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Abstract
Since its first creation, glass has always fascinated with its optical properties, its ability to let light through without being invisible. One of the most spectacular achievements of optical glass is the optical fiber for which considerable work has been done to make it as transparent as possible. However, for twenty years, contrary to this quest for transparency, nanoparticles have been inserted into optical fibres. First designed to develop new lasers and amplifiers, the lowest possible particle-induced light scattering then sought has for the last four years, on the contrary, been exacerbated in order to develop new sensors.
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1. Introduction
This International Year of Glass is an opportunity to celebrate glass and its many facets. Among its various properties, the optical ones are particularly remarkable, and noticed since they are at the very foundation of the word. Glass, verre, vetro, so many words used in different languages whose etymology comes from the relationship to its translucence and its glitter [1]. Thus, among all the characteristics of glass, its optical properties are those that first marked Humanity, which was then able to manufacture its first transparent solid material. Thanks to its properties of refraction and dispersion of light, glass was the ferment of revolutions in the 17th century: Galileo and the astronomical telescope, Robert Hooke and the microscope, Newton and the glass prism allowing him to understand the origin of colors, to name a few examples. In 1822, Fresnel lens were developed and had a great impact on the navigation. Following the work of O. Schott (glass chemist), E. Abbe (physicist) and C. Zeiss (optician), the publication of the catalog in 1886 marked a turning point by proposing new glass compositions for optics, obtained for the first time predictively and no longer empirically with respect to properties such as refractive index [2]. In parallel with the progress made on the composition, the rise of glass for optics was based on the development of new processes, in particular to improve its transparency by eliminating bubbles, inclusions, absorbing impurities, etc. The advent of fiber optics in the 1970s is a spectacular example. Indeed, following the precursor article by C.K. Kao and G. Hockman [3], the capacity to transmit 1% of the signal has thus increased from 20 m to 100 km in 10 years [4].
Optical properties have long been approximated from the overall composition of a homogeneous glass. Another way consists in introducing heterogeneities in the glass such as nanoparticles. These were used (unknowingly!) centuries ago to color stained glass red (nanoparticles of gold or copper) or yellow (nanoparticles of silver) [5]. D. Stookey’s pioneering work has made it possible to develop photosensitive glasses through the formation of silver aggregates [6]. Nanoparticles (oxides or fluorides) have also been introduced into the glass to host luminescent ions such as rare earth ions or transition metals to control their luminescence properties. This approach, which concerns this article, makes it possible to combine the advantages of the host glass and the luminescence properties specific to the composition and structure of the nanoparticles. F. Auzel et al. reported a first glass-ceramics with luminescence properties but the size of the crystals (10 ${\mathrm{\mu}}$m) made the material opaque [7]. The first transparent glass-ceramics was reported by Y. Wang and J. Ohwaki based on an oxyfluoride glass containing 20 nm nanocrystals of Pb$_x$Cd$_{1-x}$F$_2$ [8]. It is in this context that P.A. Tick published in 1998 the first article concerning optical fibers containing nanoparticles [9].
In this article, we will discuss the different fabrication techniques studied to obtain nanoparticles in optical fibers. Three approaches will be presented, based on the incorporation of particles in the molten glass, the heat treatment of the optical fibers and the in situ growth of particles in the preform. All fabrication Processes will not be described in detail but the reader can refer to previous reviews [10–15]. Recent advances offered by increased control of drawing conditions will also be discussed. Initially designed for laser and amplifier applications, the leitmotiv was to develop fibers with the smallest possible particles in order to minimize the optical losses induced by light scattering. However, since 2018, light scattering has on the contrary been exploited to produce new sensors, in particular allowing spatial multiplexing to be carried out. The second part of the article is devoted to these very promising new applications.
