The high order cladding modes of standard single mode optical fiber appear in quasi-degenerate pairs corresponding to mostly radially or mostly azimuthally polarized light. In this work, we demonstrate that, in the presence of a high-refractive-index coating surrounding the fiber outer surface, the wavelength spacing between the orthogonally polarized cladding modes families can be drastically enhanced. This behavior can be advantageously exploited for refractometric sensing purposes. For this, we make use of tilted fiber Bragg gratings (TFBGs) as spectral combs to excite the orthogonally polarized cladding modes families separately. TFBGs were coated with a nanometer-scale transparent thin film characterized by a refractive index value close to 1.9, well higher than the one of pure silica. This coating brings two important assets: an ~8-fold increase in refractometric sensitivity is obtained in comparison to bare TFBGs while the sensitivity is extended to surrounding refractive index (SRI) values above 1.45.
© 2013 Optical Society of America
Surrounding refractive index (SRI) sensing has a wide range of applications in various areas such as quality control, environment, industrial processing or life sciences. Fiber gratings yield miniaturized sensors that offer remote operation in very small volumes of analytes. In practice, uniform fiber Bragg gratings (FBGs) can be used for localized SRI measurements only if the fiber geometry is altered [1,2], either etched or side-polished. Long period gratings (LPGs) or tilted FBGs (TFBGs) are more convenient to use in practice, as they preserve the optical fiber structural integrity [3,4]. TFBGs present the additional benefit of yielding intrinsic temperature-insensitive measurements. These features result from the presence of the core mode resonance and several tens of cladding mode resonances in the transmitted spectrum of TFBGs. Indeed, the core mode resonance is confined in the fiber core and is therefore only sensitive to temperature and strain variations. As they propagate towards the surrounding medium interface, the cladding mode resonances can be used for refractometry purposes. Keeping TFBGs unstrained, temperature-insensitive surrounding refractive index measurements are obtained by referencing the cladding mode resonance shifts to the core mode one, as done in [4–7]. The two kinds of couplings are induced by the refractive index modulation slightly tilted with respect to the perpendicular to the optical fiber axis. An important property of TFBGs is that they can separately excite radially (P-polarized mode) and azimuthally (S-polarized mode) polarized cladding mode resonances .
Over the past few years, there has been a rising interest in using TFBGs for SRI sensing [4–7] and for sensing chemical or biochemical quantities [8,9]. Several demodulation methods have been reported, based on whole spectrum analysis [4,6], on single cladding mode resonance tracking , on transmission power measurement  or polarization dependent loss measurement . Experimental investigations show that bare TFBGs can be used as refractometers accurate to 10−4 RIU (refractive index unit) both in the 1.33 – 1.45 range [4–7] (between water and silica refractive index) and above 1.50 [12,13]. The range between 1.45 and 1.50 cannot be addressed by bare TFBGs, as their transmitted spectrum is completely smooth due to the fact that all the cladding mode resonances are radiated. This limitation has been alleviated thanks to the use of a nanometric-scale buffer coating deposited around the TFBGs [14–16].
Besides these works on TFBG refractometers, numerous studies have been carried out on the impact of high-refractive-index coatings on the refractometric response of LPGs [17–19] and heterogeneous structures . In particular, a transition between cladding modes and coating modes has been demonstrated, resulting in an enhanced SRI sensitivity. These studies have been conducted only on the few cladding modes that LPGs are able to excite. As TFBGs behave as dense spectral combs, their great benefit in comparison to LPGs is their ability to reveal the complete behavior of the cladding modes supported by the optical fiber. They are able to resolve the differential evolution between P- and S-polarized modes, as done in the case of gold-coated TFBGs [21–23].
