1. Introduction

Bragg gratings generated during the drawing of a photosensitive fiber are well known as highly reliable sensing elements in the field of Bragg- grating-based sensing systems. Because of their potentially cost-effective fabrication and the retention of high fiber strength, they are suitable for many high-performance sensing applications. The first reported experiments of writing gratings during the drawing process of a fiber dates back to 1993 [1, 2]. The continuous drawing of the optical fiber allows only single-pulse exposure to generate the grating. Depending on the energy density applied, one can generate two types of Bragg gratings with respect to refractive index modification and the resulting thermal stability. Type II single-pulse Bragg gratings [3] can also be produced during the drawing process [1, 2, 4] and have been reported to be temperature-stable up to $>800°\mathrm{C}$. These gratings are also known as damage gratings and provide high reflectivity, although they are limited in use because of poor spectral profiles and high transmission losses [3, 4]. If the energy density is below the damage threshold, one can also fabricate commonly known type I gratings. Such gratings result from localized polarizability changes around color center changes; much of the index change can be determined by Kramers–Kronig analysis of the modified absorption spectra [5]. They are only suitable for maximum operating temperatures around $300°\mathrm{C}$, because they decrease rapidly from their initial refractive index modulation after a few minutes. It would be extremely attractive to produce gratings online that can be stabilized for much higher temperatures, approaching $1000°\mathrm{C}$ or more.

Recently there has been exploration of a number of high-temperature grating types such as tailored type IIa gratings [6, 7] (up to $500–800°\mathrm{C}$), stabilized type I gratings [8] (up to $500–600°\mathrm{C}$), femtosecond laser-written gratings [9, 10], so-called chemical composition gratings [11] (up to $1000°\mathrm{C}$), and regenerated gratings [12, 13] (up to $1300°\mathrm{C}$ with ${\mathrm{H}}_2$ loading). Out of all these concepts, the idea of regenerating gratings from initial seed gratings is most attractive for draw tower gratings. Earlier we reported about regeneration of fiber Bragg gratings without hydrogen loading using $248\mathrm{nm}$ irradiation [14], indicating that post-thermal processing of single-pulse draw tower gratings (DTGs) may be possible. Arrays of these gratings have also been produced [15], and it has been shown that they are equivalent to thermally generated type IIa gratings [16]. However, regeneration without hydrogen loading is, in most cases, too weak to generate useful gratings. In this article, we report on an alternative approach to regeneration by postloading online fabricated type I gratings with hydrogen and then undertaking thermal processing. This postloading and annealing approach has produced regenerated gratings with refractive indices stabilized up to temperatures of more than $800°\mathrm{C}$.

2. DTG Inscription

Draw tower grating inscription is a sensitive technological process in which the fiber drawing process has to be synchronized with the grating inscription. The draw tower grating inscription setup is shown in Fig. 1. For generating the gratings, we use a highly photosensitive fiber preform containing $18\mathrm{mol}\mathrm{.}\%$ ${\mathrm{GeO}}_2$. The fiber preform is heated in a furnace up to a temperature of more than $2000°\mathrm{C}$ and is then drawn with a velocity of approximately $10\mathrm{m}/\min$ into a fiber with a final diameter of $125\mathrm{\mu m}$. Before the fiber is coated, Bragg gratings are inscribed using single UV KrF excimer laser pulses with a wavelength of $248\mathrm{nm}$ with a modified Talbot interferometer configuration. The excimer (exciplex) laser for grating generation is from Lambda Physik (Compex150) and features pulse durations of $20\mathrm{ns}$ and an energy of $150\mathrm{mJ}$ per pulse. The master oscillator and power amplifier laser configuration in two tubes in combination with a narrow band unit for improving the coherence length of the laser enables good writing conditions. Because of the geometry of the interferometer, one can continuously adjust the Bragg wavelength by moving two rotation mirrors in order to generate arrays of different Bragg grating wavelengths with customized spatial separation. The minimum spatial grating separation is below $10\mathrm{mm}$. The energy density is adjusted by means of a cylindrical focusing lens and optimized for optimum grating reflectivity (around 20%) while avoiding type II gratings.

Directly after the grating inscription, the fiber is coated and the coating cured under UV light. Besides the commonly known acrylate coating, the DTG fibers may also be coated with Ormocer. Apart from the standard diameter of $125\mathrm{\mu m}$, the fiber has a core of approximately $4.5\mathrm{\mu m}$ and a numerical aperture of around 0.26. For measuring the gratings, a standard setup with an erbium amplified spontaneous emission source from HP (spectral range from $1510\mathrm{nm}$ to $1570\mathrm{nm}$), a $3\mathrm{dB}$ coupler, and an optical spectrum analyzer from Ando (AQ6317) were used.

