Bidirectional multiband radio-over-fiber system based on polarization multiplexing and wavelength reuse Ting Su,1,2,* Jianyu Zheng,2 Zhongle Wu,1 Min Zhang,1 Xue Chen,1 and Gee-Kung Chang2 1
State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing, 100876, China 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA * [email protected]
Abstract: A polarization multiplexing technique based on phase-shiftinduced polarization modulation-to-intensity modulation (PloM-to-IM) convertor and a Mach-Zehnder modulator (MZM) is proposed to generate multi-band signals. Successful transmission of the traditional radio frequency, microwave (MW) and millimeter wave (MMW) signals is simultaneously achieved. Meanwhile, the intensity-constant optical carrier (OC) is reused for upstream 25-km transmission. ©2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.5060) Phase modulation; (060.2360) Fiber optics links and subsystems.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
P. Xia, A. V. Garcia, and S. K. Yong, 60 GHz Technology for Gbps WLAN and WPAN: From Theory to Practice (Wiley, 2011). R. C. Daniels, J. N. Murdock, T. S. Rappaport, and R. W. Heath, “60 GHz wireless: up close and personal,” IEEE Microw. Mag. 11(7), 44–50 (2010). L. Zhang, X. Hu, P. Cao, T. Wang, and Y. Su, “A bidirectional radio over fiber system with multiband-signal generation using one single-drive MZM,” Opt. Express 19(6), 5196–5201 (2011). M. Zhu, A. Yi, Y. T. Hsueh, C. Liu, J. Wang, S. C. Shin, J. Yu, and G. K. Chang, “Demonstration of 4-band millimeter-wave radio-over-fiber system for multi-service wireless access networks,” in Proceedings of OFC/NFOEC 2013, Anaheim, CA, United States, Paper OM3D.4. L. Zhang, C. Ye, X. Hu, Z. Li, S. Fan, Y. Hsueh, Q. Chang, Y. Su, and G. K. Chang, “Generation of multiband signals in a bidirectional wireless over fiber system with high scalability using heterodyne mixing technique,” IEEE Photon. Technol. Lett. 24(18), 1621–1624 (2012). J. Zheng, H. Wang, L. Wang, N. Zhu, J. Liu, and S. Wang, “Implementation of wavelength reusing upstream service based on distributed intensity conversion in ultrawideband-over-fiber system,” Opt. Lett. 38(7), 1167– 1169 (2013). T. Shao and J. Yao, “Wavelength reuse in a bidirectional UWB over fiber system,” Opt. Express 21(10), 11921– 11927 (2013). M. Morant, J. Prat, and R. Llorente, “Radio-over-fiber optical polarization-multiplexed networks for 3GPP wireless carrier-aggregated MIMO provision,” J. Lightwave Technol. 32(20), 3721–3727 (2014). J. Zheng, H. Wang, J. Fu, L. Wei, S. Pan, L. Wang, J. Liu, and N. Zhu, “Fiber-distributed ultra-wideband noise radar with steerable power spectrum and colorless base station,” Opt. Express 22(5), 4896–4907 (2014). T. Su, J. Zheng, J. Wang, M. Zhu, Z. Dong, M. Xu, M. Zhang, X. Chen, and G. K. Chang, “Multi-service wireless transport over RoF link with colorless BS using PolM-to-IM convertor,” IEEE Photon. Technol. Lett. 27(4), 403–406 (2015). J. Zheng, H. Wang, W. Li, L. Wang, T. Su, J. Liu, and N. Zhu, “Photonic-assisted microwave frequency multiplier based on nonlinear polarization rotation,” Opt. Lett. 39(6), 1366–1369 (2014). M. Lawrence, “Lithium niobate integrated optics,” Rep. Prog. Phys. 56(3), 363–429 (1993). J. Zhang, X. Yuan, M. Lin, J. Tao, Y. Zhang, M. Zhang, and X. Zhang, “Transmission of 112Gb/s PM-RZDQPSK over 960 km with adaptive polarization tracking based on power difference”, in Proceedings of ECOC, 2010, Torino, Italy, Paper P2. 09. B. Koch, R. Noé, V. Mirvoda, and D. Sandel, “1-THz bandwidth of 70-krad/s endless optical polarization control”, in proceedings of OFC/NFOEC, 2014, San Francisco, CA, United States, Paper Th2A.1. M. Yagi, S. Satomi, and S. Ryu, “Field trial of 160-Gbit/s, polarization-division multiplexed RZ-DQPSK transmission system using automatic polarization control”, in proceedings of OFC/NFOEC, 2008, San Diego, CA, United States, Paper OThT7.