2. Quest for optical fibers containing "scattering-free" nanoparticles
The first motivation to develop optical fibers containing nanoparticles was to obtain new luminescence properties to develop lasers or amplifiers [16–18]. For such fibers, silica glass is generally preferred because it has many advantages (transparency, cost, optical damage threshold, etc.) but some of these characteristics can be detrimental to the properties of luminescent ions such as rare earth and transition metals ions. The high phonon energy of silica glass causes some transitions to have a very high probability of non-radiative decay. This can be illustrated by the case of the de-excitation of the $^{3}$H$_4$ level of Tm$^{3+}$. This level can emit at 1.47 ${\mathrm{\mu}}$m (transition of interest for the telecom S band) towards the $^{3}$F$_4$ level but the probability of non-radiative desexcation is very high because of the $^{3}$H$_5$ level lying at 4000 cm$^{-1}$, i.e. approximately 4 phonons [19]. A fluorinated environment, whose phonon energy is $\approx$2 times lower, is then preferable since the probability of radiative de-excitation is then practically 100%. Compared to an amorphous environment, a crystalline environment has the advantage of (i) increasing the absorption and emission cross sections of luminescent ions and (ii) making the transition metals optically active. The idea is then to encapsulate the luminescent ions in nanoparticles of composition and structure different from silica in order to obtain emission properties which would not appear in silica. In addition, a local over concentration of rare earth ions would promote energy transfer as in the case for example of Yb$^{3+}$-Er$^{3+}$ ions or between rare earth ions to promote up-conversion mechanisms and emit in the visible. However, the introduction of nanoparticles into the core of the fiber introduces heterogeneities in composition which induce light scattering, harmful for lasers and amplifiers. In order to minimize such losses, four requirements have been proposed by P.A. Tick: i) the particle size must be less than $\approx$15 nm, ii) the distance between particles must be comparable to the particle size, iii) the size distribution must be narrow, and iv) the particles must not cluster together [9]. These requirements have since formed the leitmotif of the design of optical fibers containing nanoparticles: to obtain the smallest possible particles in the fiber. To achieve this goal, several manufacturing processes have been studied and are now discussed.
The process that would seem the simplest one would be to adopt the approach of composite materials, namely to prepare nanoparticles and insert them into the molten glass (still sufficiently viscous) to keep them in the optical fiber. This approach is in fact complex because the glass making process involves high temperatures which can harm the integrity of the nanoparticles throughout the manufacturing process. In order to limit the potential degradation of nanoparticles, M.R. Henderson et al. prepared an unstructured fiber from a tellurite glass [20]. The preparation of the glass consists of mixing the powders (TeO$_2$, ZnO, La$_2$O$_3$ and Na$_2$CO$_3$) at 900$^{\circ }$C then lowering the temperature to 700 $^{\circ }$C to incorporate nanodiamonds (Fig. 1). This temperature was chosen to avoid degrading the nanodiamonds while maintaining a glass viscosity high enough to disperse the nanodiamonds. The preform was obtained by extrusion at 355 $^{\circ }$C and drawn into fiber at 400 $^{\circ }$C. It has been estimated that about 1% of nanodiamonds survive during this process, in particular because of the reaction with oxygen [21]. This approach has also been studied in the context of the Modified Chemical Vapor Deposition (MCVD) process. Briefly, this process, widely used in the industrial field for the manufacture of specialty optical fiber preforms, consists of depositing a porous layer by chemical reaction in the gas phase inside a silica tube. The porous layer is then soaked in a hydro or alcoholic solution containing the ions of interest. The porous layer is then sintered and the tube is collapsed to form a preform whose diameter is typically around one cm. During the solution doping, LaF$_3$:Tm$^{3+}$ nanoparticles were introduced but during the process, the fluorine evaporates following a reaction with the silicon to form gaseous SiF$_4$ and the particles identified in the fiber are lanthanum silicates [22]. The introduction of Al$_2$O$_3$ nanocrystals (<50 nm) via the solution doping leads to the presence of amorphous nanoparticles (size 30-200 nm) in the fiber, without the composition having been able to be determined [23]. However, the authors also showed that at the same average aluminum concentration in the fiber (5 mol% Al$_2$O$_3$), amorphous particles of the same size are observed for standard doping with AlCl$_3$. The Rod-in-Tube process (rod of Al$_2$O$_3$ in a silica tube) and the molten core method (powder of Al$_2$O$_3$ in a silica tube) have made it possible to reach average concentrations of Al$_2$O$_3$ above 20 mol%, resulting in spinodal phase separation in the core [24,25]. Finally, a mixture of micron-size powders of SiO$_2$ and Y$_3$Al$_5$O$_{12}$ (YAG) was inserted into a silica tube and stretched at 1950 $^{\circ }$C. Again, the YAG crystals disappear in favor of a spinodal type phase separation with an amorphous phase of the YAS type (Y$_2$O$_3$-Al$_2$O$_3$-SiO$_2$) and of characteristic dimension less than 50 nm [26].
