The deformations due to thermal expansion of a binary diffractive grating in fused silica under near infrared high power laser exposure are estimated by a Finite Element Method. They are compared to the geometrical tolerances obtained by a rigorous electromagnetic method which is used to calculate the optical performances. It is shown that the grating deformations remain below the geometrical tolerances for an average laser power of up to I kW and an exposition of up to 100 s. (c) 2007 Elsevier B.V. All rights reserved.