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9772
1. Introduction The traditional low radio frequency (RF) wireless technologies, such as Wi-Fi (IEEE 802.11) and LTE (Long Term Evolution), have achieved much popularity for their accessibility and convenience. On the other hand, the development of innovative video-intensive multimedia services calling for the network is capable of transmitting high throughput signals. The microwave (MW) and millimeter wave (MMW), especially 60-GHz MMW radio-over-fiber (RoF) technology is embraced for its available unlicensed bandwidth and integration of both optical fiber and wireless broadband, which can decrease transmission loss and enhance mobility [1, 2]. A future RoF technique, multiband optical transmission technology, which can simultaneously deliver existing traditional wireless services, carried by MW and MMW signals, is strongly desired. It enables diverse applications operating at different RF bands in an integrated access network platform. Several recent reports on successfully generating and transmitting multiband signals in RoF systems have been demonstrated using only one MachZehnder modulator (MZM) [3–5]. However, these published results were not compatible with the existing traditional wireless services because vector wireless signal could not be transmitted in such system [3], or wavelength-dependent optical filters were used which would increase the cost of the base stations (BSs) [4, 5]. In [6–8], only ultra-wide band (UWB) signal [6, 7] or 3GPP carrier aggregated multiple-input multiple-output (MIMO) signals [8] for the downstream data were generated by polarization modulation. It remains to be a challenge on simultaneously transmitting both multiband signals and upstream signal in bidirectional RoF systems without using wavelength-dependent optical filters. In this paper, we propose and demonstrate a new architecture of 3-band RoF system that simultaneously supports traditional wireless services (e.g. Wi-Fi, and LTE), 15-GHz MW and 60-GHz MMW wireless services. A polarization multiplexing technique based on phase-shiftinduced polarization modulation-to-intensity modulation (PloM-to-IM) convertor and a MZM is used to generate 3-band signals. 0.1-Gb/s orthogonal frequency-division-multiplexing (OFDM) low-RF signal, independent 1.25-Gb/s non-return to zero (NRZ) MW and frequency-doubled MMW signal are achieved without wavelength-dependent filter in remote access unit (RAU). Meanwhile, the intensity-constant optical carrier (OC) with slight phase fluctuation is reused for upstream 25-km transmission successfully. 2. Experimental setup
Fig. 1. Experimental setup of 3-band RoF system based on polarization multiplexing and wavelength reuse. LO: local oscillator; RAU: remote access unite; FD: frequency doubler; FQ: Frequency quadrupler; PD: photo detector.
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9773
The distributed PolM-to-IM converter, which is consisted of a lithium-niobate (LiNbO3) phase modulator (PM), a polarization controller (PC) and a polarizer (PL), is the core device of the proposed 3-bands RoF system. The connector is oriented at an angle of 45° to the slowaxis of the polarization-maintaining optical fiber (PMF), to make the polarization direction of the OC1 from the first stream before PM orient at 45° to the principal axis of the LiNbO3 crystal. The traditional low RF signal and 60-GHz wireless signal transmission can be implemented by PolM-to-IM converter. Meanwhile, the polarization direction of the 15-GHz sub-band carrier which is provided by MZM can be adjusted by PC2. We assume that the optical output after laser diode (LD) is Ein (t ) = 2 E0 exp( jω0 t ) (1) where ω0 is the frequency of LD. After the 1 × 2 optical splitter, the OC is divided into two beams. For the second beam, we have ∞ π k +1 π EMZM (t ) = E0 exp[ jω0 (t )]{ J k ( β ) exp[ jk (ωm1t + )]cos( ⋅ π ) exp( j )} (2) 2 2 2 k =−∞
where ωm1 is the angular frequency of the microwave signal, and Jk is the kth order Bessel function of the first kind. Equation (2) can be simplified as EMZM (t ) ≈ E0 exp( jω0 t ){J1 ( β ) exp( jωm1t ) + J −1 ( β ) exp(− jωm1t )} (3) Therefore, the optical carrier suppression (OCS) modulation is implemented. The optical field at the output of the PM is [9–11] Ex 2 jw t EPM (t ) = = e 2 Ey 0