Fig. 1. Diamond NV-Photoluminescence intensity scanning confocal maps of cleaved unstructured tellurite fiber endfaces. Fibers were fabricated from billets with (a) 28 mg nanodiamonds and (b) 0.94 mg nanodiamonds in 100 g tellurite. [20]
A second approach consists of preparing a fiber with a homogeneous composition and applying a heat treatment to it. Like glass-ceramics, this heat treatment (generally consisting of a single temperature) ensures the nucleation of the particles and their growth. This is the approach that was historically used first [9,17,18]. The treatment temperature corresponds to a temperature slightly higher than T$_g$, typically +50 $^{\circ }$C. Examples of nanoparticles identified in the fibers prepared by post heat treatment are reported in Table 1. The specificity of this process is to obtain crystalline particles. Their structures can thus be identified by X-Ray Diffraction (XRD) and Selected Area Electron Diffraction (SAED) techniques. The reported nanocrystals are essentially fluorinated or oxide matrices, exceptionally sulfur-based, used for luminescence applications. The fluoride nanocrystals concern doping with rare earth ions (Er$^{3+}$, Yb$^{3+}$, Ho$^{3+}$, Nd$^{3+}$) to benefit from the low phonon energy of this composition. Oxide nanocrystals have been developed to make the emission of transition metals in silicate fibers efficient. Ba$_2$TiSi$_2$O$_8$ nanocrystals have been prepared to exacerbate non-linear properties. The heat treatment of the fibers remains to this day the only way which makes it possible to obtain in an optical fiber crystalline nanoparticles doped with luminescent ions. However, the heating can only be applied to a finite length of the fiber. In addition, it is necessary to remove the polymer cladding to apply the heating and then to recoat the fiber. Finally, the temperatures involved can be high, in particular for the formation of oxide particles, which can degrade the mechanical properties of the fiber. For example, at a temperature of 1000 $^{\circ }$C, silica glass transforms into a crystalline phase, cristobalite, which would weaken the fibre.
Table 1. Nanocrystals identified in optical fibers after their heat treatment. MiT and PiT stands for Melt-in-Tube and Powder-in-Tube techniques.
In order to overcome the post heat treatments of the fiber, a third way consists in obtaining the nanoparticles directly during the manufacture of the preform [40]. This in situ growth relies on phase separation mechanisms. When an element such as magnesium is added to silica, the phase diagram of this binary SiO$_2$-MgO mixture shows a demixing zone leading spontaneously to the formation of a silica-rich phase and another phase rich in phase separating elements (Mg) [41–43]. This domain appears for temperatures above 1700 $^{\circ }$C because the phase diagrams take into account the crystalline phases. This temperature of 1700 $^{\circ }$C corresponds to the melting point of $\beta$-cristobalite. In the case of a glass, this immiscibility domain can therefore appears at lower temperatures. These phase separation mechanisms have been performed with the MCVD process [40]. The main ions reported to initiate phase separation are the alkaline earth ions (Mg, Sr, Ca) and La [44–46]. All these ions are inserted through the solution doping technique. The phase diagrams of these binary mixtures show the existence of a domain of immiscibility for concentrations of phase separating elements below 40 mol% but their average concentrations are usually on the order of few mol%. Contrary to the previous approach, the heat treatments are here imposed by the MCVD process during which the vitreous layer is sintered then the tube is collapsed. An example of heat treatment is shown in Fig. 2. Temperatures were measured using a pyrometer pointing at the surface of the silica tube. Each cycle corresponds to the passage of the burner over this zone. The temperatures are around 1000 $^{\circ }$C for drying, 1500-1800 $^{\circ }$C for sintering and 2000 $^{\circ }$C for collapsing stage. In agreement with the phase diagrams, these high temperatures make it possible to initiate the phase separation mechanisms and to observe the formation of particles in the preform. The size of the particles in the preform depends on the phase separating element and its concentration in the doping solution. For example, for the same concentration in the doping solution, calcium leads to larger particles than by introducing magnesium [44]. Increasing the concentration of phase separating element makes it possible to increase the size of the particles (Fig. 3).
Fig. 2. Temperature measured with a pyrometer pointing on the surface of the silica tube all along the MCVD process after the solution doping step. Each cycle corresponds to a pass of the burner.
Fig. 3. SEM images of MCVD core preforms doped with lanthanum as phase separating agent. La concentration in the doping solution is 0.7 (a) and 0.175 mol/l (b). The scale bar is the same for (a) and (b) images.
