John D. Minelly

3kW single-mode direct diode-pumped fiber laser

We present a 3kW single-mode fiber laser based on an Yb-doped LMA fiber operating at 1080nm. The laser which is pumped by 9xxnm diode bars stacks. It is believed to be the highest power direct diode pumped single-mode fiber laser oscillator to date.

Extremely low NA Yb doped preforms (<0.03) fabricated by MCVD

We report the fabrication of extremely low NA preforms (<0.03), highly doped with Yb using a conventional Modified Chemical Vapor Deposition (MCVD) system. Our lowest NA preform (0.025 NA) was drawn to a 52um core step-index double-clad fiber operating in a single mode regime (M2=1.04). The fiber had a mode field diameter (MFD) and an effective area (Aeff) greater than 35um and 1000um2 respectively. In a fiber laser configuration, the efficiency was greater than 85% without any sign of photodarkening. To the best of our knowledge, by using our extremely low NA preforms we have demonstrated the largest MFD and Aeff to date for a single-mode step index double–clad Yb doped fiber without involving any micro-structuration.

Al-glass kW fibre laser end-pumped by MCCP-cooled diode stacks

High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power aggregation and the fibre laser cavity being achieved in a monolithic “all-bibre” architecture comprising fused fiber bundles, fiber Bragg grating reflectors and numerous splices. [1]. The gain fiber utilizes a low index polymer coating to provide the wave-guiding for the multimode pump as well as for compatibility with the NA increase (typically 0.22–0.45) which occurs in the fused taper combiners. While this all fibre approach has been shown to be viable, it is not trivial to implement at power levels in excess of several hundred watts Issues include polymer coating degradation, transverse mode-coupling induced instability at splices or FBGs, grating walk-off, and modal instability, [2]. The latter issue arises because these fiber laser designs are focused on single-transverse-mode operation, [3.4], even though the fibres themselves are multimode to avoid nonlinear impairments. This is despite the face that most cutting and welding applications actually utilize a multimode fibre for delivery to the cutting head. The BBP of such systems is typically 2.5mm.mRad at a wavelength around 1080nm. However, single mode operation allows power scaling by incoherently combining several lower power fiber lasers into a single beam with that BPP.

915 nm laser bar-based high-performance sources for fiber laser pumping

Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.

High peak power and high energy fiber amplifiers

Yb fiber lasers have to date produced several mJ pulse energy, beam quality and several MW peak power but largely due to materials issues Er based fiber laser systems underperform in comparison. Relevant technologies are reviewed

Cladding-Pumped EDFAs: Yb Free or Co-doped

Yb-free high-power erbium amplifiers can overcome the compositional restrictions of Yb-Er co-doped fibers. Some advantages include, higher energy storage, improved beam-quality and absence of parasitic 1060 nm lasing. Progress in both approaches is described.

Technology trends for high power fiber lasers

In recent years high power fiber lasers have increasingly replaced other types of solid state lasers. This has been driven by a number of inherent advantages including compactness, high efficiency, better heat dissipation and outstanding beam quality. Fiber lasers were initially thought of as little more than a curiosity but the demonstration of erbium doped fiber amplifiers in the late 1980s quickly changed that perception. It is interesting to note however that the touted advantages of the fiber host hardly apply to the types of fiber laser being developed for cutting, welding, marking, ranging, imaging etc. The fiber laser revolution has rather been driven by relentless improvements in semiconductor pump power and pump brightness complemented by fiber techniques which allow single mode operation at high effective area.

Micro-structured high power fiber lasers & amplifiers

Microstructuring techniques, initially developed for photonic band gap fibers are being increasingly adapted for active fiber systems due to superior design space in effective area, polarization maintenance and dispersion control.