Results of steady periodic finite element beam propagation (SP-FE-BPM) simulations of transverse mode instability (TMI) in representative coiled step index and large pitch photonic crystal fiber amplifiers with pump noise and modulation are presented and discussed. Modulating the pump power of fiber amplifiers near the TMI threshold complicates the conceptual picture due to the introduction of the additional dynamical time scale of the pump modulation. The time dependent mode field throughout the length of the amplifier calculated with the SP-FE-BPM was used to synthesize spatially and temporally resolved imaging measurements including spatial distribution of the amplitude and phase of different frequency components as a means to characterize the effects of pump modulation on TMI dynamics. Pump modulation was found to affect the threshold and dynamics of TMI in two amplifier configurations and a range of modulation cases. The step index fiber (SIF) amplifier was found on average to be approximately 20% more susceptible to pump noise than the large pitch fiber (LPF) amplifier. Furthermore a TMI dynamic not associated with stimulated thermal Rayleigh scattering that has been experimentally observed was observed in both SIF and LPF SP-FE-BPM simulations.
Results of numerical simulations of transverse mode instability (TMI) in Ytterbium-doped coiled large mode area step index fiber amplifiers are presented and discussed. The TMI thresholds for counter-pumped amplifiers were found to be significantly greater than for co-pumped amplifiers. A greater dependence of the TMI threshold on amplifier length was observed in the counter-pumped case than in the co-pumped case. Furthermore the onset of TMI was found to be localized in a specific critical position along the amplifier in the co-pumped case. In this configuration, amplifiers could be pushed slightly beyond the TMI threshold and still produce stable output due to bend loss filtering of the higher order modes. The TMI thresholds were found to have exhibit a non-monotonic behavior with respect to coiling diameter and only weakly dependent on core launch misalignment. Power stripped from the core into the cladding was in some cases found to interfere with the signal within the core leading to small power fluctuations that appear unrelated to the TMI process. In some co-pumped cases the beam was distorted at the amplifier output only during a small window within the overall simulated time period. This indicates that all of the different components of the broadened frequency spectrum are coherent. At this time, the physical explanation for this effect is unknown.
Two approaches are presented for detailed analysis of transverse mode instability in fiber amplifiers based on a scalar finite element beam propagation method (BPM). The first employs two beams: one propagating at a fundamental frequency and one de-tuned to the middle of the stimulated thermal Rayleigh scattering (STRS) gain peak. This method was found to suffer from a computational artifact causing it to converge in some cases to an unphysical solution. The second was based on the steady periodic method. This required more computational resources but was found to be reliable and not susceptible to the artifact mentioned above. This method was used to simulate step-index fiber amplifiers, large pitch photonic crystal fiber amplifiers, and a hybrid large pitch photonic bandgap fiber amplifier with reduced symmetry. Results for reference step index fiber amplifiers were found to be consistent with those obtained by other methods. The simulated instability threshold values all fell between 200 and 310 Watts showing relatively little variation among designs. Some areas for improvement in the method are discussed.
Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30μm) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 μm annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth ~ 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92×10-11 m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers
This paper describes a numerical method of accurately modeling large pitch rod type photonic crystal fiber amplifiers taking into account a converged solution to the thermo-optic feedback loop. This method also accounts for the possibility of asymmetric doping profiles and directly treats higher order mode stimulated thermal Rayleigh scattering gain competition along the entire length of the amplifier. Example applications are described. This approach enables further fiber design optimization to increase peak and average amplifier power outputs.
Numerical analysis of the onset of modal instability in fiber amplifiers is presented. Specifically calculations of the evolution of the intensity fluctuation spectrum along the fiber for a sampling point offset from the core center are presented for different instability onset conditions. These include seeding with LP01 only, seeding with LP01 and LP11 at the same frequency, seeding with LP01 and LP11 at offset frequencies, and seeding taking quantum shot noise into account. The position dependent spectra are shown to be very similar for each of these cases suggesting a common instability mechanism.
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 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 report on the performance of monolithic, polarization maintaining, Er-doped photonic crystal fibers (PCF) and
amplifiers operating in the eye-safer wavelength regime from 1.55-1.6 um. As part of this effort, we have developed
novel 6x1+1 pump/signal combiners for air-clad photonic crystal fibers with six 0.22 NA, 200/220 um pump input fibers
and a 25/250 PM signal fiber that allow efficient pump and signal coupling to the air-clad Er-doped PCF. These etched
air taper combiners have been demonstrated at the kilowatt level under 976 nm pumping and perform an efficient
brightness transformation from 0.19 NA, 1532 nm fiber coupled diode pumps into the 0.6 NA air-clad Er-doped PCF
with a measured pump throughput efficiency of 88-92% and a signal throughput of 65-80% with a PER of <18 dB.
