We proposed and experimentally demonstrated a novel radio-over-fiber architecture using an electrical mixer and an optical intensity modulator based on double side-band modulation scheme to generate dense wavelength-division multiplexing (DWDM) optical millimeter for carrying downstream data and centralized lightwave for carrying upstream data. Since the remaining optical carriers with high power have been reused, the optical power is effectively utilized; therefore the system cost can be reduced.
© 2007 Optical Society of America
The radio-over-fiber (RoF) has been studied for many years as a promising technique for providing wireless broad-band service. Millimeter-wave (mm-wave) is a promising frequency resource for future broadband communication [1–13]. To reduce the total cost of the central station (CS) and base station (BS) and connect as many users as possible, the dense wavelength division multiplexing (DWDM) technique is a strong candidate to better support the connections between CSs and BSs [3, 4]. For WDM RoF systems, it is necessary that the base-band signals of the downlink data for each channel are up-converted to the millimeter-wave frequency using a cost-efficient and high performance scheme. There have been several reports on WDM RoF system in order to reduce the complexity and cost of the system. These techniques are based on optical frequency interleaving , non-linear fiber (NLF) based on four-wave mixing (FWM) and cross-phase modulation (XPM) . In this paper, we proposed and experimentally demonstrated a novel and simple method to generate the DWDM mm-wave signals by using a single electrode Mach-Zehnder modulator (MZM). In the conventional external intensity modulation scheme for the mm-wave generation, the base-band signal is modulated onto the optical carrier by a single-electrode MZM biased at quadrature and then up-converted using a dual-arm MZM biased at the minimum transmission point to generate mm-wave . In our proposed scheme, only one single-electrode MZM is needed to generate DWDM mm-wave based on double sideband (DSB) modulation. The optical carrier and the mm-wave are fed to the base station and separated by an optical interleaver. The separated optical carrier is reused for uplink connection. Because no additional lightwave source is used in the BS, this RoF system is more compact and cost-effective relative to the previous schemes.
Figure 1 shows the principle to generate a DWDM optical mm-wave by using DSB modulation and wavelength reuse for uplink connection. In the central office, multiple channel continuous wave (CW) lightwaves were generated by a distributed feedback laser diodes (DFB-LD) array. The base-band signals of the downlink data are up-converted with the RF carrier. Each CW light wave is modulated via an external intensity modulator based on DSB modulation scheme to generate DSB signals. These DSB signals including the optical carriers and the first order sidebands are combined by a MUX and delivered to the base station by the downlink fiber. In the base station, an optical interleaver with two output ports is used to separate the optical central optical carriers and first order sidebands, the optical carrier of each channel will be separated from the first order sidebands after the WDM lightwave source passes through one optical interleaver . After the optical interleaver, the first sidebands of desired channel are selected by a tunable optical filter (TOF1). The two peak modes of the first order sidebands will be beat to generate an optical mm-wave with a frequency-doubled RF signal when they are detected by a downlink receiver. Our optical IL is an athermal component without any additional control system; therefore our system will be stable and cheap. The remaining optical carrier of the desired channel is selected by another tunable optical filter (TOF2) before it is used for uplink connection. The base-band uplink data is used to drive an external modulator to generate optical uplink signal before it is transmitted over the fiber to the central office. In the real full-duplex radio-over-fiber system, we can use self-mixing to down-convert the data, therefore the local oscillator can be removed . Because the uplink data rate is low, we can use low-speed electrical mixer . So the base station cost is very low.
3. Experimental setup and results
Figure 2 shows the experiment setup for DWDM optical mm-wave generation and wavelength reuse for up-link connection. A laser array with four DFB-LDs was used to generate four wavelength CW light-waves from 1538.19 to 1542.94nm with 200-GHz spacing. An arrayed waveguide (AWG) was used to combine the four CW light-waves. The 2.5 Gb/s base-band electrical signal of the downlink data with a pseudorandom binary sequence (PRBS) length of 231-1 is up-converted with the 20GHz RF carrier by an electrical mixer. The four CW light-waves of DWDM channel were modulated via a LiNbO3 (LN) MZM modulator biased at 0.5vπ based on DSB modulation scheme to generate DSB signals. The optical spectrum after the intensity modulator is inserted in Fig. 2 as inset (i), while the eye diagram is shown in Fig. 3(a). The second-order sideband of each channel is 20 dB lower than the first-order sideband; therefore, the second-order sidebands have little effect on the transmission of the optical mm-wave in SMF fibers. After transmission over 20km SMF, the optical spectrum and the eye diagram are shown in Fig. 2 as inset (ii) and Fig. 3(b), respectively.
In the base station, a 25/50GHz optical interleaver with two outputs was used to separate the optical carriers and optical mm-waves of DWDM channels. This interleaver has a 3dB passing band-width of 0.15nm, insert loss of 2dB and no polarization sensitivity. After the optical interleaver, the carrier suppression ratio of all channels is larger than 15 dB, and the repetitive frequency of the optical mm-wave is 40 GHz as shown inset (iii) in Fig. 2. The optical spectrum of the desired channel is inserted in Fig. 2 as insert (iv), which was selected by TOF1 with a 3dB bandwidth of 0.5 nm before O/E conversion via a PIN PD with a 3-dB bandwidth of 60 GHz. The detail information about the demodulation of the mm-wave signal can be found in Ref. 11. After optical interleaver, the optical spectrum of the remaining optical carrier of four channels is showed in Fig. 2 as inset (v). We employed TOF2 to select the optical carrier of the desired channel, whose optical spectrum is shown in Fig. 2 as inset (vi). As an example, the eye diagram and optical spectrum of channel 3 at 1539.79nm are shown. After TOF2, the optical carrier of the desired channel was modulated via a LN-MOD driven by 2.5Gb/s uplink PRBS data with a word length of 231-1. The optical spectrum of the uplink optical carrier with 2.5Gb/s uplink data is shown as inset (vii) in Fig. 2 before it was boosted by an EDFA and transmitted over another 20km SMF-28. The input power into the SMF-28 is set to be 3dBm in order to avoid nonlinear effects. The eye diagrams for the uplink data before and after transmission over 20km uplink fiber were shown in Figs. 3(c) and 3(d), respectively. The BER performance of the uplink data was evaluated by a BER tester after O/E conversion by a commercial 2.5Gb/s receiver with an optical pre-amplifier. The measured BER performance for the downlink data and uplink data for the chosen channel is shown in Fig. 4. Both for downlink and uplink data, after transmission over 20km, the power penalty of all channels are all less than 0.2dB.
