The use of upconversion to attenuate/modulate optical signals is proposed, and experimental results are presented. In this light controlling light approach, a 711.2 nm, 700 mW beam propagating in a Tm(3+)-doped fluorozirconate optical fiber was attenuated down to 150 mW by copropagating a 1117 nm beam. Further characterizations concerning time response of the transmittance revealed the limits and potential of the scheme. The maximum attenuation of a 45 cm fiber, 75%, was reached up to 400 Hz, and at least 37% modulation was measured at 770 Hz. Potential applications include Q switching of lasers and in-fiber light-controllable optical attenuation.