7.1. Glass-Ceramics Fibers
One of the first GC fibers was successfully obtained by Zheng, Hu, and Mackenzie in Bi–Ca–Sr–Cu–O system [
149,
150]. Slight modification of the glass properties by co-coping with Al or V decreased the crystallization tendency, whereas the drawing from a glass preform at the temperature above the crystallization temperature allowed the authors to avoid full devitrification of the materials during this step. The devitrification occurring during the drawing led to precipitation of only very fine particles inside the fibers, whereas a large number of spherical Bi
2(Sr, Ca)
2CuO
x crystals with an average size of 0.5 µm formed on the surface of the fibers [
150]. These crystals did not affect the fiber drawing process. The final ceramization was done by heat-treatment of the fibers.
Heat treatment of the Al-containing fiber at 825 °C for 12 h in air atmosphere led to the formation of the Bi4Ca3Sr3Cu4Oy crystals.For optical applications, silicate GCs are the most popular among GC materials due to their unique thermo-mechanical properties, such as very low thermal expansion coefficients, combined with outstanding optical characteristics, as it is possible to stabilize small enough nanocrystals and in this way avoid the light scattering and achieve high optical transmission even in the UV-blue region. Examples of typical crystals formed in silicate glasses can be found in [
71]. The first report on an GC silicate fiber laser was reported in 2001 by Samson et al. [
151]. Using the double-crucible and rod-in-tube techniques, NdF
3-doped GC fiber laser within the SiO
2–AlO
3/2–CdF
2–PbF
2–YF
3–ZnF
2 system was successfully prepared. It was possible to obtain the Nd-doped fluoride crystals in the core of the single-mode optical fiber by its heat-treating at 450 °C for 30 min. The presence of the Nd-doped crystals in the fiber compromised the efficiency of the GC fibers. Such GC fibers could be obtained with low scattering losses as the crystal size was ~10 nm. The work was then focused on transition-metal-doped GC fibers and especially on Cr
4+-doped forsterite nanocrystalline fiber which displayed luminescence similar to that of a single crystal [
152]. This GC combines the radiative properties of Cr
4+-doped crystals with the advantages of glasses. Later, Samson et al. developed novel Ni
2+-doped nanocrystalline GC fibers within the SiO
2–Ga
2O
3–Al
2O
3–K
2O–Na
2O–Li
2O system [
153]. The preform was obtained using the rod-in-tube method. Heat treatment of the glass fiber at 850 °C for 2 h led to the precipitation of gallate-rich aluminogallate spinel crystals homogeneously dispersed in the silicate glass matrix. In 2015, Ni
2+ doped GC fiber was successfully prepared using the melt-in-tube method [
143]. The temperature of 1830 °C was used to fabricate the fiber; the core glass melted while the clad glass was softened. No signs of element inter-diffusion or crystallization were found in the as-drawn glass fibers and the GC fiber was obtained by heat-treating the glass fibers; LiGa
5O
8 crystals were found to precipitate in the fiber using a heat treatment at 800 °C for 10 h.
The melt-in-tube method was used recently to prepare GC optical fiber containing Ba
2TiSi
2O
8 crystals as the fabrication of such GC fiber using other techniques is challenging [
154]. The authors noted that it was impossible to draw the fiber using the rod-in-tube technique due to the high crystallization rate of the materials (see
Figure 5, left panel). In the case of the amorphous fiber, Ba
2TiSi
2O
8 nanocrystals could precipitate in the fiber core evenly with a diameter between 1.0 and 6.0 nm to ensure low transmission loss using a heat treatment at a temperature of about 850 °C for 5 h (see
Figure 5, right panel). These studies clearly show that the melt-in-tube fabrication method is a promising new process to obtain GC fibers, which cannot be fabricated using the conventional rod-in-tube method.
Yb-doped GC fiber was also successfully drawn from yttria–alumina–silica GC preform prepared using MCVD [
118]. The collapsing step led to the formation of a phase-separated glass with two regions of different compositions. A heat treatment above 1300 °C led to the precipitation of nano-sized particles of YbPO
4, which were evidenced by X-ray absorption spectroscopy and by the shape of the emission band which was typical of a structurally ordered phase.
