Four-wave mixing in a single-walled carbon-nanotube-deposited D-shaped fiber and its application in tunable wavelength conversion

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1 Four-wave mixing in a single-walled carbon-nanotube-deposited D-shaped fiber and its application in tunable wavelength conversion K. K. Chow * and S. Yamashita Department of Electrical Engineering and Information Systems, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan * kkchow@sagnac.t.u-tokyo.ac.jp Abstract: We report the first experimental observation of four-wave mixing (FWM) in single-walled carbon nanotubes (SWCNTs) deposited on a D- shaped fiber. FWM-based tunable wavelength conversion of a 10 Gb/s nonreturn-to-zero signal is demonstrated using a 5-centimeter-long CNT-deposited D-shaped fiber. A power penalty of 4 db power is obtained in the 10 Gb/s biterror-rate measurements Optical Society of America OCIS codes: ( ) Nonlinear optical materials; ( ) Nonlinear optics, four-wave mixing References and links 1. S. Iijima and T. Ichihashi, Single shell carbon nanotubes of one nanometer diameter, Nature 363, (1993). 2. A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu,Y. H. Lee, S. G. Kim, D. T. Colbert, G. Scuseria, D. Tománek, J. E. Fischer, and R. E. Smalley, Crystalline ropes of metallic carbon nanotubes, Science 273, (1996). 3. Ph. Avouris, M. Freitag, and V. Perebeinos, Carbon Nanotube Optics and Optoelectronics, Nat. Phton. 2, (2008). 4. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, Ultrafast fiber pulsed lasers incorporating carbon nanotubes, IEEE J Select. Top. Quantum Electron. 10, (2004). 5. S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, M. Jablonski, and S. Y. Set, Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates/fibers and their applications to mode-locked fiber lasers, Opt. Lett. 29, (2004). 6. Y. C. Chen, N. R. Raravikar, L. S. Schadler, P. M. Ajayan, Y. P. Zhao, T. M. Lu, G. C. Wang, and X. C. Zhang, Ultrafast optical switching properties of single-walled carbon nanotube polymer composites at 1.55 µm, App. Phys. Lett. 81, (2002). 7. S. Tatsuura, M. Furuki, Y. Sato, I. Iwasa, M. Tian, and H. Mitsu, Semiconductor carbon nanotubes as ultrafast switching materials for optical telecommunications, Adv. Mater. 15, (2003). 8. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, A noise suppressing satiable absorber at 1550 nm based on carbon nanotube technology, in Proc. OFC 2003, paper FL2, Atlanta, GA, USA (2003). 9. Vl. A. Margulis, E. A. Gaiduk, and E. N. Zhidkin, Third-order optical nonlinearity of semiconductor carbon nanotubes: third harmonic generation, Diamond Relat. Mater. 8, (1999). 10. Vl. A. Margulis, E. A. Gaiduk, and E. N. Zhidkin, Optical third-harmonic generation from an array of aligned carbon nanotubes with randomly distributed diameters, Diamond Relat. Mater. 10, (2001). 11. Y. W. Song, S. Y. Set, and S. Yamashita, Novel Kerr shutter using carbon nanotubes deposited onto a 5-cm D-shaped fiber, in Proc. CLEO 2006, paper CMA4, Long Beach, CA, USA (2006). 12. K. K. Chow, S. Yamashita, and Y. W. Song, "A widely tunable wavelength converter based on nonlinear polarization rotation in a carbon-nanotube-deposited D-shaped fiber," Opt. Express 17, (2009), K. Kashiwagi, S. Yamashita, H. Yaguchi, C. S. Goh, and S. Y. Set, All optical switching using carbon nanotubes loaded planar waveguide, in Proc. CLEO 2006, paper CMA5, Long Beach, CA, USA (2006). 14. K. C. Jena, P. B. Bisht, M. M. Shaijumon, and S. Ramaprabhu, Study of optical nonlinearity of functionalized multi-wall carbon nanotubes by using degenerate four wave mixing and Z-scan techniques, Opt. Commun. 273, (2007). 15. G. P. Agrawal, Nonlinear Fiber Optics 3rd ed. (New York: Academic, ). (C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15608

