A full-duplex CATV/wireless-over-fiber lightwave transmission system Chung-Yi Li,1 Hai-Han Lu,1,* Cheng-Ling Ying,2 Chun-Jen Cheng1 Che-Yu Lin,1 Zhi-Wei Wan,1 and Jian-Hua Chen1 2
1 Institute of Electro-Optical Engineering, National Taipei University of Technology, Taipei, 106 Taiwan Department of Electronic Engineering, Jinwen University of Science and Technology, New Taipei City, 231, Taiwan *[email protected]
Abstract: A full-duplex CATV/wireless-over-fiber lightwave transmission system consisting of one broadband light source (BLS), two optical interleavers (ILs), one intensity modulator, and one phase modulator is proposed and experimentally demonstrated. The downstream light is optically promoted from 10Gbps/25GHz microwave (MW) data signal to 10Gbps/100GHz and 10Gbps/50GHz millimeter-wave (MMW) data signals in fiber-wireless convergence, and intensity-modulated with 50-550 MHz CATV signal. For up-link transmission, the downstream light is phaseremodulated with 10Gbps/25GHz MW data signal in fiber-wireless convergence. Over a 40-km single-mode fiber (SMF) and a 10-m radio frequency (RF) wireless transport, bit error rate (BER), carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are observed to perform well in such full-duplex CATV/wirelessover-fiber lightwave transmission systems. This full-duplex 100-GHz/50GHz/25-GHz/550-MHz lightwave transmission system is an attractive alternative. This transmission system not only presents its advancement in the integration of fiber backbone and CATV/wireless feeder networks, but also it provides the advantages of a communication channel for higher data rates and bandwidth. ©2015 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2360) Fiber optics links and subsystems; (350.4010) Microwaves.
References and links 1.
C. Y. Ying, C. Y. Li, H. H. Lu, C. H. Chang, J. H. Chen, and J. R. Zheng, “A hybrid WDM lightwave transport system based on fiber-wireless and fiber-VLLC convergences,” IEEE Photon. J. 6(6), 7200709 (2014). . C. Tang, X. Li, F. Li, J. Zhang, and J. Xiao, “A 30 Gb/s full-duplex bi-directional transmission optical wirelessover fiber integration system at W-band,” Conf. on Opt. Fiber Commun. (OFC) W2A.4 (2014). 3. C. Lim, Y. Yang, and A. Nirmalathas, “Wireless signals transport in fiber-wireless links: digitized versus analog,” Conf. on Opt. Internet (COIN) TC2–1 (2014). 4. C. Ye, L. Zhang, M. Zhu, J. Yu, S. He, and G. K. Chang, “A bidirectional 60-GHz wireless-over-fiber transport system with centralized local oscillator service delivered to mobile terminals and base stations,” IEEE Photon. Technol. Lett. 24(22), 1984–1987 (2012). 5. C. Y. Lin, H. H. Lu, C. Y. Li, P. Y. Wu, P. C. Peng, T. W. Jhang, and C. Y. Lin, “Employing injection-locked FP LDs to set up a hybrid CATV/MW/MMW WDM light wave transmission system,” Opt. Lett. 39(13), 3931– 3934 (2014). 6. L. M. Pessoa, D. Coelho, and H. M. Salgado, “Experimental evaluation of a digitized fiber-wireless system employing sigma delta modulation,” Opt. Express 22(14), 17508–17523 (2014). 7. C. Y. Li, H. H. Lu, C. H. Chang, C. Y. Lin, P. Y. Wu, J. R. Zheng, and C. R. Lin, “Bidirectional hybrid PMbased RoF and VCSEL-based VLLC system,” Opt. Express 22(13), 16188–16196 (2014). 8. J. Yu, Z. Jia, L. Xu, L. Chen, T. Wang, and G. Kung Chang, “DWDM optical millimeter-wave generation for radio-over-fiber using an optical phase modulator and an optical interleaver,” IEEE Photon. Technol. Lett. 18(13), 1418–1420 (2006). 9. R. Leners, P. Emplit, D. Foursa, M. Haelterman, and R. Kashyap, “6.1-GHz dark-soliton generation and propagation by a fiber Bragg grating pulse-shaping technique,” J. Opt. Soc. Am. B 14(9), 2339–2347 (1997). 10. M. Aamer, A. Griol, A. Brimont, A. M. Gutierrez, P. Sanchis, and A. Håkansson, “Increased sensitivity through maximizing the extinction ratio of SOI delay-interferometer receiver for 10G DPSK,” Opt. Express 20(13), 14698–14704 (2012).
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Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9221
11. P. Y. Wu, H. H. Lu, C. L. Ying, C. Y. Li, and H. S. Su, “An up-converted phase modulated fiber optical CATV transport system,” J. Lightwave Technol. 29(16), 2422–2427 (2011). 12. H. Olesen and G. Jacobsen, “A theoretical and experimental analysis of modulated laser fields and power spectra,” IEEE J. Quantum Electron. 18(12), 2069–2080 (1982). 13. C. H. Chang, P. C. Peng, H. H. Lu, C. L. Shih, and H. W. Chen, “Simplified radio-over-fiber transport systems with a low-cost multiband light source,” Opt. Lett. 35(23), 4021–4023 (2010). 14. W. Y. Lin, C. Y. Chen, H. H. Lu, C. H. Chang, Y. P. Lin, H. C. Lin, and H. W. Wu, “10m/500 Mbps WDM visible light communication systems,” Opt. Express 20(9), 9919–9924 (2012). 15. D. Menamara, Y. Fukasawa, Y. Wakabayashi, Y. Shirakawa, and Y. Kakuta, “750MHz power doubler and pushpull CATV hybrid modules using Gallium Arsenide,” NCTA Technical Papers. 19–26 (1996). 16. M. Jeffers, NCTA Recommended Practices for Measurements on Cable Television Systems, NCTA, (1989).
