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© 2009 OSA/OFC/NFOEC 2009
Modulation Format Transparent Subcarrier reuse by Feed Forward Current Injection in a Reflective SOA M. Presi, A. Chiuchiarelli, G. Contestabile and E. Ciaramella Scuola Superiore Sant’Anna, Via G. Moruzzi 1, I-56124 Pisa, Italy, e-mail:
[email protected]
L. Giorgi Ericsson, Via G. Moruzzi 1, I-56124 Pisa, Italy. e-mail:
[email protected]
Abstract: We experimentally demonstrate remodulation of a 2.4 GHz microwave channel by using feed-forward current injection in a Reflective-SOA. We tested the scheme with different modulation formats (BPSK, 16-QAM, 64-QAM) at 155 Mbaud.
c 2009 Optical Society of America
OCIS codes: (060.2330) Fiber optics communications; (060.5625) Radio Frequency Photonics
1
Introduction
Future WDM optical access networks will benefit from colorless optical network units (ONUs) able to transparently remodulate optical signals, thus allowing to use the same optical carrier for both downlink and uplink full-duplex communications. Remodulation consists of two simultaneous steps: cancellation of the downlink modulation and writing of uplink data. This task could be obtained by various techniques based on colorless amplifying remodulators, such as Reflective Semiconductor Optical Amplifiers (R-SOA), as discussed in [1]. Some techniques exploit different spectral regions of the optical signal to share the same optical carrier, e.g. sub-carrier modulation for downlink and baseband modulation for uplink [2]. However, to the best of our knowledge, full-duplex remodulation of a sub-carrier modulated signal onto the same sub-carrier, transparently to the modulation format, has not yet been reported. Radio-over-fiber systems integrated in WDM access networks [3,4] can natively benefit of the optical carrier reuse thanks to radio communication protocols. Time slotted communications, half-duplex and frequency division duplexing are can be used to this aim. However, a protocol transparent approach allowing for full-duplex reusage of a subcarrier modualted signal is highly desirable, as it would add degrees of freedom in the development of advanced communication protocols. In addition, also Sub-Carrier-Multiplexed access systems [5] may benefit from this approach. In this paper we demonstrate the full-duplex remodulation of a microwave carrier. It may allow for bandwidth symmetric bidirectional optical links. It is performed by using a feed forward current injection scheme [6]. The system has been tested with 2.4 GHz signals at 155 Gbaud and various modulation formats: BPSK, 16-QAM and 64-QAM.
Fig. 1. Experimental Setup of the proposed architecture. TL: Tunable Laser; AWG: Arbitrary Waveform Generator; IM: Intensity Modulator; OC: Optical Circulator; VSA: Vector Signal Analyzer; PD: Photodiode; EBPF: Electrical Band Pass Filter; EA: Electrical Amplifier.
2
Operating Principle and Experiment
Feed-Forward-Current-Injection (FFCI) in R-SOAs was suggested in the past to allow the remodulation of digital baseband OOK signals with reduced modulation index [6]. FFCI consists of modulating the gain of the R-SOA using a feed-forward signal that changes in the opposite direction in respect of the intensity of the optical signal at the input of the R-SOA. The feed-forward signal can be obtained using a photodiode (PD) with differential output, which provides both the detected signal D (to be delivered to the receiver) and its inverted logic copy D¯ (to be used as feed-forward
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a1016_1.pdf JWA71.pdf JWA71.pdf
© 2009 OSA/OFC/NFOEC 2009
Fig. 2. Left: constellations of input signal (1st col.), remodulated signal without FFCI (2nd col.) and with FFCI (3rd col.); Right: RF spectrum cancellation for 64-QAM signal
signal), as illustrated in fig. 1. Full remodulation is then achieved by adding the uplink data to the feed-forward signal. A correct remodulation requires a proper control of the amplitudes of the feed-forward and of the R-SOA dc-bias. The experimental setup used to validate the approach is illustrated in fig. 1. The downlink RF signal at a frequency of 2.4 GHz was created by means of an Arbitrary Waveform Generator (AWG), capable of generating signals with up to 10 Gbaud. The symbol rate was 155 Mbaud. Three modulation formats were investigated: BPSK, 16-QAM and 64-QAM (giving a total capacity of 155, 620 and 1240 Mb/s, respectively). In all cases, the signal was Nyquist filtered. In downlink a single drive Mach-Zehnder Intensity Modulator (IM) was used to generate the radio-over-fiber optical signal onto a laser at λ = 1549 nm. The signal was then delivered to the remote optical network unit (ONU) where it was split by a 70 − 30% coupler. 30% of the signal was injected into the R-SOA for remodulation; the other fraction was sent to the PD. The signal from the inverting output port was first amplified by wideband (10 GHz) electrical amplifiers, and served as the feed forward signal. An electrical bandpass filter, centered at 2 GHz with 1.5 GHz bandwidth was inserted after the electrical amplifiers. The non-inverted PD output was used to characterize the downlink performance, by means of Error Vector Magnitude (EVM) measurements. As the uplink electrical signal we used a decorrelated copy of the downlink signal. The uplink electrical signal was also amplified by a wideband amplifier and combined to the feed-forward signal by using a 3 dB electrical coupler and electrical isolators (in order to avoid spurious reflections). The composite signal was used to drive the R-SOA. The R-SOA was a commercially available device with 1.5 GHz modulation bandwidth, a small signal gain of 10 dB @ 50 mA bias, and 0 dBm output