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Luminescent Properties and Optical Amplification of Erbium-Doped Nano-Engineered Scandium-PhosphoYttria-Alumina-Silica Glass Based Optical Fiber Pinninty Harshavardhan Reddy, Shyamal Das, Debjit Dutta, Anirban Dhar, Alexander V. Kir’yanov, Mrinmay Pal, Shyamal Kumar Bhadra, and Mukul Chandra Paul* 1. Introduction In this work, a new erbium (Er) doped nano-engineered scandium-phospho-yttriaalumina-silica (SPYAS) glass-based optical fiber is reported, fabricated through the MCVD process, and solution doping technique, followed by a suitable thermal annealing of pristine preform. The fabrication process comprises the incorporation of erbium doped phase-separated scandium-yttria rich crystalline nanoparticles into the core region through in situ process based on phase-separation and crystal growth phenomena. The material characterization results, obtained from transmission electron microscopy, electron diffraction pattern, energy dispersive X-ray, electron probe micro analysis, and X-ray fluorescence, confirm the formation of Er2O3 doped crystalline phase-separated scandium-yttria rich nanoparticles in the core region. The formation of scandium-ultra rich nanocrystalline environment, possessing low photon energy around the erbium ions, enhanced the fluorescence intensity. Such kind of nano-engineered glass reduces the noise figure around 4.35 dB, and provides broadband optical flat gain with an average value of 38.675 dB, varied by less than 0. 7 dB spanning over a broad wavelength region of 1530–1590 nm compared to the pristine and Sc free Er-doped fibers. Such kind of nano-engineered glass based Er doped fiber will be useful for making highly efficient optical amplifiers, suitable for present broadband optical communication systems.

P. H. Reddy, Dr. S. K. Bhadra, Dr. M. C. Paul Academy of Scientific and Innovative Research (AcSIR) CSIR-CGCRI Campus Kolkata, India E-mail: [email protected] P. H. Reddy, Dr. S. Das, D. Dutta, Dr. A. Dhar, Dr. M. Pal, Dr. M. C. Paul Fiber Optics & Photonics Division CSIR-Central Glass & Ceramic Research Institute 196, Raja S.C.M. Road, Kolkata-700032, India Prof. A. V. Kir’yanov Centro de Investigaciones en Optica Loma del Bosque 115, Col. Lomas del Campestre, Leon 37150, Guanajuato, Mexico Dr. S. K. Bhadra Raman Centre for Atomic, Molecular, and Optical Sciences Indian Association for the Cultivation of Science Kolkata 700032, India The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssa.201700615.

DOI: 10.1002/pssa.201700615

Phys. Status Solidi A 2018, 1700615

The selection of suitable glass host for fabricating erbium doped fiber (EDF) is an essential requirement for higher optical amplification efficiency. Over the past several years, extensive research has been carried out on the incorporation of Er in various glass host materials such as silica, alumina, telluride, phosphate, chalcogenide, bismuthate, and fluorozirconate, lithium niobate, lanthanum glasses, etc.[1–3] in order to make fiber amplifier covering C-band (1525–1565 nm) and L-band (1565–1610 nm) with low noise, low loss, and compatibility with fiber lightwave systems. Such systems are useful for optical communication, range finding, remote sensing, ultra-high bit-rate telecom transmission systems, free-space communications, and wavelength multiplexing to address the data traffic. Recently, the effect of temperature on the active properties of normal Er-doped optical fibers[4] and highly efficient Yb-free air-La-Al doped fiber laser[5] has been explored to reduce concentration quenching and clustering effects. To improve the spectroscopic and laser performances of rareearths (RE) doped materials, considerable work has been carried out for incorporation of RE-oxide nano-crystallites into different glass hosts.[6–7] Different techniques such as cosputtering, pyrolysis, ion-implantation, laser ablation, sol-gel and direct nano-particle deposition (DND) processes have been developed for the incorporation of RE nano-particles.[7] A lot of research has been also reported for development of suitable silica and non-silica based glass host doping with REions to improve their lasing and amplification properties. In most of these cases, core glass was prepared by double crucible furnace melting procedure, but they are not reliable for the system compared to the high silica based RE-doped fibers. However, a new route has been developed using Modified Chemical Vapor Deposition (MCVD) and nanoparticle doping technique through the solution soaking process. These silica glass based fibers have several advantages such as high optical

