A laser speckle method for measuring ultra-low thermal expansion

A method based on objective laser speckles for the measurement of ultra-low thermal expansion is described. The sensitivity of α = 10^-8 C^-1 can easily be achieved with a 25 C temperature change of a 2°5 cm long sample. The sensitivity is obtained by the rotation of a probe due to differential thermal expansion of the sample and reference rods. The method is discussed in detail along with the effects of some user-controlled parameters like speckle size, laser beam width, unwanted thermal expansions and the spectral width of the laser beam.