π V (t ) exp( j V ) π ,x π V (t ) + jϕ ) exp( j V π,y
(4)
where Ex and Ey are the orthogonal components of EPM, φ is the phase difference between the x-axis and y-axis of OC1. V(t) is the electrical signal applied to PM, Vπ,x and Vπ,y are the halfwave voltage of the PM for x and y polarization states, respectively. For LiNbO3, η = Vπ,x / Vπ,y ~1/3 [12]. At the RAU, the PL whose principal axis is aligned at α to the polarization direction of the OC1 is utilized before photo detector (PD). Note that α and φ can be controlled and adjusted by the PCs at the RAU. Assuming α = 0° and φ = π/2 by adjusting the PC4, as can be seen from Eq. (5)-(7) in [10], the low-RF signal can be received after the low frequency PD2. As φ = π by adjusting the PC3, only odd-order sidebands are presented. According to Eq. (8)-(10) in [10], the OC1 and even-order sidebands are suppressed. It can be seen the OCS modulation is implemented and the frequency-doubled signal can be generated in this case. As α = 45° by adjusting PC5, the OC1 can be re-used directly for the uplink transmission without additional optical filtering, as seen from Eq. (11) in [10]. Meanwhile, we can receive the MW signal after PD3. The experimental setup of 3-band RoF system by utilizing polarization multiplexing and wavelength reuse is illustrated in Fig. 1. A light wave from central office (CO) at 1553.8 nm is divided into two streams. For the first stream, the OC1 is sent to a PM via connectors whose polarization direction could be aligned at an angle of 45° relative to the principle axis of the LiNbO3 crystal in the modulator. The sinusoidal wave frequency which is provided by local oscillator (LO) is up-converted to 30-GHz by frequency doubler (FD). Data1 and data3, 1.25-Gb/s pseudorandom binary sequence (PRBS) data, are mixed with 30-GHz and 15-GHz sinusoidal wave, respectively. Data2, generated by an arbitrary waveform generator (AWG) at 2.6-GSa/s sample rate, is a 0.1-Gb/s QPSK-OFDM signal carried at 0.3 GHz. The frequency up-converted data1 and original data2 are coupled by a 40-GHz electrical combiner and fed to #233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9774
the PM. The OC2 from the second stream is injected into MZM via a PC1. By biasing the MZM near Vπ, the OCS modulation is realized. The polarization direction of the 15-GHz subcarriers can be adjusted by the PC2. The two optical streams are combined together and transmitted over a 25-km standard single-mode-fiber (SSMF) to a RAU. At the RAU, the modulated light is split into three parts by a 1 × 3 optical splitter. For the first part, PC3 is applied to adjust α and φ of the light-wave to 0° and π. By adjusting PC2 to 90° relative to that of the OC1, the data1 working at 60-GHz is obtained after PD1 (3-dB bandwidth: 60GHz) because the PolM-to-IM converter works at OCS region. After transmitting through 1.2-meter air via the antennas, data 1 is down-converted to baseband for offline processing. For the second part, as α and φ are set at 0°and π/2 by adjusting PC4, the data2 is received by PD2 (3-dB bandwidth: 10-GHz) due to the fact that the PolM-to-IM convertor works in the state of linear region. The OFDM signal is then sent to a real-time digital storage oscilloscope for offline demodulation. The data1 and data3 are not recovered in this part because the bandwidth limitation of PD2. For the third part, we set α = 45° by adjusting PC5. The data3 which is carried by 15-GHz is obtained by PD3 (3-dB bandwidth: 60-GHz) and downconverted to baseband. Meanwhile, the third beam with insignificant phase fluctuation is intensity modulated by MZM with 1.25-Gb/s PRBS data 4. Finally, the upstream signal is transmitted back to CO through the 25-km SMF and tested by a bit error rate tester. 3. Experimental results and discussions
Fig. 2. Optical spectrum (a) was recorded at point e as α = 0° and φ = π; (b) was recorded at point f as α = 0° and φ = π/2; (c) was recorded at point g as α = 45°, respectively.
