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Experimental demonstration of polarization multiplexing for simultaneously providing broadband wireless and wired access
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Optics Communications 281 (2008) 2806–2810 www.elsevier.com/locate/optcom
Experimental demonstration of polarization multiplexing for simultaneously providing broadband wireless and wired access Jianqiang Li a,*, Kun Xu a, Yang Jing Wen b, Songnian Fu c, Ming Tang c, Ping Shum c, Jian Wu a, Jintong Lin a a
Key Laboratory of Optical Communication and Lightwave Technologies, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, China b Institute for InfoComm, Singapore 119613, Singapore c Lightwave Technology Group, Network Technology Research Centre, Nanyang Technological University, Singapore 637553, Singapore Received 20 November 2007; received in revised form 17 January 2008; accepted 17 January 2008
Abstract A fiber-based wavelength-division-multiplexing (WDM) network utilizing polarization multiplexing (PolMUX) is proposed to simultaneously provide broadband wireless and wired services. In such a dual-service access network, the wireless and wired services are separately delivered in two orthogonal states of polarization with well independence in a single WDM channel. The impact of several polarization-dependent interferences becomes insignificant due to the relatively short transmission distance in access networks. The feasibility of PolMUX is experimentally demonstrated with a power penalty at BER = 10–9 of about 0.5 dB and 1 dB for 2.5 Gb/s wired and wireless downstream services, respectively. The proposed system is compatible with the current reported techniques in either WDM passive optical networks (WDM-PON) or radio-over-fiber (ROF) systems. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Access networks; Polarization multiplexing; Passive optical networks (PON); Radio-over-fiber (ROF)
1. Introduction It is essential and necessary to upgrade or redesign access networks for satisfying the increasing demand of broadband wireless and wired access. Wavelength-division-multiplexing passive optical networks (WDM-PON) have been widely investigated as a promising solution to the realization of fiber-to-the-home (FTTH), where the huge bandwidth of optical fiber is explored to achieve up to multi-gigahertz wired access [1,2]. Taking into account the requirements on flexibility and mobility, radio systems operating at microwave frequency play a crucial role in wireless access networks. However, the conventional wire*
Corresponding author. E-mail address: [email protected] (J. Li).
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.01.027
less communication systems are confronted with a series of challenges, such as the high loss of coaxial cable, frequency congestion, and limited capacity. Recently, millimeter wave (mm-wave) radio-over-fiber (ROF) systems and opticalwireless networks emerge to integrate wireless communications with beneficial fiber-optic technology, which makes it feasible to cost-effectively support both wireless and wired services over the same fiber-optic infrastructure [3,4]. 2. The utilization of PolMUX in fiber-based access networks In the most previous works, WDM-PON and ROF systems were studied separately for supporting either wired or wireless services [1,2,5–8]. Many techniques have been proposed to implement low-cost, flexible, and functioncentralized full-duplex operations for either WDM-PON
J. Li et al. / Optics Communications 281 (2008) 2806–2810
or ROF systems, such as colorless optical network units (ONUs) in WDM-PON [1,2], architecture design [5,6], all-optical frequency up-conversion [7], and wavelength reuse for ROF systems [8]. Although the previous demonstrations have testified that ROF systems are capable of sharing the fiber-based infrastructure with WDM-PON, it is still a challenge to simultaneously guarantee broadband dual-service operations for both wireless and wired access in a single WDM channel. A novel ROF architecture was reported to provide broadband dual-service in one WDM channel [4]. However, this scheme is only suitable for point-to-multi-points applications due to the dependence between the delivered wireless and wired services. Two other methods were demonstrated to simultaneously deliver multiple independent wireless and wired services in one WDM channel [9,10]. In the two schemes, the optical
2807
spectrum is extensively occupied by different services in a single WDM channel. The separation of these services requires suitable narrow-band bandpass filters either in the optical or electrical domain. These filters are difficult to fabricate and usually expensive. Furthermore, only the downstream services are considered in [9,10]. When taking into account the bidirectional full-duplex operation, the extensively-occupied spectrum has no more free space to accommodate upstream services. Therefore, the technique aimed at further capability enhancement and easier service-separation is desired in a single wavelength. On the other hand, polarization multiplexing (PolMUX) can double the capacity in one WDM channel, and has been successfully demonstrated in the optical transmission experiments by transmitting independent signals in two orthogonal states of polarization (SOPs) [11]. Therefore,
Fig. 1. Proposed scenario for fiber-based access networks utilizing PolMUX.