1. Introduction With the rapid development of CATV, radio frequency (RF) wireless, and lightwave transmission technologies, the increasing requirements raise the demands for high-speed and high bandwidth applications, not only for the fiber-based backbone network, but also for the CATV/RF wireless-based feeder ones. A network that can provide both wired and wireless communications simultaneously is required because of the increased demands for large bandwidth and high-speed data rate [1–4]. By combining the capacity of the optical fiber network with the ubiquity and mobility of wireless and CATV networks, CATV/wirelessover-fiber lightwave transmission systems form a powerful platform for the support and creation of future applications and services [5,6]. In this paper, a full-duplex CATV/wirelessover-fiber lightwave transmission system consisting of one broadband light source (BLS), two optical interleavers (ILs), one intensity modulator, and one phase modulator is proposed and experimentally demonstrated. This transmission system transports downstream intensitymodulated 100-GHz millimeter-wave (MMW)/50-GHz MMW/550-MHz CATV signals and upstream phase-remodulated 25-GHz microwave (MW) signal. A BLS, comprising a distributed feedback laser diode (DFB LD) with a multi-sideband output and an optical signal-to-noise ratio (OSNR) enhancement scheme, is utilized in a full-duplex CATV/wireless-over-fiber lightwave transmission system for 100-GHz/50-GHz/25-GHz/550MHz signal transport. Employing a multi-sideband coherent-lightwave generator can support various transmissions concurrently without any signal mixing process in an electrical domain. Two optical ILs are deployed in the system to separate even and odd optical sidebands of an optical signal. The optical IL has two output ports; one output port provides the optical signal only with the even sidebands, and the other output port provides the optical signal only with the odd sidebands [7,8]. The −2 and + 2 sidebands (even sideband) are used for 100-GHz MMW down-link transmission, and the −1 and + 1 sidebands (odd sideband) are used for 50GHz MMW down-link transmission. Furthermore, the central carrier is employed for CATV down-link transmission and 25-GHz MW up-link transmission. This paper is the first to use one BLS, two optical ILs, and one phase modulator in a full-duplex CATV/wireless-overfiber lightwave transmission system. The downstream light is optically promoted from 10Gbps/25GHz MW data signal to 10Gbps/100GHz and 10Gbps/50GHz MMW data signals, and intensity-modulated with 50-550 MHz CATV signal. Moreover, the downstream light is successfully phase-remodulated with 10Gbps/25GHz MW data signal for upstream light. For up-link transmission, an optical phase modulation (PM)-to-intensity modulation (IM) conversion is developed by reflecting the phase-modulated signal at the slope of the fiber Bragg grating (FBG) reflection spectrum [9]. Consequently, the optical signal can be detected by a photodiode (PD). Thereby, employing a sophisticated delay interferometer (DI) is unnecessary to make a PM-to-IM conversion [10,11]. A DI consists of an optical splitter, a delay line, and an optical combiner. When a phase-modulated optical signal is fed into the DI, the optical signal will be split into two equal-intensity signals, in which one signal is delayed by a delay line. After recombination, the two optical signals interfere with each other constructively or destructively. The interference works as a phase demodulator if two optical signals interfere with each other constructively. The parameter bit error rate (BER) is used to measure and analyze the performance of downstream 10Gbps/100GHz and 10Gbps/50GHz MMW data signals and upstream 10Gbps/25GHz MW data signal. Meanwhile, the #232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9222
parameters carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are used to measure and analyze the performance of downstream CATV signal. An in-depth investigation of full-duplex 100-GHz/50-GHz/25-GHz/550-MHz CATV/wireless-over-fiber lightwave transmission system reveals that BER, CNR, CSO, and CTB perform well over a 40-km single-mode fiber (SMF) and a 10-m RF wireless transport. This transmission system is attractive not only because of its advancement in the integration of fiber backbone and CATV/wireless feeder networks, but also it provides the advantages of a communication link for higher data rates and bandwidth. 2. Experimental setup The configuration of the proposed full-duplex CATV/wireless-over-fiber lightwave transmission systems consisting of one BLS, two optical ILs, one intensity modulator, and one phase modulator is shown in Fig. 1. And further, Downstream/Transmitting Site, Downstream/Receiving Site, Upstream/Transmitting Site, and Upstream/Receiving Site have been labeled to identify the blocks of Fig. 1. The BLS is modulated at 10 Gbps data stream mixed with 25 GHz RF carrier. To provide various services simultaneously, the BLS generates multiple equal-space optical sidebands. The optical output of the BLS is fed into an optical IL that can separate the optical signal into even and odd channels. Following the optical IL output with even sidebands [point (b) of Fig. 1], the optical signal is separated by a 1 × 2 optical splitter. One optical signal passes through an optical band-rejection filter (OBRF). The OBRF, which is centered at 1540.16 nm and with a 3-dB bandwidth of 0.42 nm, is used to suppress the central optical sideband. The other optical signal is selected by an optical circulator (OC) combined with a FBG (λc = 1540.16 nm), and launched into an intensity modulator to modulate the CATV signal. A total of 77 channels (CH2-78; 50-550 MHz) from a multiple signal generator are used to generate the analog CATV channels. Furthermore, following the optical IL output with odd sidebands [point (e) of Fig. 1], the optical signal passes through an optical band-pass filter (OBPF). The OBPF, with a 3-dB bandwidth of 0.44 nm, is utilized to remove the outer optical sidebands. All optical signals are then combined by a 3 × 1 optical combiner and boosted by an erbium-doped fiber amplifier (EDFA). The output power and noise figure of EDFA are 17 dBm and 4.5 dB at an input power of 0 dBm, respectively. The variable optical attenuator (VOA) is introduced after the EDFA, which will result in less distortion as the optical power launched into the fiber is less. Over a 40-km SMF link, the optical signal is fed into an optical IL to separate the optical signal into even and odd sidebands. Following the optical IL output with even sidebands, the optical signal is separated by a 1 × 2 optical splitter. One optical signal passes through an OBRF; thus, the 10Gbps/25GHz MW data signal is optically promoted to the 10Gbps/100GHz MMW signal. Two optical sidebands spaced by 100 GHz (−2 and + 2 sidebands) are generated by using an OBRF. Here, the function of the OBRF is to suppress the central optical sideband (0 sideband). The 10Gbps/100GHz MMW data signal is then detected by a 100-GHz PD, boosted by a 100-GHz power amplifier (PA), and wirelessly transmitted by