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Single-source bidirectional free-space optical communications using reflective SOA-based amplified modulating retro-reflection
Optics Communications 387 (2017) 43–47
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Single-source bidirectional free-space optical communications using reflective SOA-based amplified modulating retro-reflection Xiaoyan Wanga, Xianglian Fenga, Peng Zhangb, Tianshu Wangb, Shiming Gaoa,
crossmark
⁎
a
Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310058, China National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun 130022, China
b
A R T I C L E I N F O
A BS T RAC T
PACS: 42.68.Ay 42.79.Sz
A novel amplified modulating retro-reflector (AMRR) based on a reflective semiconductor optical amplifier (RSOA) is proposed and a bidirectional free-space optical communication (FSO) system including both downstream and upstream links is experimentally demonstrated with only a single light source using this AMRR. The RSOA-based AMRR can provide a net gain more than 4 dB and support the modulation bit rate up to 1.25 Gbit/s. The bidirectional FSO transmission performance is evaluated by observing eye diagrams and measuring bit error rate (BER) results of both 10-Gbit/s DPSK downstream and 1.25-Gbit/s OOK upstream signals. The factors that limit the modulation bit rate and transmission quality are analyzed. The power penalties of both links are less than 0.69 dB in the bidirectional FSO system at the BER of 1×10-3.
Keywords: Free-space optical communications Modulating retro-reflection Bidirectional transmission Semiconductor optical amplifiers
1. Introduction Free-space optical communication (FSO) has been considered as a promising technology in response to a growing need for high-speed and tap-proof communication systems to transfer information between mobile and stationary terminals (aircrafts, satellites, and ground platforms, etc.), which exhibits many potential advantages in security, strong anti-jamming ability, high bandwidth, and large available spectrum [1,2]. In the past, several kinds of typical FSO transmissions have been demonstrated between two satellites [3], between the moon and the earth [4], and between an aerobat and a ground terminal [5]. In laboratory conditions, high-speed (gigabit and even terabit) FSO links between two fixed short-distance points have also been implemented by employing multiple-dimensional multiplexing of orbital angular momentum, polarization, and wavelength [6–8]. However, these active FSO systems with high pointing accuracy are often large, heavy, complex, and of high power consumption, which becomes challengeable when the terminals have strict payload and power limits [9]. For such implementations, modulating retro-reflectors (MRRs) have exhibited more competitions. A MRR combines an optical retro-reflector with a modulator to reflect modulated optical signals directly back to the transceiver and shifts most of the power, weight and pointing requirements onto the base station of the FSO systems [10,11]. MRRs have been demonstrated through different ways such as frustrated total internal reflectors [12], electro-optic
⁎
Corresponding author. E-mail address: [email protected] (S. Gao).
http://dx.doi.org/10.1016/j.optcom.2016.11.019 Received 20 October 2016; Accepted 10 November 2016 0030-4018/ © 2016 Elsevier B.V. All rights reserved.
phase modulators [13], liquid crystal modulators [14,15], and multiple quantum well devices [16,17]. In above schemes, the modulation and reflection elements are all discrete and of large volumes, and the modulation speed is relatively low. As a powerful signal-processing device, reflective semiconductor optical amplifier (RSOA) shows robust performances. An integrated RSOA with pigtail has many advantages such as small volume, lightweight, optical power amplification, and of collinear optical path. Moreover, RSOA can be used as a reflector and a modulator simultaneously, which has been widely demonstrated in wavelength-division multiplexing passive optical networks (WDMPONs) [18–20]. Therefore, an amplified MRR (AMRR) for FSO systems is possible to be realized by using the RSOA as an amplifier, modulator, and reflector simultaneously, since an amplified, modulated, and retro-reflected light can be obtained when an interrogation light is injected into the RSOA. In this paper, we propose and experimentally demonstrate, for the first time to our best knowledge, a RSOA-based AMRR and a bidirectional FSO system for 10-Gbit/s downstream differential phase shift keying (DPSK) signal and 1.25-Gbit/s upstream on-off keying (OOK) signal with a single light source using this AMRR. In the bidirectional FSO system, only a single optical carrier is used, which is arranged at the transceiver. The uplink signal is modulated on the reflected optical carrier at the terminal. Eye diagrams and bit error rates (BERs) are measured for both downlink and uplink signals. At the BER of 1×10-3, the power penalties are measured to be 0.69 and 0.47 dB for the 10-
Optics Communications 387 (2017) 43–47
X. Wang et al.
Fig. 1. Schematic configruations of (a) a coventional MRR structure using semicondutor multi-quantum-well modualtor and a corner-cube prism retro-reflector and (b) our proposed RSOA-based AMRR by using the RSOA as the modulator, amplifier, and rectro-reflector simultenously.
Gbit/s downstream DPSK signal and the 1.25-Gbit/s upstream OOK signal.
