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OPTICS LETTERS / Vol. 39, No. 12 / June 15, 2014
Reflection noise reduction effect of graded-index plastic optical fiber in multimode fiber link Azusa Inoue,1,3 Rei Furukawa,2 Motoharu Matsuura,2 and Yasuhiro Koike1,4 1
Graduate School of Science and Technology, Keio University, 7-1 Shin-kawasaki, Saiwai-ku, Kawasaki 212-0032, Japan 2 Center for Frontier Science and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan 3 4
e-mail: [email protected] e-mail: [email protected]
Received March 10, 2014; revised May 14, 2014; accepted May 14, 2014; posted May 15, 2014 (Doc. ID 207950); published June 12, 2014 We experimentally demonstrate that a graded-index plastic optical fiber (GI POF) can significantly reduce reflection noise in a multimode fiber link with a vertical-cavity surface-emitting laser (VCSEL). By directly observing beams backreflected to the VCSEL, we show that the noise reduction effect is closely related to random mode coupling because of light scattering by microscopic heterogeneities in the GI POF core material. This suggests that intrinsic mode coupling can lower the self-coupling efficiency of the light backreflected to the VCSEL cavity through beam quality degradation. Using GI POFs, low-cost radio-over-fiber systems for indoor networks can be realized without optical isolators or fiber end-face polishing. © 2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.2290) Fiber materials; (160.5470) Polymers; (290.5840) Scattering, molecules; (060.5625) Radio frequency photonics. http://dx.doi.org/10.1364/OL.39.003662
Graded-index plastic optical fiber (GI POF) is a promising transmission cable for short-haul communication networks because of its flexibility and low installation cost. The bandwidths of such fibers have expanded with the development of low-dispersion materials and GI profile control techniques, allowing for the demonstration of 40 Gbps transmissions over a 100 m GI POF [1,2]. It has also been reported that the transmission characteristics of GI POFs are significantly affected by much stronger mode coupling than glass multimode fibers (MMFs) [3–5], whereas the underlying physics are not well understood. Recently, we theoretically demonstrated that the strong mode coupling is closely related to intrinsic polymer properties of the microscopic heterogeneities of the GI POF core [6,7]. This suggests that the GI POFs inherently have different optical characteristics from those of the glass MMFs. Nevertheless, the microscopic material properties have been overlooked in previous studies of the GI POF. Radio-over-fiber (RoF) technology has attracted much interest for use in indoor networks, which can be enriched by transmission of various radio-frequency signals over optical fibers [8–11]. In home area networks (HANs), CATV and wireless signal transmission over fiber can simplify conventional networks with coaxial and twisted-pair cables. In addition, an RoF-based distributed antenna system can be realized for improvement of coverage and throughput of a wireless local-area indoor network. For these short-reach applications, a practical RoF technology using a single-mode fiber is too expensive. Therefore a cost-effective RoF system has been studied using an MMF with a vertical-cavity surface-emitting laser (VCSEL). MMF optical links have suffered from complex noise problems due to reflection noise and modal noise [12], which are more pronounced for shorter links such as HANs. The reflection noise can increase system noisefloor levels through laser intensity fluctuations due to 0146-9592/14/123662-04$15.00/0
optical feedback from some discontinuities such as a fiber end-face or a photodiode (PD) [13–15], lowering the carrier-to-noise ratio (CNR) in an RoF system. This can be improved by using an optical isolator and an obliquely polished optical fiber. However, such components cannot be employed in indoor networks because of their high installation costs. Moreover, the effect of the reflection noise on the CNR becomes stronger for more multiplexed channels, which require lower signal levels for each channel for linear laser operation [16]. Here we show that the GI POF can inherently reduce the reflection noise in MMF links because of microscopic material properties of the GI POF core. This suggests that GI POFs can allow for low-cost RoF systems without optical isolators and obliquely polished fibers in indoor networks. Figure 1 shows the experimental setup for evaluation of reflection noise in MMF links. The laser is a multimode VCSEL (Optowell, PM67–F1P1N) with an oscillation wavelength of ∼670 nm. The photodetector is a silicon PIN PD (New Focus, 1601-AC) with a −3 dB bandwidth of ∼1 GHz. The collimated output beam from the VCSEL was focused on a fiber end-face using an antireflection-
Fig. 1. Experimental setup. DM and NBS are the dichroic mirror and nonpolarizing beam splitter, respectively. The inset shows a microscopic image of an incident beam on an end-face of the GI POF. © 2014 Optical Society of America
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Fig. 2. Photograph of the output beam pattern of (a) a 2 m GI POF and (b) a 2 m glass GI MMF imaged on the Si PIN PD.
