1. Introduction

Pump-induced photodarkening (PD) is a detrimental process limiting the performance of Yb laser fibers in many applications. This feature of the fiber core material was attributed to the excitation of Yb ions [1] and is often characterized by accelerated PD experiments at high Yb inversion, thereby determining the kinetics of the PD loss from transmission measurements of short fiber pieces [2, 3]. Transverse variations of the PD loss were studied with both core and cladding pumping and under laser operation [4]. Possible routes to PD are being discussed including observations of similar color centers generated by UV irradiation [5, 6] or two photons of 488 nm [7]. The characterization of defects in the silica glass [8] that could act as precursors is an important contribution to understand the PD process.

A reduction of pump-induced PD was qualitatively described for fibers with enhanced Al-codoping [9]. Very low values of the PD loss and rate constant were measured in fibers purely codoped with a high P content (> 6 mol% P₂O₅) [2, 10] instead of Al. But, P evaporation causes a harmful central dip in the refractive index profile and Yb distribution of such fibers; other disadvantages of the P-codoping are the high core NA that complicates the preparation of large-mode-area (LMA) fibers, a high core background loss of > 300 dB/km [10] as well as the reduction of the Yb absorption and emission cross sections [11]. Therefore, the laser performance of such fibers is impaired in comparison to the “standard” Yb fibers with Al-codoping [10].

In this paper, we investigate the influence of combined codoping with Al and P on the photodarkening parameters and other fiber properties.

2. Fiber fabrication and standard characterization

Preforms and fiber samples have been prepared by MCVD (Modified Chemical Vapor Deposition) and solution doping according to a route with carefully controlled process steps described in [12, 13]. The goal of the two series prepared here was to provide similar fibers doped with nearly constant amount of Yb₂O₃ (about 0.45 mol%) and specific variation of the codopants. The fibers of the series #Al differ primarily in their aluminum content (Tab. 1), while the phosphorus content is kept low and constant (0.5 mol% P₂O₅). In the series #P with constant aluminum content (about 4.5 mol% Al₂O₃), the phosphorus concentration is stepwise enhanced up to a molar ratio P/Al of 1 (Tab. 2). All preforms have been collapsed in oxygen atmosphere.

For the standard characterization and the photodarkening experiments, fiber samples with a cladding diameter of 125 µm were drawn; the core diameter is approximately 10 µm. From the preform #4P, a double-cladding fiber with core and cladding diameters of 20 µm and 250/200 µm (double D-shaped) was prepared to perform laser experiments.

The samples were characterized by X-ray microprobe analysis, refractive index profiling, and absorption and fluorescence measurements in VIS and NIR. Important properties are summarized in the Tables 1 and 2. All specified concentrations correspond to the maximum values of the measured concentration profiles. Contrary to fibers with P excess [10], a very low core background loss α_b was measured at all pristine fibers discussed in this paper (Tables 1 and 2).

Refractive index profiles (measured on the preforms but displayed vs. the fiber radius r) are shown in Fig. 1. In Al-codoped fibers, the refractive index is growing with increasing Al content. In the series #P, the refractive index is lowered despite of the increasing P addition. This anomaly is caused by the interaction of Al and P ions and was already described for fibers without Yb [14]. All samples are free from a central index dip.

 

Fig. 1. Refractive index profiles of the samples from series #Al (a) and series #P (b).

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The fluorescence measurements were performed at the fibers with an improved double-perpendicular technique (both, the pump excitation with 980 nm and the collection of fluorescence light were carried out perpendicularly to the fiber axis [10]) to ensure identical measurement conditions and nearly saturated Yb inversion for all investigated fibers. The Yb³⁺ near infrared (NIR) fluorescence lifetime was detected to be almost equal in the fibers (810 to 840 µs) except for the fiber #4P with the highest P content (880 µs). The spectra of the cooperative luminescence around 500 nm are shown in Fig. 2; the reduction of the intensity by the enhanced Al and P content, respectively is evident despite of slightly reduced Yb concentrations in the fibers #4Al and #5Al.