These novel combiners have been efficiently spliced to 40 um core, 200 um pump cladding Er-doped PCFs providing
high efficiency resonantly pumped, monolithic, eye-safer PCF fiber lasers and amplifiers. Using grating stabilized 1532
nm pump diodes, our current experiments have demonstrated single transverse mode operation of both monolithic eyesafer
PCF lasers and amplifiers at the multi-Watt level with slope efficiencies of over 55%.
We report on the development and performance of a key components that enable the construction of multi-kW fiber
amplifiers for government and industrial applications that are both reliable and highly affordable. The usefulness of these
components span the range from single frequency near diffraction limited kW class fiber lasers to multimode wide
linewidth fiber lasers for welding and cutting applications. The key components for these amplifiers are a novel multi
fiber-coupled laser diode stack and a monolithic 6+1x1 large fiber pump/signal multiplexer. The precisely aligned 2-D
laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror
reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber
combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a
high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant
core size through the taper for efficient signal coupling and throughput. The fiber combiner has 6 400/440/0.22
core/clad/NA pump delivery fibers and a 20/440 PM step-index signal delivery fiber on the input side and a 40/525 PM
undoped PCF on the output side. The etched air taper transforms the six 400/440 μm 0.22 NA pump fibers to the 500
μm 0.55 NA core of the PCF fiber with a measured pump combining efficiency of 92% with zero brightness drop. The
combiner also operates as a stepwise mode converter via a 30 μm intermediate core region in the combiner between the
20 μm core of the input fiber and the 40 μm fiber core of the PCF with a measured signal efficiency of 90% while
maintaining polarization with a measured PER of 20 dB. We report the signal coupling efficiency and power handling
capability as well.
There is demand for improved deep-space satellite communications links with increased data rates to accommodate
new sensor technologies and increased sensor payloads on spacecraft. It is imperative that new solutions be compact
in size, light in weight, be high speed, and highly power efficient. Optical links offer potential improvements in
power, size and weight due to a substantially narrower beam and smaller components. Solutions using fiber-laser
transmitter master-oscillator power-amplifiers (MOPA) have been investigated previously, but methods for improving
the system power efficiency are needed. In this paper we will present recent results for a 1.5um fiber MOPAs for
deep-space communications. A high-power, broad linewidth 1550 nm seed laser is combined with 3 stages of
amplification. Each of these stages is partially designed based on the availability of the various components An
amplifier operating at 1532 nm in principle has the advantage of commercial off-the-shelf components with
demonstrated reliability though the availability of high power broad area pump diodes at 1532nm with good efficiency
is limited. The final power amplifier stage was constructed using both step index fiber (SIF) and photonic crystal fiber
(PCF).
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 report on the performance of a monolithic 6+1X1 fiber pump signal multiplexer for use in fiber amplifiers.
The key component of this coupler design is an etched taper that transforms the low-numerical aperture large
diameter pump radiation into a high numerical aperture small diamter format for injection into the pump
cladding of an air-clad fiber while maintaining a constant refractive index profile in the core for efficient signal
coupling. This taper was then fused onto the 6+1 fiber bundle at the large end and to the air-clad large mode
area polarization maintaining photonic crystal fiber at the small end. We employed 6 pump delivery fibers in
a 200/220/0.22 core/clad/NA format and a 25/250 polarization maintaining step index signal delivery fiber for
the bundle. The large end of the taper had a cladding diameter of 650 μm while the small end had a cladding
diameter of 300 μm to match the pump cladding diameter of the PCF which was 314 μm. The core within
the taper had a constant diameter of 40 μm and NA of 0.07 achieved through a step index profile. The mode
field diameter of the PCF was 54 μm. Signal coupling efficiency at 1550 nm was measured to be 90% with a
polarization extinction ratio > 20dB while pump coupling efficiency was measured to be 87% at 1532nm. The
low pump coupling efficiency was found to be due to pump delivery fibers that had a numerical aperture of 0.24,
higher than the specification of 0.22. A simple calculation shows that with 0.22 NA pump fibers, the pump
coupling efficiency would increase to 94%.
Non-axially symmetric micro-structured fibers including photonic crystal fibers and leakage channel fibers have
been investigated as possible means to enable mode field area scaling while retaining low-loss fundamental mode
propagation. In this paper we report on the theoretical and experimental investigation of a photonic crystal fiber
designed for single-polarization operation in a coiled configuration incorporating a resonant higher-order mode
suppressing structure within the cladding. We performed spatially and spectrally resolved (S2) imaging of the
output of a 18.6 meter length of fiber when seeded with a broadband ASE source. We observed no trace of any
localized or distributed scattering to higher order modes in the spatially resolved Fourier transformed output
spectra indicating robust single-transverse-mode output.
