We report on two types of modal instabilities observed in high power Yb amplifiers based on Large Mode Area Fibers. The first is observed to occur at a Threshold Power, which we refer to as Threshold Power Modal Instabilities (TPMI). The modal instability is observed as a decrease in beam quality or reduced core light output as higher order modes leak into the fiber cladding. In PM 25/400 fiber amplifiers, we observe the threshold for the modal instability to vary depending on pump wavelength detuning, with the onset occurring at approximately 15 W/m peak heat load. In PM 20/400 and 25/400 fiber amplifiers without stress rods or other polarization control, we can achieve 1 kW output, limited by available pump power, without modal instabilities. The second type of modal instability is observed for certain cases where the fiber initially operates without any sign of MI but then degrades over an extended operating time, leading to a similar behavior as the TPMI. We refer to the second class as Fiber Degradation Modal Instabilities (FDMI). For these degraded fibers, we observe that fiber performance is unchanged below the critical power for modal instabilities. Experiments on degraded fiber show a wavelength dependent permanent change in the degraded fiber with a memory of the original operating wavelength.

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We report on an eye safe fiber laser generating >5 Watts of average power at 50 kHz packaged in a cylinder measuring 6" in diameter and 3.75" in length to show compatibility with advanced seeker concepts. To our knowledge, this represents the highest average power per unit volume from an eye safe pulsed fiber laser generating multi-Watts of average power.Keywords: high power, eye safe, fiber laser, advanced seeker, LADAR IntroductionCompact, efficient, and cost effective short pulse laser sources that emit radiation in the eye safe wavelength region are required for imaging laser radar (LADAR) systems under development in the DoD. Conventional eye safe laser sources using diode-pumped solid-state laser (DPSSL) and optical parametric oscillator (OPO) technologies are too large, inefficient, and unreliable to be deployed on future autonomously-guided ground and air platforms. It has been demonstrated that pulsed eye safe fiber laser sources provide critical performance advantages for LADAR systems because they can generate short pulses (<10 ns) at high repetition rates (>20 kHz) that enable increased range resolution and rapid data collection, all in a very compact and robust package compared to other laser technologies [1]. Fiberbased laser transmitters also leverage known benefits of fiber-based sources for field deployment, which include high efficiency, small footprint, flexible form factor for packaging and the elimination of large bulk optics and their associated alignment / stability challenges.In this paper, we report a novel pulsed eye safe fiber-based laser transmitter in a package suitable for LADAR seekers in advanced munitions. It is packaged in a cylinder measuring 6" in diameter and 3.75" in length (106 cubic inches) and generates over 5 Watts of average power at a repetition rate of 50 kHz. The following section describes the laser architecture and the mechanical packaging of the unit. The final section shows the units performance at repetition rates of 20 and 50 kHz. This work represents the highest average power per unit volume from a Watt-level eye safe pulsed fiber-based laser and demonstrates the flexible, modular packaging possible when using a fiber laser transmitter.