We experimentally investigate the Lamb coupling constant C in InGaAlAs Quantum Wells active medium. An Optically-Pumped Vertical-External-Cavity Surface-Emitting Laser emitting at 1.54 mu m is designed to sustain the oscillation of two orthogonally polarized modes sharing the same active region while separated by 1 mm in the rest of the cavity. It enables to tune independently the two wavelengths and to apply differential losses in order to directly measure the coupling constant. We found C to he almost constant and equal to 0.839 +/- 0.023 for a wavelength difference between the two eigenmodes ranging from 0.36 nm up to 10.8 nm.