The power available from narrow-linewidth single-transverse-mode fiber amplifiers is primarily limited by the
onset of stimulated Brillouin scattering. One approach for increasing the SBS threshold that has shown recent
promise is to tailor the acoustic velocity within the fiber cross-section to suppress Brillouin gain. Relating the
SBS threshold to an acousto-optic effective area has yielded a theory which contradicts experimental measurements
that indicate the nonlinear optical effective area of the tested SBS suppressing and Higher Order Mode
(HOM) fibers is of primary importance in the nonlinear process. In this work, we present a new formalism for
determining the Brillouin gain in fibers with inhomogeneous acoustic velocity which may be implemented with a
wide variety of computational methods. We find that the Brillouin gain amplitude and spectrum are independent
of the acousto-optic effective area and that they reduce to the bulk result for conventional step-index fibers.
Implementing a finite-element method, we find that an SBS-suppressing design employing a negative focal length
acoustic lens exhibits a broadened gain spectrum and reduced gain amplitude relative to step-index fibers. The
SBS threshold of this fiber is increased by 8.4 dB relative to a standard large mode area fiber, each with an
identical 6 meter length. Designs that further flatten the Brillouin gain spectrum have the potential to further
increase the SBS threshold leading to higher single-frequency output power from devices incorporating these
fibers.
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.
A novel high accuracy all electronic technique for phase locking arrays of optical fibers is demonstrated. We report the first demonstration of the only electronic phase locking technique that doesn't require a reference beam. The measured phase error is λ/20. Excellent phase locking has been demonstrated for fiber amplifier arrays.
In this paper, we present the results of a frequency domain analysis of the effect of coiling induced macro-bending on the phasing of the modes of 6 and 7-core large mode area photonic crystal fibers. These fibers may enable fiber laser CW power and pulse energy scaling due to their large effective mode area (~4000 square microns), however; phase differences between the cores degrade the output beam quality. We study the effects of bend axis, bend radius, and core geometry on the phasing of the fiber modes. Coiling causes the signals in the individual cores to suffer de-phasing relative to each other in a predictable manner. This allows the possibility of recovering phased output, and thus near diffraction limited beam quality, from amplifiers employing these fibers. Preliminary experimental results are also presented.
We report a novel coherent beam combining technique. This is the first actively phase locked optical fiber array that eliminates the need for a separate reference beam. In addition, only a single photodetector is required. The far-field central spot of the array is imaged onto the photodetector to produce the phase control loop signals. Each leg of the fiber array is phase modulated with a separate RF frequency, thus tagging the optical phase shift for each leg by a separate RF frequency. The optical phase errors for the individual array legs are separated in the electronic domain. In contrast with the previous active phase locking techniques, in our system the reference beam is spatially overlapped with all the RF modulated fiber leg beams onto a single detector. The phase shift between the optical wave in the reference leg and in the RF modulated legs is measured separately in the electronic domain and the phase error signal is feedback to the LiNbO3 phase modulator for that leg to minimize the phase error for that leg relative to the reference leg. The advantages of this technique are 1) the elimination of the reference beam and beam combination optics and 2) the electronic separation of the phase error signals without any degradation of the phase locking accuracy. We will present the first theoretical model for self-referenced LOCSET and describe experimental results for a 3 x 3 array.
Stimulated Brillouin Scattering (SBS) is a polarization-dependent, nonlinear process that is often the limiting factor for high-power fiber laser applications. We report the results of experiments measuring the SBS thresholds and the SBS gain bandwidths in several passive optical fibers. Fibers with nearly identical mode-field diameters and loss coefficients from different manufacturers were selected. Observations from these experiments indicate that the SBS gain coefficient for fibers from different manufacturers varied significantly resulting in a 70% deviation in SBS threshold. Also, polarization-maintaining (PM) fiber exhibited a significant increase in the SBS threshold for a linearly polarized pump beam that is launched into the PM fiber at 45° relative to the fiber's slow axis. This increase in threshold was not mirrored in non-PM fiber. These results suggest that the polarization multiplier in the SBS threshold equation may be highest when a PM fiber is used with the appropriate launch conditions, rather than a non-PM conventional single-mode fiber. We will present the experimental results and a theoretical model demonstrating the polarization dependent gain properties in both PM and non-PM fiber.
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