Mikko Söderlund

Measuring photodarkening from single-mode ytterbium doped silica fibers

Photodarkening is recognized as a potentially important limiting factor on the lifetime and reliability of many Yb-doped fiber lasers and amplifiers. In particular, a photodarkening process attributed to the formation of photoinduced structural transformations can induce excess loss in the doped glass core of the fiber, resulting in reduced output power efficiency. Yet, quantifiable measurement techniques of this phenomenon have been scarce in the literature to date. Here we present a fast, simple and repeatable method to measure and compare the photodarkening rate caused by the formation of photoinduced structural transformations from Yb-doped single-mode fibers. The method relies on quantifying observations of transmission changes at visible wavelengths as an indicative measure of photodarkening at the signal wavelengths. Preliminary measurement results are presented supporting the utility of the technique for benchmarking the photodarkening behavior of different Yb-doped fibers.

Photodarkening rate in Yb-doped silica fibers.

Yb-doped fibers are widely used in laser applications requiring high average output powers and high-peak-power pulse amplification. Photodarkening (PD) is recognized as one limiting factor in these fibers when pumped with high-intensity radiation. We describe an approach for performing quantitative PD studies of fibers, and we present measurements of the rate of PD in Yb-doped single-mode fibers with varying inversion levels. The method is applicable to large-mode-area fibers. We observed a seventh-order dependence of the PD rate on the excited-state Yb concentration for two different fibers; this result implies that PD of a Yb-doped fiber source fabricated using a particular fiber will be strongly dependent on the configuration of the device.

Photodarkening Measurements in Large-Mode-Area Fibers

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.

Ytterbium-doped fibers fabricated with atomic layer deposition method

We report on a new fabrication method of producing ytterbium doped fibers by atomic layer deposition (ALD) in combination with the conventional modified chemical vapor deposition (MCVD) technique. An MCVD soot-preform with a porous layer of SiO(2) is coated with layers of Yb(2)O(3) and Al(2)O(3) prior to sintering, using the gas-phase ALD method. An SEM/EDS material analysis study shows that the dopants successfully penetrate the full thickness of 320 µm of the soot layer. An Yb-doped fiber fabricated by this technique shows a background attenuation of 20 dB/km, a uniform longitudinal Yb-doping profile, and good laser characteristics with a slope efficiency of 80%. Furthermore, we present a comparison in terms of photodarkening between the MCVD-ALD fiber and a solution doped fiber, fabricated with the same MCVD recipe. The new MCVD-ALD fiber appears to be more photodarkening resistant.

Heat-induced darkening and spectral broadening in photodarkened ytterbium-doped fiber under thermal cycling

We study thermal bleaching of photodarkening-induced loss in a 20-microm core diameter, large-mode-area ytterbium-doped silica fiber. Pristine and photodarkened samples are subjected to thermal cycling pulses. Recovery of the photodarkened fiber absorption coefficient initiates at approximately 350 degrees C and complete recovery is reached at approximately 625 degrees C. However, prior to recovery, the photodarkened fiber exhibits further heat-induced increase of absorption loss. This increase of loss is attributed to both a permanent increase of loss-inducing color centers and a temperature-dependent broadening of the absorption spectrum. Post-irradiation heat-induced formation of color centers suggests the presence of an intermediate energy state in the near-infrared photochemical mechanism for photodarkening.

Thermal bleaching of photodarkening-induced loss in ytterbium-doped fibers.

We study the thermal bleaching process of photodarkening-induced loss in ytterbium-doped fibers. The observed power-law time dependence of the normalized absorption coefficient is well represented by the thermal decay model of Erdogan et al. [J. Appl. Phys. 76, 73 (1994)]. Using this model, we derive the activation energy associated with thermal recovery of the induced loss to prephotodarkened state. For the studied commercial 20 microm core diameter large-mode-area fiber, the energy distribution consists of a single peak, located at 1.32 eV with a FWHM of approximately 0.31 eV. The results presented are of particular importance to understanding photodarkening and bleaching processes in high-power fiber lasers.

Mode-induced transverse photodarkening loss variations in large-mode-area ytterbium doped silica fibers

Transverse photodarkening loss variations in an LMA Yb-DCF are experimentally studied. Photodarkening rate depends on inversion, which is affected by the pump induced increase and signal induced depletion of the inversion. In double-clad fibers, intensity distributions of the pump and signal modes and their overlap with the core are significantly different, leading to transverse differences in inversion within the core. Moreover, practical fiber laser configurations aim at generating and preserving only the fundamental transverse-mode thus creating a high contrast in inversion within the core. Therefore, dramatically different rates of photodarkening across the core of the active fiber can be expected. We demonstrate the existence of transverse mode-induced photodarkening loss variations in an LMA Yb-DCF laser and discuss its implications. Composition-related transverse photodarkening loss variations are measured to be negligible in the studied Yb-DCF.

Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber

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.

Deterministic nanosecond laser-induced breakdown thresholds in pure and Yb3+ doped fused silica

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 found none.

Benchmarking and measuring photodarkening in Yb doped fibers

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.