Publisher’s Note: This paper, originally published on 2/26/18 was replaced with a corrected/revised version on 5/25/18. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
The thermal grating (TG) and inversion grating (IG) TMI gain dependence on the light beating intensity spectrum is investigated. TMI gain is restricted to intensity bandwidths comparable to the thermal gain bandwidth of ∼20 kHz. Seed laser phase noise generates intensity spectra determined by the laser linewidth and the relative group delay time of the gain fiber. These spectral bandwidths exceed the thermal gain bandwidth by orders of magnitude in both the coherent and incoherent regimes, making them unlikely sources of TMI. It is suggested that phase noise generated in the gain fiber due to external perturbations may be the source of the TMI.
The stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) threshold powers in passive small mode
area optical fibers (SMAFs) and large mode area optical fibers (LMAFs) are determined from a distributed light scattering model.
The model employs the pump and Stokes rate equations together with the equivalent thermal Brillouin and Raman noise powers to
provide a numerical solution for the evolution of the pump and Stokes powers along the fiber length without approximation and is
applicable in the non-depleted and depleted pump regimes. A simple transcendental equation for the threshold powers is obtained in
the non-depleted pump, low fiber loss regime that is of similar form to the standard threshold relations first derived by Smith:
Pth=κAeff/gLeff where Pth is the threshold power, g is the non-linear gain coefficient and Leff is the fiber effective interaction length. The
numerical coefficient κ=ln(PS/PN) is found to be a weak function of the Stokes power PS and the thermal noise power PN and differs
for the two types of fibers due to the differing order-of-magnitude Stokes powers and noise powers. It is found that the SRS threshold
power of the LMAF exceeds that first derived by Smith by 1.7 dB and that the SBS threshold power in the SMAF is 1.2 dB less than
that derived by Smith. The SRS threshold in SMAF and the SBS threshold in the LMAF are in agreement to within <1.0 dB with the
Smith relations. The transcendental approximation is useful for estimating the threshold powers in all fibers at any pump power and
pump-to-Stokes conversion efficiency η<0.1. It is found that the Smith relations do not apply to the pump-depletion regime although
they are defined for conversion efficiencies η of 0.5 and 1.0.
This paper discusses the transverse acoustic index design of Yb-doped large mode area (LMA) LP01 optical fibers that
provide ~10 dB of SBS threshold suppression relative to conventional LMA fibers with homogeneous elastic properties and
equivalent optical effective areas Aeff. SBS suppression is achieved with a ramp-like negative acoustic lens structure that refracts the
electrostrictive density fluctuations away from the optical mode thereby reducing the acousto-optic interaction that generates the
stimulated light scattering. The fundamentals of the SBS process and its mitigation are briefly reviewed. Two figures-of-merit
(FsOM) are identified to quantify the SBS suppression capability; the SBS reflectivity RSBS and the SBS threshold power Pth. An
initial design of an SBS suppressing Yb-doped double clad fiber is incorporated in the power amplifier stage of a 200 W cw singlefrequency
(SF) four stage master-oscillator power-amplifier (MOPA). The MOPA is also exercised as a pulsed amplifier and is
utilized to measure the SBS reflectivities and thresholds in passive (i.e. un-pumped) fibers with ~100 ns pulses exhibiting peak powers
up to ~250 W. In separate experiments, the SBS suppressing fibers are incorporated into the final stage of the pulsed four stage
MOPA and the SBS reflectivities of the active gain fibers are measured. Pulsed SF MOPAs with peak pulsed power outputs ~900 W
are demonstrated and confirm the kilowatt SF performance capability of these SBS suppressing fibers.
11.2 dB suppression of stimulated Brillouin scattering (SBS) in an Yb-doped, Al/Ge
co-doped large mode area (LMA) gain fiber is demonstrated with a ramp-like acoustic index
profile exhibiting an acoustic index contrast of 0.09 and acoustic index slope of 0.01/μm.
A four-stage all-fiber single-frequency single-mode continuous-wave (cw) master-oscillator power-amplifier (MOPA) at 1083 nm is presented. Small mode area (SMA) and large mode area (LMA) amplifier stages are mode matched with a fiber mode converter (MC) and the signal and pumps are combined with tapered fiber bundles (TFBs). The final power stage uses a LMA Yb doped SBS-suppressing fiber. A single-frequency output power of 194 W is demonstrated with optical net and slope efficiencies of 73% and 80%, respectively. Numerical simulations for the signal output power and the SBS-induced Stokes backscattered power in the 4th stage amplifier agree with the experimental results. Pulse amplifier measurements
showed a 400 W peak power output that was limited by the forward output ASE. The SBS
reflectivity at 400 W output was only 2.75 x 10-4.
Noise can limit the power in a fiber amplifier by competing with the signal for gain. Disributed filtering offers a
way of overcoming current high-power limitations by selectively removing noise before it is amplified. Resonant
mode suppression is a flexible strategy for wavelength filtering as well as suppression of unwanted transverse
modes. An Yb-doped gain fiber with distributed filtering of stimulated Raman scattering (SRS) was demonstrated
in a cladding-pumped amplifier. Substantial suppression of SRS was demonstrated in this fiber, with a
core and resonant Ge-doped ring made using standard solid-fiber fabrication. Simulations explore generalizations
to large-mode-area and filters with multiple band-stop features.
High power fiber lasers and amplifiers have received a significant amount of attention in recent years. Due to their stable beam characteristics, good thermal dissipation, and reduced weight compared to diode-pumped solid state lasers, they are finding more application spaces. One subset of this broad category of devices is pulsed sources operating in the mJ regime, which have a wide range of sensing and materials applications.
A tapered fiber bundle is one of the leading approaches to coupling pump light into cladding-pumped fibers. This article compares it to other pump coupling schemes, and describes the tapered fiber bundle in detail. In addition tapered fiber bundles which maintain the polarization state of the signal and transform the size of the mode are described.
This article describes Raman fiber lasers and their application as pumps to a Raman amplified optical communication system. Single wavelength, multiwavelength and dual-order devices are described. The advantages of Raman fiber lasers compared to semiconductor diodes are also discussed.
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