a 100-GHz horn antenna (HA). Over a 10-m RF wireless transmission, the 10Gbps/100GHz MMW data signal is received by a 100-GHz HA, amplified by a 100-GHz low-noise amplifier (LNA) with a noise figure of around 4.2 dB, and down-converted by a 100-GHz local oscillator (LO) and mixer. Data are recovered by a data recovery scheme and fed into a BER tester (BERT) for BER performance analysis. The other optical signal is selected by an OC combined with a FBG (λc = 1540.16 nm), split by a 1 × 2 optical splitter, and applied to a CATV receiver and a phase modulator. Following the CATV receiver, the CATV signal is supplied into a push-pull scheme for distortion elimination, and fed into an HP-8591C CATV analyzer for CNR, CSO, and CTB performance analysis. For up-link transmission, the 10Gbps/25GHz MW data signal is fed into a phase modulator for phase remodulation, amplified by an EDFA, attenuated by a VOA, and delivered by another 40-km SMF link. Over a 40-km SMF link, the optical signal passes through a FBG tilt filter-based PM-to-IM converter, detected by a 25-GHz PD, boosted by a 25-GHz PA, and wirelessly transmitted by a 25-GHz HA. Over a 10-m RF wireless
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9223
transmission, the 10Gbps/25GHz MW data signal is received by a 25-GHz HA, amplified by a 25-GHz LNA with a noise figure of approximately 2.2 dB, and down-converted by a 25GHz LO and mixer. Data are recovered by a data recovery scheme and fed into a BERT for BER performance evaluation. Following the optical IL output with odd sidebands, the 10Gbps/25GHz MW data signal is optically promoted to the 10Gbps/50GHz MMW data signal by generating two optical sidebands which are separated by a spacing of 50 GHz (−1 and 1 sidebands). The 10Gbps/50GHz MMW data signal is subsequently detected by a 50-GHz PD, boosted by a 50-GHz PA, and wirelessly transmitted by a 50-GHz HA. Over a 10-m RF wireless transmission, the 10Gbps/50GHz MMW data signal is received by a 50-GHz HA, amplified by a 50-GHz LNA with a noise figure of approximately 3.4 dB, and down-converted by a 50GHz LO and mixer. Data are recovered by a recovery scheme. Eventually, the data signal is supplied to a BERT for BER performance evaluation. As presented in Fig. 2(a), the BLS comprises a DFB LD and an OSNR enhancement scheme. The DFB LD, which has a central wavelength of 1540.16 nm, is directly modulated with a 10Gbps/25GHz RF signal. With a proper driving RF signal on the DFB LD, multiple optical sidebands are generated with a channel spacing of 25 GHz (0.2 nm). Subsequently, the generated multiple optical sidebands are launched into an OSNR enhancement scheme to improve the OSNR value. The OSNR enhancement scheme is composed of an OC, a DI, and a reflective semiconductor optical amplifier (RSOA). The optical spectra of the generated lightwaves before and after the OSNR enhancement scheme are shown in Fig. 2(b). With the application of OSNR enhancement scheme, a 7 dB to 8 dB OSNR value improvement is achieved for the optical sideband.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9224
Fig. 1. The configuration of the proposed full-duplex CATV/wireless-over-fiber lightwave transmission systems consisting of one BLS, two optical ILs, one intensity modulator, and one phase modulator.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9225
Fig. 2. (a) The configuration of the proposed BLS. (b)The optical spectra of the generated lightwaves before and after the OSNR enhancement scheme.
3. Experimental results and discussions In an optical direct modulation system, the electric field of modulating a RF signal can be described using the following small signal approximation [12]:
{
}
E (t ) = 1 + M sin(2π f m t ) exp j β cos 2π f m t + φ f ( I o , f m )
(1)
where M is the optical modulation index (OMI), fm is the modulation frequency, β is the FM index which is the ratio of the peak frequency deviation caused by the modulation and the fm. φ is the phase delay between the intensity and phase modulation, and is determined by the bias current Io and the fm. This equation proves that directly modulating a DFB laser will simultaneously cause intensity modulation (square root items) and phase modulation (exponent items) which is known as chirping effect. By boosting up the effect of phase modulation, achieving multi-sideband output for full-duplex lightwave transmission systems is feasible. The spectrum of the output light from a directly modulated DFB LD using a small signal approximation is given by the following [13]:
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9226
2
M Δf Δf Δf S ( f ) = Σ J n ( ) − J n +1 ( )e jφ + J n −1 ( )e − jφ ⋅ δ ( f − ( f 0 + nf m ) ) 4 fm fm fm
(2)
where Jn(x) is the nth order Bessel function of the first kind, n is the number of side modes, f 0 is the optical frequency under CW operation, Δf is the peak frequency deviation caused Δf is the FM index. From the Eq. (2), we determine that the fm amplitude of each sideband is mainly affected by the OMI value. The sideband optical power proportionally increases with increasing OMI value. A large OMI allows a directly modulated DFB LD to obtain a multi-sideband output with a flat power level. With a large OMI on the DFB LD, multiple optical sidebands are generated with fixed channel spacing. The optical spectra of different optical signals at several interesting points in the optical path are presented in Fig. 3(a) to Fig. 3(g) [insert (a) to (g) of Fig. 1]. The optical spectrum measured at the BLS output [insert (a) of Fig. 1] is shown in Fig. 3(a). The optical spectrum measured at the optical IL output with even sidebands [insert (b) of Fig. 1] is presented in Fig. 3(b). The optical spectrum measured at the OBRF output with −2 and + 2 sidebands [insert (c) of Fig. 1] is presented in Fig. 3(c). The optical spectrum measured at the OC output port with central carrier [insert (d) of Fig. 1] is shown in Fig. 3(d). The optical spectrum measured at the optical IL output with odd sidebands [insert (e) of Fig. 1] is presented in Fig. 3(e). The optical spectrum measured at the OBPF output with −1 and + 1 sidebands [insert (f) of Fig. 1] is presented in Fig. 3(f). The optical spectrum measured at the EDFA input port [insert (g) of Fig. 1] is shown in Fig. 3(g). The reflection spectrum of the FBG (λc = 1540.05 nm) employed in the experiment is shown in Fig. 4. The wavelength of position A is 1540.16 nm, which corresponds to the central wavelength of the phase-modulated optical signal. The wavelength of the optical carrier is located at position A. Thus, the FBG function serves as a tilt filter by which the optical carrier and two sidebands of the phase-modulated optical signal are suppressed with different ratios. By using such a wavelength alignment scheme, the upper sideband with a different phase can be deleted (blue dash line). Therefore, an optical PM-to-IM conversion is achieved successfully.
by the modulation, and
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9227
Figs. 3(a) - 3(g) The optical spectra of different optical signals at several interesting points in the optical path [insert (a) - (g) of Fig. 1].