2. Principle A MRR structure needs to include the elements that can realize optical retro-reflection and modulation. As shown in Fig. 1(a), in the conventional MRR configuration, the interrogation light is modulated by the driving data sequence using the modulator such as a semiconductor multi-quantum-well modulator. And then, it is reflected by the retro-reflector using such as a corner-cube prism. Since a RSOA can realize the modulation and retro-reflection simultaneously, the MRR function can be utilized using a single RSOA. Fig. 1(b) shows the configuration of our proposed RSOA-based MRR. The interrogation light is collected using an optical antenna and coupled into an optical fiber. And then, the interrogation light is injected into the RSOA, where it is modulated by the driving data and reflected backward to the optical fiber. After coupling from the fiber to the optical antenna, the retro-reflected light is transmitted back to the transceiver from the MRR via free space. Compared with the conventional MRR, our proposed RSOA-based MRR can support higher modulation bit rate, which is up to the level of GHz, and has a small volume. Also, the RSOA has the ability of amplification and the incident interrogation light will be amplified when it is reflected as the retro-reflected light. It means that the RSOA-based MRR is an AMRR. For an optical carrier, multiple physical quantities can be used for data modulation. According to the used physical quantities, the signals are classified as amplitude-shift keying (ASK), phase-shift keying (PSK), and polarization-shift keying (PolSK) signals, etc. In order to realize high-quality bidirectional transmission using a single light source, the influence between the downstream and upstream signals should be effectively eliminated. Since the amplitude can be kept the same when a PSK signal is modulated on the phase of the optical carrier, the PSK optical signal is expected to be amplitude modulated once more to transmit a ASK signal without crosstalk. The principle is shown in Fig. 2. For the downstream link, DPSK modulation format is selected, which is only modulated on the phase of the optical carrier at the transceiver, and the amplitude of the optical carrier is constant, as shown in Fig. 2(a). When the downstream DPSK optical signal is transmitted to the terminal, one part is used for demodulation, and the other serves as the interrogation light of the RSOA-based AMRR to generate the upstream link. Here, the interrogation light serves as the optical carrier and it is amplitude modulated by the upstream OOK format through the RSOA, as shown in Fig. 2(b). After modulated, the upstream optical signal is amplified and retro-reflected back to the transceiver. As the OOK signal only changes the optical carrier amplitude, which is independent on the carrier phase. Therefore, the existence of the downstream DPSK signal will not affect the modulation and detection of the upstream OOK signal. In this way, a high-quality bidirectional FSO system with a single light source is expected to be realized with low crosstalk using our proposed RSOA-based AMRR.
Fig. 2. Principle for choosing the modulation formats of the bidirectional FSO system.
3. Experimental setup Fig. 3 shows the experimental setup of the proposed bidirectional FSO system with a single light source using the RSOA-based AMRR. The light source is provided by a continuous-wave (CW) tunable laser (Agilent 81940) with a 1.12-dBm power at 1550.0 nm. At the transceiver, it is modulated by a 10-Gbit/s DPSK sequence via a phase modulator and launched into free space through a collimator (Collimator 1, Thorlabs F280FC-1550) whose divergence angle is 0.032°. After 0.5-m transmission, it is coupled back to an optical fiber at the terminal for detection and upstream modulation via another collimator (Collimator 2, Thorlabs A397TM-C), which serves as the optical antenna of the AMRR. Here a relatively short distance is transmitted since we mainly focus on the proof-of-concept demonstration in lab. If high-quality collimation optical systems are used for transmission, the distance has the ability to be effectively enhanced [3]. In our experiment, the field-of-view (FOV) of the RSOA-based AMRR is determined by the acceptance angle of Collimator 2, which is about 34.9°. If larger FOV is required, the conventional way is setting the optical antenna on a servo turntable and the orientation of the optical antenna can be changed by tuning the servo turntable to collect the interrogation light within a wider angle region [21]. At the terminal, the downstream DPSK optical signal is split into two branches. One branch (10% power) is sent into a one-bit delay interferometer for demodulation, where the DPSK signal is converted to a sequence of amplitude modulated signal. The converted signal is detected using a photodetector (PD1), the eye diagram is observed using an oscilloscope, and the BER is measured using a BER tester. The other branch of the downstream optical signal is sent into a RSOA (SOA-RL-OEC-1550) as the interrogation light. The transmission and coupling losses are 2.72 dB, and the optical power injected into the RSOA is −1.60 dBm. The upstream sequence, a 1.25-Gbit/s 271 pseudorandom binary sequence (PRBS) is modulated via the RSOA with a 55-mA driving current. After being modulated and amplified by the RSOA, the retro-reflected optical signal is measured to be 44
Optics Communications 387 (2017) 43–47
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Fig. 3. Experimental setup of the bidirectional FSO system using a RSOA-based AMRR.
2.42 dBm, and the signal is amplified by 4.02 dB due to the gain of the RSOA, which verifies that our proposed AMRR has the ability of amplification. The retro-reflected optical signal, that is, the upstream OOK optical signal, transmits back to the transceiver through the same free-space optical path and received by another photodetector (PD2). The eye diagram and BER of the upstream signal are also measured. Due to the limitation of the low bandwidth and high chirp characteristics of the commercially RSOA we used, whose modulation bandwidth is less than 1.5 GHz [22], we only modulate a relatively low upstream bit rate as 1.25 Gbit/s. Higher upstream bit rate is permitted in this duplex FSO system as a RSOA with broader bandwidth is adopted. 4. Results and discussion 4.1. RSOA-based AMRR In order to verify the performance of the RSOA-based AMRR, we first realize a unidirectional FSO link without the downstream DPSK signal modulated. The unmodulated light source serving as the interrogation light is first transmitted through free space to the RSOA, where the 1.25-Gbit/s upstream OOK signal is uploaded and retro-reflected. Fig. 4(a) and (b) show the eye diagrams before (backto-back, B2B) and after free-space transmission. The eye opening decreases a little after transmission due to the noises introduced in the optical paths, including the reflected lights from the collimators and the noises from the PD, which are dominated by the thermal noise. The BER results are shown in Fig. 5, where the required received powers for the 1.25-Gbit/s OOK signal before and after transmission are −13.41 and −11.66 dBm at the BER of 1×10-3, which is low enough to be corrected using forward error correction (FEC) algorithms. A power penalty of 1.75 dB is measured for the OOK signal transmission. As a result, the sensitivity of the proposed RSOA-based AMRR can be obtained as −11.66 dBm at this BER level. The sensitivity will increase if lower BER is required. In the eye diagrams of Fig. 4, one can see that large jitters are introduced in both bits “1” and “0”, especially on the top of “1”. In order to find the origination, we measure the waveform that reflects from the RSOA when the 1.25-Gbit/s PRBS sequence is modulated, as shown in Fig. 6(a). The top and bottom of the waveform become uneven and large gain slopes can be clearly observed when multiple bits “1” or “0” emerge adjacently each other. It can be explained by the intensity variation of free carriers in the RSOA. When multiple continuous bits of “1” or “0” input, the front bits “1” or “0” will deplete the free carriers in the RSOA to acquire gains. As a result, the intensity
Fig. 5. BER results of the unidirectional FSO link before and after transmission.
