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The first observation of cooling by anti-Stokes pumping in nanoparticle-doped silica fibers is reported. Four Yb-doped fibers fabricated using conventional modified chemical vapor deposition (MCVD) techniques were evaluated, namely, an aluminosilicate fiber and three fibers in which the Yb ions were encapsulated in CaF2, SrF2, or BaF2 nanoparticles. The nanoparticles, which oxidize during preform processing, provide a modified chemical environment for the Yb3+ ions that is beneficial to cooling. When pumped at the near-optimum cooling wavelength of 1040 nm at atmospheric pressure, the fibers experienced a maximum measured temperature drop of 20.5 mK (aluminosilicate fiber), 26.2 mK (CaF2 fiber), and 16.7 mK (SrF2 fiber). The BaF2 fiber did not cool but warmed slightly. The three fibers that cooled had a cooling efficiency comparable to that of the best previously reported Yb-doped silica fiber that cooled. Data analysis shows that this efficiency is explained by the fibers’ high critical quenching concentration and low residual absorptive loss (linked to sub-ppm OH contamination). This study demonstrates the large untapped potential of nanoparticle doping in the current search for silicate compositions that produce optimum anti-Stokes cooling.

Single-frequency lasers are essential for high-resolution spectroscopy and sensing applications as they combine high-frequency stability with low noise and high output power stability. For many of these applications, there is increasing interest in power-scaling single-frequency sources, both in the near-infrared and visible spectral range. We report the second-harmonic generation of 670 µJ at 532 nm of a single-frequency fiber amplifier signal operating in the quasi-continuous-wave mode in a 10-mm periodically poled Mg-doped lithium niobate (MgO:PPLN) crystal, while increasing compactness. To the best of our knowledge, this is the highest pulse energy generated in this crystal, which may find applications in the visible and UV such as remote Raman spectroscopy.

In recent years, there has been a growing interest towards compact high peak-power pulsed laser sources for applications such as LIDAR, range findings, remote sensing, communications and material processing. A common laser architecture used to realize these sources is the Master Oscillator Power Amplifier (MOPA), in which a master oscillator produces a highly coherent beam and a fiber amplifier boosts the output power, while preserving its main spectral properties. Phosphate glasses are recognized to be an ideal host material for engineering the amplification stage of a pulsed MOPA since they enable extremely high doping levels of rare-earth ions to be incorporated in the glass matrix without clustering, thus allowing the fabrication of compact active devices with high gain per unit length. With the aim of realizing compact optical fiber amplifiers operating at 1 and 1.5 µm, a series of highly Yb3+- and Yb3+/Er3+-doped custom phosphate glass compositions were designed and fabricated to be used as active materials for the core of the amplifiers. Suitable cladding glass compositions were explored and final core/cladding glass pairs were selected to realize single-mode and multi-mode optical fibers. Core and cladding glasses were synthesized by melt-quenching technique. The core glass was then cast into a cylindrical mold to form a rod, while the cladding glass was shaped into a tube by rotational casting method or extrusion technique. The latter has been extensively employed for the manufacturing of tellurite and germanate glass preforms, but only recently the first example of active phosphate fiber preform fabricated by this method has been reported by our research team. Phosphate fibers were then manufactured by preform drawing, with the preform being obtained by the rod-in-tube technique. Preliminary results of pulsed optical amplification at 1 and 1.5 µm are presented for a single-stage MOPA.

We will report the first demonstration of an optically cooled fiber amplifier. The fiber was made of silica, had a core doped with Yb, and was core-pumped to achieve both gain and cooling in the core via anti-Stokes fluorescence. Gains larger than 10 dB were measured while maintaining a negative average temperature change along the fiber.

We report what we believe to be the first radiation-balanced fiber amplifier - a device that provides optical gain while experiencing no temperature rise. The gain medium is a silica fiber with a 21 um-diameter core highly doped with Yb3+ (2.52 wt.%) and co doped with 2.00 wt.% Al to reduce concentration quenching. The amplifier was core-pumped with 1040 nm light to create anti-Stokes fluorescence (ASF) cooling and gain in the core at 1064 nm. Using a custom slow-light FBG sensor with mK resolution, temperature measurements were performed at multiple locations along the amplifier fiber. A 4.35-m fiber pumped with 2.62 W produced 17 dB of gain while the average fiber temperature remained slightly below room temperature. This advancement is a fundamental step toward the creation of ultra-stable lasers necessary to many applications, especially low-noise sensing and high-precision metrology., 8 pages including the supplementary material (SM). 5 figures in the main text, 2 in the SM

