We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations.
Fiber dispersion plays a significant role in spectral broadening of incoherent continuous-wave light. We develop a self-consistent stochastic model for spectral broadening of incoherent continuous-wave light through nonlinear wave mixing and apply this model to numerical simulations of spectral broadening in a continuous-wave fiber Raman laser. The results of these numerical simulations agree very well with carefully conducted laboratory measurements. Under a wide range of operating conditions, these numerical simulations also exhibit striking features, such as damped oscillatory spectral broadening (during the initial stages of propagation) and eventual convergence to a stationary, steady-state spectral distribution at sufficiently long propagation distances. We analyze the important role of fiber dispersion in such phenomena. We also derive an analytical rate equation expression for spectral broadening, whose numerical evaluation is far less computationally intensive than the fully stochastic simulation, and a mathematical criterion for the applicability of this analytical expression.
We study thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers. Post-irradiation heating
of a photodarkened fiber is shown to result in further increase the loss which is attributed to both a permanent
increase of loss-inducing color centers and a temperature-dependent broadening of the absorption spectrum. The
permanent heat-induced increase of loss is believed to indicate presence of an intermediate energy state in the
NIRpho tochemical mechanism for photodarkening. Further, we apply the demarcation energy curve approach
to derive the thermal activation energy of the induced defects. For the studied commercial 20-μm-core-diameter
LMA fiber, the energy distribution consists of a single peak, located at 1.3 eV with a FWHM of 0.31 eV.
We study the temperature of an Yb-doped large-mode-area (LMA) fiber during an accelerated photodarkening
experiment. In these measurements, photodarkening is optically induced by IR irradiation (i.e. 915 nm) while the fiber
temperature is measured by a thermal camera. Fiber temperature is observed to exceed 120 °C under conditions of 10.5
W of pump power and unforced air cooling. We show evidences that this temperature increase is caused by the lost
pump power due to photodarkening. A thermal model is used to explain the fiber temperature in terms of pump power
absorbed by photodarkening-induced defects. Furthermore, the effect of temperature on the rate of photodarkening and
saturation of the losses is studied. Both the photodarkening saturation level and the photodarkening rate are observed to
show significant temperature dependence that result on a variation of the photodarkening rate ion dependency. The use
of an air cooling system and low inversion measurements is shown to reduce the ion dependency from 7 to 4.5.
A combined photodarkening and thermal bleaching measurement of a large-mode-area (LMA) ytterbium-doped fiber
(YDF) is presented. Photodarkened YDF sample is recovered to pre-photodarkened state by thermal annealing. As a
result, this approach enables repeated measurements with the same sample and therefore eliminates uncertainties related
to changing of the sample (such as sample length and splice losses). Additionally, our approach potentially improves the
accuracy and repeatability of the photodarkening rate measurement, and also allows automation of the measurement
procedure.
Sandia National Laboratories' program in high-power fiber lasers has emphasized development of enabling technologies
for power scaling and gaining a quantitative understanding of fundamental limits, particularly for high-peak-power,
pulsed fiber sources. This paper provides an overview of the program, which includes: (1) power scaling of diffraction-limited
fiber amplifiers by bend-loss-induced mode filtering to produce >1 MW peak power and >1 mJ pulse energy
with a practical system architecture; (2) demonstration of a widely tunable repetition rate (7.1-27 kHz) while
maintaining constant pulse duration and pulse energy, linear output polarization, diffraction-limited beam quality, and
<1% pulse-energy fluctuations; (3) development of microlaser seed sources optimized for efficient energy extraction; (4)
high-fidelity, three-dimensional, time-dependent modeling of fiber amplifiers, including nonlinear processes; (5)
quantitative assessment of the limiting effects of four-wave mixing and self-focusing on fiber-amplifier performance; (6)
nonlinear frequency conversion to efficiently generate mid-infrared through deep-ultraviolet radiation; (7) direct diode-bar
pumping of a fiber laser using embedded-mirror side pumping, which provides 2.0x higher efficiency and much
more compact packaging than traditional approaches employing formatted, fiber-coupled diode bars; and (8)
fundamental studies of materials properties, including optical damage, photodarkening, and gamma-radiation-induced
darkening.