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9228
Fig. 4. The reflection spectrum of the FBG (λc = 1540.05 nm) employed in the experiment.
A schematic diagram of the push-pull scheme is illustrated in Fig. 5. Since the even-order harmonic distortions of systems can be eliminated dramatically by a push-pull scheme [14,15], the push-pull scheme output is given by the following: Ao = a1 ⋅ Ai + a3 ⋅ Ai3 + a5 ⋅ Ai5
(3)
where Ao is the push-pull scheme output, Ai is the push-pull scheme input, and a1 , a3 , and a5 are the amplitude coefficients ( a3 and a5 are coefficients that characterize nonlinearities). A CATV subcarrier transmission system with second- and third-order nonlinear distortions can be expressed as follows: Po = c1 ⋅ Pi + c2 ⋅ Pi 2 + c3 ⋅ Pi 3
(4)
where Po is the output of a system detected from PD, Pi is the input of system, and c1 , c2 , and c3 are the amplitude coefficients ( c2 and c3 are coefficients that characterize nonlinearities). Clearly, Po is equal to Ai . Substituting Eq. (4) into Eq. (3) and neglecting higher order nonlinear terms result in the following:
Ao = (a1 ⋅ c1 ) Pi + (a1 ⋅ c2 ) Pi 2 + (a1 ⋅ c3 + a3 ⋅ c13 ) Pi 3
(5)
Achieving linearity means deleting the nonlinear terms, but a push-pull scheme would have to delete the third-order nonlinear term. The following equation sets the appropriate nonlinear coefficient to delete the third-order nonlinear term: a1 ⋅ c3 = −a3 ⋅ c13
(6)
Subsequently, the Eq. (5) can be modified as follows: Ao = (a1 ⋅ c1 ) Pi + (a1 ⋅ c2 ) Pi 2
(7)
As shown by Eq. (7), the third-order nonlinear distortion can be eliminated by proper adjustment of the nonlinear coefficient. A small third-order nonlinear distortion is related to a high CTB value. The distortion performance of the directly modulated laser transmitter is limited by CTB rather than by CSO. However, the CTB is adjusted to obtain a higher value. The measured CNR/CSO/CTB values under NTSC channel number (CH2-78), with and without a push-pull scheme, respectively, are shown in Fig. 6. The CNR value of the system with a push-pull scheme is degraded by about 1 dB compared with system without a push-
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9229
pull scheme. This CNR degradation is due to the insertion loss of the push-pull scheme. Nevertheless, the CNR value of system with a push-pull scheme still meets the CATV CNR demand at the optical node (≥50 dB) [16]. By contrast, the CSO and CTB values of the system with push-pull scheme are improved, particularly the CTB value. The CSO and CTB values of the system with a push-pull scheme are higher than 62 and 63 dB, respectively. They satisfy the CATV CSO/CTB requirements at the optical node (≥60/60 dB) [16]. These results on improvement can be attributed to the use of the push-pull scheme to eliminate the nonlinear distortions.
Fig. 5. A schematic diagram of the push-pull scheme.
Fig. 6. The measured CNR/CSO/CTB values with and without push-pull scheme.
The measured BER curves of 10Gbps/100GHz MMW data signal for back-to-back (BTB) and over a 40-km SMF and a 10-m RF wireless transport scenarios are presented in Fig. 7. A large power penalty of 5.2 dB is observed between BTB and 40 km SMF and 10 m RF wireless transport scenarios at a BER of 10−9. BLS is generated by direct modulation of a 10Gbps/25GHz RF data signal. Thus, the generated optical sidebands remain coherent to each other, and the channel spacing between the adjacent sidebands is fixed at 25 GHz (0.2 nm). The −2 and + 2 sidebands are obtained from the BLS. Thus, the 10Gbps/25GHz RF data signal is optically promoted to the 10Gbps/100GHz MMW one. However, RF power degradation induced by fiber dispersion is generated. Over a 40-km SMF transport, fiber
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9230
dispersion degrades the transmission performance because of the natural characteristics of the two optical sidebands. Moreover, over a 10-m RF wireless transport, the fading effect leads to amplitude and phase fluctuations in the received signal and results in BER performance degradation. The measured BER curves of 10Gbps/50GHz MMW signal for BTB and over a 40-km SMF and a 10-m RF wireless transport scenarios are presented in Fig. 8. At a BER of 10−9, a power penalty of 4.6 dB is presented between BTB and 40 km SMF and 10 m RF wireless transport scenarios. The use of LNA amplifies the 10-Gbps data stream while adding as little noise and distortion as possible, and the use of data recovery scheme suppresses the amplitude and phase fluctuations. Given that LNA and data recovery are used simultaneously, error-free transmission is achieved, thereby demonstrating the feasibility of setting up a CATV/wireless -over-fiber lightwave transmission system. For up-link transmission, the measured BER curves of 10Gbps/25GHz MW data signal for BTB and over 40 km SMF and 10 m RF wireless transport scenarios are shown in Fig. 9. A small power penalty of 4.1 dB is obtained between BTB and 40 km SMF and 10 m RF wireless transport scenarios at a BER of 10−9. This finding can be due to the use of PM scheme to reduce the distortions induced by systems. PM scheme utilizes optical phase shift to record signal state, thereby providing high robustness against fiber nonlinearity. Constant power operation on PM reduces the distortions, thereby resulting in systems with better BER performance. To show a more direct association with downstream intensity-modulated CATV signal and upstream phase-remodulated 10Gbps/25GHz MW data signal, we turn off the downstream intensity-modulated CATV signal and measure the BER performances of the 10Gbps/25GHz MW data signal. Nearly no difference is observed between the BER curves for CATV on and CATV off scenarios. The 10-Gbps digital baseband signal is contained in the optical sideband (lower sideband), whereas the 50-550 MHz CATV analog signal is contained in the optical carrier. No interference exists between these two signals.