Figure 2 displays the optical spectra measured at point e, f, and g in RAU (See Fig. 1). The optical spectrum illustrated in Fig. 2(a) shows that the OCS modulation was achieved by PolM-to-IM converter and the optical carrier suppression ratio (OCSR) of the 60-GHz RF signal is around 10 dB. The 15-GHz subcarriers cannot be observed due to their polarization direction after PC2 is orthogonal to that of the OC1 from the first stream. In Fig. 2(b), the baseband and 30-GHz sub-bands are obtained as the distributed PolM-to-IM converter working at linear region, additionally, the 15-GHz sub-bands are appeared because their polarization direction is not orthogonal to that of the center carrier after adjusting PC3. The optical spectrum displaying in Fig. 2(c) indicates that the conspicuous modulated sub-bands of the first stream are disappeared with α = 45° owing to the fact that the modulation at Y-aixs of the PM is so small. Meanwhile, we can see the 15-GHz sub-bands because the angle between the sub-bands polarization direction and Y-aixs is not 90°. Figure 3(a) displays the BER as function of received optical power for data1 signal in back-to-back (BTB) and 25-km fiber transmission. The power penalty caused by SFM and wireless transmission is around 1.0 dB. The error vector magnitude (EVM) for the data2 signals versus received optical power at BTB or after 25-km fiber is illustrated in Fig. 3(b). As we can observe, the power penalty due to fiber transmission is about 1.1 dB as EVM is 0.2. The insets in Fig. 3(b) show the clear constellations of the demodulated data2 whose EVMs are 0.04 and 0.26. In Fig. 3(c), the BERs versus received optical power for data3 are measured. Optical power of around −14 dBm at BER = 10−9 is received after 25-km fiber transmission. The eye diagrams for data1, data3 and data4 with other channels on are shown in the Figs. 3(a)-3(d). We also transmit only upstream data4 without other channels as the
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9775
black and red curves in Fig. 3(d) indicate. A power penalty of 0.5 dB and 0.3 dB at BER = 10−9 can be observed due to the crosstalk from the interchannel in both BTB and 25-km fiber transmission. The slight crosstalk results from the phase fluctuation of downstream signals, especially the data2 with high peak-to-average power ratio. Around 1.4 dB power penalty (the distance between the red line and the yellow line as BER = 10−9) is shown due to the crosstalk results from the chromatic dispersion induced PM-IM conversion and the interference from data3 channel.
Fig. 3. (a) BER vs. received optical power for Data 1 NRZ signal (operating at 60-GHz) over both fiber and air transmission. (b) EVM vs. received optical power for Data 2 QPSK-OFDM signal (operating at 0.3-GHz) with and without 25-km SSMF transmission. (c) BER vs. received power for Data3 NRZ signal (operating at 15-GHz) at BTB/25-km fiber. (d) BER vs. received power for upstream signal Data4 at BTB/25-km fiber with and without DS signals.
The major problem with the practical use of polarization multiplexing over fiber-optic transmission systems are the drifts in polarization state that accumulate continuously over time results from physical changes in the fiber. However, these drifts can be ignored in our short-distance system because it will not result in rapid and erratic rotation of the polarized light’s Jones vector over the entire Poincaré sphere. It is worth mentioning that so many reports about automatic polarization trackers have been proposed and demonstrated for the high-speed and long-distance system [13-15]. Therefore, with the automatic polarization technology matures, it is becoming more and more popular in the future. 4. Conclusion We have proposed and successfully demonstrated a new architecture of 3-band RoF system with wavelength reuse to deliver multi-services for different end users. The traditional RF signal, 15GHz MW signal and 60-GHz MMW signal were attained by adjusting PCs. Meanwhile, the intensity-constant OC with insignificant phase fluctuation was re-modulated for the upstream transmission successfully. In addition, the PC we used in BS could be replaced by a commercial automatic polarization tracker to further improve the stability of the device. Acknowledgments This study is supported by NSFC Project No. 61372119, 863 Program No. 2011AA01A104, Doctoral Scientific Fund Project of the Ministry of Education of China (No. 20120005110010).
