In contrast to Yb/Al-doped fibers, the influence of very low Tm2O3 concentrations (≥ 0.1 mol-ppm) on photodarkening (PD) is clearly detectable in Yb/P-doped fibers that are known to show little degradation effects. For Tm2O3 additions of more than 50 mol-ppm, the measured PD loss is even similar to Yb/Al-doped fibers with comparable rare earth concentrations. Our work reveals the risk of color center generation by pumping at wavelengths of 915 nm or 976 nm even in Al-free Yb-doped fibers and emphasizes the importance of high purity of raw materials for the preparation of Yb laser fibers with expected very low PD.
©2013 Optical Society of America
The understanding of the photodarkening (PD) process in Yb-doped fibers pumped with low-energy photons (λ > 900 nm) is still a challenging task. Compared to this, the degradation by ultraviolet (UV)  or even 488 nm  irradiation is plausible, because the characteristic absorption regions of Yb-doped silica can be attained by one- or two-photon processes, respectively. On the other hand, photobleaching by UV , visible (VIS) [4–6], and even by the pump wavelengths themselves  has been reported. Therefore, one can assume that the PD kinetics and an eventually achieved equilibrium PD loss are always affected by all wavelengths interacting with the core material.
A permanent optical attenuation was also observed in Tm-doped fibers exposed to 1064 nm radiation and explained by the stepwise absorption or cooperative upconversion processes . On the other hand, codoping of Tm-doped silica fibers with Yb was proposed to profit from energy transfer processes and gain improvement in fiber amplifiers by means of an auxiliary pump at 980 nm ; however, photodarkening was not investigated in this paper.
Tm trace impurities in Yb-doped fibers are introduced by the raw material used for preform fabrication and amount to a value proportional to the Yb concentration in the prepared core material. The key issue whether Tm trace impurities could completely explain PD effects in Yb laser fibers was first addressed by Peretti et al. . The harmful impact of UV photons generated by energy transfer and upconversion processes in case of Tm codoping was shown for one Yb fiber and reiterated in . However, we have recently shown that PD in common, widely used Yb/Al fibers is an intrinsic feature of this core material and not caused by unintentional contamination with Tm ions . Because of the generally high PD loss in this fiber type (≈0.45 mol% Yb2O3, ≈4.5 mol% Al2O3), an assumed impact of Tm ions could not be detected up to concentrations of 10 mol-ppm Tm2O3. The acceleration and enhancement of PD in case of Tm additions of more than 100 mol-ppm as described in [10, 11] was confirmed by our investigations but interpreted as a second path of color center formation caused by the discussed UV photons. The effect of this “Tm path to UV” seemed to be obscured by the intrinsic PD loss in Yb/Al fibers with only Tm trace impurities.
Yb fibers codoped with phosphorus (P) are known to show very weak PD effects  and should be suited to reveal the impact also of weak Tm contaminations. Therefore, we prepared a fiber series with similar Yb concentration but P codoped instead of Al and with variation of the Tm content in a wide range. In this paper, the influence of Tm additions on Yb lifetime as well as Yb and Tm fluorescence intensities is discussed for this series of Yb/P fibers in connection with the PD results. This work contributes to the understanding of PD in Yb fibers with different codopants.
Preform samples were prepared by MCVD (Modified Chemical Vapor Deposition) and solution doping and collapsed in an O2/Cl2 atmosphere. For characterization purposes, fiber samples with a cladding diameter of 125 µm and a core diameter of about 11 µm were drawn. All fibers were doped with nearly constant amounts of Yb2O3 (≈0.4 mol%) and P2O5 (≈7.5 mol%). The concentration of Tm2O3 was varied between ≈0.01 mol-ppm (unintended contamination by the raw material despite extreme purity of > 99.999% or 5N) and 680 mol-ppm as specified in Table 1 . The preform samples were characterized by electron probe microanalysis (EPMA) with a detection limit of 20 mol-ppm and a measurement error of ± 6% for concentrations ≥ 100 mol-ppm. All specified concentrations correspond to the maximum values of the measured profiles. Concentrations of Tm2O3 below 20 mol-ppm were quantified from core absorption measured at 1200 nm for the fibers #1 to #8 (fiber length between 100 m and 10 cm depending on the Tm content) in comparison to the EPMA results for the higher concentration values. Fibers #1 to #3 were prepared from same raw material. From the observed differences in core absorption an accuracy of ± 50% was estimated for the quantified extremely low Tm concentrations. A consecutive reduction of this measurement error is expected as the Tm content approaches values measurable by EPMA.