We have numerically compared the performance of various designs for the core refractive-index (RI) and
rare-earth-dopant distributions of large-mode-area fibers for use in bend-loss-filtered, high-power
amplifiers. We first established quantitative targets for the key parameters that determine fiber-amplifier
performance, including effective LP01 modal area (Aeff, both straight and coiled), bend sensitivity (for
handling and packaging), high-order mode discrimination, mode-field displacement upon coiling, and
index contrast (manufacturability). We compared design families based on various power-law and hybrid
profiles for the RI and evaluated confined rare-earth doping for hybrid profiles. Step-index fibers with
straight-fiber Aeff values > 1000 &mgr;m2 exhibit large decreases in Aeff and transverse mode-field displacements
upon coiling, in agreement with recent calculations of Hadley et al. [Proc. of SPIE, Vol. 6102, 61021S
(2006)] and Fini [Opt. Exp. 14, 69 (2006)]. Triangular-profile fibers substantially mitigate these effects,
but suffer from excessive bend sensitivity at Aeff values of interest. Square-law (parabolic) profile fibers are
free of modal distortion but are hampered by high bend sensitivity (although to a lesser degree than
triangular profiles) and exhibit the largest mode displacements. We find that hybrid (combined power-law)
profiles provide some decoupling of these tradeoffs and allow all design goals to be achieved
simultaneously. We present optimized fiber designs based on this analysis.
Yb-doped fibers are widely used in applications requiring high average output powers and high power pulse
amplification. Photodarkening is one limiting factor in these fibers. In this paper, characterization of photodarkening in
large-mode-area (LMA) fibers is presented building upon our previous work, which indicated that meaningful
comparison of photodarkening properties from different fibers can be made as long as care is taken to equalize the
excited state Yb concentration between samples. We have developed a methodology that allows rapid and reproducible
photodarkening measurements to be performed and that enables quantitative comparison of the photodarkening
propensity among fibers with different compositions and under different operating conditions. We have shown that this
measurement technique can be used effectively for LMA fibers by employing cladding pumping rather than the more
standard core pumping. Finally, we observe a seventh-order dependence of the initial photodarkening rate on the excited-state
Yb population for two different Yb-doped fibers; this result implies that photodarkening of a Yb-doped fiber source
fabricated using a particular fiber will be strongly dependent on the device configuration.
We demonstrate direct diode-bar side pumping of a Yb-doped fiber laser using embedded-mirror side pumping (EMSP).
In this method, the pump beam is launched by reflection from a micro-mirror embedded in a channel polished into the
inner cladding of a double-clad fiber (DCF). The amplifier employed an unformatted, non-lensed, ten-emitter diode bar
(20 W) and glass-clad, polarization-maintaining, large-mode-area fiber. Measurements with passive fiber showed that
the coupling efficiency of the raw diode-bar output into the DCF (ten launch sites) was ~84%; for comparison, the net
coupling efficiency using a conventional, formatted, fiber-coupled diode bar is typically 50-70%, i.e., EMSP results in a
factor of 2-3 less wasted pump power. The slope efficiency of the side-pumped fiber laser was ~80% with respect to
launched pump power and 24% with respect to electrical power consumption of the diode bar; at a fiber-laser output
power of 7.5 W, the EMSP diode bar consumed 41 W of electrical power (18% electrical-to-optical efficiency). When
end pumped using a formatted diode bar, the fiber laser consumed 96 W at 7.5 W output power, a factor of 2.3 less
efficient, and the electrical-to-optical slope efficiency was lower by a factor of 2.0. Passive-fiber measurements showed
that the EMSP alignment sensitivity is nearly identical for a single emitter as for the ten-emitter bar. EMSP is the only
method capable of directly launching the unformatted output of a diode bar directly into DCF (including glass-clad
DCF), enabling fabrication of low-cost, simple, and compact, diode-bar-pumped fiber lasers and amplifiers.
KEYWORDS: Doping, Ytterbium, Erbium, Optical simulations, Manufacturing, Beam shaping, Refractive index, High power fiber lasers, Photonic crystal fibers, Control systems
Many high power fiber laser applications require doped fibers having large mode area but still working in the single mode regime. The most common techniques to keep a large mode area fiber in the single mode regime are to reduce the core numerical aperture, to strip the high order modes by coiling the fiber, to launch only a single transverse mode, or to use photonic crystal fibers. All these methods have limits and disadvantages.
