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Abstract
This article reports the spectral properties of a Ge2Sb2Se4Te1 (GSST) -coated tilted fiber Bragg grating to the changes of surrounding refractive index (SRI). The variations of the cladding mode resonance amplitudes and wavelengths under two orthogonal polarization modes are analyzed and validated with the simulation results. The introduction of the high index coating has induced non-degeneracy to the cladding modes, but the effect gradually diminishes with increasing SRI. The SRI sensitivities of the s-polarized modes have been substantially suppressed and became 20 times lower than that of p-polarized modes. The findings also reveal the association between the cladding resonance amplitudes, the imaginary part of the effective refractive indices and the evanescent field intensity distributions.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Refractometry is important in various applications ranging from food and beverage, chemical process and the manufacturing industry. Fiber Bragg grating (FBG) is one of the commonly used optical fiber components for refractometric applications due to its compactness, linear sensitivity and wavelength division multiplexing (WDM) capability. Measurement of refractive index change can be achieved by inscribing a tilted grating (also known as tilted fiber Bragg grating (TFBG)) in which the modulation direction is tilted relative to the fiber axis to couple core-guided light to backpropagating cladding modes. It is a suitable optical component for lab-on-chip which has the advantages of low-volume analyte requirement, portability and in-situ measurement. Consequently, the fiber device has a broad spectrum with multiple transmission dips below the Bragg wavelength. Also, TFBG is known for its temperature-insensitive capability by referring the individual cladding mode shift to the core mode shift. The measurement range for the surrounding refractive index (SRI) can be extended by depositing a high refractive index (HRI) material around the surface of TFBG. [1] A previous study has reported a dielectric coated-TFBG with a 200 nm thick zinc oxide (ZnO) layer (RI of 1.9, higher than that of pure silica glass) to induce non-degeneracy between s- and p-polarized cladding modes over a larger range of SRI. The differential sensitivity to SRI between the two polarization states has been greatly enhanced three-fold [2]. GSST (Ge-Sb-Se-Te) is a type of chalcogenide glass that has been extensively studied for various photonic applications, due to its high refractive index (2.7–2.9 RIU) and low optical loss [3]. The wide IR transparency of this material makes it a favorable choice for the fabrication of optical devices such as planar and photonic crystals waveguides. On the other hand, the required activation power of the radio frequency (RF) is lower for the deposition of GSST than other dielectric materials Eg. ZnO and TiO2. Hence, the induced heat and temperature to the TFBGs during the deposition process are moderately lower and safer for the TFBGs that are sensitive to the high temperature.
In this work, the spectral response of a GSST-coated TFBG is presented. The variations of cladding mode resonance wavelengths, effective refractive indices and amplitude to the SRI changes under different polarization conditions (s-polarized and p-polarized) are analysed and compared against the simulation results. Furthermore, the impacts of the high-index coating to the field intensity profiles of the s-polarized and p-polarized cladding modes are investigated. Notably, the different SRI sensitivities of resonance wavelength and amplitude can be associated with the evanescent field intensity outside of the coating. This article also explains the idle or small varying s-polarized resonance wavelengths and amplitudes with SRI changes.
2. Methods
The TFBG was inscribed in hydrogenated photosensitive fiber (GF-1, Thorlabs) by using an ArF excimer laser (Optosystems Ltd, Series CL 5000, irradiation wavelength 193 nm) and a phase mask. The 7° tilted grating has a Bragg wavelength of 1580.1 nm and a wideband of cladding mode resonances that cover the range from 1535 nm to 1575 nm. After that, the TFBG was left in the laboratory for a week to enable the out-diffusion of excess hydrogen content from the fiber.
The TFBGs were treated using Piranha solution to remove all traces of organic residue from the grating surface. Following that, the TFBGs were coated with a thin layer of GSST film based on the RF sputtering method. The GSST target is a 5 mm thick × 2 inches diameter disc-shaped block with a purity of 99.99%. It is indium bonded to a 2 inches diameter × 3 mm thick copper gasket plate. First, the RF sputtering chamber was pumped down to a working pressure of 7 mTorr before an RF power of 25 W was initiated to drive the sputtering process. The deposition thickness is controlled based on the deposition duration. A 130 nm thick deposition was completed within ∼6 minutes (deposition rate ∼ 21 nm/min).
