Albuquerque, NM, United States
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Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Optics Express | Year: 2013

We present a detailed description of the methods used in our model of mode instability in high-power, rare earth-doped, large-mode-area fiber amplifiers. Our model assumes steady-periodic behavior, so it is appropriate to operation after turn on transients have dissipated. It can be adapted to transient cases as well. We describe our algorithm, which includes propagation of the signal field by fast-Fourier transforms, steadystate solutions of the laser gain equations, and two methods of solving the time-dependent heat equation: alternating-direction-implicit integration, and the Green's function method for steady-periodic heating. © 2013 Optical Society of America.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Optics Express | Year: 2011

Using a beam propagation model of Yb3+ doped, CW fiber amplifiers we show that gain saturation by a strong fundamental mode significantly suppresses the growth of higher order modes with parallel polarization, but enhances the growth of higher order modes with perpendicular polarization. We quantify this effect in straight and bent fibers, with full core or restricted area doping. © 2011 Optical Society of America.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Optics Express | Year: 2011

For powers exceeding a sharp threshold in the vicinity of several hundred watts the beam quality from some narrow bandwidth fiber amplifiers is severely degraded. We show that this can be caused by transverse thermal gradients induced by the amplification process. © 2011 Optical Society of America.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

We show by detailed numerical modeling that stimulated thermal Rayleigh scattering can account for the modal instability observed in high power fiber amplifiers. Our model illustrates how the instability threshold power can be maximized by eliminating amplitude and phase modulation of the signal seed light and the pump light and by careful injection of the signal seed light. We also illustrate the influence of photodarkening and mode specific loss. © 2013 Copyright SPIE.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Optics Express | Year: 2012

Using numerical simulations of thermally induced mode coupling we show how the instability threshold can be substantially reduced if the pump or injected signal is modulated in the kHz range. We also show how the mode coupling gain varies with the frequency offset of the parasitic mode. We model thresholds when the source of detuned light is quantum background, amplitude modulation of the pump power, and amplitude modulation of the signal seed. We suggest several key experimental and modeling tests of our model. © 2012 Optical Society of America.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
IEEE Journal on Selected Topics in Quantum Electronics | Year: 2014

We offer an overview of the concepts, numerical methods, and applications of our model of stimulated thermal Rayleigh scattering in large mode area fiber amplifiers. This overview is intended to consolidate and summarize our studies of modal instability thresholds and how to maximize them. Several movies of thermal grating evolution and modal power evolution are presented for the first time. © 1995-2012 IEEE.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Optics Express | Year: 2016

We use a detailed numerical model of stimulated thermal Rayleigh scattering to compute mode instability thresholds in Tm3+-doped fiber amplifiers. The fiber amplifies 2040 nm light using a 790 nm pump. The cross-relaxation process is strong, permitting power efficiencies of 60%. The predicted instability thresholds are compared with those in similar Yb3+-doped fiber amplifiers with 976 nm pump and 1060 nm signal, and are found to be higher, even though the heat load is much higher in Tm-doped amplifiers. The higher threshold in the Tm-doped fiber is attributed to its longer signal wavelength, and to stronger gain saturation, due in part to cross-relaxation heating. © 2016 Optical Society of America.


Smith J.J.,AS Photonics LLC | Smith A.V.,AS Photonics LLC
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015

We show how signal bandwidth affects the gain of stimulated thermal Rayleigh scattering (STRS) which is responsible for a transverse mode instability in fiber amplifiers. The gain is reduced if the coherence time of the signal is less than the group-velocity-induced walk off between the two interacting modes, usually LP01 and LP11. We derive expressions for the bandwidth required to suppress gain for short pulses, for periodically chirped continuous waves, and for general periodically modulated cases. © 2015 SPIE.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Optics Express | Year: 2013

We show by numerical modeling that saturation of the population inversion reduces the stimulated thermal Rayleigh gain relative to the laser gain in large mode area fiber amplifiers. We show how to exploit this effect to raise mode instability thresholds by a substantial factor. We also demonstrate that when suppression of stimulated Brillouin scattering and the population saturation effect are both taken into account, counter-pumped amplifiers have higher mode instability thresholds than co-pumped amplifiers for fully Yb3+ doped cores, and confined doping can further raise the thresholds. © 2013 Optical Society of America.


Smith A.V.,AS Photonics LLC | Smith J.J.,AS Photonics LLC
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2016

We use a detailed numerical model of stimulated thermal Rayleigh scattering to compare mode instability thresholds in cladding pumped Tm3+-doped and Yb3+-doped fiber amplifiers. The Tm-doped fiber amplifies 2040 nm light using a 790 nm pump; the Yb-doped fiber amplifies 1060 nm light using a 976 nm pump. The predicted instability threshold of the Tm-doped fiber is found to be higher than that of the Yb-doped fiber, even though its heat load is much higher. We attribute the higher threshold in part to its longer signal wavelength, and in part to stronger gain saturation. © 2016 SPIE.

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