We report on a single-frequency fiber master oscillator power amplifier utilizing a polarization-maintaining step-index fiber with an Al/Ce/F core-glass composition doped with a very high Yb concentration (0.25 at.%). This design made it possible to use a very short fiber (~1 m) and to coil it in a tight radius (4 cm in the amplifier, while 2 cm gave similarly negligible bending loss) so that the packaged system is one of the most compact reported to date (~0.6 L). The use of a short fiber increased the threshold for stimulated Brillouin scattering well above 100 W while maintaining near-ideal beam quality. The fiber was pumped with a diode-pumped solid-state laser and cooled passively by spooling it on a grooved aluminum mandrel. The amplifier produced a strongly linearly polarized output at 1064 nm in the fundamental mode (M2 ≤ 1.2) with a 150 kHz linewidth and a power of 81.5 W for 107 W of launched pump power. No deleterious effects from the elevated thermal load were observed. The residual photodarkening loss resulting from the high Yb concentration, found to be small (~0.7 dB/m inferred at 1064 nm) with accelerated aging, reduced the output power by only ~20% after 150 h of operation.

In optically pumped lasers, heat generated by the quantum defect causes detrimental fluctuations in the output mode, frequency, and power. Common heat-mitigation techniques use bulky mechanical coolers that introduce vibrations, leading to laser frequency and amplitude noise. Here, we present a radiation-balanced fiber laser, optically cooled by anti-Stokes fluorescence (ASF). The gain medium is a silica fiber with a 21-µm-diameter core doped with 2.06 wt. % Y b 3 + and co-doped with A l 2 O 3 and F- to reduce concentration quenching. The laser was core-pumped at 1040 nm to create both gain at 1065 nm and ASF cooling at atmospheric pressure. We demonstrate a maximum output power of 114 mW with a slope efficiency of 41% while maintaining near-zero average temperature change. This result could enable the development of fiber lasers with unprecedented coherence and stability.

Laser cooling in silica has recently been demonstrated, but there is still a lack of understanding on how fiber composition, core size, and O H − contamination influence cooling performance. In this work, six Yb-doped silica fibers were studied to illuminate the influence of these parameters. The best fiber cooled by − 70 m K with only 170 mW/m of absorbed pump power at 1040 nm, which corresponds to twice as much heat extracted per unit length compared to the first reported laser cooling in silica. This new fiber has an extremely low O H − loss and a higher Al concentration (2.0 wt.% Al), permitting a high Yb concentration (2.52 wt.% Yb) without incurring significant quenching. Strong correlations were found between the absorptive loss responsible for heating and the loss measured at 1380 nm due to absorption by O H − .

Anti-Stokes fluorescence cooling in a silica-based fiber is reported for the first time. The fiber had a core with a 20-μm diameter doped with 2.06 wt.% Yb and co-doped with 0.86 wt.% Al and 0.88 wt.% F. Core-pumping the fiber with 1040- nm light, temperature changes as large at -50 mK were measured at atmospheric pressure. Temperature measurements were performed at 12 pump wavelengths, and the measured dependence of the temperature change as a function of pump wavelength was in excellent agreement with a previously reported model. With this model, the absorptive loss in the fiber was inferred to be less than 15 dB/km, and the critical quenching concentration to be ~15.6 wt.% Yb. This combination of low loss and high quenching concentration (a factor of 16 times higher than the highest reported values for Yb-doped silica) is what allowed the observation of cooling. The temperature measurements were performed at atmospheric pressure using a custom slow-light fiber Bragg grating sensor with an improved thermal contact between the test fiber and the FBG. The improved method involves isopropanol to establish a good thermal contact between the two fibers. This eliminated a source of heating and enabled more accurate measurements of the cooled-fiber temperature. This improved temperaturemeasurement set-up also led to a new cooling record in a multimode Yb-doped ZBLAN fiber at atmospheric pressure. When pumped at 1030 nm, the fiber cooled by -3.5 K, a factor of 5.4 times higher than the previous record.