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transparency, low cost, strong chemical, and mechanical durability. This new technique has attracted a lot of interest as it provide the formation of embedded phase-separated nanoparticles with crystalline or amorphous nature into REdoped SiO2 host-matrix to enhance their spectroscopic performance and promises for high-power optical amplifiers, modelocked, and Q-switched fiber lasers, and broadband SC sources.[8–23] In this paper, we are, for the first time to report on the formation of Er doped nano-engineered Scandium-Phospho-Yttria-AluminaSilica (SPYAS) glass based optical fiber using the MCVD process, coupled with solution doping technique followed by thermal annealing of the original preform. Scandium oxide used as one of the sesquioxides is a cubic crystal belonging to the Ia3 space group (space group number 206).[24] On the other hand Er3þ:Sc2O3 sesquioxide is a promising candidate for high energy eye-safe solid state lasers as it provides a low quantum defect and high thermal conductivity (higher than that of YAG and other sesquioxides).[25] Although development of solid state Er:Sc2O3 lasers has started earlier,[24,26] relevant spectroscopic investigations of Er:Sc2O3 are still ongoing and not reported as a host of optical fiber. The main novelty here is the development of Er-doped nano-engineered Scandium-Phospho-Yttria-Alumina-Silica (SPYAS) glass-based optical fiber, which shows strong luminescence property, better optical amplification performances from the standpoint of low noise figure, high optical gain and broadband flat gain spectrum covering 1530–1590 nm wavelength region compared to standard Sc free Er doped fiber (EDF) and pristine SPYAS glass based fiber. The luminescence properties of the fibers drawn from the pristine and the annealed preforms are studied from the viewpoints of material, geometry, absorption and fluorescent characteristics. The amplification performance of the fibers is also characterized in terms of optical gain and Noise Figure (NF).

2. Experimental Section 2.1. Fiber Fabrication The Er doped SPYAS glass based fiber preform was fabricated using MCVD process coupled with solution doping technique[20,21,23] by passing SiCl4 and POCl3 vapors through a slowly rotating high-purity silica glass tube of outer diameter of 20 mm and inner diameter of 17 mm. An external flame source was moved along the length of the rotating tube, heating it up to optimal temperature of around 1550 10 C, monitored by a synchronously moved IR pyrometer when halide precursor oxidized and deposited inner surface of the tube, resulting in deposition of porous silica layers. The glass formers, SiO2 and P2O5, were incorporated in the core matrix of the optical fiber preform through the MCVD process, where P2O5 served as a nucleating agent that facilitates phase separation[10,11] and assists generation of Er-enriched nano-crystallites.[21,27] Selection of Sc2O3 and Y2O3 allows enhancing radiative transitions between electronic levels of RE’s ions due to their low phonon energy compared to silica. Glass modifiers, namely Al2O3, Er2O3, Sc2O3, and Y2O3, were added via a solution doping process in an alcohol/water mixture of suitable strength (1:5) using AlCl3.6H2O, ErCl3.6H2O,