Sakamoto and Yamamoto demonstrated that for Li
2O–Al
2O–SiO
2 it is possible to draw the fibers using pre-crystallized preforms [
155]. In this work, it was shown that the drawing step could change the crystalline phase (from β-spodumene to β-quartz) and could reduce the crystallinity. Nevertheless, under optimized parameters, it was possible to get a GC fiber of good quality with a high amount of the crystalline phase.
Despite many successes for the silica glass, REEs do not incorporate well into the silicate glass structure. The low solubility of REE in silica glass directly results in a high probability of phase aggregation and even crystallization [
156]. Clusters give rise to luminescence quenching at REE concentrations greater than 100 ppm [
157]. Silica glass has relatively high phonon energy that increases the probability of non-radiative relaxation of the luminescent ions. As a consequence, this may decrease the emission quantum yield of certain optical transitions [
158]. Then, several glasses were developed over time [
159], such as, chalcogenides [
160] and fluorides [
161] which exhibit improved REE solubility and lower phonon energy. However, these glasses are prone to crystallization during the fiber making process. Additionally, they are not as compatible as silicates with the current common optical fiber cable network. In this context, oxyfluoride GC fibers have been also of great interest for the development of efficient amplifiers or lasers in glass-ceramics, as the fluoride environment is beneficial for the rare-earth ions keeping the chemical and thermal stability and the good mechanical properties of the oxide matrix [
162]. The first oxyfluoride GC in bulk form having high transparency was reported for the composition 30SiO
2-15AlO
1.5-24PbF
2-20CdF
2-10YbF
3-1ErF
3. This GC was found to possess efficient up-conversion, which was up to several orders higher when compared to the mother fluoride [
9]. Since then, there has been much interest in developing novel transparent oxyfluoride glass-ceramics containing REE-doped fluoride crystals. Reviews on transparent oxyfluoride GCs in bulk form can be found in [
163] and [
108]. Some of the first transparent oxyfluoride GC fibers were with the composition of 48SiO
2-11Al
2O
3-7Na
2CO
3-10CaO-10PbO-11PbF
2-3ErF
3 and 48SiO
2-11Al
2O
3-7Na
2CO
3-10CaO-10PbO-10PbF
2-3YbF
3-1ErF
3 [
105]. They were obtained by heat-treating of the glassy fiber at 700°C for various periods of time. The transparent GCs could be obtained when using the treatment shorter than 32 h. Transparent GCs fibers with SrF
2 and LaF
3 crystals possessing intense Nd
3+ emission were obtained within SiO
2–Al
2O
3–ZnO–Na
2O–SrF
2 and SiO
2–Al
2O3–ZnF
2–Na
2O–LaF
3 systems [
164]. In this work, Reben et al. showed that the most important parameters to control during fiber drawing were temperature and heating time during the drawing, leading to precipitation of SrF
2 nanocrystals in the core of the fiber.
Recently, transparent oxyfluoride Nd
3+ doped fibers with the composition 55SiO
2–20Al
2O
3-15Na
2O-10LaF
3 GCs were successfully prepared using a single crucible method [
165]. After drawing, fibers of 5–10 cm in length were heat-treated between 620 and 680 °C for 5 to 120 h in order to precipitate Nd
3+ doped LaF
3 crystals with a size of 10–20 nm. The fibers exhibit a slower crystallization rate than the parent bulk glass, and therefore, the fibers needed to be heat treated at a higher temperature than the parent bulk glass in order to precipitate crystals with similar size and similar crystal fraction. After, the GC fibers were covered with SiO
2 cladding using the sol-gel method. In this way, a multimode fiber with losses of about 20 dB/m at 633 nm at the doping level of 0.1% NdF
3 was obtained.
Ytterbium-doped oxyfluoride GC fibers were also obtained within the SiO
2-Al
2O
3-CdF
2-PbF
2-YF
3 [
166]. It was shown that partial devitrification of the glass occurs during the conventional glass preform drawing process. Alternatively, the glass fiber could be obtained directly from the melt, whereas the crystallization was done by an additional controlled heat-treatment step. The latter approach was found to be preferable to produce transparent GCs with nanocrystals homogeneously distributed along the fiber, as the drawing of a GC preform did not allow fine tailoring of the crystals’ sizes, shapes, and distribution along the fiber. Pb
1−x−y−zCd
xY
yYb
zF
2 (x + y + z ≈ 0.3–0.4) nanocrystals with an average size of ~10 nm were found to precipitate in the fiber heat-treated at 460 °C for 20 h, increasing the photoluminescence quantum yield in the near-infrared region compared with the glass-fiber.