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9776
State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing, 100876, China 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA * [email protected]
Abstract: A polarization multiplexing technique based on phase-shiftinduced polarization modulation-to-intensity modulation (PloM-to-IM) convertor and a Mach-Zehnder modulator (MZM) is proposed to generate multi-band signals. Successful transmission of the traditional radio frequency, microwave (MW) and millimeter wave (MMW) signals is simultaneously achieved. Meanwhile, the intensity-constant optical carrier (OC) is reused for upstream 25-km transmission. ©2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.5060) Phase modulation; (060.2360) Fiber optics links and subsystems.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
P. Xia, A. V. Garcia, and S. K. Yong, 60 GHz Technology for Gbps WLAN and WPAN: From Theory to Practice (Wiley, 2011). R. C. Daniels, J. N. Murdock, T. S. Rappaport, and R. W. Heath, “60 GHz wireless: up close and personal,” IEEE Microw. Mag. 11(7), 44–50 (2010). L. Zhang, X. Hu, P. Cao, T. Wang, and Y. Su, “A bidirectional radio over fiber system with multiband-signal generation using one single-drive MZM,” Opt. Express 19(6), 5196–5201 (2011). M. Zhu, A. Yi, Y. T. Hsueh, C. Liu, J. Wang, S. C. Shin, J. Yu, and G. K. Chang, “Demonstration of 4-band millimeter-wave radio-over-fiber system for multi-service wireless access networks,” in Proceedings of OFC/NFOEC 2013, Anaheim, CA, United States, Paper OM3D.4. L. Zhang, C. Ye, X. Hu, Z. Li, S. Fan, Y. Hsueh, Q. Chang, Y. Su, and G. K. Chang, “Generation of multiband signals in a bidirectional wireless over fiber system with high scalability using heterodyne mixing technique,” IEEE Photon. Technol. Lett. 24(18), 1621–1624 (2012). J. Zheng, H. Wang, L. Wang, N. Zhu, J. Liu, and S. Wang, “Implementation of wavelength reusing upstream service based on distributed intensity conversion in ultrawideband-over-fiber system,” Opt. Lett. 38(7), 1167– 1169 (2013). T. Shao and J. Yao, “Wavelength reuse in a bidirectional UWB over fiber system,” Opt. Express 21(10), 11921– 11927 (2013). M. Morant, J. Prat, and R. Llorente, “Radio-over-fiber optical polarization-multiplexed networks for 3GPP wireless carrier-aggregated MIMO provision,” J. Lightwave Technol. 32(20), 3721–3727 (2014). J. Zheng, H. Wang, J. Fu, L. Wei, S. Pan, L. Wang, J. Liu, and N. Zhu, “Fiber-distributed ultra-wideband noise radar with steerable power spectrum and colorless base station,” Opt. Express 22(5), 4896–4907 (2014). T. Su, J. Zheng, J. Wang, M. Zhu, Z. Dong, M. Xu, M. Zhang, X. Chen, and G. K. Chang, “Multi-service wireless transport over RoF link with colorless BS using PolM-to-IM convertor,” IEEE Photon. Technol. Lett. 27(4), 403–406 (2015). J. Zheng, H. Wang, W. Li, L. Wang, T. Su, J. Liu, and N. Zhu, “Photonic-assisted microwave frequency multiplier based on nonlinear polarization rotation,” Opt. Lett. 39(6), 1366–1369 (2014). M. Lawrence, “Lithium niobate integrated optics,” Rep. Prog. Phys. 56(3), 363–429 (1993). J. Zhang, X. Yuan, M. Lin, J. Tao, Y. Zhang, M. Zhang, and X. Zhang, “Transmission of 112Gb/s PM-RZDQPSK over 960 km with adaptive polarization tracking based on power difference”, in Proceedings of ECOC, 2010, Torino, Italy, Paper P2. 09. B. Koch, R. Noé, V. Mirvoda, and D. Sandel, “1-THz bandwidth of 70-krad/s endless optical polarization control”, in proceedings of OFC/NFOEC, 2014, San Francisco, CA, United States, Paper Th2A.1. M. Yagi, S. Satomi, and S. Ryu, “Field trial of 160-Gbit/s, polarization-division multiplexed RZ-DQPSK transmission system using automatic polarization control”, in proceedings of OFC/NFOEC, 2008, San Diego, CA, United States, Paper OThT7.
