Open Access
Add to BibSonomyAbstract
A full-duplex lightwave transport system employing wavelength-division-multiplexing (WDM) and optical add-drop multiplexing techniques, as well as optical free-space transmission scheme is proposed and experimentally demonstrated. Over an 80-km single-mode fiber (SMF) and 2.4 m optical free-space transmissions, impressive bit error rate (BER) performance is obtained for long-haul fiber link and finite free-space transmission distance. Such a full-duplex lightwave transport system based on long-haul SMF and optical free-space transmissions has been successfully demonstrated, which cannot only present its advancement in lightwave application, but also reveal its simplicity and convenience for the real implementation. Our proposed systems are suitable for the lightwave communication systems in wired and wireless transmissions.
© 2013 Optical Society of America
1. Introduction
Wireless communication network is a promising technology to provide broadband services to the premises. However, due to high attenuation in the wireless band, employing wireless communication to connect the head-end and the access point (AP) will place serious limitations on the allowed repeaterless distance. It is expected that fiber link may be a solution to the issue. In optical fiber integration with optical free-space transmission systems, the signal is converted into the optical signal and delivered to the remote APs by fiber link, in which providing broad bandwidth and low attenuation characteristics [1,2]. Optical free-space transmission scheme is presently developed by researchers and engineers to create high-speed and long-distance free-space link. It can provide many benefits, like providing communication link in specific areas in which RF communication is prohibited, such as in the hospital or aircraft [3–6]. Optical free-space transmission scheme uses optical light emitted by a variety of optical sources, primarily through the use of light emitting diode (LED) [7,8] or laser pointer laser [9]. However, the performance of optical free-space transmission systems can be improved further by using high optical output power distributed feedback laser diode (DFB LD) as the optical source. Optical free-space transmission systems can be divided into two categories: the divergence category and the convergence one [10,11]. The former uses the divergence beam to provide the mobile service to the end-user. However, it is difficult to obtain good free-space transmission performance due to large service area and low optical power per unit area. On the other hand, the latter uses the convergence beam to provide the mobile service to the end-user. Nevertheless, it is also difficult to obtain good free-space transmission performance due to narrow light beam and laser light misalignment problem between the transmitter and the receiver. Whatever, there is a trade-off between these two categories. To guarantee successful design of a full-duplex lightwave transport system, system designer will have to optimize the overall architecture to obtain the best transmission performances. In this paper, a novel full-duplex lightwave transport system based on wavelength-division-multiplexing (WDM) and optical add-drop multiplexing techniques, as well as optical free-space transmission scheme is proposed and experimentally demonstrated. WDM and optical add-drop multiplexing techniques can simplify the network architecture and upgrade the deployment of AP since they enable full-duplex operation on one fiber [12]. In order to take advantage of both optical free-space transmission categories, high optical output power DFB LD is employed as the optical source and fiber transmitter is employed as the optical transmitter at the fiber end. Over an 80-km single-mode fiber (SMF) and 2.4 m free-space transmissions, impressive bit error rate (BER) values were obtained for both down-link and up-link in our proposed full-duplex lightwave transport systems. For the applications at the end-user, the laser light is necessary for sending out data signal wirily and wirelessly to integrate the fiber and free-space transmission schemes [13]. This proposed full-duplex lightwave transport system is shown to be a prominent one not only to present its convenience in fiber integration with free-space transmissions, but also to reveal its potential for the real implementation. Our proposed full-duplex lightwave transport systems are suitable for the integration of long-haul fiber and finite free-space transmission links.
