We investigated the laser-energy-density dependence of absorption changes and paramagnetic centers induced by a cw Ar+ laser operating at 5.1 eV, in both unloaded and H2-loaded singlemode Ge-doped optical fibers. The induced absorption is measured in the blue and near ultraviolet spectral range by using the 3.1 eV photoluminescence, ascribed to Ge lone pair center (GLPC), as an in situ probe source. We find that the Ge(1) center (Ge) is induced upon UV exposure by electron trapping on GeO4 precursors, where the free electrons are most likely produced by ionization of GLPC. Ge(1) is responsible of optical transmission loss of the fiber in the investigated range. Hydrogen loading strongly influences the generation efficiency of the several observed paramagnetic defects, leading in particular to passivation of radiation-induced Ge(2) centers.
©2006 Optical Society of America
The effects of ultraviolet (UV) exposure on germanosilicate glasses are nowadays an open research issue strongly motivated by the many applications of these materials in photonics, ranging from standard optical fibers for telecommunications to Bragg gratings , nonlinear optical devices , etc.. In particular, in the last decade many studies have focused on transparency loss and photosensitivity induced by UV exposure. Considerable effort has been devoted to enhance photosensitivity in view to satisfy the practical requirements, and it has been reported that a simple but highly effective technique for achieving high UV photosensitivity is low temperature hydrogen loading prior to UV exposure .
Several works, both experimental [4–14] and computational [15–19], have pointed out that the UV induced change of the refractive index is related to the induced optical absorption (OA) through the Kramers-Kronig relations, and that these effects are due to generation and transformation of point defects embedded in glass matrix. Hence, the identification of defects responsible of these processes in Ge-doped glasses is a clue to make clear the microscopic origin of transparency loss and photosensitivity, in particular for the first stages of these processes occurring under very low-dose cw UV exposure (~J/cm2).
Till now, most investigations have been performed on fiber preforms  or bulk glasses [4–8, 10] by the combined use of OA and electron spin resonance (ESR) techniques. The latter has provided for the structural identification of several Ge-related paramagnetic centers which contribute to the absorption over a wide region extending from visible to vacuum UV [4–13]. At variance, OA measurements on singlemode optical fibers remain difficult to perform owing to technological and physical limitations. Since these kind of fiber is used in many fields it seems important to directly examine their radiation response instead of extrapolating it from bulk and preform samples. To overcome these difficulties, we used a moving-fiber technique, where the 3.1 eV photo-luminescence (PL) [20, 21], ascribed to GLPC , excited by UV laser light during exposure, is exploited as an in situ probe source to measure OA spectrum.
In the present study, we investigated OA and paramagnetic centers induced by laser exposure at different energy densities in both unloaded and H2-loaded singlemode Ge-doped optical fibers. Our purpose is to get a deeper clarification of the microscopic origin of the photoinduced structural changes observed in these materials.
2. Experimental procedures
The measurements reported here were carried out at room temperature in singlemode Ge-doped optical fibers with core diameter of 4 µm. The Ge content in the fiber core is 4 mol% and the cladding is pure silica. Hydrogen loading was carried out in a vessel under a H2 pressure of 140 atm for three weeks at room temperature, leading to a saturated concentration of 1.7 mol%.
The polymer coating was removed before transverse UV irradiation by 5.1 eV photons from a cw frequency-doubled Ar+ laser, focused on an elliptic 60×800 µm2 spot, with power density P from 17 to 160 W/cm2. The induced OA was measured in situ during irradiation by an innovative technique in which the laser-excited PL band centered around 3.1 eV is exploited as an internal source of probe light (see Fig.1). During the transverse irradiation, the fiber is moved with constant velocity V (parallel to the shortest axes of the laser spot), between 0.2 and 1 cm/s, towards a detection system which consists of a Chromex spectrometer coupled to an Andor CCD camera (detection 2) cooled to 223 K, so that the PL propagates within the irradiated part of the fiber where it is partially absorbed. The detected amplitude reduction yields to the induced absorption coefficient over the spectral region covered by the PL band. The exposure time of a single point of the fiber to laser light is te =60 µm/V, of the order of a few ms. To study the dependence on irradiation dose, we performed many sessions of exposure and in situ measurement, at different values of the laser energy density F=P×te . This measuring method allows to combine two features: (i) irradiation of long-enough portions of fiber yields sufficient optical density permitting to obtain high-quality OA measurements and (ii) the irradiated length receives a relatively low total dose (F ~J/cm2).
