We measured details of the strain/stress fields produced in GaAs(100) and InP(100) substrates by the presence of narrow dielectric stripes processed from thin films obtained by plasma-enhanced chemical vapor deposition with a residual and controlled built-in compressive or tensile stress. Micro-photoluminescence techniques were used, measuring either the spectral shift of the luminescence peak or the degree of polarization (DOP) of the spectrally integrated signal. These techniques provide information on different parts of the strain tensor (isotropic and anisotropic). The anisotropic deformation was found to change with the magnitude and sign of the initial built-in stress, and also with the stripe width. Using an analytical model, we were able to determine accurately several physical parameters which describe the stress/strain situation. The localized stress at the edges, expressed within an edge force concept, is shown to follow the expected initial built-in stress and also a stress reduction when the stripe width is decreased. This is interpreted as an evidence of some strain relaxation occurring near the stripe edges. This relaxation also impacts the shape of the DOP curves near the edges. The other important conclusion is the observation that the strain does not return to an isotropic situation (as in the case of an infinite thin film) in the central part of the stripes, even if the widths of these stripes are large (100 μm). The analytical model is developed and explained step-by-step. This analytical model produces quantitative data that describe the different effects observed. These data can be very helpful in the design and optimization of photonic devices when the photo-elastic effect can be significant, such as waveguides. The μPL measurements coupled with the model can also provide feedback to allow better control of the processing of such thin film devices.