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Abstract
In this work, a partially etched chirped fiber Bragg grating (pECFBG) is introduced, as a compact sensor for multi-parametric measurement of temperature, thermal gradients over the active length, and refractive index. The sensor is fabricated by wet-etching a portion of a 14-mm linearly chirped FBG with linear chirp profile. The resulting device has two active areas: the unetched part of the grating (2 mm) can be used either as a uniform temperature sensor, or to detect thermal gradients experienced through the grating length by means of a spectral reconstruction technique; the etched part (12 mm), besides having a similar thermal sensitivity, is exposed to refractive index sensing through the introduction of a sensitivity to external refractive index. Overall, the pECFBG structure behaves as a compact sensor with multi-parameter capability, that can both measure temperature and refractive index on the same grating, but also spatially resolve temperature detection through the grating section. The results have been validated through both a model and experimental setup, showing that the mutual correlation algorithm applied to different spectral parts of the grating is able to discriminate between uniform and gradient-shaped temperature profiles, and refractive index changes.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber optic sensors based on Fiber Bragg Gratings (FBGs) are gaining significant interest in measurement science and technology thanks to their advantageous properties [1]: lightweight and compact size, immunity to electromagnetic interference, possibility to create sensor networks using time/wavelength division multiplexing, clear definition of the active sensing region, sensitivity to strain and temperature, biocompatibility (ISO 10993 standard), easiness to demodulate the FBG spectra.
The possibility to use FBG devices in biomedical applications [2] such as real-time diagnostic with spatial resolution capability [3, 4], supporting minimally invasive treatments [5,6], implement a guidance and feedback for medical catheters [7, 8] and biosensing [9, 10], is progressively requiring advanced sensing capability that standard uniform gratings do not achieve. The opportunity to modify the pattern of the refractive index modulation of the FBG, or modifying the optical fiber hosting the grating, improves the sensing capability provided by the Bragg grating structure [1]. Among others, tilted FBGs have been used as biosensors as they introduce cladding modes dependent in amplitude and wavelength on the external refractive index [11, 12]; FBGs inscribed on highly birefringent fibers enable the discrimination between temperature and strain [13]; FBGs inscribed in polymethyl methacrylate fibers yield a ~10x improve on thermal sensitivity [14].
Most notably, etched or thinned FBGs (EFBGs) are optical gratings in which the cladding surrounding the fiber core is depleted exposing the FBG to a change of effective refractive index [15, 16]: this structure has been used for refractive index sensing [15], and it has the advantage of working in reflection (as opposite to tilted FBG or surface plasmon resonance that work in transmission) and does not require a polarization control [17]. On the other side, chirped FBGs (CFBGs) are gratings having a spatially varying refractive index modulation period [18] and behave as broadband FBGs. The recent work by [5] Korganbayev et al. has shown the possibility of using a CFBG to detect and spatially resolved temperature variations in both space and time, labeled as thermal maps [2,5]; in this work, thermal maps have been demonstrating over a length of 15 – 50 mm.
In some biomedical applications it is desirable to combine these sensing features into an individual multi-parametric compact probe, in order to expand the measurement capacity in a short length. In particular, the combination of thermal pattern detection with millimeter-level spatial resolution with refractive index sensing can have significant impact in minimally invasive thermo-therapies such as radiofrequency [18] or laser [19] ablation, whereas a percutaneous heat source introduces a thermal heat field with steep spatial and temporal gradients in the center of a localized tumor, and at the same time the refractive index detection is used to discriminate the tumor borders from health tissue in the same location [20]. Another application field is the real-time measurement of blood temperature during angioplasty for revascularization, in which the blood temperature gradient in the vessel and the refractive index of blood are measured [21]. Similar considerations are pertinent to catheterizations for epidural anesthesia, whereas strain maps and refractive index detection can return significant information on the advancement of the hemodynamic catheter to the epidural space [2,8].
Thus, the possibility of combining the properties of an EFBG and a CFBG can significantly enhance the sensing potential. The first attempts to control both the chirp rate and the fiber profiles were not aimed at sensing, but rather to tune the group delay in dispersion compensating fiber elements [22, 23]. More recently, Chang et al. used an etched chirped FBG as a liquid-level or inclination sensor [24], [25]: the principle of operation is to detect a step-size refractive index function by analyzing the transitions occurring in the CFBG spectrum. In addition [26], Lee et al. developed a strain, temperature, and refractive index sensor using an etched grating. The methodology introduced in [5] opens a new perspective for the CFBG-based detection of temperature (or strain) patterns recorded over the grating length; the grating is modeled by using the coupled-mode theory [27], and an optimization algorithm is used to track the temperature pattern that, applied to the CFBG, matches the simulated and measured spectra. In addition, recent advances in polymer CFBGs, inscribed on a step-index fiber [14] and microstructured fiber [28], allow a significant increase (6 to 12 times) of the thermal sensitivity of the grating, allow the bandwidth of polymer CFBGs is limited to few nanometers.
