This paper proposes a low-cost RoF transceiver for multichannel SCM/WDM signal distribution suitable for future broadband access networks. The transceiver is based on the phase modulation of an optical broadband source centered at third transmission window. Prior to phase modulation the optical broadband source output signal is launched into a Mach-Zehnder interferometer structure, as key device enabling radio signals propagation over the optical link. Furthermore, an optical CWDM is employed to create a multichannel scenario by performing the spectral slicing of the modulated optical signal into a number of channels each one conveying the information from the central office to different base stations. The operation range is up to 20 GHz with a modulation bandwidth around of 500 MHz. Experimental results of the transmission of SCM QPSK and 64-QAM data through 20 Km of SMF exhibit good EVM results in the operative range determined by the phase-to-intensity conversion process. The proposed approach shows a great suitability for WDM networks based on RoF signal transport and also represents a cost-effective solution for passive optical networks.
©2010 Optical Society of America
It is widely believed that for future optical access networks the combination between Radio over Fibre (RoF) technology and optical Wavelength Division Multiplexing (WDM) networking represents the most efficient solution for providing broadband wireless and wired services everywhere through a Passive optical Network (PON) [1–4]. Whereas features such as wavelength-per-service distribution and co-existence of analog and digital signals over the same network satisfy the bandwidth requirements and guarantee a high Quality of Service (QoS),the reduction of costs and complexity of the whole network is an important issue for a practical deployment. In this context the use of Optical Broadband Sources (OBSs) has been largely suggested [5–7] as a cost-effective alternative to the expensive use of multiple lasers as WDM light sources. Since the incoherent light of a single OBS can be shared among many users by slicing its spectra into a number of narrower WDM channels there is no need to equip both central office and base station sides with multiple and high-cost lasers. In this way both implementation and maintenance costs are reduced as well as temperature-stabilization controlling is relaxed. Although high quality of broadband light suitable for spectrum-sliced WDM applications can be easily obtained by use of conventional ASE sources, fiber dispersion reduce drastically the electrical bandwidth of the OBS to few MHz’s. In order to surmount this problem the authors proposed in  an RoF scheme based on the combination of a low-cost 80 nm ASE light source centred at third transmission window with a Mach-Zehnder interferometric structure (MZI). Experimental results shown that by inserting a MZI structure with a tunable delay line, the frequency operative range increased spectacularly up to tens of GHz avoiding the Carrier Suppression Effect (CSE) even when Double Side Band (DSB) modulation format was adopted.
In this paper, the transceiver system adopts the phase modulation (PM) instead of amplitude modulation (AM) of the optical carrier by the electrical RF subcarrier with a further reduction of the system cost. Furthermore, as previously demonstrated in  the dispersive nature of the Singlemode Fiber (SMF) serving as a transmission media between the Central Office (CO) and a number of Base Station Groups provides the phase-to-intensity modulation conversion The paper is structured as follows: section 2 describes the proposed scheme, in section 3 we evaluate the system performance in terms of EVM measurements when digital signals at QPSK and 64-QAM modulation formats are transmitted using the SCM technique over each channel, and section 4 sumarices the main conclusions of the paper.
2. Experimental description of the transmission link
The RoF transmission system employed in the experimental analysis is depicted in Fig. 1 . In the CO, an 80 nm-width ASE source centered at the third transmission window with a total optical power of 19 dBm provides the optical carrier whose phase is modulated by the electrical subcarrier coming from a vector signal generator. Inset (a) of Fig. 1 plots the optical spectrum of the OBS. Prior to the phase modulation a MZI structure with a Variable Delay Line (VDL) in one of the two arms is inserted. The MZI is formed by two 50:50 fiber couplers and the VDL has 1 dB of insertion losses and permits a maximum optical time delay Δτ of 330 ps between the two optical paths. As example, Fig. 1(b) shows the corresponding spectral slicing of the OBS after the MZI for a given optical delay. The phase modulated optical carrier is then launched into a Coarse Wavelength Division Multiplexer (CWDM) after propagation over 20 Km of SMF dispersive link. The optical CWDM is used to slice the optical spectra into four 17 nm-width channels separated by 20 nm as shown in inset (c) of Fig. 1. The central wavelength of the channels are 1531 nm, 1551 nm, 1571 nm and 1591 nm and each channel connects the transceiver at the CO with its correspondent remote BSG where a photodetector (PD) is used to detect the RF subcarrier. The phase modulator (PM) has a 3dB-RF bandwidth of 15 GHz and after propagation the light is photodetected using a 23 GHz-bandwidth PIN photodiode.
