The sensitivity of a liquid level sensor based on a single-mode-multimode-single-mode (SMS) fiber structure has been increased by hydrofluoric acid etching. The etching process was analyzed and monitored both theoretical and experimentally, which permitted to observe that a sinusoidal spectrum can be obtained for low diameters. As an example, a 2.77 fold sensitivity increase was attained by etching from diameter 125 to 50 µm. Moreover, the sinusoidal shape of the optical spectrum permitted to monitor liquid level changes both in wavelength and in phase. The cross sensitivity of the sensor to refractive index and temperature was also studied.