3.1. Transmission Spectra of BDF and BEDF Before and After UV Irradiation
The measured irradiation-induced transmission changes with the 35-mW laser power, the slight broadening of transmission intensity in BDF, and the enhanced characteristic bands in BAC-Al, as shown in
Figure 2a, were all related to typical electronic transitions of bismuth ions.
In near-infrared and visible regions, a substantial variation in transmission was observed. Variations in transmission after irradiation of the fibers were because of structural modifications, which resulted from the damage or defects in glass. It can be assumed that photobleaching occurred in the region. In addition, it can also be assumed that photodarkening occurred in BAC-Si, where the intensity decreased after exposure to irradiation.
The mechanism for varying intensities indicated that photobleaching process influenced saturation at a slower rate than by the photodarkening rate, although its basis was not evident. The transmission loss reduced by about 0.24 dB with only 2.5 cm in fiber length (equivalent to 9.7 dB/m) at near-infrared region with the UV laser of 35 mW for 60 s. However,
Figure 2b describes the variation of the optical transmittance intensity of BEDF in a region within the proximity 850 nm band as a function of the irradiation time. A considerable variation in transmission can be observed in that region, which indicated that there was no significant change in first excitation and ground states. However, the excitation of the state of bismuth ion population occurred after UV irradiation. The experimental evidence obtained after irradiation indicated that photobleaching was not involved to the first excitation, and the ground state transition of bismuth ions contributed to the creation of bismuth active centres. It was possible that the ground state of bismuth ion population changes, which contributed to the creation of BAC, was influenced by the irradiation [
16].
3.2. Luminescence Spectra of BDF and BEDF Before and After UV Irradiation
The measurement of luminescence intensities were performed at different time intervals. Before irradiation was started, the luminescence intensity of the ideal fiber was measured through the excitation of the pump source. The irradiation effect of UV on BDF showed that at ~1100 nm the luminescence intensity (BAC-Al) was increasing firstly and then decreasing, whereas at ~1420 nm (BAC-Si) the intensity kept reducing, as shown in
Figure 3a–c. Thus, that the variations observed under UV laser irradiation suggested the results were because of the two-photon process, where a number of blue photons had enhanced energies to match the energy of one UV photon. If it is supposed that the variation was because of the two-photon process, then the quadratic dependency of the amount of irradiation on the laser power intensity was probably the characteristic time of the irradiation and number of photons [
17]. However, because the UV irradiation process is lower than the band gap of silica glass, exciting an electron from the valence band to the conduction band through a one-photon process is insufficient by the pulsed UV irradiation. When high-photon density and coherency absorptions because of multiple photons were considered, with a 6.4 eV photon energy, the laser operated at 50 Hz repetition rate and the laser energy was 4.1–4.2 mJ. Recent studies revealed that the irradiation of silica glasses by an ArF laser created E’ centres having intrinsic defects, which exhibited a characteristic behavior. The creation of E’ centres were saturated with a small dose of light. At room temperature the induced centres reduced quickly [
18,
19].
The change in the intensity can be ascribed to centres in the presence of Bi and Al ions, the luminescence at ~1100 nm corresponded to the BAC-Al was increased by 0.6 dB with only 2.5 cm in the fiber length (equivalent to 24 dB/m) at ~1100 nm by the UV irradiation for 60 s and the fluorescence intensity of BAC-Si at ~1420 nm was decreased obviously. In addition, it can be emphasized that the luminescence at the ~1100 nm band was allocated to the ³P₁, ³P₂→³P₀ transition of Bi⁺ and ²D
3/2 →
4S
3/2 transition of Bi
0, and the luminescence at ~1420 nm band was assigned to the mixed valence states of Bi
3+/Bi⁵⁺. Moreover, it was also observed that the intensity at ~1100 nm could be enhanced by changing the length of fiber and pump power, compared to that when the band was ~1420 nm [
14,
20].
The irradiation changes can be expressed in relation to the decay curve of the luminescence intensity by considering a stretched exponential function. This function is physically interpreted as a continuous sum of a number of single exponential relaxation systems and extensively used to fit an integrated relaxation process in the disordered electronic and molecular system. It can be expressed by the following equation:
where
, ∞ represents for the bleachable part of the luminescence, IA(t) and I(A); β is the stretched parameter; τ is the time constant and ∞ is the luminescence intensity at the time when the irradiation effect is saturated under the radiation power, P.
All measured luminescence spectra have the same exponential decay trend. However, the speed and degree of decay vary with the improvement of the induced power; the irradiation ratio increases and the time constant drops, exhibiting a faster and stronger irradiation effect.
After irradiation, BAC-Al and BAC-Si gained the same changes in BEDF fibers as shown in
Figure 4. It can be observed from the figure that both irradiation ratio and decay rate were inclined to be saturated at a higher irradiation rate. Therefore, we related these two parameters with the ~1100 nm luminescence intensity in relation with the pump power in FUT. Furthermore, it was also observed the luminescence intensity was increased by 1.53 dB with only 2.5 cm in fiber length (equivalent to 61.32 dB/m) when the irradiation time was 60 s and tended to saturate, subsequently. And the peak at 1420 nm, which belonged to BAC-Si, dropped markedly with the increasing of the irradiation time. The trend can also be compared with the irradiation ratio and decay rate of photobleaching. The possible energy levels of BAC-Al and BAC-Si are shown in
Figure 5a,b, respectively. These evidences allow the assumption that the principal mechanism responsible for effect on the BAC-Si was its loss of an excited electron. It can be stated that, firstly, BAC-Si absorbed the 193-nm photon and was agitated towards the second excitation level. Secondly, some of the BAC-Si fell to the lower excited state through the non-radiative transition and another release the electron to the acceptor site.
Accordingly, thermal vibrational energy generated by the laser beam aided in implementing the decay of luminescence and ground state absorption [
20]. Finally, supported by thermal energy, the trapped electron still had possibility to returning to the bismuth ion, which can cause the recovery of photobleaching. All the acquired results suggest that the existence of the luminescence of Bi-doped fibers could not be entirely ascribed to some optical transitions of bismuth ion. The most substantial hypotheses for the source of the BAC with laser-active transitions were bismuth ions related to a structural defect, as previously proposed. The defect had greater probability of becoming an ODC with an absorption which approximated 5 eV band, creating an environment for the Bi
(n+) ion. The schematic structure of BAC can be denoted as (Bi
(n+) + ODC). The two types of ODC defects that are approximately twofold are as follows: corresponding silicon (Si + ODC) and corresponding germanium (Ge + ODC) atoms, which are similar to Si/Ge. The aforementioned possibly caused the formation of two types of BACs, namely, BAC-Si and BAC-Ge. The photoelectron progression because of the photoionization process can be confined by bismuth ions. However, no manifestation of new absorption or luminescence band was observed that could be related with bismuth ions. In the thermally-stimulated luminescence (TSL) of Bi-doped fibers, it was observed that bismuth related trap condition in germino-silicate glass fibers was not created. Although ODC was transformed in to an E’ centre, the structural model was not a paired spin on a sp
3-hybridized molecular orbital of a threefold corresponding silicon/germanium atom. The transformation was most possibly shaped by the local structural readjustment of the glass caused the variation of the bismuth ion atmosphere [
21].