Fig. 6. Performance comparison of the PRBS sequence and the alternative “1” and “0” sequence modualted via the RSOA. (a) B2B waveform of a PRBS sequence; (b) B2B waveform of an alternative sequence; (c) B2B eye diagram of the PRBS sequence; (d) B2B eye diagram of the alternative sequence; (e) eye diagrams of the PRBS sequence after FSO transmission; (f) eye diagrams of the alternative sequence after FSO transmission (160 ps/div in (c)–(f)).
of free carriers will drop. When the following bits “1” or “0” propagate in the RSOA, their gains that can be obtained will be reduced [23]. For comparison, we modulate a 1.25-Gbit/s OOK sequence with alternative “1” and “0”, which has no continuous bits, on the ROSA to verify our analysis. The corresponding reflected waveform from the RSOA is shown in Fig. 6(b). One can observe that the bit jitters become small in the alternative OOK sequence since the incident powers are all
Fig. 4. Eye diagrams of (a) B2B and (b) after free-space transmission for the 1.25-Gbit/s upstream OOK signal of the unidirectional FSO link (500 ps/div).
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Fig. 8. Eye diagrams of (a), (b) B2B and (c), (d) after free-space transmission for the 10Gbit/s downstream DPSK and 1.25-Gbit/s upstream OOK signals in the bidirectional FSO system (50 ps/div for the DPSK signal in (a), (c), and 500 ps/div for OOK signal in (b), (d)).
Fig. 7. BER performance of the PRBS and alternative “1” and “0” sequences before and after the FSO transmission.
different between two arbitrary adjacent bits. In this case, the depletions of free carriers along all the sequence are almost the same and the gain is stable in time domain. The difference can be evaluated through eye diagrams. Fig. 6(c) and (d) show the eye diagrams reflected from the RSOA for the PRBS and alternative sequences, respectively. The eye opening of the alternative sequence in Fig. 6(d) is much larger than that of the PRBS sequence in Fig. 6(c). Correspondingly, the transmitted eye diagram is also improved, which is verified by comparing the eye diagrams of the PRBS and alternative sequences at the transceiver, as shown in Fig. 6(e) and (f). The improvement is quantitatively evaluated by measuring the BERs for these two kinds of sequences. Fig. 7 shows the BERs before and after transmission in the free space for the PRBS and alternative sequences, respectively. One can see that the required received power is reduced by 1.24 dB at a BER of 1×10-3 if the PRBS is replaced by the alternative sequence. Moreover, the power penalty of transmission is also reduced from 1.75 to 1.5 dB. Above analysis shows that the degenerate of the modulated signal is due to the depletion of free carriers in the RSOA, and better signal quality can be expected using a RSOA with fast recovering performance.
Fig. 9. BER results before and after transmission for the downstream and upstream signals in the bidirectional FSO system.
signal of the bidirectional FSO system before and after transmission are both a little bigger than those of the unidirectional FSO link. It seems that the bidirectional FSO system has a little worse noise tolerance. The power penalties for the downstream and upstream signals are 0.69 and 0.62 dB, which are not larger than the unidirectional transmission and the validity of the bidirectional FSO transmission based on a single light source and a single RSOA is verified. For a real outdoor FSO transmission, the received optical signals will be affected by the atmospheric turbulence, which will cause the power attenuation and fluctuation of the signal. Several kinds of theoretical models have been presented to describe the influence of atmospheric turbulence, such as Gamma-Gamma distribution [24] and lognormal distribution [25], etc. As a result, the measured BER will also fluctuate with the signal power. Conventionally, an average BER will be used to evaluate the quality of a real FSO transmission link.
4.2. Single-source bidirectional FSO system using RSOA-based AMRR The bidirectional FSO system with a single light source is demonstrated and measured based on the AMRR using a single RSOA. The downstream link with the interrogation light is modulated by a 10Gbit/s DPSK signal, and the upstream link with the retro-reflected light is modulated by a 1.25-Gbit/s OOK signal. Fig. 8 shows the eye diagrams before [Fig. 8(a) and (b)] and after free-space transmission [Fig. 8(c) and (d)] for the downstream DPSK and upstream OOK signals, respectively. The B2B eye diagram of the upstream OOK signal is measured by being modulated on the optical carrier carrying the downstream DPSK signal. Comparing the eye diagrams of the upstream OOK signal in Fig. 8(b) with that of the OOK signal in the unidirectional link in Fig. 4(a), one can see that their qualities are similar, which demonstrates that the crosstalk between the upstream and downstream signals is low. The eye openings of both links are reduced since some noises are introduced during the transmission. Fig. 9 illustrates the BER results for the downstream DPSK and upstream OOK sequences. The required received powers for the downstream DPSK sequence before and after free-space transmission at the BER of 1×10-3 are −13.50 and −12.81 dBm respectively, while they are −11.62 and −11.03 dBm for the upstream OOK sequence. At the BER of 1×10-3, the needed received powers for the upstream OOK
5. Conclusion A novel RSOA-based AMRR has been proposed and demonstrated and a bidirectional FSO transmission system with a single light source has been realized using it. The RSOA-based AMRR is measured to provide an amplification of 4.02 dB and support the modulation bit rate up to 1.25 Gbit/s. For the bidirectional FSO transmission, the downstream and upstream links are modulated by 10-Gbit/s DPSK sequence and 1.25-Gbit/s PRBS OOK sequence, respectively. The transmission performances are tested via eye diagrams and BER 46