In this work, we propose the use of a partially etched chirped FBG (pECFBG). The pECFBG is achieved by wet-etching a portion (85%) of a 14-mm linearly chirped FBG. In the unetched part, the grating maintains the CFBG properties, including the capability to resolve temperature profiles. In the etched portion, the pECFBG is exposed to a dual chirp, since both the grating period and the effective refractive index are spatially varying, and the latter is dependent upon the refractive index external to the fiber. Overall, it is possible to observe that a pECFBG is a grating structure in which the reflection spectrum changes in a different way depending on whether refractive index and/or temperature patterns are experienced. This result significantly extends over previous works, as it enables a spatially resolved measurement associated with temperature, and to refractive index. The use of a pECFBG contributes to a step forward in fiber optic sensing capability, because it brings the dual-sensing capacity of a grating, which is encoded in tilted gratings [29] and in thinned gratings [30], through each section of the sensing region at the millimeter scale.
The paper is arranged as follows: Sect. 2 describes the working principle of the pECFBG using a transmission-line equivalent of the grating; Sect. 3 describes the pECFBG fabrication and experimental setup; Sect. 4 shows the experimental results obtained when the grating is exposed to temperature variations, changes of thermal gradient, and refractive index variations; finally, Sect. 5 draws the conclusions.
2. pECFBG working principle
The principle of operation of a pECFBG can be drawn by extending the coupled mode theory (CMT) [27, 31] to the specific grating structure, and is sketched in Fig. 1: the grating is simulated as a cascade of discrete grating elements, similar to [5], but each grating element is subjected to both temperature and refractive index variations, and it has a different attenuation.
Fig. 1 Sketch of the pECFBG device. (a) Working principle of the pECFBG, based on two distinct etched and unetched regions, and having the shortest wavelength on the etched tip; (b) CMT equivalent of the pECFBG, whereas the grating is discretized in short gratings each with length Lg.
The grating having length L is discretized into a set of M gratings along the fiber axis z, each grating slice having length Lg = L/M and local grating strength coefficient kLg. In reference conditions, i.e. grating maintained at room temperature and reference refractive index, each i-th grating of the array has Bragg wavelength given by:
where neff,i is the refractive index and Λi is the period of the refractive index modulation, respectively of the i-th layer. Using Erdogan’s CMT [27], the reflectivity of each layer can be calculated as:where λ is the wavelength. The term σi can be expressed as [27]:where: δneff is the amplitude of the modulation of the refractive index of the grating, and λi is the Bragg wavelength of each i-th layer. In Eqs. (2) and (3), we assume that the grating strength coefficient kLg and the amplitude of the index modulation δneff are spatially constant.As suggested in Fig. 1, in a pECFBG both the Λi and neff,i terms are spatially dependent, and therefore their expression depends on the layer number. Concerning the modulation period, for a linearly chirped FBG [5] it is possible to express:
in which the term ψ represents the chirp rate coefficient, that describes the rate of spatial change of Bragg wavelength within the grating. For a grating as sketched in Fig. 1, having the longest wavelengths on the input side and the shortest ones on the tail, ψ has a negative sign. The additional chirp induced to the effective refractive index is instead dependent upon the etching profile. In a simple approximation, it is assumed that P sections of the grating in proximity of the tip are uniformly etched, while the (M-P) sections near the input end remain unaltered. This allows expressing the effective refractive index modulation as a step function:The refractive index variation Δneff obtained by reducing the cladding thickness depends on the diameter of the etched fiber. The etching process results in a change of neff that gets significant when the fiber diameter decreases below 20-30 μm; in this design, the condition to be respected is that the fiber maintains its single-mode profile such that only the propagation of the fundamental mode is significant for the analysis.As suggested in Fig. 1, in a pECFBG the longest wavelengths are backreflected in the first part of the grating, that has no etching and therefore no evanescent waves, while the shortest wavelengths travel through the lossy part of the grating, and are therefore attenuated. This can be accounted by introducing an attenuation coefficient αi for each i-th section; under the assumption of Eq. (5), the attenuation can be expressed as a step function:
from Eqs. (1)-(5) it possible to evaluate the spectrum of each i-th layer of the grating; the overall pECFBG spectrum can be accounted as [5]:
