A PbS quantum dots (QDs) fiber amplifier was fabricated and characterized by using a standard single mode fiber (SMF) coupler. The fiber amplifier was fabricated by coating PbS QDs doped sol-gel films onto the tapered SMF coupler. Through the evanescent wave, the PbS quantum dots were excited. With a 980 nm wavelength laser diode (LD) as the pump, the fiber amplifier exhibited a wide band optical gain at 1310 nm with the largest gain as high as 10 dB. The amplified spontaneous emission (ASE) noise is very low resulted from the amplifier configuration of evanescent wave exciting, which is critical to improve the signal-to-noise ratio. Therefore the proposed fiber amplifier will find great potential in the fiber-optic communication systems.
©2010 Optical Society of America
Semiconductor quantum dots (QDs) have received much attention due to their unique optical and electronic properties, and have found a wide range of potential applications in biosensors, chemical sensors, photovoltaic cells, light amplifiers, and light emitting diodes etc [1,2]. The absorption and photoluminescence (PL) of QDs present strong size dependent properties which are different from the corresponding bulk materials due to the quantum confinement effect [3, 4]. The bandwidth, position, and profile of the PL of QDs can be conveniently tailored by varying the size and distribution for various requirements. Therefore, semiconductor QDs have become a promising candidate material for components in optical fiber communication amplification . Especially for high-speed and wide band optical fiber communication, optical amplifiers using QDs have exhibited many advantages including greatly expanded bandwidth, fast gain response, high saturation power, distortion-free amplification and low noise figure etc [6–9].
To utilize semiconductor QDs in optical fiber amplifiers, a variety of technologies have been developed to synthesize QDs and fabricate optical amplifier components. T. Akiyama synthesized InAs QDs on InP substrate using molecular beam epitaxy (MBE) method, and fabricated a semiconductor optical amplifier which demonstrated >25 dB gain at 1550 nm . By using PbS QDs doped glass, K. Wundke et al. demonstrated the first directly measured gain dynamics in PbS quantum dots with a pump at 980 nm and gain tuning from 1317 to 1352 nm. The tunability was resulted from the dot-size selective excitation . J. M. Auxier utilized ion-exchange method to fabricate a channel waveguide in glass doped with PbS QDs  which exhibited a wide band photoluminescence at 1300nm. However, the above technologies are based on conventional planar waveguide structure, which requires complex fabrication process and fiber to chip coupling. To solve this problem, QDs were directly doped into specialty optical fibers. For example, 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 inherent high temperature of MCVD degraded the quality of QDs, thus lowering the doping efficiency. S. Kawanishi proposed to fill solution-based PbSe QDs into a photonic bandgap fiber which demonstrated an ASE at 1554 nm . However it also inevitably has the connecting problem with single mode fibers (SMFs).
In this paper, a novel semiconductor quantum dots fiber amplifier (SQDFA) was proposed and the optical properties were investigated. The SQDFA directly utilizes the standard SMF used in optical fiber communication systems. It is made of a tapered twin SMF coupler coated with a PbS QDs doped film. At the tapered region, light wave interacts with the doped QDs through the evanescent wave. The twin fiber structure allows a pump to be injected into the tapered region simultaneously along with a light wave, which realizes a very compact device size. Compared with the reported semiconductor QDs optical amplifiers, the proposed SQDFA has distinct advantages of simpler fabrication process and more compatibility with standard optical fiber communication system. Moreover, since the QDs doped film is on the surface of the fiber where most of the ASE light will not be gathered into the optical fiber, this amplification configuration has a good suppression to the ASE noise.
2. Device structure
As depicted schematically in Fig. 1 , the SQDFA consists of a tapered twin fiber coupler and a film coating doped with the QDs around the tapered region. H.S.Mackenzie has demonstrated that an active material could be excited effectively by a tapered single fiber through evanescent wave . In that work, the tapered single fiber was emerged in a dye solution and a 20 dB gain at 750 nm was obtained with an optical pump. Optical pump method is quite convenient for optical fiber amplifiers. For the tapered single fiber amplifier technique, a WDM combiner is usually required to combine the light wave and the pump wave. In this paper, a tapered twin fiber coupler is proposed to construct an optical fiber amplifier. A semiconductor QDs doped film is coated around the tapered region as the active material to realize optical amplification. The QDs doped film can be synthesized with solution processing methods. With this twin fiber structure, a signal and a pump can be injected into the active region simultaneously. At the tapered region, the pump will excite the doped QDs through evanescent wave. Meanwhile, the signal interacts with excited quantum dots 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 the QDs doped film must be lower than that of silica to ensure the confinement of light wave.