These preforms are then drawn at a typical temperature of 2000 $^{\circ }$C. Contrary to the case of homogeneous fibers for which the drawing corresponds to a homothetic transformation of the preform, in the case of a fiber containing heterogeneities it has recently been shown that the particles undergo a complex evolution [47–49]. Indeed, they can elongate or even fragment into smaller particles (Fig. 4). This deformation is explained by known mechanisms in the field of rheology and involves a competition between the viscous flow which tends to deform the particles and the surface tension which tends to maintain a spherical shape. This ability to deform is characterized by the capillary number Ca
where $\eta$ is the viscosity, $\dot {\epsilon }$ is the deformation rate, R$_{NP}$ is the radius of the particle and $\gamma$ is the surface tension between the particle and the matrix [47]. Beyond the critical capillary number, the elongated particles fragment via Plateau-Rayleigh instabilities. This mechanism has been exploited to continuously manufacture nanoparticles of controlled size by drawing a rod of chalcogenide glass (As$_2$Se$_3$) coated with a PES polymer cladding [50]. This use of Plateau-Rayleigh instabilities is also a clin d’oeil to the history of optical fibers which started with Jean Daniel Colladon when he discovered the luminous fountains while wanting to present the recent discovery by Félix Savart of the instabilities of a jet of water, evolving from a continuous jet into droplets, laterly studied by Joseph Plateau and then Lord Rayleigh [51]. The use of these mechanisms opens very promising perspectives because it becomes possible, from an initial particle size in the preform, to design via a top-down approach the characteristics of the particles in the fiber by modifying the drawing conditions. For example, by changing the stretching temperature, it is possible to obtain very small particles for drawing at high temperature (2050 $^{\circ }$C) or on the contrary very elongated and fragmented particles for drawing at lower temperature (1850 $^{\circ }$C) (Fig. 4). Drawing is therefore a key step in controlling fiber transparency. The presence of elongated particles (or air bubbles) could also be of interest for particular properties of light transport as in the case of Anderson localization random lasers [52,53]. Finally, from a more fundamental point of view, the in-depth analysis of the elongation/fragmentation mechanisms could be a way to determine values that are difficult to access, such as the surface tension between two glasses.Fig. 4. (a) 3D rendering of the core of an optical fiber analyzed by FIB and SEM [49]. Particles are obtained by introducing magnesium. Smaller spherical particles, randomly distributed, are not represented. (b,c) SEM images of longitudinal sections of fibers drawn from the same preform (doped with La as phase separating agent) at two temperatures: 2050 $^{\circ }$C (b), 1850 $^{\circ }$C (c). The scale bar is the same for (b) and (c) images. For all the images, the drawing axis is horizontal.
From the applications point of view, the first idea for the development of these fibers was to produce lasers and amplifiers. However, in two decades, only a few lasers and amplifiers have been reported despite the a priori potential offered by environmental control of luminescent ions. One of the limitations comes from the losses induced by the diffusion of light linked to the presence of the nanoparticles. Reducing these losses also imposes a compromise on the size and the volume fraction of nanoparticles, and therefore the quantity of luminescent ions which can be incorporated. Another point that has been highlighted recently concerns an effect that has been neglected until now: the variation in composition of amorphous particles as a function of their size [43]. This effect is not described within the framework of the classical theory of nucleation for which, once the critical size has been exceeded, the particle grows while retaining the same properties, identical to those of the macroscopic phase [54]. This variation in composition is on the other hand predicted in the generalized approach of Gibbs [55]. The variation in composition implies that the smallest particles have very little changed composition and therefore potentially offer very little alteration in luminescence properties. Thus, in the case of amorphous particles obtained by phase separation, the doxa can no longer be to choose the smallest particles possible, but to opt for the best compromise between a size small enough to limit losses by light scattering and a size large enough to obtain a modification of the composition leading to a noticeable change in the luminescence properties. In addition, this variation in composition should also lead to a variation in the refractive index as a function of particle size. However, in light scattering models such as Rayleigh’s one, nanoparticles have only one index regardless of their size. Assuming a linear variation of the refractive index [56], an increase in the concentration of 5 to 35 mol% MgO in SiO$_2$ leads to an increase in refractive index of 0.06. Thus, for small particles (those containing 5 mol% MgO), the Rayleigh scattering coefficient by considering a single index for all particles would be an order of magnitude higher than that obtained by considering a variable composition. The study of the composition variation of the particles therefore deserves a particular interest for the development of fibers. This study is however made difficult by the ability to measure compositions with a nanometric spatial resolution but a technique such as Atom Probe Tomography offers very interesting perspectives from this point of view [43].
The original approach to nanoparticles in optical fibers was to minimize their impact on light scattering. Very recently, the opposite of this approach was taken and made it possible to show that light scattering could on the contrary be exploited to produce sensors. We will focus on these applications in the rest of the manuscript.
3. Sensing applications
3.1 Scattering-level multiplexing framework
The use of nanoparticle-doped fibers (NPDFs) [57,58], as well as other enhanced backscattering fibers (EBFs) [59,60] or ultra-weak and random gratings [61] finds one of the most promising and established applications in the development of distributed fiber-optic sensing networks [58].