In this paper we demonstrate by simulation the effectiveness of another method to suppress the high order modes in large mode area active fibers by optimizing the rare earth dopant concentration across the core while keeping the step index structure of the core of the fiber. This method was not previously employed because the traditional doped fiber manufacturing technologies do not have the required capability to radially control the dopant concentration. However, Direct Nanoparticle Deposition (DND) can be used to manufacture large mode area fibers having any radial distribution of active element concentration and any refractive index profile. Thus, DND fibers can be designed to benefit from this high order mode suppression technique.
The simulation results presented in this paper have been obtained using Liekki Application Designer v3.1, a software simulator for fiber lasers and amplifiers.
Formatted diode bars are currently the preferred pump source for high-power fiber laser systems. In a formatted diode bar, the raw output of the diode bar is reformatted into a beam waist with a
divergence, aspect ratio, and fill factor suitable for coupling into a conventional multimode fiber pigtail. The main drawbacks to formatted diode bar pump sources are cost, complexity and an
inevitable tradeoff between coupling efficiency and source brightness. In this paper we describe how the Embedded Mirror Side Pumping (EMSP) technique allows the raw output of an unformatted diode bar to be used directly for side pumping double clad fiber (DCF) amplifiers. The EMSP technique allows the raw output of an unformatted diode bar to be coupled directly into the DCF inner cladding, without the same penalties in cost, complexity, coupling efficiency and loss of brightness. In particular, for applications at the 10 to 100 Watt power level and applications that require arrays of DCF amplifiers, diode bar EMSP will provide a very attractive alternative to systems based on formatted diode bar pump sources.
Single-mode (SM) fiber lasers and amplifiers are constrained to low output powers by fundamental physical limitations of the fiber, specifically, by low energy storage and by the onset of nonlinear processes in the fiber. The simplest way to overcome both limiting factors is to increase the core size, but maintaining SM operation imposes an upper limit. Further power scaling is possible with multimode (MM) fiber, but the poor beam quality generally associated MM fiber is unacceptable for most applications. We have developed a technique (bend-loss-induced mode filtering) that allows the core size to be increased significantly beyond the SM limit while maintaining diffraction-limited beam quality and high efficiency. In this method, coiling of the fiber is used as a form of distributed spatial filtering to suppress all but the fundamental mode of a highly MM fiber amplifier. Unlike conventional spatial filtering, in which high-order modes are discarded, the mode-filtering technique does not result is substantial loss of efficiency because high-order modes are suppressed along the entire length of the amplifier and are thus prevented from building up significant intensity. We will review this method and recent experimental results for both cw and pulsed fiber sources, including nonlinear frequency conversion of mode-filtered fiber lasers. Optical damage issues will also be discussed.
Single-mode (SM) fiber lasers and amplifiers are constrained to low output powers by fundamental physical limitations of the fiber, specifically, by low energy storage and by the onset of nonlinear processes in the fiber. The simplest way to overcome both limiting factors is to increase the core size, but maintaining SM operation imposes an upper limit. Further power scaling is possible with multimode (MM) fiber, but the poor beam quality generally associated MM fiber is unacceptable for many applications. We have developed a technique (bend-loss-induced mode filtering) that allows the core size to be increased significantly beyond the SM limit while maintaining diffraction-limited beam quality and high efficiency. In this method, coiling of the fiber is used as a form of distributed spatial filtering to suppress all but the fundamental mode of a highly MM fiber amplifier. Unlike conventional spatial filtering, in which high-order modes are discarded, the mode-filtering technique does not result is substantial loss of efficiency because high-order modes are suppressed along the entire length of the amplifier and are thus prevented from building up significant intensity. We will review this method and recent experimental results for both cw and pulsed fiber sources, including nonlinear frequency conversion of mode-filtered fiber lasers. Optical damage issues will also be discussed.
We report on the effectiveness of bend-loss mode filtering for power scaling of large-core, rare-earth-doped fibers. The technique affords a diffraction-limited beam quality from multimode fibers, which is crucial for practical applications of fiber sources. We demonstrate the technique in a >300-kW-peak-power, <1-ns-pulse-duration Yb-doped fiber amplifier, which we used for multiple harmonic generation.