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(2016) 622–625. [9] H. Henniger, O. Wilfert, An introduction to free-space optical communications, Radioengineering 19 (2010) 203–212. [10] P.G. Goetz, W.S. Rabinovich, R. Mahon, J.L. Murphy, M.S. Ferraro, W.R. Smith, B. B. Xu, H.R. Burris, C.I. Moore, W.W. Schultz, Modulating retro-reflector lasercom systems at the naval research laboratory, in: Proceedings of the IEEE Military Communications Conference, 2010, pp. 1601–1606. [11] G.C. Gilbreath, W.S. Rabinovich, T.J. Meehan, M.J. Vilcheck, R. Mahon, R. Burris, M. Ferraro, I. Sokolsky, J.A. Vasquez, C.S. Bovais, Large-aperture multiple quantum well modulating retroreflector for free-space optical data transfer on unmanned aerial vehicles, Opt. Eng. 40 (2001) 1348–1356. [12] E. Rosenkrantz, S. Arnon, An innovative modulating retro-reflector for free-space optical communication, Proc. SPIE 8874 (2013) 88740D. [13] T. Mikaelian, M. Weel, A. Kumarakrishnan, P. Battle, R. Swanson, A high-speed retro-reflector for free-space communication based on electro-optic phase modulation, Can. J. Phys. 81 (2003) 639–650. [14] D. O'brien, J. Liu, G. Faulkner, S. Sivathasan, W. Yuan, S. Collins, S. Elston, V. Pithamiron, Design and implementation of optical wireless communications with optically powered smart dust motes, IEEE J. Sel. Area Commun. 27 (2009) 1646–1653. [15] C.M. Swenson, C.A. Steed, A. Imelda, R.Q. Fugate, Low-power FLC-based retromodulator communications system, Proc. SPIE 2290 (1997) 296–310. [16] W.S. Rabinovich, R. Mahon, P. Goetz, E. Waluschka, D. Katzer, S. Binari, G. Gilbreath, A cat's eye multiple quantum-well modulating retro-reflector, IEEE Photon. Technol. Lett. 15 (2003) 461–463. [17] S. Junique, D. Agren, Q. Wang, S. Almqvist, B. Noharet, J.Y. Andersson, A modulating retro-reflector for free-space optical communication, IEEE photon. Technol. Lett. 18 (2006) 85–87. [18] H. Takesue, T. Sugie, Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized light sources, J. Lightwave Technol. 21 (2003) 2546–2556. [19] J. Kang, Y. Won, S. Lee, S. Han, Modulation characteristics of RSOA in hybrid WDM/SCM-PON optical link, in: Proceedings of the National Optic Engineers Conference, paper JThB6, 2010. [20] Z. Xu, Y.J. Wen, W.-D. Zhong, T.H. Cheng, M. Attygalle, X. Cheng, Y.-K. Yeo, Y. Wang, C. Lu, Characteristics of subcarrier modulation and its application in WDM-pons, J. Lightwave Technol. 27 (2009) 2069–2076. [21] J.M. Lopez, K. Yong, Acquisition, tracking, and fine pointing control of space-based laser communication systems, in control and communication technology in laser systems, Proc. SPIE 295 (1981) 100–114. [22] S.-C. Lin, S.-L. Lee, C.-K. Liu, Simple approach for bidirectional performance enhancement on WDM-PONs with directmodulation lasers and RSOAs, Opt. Express 16 (2008) 3636–3643. [23] I. Papagiannakis, M. Omella, D. Klonidis, J.A. Lázaro Villa, A.N. Birbas, J. Kikidis, I. Tomkos, J. Prat, Design characteristics for a full-duplex IM/IM bidirectional transmission at 10 Gb/s using low bandwidth RSOA, J. Lightwave Technol. 28 (2010) 1094–1101. [24] H.E. Nistazakis, G.S. Tombras, On the use of wavelength and time diversity in optical wireless communication systems over gamma–gamma turbulence channels, Opt. Laser Technol. 44 (2012) 2088–2094. [25] S. Arnon, Effects of atmospheric turbulence and building sway on optical wirelesscommunication systems, Opt. Lett. 28 (2003) 129–131.
measurements. The origination of the gain jitter on the PRBS signal is analyzed by comparing it with an alternative “1” and “0” sequence. The power penalties are measured to be less than 0.69 dB at the BER of 1×10-3 for the downstream and upstream sequences in the bidirectional FSO system. The transmission bit rate and quality are expected to be improved by using a RSOA with broad modulation band and short recovering time. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 61675177 and 61475138), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130101110089), and the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY14F050006). References [1] V.W.S. Chan, Free-space optical communications, J. Lightwave Technol. 24 (2006) 4750–4762. [2] J.C. Juarez, A. Dwivedi, A.R. Hammons, S.D. Jones, V. Weerackody, R.A. Nichols, Free-space optical communications for next-generation military networks, IEEE Commun. Mag. 44 (2006) 46–51. [3] M. Gregory, F. Heine, H. Kämpfner, R. Lange, M. Lutzer, R. Meyer, Commercial optical inter-satellite communication at high data rates, Opt. Eng. 51 (2012) 031202. [4] D.M. Boroson, B.S. Robinson, D.A. Burianek, D.V. Murphy, A. Biswas, Overview and status of the lunar laser communications demonstration, Proc. SPIE 8246 (2006) 82460C. [5] R.M. Sova, J.E. Sluz, D.W. Young, J.C. Juarez, A. Dwivedi, N.M. Demidovich III, J.E. Graves, M. Northcott, J. Douglass, J. Phillips, D. Driver, A. McClarin, D. Abelson, 80 Gb/s free-space optical communication demonstration between an aerostat and a ground terminal, Proc. SPIE 6304 (2006) 630414. [6] J. Wang, J.Y. Yang, I.M. Fazal, N. Ahmed, Y. Yan, H. Huang, Y. Ren, Y. Yue, S. Dolinar, M. Tur, A.E. Willner, Terabit free-space data transmission employing orbital angular momentum multiplexing, Nat. Photon. 6 (2012) 488–496. [7] H. Huang, G. Xie, Y. Yan, N. Ahmed, Y. Ren, Y. Yue, D. Rogawski, M.J. Willner, B.J. Erkmen, K.M. Birnbaum, S.J. Dolinar, M.P.J. Lavery, M.J. Padgettm, M. Tur, A.E. Willner, 100 Tbit/s free-space data link enabled by three-dimensional multiplexing of orbital angular momentum, polarization, and wavelength, Opt. Lett. 39 (2014) 197–200. [8] Y. Ren, Z. Wang, P. Liao, L. Li, G. Xie, H. Huang, Z. Zhao, Y. Yan, N. Ahmed, A. Willner, M.P.J. Lavery, N. Ashrafi, S. Ashrafi, R. Bock, M. Tur, I.B. Djordjevic, M.A. Neifeld, A.E. Willner, Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m, Opt. Lett. 41
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Single-source bidirectional free-space optical communications using reflective SOA-based amplified modulating retro-reflection Xiaoyan Wanga, Xianglian Fenga, Peng Zhangb, Tianshu Wangb, Shiming Gaoa,
crossmark
⁎
a
Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310058, China National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun 130022, China
b
A R T I C L E I N F O
A BS T RAC T
PACS: 42.68.Ay 42.79.Sz
A novel amplified modulating retro-reflector (AMRR) based on a reflective semiconductor optical amplifier (RSOA) is proposed and a bidirectional free-space optical communication (FSO) system including both downstream and upstream links is experimentally demonstrated with only a single light source using this AMRR. The RSOA-based AMRR can provide a net gain more than 4 dB and support the modulation bit rate up to 1.25 Gbit/s. The bidirectional FSO transmission performance is evaluated by observing eye diagrams and measuring bit error rate (BER) results of both 10-Gbit/s DPSK downstream and 1.25-Gbit/s OOK upstream signals. The factors that limit the modulation bit rate and transmission quality are analyzed. The power penalties of both links are less than 0.69 dB in the bidirectional FSO system at the BER of 1×10-3.
