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Photoinduced reduction of absorption (photobleaching) in bismuth-doped germanosilicate fibers irradiated with 532-nm laser has been observed for the first time. It was demonstrated that bismuth-related active centers having the absorption bands at wavelengths of 1400 and 1700 nm degrade under photoexcitation at 532 nm. The photobleaching process rate was estimated using conventional stretched exponential technique. It was found that the photobleaching rate in bismuth-doped germanosilicate fibers does not depend on type of bismuth-related active center. The possible underlying mechanism of photobleaching process in bismuth-doped fibers is discussed.
© 2015 Optical Society of America
1. Introduction
Bismuth-doped optical fibers are promising laser media exhibiting many important properties such as intense broadband near infrared emission extending over 150 nm with a long lifetime of about 1 ms [1].
Up till now, continuous-wave as well as pulsed lasing has been obtained using fibers doped with the extremely low concentration of bismuth (no more than 0.1 mol.%) (e.g [2]. and references therein). It has been repeatedly observed that the dopant concentration exceeding this level leads to a high level of unsaturated optical absorption that is a real problem for development of the efficient bismuth-doped laser [3]. Unfortunately, unambiguous experiments allowing one to reveal the exact structure of the bismuth-related active centers (BAC) and the centers associated with unsaturated absorption are yet to be carried out. Therefore, understanding the nature of the active centers is still a very important problem whose solution can lead to the improvement of laser characteristics of bismuth-doped materials.
Since the discovery of NIR luminescence from Bi-doped glasses [4] there has been considerable amount of research on the understanding of its mechanism [5, 6]. However, the results of the most investigations showed that the determination of the nature of these centers is very complicated task. As a result, many different models of BAC were proposed (e .g [5]. and references therein), but none of them has been directly confirmed. In the recent years, a bismuth ion located near the oxygen-deficiency defect of host glass has been considered as the most probable model of the laser-active centers related to bismuth. At the present time, the experimental and theoretical results obtained evidence for the proposed model [7–10]. In particular, the photoinduced disappearance of the absorption and emission bands of BACs under 244-nm radiation was recently revealed. It was confirmed that the degradation of BAC is a result of the photoionization of oxygen-deficiency center (ODC) [10].
We performed a series of experiments with the purpose of understanding the role of ODC in bismuth-related active center formation. In this paper we will report and discuss the experimental data on photoinduced changes in the absorption spectra of BACs under 532-nm radiation.
2. Experimental samples and measurement scheme
The samples selected for study were bismuth-doped germanosilicate fibers drawn from preforms fabricated by the modified chemical vapor deposition technique. The detailed core glass compositions and second order mode cut-off wavelengths of the investigated fibers with silica cladding are listed in Table 1. The outer diameter of all fibers was 125 μm. The total bismuth concentration in fiber core was less than 0.1 mol%. Absorption spectra of the optical fibers in the spectral region of 450-1800 nm were measured by the well-known cut-back technique using a halogen lamp (DH 2000 Mikropack) and a spectrum analyzer.
Table 1. Characteristics of developed Bi-doped fibers
The experimental setup for registration of photobleaching effect is demonstrated in Fig. 1.
Fig. 1 Experimental setup for photobleaching measurements in bismuth-doped fibers. Configurations 1 and 2 were applied for transmission spectra measurements and the laser irradiation of the tested fiber correspondingly.
Transmission spectra of a bismuth-doped (active) fiber were recorded before and after 532-nm irradiation (the configuration 1 in Fig. 1). Fiber coupled above-mentioned halogen lamp was used as a wideband light source (WBS). Radiation of WBS was launched into the active fiber core through the splice with a standard single-mode optical fiber (SMF) and detected using the optical spectrum analyzer (OSA) operating in a spectral region of 750 – 1700 nm. Photobleaching was realized in an experimental scheme in Fig. 1 (the configuration 2). A 532 nm irradiation was provided by frequency-doubled solid-state Nd:YAG laser (SSL). Radiation was collected by a microscope objective into SMF and launched into the active fiber core through the splice with SMF. Intensity of pumping radiation in fiber core was varied from 0.6 to 1.2 MW/cm2. Depending on a particular experiment being carried out the laser diode emitting at wavelength of 1555 nm can be either turned on or off. It should be emphasized that switching from the configuration 1 to 2 (Fig. 1) involves only a change in SMF-SMF splices.
3. Results and discussion
The typical loss spectra of fibers #1 and #2 are presented in Fig. 2(a). No distinct peaks were observed in loss spectrum of fiber without bismuth (fiber #1). It is also seen that the background loss increases with the decreasing of wavelength. Minimal loss level of this fiber is ~0.03 dB/m at the wavelength of 1650 nm.
