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Full-duplex radio-over-fiber system with photonics frequency quadruples for optical millimeter-wave generation
Optical Fiber Technology 15 (2009) 290–295
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Full-duplex radio-over-fiber system with photonics frequency quadruples for optical millimeter-wave generation J. He ∗ , L. Chen, Z. Dong, S. Wen, J. Yu Key Laboratory for Micro/Nano Opto-Electronic Devices of Ministry of Education, School of Computer and Communication, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 2 September 2008 Revised 4 December 2008 Available online 31 January 2009 Keywords: Wavelength reuse Mach–Zehnder modulator Optical millimeter-wave with photonics frequency quadruple Radio-over-fiber
a b s t r a c t We have experimentally investigated two different schemes (schemes A and B) to generate optical millimeter-wave using optical frequency quadrupling with a Mach–Zehnder modulator (MZM), and wavelength reuse for uplink connection in the radio-over-fiber (ROF) systems. For scheme A, only one MZM is used for both the optical millimeter-wave generation and signal modulation. For scheme B, two MZMs are used. In this scheme, one of MZMs is used to generate optical millimeter-wave for frequency quadrupling, and another one is used for signal modulation. In both schemes, at the base station (BS), the optical carrier can be reused to carry upstream data and delivered to the central station (CS). By experimentally comparing the performance of downstream and upstream transmission in two schemes, it can be seen that scheme B can overcome the crosstalk between the upstream and downstream signals, but scheme A cannot. Meanwhile we also show that the millimeter-wave generated in scheme B has better quality and is almost robust to fiber chromatic dispersion. Crown Copyright © 2009 Published by Elsevier Inc. All rights reserved.
1. Introduction Recently radio-over-fiber (ROF) techniques have become attractive solutions in realizing future broadband wireless networks because ROF technique can be used for the distribution of wireless signals. Optical millimeter-wave signal generation and simple configuration of base station are key techniques to realize low cost and high transmission performance in the ROF-based optical wireless access networks. To realize optical millimeter-wave generation by frequency up-conversion, many techniques have been reported, such as the frequency up-conversions using four-wave mixing [1], optical heterodyne detection with optical interleaving [2] and cross-gain modulation in a semiconductor amplifier [3–5], frequency doubling using an optical carrier suppression modulation [6,7], and frequency quadrupling and sextupling using optical frequency multiplication technique [8]. Because a low cost RF oscillator can be used to generate optical millimeter-wave signal with frequency quadrupling and sextupling, it has been considered to be a cost-effective solution. In order to simplify the base station, it has proposed centralized lightwave at the central station or wavelength reuse in the base station. To realize wavelength reuse, different schemes have been proposed [9–13]. It employed a FBG to reflect the optical carrier and reuse it for uplink connection [9], but no any experiment has demonstrated how to realize this function for downstream
*
Corresponding author. E-mail address: [email protected] (J. He).
1068-5200/$ – see front matter Crown Copyright doi:10.1016/j.yofte.2008.12.006
© 2009 Published
data transmission. In [10], it utilized optical carrier suppression (OCS) to generate optical millimeter-wave, and then recombined the same optical carrier with the optical millimeter-wave before the upstream signals are transmitted to the base station. However, the electrical local oscillator (LO) frequency to generate the optical millimeter-wave is half of the spacing between the two first-order sidebands. For example, for the 40 GHz optical millimeter-wave generation, the LO frequency is 20 GHz. In the paper, we have experimentally demonstrated two different schemes to generate optical millimeter-wave using optical frequency quadrupling and wavelength reuse for uplink connection in the ROF systems. In the two schemes, a 40 GHz millimeter-wave signal was generated by a 10 GHz RF signal. The MZM driven by a large RF signal will lead to a large modulation depth; therefore the remained optical carrier is large. Hence this remained optical carrier can be reused to carry upstream data. In this way, it can effectively utilize optical power and reduce cost of system. We have also investigated the quality of the signals and transmission performance of downstream and upstream data considering the impact of fiber chromatic dispersion in the two schemes. 2. Principle Fig. 1 shows the principle of full-duplex ROF architecture in two different schemes to generate optical millimeter-wave with frequency quadruple and to realize wavelength reuse for upstream connection. The principle in scheme A based on sub-carrier multiplexing (SCM) technique is shown as Fig. 1(a). In the CS, the baseband downstream data are up-converted with the RF signal from
by Elsevier Inc. All rights reserved.