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9772
1. Introduction The traditional low radio frequency (RF) wireless technologies, such as Wi-Fi (IEEE 802.11) and LTE (Long Term Evolution), have achieved much popularity for their accessibility and convenience. On the other hand, the development of innovative video-intensive multimedia services calling for the network is capable of transmitting high throughput signals. The microwave (MW) and millimeter wave (MMW), especially 60-GHz MMW radio-over-fiber (RoF) technology is embraced for its available unlicensed bandwidth and integration of both optical fiber and wireless broadband, which can decrease transmission loss and enhance mobility [1, 2]. A future RoF technique, multiband optical transmission technology, which can simultaneously deliver existing traditional wireless services, carried by MW and MMW signals, is strongly desired. It enables diverse applications operating at different RF bands in an integrated access network platform. Several recent reports on successfully generating and transmitting multiband signals in RoF systems have been demonstrated using only one MachZehnder modulator (MZM) [3–5]. However, these published results were not compatible with the existing traditional wireless services because vector wireless signal could not be transmitted in such system [3], or wavelength-dependent optical filters were used which would increase the cost of the base stations (BSs) [4, 5]. In [6–8], only ultra-wide band (UWB) signal [6, 7] or 3GPP carrier aggregated multiple-input multiple-output (MIMO) signals [8] for the downstream data were generated by polarization modulation. It remains to be a challenge on simultaneously transmitting both multiband signals and upstream signal in bidirectional RoF systems without using wavelength-dependent optical filters. In this paper, we propose and demonstrate a new architecture of 3-band RoF system that simultaneously supports traditional wireless services (e.g. Wi-Fi, and LTE), 15-GHz MW and 60-GHz MMW wireless services. A polarization multiplexing technique based on phase-shiftinduced polarization modulation-to-intensity modulation (PloM-to-IM) convertor and a MZM is used to generate 3-band signals. 0.1-Gb/s orthogonal frequency-division-multiplexing (OFDM) low-RF signal, independent 1.25-Gb/s non-return to zero (NRZ) MW and frequency-doubled MMW signal are achieved without wavelength-dependent filter in remote access unit (RAU). Meanwhile, the intensity-constant optical carrier (OC) with slight phase fluctuation is reused for upstream 25-km transmission successfully. 2. Experimental setup
Fig. 1. Experimental setup of 3-band RoF system based on polarization multiplexing and wavelength reuse. LO: local oscillator; RAU: remote access unite; FD: frequency doubler; FQ: Frequency quadrupler; PD: photo detector.
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9773
The distributed PolM-to-IM converter, which is consisted of a lithium-niobate (LiNbO3) phase modulator (PM), a polarization controller (PC) and a polarizer (PL), is the core device of the proposed 3-bands RoF system. The connector is oriented at an angle of 45° to the slowaxis of the polarization-maintaining optical fiber (PMF), to make the polarization direction of the OC1 from the first stream before PM orient at 45° to the principal axis of the LiNbO3 crystal. The traditional low RF signal and 60-GHz wireless signal transmission can be implemented by PolM-to-IM converter. Meanwhile, the polarization direction of the 15-GHz sub-band carrier which is provided by MZM can be adjusted by PC2. We assume that the optical output after laser diode (LD) is Ein (t ) = 2 E0 exp( jω0 t ) (1) where ω0 is the frequency of LD. After the 1 × 2 optical splitter, the OC is divided into two beams. For the second beam, we have ∞ π k +1 π EMZM (t ) = E0 exp[ jω0 (t )]{ J k ( β ) exp[ jk (ωm1t + )]cos( ⋅ π ) exp( j )} (2) 2 2 2 k =−∞
where ωm1 is the angular frequency of the microwave signal, and Jk is the kth order Bessel function of the first kind. Equation (2) can be simplified as EMZM (t ) ≈ E0 exp( jω0 t ){J1 ( β ) exp( jωm1t ) + J −1 ( β ) exp(− jωm1t )} (3) Therefore, the optical carrier suppression (OCS) modulation is implemented. The optical field at the output of the PM is [9–11] Ex 2 jw t EPM (t ) = = e 2 Ey 0