ESR measurements were performed by a Bruker EMX spectrometer working at 9.7 GHz. Signal was detected on a few meters long irradiated fibers, after cutting them in portions of ~0.5 cm. A microwave power P=1.6 mW and a 100 kHz modulation field of peak-to-peak amplitude Bm =0.1 mT were used, these values being properly chosen to avoid saturation and distortion effects in revealing the induced paramagnetic centers. Concentration of paramagnetic centers was determined by comparing the double-integrated ESR spectra with that of Si-E ’ centers , in a reference sample where their absolute density was measured with accuracy of ±20% by spin-echo technique .
Figure 2(a) evidences the evolution of the PL centered around 3.1 eV transmitted within the laser-irradiated length of the unloaded optical fiber. The inset shows the decay of the PL intensity at three fixed spectral positions as a function of the fiber-irradiated length. The linear-best fitting slope gives the corresponding induced OA coefficient, which is plotted in Fig. 2(b) over the 2.56–3.44 eV range, after irradiation at three different energy densities. The induced OA increases on increasing the energy, suggesting that these curves represent the tail of an absorption band peaked at higher energy, the intensity of the band depending on energy density.
Figure 3 depicts a log-log plot of the induced absorption coefficient measured at 3.1 eV, Δα 3.1, in unloaded and H2-loaded fibers as a function of the laser-energy density ranging from 0.1 up to 5 J/cm2. We see that the addition of hydrogen results in a reduction of the initial growth rate and in an increase of the saturation value reached at high energy densities.
ESR spectra detected in the two kinds of fibers, both exposed to a energy density of 0.8 J/cm2, are shown in Fig. 4. It is apparent that these spectra are composite signals, resulting from the overlap of different contributions. As regards the ESR lineshapes measured in unloaded fibers [panel (a)], we found that they can be least-square-fitted, after irradiation at all energy densities, by a linear combination of the lineshapes associated with the following Ge-related paramagnetic point defects: (i) Ge(1), an electron trapped in a fourfold coordinated Ge atom, (GeO4)- , (ii) GeE ’, an unpaired electron on a threefold coordinated Ge, ≡Ge’ , and iii) Ge(2). The structure of the defect responsible of the latter ESR signal is still questioned between models consisting either in a trapped electron center [10, 26] or in a hole center [8, 27, 28]. Even the circumstance that the g value of Ge(2) is smaller than 2.0023 does not permit its conclusive assignment to a trapped electron center, because this line of reasoning is rigorously valid only for very simple paramagnetic centers, and generally cannot be extended to point defects in silica .
The normalized ESR lineshapes of these three defects, used in the best fitting procedure, are shown separately in panel (c) of the same figure. Ge(1) and GeE ’ lineshapes are experimental curves measured in γ-irradiated Ge-doped samples , where it has been possible to single out the two defects. At variance, the Ge(2) lineshape is a curve taken from literature, obtained by Friebele et al.  by a simulation procedure aimed to fit ESR lineshapes observed in γ-irradiated multimode Ge-doped fibers.
We point out that fitting an ESR signal with a linear combination of single-defect lineshapes is founded on the assumption that the centers are sufficiently far from each other that their lineshape is not influenced by mutual interactions. This condition is usually satisfied at defects concentrations up to 1018cm-3 . Moreover, the residual discrepancies between best fit curves and experimental data are probably due both to the accuracy limit with which the three base lineshapes of panel (c) were singled out and to the presence of furhter minor contributions to the overall spectrum.
