We present results on the first all-semiconductor, vertical Fabry-Perot-cavity optical transmission modulator. This device combined mechanically stable strained and unstrained (In,A1,Ga)As multilayers to achieve operation at 1.06 mum. Transmission-mode operation allows the resonant wavelength of the cavity to be finely tuned by varying the angle of incidence, providing, for the first time, a means of compensating for small inaccuracies in growth parameters. Using the modulator in double-pass operation with a corner-cube retroreflector, we demonstrate a tunable reflectance modulator with a fractional modulation of 25% at 3-V bias, suitable for applications in free-space communication.