Title

Continuously tunable, single-frequency, fiber-laser source/analyzer for communications and sensors

Date of Completion

January 1993

Keywords

Engineering, Electronics and Electrical|Physics, Optics

Degree

Ph.D.

Abstract

This dissertation presents the design constraints required to achieve single longitudinal mode operation from a standing-wave, homogeneously broadened, three-level fiber laser which utilizes intra-core Bragg reflectors for cavity feedback. In particular, a closed-form solution to the laser rate equations has been found which bounds the region in which only single longitudinal mode operation is supported. The principal resonator geometry to be considered will be a two reflector cavity which utilizes only Bragg reflectors for feedback. Analytical and experimental results will be discussed.^ The single-frequency fiber laser analysis was applied to the design of three erbium doped fiber lasers. The three fiber lasers were designed to operate in the 1.55 $\mu$m optical communications band and were based on three different erbium doped fibers having different gain properties. Of the three fiber laser designs, two of the lasers fabricated in the higher gain-per-unit-length fiber were characterized to run robustly in a single frequency. The third fiber laser, fabricated in the lowest gain-per-unit-length fiber, operated in a single frequency over 70% of its free spectral range. The lasing linewidths of the two continuous wave fiber lasers were less than 20 kHz. In addition, thermal wavelength tuning over a 2 nm range and strain tuning over a 4.8 nm range for the short fiber laser was demonstrated.^ In an effort to make practical use of the single-frequency fiber laser, a calibrated, narrow-linewidth, continuously wavelength-tunable erbium fiber laser was used to interrogate a multipoint Bragg grating temperature sensor. The fiber laser was wavelength tuned, through an array of three fiber Bragg grating sensors, to determine the temperature of each individual grating. The temperatures of the three gratings were measured as a function of grating Bragg wavelength. The minimum wavelength resolution, due to electro-mechanical repeatability, of the fiber laser source/analyzer was determined to be $\approx$2.3 pm. This corresponds to a grating temperature resolution of 0.2$\sp\circ$C. ^