Fig. 2 a) Absorption spectra of fibers #1 and #2. b) Representative emission (1, 2) and excitation (1*, 2*) spectra of BAC-Ge and BAC-Si, respectively.
In contrast with silica glass fibers, the MCVD germanosilicate fibers have a high level of background losses (>0.03 dB/m) caused by the optical inhomogeneity of core glass (more defect glass structure). The occurrence of the absorption edge in the short-wavelength region of visible spectrum and long tail stretching into the near infrared is due to formation of glass defects (e.g. oxygen-deficiency centers (ODC)) (e.g [11]. and references therein).
Characteristic bands arising from bismuth-related active centers can be found in loss spectrum of fiber #2 [Fig. 2(a)]. It is well-known that two types of BACs form in bismuth-doped germanosilicate fibers [12, 13]. First one having emission at 1400 is the bismuth-related active center associated with Si (BAC-Si), second one having emission at 1700 nm is the bismuth-related active center associated with Ge (BAC-Ge) [Fig. 2(b)]. BAC-Si and BAC-Ge are responsible for absorption and excitation bands peaking at 1400, 820, 420 and 1600, 950, 460 nm, correspondingly [Figs. 2(a) and 2(b)]. Apart from above-mentioned absorption bands, unidentified absorption shoulder near 500 nm can be clearly observed in the loss spectra of active fibers. It should be noted the near infrared emission bands from bismuth-doped germanosilicate fibers were not detected under the 532-nm excitation.
Figure 3 shows the difference between the transmission spectra of the investigated fibers #1 and #2 after irradiation at 532 nm with 1 MW/cm2 and before irradiation (photoinduced change in transmission). The measured change in the optical transmission ΔT(t) is determined by expression:
where L is a fiber length, T0 and T(t) are optical transmittance in the initial state and after exposition time t, respectively.Fig. 3 Photoinduced changes in transmission spectrum of fibers #1 and #2 under 532-nm irradiation with the power density of 1 MW/cm2. The duration of the irradiation process was equal to 1 h.
The obtained curve represents the photobleaching effect in the spectral region where the values are positive and the photodarkening effect where values are negative. The appreciable change in transmission of fiber #1 without Bi ions (Fig. 3) can be seen in visible region and no changes are in the near-infrared region. Structural rearrangement resulting from the destruction of glass defects are responsible for the observed changes in the transmission of the irradiated fiber #1 [14].
The photoinduced changes in transmission for the Bi-doped fiber #2 are different from the observed changes for fiber #1. Photobleaching is observed in a spectral region of 1370 – 1800 nm and 750 – 1000 nm, whereas the photodarkening occurs in 1000-1370 nm range. It is clearly seen that bleaching takes place in regions of BAC absorption. In contrast with photobleaching, photodarkening occurs in the spectral regions where BACs in germanosilicate fibers have not any emission and absorption bands. Moreover, the detailed analysis of process dynamics presented in Fig. 4(a) shows that photodarkening in a region of 1100-1300 nm reaches saturation by an order of magnitude faster than photobleaching process in a region of 1370-1700 nm. In present paper we will focus on the study of the photobleaching phenomenon of BACs. The origin of the photodarkening effect is still unknown.
Fig. 4 a) Spectral changes in transmission versus time exposition. b) The temporal dependence of photoinduced bleaching effect at 1400 (rhombus) and 1600 nm (circle). Solid line is the fitting curve of the stretched exponential function with indicated parameters. Experimental sample is fiber #2.
Figure 4(a) shows the changes of optical transmission spectrum in a region of 1000-1700 nm as a function of the exposition time. The temporal dependences of the photobleaching at 1400 and 1600 nm are demonstrated in Fig. 4(b).
The obtained temporal dependence of photobleaching effect allows us to determine the photobleaching time constants using the stretched exponential analysis. Earlier stretched exponential function was successfully used in study of photoinduced processes in Yb and Tm–doped fibers [15]. In this case the time evolution of photoinduced effects can be represented as:
where A is the transmission at the final equilibrium state, β (0≤β≤1) is stretch parameter, and τ is the photobleaching time constant for each measurement. Fitting of Eq. (2) to the experimental data produces parameters τ and β being equal to 40 min and 0.5 for λ = 1600 nm and 50 min and 0.7 for λ = 1400 nm. The inverse of the obtained characteristic time (1/τ) can be considered as the rate of the photobleaching [15].We also measured the photobleaching rates at various pump power levels. For that purpose a series of fibers #2 and #3 were irradiated during 1 hour at different pump power. The length of each fiber was chosen to be equal to 2 meters in order to ensure correct measurements of changes in transmission spectra in the 1300-1700 nm spectral range.
