Optical Freouency Shifting in Two-Mode Optical Fibers by Flexural Acoustic Waves

Abstract

A novel all-fiber optical frequency shifter, using coupling between the two optical spatial modes in a two-mode glass fiber, is reported. The mode coupling is performed by acoustic flexural waves traveling along the fiber together with the optical waves. The acoustic wavelength for coupling to occur is equal to the beat length between the two optical spatial modes. Flexural waves of high amplitude are excited on the fiber by bonding the thin end of a tapered glass rod to the fiber while a piezoelectric transducer is bonded to the other end of the rod, which has a much larger cross section. A discussion of the acoustic behavior of the system is given. The calculated dispersion of the flexural waves guided by the fiber is compared with experimental measurements. Calculations prescribe the optimum excitation distribution at the thick end of the tapered rod. Experiments have shown 100 percent mode conversion at 8 MHz for 100 mW of electric input power. Measured suppression of both carrier frequency and image sideband was 35 dB.