J. He et al. / Optical Fiber Technology 15 (2009) 290–295
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Fig. 1. Principle of two schemes of full-duplex ROF system with photonics frequency quadruple millimeter-wave. (a) Scheme A, (b) scheme B.
on the optical millimeter-wave by using an intensity modulator, which can be realized by another MZM. The central optical carrier will be combined with the downstream optical millimeter-wave signal by using a 3 dB optical coupler before they are transmitted to the BS. The BS is the same as that of scheme A. 3. Experimental setup and results
Fig. 2. MZM transmission curve.
the LO by an electrical mixer. The continuous wave (CW) lightwave is modulated via a LiNbO3 Mach–Zehnder modulator (MZM) driven by the up-converted signal. The MZM has two functions: (1) to generate high-order optical harmonic; (2) to modulate optical signal. If the MZM is DC biased at the top peak output power when the LO signal are removed, the odd-order sidebands can be suppressed. Therefore, the optical millimeter-wave signals with four times of LO frequency is generated after they are separated from the optical carrier by using a FBG [14,15]. The millimeter-wave signal is generated when the two second-order optical sidebands are beat at an optical/electrical (O/E) converter at the base station. The millimeter-wave signal will be broadcasted by an antenna after O/E conversion, while the reflected optical carrier from FBG is acted as the continuous wave and modulated by an intensity modulator driven by the upstream data. The upstream optical data will be sent back to the CS, where a low-cost low-frequency receiver detects the upstream data. The principle of scheme B is shown as Fig. 1(b). In scheme B, the CW lightwave is modulated via one MZM driven by the RF signal. The MZM transmission curve is shown as Fig. 2. The MZM is DC biased at the top peak output power when the LO signal are removed, which is the same as that of scheme A. After the MZM, the odd-order sidebands can be suppressed. Then an optical circulator and FBG are used to separate the central optical carrier and the second-order sidebands. The baseband downstream data is added
Fig. 3 shows the experimental setup of scheme A. The 2.5-Gb/s baseband signal of the downstream data with a pseudorandom binary sequence (PRBS) length of 231 –1 is up-converted with the 10 GHz RF signal by an electrical mixer. The CW at 1543.72 nm is modulated by a MZM driven by the up-converted signal. When the DC bias is 3.45 V and the peak-to-peak voltage of microwave signal for the electrical narrowband amplifier is 7.62 V, the odd-order sidebands are suppressed completely. The half-wave voltage of the MZM is 7.28 V. The optical spectrum after SCM technique modulation is inserted in Fig. 3(a). The wavelength spacing between the two second-order sidebands is 0.32 nm (40 GHz). The secondorder sideband is 12 dB lower than the optical carrier while the first-order sideband is suppressed completely. After transmission over 20 km SMF with a dispersion of 17 ps/nm/km, a FBG with the central wavelength at 1543.72 nm is used to reflect the optical carrier while pass the optical millimeter-wave signal to O/E receiver. The optical spectrum after passing through the FBG is inserted in Fig. 3(b). The 40 GHz millimeter-wave signal with four times of RF frequency is generated by beating the two second-order optical sidebands at the O/E receiver. The optical spectrum of the optical carrier reflected by the FBG is inserted in Fig. 3(c). For the uplink, the reflected optical carrier from the FBG is modulated by another intensity modulator driven by the 2.5-Gb/s upstream data with a PRBS length of 231 –1. After modulation, the optical spectrum is inserted in Fig. 3(d). Then the upstream optical signal is injected into another 20 km SMF and transmitted back to the CS. In real networks, one duplexer to connect the antenna can be used to circulate the transmitting and receiving signal at the BS. The baseband upstream signals would be obtained after down-conversion of end user’s information coming from the duplexer in the BS. The measured eye diagrams of the millimeter-wave with downstream data before and after transmission over 20 km SMF are shown in Figs. 4(a) and 4(b). After a PIN photodiode, the converted electrical signal is amplified by an electrical amplifier with
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Fig. 3. Experimental setup for scheme A: optical millimeter-wave generation by using SCM modulation based on frequency-quadrupled. MZM: LiNbO3 Mach–Zehnder modulator; FBG: fiber Bragg grating; Cir: circulator.
Fig. 4. Measured eye diagrams of millimeter-wave signals. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 5. Measured eye diagrams of downstream data. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 6. Measured eye diagrams of upstream data. (a) Back-to-back, (b) transmission 20 km SMF.
a bandwidth of 10 GHz. The measured eye diagrams of the demodulated downstream data before and after transmission over 20 km SMF are shown in Figs. 5(a) and 5(b), respectively. The power fluctuation of the 40 GHz modulation arises from the chromatic dispersion. The remained optical carrier reflected from the FBG is re-modulated by a 2.5-Gb/s upstream data. The measured eye diagrams of the upstream data before and after transmission
over 20 km SMF are shown in Figs. 6(a) and 6(b), respectively. It can be seen that a part of downstream data are transmitted over the upstream SMF to the CS. The reason is that a part of downstream signals have been carried by the optical carrier reflected from the FBG at BS. Once the optical carrier is reused to modulate upstream data, the downstream data will be existence. As the DC bias is decreased, the power of the second-order sideband and the
J. He et al. / Optical Fiber Technology 15 (2009) 290–295
Fig. 7. BER curves for both downstream and upstream data.
