sensor technology, quantum technologies and communication. Photonics Integrated Circuits (PICs) are an essential tool for this. Up to now, PICs have been developed mainly for optical data transmission, driven by the high volumes that allowed high development costs. The HHI has worldwide expertise in this area, especially for the material platform indium phosphide (InP). InP is ideal for optical communication, but cannot be used for applications such as sensor technology with wavelengths <1250 nm. New PIC technologies must be developed for this spectral range.

The goal of RAINBOW is to open up new wavelength ranges for PICs to enable a variety of new applications outside of data transmission. Of particular interest are wavelengths up to the short-wave edge of the visible spectral range at λ = 450 nm. In this spectral range, there are essential sensor technologies such as LIDAR (980 nm), OCT (1060 - 1300 nm) or ultra-sensitive magnetic field sensor technologies in medical applications based on NV centres in diamond nanocrystals (637 nm). Medical diagnostic applications also rely on optically detectable biomarkers, which typically fluoresce at wavelengths <670 nm.

The main challenge for extending the wavelength range of PICs is the lack of availability of a uniform material and component base for the key integrated-optical functions. In the RAINBOW project, this gap will be closed by a hybrid combination of different material platforms, which will allow the full potential of highly integrated optics to be exploited in the spectral range from 450 nm to 1650 nm. To this end, three key technologies are being advanced: the establishment of an integrated-optics platform based on new lithium niobate (LiNbO₃ abbreviated: LN-on-Insulator (LNoI) materials, the expansion of the spectral range of Si₃N₄-integrated optical waveguides, as well as their hybrid integration with other active and passive components to form compact PICs. The aforementioned materials enable the development of this very large spectral range, and the hybrid integration ensures direct connection to established systems based on silicon (Si), InP or GaAs.
The result is a novel and highly flexible technology platform whose application potential, which has been substantially expanded compared to established approaches, is underlined in the project by three demonstrators. These realise sophisticated demonstrators such as a chip-integrated frequency conversion for sensor technology, PICs for a broadband 50 GHz modulator for data transmission, and a chip-based, cross-octave frequency comb for metrology.