The optimal acousto-optic interaction length in Raman-Nath diffraction is investigated by using deliberately designed apparatus working with pure water. Then the optimum interaction length, the suitable dimensions of the transducer, and the required index difference dominated by the ultrasonic frequency and power for achieving the ultimate efficiency are analyzed. The portable device analogy to the solid appliance is designed based on the optimization. Taking advantage of the device the highest diffraction efficiency of more than 98% is obtained. The feasibility of using the portable device to act as a Q-switch for ultraviolet and visible lasers is discussed.