GCs oxyfluoride fibers with remarkably enhanced Er
3+ 2.7 µm emission under 980 nm excitation were first reported in [
167]. The fibers with the core composition 40B
2O
3-25SiO
2-18Na
2O-7NaF-10YF
3-2ErF
3 were obtained using melt in tube technique. Heat treatment at 470–500 °C for 5 h led to precipitation of NaYF
4: Er
3+ nanocrystals in the core. The increase in the transmission loss values at 1310 nm from 7.44 to 11.81 dB/m after heat treatment was related to the scattering caused by the precipitated nanocrystals.
Efforts have been also focused on the preparation of GCs transparent up to 12 µm using chalcogenide glasses. The crystallization of chalcogenide glasses has been of great interest [
168,
169,
170,
171,
172,
173,
174]. However, the crystallization of these glasses is difficult to control, as reported in [
173]; Zhang et al. reported that, as for oxide glasses, the control of the nucleation and crystal growth is very sensitive to temperature and to the glass composition. The temperature of the heat treatment should not be too high so the crystal growth can be controlled. If not controlled, the GC loses its transparency. A reasonable nucleation rate was obtained by varying the duration of the treatment, which could last for up to 15 days in order to observe a significant decrease of the expansion coefficient of the final glass-ceramics. In [
175], Zhang et al. reported that it is possible to draw the GC with the composition Ga
5Sb
10Ge
25Se
60 into infrared transmitting GC fibers. During the fiber drawing, they noticed that the crystals in the GCs continue to grow. Although the scattering losses in short wavelengths increased, the transmission of the GC fibers beyond 6 µm remained unchanged despite the growth of the crystals during the fiber drawing.
The other technique developed to control the rare-earth optical response independently of the host glass composition involves the incorporation of rare-earth-doped nanoparticles in a glass. This new nanoparticle doping MCVD-process was implemented to fabricate REE-doped fibers. Use of erbium-doped nanoparticles allows one to control better Er
3+ local environment, decreasing the probability of the pair-formation and up-conversion level, providing longer luminescence lifetimes; achieving high homogeneity of the dopant along the fiber length erbium-doped fibers; and reducing light attenuation level in the fiber [
176,
177,
178,
179]. This approach also works for other REEs [
179,
180]. Fibers have been prepared by adding other NPs, such as REE-doped Y
3Al
5O
12 (YAG) [
181] and LaF
3 [
182,
183] nanocrystals. These nanocrystals, used as precursors, allow one to improve the REE luminescence properties. However, direct proof of the existence of the initial nanocrystals in the final fiber is rarely presented. In the case of LaF
3 nanoparticles, it has been clearly demonstrated that the integrity of these nanocrystals is not maintained during the MCVD process [
183]. Fluor ions react with silicon and form the gaseous SiF
4 compound. Even if the nanoparticles could be imaged in the fiber, their composition was La-silicate, not LaF
3. Summary information about various nanoparticles suitable host material composition and their characteristics can be found in [
184].
7.2. Phase-Separated Fibers
Fabrication of phase-separated fibers is a quite novel area, still having a lot of things to discover. Advances in this area were achieved by the group of W. Blanc. During the fabrication of the silica core using the MCVD process, the sintering of the soaked porous core layer, the collapsing of the tube, and the preform drawing steps require many thermal cycles with temperatures up to more than 2000 °C. These thermal cycles last for only a few seconds to a few minutes at each pass or step at specific point of the fiber. Nevertheless, due to this thermal treatment, inherent to the MCVD process, nanoparticles can be formed
in situ through the phase separation mechanism by introducing, through the doping solution, alkaline earth elements, such as magnesium [
185], calcium [
186], or scandium and yttrium ions [
187], resulting in the particles of nm-range size. Later, phase separation was obtained for Mg and La-doped silicate fibers [
188,
189]. The small size of the nanoparticles allows one to ensure low scattering losses, and at the same time, a significant broadening of the Er
3+ emission spectrum was observed (
Figure 6a). It is important to note, that the formation of the nanoparticles in these works did not require additional post-heat treatment of the fibers as the nanoparticles formed during the collapsing step of the MCVD process [
190]. The particles survive the high-temperature drawing process. However, from the geometrical considerations, it is clear that their shape should evolve. Recently, to shed more light on this question, Vermillac et al. studied the morphologies of oxide particles in optical fibers [
191,
192]. It was found that during the drawing step, the spherical particles present in the preform could be elongated and even broken up into smaller particles (
Figure 6b). The break-up phenomenon can be favoured by decreasing the drawing temperature (increasing drawing tension). This top-down approach (starting from “big” particles in the preform and finishing with “small” nanoparticles in the fiber) opens a new strategy to control the final size of the nanoparticles in the fibers.