To explain the relationship between resonance wavelength shifts and the surrounding refractive index (SRI) from the experiment data, the effective indices, neff of all the cladding modes were determined from the dispersion relations for a 4-layer model with complex roots based on Chen’s approach that involves transfer matrix method and Secant method [4]. The real part of the complex root represents the effective index of the cladding mode (Re(neff)) whereas the imaginary part is associated with the optical loss of that cladding mode (Im(neff)). For a better representation of the cladding modes in TFBGs, the exact modes such as Transverse electric (TE), transverse magnetic (TM) and the hybrid HE/EH modes are considered [5]. Generally, the electric ($\vec{E}$) and magnetic ($\vec{H}$) field couple with each other in TFBG, hence the current work focuses on hybrid HE/EH modes. The cladding modes with both azimuthal numbers (v = 1, 2) were considered [6]. The following are the parameters used in the investigation: The core RI, n1 = 1.448702, cladding RI, n2 = 1.444024, the RI of GSST, n3 = 2.8, core radius, r1 = 4.15 µm, cladding radius, r2 = 62.5 µm. The grating length is 1 cm and the grating period, Λ = 545.48 nm. The fourth layer of the model is the glucose solution with varying Re(SRI) from 1.33 RIU to 1.45 RIU and a constant Im(SRI) of 8.55 × 10−5.
From the experiment, the effective indices of the cladding modes are extracted from the cladding mode resonance wavelength, λC, and Bragg wavelength, λB [7]:
where $\mathrm{\Lambda }$ =$\; {\mathrm{\Lambda }_g}/cos\theta $ is the resultant grating period at the tilted angle $\theta $ and the untilted grating period ${\mathrm{\Lambda }_g}$, $n_{Cl}^{eff}$ is the effective index of the phase-matched cladding mode at ${\lambda _C}$, ${\lambda _B}$ is the Bragg wavelength, $n_{CO}^{eff}$ is the effective index of core mode at the same wavelength. The HE and EH of similar modes (e.g., the HE1,19/EH1,18) have the closest effective RI and they are analysed in pairs [6].3. Results and discussion
Figure 1(a) shows the transmission spectra of a 130 nm GSST-coated TFBG immersed in distilled water (SRI = 1.3159 RIU) under different polarization conditions: radially p-polarized and azimuthally s-polarized modes. Noticeable wavelength separations between the orthogonal polarization state can be observed from mode pairs from the s-polarized and p-polarized spectra in which the wavelength separation for the higher order mode is larger than that of lower order mode. For instance, the wavelength separation at 1541 nm (higher-order) is ∼400pm whereas the wavelength separation at 1568 nm (lower-order) is ∼200pm. The s-polarized modes (grey curve) have larger peak-to-peak amplitude due to their azimuthal vector fields which are fully shielded by the GSST coating, they suffer lesser loss than the p-polarized (blue curve) spectrum. The high penetration depth of p-polarized modes have stronger interactions with the surrounding medium, hence they are more vulnerable to scattering and absorption loss. Figs.1(b) and 1(c) show the SRI spectral responses of the GSST-coated TFBG under p- and s-polarizations respectively. The p-polarized modes have shown progressive spectral shift while the s-polarized modes are practically idle with increasing SRI. To the best of our knowledge, this is the first report on idle s-polarized modes in dielectric coated TFBG.