As demonstrated in  the MZI is the key component that allows the use of an optical broadband source in a RoF system. In fact, only when the MZI is employed a bandpass window is generated in the electrical transfer function measured at the correspondent BS’s receiver. In our approach, the central frequency fCHi of the electrical bandpass window at channel i for a given optical delay Δτ depends on the chromatic dispersion coefficient at the optical frequency ωCHi and the optical fiber length connecting the transceiver and the receiver, as following:Eq. (1).
In order to explain the electrical behavior of the transceiver, we have measured the electrical transfer function of the system by using a vector network analyzer as electrical subcarrier. Figure 2(a) represents the amplitude response measured at the PD of the first base station BSG1 whose channel central wavelength λ1 is located at 1531 nm. In this experiment, we suppose that the BSG1 is composed only by one Base Station (BS). The extra length of optical SMF employed in the experiment is L1 = 5 Km. The dashed curve refers to the phase-to-intensity conversion process measured with a laser diode emitting at the same wavelength of the considered channel, 1531 nm along 20 Km of SMF. The high-pass filtering effect observed in the phase-to-intensity conversion curve is caused by the presence of the fiber dispersive link. The curve presents a 3dB bandwidth of 8.6 GHz around a central frequency of 10 GHz where the maximum amplitude level is reached. In the case shown in Fig. 2(a) three different optical time delays are selected by means of tuning the VDL to generate three transmission windows at 6 GHz, 10 GHz and 14 GHz placed in the high-pass region. Being the phase-to-intensity curve not flat over the frequency, the transmission window generated at 10 GHz is the one showing the maximum peak amplitude level, whereas the peak of the other two transmission windows at 6 GHz and 14 GHz has a 6dB lower value. A second experiment has been performed considering the others channels at the output of the CWDM. Here the tunable delay line was set in the four cases to generate the bandpass window centred at 10 GHz which is the most favourable frequency according to the electrical transfer function of Fig. 2(a). In this way, Fig. 2(b) plots the amplitude responses measured at each BSG. Small differences are observed in the transmission windows of the three first BSs comparing with BS4. This effect is due to the power distribution of the OBS which is not flat with the wavelength. Since the shape and total power of the optical carrier sliced by the CWDM is not the same for each channel and the difference is clearly reflected on the electrical transfer function of the BS. The results obtained in this section confirm that the phase modulation of the OBS combined with the MZI structure is highly compatible with RoF signal distribution and optical WDM networks in the frequency range permitted by the phase-to-intensity conversion process. Note the operation range of the system is around 20 GHz which is determined mainly by the bandwidth of the PM. Moreover, the bandwidth of the amplitude responses of Fig. 2(b) for each BSG is around 500 MHz. Therefore, our system support data rate up to hundreds of Mb/s over an electrical subcarrier close to 20 GHz.
3. Experimental evaluation of digital SCM signal transmission
In this section the experimental validation of the system performance is presented considering the transmission from the CO to all BSGs through 20 Km of SMF link. Following the scheme of Fig. 1, SCM technique is employed in the electrical domain to transport QPSK and 64-QAM digital sequences at 5 Mb/s over electrical subcarriers using an Agilent E8267D PSG vector signal generator. The incoming data are finally demodulated by a N9020A-526 MXA Agilent signal analyser with a maximum demodulation bandwidth of 25 MHz. A Maximum 12.5% Error Vector Magnitude (EVM) for QPSK data and 5.6% EVM for 64-QAM data are assumed as standards criteria for signal quality evaluation .