The characterization of fluorescence intensity and decay was done with continuous-wave (CW) or chopped (pulse width of 0.5 ms, fall time 10 µs, repetition rate 10 Hz) excitation, respectively, using a MOPA system emitting at 976 nm. The laser beam is focused perpendicularly to the fiber core to excite a very small volume (fiber core with diameter 10 µm) thus avoiding ASE (amplified spontaneous emission) and reabsorption effects. A collecting fiber (diameter 400 µm), which guides the fluorescence light to the spectrometer or to the detector, is oriented perpendicularly to both, the exciting beam and the fiber axis. This twice-perpendicular measurement method is described more detailed in . The fluorescence decay of Yb3+ after the end of exciting 976 nm pulses was characterized in the near-infrared (NIR) region (λ > 1 µm) to determine the Yb3+ fluorescence lifetime. The discussed values of the Yb3+ (at 1075 nm) and the Tm3+ (at 473 nm) fluorescence intensities were read out from the measured emission spectra under 976 nm excitation (CW). The Yb inversion was kept low (≈0.1) and the measurement time short (below 10 s) to avoid PD effects during the characterization of fluorescence properties [14, 15].
To analyze PD effects, the accelerated measurement method was used. Short fiber pieces (1 to 2 cm) were core-pumped with 250 mW at 976 nm (Yb inversion ≈0.46) or cladding-pumped with 16 W at 915 nm (Yb inversion ≈0.7), respectively, in a setup schematically shown in Fig. 1 . A wavelength division multiplexing (WDM) or a tapered fiber bundle (TFB) coupler with signal feed-through was used to couple the pump power and the probe light (power < 1 nW) into the fiber under test, which was laid into a water bath to keep the temperature near room conditions. The probe light was modulated by a chopper (170 Hz) and analyzed with a low-noise receiver and lock-in amplifier. All non-modulated fluorescence emission wavelengths and the residual pump light were, for the most part, removed by filters to avoid the saturation of the receiver. The evolution of fiber transmission at 633 nm was measured during pumping to determine PD loss as a function of time .
Some fiber samples were irradiated transversely with UV pulses (266 nm, 250 fs, repetition rate 1 kHz, average power 100 mW, focused to a line of 1 cm in length) using the setup described in [16, 17] but performing a homogeneous irradiation without inscription of a fiber grating by blocking one interferometer arm. The resulting excess loss spectra were determined from transmission measurements before and after degradation with a white light source and an Optical Spectrum Analyzer (OSA) in the wavelength range from 300 nm to 800 nm. These spectra are compared with those from fibers after pump-induced PD.
3. Results and discussion
Typical radial concentration profiles in an Yb/Tm/P-doped preform sample are shown in Fig. 2 . The central dip is caused by evaporation of phosphorus during preform collapsing and affects the distributions of rare earth ions, too . Because of comparable P concentrations in all fibers, similar rare earth distributions are expected also in the fibers with lowest Tm content. From these concentration profiles, strongly inhomogeneous densities of excited Yb3+ and Tm3+ ions during pumping can be expected followed by PD loss and rates varying across the fiber core. The radial refractive index profile is similar in all investigated fibers and mainly determined by the distributions of P and Yb; the numerical aperture (NA) amounts to 0.14. Thus, in a fiber core of diameter 11 µm, more than 10 modes may propagate at the probe wavelength of 633 nm, which will “measure” at any time of observation an effective value of PD loss averaged over the core cross section.
When excited at 976 nm, the characteristic signatures of Tm3+ fluorescence are found in all fibers of the prepared series due to non-radiative energy transfer from Yb3+ ions and excited state absorption (ESA) of pump photons . Figure 3(a) clearly demonstrates that UV emission occurs, which is suspected to cause photodarkening [10, 11], and even fluorescence at wavelengths below 300 nm was detected. At least the first step of Tm3+ excitation from the ground state must be initiated by energy transfer from a neighbored Yb3+ ion because we did not found any Tm3+ fluorescence or photodarkening by the pump wavelengths used in Yb-free Tm fibers.