3. Synthesis and characterization of PbS QDs
PbS QDs are typically used to obtain the optical fiber communication bands (1310 nm or 1550 nm) because PbS QDs have a comparatively large excitation Bohr Radius (18 nm) [16,17], and the quantum confinement effect can be easily realized. Additionally, bulk PbS semiconductor has a small bandgap energy (0.41 eV), thus the emission wavelength can be tuned to the optical fiber communication bands conveniently by varying its size. Because the refractive index of bulk PbS is much higher than that of silica, PbS QDs must be doped into a proper host to achieve a lower refractive index.
In this paper, a combined colloidal and sol-gel technique was adopted to synthesize PbS QDs in which an organic-inorganic silica sol-gel material was used as the host. The refractive index was adjusted by methyl-dopants . First, two precursors, methyltriethoxysilane (MTES) and tetraethylorthosilicate (TEOS), were added into ethanol. The molar ratio of the precursors to ethanol was taken to 0.65:1 for total silicon species. Second, 0.7 equivalent (relative to ethanol) of acidified water (0.04 M, hydrochloric acid) was added. The solution mixture was magnetically stirred at 50 °C for 24 hours in a sealed flask under refluxing conditions. After that, lead acetate was dissolved into methanol, to which acetic acid and the capping agent 3-mercaptopropyltrimethoxysilane (MPTMS) were added. The molar ratio of the reactants was controlled to be Pb(CH3COO)2/CH3OH/CH3COOH/MPTMS = 1:100:3.5:0.5. The mixed solution was stirred at room temperature for 3 min. Then the solution was added to the previous silica sol where the ratio of Pb to Si was controlled to be 0.03. Finally, thioacetamide (TA) in methanol was added into the above solution dropwise. The molar ratio of Pb(CH3COO)2/TA/CH3OH was controlled to be 1:1:50. The color of the solution gradually changed from yellow to dark brown, indicating the formation of PbS QDs. The PbS QDs solution was centrifuged at 10,000 r/min for 5 min.
The PbS QDs doped sol was characterized by a transmission electronic microscope (TEM, JSM-2010F). As shown in Fig. 2 , the PbS QDs was homogeneously dispersed in the sol-gel host, and the size of QDs was estimated to be 6 nm. The crystalline structure of PbS were characterized by X-ray diffraction (XRD, DLmax-2200X) (Fig. 3 ), and all the diffraction peaks can be indexed to the cubic rock salt phase of PbS (JCPDS 5-592).
To characterize the absorption and PL properties, the PbS QDs doped sol was spin-coated onto a quartz glass slide. The absorption spectrum taken on a spectrophotometer (Lambda 900UV/VIS/NIR) as relative absorption values is shown in Fig. 4 . The PbS QDs absorption edge (around 1450 nm) was blue-shifted compared with that of the corresponding bulk material (around 3100 nm). An absorption shoulder was observed near 1250 nm due to the 1s-1s inter-band stimulated absorption. The exciton peak was less prominent owing to the polydispersity of the QDs. According to the band gap equation proposed by Y. Wang , with the effective mass (m*) approximation, the band gap of a nanoparticle was given as a function to the QD’s diameter. Thus, the excition absorption wavelength λ can be derived as:Fig. 5 . The first exciton absorption peak (1s-1s) is blue-shifted with decreasing size of the QDs. The exciton peak appears around 1200 nm when the QDs size is about 6 nm, which is consistent with the measured absorption spectrum.
By using a 980 nm laser diode (LD) pump with power of 800 mW, which was launched directly on the sample slice, a wide band PL spectrum with an emission peak around 1340 nm was measured, as shown in Fig. 4. The PL spectrum presented a primary and a secondary peak which was resulted from the polydispersity of the QDs size as shown in the TEM image.
4. Fabrication and characteristics of optical fiber evanescent wave amplifier
Utilizing the PbS QDs doped sol, the SQDFA was fabricated. As depicted in Fig. 1, the SQDFA was made by coating a QDs doped film onto a tapered fiber coupler at the tapered region. The tapered fiber coupler was fabricated by using a fusion pulling machine. The tapered region was about 2 cm long. The waist was about 15 μm in diameter. The QDs film coating was deposited by a dip-coating technique. After coated with the film, the SQDFA was annealed in a temperature-controlled chamber at 100°C for two hours.
