Novel optical fiber components based on thin-film coatings and intra-core fiber gratings

Abstract

We have designed and developed a new class of optical fiber components based on thin-film coatings and fiber gratings. In our design, the function of the thin-film fiber coating is to provide an electrical means to regulate the optical response of the fiber section underlying the coating. Using this principle, we were able to implement both wavelength tunable filters and optical modulators. We implemented wavelength tunable filters by running an electrical current through a metallic coating. The effect of the current is to directly heat the fiber (which hosts a fiber grating), and the resulting temperature change tunes the filter resonance wavelength(s). The main characteristic of this device are: a) the tuning efficiency is inversely proportional to the length of the heated coating; b) thermal tuning does not induce any spectral profile changes; c) the temperature induced wavelength modulation depends on the packaging and is limited to low frequencies. To produce fiber optic phase modulators, we exploited the change in the refractive index determined by a thin-film piezoelectric coating sandwiched between metal electrodes. The induced strains are proportional to the electric field applied to the piezoelectric layer (zinc oxide or lead titanate zirconate sputtered thin-films). We found that the response is flat at low frequencies and dominated by strong radial mode resonances above a few MHz up to GHz frequencies. We also investigated piezoelectric coated fiber Bragg gratings for intensity and Bragg wavelength modulation. Our results indicate that, in these devices, the refractive index of the fiber grating is changed by the straining action of the piezoelectric film. Finally, we operated an all-fiber Mach-Zehnder interferometer, based on two long period fiber gratings, as an intensity modulator. In this case, a zinc oxide piezoelectric actuator modulates the phase difference between the arms of the interferometer and thus the transmitted intensity. Piezoelectric actuators allowed for wavelength and intensity modulation up to GHz frequencies, with a response similar to that of the phase modulators. In conclusion, the all-fiber modulators and wavelength tunable filters that we have designed and implemented are characterized by low insertion loss, high electrical power efficiency, small size, and high optical power damage threshold. We believe that such novel devices represent a useful addition to the tool kit of the fiber-optic system designer.