3.1. Transmission evaluation as a function of the subcarrier frequency for different BSGs
First the impact of the electrical subcarrier frequency on the quality of the signal received at BSG1 is experimentally evaluated. Figure 3 (a) plots the EVM measurements for QPSK (■) and 64-QAM (●) codification formats. As predicted by the phase-to-intensity conversion curve and the amplitude responses of Fig. 2 (a), the EVM increases at lower and higher frequencies and reaches minimum values around 10 GHz for both type of codifications. In case of QPSK the EVM is always below standard 12% in all the frequency range with values below 3.1% around 10 GHz. The highest EVMs, 6.8% and 10.5%, were achieved at the extremes of the frequency range considered, 4 GHz and 17 GHz respectively. Using 64-QAM a good signal quality is obtained for subcarrier frequencies up to 16 GHz. In fact, the worst case is observed at 17 GHz with an EVM of 7.4% however EVM not exceeding 2.2% are measured at subcarrier frequencies close to 10 GHz. To investigate the influence of the optical wavelength of the four CWDM channels on the quality of the demodulated signal, 5 Mb/s QPSK and 64-QAM data are broadcasted from the CO transceiver to each BSG after tuning the electrical transmission window at 10 GHz in all cases. Figure 3 (b) summarizes the experimental results obtained in terms of EVM. Observing the figure the received signal verifies the EVM standards for both QPSK (■) and 64-QAM (●) codification formats. The best results are achieved by BSG1 and BSG2 with EVM around 3.1% for QPSK and 2.1% for 64-QAM. According with the electrical transfer functions of Fig. 2 (b), EVM values increase with the optical wavelength. From inset (c) of Fig. 1, we can observe that each optical channel has different optical spectrum with a given optical power which introduces small variations in the corresponding electrical transfer function. The worst case is measured for the BSG4 who has the lower peak amplitude level resulting in an EVM of 4.1% (QPSK) and 3.0% (64-QAM). However even for the BSG with the poorest performance, the quality of the demodulated symbols is below the EVM standard limits
The constellation diagrams depicted in Fig. 4 refers to the best and the worst case of the results presented in Fig. 3 (b). They demonstrate that even in the worst case (BSG4) no significant errors are observed in the received symbols when the transmission window is tuned at the central frequency of 10 GHz.
3.2. Transmission performances as a function of the number of BSs
In this last experiment the total number of BSs per BSG is experimentally estimated in terms of splitting the total optical power received at a BSG into a multiple number of BS. The experiment is performed considering SCM transmission of digital sequences on a subcarrier frequency of 10 GHz from the transceiver to the BSG1 and BSG4 which corresponds to the best and the worst channel respectively as predicted by Fig. 2(b) and Fig. 3(b). In Fig. 5(a) the transmitted data are 5 Mb/s QPSK and the EVM is plotted as a function of the number of the base stations which can be fed by splitting the optical power received at BSG1 (●) and BSG4 (■). An increase of the EVM is observed with the number of substations for both BSGs. In case of using QPSK, the EVM is below standard 12% for a number of 8 base stations per BSG. This means that the system allows a transmission with good performances for a total number of 32BSs. Beyond 8 BSs any splitting of the optical power produces a several degradation of the received signal at BSG4 (■) which is the one that mostly determinates the system feeding capabilities. When the codification format 64-QAM is employed Fig. 5(b) shows an EVM increase with the number of base stations as well as in the previous case but the standard of 5.6 maximum EVM is achieved by BSG1 (●) and BSG4 (■) only for a number of 4 BSs per BSG that means a total number of possible 16 BS . In case of more than 4 BS the available optical power is not able to guarantee a good broadcast transmission at the same time over the link.
