An optical tunable notch filter using an ultrasonically induced long period fiber grating

A long period optical fiber grating is obtained by ultrasonic bending vibration propagating along the fiber. The rejection optical wavelength is tunable by the ultrasonic frequency, since it is determined by the period of refractive index modulation due to the vibration. The rejection center wavelength and the rejection ratio are experimentally investigated as functions of the frequency, intensity and length of the ultrasonic vibration excitation. The rejection ratio is changed by the ultrasonic vibration amplitude, while the rejection bandwidth depends on the fiber length where the vibration is excited. The wide tunable range of 160 nm (from 1490 to 1650 nm) was available for ultrasonic frequency range of 300 kHz. The rejection ratio of 17 dB was achieved for the vibration velocity of 0.25 m/s. A method to synthesize the rejection band shape is also discussed.