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ScCl3.6H2O, YCl3.6H2O as precursors. A small amount of Y2O3 served to slow down or stop the changes in the formation of larger sized crystallites and, eventually, preserve the mechanical strength and integrity of the preform and final fiber. The soaked layer was dehydrated with the flow of Cl2 gas at around 900 C to minimize contamination with OH groups. Subsequently, the soaked porous soot layer was oxidized, sintered, and collapsed to get the final preform of 10.5 mm diameter. A “standard” Er-doped preform with the same solution composition but without ScCl3.6H2O was made by analogous way, for comparison. The fabricated preform was then split into two portions. The first portion was drawn into a fiber strand with diameter 125.0 0.5 mm using a conventional fiber drawing tower. The second portion was heated under optimum annealing conditions at 1200 C for 3 h withheating/cooling rate of 15 C min1 in a closed furnace, followed by drawing of fiber. The thermal annealing was the most critical step to achieve the formation of the nano-engineered SPYAS glass based optical fiber. The final fiber was coated with primary and secondary coatings; note that the drawing parameters were optimized to ensure a high-quality fiber.[28]

2.2. Material and Optical Characterizations The microscopic images of different fiber cross-sections were obtained using a high-resolution optical microscope (Olympus BX51), connected to a high-resolution digital camera. Morphology of the nanoparticles (NPs) in the fiber core was studied by means of high-resolution Transmission Electron Microscopy (TEM) (Tecnai G2 30ST, FEI Company, USA) to capture TEM bright-field images as well as to analyze Energy Dispersive-X-ray (EDX) and Electron Diffraction (ED) patterns for both the pristine and nano-engineered fibers. To evaluate the formation of phase-separated particles by the thermal annealing process, we focused electron beam on and outside of the particles during TEM-EDX analyses. The details about the fiber sample preparation for TEM analyses were described in detail earlier.[11] The average doping distributions in the fiber core region were measured by means of Electron Probe Micro Analysis (EPMA) using a JEOL EPMA instrument. X-ray photoelectron spectroscopy (XPS) was utilized to determine surrounding environment of phase-separated NPs in terms of the corresponding binding energies for Si, O, Sc, Er, Al, and Y in the SPYAS glass based fiber preform under ultra-high vacuum (UHV) conditions in a PHI 5000 Versaprobe II scanning XPS microprobe. The Refractive Index (RI) profile of both the SPYAS glass based and Sc free standard Er doped fibers were measured using a Fiber Analyzer (NR-9200, EXFO, Canada). The absorption spectra of the fabricated fibers were obtained using a white-light source with fiber output and optical spectrum analyzer (OSA, AQ-6315 Ando) with 0.5 nm resolution. The fluorescence spectra were measured using 980 nm pumping (pump power 350 mW), in lateral geometry. A fiber-coupled laser diode at 980 nm was used as a pump source; it was connected to a 980-nm arm of a 980/ 1550-nm WDM coupler. A short piece of fiber (around 4.5 cm) was fusion-spliced to the 980/1550 nm arm of the WDM; its exit end was broken and immersed in an index-matching fluid to diminish the Fresnel reflection. The fluorescence from the fiber

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was collected with the OSA and recorded using a computer. The decay of the intensity of the luminescence (4I13/2 to 4I15/2 of the Er3þ) was also carefully measured with a 980-nm core-pumped 4.5 cm-long fiber sample. The experimental setup, employed to investigate the gain and noise figure of the fabricated SPYAS based EDFs, is shown in Figure 1. In the experiment, tunable laser source (TLS) as an input signal provided wavelength variation from 1520 to 1620 nm.

3. Results and Discussions

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Figure 1. Experimental set up for measuring the gain and noise figure of the SPYAS glass based fiber.

found to be 0.125 wt.% to achieve phase-separated NPs of diameter 8–12 nm under optimized annealing condition, mentioned earlier. The fiber drawn from the annealed preform did not produce better optical amplification due to the formation of larger NP of diameter 25–40 nm. Therefore, all our experiments were performed using the fiber fabricated under optimized conditions before and after post-thermal annealing of the preform. The microscopic views of Er doped SPYAS fibers drawn from the pristine and annealed preforms, as well as “standard” Sc-free Er doped fiber are shown in Figures 2(a) and (b), and 3(a), respectively. The measured core diameter of the Er-doped pristine, nano-engineered SPYAS and Sc free standard erbium doped fibers were found to be 8.5, 9, and 9.25 μm respectively. There were no core-clad imperfections which otherwise might lead to certain bending loss. As seen from Figure 3(a), the color of core glass is slightly brownish-yellow in the nano-engineered fiber drawn from the annealed preform. The ED pattern of this fiber (discussed below) indicates the formation of phaseseparated NPs with crystalline nature.

