A cw-244-nm-Ar+ laser was used to fabricate Bragg gratings in pristine and H2-loaded Bi-Al-SiO2 optical fibers with index changes as high as 3.6 × 10−4 and 19.3 × 10−4, respectively. For comparison, fiber Bragg gratings in pristine and H2-loaded SMF-28e showed index changes of 13.6 × 10−4 and 63.3 × 10−4. Continuous isochronal thermal annealing revealed higher thermal stability for the H2-loaded Bi-Al-SiO2 fiber compared to the pristine one. The SMF-28e fibers, with and without hydrogen, were more stable than the Bi-Al-SiO2 fibers.
© 2011 OSA
Bi-doped optical fibers exhibit luminescence in the near infrared and the visible spectrum making them appealing for use in integrated tunable lasers, amplifiers, or broadband light sources for the telecommunications or the medical industry. Functional devices have already been demonstrated in Bi-Al co-doped fibers, like fiber lasers , fiber amplifiers  or fiber Bragg gratings fabricated in H2-loaded bismuth-doped silica fibers using ArF excimer  or cw-frequency doubled Ar+ laser . The efficiency of the Bi-Al-doped fiber lasers does not typically exceed 30% at room temperature  for two reasons: a) the nature of the bismuth active center is not known limiting the optimization of the technology and b) FBGs are fabricated in Ge-doped fibers spliced to the active fiber. The application of gratings in fiber lasers or in amplifiers requires sufficient index changes and thermal stability . As H2-loading may change the absorption and luminescence properties of Bi-doped fibers  it is important to be able to fabricate FBG in non-H2-loaded, pristine fibers. Moreover, the thermal stability of these gratings is important for future applications.
In this work, the fabrication of Bragg gratings in pristine Bi-Al co-doped and SMF-28e optical fibers  using a cw-244-nm laser is presented. Using the continuous thermal annealing method [9, 10] the stability of gratings in pristine bismuth doped fiber and SMF-28e is reported for the first time. Using the annealing data the defect energy distribution was calculated, assuming first order reaction kinetics . In addition to the original work published in the ECOC 2011 proceedings , results on fabrication and thermal annealing of H2-loaded Bi-Al co-doped and SMF-28e fibers were included.
Fiber Bragg gratings in both Bi-Al co-doped and standard telecommunications single mode (SM) fiber were fabricated using a frequency doubled cw-Ar+ laser operating at 244 nm with a 1/e2 beam waist of 700 μm. The periodic refractive index modulation in the fiber core was formed using a phase mask with a grating pitch of 1066.33 nm. The laser beam was focused perpendicular to the fiber using a cylindrical lens (f = 54 mm) to a 1/e2 size of 40 μm. The irradiation power density was ~500 W/cm2. The grating fabrication process was monitored online using a commercial FBG interrogator. From the spectral data, mean and modulated refractive index changes in the fiber core were calculated. The Bi-Al co-doped silica optical fiber used (Bi#10) has a silica cladding, 2 mol% Al2O3 and less than 0.02 mol% Bi concentration in the core. The standard telecommunications fiber is a commercially available fiber manufactured by Corning (SMF-28e) with a GeO2 concentration in the core of ~3 mol%. The length of the FBGs was 0.7 mm except for those in pristine Bi-Al co-doped which was 1.4 mm. Gratings longer than the corresponding 1/e2 diameter of the UV beam (0.7 mm) were realized by translating the beam along the fiber core axis. Hydrogen loading of the fibers was performed under a pressure of ~150 bars for 2 weeks at room temperature.
Thermal annealing of the FBGs was realized using a continuous isochronal annealing method . FBG transmission/reflection spectra were acquired using a broadband light source and an optical spectrum analyzer (OSA). A split tube furnace, an additional thermocouple placed next to the FBG, and the OSA were interfaced using a personal computer to generate linear ramp rates and automatic data acquisition. The fastest heating rate was determined by the furnace specifications and the lowest rate was limited by practical considerations such as total run time (< 3 days). The three heating rates cover 2 orders of magnitude and range from 0.25 to 0.0038 K/s. Gratings subjected to annealing were not pre-annealed. The spectra of this continuous isochronal annealing method were analyzed in a way similar to that proposed by Rathje et al. , to obtain the thermal stability of the gratings.