 

Fig. 2. Measured spectra of the NIR-excited cooperative luminescence of the fibers from series #Al (a) and series #P (b).

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Table 1. Fiber series #Al (0.5 mol% P2O5, variation of the Al content): sample number, ytterbium content, aluminum content, fiber background loss αb at 1200 and 633 nm, respectively, and determined photodarkening parameters: equilibrium loss αeq and time constant τ.

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Table 2. Fiber series #P (about 4.5 mol% Al2O3, variation of the P content): sample number, ytterbium content, phosphorus content, fiber background loss αb at 1200 and 633 nm, respectively, and determined photodarkening parameters: equilibrium loss αeq and time constant τ.

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Core absorption spectra were measured at thin preform slices to determine the absorption cross section of the Yb³⁺ions; in Fig. 5(a), results for samples codoped merely with Al and merely with P [15] are compared with the fiber #4P of this paper (equal content of Al and P). Generally, the Yb³⁺ absorption cross section spectra for the fibers of the series #Al and #P were found to be almost equal except for the fiber #4P the values of which are lower (about 25% at the main pump wavelengths around 915 nm and 976 nm).

3. Photodarkening measurements

The transmission of probe light (633 nm) was measured in-situ during long-term Yb³⁺ excitation by cladding pumping of the fibers with 915 nm. The use of a stable, broadband light source (halogen lamp) and the chopper/lock-in technique facilitates low-noise measurements of the photodarkening kinetics. Special care was taken to measure the true core excess loss without falsification by probe light guided in the cladding of the fiber under test without attenuation. A detailed description of the experimental set-up is given in [3]. Very short fibers (lengths 1 to 2 cm) were examined to avoid ASE (amplified spontaneous emission) and to permit also the evaluation of high PD losses induced in some fiber types with the applied pump power of 6.1 to 13.2 W.

The pump-induced Yb inversion was estimated from the launched pump power, the saturation power and the cross sections of Yb absorption and emission at 915 nm. The density of excited Yb ions [Yb*] was calculated as the product of the inversion and the density of Yb ions (Yb content see Tables 1 and 2). The pump power was adjusted to reach similar density of excited Yb ions [Yb*] within each fiber series. Therefore, the fibers #1 to #3 of the series #Al were pumped with 6.1 W (Inv. 0.55), but the fibers #4 and #5 with 13.2 W (Inv. 0.69). All fibers of the series #P were pumped with 10.1 W (Inv. 0.66, but 0.62 for fiber #4P).

 

Fig. 3. Temporal development of the photodarkening loss of the fibers from series #Al (a) at [Yb*] ≈ 1.1· 10²⁶ m^-3 and series #P (b) at [Yb*] ≈ 1.3· 10²⁶ m^-3. The stretched exponential functions determined by the fitting procedure are shown as thin dashed lines.

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The time-dependence of the core excess loss α (t)=-10·log(T(t))/L in units of dB/m (L is the fiber length) was calculated from the measured probe transmission T(t)=P(t)/P₀ (P₀ and P(t) are the transmitted probe powers before and during photodarkening, respectively). The results are shown in Fig. 3(a) for the series #Al ([Yb*] ≈ 1.1·10²⁶ m^-3) and in Fig. 3(b) for the series #P ([Yb*]=1.4·10²⁶ m^-3, but 1.2·10²⁶ m^-3 for the fiber #4P).

The photodarkening parameters, i.e. the equilibrium loss α_eq and the time constant τ, were determined by a fitting procedure with a stretched exponential function defined in [3]; these curves are also shown in Fig. 3 as thin dashed lines. The obtained results are summarized in the Tables 1 and 2 for all investigated fibers. For the series #Al, the values of α_eq and the rate constant τ^-1 are also shown in Fig. 4 as a function of the Al content.

 

Fig. 4. Photodarkening loss α_eq (a) and rate constant τ^-1 (b) in dependence on the aluminum content (fiber series #Al, [Yb*] ≈ 1.1· 10²⁶ m^-3).