coated (AR-coated) lens with an NA of 0.26 and a focal length of 11 mm, monitored with a CCD camera to confirm normal light incidence along the fiber axis (shown in the inset in Fig. 1). Under this restricted mode-launching condition, we measured the noise-floor spectra of the MMF links with an unmodulated VCSEL, which was operated with a drive current of 3.0 mA. In this study, highfrequency intensity noise [13–15] due to optical feedback from the fiber output face and the PD were mainly evaluated because refection noise can appear in the frequency bands for CATV and wireless signals. We did not evaluate the optical feedback from an MMF input face, which typically results in low-frequency intensity noise owing to the close distance between an MMF and a VCSEL in practical optical modules [17]. The modal noise was eliminated by monitoring the PD irradiated with output beams from MMFs to detect all of the output beams with the PD, as shown in Fig. 2. This allows for evaluation of the reflection noise without the modal noise effect on the detected power fluctuation, which results from partial detection of the beam that has the fluctuating speckle pattern [18]. We developed a POF core material based on a partially halogenated acrylic polymer, which inherently has microscopic heterogeneities in its higher-order structures. Using the core material in the co-extrusion process, we fabricated a GI POF with a core diameter of 200 μm and an NA of 0.3. At a wavelength of 670 nm, the GI POF has an attenuation of 154 dB∕km, which includes higher scattering loss (50 dB∕km) than a glass GI MMF. This suggests that the GI POF can have much stronger mode coupling than glass GI MMFs. Figures 2(a) and 2(b) show the output beams imaged on the PD for the 2 m GI POF and a 2 m glass GI MMF, which has a core diameter of 200 μm, an NA of 0.2, and an attenuation of ∼10 dB∕km at 670 nm, under the center-launching condition (shown in the inset in Fig. 1). The GI POF has a highly speckled beam pattern with a larger width than the glass GI MMF. This suggests that strong mode coupling results in a pronounced power transition from the launched modes to the other guided modes. In the glass GI MMF with weak mode coupling, however, the output mode power distribution is almost the same as the initial distribution for launched modes, resulting in a near-field pattern in which the power is spatially localized around the center of the core. Figure 3 shows the noise floor spectra of optical links using the GI POF and the glass GI MMF. We also evaluated a glass GI MMF with an obliquely polished output face at 8°, where the guided light is almost reflected out of the core. For the glass GI MMF, the spectrum
Fig. 3. Noise floor spectra of optical links with the 2 m GI POF and 2 m glass GI MMFs under the center-launching condition. The input faces of all the fibers are polished flat.
has peaks with an equal spacing of ∼50 MHz, which corresponds to the round-trip frequencies of the external cavities with the reflectors at the fiber output face and at the PD. This suggests that the spikes are longitudinal-mode beat notes of the external cavity of the VCSEL [13–15]. On the other hand, spikes and reflection noise were not observed in the spectrum of the GI POF. Moreover, the floor levels of the GI POF were lower than those of the obliquely polished glass MMF, which only exhibited optical feedback from the PD. This suggests that the MMF links using the GI POF are barely affected by the backreflected light from both the output face and the PD. The 2 m GI POF has a higher attenuation (∼0.3 dB) than the negligible attenuation (∼0.02 dB) of the 2 m glass GI MMF, whereas their reflectivities at the fiber/ air interfaces were comparable because of the similar refractive indices of their cores. The high attenuation can reduce reflection noise by power reduction of the backreflected light from the fiber output face and the PD. To evaluate the fiber attenuation effect on the reflection noise, an attenuator consisting of an AR-coated neutral density (ND) filter was inserted to adjust the reflected power without changing the light path (as shown in Fig. 1). Figure 4 shows the effect of a 2 dB attenuator on the noise floor spectrum of the flat-polished glass GI MMF. The ND filter has a higher absorption than
Fig. 4. Noise floor spectra for a GI POF, a glass GI MMF, and a glass GI MMF with 2 dB attenuators. All the fibers have flatpolished input and output faces.