π V (t ) exp( j V ) π ,x π V (t ) + jϕ ) exp( j V π,y
(4)
where Ex and Ey are the orthogonal components of EPM, φ is the phase difference between the x-axis and y-axis of OC1. V(t) is the electrical signal applied to PM, Vπ,x and Vπ,y are the halfwave voltage of the PM for x and y polarization states, respectively. For LiNbO3, η = Vπ,x / Vπ,y ~1/3 [12]. At the RAU, the PL whose principal axis is aligned at α to the polarization direction of the OC1 is utilized before photo detector (PD). Note that α and φ can be controlled and adjusted by the PCs at the RAU. Assuming α = 0° and φ = π/2 by adjusting the PC4, as can be seen from Eq. (5)-(7) in [10], the low-RF signal can be received after the low frequency PD2. As φ = π by adjusting the PC3, only odd-order sidebands are presented. According to Eq. (8)-(10) in [10], the OC1 and even-order sidebands are suppressed. It can be seen the OCS modulation is implemented and the frequency-doubled signal can be generated in this case. As α = 45° by adjusting PC5, the OC1 can be re-used directly for the uplink transmission without additional optical filtering, as seen from Eq. (11) in [10]. Meanwhile, we can receive the MW signal after PD3. The experimental setup of 3-band RoF system by utilizing polarization multiplexing and wavelength reuse is illustrated in Fig. 1. A light wave from central office (CO) at 1553.8 nm is divided into two streams. For the first stream, the OC1 is sent to a PM via connectors whose polarization direction could be aligned at an angle of 45° relative to the principle axis of the LiNbO3 crystal in the modulator. The sinusoidal wave frequency which is provided by local oscillator (LO) is up-converted to 30-GHz by frequency doubler (FD). Data1 and data3, 1.25-Gb/s pseudorandom binary sequence (PRBS) data, are mixed with 30-GHz and 15-GHz sinusoidal wave, respectively. Data2, generated by an arbitrary waveform generator (AWG) at 2.6-GSa/s sample rate, is a 0.1-Gb/s QPSK-OFDM signal carried at 0.3 GHz. The frequency up-converted data1 and original data2 are coupled by a 40-GHz electrical combiner and fed to #233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9774
the PM. The OC2 from the second stream is injected into MZM via a PC1. By biasing the MZM near Vπ, the OCS modulation is realized. The polarization direction of the 15-GHz subcarriers can be adjusted by the PC2. The two optical streams are combined together and transmitted over a 25-km standard single-mode-fiber (SSMF) to a RAU. At the RAU, the modulated light is split into three parts by a 1 × 3 optical splitter. For the first part, PC3 is applied to adjust α and φ of the light-wave to 0° and π. By adjusting PC2 to 90° relative to that of the OC1, the data1 working at 60-GHz is obtained after PD1 (3-dB bandwidth: 60GHz) because the PolM-to-IM converter works at OCS region. After transmitting through 1.2-meter air via the antennas, data 1 is down-converted to baseband for offline processing. For the second part, as α and φ are set at 0°and π/2 by adjusting PC4, the data2 is received by PD2 (3-dB bandwidth: 10-GHz) due to the fact that the PolM-to-IM convertor works in the state of linear region. The OFDM signal is then sent to a real-time digital storage oscilloscope for offline demodulation. The data1 and data3 are not recovered in this part because the bandwidth limitation of PD2. For the third part, we set α = 45° by adjusting PC5. The data3 which is carried by 15-GHz is obtained by PD3 (3-dB bandwidth: 60-GHz) and downconverted to baseband. Meanwhile, the third beam with insignificant phase fluctuation is intensity modulated by MZM with 1.25-Gb/s PRBS data 4. Finally, the upstream signal is transmitted back to CO through the 25-km SMF and tested by a bit error rate tester. 3. Experimental results and discussions
Fig. 2. Optical spectrum (a) was recorded at point e as α = 0° and φ = π; (b) was recorded at point f as α = 0° and φ = π/2; (c) was recorded at point g as α = 45°, respectively.