2808
J. Li et al. / Optics Communications 281 (2008) 2806–2810
the introduction of PolMUX will be potential and favorable in dual-service fiber-based access networks due to high spectrum-efficiency and convenience to separate different services. Furthermore, the impact of several polarizationdependent interferences becomes insignificant due to the relatively short transmission distance in access networks, such as polarization-mode dispersion (PMD) and polarization-dependent loss (PDL), etc. Although a feed-back setup typically comprising an automatic polarization controller (APC) is required at the receiver for practical applications, the overall system is expected to be ultimately costeffective considering the doubly enhanced capability. In this paper, we proposed and experimentally demonstrated a hybrid fiber-based WDM system by introducing PolMUX to provide broadband wireless and wired services. As shown in Fig. 1, a central station (CS) and several base stations (BSs) are interconnected by a fiber-based WDM access network. In the N parallel downlink transmitters (Tx) located at CS, there are N laser sources at k1, k2,. . .,kn to generate N optical carriers for N BSs, which means N WDM channels. Each original optical carrier is split into two parts by an optical coupler and then loaded with the base-band data for wireless and wired services, respectively. As can be seen there are two outputs for each downlink Tx, one of which corresponds to the wired service and the other corresponds to the wireless service. These output signals are combined into two branches by WDM for wireless and wired services, respectively. For the upper branch aimed at wireless service, photonic frequency upconversion techniques can be suitably fit into this system with good compatibility. Before PolMUX, the optical powers of the two branches necessarily require a tradeoff, which is important to obtain a balanced transmission performance for both services. The optical signal after PolMUX will possess two kind of independent information in one WDM channel, which means a dramatic capability improvement in one wavelength. The N WDM channels are routed out of the access network and then selectively fed to the target BSs by a WDM at remote node (RN). At each BS, the separation of wireless and wired services is performed by demultiplexing the two orthogonal SOPs. For wireless service, the final RF signal will be sent to the antenna after photodetection by a high-speed PIN. For wired service, the demultiplexed optical signal is detected by a receiver and then fed to the concerned fixed terminals. Referring to the uplink, the wireless and wired services, respectively from the antennae and fixed terminals can also be assembled in a WDM channel by PolMUX at BS. After WDM at RN, the upstream signals are transported to the CS over the fiber-based access network and received at CS. Besides the traditional function in wireless systems, each BS acts another role similar to an ONU in WDM-PON for wired access. Therefore, the system using PolMUX serves as a shared delivery platform for wireless and wired services in two isolated polarization channels. The isolation determines the well compatibility with the current techniques which have been separately investigated
in ROF systems and WDM-PON. The effective combination of these reported techniques in WDM-PON and ROF systems will lead to establishment of a cost-effective and function-centralized multi-service fiber-optic access network. The experiment was carried out only for downlink to verify the proposed scheme. The technique PolMUX can also be employed in uplink. 