The fabrication of Er doped nano-engineered glass based optical fiber based on the incorporation of Al2O3, Y2O3, and Sc2O3 into silica glass made to reduce RE clustering compared to pure silica glass along with improving fiber’s spectroscopic properties. Insulation of REs ions from matrix vibrations is essential by appropriate material design of the surrounding ion site through the formation of phase-separated NPs.[10,11,19–21] This involves encapsulation of dopants inside glassy or crystalline NPs, embedded in the fiber glass. The selection of different components of the present glass is discussed in details in the experimental section. In such nano-engineered glass based optical fiber, the basic silica material serves as a support for providing the optical and mechanical properties, whereas the spectroscopic properties would be controlled by the selected composition along with the type of incorporated NPs. The heat treatment stage involved annealing of the preform, usually above the glass-transition temperature, found to be around 980 C for such glass determined from DTA measurement.[11] Growth of Er2O3 doped nano-crystallites depends on the metastable nature of Sc2O3-Y2O3-Al2O3 silicate glass with the transformation of the stable crystalline state if enough thermal energy is available at twostep nucleation and crystal growth process[6]; the detailed mechanism has been reported earlier.[6,11] As scandium-yttria–alumino silicate glass is heated above 1000 C crystal nuclei initially, tiny regions in the glass structure begin to form. With further temperature rise, the rate of nucleation increases, reaching maximum at around 1200–1300 C, depending on the composition. The effect of P2O5 doping levels and thermal annealing condition on the nature and sizes of the NPs within the fiber core was evaluated for optimization of the glass composition. Experimental results revealed that with increasing P2O5 content the growth of formation of phase separated particles accelerate through thermal perturbation, where P2O5 serves as a nucleating agent owing to the higher field strength difference (>0.31) between Si4þ and P5þ.[29] The size of NPs increased with increasing P2O5 content Figure 2. Microscopic views of the pristine SPAYS (a) and Sc free standard (b) Er doped fibers; and the optimum doping level of P2O5 was ED pattern (c), and TEM picture (d) of the SPAYS fiber.


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of the nano-engineered fiber may stem from the segregation of minor amounts of YAG along with ErxSc2xSi2O7 and ErxY2xSiO5 inclusions in the polycrystalline phase. The composition of phase-separated crystalline NPs was evaluated from the EDX data by focusing electron beam precisely outside such particles and onto them, as shown in Figure 3(e) and (f), respectively. The EDX spectra revealed that both phases separated and non-phase-separated regions contain Sc, Er, Al, and Y. However, the signals related to Sc, Y, and Er inside phaseseparated crystalline NPs are much stronger than the one’s adherent to these elements being outside the phase-separated particles (except Al, showing the opposite behavior). The presented analysis indicates that most of Er is localized in the phase-separated crystalline Sc-Y rich environments, whereas Al dominates in the non-phase-separated region. The exact core composition of the nano-engineered Er-doped SPYAS fiber, measured by EPMA (see Figure 4), was found to be as follows: 1.256 wt.% of Sc2O3, 0.125 wt.% P2O5, 5.85 wt.% of Al2O3, 0.567 wt.% of Y2O3, and 0.285 wt.% of Er2O3. Er doping level in the core of the Sc free standard EDF, fabricated for comparison, was found to be around 0.282 wt.%, determined from the absorption loss curve. The XPS spectra of O 1s, Si 2p, Al 2p, Y 3p, Er 4d, and Sc 2p, acquired from the Er-doped pristine and nano-engineered SPYAS preform samples (see Figure 5) revealed the formation of phase-separated NPs that contain Er ions mostly linked to Sc and Y. Such kind of Sc and Y rich phase-separated nanoenvironment surrounding Er-ions of nano-engineered SPYAS fiber preform can be explained from the spectral broadening as well as shifting of the O1s, Si2p, Y 3p1/2 and Y 3p3/2, Sc 2p3/2 and Sc 2p1/2 along with Er 4d peaks, as compared to the pristine preform. Particularly, the O 1s and Si 2p core region’s spectra shown in Figure 5(A) and Figure 5(B) exhibit significant differences in spectral shape as well as in binding energies due to binding of Si and O with other elements such as Sc, Er, and Y in the nanoengineered preform sample, which shifts the binding energy of both Si 2p and O 1s toward lower side as compared to the pristine SPYAS preform. This result indicates the formation of more ionic networks around Er ions in phase-separated NPs, dispersed into silica