The size control of nanophases in fibers provides a great opportunity for new applications. One of the most trivial examples is the development of the light-diffusing fibers used for illumination [
193]. In these fibers, the scatterers (which can be air bubbles with a typical size of 50–500 nm) are located in a ring around the fiber core. Despite the apparent simplicity of this approach, one of the main issues relies on the ability to keep the scattered light intensity constant all along the fiber [
194]. Sensors were recently developed, based on the enhanced nanoparticle-induced backscattered light and the optical backscatter reflectometry. The fabrication method for these sensors is simplified since there is no need for inscribing reflective elements (such as fiber Bragg gratings) or fabricating microstructures in the fiber. A fiber-optic refractive index sensor was demonstrated by simply etching a high-scattering nanoparticle-doped fiber in hydrofluoric acid [
195,
196]. A setup for multiplexed distributed optical fiber sensors, which are capable to measure temperature in the range of up to 140 °C (of interest for biomedical applications such as thermo-therapy) with a spatial resolution of 2.5 mm over the several fibers simultaneously, has been reported in [
197]. Finally, this approach is also applicable for strain measurements and 3D shape sensing [
198,
199].
7.3. MeNP-Containing Fibers
To date, most of the efforts have been focused on the fabrication of MeNP-doped silica fibers. Temperatures of about 2000 °C used for the drawing of high silica fibers are lower than the boiling point but high above the melting point of most of the metals. Therefore, MeNPs can survive, and the precipitation of the MeNPs is often performed by heat-treating the preform before the drawing; however, the drawing can change the nanoparticle characteristics dramatically. It is complicated to control the particle size and distribution, making the fabrication of the fibers with particular optical properties more challenging. Ju et al. demonstrated that the MCVD technique can be used for fabrication of Au-NP-doped silica-fibers, but, a shift of the Au-resonance absorption band was observed after the drawing when compared to the preform [
200]. This shift was attributed to the recrystallization of the Au metal at high temperatures. Recently de Oliveira et al. studied different ways to produce Au-NP-doped silica fibers in detail, and mentioned that the drawing of a preform, that already has MeNPs inside (see
Figure 7a), could result in the fluctuations in the NPs concentration over the length of the fiber, with the formation of regions with higher concentrations of gold clusters. These gold clusters are highly scattered and increase optical losses [
201]. Several studies showed that this problem could be solved by co-doping with Al, which was found to lead to a better dispersion of Au-NPs in the final fiber and so to better optical performance [
200,
201,
202].
In the recent work of de Oliveira et al. [
201] it was demonstrated that annealing of the fiber under reducing conditions results in the particles similar to the case for nanoparticles formed in the preform, allowing one to reach the concentration of the Au-NPs with resonance absorption band intensity >800 dB/m. At the same time, the fiber is fully spliceable and can be handled in a common way. Better control of the NPs growth process can be achieved if the fiber is exposed to local heating using a CO
2 laser instead of being heat-treated in a furnace. The IR irradiation is well absorbed by silicate glasses, allowing one to locally change the temperature and to achieve resolution in the NPs nucleation regions of about 100 μm in fiber length. Changes in the laser power and in the number of scans can be used to precisely control the process of the MeNPs formation via one or two-step procedure, producing fibers with different NP concentrations and size distributions (see
Figure 7b).