3. Results and discussion
The evolution of mean and amplitude refractive index fabricated in the Bi-Al co-doped fiber and the SMF-28e fiber are shown in Fig. 1 . The figure insets show the corresponding reflection spectrum at the end of the irradiation process. Mean refractive index changes () were calculated from the Bragg wavelength shift and modulated index changes () were calculated using the coupled mode equations for an equivalent top hat index profile using the Gaussian-FWHM length of 412 μm. The error in the calculated refractive index changes were estimated from the level accuracy of the OSA ( ± 0.3 dB) to ± 5 × 10−5. The total irradiation doses were 1.8 and 4.3 MJ/cm2 for the SMF-28e and the Bi-Al co-doped fiber with corresponding maximum mean index changes of 13.6 × 10−4 and 3.6 × 10−4. The short gratings fabricated in the bismuth doped silica fiber had a peak reflectivity of ~22%, while the SMF gratings ~43%. The higher irradiation dose and the lower index changes achieved in Bi#10 fiber indicate a lower photosensitivity compared to the Ge-doped SMF-28e.
Three similar Bragg gratings were fabricated in each fiber for the continuous isochronal annealing experiment. The temperature range covered during the annealing process was from room temperature (T0 ~23°C) to 1000°C. The results of the isochronal annealing are presented in Fig. 2 for the Bi-Al co-doped and for the SMF-28e fiber, where the normalized integrated coupling constant (ICCnorm) is plotted as a function of temperature for the three different heating rates. The integrated coupling constant (ICC) is obtained through the measured grating reflectivity R by . By normalizing to the values before annealing, the temperature dependent reflects the changes of the refractive index amplitude with temperature. FBGs in SMF-28e and bismuth co-doped fiber start decaying (ICCnorm ≤ 0.99) at ~350°C and ~200°C, respectively. At 800°C the gratings in the SMF-28e retain ≥ 40% of their initial strength while the gratings in the Bi-Al co-doped fiber have been erased.
For a first order reaction, the annealing data for the UV-induced defects can be plotted as a function of the demarcation energy using a common decay parameter ν0 for all three ramp rates (Fig. 3 , left) , where T is the temperature in Kelvin, k the Boltzmann constant, t the time, and ν0 is measured in Hz . A decay parameter of 109 ± 2 and 1013 ± 1 Hz was obtained for the Bi-Al and the SMF-28e fiber, respectively. Maximum demarcation energies of 3.0 and 4.3 eV were observed for Bi-Al and the SMF-28e, respectively. The aging curves (Fig. 3, left) were averaged and fitted to a high-order polynomial function. The energy density distributions for each fiber were then obtained from the differentiation of the polynomial fit with respect to Ed . The resulting defect energy density is presented in Fig. 3 (right). The error was estimated from the uncertainty of the grating reflectivity measurements, the standard deviation of the averaged decay curves, the uncertainty in ν0, and the error propagation due to differentiation. With increasing energy, i.e. temperature, the reflectivity decreases, which leads to an increase of uncertainty in the grating strength. The energy density distribution in Bi-Al fiber exhibits larger errors, due to the lower grating reflectivity. A superposition of two energy distributions with maxima at 1.2 ± 0.1 and 2.3 ± 0.2 eV was observed for the Bi-Al fiber. The distribution at higher energy has a larger bandwidth. The SMF-28e is composed of several energy distributions. Two distributions can clearly be discerned with energy maxima at 3.1 ± 0.2 and 4.2 ± 0.2 eV. However, the spectrum between 1 and 2.5 eV contains additional distributions.
At the initial stage of annealing, a 10% and 4% increase in ICCnorm can be observed for the Bi-Al fiber and the SMF-28e (see e.g. Figure 3). The origin of the effect is unclear and has been observed only in non-H2-loaded fibers. The relative refractive index increases linear with temperature [9, 13, 14], and is more pronounced in high Ge-doped fibers .