High-power, single-frequency, pulsed fiber amplifiers are required in light detection and ranging, coherent laser detection, and remote sensing applications to reach long range within a short acquisition time. However, the power-scaling of these amplifiers is limited by nonlinearities generated in the optical fibers, in particular by stimulated Brillouin scattering (SBS). In this regard, the use of multicomponent phosphate glasses maximizes the energy extraction and minimizes nonlinearities. Here, we present the development of a single-stage, hybrid, pulsed fiber amplifier using a custom-made multicomponent Yb-doped phosphate fiber. The performance of the phosphate fiber was compared to a commercial Yb-doped silica fiber. While the latter showed SBS limitation at nearly 6.5 kW for 40 cm length, the maximum achieved output peak power for the multicomponent Yb-doped phosphate fiber was 11.7 kW for 9 ns pulses using only 20 cm with no sign of SBS.

Metallic nanowire-based transparent electrodes (TEs) are potential alternatives to indium tin oxide (ITO). To achieve a high performance [sheet resistance (Rs) 90%]...

The performance of optical fibers is dependent on both the fiber design and the materials from which it is made. While much of the development over the past few decades has focused on fiber geometry and microstructuring, more recent analyses have shown clear benefits of addressing parasitic nonlinearities at the origins of their light–matter interactions. Reported here are results on intrinsically low Brillouin and thermo-optic core fibers, fabricated using modified chemical vapor deposition. Specifically, fibers in the Yb-doped A l 2 O 3 − P 2 O 5 − B 2 O 3 − S i O 2 system are developed based on how each glass constituent affects the material parameters that enable both stimulated Brillouin scattering (SBS) and transverse mode instability (TMI). One fiber, developed to be very heavily doped, exhibited thermo-optic and Brillouin gain coefficients up to ∼ 3 d B and 6 dB below conventional laser fibers, respectively. A second fiber, designed to approximate a commercial double-clad laser fiber, which necessitated lower doping levels, was output power scaled to over 1 kW with an efficiency over 70% and no observed photodarkening under conventional testing. Design curves for the enabling material properties that drive TMI and SBS also are provided as functions of compositions as a tool for the community to further study and develop intrinsically low-nonlinearity fiber lasers.

Interest in compact, single-frequency fiber amplifiers has increased within many scientific and industrial applications. The main challenge is the onset of nonlinear effects, which limit their power scaling. Here we demonstrate a compact, highpower, single-frequency, polarization-maintaining, continuous-wave fiber amplifier using only one amplification stage. We developed the fiber amplifier using a master oscillator fiber amplifier architecture, where a low-noise, singlefrequency, solid-state laser operating at 1064 nm was used as a seed source. We evaluated the amplifier's performance by using several state-of-the-art, small-core, Ytterbium (Yb)-doped fibers, as well as an in-house-made, highly Yb-doped fiber. An output power of 82 W was achieved with no sign of stimulated Brillouin scattering. A good beam quality and a polarization extinction ratio (PER) of < 25 dB were achieved. The compact fiber amplifier can be a competitive alternative to multi stage designed fiber amplifiers.

Ytterbium (Yb) doped silica fibers are widely used in high power fiber lasers where co-doping the silica core material with other elements is pivotal for high efficiency, low detrimental effects, and reliable optical properties. Aluminum (Al) is one of the most preferred co-dopants, yet, purely Yb/Al-doped silica fibers suffer from relatively high levels of photodarkening (PD) when used in laser applications. A slightly improved PD-resistance has been reported for increasing Al-doping concentration. However, the source of this improved performance is still unknown. In this article, we present the origin of the improved PD-resistance observed in Yb-doped silica fibers with high Al-concentration. It is found that a high Al-co-doping concentration reduces the interaction strength between the Yb-ion and nearby oxygen ions, resulting in a significant PD-resistance with negligible induced loss in the entire visible spectral range. A negligible PD is observed even for significantly higher Yb-concentrations than commonly used in commercially available Yb-doped silica fibers.

Transparent electrodes (TEs) made of metallic nanowires, such as Ag, Au, Cu, and Ni, have attracted rising attention for several reasons: 1) they can act as a substitute for tin oxide-based TEs such as indium-tin oxide (ITO) and fluorine-doped tin oxide (FTO); 2) various methods exist for fabricating such TEs such as filtration, spraying and meyer bar coating; 3) greater compatibility with different substrates can be achieved due to the variety of fabrication methods; and 4) extra functions in addition to serving as electrodes, such as catalytic abilities, can be obtained due to the metals that compose the TEs. There are a large number of applications for TEs, ranging from electronics and sensors to biomedical devices. This short review is a summary of recent progress, mainly during the past five years, on silver nanowire-based TEs. The focus of the review will be on theory development, mechanical, chemical and thermal stability and optical properties. The many applications of TEs are outside the scope of this review., 9 pages, 10 figures, review