Keywords: Free-space optical communications Modulating retro-reflection Bidirectional transmission Semiconductor optical amplifiers
1. Introduction Free-space optical communication (FSO) has been considered as a promising technology in response to a growing need for high-speed and tap-proof communication systems to transfer information between mobile and stationary terminals (aircrafts, satellites, and ground platforms, etc.), which exhibits many potential advantages in security, strong anti-jamming ability, high bandwidth, and large available spectrum [1,2]. In the past, several kinds of typical FSO transmissions have been demonstrated between two satellites [3], between the moon and the earth [4], and between an aerobat and a ground terminal [5]. In laboratory conditions, high-speed (gigabit and even terabit) FSO links between two fixed short-distance points have also been implemented by employing multiple-dimensional multiplexing of orbital angular momentum, polarization, and wavelength [6–8]. However, these active FSO systems with high pointing accuracy are often large, heavy, complex, and of high power consumption, which becomes challengeable when the terminals have strict payload and power limits [9]. For such implementations, modulating retro-reflectors (MRRs) have exhibited more competitions. A MRR combines an optical retro-reflector with a modulator to reflect modulated optical signals directly back to the transceiver and shifts most of the power, weight and pointing requirements onto the base station of the FSO systems [10,11]. MRRs have been demonstrated through different ways such as frustrated total internal reflectors [12], electro-optic
⁎
Corresponding author. E-mail address: [email protected] (S. Gao).
http://dx.doi.org/10.1016/j.optcom.2016.11.019 Received 20 October 2016; Accepted 10 November 2016 0030-4018/ © 2016 Elsevier B.V. All rights reserved.
phase modulators [13], liquid crystal modulators [14,15], and multiple quantum well devices [16,17]. In above schemes, the modulation and reflection elements are all discrete and of large volumes, and the modulation speed is relatively low. As a powerful signal-processing device, reflective semiconductor optical amplifier (RSOA) shows robust performances. An integrated RSOA with pigtail has many advantages such as small volume, lightweight, optical power amplification, and of collinear optical path. Moreover, RSOA can be used as a reflector and a modulator simultaneously, which has been widely demonstrated in wavelength-division multiplexing passive optical networks (WDMPONs) [18–20]. Therefore, an amplified MRR (AMRR) for FSO systems is possible to be realized by using the RSOA as an amplifier, modulator, and reflector simultaneously, since an amplified, modulated, and retro-reflected light can be obtained when an interrogation light is injected into the RSOA. In this paper, we propose and experimentally demonstrate, for the first time to our best knowledge, a RSOA-based AMRR and a bidirectional FSO system for 10-Gbit/s downstream differential phase shift keying (DPSK) signal and 1.25-Gbit/s upstream on-off keying (OOK) signal with a single light source using this AMRR. In the bidirectional FSO system, only a single optical carrier is used, which is arranged at the transceiver. The uplink signal is modulated on the reflected optical carrier at the terminal. Eye diagrams and bit error rates (BERs) are measured for both downlink and uplink signals. At the BER of 1×10-3, the power penalties are measured to be 0.69 and 0.47 dB for the 10-
Optics Communications 387 (2017) 43–47
X. Wang et al.
Fig. 1. Schematic configruations of (a) a coventional MRR structure using semicondutor multi-quantum-well modualtor and a corner-cube prism retro-reflector and (b) our proposed RSOA-based AMRR by using the RSOA as the modulator, amplifier, and rectro-reflector simultenously.
Gbit/s downstream DPSK signal and the 1.25-Gbit/s upstream OOK signal.
2. Principle A MRR structure needs to include the elements that can realize optical retro-reflection and modulation. As shown in Fig. 1(a), in the conventional MRR configuration, the interrogation light is modulated by the driving data sequence using the modulator such as a semiconductor multi-quantum-well modulator. And then, it is reflected by the retro-reflector using such as a corner-cube prism. Since a RSOA can realize the modulation and retro-reflection simultaneously, the MRR function can be utilized using a single RSOA. Fig. 1(b) shows the configuration of our proposed RSOA-based MRR. The interrogation light is collected using an optical antenna and coupled into an optical fiber. And then, the interrogation light is injected into the RSOA, where it is modulated by the driving data and reflected backward to the optical fiber. After coupling from the fiber to the optical antenna, the retro-reflected light is transmitted back to the transceiver from the MRR via free space. Compared with the conventional MRR, our proposed RSOA-based MRR can support higher modulation bit rate, which is up to the level of GHz, and has a small volume. Also, the RSOA has the ability of amplification and the incident interrogation light will be amplified when it is reflected as the retro-reflected light. It means that the RSOA-based MRR is an AMRR. For an optical carrier, multiple physical quantities can be used for data modulation. According to the used physical quantities, the signals are classified as amplitude-shift keying (ASK), phase-shift keying (PSK), and polarization-shift keying (PolSK) signals, etc. In order to realize high-quality bidirectional transmission using a single light source, the influence between the downstream and upstream signals should be effectively eliminated. Since the amplitude can be kept the same when a PSK signal is modulated on the phase of the optical carrier, the PSK optical signal is expected to be amplitude modulated once more to transmit a ASK signal without crosstalk. The principle is shown in Fig. 2. For the downstream link, DPSK modulation format is selected, which is only modulated on the phase of the optical carrier at the transceiver, and the amplitude of the optical carrier is constant, as shown in Fig. 2(a). When the downstream DPSK optical signal is transmitted to the terminal, one part is used for demodulation, and the other serves as the interrogation light of the RSOA-based AMRR to generate the upstream link. Here, the interrogation light serves as the optical carrier and it is amplitude modulated by the upstream OOK format through the RSOA, as shown in Fig. 2(b). After modulated, the upstream optical signal is amplified and retro-reflected back to the transceiver. As the OOK signal only changes the optical carrier amplitude, which is independent on the carrier phase. Therefore, the existence of the downstream DPSK signal will not affect the modulation and detection of the upstream OOK signal. In this way, a high-quality bidirectional FSO system with a single light source is expected to be realized with low crosstalk using our proposed RSOA-based AMRR.
