Bragg gratings were inscribed in H2-loaded SMF-28e optical fibers and measured for axial stress changes for various exposure doses. Mean refractive index changes as high as 7.5 × 10−3 were observed under cw-244 nm irradiation of 143 W/cm2. Bragg grating reflectivity >99% was achieved for 0.7 mm long (1/e2) gratings. Axial stress measurements realized before and after UV exposure of the fibers, show two competing dose-dependent photosensitivity mechanisms: Negative stress changes at the early stages of exposure and positive stress changes for high exposures.
©2012 Optical Society of America
Hydrogen loading of optical fibers is a commonly used photosensitization method, realized prior to UV exposure of fiber Bragg grating (FBG) fabrication . Despite the extended research in optical fiber photosensitivity, the exact nature of the underlying physical mechanisms is still not yet fully understood. In pristine fibers the dominating mechanisms are color center changes , and glass compaction [3,4]. Local heating and cooling may relax stresses, induce dopant diffusion , or change the glass history . Glass compaction leads to changes of the core stress which can therefore be used to trace the presence of compaction . Tensile stress increase due to compaction was observed in pristine fibers using different low and high intensity lasers with different total dose: 242 pulse dye laser , femtosecond laser , and cw-Ar+ laser .
In general it is believed that H2 molecules react with a UV-laser excited regular Ge—O bond producing SiOH or GeOH and a GeE’ center , as well as other Ge-related defects . For fibers irradiated with small femtosecond exposure dose and total refractive index (RI) changes up to 10−3, a photosensitivity without stress changes was observed . This confirmed the idea that H2-loaded fibers show color center changes only if no thermal annealing is involved. Recently, in Bi-Al-doped hydrogen loaded fibers a positive stress change was reported , which was assigned to compaction as in GeO2-doped fibers .
On the other hand research on the interaction of silica glass containing different OH content with radiation started more than thirty years ago. The use of silica lenses for semiconductor lithography equipment that operates at 193 nm (ArF excimer laser) triggered a lot of research lead by the silica glass manufacturers on the interaction with pulsed lasers operating at 248, 193, and 157 nm [13–16]: A superposition of two different effects, i.e. rarefaction and compaction that depend on laser photon energy, laser intensity, dose, and OH concentration was reported [15,16].
In this work, a cw-244-nm-Ar+ laser was used to fabricate Bragg gratings in H2-loaded SMF-28e optical fibers using various exposure doses. Each fabricated grating was measured for axial stress changes and compared to a non-irradiated H2-loaded fiber sample . The evolution of core stress changes as a function of exposure dose provides insight in the photosensitivity mechanisms related to compaction and rarefaction.
FBGs were fabricated in H2-loaded SMF-28e optical fibers using a cw-Ar+ laser (244 nm) with a laser intensity of ~143 W/cm2 at the fiber core. In total 5 Bragg gratings were produced with irradiation times from 1 to 36 minutes corresponding to a total UV fluence of 9 to 307 kJ/cm2. The produced FBGs had a Gaussian profile and their 1/e2 length was 0.7 mm. The beam was focused perpendicular to the fibers using a cylindrical lens of f = 102 mm to achieve a 1/e2 size of 71 μm. A phase mask (Λ = 1066.33 nm) was used to create the necessary interference pattern. Grating fabrication was monitored online using a commercial FBG interrogator which provided the reflection and transmission spectra. The mean refractive index changes were calculated from the acquired grating spectra. Hydrogen loading was performed under a pressure of ~150 bars for 2 weeks at room temperature prior to irradiation. The axial core stresses of the optical fibers were measured using a polariscope (Fig. 1 ) which consists of a polarization controller, a de Sénarmont compensator and imaging optics . The detector is a 12-bit camera with 1392 × 1040 pixels, each having a size of 6.45 × 6.45 μm. An image consists of 1024 × 1024 pixels. The spatial resolution perpendicular to the fiber axis of 0.7 μm is given by the diffraction limit due to objective ( × 20) and the laser used (632.8 nm). The stress is averaged along the fiber axis using all 1024 lines. This leads to a spatial resolution of about 240 μm. This improves the resolution of the measured retardation to below 1 nm. The stress values have a larger standard deviation at the core center due to the Abel inversion. However, by averaging the stress over the core the core stress error is estimated to be below ± 1 MPa. The measured birefringence is composed of elastic stress and drawing induced inelastic strain birefringence [19,20], which was separated by assuming the inelastic strain to be constant over the fiber diameter and the elastic stress area integral to be zero [12,21].