crosstalk effect from the downlink will be decreased. However, we have not realize the frequency quadrupled modulation when the DC bias is too far away from the idea value to generate quadrupled modulation. The measured bit-error-rate (BER) performance for both downstream and upstream signals is shown in Fig. 7. After 20 km transmission, the power penalty caused by fiber chromatic dispersion is 1 dB for the downstream data, meanwhile the power penalty after transmission over 20 km SMF for the upstream data is smaller than 1 dB. Therefore, the perfect BER performance for both downstream and upstream can be obtained by properly adjusting the DC bias of the MZM in scheme A. Fig. 8 shows the experimental setup of scheme B. The optical millimeter-wave is generated using a MZM along with a FBG. The MZM with a half-wave voltage of 7.28 V is driven by a 10 GHz RF sinusoidal wave with a peak-to-peak voltage of 9.51 V. The optical spectrum after modulation is inserted in Fig. 8(a). The power of the optical carrier is 10 dB larger than that of the second-order side-
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band. As the DC bias is 3.6 V, the MZM is biased at the maximum output power so that the odd-order sidebands can be suppressed. Two second-order sidebands are obtained after using a circulator and a FBG to filter out optical carrier. In this way, the optical millimeter-wave with two second-order sidebands has four times the RF frequency. Then the generated optical millimeter-wave is amplified by an EDFA and modulated by an intensity modulator driven by 2.5-Gb/s electrical signal. The optical spectrum of the optical millimeter-wave signal with downstream data is inserted in Fig. 8(b). We can see that the optical carrier and odd-order sidebands are removed. The remained optical carrier is reflected from another output of FBG, and the optical spectrum is inserted in Fig. 8(c). After combined by a 3 dB optical coupler, the optical millimeter-wave signal and the remained optical carrier are transmitted over 20 km SMF with dispersion of 17 ps/nm/km before reaching the BS. At the BS, another circulator and FBG are used to separate the optical millimeter-wave signal and the remained optical carrier. The FBG filter has a 3 dB reflection bandwidth of 0.15 nm and reflection ratio larger than 25 dB at the reflection peak wavelength, which is used to separate the optical second-order sidebands (downstream signals) and the optical carrier. The separated downstream signals are detected via a PIN photodiode with a 3 dB bandwidth of 60 GHz after they are boosted by a regular EDFA with a small-signal gain of 30 dB, and then filtered by a tunable optical filter with the bandwidth of 0.5 nm to suppress amplified spontaneous emission noise. The converted electrical signal is amplified by an electrical amplifier with a bandwidth of 10 GHz centered at 40 GHz. In our experiment, the downstream and upstream data are two 2.5-Gb/s pseudorandom bit sequence electrical signals with a word length of 231 –1 generated from different sources. The optical spectrum of the upstream optical signal is shown in inserted in Fig. 8(d). At the CS, the upstream data is detected by a commercial PIN receiver.
Fig. 8. Experimental setup for scheme B: optical millimeter-wave generation by using OCS modulation based on frequency-quadrupled. MZM: LiNbO3 Mach–Zehnder modulator; FBG: fiber Bragg grating; Cir: circulator.
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Fig. 9. Measured eye diagrams of millimeter-wave signals. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 10. Measured eye diagrams of downstream data. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 11. Measured eye diagrams of upstream data. (a) Back-to-back, (b) transmission 20 km SMF.
Here we compare the transmission performance of downstream and upstream data in the two schemes. In scheme A, the millimeter-wave with frequency quadruple is generated only by one MZM; hence this configuration in CS is simpler. But the upstream data contains a part of downstream data as shown in Figs. 6(a) and 6(b). In scheme B, the configuration is more complicate and expensive since two MZMs are used. However, the upstream signal has better performance, which can be seen that the eye of the upstream signals still keeps open despite transmission over 20 km SMF in Fig. 11. It means that the remained optical carrier separated from the downstream signals in this scheme has negligible effect on the transmission performance of millimeterwave with downstream data. Fig. 12. BER curves of upstream data at 2.5-Gb/s.