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Fig. 5. Microscopic images of backreflected beam patterns for the center-launching condition (top) and off-axis launching condition (bottom). (a) and (d) GI POF with a flat-polished output face. (b) and (e) Glass MMF with a flat-polished output face. (c) and (f) Glass MMF with an obliquely polished output face.
the attenuation of the GI POF, reducing the backreflected light power and the detected light power. Nevertheless, the GI POF has lower floor levels than the glass MMF with the 2 dB attenuators. This indicates that the significant reduction in the reflection noise cannot be mainly attributed to the high attenuation of the GI POF compared to the glass GI MMFs. To investigate the mechanism of the significant reflection noise reduction, the backreflected beams from the fibers were microscopically observed using a laser diode with higher power than the VCSEL in the experimental setup of Fig. 1. The microscopic images were captured with different CCD sensitivities for the fibers with different reflected-light intensities. Figures 5(a)–5(c) show the reflected light from the fibers under a center-launching condition. All the images have incident beam spots at the centers of the cores on the input faces, which can be clearly observed for an obliquely polished glass GI MMF with a weak backreflected beam [Fig. 5(c)]. For the fibers with flat-polished output faces, we were able to observe backward-guided light that was reflected from the output faces. In the flat-polished glass GI MMF with weak mode coupling, the mode power distribution of the backreflected light is similar to that for the initially launched modes. Therefore the backreflected light can interfere with the incident light, as shown in Fig. 5(b). This suggests that backreflected light from the glass GI MMF can be self-coupled into the VCSEL cavity, resulting in high-frequency intensity noise, as shown in Fig. 3. On the other hand, the flat-polished GI POF has reflected light with highly speckled patterns filling the entire core, as shown in Fig. 5(a). This indicates that mode coupling results in pronounced power transitions from the launched modes to almost all the guided modes for the round-trip propagation in the GI POF, even for a length of 2 m. Therefore, the MMF link using the GI POF can have lowered self-coupling efficiencies into the VCSEL cavity because of its different reflected beam properties from the VCSEL cavity modes, resulting in the significant reduction of the reflection noise. Figures 5(d)–5(f) show the reflected light from the fibers under an off-axis launching condition with an offset of 25 μm. As shown in Fig. 5(e), the flat-polished glass GI MMF has a backward guided wave with modes of higher order than those for the center-launching condition,
whereas the reflection was not observed for the obliquely polished glass GI MMF [Fig. 5(f)]. For the flat-polished GI POF, however, the reflected light has a highly speckled beam pattern similar to that for the center-launching condition, as shown in Fig. 5(d). Therefore the mode power distribution of the backreflected guided light in the GI POF barely depends on the launching conditions. This suggests that the mode-coupling effect almost reaches the steady state even for round-trip propagation in the 2 m GI POF. This strong mode coupling is closely related to the different mode coupling mechanism from the microbending-induced diffusion processes in the glass GI MMFs [6,7]. In the GI POF, random power transitions between all the guided modes can be induced because of forward light scattering by microscopic heterogeneities in the core. Therefore, the pronounced beam quality degradation of the reflected beam is likely due to the efficient mode mixing due to the microscopic material properties of the GI POF core. This suggests that we can control the noise reduction effect by changing the microscopic heterogeneous properties of the GI POF to reduce the noise floor level in the MMF-based RoF link. In conclusion, we have experimentally demonstrated that a GI POF can significantly reduce high-frequency reflection noise in an MMF link with a VCSEL. By directly observing backreflected beams from the GI POF to the VCSEL, we showed that the noise reduction effect is closely related to random mode coupling because of light scattering by microscopic heterogeneities in the GI POF core. These results suggest that the noise floor levels of the MMF-based RoF systems can be reduced by controlling the spatial correlation characteristics of microscopic core heterogeneities. We are currently investigating noise reduction effects in GI POFs with various core materials to quantitatively correlate the noise reduction effect with the microscopic material structure. This will allow for low-cost practical RoF systems without an optical isolator and fiber end-face polishing for indoor networks. This research is supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)”. References 1. A. Polley, R. J. Gandhi, and S. E. Ralph, in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference (OFC2007), OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper OMR5. 2. S. R. Nuccio, L. Christen, X. Wu, S. Khaleghi, O. Yilmaz, A. E. Willner, and Y. Koike, in Proceedings of the 34th European Conference and Exhibition on Optical Communication (ECOC2008) (IEEE, 2008), paper We.2.A.4. 3. R. F. Shi, C. Koeppen, G. Jiang, J. Wang, and A. F. Garito, Appl. Phys. Lett. 71, 3625 (1997). 4. S. E. Golowich, W. White, W. A. Reed, and E. Knudsen, J. Lightwave Technol. 21, 111 (2003). 5. A. Polley and S. E. Ralph, IEEE Photon. Technol. Lett. 19, 1254 (2007). 6. A. Inoue, T. Sassa, K. Makino, A. Kondo, and Y. Koike, Opt. Lett. 37, 2583 (2012).
