wdm optical interfaces for future fiber radio systems phần 9 pot

Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks performance significantly. In addition, the optical spectrum in Fig. 5.31(b) shows that the uplink signal is contaminated by the out-of-band reflected crosstalk from the downlink direction, which is approximately -17 dB. This unwanted power can be removed (as shown in Fig. 5.31c) by the suitable selection of an optical BPF that follows the EDFA in order to minimise the out-of-band ASE noise as shown in Fig. 5.27. Also, in a practical network each of the WI-DWDM uplink signals will be demultiplexed at the CO before detection, therefore the out-of-band crosstalk from the downlink path does not require any special attention, and will merge with the typical crosstalk caused by the filtering characteristic of the demultiplexer. To measure the BER, the filtered uplink signal was subsequently detected and data was recovered using the data recovery circuit previously described in the downlink path. Fig. 5.32 shows the measured BER curves for the back-to-back condition (with the MUX/DEMUX scheme but no transmission fibre) and after transmission over 10 km of SMF for the signal, (S U3 , C U3 ). The result exhibits a negligible 0.3 dB power penalty at a BER of 10 -9 which can be attributed to experimental errors. Therefore, the recovered optical spectra and the BER curves -6 -7 -8 -9 -19.6 -19.2 -18.8 -18.4 -18 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) with 0.0 KM SMF with 10 KM SMF λ UL = λ DL -5×FSR -6 -7 -8 -9 -19.6 -19.2 -18.8 -18.4 -18 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) with 0.0 KM SMF with 10 KM SMF λ UL = λ DL -5×FSR with 0.0 KM SMF with 10 KM SMF λ UL = λ DL -5×FSR Fig. 5.32: Measured BER curves as a function of received optical power for the multiplexed uplink signal, (S U3 , C U3 ) after transmission over 10 km of SMF with the back-to-back (0.0 km SMF) curve as a reference. The uplink signal was generated using an optical carrier separated by 500 GHz from the downlink carrier. 225 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks clearly demonstrate the functionality of the proposed DEMUX/MUX scheme in multiplexing the uplink signals with optical carriers at wavelengths equal to the difference between the downlink optical carriers and 5 × FSR. 5.7.4.2 Uplink by Reusing Downlink Optical Carrier Fig. 5.33(a) shows the measured optical spectrum of the downlink signal after recovering 50% of the carrier, while Figs. 5.33(b) – (c) present the optical spectra for the recovered optical carrier and the generated uplink (S U3 , C U3 ) before entering the 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) 50%C D3 -60 -40 -80 -20 0 S D3 (a) 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) 50%C D3 -60 -40 -80 -20 0 (b) 1555.9 1556.3 1556.7 Wavelength (nm) C U3 -60 -40 -80 -20 S U3 (c) 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) 50%C D3 -60 -40 -80 -20 0 S D3 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) 50%C D3 -60 -40 -80 -20 0 S D3 (a) 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) 50%C D3 -60 -40 -80 -20 0 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) 50%C D3 -60 -40 -80 -20 0 (b) 1555.9 1556.3 1556.7 Wavelength (nm) C U3 -60 -40 -80 -20 S U3 1555.9 1556.3 1556.7 Wavelength (nm) C U3 -60 -40 -80 -20 S U3 (c) Fig. 5.33: Measured optical spectra of: (a): the downlink signal, (S D3 , C D3 ) after recovering 50% carrier, (b): the recovered optical carrier, and (c): the uplink signal, (S U3 , C U3 ) generated using the recovered carrier. 