Figs. 9(a) and 9(b) show the eye diagrams of optical millimeterwave before and after transmission over 20 km SMF, respectively. Fiber dispersion re-shapes the optical millimeter-wave signal, which leads a different shape compared with the original millimeter-wave signal without transmission fiber. The downconverted eye diagrams of downstream data before and after transmission over 20 km SMF are shown in Fig. 10. It shows the fiber chromatic dispersion impacts on eye diagram of downstream data after transmission over 20 km SMF. But it is clearly seen that the eye diagrams still keep open after transmission over 20 km SMF. The eye diagram of the upstream data for back-to-back and transmission over 20 km SMF are shown in Fig. 11. It can be seen from the eye diagrams that there have small amplitude fluctuation caused by the wide bandwidth of the FBG. The BER performance of the upstream data for scheme A and scheme B are shown in Fig. 12. For upstream data in scheme B, the power penalty is 0.1 dB at a BER of 10−9 . It can be seen that receiver sensitivity of scheme B is better than that of scheme A. Therefore scheme B can overcome the crosstalk between the upstream and downstream signals obviously. Meanwhile it also show that the millimeter-wave generated in scheme B has better quality and is almost robust to fiber chromatic dispersion.
4. Conclusions We have experimentally demonstrated two schemes of fullduplex ROF systems with frequency-quadrupled optical millimeterwave generation and wavelength reuse for uplink connection. In the two schemes, as a MZM driven by an RF at 10 GHz, 40 GHz millimeter-wave can be generated, which can greatly reduce the bandwidth of microwave component and modulator. At the same time, since the remained optical carrier can be reused to carry upstream data at BS, it can effectively utilize optical power and reduce cost of system. We also compared two schemes considering the impact of fiber chromatic dispersion. For scheme A, the millimeter-wave with frequency quadruple was generated only by one MZM. However, the generated millimeter-wave and optical carrier will be contained in downstream data at CS. Once the optical carrier reflected from the FBG at BS is reused to modulate upstream data, the crosstalk effect from downstream data will be existence. Therefore the millimeter-wave power fluctuates due to constructive and destructive interaction between the two beatings induced by chromatic dispersion. For scheme B, only the generated millimeter-wave is modulated by downstream data, the optical carrier is not. So the upstream data carried by the optical carrier have better performance. By experimentally comparing the performance of downstream and upstream transmission in two schemes, it can
J. He et al. / Optical Fiber Technology 15 (2009) 290–295
be seen that scheme B is better in the quality of the generated millimeter-wave, and the generated millimeter-wave signal with downstream data is almost robust to fiber chromatic dispersion in scheme B. Acknowledgments This work is partially supported by National 863 High Technology Research and Development Program of China (Grant No. 2007AA01Z263), the Hunan Provincial Natural Science Foundation of China (Grant No. 06JJ50108) and the Open Fund of Key Laboratory of Optical Communication and Lightwave Technologies (Beijing University of Posts and Telecommunications, Ministry of Education, PR China). References [1] J. Yu, J. Gu, X. Liu, Z. Jia, G. Chang, Seamless integration of an 8 × 2.5 Gb/s WDM-PON and radio-over-fiber using all-optical up-conversion based on Raman-assisted FWM, IEEE Photon. Technol. Lett. 17 (2005) 1986–1988. [2] T. Kuri, K. Kitayama, Y. Takahashi, A single light-source configuration for fullduplex 60-GHz-band radio-on fiber system, IEEE Trans. Microwave Theory Tech. 51 (2003) 431–439. [3] J. Seo, Y. Seo, W. Choi, 1.244 Gb/s data distribution in 60 GHz remote optical frequency upconversion systems, IEEE Photon. Technol. Lett. 18 (2005) 1389– 1391. [4] J. Yu, Z. Jia, G. Chang, All-optical mixer based on cross-absorption modulation in electroabsorption modulator, IEEE Photon. Technol. Lett. 18 (2005) 2421– 2423.
