An ArF excimer laser was used to fabricate Bragg gratings in fibers with Bi-SiO2 core and microstructured or F-doped claddings without fiber presensitization. Average and modulated refractive index changes of 2.7 × 10−4 and 1.0 × 10−4 were induced in pristine microstructured fiber while 1.0 × 10−4 and 0.7 × 10−4 were observed in the F-doped-cladding fiber. Fiber luminescence was also measured under 1064 nm pumping for both fibers. Photosensitivity and luminescence were compared to a Bi-Al2O3-SiO2 core optical fiber.
© 2012 OSA
Bismuth doped optical fibers exhibit luminescence over a wide wavelength range . This makes them prime candidates for fiber devices such as lasers, amplifiers and light sources [2, 3]. Lasers have been realized in Bi-Al-co-doped SiO2 optical fibers with slope efficiencies up to 14% at 1215 nm (pumped at 1064 nm)  and in Bi-Ge-co-doped SiO2 fibers with a slope efficiency up to 60% at 1460 nm (pumped at 1340 nm) . Recently fiber lasing action was demonstrated in pure step index (SI) Bi-SiO2-core, F-doped inner cladding fiber , while luminescence [5, 6] and lasing  were observed in microstructured Bi-SiO2 optical fibers as well. Despite the fact that fiber Bragg gratings were realized in a core-size matched germanosilicate fiber, the splicing of microstructured and standard single mode fibers resulted in variable laser slope efficiency . Therefore it is of prime interest to realize fiber Bragg grating mirrors in the micro-structured fiber as well.
Photosensitivity in Ge-doped microstructured optical fiber (MOF) has been reported as early as 1999 . Photosensitivity in pure SiO2 fiber under 193 nm irradiation has been reported for step index  and microstructured fibers  and recently for Bi-Al2O3 silica based fibers . Without germanium doping the photosensitivity was attributed to a great part to the presence of bismuth, without excluding an aluminum contribution. The thermal stability of FBGs in pristine Bi-Al-SiO2 doped fibers was reported just recently .
The photosensitivity can be enhanced using H2-loading (presensitization) leading also to higher refractive index changes in Bi-Al2O3 silica fibers . On the other hand it was shown that H2-loading increases both luminescence and absorption. So, it is important to investigate the photosensitivity of pristine fibers .
In this work the photosensitivity of a pure Bi-SiO2 MOF  without prior fiber presensitization was examined under 193 nm laser irradiation by fabrication of Fiber Bragg gratings (FBG) and compared to the photosensitivity in Bi-Al2O3-SiO2 fibers. In addition to the original work in ECOC  a pure Bi-SiO2 step index fiber with F-doped cladding  was examined for photosensitivity. The luminescence of both pure Bi-SiO2 core fibers was examined under 1064 nm pumping and compared to Bi-Al2O3-SiO2 co-doped step index fiber.