Figure 3. Microscopic view (a), TEM picture (b), high-resolution TEM image (c), ED pattern (d), and EDX spectra taken out of the particles (e) and on the particles (f), all obtained for the Er doped nano-engineered SPYAS glass based optical fiber, drawn from the annealed preform.

However, it is hard to detect the exact type of crystallites which formed through the annealing process due to the presence of Y2O3. It may be a combination of ErxSc2-xSi2O7 and ErxY2xSiO5[30–31] in the polycrystalline phase, but the presence of Y3Al5O12 (YAG)[11,16] (in minor amount) cannot be excluded too, as the host is co-doped with 5.85 wt.% of Al2O3 and 0.567 wt.% of Y2O3. In turn, the brownish-yellow coloration of the doping host


Figure 4. Elemental analysis of the Er-doped nano-engineered SPYAS glass based optical fiber.

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matrix. Furthermore, in the nano-engineered SPYAS preform sample, the peak position of Y 3p1/2 (314.9 ev) and Y 3p3/2 (304.0 eV) signals are shifted to higher values compared to pure Y2O3: Y 3p1/2 (310.6 eV) and Y 3p3/2 (298.8 eV)[32] as well as to the pristine preform sample, shown in Figure 5(C) because of the presence of electron density deficiency around Y ions due to strong ionic bonding with Sc and Er ions. At the same time, the Y 3p spectra of the nano-engineered SPYAS preform become slightly broader compared to the pristine preform due to different bonding of Y with other elements, such as Sc and Er, within the phase-separated NPs. This kind of changes in surroundings of Al both in the nano-engineered and pristine SPYAG preform samples did not occur, as shown in Figure 5(D). The formation of Er-Sc-Y in the nano-engineered SPYAS preform sample can be clearly explained from their Sc 2p and Er 4d spectra, shown in Figure 5(E) and (F), respectively. In the nano-engineered SPYAS preform sample under suitable thermal annealing process, the peak positions of Sc 2p3/2 (400.9 eV), Sc2p1/2 (405 eV), and Er 4d (170.2 eV) are shifted to higher values as compared to their peaks in the pristine preform and broaden significantly, as shown in Figure 5(E) and (F), respectively. In overall, the XPS study indicates that the phase-separated NPs are composed of mainly Er, Sc, and Y, where the surrounding environment of Er is greatly influenced by Sc and Y. The TEM, ED and EDX analyses, discussed earlier, give the same idea about the formation of phase-separated NPs, composed of Sc, Er, and Y rich region, of crystalline nature. Thus, both the EDX and XPS results provide clear evidence about the presence of Er ions in the nano-phase-separated Sc- and Y- rich sites, embedded in silica glass matrix. From the RI profile shown in Figure 6, the average Numerical Aperture (NA) of the Er doped nano-engineered fiber and Sc free standard Er doped fiber (core diameter: 9.25 μm) was estimated to be 0.18 and 0.19, respectively. The RI profile found to be slightly Gaussian in shape, not perfectly step index along the core diameter due to non-uniform penetration of index-rising elements (Al, Sc, and Al) caused by inhomogeneous porosity of the deposited porous layer during preform fabrication process. From the optical absorption spectra shown in Figure 7A and B, note that the peak absorption values (at 980 and 1530 nm, adherent to the well-known Er3þ absorption 1530 nm, adherent to the well-known Er3þ absorption transitions: 4I15/2 ! 4I11/2 and 4 I15/2 ! 4I13/2) in all the fibers are virtually the same, signifying almost the same level of doping with Er. The absorption spectrum of the nano-engineered SPAYS fiber reveals multiple sharp peaks within the 980 and 1530 nm absorption bands (see Figure 7(B), which may relate to the fact that Er ions are present in it mostly on the crystalline environment of phaseseparated NPs. The nano engineered Er doped SPYAS glass based fiber shows a very broad fluorescence spectrum as well as strong emission signal at 1550 nm with increasing pump power at 980 nm compared to the Er-doped pristine SPYAS glass based optical fiber and Sc-free standard Er doped optical fiber shown in Figures 8 and 9. It was observed that, with increasing pump power upto 350 mW at 980 nm, the intensity of 1.5-μm emission increased significantly in the case of the nano-engineered SPYAS fiber, as evident from Figure 9. The enhancement of fluorescence