Figure 4 shows the photo-induced refractive index changes of H2-loaded Bi-Al-fiber and SMF-28e obtained using cw-Ar+ laser. In both fibers ac-refractive index changes larger than 1 × 10−3 have been observed. Maximum mean (dc) and amplitude (ac) changes are summarized in Table 1 . Interestingly in SMF-28 the dc saturation index change was twice the ac value. This difference cannot be attributed to setup stability issues due to the fast irradiation time (30 seconds), because the irradiation of Bi-Al fiber showed excellent mechanical stability for much longer irradiation times (~30 minutes). The SMF ac-index changes are higher than the Bi-Al fiber values.
The results of the isochronal annealing of the H2-loaded fibers are presented in Fig. 5 . As for the pristine fibers, the normalized ICC is plotted as a function of temperature for three similar Bragg gratings annealed using three different ramp rates. Comparing the stability of the two H2-loaded fibers for the 0.25 K/s ramp rate, any photo-induced index change is bleached at a temperature near 950 °C. A 50% of the initial photo-induced refractive index change is retained up to temperatures of 678 and 781°C for the H2-loaded bismuth and the SMF-28 fiber, respectively (see Table 1). Thus, FBGs in H2-loaded SMF-28e are considered as more stable than FBGs in H2-loaded Bi-Al co-doped fiber. Smelser at al. reported thermal stability measurements for FBGs fabricated in H2-loaded SMF-28 fibers using a cw-Ar+ laser . The stability observed was very different for high (10−3) and low index changes (10−4). Our data agree quite well with their annealing data for high index changes (Fig. 5, right).
The stability of the photo-induced index change in H2-loaded Bi-Al co-doped fiber is higher than in the pristine fiber: Fifty percent of the refractive index change is retained up to 580°C and 678°C for the pristine and the H2-loaded fiber (0.25 K/s ramp rate). The grating erasure temperatures are higher for all three different annealing ramp rates.
The stability of the photo-induced index change in pristine and H2-loaded SMF-28e is quite similar up to ~500°C. Differences appear at higher temperatures: Fifty percent of the UV-induced index change is retained up to 862 and 781°C for the pristine and the H2-loaded fiber.
Similar to the pristine fibers and the analysis followed by Rathje et al. , the decay parameter was determined for the H2-loaded bismuth and standard single mode fibers to 1021 ± 1 and 1018 ± 1 Hz. However, these decay parameters are several orders of magnitude higher than the values for the pristine fibers (Table 1) and commonly reported values (see e.g. Refs [6, 9].). These high values are probably an indication that the first order reaction kinetics model commonly used for annealing analysis does not apply for all kinds of photo-induced index changes and – for the H2-loaded fibers in this work– higher order reaction kinetics have to be considered to obtain the correct decay parameter value.
In conclusion it was shown for the first time that fiber Bragg grating fabrication is possible in H2-free Bi-Al co-doped silica optical fibers using a frequency doubled cw-244-nm-Ar+ laser. Maximum refractive index changes of 3.6 × 10−4 were achieved. For the case of H2-loaded Bi-Al co-doped fiber maximum refractive index changes of up to 19.3 × 10−4 were observed. For comparison FBGs were fabricated in pristine and H2-loaded SMF-28e with maximum refractive index changes of 13.6 × 10−4 and 63.3 × 10−4, respectively. The thermal stability of the gratings was investigated through continuous isochronal thermal annealing. UV-induced index changes in H2-loaded Bi-Al fiber were more stable than in pristine fiber, but less stable than in pristine or H2-loaded SMF. While demarcation energy mapping based on first order reaction kinetics could be applied to UV-induced refractive index changes in pristine bismuth and standard telecom fibers, the method failed for the FBG fabricated in H2-loaded fibers. The demarcation energy mapping in pristine fibers revealed two distinct distributions for the bismuth fiber and a sum of several unresolved distributions for the SMF.
The authors thank Dr. A. A. Umnikov and Prof. A. N. Guryanov (Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences) for the Bi-Al fiber fabrication and Dr. S. A. Vasiliev (FORC RAS) for critical remarks. Georgios Violakis acknowledges financial support from SNSF project 200020-126900.