The cladding mode resonances in the bare TFBG can be categorized according to S- and P- polarization modes, which are of azimuthal vector (TE0n and HEmn) and radial vector (TM0n, EHmn), respectively, as shown in Fig. 2. The Re(neff) and Im(neff) curves of HE- and EH- mode share similar curve characteristics as depicted by the orange curves in Figs. 2(a) and 2(b). The difference between the modal indices is small and even smaller at higher SRI regions. The apparent changes can be observed from the decreasing cladding resonance amplitude with increasing SRI, especially when the SRI approaches the Re(neff) of the cladding mode. In the case of HE1,19 and EH1,18 modes, the Im(neff) increases rapidly when SRI approaches Re(neff) ∼ 1.4275 RIU and this is the cut-off point of these cladding modes. This can be attributed to the weakening guiding condition for the cladding modes in the fiber and the growing coupling with the leaky modes as indicated by the increasing Im(neff) in the high SRI region. More cladding modes will diminish from the fiber and eventually, the transmission spectrum will be smoothened with minimum traces of cladding mode resonances except for the core mode when the SRI equals or exceeds the cladding refractive index, which is commonly accepted as the upper detection limit of a bare TFBG. [8]
The GSST-coated TFBG was characterised by using a set of glucose solutions with different concentrations to provide a range of SRI from 1.33 RIU to 1.435 RIU. The reference SRIs of the glucose solutions were measured by using an Abbe refractometer (calibration wavelength 532 nm). In the analysis, a comparative study between two groups of cladding modes, namely the EH1,19/HE1,20 and EH2,20/HE2,19 that correspond to the cladding modes resonances in the vicinity of ∼1566 nm against the higher order modes, EH1,28/HE1,29 and EH2,29/HE2,28 modes in the vicinity of ∼1553 nm. Their effective indices were extracted from the transmission spectra and fitted with the simulated results as presented in Fig. 3. It can be seen that the demonstrated GSST-coated TFBG shows the extreme polarization-dependent spectra in response to the varying SRI. The presence of high index GSST coating has altered the Re(neff) and Im(neff) curves of HE- and EH- modes. The most apparent change is the separation between the two Re(neff) of HE mode (s-polarized mode) and EH mode (p-polarized mode) as depicted by the solid and dotted curves in Figs. 3(a) and 3(d). In Fig. 3(a), the EH modes in GSST-coated fiber remain sensitive to the variation of SRI, in which its Re(neff) increases from 1.4251 RIU to 1.4254 RIU and it becomes constant after SRI > 1.426 RIU. The Im(neff) of the EH mode in GSST-coated fiber is within the vicinity of 10−7. Similar to the cladding modes of bare fiber, Im(neff) increases rapidly when SRI approaches the Re(neff) of the modes. On the other hand, the high-index coating limits the coupling between the HE modes and the leaky modes. This can be seen from the constant characteristic curve of Re(neff) (see the dotted curve in Fig. 3(a)) and the low Im(neff) which is well below 10−8 until 1.426 RIU (see the dotted curve in Fig. 3(b)). This explains the unchanged amplitudes of the cladding mode resonances with increasing SRI (see Fig. 1(c)).
Fig. 1. (a) Transmission spectra of the GSST-coated 7°-TFBG (in the distilled water) under different polarization conditions: p-polarized and s-polarized. The spectral responses of the (b) p-polarized and (c) s-polarized transmission spectra with SRI changes.
Fig. 3. (a),(d) The comparison between the simulated data and the experimental data for Re(neff) with varying SRI (1.33-1.435) for (b),(e) The variation of Im(neff) with varying SRI (c),(f) the measured amplitude changes of the cladding mode resonance with varying SRI.
Over the range of 1.33–1.435RIU, the measured linear sensitivities of the resonance wavelengths of HE2,19, HE1,20, HE2,28 and HE1,29 are 0.095 nm/RIU (R2 = 0.5186), 0.110 nm/RIU (R2 = 0.5425), 0.150 nm/RIU (R2 = 0.7875) and 0.177 nm/RIU (R2 = 0.7703) respectively, whereas the measured sensitivities for EH2,20, EH1,19, EH2,29 and EH1,28 are 2.067 nm/RIU (R2 = 0.9739), 2.017 nm/RIU (R2 = 0.9648), 3.584 nm/RIU (R2 = 0.9666) and 3.500 nm/RIU (R2 = 0.9615) respectively. HE modes have shown substantially lower SRI sensitivity (∼20 times lower) than those of EH modes. It can be attributed to the fact that the EH modes are radial modes that have greater penetration into the surrounding medium whereas the HE modes are azimuthal that are well-confined within the fiber, hence the lower interaction with the surrounding medium. This property of separation between the sensitive modes (EH) and the insensitive modes (HE) offers an intriguing feature for refractory through differential measurements based on the HE / EH mode pairs, where HE modes serve as references.