2 1. Introduction Single-walled carbon nanotube (SWCNT) technology has recently drawn much research attention owing to its unique optical properties [1-3]. Carbon nanotubes are mainly categorized as SWCNTs and multi-walled carbon nanotubes (MWCNTs). The structure of a SWNT can be simply understood by wrapping a one-atom-thick layer of graphene into a cylinder. A MWCNT is two or more SWCNTs nested within one another in a coaxial form. As the properties of MWCNTs are determined by contribution of all individual shells with different structures, they are usually more defective than SWCNTs. In the case of SWCNTs, due to their pure one-dimension properties and well-defined structures, they exhibit optical properties that are not shared by MWCNTs [1]. Much research effort had been focused on various photonics applications and studies of SWCNTs including mode-locked lasers [4, 5], ultra-fast optical response [6, 7], and optical noise suppression [8]. In particular, SWCNTs can be employed as an optical nonlinearity medium due to the theoretically estimated ultra-high third-order nonlinearity [9, 10]. The third-order nonlinearity of the SWCNTs is believed to be originated from the inter-band transitions of the π-electrons causing nonlinear polarization. In this respect the SWCNTs are similar to other organic optical materials such as polyacetylene or polydiacetylenes which exhibit extremely high third-order nonlinearity [9]. Recently, our group had reported Kerr shutter based optical switching using a few centimeters of SWCNTdeposited D-shaped fiber [11, 12] as well as optical loop mirror incorporated with SWCNTloaded planar waveguide [13], which initiated the possibilities of practical nonlinear SWCNTbased devices. There was previous report on studying four-wave mixing (FWM) generated in MWCNTs [14]. However, currently there is still no report on direct observation of nonlinear effects generated in the SWCNTs due to the difficulties in adopting suitable SWCNTs and designing suitable configurations for SWCNT-light interaction. In this paper, we report, for the first time to the best of our knowledge, experimental observation of FWM generated in the SWCNTs. The SWCNTs are sprayed on a polished D- shaped fiber and the one-dimensional nature as well as the bandgap properties of the deposited SWCNTs can enhance the nonlinearity. With the light interaction in the SWCNTdeposited 5-cm long D-shaped fiber, FWM spectra with obvious idlers are obtained. The FWM properties of the SWCNT-deposited D-shaped fiber are further studied by performing FWM-based wavelength conversion experiment. The existence of FWM is verified by investigating the wavelength tunability of the converted signal and a tunning range of around 6 nm with a peak conversion efficiency of -31 db is measured. A power penalty of 4 db is measured for 10 Gb/s wavelength-converted non-return-to-zero (NRZ) signal in the bit-errorrate (BER) measurements. 2. Design and fabrication of SWCNT-deposited D-shaped fiber The working principle of the SWCNT-deposited D-shaped fiber is based on the interaction of SWCNTs with evanescent field of the propagating light in the fiber. In our experiment, the SWCNTs are made by a bulk production method called high-pressure CO conversion (HiPCO). Since the isolation of individual SWCNT is critical to obtain the maximum nonlinearity, the diameter and the diameter distribution of the SWCNTs are well controlled in the process. The SWCNTs are then dispersed in dimethylformamide (DMF), an effective solvent for separating and suspending SWCNTs. The SWCNT in DMF solution is finished by taking only the homogeneous part after centrifugal separation. Fig. 1(a) shows the absorption spectrum of the SWCNTs measured by a spectrophotometer scanning from 400 nm to 2000 nm. By controlling the HiPCO process thus the nanotube diameters and the diameter distribution, the SWCNTs show a desirable absorption peak near 1550 nm. In our experiment, such properties of the SWCNTs with an absorption near 1550 nm is found to be highly suitable for generating nonlinear effects such as FWM at 1550 nm wavelength range. The D- shaped fiber then is prepared by polishing a segment of standard SMF held by a V-grooved block. The fiber together with the V-grooved block is polished with 4 steps in order to ensure the non-cracked and smooth surface of the D-shaped area, thus minimize the beam scattering through the polished face. The insertion loss is monitored during polishing therefore the (C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15609