3. Results and discussion
In Fig. 2(a) the UV beam exposure of one of the 5 fabricated FBGs is presented as the UV-induced mean index change versus exposure dose. The FBG with the longest exposure duration was chosen. The refractive index evolution during UV irradiation can be roughly split into two parts. In the first part a rapid increase is observed which is followed by an almost saturated evolution in the second part. Within the first 2 minutes of irradiation a mean RI change of 5 × 10−3 was achieved. As the exposure continued, the signal saturated to a value close to 7 × 10−3. In Fig. 2(b) a typical transmission spectrum of the fabricated gratings at high total fluence is presented. The refractive index amplitude was estimated to 5 × 10−3.
Figure 3 shows the axial stress distribution of pristine and H2-loaded SMF-28e optical fibers. H2-loading has only a small effect on the stress distribution in contrary to the SMF-28 fiber investigated in . The stress distribution of the pristine fiber is a superposition of thermal induced stress (due to the different thermal expansion coefficients of core and cladding) and the drawing induced stress which leads to a negative (compressive) stress of −21 MPa in the fiber core. The core stress value was calculated as the average over the core area and the error is less than ± 1 MPa. The positive stress in the cladding compensates the negative core stress. The average core stress was found to be −19 MPa and −21 MPa for the pristine and the H2-loaded fiber, respectively, while the inelastic strain remained practically unchanged.
In Fig. 4 the evolution of the axial stresses is presented for different exposure doses using a UV beam intensity of ~143 W/cm2. This intensity corresponds to the evolution of grating refractive index change as shown in Fig. 2(a). At the beginning of the irradiation the compressive core stress () increases () with dose. The unexposed (pristine) fiber exhibits an average core stress value of −21 MPa, which is reduced down to −47 MPa after 3 minutes of irradiation (28 kJ/cm2). As the core is attached to the cladding, the effect is due to an expansion of the irradiated photosensitive core. The inner cladding area (extending around 4 μm away from the fiber core) is also exhibiting stress changes, which follow the same trend as the core stress changes (initially, stress reduction, followed by an increase), but are much less in magnitude, which leads to the conclusion that UV irradiation changes the stresses mainly in the photosensitive fiber core. The inelastic strain of the pristine fiber was calculated from the birefringence data to be 5.5 MPa and remained practically unaltered for every FBG fabricated in this work. As the inelastic strain can be thermally annealed, this is an indication that the temperature during UV irradiation stayed below the annealing point. For fibers irradiated with substantially less exposure doses and which exhibited less total refractive index changes (up to 10−3), color center photosensitivity without stress changes was observed .
Figure 5 shows the evolution of the axial core stress change with UV-dose. The negative core stress at the beginning of irradiation corresponds to rarefaction, while the positive stress that seems to saturate corresponds to compaction of the fiber core. Two different mechanisms with different dose dependence and amplitude are superposed with the compaction mechanism dominating for high dose. It is interesting to note that such a superposition of initial rarefaction followed by compaction was observed in silica containing OH under pulsed laser irradiation (see [15,16] and references therein).
The fact that compaction dominates at higher dose means that rarefaction saturates quickly. The saturation value is on the order of the maximum negative stress change observed (−25 MPa). Assuming that both stress changing effects superpose independently the compaction contribution at 300 kJ/cm2 is estimated to 60 MPa. This increase in compressive stress (negative stress change) is only observed in the H2-loaded fiber. Without hydrogen, the stress change is only positive [3–7]. The stress change in pristine SMF-28e is around 110 MPa for an irradiation with 177 W/cm2 and a total dose of 2 MJ/cm2 . A rough estimation of the compaction in pristine fiber for 140 W/cm2 and a total dose of 300 kJ/cm2 would be close to the above estimated compaction value of 60 MPa.
The origin of the positive and the negative stress changes is different. While compaction is most probably due to the collapse of higher order ring structures  the rarefaction which is the decrease in density due to an expansion of the material may be caused by the formation of SiOH [13,14] or similarly by the formation of GeOH, which is produced by laser irradiation in H2-loaded fibers .
The photoelastic contribution to the index change (Δnpe) is presented as a function of exposure dose, along with the total mean index change (Δndc) in Fig. 5 as well. The photoelastic contribution was calculated using:23] and Δσz,core is the axial core stress change (Pa). The ratio of photoelastic index changes to the total mean index changes, yields a percentage of only 2 – 3%, indicating that the photoelastic contribution to the total index change is almost negligible.
In conclusion, it was shown that in addition to the well-known photolytic process two competing photosensitivity mechanisms take place during 244 nm cw-Ar+ exposure of H2-loaded SMF-28e optical fibers. During the early stages of exposure the prevalent mechanism is a dilation inducing process. With prolonged exposure, a compaction inducing process takes over leaving the fiber core with positive stress changes. The refractive index change contribution due to photoelasticity changes is less than 3%.
G. Violakis and N. Aggarwal acknowledge financial support from SNSF projects 200020-126900, 200020-138012 and 200020-127183, respectively.
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