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Abstract
This work contains a study of the mechanisms of emerging and dissociation of UV-induced defects responsible for the degradation of the gain in Er- and Al- codoped silica glass. We investigate absorption spectra changes at the visible and ultraviolet range under UV irradiation at 193 nm and show the role of molecular hydrogen in this process. This includes a detailed description of the observed processes in the glass in terms of photochemical reactions. We also considered the effect of photobleaching of defects under 976 nm irradiation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber optic telecommunications and sensorics have been actively developed over the past decades. Such a dynamic development requires the creation of efficient light sources and amplifiers operating in the main telecommunication window of about 1.5 µm. Fiber lasers and amplifiers are the most convenient for telecommunication applications. These include devices based on erbium-doped silica optical fibers [1]; in addition, fiber sensorics is being developed. In this field, development of effective single-frequency fiber lasers is especially relevant. The design and features of such lasers involve the use of heavily erbium-doped silica glasses [2].
A fiber laser cavity is usually represented by two mirrors on the borders of the gain medium — a fiber with a rare-earth element-doped core. These mirrors are most commonly fiber Bragg gratings (FBGs) inscribed in the fiber core [3]. These gratings can be inscribed in passive fibers, which are subsequently spliced with active fiber on both sides. Another way is to inscribe the gratings directly in the active fiber to minimize intracavity losses [4]. Of particular interest are distributed-feedback (DFB) single-frequency fiber lasers [5]. The cavity of such a laser is formed by a phase-shifted FBG inscribed along the whole of its length [6]. In this case, the FBG inscription directly in the core of the active fiber is the only option for laser cavity formation.
Laser UV irradiation through a special phase mask is the most common method of FBG inscription in optical fibers [3]. Pulsed excimer lasers based on KrF (248 nm) and ArF (193 nm) are ordinarily used as UV light sources for such irradiation. Refractive index changes of the glass during the FBG inscription are caused by the emergence of various kinds of photoinduced defects of the fiber core glass. There are a great number of defect types being studied and explained in literature [7–9]. The most commonly observed defects are hydroxyl groups (Ge-OH in Ge-codoped fibers and Al-OH in Al-codoped fibers), which appear in preliminary hydrogen-loaded fibers. The emergence of these defects is due to the interaction of hydrogen molecules with active centers resulting from UV-induced dissociation of structural elements of the glass network associated with the presence of aluminum and germanium (non-bridging oxygen hole centers) [9]. Due to the high efficiency of such reactions, H2 loading is one of the main methods of increasing fiber photosensitivity to UV irradiation for FBG inscription [10].
A number of articles [11–13] have shown that codoping of active erbium optical fibers with germanium leads to a deterioration of its luminescent properties. In this regard, the use of Al-doped core fibers is preferred to form the necessary fiber-refractive index profile. Article [2] demonstrated the possibility of FBG inscription in the core of Er- and Al-doped H2-loaded fiber without Ge-codoping. At the same time, it was demonstrated in [14] that 193 nm UV-irradiation of such a fiber causes significant degradation of its gain properties. The level of this degradation depends on the presence of hydrogen in the glass network of the fiber. Some of the induced defects are not involved in FBG formation but significantly affect the gain properties of the fiber. There are various methods to deal with such defects, including high-temperature annealing of the fiber [15], photobleaching [16], or partial “healing“ of defects by hydrogen atoms [17]. However, thermal annealing of a fiber with a grating in it at a high (up to 800 °C) temperature results in a significant degradation of FBG [18,19] and the durability of the fiber. The presence of the molecular hydrogen in the active fiber itself seriously damages its luminescent properties [20]. It should be noted that attempts to compensate for the negative effect of UV irradiation of a fiber on its gain properties should not lead to the degradation of the FBG inscribed in it.
Our previous work [21] demonstrated the gain restoration of a UV-irradiated Er-doped active fiber as a result of exposure to a high-power density (∼ 150 W/cm2) 976 nm radiation. No significant fiber core temperature growth was observed during this experiment. The main mechanism of the observed restoration was obviously the photobleaching of UV-induced defects under exposure at 976 nm. At the same time, the nature of the defects causing negative influence on the gain and the mechanism of its photobleaching remain unclear. In this paper, we study the formation of defects under UV irradiation in Er- and Al-codoped silica optical fiber as well as the partial decay of these defects under 976 nm exposure with the help of visible and near-UV spectroscopy.