the CMT-based model of a pECFBG provides the spectrum of the grating as a function of the chirp and etching profiles, that are the two parameters controlling the properties of the device. The sensitivity arises from the dual dependence of the i-th Bragg wavelengths on both the variations of temperature and refractive index profiles [15]:
The sensitivity terms are sT (thermal coefficient) and sn,i (refractive index sensitivity), and respectively relate to the linear shift of the Bragg wavelength experienced for a temperature variation ΔTi and change of the refractive index external to the fiber probe Δni. The chirped profile of the grating inherently results in a change of the pECFBG spectrum that is dependent on the temperature in each i-th spectral slice, thus making the device suitable for the detection of thermal profiles. A similar consideration can be drawn for the refractive index term; in this demonstrative work, we assume that the refractive index is spatially homogeneous, thus Δni = Δn for each i. For a grating inscribed in a silica optical fiber and operating in the third optical window around 1550 nm, the thermal sensitivity is ≈10 pm/°C [1]. Conversely, the sensitivity to refractive index variation under the step-etching assumption of Eq. (5), is also a step function:whereas in a non-etched region of the grating the sensitivity is zero, due to the almost ideal confinement factor, while in the etched region the sensitivity sn depends on the fiber thickness. In [32], with a shallow fiber etching similar to the present work, a sensitivity of about 0.8 nm/RIU (refractive index units) has been observed.The principle of operation of the pECFBG is shown in Fig. 2, whereas the spectrum of a partially etched linearly chirped FBG having parameters similar to the grating used in experiments is illustrated for different types of temperature and refractive index changes. The parameters used for the model are listed in Table 1: temperature sensitivity and the chirp design are derived from [5], flipping the grating in order to have the longest wavelengths on the tail and therefore a negative chirp rate; refractive index sensitivity values have been estimated from [15,32].
Table 1. Parameters of the pECFBG used in CMT simulations, reproducing experimental values.
The spectrum of the grating observed in Fig. 2 shows the etching effect applied to the chirped FBG. The rightmost part of the grating, unetched, maintains its original reflectivity typical of a short chirped FBG, with a floor close to the maximum reflectivity value. Conversely, the dual effect of evanescent waves and wavelength shift contribute to separate the leftmost part of the grating, that experiences both a reduction of the reflectivity down to ~40% and a wavelength shift with increase of ripples in bandwidth. The spectrum clearly highlights the two distinct regions, distinguishing between etched and unetched spectral portions; it is possible to observe that even though the pECFBG is etched to a 75% portion, the resulting width of the etched spectral region is approximately 20% larger than the unetched spectral region. This is due to the fact, even assuming a step-function abrupt etching, the spectrum of each grating slice is broad enough to smooth the overall spectral envelope, creating a soft transition between the two regions of about 5 nm width.
The changes or temperature and refractive index affect the spectrum of the pECFBG in different ways, as illustrated in Fig. 3; each change (temperature, temperature profile, and refractive index) induces a different change in the spectrum that can be demodulate using a different method, similarly to a signature change. A temperature variation, as in Fig. 3(a), induces a shift of the whole pECFBG spectrum that is uniform on each region; it results in a linear shift of both the left and side edges of the grating, and can also be recorded in proximity of the inner transition region at 1560 nm. The response to a thermal gradient is instead different, and follows the observations in [5,19]. When a thermal spatial gradient 0°C → 100°C (left → right) is experienced across the pECFBG the result is symmetrical with respect to the opposite 100°C → 0°C gradient. In the first case, we observe a maximum shift of the right edge of the grating, while the left part of the spectrum does not change; conversely, in the second case the left part of the grating experiences the largest shift, and the right part of the grating remains close to the reference spectrum. Since the central region is located approximately at half of the grating, the spectra observed in both thermal gradients appear to coincide in the grating center.
Fig. 3 Response of the pECFBG spectrum to the variations of different parameters. (a) Constant temperature variations of 20°C and 100°C; (b) temperature gradient 0°C → 100°C and 100°C → 0°C recorded from the left (longest wavelengths) to the right (shortest wavelengths) sides of the grating; (c) variation of refractive index of + 0.1 RIU and −0.33 RIU.