June 15, 2014 / Vol. 39, No. 12 / OPTICS LETTERS 7. A. Inoue, T. Sassa, R. Furukawa, K. Makino, A. Kondo, and Y. Koike, Opt. Express 21, 17379 (2013). 8. M. Sauer, A. Kobyakov, and J. George, J. Lightwave Technol. 25, 3301 (2007). 9. N. J. Gomes, A. Nkansah, and D. Wake, J. Lightwave Technol. 26, 2388 (2008). 10. A. M. J. Koonen and M. García Larrodé, J. Lightwave Technol. 26, 2396 (2008). 11. R. Gaudino, D. Cardenas, M. Bellec, B. Charbonnier, N. Evanno, P. Guignard, S. Meyer, A. Pizzinat, I. Mollers, and D. Jager, IEEE Commun. Mag. 48, 39 (2010). 12. K. Peterman, Laser Diode Modulation and Noise (Kluwer Academic, 1991).
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13. Y. C. Chen, Appl. Phys. Lett. 37, 587 (1980). 14. O. Hirota, Y. Suematsu, and K. Kwok, IEEE J. Quantum Electron. 17, 1014 (1981). 15. J. W. Bae, H. Temkin, S. E. Swirhun, W. E. Quinn, P. Brusenbach, C. Parsons, M. Kim, and T. Uchida, Appl. Phys. Lett. 63, 1480 (1993). 16. M. Maeda and M. Yamamoto, IEEE J. Sel. Areas Commun. 8, 1257 (1990). 17. O. Hirota and Y. Suematsu, IEEE J. Quantum Electron. 15, 142 (1979). 18. R. E. Epworth, in Proceedings of the 4th European Conference and Exhibition on Optical Communication (ECOC’78) (IIC, 1978), 492.
OPTICS LETTERS / Vol. 39, No. 12 / June 15, 2014
Reflection noise reduction effect of graded-index plastic optical fiber in multimode fiber link Azusa Inoue,1,3 Rei Furukawa,2 Motoharu Matsuura,2 and Yasuhiro Koike1,4 1
Graduate School of Science and Technology, Keio University, 7-1 Shin-kawasaki, Saiwai-ku, Kawasaki 212-0032, Japan 2 Center for Frontier Science and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan 3 4
e-mail: [email protected] e-mail: [email protected]
Received March 10, 2014; revised May 14, 2014; accepted May 14, 2014; posted May 15, 2014 (Doc. ID 207950); published June 12, 2014 We experimentally demonstrate that a graded-index plastic optical fiber (GI POF) can significantly reduce reflection noise in a multimode fiber link with a vertical-cavity surface-emitting laser (VCSEL). By directly observing beams backreflected to the VCSEL, we show that the noise reduction effect is closely related to random mode coupling because of light scattering by microscopic heterogeneities in the GI POF core material. This suggests that intrinsic mode coupling can lower the self-coupling efficiency of the light backreflected to the VCSEL cavity through beam quality degradation. Using GI POFs, low-cost radio-over-fiber systems for indoor networks can be realized without optical isolators or fiber end-face polishing. © 2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.2290) Fiber materials; (160.5470) Polymers; (290.5840) Scattering, molecules; (060.5625) Radio frequency photonics. http://dx.doi.org/10.1364/OL.39.003662
Graded-index plastic optical fiber (GI POF) is a promising transmission cable for short-haul communication networks because of its flexibility and low installation cost. The bandwidths of such fibers have expanded with the development of low-dispersion materials and GI profile control techniques, allowing for the demonstration of 40 Gbps transmissions over a 100 m GI POF [1,2]. It has also been reported that the transmission characteristics of GI POFs are significantly affected by much stronger mode coupling than glass multimode fibers (MMFs) [3–5], whereas the underlying physics are not well understood. Recently, we theoretically demonstrated that the strong mode coupling is closely related to intrinsic polymer properties of the microscopic heterogeneities of the GI POF core [6,7]. This suggests that the GI POFs inherently have different optical characteristics from those of the glass MMFs. Nevertheless, the microscopic material properties have been overlooked in previous studies of the GI POF. Radio-over-fiber (RoF) technology has attracted much interest for use in indoor networks, which can be enriched by transmission of various radio-frequency signals over optical fibers [8–11]. In home area networks (HANs), CATV and wireless signal transmission over fiber can simplify conventional networks with coaxial and twisted-pair cables. In addition, an RoF-based distributed antenna system can be realized for improvement of coverage and throughput of a wireless local-area indoor network. For these short-reach applications, a practical RoF technology using a single-mode fiber is too expensive. Therefore a cost-effective RoF system has been studied using an MMF with a vertical-cavity surface-emitting laser (VCSEL). MMF optical links have suffered from complex noise problems due to reflection noise and modal noise [12], which are more pronounced for shorter links such as HANs. The reflection noise can increase system noisefloor levels through laser intensity fluctuations due to 0146-9592/14/123662-04$15.00/0