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[5] H. Song, J. Lee, Error-free simultaneous all-optical upconversion of WDM radioover-fiber signals, IEEE Photon. Technol. Lett. 17 (2005) 1731–1733. [6] J. Yu, Z. Jia, L. Yi, Y. Su, G. Chang, Optical millimeter-wave generation or upconversion using external modulators, IEEE Photon. Technol. Lett. 18 (2006) 265–267. [7] J. Yu, Z. Jia, L. Xu, L. Chen, T. Wang, G. Chang, DWDM optical millimeter-wave generation for radio-over-fiber using an optical phase modulator and an optical interleaver, IEEE Photon. Technol. Lett. 18 (2006) 1418–1420. [8] J.J. O’Reilly, P.M. Lane, Fiber-supported optical generation and delivery of 60 GHz signals, Electron. Lett. 30 (1994) 1329–1330. [9] A. Kaszubowska, L. Hu, L.P. Barry, Remote down-conversion with wavelength reuse for the radio/fiber uplink connection, IEEE Photon. Technol. Lett. 18 (2006) 562–564. [10] Z. Jia, J. Yu, Chang, G. Chang, A full-duplex radio-over-fiber system based on optical carrier suppression and reuse, IEEE Photon. Technol. Lett. 18 (2006) 1726–1728. [11] J. Yu, M. Huang, Z. Jia, T. Wang, G.K. Chang, A novel scheme to generate singlesideband millimeter-wave signals by using low-frequency local oscillator signal, IEEE Photon. Technol. Lett. 20 (2008) 478–480. [12] J. Yu, Z. Jia, T. Wang, 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 (2007) 140– 142. [13] J. Yu, Z. Jia, T. Wang, G.K. Chang, Centralized lightwave radio-over-fiber system with photonic frequency quadrupling for high-frequency millimeter-wave generation, IEEE Photon. Technol. Lett. 19 (2007) 1499–1501. [14] M. Attygalle, C. Lim, G. Pendock, A. Nirmalathas, G. Edvell, Transmission improvement in fiber wireless links using fiber Bragg gratings, IEEE Photon. Technol. Lett. 17 (2005) 190–192. [15] M. Bakaul, A. Nirmalathas, C. Lim, Efficient multiplexing scheme for wavelength-interleaved DWDM millimeter-wave fiber-radio systems, IEEE Photon. Technol. Lett. 17 (2005) 2718–2720.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Full-duplex radio-over-fiber system with photonics frequency quadruples for optical millimeter-wave generation J. He ∗ , L. Chen, Z. Dong, S. Wen, J. Yu Key Laboratory for Micro/Nano Opto-Electronic Devices of Ministry of Education, School of Computer and Communication, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 2 September 2008 Revised 4 December 2008 Available online 31 January 2009 Keywords: Wavelength reuse Mach–Zehnder modulator Optical millimeter-wave with photonics frequency quadruple Radio-over-fiber
a b s t r a c t We have experimentally investigated two different schemes (schemes A and B) to generate optical millimeter-wave using optical frequency quadrupling with a Mach–Zehnder modulator (MZM), and wavelength reuse for uplink connection in the radio-over-fiber (ROF) systems. For scheme A, only one MZM is used for both the optical millimeter-wave generation and signal modulation. For scheme B, two MZMs are used. In this scheme, one of MZMs is used to generate optical millimeter-wave for frequency quadrupling, and another one is used for signal modulation. In both schemes, at the base station (BS), the optical carrier can be reused to carry upstream data and delivered to the central station (CS). By experimentally comparing the performance of downstream and upstream transmission in two schemes, it can be seen that scheme B can overcome the crosstalk between the upstream and downstream signals, but scheme A cannot. Meanwhile we also show that the millimeter-wave generated in scheme B has better quality and is almost robust to fiber chromatic dispersion. Crown Copyright © 2009 Published by Elsevier Inc. All rights reserved.
1. Introduction Recently radio-over-fiber (ROF) techniques have become attractive solutions in realizing future broadband wireless networks because ROF technique can be used for the distribution of wireless signals. Optical millimeter-wave signal generation and simple configuration of base station are key techniques to realize low cost and high transmission performance in the ROF-based optical wireless access networks. To realize optical millimeter-wave generation by frequency up-conversion, many techniques have been reported, such as the frequency up-conversions using four-wave mixing [1], optical heterodyne detection with optical interleaving [2] and cross-gain modulation in a semiconductor amplifier [3–5], frequency doubling using an optical carrier suppression modulation [6,7], and frequency quadrupling and sextupling using optical frequency multiplication technique [8]. Because a low cost RF oscillator can be used to generate optical millimeter-wave signal with frequency quadrupling and sextupling, it has been considered to be a cost-effective solution. In order to simplify the base station, it has proposed centralized lightwave at the central station or wavelength reuse in the base station. To realize wavelength reuse, different schemes have been proposed [9–13]. It employed a FBG to reflect the optical carrier and reuse it for uplink connection [9], but no any experiment has demonstrated how to realize this function for downstream
*
Corresponding author. E-mail address: [email protected] (J. He).
