Evanescent Field Devices Using Single-Mode Optical Fibers

Single-mode optical fibers are finding increased uses in areas such as long haul communications, local area networks, environmental sensing and optical signal processing. As these systems become more complex there is a growing need for efficient low-loss optical fiber components. This thesis explores using the evanescent field of an optical fiber to construct several different devices. The evanescent field is the portion of the guided mode found outside the core region of an optical fiber. Access to this field is obtained using a grinding and polishing technique to remove a section of the fiber cladding. Since the signal is not removed from the fiber, insertion losses for these devices can be very small. By placing a laser dye solution onto the removed cladding region, a single-mode fiber optical amplifier is demonstrated. By replacing the laser dye solution with a saturable absorbing dye, an intensity dependent nonlinear transmission is achieved. These two devices are important for the stable storage of optical pulses in recirculating fiber loops. By incorporating the evanescent amplifier into a fiber ring resonator, an all-fiber ring dye laser is also demonstrated. A wavelength sensitive reflector is constructed by placing a metal diffraction grating over the exposed evanescent field. Due to the metal surface of the grating both reflected and transmitted signal are highly polarization sensitive. By using two dissimilar fibers in a direction coupler, an efficient modal filter is demonstrated for use with double-mode fibers. This device has application in the development of frequency shifters and narrow-band wavelength filters. A measurement technique is also demonstrated which allows accurate (fractional error better than 10(' -4)) measurement of modal propagation constants. This measurement technique is used to accurately determine core diameter and index values for a step-index fiber. An analysis is provided using a slab-waveguide model. Due to the difference in geometry between the model and an actual fiber, the results are intended to provide only a qualitative understanding of how evanescent field interactions effect the properties of the guided mode.