13 Gbit/s WDM-OFDM PON using
RSOA-based colourless ONU with seeding
light source in local exchange
C.W. Chow, C.H. Yeh, Y.F. Wu, H.Y. Chen, Y.H. Lin,
J.Y. Sung, Y. Liu and C.-L. Pan
Optical-carrier-distribution using
reflective-semiconductor-optical-amplifier (RSOA)-based colorless optical-networking-unit (ONU) is a promising candidate for the wavelength-division-multiplexed passive-optical-network (WDM-PON). However, commercial available RSOA
has a typical modulation-bandwidth of1 GHz, which is not enough
for the future bandwidth demand. Here we demonstrate a WDM-PON using 1 GHz bandwidth RSOA-based ONU, which can be operated up to 13 Gbit/s. Experimental results show that 20 km singlemode-fibre transmission of the 13 Gbit/s orthogonal-frequency-division-multiplexed (OFDM) signal can be achieved with bit-error rate lower than the forward error correction threshold.
Introduction: The wavelength division multiplexed passive optical network (WDM-PON) is a promising candidate for the future fibre-to-the-home (FTTH). Optical carrier distribution with a colourless optical networking unit (ONU) is one of the cost-effective implementations
of the WDM-PON[1]. The reflective semiconductor optical amplifier
(RSOA)-based ONU is important for the WDM-PON owing to its
compact size and low power consumption [2 – 5]. Recently, using
optical orthogonal frequency division multiplexing (OFDM) modulation
in PONs has attracted much attention[6]. As the OFDM quadrature
amplitude modulation (QAM) signal is very spectrally efficient, the data rate of the PON can be increased while low bandwidth optical
com-ponents optimised for the present PON can still be used[7]. By using
OFDM access (OFDMA), one more degree of freedom can be obtained for bandwidth sharing and dynamic bandwidth allocation apart from time division multiple access (TDMA) and WDM access (WDMA).
Commercially available RSOA has a typical modulation bandwidth of 1 GHz. It is desirable to operate the RSOA-based ONU .10 Gbit/s for the future WDM-PON. Recently, there have been several demon-strations to increase the RSOA operation speed to 10 Gbit/s by using
adaptively modulated optical OFDM (AMOOFDM)[2], offset optical
filtering and electronic equalisation[3], special design structure[4], or
adding a delay interferometer [5]. In this Letter, we demonstrate a
WDM-PON using a commercially available 1.2 GHz bandwidth RSOA-based ONU, which can be operated up to 13 Gbit/s. In this dem-onstration, the optical seeding light source is located in the local exchange (LE) for providing relatively higher injection power to enhance the relaxation oscillation speed of the RSOA. The proposed scheme does not require complicated OFDM subcarrier-level power management and analysis when compared with the AMOOFDM. It also does not require the optical filtering or delay interferometer. 20 km singlemode-fibre (SMF) transmission of the 13 Gbit/s OFDM signal can be achieved with a bit-error rate (BER) lower than the forward error correction (FEC) threshold.
Experiment: Fig. 1shows the experimental setup. A continuous-wave
(CW) optical signal produced by a tunable laser at wavelength of 1550 nm with different injection powers was launched into the RSOA via an optical circulator. The RSOA (standard product by CIP) has a 3 dB bandwidth of 1.2 GHz when DC-biased at 80 mA, and has a small signal gain of 20 dB. The noise figure and the polarisation depen-dent gain are 7 and 1 dB, respectively. The RSOA was directly modu-lated by a baseband electrical OFDM upstream signal generated by an arbitrary waveform generator (AWG). The data was packed into 93 OFDM subcarriers; each was in a 128-QAM. The total data rate of 15.3 Gbit/s can be provided by these 93 OFDM subcarriers. By using inverse fast Fourier transform (IFFT), this signal was converted to a real-valued time-domain waveform. Then the digital-to-analogue verter (DAC) (AWG having the sampling rate of 4 Gsample/s) con-verted the digital data to an analogue signal for the RSOA. It has the resolution of 6-bit. The cyclic prefix (CP) was 1/64. The electrical signal was then applied to the RSOA via a bias-tee.
Then, the optical upstream signal was transmitted in 20 km SMF without dispersion compensation, and was directly received by a 2.5 GHz pin photodiode (PD) at the central office (CO) without using
an optical preamplifier. As shown in Fig. 1, the received OFDM
signal was captured by a real-time oscilloscope (RTO) with 12.5 GHz bandwidth. The RTO performed the analogue-to-digital conversion (ADC). For signal analysis, offline digital signal processing (DSP) was used. It consisted of synchronisation, FFT and QAM symbol decod-ing. Finally the BER was calculated using the measured signal-to-noise ratio (SNR). RSOA PD CW DAC IFF T P/ S S/ P Q A M m apper data ONU LE CO 20km SMF ADC s y nchroni se r FFT equal iser QA M-demod u lato r data
Fig. 1 Experimental setup of WDM-PON with seeding light source at local exchange (LE)
S/P: serial-to-parallel; P/S: parallel-to-serial; IFFT: inverse fast Fourier trans-form; FFT: fast Fourier transtrans-form; DAC: digital-to-analogue converter; ADC: analogue-to-digital converter
Results and discussion: Fig. 2shows the measured SNR of each OFDM
subcarrier from frequencies of 0.1 to 2.3 GHz under different injection powers. The RSOA was DC-biased at 80 mA in all cases. We can observe the frequency ripple problem of the RSOA under low optical injection powers. This produces relatively low SNR at some frequencies, such as at frequencies of 0.19 and 0.94 GHz. The frequency ripple can be improved under high optical injection powers, as can be seen in
Fig. 2. Insets show the corresponding measured constellation diagrams
of the 128-QAM OFDM signal including all the 93 OFDM subcarriers at different injection powers. We can clearly observe that the overall per-formance of the upstream signal can be improved by increasing the optical injection power.
