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Abstract
We investigate the microstructural characteristics and optical properties of PbS quantum dots-doped silica fiber (PQDF), prepared by atomic layer deposition (ALD) doping technique. The fiber exhibits ultra-wideband luminescence and flat-gain with 3 dB bandwidth of 300 nm. The on-off gain and net gain can reach to 7.1-15.0 dB and 6.0-9.2 dB at 1050-1350 nm, respectively. The results of high-resolution transmission electron microscopy (HRTEM) further reveal the effects of PbS QDs doping in PQDF. The broadband luminescence spectrum originating from three active centers (1086, 1179, and 1304 nm), can be attributed to the dimension effect of PbS QDs (3.7, 4.0, and 4.3 nm, respectively). Moreover, the calculation results indicate that the multi-sized PbS QDs concentrated at 3.65-4.45 nm make the 3 dB gain bandwidth increase, which is six times wider than that of traditional erbium-doped fiber (EDF). Therefore, this type of PQDF is a promising gain medium for optical amplifiers and broadband light sources.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Increasing the capacity of optic communication system necessitates doped fiber working in the spectrum of 1100-1600 nm [1]. The gain bandwidth of Er-doped fiber (EDF) is restrictive of 35-40 nm owing to the electronic transition of erbium ions [2]. Recently, Bi-doped fiber also have shown promising applications. However, the nature of emitting Bi active centers in near-infrared is controversial [3–5]. Therefore, there is great demand for special active materials that can improve the optical radiation bandwidth, amplification efficiency and gain of the doped fiber. Quantum dots (QDs) have gained most attention on account of their highly symmetrical band structure, large stokes-shift, wide and continuous excitation spectrum, high fluorescence intensity, and spectral stability [6].
Lead sulfide (PbS) QDs are of extensive interest in infrared optical communication because of their particular electrical and optical properties, with large exciton Bohr radii (∼18 nm), small bandgap (∼0.4 eV), nearly equivalent electron and hole effective masses [7,8]. Due to the quantum dimension effect, the low-loss communication at the wavelengths of 1310 nm (0.95 eV) and 1550 nm (0.8 eV) can be achieved by adjusting the luminescence wavelength of PbS QDs through precise size control [9,10].
Until now, a lot of research have been exploited with an aim to improve the luminescence and optical gain properties of doped fiber by utilizing PbS QDs in optical fiber. Wu et al. fabricated a PbS QDs fiber amplifier with a gain of 5.6 dB at 1550 nm by coating the tapered single mode fiber (SMF) coupler with PbS QDs, using evanescent wave exciting [11]. However, the technology have some disadvantages such as poor stability and high sealing requirements. Moreover, QD-doped glass and QDs-doped fiber (QDF) have obtained excellent results in recent years [12–14]. K. Wundke et al. firstly obtained the dynamic gain of PbS QDs doped glass tuning at 1317-1352 nm range with 980 nm pumping, owing to the dot-size selective excitation [15]. Huang et al. produced all solid-state PbS QDs-doped glass precursor fibers by melt-in-tube technique. The luminescence bands were tunable through changing the size of QDs by heat treatment [16]. Dong et al. made PbS QDs-embedded silicate glass fiber by using a glass melt-quenching method. Excited by an 808 nm laser, tunable near-infrared (NIR) luminescence bands were obtained [17]. Although the luminescence bands of QDs-doped glass fibers can be adjusting through changing the size of QDs by heat treatment, their application in optical communication is still limited due to high splice loss, narrow bandwidth and complex mixture. Based on previous research, in order to improve the stability and bandwidth of doped fiber, we propose PbS quantum dots-doped silica fiber (PQDF) by using modified chemical deposition (MCVD) method combined with atomic layer deposition (ALD) technique [18]. ALD is a self-limiting chemical vapor deposition technology with significant advantages of good uniformity, high coverage and controllable doping concentration [19]. It can deposit high-purity and good shape nanomaterials on silica fiber substrate, which can meet the high-quality preparation requirements of doped fiber.