Despite the difficulties in the fabrication of MeNPs containing fibers, potential enhancement in the luminescence properties inspires scientists to improve the spectroscopic properties of REE-doped fibers using MeNPs. Significant efforts in the development of MeNP-doped optical fibers were performed by the group of W.-T. Han [
203,
204,
205]. In particular, it was demonstrated that co-doping of Er
3+-doped germano-silicate optical fiber with Au-NPs allowed increasing the emission intensity under 980 nm excitation about 20% [
203]. Authors ascribed the observed enhancement to the absorption of hydroxyl groups by Au-NPs. However, the MeNPs co-doping also can lead to a decreased radiative emission rate; i.e., suppressing the REE luminescence [
206]. The other possible reasons for this are the extensive scattering and the additional losses caused by the high MeNPs concentration [
206,
207].
In comparison to traditional surface sensing systems based on the plasmon resonance effect and including an optical prism in the setup, sensing configuration based on an optical fiber allows one to fabricate a small sensing element with a simplified optical design, which requires only a small sample volume and has great potential for use as disposable fiber-optic sensors and the capability for use in remote sensing [
208]. For a sensor fabrication, an unclad region of the fiber can be coated with metal-layer or MeNPs. Commonly, the sensing is based on the evanescent light field absorption in the fiber, which is analogous to attenuated total reflection (ATR) spectroscopy. The sensitivity of the method depends on the length of the sensing (modified) part of the fiber [
145] and the morphology of the deposited nanoparticles [
209]. A LSPR-sensor based on a modified standard telecommunication single-mode fiber can achieve the sensitivity of about 10
−5 RIU [
210].
As an alternative to the outer surface, the MeNPs can be deposited on the inner surfaces of a microstructured fiber—the theoretical consideration of such a configuration has predicted its high sensitivity [
211]. This approach was successfully realized in the work of A. Csaki et al. [
146]. In this study, Ag and Au nanoparticles of different sizes and shapes were deposited inside the channels of microstructured optical fibers with high coating uniformity (see
Figure 8). Experimental results agreed well with the theoretical calculations. Moreover, a proof-of-principle sensing experiment demonstrated high sensitivity (up to 78 nm RIU
−1) of the obtained sensor and its potential.
Nowadays NIR lasers based on different REEs dominate various practical applications. In particular, Er-doped lasers and amplifiers emitting at around 1.55 µm are used for telecommunications—the application area where the MeNP-doped fibers could be used for optical switching. It is important to note here that the enhancement of the nonlinear properties is the most pronounced for the laser wavelengths, which are in resonance with the plasmon modes of the MeNPs [
212]. For the most common Au and Ag particles, the resonance band is typically observed in the visible range. The position of the resonance band can be controlled by the size of the MeNPs and the precipitation of bigger particles can shift the resonance band towards lower energies. The broader tunability range of the resonance can be achieved by an extensive control of the particles geometry and using multipolar plasmon oscillations. In particular, it was demonstrated that the resonance band in the region up to 1 µm can be achieved with a change in length of the Ag nanorods [
213]. Nevertheless, even far from the resonance frequencies, e.g., at 1064 nm, nonlinear susceptibilities of the MeNPs in colloids are in order of 10
−14 ESU [
212], which is about two orders higher when compared to amorphous SiO
2 at the same wavelength [
214]. Therefore, the properties of the MeNPs will determine the final nonlinear susceptibility when embedded in commonly used optical glasses, such as silica.
In practice, it was demonstrated that the formation of Au-NPs in silica glass fiber permits one to achieve the nonlinear refractive index (n
2) of about 10
−16 m
2/W, which is several times higher in comparison to the reference fiber without the MeNPs [
215]. To date, it has been shown that the final nonlinearity depends on the chosen metal, the concentration, and the dispersion of the NPs [
202,
216,
217]. As was already mentioned above, co-doping with Al usually improves the dispersion of the MeNPs in the fiber and can enhance the nonlinear properties [
202]. However, this is not always the case. According to Lin et al., high resonant nonlinearity in Ag-NP-doped fibers can be obtained in Al-free fibers [
216]. Later, the authors demonstrated that this level of nonlinearity is enough for all-optical switching applications at 1.55 µm [
217]. In more recent study on Au-NP-doped fibers it was demonstrated that a higher concentration of the nanoparticles can be achieved in the fiber, resulting in n
2 = (6.75 ± 0.55) × 10
−15 m
2/W; i.e., five orders of magnitude higher than that of silica glass [
201].