3. Experiment demonstration of PolMUX in fiber-based access networks The experiment setup is shown in Fig. 2, where two WDM downlink channels were demonstrated. Due to our hardware constraint, the RF carrier frequency is assumed at 20 GHz band for wireless access. The RF operation band can be easily extended to higher frequency. Two tunable laser sources (TLSs) followed by two polarization controllers (PCs) were used to generate a pair of optical carriers at 1556.55 and 1557.35 nm, respectively. After a WDM with 0.8 nm spacing, the two optical carriers were both modulated in a LiNbO3 Mach–Zehnder modulator (MZM) by a 2.5 Gb/s pseudo-random bit sequence (PRBS) electrical signal with a word length of 223–1 from a pulse pattern generator (PPG). Note that the same electrical signal was loaded to emulate the base-band data of both wireless and wired services for simplicity. The output of the MZM was split into two parts by a 3 dB coupler. One part was injected into another MZM biased at Vp and driven by a 10 GHz electrical clock to realize all-optical up-conversion via optical carrier suppression (OCS) [7]. As shown in the inset of Fig. 2, the carrier suppression ratio is larger than 25 dB. The optical power of the other part was adjusted by a variable attenuator to ensure that the optical powers of the two parts were identical before PolMUX. This action will avoid quality imbalance between different services after their separation at the receiver end. Two PCs were inserted before the polarization beam splitter (PBS) to allow independently aligning both SOPs. Then, the optical signal comprising two WDM channels with PolMUX was amplified by an erbium-doped fiber amplifier (EDFA) to obtain a total power of 9 dBm before transmission over a certain length of standard single-mode fiber (SSMF) with a dispersion of 17 ps/nm/km. At RN, another WDM with 0.8 nm spacing was used to separate the two WDM channels and meanwhile to suppress the amplified spontaneous emission (ASE) noise. The optical signal in each WDM channel was fed to its target BS and then polarization-demultiplexed using a PBS after adjusting the PC to eliminate the presence of unwanted signal in a certain polarization. The insets at BSs in Fig. 2 correspond to the measured spectra after a 25 km fiber transmission. As can be seen the polarization-demultiplexed wireless signal also exhibits a 20 dB carrier suppression ratio. The optical signal for wired service was directly detected by a 12.5 Gb/s PIN photodiode (PD) and finally sent to the bit-error-rate tester (BERT). For wireless service, the detected RF signal by a PIN PD with a 3 dB bandwidth
J. Li et al. / Optics Communications 281 (2008) 2806–2810
2809
Fig. 2. Experiment setup and the measured spectra.
of 45 GHz was amplified by an electrical amplifier (EA) centered 20 GHz. The electrical spectrum of the detected RF signal was also inserted in Fig. 2. A 20 GHz local oscillator (LO) signal by frequency-doubling from 10 GHz was adjusted by a phase shifter and then fed into a 26 GHz electrical mixer to down-convert the detected RF signal. After filtering by a 6 GHz electrical low-pass filter (LPF), the base-band electrical signal after down-conversion was sent to the BERT. The BER measurement results are summarized in Fig. 3. The two WDM channels for different BSs (labeled CH1 and CH2 in Fig. 3) show similar transmission performance in each case for both wired and wireless services, meaning that WDM and PolMUX can be used at the same time without conflict. By comparing the cases in which only
one separate service are delivered and both services are delivered by PolMUX, the utilization of PolMUX technique will lead to a power penalty of about 0.5 dB and 1 dB at a BER of 10–9 for wired and wireless services, respectively. This result indicates that the utilization of PolMUX makes insignificant mutual impact on the delivery of both wireless and wired services, which exhibits the well feasibility of PolMUX in dual-service fiber-based WDM access networks. Please note that the above experimental results are based on the precondition that the two services have been separated after polarization control. Thus the successful implementation of the proposed scheme requires more practical automatic polarization controllers. Considering the fiber transmission employing PolMUX, the power penalty caused by fiber transmission is negligible for wired service, which can be observed from the left cluster of BER curves in Fig. 3. The right cluster of BER curves in Fig. 3 shows that the power penalty keeps within 1 dB for wireless service even after 25 km fiber transmission by employing PolMUX, due to the well tolerance of OCS modulation to fiber dispersion [7]. 4. Conclusion
Fig. 3. Measured BER versus received power.
