We have discovered that in ytterbium-doped silica fibres the excited state lifetime of a fraction of the Yb ions can be quenched to a very small value, leading to a strong unbleachable loss. This unexpected behaviour seems to be caused by some, yet unidentified, impurity or structural defect. It is of considerable relevance for various Yb doped lasers and-amplifiers including Er:Yb codoped fibres as used in telecommunication amplifiers although it should also be emphasized that fibres can be produced that are free from the quenching effect

Barber, P.R.

Caplen, J.E.

Hanna, D.C.

Nilsson, J.

Paschotta, R.

Tropper, A.C.

English

Southampton (e-Prints Soton)

1Southampton SO17 1BJ,  UKTelephone: +44 1703 593150e-mail:   light@orc.soton.ac.ukOptoelectronics Research CentreUniversity of SouthamptonFig. 1. Absorption (solid) and emission (dotted)cross-sections (in 10  m ) of Yb  in germanosilicate-27 2   3+glass. Lifetime Quenching in Yb Doped FibresR.Paschotta, J.Nilsson, P.R.Barber, J.E.Caplen, A.C.Tropper and D.C.HannaAbstractWe have discovered that in ytterbium-doped silica fibres the excited state lifetime of a fraction of the Yb ionscan be quenched to a very small value, leading to a strong unbleachable loss. This unexpected behaviour seemsto be caused by some, yet unidentified, impurity or structural defect. It is of considerable relevance for variousYb doped lasers and amplifiers including Er:Yb codoped fibres as used in telecommunication amplifiersalthough it should also be emphasized that fibres can be produced that are free from the quenching effect. IntroductionYtterbium doped silica fibres offer a wide range of applications as fibre lasers and amplifiers [1].Gain is available between ~976nm and ~1200nm using pump wavelengths in the very wide rangefrom ~800nm toat least 1064nm, as available e.g. from AlGaAs and InGaAs diode lasers anNd:YAG lasers. Highly efficient wavelength conversion, high power operation in a claddingpumped scheme [2], and single-frequency lasing have all been demonstrated already with Ybdoped fibres [1]. The simple level structure of ytterbium avoids problems like multiphonon decay,excited state absorption, and concentration quenching. However, we have recently observed effects which seriously affect the performance in someYb-doped fibres. For a list of the fibres see Tab. 1. The first observation was made when we triedto achieve lasing from the reflection of two bare fibre ends with various fibres, pumped with aTi:sapphire laser with wavelengths between 940nm and 980nm. A 3m piece of fibre 1 (1200 ppmYb) showed lasing around 1035nm with threshold powers around40 mW launched into the fibre (asexpected from a numerical modelbased on data in Ref. 1) when thepump wavelength was between940nm and 960nm. However, asthe pump wavelength approachedthe main absorption peak at 976nm (see Fig. 1) the thresholdincreased rapidly while both thefluorescence output and theresidual pump power at the fibreexit (for a given launched pumppower) decreased dramatically;with a 976 nm pump no lasing,very little transmitted pump andvery much reduced fluorescence were observed even at high pump powers (up to ~120mW). Asessentially the whole pump power could be absorbed in this length of fibre, no significantwavelength dependence of the threshold power had been expected.2In another experiment a 2 m piece of fibre 2 (4200 ppm Yb) was pumped by a Ti:sapphire laserbetween 850nm and 940nm. Lasing around 1035nm was achieved only for pump wavelengthsbetwen 855 nm and 890 nm, despite essentially total pump absorption for longer wavelengths.Fibre 3 with still higher Yb concentration (10,000ppm) did not lase with any pump wavelength,using various lengths of fibre. The same applies to fibre 4 which has similar Yb concentration butan alumino-phospho-silicate host material (without germanium). Measurements of the decay ofside-light fluorescence did not reveal any anomalies; simple exponential decays with timeconstants between 843 µs and 1350 µs were obtained (see table 1), with only small deviationscaused by some inhomogeneity. The variations could be explained by the differences in hostmaterials. Table 1. Parameters of the fibres investigated. no Yb conc. host cut-off NAmeasuredlifetime1 1200 ppm germano-silicate + boron900 nm 0.15 945 µs2 4200 ppm germano-silicate 850 nm 0.3 900 µs a3 10000 ppm germano-silicate 1180 nm 0.2 843 µs 4 10000 ppm alumino-phosphate-silicate>1050 nm 0.21 1350 µs5 3000 ppm germano-silicate 930 nm 0.21 760 µs 6 2200 ppm germano-silicate 700 nm 0.31 720 µs  The weak fluorescence of fibre 2 showed significant non-exponential decay; for lowaexcitation the decay constant was about 900 µs.Due to the simple level structure of ytterbium we did not expect any concentration quenching tobe present. So, to explain the observed behaviour, we first considered the po sibility that theremight be an induced loss mechanism similar to one which we had observed in Tm doped ZBLANfibres [3]. We set up an experiment in which the pump beam