2. Samples and experiment
Optical absorption spectroscopy is one of the methods for studying the properties of silica glass defects. Spectroscopy in the UV and visible spectral ranges is the most informative [9]. The main object of our experiments was an optical fiber with a silica glass core doped with aluminum and erbium oxides. This fiber was fabricated by SPCVD technology [22]. The core refractive index profile (Fig. 1 a) was formed mainly by the presence of aluminum oxide in the glass network. In addition, the presence of aluminum in the network of silica glass increases the solubility of rare-earth elements, reducing the degree of clustering at high concentrations [23]. The concentration of erbium in the fiber core was determined to be 3.74×1020 cm−3 by x-ray microstructural analysis. The Al2O3 concentration was calculated to be 6 mol.% according to known refractivity of aluminum oxide in silica glass [24]. The absorption and IR luminescence spectra of the fiber under study are shown in Fig. 1 b).
Fig. 1. Refractive index profile (a) Er3+-doped fiber absorption spectrum, luminescence spectrum on the inset (b).
To identify and obtain additional information on glass network defects that are not structurally related to erbium ions, in this work, we also studied 0.2 mm-thick slices of a fiber preform with a core of erbium-free aluminosilicate glass with an aluminum oxide concentration of 4.3 mol. %.
The measurement range of the transmission spectra of the samples was limited by the sensitivity of the spectrometric equipment. We investigated hydrogen-loaded and pristine fiber as well as preform samples. Hydrogen loading was held by means of a self-made gas loading chamber that allowed us to load samples with hydrogen isobarically at a pressure of about 130 atm. and temperature of 100 °C. The samples were saturated over the course of 24 hours, resulting in quasi-equilibrium hydrogen concentration in the fiber core of about 1.04×1020cm−3 [25].
In our previous work [21], we demonstrated restoration of a weak signal gain at 1550 nm in a UV-irradiated, H2-loaded, Er-doped active optical fiber under 976 nm exposure. This restoration was not associated with the release of molecular hydrogen from the glass network or its heating. The growth dynamics of the amplified signal is shown in Fig. 2:
Fig. 2. Evolution of the signal, amplified in an H2-loaded sample, placed into room-temperature water under the influence of pump radiation (Ref. [21], Fig. 9).
For a deeper understanding of the mechanism of the gain deterioration as well as its subsequent restoration, we studied the evolution of the absorption spectra of fiber samples as a result of their irradiation with a pulsed UV 193 nm laser with corresponding quantum energy 6.4 eV. In addition, we studied the evolution of the shape of the photo-induced absorption spectra in the fiber samples as a result of subsequent 976 nm (quantum energy: 1.27 eV) exposure with pump laser radiation.
The experiment consisted of two parts. In the first part (Fig. 3(a)), we studied the evolution of the transmission spectrum under UV irradiation of 1 cm of active fiber 2. A combined deuterium/halogen lamp 1 with FC/APC fiber output served as a light source in the 200–1000 nm range. We used a PerkinElmer Lambda 900 spectrometer 3 for spectra measurement in the 200–850 nm range. This experiment was conducted in two stages, using the deuterium and with halogen lamp separately to avoid the influence of the second order of diffraction on the spectrum form. It should be noted that absorption in short wavelength range in the fibers of the experimental setup allowed us to measure spectra only in 330–850 nm range for fiber samples. UV irradiation of the samples was carried out with the help of an ArF-excimer laser Coherent CompexPRO 110F with a radiation wavelength of 193 nm. The pulse duration and repetition rate were 20 ns and 10 Hz, respectively. Pulse power density in the exposed area was 150 mJ/cm2. In the second part of the experiment (Fig. 3(b)) UV-irradiated samples of the active fiber were exposed by the 976 nm pump laser radiation. The radiation was launched into the fiber core. During this exposure, the fiber sample was placed into the cuvette 4 with room-temperature water to avoid significant heating of the fiber core due to the absorption of pump radiation. In our previous paper [26], we demonstrated that in such conditions, the fiber core temperature increases by no more than 10 °C. The output connector of the fiber line was removed from the spectrometer, and the light source was replaced with a 976 nm semiconductor laser. The laser diode 5 Gooch and Housego D1401015 with fiber output was a source of 976 nm radiation. The diode current was controlled by the Thorlabs ITC4001 driver. The actual output power of the laser diode was monitored by a JDSU fiber power meter connected through a 99/1 splitter 7. The time of the 976 nm exposure was 5 minutes, with a radiation power of 100 mW launched into the fiber core. After the 976 nm exposure, the absorption spectrum of the fiber was measured again with the initial experimental setup (Fig. 3(a)). Evolution of the absorption spectra of the fiber preform slice under UV irradiation was measured separately (Fig. 3(c)) under similar conditions.
