We present experimental and theoretical studies on the stimulated Brillouin scattering (SBS) threshold in fiber amplifiers seeded with a spectrally broadened single-frequency laser source. An electro-optic phase modulator is driven with various pseudo-random binary sequence (PRBS) patterns to highlight the unique characteristics of this linewidth broadening technique and its facility in SBS mitigation. Theoretical predictions show a variation in SBS suppression based on PRBS pattern and modulation frequency. These predictions are experimentally investigated in a kilowatt level monolithic fiber amplifier operating with near diffraction-limited beam quality. We also show Rayleigh scattering and other sources of back reflected light in phase modulated signals can seed the SBS process and significantly reduce the nonlinear threshold.
We present experimental studies of PM Yb-doped photonic crystal fibers possessing acoustic and Yb-ion concentration
tailoring. In the initial design, the concentration of dopants in two regions of the core were selected such that the
corresponding Brillouin shifts were sufficiently separated to allow for further stimulated Brillouin scattering suppression
through thermal effects. The Yb-ion concentration was maintained uniformly throughout the entire core. When this
fiber was utilized in a counter-pumped amplifier configuration, ~500 W of single-frequency (kHz linewidth) output was
obtained in a 10 m long fiber. Further power scaling with good beam quality beyond 500 W was limited by modal
instabilities. A second fiber design was developed in which the Yb-ion concentration was modified to have preferential
overlap with the fundamental mode as well as reduced pump absorption. The onset of the modal instabilities was
sufficiently suppressed to allow for an output of 990 W with a nominal linewidth of 300 MHz and good beam quality.
We present experimental results of a single-stage Raman fiber amplifier (RFA) for guide star application. SBS suppression was
achieved through the acoustic tailoring of the core of a polarization-maintaining single-mode conventional fiber. The core was also
doped for enhanced Raman gain. This fiber was utilized in a counter-pumped amplifier configuration to generate 1178 nm light with a
linewidth < 2 MHz for frequency doubling into the D2 sodium line. Due to the SBS suppressing characteristics of the fiber, the RFA
provided 11.2 W of 1178 nm signal when seeded with 15 mW. Application of a thermal gradient allowed for further power scaling
leading to 18.3 W. Our measurement of the linewidth at the highest output power indicated no spectral broadening.
We present theoretical and experimental results of a 130 W continuous-wave (CW), single-frequency, 7 m, polarizationmaintaining
(PM) Yb:doped fiber (25/400) μm amplifier simultaneously seeded with a combination of broadband and
narrow-line signals. Experiments were performed for two thermal configurations and the SBS threshold of the doubly
seeded amplifier is compared to the singly seeded case. In the first configuration, the fiber was wrapped around a cold
spool held at 12° C to diminish thermally induced shifts in the acoustic resonance of the fiber, which is known to
suppress stimulated Brillouin scattering (SBS). In this case, over 80 W of single-frequency output was obtained
demonstrating an enhancement of 3 dB in the SBS threshold compared to the single-tone case whereby the SBS
threshold was 40 W. In the second thermal configuration, 6 m of the fiber is wrapped around the same cold spool, but
approximately 1 m of the fiber is left to cool in ambient conditions. In this case, an optically induced thermal gradient
was formed due to the quantum defect heating associated with power transfer from the pump and broadband seed signals
into the single-frequency signal at the output end of the fiber. Over 130 W of single-frequency output was demonstrated
yielding an effective increase of ~5 dB in the SBS threshold when compared to the single-tone case.
We present theoretical and experimental results on the scalability of amplifying a single-frequency diode laser operating
at 1178 nm through the utilization of a core pumped Raman fiber amplifier. A model that accounts for stimulated Raman
scattering (SRS) and stimulated Brillouin scattering (SBS) in both co-pumped and counter-pumped configurations is
developed. In order to mitigate the SBS process, a multi-step temperature distribution that is spatially optimized along
the length of fiber is investigated numerically. We also present preliminary experimental results on a co-pumped Raman
amplifier with an output power of 12 W and a measured Brillouin gain coefficient of approximately 1.2×10-11 m/W.
We describe a time-dependent model that describes the evolution of stimulated Brillouin scattering (SBS) in fibers under
phase-modulated pump conditions. In order to accurately model fast modulations, the triply-coupled system of
differential equations describing the interaction of SBS through optical and phonon fields is solved numerically. SBS is
initiated from noise by using a Langevin term. We initially consider single-frequency sinusoidal modulations as a
function of modulation amplitude and frequency. We then investigate the effects of SBS mitigation when a single-frequency
seed is phase modulated with a broad-band white-noise source (WNS).
