Nonlinear frequency conversion in fiber lasers

The concept of nonlinear frequency conversion entails generating light at new frequencies other than those of the source light. The emission wavelength of typical fiber laser systems, relying on rare-earth dopants, are constrained within specific bands of the infrared region. By exploiting nonlinear processes, light from these specific wavelength bands can be used to generate light at new frequencies otherwise not obtainable by rare-earth elements. This thesis describes work covering Raman fiber lasers (RFLs) and amplifiers for nonlinear frequency down-conversion, and also the method of fiberoptic Cherenkov radiation (FCR) using ultrafast pulses as a means for generating tunable visible (VIS) light at higher frequencies. Two different polarization maintaining (PM) RFL cavities are studied with an emphasis on stability and spectral broadening. The cavities are formed by inscription of fiber Bragg gratings (FBGs) in a PM Raman fiber. Active temperature control feedback of the cavity resonators is investigated as a means for obtaining a high degree of power and spectral stability. The impact of accurate cavity resonator alignment upon the RFL stability and emission characteristics is investigated and a highly stable PM RFL emitting at 1679 nm with a narrow spectral emission bandwidth is demonstrated. A driftless output was obtained for an output power of 680 mW at a 29 pm line width while having high output power and spectral stability; with a sub-pm standard deviation in the emission wavelength and line width. Subsequently, the RFL is used for the demonstration of a Raman amplifier, for which both the gain and noise characteristics in the vicinity of 1800 nm wavelength are examined. The VIS FCR source can be considered for a broad range of applications in the field of biophotonics. FCR emission is characterized by a high temporal and spatial coherence, short temporal pulse duration, a tunable emission wave length in the tens of nanometer range, along with a potential for having very low noise properties. The pursuit of a compact, portable, and robust VIS FCR source, suitable for applications outside of the optical lab, defined the work on an all fiber based system. Experimentally, the generation of VIS light using the FCR process is demonstrated in both uniform and tapered nonlinear fibers. VIS emission from the blue to the red parts of the VIS spectrum is demonstrated; extending across a 430 to 610 nm wavelength range for output powers up to2 mW. Utilization of tapered nonlinear fibers resulted in a substantial increase in the obtainable wavelength tunability, with an increase from 20 to 118 nm, when compared to the results obtained for uniform nonlinear fibers.