For the first time to the best of our knowledge, this paper studies the differential behavior between P- and S-polarized modes in the presence of a high-refractive-index coating (real refractive index well higher than 1.5) surrounding a TFBG. Numerical simulations are first conducted using a complex mode solver. Experiments are then reported on TFBGs coated with nanometer-scale dense thin films of Zinc oxide (ZnO). This material was chosen for its transparency and for its refractive index value (~1.9) well above the one of silica. We will show in the following that the presence of this coating drastically decreases the degeneracy between P- and S-polarized cladding mode resonances families. As a result, a pronounced wavelength separation between the orthogonal P- and S-polarized modes is obtained in the spectral evolution. This outstanding behavior occurs in air (favorable case for which the refractive index contrast between the optical fiber cladding and the surrounding medium is the highest) and also in liquids, as the wavelength spacing is so high that the orthogonal modes keep well separated, which is not the case for bare TFBGs. We also give insight into the influence of the coating thickness on the differential evolution between P- and S-polarized modes. Using polarized light injected into ZnO coated TFBGs, we experimentally demonstrate that the presence of the coating brings two important benefits for refractometric sensing: an ~8-fold increase in sensitivity is obtained in comparison to bare TFBGs while the response is extended to SRI values beyond 1.45. This is particularly useful for accurate refractometry in dense liquids or when a buffer overlay is used between the fiber and another sensitive film of higher refractive index.
2. Polarization dependency in bare TFBGs
Light polarization modifies the transmitted spectrum of a TFBG as its refractive index modulation presents a well-defined orientation in space, which breaks the fiber symmetry according to the tilt direction [5,16,24]. Figure 1 depicts the typical transmitted spectrum of a 1 cm long 6° TFBG measured for two different linear states of polarization corresponding to the P- and S-polarized modes. Several tens of cladding mode resonances appear at the left side of the Bragg wavelength. Each one corresponds to a backward light coupling between the core mode and a cladding mode and responds to the following phase matching condition: λclad,i = (neff,core + neff,clad,i)Λ where λclad,i is the resonance wavelength of the ith cladding mode while neff,core and neff,clad,i are the effective refractive indices of the core mode and the ith cladding mode, respectively. Unlike the Bragg wavelength that shows negligible variation with the input state of polarization (as it corresponds to two degenerated LP01 modes), the wavelength separation (Δλi(S-P)) between corresponding orthogonally polarized resonances increases with the cladding mode order. The two insets show this gradation, with a separation of ~10 pm at 1570 nm (~20 nm away from the Bragg wavelength) and ~200 pm at 1540 nm (~50 nm below the Bragg wavelength). This results from the fact that the fiber cladding is not a weakly guiding structure for most practical surrounding media ranging from gases to liquids. Therefore, for higher order modes, the modes are no longer degenerated and vector modes (radial EHmn and azimuthal HEmn) components appear as closely split resonances.
3. Numerical simulations of modal distribution in thin film coated optical fibers
Numerical simulations were conducted using a finite-difference complex mode solver in cylindrical coordinates (FIMMWAVE from Photon Design Inc.). With them, we intend to show the influence of a coating of real refractive index value (both in terms of refractive index value and thickness) on the wavelength separation between P- and S-polarized cladding mode resonances (Δλi(S-P)). For this, a four-layer model was considered for the optical fiber: a Germanium-doped core (8 µm) was surrounded by a pure silica cladding (125 µm), a coating (whose refractive index and thickness can vary) and an infinite external medium (air in this case). Simulations yield the effective refractive indices of the modes present in the considered configuration. For each mode computed by the solver, we use the TFBG phase matching condition to estimate the corresponding resonance wavelength. This is obtained by considering 1.447 for the effective refractive index of the core mode (neff,Bragg) and 557 nm for the grating period (Λ). The wavelength spacing is computed as the difference in wavelength between corresponding P and S modes. FIMMWAVE also provides the distribution of the modes in the fiber cross-section, allowing to unambiguously distinguish P- and S-polarized modes.
A first round of simulations has been carried out to refine the range of thickness and refractive index values suited to increase the wavelength spacing between P- and S-polarized modes. The ranges of interest (i.e. for which the wavelength spacing between both orthogonally polarized modes continues to increase) are bounded to 200 nm and 2.0 RIU respectively for the thickness and the refractive index. Beyond these values, there is no further improvement for parameter values. Figure 2(a) presents the results of simulations conducted to evaluate the effect of the refractive index value of the coating for a constant thickness of 200 nm. The refractive index was changed between 1.0 and 2.0 by steps of 0.1 RIU. Looking to the behavior of a bare TFBG (nLayer = 1.0), simulations confirms the Vernier effect capability, i.e. Δλi(S-P) increases with the mode order, as reported in . It reaches ~250 pm, 60 nm away from the Bragg wavelength.