Environmental monitoring of land, water and air, is an area receiving greater attention because of human health and safety concerns. Monitoring the type of pollution and concentration levels is vital, so that appropriate contingency plans can be determined. To effectively monitor the environment, there is a need for new sensors and sensor systems that suits these type of measurements. However, the diversity of sensors suitable for low, battery powered- and large area sensor systems are limited. We have manufactured and characterized a flexible pH sensor using laser processing and blade coating techniques that is able to measure pH between 2.94 and 11.80. The sensor consists of an interdigital capacitance with a pH sensitive hydrogel coating. Thin sensors can reach 95% of their final value value within 3 min, and are stable after 4 min. Good repeatability was achieved in regard to cycling of the sensor with different pH and multiple measurements from dry state. We have also studied the relation between an interdigital capacitance penetration depth and hydrogels expansion. We believe that our passive sensor is suitable to be used in low power and large area sensor networks.

Development of cost-effective and environmentally friendly manufacturing methods will enable important advances for the production of large-scale flexible electronics. Laser processing has shown to be a promising candidate that offers a fast and non-destructive way to produce highly conductive patterns on flexible substrates such as plastics. However, an emerging option with a lower environmental impact is instead the use of cellulose-based flexible substrates, such as paper. In this work we investigate the use of laser sintering of silver nanoparticle inks, which were inkjet-printed on three different types of paper. Patterns with a high conductivity could be manufactured where a special care was taken to prevent the substrates from damage by the intense laser light. We found that the best results was obtained for a photopaper, with a conductivity of 1.63 ∗ 107 S/m corresponding to nearly 26% of the bulk silver conductivity. In addition, we demonstrate laser sintering to fabricate a fully functional near field communication tag printed on a photopaper. Our results can have an important bearing for the development of cost-effective and environmentally friendly production methods for flexible electronics on a large scale.

Networks of metallic nanowires are promising candidates for transparent and flexible electrodes, as alternatives to metal oxide based electrodes such as indium tin oxide (ITO). Such nanowire electrodes can have excellent performance, however, the mechanical and chemical stability of the electrodes are usually week. Here, we report a simple method for fabricating transparent and flexible gold electrodes that have excellent mechanical and chemical stability. Gold nanoparticles were synthesized and subsequently aggregated by adding sodium hydroxide into the solution. The aggregated nanoparticles were deposited on a polycarbonate (PC) filter membrane followed by flash light sintering (FLS). By using this method, transparent and flexible gold electrodes were produced, with electric properties found to be stable under mechanical treatment such as folding, peeling and rubbing and under chemical treatment such as water, ethanol, nitric acid and sodium hydroxide. A nanosecond pulsed (7-9 ns), solid state laser (SSL) at 355 nm coupled to a galvo-scanning mirror system was used for laser assisted processing (patterning and ablation) of the gold nanoparticle thin films to further increase the transparency. By careful evaluation of laser (pulse energy, repetition rate etc.) and scanning parameters (speed, pitch etc.) a resolution better than 10 um could be realized by this laser scanning system, which are used to optimize the transmission and conductivity of the FLS gold nanoparticle layer. It is found that this method have potential to produce mechanically and chemically stable electrodes with transmittance over 90% and sheet resistance less than 100 Ohm/sq.

Long and short path length differences interferometric sensing modalities have been combined based on immobilizing hydrogel on thin-core optical fiber end face. Dual parameter sensing of hydrogel swelling and refractive index was demonstrated.

MoS2 has been studied intensively during recent years as a semiconducting material in several fields, including optoelectronics, for applications such as solar cells and phototransistors. The photoresponse mechanisms of MoS2 have been discussed but are not fully understood, especially the phenomenon in which the photocurrent slowly increases. Here, we report on a study of the photoresponse flash-light-processed MoS2 films of different thicknesses and areas. The photoresponse of such films under different light intensities and bias voltages was measured, showing significant current changes with a quick response followed by a slow one upon exposure to pulsed light. Our in-depth study suggested that the slow response was due to the photothermal effect that heats the MoS2; this hypothesis was supported by the resistivity change at different temperatures. The results obtained from MoS2 films with various thicknesses indicated that the minority-carrier diffusion length was 1.36 µm. This study explained the mechanism of the slow response of the MoS2 film and determined the effective thickness of MoS2 for a photoresponse to occur. The method used here for fabricating MoS2 films could be used for fabricating optoelectronic devices due to its simplicity.