from a Ti:sapphire laser waslaunched through a dielectric beam splitter into a 50 cm length of fibr  1, while chopped whitelight (probe) from a bulb was launched into the opposite end. A part of this probe light wasextracted from the pum  input end via the beam splitter and monitored using a monochromator,a silicon photodetector, and a lock-in amplifier. The fibre ends were angle cleaved in order toprevent reflections of pump light. The set-up allowed measurement of the transmission at variousprobe wavelengths with and without pump power, for various pump wavelengths. If pump-induced absorption were the culprit then a decrease in transmission at the fluorescence(gain) wavelengths would be expected as the pump is tuned towards 976 nm. However, no suchevidence was found. 3Fig. 2. Absorption spectra of fibre 1. Solid curve: absorption for maximumavailable power (e.g. 120mW at 976nm); dashed curve: unsaturableabsorption, calculated by subtracting the expected absorption of the notquenched ytterbium; dotted curve: absorption of quenched ytterbiumwith an assumed concentration of 220 ppm, calculated from thecross-sections as used in the model. We then carefully measured the absorption of the pump itself. The pump light was launched intoone arm of a fibre coupler. Most of the pump light then entered a length of Yb-doped fibre whichwas fusion spliced to one output port of the fibre coupler while a g manium detector at the otheroutput port was used to provide a measure of the launched pump power independent of anychanges in launch efficiency. An angled cleave at the end of the doped fibre was used to avoidreflections and thereby suppress lasing; in addition it was necessary to use short fibre lengths toavoid significant gain at 976 nm. First measurements on fibre 1 revealed that there is an unsaturable absorption which is especiallystrong around 976nm, the highest absorption peak of ytterbium: even for high input powers(100mW, i.e. much higher than the saturation power of 0.3mW) the pump transmission wassignificantly lower than at longer wavelengths (e.g. 1050nm). In order to obtain a spectrum of theunsaturable absorption we measured the power transmission for relatively high input powers(around 100mW) at various wavelengths and used a computer program (propagating the pumpwave through the fibre) to determine the amount of unsaturable absorption which was needed toaccount for the measured transmitted pump power. The computer model used cross-sections asmeasured earlier for a similar Yb doped silica fibre; the actual cross-sections in the fibres used areexpected to differ slightly from these values, but these deviations are not very significant as thecorrection for saturable absorption was in most cases small compared to the peak of theunsaturable absorption. The spectrum of unsaturable absorption which we obtainedin this way for fibre 1 is shown in Fig.2 (dashed curve). The shape of this spectrum strongly suggests that the unsaturable absorptionis caused by a part of the Yb population for which the upper state lifetime is reduced to a very4Fig. 3. Absorption spectrum of fibre 2, measured formaximum available power3small value. Fig. 2 also shows the calculated absorption of an assumed population of 220 ppmof quenched ytterbium (dotted curve). The spectral shape of the unsaturable absorption seems todiffer slightly from typical ytterbium absorption spectra; some broad absorption background inaddition to the typical ytterbium features seems to suggest additional absorption by some impurityor structural defect which may also be respnsibl  for quenching of the ytterbium ions: excitationenergy is transferred from an Yb io to a defect centre and then dissipated non-radiatively by thelatter one. Measurement errors as well as errors in the fibre parameters and cross-sections usedin the model are not large enough to explain the discrepancy of the spectrum from the usualytterbium absorption spectrum. We conclude that in fibre 1 roughly 20% of the Yb ions (i.e. about 220 ppm of the total 1200ppm) are quenched by some impurity or structural defect while the other 80 % of the ions are notaffected at ll (and display the normal fluorescence with a measured decay time of 945 µs). Thelifetime of the quenched population can be at most a few microseconds, since we could not detectthis fast decay with a time resolution of our fluorescence measurement of ~5 µs. Similar unsaturable absorption spectra were found for other fibres. Fig. 3 shows the spectrum forfibre 2 in which we calculate that roughly 90% of the Yb ions are quenched. In this case we didnot subtract the saturable absorption because this correction is small. Again we see the ytterbiumfeatures appearing on a broad background rising towards shorter wavelengths. The strongquenching explains why lasing of this fibre could only be observed in a region of relatively smallpump absorption cross-sections (see above): in regions of high absorption cross-sections, theuseful absorption is saturated and the unsaturable