We have proposed a novel hybrid system for fiber-based access networks to simultaneously provide wired and wireless services by PolMUX. Although the PolMUX technique has been employed, the impact of several polarization-dependent interferences becomes insignificant due to the relatively short transmission distance in access networks, rather than in long-haul transmission systems. This scheme has unique advantages in terms of large capacity in a single wavelength, independent dual-service opera-
2810
J. Li et al. / Optics Communications 281 (2008) 2806–2810
tion and compatibility with the current techniques in either WDM-PON or ROF systems. On the other hand, the main weakness of the propose system lies in the relatively complex polarization control during multiplexing and demultiplexing, which necessitates more practical automatic polarization controllers. The experiment results show a power penalty of 0.5 dB and 1 dB at a BER of 10–9 for 2.5 Gb/s wired and wireless services, respectively after PolMUX, which indicates that PolMUX is feasible in fiberbased access networks. Acknowledgements This work was partially supported by the National 863 Program of China (2007AA01Z264 and 2006AA01Z256), the National Natural Science Foundation of China (60702006 and 60736002), the New Century Excellent Talent Project in Ministry of Education of China (NCET-060093), and the 111 Project (B07005). References [1] K. Iwatsuki, J. Kani, H. Suzuki, M. Fujiwara, Access and metro networks based on WDM technologies, J. Lightwave Technol. 22 (11) (2004) 2623. [2] W. Lee, M.Y. Park, S.H. Cho, J. Lee, C. Kim, G. Jeong, B.W. Kim, Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 17 (11) (2005) 2460.
[3] D. Wake, M. Webster, G. Wimpenny, K. Beacham, L. Crawford, Radio over fiber for mobile communications, in: IEEE Int. Topical Meeting Microwave Photon, (MWP), October 4–6, 2004, p. 157. [4] G.-K. Chang, J. Yu, Z. Jia, J. Yu, Novel optical-wireless access network architecture for simultaneously providing broadband wireless and wired services, in: Proc. OFC 2006, Anaheim, CA, Paper OFM1. [5] G.H. Smith, D. Novak, C. Lim, A millimeter-wave full-duplex fiberradio star-tree architecture incorporating WDM and SCM, IEEE Photon. Technol. Lett. 10 (11) (1998) 1650. [6] X. Zhang, B. Liu, J. Yao, K. Wu, R. Kashyap, A novel millimeterwave-band radio-over-fiber system with dense wavelength-division multiplexing bus architecture, IEEE Trans. Microwave Theory Technol. 54 (2) (2006) 929. [7] J. Yu, Z. Jia, L. Yi, Y. Su, G.-K. Chang, T. Wang, Optical millimeterwave generation or up-conversion using external modulators, IEEE Photon. Technol. Lett. 18 (1) (2006) 265. [8] A. Nirmalathas, D. Novak, C. Lim, R.B. Waterhouse, Wavelength reuse in the WDM optical interface of a millimeter-wave fiber-wireless antenna base station, IEEE Trans. Microwave Theory Technol. 49 (10) (2001) 2006. [9] Z. Jia, J. Yu, A. Chowdhury, G. Ellinas, G.-K. Chang, Simultaneous generation and delivery of independent wired and wireless services in radio-over-fiber systems using a single modulator, in: Proc. ECOC, Berlin, Germany, September 2007, Paper Tu3.3.2. [10] K. Ikeda, T. Kuri, K. Kitayama, Simultaneous three-band modulation and fiber-optic transmission of 2.5 Gb/s baseband, microwave-, and 60 GHz-band signals on a single wavelength, J. Lightwave Technol. 21 (12) (2003) 3194. [11] A.R. Chraplyvy, A.H. Gnauck, R.W. Tkach, J.L. Zyskind, J.W. Sulhoff, A.J. Lucero, Y. Sun, R.M. Jopson, F. Forghieri, R.M. Derosier, C. Wolf, A.R. McCormick, 1-Tb/s transmission experiment, IEEE Photon. Technol. Lett. 8 (9) (1996) 1264.