When the refractive index of the coating grows and reaches 1.45 (the refractive index value of silica), Δλi(S-P) decreases. In this case, modes are guided and confined into the cladding as their effective refractive index is smaller than 1.45. As the contrast between the refractive indices of both materials decreases, the wavelength separation also decreases. We can refer to the Goos-Hänchen shift to confirm this behavior .
For refractive indices of the coating higher than 1.45, all cladding modes propagate in the coating as their effective refractive index is smaller than 1.45 (they do not “see” anymore the cladding-surrounding medium interface) and a differential behavior is obtained as TFBGs break the optical fiber cylindrical symmetry by coupling light from the core to the cladding in a privileged direction, close to the perpendicular to the grating planes. As already mentioned, they couple light into P-polarized (TM0n, EHmn) and S-polarized (TE0n and HEmn) mode families. In practice, S-polarized modes are tangentially polarized at the outer surface boundary and therefore, they penetrate it with more difficulty. This is not the case for P-polarized modes that are radial. These ones readily localize in the high refractive index coating, which increases their effective refractive index. As a result, P-polarized modes resonances are shifted to higher wavelengths with respect to S-polarized ones, which is spectrally manifested by a pronounced spacing between both mode families. And so, Δλi(S-P) increases with the refractive index of the coating until a saturation is obtained. For nLayer = 1.9, the wavelength spacing reaches ~750 pm, 60 nm away from the Bragg wavelength. It cannot increase further, as the wavelength spacing between neighboring cladding mode resonances does not exceed 850 pm at these wavelengths.
Figure 2(b) displays the influence of the coating thickness on Δλi(S-P). Here, the refractive index of the coating was set to the upper investigated value, i.e. 1.9. Again, the effect of the thickness is to increase Δλi(S-P) up to a saturation at ~750 pm, 60 nm away from the Bragg wavelength.
Similar simulations were conducted in liquid surrounding media (refractive index value varying between 1.31 and 1.40). Figure 3 depicts the results obtained in water (refractive index taken equal to 1.315) and confirms that the orthogonal polarization modes overlap for bare TFBGs (wavelength spacing ranging between 0 and 25 pm) while they remain well separated for coated TFBGs, with wavelength spacing up to 210 pm. Let us note that the horizontal axis is here limited to – 60 nm, roughly corresponding to the cut-off. Below this value, higher order modes are no longer guided.
4.1 Grating manufacturing and thin film synthesis
1 cm long TFBGs were manufactured into hydrogen-loaded single mode optical fiber (SMF28 from CorningTM) by means of a 1095 nm uniform phase mask and a frequency-doubled Argon-ion laser emitting at 244 nm. The phase mask was slanted in the plane perpendicular to the incident beam. To ensure strong couplings to cladding modes characterized by an effective refractive index close to 1.33 (characteristic of aqueous solutions), an external tilt angle of 6° was chosen. Gratings were annealed at 100 °C during 12 hours.
TFBGs were then covered with Zinc oxide (ZnO) by radio-frequency sputtering at room temperature. ZnO was chosen for its high refractive index value (close to 1.9) in order to maximize the wavelength separation between P- and S-polarized modes, as observed in the simulations. Zinc oxide (Sigma-Aldrich, purity > 99.9%) powder was pressed in a matrix to produce Ø 46 mm targets. Then, the targets were sintered at 900 °C for 1 hour in air. In the sputtering chamber, the distance between the fibers and the target was set to 75 mm. During the deposition of the ZnO films, the radio-frequency power was set to 200 W, yielding an average deposition rate of 50 nm per hour. The pressure in the chamber during deposition was set to 2 Pa, always with a gas composition of 90% Ar and 10% O2. After the first deposition, the fibers were rotated by 180° in order to deposit films on the other face. The film thickness was measured by means of the step method using an optical profilometer on samples sputtered on reference glass substrates.
