We exposed H2-loaded optical fibers to cw UV light and simultaneously measured the intensity of the blue luminescence from the fiber core. The UV-induced blue luminescence experiences a non monotonous evolution and thus cannot be correlated to the refractive index changes. However, a quasi-linear relationship has been found between the increase of the blue luminescence and the refractive index changes in the range 5 10-4 < Δnmean (or Δnmod) < 2.5 10-3. Using this property, we analyze a fiber Bragg grating by focusing a UV beam probe onto the fiber core and we record the UV-induced blue luminescence at the end of the fiber. By scanning the UV beam along the fiber, we measure thus the axial profile of the refractive index changes with a spatial resolution of 1 μm.
©2005 Optical Society of America
The photosensitivity of germanosilicate optical fibers is related to the bleaching of the 240 nm absorption band, associated with Germanium Oxygen Deficient Centers (GODC), when the fiber is exposed to UV light of the same wavelength. This permanent bleaching is accompanied by a change in the refractive index of the fiber and it is now routinely used to write Bragg gratings which cover a very large area of applications . When the fiber is irradiated with UV light (at 244 nm) we can observe an emission of broadband blue fluorescence from 390 to 450 nm [2–3]. The former has been the subject of numerous papers. However, attempts to measure and correlated the fluorescence to the refractive index changes, especially at 400 nm, as a function of UV exposure have generated inconsistent results. Indeed, some authors have correlated the UV-induced refractive index modulation and the evolution of the integrated blue luminescence intensity. These correlations have been studied for both hydrogen loaded or not loaded Ge-doped fibers under exposure to pulsed or continuous light at 244nm or even to pulsed laser light at 266nm [3–8] but this does not stand for any conditions. For instance, Mizrahi and Atkins  reported no change in the fluorescence from the Ge-doped optical fibre during a pulsed UV exposure that produced large changes in the index of refraction and in the UV absorption spectrum. Lauzon et al.  reported constant fluorescence from Ge-doped optical fiber exposed to 240 nm UV light, and decreasing fluorescence when exposing the fibre to 266 nm light. Hosono et al.  saw no change in the photoluminescence spectrum of bulk Ge-doped glass before and after exposure to broadband continuous-wave (cw) UV light. In this paper, we report a full study (a few J/cm2 - 100 kJ/cm2) of the kinetics of the UV-induced blue luminescence linked with the ones of the refractive index changes in standard telecommunication fibers.
The need to be able to monitor and subsequently characterize waveguide Bragg gratings filters with sufficient spectral and spatial resolution has caused the development of improved and new characterization techniques. In response to such issues, techniques such as side Rayleigh scattering profiling , optical low coherence reflectometry [13 and 14], side diffraction reported by P. Krug et al.  or side diffraction interference technique  have been developed to measure the spatial properties of fiber gratings. However for most of them, the primary limitation is the inability (or the poor precision) to measure directly the average index along the grating. This parameter is extremely important since it determines the spatial uniformity of a grating as well as the spectral shape of the transmission and reflection profiles. For instance, Δnmean must be constant in the case of apodized Bragg gratings, therefore the knowing of Δnmean is critical for high quality apodized Bragg gratings. Furthermore, most of these techniques are not applicable for long gratings or for low diffraction efficiency gratings.
In the present paper, we report a full study of the kinetics of the blue luminescence linked with the one of photo-induced refractive index changes in H2-loaded germanosilicate fibers. Then using these results, we present a new characterization technique to obtain direct, in situ, on line monitoring as well as post-fabrication measurements of fiber grating mean index. This method allows measuring and analyzing the axial distribution of local photo-induced refractive index changes (both mean index and modulation index) with a resolution of 1 micron.
2.1 Bragg Grating (BG) photo-inscription and blue luminescence record
The experiments were performed in pristine or hydrogen loaded standard telecommunication fibers (SMF 28 from Corning). The fibers were hydrogen loaded at 140 atmospheres for one month at room temperature. The pump source was a continuous laser beam emitting at λp = 244nm. A phase-mask was used to split the laser beam into two probe beams (diffraction order ±1). These two probe beams have equal path distances, and they are focused using two mirrors onto the fiber with an angle of incidence near to 27°. As the 0th order of the phase-mask was blocked, the visibility of the fringe pattern was then equal nearly to 1. The gratings in this facility were written by exposure to a continuous-wave fringe pattern moving with the fiber, which is translated with an interferometer-controlled translator stage. The relative position of the fringe pattern and fiber was measured with an accuracy of 1nm using an interferometric scale. For the experiments described below, the gratings were 200 microns long and the power density was fixed at 15W/cm2. The reflection and transmission spectra of the Bragg gratings were recorded in the course of UV exposure by means of a tunable mono-frequency laser (TUNICS of Photonetics SA) and of an optical power meter. In situ photoluminescence measurements were performed while the gratings were being written. The output light at the end of the fiber was focused by a lens onto a photomultiplier tube detector (linked with a lock-in amplifier). An electronic low-pass filter was used to reduce the amount of noise from the photomultiplier.