References and links
1. I. A. Bufetov and E. M. Dianov, “Bi-doped fiber lasers,” Laser Phys. Lett. 6(7), 487–504 (2009). [CrossRef]
2. M. A. Melkumov, I. A. Bufetov, A. V. Shubin, S. V. Firstov, V. F. Khopin, A. N. Guryanov, and E. M. Dianov, “Bismuth-Doped Optical Fiber Amplifier for 1430 nm Band Pumped by 1310 nm Laser Diode,” in OFC 2011 Optical Fiber Communication Conference and Exposition (OFC) and National Fiber Optic Engineers Conference (NFOEC) (Optical Society of America, Washington, DC, 2011), OMH1.
3. C. Ban, H. G. Limberger, V. Mashinsky, V. Dvoyrin, and E. Dianov, “UV-Photosensitivity of Germanium-free Bi-Al Silica Fibers,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides BGPP: OSA Topical Meeting, 2010), BWD3.
4. G. Violakis, H. G. Limberger, V. Mashinsky, and E. Dianov, “Strong fiber Bragg gratings in Bi-Al co-doped H2-loaded optical fibers using CW-Ar+ laser,” in OFC 2011 Optical Fiber Communication Conference and Exposition (OFC) and National Fiber Optic Engineers Conference (NFOEC) 2011), OTuC3.
5. V. V. Dvoyrin, V. M. Mashinsky, and E. M. Dianov, “Efficient Bismuth-Doped Fiber Lasers,” IEEE J. Quantum Electron. 44(9), 834–840 (2008). [CrossRef]
6. T. Erdogan, V. Mizrahi, P. J. Lemaire, and D. Monroe, “Decay of ultraviolet-induced fiber Bragg gratings,” J. Appl. Phys. 76(1), 73–80 (1994). [CrossRef]
7. C. Ban, L. I. Bulatov, V. V. Dvoyrin, V. M. Mashinsky, H. G. Limberger, and E. M. Dianov, “Infrared Luminescence Enhancement by UV-Irradiation of H2-loaded Bi-Al-doped Fiber,” in ECOC 2009 – 35th European Conference and Exhibition on Optical Communication, 2009), paper 6.1.5.
8. H. G. Limberger and G. Violakis, “Formation of Bragg gratings in pristine SMF-28e fibre using cw 244-nm Ar+-laser,” Electron. Lett. 46(5), 363–365 (2010). [CrossRef]
9. J. Rathje, M. Kristensen, and J. E. Pedersen, “Continuous anneal method for characterizing the thermal stability of ultraviolet Bragg gratings,” J. Appl. Phys. 88(2), 1050–1055 (2000). [CrossRef]
10. S. A. Vasiliev, O. I. Medvedkov, A. S. Bozhkov, and E. M. Dianov, “Annealing of UV-induced fiber gratings written in Ge-doped fibers: investigation of dose and strain effects ” in BGPP'03, Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides: OSA Topical Meeting, (OSA, 2003), MD31.
11. P. A. Redhead, “Thermal-Desorption of Gases,” Vacuum 12(4), 203–211 (1962). [CrossRef]
12. G. Violakis, H. G. Limberger, V. Mashinsky, and E. Dianov, “Fabrication and thermal decay of fiber Bragg gratings in Bi-Al co-doped optical fibers,” in European Conference and Exhibition on Optical Communication (ECOC) (Optical Society of America, Washington, DC,2011), Tu.3.LeCervin.2.
13. A. Hidayat, Q. Wang, P. Niay, M. Douay, B. Poumellec, F. Kherbouche, and I. Riant, “Temperature-induced reversible changes in the spectral characteristics of fiber Bragg gratings,” Appl. Opt. 40(16), 2632–2642 (2001). [CrossRef]
14. P. I. Gnusin, S. A. Vasil'ev, O. I. Medvedkov, and E. M. Dianov, “Reversible changes in the reflectivity of different types of fibre Bragg gratings,” Quantum Electron. 40(10), 879–886 (2010). [CrossRef]