Optical fiber amplifiers based on PbS/CdS semiconductor quantum dots (QDs) modified by an amphiphilic polymer were demonstrated. Well-defined QDs and an amphiphilic copolymer were first prepared and the amphiphilic copolymer was then used to disperse the QDs into silica sol to allow uniform and reproducible incorporation of QDs into the silica coating of the optical fibers. QD-doped silica sol was deposited on the fusion tapered fiber coupler via dip-coating. A 1550 nm semiconductor light emitting diode as the signal source and a 980 nm laser diode as the pump source were injected into the fiber coupler simultaneously. Through evanescent wave excitation, a signal gain as high as 8 dB was obtained within the wavelength range between 1450 and 1650 nm. In addition, the optical fiber amplifiers based on PbS/CdS QDs showed enhanced thermal stability when compared to amplifiers based on PbS QDs.
©2013 Optical Society of America
Semiconductor quantum dots (QDs) in the near infrared (NIR) regime are very attractive in a variety of potential applications such as photovoltaic devices and biological labeling due to their size-tunable optical properties, arising from quantum conðnement effect [1–3]. In particular, PbS QDs can be tuned to absorb and emit in the spectral ranges of 700–900 nm and 1200–1600 nm, respectively . For practical applications, QDs are required to exhibit high quantum yields (QYs) and to have good photo- and thermal stability. PbS QDs display spectral shift of photoluminescence (PL) band and reduction of QYs mostly due to surface oxidation at high temperatures while sometimes even under normal conditions [5–8]. Recent studies have revealed that core/shell structure can stabilize and maximize the QYs of the core QDs by not only passivating the surface of the core nanocrystals but also burying the core semiconductor in a potential energy well [9–12], which makes it a more promising candidate material for components in optical fiber communication amplification.
To utilize QDs in optical fiber amplifiers, different kinds of technologies have been developed to fabricate optical amplifier components. P. R. Watekar doped PbSe QDs into silica optical fibers using modified chemical vapor deposition (MCVD) technology, which showed an amplified spontaneous emission (ASE) at 1537 nm . However, the high temperature of MCVD process affected the characteristics of QDs and the doping concentration. S. Kawanishi proposed to fill solution-based PbSe QDs into a photonic bandgap fiber, which demonstrated PL at 1554 nm . Similarly, Ali Hreibi fabricated and characterized a liquid-core fiber incorporating PbSe QDs. Using a pump power as low as 5 mW, a stable spontaneous emission centered at 1220 nm (FWHM 120 nm) was observed . But all of the above nevertheless have the inevitable connecting problem with single mode fibers (SMFs). By coating PbS QDs doped film onto tapered twin SMF coupler, our laboratory has proposed a new amplifier structure which can be well connected with SMFs . However, the PbS QDs were synthesized in situ during the sol-gel process, in which the control of the size and quality of QDs was difficult, and it was also impossible to synthesize core/shell QDs via this method.
In this paper, we present a different strategy to prepare high-quality optical fiber amplifiers. We decoupled the synthesis of QDs from the coating process such that high quality QDs with tunable size and core/shell structure can be separately prepared. Dispersion of QDs into silica sol was realized by modifying the QDs with well-defined amphiphilic comb polymers, which allowed successful transfer of QDs from organic solvents to polar solutions and also promoted uniform distribution of QDs in the silica coating. This method can retain the high QYs and good photostability of QDs . The fiber amplifier was fabricated by depositing silica sol containing PbS/CdS QDs onto the tapered region of a tapered twin SMF coupler. A 1550 nm semiconductor light emitting diode (SLED) as the signal source and a 980 nm laser diode (LD) as the pump were injected into the fiber coupler simultaneously. The QDs were excited by evanescent wave to achieve amplified signals.
2. Synthesis and characterization of PbS/CdS QDs
PbS/CdS QDs were synthesized according to literature [17, 18] and were characterized with a JEM 2100F HRTEM equipped with EDX. As shown in Fig. 1 , the diameter of PbS/CdS was estimated to be 7.5 nm, consisting of a PbS core of 6.5 nm and a CdS shell of 1 nm. A core/shell structure can be clearly discerned at higher magnification (Fig. 1(b)). EDX further confirmed the existence of the shell.
The PbS/CdS QDs synthesized in organic solvents was hydrophobic. In order to disperse the hydrophobic QDs into polar solvents, the QDs were modified with tailor-made amphiphilic copolymers. Figure 2 shows the schematic process of QD modification. After modification, the QDs were packaged with polymers which can make the QDs water soluble and retain high QYs. To obtain a SiO2 sol-gel solution, 6.32 mL of TEOS, 5 mL of ethanol, 0.5 mL of hydrochloric acid (0.04 M) were added into a flask. The mixture was heated to reflux for 2.5 hours at 80°C under N2 protection. 0.021 g of water-soluble PbS/CdS QD was dispersed in 5 mL of alcohol to get a QD solution. Then the PbS/CdS QD solution and the SiO2 sol-gel solution were mixed at a volume ratio of 4:3. The mixture after ultrasound treatment for 10 minutes was coated onto the tapered region of fiber coupler to obtain QD doped films.