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Figure 5. XPS spectra of O1s (a), Si2p (b), Y3p (c), Al2p (d), Sc2p (e), and Er4d (f) of the Er doped nano-engineered SPYAS glass based optical fiber, drawn from annealed and pristine preforms.

intensity along with spectral broadening for the nano-engineered SPYAS fiber is associated with the presence of Sc/Y surrounding crystalline environment around Er ions induced by thermal annealing process. Such process promotes the accommodation of Er ions, at least partially, in nano-phase

Figure 6. RI profile of the Er-doped nano-engineered SPYAS glass based (a) and Sc free standard Er doped fiber (b).

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Figure 7. Absorption spectra of the Sc-free standard Er-doped fiber/Erdoped pristine SPYAS glass based fiber (A) and nano-engineered Erdoped SPYAS based fiber (B).

Figure 8. Fluorescence spectra of the Er-doped pristine, nano-engineered SPYAS glass based optical fibers and Sc-free standard Er doped optical fiber.

separated crystalline SPYAS glass host compared to the normal SPYAS and Sc free standard fibers. The fluorescence intensity of the nano-engineered SPYAS fiber is found to be a very strong due to the presence of Y and Sc rich crystalline NPs, where Sc enhances the crystal field strength in Er ions sites as the ionic radius of Sc is smaller than that of Er ions.[33] Another reason

Figure 9. The effect of 980 nm pump power on 1550 nm emission signal intensity of the Er-doped pristine, nano-engineered SPYAS glass based optical fibers, and Sc-free standard Er doped optical fiber.