1068-5200/$ – see front matter Crown Copyright doi:10.1016/j.yofte.2008.12.006
© 2009 Published
data transmission. In [10], it utilized optical carrier suppression (OCS) to generate optical millimeter-wave, and then recombined the same optical carrier with the optical millimeter-wave before the upstream signals are transmitted to the base station. However, the electrical local oscillator (LO) frequency to generate the optical millimeter-wave is half of the spacing between the two first-order sidebands. For example, for the 40 GHz optical millimeter-wave generation, the LO frequency is 20 GHz. In the paper, we have experimentally demonstrated two different schemes to generate optical millimeter-wave using optical frequency quadrupling and wavelength reuse for uplink connection in the ROF systems. In the two schemes, a 40 GHz millimeter-wave signal was generated by a 10 GHz RF signal. The MZM driven by a large RF signal will lead to a large modulation depth; therefore the remained optical carrier is large. Hence this remained optical carrier can be reused to carry upstream data. In this way, it can effectively utilize optical power and reduce cost of system. We have also investigated the quality of the signals and transmission performance of downstream and upstream data considering the impact of fiber chromatic dispersion in the two schemes. 2. Principle Fig. 1 shows the principle of full-duplex ROF architecture in two different schemes to generate optical millimeter-wave with frequency quadruple and to realize wavelength reuse for upstream connection. The principle in scheme A based on sub-carrier multiplexing (SCM) technique is shown as Fig. 1(a). In the CS, the baseband downstream data are up-converted with the RF signal from
by Elsevier Inc. All rights reserved.
J. He et al. / Optical Fiber Technology 15 (2009) 290–295
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Fig. 1. Principle of two schemes of full-duplex ROF system with photonics frequency quadruple millimeter-wave. (a) Scheme A, (b) scheme B.
on the optical millimeter-wave by using an intensity modulator, which can be realized by another MZM. The central optical carrier will be combined with the downstream optical millimeter-wave signal by using a 3 dB optical coupler before they are transmitted to the BS. The BS is the same as that of scheme A. 3. Experimental setup and results
Fig. 2. MZM transmission curve.
the LO by an electrical mixer. The continuous wave (CW) lightwave is modulated via a LiNbO3 Mach–Zehnder modulator (MZM) driven by the up-converted signal. The MZM has two functions: (1) to generate high-order optical harmonic; (2) to modulate optical signal. If the MZM is DC biased at the top peak output power when the LO signal are removed, the odd-order sidebands can be suppressed. Therefore, the optical millimeter-wave signals with four times of LO frequency is generated after they are separated from the optical carrier by using a FBG [14,15]. The millimeter-wave signal is generated when the two second-order optical sidebands are beat at an optical/electrical (O/E) converter at the base station. The millimeter-wave signal will be broadcasted by an antenna after O/E conversion, while the reflected optical carrier from FBG is acted as the continuous wave and modulated by an intensity modulator driven by the upstream data. The upstream optical data will be sent back to the CS, where a low-cost low-frequency receiver detects the upstream data. The principle of scheme B is shown as Fig. 1(b). In scheme B, the CW lightwave is modulated via one MZM driven by the RF signal. The MZM transmission curve is shown as Fig. 2. The MZM is DC biased at the top peak output power when the LO signal are removed, which is the same as that of scheme A. After the MZM, the odd-order sidebands can be suppressed. Then an optical circulator and FBG are used to separate the central optical carrier and the second-order sidebands. The baseband downstream data is added
Fig. 3 shows the experimental setup of scheme A. The 2.5-Gb/s baseband signal of the downstream data with a pseudorandom binary sequence (PRBS) length of 231 –1 is up-converted with the 10 GHz RF signal by an electrical mixer. The CW at 1543.72 nm is modulated by a MZM driven by the up-converted signal. When the DC bias is 3.45 V and the peak-to-peak voltage of microwave signal for the electrical narrowband amplifier is 7.62 V, the odd-order sidebands are suppressed completely. The half-wave voltage of the MZM is 7.28 V. The optical spectrum after SCM technique modulation is inserted in Fig. 3(a). The wavelength spacing between the two second-order sidebands is 0.32 nm (40 GHz). The secondorder sideband is 12 dB lower than the optical carrier while the first-order sideband is suppressed completely. After transmission over 20 km SMF with a dispersion of 17 ps/nm/km, a FBG with the central wavelength at 1543.72 nm is used to reflect the optical carrier while pass the optical millimeter-wave signal to O/E receiver. The optical spectrum after passing through the FBG is inserted in Fig. 3(b). The 40 GHz millimeter-wave signal with four times of RF frequency is generated by beating the two second-order optical sidebands at the O/E receiver. The optical spectrum of the optical carrier reflected by the FBG is inserted in Fig. 3(c). For the uplink, the reflected optical carrier from the FBG is modulated by another intensity modulator driven by the 2.5-Gb/s upstream data with a PRBS length of 231 –1. After modulation, the optical spectrum is inserted in Fig. 3(d). Then the upstream optical signal is injected into another 20 km SMF and transmitted back to the CS. In real networks, one duplexer to connect the antenna can be used to circulate the transmitting and receiving signal at the BS. The baseband upstream signals would be obtained after down-conversion of end user’s information coming from the duplexer in the BS. The measured eye diagrams of the millimeter-wave with downstream data before and after transmission over 20 km SMF are shown in Figs. 4(a) and 4(b). After a PIN photodiode, the converted electrical signal is amplified by an electrical amplifier with
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Fig. 3. Experimental setup for scheme A: optical millimeter-wave generation by using SCM modulation based on frequency-quadrupled. MZM: LiNbO3 Mach–Zehnder modulator; FBG: fiber Bragg grating; Cir: circulator.