A 15 ns 193 nm excimer laser was used to fabricate the Bragg gratings in the pristine fibers using the phase mask technique. A phase mask of 1060 nm pitch was used to create the required interference pattern in the fiber core and the light was focused to a 10 × 2 mm2 area using a cylindrical lens of 31 cm focal length. The energy density used to inscribe the gratings was about 100 mJ/cm2 and the laser repetition rate was f = 10 Hz. Fiber transmission and reflection spectra were monitored online using a commercial fiber Bragg grating interrogator. Average (Δndc) and modulated (Δnac) refractive index changes were calculated from the acquired spectra considering the respective overlap integral. The step index Bi-Al co-doped optical fiber has a Bi concentration of less than 0.02 at.%. The Al2O3 concentration of 2.6 mol.% provides sufficient index change in the core to allow for wave guiding . The microstructured Bi-SiO2 fiber has a bismuth concentration of ~0.03 at.% in the core and a 6.3 μm diameter silica core. Two concentric rings of 6 holes each around the fiber core (see SEM image in Fig. 1(a) ) reduce the effective index of the cladding and allow light guidance. A summary of all the fiber specifications used in this work is presented in Table 1 . A more detailed description of the microstructured fiber and its fabrication can be found in . The MOF fiber is multi-mode. A custom mode solver for an equivalent step-index fiber was used to confirm that the measured mode was the fundamental one and to estimate the confinement factor of the mode for the evaluation of the refractive index changes. For every fiber the effective index of the mode neff was obtained from the Bragg wavelength resonance (Table 2 ). The step index pure Bi-SiO2 core fiber had a Bi concentration below the detection threshold (as the Bi-Al step index fiber) and light guidance was achieved by introducing fluorine in the cladding which reduces its refractive index (see also ). The core diameter of this fiber was measured to be dcore = 6.8 ± 0.3 μm using a microscope. The fiber parameters of the Bi-Al2O3 silica based fibers can be found in . Assuming an equivalent step-index fiber it is possible to calculate the V-number and the overlap integral, where dcore is the core and MFD the mode field diameter for the step index fibers. For the MOF fiber which is multimode the overlap integral is estimated to ~0.95 (see Table 2).
A commercial ray tracing software  was used to calculate the amount of light that is scattered from the holes in the microstructured fiber to estimate the light intensity for different fiber rotation angles α (Fig. 1(b)). The results were compared to a step index fiber. A virtual power detector was placed at the fiber core to estimate the power density that reaches the fiber core. To simplify the MOF holes were considered circular (instead of elliptic) with a diameter of d = 6.3 μm and a diameter to pitch ratio (d/Λ) of 0.9. Figure 1(b) shows the situation with 500 layout rays, while simulation was performed using 2’000’000 rays.
Luminescence was measured in all fibers using a cw-Yb-fiber laser operating at 1064 nm for pumping. The light from the laser was coupled to the core of a standard single mode fiber (SMF-28e) with a coupling efficiency of ~35%. At the exit of the single mode fiber, bare fiber adaptors were used to couple light into the step index or the microstructured fiber. Luminescence was measured from the side of the optical fibers using a 910 μm core diameter, 0.22 NA probe fiber that was coupled to an optical spectrum analyzer (OSA).
3. Results and discussion
The MOF used in these experiments has a hexagonal symmetry and in the ray tracing simulations two different laser incidence angles were examined: α = 0° and α = 30° covering the two extremes of the fiber’s symmetry. At α = 0° incidence, the power density in the MOF core is 22% less than that in a step index fiber. At 30° incidence (see Fig. 1(b) the power density reaching the core is 12% less. The difference between the two orientations is 10% and gives the error in the energy density calculation for this fiber, as the rotation angle of the fiber was random during the UV irradiation. The Bi-SiO2 MOF was irradiated for a total time of t = 75 minutes with an energy density per pulse of Fp = 110 mJ/cm2. The average and modulated refractive index change evolution is presented in Fig. 2(a) . At the end of the exposure the fiber temperature increased by ~6 K (−60 pm wavelength shift) and the index evolution data were corrected for it. After a total dose of F = Fp × f × t = 5.3 kJ/cm2 the average and modulated index changes observed were 2.7 × 10−4 and 1.0 × 10−4, respectively. The observation of the Bragg wavelength after a small number of pulses allowed for the determination of the effective index of this fiber. Taking into account the minimal amount of Bi in the core, the refractive index of the core is considered equal to that of a pure SiO2 core.
The 10 mm long Bragg grating inscribed in the MOF exhibited a reflectivity of 93.6% (Fig. 2(b)). Doubling the length would result in a 99.9% reflectivity indicating the possibility to fabricate high reflectivity laser mirrors in this fiber. The background ripples were the result of a Fabry-Perot cavity formed between the SMF and the MO fiber, connected by bare fiber adaptors, without index matching gel. Splicing the MOF causes the collapse of the cladding holes and significantly increases the coupling losses. The bare fiber adaptors provided a smoother connection of this fiber to a standard step index single mode fiber.