By using a semiconductor light emitting diode (SLED) with center wavelength of 1310 nm as the signal source and a 980 nm LD as the pump, the amplification properties of the SQDFA were characterized. The amplified spectrum was recorded with an optical spectrum analyzer (OSA, Ando, AQ6315). First, the input signal and the ASE spectrum with injecting the pump only were recorded respectively. Then, the amplified output spectrum was recorded when both the signal and the pump light wave were injected simultaneously. Finally, the gain was obtained by subtracting the ASE from the amplified signal then being divided by the input signal. As shown in Fig. 6 , the output spectra of the signal only, the pump only and the signal with pump were recorded. We observed that the ASE at 1310 nm is fairly low. It can be illustrated more clearly when the spectrum is converted from a log coordinate into a linear one, as shown in the inset of Fig. 6. In our proposed SQDFA, the QDs were excited through the evanescent wave and the ASE was excited within the QDs film. Because the refractive index of the tapered silica was higher than that of the QDs doped coating, very little ASE light wave could be gathered into the tapered fiber when the pump was injected. Therefore, the output ASE noise is very low and can be neglected. Compared with conventional fiber amplifiers, this is a very good performance in practice because the low ASE noise is very significant to improve the signal-to-noise ratio and the detecting sensitivity. A 10 dB gain was obtained at 1310 nm with 140 mW of pump power. As shown in Fig. 7 , within the wavelength range of 1200 to 1400 nm, the overall gain was going up gradually with increasing pump power. The gain bandwidth covers a wide spectral range, consistent with the nature of the PL of the PbS QDs doped film shown in Fig. 4, which results from the polydispersity of the QDs size.
To demonstrate the change of the gain more clearly, the dependence of the gain on the pump power was plotted in Fig. 8 . With increasing pump power, the gain increased gradually when the pump power was in the range of 10 to 120 mW with the maximum gain of 10dB. The gain was finally saturated starting at 120 mW, suggesting a maximum amplification was obtained at 120 mW. This result has also been discussed theoretically by C. Jiang . When the pump power increased, the population number increased gradually and so did the gain. However, 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 was saturated when the pump power reached a large enough value.
In summary, a wide band optical amplifier based on PdS QDs was fabricated and studied. The PbS QDs doped sol was synthesized via a simple colloidal technique combined with sol-gel process. Using the PbS QDs doped film, the signal and the pump interacted with the QDs in the coating at the tapered coupler region through evanescent wave. The gain increased with the pump power within the 10 to 120 mW range, and then reached a saturation regime at 120 mW. Up to 10 dB gain was achieved when the pump power was about 120 mW. With the evanescent wave exciting amplifier configuration, the ASE was successfully suppressed. Due to the all-fiber structure and the good PL performance of the QDs, the proposed SQDFA will find great potential in wide band and high speed fiber-optic communication.
This work was funded by the State Key Program of National Natural Science of China (60937003), the National Natural Science Foundation of China (60807031), Science and Technology Commission of Shanghai Municipality (STCSM) (0952nm06800, 09520702400 and 08DZ2231100), Shanghai Leading Academic Discipline Project (S30108), Supported by Innovation Program of Shanghai Municipal Education Commission (10YZ12).
References and links
1. V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. J. Eisler, and M. G. Bawendi, “Optical Gain and Stimulated Emission in Nanocrystal Quantum Dots,” Science 290(5490), 314–317 (2000). [CrossRef] [PubMed]
2. M.-S. Bakshi, P. Thakur, G. Kaur, H. Kaur, T.-S. Banipal, F. Possmayer, and N. O. Petersen, “Stabilization of PbS Nanocrystals by Bovine Serum Albumin in its Native and Denatured States,” Adv. Funct. Mater. 19(9), 1451–1458 (2009). [CrossRef]
3. A. L. Efros and A. L. Efros, “Interband absorption of light in a semiconductor sphere,” Sov. Phys. Semicond. 16(7), 772–775 (1982).