Fig. 3. Schemes of the experiments. 1– combined (halogen / deuterium lamp) light source, 2– 1 cm active fiber under study, 3– spectrometer for the UV and visible range, 4 - cuvette with room temperature water, 5– 976 nm semiconductor laser diode, 6– 976 nm insulator, 7– 99/1% splitter at 976 nm, 8– power level meter, 9– lenses, 10– bulk sample.
3. Results and discussion
In the first part of the experiment, we investigated the dependence of the induced absorption spectra on the UV exposure dose. The results of the experiment are presented in Fig. 4. Figure 5 shows photoinduced absorption spectra of the preform samples (total UV dose 2kJ, 200 mJ/cm2).
Fig. 4. Induced absorption spectra of the fiber samples in the UV and visible spectral range for pristine (a) and H2-loaded (b) fiber depending on the number of UV laser pulses. Evolution of spectral shape in 20 minutes at room temperature after the UV irradiation and consequent 976 nm exposure of the sample without H2 (c) and H2-loaded fiber (d).
Fig. 5. UV-induced absorption spectra of the pristine (a) and hydrogen-loaded (b) preform slice samples.
The abscissa axis of Fig. 4 is graded with eV for convenience of data interpretation. Absorption growth under UV irradiation is shown in Fig. 4 for pristine (a) and H2-loaded (b) fiber. Figures 4 c and d show how the shape of the induced absorption spectra changes after 20 minutes of “relaxation” of the irradiated fiber at room temperature and 5 minutes of exposure at 976 nm with 100 mW for hydrogen-unloaded (c) and H2-loaded (d) fiber, respectively. These spectra were obtained by normalizing the transmission spectra of the irradiated fiber to the transmission spectrum of the fiber before irradiation. It is necessary to note that narrow absorption lines in Er-doped fiber at 2.35 and 3.27 eV are artifacts and caused by an almost-zero signal in the transmission spectra. These lines are almost absent in absorption spectra of the Er-undoped preform (Fig. 5 a, b).
According to well-known literature, intense absorption peaks with maxima of 2.35 and 3.27 eV are associated with the resonance transitions of Er3+ ions in a 2H11/2 → 4I15/2 and 4G11/2 → 4I15/2 silica glass network [27,28].
It has been shown [7,8] that 193 nm UV irradiation of Al-doped silica glass results in the emergence of point defects (color centers) in its network. These defects can be divided into the following main groups: aluminum oxygen-hole centers (Al-OHC with resonance absorption at 2.3 and 3.2 eV and oscillator strength of 0.06 and 0.124, respectively), aluminum E'-centers (AlE’ with 4.1 eV energy and 0.214 oscillator strength), and silicon E’ centers (SiE’ with 5.5 eV energy and 0.2 oscillator strength). In fiber samples, measurement range was limited to photon energy of approximately 3.6 eV due to high absorption in the UV part of the spectrum, so we could correctly take into account only the absorption bands of Al-OHC centers with maxima of 2.3 and 3.2 eV (Fig. 4(a)-(d)). Relatively small thickness of the preform slice sample permits to measure the absorption in a wider spectral range from 1.5 to 4.5 eV (Fig. 5 a, b).
To interpret the obtained results, we analyzed quantitative estimation absorption spectra by means of three Gaussian functions with character energies 2.3, 3.2, and 4.1 eV. Since preform slices allowed us to measure induced absorption in a wider spectral range with less noise, such Gaussian approximation is more visible in Fig. 5 (additional function with 5.5 eV energy was considered for bulk sample). The dynamics of intensity changes for each of the peaks in various fiber samples during UV irradiation and subsequent photobleaching under 976 nm exposure is shown in Fig. 6. This graph is built on a logarithmic scale on both axes for a more visual presentation of the results. Vertical axis is graded in cm−1 units for the more convenient comparison between band intensities. One can see that the dependence of the absorption peak intensity on the exposed UV dose has a power-law activation nature with saturation at the maximum exposed dose, which is typical for photoionization processes. The arrows on the right side of the graphs show the decrease in the intensity of the corresponding absorption peaks under the influence of 976 nm exposure. It should be noted that the 3.2 eV peak intensity in the hydrogen-loaded sample decreases almost to zero. Analysis shows that the contribution of the short-wavelength edge of the absorption band of AlE’ centers with a 4.1 eV maximum has the greatest influence on the shape of the spectra. The same absorption band also undergoes maximum decay during 976 nm exposure.
