Optical fibers with interferometric path length stability by controlled   heating for transmission of optical signals and as components in frequency   standards
  We present a simple method to stabilize the optical path length of an optical fiber to an accuracy of about 1/100 of the laser wavelength. We study the dynamic response of the path length to modulation of an electrically conductive heater layer of the fiber. The path length is measured against the laser wavelength by use of the Pound-Drever-Hall method; negative feedback is applied via the heater. We apply the method in the context of a cryogenic resonator frequency standard. 
Holger Mueller, Achim Peters, and Claus Braxmaier