Finally, in case of a refractive index change, the unetched part of the grating does not experience any shift, while this is observed on the left part of the grating. A positive change of + 0.1 RIU and a negative change of −0.33 RIU (correspondent, for example, to a transition between water and air) result in the left part of the grating shifting towards the longer and shorter wavelengths respectively. In the first case, a partial spectral shift is also observed on the inner grating part, while in the second case in correspondence of a large negative RIU change the spectrum deforms in the center, in correspondence of the transition region.
The results in Figs. 2 and 3 justify the choice of a partially etched chirped FBG, rather than a completely etched chirped FBG [24,25]. The distinction between two regions allow encoding refractive index changes only in one portion of the grating, while temperature changes (uniform or spatially varying) are encoded in the whole grating bandwidth, obtaining a split demodulation. In addition, the results suggest that the application of a spectral correlation in the different parts of the grating can be used to detect each specific parameter.
3. Setup and fabrication
The experimental setup of the pECFBG is shown in Fig. 4 and has been used to fabricate, interrogate, and calibrate the sensor. The instrument used for the analysis is a Luna OBR4600 (Luna Inc., Roanoke, VA, US), which has been set to detect the spectrum of the pECFBG over the 1525-1610 nm window. The OBR has been used to detect the spectrum with 10 pm wavelength resolution, by means of a scanning-laser interferometer [33].
Fig. 4 Schematic (a) and photographic view (b) of the pECFBG sensing setup, based on OBR system. The system is shown during thermal calibrations, using a temperature bath in water that allows controlling the thermal gradient with a reference thermometer.
The pECFBG has been fabricated by means of wet-etching in hydrofluoric acid (HF). A linearly chirped FBG (Technica S.A., Beijing, China) has been used as a starting point, having 15 mm length and 2.5 nm/mm chirp rate, having ~95% reflectivity and written on a standard SMF-28 fiber. After removing the acrylate coating, the grating has been cleaved on the tip in correspondence of the shortest wavelengths, reducing the grating length to 14 mm in order to ensure that the etching process is effective on the tip. Subsequently, the grating has been etched by immersing the tip of the fiber in HF (48%) for 40 minutes. This process has been performed in a chemical fume hood (Waldner Secuflow airflow controller) under constant temperature ( ± 0.1°C) in order to stabilize the etching rate. During etching, the fiber has been immersed only on the tip side, in order to have approximately 85% of the chirped grating etched, while the tail part (longest wavelengths) has been kept out from etched region.
The final spectrum of the grating is shown in Fig. 5. The spectrum of the grating is similar to Figs. 2 and 3, having a narrow region (1575-1580 nm) that has a high reflectivity, while the leftmost part of the spectrum has a reflectivity reduced to 15-25%. Compared to the CMT model, losses are slightly higher and the spectral flatness in the etched region is inferior; this is due to the step-function approximation for the etching pattern, while the fabricated pECFBG has a smoother contrast between the two regions, due in part to the partial evaporation of the HF during etching and to the non-linearity in the etching rate. The effect of etching is shown also in the group delay: while a linearly chirped FBG has a linear group delay, that implies a constant dispersion in the FBG bandwidth [34] the group delay trace for a pECFBG has a different profile, and can be divided into 3 regions: the first region 1543-1549 nm has a dispersion of −2.30 ps/nm, the inner region 1549-1578 nm has a dispersion of −6.65 ps/nm, while the last narrow flat region 1578-1582 nm has a dispersion of −0.35 ps/nm. We observe that when the pECFBG is etched in HF, the etching pattern differs from the ideal step-function implied in Sect. 2, resulting in a broader spectrum of the etched region.
Fig. 5 Spectrum of the pECFBG fabricated by wet-etching, measured by OBR4600 in air at room temperature; left: reflectivity; right: group delay.
The sensitivity of the pECFBG to refractive index has been evaluated by maintaining the whole pECFBG at room temperature, at different values of refractive index using different concentrations of water/sucrose: the refractive index change is 1.85 × 10−3 RIU for each 1% of sucrose up to 50% [35]. This method allows obtaining refractive index changes in water solutions, that is the most important case scenario for biosensing [11,17]. For this task, solutions of sucrose of 0% (reference condition), 1.563%, 3.125%, 6.25%, 12.5%, 25% have been used. Measurements have been performed in the temperature-stabilized fume hood.