Fig. 2. Principle for choosing the modulation formats of the bidirectional FSO system.
3. Experimental setup Fig. 3 shows the experimental setup of the proposed bidirectional FSO system with a single light source using the RSOA-based AMRR. The light source is provided by a continuous-wave (CW) tunable laser (Agilent 81940) with a 1.12-dBm power at 1550.0 nm. At the transceiver, it is modulated by a 10-Gbit/s DPSK sequence via a phase modulator and launched into free space through a collimator (Collimator 1, Thorlabs F280FC-1550) whose divergence angle is 0.032°. After 0.5-m transmission, it is coupled back to an optical fiber at the terminal for detection and upstream modulation via another collimator (Collimator 2, Thorlabs A397TM-C), which serves as the optical antenna of the AMRR. Here a relatively short distance is transmitted since we mainly focus on the proof-of-concept demonstration in lab. If high-quality collimation optical systems are used for transmission, the distance has the ability to be effectively enhanced [3]. In our experiment, the field-of-view (FOV) of the RSOA-based AMRR is determined by the acceptance angle of Collimator 2, which is about 34.9°. If larger FOV is required, the conventional way is setting the optical antenna on a servo turntable and the orientation of the optical antenna can be changed by tuning the servo turntable to collect the interrogation light within a wider angle region [21]. At the terminal, the downstream DPSK optical signal is split into two branches. One branch (10% power) is sent into a one-bit delay interferometer for demodulation, where the DPSK signal is converted to a sequence of amplitude modulated signal. The converted signal is detected using a photodetector (PD1), the eye diagram is observed using an oscilloscope, and the BER is measured using a BER tester. The other branch of the downstream optical signal is sent into a RSOA (SOA-RL-OEC-1550) as the interrogation light. The transmission and coupling losses are 2.72 dB, and the optical power injected into the RSOA is −1.60 dBm. The upstream sequence, a 1.25-Gbit/s 271 pseudorandom binary sequence (PRBS) is modulated via the RSOA with a 55-mA driving current. After being modulated and amplified by the RSOA, the retro-reflected optical signal is measured to be 44
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Fig. 3. Experimental setup of the bidirectional FSO system using a RSOA-based AMRR.
2.42 dBm, and the signal is amplified by 4.02 dB due to the gain of the RSOA, which verifies that our proposed AMRR has the ability of amplification. The retro-reflected optical signal, that is, the upstream OOK optical signal, transmits back to the transceiver through the same free-space optical path and received by another photodetector (PD2). The eye diagram and BER of the upstream signal are also measured. Due to the limitation of the low bandwidth and high chirp characteristics of the commercially RSOA we used, whose modulation bandwidth is less than 1.5 GHz [22], we only modulate a relatively low upstream bit rate as 1.25 Gbit/s. Higher upstream bit rate is permitted in this duplex FSO system as a RSOA with broader bandwidth is adopted. 4. Results and discussion 4.1. RSOA-based AMRR In order to verify the performance of the RSOA-based AMRR, we first realize a unidirectional FSO link without the downstream DPSK signal modulated. The unmodulated light source serving as the interrogation light is first transmitted through free space to the RSOA, where the 1.25-Gbit/s upstream OOK signal is uploaded and retro-reflected. Fig. 4(a) and (b) show the eye diagrams before (backto-back, B2B) and after free-space transmission. The eye opening decreases a little after transmission due to the noises introduced in the optical paths, including the reflected lights from the collimators and the noises from the PD, which are dominated by the thermal noise. The BER results are shown in Fig. 5, where the required received powers for the 1.25-Gbit/s OOK signal before and after transmission are −13.41 and −11.66 dBm at the BER of 1×10-3, which is low enough to be corrected using forward error correction (FEC) algorithms. A power penalty of 1.75 dB is measured for the OOK signal transmission. As a result, the sensitivity of the proposed RSOA-based AMRR can be obtained as −11.66 dBm at this BER level. The sensitivity will increase if lower BER is required. In the eye diagrams of Fig. 4, one can see that large jitters are introduced in both bits “1” and “0”, especially on the top of “1”. In order to find the origination, we measure the waveform that reflects from the RSOA when the 1.25-Gbit/s PRBS sequence is modulated, as shown in Fig. 6(a). The top and bottom of the waveform become uneven and large gain slopes can be clearly observed when multiple bits “1” or “0” emerge adjacently each other. It can be explained by the intensity variation of free carriers in the RSOA. When multiple continuous bits of “1” or “0” input, the front bits “1” or “0” will deplete the free carriers in the RSOA to acquire gains. As a result, the intensity
Fig. 5. BER results of the unidirectional FSO link before and after transmission.