3 amount of optical power leakage through the polished face can also be monitored. The SWCNTs are then deposited on the D-shaped surface by spray method, and the fiber sample is finalized by adding a protection layer above the deposited SWCNTs. Fig. 1(b) shows the schematic illustration of the D-shaped fiber and the corresponding SEM image with 50K magnification of the deposited SWCNTs. Fig. 1(c) is photo of the finished device in a V- grooved block with the SMF pigtails. The overall insertion loss of the SWCNT-deposited D- shaped fiber adopted in our experiment is 12 db with a SWCNT-light interaction length of around 5 cm. Note that the V-grooved block is a few cm longer as a buffer for protecting the junctions between the D-shaped region and the SMF pigtails. Since the SWCNTs are randomly sprayed on the D-shaped area, the fiber is polarization sensitive with around 4-dB power variation to the polarization-dependent resonance of individual SWCNTs. It is worth noting that since the splicing loss of the device to sub-systems or laser cavities can be nearly neglected as the D-shaped fiber is made by standard SMF. Fig. 1. (a) Schematic illustration of D-shaped fiber with single-walled carbon nanotubes (SWCNTs) deposited on the polished surface (SEM image: magnification 50K); (b) absorption spectrum of the deposited SWCNTs measured by a spectrophotometer; and (c) photo of the finished SWCNT-deposited D-shaped fiber in a V-grooved block with single-mode fiber pigtails. 3. Four-wave mixing in SWCNT-deposited D-shaped fiber The experimental setup on FMW in SWCNT-deposited D-shaped fiber is shown in Fig. 2. In this session, the signal modulation (dotted line part) is initially removed and the continuouswave (cw) output of the external cavity laser (ECL1) serves as one of the cw pumps (S) for the FWM effect. The light is combined with the cw output (P) of another external cavity laser (ECL2) using a 3-dB coupler. The combined light is then launched on an erbium-doped fiber amplifier (EDFA) followed by a segment of 5 cm long SWCNT-deposited D-shaped fiber. The launched pump (P) power into the fiber device is estimated to be +30 dbm. The amplified light S and P then undergo FWM effect and new wavelength components are generated. ECL1 Intensity Modulator 3 db Coupler ` CNT-Deposited D-shaped Fiber OSA 10 Gb/s Pattern Generator Signal Modulation ECL2 EDFA Receiver/ BERT Optical Filter Fig. 2. Experimental setup on four-wave mixing in a SWCNT-deposited D-shaped fiber. ECL: external cavity laser; EDFA: erbium-doped fiber amplifier; OSA: optical spectrum analyzer; BERT: bit-error rate test set. (C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15610

4 Spectral Intensity (10 db/div) C P S Wavelength (nm) Fig. 3. Continuous-wave four-wave mixing spectrum obtained after the SWCNT-deposited D- shaped fiber. Figure 3 shows the FWM spectra obtained after the SWCNT-deposited D-shaped fiber. In our experiment the pump (P) is fixed at nm and different converted wavelengths are obtained by tuning the signal (S) wavelength. The same experiment is repeated with bare D- shaped fibers to ensure that the FWM effect is generated from the SWCNTs only. It is observed that a strong idler (C) at 1548 nm is generated corresponding to the S wavelength at 1552 nm. We define the conversion efficiency as the ratio of the idler (C) power to the signal (S) power inside the SWCNT-deposited D-shaped fiber. Assuming the pump wavelengths are close enough and the propagation length is short, the conversion efficiency η can be approximately expressed as [15]: 2 η ( L) = ( γpl) (1) where L is the light propagation distance, γ is the effective nonlinear coefficient, and P is the pump (P) power. Form Eq. (1) the effective nonlinear coefficient of the SWCNT-deposited D- shaped fiber in our experiment is calculated to be as high as W -1 km -1. It is reported that the distributed diameter of the SWCNTs is critical for obtaining high third-order nonlinearity. One can possibly further enhance the effective nonlinear coefficient using SWCNTs with more precisely engineered average tube diameter [9, 10]. 4. FWM-based wavelength conversion using SWCNT-deposited D-shaped fiber The FWM properties of the SWCNT-deposited D-shaped fiber are further investigated by performing FWM-based wavelength conversion experiment. The signal modulation is turned on and the ECL1 output is modulated to be a bits pseudorandom NRZ signal at 10 Gb/s as shown in Fig. 2. The modulated light is then amplified by a low noise EDFA with ASE filtering to compensate the loss of the modulator. Afterwards, the 10 Gb/s signal is combined with the cw light from the ECL2 and amplified together with the EDFA followed by launching into the SWCNT-deposited D-shaped fiber. Fig. 4(a) shows the output FWM spectrum obtained after the SWCNT-deposited D-shaped fiber where Fig. 4(b) and 4(c) depict the close-up views of the input signal and the converted signal, respectively. From Fig. 4(c) it is observed that the generated converted signal is spectrally broadened to have a 10 Gb/s modulation characteristics corresponding to the input signal, thus confirming the generation of the converted signal is the result of wave mixing between the input signal and the pump. (C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15611

