A three—port fiber—optic switch using a 2×2 and a 3×3 directional coupler to form Mach—Zehnder interferometer is presented. In the consecutive fabrication of two fused couplers,not only the equal splitting ratio for both couplers, but the equal length of two single—mode fiber arms have been fabricated, which improved the switch performance a lot. The swich is PZT electrically controlled and has broad band. The mininum cross—talk reaches to 30 dB and the insertion loss is 0.23 dB.