Spectral Beam Combining (SBC) of fiber lasers provides a simple, robust architecture for high brightness power scaling beyond the limit of a single fiber. We review recent progress in power scaling and describe what we believe is the highest power SBC fiber demonstration and largest number of fiber lasers combined to date. Here we report results on a fiber SBC system where we achieved > 30 kW by combining 96 individual fiber lasers into a single high brightness beam with a beam quality of M2 = 1.6 x 1.8. The potential for further power scaling at the system level is highlighted with examples of beam combinable fiber laser power scaling.
White noise phase modulation is an effective technique capable of increasing the SBS threshold in high power fiber amplifiers. Theoretical models predict the enhancement factor as a function of linewidth and fiber length, but have yet to be experimentally verified over wide ranges of these variables. We present results on a cut-back experiment performed on a passive fiber with a white-noise broadened laser, measuring the SBS enhancement factor as a function of fiber length and bandwidth. In addition, the experimental results will be compared to phase modulation models of the SBS process in optical fibers.
A novel acoustic and gain tailored Yb-doped photonic crystal fiber is used to demonstrate over 800 W single-frequency output power with excellent beam quality at 1064 nm. The large mode area fiber core is composed of 7 individually doped segments arranged to create three distinct acoustic regions and preferential gain overlap with the fundamental optical mode. This design leads to suppression of both stimulated Brillouin scattering and modal instability. To the best of our knowledge, the output power represents the highest power ever reported from a near diffractionlimited single-frequency fiber laser. Furthermore, we show that by using a broadband seed, 1.22 kW of output power is obtained without the onset of the modal instability.
We present experimental results of SBS suppression in high power, monolithic, Yb-doped fiber amplifiers via phase modulated laser gain competition. To narrow the linewidth, two-tone laser gain competition between broad (1036 nm) and narrow linewidth (1064 nm) laser signals is investigated in conjunction with phase modulation and yields pump limited output powers of 600 W. Here integration of both two-tone and pseudo random bit sequence (PRBS) phase modulation concepts, generated SBS enhancement factors of greater than 17x at a modulation frequency of 500 MHz, without reaching the SBS threshold. Significantly, the results represent a near order of magnitude reduction in linewidth over current high-power, monolithic, Yb-doped fiber amplifiers.
We present a detailed time-dependent numerical model of the modal instability phenomenon observed in Yb-doped fiber amplifiers. The thermal effects are captured by solving the heat equation in polar coordinate using a 2D, second-order, time-dependent, alternating direction implicit (ADI) method. The model captures the three power-dependent regions that are characteristic of the transfer of energy between the fundamental mode and the higherorder mode as a function of time. It is also shown that for the fiber configuration investigated, the modal instability threshold scales linearly over a wide range with the seed power. Furthermore, we present numerical results indicating that gain tailoring can increase the threshold. Two different gain-tailored fiber designs are simulated and compared.
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 a detailed study of power scaling in polarization-maintaining Raman fiber amplifiers operating at 1178 nm since this wavelength can be frequency doubled into 589 nm for sodium guide star applications. We confirm experimentally that the optimized output signal at SBS threshold scales linearly with the pump power. We also present results from numerical and experimental studies investigating the scalability of Raman fiber amplifiers with seed power. Both co-pumped and counter pumped two-stage amplifiers utilizing acoustically tailored fiber for SBS suppression were constructed and studied. For the former configuration spectral broadening was observed, while the latter configuration provided 22 W of single-frequency output. Finally, we show results of a phase-modulated amplifier generating multiple spectral lines separated by 886 MHz, which corresponds to the spectral separation of the sodium D2a, and D2b lines after frequency doubling in a nonlinear cavity.
We present detailed studies of the effect of sinusoidal phase modulation on stimulated Brillouin scattering (SBS) in
ytterbium-doped fiber amplifiers. Based on a time-dependent SBS model, SBS enhancement factor versus pump
linewidth for different modulation depths ranging from 0 to π , and modulation frequencies ranging from 30 MHz to
500 MHz were analyzed. In addition, experimental validation of SBS suppression via sinusoidal phase modulation is
presented with experimental results agreeing well with the model and simulations. Furthermore, narrow linewidth
coherent beam combining (CBC) of 5 sinusoidal phase modulated lasers is demonstrated via LOCSET.
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 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 present experimental studies of a novel polarization-maintaining (PM) Yb-doped photonic crystal fiber (PCF)
possessing a two-segment transverse acoustic profile and a mode field diameter (MFD) of approximately 30 μm. The
concentrations of the dopants (fluorine, aluminum, germanium) in the two segments were selected such that the
corresponding Brillouin shifts were sufficiently separated to allow for the introduction of a large thermal gradient for
further SBS suppression. A pump-probe experiment was conducted in order to characterize the Brillouin gain spectrum
(BGS) and to confirm the existence of two narrow Brillouin gain peaks. The separation of the two peaks was
approximately 220 MHz and the bandwidth of each was estimated to be 50 MHz. The application of a step temperature
profile resulted in the BGS displaying four distinct peaks; thus demonstrating further SBS suppression through a thermal
gradient. By utilizing the thermal gradient obtained through quantum defect heating, we obtained 494 W of output power
in a counter-pumped configuration without the onset of SBS. Measurements of the beam quality at various power levels
and up to the highest reported power consistently indicated an M2 of less than 1.3.
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.
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.
A new technique for mitigating stimulated Brillouin scattering (SBS) effects in narrow-linewidth Yb-doped fiber
amplifiers is demonstrated with a model that reduces to solving an 8×8 system of coupled nonlinear equations with the
gain, SBS, and four-wave mixing (FMW) incorporated into the model. This technique uses two seed signals, or 'two-tones',
with each tone reaching its SBS threshold almost independently and thus increasing the overall threshold for SBS
in the fiber amplifier. The wavelength separation of these signals is also selected to avoid FWM, which in this case
possesses the next lowest nonlinear effects threshold. This model predicts an output power increase of 86% (at SBS
threshold with no signs of FWM) for a 'two-tone' amplifier with seed signals at 1064nm and 1068nm, compared to a
conventional fiber amplifier with a single 1064nm seed. The model is also used to simulate an SBS-suppressing fiber
amplifier to test the regime where FWM is the limiting factor. In this case, an optimum wavelength separation of 3nm to
10nm prevents FWM from reaching threshold. The optimum ratio of the input power for the two seed signals in 'two-tone'
amplification is also tested. Future experimental verification of this 'two-tone' technique is discussed.
Photonic crystal fiber designs for power scaling of single-polarization amplifiers are
presented. These fibers incorporate a core with a refractive index slightly lower than that of pure
silica and borosilicate stress rods embedded within the photonic crystal lattice. They are
realizable as rod-type as well as conventional photonic crystal fibers. The core index change
results in a fundamental mode profile that is flattened relative to that of standard photonic crystal
fibers. A continuum of mode profiles ranging from approximately Gaussian to approximately
top-hat can be achieved in this manner. The optimal parameters of these designs depend on the
desired coiling radius, birefringence, and mode area. Preliminary experimental results are
presented.
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