From the best fit coefficients appearing in the linear combination, we calculated the concentrations of the three defects which contribute to the overall signal measured upon exposure at each energy density.
As regards the H2-loaded samples [panel (b)], the Ge(2) center is not present, as inferred from the absence of its characteristic negative peak at about 348.5 mT. So, the central portion of the spectrum mainly consists of the overlap of Ge(1) and GeE ’ signals whose concentration is calculated by a best fitting procedure as above described. Moreover, these samples exhibit the well-known 11.8 mT doublet [inset of panel (b)] associated with the presence of H(II) centers, =Ge’-H . The concentration of H(II) has been evaluated by numerical double integration of the signal.
The obtained concentration of paramagnetic species as a function of energy density is reported in Fig. 5(a) for virgin fibers, and in Fig. 5(b) for H2-loaded fibers. In the virgin fibers, the concentrations of Ge(1) and Ge(2) grow respectively to ~1×1017 cm-3 and ~2×1017 cm-3 on increasing the energy density to about 0.6 J/cm2. The concentration [GeE’] remains much lower than the other two species. At variance, in the H2-loaded samples, [GeE ’] is much higher than in the virgin fiber, whereas the concentration of Ge(1) has the same order of magnitude. Finally, [H(II)] is almost constant in the investigated energy density range, suggesting its growth process to be completed at lower energy densities.
From the above reported results, we note that only Ge(1) and GeE ’ centers are induced in both fibers. However, while Ge(1) centers are induced in comparable amount, the concentration of GeE ’ is increased in presence of H2 by a factor ~40. On the basis of these findings, the Ge(1) center is the only candidate to be responsible for the induced absorption in the investigated spectral range among the several observed paramagnetic centers.
To better investigate this issue, Fig. 6(a) plots the dependence between Δα 3.1 and the Ge(1) concentration measured in our fibers exposed to various energy densities. Data are fitted by a linear function with zero intercept, whose slope is found to be (3.5±0.5)×10-18 cm2, the linear correlation coefficient being r=0.82.
The linear relationship allows to infer that the induced absorption is mainly due to Ge(1) center; this finding is consistent with previous literature studies, since the observed spectral profile (Fig. 2) can be interpreted as the tail of the absorption band ascribed to Ge(1), peaked at 4.5 eV with full width at half maximum of 0.9–1.3 eV. The slope represents the absorption cross section σ at 3.1 eV, σ 3.1=Δα 3.1/N, where N is the total concentration [8, 11, 17, 22, 23, 33]. We acknowledge that the agreement between fit and experimental data may be limited by the presence of other small contributions to induced absorbtion, not taken into account since we have fixed the intercept to zero. This may influence the measured value of σ but does not invalidate the attribution of the observed absorption to Ge(1).
Given that the formation of Ge(1) is the cause of transmission loss under UV irradiation of the singlemode fibers in the investigated spectral range, we examine now the generation mechanism of this defect; this will lead us to discuss, more in general, the generation and transformation processes of the several observed Ge-related species.
As known from ESR studies, Ge(1) is induced by trapping of an electron e- on a fourfold coordinated Ge (GeO4) [8, 10]. Hence, the generation of Ge(1) requires ionization of an donor which makes available the electron to be trapped on GeO4 site. A hint on the generation mechanism of Ge(1) comes from Fig. 6(b), where we report the concentration of Ge(1) as a function of Ge(2) as observed in virgin fibers. We see that the growths of the two centers are linearly correlated (r=0.98) with slope ~0.5. This finding, which is peculiar to virgin fibers, suggests that the generation processes of the two defects are actually linked in these materials. The two alternative models proposed for Ge(2) center give rise to two different interpretations for this correlation.