Distributed sensors interrogate the backreflections occurring in an optical fiber due to scattering events [62]. Long-range detection systems based on time-domain operation can interrogate up to hundreds of kilometers of fibers, mainly detecting Raman scattering. However, the recent focus has shifted on frequency-based methods such as optical backscatter reflectometry (OBR) [63], and other optical frequency domain reflectometry (OFDR) methods, in order to interrogate Rayleigh scattering for physical sensing [62], [63] and Brillouin scattering for acoustic sensing [64]. OBR methods can work over tens or hundreds of meters, with spatial resolution as low as 10 $\mu$m (theoretical) to 1-2 mm (practical). The possibility to detect and localize physical or biological parameters along a fiber extends multi-point sensing up to architectures that have hundreds of sensors, arranged in maps that report the measurand as a function of time and distance [65,66].
There is a growing interest in applying distributed sensors to biomedical applications, which implies working with narrow spatial resolution, multiple fibers, short interrogation range (few centimeters), and real-time detection [67]. In order to fully take advantage of distributed sensors, and approach the resolution and functionalities of imaging methods, 2D/3D distributed sensing architectures are a key asset for diagnostic and supporting therapeutic tasks [66–68]. The key enabling technology for achieving multi-fiber, 2D/3D sensing in medical devices is represented by scattering-level multiplexing (SLMux), which is a subset of spatial division multiplexing specifically applied to distributed sensing [58].
The SLMux is implemented by modulating the scattering levels in each portion of a distributed fiber sensing network, using fibers with low scattering (like the commercial single-mode fibers, SMFs) having only interconnection purposes, while each sensing element is formed by an EBF having a high scattering increment [69].
Figure 5 shows the implementation of a SLMux system. The OBR interrogator is a swept-wavelength interferometer, having a sensing (upper) branch separated from the trigger (lower) part; it is designed to interrogate a single fiber, but it can be adapted to the SLMux with no change of the hardware or software internal to the system (e.g. no switch to commute between channels) [69]. A 1xN splitter (or a cascade of splitters) is used to multiplex from 1 to N channels; each channels has a SMF extender having different length, and arranged such that none of the sensing spans overlaps with each other. Each sensing fiber has high scattering, and can be placed in 2D [69] or 3D [66] geometries; the SMF and splitters instead form the distribution network.
Fig. 5. Architecture and principles of a scattering-level multiplexing (SLMux) distributed sensing system. (a) Schematic of an OBR detection system; (b) Sensing zone of a SLMux system with N fibers (red fibers = SMF; blue fibers = EBF). (c) Backscattering trace for the SLMux system; the chart displays the scattering gain (G) and the signal-to-interference ratio (SNR); (d) Example of a 4-fiber SLMux trace, with 4 NPDF having 20 cm length [69]
In Fig. 5(c-d) we show a sketch and an experimental measurement of the backscattering trace for a set of NPDF fibers. The sensing fibers have a scattering gain (G), which is defined as the ratio between the power levels corresponding to the NPDF and SMF. In each i-th sensing location, we define as the signal-to-interference ratio (SIR) the ratio between the power measured from the NPDF and the whole interference power originated from the (N-i) overlapping SMF fibers. The SLMux can work with any EBF or weak/random grating [58], provided that the SIR at any sensing point is adequate ($\geqslant$ 20 dB [65]). NPDF however own two substantial advantages: (1) they are optical fibers, that can be spooled and spliced to telecom-grade SMF fibers using a standard telecom splicer with negligible losses, making it very simple to arrange a sensing network; (2) they have a high gain, over 50 dB when optimizing the nanoparticles density and fiber design, while having losses as low as 15-30 dB/m which enables sensing spans compatible with the tens of centimeters required for medical devices [70]. Alternative methods require tagging each sensing region with a weak [71] or random [61] distributed grating, or exposing SMF fibers to intense UV light [72].
3.2 Biomedical physical sensors
The SLMux concept has provided a significant step forward in biophysical sensing, allowing the interrogation and sensing of medical devices with hundreds of sensing points stacked in few mm$^{2}$, with resolution that approach thermal imaging [69] or pixel-based shape reconstructions [68]. Figure 6 shows the timeline of multi-point biomedical sensors, in terms of the state-of-the-art of sensing distributions [67]. The first generation of multi-point sensors makes use of fiber Bragg gratings (FBGs) or interferometers interrogated as in-line sensors [73,74]. FBGs achieve centimeter-level spatial resolution [73], but also require the inscription of each sensing element which makes the sensor much more expensive than a bare fiber. The OBR allowed evolving from few points of sensing up to a continuous sensing fiber, interrogating the Rayleigh scattering profile of SMF fibers; this has been successfully used for 1D measurements of temperature [75] and strain [76] in medical devices. As NPDFs enabled 2D/3D sensing networks, the present generation of sensors makes full use of multiple fibers, arbitrarily positioned in configurations such as grids [69], cylindrical shapes [68], or full-3D grids with up to 12 fibers [66].
