The crucial parts of a fiber-optic sensor (FOS) are 3dB couplers. It is a novel idea that a holographic lens is made as the coupler for a fiber-optic gyro. Theoretically we explored the possibility of this way and achieved it experimentally. According to Kogelnik s Coupled Wave Theory, under the Bragg condition, the diffraction efficiency of transmission holographic gratings, whose recording material is fit for certain conditions, theoretically, is up to 100%.