Metrology

The optical path length (OPD) provided by the DDL, hence the position of the cat's eye is accurately measured by a dedicated laser interferometry metrology system, based on Agilent products. The same laser head will be used for the metrology of four DDLs. A folding 45° mirror directs the beam into the vacuum vessels. The beams are then splitted in several arms by means of the “Beam Directing Optics” composed of beamsplitters and beam benders.

Each beam feeds polarizing interferometers, in a Mach-Zehnder configuration. The mechanical structures which holds the two polarizing beamsplitters PBS1 and PBS2, referred hereafter as the “Metrology Towers”, has been custom designed for this application. The interference signals are detected on top of the metrology tower by means of Agilent receivers. The receiver head is linked to the main receiver electronics by an multimode optical fiber. The fibers pass through the vessels via a “Fiber Optic Vacuum Feedthrough”.