Figure 2 displays the optical spectra measured at point e, f, and g in RAU (See Fig. 1). The optical spectrum illustrated in Fig. 2(a) shows that the OCS modulation was achieved by PolM-to-IM converter and the optical carrier suppression ratio (OCSR) of the 60-GHz RF signal is around 10 dB. The 15-GHz subcarriers cannot be observed due to their polarization direction after PC2 is orthogonal to that of the OC1 from the first stream. In Fig. 2(b), the baseband and 30-GHz sub-bands are obtained as the distributed PolM-to-IM converter working at linear region, additionally, the 15-GHz sub-bands are appeared because their polarization direction is not orthogonal to that of the center carrier after adjusting PC3. The optical spectrum displaying in Fig. 2(c) indicates that the conspicuous modulated sub-bands of the first stream are disappeared with α = 45° owing to the fact that the modulation at Y-aixs of the PM is so small. Meanwhile, we can see the 15-GHz sub-bands because the angle between the sub-bands polarization direction and Y-aixs is not 90°. Figure 3(a) displays the BER as function of received optical power for data1 signal in back-to-back (BTB) and 25-km fiber transmission. The power penalty caused by SFM and wireless transmission is around 1.0 dB. The error vector magnitude (EVM) for the data2 signals versus received optical power at BTB or after 25-km fiber is illustrated in Fig. 3(b). As we can observe, the power penalty due to fiber transmission is about 1.1 dB as EVM is 0.2. The insets in Fig. 3(b) show the clear constellations of the demodulated data2 whose EVMs are 0.04 and 0.26. In Fig. 3(c), the BERs versus received optical power for data3 are measured. Optical power of around −14 dBm at BER = 10−9 is received after 25-km fiber transmission. The eye diagrams for data1, data3 and data4 with other channels on are shown in the Figs. 3(a)-3(d). We also transmit only upstream data4 without other channels as the
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9775
black and red curves in Fig. 3(d) indicate. A power penalty of 0.5 dB and 0.3 dB at BER = 10−9 can be observed due to the crosstalk from the interchannel in both BTB and 25-km fiber transmission. The slight crosstalk results from the phase fluctuation of downstream signals, especially the data2 with high peak-to-average power ratio. Around 1.4 dB power penalty (the distance between the red line and the yellow line as BER = 10−9) is shown due to the crosstalk results from the chromatic dispersion induced PM-IM conversion and the interference from data3 channel.
Fig. 3. (a) BER vs. received optical power for Data 1 NRZ signal (operating at 60-GHz) over both fiber and air transmission. (b) EVM vs. received optical power for Data 2 QPSK-OFDM signal (operating at 0.3-GHz) with and without 25-km SSMF transmission. (c) BER vs. received power for Data3 NRZ signal (operating at 15-GHz) at BTB/25-km fiber. (d) BER vs. received power for upstream signal Data4 at BTB/25-km fiber with and without DS signals.
The major problem with the practical use of polarization multiplexing over fiber-optic transmission systems are the drifts in polarization state that accumulate continuously over time results from physical changes in the fiber. However, these drifts can be ignored in our short-distance system because it will not result in rapid and erratic rotation of the polarized light’s Jones vector over the entire Poincaré sphere. It is worth mentioning that so many reports about automatic polarization trackers have been proposed and demonstrated for the high-speed and long-distance system [13-15]. Therefore, with the automatic polarization technology matures, it is becoming more and more popular in the future. 4. Conclusion We have proposed and successfully demonstrated a new architecture of 3-band RoF system with wavelength reuse to deliver multi-services for different end users. The traditional RF signal, 15GHz MW signal and 60-GHz MMW signal were attained by adjusting PCs. Meanwhile, the intensity-constant OC with insignificant phase fluctuation was re-modulated for the upstream transmission successfully. In addition, the PC we used in BS could be replaced by a commercial automatic polarization tracker to further improve the stability of the device. Acknowledgments This study is supported by NSFC Project No. 61372119, 863 Program No. 2011AA01A104, Doctoral Scientific Fund Project of the Ministry of Education of China (No. 20120005110010).
#233051 - $15.00 USD © 2015 OSA
Received 4 Feb 2015; revised 27 Mar 2015; accepted 27 Mar 2015; published 8 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009772 | OPTICS EXPRESS 9776