In this work we have demonstrated experimentally a novel photonic transceiver based on the phase modulation of an OBS suitable for RoF signal transport over 20 Km of SMF. The incorporation of a tunable MZI structure in the transceiver scheme between the light source and the phase modulator offers the possibility to transmit SCM electrical signals in a frequency range established by the phase-to-intensity conversion response. Furthermore the insertion of an optical CWDM makes the proposed solution compatible with a conventional RoF-WDM scenario since the spectral slicing of the optical broadband source enables multichannel transmission in a very cost-effective way. Experimental measurements have been carried out over 20 km of SMF to evaluate the quality of transmission of digital QPSK and 64-QAM data at each BSG. The EVM results largely satisfy the standards for all the channels when the transmission window is generated in the phase-to-intensity conversion region although some little variations have been observed depending on the optical channel wavelength due to a different power distribution of the OBS. We experimentally are limited to a modulation of few MHz but the electrical bandwidth which is available is around 500 MHz. Therefore, our system support data rate up to hundreds of Mb/s over an electrical subcarrier close to 20 GHz.
From the evaluation of system feeding capabilities, it results that the number of BSs permitted by splitting the optical power received at each BSG depends on the power distribution of the channels. In this context, our proposal shows a low cost solution since the proposed technique based on the spectral slicing of optical broadband sources permits to reduce the cost of WDM systems avoiding sources selected within a particular wavelength range. The slicing of an OBS relaxes precise tuning requirements since it avoids the need for well-defined wavelengths, that is, specify tightly the center wavelength of the source. Furthermore, the location of the MZI in the CO and the use of phase modulation imply an important cost reduction per BSG. Firstly, the cost of the MZI can be shared by a large numbers of users and secondly, PM avoids the conventional use of amplitude modulation.
The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7) under project 212 352 ALPHA “Architectures for fLexible Photonic Home and Access networks”. Also the authors acknowledge PROMETEO 2008/092 MICROWAVE PHOTONICS, a research programme of excellency, supported by The Generalitat Valenciana.
References and links
1. C.-H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the home using a PON infrastructure,” J. Lightwave Technol. 24(12), 4568–4583 (2006). [CrossRef]
2. R. Lin, “Next generation PON in emerging networks,” in Proceedings of Optical Fiber Communication Conf (OFC2008), San Diego (CA), Feb. 2008, OWH1.
3. A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, and B. Mukherjee, “Wavelength-division-multiplexed passive optical network (WDM-PON) technologies for broadband access: a review [Invited],” J. Opt. Netw. 4(11), 737–758 (2005). [CrossRef]
4. L. G. Kazovsky, S. W. T. Shaw, D. Gutierrez, N. Cheng, and S. W. Wong, “Next-generation optical access networks,” J. Lightwave Technol. 25(11), 3428–3442 (2007). [CrossRef]
5. J. Cho, J. Kim, D. Gutierrez, L.G. Kazovsky, “Broadcast transmission in WDM-PON using a broadband light source,” in Proceedings of Optical Fiber Communication Conf (OFC2007), Anaheim (CA), March 2007, OWS7.
6. K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, “Bidirectional WDM PON using light-emitting diodes spectrum-sliced with cyclic arrayed-waveguide grating,” IEEE Photon. Technol. Lett. 16(10), 2380–2382 (2004). [CrossRef]
7. P. K. J. Park, S. B. Jun, H. Kim, D. K. Jung, W. R. Lee, and Y. C. Chung, “Reduction of polarization-induced performance degradation in WDM PON utilizing MQW-SLD-based broadband source,” Opt. Express 15(21), 14228–14233 (2007). [CrossRef] [PubMed]
8. F. Grassi, J. Mora, B. Ortega, and J. Capmany, “Subcarrier multiplexing tolerant dispersion transmission system employing optical broadband sources,” Opt. Express 17(6), 4740–4751 (2009). [CrossRef] [PubMed]
9. J. Yao, G. Maury, Y. Le Guennec, and B. Cabon, “All-optical subcarrier frequency conversion using an electrooptic phase modulator,” IEEE Photon. Technol. Lett. 17(11), 2427–2429 (2005). [CrossRef]
10. C. Lethien, C. Loyez, and J.-P. Vilcot, “Potentials of radio over multimode fiber systems for the in-buildings coverage of mobile and wireless LAN applications,” IEEE Photon. Technol. Lett. 17(12), 2793–2795 (2005). [CrossRef]