Fig. 1
From: Demonstration of a length control system for ALPS II with a high finesse 9.2 m cavity
Experimental setup. Length control of the RC: The cavity consists of the mirrors RCI and RCO as well as a PDH feedback control loop. A high bandwidth length actuator is attached to RCO. The laser beam from laser L1 is frequency doubled in a temperature controlled SHG. An EOM imprints phase modulation sidebands on the laser beam and the photodetector PDr_g is used to sense the PDH error signal. Laser frequency feedback: For the high finesse cavity operation with 1064 nm light the frequency of laser L2 follows the RC. The feedback control loop uses PDr_ir as a sensor and photodetector PDt in transmission of the cavity is used to measure the storage time. Beatnote frequency measurement: The beatnote signal between L1 and L2 is measured with a high bandwidth photodetector (PDbn)