saturated power. Proper synchronization between downlink and uplink signals inside the R-SOA was obtained by an optical delay line (ODL). The ODL line had 5 dB insertion loss. In a real deployment, synchronization (which is fixed, and depends only on the relative optical paths inside the remote base station) should be achieved by electronic means, so that this extra optical loss can be avoided. EVM measurements were carried out by means of a Vector Signal Analyzer with 12 GHz bandwidth. In fig. 2 we report the signals constellations in three different conditions, for each modulation format considered: the leftmost figures indicate the constellation of the signals at the R-SOA input (downstream signal). The central column refers to the constellations at the R-SOA output, when the R-SOA is only driven by the uplink signal (i.e., when the FFCI circuit is disabled). Clearly the constellations cannot be recognized. When the FFCI circuit is turned on, the constellations of the uplink signals are recovered (3rd column). In order to prove the effective cancellation of the incoming modulation, we report in fig. 2 also the RF spectrum of a 64-QAM signal at the R-SOA output when the uplink signal is disabled: in this case (top figure), the R-SOA reflects back the input signal, but when the FFCI circuit is enabled (bottom figure), the RF spectrum is erased, confirming the cancellation of the modulation. We note a small residual of the input spectrum when the FFCI circuit is enabled. We attribute it to the non-ideal driving amplifier used in the FFCI circuit, which introduces small distortions and does not allow for an exact cancellation. Similar results
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Fig. 3. EVM vs power penalty for the various modulation formats
are obtained when BPSK or 16-QAM modulation format are used. In order to achieve a good cancellation of the incoming modulation, it is important to operate the R-SOA in the linear regime (i.e. far from the saturation): indeed, if the R-SOA is operated in saturation, the microwave input signal is distorted by pattern effects, while the FFCI signal is not; this would cause a mismatch between the two inverted signals, leading to strong remodulation impairments. In our case, we found that the optimal input power at the R-SOA was arounf −15 dBm. In fig. 3 we report the EVM measurements for the 3 modulation formats. In each case, we report the EVM of the input signal, at the R-SOA input (label “Mach-Zehnder”), and after the remodulation by means of FFCI (label “FFCI-ON”). For a better comparison, we also report the EVM vs. power in the back-to-back case in which the RSOA modulates a CW signal with the same input power of −15 dBm (label “B2B”). In all cases, the remodulation process introduce an EVM penalty < 1% in respect of the CW modulation, mostly due to the not-perfect cancellation caused by the non-ideal electrical amplifiers, as discussed above. We observed an EVM floor at values around 4.5% for both 16-QAM and 64-QAM and 6% for the BPSK case: however, this is not due to the remodulation process (as it is observed also in the case in which the R-SOA modulates a CW). We attribute this floor to the driving amplifier used to modulate the R-SOA, which is not suitable for this application: indeed, the minimum EVM achievable at the driver output was 4% for all the modulation format considered. This is evident also in the constellations reported in fig. 2, in which a distortion in the constellation pattern can be observed. 3 Conclusions We experimentally characterized and realized the remodulation of a subcarrier modulated optical signal by using Feed-Forward-Current Injection in a Reflective SOA. Notwithstanding the non-linear electro-optical response of the R-SOA, the FFCI approach allowed the remodulation of signals with different modulation formats, both constant envelope (such BPSK) and multi-level (16-QAM and 64-QAM) for a total capacity up to 1240 Mb/s. This scheme is not based on any optical filtering, thus it is fully wavelength agnostic. The main limitations found in this characterization come from the use of non-optimal driving amplifier and we believe they can be overcome by means of appropriate components. The proposed solution can found applications in radio-over-fiber and sub-carrier-multiplexed systems. References 1. J. Yu, N. Kim, and B. Kim, “Remodulation schemes with reflective SOA for colorless DWDM PON,” Journal of Optical Networking, vol. 6, no. 8, pp. 1041–1054, 2007. 2. J.M. Fabrega, E.T. Lopez, J.A. Lazaro, M. Zundhi, and J. Prat, “Demonstration of a full duplex PON feturing 2.5 Gbps Sub Carrier Multiplexing downstream and 1.25Gbps upstream with colourless ONU and simple optics,” ECOC 2008, vol. We.1.F.6, 2008. 3. G. Shen, R. Tucker, and C. Chae, “Fixed Mobile Convergence Architectures for Broadband Access: Integration of EPON and WiMAX,” IEEE COMMUNICATIONS MAGAZINE, vol. 45, no. 8, p. 44, 2007. 4. P. Chanclou, Z. Belfqih, B. Charbonnier, T. Duong, T. Frank, T.Genvay, T. Huchard, A. Pizzinat, H. Ramanitra, F. Saliou, S. Durel, A. Othmani, P. Urvoas, M. Ouzzif, and J. LeMasson, “Optical access evolutions and their impact on the metropolitan and home networks,” in ECOC 2008, vol. 3, 2008. 5. J. Ha, A. Wonfor, R. Penty, I. White, and P. Ghiggino, “Spectrally Efficient 10× 1 Gb/s QPSK Multi-User Optical Network Architecture,” in Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007. OFC/NFOEC 2007. Conference on, pp. 1–3, 2007. 6. W. Lee, S. Cho, M. Park, J. Lee, C. Kim, G. Jeong, and B. Kim, “Optical Transceiver employing an RSOA with Feed-Forward Current Injection,” in Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007. OFC/NFOEC 2007. Conference on, pp. 1–3, 2007.