2. Experimental setup
The experimental configuration of our proposed full-duplex lightwave transport systems employing WDM and optical add-drop multiplexing techniques, and optical free-space transmission scheme is shown in Fig. 1. For down-link transmission, the head-end is composed of two DFB LDs, two polarization controllers (PCs) with insertion loss of 0.3 dB, and one 2 × 1 optical coupler. The PCs are required to optimize the optical output power of the DFB LDs. The DFB LDs, with central wavelengths/output powers of 1545.32nm(λ1)/17dBm and 1553.33nm(λ2)/17dBm, are directly modulated with 1~10 Gbps data stream. To transmit the optical wavelengths over a long-haul fiber link, the optical power is conventionally amplified by the erbium-doped fiber amplifier (EDFA). In this experiment, however, we use the high optical output power DFB LD to replace the EDFA. Since our proposed system does not use the EDFA, it reveals a prominent one with simpler and more economic advantages. Signal is generated at the head-end and then distributed to the remote APs by the optical fiber (red line in Fig. 1). Each fiber span is 40 km, i.e., from the head-end to the AP1, from the AP1 to the AP2, and from the AP2 to the head-end, each fiber transmission distance is 40 km. Every AP is addressed by individual wavelength for an optical add-drop multiplexer (OADM). Such architecture is attractive because it enables a large number of APs to share the LDs remotely located at the head-end. The appropriate wavelengths are dropped by the OADM in AP1 and AP2, as shown in Fig. 2. The OADM is employed to drop the downstream wavelength to the dedicated AP, and add the upstream wavelength to the fiber backbone. The OADM, with >40 dB add/drop channel isolation, consists of one fiber Bragg grating (FBG) located between two optical circulators (OCs). Such a high channel isolation characteristic provides excellent add/drop ability to prevent crosstalk from the drop/add channel. The downstream wavelength is transmitted by a fiber transmitter, and detected by a broadband photodiode (PD). The broadband PD has a detection wavelength range of 400-1560 nm, a responsivity of 0.95 mA/mW (at 1550 nm), a small active area (0.6 mm diameter), and a small numerical aperture (NA) (less than 0.05). A small NA is related to a large beam convergence. The fiber transmitter consists of a fiber end, a lens, and a coupling subsystem. The lens is used to increase the divergence of the optical beam to cover a certain area. Nevertheless, the coupling subsystem is used to increase the convergence of the optical beam to focus into a point. The detected optical signal, in which converted into the data signal, is then amplified by a push-pull amplifier with low noise figure of around 4.5 dB, and passed through an adaptive filter for error correction. Finally, the data signal is fed into a BER tester (BERT) for BER analysis.
Fig. 1 The experimental configuration of our proposed full-duplex lightwave transport systems employing WDM and optical add-drop multiplexing techniques, and optical free-space transmission scheme.
Simultaneously, 1~10 Gbps upstream data stream is added to the fiber backbone and transmitted to the head-end. From the AP1 to the head-end (blue line in Fig. 1), the fiber transmission distance is 80 km; from the AP2 to the head-end (blue line in Fig. 1), the fiber transmission distance is 40 km. The upstream wavelength is selected by using a tunable optical band-pass filter (TOBPF), transmitted by a fiber transmitter, and detected by a broadband PD. The detected optical signal is then amplified by a push-pull amplifier, and passed through an adaptive filter for error correction. Eventually, the data signal is fed into a BERT for BER analysis.
3. Experimental results and discussions
In implementation the adaptive filter, first the transmitter send a data pattern with an arbitrary data length as a protocol; and at the receiving site, the adaptive filter has a stored copy of data signal before starting communication. Let has an amplitude and phase :
After transmission through a free-space link, the received signal has a distorted amplitude and phase error :The adaptive filter has to estimate from by error feedback. For amplitude compensation, the output of the amplitude compensator is compared with a stored copy of to create an amplitude error. For phase compensation, the output of the phase compensator is compared with a stored copy of to create a phase error. The use of adaptive filter offers significant amplitude and phase error compensations.The measured down-link BER curves of 1 Gbps data channel from the head-end to the AP2 (over an 80-km SMF transmission and transmitted by a fiber transmitter; red line in Fig. 1) are presented in Fig. 3(a). At a free-space transmission distance of 2.4 m; without employing the push-pull amplifier and the adaptive filter, the BER is about 10−5; with employing the push-pull amplifier and the adaptive filter, the BER is reached down to 10−9. It is clear that as the push-pull amplifier and the adaptive filter are employed simultaneously, large BER performance improvement (104 order) can be achieved. In order to know how much BER performance improvement is based on each scheme, systems only employing the push-pull amplifier or only employing the adaptive filter have been used to evaluate the BER performance. At a free-space transmission distance of 2.4 m; with only employing the push-pull amplifier, the measured BER is about 10−7; with only employing the adaptive filter, the measured BER is also about 10−7. It means that systems only employ the push-pull amplifier or only employ the adaptive filter to improve the BER performance, the BER performance improvement is limited (just only 102 order). The results indicate that both the push-pull amplifier and the adaptive filter schemes play important roles for error correction functions. The BER is given by [14]:
The push-pull amplifier and the adaptive filter schemes can increase systems’ signal-to-noise ratio (SNR) value, resulting in system with better BER performance. If both the push-pull amplifier and the adaptive filter schemes are employed simultaneously, systems’ SNR value is increased greatly, leading to large BER performance improvement. And further, the measured up-link BER curves of 1 Gbps data channel from the AP1 to the head-end (over an 80-km SMF transmission and transmitted by a fiber transmitter; blue line in Fig. 1) are presented in Fig. 3(b). At a free-space transmission distance of 2.38 m; without employing the push-pull amplifier and the adaptive filter, the BER is around 10−5; with employing the push-pull amplifier and the adaptive filter, the BER is reached down to 10−9. A huge improvement of 104 order is obtained as push-pull amplifier and adaptive filter are employed simultaneously.Fig. 3 (a) The measured down-link BER curves of 1 Gbps data channel from the head-end to the AP2, (b) The measured up-link BER curves of 1 Gbps data channel from the AP1 to the head-end.