In this study, a novel PQDF were fabricated by combining MCVD technology with ALD process and its optical properties were investigated. In addition, refractive index analysis, high resolution transmission electron microscopy (HRTEM) images, energy dispersive spectrometer (EDS) measurements and selected-area electron diffraction (SAED) patterns were used to characterize the distribution, composition and microstructure of PbS materials in the fiber. These analysis provided a better evidence of the relationship between size of QDs and ultra-wideband optical properties of PQDF.
2. Experimental section
The fabrication process of PQDF was shown in Fig. 1(a) [20,21]. During the fabrication processes, firstly, the MCVD technique was used to introduce a SiO2 porous soot layer into inwall of silica substrate tube by chemical reactions. Secondly, PbS films were deposited into the soot layer using ALD process (Beneq TFS-200, Finland). Herein, we choose Pb(tmhd)2 (Bis(2,2,6,6-tetramethyl-3, 5-heptanedionato) lead(II)) as Pb precursor, and a gas mixture of 10% H2S in high purity N2 (99.99 %) as S precursor. In this process, we optimized the parameters of ALD. When the parameters are Pb source pulse (500 ms), H2S pulse (200 ms), reaction chamber temperature (170-190 °C) and Pb precursor source (115°C), high-quality PbS nanofilm could be deposited. Thirdly, semi-transparent doped glass was formed at 1600 °C and SiO2 mixed with GeO2 material was taken as the core layer by MCVD. Then the doped fiber preform was fabricated by collapsing process at heating reach to 2000 °C. Finally, the PQDF was prepared using drawing tower to draw fiber preform. The cross section and refractive index difference (RID) of the fiber were analyzed using an optical fiber index analyzer based on digital hologram technology (SHR-11202, Shanghai University, China), as presented in Fig. 1(b). The diameters of the fiber core is 10.2 µm and cladding is 127.3 µm. The RID between the fiber core and cladding is approximately 1.51 %.
Fig. 1. (a) Fabrication process of PbS quantum dots-doped silica fiber (PQDF); (b) refractive index difference (RID) of the PQDF, (inner) cross-section of the fiber.
The cut-back method was used to analyze the optical loss spectrum of PQDF. The optical properties measurement system was schematically depicted in Fig. 2. A white light source (AQ-4305) and a 980 nm laser diode (LD) were used as signal and pump sources, respectively. The luminescence spectrum of the PQDF was measured using a backward pumping setup when the signal source was turned off. The measurement system for optical gain was based on the same setup. The 980 nm pump excited the doped PbS QDs at the fiber. The white light source signal interacted with excited QDs and could be amplified. The amplified signal was tested by an optical spectrum analyzer (OSA, Yokogawa AQ-6315A). The isolator was used to protect the signal source from the pump. The length of the experimental sample was 2 m.
Furthermore, the HRTEM (JEM-2100F, Japan) combining with SAED patterns and EDS (OXFORD, England) are used to examine and analyze the existential state and distribution of PbS materals in the fiber core. The HRTEM sample were prepared by focused ion beam (FIB) micro-dissection technology (1200i, FEI Hongkong Co., LTD, Czekh).
3. Results and discussion
3.1 Spectral properties
The optical loss spectrum of PQDF was measured by cut-back method with a broadband white light source (AQ-4305) [22]. There are three distinct absorption bands at 803, 972, and 1171 nm in visible and near infrared bands, with corresponding absorption coefficients of 0.3, 1.4, and 0.65 dB/m, respectively, as shown in the Fig. 3. These absorption bands are fundamentally consistent with the typical absorption bands of PbS materials [23–25]. Besides, based on the band gap equation [26–28], when the absorption bands are 803, 972, and 1171 nm, the corresponding average diameters of the PbS QDs are 3.2, 4.0, and 5.0 nm, respectively. In addition, the background attenuation is approximately 0.08 dB/m. Besides, the absorption peak at 1380 nm results from the role of the [OH] group [29].