Fig. 6. Intensity of the Gaussian peaks of the induced absorption spectra for a fiber without hydrogen (a) and hydrogen-loaded (b). The arrows show the effect of the 976 nm exposure.
Let us analyze the induced absorption spectra in more detail. The Al-OHC absorption band with a peak at 2.3 eV is well distinguished in the fiber, and the preform induced absorption spectra for samples without hydrogen loading (Fig. 4(a), 5(a)). The preform absorption spectra (Fig. 5) also present a high-intensity 4.1 eV band corresponding to the AlE’ center. The second absorption band of Al-OHC centers with 3.2 eV maximum is strongly overlapped by its neighboring bands of 2.3 eV and 4.1 eV. Therefore, it cannot be identified except by using the Gaussian function approximation of the spectrum curve of the induced absorption [7,29]. An SiE’ center absorption band with 5.5 eV maximum can be identified in the same way (green curves in Fig. 5).
Quantitative comparison of Gaussian peaks intensities between Er-doped fiber samples and Er free bulk samples are shown in Table 1:
Table 1. Quantitative comparison of induced absorption peak intensities for H2 loaded and unloaded bulk and fiber samples at the maximal UV exposed dose
Comparing the induced absorption spectrum shapes for Er-doped fiber samples and an Al:SiO2 preform without Er allows us to find their identity and suggest that all the groups of photoinduced defects characteristic of irradiated aluminosilicate glass, namely Al-OHC, AlE’, and SiE’ centers, are also formed in the glass of the fiber core, additionally doped with erbium oxide. There is no additional absorption associated with the presence of Er ions. The mechanism of each type of center formation can be explained by equations of photochemical reactions. According to the understanding of the aluminosilicate glass structure, the basic network elements of this glass are tetrahedral. Each tetrahedron is a 4-coordinated silicon or aluminum atom associated with 4 oxygen atoms: ≡Al-O-Al≡, ≡Al-O-Si≡, or ≡Si-O-Si≡ [30,31]. Erbium doping of aluminosilicate glass significantly complicates the structure of the glass network. Thus, according to [31], the main state of erbium in the aluminosilicate glass network is the 6-coordinated Er3+ ion, which is coordinated to 6 oxygen atoms, forming the nearest-neighbor environment (we denote its structure as (O)3≡Er3+≡(O)3). In [32], it was also noted that 5- and 6-coordinated aluminum atoms (≡Al≡), interconnected by oxygen bridges, prevail in the second-neighbor environment of Er3+ ions.
The influence of 6.4 eV UV quanta on an aluminosilicate glass network results in ionization of its oxygen bridges (mainly Al-O-Al and AlO-Si). The main result of such ionization is the formation of Al-oxygen hole centers (OHC) (structurally referred to as *O-Al≡, where “*” corresponds to a hole localized on the oxygen atom) and AlE’ and SiE’ electron centers with the structure of 3-coordinated aluminum and silicon atoms, respectively (≡Al• and ≡Si•, where the “•” signifies an unpaired electron). These groups of UV-induced point defects of aluminosilicate glass were previously detected and identified by the electron spin resonance (ESR) [7,8]. The possibility of the simultaneous emergence of pairs of defects—hole Al-OHC and electron SiE’ centers during the ionization of Al-O-Si bridges in an aluminosilicate glass network was previously confirmed by quantum chemical calculations [33]. Other photoinduced network transforms are possible in the environment of Er3+ ions. In [34], the appearance of Er2+ ions was noted under the influence of γ-quanta on a network of erbium-doped aluminosilicate glass. According to the assumptions made in [35], the formation of the divalent state of Er2+ ions from the initial Er3+ occurs simultaneously with the formation of Al-OHC centers in the closest environment as a result of the electron transfer of the first coordination sphere oxygen atoms to excited erbium ions. Based on the results of [32,34], we can compose equations for the main photochemical reactions involving the elements of the erbium-aluminosilicate glass network and 193 nm quanta with 6.4 eV energy:
It is noteworthy that these photochemical reactions can be partially reversible even under normal conditions. Partial relaxation of induced absorption in the irradiated fiber core 20 minutes after termination of UV exposure (Fig. 4(c)) indicates the decay of a certain number of induced centers and the restoration of the initial elements of the glass network. Also it should be noted that in aluminosilicate glass, erbium ions are predominantly surrounded by aluminum atoms [32]. And since all reactions in our case mainly occur near erbium ions, the number of induced Si-OHC will be negligible compared to Al-OHC.