Crosstalk (XT) from the downstream (upstream) optical signal to the upstream (downstream) one may influence the BER performance, because the same data stream is used for both down-link and up-link. Crosstalk that arises from the incomplete isolation of the add/drop channel at the OADM output can be expressed as [15,16]:
where Pa is the is the optical power of add channel, and Pd is the optical power of drop channel. The effective isolation factor, Kad, is the ratio of the power transmission of drop channel to the add channel. Using an optical spectrum analyzer (OSA) to measure the crosstalk, the crosstalk from drop (add) channel to add (drop) channel is less than 40 dB. Since the crosstalk is very low, crosstalk from the downstream (upstream) optical signal to the upstream (downstream) one will not influence the BER performance. No BER performance degradation due to crosstalk is observed. In order to guarantee successful design of a full-duplex lightwave transport system, system designer must address the minimum add/drop channel isolation property of the OADM.The received signals of 1 Gbps and 10 Gbps data channels from the head-end to the AP2, with respect to different distance from beam center are shown in Fig. 4(a). The distance from beam center refers to longitudinal distance from the beam center. It is clear that, for both cases, as the distance from beam center increases the BER value increases as well. An error-free operation of 10−9 can be achieved when the distance from beam center is smaller than 0.38m (10Gbps) and 2.4m (1Gbps), respectively. In addition, the received signals of 1 Gbps and 10 Gbps data channels from the head-end to the AP2, with respect to different beam radius are present in Fig. 4(b). It is obvious that, for both cases, as the beam radius increases the BER value increases as well. An error-free operation of 10−9 can be obtained as the beam radius is smaller than 0.28m (10Gbps) and 0.6m (1Gbps), respectively. These distance (0.38m → 2.4m) and radius (0.28m → 0.6m) improvements are due to the reduction of data stream (10Gbps → 1Gbps). The distance from beam center and beam radius under different data stream, at an error-free operation of 10−9, is present in Fig. 5. It can be seen that there is a trade-off between the distance from beam center/beam radius and the data stream. Error-free operation can always be achieved for different data stream with different distance from beam center and beam radius. Thereby, different data stream, distance from beam center, and beam radius can be selected according to system requirement.
Fig. 4 (a) The received signals of 1 Gbps and 10 Gbps data channels with respect to different distance from beam center. (b) The received signals of 1 Gbps and 10 Gbps data channels with respect to different beam radius.
Fig. 5 The distance from beam center and beam radius under different data stream, at an error-free operation of 10−9.
4. Conclusions
A novel full-duplex lightwave transport system based on WDM and optical add-drop multiplexing techniques, as well as optical free-space transmission scheme is proposed and demonstrated. Impressive BER operation is achieved for long-haul fiber and finite free-space transmissions. This proposed that such a full-duplex lightwave transport system has been successfully demonstrated, which cannot only present its advancement in optical fiber integration with optical free-space applications, but also reveal its simplicity and convenience for the implementation. Our proposed systems are suitably applicable to the optical communication systems in optical fiber integration with optical free-space transmission schemes.