For temperature calibration, the setup is shown in Fig. 4. A water bath has been prepared, using a calibrated hot plate (IKA magnetic stirrer/C-Mag HS4), with temperature controlled by a reference thermometer (IKA electronic contact thermometer/ETS D-5, accuracy ± 0.2°C). For spatially uniform temperature measurements, the pECFBG is located on the bottom of the bath, measuring the reference temperature of the bath with the thermometer during a heating cycle. For thermal gradients, the pECFBG is placed in proximity of the surface, using the reference thermometers to measure the temperature in proximity of the tip and of the tail of the pECFBG.
4. Experimental results
4.1 Wavelength shifts estimation
As suggested in Sect. 2, the proposed demodulation method is based on mutual correlation, applied to different portions of the spectrum [36,37]. We define λ1, λ2, …, λN, the discrete wavelength axis used by the instrument for detection, on an equally spaced wavelength grid step δλ such that λk = λ1 + (k-1)⋅δλ; on the Luna OBR4600, with the settings used for the analysis, δλ = 5.2 pm. The measured and reference reflection spectra Rmeas(λ) and Rref(λ) are regarded as digital signals Rmeas(λk) and Rref(λk), respectively. We introduce the cross-correlation operator [36]:
For the analysis, we calculate the autocorrelation spectrum on the left side of the pECFBG in the region 1534.9 – 1547.6 nm that corresponds to the most sensitive part of the grating to RIU changes, in the central part of the grating 1572.5 – 1576.1 nm that corresponds to the transition region between the etched and unetched parts, and on the right side of the grating 1578.8 – 1581.0 nm that corresponds to the window in which the grating is sensitive to temperature. The window selection is computed by calculating the correlation function P on the appropriate spectral slice, selecting the limits of the summation N1 and N2 in Eq. (10).
Since the correlation is limited in terms of resolution to the grid δλ, in order to further refine the wavelength shift estimation it is possible to use the centroid algorithm applied to the autocorrelation [37]:
4.2 Refractive index sensing
The process of demodulation and the obtained results for refractive index sensing are shown in Fig. 6. The first chart shows the cross-correlation Pm as a function of the wavelength shift, for different percentages of sucrose concentration. As the refractive index increases, the cross-correlation function exhibits a right shift, and a decrease of peak value. In Fig. 6(b), the algorithm in Eq. (11) has been computed on the left side of the spectrum to evaluate the wavelength shift of the pECFBG. It is possible to observe that a positive variation of the refractive index corresponds to a shift of the refractive index towards the longer wavelengths. The results are not linear for small refractive index values, and this can be attributed both to the fact that the etching is not an ideal step-function, and to the poor sensitivity (that ranges from 0.10 to 1.49 nm/RIU). The correlation applied to the central part of the spectrum does not return any measurable wavelength shift, due to the temperature stability during the refractive index measurement. Overall, the results in Fig. 6 show the possibility of measuring a refractive index variation with the pECFBG by applying the correlation to the left part of the spectrum.
Fig. 6 Refractive calibration of the pECFBG. (a) The chart reports the cross-correlation Pm for different values of refractive index changes, reported as a percentage of sucrose in water solution. (b) Calibration curve, reporting the wavelength shift measured by the centroid algorithm as a function of the refractive index change on the left window of the grating (1534.9 – 1547.6 nm).
4.3 Temperature sensing
The measurement of a spatially constant temperature profile, performed in a water bath maintaining a constant refractive index, is shown in Fig. 7. It is worth noting that the refractive index of the water bath depends on the temperature, with a measured thermo-optic coefficient that is around −7.66 × 10−5 at 1550 nm, according to [38] Lee et al. Nevertheless, by considering the coefficients of temperature sensitivity and refractive index sensitivity, respectively ST and Sn, reported in Table 1, it appears evident that the wavelength shift induced by the refractive index variation is poorly relevant with respect to the one induced by the temperature variation. For this reason, the refractive index of the water has been considered a constant in the context of the experiment. The spectral analysis shows that, in agreement with Fig. 3(a), the whole spectrum linearly shifts when a uniform temperature variation is applied; by varying the temperature from 31.5°C to 63°C, we observe a shift of the pECFBG spectrum on each of its sections. Since the refractive index is considered constant and each section of the grating has the same temperature coefficient (10.2 pm/°C [5], when the correlation is applied to each section of the grating (left, center, and right) we expect to have the same response. This is confirmed in Fig. 7(b), that compares the water bath temperature measured with the reference thermocouple with the temperature estimated with the correlation-centroid in Eqs. (10) and (11). The three temperature curves are overlapping, confirming that all the portions of the grating are detecting the same temperature; the maximum discrepancy observed between left and right portions of the spectrum is 0.5°C, which can be attributed to the accuracy of the reference sensor.