The Tm3+ fluorescence intensity is growing at all characteristic wavelengths proportionally to the Tm concentration up to 30 mol-ppm; the results for the peak at 473 nm are shown in Fig. 3(b). The reduction of Yb3+ fluorescence intensity and lifetime due to energy transfer to Tm3+ ions becomes measurable for Tm concentrations above 1.7 mol-ppm. In Fig. 4(a) , the normalized NIR fluorescence decay after the end of exciting 976 nm pulses is shown for selected fibers. With increasing Tm content, the decay curves deviate progressively from a single-exponential behavior indicating inhomogeneous effects on the Yb3+ fluorescence. It is to be mentioned that processes faster than the fall time of the exciting pulse (10 µs) cannot be detected. A detailed examination of the non-radiative energy transfer from Yb3+ to Tm3+ ions in these fibers is out of the scope of this paper.
For a straightforward discussion of the investigated fiber series, the 1/e-values were extracted from the normalized NIR fluorescence decay curves measured at all fibers. These lifetime values are depicted in Fig. 4(b) together with the normalized Yb fluorescence intensities at 1075 nm. The strong decrease of Yb3+ fluorescence in fibers with high Tm content obviously prevents a further increase of Tm3+ fluorescence intensity for concentrations of 100 mol-ppm or higher as concluded from Fig. 3(b). The reduction of Yb fluorescence values for Tm concentrations higher than 10 mol-ppm may partially be caused by PD effects [14, 15] despite the very low Yb inversion of ≈0.1 applied and the short measurement time during fluorescence analysis with 976 nm excitation. The acceleration of PD by this pump wavelength is discussed below.
The temporal evolution of PD loss in the prepared Yb/Tm/P-doped fibers is shown in Fig. 5(a) and 5(b) for core and cladding pumping, respectively. The enhancement of PD loss with increasing Tm content is much stronger in case of core pumping at 976 nm than with cladding pumping at 915 nm despite the higher Yb inversion in the latter case. The diminished PD loss in fibers with very high Tm content can be attributed to the reduction of Yb inversion due to the energy transfer to Tm ions. This reduction has obviously a stronger impact in case of 915 nm pumping. The velocity of PD evolution seems to be similar for Tm concentrations of 28 mol-ppm or higher apart from the jump of loss observed in case of 976 nm pumping immediately after switching on the pump power. In the inset of Fig. 5(b), the PD kinetics during the first minutes of pumping is compared for both pump regimes with one fiber serving as example (310 mol-ppm Tm2O3). If the pump power is switched on, the sudden increase of Tm3+ fluorescence (not completely removed by filters) in case of 976 nm pumping interferes with the analysis of modulated probe light by the receiver. However, after a dead time of some seconds (dotted line), the loss is measured correctly again. If the pump power is switched off, a small part of PD loss is bleached immediately.
The jump in PD loss observed at the start of measurement is caused by fast color center generation via the “Tm path to UV”, which obviously works more efficiently with 976 nm photons, because the excited state absorption (ESA) of Tm3+ is increasing with wavelength . This fast change of PD loss may be attributed to the highly-doped part of fiber core, whereas the further slow increase is assumed to be rather caused by the region around the central dip with low rare earth concentrations (Fig. 2) resulting in a lower probability of energy transfer from Yb3+ to Tm3+ ions. Because of the strongly inhomogeneous dopant profiles, the determination of meaningful PD rate constants is impossible for Yb/P-doped fibers prepared with the MCVD technology.
The PD curves of Fig. 5(a) represent the temporal evolution of the averaged core excess loss. Only the loss values of PD equilibrium were estimated by the common fit procedure with a stretched exponential function  and depicted in Fig. 6(a) . Actually, we did not detect a PD loss higher than 1.2 dB/m (at 633 nm) in the Yb/P fibers prepared from extremely pure raw material; Tm trace impurities around 0.015 mol-ppm were estimated for these fibers from core absorption measured at 1200 nm. However, in fibers with Tm concentrations around 1 ppm-mol, PD is no longer negligible. The color center generation on the “Tm path to UV” as described in [10, 11] is clearly revealed in our fiber series. In the log-log plot of Fig. 6(a), a power-law dependence of PD loss on Tm concentration with an exponent of 0.67 is observed in the region up to 100 mol-ppm, which can be understood as a summary of all effects of photodarkening and -bleaching with UV and VIS photons generated by Tm3+ ions in these pumped Yb/P fibers. With 100 mol-ppm Tm2O3 or more, loss values similar to Yb/Tm/Al fibers of the same Yb concentration  are observed.