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may be the reduced up-conversion phenomena as Y cations substitute Er ions due to similarity of ionic radii of Er and Y ions.[34] To measure the fluorescence lifetime, a fiber sample was the angle cleaved to prevent any feedback. The pump laser diode was modulated to achieve square-shaped pulses of ms-width with a sharp rise and fall edges with a time resolution of 8 ms. To diminish pump background in the measured signal, a long-pass optical filter with cutoff wavelength at 1000 nm (Thorlabs FEL1000) was placed between an Er doped fiber sample and photo-detector. The Er3þ fluorescence lifetime of the nano-engineered SPYAS fiber was found to be larger than that of the normal SPYAS and Sc free standard Er-doped fibers, as shown in Figure 10. This may be due to the presence of two kinds of Er ions having different surrounding environments in the fibers: one, in the amorphous silica glass, and others, mostly in Sc with Y rich crystalline and non-crystalline nano-phase separated regions, of the fibers drawn from the annealed and pristine preforms respectively. In addition, the measured fluorescence decay curve showed a single exponential behavior, thus revealing negligible concentration quenching in such kind of SPYAS glass-based fibers. The measured data were fitted to an exponential curve to determine the effective lifetimes (see insets in A, B, and C). The fluorescence lifetime of the 4I13/2 ! 4I15/2 emission of the Er doped nano-engineered, normal SPYAS, and Sc free standard Er doped fibers were found to be 11.30, 10.16, and 9.96 ms respectively. The long lifetimes of the 4I13/2 level, together with the enhanced intensity of the 4I13/2 ! 4I15/2 emission in the nano-engineered SPYAS fiber sample compared to the pristine SPYAS fiber, suggest a Sc and Y rich nano-crystalline environment surrounding Er3þ ions. The low phonon energy based Sc2O3[35] and Y2O3[6,11] rich surrounding crystalline environment of Er ions favors enhancing Er3þ fluorescence lifetime.

Figure 10. Fluorescence decays of the Er-doped printine (a), nanoengineered (b) SPYAS glass based optical fibers and Sc-free standard Er doped optical fiber (c).

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Figure 11. The optical gain spectra of the Er-doped pristine, nanoengineered SPYAS glass based fibers, and Sc-free Er doped fiber.

The variable optical attenuator (VOA) was used to obtain the specific and accurate input power, delivered to the cavity to study the optical amplification properties, as shown in Figure 1. An isolator was placed into the scheme to prevent backward ASE noise from entering the first stage and parasitic lasing which would otherwise cause degradation of the optical gain of amplifiers. 15-m length of the nano-engineered SPYAS based EDFs along with 16.5-m length of the Sc-free standard Er-doped fiber was used as gain media. The EDF was forward-pumped by a 980-nm laser diode via an 980/1550-nm WDM-coupler. The optimized length and signal power used for the nano-engineered and normal SPYAS fibers were found to be 15.0 m and 10 dBm, respectively. Accordingly, for the standard Sc-free EDF, these parameters were 16.5 m and 10 dBm. The results are demonstrated in Figure 11 (for gain) and Figure 12 (for NF), respectively. The flat gain region of Er-doped nano-engineered SPYAS fiber (see red curve in Figure 11) was found to span the range 1530– 1590 nm, with variations of less than 0.70 dB. In contrast, the standard Sc-free EDF as well as the normal pristine SPYAS fiber demonstrate (see blue and black curves in Figure 11) much narrower flat-gain regions from 1530 to 1565 nm and 1530 to 1572 nm, respectively. Such nano-engineered Er-doped SPYAS glass based fiber, showing a flat gain region that covers both C and L bands, seems to be attractive for broadband optical communication systems. The measured maximum gain value of the nano-engineered optical fiber at 1560 nm was found to be 39.45 dB, which becomes higher than that of pristine Er doped SPYAS glassbased fibers and the Sc-free EDF. Furthermore, the nanoengineered optical fiber shows an average gain of 38.675 dB

Figure 12. The noise figure spectra of the Er-doped pristine, nanoengineered SPYAS glass based fibers, and Sc-free Er doped fiber.