Fig. 7. The measured BER curves of 10Gbps/100GHz MMW data signal for BTB and over a 40-km SMF and a 10-m RF wireless transport scenarios.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9231
Fig. 8. The measured BER curves of 10Gbps/50GHz MMW data signal for BTB and over a 40-km SMF and a 10-m RF wireless transport scenarios.
Fig. 9. The measured BER curves of 10Gbps/25GHz MW data signal for BTB and over a 40km SMF and a 10-m RF wireless transport scenarios.
4. Conclusions
A full-duplex CATV/wireless-over-fiber lightwave transmission system based on one BLS, two optical ILs, one intensity modulator, and one phase modulator is proposed and experimentally demonstrated. Through an in-depth investigation, BER, CNR, CSO, and CTB have been observed to perform well over a 40-km SMF and a 10-m RF wireless transport. Such a full-duplex CATV/wireless-over-fiber lightwave transmission system is attractive not only to present its advancement in the integration of fiber backbone and CATV/wireless feeder networks, but also to provide the advantages of a communication link for higher data rates and bandwidth.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9232
1 Institute of Electro-Optical Engineering, National Taipei University of Technology, Taipei, 106 Taiwan Department of Electronic Engineering, Jinwen University of Science and Technology, New Taipei City, 231, Taiwan *[email protected]
Abstract: A full-duplex CATV/wireless-over-fiber lightwave transmission system consisting of one broadband light source (BLS), two optical interleavers (ILs), one intensity modulator, and one phase modulator is proposed and experimentally demonstrated. The downstream light is optically promoted from 10Gbps/25GHz microwave (MW) data signal to 10Gbps/100GHz and 10Gbps/50GHz millimeter-wave (MMW) data signals in fiber-wireless convergence, and intensity-modulated with 50-550 MHz CATV signal. For up-link transmission, the downstream light is phaseremodulated with 10Gbps/25GHz MW data signal in fiber-wireless convergence. Over a 40-km single-mode fiber (SMF) and a 10-m radio frequency (RF) wireless transport, bit error rate (BER), carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are observed to perform well in such full-duplex CATV/wirelessover-fiber lightwave transmission systems. This full-duplex 100-GHz/50GHz/25-GHz/550-MHz lightwave transmission system is an attractive alternative. This transmission system not only presents its advancement in the integration of fiber backbone and CATV/wireless feeder networks, but also it provides the advantages of a communication channel for higher data rates and bandwidth. ©2015 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2360) Fiber optics links and subsystems; (350.4010) Microwaves.
References and links 1.
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11. P. Y. Wu, H. H. Lu, C. L. Ying, C. Y. Li, and H. S. Su, “An up-converted phase modulated fiber optical CATV transport system,” J. Lightwave Technol. 29(16), 2422–2427 (2011). 12. H. Olesen and G. Jacobsen, “A theoretical and experimental analysis of modulated laser fields and power spectra,” IEEE J. Quantum Electron. 18(12), 2069–2080 (1982). 13. C. H. Chang, P. C. Peng, H. H. Lu, C. L. Shih, and H. W. Chen, “Simplified radio-over-fiber transport systems with a low-cost multiband light source,” Opt. Lett. 35(23), 4021–4023 (2010). 14. W. Y. Lin, C. Y. Chen, H. H. Lu, C. H. Chang, Y. P. Lin, H. C. Lin, and H. W. Wu, “10m/500 Mbps WDM visible light communication systems,” Opt. Express 20(9), 9919–9924 (2012). 15. D. Menamara, Y. Fukasawa, Y. Wakabayashi, Y. Shirakawa, and Y. Kakuta, “750MHz power doubler and pushpull CATV hybrid modules using Gallium Arsenide,” NCTA Technical Papers. 19–26 (1996). 16. M. Jeffers, NCTA Recommended Practices for Measurements on Cable Television Systems, NCTA, (1989).
1. Introduction With the rapid development of CATV, radio frequency (RF) wireless, and lightwave transmission technologies, the increasing requirements raise the demands for high-speed and high bandwidth applications, not only for the fiber-based backbone network, but also for the CATV/RF wireless-based feeder ones. A network that can provide both wired and wireless communications simultaneously is required because of the increased demands for large bandwidth and high-speed data rate [1–4]. By combining the capacity of the optical fiber network with the ubiquity and mobility of wireless and CATV networks, CATV/wirelessover-fiber lightwave transmission systems form a powerful platform for the support and creation of future applications and services [5,6]. In this paper, a full-duplex CATV/wirelessover-fiber lightwave transmission system consisting of one broadband light source (BLS), two optical interleavers (ILs), one intensity modulator, and one phase modulator is proposed and experimentally demonstrated. This transmission system transports downstream intensitymodulated 100-GHz millimeter-wave (MMW)/50-GHz MMW/550-MHz CATV signals and upstream phase-remodulated 25-GHz microwave (MW) signal. A BLS, comprising a distributed feedback laser diode (DFB LD) with a multi-sideband output and an optical signal-to-noise ratio (OSNR) enhancement scheme, is utilized in a full-duplex CATV/wireless-over-fiber lightwave transmission system for 100-GHz/50-GHz/25-GHz/550MHz signal transport. Employing a multi-sideband coherent-lightwave generator can support various transmissions concurrently without any signal mixing process in an electrical domain. Two optical ILs are deployed in the system to separate even and odd optical sidebands of an optical signal. The optical IL has two output ports; one output port provides the optical signal only with the even sidebands, and the other output port provides the optical signal only with the odd sidebands [7,8]. The −2 and + 2 sidebands (even sideband) are used for 100-GHz MMW down-link transmission, and the −1 and + 1 sidebands (odd sideband) are used for 50GHz MMW down-link transmission. Furthermore, the central carrier is employed for CATV down-link transmission and 25-GHz MW up-link transmission. This paper is the first to use one BLS, two optical ILs, and one phase modulator in a full-duplex CATV/wireless-overfiber lightwave transmission system. The downstream light is optically promoted from 10Gbps/25GHz MW data signal to 10Gbps/100GHz and 10Gbps/50GHz MMW data signals, and intensity-modulated with 50-550 MHz CATV signal. Moreover, the downstream light is successfully phase-remodulated with 10Gbps/25GHz MW data signal for upstream light. For up-link transmission, an optical phase modulation (PM)-to-intensity modulation (IM) conversion is developed by reflecting the phase-modulated signal at the slope of the fiber Bragg grating (FBG) reflection spectrum [9]. Consequently, the optical signal can be detected by a photodiode (PD). Thereby, employing a sophisticated delay interferometer (DI) is unnecessary to make a PM-to-IM conversion [10,11]. A DI consists of an optical splitter, a delay line, and an optical combiner. When a phase-modulated optical signal is fed into the DI, the optical signal will be split into two equal-intensity signals, in which one signal is delayed by a delay line. After recombination, the two optical signals interfere with each other constructively or destructively. The interference works as a phase demodulator if two optical signals interfere with each other constructively. The parameter bit error rate (BER) is used to measure and analyze the performance of downstream 10Gbps/100GHz and 10Gbps/50GHz MMW data signals and upstream 10Gbps/25GHz MW data signal. Meanwhile, the #232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9222