absorption (which does not create significantexcited state population and gain) dominates. Note that lasing around 1035 nm is possible despitethe strong quenching effects because there is only low absorption at this wavelength. Of the fibres tested only fibre 5 (the most recently fabricated) was found to have nearly nunsaturable absorption (only about 1% of the 3000 ppm Yb doping). This fibre showed lasingfrom bare ends for a brod ange of pump wavelengths with thresholds around 40 mW launched5as expected from calculation based on the usual Yb data [1]. The threshold power increases onlyby about 15% when the pump laser is tuned to the 976nm peak, and the absorption at 976nm canbe bleached almost completely. Note that lasing with the reflections from bare ends and a 976 nmpump constitutes the case where the quenching has the strongest influence on performance. The nature of the observed quenching process is still not fully understood although it seems likelythat the problem is caused by some impurity or structural defect as mentioned above. Energyexchange between Yb ions can clearly not explain the effect since the simple level structure of Ybions only allows energy migration but no lifetime shortening. Transfer of energy to otherrare-earth impurities would be expected to result in fluorescence at other wavelengths, e.g. around1.55 µm if it were erbium. However, no other fluorescence (besides the usual Yb emission) hasbeen detected in the whole range from 400 nm to 1800 nm. (Only when a sample of fibre 6 wasirradiated with 488nm light from an argon ion laser, some very weak broad-band fluorescencebetween about 575nm and 780nm has been observed; this may be caused by Ge-O defects [4].)At higher Yb concentrations (e.g. 10,000ppm), the rapid energy migration between Yb ions couldresult in most of the ions being quenched even for low d fect (trap) concentrations. However, thefact that fibre 5 does not display any quenching poi ts to the possibility of changes in the preformproduction process, i.e. either that some impurity in the chemicals used earlier was no longerpresent in later batches or that modifications in the process parameters have eliminated thecreation of certain structural defects. We note that Ref. 5 reports about defects with a similarabsorption spectrum in phosphate-silicate fibres which have been identified as colour centres. Whatever the origin of the effect, the unwelcome consequences of the unsaturable absorption forYb doped fibre lasers and amplifiers are obvious. Especially if the pump wavelength is close tothe 976 nm peak (as requi ed in some applications) the unsaturable absorption will cause a largeloss of pump power. We also expect that fibres with Yb codoping - in particular the Er:Yb fibresas used in telecommunication amplifiers - could be adversely affected in a similar way. Indeedthere has been a, hitherto unexplained, observation that maximum efficiency of some Er:Yb fibreamplifiers has required detuning of the pump slightly from the 976 nm absorption peak [6]. Aneven more severe problem will occur for lasers operating at the wavelength of the 976 nm peak(being pumped around 910 nm) where according to our computer model the slope efficiency canbe significantly reduced if even only 1 % of the ytterbium is quenched; such lasers are beingconsidered as powerful pump sources for erbium-doped amplifiers, and high efficiency isespecially important in this case. In conclusion, we have discovered that in various Yb doped fibres the excited state lifetime of apart of the Yb population is quenched by an, as yet unidentified, impurity or defect. This effectcan severely affect the performance of Yb doped fibre lasers and amplifiers. However, it is alsoconfirmed that fibres can be prepared in which this problem is much reduced, although theprecautionary procedures required have not been explicitly identified. Acknowledgments We thank W.S.Brocklesby for performing Raman and fluorescence measurements on our fibres.P.R.Barber acknowledges the UK EPSRC for financial support. 6References 1. H.M.Pask, R.J.Carman, D.C.Hanna, A.C.Tropper, C.J.Mackechnie, P.R.Barber, andJ.M.Dawes, IEEE J. of Sel. Topics in Quant. Electron. 1, 2 (1995). 2. H.M.Pask, J.L.Archambault, D.C.Hanna, L.Reekie, P.St.J.Russel, J.E.Townsend, andA.C.Tropper, Electron. Lett. 30, 863 (1994). 3. P.R.Barber, R.Paschotta, A.C.Tropper, and D.C.Hanna, Opt. Lett. 20, 2195 (1995). 4. W.S.Brocklesby, private communication. 5. A.L.G.Carter, "High phosphate content silicate optical fibres fabricated viaflash-condensation", Ph.D. thesis, University of Sydney (1994). 6. R.I.Laming, private communication.  

D.C.Hanna, A.C.Tropper, C.J.Mackechnie, P.R.Barber, and J.M.Dawes,

D.C.Hanna, L.Reekie, P.St.J.Russel, J.E.Townsend, and A.C.Tropper,

High phosphate content silicate optical fibres fabricated via flash-condensation&quot;, Ph.D. thesis,

Lifetime quenching in Yb doped fibres

Lifetime quenching in Yb doped fibres

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