4. J.-S. Steckel, S. Coe-Sullivan, V. Bulovic, and M. G. Bawendi, “1.3mm and 1.55mm Tunable Electroluminescence from PbSe Quantum Dots Embedded within an Organic Devices,” Adv. Mater. 15(21), 1862–1866 (2003). [CrossRef]
5. P. Bhattacharya and Z. Mi, “Quantum-Dot Optoelectronic Devices,” Proc. IEEE 95(9), 1723–1740 (2007). [CrossRef]
6. T. Akiyama, M. Sugawara, and Y. Arakawa, “Quantum-Dot Semiconductor Optical Amplifiers,” Proc. IEEE 95(9), 1757–1766 (2007). [CrossRef]
7. T. Erneux, E. A. Viktorov, P. Mandel, T. Piwonski, G. Huyet, and J. Houlihan, “The fast recovery dynamics of a quantum dot semiconductor optical amplifier,” Appl. Phys. Lett. 94(11), 113501 (2009). [CrossRef]
8. J. Kim, M. Laemmlin, C. Meuer, D. Bimberg, and G. Eisenstein, “Theoretical and Experimental Study of High-Speed Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers,” IEEE J. Quantum Electron. 45(3), 240–248 (2009). [CrossRef]
9. O. Qasaimeh, “Effect of Doping on the Optical Characteristics of Quantum-Dot Semiconductor Optical Amplifiers,” IEEE J. Lightw. Technol. 27(12), 1978–1984 (2009). [CrossRef]
10. T. Akiyama, M. Ekawa, M. Sugawara, K. Kawaguchi, H. Sudo, A. Kuramata, H. Ebe, and Y. Arakawa, “An Ultrawide-Band Semiconductor Optical Amplifier Having an Extremely High Penalty-Free Output Power of 23 dBm Achieved With Quantum Dots,” IEEE Photon. Technol. Lett. 17(8), 1614–1616 (2005). [CrossRef]
11. K. Wundke, J. M. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “Room-temperature gain at 1.3 um in PbS-doped glasses,” Appl. Phys. Lett. 75(20), 3060–3062 (1999). [CrossRef]
12. J. M. Auxier, M. M. Morrell, B. R. West, S. Honkanen, A. Schülzgen, N. Peyghambarian, S. Sen, and N. F. Borrelli, “Ion-exchanged waveguides in glass doped with PbS quantum dots,” Appl. Phys. Lett. 85(25), 6098–6100 (2004). [CrossRef]
13. P. R. Watekar, A. Lin, S. Ju, and W. T. Han, “1537 nm Emission Upon 980 nm Pumping in PbSe Quantum Dots Doped Optical Fiber,” OFC, OWO1 (2008).
14. S. Kawanishi, T. Komukai, M. Ohmori and H. Sakaki, “Photoluminescence of semiconductor nanocrystal quantum dots at 1550 nm wavelength in the core of photonic bandgap fiber,” CLEO, CTuII4(2007).
15. H. S. Mackenzie and F. P. Payne, “Evanescent field Amplification in a Tapered Single-Mode Optical Fiber,” Electron. Lett. 26(2), 130–132 (1990). [CrossRef]
16. V. Sukhovatkin, S. Musikhin, I. Gorelikov, S. Cauchi, L. Bakueva, E. Kumacheva, and E. H. Sargent, “Room-temperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films,” Opt. Lett. 30(2), 171–173 (2005). [CrossRef] [PubMed]
17. L. Bakueva, S. Musikhin, M. A. Hines, T.-W. F. Chang, M. Tzolov, G. D. Scholes, and E. H. Sargent, “Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer,” Appl. Phys. Lett. 82(17), 2895–2897 (2003). [CrossRef]
18. F. Pang, X. Han, F. Chu, J. Geng, H. Cai, R. Qua, and Z. Fang, “Sensitivity to alcohols of a planar waveguide ring resonator fabricated by a sol-gel method,” Sens. Act. B 120(2), 610–614 (2007). [CrossRef]
19. Y. Wang, A. Suna, W. Mahler, and R. Kasowski, “PbS in Polymers. From molecules to bulk solids,” J. Chem. Phys. 87(12), 7315–7322 (1987). [CrossRef]
20. C. Jiang, “Ultrabroadband Gain Characteristics of a Quantum-Dot-Doped Fiber Amplifier,” IEEE J. Sel. Top. Quantum Electron. 15(1), 140–144 (2009). [CrossRef]