A new concept for dual parameter fiber optic sensor has been developed and characterized, both in a two-fiber and single-fiber configuration. The liquid sensor measures ethanol concentration with a stimuli responsive hydrogel which constitutes a low finesse Fabry-Perot cavity, and refractive index with a Mach-Zehnder type fiber optic interferometer. The two-fiber configuration utilize a fiber optic 1310/1550 nm wavelength division multiplexer to separate the two parameters, while a versatile filtering algorithm extracts and separates the two parameters in the single-fiber configuration. No cross talk were observed for the measured parameters of free spectral range and refractive index for the two-fiber configurations, while for the single fiber configuration cross-talk were observed. The two configurations prove to be versatile dual parametric fiber optic sensor concepts for accurate detection of specific parameters, based on stimuli responsive hydrogels. © 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

The sensing of body motions is of great importance in areas such as healthcare, rehabilitation, and human-computer interactions. Different methods have been developed based on visual or electrical signals. However, such signals are acquired by external devices and are not intrinsic signals that are created on the body. Here, we report a new universal body motion sensor (UBS) to detect motions based on the intrinsic contact electrification (CE) of the skin or electrical induction (EI) of the body. The CE or EI generates charges on the body, leading to potential differences between the body and ground that can be measured to identify different body motions, such as motions of the head, arms, fingers, waist, legs, feet and toes. Proof-of-concept experiments have demonstrated that the UBS can be used to monitor the conditions of people with Parkinson's disease (PD) and to quantitatively monitor the recovery of those with a leg injury, suggesting great potential for healthcare applications.

In this work we present a promising method for fabrication of conductive tracks on paper based substrates by laser assisted reduction of Graphene Oxide (GO). Printed electronics on paper based substrates is be coming more popular due to lower cost and recyclability. Fabrication of conductive tracks is of great importance where metal, carbon and polymer inks are commonly used. An emerging option is reduced graphene oxide (r-GO), which can be a good conductor. Here we have evaluated reduction of GO by using a 532 nm laser source, showing promising results with a decrease of sheet resistance from >100 M Ω/Sqr for unreduced GO down to 126 Ω/Sqr. without any observable damage to the paper substrates.

In this work we present an ink-jet printed temperature sensor consisting of PEDOT:PSS printed on paper suitable for packaging, flexible electronics and other printed applications. The substrate showed to have a large influence on both the resistance as well as the temperature sensitivity of the PEDOT:PSS ink. This effect is most likely due to NaCl content in the photo paper coating, which reacts with the PEDOT:PSS. The temperature coefficient of a prepared device of α = −0.030 relative to room temperature (22oC) was measured, which is higher than compared to for example Silicon α = −0.075.

In this work we have investigated the use of laser sintering of different ink-jet printed nano-particle links (NPIs) on paper substrates. Laser sintering is shown to offer a fast and non-destructive way to produce paper based printed electronics. A continuous wave fiber laser source at 1064 nm is used and evaluated in combination with a galvo-scanning mirror system. A conductivity in order of 2.16 * 107 S/m is reached for the silver NPI structures corresponding to nearly 35 % conductivity compared to that of bulk silver and this is achieved without any observable damage to the paper substrate.

A structural investigation of a synthesized precursor in a silica glass matrix is performed. Silica soot samples are doped with the heterobimetallic precursor ErNb2(OPri)13 by using a conventional solution doping technique and heat treatments to different temperatures. The precursor has also been introduced into a silica fiber preform by using the modified chemical vapor deposition technique. Analyses are made by using ultraviolet–visible–near infrared absorption spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and powder X-ray diffraction. It is concluded that an immiscible system of ErNbO4 crystallites and Nb2O5 is formed in the silica soot samples at high temperatures. Colloidal particles of ErNbO4 are also formed in the silica glass fiber preform showing interesting features.