optical feedback from some discontinuities such as a fiber end-face or a photodiode (PD) [13–15], lowering the carrier-to-noise ratio (CNR) in an RoF system. This can be improved by using an optical isolator and an obliquely polished optical fiber. However, such components cannot be employed in indoor networks because of their high installation costs. Moreover, the effect of the reflection noise on the CNR becomes stronger for more multiplexed channels, which require lower signal levels for each channel for linear laser operation [16]. Here we show that the GI POF can inherently reduce the reflection noise in MMF links because of microscopic material properties of the GI POF core. This suggests that GI POFs can allow for low-cost RoF systems without optical isolators and obliquely polished fibers in indoor networks. Figure 1 shows the experimental setup for evaluation of reflection noise in MMF links. The laser is a multimode VCSEL (Optowell, PM67–F1P1N) with an oscillation wavelength of ∼670 nm. The photodetector is a silicon PIN PD (New Focus, 1601-AC) with a −3 dB bandwidth of ∼1 GHz. The collimated output beam from the VCSEL was focused on a fiber end-face using an antireflection-
Fig. 1. Experimental setup. DM and NBS are the dichroic mirror and nonpolarizing beam splitter, respectively. The inset shows a microscopic image of an incident beam on an end-face of the GI POF. © 2014 Optical Society of America
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Fig. 2. Photograph of the output beam pattern of (a) a 2 m GI POF and (b) a 2 m glass GI MMF imaged on the Si PIN PD.
coated (AR-coated) lens with an NA of 0.26 and a focal length of 11 mm, monitored with a CCD camera to confirm normal light incidence along the fiber axis (shown in the inset in Fig. 1). Under this restricted mode-launching condition, we measured the noise-floor spectra of the MMF links with an unmodulated VCSEL, which was operated with a drive current of 3.0 mA. In this study, highfrequency intensity noise [13–15] due to optical feedback from the fiber output face and the PD were mainly evaluated because refection noise can appear in the frequency bands for CATV and wireless signals. We did not evaluate the optical feedback from an MMF input face, which typically results in low-frequency intensity noise owing to the close distance between an MMF and a VCSEL in practical optical modules [17]. The modal noise was eliminated by monitoring the PD irradiated with output beams from MMFs to detect all of the output beams with the PD, as shown in Fig. 2. This allows for evaluation of the reflection noise without the modal noise effect on the detected power fluctuation, which results from partial detection of the beam that has the fluctuating speckle pattern [18]. We developed a POF core material based on a partially halogenated acrylic polymer, which inherently has microscopic heterogeneities in its higher-order structures. Using the core material in the co-extrusion process, we fabricated a GI POF with a core diameter of 200 μm and an NA of 0.3. At a wavelength of 670 nm, the GI POF has an attenuation of 154 dB∕km, which includes higher scattering loss (50 dB∕km) than a glass GI MMF. This suggests that the GI POF can have much stronger mode coupling than glass GI MMFs. Figures 2(a) and 2(b) show the output beams imaged on the PD for the 2 m GI POF and a 2 m glass GI MMF, which has a core diameter of 200 μm, an NA of 0.2, and an attenuation of ∼10 dB∕km at 670 nm, under the center-launching condition (shown in the inset in Fig. 1). The GI POF has a highly speckled beam pattern with a larger width than the glass GI MMF. This suggests that strong mode coupling results in a pronounced power transition from the launched modes to the other guided modes. In the glass GI MMF with weak mode coupling, however, the output mode power distribution is almost the same as the initial distribution for launched modes, resulting in a near-field pattern in which the power is spatially localized around the center of the core. Figure 3 shows the noise floor spectra of optical links using the GI POF and the glass GI MMF. We also evaluated a glass GI MMF with an obliquely polished output face at 8°, where the guided light is almost reflected out of the core. For the glass GI MMF, the spectrum
Fig. 3. Noise floor spectra of optical links with the 2 m GI POF and 2 m glass GI MMFs under the center-launching condition. The input faces of all the fibers are polished flat.