226 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks DEMUX/MUX scheme respectively. As expected, due to recovering 50% of optical carrier, the CSR of the downlink signal is reduced by 3 dB, which eventually contributes in improving the link performance, as illustrated in Section 5.4.2. Spectra of Fig. 5.33(b)-(c) show that uplink DE-MZM experiences an unusual insertion loss of 16 dB resulting in a weaker uplink signal. Such situation can be avoided by placing a suitable DE-MZM having lower OSSB+C generation loss (typical loss < 9 dB). Fig. 3.34(a) presents the multiplexed uplink signal at the CO after transmission over 10 km of SMF, while Fig. 5.34(b) presents the unwanted crosstalk at the CO from the downlink path (in the absence of uplink signal in the link). The spectra indicate that due to traversing through the AWG, the uplink signal is contaminated by the unwanted in-band and out-of-band crosstalk by the reflections from the downlink path, which is approximately -12 dB here. As before, the out-of-band crosstalk from the downlink path does not require any special attention, and will merge with typical crosstalk caused by the filtering characteristics of the demultiplexer. However, the in-band crosstalk may need to be addressed and managed when deploying such systems in practical networks. Fig. 5.34(a) also 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) C U3 -60 -40 -70 -50 -30 S U3 (a) 1555.9 1556.3 1556.7 Wavelength (nm) -60 -40 -70 -50 -20 -30 C D3 S D3S D2 S D1 C D1 C D2 (b) 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) C U3 -60 -40 -70 -50 -30 S U3 1555.9 1556.3 1556.7 Wavelength (nm) Optical Power (dBm) C U3 -60 -40 -70 -50 -30 S U3 (a) 1555.9 1556.3 1556.7 Wavelength (nm) -60 -40 -70 -50 -20 -30 C D3 S D3S D2 S D1 C D1 C D2 1555.9 1556.3 1556.7 Wavelength (nm) -60 -40 -70 -50 -20 -30 C D3 S D3S D2 S D1 C D1 C D2 (b) Fig. 5.34: Optical spectra measured at the CO for: (a): multiplexed uplink signal, (S U3 , C U3 ) after transmission over 10 KM SMF, and (b): unwanted crosstalk from the downlink path due to reflections. 227 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks confirms the CSR of the multiplexed uplink (S U3 , C U3 ) as 5 dB, although before the proposed DEMUX/MUX scheme it was shown as 14 dB (shown in Fig. 5.33c). As stated before, this reduction in CSR also improves the sensitivity of the link significantly. To quantify the signal degradation due to transmission over 10 km of SMF, uplink (S U3 , C U3 ) was detected and BER curves measured, both at the beginning (back-to- back) and at the end of the fibre link using the same PD and data recovery circuit described earlier. The recovered BER curves are presented in Fig. 5.35 and it can be seen that the uplink (S U3 , C U3 ) experiences a negligible 0.4 dB power penalty at a BER of 10 -9 , which can be attributed to experimental errors. The presented recovered optical spectra and the BER curves clearly demonstrate the functionality of the proposed DEMUX/MUX scheme in multiplexing uplink signals that are generated by employing a wavelength reuse technique which simplifies the BS by -6 -7 -8 -9 -18.5 -18 -17.5 -17 -16.5 -16 -15.5 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) with 10 KM SMF with 0.0 KM SMF Carrier Reused Uplink -6 -7 -8 -9 -18.5 -18 -17.5 -17 -16.5 -16 -15.5 l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) with 10 KM SMF with 0.0 KM SMF Carrier Reused Uplink Fig. 5.35: Measured BER curves as a function of received optical power for the multiplexed uplink (S U3 , C U3 ) after transported over 10 km SMF with the back-to- b ack (0.0 km SMF) curve as reference, where uplink signal was generated by reusing the downlink optical carrier. 