We have proposed and experimentally demonstrated a novel WDM RoF architecture. In this architecture, a novel scheme to generate optical mm-wave and simultaneously provide light wave for uplink data is proposed and experimentally demonstrated. In this scheme, only one external arm MZM is employed to generate double side-band modulation signals. In the base station, an optical interleaver is used to separate the optical carrier and optical mm-wave of DWDM signals. The power penalty of 40-GHz RF signals carrying the 2.5 Gb/s broadband signal after transmission over a 20-km SMF is less than 0.2 dB. But for 2.5Gb/s uplink data carried by the reused light wave, the power penalty is also less than 0.2dB after transmission over a 20-km SMF. Because only a single arm MZM is used to generate the optical mm-wave and centralized lightwave, this RoF system is more compact and cost-effective.
Authors would like to thank Dr. J. Yu for his useful discussion and encouragement, and his assistance in part of the experiments. This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 10576012 and 60538010), the program of the Ministry of Education of China for New Century Excellent Talents in University, the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20040532005), and the Hunan Provincial Natural Science Foundation of China (Grant No. 06JJ50108).
References and links
1. A. Kaszubowska, L. Hu, and L. P. Barry, “Remote downconversion with wavelength reuse for the radio/fiber uplink connection,” IEEE Photon. Technol. Lett. 18, 562–564 (2006). [CrossRef]
2. A. Wiberg, P. P. Millan, M. V. Andres, P. A. Andrekson, and P. O. Hedkvist, “Fiber-optic 40GHz mm-wave link with 2.5Gb/s data transmission,” IEEE Photon. Technol. Lett. 17, 1938–1940 (2005). [CrossRef]
3. M. Attygalle, C. Lim, and A. Nirmalathas, “Extending optical transmission distance in fiber wireless links using passive filtering in conjunction with optimized modulation,” J. Lightw. Technol. 24, 1703–1709 (2006). [CrossRef]
4. J. J. O’ Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, “Optical generation of very narrow linewidth millimeter wave signals,” Electron. Lett. 28, 2309–2311 (1992).
5. J. Yu, Z. Jia, L. Yi, Y. Su, G. K. Chang, and T. Wang, “Optical Millimeter-Wave Generation or Up-Conversion Using External Modulators,” IEEE Photon. Technol. Lett. 18, 265–267 (2006). [CrossRef]
6. J. Yu, Z. Jia, L. Xu, L. Chen, T. Wang, and G. K. Chang, “DWDM optical millimeter-wave generation for radio-over-fiber using an optical phase modulator and an optical interleaver,” IEEE Photon. Technol. Lett. 18, 1418–1420 (2006). [CrossRef]
7. A. Nirmalathas, D. Novak, C. Lim, and R. B. Waterhouse, “Wavelength reuse in the WDM optical interface of a millimeter-wave fiber-wireless antenna base station,” IEEE Trans. Microwave Theory Tech. Part 2 , 49, 2006–2009 (2001). [CrossRef]
8. J. Yu, J. Gu, Z. Jia, and G. K. Chang, “Seamless integration of an 8× 2.5Gb/s WDM-PON and Radio-Over-Fiber using all-optical up-conversion based on Raman-assisted FWM,” IEEE Photon. Technol. Lett. 17, 1986–1988 (2005). [CrossRef]
9. G. H. Smith, D. Novak, and Z. Ahmed, “Overcome chromatic-dispersion effects in fiber-wireless systems incorporating external modulators,” IEEE Trans. Microwave Theory Tech. 45, 1410– 1415 (1997). [CrossRef]
10. G. Qi, J. Yao, J. Seregelyi, S. Paquet, and C. Belisle, “Optical generation and distribution of continuously tunable millimeter-wave signals using an optical phase modulator,” J. Lightw. Technol. 23, 2687–2695(2005). [CrossRef]
11. L. Chen, H. Wen, and S. Wen, “A radio-over-fiber system with a novel scheme for millimeter-wave generation and wavelength reuse for up-link connection,” IEEE Photon. Technol. Lett. 18, 2056–2058 (2006). [CrossRef]
12. Z. Jia, J. Yu, and G. K. Chang, “A full-duplex radio-over-fiber system based on optical carrier suppression and reuse,” IEEE Photon. Technol. Lett. 18, 1726 –1728 (2006). [CrossRef]
13. J. Yu, Z. Jia, T. Wang, and G. K. Chang, “A Novel Radio-Over-Fiber Configuration Using Optical Phase Modulator to Generate an Optical mm-Wave and Centralized Lightwave for Uplink Connection,” IEEE Photon. Technol. Lett. 19, 140–142 (2007). [CrossRef]