The obtained photobleaching rates are plotted against corresponding pump power in Fig. 5 (in the log-log scale). In both cases (fiber #2 and #3) linear fittings of data obtained provides slope being close to unity. However, photobleaching could not be surely declared as one-photon process [15] because of non-uniform distribution of 532-nm irradiation power along the fibers. Using shorter pieces of active fibers did not provide a required level of absorption in a spectral region 1300-1700 nm for the detection of photobleaching effect because of significant difference of loss values near 532 nm and in a region of 1300-1700 nm.
We obtained experimental evidence that photobleaching process does not involve the optical transition from the ground and the first excited state of bismuth ions participating in the formation of BACs. The absence of excitation bands peaking at 530 nm allows us to conclude that no significant change of the population of ground and first excited (metastable) states of bismuth ions under pumping of 532-nm radiation took place. It was expected that the change of the population of the ground state of bismuth ions participating in the formation of BACs may effect on photobleaching rate. For this purpose, we carried out the measurement of photoinduced transmission changes under simultaneous pumping at 532 and 1555 nm. The 30 mW output power of laser diode at wavelength of 1555 nm was enough to provide the sufficient population of the first excited state. Photoinduced optical transmission changes in a spectral region of 1150-1700 nm under pumping at 532 nm only and under simultaneous pumping at wavelengths of 532 and 1555 nm are presented in Fig. 6. The photoinduced transmission spectra obtained under one- and two-wavelength pumping are almost identical. The lack of sufficient differences between photoinduced transmission spectra obtained allows us to exclude the direct participation of the bismuth ions in photobleaching process, in particular, photoionization process of the bismuth ions.
Fig. 6 Change in transmission in spectral region of 1150-1700 nm under pumping at 532 nm and (532 + 1555) nm simultaneously.
As we have shown in [10] BAC consists of a bismuth ion and adjacent oxygen deficiency center. In our opinion, photobleaching process is caused by the destruction (photoionization) of the ODCs(II) which participate in formation of BACs. Photobleaching of these ODCs (II) under 532-nm radiation is well known and firmly experimentally established fact [14]. It can be schematically represented as it is shown in Fig. 7. Two-photon absorption of the 532-nm radiation excites ODC(II) from its singlet ground state S0 to short living singlet state S1. As it is known, lifetime of S1 level does not exceed 10 ns and therefore excited state absorption process from that state has low probability. In accordance with earlier experimental data, during the singlet–singlet excitation of ODC more than 95% of the excited centers reach the long-lived (100 us for GeODC(II), and 10 ms for SiODC(II), see [16]) triplet state T1 through nonradiative transition [17]. Hence photoionization from triplet state under 532-nm radiation with intensity near 0.5 MW/cm2 and more is quite probable process [14]. It is seen that the photobleaching effect is the same for BAC-Si and for BAC-Ge, hence, do not depend on the type of BACs. It confirms that two type of bismuth related IR active centers presented in the samples studied have similar structure. Therefore, photoionization of ODC(II) is the most probable cause to quenching near IR activity of BAC.
Fig. 7 Schematic representation of photoionization of ODC(II) under 532 nm excitation. Conduction and valence bands are designated by CB and VB, correspondingly.
Prolonged (more than 15 hours) irradiation of 5-m length fiber #3 was also carried out for complete suppression of BACs. Fiber #3 after such treatment has not any IR luminescence. The obtained loss spectra of the pristine and the 532-nm irradiated fiber are represented in Fig. 8. The bleaching of overlapping absorption bands of BAC-Ge and BAC-Si which cover visible and IR region led to the decreasing of loss in whole spectral region. Loss spectrum of the irradiated fiber still has unstructured absorption in the region of 500-700 nm and tail extends up to the near IR range. It is required to note, that prolonged irradiation leads to reduction of the photoinduced absorption in the range of 1000-1300 nm (Figs. 3 and 4). Probably, bismuth-related non-active centers are responsible for the residual loss observed in Fig. 8.
4. Conclusion
In summary, irreversible photoinduced reduction of absorption (photobleaching) of bismuth-related active centers under photoexcitation at 532 nm was observed for the first time. We investigated the temporal evolution of the photobleaching phenomenon under exposure to green laser radiation for different active centers (BAC-Si and BAC-Ge). It was revealed that origin of photobleaching effect is the same regardless of the type of the active centers. The photobleaching mechanism is explained by the photoionization process of the oxygen-deficiency center (ODC(II)) participating in the formation of the BAC. The obtained data are entirely consistent with our previous results on the photobleaching under 244-nm irradiation [10]. Moveover, the absorption spectrum of a bismuth-doped germanosilicate fiber after complete destruction of bismuth-related active centers was also presented.
Acknowledgments
This work was supported by the Russian Foundation for Basic Research (Grant 15-32-20234).
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