In Eq. (1a) and Eq. (1b), η is the proportion of the LP01 mode power propagating along the fiber core, V is the normalized fiber frequency and L is the grating length. As the Bragg wavelength shift (ΔλB) can only be measured by reference to the Bragg wavelength at the early stage of the grating formation (t>10 s), the values of ΔλB from the origin t=0 were calculated by assuming that the change in mean refractive index is equal to that in the modulation at the early formation of the grating.
3. Results and discussion
3.1 Dynamics of the UV-induced blue luminescence and refractive index changes in H2-loaded SMF 28 fibers
Figure 2 shows the kinetics of growth of the refractive index changes and of the intensity of non dispersed blue luminescence according to the UV exposure time. The open circles are for the blue luminescence while the open squares and the full squares are respectively for Δnmean and Δnmod. The cumulated UV dose used to perform the UV exposure is the parameter of the experiment. As shown in Fig. 2, the grating growths are monotonous as a function of the UV exposure time whereas it is not the case for the blue luminescence intensity. Indeed, we can observe three different behaviors according to the cumulated fluence (Fc). At low accumulated fluence (Fc < 150 J/cm2), we observe a fast decrease (see the inset in Fig. 2) as when the fiber is not hydrogenated (see Ref 3). The inset of Fig. 2 shows this typical behavior of the kinetics at the beginning (between 0s and 500s) of the UV exposure. For higher cumulated fluence, the intensity of blue luminescence presents a growth until a fluence of 45kJ/cm2 and then a slow decrease. This previously unreported phenomenon suggests that H2 catalyses a new cycle in the UV absorption and photoluminescence processes. As it is reported , the photo darkening can mask the temporal behavior of the luminescence signal. We have thus verified that this effect does not play a significant role in our experiment.
Analysis of the kinetics allows the measurement of the evolutions of the two quantities: index changes (Δnmean or Δnmod) and blue luminescence. The evolutions of Δnmod and Δnmean are thus shown in Fig. 3 according to the intensity of the blue luminescence. The parameter of the experiment is the UV accumulated fluence. In this figure, the symbols have the same meaning as those in Fig 2. As it can be seen, the refractive index changes and luminescence intensity evolve on the same time scale (a few 10 kJ/cm2) but their dynamics are quite different. In contrast to the results obtained in non H2-loaded fibers , it is worth noticing that no correlation could be observed between the two quantities. Indeed, at first, the luminescence is strongly bleached with a couple of 150 J/cm2 whereas the refractive index changes are small. After that period, we can notice a quasi-linear proportionality in the range 5 10-4 to 2.5 10-3 between the photo-induced refractive index changes (both mean index and modulation index) and the blue luminescence intensity change. Indeed, we can observe that Δnmean and Δnmod coincide during the initial UV exposure (Δn < 2.5 10-3). Finally for refractive index changes higher than 2.5 10-3, the luminescence experiences a decrease while the refractive index changes continue to growth.
To conclude, after a preliminary calibration, the blue luminescence intensity may be used for photo-induced refractive index changes monitoring in the range 5 10-4 < Δnmean or Δnmod < 2.5 10-3. Also, we can note that due to the fact that a luminescence grating appears, it is possible to use it for fitting the interference pattern for a second irradiation step. Indeed, the method allows the user to place the grating in a correct position for a new inscription after a translation of the fiber along oz, provided the translation is small enough to keep a spatial overlap between a part of the grating and the UV fringe pattern. Especially, grating concatenation can take profit of that property either in pristine  or in H2-loaded fibers [19, 20].
3.2 Application: side reading of Bragg gratings using the blue luminescence
Figure 4 shows the experimental setup proposed and demonstrated in this paper for analyzing fiber Bragg gratings. When characterizing a grating, the UV fringe pattern is kept fixed and the power of the UV source turned down using a neutral density filter in order to reduce the UV power density to less than 0.1W/cm2. The UV probe was focused onto the fiber core through the cladding by means of a microscope objective. As a result, the grating will be evenly exposed to UV light, causing a slight change of the average mean index. If the refractive index change in the fiber is close to saturation, an increased UV dose will also start erasing the grating. Additionally, when interrogated the gratings, in order to minimize the effect of an additional exposure, a high scanning speed was used for the Δnmean (or Δnmod) measurement. The UV beam was not circular at the grating place: it was like a “slice” of UV light perpendicular to the fiber axis with a thickness of 1 micron. The thickness of the focusing point determined the precision of the characterization. The UV slice was then scanned along the fiber axis in order to excite the local blue luminescence at the grating place. The level of blue luminescence is recorded by means of a broadband photomultiplier tube detector and a lock-in amplifier. We call this method: the “side blue luminescence reading”.