One of the main applications of NPDFs is in temperature sensing during thermal ablation of cancer, as a mini-invasive thermotherapy [77,78]. Such therapeutic treatments are based on delivering electromagnetic energy from a radiofrequency, microwave, or laser source onto a tissue, selectively heating the cancer cells over cytotoxic values while safeguarding the peripheral healthy tissues. Steep spatial and temporal gradients are measured during thermal ablation [75], and they can be estimated with a multi-fiber architecture [66,69,79]. Figure 7 shows examples of temperature sensing during ex-vivo thermal ablation, using setups that allow in situ detection. The first charts shows an example of thermal map, measured during a laser ablation on the surface of a porcine tissue, and its corresponding cytotoxicity map that shows the regions exposed to 60 $^{\circ }$C (instantaneous protein coagulation) and >42 $^{\circ }$C (cytotoxic region) [23]. In this setup, 4 fibers record temperature data on a 600 mm$^{2}$ surface, with resolution 2.5 mm $\times$ 5.0 mm (0.11 sensors/mm$^{2}$). The third chart shows the wavelength shift measured for each temperature, estimating the temperature coefficient as 10.25 pm/$^{\circ }$C, as the same coefficient for FBGs. The last thermal map reports the first-ever 3D thermal map recorded for fiber-optic distributed sensors, using 12 fibers arranged on a square shape of 16 $\times$ 16 mm and 2.5 mm resolution along the fiber direction [66]; in this architecture, sensors allow mapping the temperature over a volume of 6400 mm$^{3}$ with 132 data points (0.021 sensors/mm$^{3}$), and the thermal map is shown using a plane-by-plane visualization (removing several intermediate planes for a better visual rendering). Data reproduced in the figure have been adapted from [66,79], where the temporal evolutions of the thermal map can be also viewed.
Fig. 7. Application of NPDF-based SLMux to thermal sensing, for real-time monitoring of thermal ablation. (a) 2D surface maps, obtained with 4 NPDF (2.5 x 5.0 mm pixel size) recorded during a laser ablation of porcine tissue; (b) contour of the high-mortality zone (red, >60 $^{\circ }$C) and cytotoxic region (yellow, >42 $^{\circ }$C). (c) Calibration of a NPDF fiber estimating the temperature coefficient as 10.25 pm/$^{\circ }$C (-1.280 GHz/$^{\circ }$C). (d) 3D temperature map, obtained by 12 NPDF fibers over a 16x16 surface during microwave ablation heating; each plane shows the temperature recorded on the plane perpendicular to the fiber. Images adapted from [79] (a-b) and [66] (c-d)
The use of NPDF fibers allows also exploring the strain sensitivity of the fiber ($\approx$1 pm/$\mu$ $\epsilon$) in order to provide 3D shape sensing of medical devices [80]. The design reported in [81,82] is based on 4 NPDFs externally mounted to a 18-Gage epidural needle: each sensing point, with 2 mm resolution, detects a strain that depends on the amount of directional bending. As shown in Fig. 8 (images from [81] and [82]), by applying a reconstruction algorithm optimized for rigid needles (in a low-bending approximation), we can reconstruct the needle silhouette in a 3D geometry, and evaluate its cinematic during the insertion motions. These results have been reported in [81], acquiring statistics over 25 different epidural insertions in a phantom. An experimental framework for shape validation has been reported in [82], comparing the reconstructed shape with a 3D-calibrated ground truth and showing an evaluation of the silhouette of the needle with <1% error.
Fig. 8. 3D shape sensing of a medical epidural needle, based on NPDF fibers. (a) Top-view of the fibers mounted externally to the needle. (b) Sketch of the algorithm for shape reconstruction. (c) Sequence of bending patterns observed for an epidural insertion. (d) Image showing the reconstructed shapes for a needle progressively bent along its horizontal direction. (e-f) Real-time sensing, showing the applied deformation (e) and its corresponding shape estimation (f). Images adapted from [81] (a-c) and [82] (d-f).
