Abstract: A device (10), for allowing axial displacement of an optical element (12) within a laser cavity (14), so as to control the optical path length in the cavity, includes a generally cylindrical container (16) arranged to be connected to an end of the laser cavity where the optical element is located, and a piezoelectric stack (38) mounted in the container so that the length of the stack in the axial direction can be varied by an amount corresponding to the desired optical element movement in response to an electric field applied to the stack. One end of the cylindrical container comprises a belleville shaped diaphragm (26) to which is mounted the optical element within the laser cavity. A very stiff threaded cap (20) forming the end of the cylindrical container preloads the stack and diaphragm by a prescribed torque to allow the device to perform satisfactorily when subjected to extreme environmental conditions (e.g., above 5 KHz).