Acknowledgment
The authors would like to thank the financial support from the National Science Council of the Republic of China under Grant NSC 100-2221-E-027-067-MY3, NSC 101-2221-E-027-040 -MY3, and NSC 102-2218-E-027-002.
References and links
1. C. Y. Li, H. S. Su, C. H. Chang, H. H. Lu, P. Y. Wu, C. Y. Chen, and C. L. Ying, “Generation and transmission of BB/MW/MMW signals by cascading PM and MZM,” J. Lightwave Technol. 30(3), 298–303 (2012). [CrossRef]
2. C. Y. Li, H. S. Su, C. Y. Chen, H. H. Lu, H. W. Chen, C. H. Chang, and C. H. Jiang, “Full-duplex lightwave transport systems employing phase-modulated RoF and intensity-remodulated CATV signals,” Opt. Express 19(15), 14000–14007 (2011). [CrossRef] [PubMed]
3. Y. Wang, Y. Wang, N. Chi, J. Yu, and H. Shang, “Demonstration of 575-Mb/s downlink and 225-Mb/s uplink bi-directional SCM-WDM visible light communication using RGB LED and phosphor-based LED,” Opt. Express 21(1), 1203–1208 (2013). [CrossRef] [PubMed]
4. C. H. Yeh, Y. F. Liu, C. W. Chow, Y. Liu, P. Y. Huang, and H. K. Tsang, “Investigation of 4-ASK modulation with digital filtering to increase 20 times of direct modulation speed of white-light LED visible light communication system,” Opt. Express 20(15), 16218–16223 (2012). [CrossRef]
5. C. W. Chow and Y. H. Lin, “Convergent optical wired and wireless long-reach access network using high spectral-efficient modulation,” Opt. Express 20(8), 9243–9248 (2012). [CrossRef] [PubMed]
6. Y. F. Liu, Y. C. Chang, C. W. Chow, and C. H. Yeh, “Equalization and pre-distorted schemes for increasing data rate in-door visible light communication system,” In Proc. Opt. Fiber Commun. (OFC), JWA83 (2011).
7. F. M. Wu, C. T. Lin, C. C. Wei, C. W. Chen, Z. Y. Chen, and H. T. Huang, “3.22-Gb/s WDM visible light communication of a single RGB LED employing carrier-less amplitude and phase modulation,” In Proc. Opt. Fiber Commun. (OFC), OTh1G4 (2013).
8. C. W. Chow, C. H. Yeh, Y. F. Liu, and Y. Liu, “Improved modulation speed of LED visible light communication system integrated to the main electricity network,” Electron. Lett. 47(15), 867–868 (2011). [CrossRef]
9. 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). [CrossRef] [PubMed]
10. F. Alsaadi and J. Elmirghani, “Performance evaluation of 2.5 Gbit/s and 5 Gbit/s optical wireless systems employing a two dimensional adaptive beam clustering method and imaging diversity detection,” IEEE J. Sel. Areas Comm. 27(8), 1507–1519 (2009). [CrossRef]
11. J. Fadlullah and M. Kavehrad, “Indoor high-bandwidth optical wireless links for sensor networks,” J. Lightwave Technol. 28(21), 3086–3094 (2010).
12. D. C. O'Brien, “Visible light communications: challenges and potential,” In Proc. IEEE Photon. Conf., 365–366 (2011).
13. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed optical wireless communication system for indoor applications,” IEEE Photon. Technol. Lett. 23(8), 519–521 (2011). [CrossRef]
14. H. H. Lu, S. J. Tzeng, and Y. L. Liu, “Intermodulation distortion suppression in a full-duplex radio-on-fiber ring network,” IEEE Photon. Technol. Lett. 16(2), 602–604 (2004). [CrossRef]
15. M. R. Phillips and D. M. Ott, “Crosstalk caused by nonideal output filters in WDM lightwave systems,” IEEE Photon. Technol. Lett. 12(8), 1094–1096 (2000). [CrossRef]
16. M. R. Phillips and D. M. Ott, “Crosstalk due to optical fiber nonliearities in WDM CATV lightwave systems,” J. Lightwave Technol. 17(10), 1782–1792 (1999). [CrossRef]