In the preliminary hydrogen-loaded samples, photochemical reactions will proceed with the active participation of H2 molecules. Free (chemically unbound to the network) hydrogen in silica glass has high chemical activity and often acts as a stabilizer of oxygen bridges ionized by UV irradiation [35]. The presence of hydrogen in the glass network during the UV irradiation initiates the emerging of hydroxyl centers (H-O-Al≡) instead of oxygen-hole centers. This may explain the significantly lower (3-4 times) absorption intensity of Al-OHC in the spectra of hydrogen-loaded samples (Fig. 4(b)) compared to unloaded ones (Fig. 4(a)). In addition to hydroxyl centers, the combined action of UV irradiation and hydrogen can lead to the formation of H2O complexes in a silica glass network [36]. Thus, taking into account the influence of hydrogen in the erbium-aluminosilicate glass network, the equations of photochemical reactions [Eq. (1-3)] take the form:
Note that “passive“ deactivation of photoinduced oxygen-hole centers by hydrogen molecules can also occur after UV irradiation stops, resulting in an additional decrease in the intensity of the induced absorption in 1.5–3.5 eV spectral range (Fig. 4(d)).
An additional deactivation mechanism of the color centers and corresponding absorption in an erbium-doped fiber is observed under the influence of continuous IR pump radiation with 1.27 eV quantum energy. Figures 4(c) and (d) show that 976 nm exposure over the course of 5 minutes resulted in doubled decrease of induced absorption in the UV-irradiated pristine fiber samples and almost no absorption in previously H2-loaded samples. To explain this effect, it is necessary to assume the participation of excited Er3+ ions in photochemical reactions leading to the decay or structural transformation of Al-OHC centers, since the quantum energy of the IR pump radiation is low compared to the minimum excitation energy (∼2 eV). It is known that a weak green line with a maximum at 483 nm under 976 nm exposure is present in the luminescence spectrum of erbium-doped fibers due to an up-conversional effect [37]. This luminescence corresponds to the 4F7/2 → 4I15/2 transitions. Despite low intensity, its quantum energy (2.5 eV) is enough to excite Al-OHC, likely resulting in photoinduced dissociation of these centers or, in the presence of close H2 molecules, their transformation into hydroxyl centers (H-O-Al≡). Thus, the most typical reactions of photobleaching of color centers in erbium-aluminosilicate fibers under the influence of IR pump exposure are:
It should be emphasized separately that the hydroxyl centers and H2O complexes are thermally stable and optically inactive in the wavelength range from 200 to 1000 nm. Therefore, almost complete irreversible photobleaching of the induced absorption bands was present in the hydrogen-loaded samples under the influence of IR pump radiation. It should also be noted that during the 976 nm irradiation of the sample 3.2 eV band visible intensity decreases more significantly than 2.3 eV band intensity (Fig. 4(c)). We suppose that in addition to Al-OHC, a significant contribution to the amplitude of the 3.2 eV band is made by absorption associated with the presence in the glass of Er2+ formed during preliminary irradiation at 193 nm (Eq. (3)). In [38] it was shown that Er2+ absorption spectrum contains rather intense and wide bands around 3 eV in CaF2 crystals. Under the exposure at 976 nm, excitation by pump quanta transfers to the Er2+ ions from neighboring Er3+ ions, resulting in their conversion to tri-valent state according to (Eq. (9)) and to decrease of the observed 3.2 eV band intensity.
4. Conclusion
In this work, we demonstrated the evolution of the absorption spectra of erbium- and aluminum-doped silica optical fibers in UV and visible ranges under the influence of 193 nm UV irradiation. We showed that, as a result of UV irradiation, aluminum and silicon OHC and E’ centers with absorption bands at 2.3, 3.2, and 4.1 eV are formed in the glass. We also showed that defects can dissociate under the influence of 976 nm exposure. In a hydrogen-loaded fiber, this process is much more effective than in an unloaded one. The observed spectrum changes (as well as their detailed explanation related to photochemical reactions occurring in the glass) allow a deeper understanding of the processes occurring in active silica glasses under the influence of UV irradiation.
Acknowledgements
This work was carried out within the framework of the state task. We thank Prof. K. M. Golant and Dr. Yu. K. Chamorovskiy for kindly providing the erbium doped fibers.
Disclosures
The authors declare no conflicts of interest.
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