parameters carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are used to measure and analyze the performance of downstream CATV signal. An in-depth investigation of full-duplex 100-GHz/50-GHz/25-GHz/550-MHz CATV/wireless-over-fiber lightwave transmission system reveals that BER, CNR, CSO, and CTB perform well over a 40-km single-mode fiber (SMF) and a 10-m RF wireless transport. This transmission system is attractive not only because of its advancement in the integration of fiber backbone and CATV/wireless feeder networks, but also it provides the advantages of a communication link for higher data rates and bandwidth. 2. Experimental setup The configuration of the proposed full-duplex CATV/wireless-over-fiber lightwave transmission systems consisting of one BLS, two optical ILs, one intensity modulator, and one phase modulator is shown in Fig. 1. And further, Downstream/Transmitting Site, Downstream/Receiving Site, Upstream/Transmitting Site, and Upstream/Receiving Site have been labeled to identify the blocks of Fig. 1. The BLS is modulated at 10 Gbps data stream mixed with 25 GHz RF carrier. To provide various services simultaneously, the BLS generates multiple equal-space optical sidebands. The optical output of the BLS is fed into an optical IL that can separate the optical signal into even and odd channels. Following the optical IL output with even sidebands [point (b) of Fig. 1], the optical signal is separated by a 1 × 2 optical splitter. One optical signal passes through an optical band-rejection filter (OBRF). The OBRF, which is centered at 1540.16 nm and with a 3-dB bandwidth of 0.42 nm, is used to suppress the central optical sideband. The other optical signal is selected by an optical circulator (OC) combined with a FBG (λc = 1540.16 nm), and launched into an intensity modulator to modulate the CATV signal. A total of 77 channels (CH2-78; 50-550 MHz) from a multiple signal generator are used to generate the analog CATV channels. Furthermore, following the optical IL output with odd sidebands [point (e) of Fig. 1], the optical signal passes through an optical band-pass filter (OBPF). The OBPF, with a 3-dB bandwidth of 0.44 nm, is utilized to remove the outer optical sidebands. All optical signals are then combined by a 3 × 1 optical combiner and boosted by an erbium-doped fiber amplifier (EDFA). The output power and noise figure of EDFA are 17 dBm and 4.5 dB at an input power of 0 dBm, respectively. The variable optical attenuator (VOA) is introduced after the EDFA, which will result in less distortion as the optical power launched into the fiber is less. Over a 40-km SMF link, the optical signal is fed into an optical IL to separate the optical signal into even and odd sidebands. Following the optical IL output with even sidebands, the optical signal is separated by a 1 × 2 optical splitter. One optical signal passes through an OBRF; thus, the 10Gbps/25GHz MW data signal is optically promoted to the 10Gbps/100GHz MMW signal. Two optical sidebands spaced by 100 GHz (−2 and + 2 sidebands) are generated by using an OBRF. Here, the function of the OBRF is to suppress the central optical sideband (0 sideband). The 10Gbps/100GHz MMW data signal is then detected by a 100-GHz PD, boosted by a 100-GHz power amplifier (PA), and wirelessly transmitted by a 100-GHz horn antenna (HA). Over a 10-m RF wireless transmission, the 10Gbps/100GHz MMW data signal is received by a 100-GHz HA, amplified by a 100-GHz low-noise amplifier (LNA) with a noise figure of around 4.2 dB, and down-converted by a 100-GHz local oscillator (LO) and mixer. Data are recovered by a data recovery scheme and fed into a BER tester (BERT) for BER performance analysis. The other optical signal is selected by an OC combined with a FBG (λc = 1540.16 nm), split by a 1 × 2 optical splitter, and applied to a CATV receiver and a phase modulator. Following the CATV receiver, the CATV signal is supplied into a push-pull scheme for distortion elimination, and fed into an HP-8591C CATV analyzer for CNR, CSO, and CTB performance analysis. For up-link transmission, the 10Gbps/25GHz MW data signal is fed into a phase modulator for phase remodulation, amplified by an EDFA, attenuated by a VOA, and delivered by another 40-km SMF link. Over a 40-km SMF link, the optical signal passes through a FBG tilt filter-based PM-to-IM converter, detected by a 25-GHz PD, boosted by a 25-GHz PA, and wirelessly transmitted by a 25-GHz HA. Over a 10-m RF wireless
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9223
transmission, the 10Gbps/25GHz MW data signal is received by a 25-GHz HA, amplified by a 25-GHz LNA with a noise figure of approximately 2.2 dB, and down-converted by a 25GHz LO and mixer. Data are recovered by a data recovery scheme and fed into a BERT for BER performance evaluation. Following the optical IL output with odd sidebands, the 10Gbps/25GHz MW data signal is optically promoted to the 10Gbps/50GHz MMW data signal by generating two optical sidebands which are separated by a spacing of 50 GHz (−1 and 1 sidebands). The 10Gbps/50GHz MMW data signal is subsequently detected by a 50-GHz PD, boosted by a 50-GHz PA, and wirelessly transmitted by a 50-GHz HA. Over a 10-m RF wireless transmission, the 10Gbps/50GHz MMW data signal is received by a 50-GHz HA, amplified by a 50-GHz LNA with a noise figure of approximately 3.4 dB, and down-converted by a 50GHz LO and mixer. Data are recovered by a recovery scheme. Eventually, the data signal is supplied to a BERT for BER performance evaluation. As presented in Fig. 2(a), the BLS comprises a DFB LD and an OSNR enhancement scheme. The DFB LD, which has a central wavelength of 1540.16 nm, is directly modulated with a 10Gbps/25GHz RF signal. With a proper driving RF signal on the DFB LD, multiple optical sidebands are generated with a channel spacing of 25 GHz (0.2 nm). Subsequently, the generated multiple optical sidebands are launched into an OSNR enhancement scheme to improve the OSNR value. The OSNR enhancement scheme is composed of an OC, a DI, and a reflective semiconductor optical amplifier (RSOA). The optical spectra of the generated lightwaves before and after the OSNR enhancement scheme are shown in Fig. 2(b). With the application of OSNR enhancement scheme, a 7 dB to 8 dB OSNR value improvement is achieved for the optical sideband.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9224
Fig. 1. The configuration of the proposed full-duplex CATV/wireless-over-fiber lightwave transmission systems consisting of one BLS, two optical ILs, one intensity modulator, and one phase modulator.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9225
Fig. 2. (a) The configuration of the proposed BLS. (b)The optical spectra of the generated lightwaves before and after the OSNR enhancement scheme.