3.3 BG characterization using “side blue luminescence reading” and comparison with the “side diffraction technique”
After the inscriptions, some gratings were “side probed” using two methods. The first one reported by P. Krug et al. allows the measurement of the photo-induced amplitude of the modulation index using a side diffraction technique . The accuracy of our measurement was nearly equal to 10 microns. The second method is the side blue luminescence reading described above (see § 3.2). The figure 5 displays the signals delivered by the detector as a function of the position of the grating along the fringe pattern (displacement along Oz). The open squares are for the side diffraction method while the full ones are for the side blue luminescence reading. As it is shown in this figure, the side diffraction technique displays a BG with a uniform rectangular profile. Whereas, using the side blue luminescence reading, one can observe some “shoulders” on each side of the grating.
This axial blue luminescence intensity profile measured for the grating under test is in good agreement with the observations reported on the experimental set up (see Fig. 1). Indeed, these “shoulders” seem to correspond to a continuous ultraviolet exposition. According to the angle between the two ultraviolet beams and the distance of the fiber to the slit, the length of the fringe pattern is close to 200 microns. The length of the fringeless exposure (not measurable by the side diffraction technique) on each side of the fringe pattern is near to 100 microns. Such conclusions are in agreement with our observations (see Fig. 5). In these conditions, blue luminescence allows to monitor both the photo-induced mean index and the modulation index whereas the side diffraction method shows the refractive index modulation amplitude profile.
Indeed, it is obvious that the knowledge of the mean index remains the main key to write high quality apodized gratings. For example, it appears that, when writing an apodized grating, although the irradiance distribution could be perfectly apodized along Oz, a distortion of the refractive index profile along Oz comes from the non-linear relationship between Δn and the fluence per pulse  or from the saturation effect. This in turn leads to a non uniform distribution of Δnmean along the grating length. The side blue luminescence reading allows us to follow the mean index during the grating writing and correct its value if necessary.
Secondly, we probe the efficiency of the method by writing two gratings with different photo-induced mean index. This in turn implies that the two gratings have different contrast. A first 1 mm long gratings (G1 and G2) were written (step 1) in H2-loaded SMF 28 fibers using a phase mask The UV exposure time was 15 min and the power density 15W/cm2. Then the laser beam was blocked and the phase mask was removed. After this step, the uniform post-exposure of the G2 grating could be achieved (step 2) during 15 min more. These two gratings were then “side probed” using the two methods. This comparison is presented in Figs. 6 and 7.
As it can be seen in Fig. 6, the modulation index profile is nearly the same for the two gratings. Whereas, the axial mean index amplitude profile (see Fig. 7) measured with the “side blue luminescence reading” for the G2 grating under test is in good agreement with our experimentation. Indeed, the Δnmean is nearly two times higher than that of the G1 grating. Repeating the experiment yields curves resembling the ones in Fig. 7 with reproducibility errors of 2 % for the level of blue luminescence. This in turn allows Δnmean (or Δnmod) measurements with an accuracy near to 2 10-5. As long as the fiber is cleaned, the same reproducibility is obtained when performing scans directly after each other or with an interval of a few days.
In contrast to what is observed in non H2-loaded fibers , we have shown that no correlation could be observed between the evolution of the refractive index changes and that of the UV-induced blue luminescence intensity. Therefore, after a preliminary calibration of the blue luminescence intensity it is possible to monitor the axial profile of both Δnmean and Δnmod in the range 5.10-4 < Δnmean (or Δnmod) < 2.5 10-3. This new method allows then to measure the axial variation of the refractive index changes and especially the mean index of an optical fiber Bragg grating with a spatial resolution of approximately 1 μm. This method is advantageous because both writing step and characterization method can be performed with the same optical bench. The technique that we used is simple and can be expected to characterize both uniform and non uniform Bragg gratings, as well as other resonant fiber devices. We just need to add an attenuator at the time of the characterization. Because of the low level of UV beam intensity; there is negligible danger of fiber damage when using this technique. As a consequence the side luminescence reading can be use in order to characterize the gratings.
This project was supported financially by the EEC PLATON contract (IST-2002-381668).