228 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks eliminating the light source from the uplink path while realising compact, low-cost and light-weight BSs. 5.8 Effects of Optical Crosstalk on the Proposed System Technologies Section 5.4.1 has described the characteristics of the 8 × 8 AWG used in demonstrating the system technologies throughout the Sections 5.4 to 5.7. The characterised results indicate that the proposed schemes incorporating such AWG are contaminated by the adjacent and nonadjacent channels crosstalk of -16 dB to -25 dB and -29 dB to -46 dB respectively. The demultiplexed results in Sections 5.6.1 and 5.7.3 also confirm presence of crosstalk from -18 to -30 dB in the demultiplexed signals. Moreover, the multiplexed results of the simultaneous MUX/DEMUX scheme described in Section 5.7.4 demonstrate that uplink signals generated by using 37.5 GHz 155Mb/s BPSK A1 A2 A3 A4 B5 B7 B8 A8 OSSB 3 +C 3 PD and Data Recovery BPF OSSB 1 +C 1 OSSB 2 +C 2 S 3 , C 3 37.5 GHz 155Mb/s BPSK A1 A2 A3 A4 B5 B7 B8 A8 OSSB 3 +C 3 PD and Data Recovery BPF OSSB 1 +C 1 OSSB 2 +C 2 S 3 , C 3 Fig. 5.36: Experimental setup used to characterise optical crosstalk effects on the performance of the optical mm-wave signals, using the proposed schemes incorporating AWG. 229 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks optical carriers spaced at 500 GHz from the downlink signals are contaminated by as much as -17 dB optical crosstalk, which increases to -12 dB with the uplink signals generated by reusing the downlink optical carriers. Therefore, there is the potential to incur performance degradation of the proposed system technologies through optical crosstalk. Fig. 5.36 shows the simplified experimental setup developed to characterise the effects of optical crosstalk while transmitting the optical mm-wave signals through the proposed system technologies incorporating AWG. Three OSSB+C modulated optical mm-wave signals, each carrying 37.5 GHz-band 155 Mb/s BPSK data, were generated by using three optical carriers at the wavelengths C 1 (1556.0 nm), C 2 (1556.2 nm) and C 3 (1556.4 nm). The modulated signals were then applied to the AWG as shown in Fig. 5.36, where signals (S 1 , C 1 ) and (S 2 , C 2 ) follow separate VOAs before being applied. The output at port B 5 was recovered in such way that the signal (S 3 , C 3 ) is contaminated by the adjacent and the nonadjacent -30 -70 -60 -50 1556 1556.2 1556.4 Wavelength (nm) Optical Power (dBm) -40 1556.6 S 3 C 3 Adjacent Crosstalk Nonadjacent Crosstalk -30 -70 -60 -50 1556 1556.2 1556.4 Wavelength (nm) Optical Power (dBm) -40 1556.6 S 3 C 3 Adjacent Crosstalk Nonadjacent Crosstalk Fig. 5.37: Measured optical spectrum of the recovered signal (S 3 , C 3 ) with adjacent and nonadjacent channel crosstalk from neighboring signals (S 2 , C 2 ) and (S 1 , C 1 ) respectively. 230 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks channel crosstalk from the signals (S 2 , C 2 ) and (S 1 , C 1 ) respectively. The VOAs are inserted to vary the optical powers of (S 1 , C 1 ) and (S 2 , C 2 ) that result in variable optical crosstalk with the recovered signal (S 3 , C 3 ). Also carrier C 3 was provisioned two loop-backs before combining with S 3 , as the optical mm-wave signals are expected to undergo two loop-backs while multiplexing (as described in Section 5.3). The spectrum of the recovered signal (S 3 , C 3 ) is shown in Fig. 5.37, where the respective crosstalk components are mentioned in the insets. In order to observe the effects of such crosstalk, the adjacent channel crosstalk is varied with a 3–dB interval from -9 dB to -24 dB and the respective BER curves were measured as shown in Fig. 5.38. From the Fig. 5.38, it can also be seen that another two BER curves were plotted with (i) adjacent channel crosstalk removed, but nonadjacent channel crosstalk present, and (ii) both adjacent and nonadjacent channel crosstalk removed. The BER curves indicate that the demonstrated schemes will endure noticeable -6 -7 -8 -9 -19 -18.5 -18 -17.5 -17 -16.5 -16 -15.5 Adj. Xtalk: 9 dB Adj. Xtalk: 12 dB Adj. Xtalk: 15 dB Adj. Xtalk: 18 dB Adj. Xtalk: 21 dB Adj. Xtalk: 24 dB NO Adj. Xtalk NO Xtalk l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) -6 -7 -8 -9 -19 -18.5 -18 -17.5 -17 -16.5 -16 -15.5 Adj. Xtalk: 9 dB Adj. Xtalk: 12 dB Adj. Xtalk: 15 dB Adj. Xtalk: 18 dB Adj. Xtalk: 21 dB Adj. Xtalk: 24 dB NO Adj. Xtalk NO Xtalk l o g l o g 1 0 1 0 ( ( B E R ) ) Received Optical Power (dBm) Fig. 5.38: Measured BER curves as a function of received optical power for various crosstalk levels contaminating the recovered signal (S 3 , C 3 ). 231 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks crosstalk induced penalties with the presence of crosstalk levels more than -21 dB, which diminishes to zero when it is less than -21 dB. In order to quantify the gradual changes in performance due to crosstalk, power penalties incurred by the signal (S 3 , C 3 ) (at a BER of 10 -9 ) at various crosstalk levels are compared and the results are plotted in Fig. 5.39. This graph shows that a power penalty of 0.5 dB is observed for an optical crosstalk level of -16 dB, which however increases to 1 dB when the crosstalk level increases to -12 dB. 0.4 0.8 1.2 1.6 -25 -20 -15 -10 Optical Crosstalk (dB) Power Penalty (dB) 0 0.4 0.8 1.2 1.6 -25 -20 -15 -10 Optical Crosstalk (dB) Power Penalty (dB) 0 Fig. 5.39: Measured crosstalk induced power penalties, with the gradual increase of crosstalk levels in the transmitted signals by the demonstrated system technologies for WI-DWDM mm- wave fibre-radio systems. 5.9 Conclusion This chapter presented novel system technologies incorporating arrayed waveguide grating filters for future wavelength-interleaved DWDM mm-wave fibre- 232 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks radio networks. WI-MUXs with the capacity to multiplex optical mm-wave signals to the wavelength interleaving schemes for these networks are proposed, which also improves the link performance by enabling reductions in CSRs of the multiplexed signals. WI-DEMUX, capable of demultiplexing wavelength interleaved signals in these networks, is also proposed. Moreover, a single MUX-DEMUX scheme for simultaneous multiplexing and demultiplexing is proposed that offers a route towards a simple network architecture by realising simplified and cost-effective CO and RNs. The proposed schemes are based on standard AWG technology, therefore, are suitable for integration with the other conventional technologies found in the optical access or metro domain. These schemes incorporating a commercially available 8 × 8 AWG are demonstrated experimentally with three optical mm-wave signals spaced at 25 GHz, each of them carrying 37.5 GHz RF signal with 155 Mb/s BPSK data. The error-free (at a BER of 10 -9 ) recovery of data confirms the functionality of the proposed schemes without significant power penalty observed while transported the signals over 10 km of SMF. The AWG characteristics affecting the performance of the demonstrated schemes have been investigated experimentally. 233 Chapter 5: Enabling Wavelength Interleaving in Millimetre-Wave Fibre-Radio Networks 5.10 References [1] H. Schmuck, R. Heidemann, and R. Hofstetter, “Distribution of 60 GHz signals to more than 1000 base stations,” Electron. Lett., vol. 30, pp. 59 – 60, Jan. 1994. [2] R. Heidemann and G. Veith, “MM-wave photonics technologies for Gbit/s-wireless-local- loop”, Proc. OECC’98, Chiba, Japan, pp. 310 – 311, 1998. [3] J. O’Reilly and P. Lane, “Remote delivery of video services using mm-waves and optics,” J. Lightwave Technol., vol. 12, no. 2, pp. 369-375, 1994. [4] M. Shibutani, T. Kanai, W. Domom, W. Emura, and J. 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