Optics Communications 281 (2008) 2806–2810 www.elsevier.com/locate/optcom
Experimental demonstration of polarization multiplexing for simultaneously providing broadband wireless and wired access Jianqiang Li a,*, Kun Xu a, Yang Jing Wen b, Songnian Fu c, Ming Tang c, Ping Shum c, Jian Wu a, Jintong Lin a a
Key Laboratory of Optical Communication and Lightwave Technologies, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, China b Institute for InfoComm, Singapore 119613, Singapore c Lightwave Technology Group, Network Technology Research Centre, Nanyang Technological University, Singapore 637553, Singapore Received 20 November 2007; received in revised form 17 January 2008; accepted 17 January 2008
Abstract A fiber-based wavelength-division-multiplexing (WDM) network utilizing polarization multiplexing (PolMUX) is proposed to simultaneously provide broadband wireless and wired services. In such a dual-service access network, the wireless and wired services are separately delivered in two orthogonal states of polarization with well independence in a single WDM channel. The impact of several polarization-dependent interferences becomes insignificant due to the relatively short transmission distance in access networks. The feasibility of PolMUX is experimentally demonstrated with a power penalty at BER = 10–9 of about 0.5 dB and 1 dB for 2.5 Gb/s wired and wireless downstream services, respectively. The proposed system is compatible with the current reported techniques in either WDM passive optical networks (WDM-PON) or radio-over-fiber (ROF) systems. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Access networks; Polarization multiplexing; Passive optical networks (PON); Radio-over-fiber (ROF)
1. Introduction It is essential and necessary to upgrade or redesign access networks for satisfying the increasing demand of broadband wireless and wired access. Wavelength-division-multiplexing passive optical networks (WDM-PON) have been widely investigated as a promising solution to the realization of fiber-to-the-home (FTTH), where the huge bandwidth of optical fiber is explored to achieve up to multi-gigahertz wired access [1,2]. Taking into account the requirements on flexibility and mobility, radio systems operating at microwave frequency play a crucial role in wireless access networks. However, the conventional wire*
Corresponding author. E-mail address: [email protected] (J. Li).
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.01.027
less communication systems are confronted with a series of challenges, such as the high loss of coaxial cable, frequency congestion, and limited capacity. Recently, millimeter wave (mm-wave) radio-over-fiber (ROF) systems and opticalwireless networks emerge to integrate wireless communications with beneficial fiber-optic technology, which makes it feasible to cost-effectively support both wireless and wired services over the same fiber-optic infrastructure [3,4]. 2. The utilization of PolMUX in fiber-based access networks In the most previous works, WDM-PON and ROF systems were studied separately for supporting either wired or wireless services [1,2,5–8]. Many techniques have been proposed to implement low-cost, flexible, and functioncentralized full-duplex operations for either WDM-PON
J. Li et al. / Optics Communications 281 (2008) 2806–2810
or ROF systems, such as colorless optical network units (ONUs) in WDM-PON [1,2], architecture design [5,6], all-optical frequency up-conversion [7], and wavelength reuse for ROF systems [8]. Although the previous demonstrations have testified that ROF systems are capable of sharing the fiber-based infrastructure with WDM-PON, it is still a challenge to simultaneously guarantee broadband dual-service operations for both wireless and wired access in a single WDM channel. A novel ROF architecture was reported to provide broadband dual-service in one WDM channel [4]. However, this scheme is only suitable for point-to-multi-points applications due to the dependence between the delivered wireless and wired services. Two other methods were demonstrated to simultaneously deliver multiple independent wireless and wired services in one WDM channel [9,10]. In the two schemes, the optical
2807
spectrum is extensively occupied by different services in a single WDM channel. The separation of these services requires suitable narrow-band bandpass filters either in the optical or electrical domain. These filters are difficult to fabricate and usually expensive. Furthermore, only the downstream services are considered in [9,10]. When taking into account the bidirectional full-duplex operation, the extensively-occupied spectrum has no more free space to accommodate upstream services. Therefore, the technique aimed at further capability enhancement and easier service-separation is desired in a single wavelength. On the other hand, polarization multiplexing (PolMUX) can double the capacity in one WDM channel, and has been successfully demonstrated in the optical transmission experiments by transmitting independent signals in two orthogonal states of polarization (SOPs) [11]. Therefore,
Fig. 1. Proposed scenario for fiber-based access networks utilizing PolMUX.
