A Precision Angle Sensor using an Optical Lever inside a Sagnac   Interferometer
  We built an ultra low noise angle sensor by combining a folded optical lever and a Sagnac interferometer. The instrument has a measured noise floor of 1.3 prad / Hz^(1/2) at 2.4 kHz. We achieve this record angle sensitivity using a proof-of-concept apparatus with a conservative N=11 bounces in the optical lever. This technique could be extended to reach sub-picoradian / Hz^(1/2) sensitivities with an optimized design. 
J. M. Hogan, J. Hammer, S.-w. Chiow, S. Dickerson, D. M. S. Johnson,   T. Kovachy, A. Sugarbaker and M. A. Kasevich
physics.optics