To stabilize the deposited film, coated gratings were further annealed at 300 °C in air for 3 hours. A small oven was used (~7 cm in length) to avoid degradation of the polymer jacket on the whole fiber. A 10 dB attenuation in the depth of the resonance peaks (peak-to-peak amplitude of ~20 dB before annealing and ~10 dB afterwards) was noticed.
The geometry and quality of ZnO coatings were examined using scanning electron microscopy (SEM). Figure 4 depicts lateral images of a 100 nm ZnO coated fiber. The coating appears homogenous with no cracks. Similar observations were made for thicker coatings.
Fiber cross sections were also investigated to estimate the actual coating thickness. Images have revealed that the coating presents an ovoid shape around the fiber transverse section. The thickness is not constant around the transverse section and is always smaller than the target value. The non-uniformity of the coating thickness is manifested by a 10 to 30% difference between minimum and maximum thickness values. To clearly illustrate this, Fig. 5 shows different pictures of the cleaved end of a ZnO coated fiber for which the target thickness was 400 nm. Figure 5(a) depicts the global vue while Fig. 5(b) and Fig. 5(c) focus respectively on the minimum and maximum thickness values measured around the cross section. In this particular case, the extreme values were equal to ~240 nm and ~340 nm, confirming the overestimation of the ZnO thickness. This non-uniformity arises from the two step deposition process but alos from the long sputtering time (current modifications appear during the sputtering process that modify the deposition rate) and the cylindrical shape of the optical fiber.
To alleviate the non-uniformity of the ZnO coating thickness, the best practice would be to continuously rotate the optical fiber during the sputtering process, as described in . Unfortunately, the volume of our sputtering chamber does not allow us to set-up such a system.
4.2 Experimental set-up
The refractometric sensitivity of ZnO coated TFBGs was investigated with polarized light. The measurement set-up consists of an Amplified Spontaneous Emission source (ASE) covering the C + L bands (depolarized source), a linear polarizer and an Optical Spectrum Analyser (OSA), as depicted in Fig. 6. Short pieces of optical fiber were used (typically ~1 m) and fixed to avoid undesired polarization instabilities. The surrounding refractive index range studied here goes from 1.328 to 1.575. This was obtained using solutions of cinnamaldehyde in methanol at different concentrations. Refractive index values of the solutions were measured by a Reichert AR 200 refractometer (accuracy: 10−4 RIU).
5. Experimental results
5.1 Analysis of the behavior in air
In all our experiments, the linear polarizer was rotated to identify the P- and S-polarized transmitted amplitude spectra that were recorded consecutively. Figure 7 depicts two zooms on a 10 nm wavelength scale (around 1535 and 1575 nm) for a 200 nm ZnO coated grating. Comparing to the insets of Fig. 1, one can readily see that Δλi(S-P) are much pronounced than in the case of bare TFBGs. At 1535 nm (57 nm away from the Bragg wavelength), it is 2.5 times higher than for bare TFBGs while at shorter wavelengths, the ratio between bare and coated configurations reaches 20.
As observed in the simulations, the thickness of the ZnO coating influences Δλi(S-P). Figure 8 presents the experimental behavior measured for 3 different configurations: bare TFBG, 100 nm and 200 nm coated TFBGs.
For bare TFBGs, the correspondence between the experimental evolution and the simulated one (Fig. 2) is excellent, with Δλi(S-P) reaching ~250 pm, 60 nm away from the Bragg wavelength. For coated TFBGs, although the thickness clearly increases the wavelength spacing, the experimental values are smaller (roughly 1.5 times) than the simulated ones. This was somehow expected since the actual thickness value is smaller (and not constant around the cross section) than the target one. Another source of discrepancy can be attributed to an uncontrolled residual porosity that could slightly decrease the refractive index of the ZnO coating, leading to smaller wavelength separations than expected.