3. Experimental results and discussions In an optical direct modulation system, the electric field of modulating a RF signal can be described using the following small signal approximation [12]:
{
}
E (t ) = 1 + M sin(2π f m t ) exp j β cos 2π f m t + φ f ( I o , f m )
(1)
where M is the optical modulation index (OMI), fm is the modulation frequency, β is the FM index which is the ratio of the peak frequency deviation caused by the modulation and the fm. φ is the phase delay between the intensity and phase modulation, and is determined by the bias current Io and the fm. This equation proves that directly modulating a DFB laser will simultaneously cause intensity modulation (square root items) and phase modulation (exponent items) which is known as chirping effect. By boosting up the effect of phase modulation, achieving multi-sideband output for full-duplex lightwave transmission systems is feasible. The spectrum of the output light from a directly modulated DFB LD using a small signal approximation is given by the following [13]:
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9226
2
M Δf Δf Δf S ( f ) = Σ J n ( ) − J n +1 ( )e jφ + J n −1 ( )e − jφ ⋅ δ ( f − ( f 0 + nf m ) ) 4 fm fm fm
(2)
where Jn(x) is the nth order Bessel function of the first kind, n is the number of side modes, f 0 is the optical frequency under CW operation, Δf is the peak frequency deviation caused Δf is the FM index. From the Eq. (2), we determine that the fm amplitude of each sideband is mainly affected by the OMI value. The sideband optical power proportionally increases with increasing OMI value. A large OMI allows a directly modulated DFB LD to obtain a multi-sideband output with a flat power level. With a large OMI on the DFB LD, multiple optical sidebands are generated with fixed channel spacing. The optical spectra of different optical signals at several interesting points in the optical path are presented in Fig. 3(a) to Fig. 3(g) [insert (a) to (g) of Fig. 1]. The optical spectrum measured at the BLS output [insert (a) of Fig. 1] is shown in Fig. 3(a). The optical spectrum measured at the optical IL output with even sidebands [insert (b) of Fig. 1] is presented in Fig. 3(b). The optical spectrum measured at the OBRF output with −2 and + 2 sidebands [insert (c) of Fig. 1] is presented in Fig. 3(c). The optical spectrum measured at the OC output port with central carrier [insert (d) of Fig. 1] is shown in Fig. 3(d). The optical spectrum measured at the optical IL output with odd sidebands [insert (e) of Fig. 1] is presented in Fig. 3(e). The optical spectrum measured at the OBPF output with −1 and + 1 sidebands [insert (f) of Fig. 1] is presented in Fig. 3(f). The optical spectrum measured at the EDFA input port [insert (g) of Fig. 1] is shown in Fig. 3(g). The reflection spectrum of the FBG (λc = 1540.05 nm) employed in the experiment is shown in Fig. 4. The wavelength of position A is 1540.16 nm, which corresponds to the central wavelength of the phase-modulated optical signal. The wavelength of the optical carrier is located at position A. Thus, the FBG function serves as a tilt filter by which the optical carrier and two sidebands of the phase-modulated optical signal are suppressed with different ratios. By using such a wavelength alignment scheme, the upper sideband with a different phase can be deleted (blue dash line). Therefore, an optical PM-to-IM conversion is achieved successfully.
by the modulation, and
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9227
Figs. 3(a) - 3(g) The optical spectra of different optical signals at several interesting points in the optical path [insert (a) - (g) of Fig. 1].
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Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9228
Fig. 4. The reflection spectrum of the FBG (λc = 1540.05 nm) employed in the experiment.
