Atomic layer deposition (ALD) was used to fabricate an ytterbium (Yb)-doped silica fiber in combination with the
conventional modified chemical vapor deposition (MCVD) method. An MCVD soot-preform with a porous layer of SiO2
doped with GeO2 was coated with layers of Yb2O3 and Al2O3 prior to sintering, using the ALD method. ALD is a surface
controlled CVD-type process enabling thin film deposition over large substrates with good thickness control, uniformity
and high conformality. A materials analysis study showed that the dopants successfully penetrated the full thickness of
320 μm of the soot layer. Preliminary preform and fiber experiments on refractive index profiles, background losses,
lifetime and the characteristic gain-loss curve were performed demonstrating the potential of this method for fabricating
Yb-doped fibers with high concentration of dopants.
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.
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.
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.
Photodarkening is a detrimental phenomenon known to affect ytterbium doped fibers. Methods to study the spectral and
temporal properties of the photodarkening induced loss were developed. The spectral shape of the photodarkening loss
measured from multiple aluminosilicate samples indicate that visible wavelength(s) could be used in benchmarking
fibers for their PD induced loss. Two principal methods, core and cladding pumping, were introduced to induce a known
and repeatable inversion to fiber samples. The photodarkening rate could be parameterized using a single variable,
inversion. More generally, the photodarkening rate was found to follow a simple power law and to be proportional to
[Yb]7±1 (the excited state Yb ion density). Two methods, stretched exponential and bi-exponential, were used to fit the
rate measurements. Both fitting methods were found suitable, with the bi-exponential method having more potential in
increasing the understanding of the mechanism(s) behind photodarkening. Coiling induced spatial changes in the
inversion and subsequent photodarkening performance were demonstrated for a large-mode-area fiber laser.
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
Accuracy at which population inversion in an ytterbium-doped fiber can be determined by modeling is studied. Here inversion refers to the percentage of ions excited to a higher energy level by the various optical fields. Knowledge of rare-earth ion inversion is crucial for determining the photodarkening behavior of a fiber, but can also be used to study the gain and noise properties of amplifiers. Sample fibers are first characterized for their optical and mechanical properties (e.g. absorption and fluorescence spectrum, excited state lifetime, rare-earth concentration and geometrical dimensions). Fiber specific absorption and emission cross-sections are then derived from the measured fiber parameters. Two methods (i.e. McCumber theory and Fuchtbauer-Ladenburg relation) are used to determine the detailed shape and the relative level of absorption to emission at different wavelengths. A full numerical model is used to simulate both core and cladding pumped YDFs. In order to validate the inversion results produced by the simulator a comparison between the measured and simulated ASE spectra is made. Sensitivity of the simulated ASE spectrum on the different parameters is investigated. Uncertainty analysis is made to show the contributions of various measured parameters on the uncertainty of the inversion. The principal contributor of uncertainty on the inversion was found to be the cross-section values.
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
Development of photodarkening in two similar large-mode-area ytterbium doped fibers from different sources is compared. The excess loss induced by photodarkening is derived from transmission loss measurements of pristine and pumped or photodarkened samples. To accelerate the photodarkening process, cladding pumping is used so as to achieve high and uniform inversion through the sample. Further, intensity profiles are measured and compared in effort to detect
possible radial variations in the induced losses.
The objective of this work is to understand catastrophic optical damage in nanosecond
pulsed fiber amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064
µm, with 7.5-nsec pulse duration, focused to a 7.45-&mgr;m-radius spot in bulk fused silica.
Our bulk damage threshold irradiance is corrected to account for self focusing. The pulse
to pulse variation in the damage irradiance in pure silica is less than 1%. Damage is
nearly instantaneous, with an induction time much less than 1 ns. These observations are
consistent with an electron avalanche rate equation model, using reasonable rate
coefficients. The bulk optical breakdown threshold irradiance of pure fused silica is
5.0x1011 ±7% Watts/cm2. We also measured the surface damage threshold irradiance of
1% Yb3+ doped fused silica preform of Liekki Yb1200 fiber, and found it is equal to that
of pure silica within 2%.
The optical damage morphology is reproducible from pulse to pulse. To facilitate the
morphology study we developed a technique for locating the laser focus based on the
third harmonic signal generated at the air-fused silica interface. This gives a very small
uncertainty in focal position (~ 10 &mgr;m) which is important in interpreting the damage
structure. The surface third harmonic method was also used to determine the laser focus
spot size and verify beam quality.
Earlier reports have claimed that the damage irradiance depends strongly on the size
of the focal spot. We varied the focal volume to look for evidence of this effect, but
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.
The objective of this work is to understand catastrophic optical damage in nanosecond pulsed fiber
amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064 &mgr;m, with 7.5-nsec pulse
duration, focused to a 7.45-&mgr;m-radius spot inside a fused silica window, to measure the single shot optical
breakdown threshold irradiances of 4.7E11 and 6.4E11 W/cm2 respectively for pure fused silica, and for a
1% Yb3+ doped fused silica preform of Liekki's Yb1200 fiber. These irradiances have been corrected for
self focusing which reduced the area of the focal spot by 10% relative to its low field value. Pulse to pulse
variations in the damage irradiance in pure silica was >2%. The damage induction time appears to be much
less than 1 ns.
We found the damage morphology was reproducible from pulse to pulse. To facilitate our morphology
study we developed a technique for locating the position of the focal waist based on the third harmonic
signal generated at the air-fused silica interface. This gives a precise location of the focal position (± 10
&mgr;m) which is important in interpreting the damage structure. The surface third harmonic method was also
used to determine the diameter of the focal waist.