2808
J. Li et al. / Optics Communications 281 (2008) 2806–2810
the introduction of PolMUX will be potential and favorable in dual-service fiber-based access networks due to high spectrum-efficiency and convenience to separate different services. Furthermore, the impact of several polarizationdependent interferences becomes insignificant due to the relatively short transmission distance in access networks, such as polarization-mode dispersion (PMD) and polarization-dependent loss (PDL), etc. Although a feed-back setup typically comprising an automatic polarization controller (APC) is required at the receiver for practical applications, the overall system is expected to be ultimately costeffective considering the doubly enhanced capability. In this paper, we proposed and experimentally demonstrated a hybrid fiber-based WDM system by introducing PolMUX to provide broadband wireless and wired services. As shown in Fig. 1, a central station (CS) and several base stations (BSs) are interconnected by a fiber-based WDM access network. In the N parallel downlink transmitters (Tx) located at CS, there are N laser sources at k1, k2,. . .,kn to generate N optical carriers for N BSs, which means N WDM channels. Each original optical carrier is split into two parts by an optical coupler and then loaded with the base-band data for wireless and wired services, respectively. As can be seen there are two outputs for each downlink Tx, one of which corresponds to the wired service and the other corresponds to the wireless service. These output signals are combined into two branches by WDM for wireless and wired services, respectively. For the upper branch aimed at wireless service, photonic frequency upconversion techniques can be suitably fit into this system with good compatibility. Before PolMUX, the optical powers of the two branches necessarily require a tradeoff, which is important to obtain a balanced transmission performance for both services. The optical signal after PolMUX will possess two kind of independent information in one WDM channel, which means a dramatic capability improvement in one wavelength. The N WDM channels are routed out of the access network and then selectively fed to the target BSs by a WDM at remote node (RN). At each BS, the separation of wireless and wired services is performed by demultiplexing the two orthogonal SOPs. For wireless service, the final RF signal will be sent to the antenna after photodetection by a high-speed PIN. For wired service, the demultiplexed optical signal is detected by a receiver and then fed to the concerned fixed terminals. Referring to the uplink, the wireless and wired services, respectively from the antennae and fixed terminals can also be assembled in a WDM channel by PolMUX at BS. After WDM at RN, the upstream signals are transported to the CS over the fiber-based access network and received at CS. Besides the traditional function in wireless systems, each BS acts another role similar to an ONU in WDM-PON for wired access. Therefore, the system using PolMUX serves as a shared delivery platform for wireless and wired services in two isolated polarization channels. The isolation determines the well compatibility with the current techniques which have been separately investigated
in ROF systems and WDM-PON. The effective combination of these reported techniques in WDM-PON and ROF systems will lead to establishment of a cost-effective and function-centralized multi-service fiber-optic access network. The experiment was carried out only for downlink to verify the proposed scheme. The technique PolMUX can also be employed in uplink. 