References and links
1. M. Douay, W. X. Xie, T. Taunay, P. Bernage, P. Niay, P. Cordier, B. Poumellec, L. Dong, J. F. Bayon, H. Poignant, and E. Delevaque, “Densification involved in the UV-based photosensitivity of silica glasses and optical fibres,” J. Lightwave Technol. 15, 8 (1997) [CrossRef]
2. Y. Duval, R. Kashyap, S. Fleming, and F. Ouellette , “Correlation between uv-induced refractive index and photoluminescence in Ge-doped fibre,” Appl. Phys. Lett. 61, 25 (1992) [CrossRef]
3. M. Poirier, S. Thibault, J. Lauzon, and F. Quellette, “Dynamic and orientational behavior of UV-induced luminescence bleaching in Ge-doped silica optical fiber,” Opt. Lett. 18, 870 (1993) [CrossRef] [PubMed]
4. H. Patrick, S. L. Gilbert, and A. Lidgard, “Decrease of Fluorescence in Optical Fiber During Exposure to Pulsed or Continuous-Wave Ultraviolet Light,” Opt. Mater. 3, 209 (1994) [CrossRef]
5. J. Martin, G. Atkins, F. Ouellette, M. Têtu, J. Deslauriers, and M.A. Duguay , “Direct correlation between UV-excited photoluminescence and refractive index change in photosensitive Ge-doped and hydrogenated optical fiber,” BGPP Technical Digest Series 22 Portland USA, paper PMA2 (1995)
6. H. Kuswanto, F. Goutaland, A. Boukenter, and Y Ourdane , BGPP 2001 Technical Digest, Stresa, Italy, paper BThC26 (2001)
7. T. Komukai and M. Nakazawa, “Fabrication of high-quality long-fiber Bragg grating by monitoring 3.1-eV radiation (400 nm) from GeO defects,” IEEE Photonics Technol. Lett. 8, 11 (1996) [CrossRef]
8. H. Kuswanto, F. Goutaland, and A. Boukenter, Ourdanen “UV influence on H2-loaded germanosilicate optical fibres defect transformation mechanism through the blue luminescence,” POWAG 2002, Saint-Petersburg, Russia 17-21 June (2002)
9. V. Mizrahi and R. M. Atkins, “Constant fluorescence during phase grating formation and defectband bleaching in optical fibres under 5.1 eV laser exposure,” Electron. Lett. 28, 2210 (1992) [CrossRef]
10. J. Lauzon, M. G. Sceats, P. A. Krug, and F. Ouellette, Tech. Dig. OFC, paper WK14, 137 (1994)
11. H. Hosono, Y. Abe, D. L. Kinser, R. A. Weeks, K. Muta, and H. Kawazoe, “Nature and origin of the 5-eV band in SiO2:GeO2 glasses,” Phys. Rev. B 46, 11445 (1992) [CrossRef]
12. J. Canning, M. Janos, and M. G. Sceats, “Rayleigh longitudinal profiling of optical resonances within waveguide grating structures using sidescattered light,” Opt. Lett. 21, 609 (1996) [CrossRef] [PubMed]
13. K. Takada, I. Yokohama, K. Chida, and J. Noda, “New measurement system for fault location in optical waveguide devices based on an interferometric technique,” Appl. Opt. 26, 1603 (1987) [CrossRef] [PubMed]
14. P. Lambelet, P. Y. Fonjallaz, H. G. Limberger, R. P. Salathe, Ch. Zimmer, and H. H. Gilgen, “Bragg Grating Characterization by Optical Low-Coherence Reflectometry,” IEEE Photonics Technol. Lett. 5, 565 (1993) [CrossRef]
16. F. El-Diasty, A. Heaney, and T. Erdogan, “Analysis of Fiber Bragg Gratings by a Side-Diffraction Interference Technique,” Appl. Opt. 40, 890 (2001) [CrossRef]
17. G. Meltz and W.W. Morey, “Bragg grating formation and germanosilicate fiber photosensitivity,” SPIE, Inter. Workshop on Photoinduced Self-Org. effects in Opt. Fib. 516, 185 (1991)
18. H. N. Rourke, S. R. Baker, K. S. Baulcomb, K.C. Byron, S. M. Ojha, and S. Clements, “Fabrication and characterisation of long, narrowband fibre gratings by phase mask scanning,” Electon. Lett. 30 (16), 1341 (1994) [CrossRef]
19. L. Pacou, M. Lancry, P. Niay, M. Douay, I. Riant, B. Poumellec, and D. Dragoe, “Dynamics of the UV blue luminescence intensity: observation of the local mean photoinduced refractive index in Bragg grating,” BGPP conference Monterey/USA, paper TuD3, 229 (2003)
20. L. Paccou, P. Niay, L. Bigot, M. Douay, and I. Riant, “Nano-positioning of a Bragg grating inscription setup for the concatenation of multiple gratings using the diffraction by the grating experienced by the ultraviolet inscription beam,” Optics and Lasers in Engineering 43, iss. 2, 143 (2005) [CrossRef]
21. B. Leconte, University of Lille, France, P.H.D. thesis n°2379, available on request (1998)