Fig. 4. Measured eye diagrams of millimeter-wave signals. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 5. Measured eye diagrams of downstream data. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 6. Measured eye diagrams of upstream data. (a) Back-to-back, (b) transmission 20 km SMF.
a bandwidth of 10 GHz. The measured eye diagrams of the demodulated downstream data before and after transmission over 20 km SMF are shown in Figs. 5(a) and 5(b), respectively. The power fluctuation of the 40 GHz modulation arises from the chromatic dispersion. The remained optical carrier reflected from the FBG is re-modulated by a 2.5-Gb/s upstream data. The measured eye diagrams of the upstream data before and after transmission
over 20 km SMF are shown in Figs. 6(a) and 6(b), respectively. It can be seen that a part of downstream data are transmitted over the upstream SMF to the CS. The reason is that a part of downstream signals have been carried by the optical carrier reflected from the FBG at BS. Once the optical carrier is reused to modulate upstream data, the downstream data will be existence. As the DC bias is decreased, the power of the second-order sideband and the
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Fig. 7. BER curves for both downstream and upstream data.
crosstalk effect from the downlink will be decreased. However, we have not realize the frequency quadrupled modulation when the DC bias is too far away from the idea value to generate quadrupled modulation. The measured bit-error-rate (BER) performance for both downstream and upstream signals is shown in Fig. 7. After 20 km transmission, the power penalty caused by fiber chromatic dispersion is 1 dB for the downstream data, meanwhile the power penalty after transmission over 20 km SMF for the upstream data is smaller than 1 dB. Therefore, the perfect BER performance for both downstream and upstream can be obtained by properly adjusting the DC bias of the MZM in scheme A. Fig. 8 shows the experimental setup of scheme B. The optical millimeter-wave is generated using a MZM along with a FBG. The MZM with a half-wave voltage of 7.28 V is driven by a 10 GHz RF sinusoidal wave with a peak-to-peak voltage of 9.51 V. The optical spectrum after modulation is inserted in Fig. 8(a). The power of the optical carrier is 10 dB larger than that of the second-order side-
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band. As the DC bias is 3.6 V, the MZM is biased at the maximum output power so that the odd-order sidebands can be suppressed. Two second-order sidebands are obtained after using a circulator and a FBG to filter out optical carrier. In this way, the optical millimeter-wave with two second-order sidebands has four times the RF frequency. Then the generated optical millimeter-wave is amplified by an EDFA and modulated by an intensity modulator driven by 2.5-Gb/s electrical signal. The optical spectrum of the optical millimeter-wave signal with downstream data is inserted in Fig. 8(b). We can see that the optical carrier and odd-order sidebands are removed. The remained optical carrier is reflected from another output of FBG, and the optical spectrum is inserted in Fig. 8(c). After combined by a 3 dB optical coupler, the optical millimeter-wave signal and the remained optical carrier are transmitted over 20 km SMF with dispersion of 17 ps/nm/km before reaching the BS. At the BS, another circulator and FBG are used to separate the optical millimeter-wave signal and the remained optical carrier. The FBG filter has a 3 dB reflection bandwidth of 0.15 nm and reflection ratio larger than 25 dB at the reflection peak wavelength, which is used to separate the optical second-order sidebands (downstream signals) and the optical carrier. The separated downstream signals are detected via a PIN photodiode with a 3 dB bandwidth of 60 GHz after they are boosted by a regular EDFA with a small-signal gain of 30 dB, and then filtered by a tunable optical filter with the bandwidth of 0.5 nm to suppress amplified spontaneous emission noise. The converted electrical signal is amplified by an electrical amplifier with a bandwidth of 10 GHz centered at 40 GHz. In our experiment, the downstream and upstream data are two 2.5-Gb/s pseudorandom bit sequence electrical signals with a word length of 231 –1 generated from different sources. The optical spectrum of the upstream optical signal is shown in inserted in Fig. 8(d). At the CS, the upstream data is detected by a commercial PIN receiver.
Fig. 8. Experimental setup for scheme B: optical millimeter-wave generation by using OCS modulation based on frequency-quadrupled. MZM: LiNbO3 Mach–Zehnder modulator; FBG: fiber Bragg grating; Cir: circulator.
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Fig. 9. Measured eye diagrams of millimeter-wave signals. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 10. Measured eye diagrams of downstream data. (a) Back-to-back, (b) transmission 20 km SMF.