5 Fig. 4. (a) Four-wave mixing spectrum obtained after the SWCNT-deposited D-shaped fiber with input signal modulated at 10 Gb/s and the corresponding close-up views of (b) input signal and (c) converted signal. The converted signal tunability is further investigated and the relationship between the conversion efficiency and the S wavelength detuning again the fixed P is plotted in Fig. 5. A tunning range of around 6 nm is obtained with a peak conversion efficiency of -31 db. It is believed that the response of SWCNT is related to high polarizability of the device. High nonlinearity comes at the expense of high material dispersion, which makes for very narrowband phase-matching. The high dispersion of the device, which is a consequence slow response time, is therefore responsible for the FWM bandwidth. Note that the optical signal to noise ratio of the converted wavelength is also maintained over 10 db throughout the whole tuning range. Conversion Efficiency (db) Wavelength Detuning (nm) Fig. 5. Plot of conversion efficiency against signal wavelength detuning. (C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15612

6 Fig. 6. Plot of bit-error rate against received optical power; inset (upper) and (lower) show the 10 Gb/s eye-diagrams of the input and the converted signal, respectively. The performance of the FWM-based wavelength conversion is further investigated by performing BER measurements. Fig. 6 plots the output BER against the received optical power with the inset showing the 10 Gb/s eye diagrams of the input signal and the converted signal. Note that the eye-diagram of the converted signal is obtained after the optical bandpass filter as shown in Fig. 2 followed by a low noise EDFA with suitable ASE filtering in order to boost up the optical power for measurements. In this measurement the wavelengths of S and C are the same with those shown in Fig. 4. The Fig. shows the results of a 4-nm downconversion and the power penalty is measured to be around 4 db at 10-9 BER level. The power penalty is believed to be mainly originated from the defect of the hand-polished D-shaped surface which causes scattering of the propagating light. Such scattering causes light reflection and oscillation of the propagating signal inside the device and generates noise mainly in the high power level, thus affects the eye opening. A better BER performance is expected with the D-shaped fiber polished by precise machining. Also, the relatively low conversion efficiency is believed to be originated from the interaction between the SWCNTs and the evanescent field of the propagating light only in the D-shaped fiber. One can further enhance the conversion efficiency by depositing the SWCNTs on suitable optical waveguides to ensure more direct SWCNT-light interaction. 5. Conclusion Four-wave mixing generated in single-walled carbon nanotubes deposited on a D-shaped fiber has been experimentally observed for the first time. With suitable SWCNTs deposited onto a D-shaped fiber for SWCNT-light interaction, FWM spectra with obvious idlers are obtained. FWM-based wavelength conversion of a 10 Gb/s NRZ signal is further demonstrated. A wavelength tunning range of 6 nm of the generated light and a peak conversion efficiency of - 31 db is measured, and a power penalty of around 4 db at 10-9 bit-error-rate level in the BER measurements is obtained. It is expected if more direct SWCNT-light interaction is realized with suitable optical waveguides, higher FWM conversion efficiency and lower power penalty can be obtained. The results are significant for SWCNT technology especially future practical SWCNT nonlinear devices for optical communications and networks. Acknowledgement This work was supported by Strategic Information and Communications R&D Promotion Programme (SCOPE) of The Ministry of Internal Affairs and Communications (MIC). (C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15613