The luminescence characteristics were tested by backward pumping method. The evolution of luminescence spectra under different pump powers are illustrated in Fig. 4. The PQDF exhibits ultra-wide luminescence band range of 1050-1350 nm. Similar active centers in the band range have been reported [30,31]. According to previous reports, in the process of drawing, the PbS QDs doped in the fiber core can be formed to different sizes after suffering an annealing process with the high temperatures [32,33]. We deduce that the broadband luminescence phenomenon is caused by the recombination of the deep levels, resulting from the introduction of PbS QDs [34,35]. Through Gaussian fitting, the luminescence bands at 1086, 1179, and 1304 nm are mainly attributed to the role of PbS QDs at 3.7, 4.0, and 4.3 nm, respectively. The luminescence intensity versus pump power of three active centers is plotted on the inset of Fig. 4. It is shown in the figure that the luminescence intensity gradually increases with the excitation of increasing pump power. Under the 980 nm laser pumping with power of 120 mW, the intensity reaches to extreme value.
Fig. 4. Luminescence spectra of the PQDF with different pump powers, (inner) luminescence intensity versus pump power of three active centers.
Furthermore, we choose the white light source as the signal source to match the broadband luminescence of PQDF. The on-off gain, Gon-off, is calculated in Eq. (1). Giving the negative effect of the amplified spontaneous source (ASE), Gnet, the net gain can be expressed using the following Eq. (2) [36].
Where Pin, Pout, PASE are the power of input signal, output signal power, ASE power respectively.The net gain spectra of PQDF under different pump powers are depicted in Fig. 5. The net gain spectra cover the wavelength region from 1050 to 1350 nm with bandwidth of approximately 300 nm.
In order to better analyze the variation rule of gain, the net gain versus pump power of three active centers (1086, 1179, and 1304 nm), is depicted in Fig. 6. In the wavelength range of 1050-1350 nm, with the increase of pump power, the gain value continues to increase. The upward trend slows down gradually, and finally approaches to a stable gain when the pump power is 120 mW. Because the gain value depends mainly on the inversion number of carriers in PbS QDs. When the pump power is low, only a part of the carriers is in transition. The carriers aggregated at high energy level increases with the augment of pump power. However, the gain will not change when all carriers in the PbS QDs are actived to high energy levels under enough large pump power. This exactly describes the gain saturation phenomenon, which has also been discussed theoretically [37,38].
Besides, the on-off gain and net gain are 7.1-15.0 dB and 6.0-9.2 dB in the range of 1050-1350 nm at gain saturation status, respectively, as presented in Fig. 7. The net gain flatness is less than 3.2 dB, which means the 3 dB bandwidth is approximately 300 nm.
3.2 Microstructure characteristics and analysis
In order to further analyze the existential state and microstructures of PbS materials in the fiber core, the HRTEM-SAED-EDS analysis are used to examine and analyze the core materials. The preparation of HRTEM samples can be involved the following steps by Focused Ion beam (FIB) micro-dissection technology (600i, FEI Hongkong Co., LTD, Czekh) . Firstly, the fiber was cut flat with a cutting knife and fixed on the sample rack. Due to the non-conducting of fiber, the sample was required to spray gold on the fiber facet, and sprayed a layer of Pt protective film on the interest region of fiber core to prevent damage during ion beam cutting, as shown in Fig. 8(a). Then, after ion beam clipping, the general size of 10×2×3 µm (length×width×height) was taken out by mechanical Pt probe depicted in Fig. 8(b) and placed on the standard sample rack in Fig. 8(c). Finally, the thickness of the trimmed TEM sample is less than 100 nm with focused Ga ion source under low voltage and low current, as presented in Fig. 8(d).
Fig. 8. Procedure of focused ion beam (FIB) machining: (a) Pt spraying layer; (b) sampling; (c) fixed connection; (e) thinning.
Under observation of HRTEM, some nanocrystals could be observed in the sample. The nanoparticles are mainly distributed in 3.65-4.45 nm, then a few distributed in 2.85-3.65 nm and 4.45-5.25 nm, as depicted in Fig. 9(a). There exists a kind of crystalline structure, which can be clearly observed in Fig. 9(b). In addition, the SAED pattern further confirm that this crystal phase structure belongs to PbS QDs as depicted in Fig. 9(c). From the diffraction rings and lattice fringes, the interplanar distances can be calculated as 2.06, 2.98, and 3.43 Å, which correspond well to that of cubic PbS in (220), (200), and (111) plane [39,40]. The detailed statistical result for maximum size distribution peak at 3.65-4.45 nm is analyzed in Fig. 9(d). Moreover, in order to confirm the nanocrystals more thoroughly, the elemental components of the fiber core sample are detected by EDS analysis, as listed in Table 1. The doping ratio of Pb and S elements are 2.34 and 2.91 at. %, respectively. Therefore, it is clear that there exists PbS materials in the fiber core, with the form of QDs. Due to the quantum size effect of these PbS QDs, the PQDF presents special spectral characteristics.