Fig. 11. Measured eye diagrams of upstream data. (a) Back-to-back, (b) transmission 20 km SMF.
Here we compare the transmission performance of downstream and upstream data in the two schemes. In scheme A, the millimeter-wave with frequency quadruple is generated only by one MZM; hence this configuration in CS is simpler. But the upstream data contains a part of downstream data as shown in Figs. 6(a) and 6(b). In scheme B, the configuration is more complicate and expensive since two MZMs are used. However, the upstream signal has better performance, which can be seen that the eye of the upstream signals still keeps open despite transmission over 20 km SMF in Fig. 11. It means that the remained optical carrier separated from the downstream signals in this scheme has negligible effect on the transmission performance of millimeterwave with downstream data. Fig. 12. BER curves of upstream data at 2.5-Gb/s.
Figs. 9(a) and 9(b) show the eye diagrams of optical millimeterwave before and after transmission over 20 km SMF, respectively. Fiber dispersion re-shapes the optical millimeter-wave signal, which leads a different shape compared with the original millimeter-wave signal without transmission fiber. The downconverted eye diagrams of downstream data before and after transmission over 20 km SMF are shown in Fig. 10. It shows the fiber chromatic dispersion impacts on eye diagram of downstream data after transmission over 20 km SMF. But it is clearly seen that the eye diagrams still keep open after transmission over 20 km SMF. The eye diagram of the upstream data for back-to-back and transmission over 20 km SMF are shown in Fig. 11. It can be seen from the eye diagrams that there have small amplitude fluctuation caused by the wide bandwidth of the FBG. The BER performance of the upstream data for scheme A and scheme B are shown in Fig. 12. For upstream data in scheme B, the power penalty is 0.1 dB at a BER of 10−9 . It can be seen that receiver sensitivity of scheme B is better than that of scheme A. Therefore scheme B can overcome the crosstalk between the upstream and downstream signals obviously. Meanwhile it also show that the millimeter-wave generated in scheme B has better quality and is almost robust to fiber chromatic dispersion.
4. Conclusions We have experimentally demonstrated two schemes of fullduplex ROF systems with frequency-quadrupled optical millimeterwave generation and wavelength reuse for uplink connection. In the two schemes, as a MZM driven by an RF at 10 GHz, 40 GHz millimeter-wave can be generated, which can greatly reduce the bandwidth of microwave component and modulator. At the same time, since the remained optical carrier can be reused to carry upstream data at BS, it can effectively utilize optical power and reduce cost of system. We also compared two schemes considering the impact of fiber chromatic dispersion. For scheme A, the millimeter-wave with frequency quadruple was generated only by one MZM. However, the generated millimeter-wave and optical carrier will be contained in downstream data at CS. Once the optical carrier reflected from the FBG at BS is reused to modulate upstream data, the crosstalk effect from downstream data will be existence. Therefore the millimeter-wave power fluctuates due to constructive and destructive interaction between the two beatings induced by chromatic dispersion. For scheme B, only the generated millimeter-wave is modulated by downstream data, the optical carrier is not. So the upstream data carried by the optical carrier have better performance. By experimentally comparing the performance of downstream and upstream transmission in two schemes, it can
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be seen that scheme B is better in the quality of the generated millimeter-wave, and the generated millimeter-wave signal with downstream data is almost robust to fiber chromatic dispersion in scheme B. Acknowledgments This work is partially supported by National 863 High Technology Research and Development Program of China (Grant No. 2007AA01Z263), the Hunan Provincial Natural Science Foundation of China (Grant No. 06JJ50108) and the Open Fund of Key Laboratory of Optical Communication and Lightwave Technologies (Beijing University of Posts and Telecommunications, Ministry of Education, PR China). References [1] J. Yu, J. Gu, X. Liu, Z. Jia, G. Chang, Seamless integration of an 8 × 2.5 Gb/s WDM-PON and radio-over-fiber using all-optical up-conversion based on Raman-assisted FWM, IEEE Photon. Technol. Lett. 17 (2005) 1986–1988. [2] T. Kuri, K. Kitayama, Y. Takahashi, A single light-source configuration for fullduplex 60-GHz-band radio-on fiber system, IEEE Trans. Microwave Theory Tech. 51 (2003) 431–439. [3] J. Seo, Y. Seo, W. Choi, 1.244 Gb/s data distribution in 60 GHz remote optical frequency upconversion systems, IEEE Photon. Technol. Lett. 18 (2005) 1389– 1391. [4] J. Yu, Z. Jia, G. Chang, All-optical mixer based on cross-absorption modulation in electroabsorption modulator, IEEE Photon. Technol. Lett. 18 (2005) 2421– 2423.
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