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4. Laser measurements

The fiber #4P was characterized in a CW laser set-up by pumping at 980 nm. The resonator has been realized by a dichroitic mirror at the pump launching fiber end (reflectivity nearly 100% for wavelengths > 1000 nm) and perpendicularly cleaved fiber ends. The fiber length has been chosen to absorb most of the pump power (L=6 m). In Fig. 5(b), the measured Yb laser power in dependence on the absorbed pump power is shown. The laser emission in the wavelength range around 1070 nm was monitored by an Optical Spectrum Analysator.

 

Fig. 5. Spectra of the Yb³⁺ absorption cross section for different codoping (a) and laser characteristics of the fiber #4P (b).

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5. Discussion

Our investigations verify the reduction of photodarkening in Yb doped fibers by the enhancement of the Al content (Fig. 3(a)). A quantitative evaluation can be deduced from the results of the fiber series #Al for both PD parameters (Fig. 4): whereas the PD equilibrium loss drops almost linearly, the rate constant shows a stronger decrease of approximately τ^-1~c(Al₂O₃)^-2. (For the fibers #4Al and #5Al with lower Yb content (Tab.1), slightly reduced PD parameter values are expected from earlier investigations [3], but this is of minor influence on the relations stated above.)

Additional codoping with phosphorus results in a more pronounced reduction of the PD loss, whereas the time constant is only weakly influenced (Tab. 2 and Fig. 3(b)). For equal concentrations of P and Al, the PD loss is almost completely suppressed.

Fortunately, the reduction of PD in the series #P is accompanied by a reduction of the core refractive index (Fig. 1(b)) due to the interaction of both codopants [14]. Moreover, the central dip of the index profile known from purely P-codoped laser fibers is avoided by this core composition.

As already described in [15], the Yb³⁺ absorption cross section σ_abs in purely P-codoped fibers is reduced to about 50% in comparison to the Al-codoped fibers (Fig. 5(a)). For the combined codoping with Al and P in the Al-excess branch, the Yb³⁺ absorption cross section is slightly reduced by no more than 25% at the main pump wavelengths around 915 nm and 976 nm. From these results and the low core background loss (Tab. 2) a favorable laser performance of the fibers of series #P could be expected and was verified by the laser examination of the fiber #4P. A laser output power of more than 100 W was demonstrated with a slope efficiency of 72% (Fig. 5(b)).

The reduction of the PD loss by the enhanced P content in the series #P is correlated with a diminished intensity of the NIR-excited cooperative luminescence of Yb³⁺ ions in the visible range (Fig. 2(b)). A similar correlation was found for the fiber series #Al of this paper (Fig. 2(a)) and for the fiber series with increasing oxygen deficiency in the core glass discussed in [12]. These observations support the assumption, that cooperative energy transfer processes from excited Yb³⁺ ions or pairs to atomic defect precursors (e.g. [8]) in the doped silica bridge the energy gap for the generation of color centers, that can otherwise be created by UV irradiation around 230 nm [5, 6] or by two photons of 488 nm [7].

6. Conclusions

We investigated the photodarkening kinetics of Yb doped fibers codoped with aluminum and phosphorus. The reduction of the PD loss α_eq and the rate constant τ^-1 by enhanced codopant concentrations could be quantified, whereof the distinct diminution of the pump-induced loss is most important for the application of Yb laser fibers.

Especially, a core composition with equal content of Al and P is very advantageous to realize Yb fibers with improved laser performance. The PD loss almost disappears, a low core background loss was found, and a low numerical aperture of the laser core can be realized despite of the high codoping. An excellent laser performance could be demonstrated for such a fiber with an output power of more than 100 W and a slope efficiency of 72%. Further optimization of the core composition to prepare LMA fibers with a core NA < 0.08 retaining the excellent characteristics of the fiber #4P will be subject of next investigations.

The obvious correlation of the PD loss with the intensity of NIR-excited cooperative luminescence encourages the supposition that efficient cooperative energy transfer processes from excited Yb³⁺ ions could also assist the formation of color centers.