Fig. 9. (a) High-resolution transmission electron microscopy (HRTEM) images of fiber core with PbS-doped materials, (inner) size distribution of PbS QDs; (b) lattice image; (c) selected-area electron diffraction (SAED) patterns; (d) detailed statistics for maximum size distribution peak.
Table 1. The content of different elements in the fiber core sample
Based on the absorption characteristics of PQDF, the corresponding average diameters of the PbS QDs, when the absorption bands are 803, 972, and 1171 nm, are 3.2, 4.0, and 5.0 nm, respectively. These analysis are essentially correspond to the distribution trend of PbS QDs in fiber core as HRTEM results show in Fig. 9(a) (mainly distributed in 3.65-4.45 nm, a few distributed in 2.85-3.65 nm and 4.45-5.25 nm). In addition, luminescence characteristics at 1050-1350 nm coming from 1086, 1179, and 1304 nm can be mainly attributed to the role of PbS QDs at 3.7, 4.0, and 4.3 nm, respectively. These results principally correspond to HRTEM images and detailed statistical result for maximum size distribution peak of PbS QDs at 3.65-4.45 nm, as evidenced in Fig. 9(d), which is consistent with the predicted results of absorption bands at 1171 nm. Therefore, the luminescence spectrum is due to the quantum dimension effect of PbS QDs in the fiber core.
Furthermore, we research the reasons on the gain bandwidth broadening up to 300 nm of PQDF. According to the Brus formula and literature, the band gap energy of QDs is determined in Eq. (3) [41–43]:
In addition, compared with optical gain properties of reported QDF (excited by evanescent wave), PDF (Pb-doped fiber), and EDF, the PQDF shows certain excellent performance. The 3 dB gain bandwidth of PQDF (300 nm) is wider than other fibers (QDF and EDF), which is especially ten times wider than that of EDF (25 nm). The pump power at gain saturation of PQDF is 120 mW, which is lower than that of QDF and PDF. Net gain flatness of PQDF (< 3.2 dB) is approximately half as flat as that of EDF. The comparison of QDF, PDF, EDF and PQDF are listed in Table 2 [11,30,44]. Therefore, the PQDF realizes the amplification of signal light in the range of 1050-1350 nm and exhibits ultra-wideband and flat-gain optical properties.
Table 2. The optical gain properties of QDF, PDF, EDF, and PQDF
4. Conclusions
In summary, we prepared PbS QDs using ALD method and fabricated PQDF by MCVD technology. The microstructural features and optical properties were investigated theoretically and experimentally to reveal the internal relation between size of QDs and ultra-wideband optical properties of fiber. The HRTEM results revealed that PbS QDs was distributed in the range of 3-5 nm, which led to the distinct absorption bands at 803, 972, and 1171 nm and broad luminescence in the range 1050-1350 nm of PQDF, in accordance with the quantum dimension effect. In addition, the broad luminescence property of three active centers (1086, 1179, and 1304 nm) could be attributed to the role of PbS QDs (3.7, 4.0, and 4.3 nm, respectively), which was basically in agreement with the size distribution of PbS QDs mainly in 3.65-4.45 nm. Furthermore, we studied the optical gain properties of the PQDF. In the spectrum range of 1050-1350 nm, the on-off gain was 7.1-15.0 dB and net gain was 6.0-9.2 dB, when pump power was only 120 mW. Moreover, the 3 dB gain bandwidth was almost 300 nm, resulting from multi-sized PbS QDs. Hence, it is suggested that this special doped silica fiber holds considerable application foreground for large-capacity optical communication network and novel fiber optic devices.
Funding
National Natural Science Foundation of China (61520106014, 61705126, 61975113, 61635006, 61675125); Pre-Research Fund Project (6140414030203).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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