In particular, if we accept the model of Ge(2) as an electron trapping center [10, 26], the correlation can be interpreted as follows: the generation of both defects is driven by trapping of electrons produced by ionization of a common center X (whose structure is unknown at this stage).
and the slope ~0.5 represents the relative probability of a released electron being trapped on the two precursors giving rise to Ge(1) or Ge(2) (fourfold coordinated Ge with one or two nearest-neighboring Ge atoms respectively).
i.e. laser-induced ionization of GLPC followed by trapping of the e- on a fourfold coordinated Ge precursor. In this scheme, the slope ~0.5 represents the probability that an electron released from GLPC encounters a GeO4 site and is trapped on it, while the extra electrons are trapped on unknown centers acting as well as electron traps. We stress that since the GLPC is known to absorb at 5.1 eV and 7.4 eV, its ionization requires at least two Ar+ laser photons. Then, the most probable ionization mechanism is two-step absorption via the intermediate long-lived triplet state, where the center can be excited by resonant 5.1 eV absorption in its B2β band followed by intersystem crossing . At variance, the possibility of direct two photon absorption from ground state to conduction band is expected to be very unlikely due to the low intensity of the cw laser radiation.
As regards the loaded fiber, we see that the addition of H2 leads to several effects, which permit to further discuss the issue of the Ge(2) structural model. In fact, the presence of hydrogen causes the disappearance of Ge(2) centers, whereas Ge(1) are detected in comparable concentrations with respect to unloaded fibers (Fig. 3). Hence, our data allow to infer that Ge(2) centers are passivated by reaction with mobile hydrogen. This observation contrasts with the model in which Ge(1) and Ge(2) are both four-fold coordinated Ge electron traps, differing only for the number of nearest-neighboring Ge atoms, because in this case the very similar structures of the two centers should lead to very similar reaction properties with hydrogen. At variance, present results can be easily interpreted on the basis of the GLPC + model for Ge(2), whose annealing is due to the following reaction with mobile hydrogen:
where the defect at the right side of reaction 3 is an ESR-insensitive diamagnetic center, as already proposed by Fujimaki et al . Then, we can suppose that in H2-loaded samples Ge(1) are still generated by reaction 2, even if Ge(2) are not observed due to reaction 3.
The presence of H2 gives rise also to the generation of H(II), known to be formed by reaction of hydrogen with GLPC. This can occur either by reaction of GLPC in ground state with H0 produced by breaking of H2 on paramagnetic centers , or by direct reaction of H2 with GLPC in excited-triplet state [14, 33]. The latter process leads to an observed reduction of lifetime of GLPC in excited triplet state in presence of H2 ; since we have supposed the generation of Ge(1) to occur by two step absorption of GLPC via the long-lived triplet state, we expect a reduction of the Ge(1) formation rate in presence of hydrogen: this is consistent with the lower initial slope of the H2-loaded curve in Fig. 3. In this interpretation, it still remains unclear the H2-induced variation of the saturation value of Δα 3.1.
A further effect of H2 addition is the strong enhancement of the GeE ’ creation. This effect has been observed in previous works  and may be important for applications as GeE ’ plays an important role in UV poling of Ge-doped silica leading to induced nonlinearity of the material . Following a model proposed by Awazu et al. , the increase in the generation efficiency of GeE ’ in presence of hydrogen may be due to the formation of Ge-H bonds, acting as precursors of the defect. It is interesting to note that the behaviour of GeE ’ in presence of hydrogen appears very different from that of SiE ’, the isoelectronic analogous on silicon of the center, which is known to be passivated, and not enhanced, by H2 diffusion and reaction .
The UV-induced conversion processes between Ge related point defects were investigated in singlemode optical fibers by the combined use of ESR and OA measurements. The latter were performed by an innovative technique in which the 3.1 eV PL arising from GLPC is used as an intrinsic probe source. The transmission loss of the fibers observed upon UV laser exposure is due to the the tail of a Ge(1)-related absorption band peaked in the UV region. Ge(1) center is generated by trapping on GeO4 sites of electrons produced by ionization of preexisting precursors, GLPC being a possible candidate. In hydrogen-loaded fibers Ge(2) centers are passivated by reaction with H2. Moreover, the addition of hydrogen enhances the generation efficiency of GeE ’, leads to formation of H(II) centers, and alters the dependence of induced OA on laser energy density.
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