High-Energy Ultrafast Ytterbium Fiber Lasers

Abstract

Within the past two decades, applications for sources of ultrafast pulses of optical radiation have expanded beyond the confines of research laboratories. Applications are too numerous to list in entirety but include uses in the fields of medicine, micromachining, high-bit-rate telecommunications, optical range-finding, and detection of trace chemicals to name a few. Currently, the market is dominated by femtosecond solid-state lasers due to their superior pulse energy and quality. However, many sources are being replaced with fiber-based lasers due to their inherent stability, compact design, and reduced cost.

This thesis summarizes research on femtosecond ytterbium-doped (Yb) fiber lasers operating in the near-infrared spectrum (~ 1-micron wavelength). There is strong interest for stable, femtosecond sources at this wavelength for biomedical imaging and noninvasive surgery. The overriding theme of this research is an attempt to better understand the limitations of short-pulse fiber lasers, with the goal of providing higher-energy, shorter-duration, and higher-quality optical pulses from fiber.

In the context of high energy, two approaches are reviewed. In the first approach, fiber nonlinearities are exploited through relatively new forms of pulse evolution which provide stable, high-power operation. In the second approach, excessive nonlinearity is avoided as much as possible, and high-energy pulse formation is stabilized with an intra-cavity frequency filter. Experiments have successfully generated the highest pulse energy (3-times improvement), highest peak power (50% improvement), and highest average power from any fiber oscillator.

Also discussed is a new regime of modelocked operation in which highly-chirped pulses are relatively unchanged during propagation in the laser. While these pulses suffer some in quality, they can attain relatively high energy and can be de-chirped external to the laser, achieving durations of a few hundred femtoseconds.

A fiber laser is explored which is capable of generating ten-cycle pulses. By compensating for higher-order dispersion inside the oscillator, record pulse durations are generated with excellent pulse quality.

Finally, an experiment is discussed in which a fiber laser is modelocked with a semiconductor saturable absorber mirror (SESAM). Femtosecond durations are achieved. This laser represents a first step toward an environmentally stable source of high-energy, short pulses.