Fig. 7 Response of the pECFBG to a uniform temperature pattern in a water bath. (a) Spectrum of the pECFBG for different values of the bath temperature; (b) Estimated pECFBG temperature using the correlation method applied to the left, central, and right part of the grating.
4.4 Temperature gradient detection
For this experiment, the pECFBG has been positioned in air, in proximity of the water bath, during a heating cycle. The tail of the grating corresponding to the right portion of the spectrum has been positioned closer to the heated water, thus having a higher temperature, while the tip has been positioned on the opposite side; the reference temperature has been measured in water. At the measurement start, the 3 temperatures coincide. When the bath is heated, we observe an initial hystheresis of the cycle, then the measurement reverses the polarity and the right temperature starts to diverge from the other values, until the bath reaches a temperature of 35°C with a discrepncy of about 8°C between the bath and pECFBG temperatures. As the bath is fully heated and heat is transferred to the grating, a temperature gradient arises and this is recorded by the pECFBG: the temperature measured at the two edges start to significantly diverge, achieving a maximum discrepancy of 7.4°C when the bath temperature is at 45°C, thus recording a gradient of 5.3°C/cm similar to values observed in [18]. After the maximum discrepancy, the slower rate of heating as the water bath overcomes 45°C contributes in reducing the gap between minimum and maximum temperature.
The comparison between the experiments in Figs. 7 and 8 shows the effectiveness of the pECFBG as a sensor measuring temperature gradients. While in Fig. 7 the whole grating is exposed to a uniform temperature, and the correlation algorithm returns the same temperature value for each portion of the grating, in Fig. 8 the grating is exposed to a non-uniform temperature profile, and the temperature estimated in each grating portion is different. Since the whole grating (both etched and unetched regions) has a temperature sensitivity, it is possible to use the whole grating as a thermal profiling sensor.
Fig. 8 Temperature detected in each portion of the pECFBG during a heating experiment, inducing a thermal gradient.
5. Conclusions
In this work, we demonstrated the capability of fiber-optic pECFBG sensor to detect temperature and refractive index variations, and also detect thermal profiles across the grating. The device has been fabricated by etching a portion of a linearly chirped FBG, obtaining two distinct regions: one in which the grating maintains its initial chirp profile, and is inherently sensitive to thermal patterns [5], and the etched region in which an additional chirp due to the exposure of the fields to the external refractive index, adding this sensitivity. Experiments have shown the sensitivity of the grating to uniform and non-uniform temperature profiles, and refractive index; since the two regions are distinct in the spectrum, the sensor is suitable for a cross-compensation of temperature and refractive index. The device can be work for both physical and biochemical detection.
The main feature of the pECFBG, in comparison to other multi-parameter fiber optic sensors [26], is that the pECFBG maintains the capability of chirped FBGs to detect spatially resolved events (temperature or strain, but also potentially the refractive index) while other approaches act as a point-based sensor, returning the information in one sensing point.. The demonstrations carried out with a 14-mm grating show the potential for sub-millimeter spatial resolution. This is an essential feature to enable sensing at the millimeter scale, particularly in healthcare: an immediate application of the pECFBG is the real-time dual detection of temperature and refractive index patterns during minimally invasive thermo-therapies for cancer care, while detecting the borders of a tumor by means of the refractive index of the tissue [2]. The spatial resolution capacity offered by the pECFBG has the potential to improve dual temperature-refractive index sensors, by adding the spatial dimension.
In addition to the application of the pECFBG in minimally invasive medical diagnostic and treatments, future work will address two tasks: (1) improving the spectral reconstruction method first proposed in [5] by extending it to a dual spatially-resolved detection of temperature and refractive index patterns; (2) use the pECFBG for an advanced wavelength-selective dispersion analysis and compensation, exploiting the capability of the refractive index modulation to change the dispersion slope in a controlled way within the grating bandwidth, as suggested by Fig. 5. The proposed architecture can also be made compatible with polymer CFBGs, having a larger thermal sensitivity and therefore a better potential to detect thermal gradients [14,39], adjusting the etching process to the plastic compound as in [28].
Funding
Nazarbayev University grant LIFESTART (ORAU programme at Nazarbayev University 2017-2019).
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