3. Experiment demonstration of PolMUX in fiber-based access networks The experiment setup is shown in Fig. 2, where two WDM downlink channels were demonstrated. Due to our hardware constraint, the RF carrier frequency is assumed at 20 GHz band for wireless access. The RF operation band can be easily extended to higher frequency. Two tunable laser sources (TLSs) followed by two polarization controllers (PCs) were used to generate a pair of optical carriers at 1556.55 and 1557.35 nm, respectively. After a WDM with 0.8 nm spacing, the two optical carriers were both modulated in a LiNbO3 Mach–Zehnder modulator (MZM) by a 2.5 Gb/s pseudo-random bit sequence (PRBS) electrical signal with a word length of 223–1 from a pulse pattern generator (PPG). Note that the same electrical signal was loaded to emulate the base-band data of both wireless and wired services for simplicity. The output of the MZM was split into two parts by a 3 dB coupler. One part was injected into another MZM biased at Vp and driven by a 10 GHz electrical clock to realize all-optical up-conversion via optical carrier suppression (OCS) [7]. As shown in the inset of Fig. 2, the carrier suppression ratio is larger than 25 dB. The optical power of the other part was adjusted by a variable attenuator to ensure that the optical powers of the two parts were identical before PolMUX. This action will avoid quality imbalance between different services after their separation at the receiver end. Two PCs were inserted before the polarization beam splitter (PBS) to allow independently aligning both SOPs. Then, the optical signal comprising two WDM channels with PolMUX was amplified by an erbium-doped fiber amplifier (EDFA) to obtain a total power of 9 dBm before transmission over a certain length of standard single-mode fiber (SSMF) with a dispersion of 17 ps/nm/km. At RN, another WDM with 0.8 nm spacing was used to separate the two WDM channels and meanwhile to suppress the amplified spontaneous emission (ASE) noise. The optical signal in each WDM channel was fed to its target BS and then polarization-demultiplexed using a PBS after adjusting the PC to eliminate the presence of unwanted signal in a certain polarization. The insets at BSs in Fig. 2 correspond to the measured spectra after a 25 km fiber transmission. As can be seen the polarization-demultiplexed wireless signal also exhibits a 20 dB carrier suppression ratio. The optical signal for wired service was directly detected by a 12.5 Gb/s PIN photodiode (PD) and finally sent to the bit-error-rate tester (BERT). For wireless service, the detected RF signal by a PIN PD with a 3 dB bandwidth
J. Li et al. / Optics Communications 281 (2008) 2806–2810
2809
Fig. 2. Experiment setup and the measured spectra.
of 45 GHz was amplified by an electrical amplifier (EA) centered 20 GHz. The electrical spectrum of the detected RF signal was also inserted in Fig. 2. A 20 GHz local oscillator (LO) signal by frequency-doubling from 10 GHz was adjusted by a phase shifter and then fed into a 26 GHz electrical mixer to down-convert the detected RF signal. After filtering by a 6 GHz electrical low-pass filter (LPF), the base-band electrical signal after down-conversion was sent to the BERT. The BER measurement results are summarized in Fig. 3. The two WDM channels for different BSs (labeled CH1 and CH2 in Fig. 3) show similar transmission performance in each case for both wired and wireless services, meaning that WDM and PolMUX can be used at the same time without conflict. By comparing the cases in which only
one separate service are delivered and both services are delivered by PolMUX, the utilization of PolMUX technique will lead to a power penalty of about 0.5 dB and 1 dB at a BER of 10–9 for wired and wireless services, respectively. This result indicates that the utilization of PolMUX makes insignificant mutual impact on the delivery of both wireless and wired services, which exhibits the well feasibility of PolMUX in dual-service fiber-based WDM access networks. Please note that the above experimental results are based on the precondition that the two services have been separated after polarization control. Thus the successful implementation of the proposed scheme requires more practical automatic polarization controllers. Considering the fiber transmission employing PolMUX, the power penalty caused by fiber transmission is negligible for wired service, which can be observed from the left cluster of BER curves in Fig. 3. The right cluster of BER curves in Fig. 3 shows that the power penalty keeps within 1 dB for wireless service even after 25 km fiber transmission by employing PolMUX, due to the well tolerance of OCS modulation to fiber dispersion [7]. 4. Conclusion
Fig. 3. Measured BER versus received power.