3.3 Reflector-less biosensors
Optical fiber biosensors provide a high-performance and miniature platform for the detection of biomolecules [83,84]; the possibility of using single-mode fibers dramatically improves the accuracy, as it enables the use of telecom-grade interrogators that have picometer-level resolution [67], which ensure low limit of detection. A biosensor is formed by designing a refractive index (RI) detector, which is then biofunctionalized through silanization or thin metallic films that can host bioreceptors, selectively binding to the target molecules [83].
Traditional biosensors make use of gratings or interferometers, to form wavelength-selective filters whose spectrum shifts when exposed to RI; for this reason, the fabrication of biosensors is bulky, as it requires the inscription of reflectors or the fabrication of multiple reflective surfaces to form a fiber-tip interferometer [84,85]. However, Rayleigh scattering already provides the reflective content that can be detected through an OBR; by etching a fiber and depleting its cladding, the change of effective refractive index of the fundamental mode would cause a wavelength shift of the scattering signature [86,87]. However, since etched fibers have high propagation losses, usually around 20 dB [87], the OBR cannot detect the output signal from an etched SMF fiber, as it would drop below its noise floor.
NPDF fibers allow solving this technological problem, as they can raise the scattering levels in a significant manner; by etching a NPDF in the sensing zone, we can form a biosensor that requires no reflection other than the Rayleigh scattering, augmented by the nanoparticles located in the core (that are not etched away) and sustaining the propagation losses throughout the thinner region. This concept, labelled as reflector-less biosensor, shows a minimalistic design that requires only wet-etching (a process very common in mass-production of electronic circuits), while working with a wavelength-sensitive reflective sensor [86].
Figure 9 shows the working principle of a reflector-less biosensor, functionalized for protein detection [86]. The structure is simply formed by a NPDF fiber etched along its length through hydrofluoric acid. A thin gold film is deposited on its surface for immobilizing protein-binding bioreceptors, using either the antibody-antigen binding as a sensing mechanism [83] or aptamers as organic bioreceptors [84]. The work reported in [86] makes use of electrode-less plating of gold film, the subsequent immobilization of thrombin-binding aptamers, and the detection of protein in concentrations ranging from 0.6 to 20 mg/ml.
Fig. 9. Reflector-less biosensors designed by etching and functionalizing a NPDF. (a) Schematic of a generic label-free biosensor: after etching a NPDF and depleting its cladding, the fiber is gold-coated and bioreceptors are immobilized on the fiber surface, binding to proteins (analyte) and causing a wavelength shift of the local scattering signature. (b) Sensitivity as a function of the fiber thickness: simulation for thick fibers, extrapolation for thin-fiber conditions, and measurement over 3 samples. (c) Report of thrombin protein sensing using a biofunctionalized sensor [86]; the linear fit shows 74.3 pm response for each 10x thrombin concentration rise.
Experimental data and simulations show that in order to get a sensitivity >1 nm/RIU (RI units), it is necessary to deplete almost the entire fiber cladding, achieving a fiber with 10 $\mu$m diameter; performances increase exponentially below this limit, although the fiber fragility would compromise the possibility of sustaining the whole biofunctionalization process. Even with a sensor having a sensitivity around this value, given the accuracy of the wavelength shift estimation and the possibility to localize the most sensitive point by using a distributed detection, it is possible to detect proteins at low limits as shown in Fig. 9(c), with a sensitivity of 74.3 pm for each 10x increase of concentration. To date, reflector-less biosensors have been reported with sensitivity up to 46 nm/RIU [87].
3.4 Radiation sensing
In the applications described in the previous sections, the NPDF plays a significant role for its substantial increase of Rayleigh scattering, which enables multiplexing configurations, or sustaining propagation losses through a sensing zone; the actual sensitivity ratings for temperature, strain, and refractive index are similar to those of SMF fibers (or FBGs inscribed on SMF), and they are not significantly affected by the presence of a high density of scatterers in the fiber core.
The work reported in [88] extends the use of distributed sensing of physical or biological parameters to the detection of ionizing radiation, demonstrating the first in situ spatially resolved detector for radiation (X-ray, dose rates varying from 2 to 700 Gy/min). In this case, the use of NPDF fibers is strongly beneficial, as they have a radiation sensitivity >10 times larger than a SMF when exposed to radiation.
Traditional fiber-optic radiation sensors use a scintillating material placed on the tip of a large-core fiber that converts the radiation into fiber-coupled UV/visible light [89], or measure the changes in transmission spectra of customized long-period gratings [90]; these systems lack both the spatial resolution factor as they are single-point detectors, and do not work well with a reflective probe.