5.2 Enhancement of the range of sensitivity in liquids
We have investigated the benefits of the ZnO coating on a large SRI scale ranging from 1.328 to 1.575. We have tracked the evolution of selected cladding mode resonances (both in amplitude and in wavelength) as a function of the SRI. In the following, the focus is made on the behavior of three cladding mode resonances centered on 1545, 1555 and 1565 nm, respectively. The latter wavelength corresponds to the center of the transmitted spectrum.
Figure 9 displays the wavelength shift evolution as a function of the SRI for P- and S-polarized modes. In practice, 100 nm coated TFBGs present evolutions very close to bare TFBGs. Hence, for the readability of the graphs, the results obtained for the 100 nm coated TFBG are not presented. Several observations can be made. First, coated gratings increase the range of sensitivity to the SRI at values higher than the refractive index of silica. For bare and 100 nm ZnO coated TFBGs, the cladding mode resonances are completely radiated (they disappear from the transmitted amplitude spectrum), rendering impossible SRI measurements in the range between 1.45 and 1.55. For 200 nm coated TFBGs, cladding modes continue to be present in the amplitude spectrum for SRI values higher than the refractive index of silica. In both cases, the range of sensitivity is increased up to 1.575. A second important result is the pronounced differential evolution between P-polarized and S-polarized modes for coated TFBGs: P-polarized light exhibits a higher sensitivity than the orthogonal state of polarization, due to their strong localization in the ZnO coating. The 200 nm coating thickness maximizes the differential wavelength shift (difference between P-polarized and S-polarized modes) as a function of the SRI. The slopes for the P-polarized and S-polarized modes are respectively 3.5 and 1.0 nm/RIU, yielding a nearly three times factor between the orthogonal states.
These results clearly demonstrate that the ZnO coating allows improving the behavior of less sensitive modes (i.e. low order modes that present a higher refractive index value) in comparison to bare TFBGs.
5.3 Enhancement of the refractometric sensitivity in liquids
Dilutions were operated at room temperature to slightly modify the refractive index of the solution in which TFBGs were immersed (total SRI change equal to 8.0 10−3 RIU around 1.39). The evolution of the cladding mode resonance located just beyond the cut-off wavelength was tracked as a function of the SRI value. Figure 10 presents the obtained results. The P- and S-polarized modes of bare TFBGs behave equally, with a refractometric sensitivity equal to 25 nm/RIU. The P- and S-polarized mode sensitivities of the 100 nm coated TFBG were computed equal to 110 nm/RIU and 75 nm/RIU, respectively. For 200 nm coated TFBGs, the behavior becomes non linear. For slight SRI change up to 2.5 10−3 RIU, sensitivities were computed equal to 210 nm/RIU and 179 nm/RIU for the P- and S-polarized modes, respectively, yielding a more than 8 times enhancement with respect to bare TFBGs. As a consequence, the limit of detection for ZnO coated TFBGs is of the order of 10−5 RIU.
In this paper, we have shown that a thin layer of material with a refractive index higher than silica considerably modifies the behavior of cladding mode resonances in response to surrounding refractive index changes. Using tilted fiber Bragg gratings as spectral combs to probe these cladding modes, we have demonstrated that a nanometric-scale ZnO coating induces a strong polarization dependency. The two orthogonal polarization states are maintained non-degenerated even when the surrounding refractive index is above 1.45. In liquids, using P-polarized state, it is possible to increase the range of sensitivity and to enhance by an 8-times factor the intrinsic sensitivity to the surrounding refractive index. Our experiments confirm that the thin film thickness is a design parameter that strongly modifies the differential behavior of coated TFBGs. Among the thickness values tested in this work, the best results were obtained with a 200 nm coating. It has yielded the best compromise in terms of differential sensitivity enhancement, on the one hand, and increase of the linear sensitivity range, on the other hand.
This study was performed in the framework of the Opti2Mat project financially supported by the Walloon region in Belgium. C. Caucheteur is supported by the Fonds National de la Recherche Scientifique (F.R.S.-FNRS) and by the ERC (European Research Council) Starting Independent Researcher Grant PROSPER (grant agreement No 280161 – http://hosting.umons.ac.be/erc-prosper). Authors are grateful to Nicolas André from the ELEN Department of the Catholic University of Louvain for his help during SEM measurements.
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