We have demonstrated operation of an optical parametric oscillator (OPO) pumped by a coiled multi-mode fiber amplifier. Coiling of multi-mode fiber amplifiers is a simple technique that allows the output from diode-pumped fibers to be substantially increased relative to that from a standard single-mode fiber, while maintaining the diffraction-limited beam quality. Pulse energies from such fiber amplifiers are high enough to drive nonlinear optical processes. Here we report efficient frequency down-shifting of such a device using an OPO.
A diode-pumped Yb-doped fiber was used to amplify 1 microjoule, 20 nanosecond pulses from a 1.064μm Nd:YVO4 laser to a level of up to 107 microjoules in a single transverse mode, polarized beam. The 6m-length fiber had a 200 micron diameter cladding and a 25 micron core. An unpolarized output of 5.6 Watts was measured for 13 Watts of 976nm diode pump input. Oscillation of a periodically-poled lithium niobate (PPLN) OPO was observed with an oscillation threshold pump pulse energy of 31 microjoules. Up to 0.63 Watts of 1.5μm OPO signal output was measured with 2.6 Watts of incident pump, at a repetition rate of 24kHz. Tuning of the OPO signal wavelength was performed between 1.5 and 1.6μm.
A 1550 nm eye-safe, free-space optical communications link has been demonstrated at rates up to 5 Mbps over a distance of two kilometers in the Chesapeake Bay, using quantum-well-based modulating retro-reflectors. In addition to waveform studies, video and audio transmission has also been carried out, as well as bit error rate measurements. Tests have been conducted under various atmospheric conditions over a time period of eight months.
We have generated the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser. The fiber amplifier employed multimode fiber (25 μm core diameter, V ~ 7.4) to provide high-peak-power pulses, but diffraction-limited beam quality was obtained by use of bend-loss-induced mode filtering. The amplifier output had a pulse duration of 0.97 ns and smooth, transform-limited temporal and spectral profiles (~500 MHz linewidth). We obtained high nonlinear conversion efficiencies using a simple optical arrangement and critically phase-matched crystals. Starting with 320 mW of average power at 1064 nm (86 µJ per pulse at a 3.7 kHz repetition rate), we generated 160 mW at 532 nm, 38 mW at 355 nm, 69 mW at 266 nm, and 18 mW at 213 nm. The experimental results are in excellent agreement with calculations. Significantly higher visible and UV powers will be possible by operating the fiber amplifier at higher repetition rates and pulse energies and by further optimizing the nonlinear conversion scheme.
The Naval Research Laboratory (NRL) has established a free-space laser communication link across 16.2 km of the Chesapeake Bay between the Chesapeake Bay Detachment of NRL and Tilghman Island. The transmitter consists of a modulated 1550 nm oscillator amplified to 2 watts in an erbium doped fiber amplifier developed at NRL. The beam is fiber coupled to a 4 inch collimating lens on a remotely controllable gimbal mount. The beam is transmitted to a retro-reflector array at Tilghman Island and back to the receiver at CBD (32.4 km round trip). The receiver consists of a 16" Meade telescope either directly or fiber coupled to a variety of fast photo-detectors. Experiments have been conducted to study the stability and quality of the link. These include: bit-error rate measurements, probability density functions, power spectrum densities, and angle of arrival measurements of the received signal. Results of these experiments are presented.
The development of compact mid-IR sources using frequency- converted diode lasers has been demonstrated to be applicable for the ultra sensitive, selective, and real time detection of many trace gas species in the infrared spectroscopic fingerprint region, which contains virtually all the fundamental vibrational modes of molecules. This development of infrared laser sources has taken advantage of recent significant technological advances of semiconductor diode lasers and solid state lasers, new nonlinear optical materials, optical fiber and novel data acquisition techniques. Such sensors are able to detect molecules at the parts-per-billion level in ambient air using infrared absorption spectroscopy either by monitoring trace gases in an open path or multi-pass cell configuration. Real world applications ranging from urban, industrial, rural emission studies to spacecraft habitat monitoring are described.
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