We report on theoretical and numerical investigations of stimulated Brillouin scattering (SBS) in optical fibers. A
theoretical framework is presented which involves a nonlinear
triply-coupled time-dependent system of equations
containing the optical, Stokes, and phonon fields. We examine short fibers where previous approximations for long
fibers cannot be made. We consider modulation frequencies and linewidths starting at approximately the Brillouin gain
bandwidth and all the way up to frequencies of the order of the Brillouin resonance frequency. We provide simulations
showing the SBS suppression factor as a function of modulation amplitude and frequency for a single-sinusoidal
modulator.
We report on experimental and theoretical investigations of single frequency high power PCF amplifiers. A model
describing the interplay among laser gain, thermals effects, and SBS was developed to study the power limitations of
single frequency amplifiers in general, and PCF amplifiers in particular. A distributed noise term was used to initiate the
SBS process with the Stokes light spanning multi-frequency channels. The use of thermal and acoustic gradients in
conjunction was considered and indicated marginal improvement. In the set of experiments, slope efficiencies as high as
77% were obtained with a maximum output of 427 W. The linewidth was measured and yielded values that were less
than 10 KHz. A pump-probe measurement of the Brillouin gain spectrum revealed secondary peaks lying at the highfrequency
side. Measurements conducted on a novel PCF, specifically designed to utilize thermal and acoustic gradients
in conjunction, showed the existence of two primary gain peaks.
We present experimental verification of a novel technique to suppress stimulated Brillouin scattering (SBS) in single
frequency fiber amplifiers. This technique relies on seeding with a combination of broadband and single frequency laser
beams to allow for efficient laser gain competition between the two signals. In the experiment, a monolithic fiber
configuration was used. Broadband 1045 nm light and single frequency 1064 nm light were coupled into an Yb-doped
gain fiber. With appropriate selection of seed power ratio, we were able to generate an output signal predominantly
comprised of 1064 nm light while simultaneously suppressing the back-scattered Stokes light. The slope efficiency for
the two-tone amplifier was approximately 78%; slightly below that of a single-tone amplifier. The SBS threshold for the
former, on the other hand, was appreciably higher than that of the latter which is in excellent agreement with the theory.
In preliminary implementation of this technique at high power, we generated close to 100 W without encountering the
SBS threshold. Finally, we show numerically that due to a favorable thermal gradient much higher powers can be
obtained.
KEYWORDS: Phase shift keying, Optical components, Photodetectors, Phase modulation, Signal processing, Modulation, High power fiber amplifiers, Optical amplifiers, Phase measurement, Signal to noise ratio
We report high power phase locked fiber amplifier array using the Self-Synchronous Locking of Optical Coherence by
Single-detector Electronic-frequency Tagging technique. We report the first experimental results for a five element
amplifier array with a total locked power of more than 725-W. We will report on experimental measurements of the
phase fluctuations versus time when the control loop is closed. The rms phase error was measured to be λ/60. Recent
results will be reported. To the best of the authors' knowledge this is the highest fiber laser power to be coherently
combined.
Two approaches to two- and three-tone seeding of high power ytterbium-doped fiber amplifiers are investigated using a
symbolic and numerical code that solves a two point boundary problem consisting of a 12×12 system of nonlinear
differential equations. Optimization of amplifier action is considered in relation to the two most dominant nonlinear
effects: stimulated Brillouin scattering and four-wave mixing. One approach uses a large wavelength separation among
the input seed beams, while in the other approach the seeds are separated by precisely twice the Brillouin shift. It is
found that for both techniques significant increase in amplifier output can be obtained, although for the latter case a
substantial amount of power is generated in the four-wave mixing sidebands.
We report high power phase locked fiber amplifier array using the Self-Synchronous Locking of Optical Coherence by
Single-detector Electronic-frequency Tagging technique. We report the first experimental results for a five element
amplifier array with a total locked power of more than 725-W. We will report on experimental measurements of the
phase fluctuations versus time when the control loop is closed. The rms phase error was measured to be λ/60. Recent
results will be reported. To the best of the authors' knowledge this is the highest fiber laser power to be coherently
combined.
We investigate theoretically multi-tone seeding of high power ytterbium-doped amplifiers using a numerical code that
solves a two-point boundary problem. A large wavelength separation among the signals is used leading to efficient
transfer of power through laser gain while increasing the stimulated Brillouin scattering threshold in the channel of
interest. Two-tone and three-tone seeding configurations are considered. For two-tone seeding, it is shown that a
combination of narrow linewidth and broadband signals employed in a co-propagating geometry can achieve the same
level of SBS suppression as counter-pumping.
Computer calculations of the beam quality and output power of nearly diffraction-limited thin-disk Yb:YAG
lasers are presented. The CW lasers have stable resonators. Our calculations include the simultaneous operation
of two transverse modes. The simulated lasers are similar to those for which experimental results have been
reported by other researchers but we have not attempted to make a precise comparison. Our calculated results
show interesting trends that can be used in designing such lasers.
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