Fig. 6. Performance comparison of the PRBS sequence and the alternative “1” and “0” sequence modualted via the RSOA. (a) B2B waveform of a PRBS sequence; (b) B2B waveform of an alternative sequence; (c) B2B eye diagram of the PRBS sequence; (d) B2B eye diagram of the alternative sequence; (e) eye diagrams of the PRBS sequence after FSO transmission; (f) eye diagrams of the alternative sequence after FSO transmission (160 ps/div in (c)–(f)).
of free carriers will drop. When the following bits “1” or “0” propagate in the RSOA, their gains that can be obtained will be reduced [23]. For comparison, we modulate a 1.25-Gbit/s OOK sequence with alternative “1” and “0”, which has no continuous bits, on the ROSA to verify our analysis. The corresponding reflected waveform from the RSOA is shown in Fig. 6(b). One can observe that the bit jitters become small in the alternative OOK sequence since the incident powers are all
Fig. 4. Eye diagrams of (a) B2B and (b) after free-space transmission for the 1.25-Gbit/s upstream OOK signal of the unidirectional FSO link (500 ps/div).
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Fig. 8. Eye diagrams of (a), (b) B2B and (c), (d) after free-space transmission for the 10Gbit/s downstream DPSK and 1.25-Gbit/s upstream OOK signals in the bidirectional FSO system (50 ps/div for the DPSK signal in (a), (c), and 500 ps/div for OOK signal in (b), (d)).
Fig. 7. BER performance of the PRBS and alternative “1” and “0” sequences before and after the FSO transmission.
different between two arbitrary adjacent bits. In this case, the depletions of free carriers along all the sequence are almost the same and the gain is stable in time domain. The difference can be evaluated through eye diagrams. Fig. 6(c) and (d) show the eye diagrams reflected from the RSOA for the PRBS and alternative sequences, respectively. The eye opening of the alternative sequence in Fig. 6(d) is much larger than that of the PRBS sequence in Fig. 6(c). Correspondingly, the transmitted eye diagram is also improved, which is verified by comparing the eye diagrams of the PRBS and alternative sequences at the transceiver, as shown in Fig. 6(e) and (f). The improvement is quantitatively evaluated by measuring the BERs for these two kinds of sequences. Fig. 7 shows the BERs before and after transmission in the free space for the PRBS and alternative sequences, respectively. One can see that the required received power is reduced by 1.24 dB at a BER of 1×10-3 if the PRBS is replaced by the alternative sequence. Moreover, the power penalty of transmission is also reduced from 1.75 to 1.5 dB. Above analysis shows that the degenerate of the modulated signal is due to the depletion of free carriers in the RSOA, and better signal quality can be expected using a RSOA with fast recovering performance.
Fig. 9. BER results before and after transmission for the downstream and upstream signals in the bidirectional FSO system.
signal of the bidirectional FSO system before and after transmission are both a little bigger than those of the unidirectional FSO link. It seems that the bidirectional FSO system has a little worse noise tolerance. The power penalties for the downstream and upstream signals are 0.69 and 0.62 dB, which are not larger than the unidirectional transmission and the validity of the bidirectional FSO transmission based on a single light source and a single RSOA is verified. For a real outdoor FSO transmission, the received optical signals will be affected by the atmospheric turbulence, which will cause the power attenuation and fluctuation of the signal. Several kinds of theoretical models have been presented to describe the influence of atmospheric turbulence, such as Gamma-Gamma distribution [24] and lognormal distribution [25], etc. As a result, the measured BER will also fluctuate with the signal power. Conventionally, an average BER will be used to evaluate the quality of a real FSO transmission link.
4.2. Single-source bidirectional FSO system using RSOA-based AMRR The bidirectional FSO system with a single light source is demonstrated and measured based on the AMRR using a single RSOA. The downstream link with the interrogation light is modulated by a 10Gbit/s DPSK signal, and the upstream link with the retro-reflected light is modulated by a 1.25-Gbit/s OOK signal. Fig. 8 shows the eye diagrams before [Fig. 8(a) and (b)] and after free-space transmission [Fig. 8(c) and (d)] for the downstream DPSK and upstream OOK signals, respectively. The B2B eye diagram of the upstream OOK signal is measured by being modulated on the optical carrier carrying the downstream DPSK signal. Comparing the eye diagrams of the upstream OOK signal in Fig. 8(b) with that of the OOK signal in the unidirectional link in Fig. 4(a), one can see that their qualities are similar, which demonstrates that the crosstalk between the upstream and downstream signals is low. The eye openings of both links are reduced since some noises are introduced during the transmission. Fig. 9 illustrates the BER results for the downstream DPSK and upstream OOK sequences. The required received powers for the downstream DPSK sequence before and after free-space transmission at the BER of 1×10-3 are −13.50 and −12.81 dBm respectively, while they are −11.62 and −11.03 dBm for the upstream OOK sequence. At the BER of 1×10-3, the needed received powers for the upstream OOK
5. Conclusion A novel RSOA-based AMRR has been proposed and demonstrated and a bidirectional FSO transmission system with a single light source has been realized using it. The RSOA-based AMRR is measured to provide an amplification of 4.02 dB and support the modulation bit rate up to 1.25 Gbit/s. For the bidirectional FSO transmission, the downstream and upstream links are modulated by 10-Gbit/s DPSK sequence and 1.25-Gbit/s PRBS OOK sequence, respectively. The transmission performances are tested via eye diagrams and BER 46
