The paper presents an investigation of deformation in the active region of GaAs laser diodes under operating conditions. Since the properties of bulk material and manufactured diode are not identical, a study of the spectral splitting and polarisation changes caused by mechanical stress in the diode is made. These results are used to calibrate instruments for a study of the deformation due to thermal stress in the diode under operating conditions. This thermal stress can exceed the shear stress required for dislocation motion in the active region of the diode.