The second Bi-SiO2 core optical fiber had a step index design with a core radius of 3.4 μm and an F-doped inner cladding radius of 16 μm. The fiber was irradiated for t = 65 minutes with an energy density per pulse of Fp = 110 mJ/cm2 and a repetition rate f = 10 Hz resulting in a total dose of: F = Fp × f × t = 4.2 kJ/cm2. Average and modulated index changes of 1.0 × 10−4 and 0.7 × 10−4 respectively were obtained (Fig. 3(a) ). The values are smaller than those for the MOF and could be attributed to a lower Bi concentration in the core. The reflection spectrum of the 10 mm long Bragg grating is presented in Fig. 3(b).
The photosensitivity of the Bi-MOF and the Bi-SI silica fibers was compared to a step index Bi-Al2O3 co-doped fiber . The fiber was irradiated with a 100 mJ/cm2 energy density per pulse for a total of 60 minutes. During the exposure the fiber temperature had increased by ~13 K which is more than double the temperature increase in the MOF and significantly higher than the increase in the SI Bi-SiO2 fiber. This is an indication of a higher absorption of 193 nm light by the Al2O3 in the step index fiber core. The index evolution (Fig. 4(a) ) was corrected for this temperature increase. A maximum average and modulated RI change of 3.4 × 10−4 and 1.8 × 10−4 respectively was achieved after a total irradiation dose of 3.48 kJ/cm2. Figure 4(b) shows the FBG transmission spectrum at the end of the irradiation. A reflectivity of 99.9% was obtained. The fringe visibility was about 55% in this case as compared to 38% for the MO fiber. The smaller fringe visibility in the case of the MOF is attributed scattering by the cladding holes. The higher mean RI change observed in the Bi-Al fiber compared to the other two must be attributed to the presence of Al2O3, as the Bi core concentration in all fibers is quite low and the pure SiO2 fibers show refractive index changes of ~2.0 × 10−6 for comparable total ArF dose (5 kJ/cm2) [8, 9].
The side luminescence measurements of the three fibers are presented in Fig. 5 . All fibers were pumped with ~1.4 W of 1064 nm laser, as measured at the output of the single mode fiber using an OSA with 2 nm wavelength resolution. Connection between the SM28e fiber and the bismuth silica fibers was realized using bare fiber adaptors. Losses between SMFand the step index Bi-SiO2 or MOF fiber were measured to 0.85 and 3.18 dB. Both Bi-SiO2 core fibers have the luminescence signal centered at 1390 nm, while in the Bi-Al fiber the luminescence is centered at 1130 nm. Compared to the Bi-SiO2 fiber luminescence measured by Neff et al. the peak maxima of the Bi-SiO2 in this work are shifted by −35 nm , which might be due to the different excitation pathways. However, the Bi-Al-doped fiber luminescence maximum is close to the value of 1140 nm reported by Neff et al. .
The luminescence in the Bi-SiO2-fibers (MOF and Bi-SI) increased with pump power up to a saturation of ~3 W, while the Bi-Al-SiO2 fiber was already saturated at 0.7 W. For comparable input power of 1.4 W the luminescence peak signal of the SI Bi-SiO2 fiber exhibited an 8 dB weaker signal than the MOF Bi-SiO2, which – along with the lower photosensitivity of this fiber – indicates a lower concentration of Bi associated centers, both absorbing at 193 nm and responsible for the near infrared luminescence.
It was demonstrated that it is possible to fabricate Bragg gratings in pure Bi-SiO2 core microstructured and step index optical fiber without any prior fiber presensitization. The photosensitivity of these fibers was attributed to the presence of bismuth. Refractive index changes would be sufficient to realize fiber lasers cavity mirrors which should improve the laser slope efficiency.
G. Violakis acknowledges financial support from SNSF projects 200020-126900 and 200020-138012.
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