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(2016) 622–625. [9] H. Henniger, O. Wilfert, An introduction to free-space optical communications, Radioengineering 19 (2010) 203–212. [10] P.G. Goetz, W.S. Rabinovich, R. Mahon, J.L. Murphy, M.S. Ferraro, W.R. Smith, B. B. Xu, H.R. Burris, C.I. Moore, W.W. Schultz, Modulating retro-reflector lasercom systems at the naval research laboratory, in: Proceedings of the IEEE Military Communications Conference, 2010, pp. 1601–1606. [11] G.C. Gilbreath, W.S. Rabinovich, T.J. Meehan, M.J. Vilcheck, R. Mahon, R. Burris, M. Ferraro, I. Sokolsky, J.A. Vasquez, C.S. Bovais, Large-aperture multiple quantum well modulating retroreflector for free-space optical data transfer on unmanned aerial vehicles, Opt. Eng. 40 (2001) 1348–1356. [12] E. Rosenkrantz, S. Arnon, An innovative modulating retro-reflector for free-space optical communication, Proc. SPIE 8874 (2013) 88740D. [13] T. Mikaelian, M. Weel, A. Kumarakrishnan, P. Battle, R. Swanson, A high-speed retro-reflector for free-space communication based on electro-optic phase modulation, Can. J. Phys. 81 (2003) 639–650. [14] D. O'brien, J. Liu, G. Faulkner, S. Sivathasan, W. Yuan, S. Collins, S. Elston, V. Pithamiron, Design and implementation of optical wireless communications with optically powered smart dust motes, IEEE J. Sel. Area Commun. 27 (2009) 1646–1653. [15] C.M. Swenson, C.A. Steed, A. Imelda, R.Q. Fugate, Low-power FLC-based retromodulator communications system, Proc. SPIE 2290 (1997) 296–310. [16] W.S. Rabinovich, R. Mahon, P. Goetz, E. Waluschka, D. Katzer, S. Binari, G. Gilbreath, A cat's eye multiple quantum-well modulating retro-reflector, IEEE Photon. Technol. Lett. 15 (2003) 461–463. [17] S. Junique, D. Agren, Q. Wang, S. Almqvist, B. Noharet, J.Y. Andersson, A modulating retro-reflector for free-space optical communication, IEEE photon. Technol. Lett. 18 (2006) 85–87. [18] H. Takesue, T. Sugie, Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized light sources, J. Lightwave Technol. 21 (2003) 2546–2556. [19] J. Kang, Y. Won, S. Lee, S. Han, Modulation characteristics of RSOA in hybrid WDM/SCM-PON optical link, in: Proceedings of the National Optic Engineers Conference, paper JThB6, 2010. [20] Z. Xu, Y.J. Wen, W.-D. Zhong, T.H. Cheng, M. Attygalle, X. Cheng, Y.-K. Yeo, Y. Wang, C. Lu, Characteristics of subcarrier modulation and its application in WDM-pons, J. Lightwave Technol. 27 (2009) 2069–2076. [21] J.M. Lopez, K. Yong, Acquisition, tracking, and fine pointing control of space-based laser communication systems, in control and communication technology in laser systems, Proc. SPIE 295 (1981) 100–114. [22] S.-C. Lin, S.-L. Lee, C.-K. Liu, Simple approach for bidirectional performance enhancement on WDM-PONs with directmodulation lasers and RSOAs, Opt. Express 16 (2008) 3636–3643. [23] I. Papagiannakis, M. Omella, D. Klonidis, J.A. Lázaro Villa, A.N. Birbas, J. Kikidis, I. Tomkos, J. Prat, Design characteristics for a full-duplex IM/IM bidirectional transmission at 10 Gb/s using low bandwidth RSOA, J. Lightwave Technol. 28 (2010) 1094–1101. [24] H.E. Nistazakis, G.S. Tombras, On the use of wavelength and time diversity in optical wireless communication systems over gamma–gamma turbulence channels, Opt. Laser Technol. 44 (2012) 2088–2094. [25] S. Arnon, Effects of atmospheric turbulence and building sway on optical wirelesscommunication systems, Opt. Lett. 28 (2003) 129–131.
measurements. The origination of the gain jitter on the PRBS signal is analyzed by comparing it with an alternative “1” and “0” sequence. The power penalties are measured to be less than 0.69 dB at the BER of 1×10-3 for the downstream and upstream sequences in the bidirectional FSO system. The transmission bit rate and quality are expected to be improved by using a RSOA with broad modulation band and short recovering time. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 61675177 and 61475138), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130101110089), and the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY14F050006). References [1] V.W.S. Chan, Free-space optical communications, J. Lightwave Technol. 24 (2006) 4750–4762. [2] J.C. Juarez, A. Dwivedi, A.R. Hammons, S.D. Jones, V. Weerackody, R.A. Nichols, Free-space optical communications for next-generation military networks, IEEE Commun. Mag. 44 (2006) 46–51. [3] M. Gregory, F. Heine, H. Kämpfner, R. Lange, M. Lutzer, R. Meyer, Commercial optical inter-satellite communication at high data rates, Opt. Eng. 51 (2012) 031202. [4] D.M. Boroson, B.S. Robinson, D.A. Burianek, D.V. Murphy, A. Biswas, Overview and status of the lunar laser communications demonstration, Proc. SPIE 8246 (2006) 82460C. [5] R.M. Sova, J.E. Sluz, D.W. Young, J.C. Juarez, A. Dwivedi, N.M. Demidovich III, J.E. Graves, M. Northcott, J. Douglass, J. Phillips, D. Driver, A. McClarin, D. Abelson, 80 Gb/s free-space optical communication demonstration between an aerostat and a ground terminal, Proc. SPIE 6304 (2006) 630414. [6] J. Wang, J.Y. Yang, I.M. Fazal, N. Ahmed, Y. Yan, H. Huang, Y. Ren, Y. Yue, S. Dolinar, M. Tur, A.E. Willner, Terabit free-space data transmission employing orbital angular momentum multiplexing, Nat. Photon. 6 (2012) 488–496. [7] H. Huang, G. Xie, Y. Yan, N. Ahmed, Y. Ren, Y. Yue, D. Rogawski, M.J. Willner, B.J. Erkmen, K.M. Birnbaum, S.J. Dolinar, M.P.J. Lavery, M.J. Padgettm, M. Tur, A.E. Willner, 100 Tbit/s free-space data link enabled by three-dimensional multiplexing of orbital angular momentum, polarization, and wavelength, Opt. Lett. 39 (2014) 197–200. [8] Y. Ren, Z. Wang, P. Liao, L. Li, G. Xie, H. Huang, Z. Zhao, Y. Yan, N. Ahmed, A. Willner, M.P.J. Lavery, N. Ashrafi, S. Ashrafi, R. Bock, M. Tur, I.B. Djordjevic, M.A. Neifeld, A.E. Willner, Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m, Opt. Lett. 41
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