The thulium fiber laser has gained interest due to its long emission wavelength, large bandwidth (~1.8 – 2.1 µm), high efficiencies (~60 %), and high output power levels both in cw as well as pulsed regimes. Applications like remote sensing, machining, medical tissue ablation, and mid-infrared generation benefit from high peak power thulium laser sources. Pulsed thulium fiber laser systems are advancing rapidly towards higher peak power levels and are becoming the preferred sources for these applications. This dissertation work describes the development of novel nanosecond pulsed thulium fiber laser systems with record high peak power levels targeting mid-infrared generation. The peak power scaling in thulium fiber lasers requires new fiber designs with larger mode field area (MFA) than commercially available step index large mode area (SI-LMA) fibers. Two different prototypes of thulium doped photonic crystal fibers (PCF) were investigated for high peak power generation. The first prototype is a flexible-PCF with MFA twice as large as SILMA fiber and the second prototype is a PCF-rod with six times larger MFA. A robust single stage master oscillator power amplifier (MOPA) source based on flexible-PCF was developed. This source provided narrow linewidth, tunable wavelength, variable pulse duration, high peak power, and high energy nanosecond pulses. The PCF-rod was implemented as a second stage power amplifier. This system generated a record level of ~1 MW peak power output with 6.4 ns pulse-duration at 1 kHz repetition rate. This thulium doped PCF based MOPA system is a state of the art laser source providing high quality nanosecond pulses. iv The single stage MOPA system was successfully implemented to pump a zinc germanium phosphide (ZGP) crystal in an optical parametric oscillator (OPO) cavity to generate 3 - 5 µm wavelengths. The MOPA source was also used to demonstrate backside machining in silicon wafer. The PCF based laser system demonstrated an order of magnitude increase in the peak power achievable in nanosecond thulium doped fiber laser systems, and further scaling appears possible. The increase in peak power will enable additional capabilities for mid-infrared generation and associated applications.