3. Thermal Regeneration

After the fabrication, the gratings were loaded with hydrogen at 200 bars and a temperature of $80°\mathrm{C}$ for more than 3 days. Because of hydrogen diffusion, the fiber is then completely loaded. This process is different from conventional hydrogen loading used to increase photosensitivity before the grating writing process to enable strong regenerated gratings [13]. Immediately after hydrogen loading, the DTGs are thermally processed for regeneration. For that purpose, a grating with an initial reflectivity of 17% was heated up in a ramp from $35°\mathrm{C}$ up to $850°\mathrm{C}$ in approximately $60\min$. With a grating reflectivity of 17% and a grating length of $8\mathrm{mm}$, one can derive an initial refractive index modulation according to [16] as $\mathrm{\Delta }n=3.5\times {10}^{-5}$, which is weak compared to other multipulse gratings. The development of grating reflectivity, R, and Bragg wavelength, ${\lambda }_B$, during heating is shown in Fig. 2. After $15\min$, at a temperature of $300°\mathrm{C}$, the grating reflectivity begins to strongly decrease. This behavior is characteristic of type I gratings because of the small refractive index change due to the single-pulse exposure. After approximately $42\min$ at a temperature close to $850°\mathrm{C}$, the grating seems to have disappeared before the regeneration process starts.

During the disappearance of the grating in the noise, it is not possible to measure the Bragg wavelength. Then the regeneration process starts, and the grating strength increases to approximately $17\mathrm{dB}$ above the noise level in reflection. The final grating reflectivity was only 0.25%, but clearly measurable. Figures 3a, 3b show the grating spectrum before and after regeneration at a temperature of $850°\mathrm{C}$. The shape of the spectrum including the longer-wavelength side lobe is very similar, indicating that the phase information of the grating stays unchanged during regeneration. This indicates that there seem to be no diffusion processes involved that would lead to a change of the spectral response. This is consistent with properties found for directly regenerated gratings [17].

Furthermore, the regenerated refractive index change achieved is approximately one magnitude lower than before regeneration with $\mathrm{\Delta }n=3.7\times {10}^{-6}$. In order to check for wavelength drift or hysteresis, we performed temperature calibration from room temperature up to $800°\mathrm{C}$. For measuring the grating, we used a commercial Bragg grating interrogator from FOS&S (FBGscan 700). The interrogator has a resolution of $1\mathrm{pm}$ and a maximum sampling rate of $50\mathrm{Hz}$. Because of the regeneration process, the grating was weak in absolute reflectivity, but being $17\mathrm{dB}$ above the noise level of the interrogator, it was well measurable. The results are shown in Fig. 4. No significant drift or hysteresis was observed during the calibration. One can determine the temperature dependence of the Bragg wavelength in the quadratic approximation $\lambda (T)=s_1·T+s_2·T^2+{\lambda }_0$, due to the combination of temperature-dependent refractive index change and thermal expansion of the periodic structure. From the fit of the graph in Fig. 4, one can determine the coefficients $s_1$, $s_2$, and ${\lambda }_0$. The values are comparable with the coefficients before the regeneration of the grating.

4. Conclusion

In summary, we have been able to extend the usable temperature range of cost-effective and highly reliable draw tower fiber Bragg gratings up to $800°\mathrm{C}$. We have demonstrated that the regeneration process can be activated with hydrogen loading after the grating writing process. For the same type of grating but without hydrogen loading we did not observe any regeneration. Experiments with postloaded multipulse gratings showed even higher reflectivity after regeneration. This is comparable with writing experiments and regeneration in preloaded fibers, where the strength of the seed grating determines the grating strength after regeneration [11]. The possibility of a regeneration process of DTGs with post-hydrogen loading is a promising, cost-attractive solution for high-temperature sensing systems with fiber grating arrays. Furthermore, postloading of a fiber containing a grating may be very attractive for other fiber types, such as microstructured fibers, where hydrogen retention is difficult to achieve for inscription due to rapid outdiffusion.

Funding by the Thuringian Ministry of Education and Cultural Affairs is gratefully acknowledged. Funding from the Australian Research Council (ARC) is also acknowledged.

 

Fig. 1 Draw tower grating inscription setup.

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Fig. 2 Bragg wavelength and grating reflectivity during the annealing process.

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Fig. 3 Grating spectrum (a) before and (b) after regeneration.

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Fig. 4 Calibration graph of a regenerated DTG.

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