In Fig. 6(b), excess loss spectra determined from fiber transmission measurements before and after degradation are shown. Practically no pump-induced PD effects occur in Yb/P fibers prepared from extremely pure raw material. However, UV irradiation results in a loss spectrum with characteristic absorption bands peaking at about 390 nm and 540 nm as already described in . The addition of Tm ions in Yb/P fibers enables the generation of the same types of color centers also by NIR pumping as concluded from the very similar loss spectra in Fig. 6(b). This observation emphasizes the role of UV photons on the “Tm path” to PD.
In Yb/Al fibers, however, color center generation can take place both with UV irradiation or NIR pumping and results in the same shape of a broad excess loss spectrum as already reported in . In Fig. 6(b), only the spectrum measured after NIR pumping is shown for such a fiber. The addition of Tm concentrations > 10 mol-ppm increases the PD loss values  but does not change the shape of the spectrum (not shown here).
Differing effects of Al or P codoping or a combination of both on the solubility of rare earth ions in silica and the mitigation of pump-induced PD in Yb fibers have been discussed for many years (e.g [13, 20–24].). Recently, the structural properties of these materials were enlightened by advanced electron spin resonance (ESR) investigations and correlated to absorption measurements after degradation by gamma or pump irradiation [23, 24]. This way, aluminium-oxygen hole centers (AlOHC) have been identified as the absorbing defects in the VIS to NIR range of darkened Yb/Al-doped preforms and fibers. Contrastingly, the solvation shells formed by P can obviously avoid clustering and pump-induced PD effects in the Yb/P-doped core material. However, color centers residing mainly in the UV to VIS range can be caused by direct UV irradiation and also in a Tm-assisted degradation process as shown by our work. Most of published investigations aimed at the clarification of structural differences and induced defect types in doped silica but did not explain the intrinsic PD effect in Yb/Al-doped fibers caused by low-energy pump photons. Up to now, only one model is known  that can describe this process based on the change of chemical bonds without the need to achieve the UV absorption bands of Yb/Al-doped silica by the energy of 4 or more simultaneously excited Yb ions in close vicinity or by the action of cooperative Yb luminescence or Tm fluorescence.
In the Yb/P-doped fibers prepared from raw material of extreme purity (5N), practically no photodarkening effect was found. From our investigations of dependence on the Tm content, it can be speculated that Yb/P-doped fibers without any Tm trace impurities could indeed be free of pump-induced PD. For distinct Tm trace impurities up to several mol-ppm, the accelerated PD measurements show still low PD loss values. In high-power Yb fiber lasers or amplifiers, however, the impact of these Tm trace impurities may be enhanced by strong ESA effects which are increasing with laser wavelength and intensity .
In case of Yb/P-doped fibers with intentional Tm addition, the pump-induced loss spectrum resembles the spectrum caused by UV irradiation in “Tm-free” fibers. This observation proves the role of UV photons generated by energy transfer from Yb to Tm ions and ESA processes also in case of NIR pumping. With only one Tm ion per 50 Yb ions, PD loss values of several 100 dB/m (at 633 nm) were observed.
In common Yb/Al-doped fibers with Tm trace impurities below 1 mol-ppm, however, the color center generation by an intrinsic PD effect surpasses the Tm impact by many orders of magnitude . The model described in  opens perspectives to understand this intrinsic PD process observed in accelerated PD measurements as well as in high-power fiber laser operation.
Financial support by the German Federal Ministry of Education and Research (BMBF) under contract 13N11972 (TEHFA) and by the Thuringian Ministry of Economics, Labor and Technology (TMWAT) under contract 2011 FGR 0104 (FG Faser-Tech) with financial support from the European Social Fund (ESF) is gratefully acknowledged.
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