The simulation for neff started from low to high SRI and it is terminated at the point where the Im(neff) is excessively high, herewith denoted as the cut-off point. Coincidently, the Re(neff) of the EH mode has risen and approached that of its HE mode pair. That is where their resonance wavelength difference and polarization dependency are minima [7]. Figures 3(c) and 3(f) show the variation of resonance amplitude starting from SRI = 1.33 RIU. The resonance amplitudes of EH modes gradually increase with increasing SRI until they reach the cut-off point (red vertical dotted line at SRI∼1.426) before they plummet after that due to rising Im(neff) at the cut-off point. A similar phenomenon has been reported in [1]. On the other hand, the HE modes are less sensitive to the SRI changes and the maximum absolute variations are less than 0.1 dB. Nonetheless, the cladding mode resonances (both EH and HE modes) can still be observed after the cut-off point.
Similar curve characteristics are observed for the higher order cladding modes, EH1,29/HE1,28 and EH2,28/HE2,29. The major difference for the higher order cladding modes are the lower cut-off point at SRI = 1.40. This can be observed from the rapid rise in their Im(neff) and the onset of the decay in the peak-to-peak amplitude (EH modes) at SRI = 1.40. Nonetheless, the cladding modes can still be sustained (the presence of the markers indicates that the resonance wavelengths are still detectable from the experiment) beyond the cut-off point as shown in Fig. 3(d). The extended SRI detection range shows the potential application of the GSST-coated TFBG for high index measurement for any liquid medium with higher RI than that of silica such as edible oil, organic solvent, etc.
Figure 4 shows field intensity distribution profiles of the HE and EH mode as a function of radial position within the vicinity of the GSST coating region (radial position ∼ 0–0.13 µm) at different SRIs. The EH modes are discontinuous at the cladding-GSST (radial position = 0 µm) and GSST-liquid medium (radial position = 0.13 µm), whereas the HE modes are continuous across the boundary. The field intensity magnitude decay exponentially starting from the coating boundary to the surrounding medium. The EH-modes have obvious variations in their field intensity profiles with SRI changes and stronger evanescent field intensity in the surrounding medium (> radial position 0.13 µm) which promotes strong coupling with the leaky modes. These explain the higher SRI sensitivities of Re(neff) and high values of Im(neff). On the other hand, the HE modes have consistent field intensity distribution profiles and they are less affected by the varying SRI condition. These explain the small variations of their resonance wavelengths and amplitudes even at the higher SRI beyond the effective RI of the modes (SRI > Re(neff)).
4. Conclusion
In summary, a GSST-coated TFBG was fabricated and characterised. The spectral responses of HE- and EH-modes with varying SRI were extracted from the wavelength shift and the peak-to-peak amplitude change of the cladding modes resonance at ∼1566 nm and ∼1553 nm. Based on the four-layer model, the complex refractive indices of the GSST-coated TFBG were calculated using Chen’s method and the results are found to be in good agreement with the experimental data. The incorporation of the GSST coating has extended the operation range of both EH and HE beyond the cut-off points and effectively suppressed the SRI sensitivity of HE modes, almost 20 times lower than that of their EH mode pairs. This can be well-explained by the low Im(neff) and consistent field intensity distributions of HE modes that have lower evanescent fields.
Funding
Fundamental Research Grant Scheme (FRGS), Ministry of Education, Malaysia (FRGS/1/2022/STG07/UM/02/6).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
References
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