3. Fabrication and characteristics of QD optical fiber amplifiers
As depicted schematically in Fig. 3 , the fiber amplifier was fabricated by dip-coating the silica sol doped with PbS/CdS QDs onto a fusion tapered fiber coupler. The tapered twin fiber coupler was fabricated by flame fused taper technique using standard SMF which can easily couple with other optical fiber devices. With this twin fiber structure, a signal and a pump can be injected into the active region simultaneously. Because of the tapered fiber shape, evanescent wave can penetrate into the outer surface. The pump will excite the doped QDs through evanescent wave. Meanwhile, the signal interacts with excited QDs through evanescent wave and then can be amplified. For the evanescent wave exciting structure, the total internal reflection condition must be satisfied at the gain region. Thus the refractive index of our QD doped film (n = 1.443) was lower than that of silica (n = 1.453) to ensure the confinement of light wave.
Figure 4 shows that the amplification of QDs fiber amplifier at 1550 nm is as high as 8 dB. The black line represents the 1550 nm SLED signal light only through an optical spectrum analyzer, the blue line represents the 980 nm pump light, and the red line is realized by injecting the pump light and the signal light simultaneously in the tapered area of QD fiber amplifier to obtain amplification with excitation through evanescent wave. In the measurement, the absorption, coupling and insertion losses were not taken individually, thus Fig. 4 shows the signal enhancement instead of net gain.
In order to demonstrate the dependence of the gain on the pump power, the power of the signal is fixed, and the change of the gain with gradually increasing the power of pump is monitored. As shown in Fig. 5 , within the wavelength range of 1450-1650 nm, the signal intensity is amplified gradually with increasing pump power. The gain bandwidth covers a wide spectral range, consistent with the nature of the PL of the QDs-doped film shown in Fig. 1(c). As shown in Fig. 6 , with increasing pump power, the gain increases from 1.3 dB to about 8 dB and starts to level off at 120 mW. For a given dopant concentration, only a limited number of QDs could be excited from the ground level to the excited level, therefore, the gain is saturated when the pump power reached a critical value.
4. Thermal stability of QD optical fiber amplifiers
In practical applications, the thermal stability of QD fiber amplifiers is a very important issue. Compared to the core-only QDs, the core/shell structure may suppress Auger recombination rate which contributes to better thermal stability [19, 20]. Therefore, we also analyzed the thermal stability of PbS/CdS QD optical fiber amplifiers and compared with that of PbS QD optical fiber amplifiers. As shown in Fig. 7 , the injection pump light power was 80 mW in both cases. The gain with the core-shell QDs was higher. And the gain difference at 25 °C, 35 °C and 45 °C was 0.61, 0.93, and 0.99 dB, respectively. The decreased PL intensity with increasing temperature may be due to easier formation of spontaneous radiation at higher temperature, resulting in a decrease of particle reversal concentration and hence the gain. However, it should be noted that the PbS/CdS QD optical fiber amplifier not only exhibits higher amplification but also has higher thermal stability than the PbS QD amplifier, clearly demonstrating the advantage of using core-shell QDs in fiber amplifier application. The addition of a robust, larger band gap inorganic shell not only passivates the surface of the core nanocrystal, but also buries the core semiconductor in a potential energy well, and thus concentrating the charge carriers in the nanocrystal core and keeping them away from the surface and surrounding environment [9–12]. Consequently, surface and environmental factors will have less impact on the PL efficiency and stability of core-shell QDs and hence the property of optical fibers based on them.
A novel strategy was demonstrated to fabricate QD optical fiber amplifiers, which decoupled the preparation of QDs from the coating process such that high quality core-shell PbS/CdS QDs could be prepared and used. Key to the decoupling strategy was the use of an amphiphilic copolymer to disperse the QDs into the coating sol to enable uniform presence of QDs in the final coating film on the fiber. With a 980 nm LD as the pump, a more than 8 dB gain was observed around 1550 nm. Both the value of the gain and the thermal stability of PbS/CdS QD optical fiber amplifiers were higher than those of PbS optical fiber amplifiers. Due to the all-fiber structure and improved thermal stability, the proposed fiber amplifier will find potential applications in wide band and high speed fiber-optic communication.
This work was funded by the National Natural Science Foundation of China (61006083, 21274084, 61205172 and 60937003) and Science and Technology Commission of Shanghai Municipality (STCSM)(10PJ1404300). Z. An is grateful for support by Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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