has peaks with an equal spacing of ∼50 MHz, which corresponds to the round-trip frequencies of the external cavities with the reflectors at the fiber output face and at the PD. This suggests that the spikes are longitudinal-mode beat notes of the external cavity of the VCSEL [13–15]. On the other hand, spikes and reflection noise were not observed in the spectrum of the GI POF. Moreover, the floor levels of the GI POF were lower than those of the obliquely polished glass MMF, which only exhibited optical feedback from the PD. This suggests that the MMF links using the GI POF are barely affected by the backreflected light from both the output face and the PD. The 2 m GI POF has a higher attenuation (∼0.3 dB) than the negligible attenuation (∼0.02 dB) of the 2 m glass GI MMF, whereas their reflectivities at the fiber/ air interfaces were comparable because of the similar refractive indices of their cores. The high attenuation can reduce reflection noise by power reduction of the backreflected light from the fiber output face and the PD. To evaluate the fiber attenuation effect on the reflection noise, an attenuator consisting of an AR-coated neutral density (ND) filter was inserted to adjust the reflected power without changing the light path (as shown in Fig. 1). Figure 4 shows the effect of a 2 dB attenuator on the noise floor spectrum of the flat-polished glass GI MMF. The ND filter has a higher absorption than
Fig. 4. Noise floor spectra for a GI POF, a glass GI MMF, and a glass GI MMF with 2 dB attenuators. All the fibers have flatpolished input and output faces.
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Fig. 5. Microscopic images of backreflected beam patterns for the center-launching condition (top) and off-axis launching condition (bottom). (a) and (d) GI POF with a flat-polished output face. (b) and (e) Glass MMF with a flat-polished output face. (c) and (f) Glass MMF with an obliquely polished output face.
the attenuation of the GI POF, reducing the backreflected light power and the detected light power. Nevertheless, the GI POF has lower floor levels than the glass MMF with the 2 dB attenuators. This indicates that the significant reduction in the reflection noise cannot be mainly attributed to the high attenuation of the GI POF compared to the glass GI MMFs. To investigate the mechanism of the significant reflection noise reduction, the backreflected beams from the fibers were microscopically observed using a laser diode with higher power than the VCSEL in the experimental setup of Fig. 1. The microscopic images were captured with different CCD sensitivities for the fibers with different reflected-light intensities. Figures 5(a)–5(c) show the reflected light from the fibers under a center-launching condition. All the images have incident beam spots at the centers of the cores on the input faces, which can be clearly observed for an obliquely polished glass GI MMF with a weak backreflected beam [Fig. 5(c)]. For the fibers with flat-polished output faces, we were able to observe backward-guided light that was reflected from the output faces. In the flat-polished glass GI MMF with weak mode coupling, the mode power distribution of the backreflected light is similar to that for the initially launched modes. Therefore the backreflected light can interfere with the incident light, as shown in Fig. 5(b). This suggests that backreflected light from the glass GI MMF can be self-coupled into the VCSEL cavity, resulting in high-frequency intensity noise, as shown in Fig. 3. On the other hand, the flat-polished GI POF has reflected light with highly speckled patterns filling the entire core, as shown in Fig. 5(a). This indicates that mode coupling results in pronounced power transitions from the launched modes to almost all the guided modes for the round-trip propagation in the GI POF, even for a length of 2 m. Therefore, the MMF link using the GI POF can have lowered self-coupling efficiencies into the VCSEL cavity because of its different reflected beam properties from the VCSEL cavity modes, resulting in the significant reduction of the reflection noise. Figures 5(d)–5(f) show the reflected light from the fibers under an off-axis launching condition with an offset of 25 μm. As shown in Fig. 5(e), the flat-polished glass GI MMF has a backward guided wave with modes of higher order than those for the center-launching condition,
