An experiment was performed to show that the coherence of a laser beam can be maintained over a 5‐cm path in a GaAs parallelepiped. The temperature coefficient of the refractive index was measured from the temperature dependence of the intensity of radiation transmitted through the sample. The value of this coefficient was found to be (5.64±0.28)×10−5(°C)−1for semi‐insulating GaAs at room temperature and 10.6‐&mgr; wavelength. The absorption coefficient of the material used was measured calorimetrically and found to be 0.012±0.002 cm−1.