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We report the observation of a strong variation of the active layer refractive index with the current in a 1.55 μm "gain-guiding" GaInAsP laser 12 μm wide. A negative refractive index change of several 10-2has been found between the center and the edge of the stripe from a transverse mode distribution analysis and a study of the wavelength shift of the longitudinal modes with the current. We have shown that this effect can explain the increase of threshold current of a gain-guiding laser for a small stripe width. We have observed, moreover, a time dependence of the transverse mode width in the transient regime in pulsed operation and a broadening of a time average spectrum of a stabilized single longitudinal mode laser under current modulation. We have also shown that these two last effects can be explained by the refraction index current dependence.