A schematic diagram of the push-pull scheme is illustrated in Fig. 5. Since the even-order harmonic distortions of systems can be eliminated dramatically by a push-pull scheme [14,15], the push-pull scheme output is given by the following: Ao = a1 ⋅ Ai + a3 ⋅ Ai3 + a5 ⋅ Ai5
(3)
where Ao is the push-pull scheme output, Ai is the push-pull scheme input, and a1 , a3 , and a5 are the amplitude coefficients ( a3 and a5 are coefficients that characterize nonlinearities). A CATV subcarrier transmission system with second- and third-order nonlinear distortions can be expressed as follows: Po = c1 ⋅ Pi + c2 ⋅ Pi 2 + c3 ⋅ Pi 3
(4)
where Po is the output of a system detected from PD, Pi is the input of system, and c1 , c2 , and c3 are the amplitude coefficients ( c2 and c3 are coefficients that characterize nonlinearities). Clearly, Po is equal to Ai . Substituting Eq. (4) into Eq. (3) and neglecting higher order nonlinear terms result in the following:
Ao = (a1 ⋅ c1 ) Pi + (a1 ⋅ c2 ) Pi 2 + (a1 ⋅ c3 + a3 ⋅ c13 ) Pi 3
(5)
Achieving linearity means deleting the nonlinear terms, but a push-pull scheme would have to delete the third-order nonlinear term. The following equation sets the appropriate nonlinear coefficient to delete the third-order nonlinear term: a1 ⋅ c3 = −a3 ⋅ c13
(6)
Subsequently, the Eq. (5) can be modified as follows: Ao = (a1 ⋅ c1 ) Pi + (a1 ⋅ c2 ) Pi 2
(7)
As shown by Eq. (7), the third-order nonlinear distortion can be eliminated by proper adjustment of the nonlinear coefficient. A small third-order nonlinear distortion is related to a high CTB value. The distortion performance of the directly modulated laser transmitter is limited by CTB rather than by CSO. However, the CTB is adjusted to obtain a higher value. The measured CNR/CSO/CTB values under NTSC channel number (CH2-78), with and without a push-pull scheme, respectively, are shown in Fig. 6. The CNR value of the system with a push-pull scheme is degraded by about 1 dB compared with system without a push-
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9229
pull scheme. This CNR degradation is due to the insertion loss of the push-pull scheme. Nevertheless, the CNR value of system with a push-pull scheme still meets the CATV CNR demand at the optical node (≥50 dB) [16]. By contrast, the CSO and CTB values of the system with push-pull scheme are improved, particularly the CTB value. The CSO and CTB values of the system with a push-pull scheme are higher than 62 and 63 dB, respectively. They satisfy the CATV CSO/CTB requirements at the optical node (≥60/60 dB) [16]. These results on improvement can be attributed to the use of the push-pull scheme to eliminate the nonlinear distortions.
Fig. 5. A schematic diagram of the push-pull scheme.
Fig. 6. The measured CNR/CSO/CTB values with and without push-pull scheme.
The measured BER curves of 10Gbps/100GHz MMW data signal for back-to-back (BTB) and over a 40-km SMF and a 10-m RF wireless transport scenarios are presented in Fig. 7. A large power penalty of 5.2 dB is observed between BTB and 40 km SMF and 10 m RF wireless transport scenarios at a BER of 10−9. BLS is generated by direct modulation of a 10Gbps/25GHz RF data signal. Thus, the generated optical sidebands remain coherent to each other, and the channel spacing between the adjacent sidebands is fixed at 25 GHz (0.2 nm). The −2 and + 2 sidebands are obtained from the BLS. Thus, the 10Gbps/25GHz RF data signal is optically promoted to the 10Gbps/100GHz MMW one. However, RF power degradation induced by fiber dispersion is generated. Over a 40-km SMF transport, fiber
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9230
dispersion degrades the transmission performance because of the natural characteristics of the two optical sidebands. Moreover, over a 10-m RF wireless transport, the fading effect leads to amplitude and phase fluctuations in the received signal and results in BER performance degradation. The measured BER curves of 10Gbps/50GHz MMW signal for BTB and over a 40-km SMF and a 10-m RF wireless transport scenarios are presented in Fig. 8. At a BER of 10−9, a power penalty of 4.6 dB is presented between BTB and 40 km SMF and 10 m RF wireless transport scenarios. The use of LNA amplifies the 10-Gbps data stream while adding as little noise and distortion as possible, and the use of data recovery scheme suppresses the amplitude and phase fluctuations. Given that LNA and data recovery are used simultaneously, error-free transmission is achieved, thereby demonstrating the feasibility of setting up a CATV/wireless -over-fiber lightwave transmission system. For up-link transmission, the measured BER curves of 10Gbps/25GHz MW data signal for BTB and over 40 km SMF and 10 m RF wireless transport scenarios are shown in Fig. 9. A small power penalty of 4.1 dB is obtained between BTB and 40 km SMF and 10 m RF wireless transport scenarios at a BER of 10−9. This finding can be due to the use of PM scheme to reduce the distortions induced by systems. PM scheme utilizes optical phase shift to record signal state, thereby providing high robustness against fiber nonlinearity. Constant power operation on PM reduces the distortions, thereby resulting in systems with better BER performance. To show a more direct association with downstream intensity-modulated CATV signal and upstream phase-remodulated 10Gbps/25GHz MW data signal, we turn off the downstream intensity-modulated CATV signal and measure the BER performances of the 10Gbps/25GHz MW data signal. Nearly no difference is observed between the BER curves for CATV on and CATV off scenarios. The 10-Gbps digital baseband signal is contained in the optical sideband (lower sideband), whereas the 50-550 MHz CATV analog signal is contained in the optical carrier. No interference exists between these two signals.
Fig. 7. The measured BER curves of 10Gbps/100GHz MMW data signal for BTB and over a 40-km SMF and a 10-m RF wireless transport scenarios.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9231
Fig. 8. The measured BER curves of 10Gbps/50GHz MMW data signal for BTB and over a 40-km SMF and a 10-m RF wireless transport scenarios.
Fig. 9. The measured BER curves of 10Gbps/25GHz MW data signal for BTB and over a 40km SMF and a 10-m RF wireless transport scenarios.
4. Conclusions
A full-duplex CATV/wireless-over-fiber lightwave transmission system based on one BLS, two optical ILs, one intensity modulator, and one phase modulator is proposed and experimentally demonstrated. Through an in-depth investigation, BER, CNR, CSO, and CTB have been observed to perform well over a 40-km SMF and a 10-m RF wireless transport. Such a full-duplex CATV/wireless-over-fiber lightwave transmission system is attractive not only to present its advancement in the integration of fiber backbone and CATV/wireless feeder networks, but also to provide the advantages of a communication link for higher data rates and bandwidth.
#232489 - $15.00 USD (C) 2015 OSA
Received 15 Jan 2015; revised 25 Mar 2015; accepted 25 Mar 2015; published 2 Apr 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.009221 | OPTICS EXPRESS 9232