within the spectral gain flatness region. Such kind of nanoengineered optical fiber shows a better gain variation of 0.70 dB within 1530–1590 nm wavelength compared to the standard Sc free Er doped fiber, which is important for a broad band optical amplifier. As seen from the above reported values, on average the quality of Er doped nano-engineered optical fiber is enhanced compared to the pristine Er doped SPYAS glass based fiber and Sc free EDF. On the other hand, the variation of the NF of the pristine Er doped fiber, nano-engineered SPYAS glass based fiber and standard Sc free Er doped fiber within the spectral gain flatness region is found to be (4.61–7.15), (4.35–6.85), and (5.5–7.67) dB, respectively shown in Table 1. The above Figure 12 shows that the values of NF decrease from the nano-engineered SPYAG glass based fiber to the standard Sc free EDF. The measured results of the nano-engineered fiber show a minimum noise figure of 4.35 dB, which make a better fiber amplifier performance in terms of overall higer gain value and flatness gain profile compared to the normal SPYAG glass based fiber and the Sc free standard EDF as demonstrated in the above Table 1. The overall improvement of gain performance of the nanoengineered Er-doped fiber was happening due to the inhomogeneous energy level degeneracy of Er ions, induced by the crystalline ligand field of the Sc/Y rich subsystem (because of site-to-site variations) known as the Stark effect, causing the widened optical transitions operating in the 1530–1600 nm region. Such kind of phenomena ought to contribute in the enhancement of fluorescence intensity as well as optical gain broadening toward the L-band region, up to 1600 nm.

Table 1. Amplification parameters of Er doped nano-engineered and printine SPYAS glass based fibers along with Er doped Sc free standard fiber. Amplification parameters Maximum gain at 1560 nm Flat-gain region Flat-gain (average value) Gain variation (dB) Noise figure (NF) (dB)


Er doped nano-engineered SPYAS fiber

Er doped normal SPYAS fiber

Er doped Sc free fiber

39.45 dB

37.35 dB

35.89 dB

1530–1590 nm

1530–1575 nm

1530–1565 nm

38.675 dB

36.765 dB

36.5 dB

0.70

0.67

0.82

4.35–6.85

4.61–7.15

5.5–7.67

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Apparently, such new Er doped nano-engineered glass based fiber may be a very good choice for making optical amplifiers with intrinsically flattened gain having a low noise figure in modern broadband optical communication systems.

4. Conclusions For the first time, we successfully made Er-doped nano-engineered Scandium-Phospho-Yttria-Alumina-Silica (SPYAS) glass-based optical fiber, applying the concepts of phase-separation and crystal growth phenomena through MCVD with solution doping technique, followed by a suitable thermal annealing of sourcing preform. The TEM images along with ED, EDX and XPS studies have been performed on the Er doped nano-engineered fibers to confirm the formation of scandium yttria rich phase-separated crystalline nano-particles having sizes 8–12 nm. The optical and spectroscopic properties of the Er doped nano-engineered SPYAS glass based optical fibers have been examined in the study of their amplification characterizations. Such kind of novel nanoengineered optical fibers show better optical amplification performances compared to standard Sc free EDF and achieved a flat-gain spectrum about 38.675 dB with variations of less than 0.70 dB over the whole range spanning 1530–1590 nm. The results suggest that such kind of Er doped nano-engineered fiber based amplifiers are expected to be useful in broadband optical communication systems. The use of such fibers also opens some interesting perspectives on the investigation of material characterizations of the RE-doped preforms and fibers, targeting novel glass composition based nano-engineered specialty optical fibers for nowadays high-power applications.

Acknowledgements The authors acknowledge the financial assistance from the DST under Nano Mission. P. Harshavardhan Reddy thanks the DST, India, for awarding a DST-INSPIRE Fellowship (IF150744) for his PhD. The authors are also thankful to Dr. K Muraleedharan (the Director of CGCRI) for his support and encouragement.

Conflict of Interest The authors declare no conflict of interest.

Keywords broadband amplification, EDFA, erbium-doped glass based optical fiber, nano-structuration, thermal annealing Received: August 23, 2017 Revised: December 30, 2017 Published online:

[1] M. Naftaly, S. Shen, A. Jha, J. Appl. Opt. 2000, 39, 4979. [2] S. Jiang, B.-C. Hwang, T. Luo, K. Seneschal, F. Smektala, S. Honkanen, J. Lucas, N. Peyghambarian, Optical Fiber Communications Conference Proceedings 2000, PD5–1.


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