Adjustable access couplers for multimode fiber-optic networks have been constructed. The 4-port fiber-coupled devices make use of voltage-controlled total internal reflection by a thin film of nematic liquid crystal. The devices can serve as tee-couplers in unidirec-tional single-strand data buses or in duplexed two-strand buses. This active-coupler approach gives low loss between any pair of addressed terminals on the bus. In the devices, multimode fibers are coupled to rod lenses, and those lenses are, in turn, coupled to flint glass prisms that confine a 6-μm liquid-crystal layer. The achromatic switches have a 1.6 dB optical insertion loss, a tap-ratio that is controllable from -48 dB to -4.6 dB, a directionality of 44 dB, and an operating voltage of 5 to 20 V rms at 1 kHz. The structure is extendable to matrix switching arrays and to higher-order switching functions.