The use of NPDF fibers interrogated via OBR, on the other hand, shows the premises for the first-ever distributed radiation detector, as the system can resolve both the intensity and the spatial width of the X-ray exposure. The experiments reported in [88] show first the results for high-levels of radiation, showing the capability of the system to detect in radiation-prone and harsh environments; at 700 Gy/min, the NPDF observes a peak wavelength shift of about 75 pm and a sensitivity of 0.011 pm/Gy, while the wavelength shift of a SMF exposed to the same conditions was negligible. Low-dose experiments, at 2 Gy/min which mimics the values observed in medical radiotherapies, show an opposite trend as the wavelength shift decreases during the exposure, reaching the peak value of -29.6 pm. The possibility to detect and localize radiation with millimeter-level spatial resolution opens up to novel types of dosimeters, particularly envisioning the optimization of the fiber to improve the specific radiation sensitivity.
3.5 Transmission-reflection analysis
An alternative use of NPDFs involves the transmission-reflection analysis (TRA) [91], which has been reported by Silveira et al. [92]; this approach allows implementing a distributed sensing approach in the presence of a single perturbation in the fiber, but with a simpler and cheaper hardware with respect to the OBR approach. The TRA method relies on the simultaneous measurement of normalized transmission and reflection coefficients for an optical fiber, subjected to a single perturbation in unknown location and illuminated by an incoherent light source; by solving an analytical system that takes into account the ratio between the reflected and transmitted power terms, the location of the perturbation can be unambiguously identified, with accuracy of few millimeters. Since NPDF fibers have high losses and substantial Rayleigh scattering, they enable a TRA analysis on shorter distances than SMFs.
Subsequently, Leal-Junior et al. [93] extended this concept to application in wearable sensing; in this work, a NPDF span was embedded in a garment, pre-tensioning the fiber in 8 locations in correspondence of specific points of interest on the chest. In absence of any perturbation, the TRA highlights the 8 regions of interest, and when mechanical disturbances caused by movements are applied, they are recorded through the backscattered power. The feasibility study reported in [93] shows the possibility of using this approach in exoskeleton systems.
4. Conclusion
Thanks to their ability to provide new luminescent properties, and more generally new functionalities, nanoparticles in optical fibers have been of growing interest for twenty years. The first motivation concerned the development of new lasers and amplifiers. Different manufacturing processes have been tested to obtain the smallest possible nanoparticles in order to limit optical losses induced by light scattering. The heat treatment of optical fibers makes it possible to achieve this objective. In particular, this process makes it possible to grow nanocrystals, which in particular opens the way to applications requiring the luminescence of transition metals. The MCVD process, already used industrially for specialty optical fibers, has also demonstrated its full potential for producing fibers containing nanoparticles while avoiding post-heat treatment which can degrade the mechanical properties of the fiber. Recent knowledge acquired on the effect of drawing on nanoparticles opens up new perspectives for custom designing nanoparticles in the fiber. Despite significant work done on processes to reduce the size of particles, the production of lasers is still rare because these components need to overcome the diffusion of light. On the other hand, light scattering open new opportunity for random fiber lasers [25,94] and it could be exploited to develop new sensors.
The use of nanoparticles in the core of single-mode optical fibers to enhance Rayleigh backscattering is a key asset in bringing distributed sensing architectures to the millimeter scale, with emphasis on biomedical applications. In such systems, the Rayleigh scattering corresponds to the signal at the detector, and its substantial increment enables novel multiplexing concepts. To date, SLMux has been reported with up to 12 fibers positioned in miniature needles or around medical applicators, for 2D/3D sensing over fiber grids, as well as introducing biosensing concepts. New sensing applications might exploit an optimization of the fiber properties, as well as studying the effect of polarization changes over NPDF which appear to have a short beat length.
From a commercial standpoint, the fabrication of an entire fiber span having enhanced backscattering is a clear advantage over other methods to increment the backreflection such as inscribing nanogratings or random gratings in the core. While these gratings have a relatively slow fabrication, they are limited to several centimeters in length, and need to be inscribed ad-hoc in a specific fiber location, the NPDF method provides users with a fiber, that can be spliced to telecom-grade SMF fibers with a common splicer, and becomes a building block for any sensing network, with the users that can set the length and position of the NPDF as well as the diversity of multiplexing with a much higher flexibility and reliability (since the fibers maintain the protective coating).
Funding
Nazarbayev University (021220FD1851, 091019CRP2117, 240919FD3908); Agence Nationale de la Recherche (ANR-17-CE08-0002-05).
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 may be obtained from the authors upon reasonable request, except Fig. 1 issued from Ref. [20].
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