Analysis of sub-harmonic response in all fiber-optic phase modulator

Abstract

All fiber phase modulator is the device which is composed of a cylindrical piezoelectric tube, a strand of optical fiber wrapped around the PZT tube. It is widely used in various fields such as the optical fiber sensor, fiber laser and polarization scrambler, etc. The major role in the phase modulation is from the length change of the fiber induced by the expansion of the PZT tube in response to the applied electrical signal. If the phase modulator is located one of the two paths in the Mach-Zehnder (MZ) interferometer, the interference signal according to the phase modulation can be measured at the output. The RF spectrum of output signal is composed of only harmonic components of a driven frequency, theoretically. However, the subharmonic response appeared at the specific condition that the modulator was driven with relative high voltage at the PZT’s fundamental resonance frequency. It can be thought that the subharmonic response results from the phase mismatch of the movement between the PZT tube and the wound fiber. In addition, it could be shown that the radial movement of the PZT tube and the wound fiber through the detection by using the Michelson Interferometer was different from each other. Resultantly, the PZT’s wall vibrated to the radial direction with harmonics of applied frequency. However, it was detected that the subharmonics as well as the harmonics on the fiber wrapped around PZT’s surface. It was thought that the free movement of coiled fiber on PZT’s surface leaded to the different movement between the PZT and the coiled fiber. In order to analyze the reason of subharmonic response, the condition for the subharmonic generation was obtained through the experiment for coiling tension and friction between the PZT and the coiled fiber. As a result, the PZT needs to vibrate with high amplitude and the fiber needs to be hold on the tensionless state. In addition, the low friction between the PZT and the coiled fiber needs to create the subharmon...