Celestial mechanical simulations from a purely classical point of view of the whole solar system, including our Moon and the Mars moons { Phobos and Deimos { are carried out for 2 millions of years before present. Within the classical approximation, the results are derived at a very high level of accuracy. Eects from general relativity for a number of variables are investigated and found to be small. For climatic studies of about 1 Myr, general relativity can safely be ignored. Three dierent and independent integrations schemes are used in order to exclude numerical anomalies. The converged results from all methods are found to be in complete agreement. For verication, a number of properties such as spin axis precession, nutation, and orbit inclination for Earth and Mars have been calculated. Times and positions of equinoxes and solstices are continously monitored. As also observed earlier, the obliquity of the Earth is stabilized by the Moon. On the other hand, the obliquity of Mars shows dramatic variations. Climatic influences due to celestial variables for the Earth and Mars are studied. Instead of using mean insolation as in the usual applications of Milankovitch theory, the present approach focuses on the instantaneous solar radiation power (insolation) at each summer solstice. Solar radiation power is compared to the derivative of the icevolume and these quantities are found to be in excellent agreement. Orbital precessions for the inner planets are studied as well. In the case of Mercury, it is investigated in detail.

In this work we present a compact, nanosecond pulsed, single frequency, single stage Yb-doped fiber amplifier by using an overall fiber core diameter of 20 μm. The key component is a custom made, compact, ultra-low noise, single frequency ring-cavity solid state laser (SSL) at 1064 nm used as a master oscillator. The SSL can be designed to provide nanosecond pulses with pulse energies in the sub-mJ range. Our ultimate goal is to develop a compact linearly polarized, single frequency, nanosecond pulsed laser source in an all-fiber format. Short (less than 1m), highly Yb-doped fibers have been used in order to suppress non-linear effects.

Excited state absorption (ESA) measurements performed on Yb-doped silica bers show the onset of a strong absorption band in the visible range. In this work, we perform experiments to investigate the possibility for ESA to be part of the induced optical losses (photodarkening) observed in Yb-doped ber lasers. Our results indicate that an ESA process, from the 2F5/2 excited state manifold in the Yb3+ ion to the charge-transfer state with absorption bands in the UV range, may constitute a transfer route for pump- and laser photons in the near-infrared range. © 2013 Copyright SPIE.

Photodarkening spectra were studied for Yb-fibers co-doped with Al, Al/Ce or Al with H2-loading and it is proposed that the pump-induced color centers in Yb/Al-doped silica fibers mainly consist of Yb2+ and AlOHC.

UV-induced absorption is investigated for three different ytterbium doped laser materials. It is found that the induced transmission loss originates from the formation of Yb2+-ions and hole-related defects.

We have recently shown that the photodarkening (PD) resistivity in Yb/Al-doped fibers can be greatly improved by adding cerium (CE) to the core glass composition. We are now further investigating the laser performance and logn term stability of Yb/Ce/Al fibers in high inversion applications such as 980nm lasers and amplifiers. Our objective is to study the limitations of Yb/Ce/Al fibers and elaborate on their potential of becoming the next generation of fibers for high power fiber lasers and applifiers.

A linearly polarized fiber laser emitting at 979 nm with an output power of 2 W is presented. By using a photodarkening-resistant Yb/Ce/Al-codoped silica fiber, degradation-free operation is achieved over several hours.

We show that the photodarkening resistivity of ytterbium-doped fiber lasers can be greatly improved by cerium codoping. It is suggested that the coexistence of the redox couple Ce(3+)/Ce(4+) in the glass provides means for trapping both hole- and electron-related color centers that are responsible for the induced optical losses in Yb-doped fiber lasers.

In this paper we present how charge transfer processes influences the induced optical losses (photodarkening) in ytterbium doped fiber lasers. The location of the charge transfer absorption band is strongly composition dependent and is correlated to the valence stability of the ytterbium ion in the silicate glass matrix. An improved photodarkening performance can in general be observed for a charge-transfer band shifted to shorter wavelengths, although other routes are also possible to reduce photodarkening. Other parameters that affect the laser performance, such as absorption and emission cross section, must also be considered.

We present an overview of quenching processes in Yb-doped lasers. Experiments made on Yb:YAG crystals and Yb-doped fiber lasers show that induced losses appear upon UV-irradiation through a charge-transfer process. The valence stability of the Yb ion is believed to be the key issue for the quenching processes in Yb-doped high power laser systems.

We present some strong evidence against the assignment of an ytterbium-related oxygen deficiency center (ODC) near 230 nm in Yb-doped aluminosilicate glass in the Letter by Yoo [Opt. Lett.32, 1626 (2007)]. The strong 230 nm absorption band is actually a charge-transfer band corresponding to the transfer of an electron from nearby oxygen ligands to the ytterbium ion and not related to ODCs.

In this paper we will present paths to reduce the extent of photodarkening in ytterbium doped fiber lasers based on the aluminosilicate glass matrix. This will enable higher powers and longer operating lifetimes.