Modern large area tracking detectors (ATLAS, CMS at CERN) require increasing precision for the monitoring of relative positions of detector components. A novel high-precision optical alignment monitoring system (ALMY system) has been developed for such applications.

Principle:

Collimated laser beams shine through the semi-transparent silicon strip sensors (ALMY sensors). The sensors provide an accurate 2D measurement of the laser beam position, indicating displacements from the nominal positions.
The structure: a 0.5 micron thick a-Si:H layer is placed between the two 100 nm thick ITO electrode layers. Electrodes are segmented into 2 orthogonal strip rows, covering the whole sensitive area of 20x20 mm². Everything is deposited on a 0.5 mm thick glass substrate with an additional anti-reflective coating on the bottom side.
Sensors' insensitivity to large magnetic fields, as well as their radiation hardness have been demonstrated in conditions corresponding to the ATLAS and CMS environment.

Sensor Properties (see the setup for the tests):

high transmittance of ~85-90% at 780 nm, allows up to 10 sensors along one laser beam
position resolution of 5 microns, uniform over the whole 20x20 mm² surface
laser beam refraction within 5 microradians
no need for calibration, due to a high-quality surface of the glass substrate