whereas the reflection was not observed for the obliquely polished glass GI MMF [Fig. 5(f)]. For the flat-polished GI POF, however, the reflected light has a highly speckled beam pattern similar to that for the center-launching condition, as shown in Fig. 5(d). Therefore the mode power distribution of the backreflected guided light in the GI POF barely depends on the launching conditions. This suggests that the mode-coupling effect almost reaches the steady state even for round-trip propagation in the 2 m GI POF. This strong mode coupling is closely related to the different mode coupling mechanism from the microbending-induced diffusion processes in the glass GI MMFs [6,7]. In the GI POF, random power transitions between all the guided modes can be induced because of forward light scattering by microscopic heterogeneities in the core. Therefore, the pronounced beam quality degradation of the reflected beam is likely due to the efficient mode mixing due to the microscopic material properties of the GI POF core. This suggests that we can control the noise reduction effect by changing the microscopic heterogeneous properties of the GI POF to reduce the noise floor level in the MMF-based RoF link. In conclusion, we have experimentally demonstrated that a GI POF can significantly reduce high-frequency reflection noise in an MMF link with a VCSEL. By directly observing backreflected beams from the GI POF to the VCSEL, we showed that the noise reduction effect is closely related to random mode coupling because of light scattering by microscopic heterogeneities in the GI POF core. These results suggest that the noise floor levels of the MMF-based RoF systems can be reduced by controlling the spatial correlation characteristics of microscopic core heterogeneities. We are currently investigating noise reduction effects in GI POFs with various core materials to quantitatively correlate the noise reduction effect with the microscopic material structure. This will allow for low-cost practical RoF systems without an optical isolator and fiber end-face polishing for indoor networks. This research is supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)”. References 1. A. Polley, R. J. Gandhi, and S. E. Ralph, in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference (OFC2007), OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper OMR5. 2. S. R. Nuccio, L. Christen, X. Wu, S. Khaleghi, O. Yilmaz, A. E. Willner, and Y. Koike, in Proceedings of the 34th European Conference and Exhibition on Optical Communication (ECOC2008) (IEEE, 2008), paper We.2.A.4. 3. R. F. Shi, C. Koeppen, G. Jiang, J. Wang, and A. F. Garito, Appl. Phys. Lett. 71, 3625 (1997). 4. S. E. Golowich, W. White, W. A. Reed, and E. Knudsen, J. Lightwave Technol. 21, 111 (2003). 5. A. Polley and S. E. Ralph, IEEE Photon. Technol. Lett. 19, 1254 (2007). 6. A. Inoue, T. Sassa, K. Makino, A. Kondo, and Y. Koike, Opt. Lett. 37, 2583 (2012).
June 15, 2014 / Vol. 39, No. 12 / OPTICS LETTERS 7. A. Inoue, T. Sassa, R. Furukawa, K. Makino, A. Kondo, and Y. Koike, Opt. Express 21, 17379 (2013). 8. M. Sauer, A. Kobyakov, and J. George, J. Lightwave Technol. 25, 3301 (2007). 9. N. J. Gomes, A. Nkansah, and D. Wake, J. Lightwave Technol. 26, 2388 (2008). 10. A. M. J. Koonen and M. García Larrodé, J. Lightwave Technol. 26, 2396 (2008). 11. R. Gaudino, D. Cardenas, M. Bellec, B. Charbonnier, N. Evanno, P. Guignard, S. Meyer, A. Pizzinat, I. Mollers, and D. Jager, IEEE Commun. Mag. 48, 39 (2010). 12. K. Peterman, Laser Diode Modulation and Noise (Kluwer Academic, 1991).
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13. Y. C. Chen, Appl. Phys. Lett. 37, 587 (1980). 14. O. Hirota, Y. Suematsu, and K. Kwok, IEEE J. Quantum Electron. 17, 1014 (1981). 15. J. W. Bae, H. Temkin, S. E. Swirhun, W. E. Quinn, P. Brusenbach, C. Parsons, M. Kim, and T. Uchida, Appl. Phys. Lett. 63, 1480 (1993). 16. M. Maeda and M. Yamamoto, IEEE J. Sel. Areas Commun. 8, 1257 (1990). 17. O. Hirota and Y. Suematsu, IEEE J. Quantum Electron. 15, 142 (1979). 18. R. E. Epworth, in Proceedings of the 4th European Conference and Exhibition on Optical Communication (ECOC’78) (IIC, 1978), 492.
