A wide-band high-sensitivity laser strainmeter

A new laser strainmeter, inspired by those used in the first gravitational waves detectors, is presented. A wide-band feedback circuit controls the optical length of the reference arm of an unequal-arm Michelson interferometer by means of a Pockels cell. No modulation is added to the Pockels cell supply, since laser power fluctuations are proved to be of negligible effect if some care is taken in balancing the two outputs of the interferometer. A reset circuit follows variations in arm length greater than λ, obscuring the servo for 50 μs. As a consequence, an analog signal combined with a digital signal allow to detect any change in the arm length greater than 10−3 λ. The frequency band extends from continuous signals to 100 kHz.