Earlier reports have claimed the damage irradiance depends strongly on the size of the focal waist. We
varied the waist size to look for evidence of this effect, but to date we have found none. We have also
studied the temporal structure of the broadband light emitted upon optical breakdown. We find it consists
of two pulses, a short one of 16 ns duration, and a long one of several hundred ns. The brightness, spectra,
and time profiles of the white light provide clues to the nature of the material modification.
Fiber lasers offer substantial advantages compared to conventional solid-state lasers due to their high efficiency,
compact size, diffraction-limited beam quality, tunability, and facile thermal management. A number of important
applications require high peak powers and pulse energies, which has generated great interest in Yb-doped, large-modearea
(LMA) fibers. Liekki has pioneered a new manufacturing technology for rare-earth-doped fibers, Direct
Nanoparticle Deposition (DND), that is capable of producing fibers uniquely well suited to power scaling.
Conventional fiber fabrication methods are characterized by poor process accuracy and flexibility due to the large
particle sizes and relatively small number of deposition layers (2-10). In contrast, DND provides independent control of
the composition of hundreds of layers that make up the core, thereby allowing previously unattainable precision,
accuracy, and uniformity in the index and rare-earth-dopant profiles. DND allows the simultaneous use of both gasphase
and liquid precursors, providing unprecedented flexibility in the glass composition. Furthermore, DND enables
fabrication of fibers with extremely high rare-earth concentrations, which minimizes the required fiber length and
correspondingly raises the threshold power for nonlinear processes. Finally, the single-step, direct-deposition process
makes manufacturing of fibers rapid and cost-effective, even for fibers with large core diameters or sophisticated
geometries and dopant distributions. DND fibers have shown high conversion efficiency (low clustering), low
photodarkening, and high damage threshold. DND thus promises to revolutionize the use of fiber lasers in applications
previously restricted to bulk, solid-state lasers and to enable new applications of high-power lasers.
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.
Ytterbium-doped fibers are widely used in applications requiring short fiber amplifiers for high peak power pulse amplification. One of the key challenges posed on the performance and reliability of such amplifiers is mitigating photodarkening of the active fiber. Photodarkening manifests itself as a temporal increase in broadband absorption centered at visible wavelengths, and varies on how the active fiber has been manufactured. The tail of the photodarkening absorption extends to the 1μm region, thus in some cases seriously degrading the fiber efficiency over time. Accurate measurement methods for characterization of photodarkening must be developed in order to better understand its causes and create techniques to eliminate it so as to secure widespread commercialization of reliable Yb-doped fibers. This paper presents a simple method to characterize photodarkening in both single-mode and double clad Yb-doped fibers. A short length of fiber is pumped using high brightness source in order to achieve high and uniform inversion. The high inversion speeds up the formation of the color centers to the high degradation level, thus reducing the analyzing time from weeks to few of hours. With this method, photodarkening can be measured even from relatively short fibers by monitoring loss at visible wavelengths, where the degradation is greatest. We have analyzed the repeatability of the measurement method against the pumping conditions and fiber sample properties. The impact of photodarkening in different applications is discussed. We present the results of recent optimization of Liekki Yb1200 product family and also compare these with some other commercially available fibers.
Large-mode-area double clad fibers offer excellent efficiency and beam quality, high output power as well as lightweight, robust and reliable packaging. The addition of polarization maintaining property though use of well-know Panda-structure has further increased the interest in double clad fibers, especially in fibers doped with ytterbium (Yb). Many material processing, military and R&D applications benefit from wavelength conversion by nonlinear effects, from IR through UV, of 1064nm Q-switched pulses through polarization maintaining large-mode-area double clad Yb-fiber amplifiers. The possibility of power scaling through coherent beam combining has also been identified by the military. The design of a polarization maintaining large modea area double clad fiber for the above mentioned applications must address several key performance parameters: provide large mode area (>300μm2), high efficiency (>80% slope PCE), high average power (>100W), high birefringence (>2*10-4) and offer good beam quality (M2 <1.5), short fiber length (<3m), as well as high reliability and good usability. Further optimization of the fiber design must take into consideration the impairment of the fiber by thermal loading as well as coiling of the fiber for elimination of higher order modes. This paper presents the key design considerations of such fibers for high-average-power pulsed amplifiers and provides the latest experimental techniques to verify the results. The design and results on high performance highly Yb-doped polarization maintaining large mode area fiber manufactured by the Direct Nanoparticle Deposition technology are presented and possibilities and opportunities brought by this technology are discussed.
Wavelength routing and reconfigurable cross connects are emerging concepts for optical multiwavelength telecommunication networks. They provide more efficient usage of the network resources, as individual wavelength channels can be added or dropped from the wavelength multiplex. However, problems arise with the erbium-doped fiber amplifiers (EDFAs), whose gain is dependent on the input power level. If the gain of the EDFA is not by some means controlled, transient effects will occur due to the EDFA's slow gain dynamics. In this paper a simple device for controlling the gain of the EDFA is studied. The gain- controlling scheme is based on a fast pump laser control. Part of the total input power to the amplifier is detected by the gain controlling circuitry, which then compensates for the changing gain by adjusting the pump laser power. In the study, transient effects due to changing number of channels in an EDFA are measured. The response time and the transient suppression of the gain-controlling device are verified through measurements. The effect of the amplifier gain tilt is also studied. A comparison against other proposed gain-controlling schemes is done.