Interferometers are used for precision measurements. The principle is based on the comparison of a light beam from the sample with a reference steel, with accuracies down to the sub-nanometer range. Solutions based on the Michelson interferometer concept require splitting and precise adjustment of the two partial beams. A significant simplification of the Michelson interferometer is the so-called standing wave interferometer (SWIF). Excitation, reflection and detection all take place along one optical axis. Reference beam paths and the associated precise alignment are not required. However, this concept only works if the detectors themselves are designed as semi-transparent components and aligned with a distance to each other defined to within a few nanometers.
These detectors are being developed by the CiS Research Institute.
To realize a defined semi-transparent detector for light wavelengths in the visible or near-infrared range required new technologies and highest accuracy to fabricate active layers of doped silicon or polysilicon only a few 10 nm thin.
The tandem configuration ,consisting of two such sensors, allows to realize standing-wave interferometers with direct resolution of the direction of motion. First demonstrators are available.
The following performance parameters for the transparent detectors are aimed for:
• Very good diode behavior (reverse current • Sufficient photosensitivity of about 2 µA / mW @633 nm
• Cut-off frequency: > 10 MHz
• Good transparency: >80 % per single diode)
• Low reflection: • Low scattering of device parameters
• Low unevenness of the layers
• High accuracy / reproducibility of layer thicknesses in the process
The research and development work described was carried out in the "Tandem Diode" research project, funded by the German Federal Ministry of Economics and Climate Protection (BMWK). (FKZ: 49VF200011)
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