We have proposed a novel hybrid system for fiber-based access networks to simultaneously provide wired and wireless services by PolMUX. Although the PolMUX technique has been employed, the impact of several polarization-dependent interferences becomes insignificant due to the relatively short transmission distance in access networks, rather than in long-haul transmission systems. This scheme has unique advantages in terms of large capacity in a single wavelength, independent dual-service opera-
2810
J. Li et al. / Optics Communications 281 (2008) 2806–2810
tion and compatibility with the current techniques in either WDM-PON or ROF systems. On the other hand, the main weakness of the propose system lies in the relatively complex polarization control during multiplexing and demultiplexing, which necessitates more practical automatic polarization controllers. The experiment results show a power penalty of 0.5 dB and 1 dB at a BER of 10–9 for 2.5 Gb/s wired and wireless services, respectively after PolMUX, which indicates that PolMUX is feasible in fiberbased access networks. Acknowledgements This work was partially supported by the National 863 Program of China (2007AA01Z264 and 2006AA01Z256), the National Natural Science Foundation of China (60702006 and 60736002), the New Century Excellent Talent Project in Ministry of Education of China (NCET-060093), and the 111 Project (B07005). References [1] K. Iwatsuki, J. Kani, H. Suzuki, M. Fujiwara, Access and metro networks based on WDM technologies, J. Lightwave Technol. 22 (11) (2004) 2623. [2] W. Lee, M.Y. Park, S.H. Cho, J. Lee, C. Kim, G. Jeong, B.W. Kim, Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 17 (11) (2005) 2460.
[3] D. Wake, M. Webster, G. Wimpenny, K. Beacham, L. Crawford, Radio over fiber for mobile communications, in: IEEE Int. Topical Meeting Microwave Photon, (MWP), October 4–6, 2004, p. 157. [4] G.-K. Chang, J. Yu, Z. Jia, J. Yu, Novel optical-wireless access network architecture for simultaneously providing broadband wireless and wired services, in: Proc. OFC 2006, Anaheim, CA, Paper OFM1. [5] G.H. Smith, D. Novak, C. Lim, A millimeter-wave full-duplex fiberradio star-tree architecture incorporating WDM and SCM, IEEE Photon. Technol. Lett. 10 (11) (1998) 1650. [6] X. Zhang, B. Liu, J. Yao, K. Wu, R. Kashyap, A novel millimeterwave-band radio-over-fiber system with dense wavelength-division multiplexing bus architecture, IEEE Trans. Microwave Theory Technol. 54 (2) (2006) 929. [7] J. Yu, Z. Jia, L. Yi, Y. Su, G.-K. Chang, T. Wang, Optical millimeterwave generation or up-conversion using external modulators, IEEE Photon. Technol. Lett. 18 (1) (2006) 265. [8] A. Nirmalathas, D. Novak, C. Lim, R.B. Waterhouse, Wavelength reuse in the WDM optical interface of a millimeter-wave fiber-wireless antenna base station, IEEE Trans. Microwave Theory Technol. 49 (10) (2001) 2006. [9] Z. Jia, J. Yu, A. Chowdhury, G. Ellinas, G.-K. Chang, Simultaneous generation and delivery of independent wired and wireless services in radio-over-fiber systems using a single modulator, in: Proc. ECOC, Berlin, Germany, September 2007, Paper Tu3.3.2. [10] K. Ikeda, T. Kuri, K. Kitayama, Simultaneous three-band modulation and fiber-optic transmission of 2.5 Gb/s baseband, microwave-, and 60 GHz-band signals on a single wavelength, J. Lightwave Technol. 21 (12) (2003) 3194. [11] A.R. Chraplyvy, A.H. Gnauck, R.W. Tkach, J.L. Zyskind, J.W. Sulhoff, A.J. Lucero, Y. Sun, R.M. Jopson, F. Forghieri, R.M. Derosier, C. Wolf, A.R. McCormick, 1-Tb/s transmission experiment, IEEE Photon. Technol. Lett. 8 (9) (1996) 1264.