–14dBm –9dBm –5dBm 3dBm 0 5 10 15 20 25 30 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 frequency, GHz SN R , d B –14 dBm –13 dBm –12 dBm –11 dBm –10 dBm –9 dBm –8 dBm –7 dBm –6 dBm –5 dBm –4 dBm –3 dBm –2 dBm –1 dBm –0 dBm 1 dBm 2 dBm 3 dBm
Fig. 2 Measured SNR of each OFDM subcarrier under different CW injec-tion powers
Insets: constellation diagrams of 128-QAM OFDM signal including all 93 OFDM subcarriers at different injection powers
We then analysed and compared the BER performance based on the
measured SNR using the equations as described in[7]. We can observe
fromFig. 3athat the BER of the upstream signal cannot achieve the
forward error correction (FEC) threshold (BER ¼ 3.8× 1023) [8]if
all the 93 OFDM subcarriers are included. Hence, at the CO, some of the low SNR OFDM subcarriers are neglected to obtain an improved upstream signal performance. By including 80 OFDM subcarriers (sub-carrier 1 to sub(sub-carrier 79), a BER lower than the FEC threshold can be achieved, providing a total upstream data rate of 13.1 Gbit/s when the injection power to the RSOA is larger than 21 dBm. Neglecting the low SNR OFDM subcarriers at the CO is much simpler than the AMOOFDM case since the ONU using the AMOOFDM requires feed-back information for the CO to control the power levels of different OFDM subcarriers. Higher performance signals can be achieved by neglecting the relatively low SNR OFDM subcarriers; however, the data rate is sacrificed. For example, if only 12 OFDM subcarriers are
included (total data rate is 2 Gbit/s), BER , 1× 1024can be achieved
with improved dynamic range of the optical injection power (injection power .24 dBm). As mentioned before, OFDMA can provide one more degree of freedom for bandwidth sharing and dynamic bandwidth
allocation. Here, we also analysed the bit rate per user for the proposed
OFDM-WDM PON. We can observe fromFig. 3bthat, when 80 OFDM
subcarriers are used, the bit rate per user can be 410 and 205 Mbit/s when the OFDM split-ratios are 32 and 64, respectively.
a FEC threshold Log (3.8 x 10-3) = -2.42 –5.0 –4.5 –4.0 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0 –14 –13 –12 –11 –10 –9 –8–7 –6 –5–4 –3 –2–1 0 1 2 3 4 5 injection power, dBm log (BER) 15.3 Gbit/s (93 subcarriers) 14.4 Gbit/s (88 subcarriers) 13.1 Gbit/s (80 subcarriers) 9.8 Gbit/s (60 subcarriers) 6.9 Gbit/s (42 subcarriers) 2.6 Gbit/s (16 subcarriers) 2.0 Gbit/s (12 subcarriers) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 OFDMA split-ratio
bit rate per user, Mbit/s
93 subcarriers 88 subcarriers 80 subcarriers 60 subcarriers 42 subcarriers 16 subcarriers 12 subcarriers b FEC threshold Log (3.8 x 10-3) = -2.42
Fig. 3 BER measurements of upstream signal with different effective data rates and under different CW injection power; and bit rate per user against OFDMA split-ratios when different numbers of OFDM subcarriers are used
a BER measurements of upstream signal b Bit rate per user against OFDMA split-ratios
Conclusions: The commercially available RSOA has a typical
modu-lation bandwidth of1 GHz. Recently, there have been several
demon-strations to increase the RSOA operation speed to 10 Gbit/s, but they require complicated AMOOFDM, special design structure or addition of optical components. We have demonstrated a WDM-PON using a commercially available 1.2 GHz bandwidth RSOA-based ONU, which can be operated up to 13 Gbit/s. In this demonstration, the optical seeding light source is located in the LE for providing relatively higher injection power for the RSOA to enhance the relaxation oscil-lation speed of the RSOA. 20 km SMF transmission of the 13 Gbit/s OFDM signal can be achieved with BER lower than the FEC threshold. Acknowledgment: This work was supported in part by the National Science Council, Taiwan under contract NSC-100-2221-E-009-088-MY3, NSC-98-2221-E-009-017-MY3.
#The Institution of Engineering and Technology 2011
30 July 2011
doi: 10.1049/el.2011.2333
One or more of the Figures in this Letter are available in colour online. C.W. Chow, Y.F. Wu, Y.H. Lin and J.Y. Sung (Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Rm 216A, Tin Ka Ping Building, Hsinchu 30010, Taiwan) E-mail: [email protected]
C.H. Yeh and H.Y. Chen (Information and Communications Research
Laboratories, Industrial Technology Research Institute (ITRI),
Hsinchu 31040, Taiwan)
Y. Liu (Hong Kong Productivity Council (HKPC), Hong Kong) C.-L. Pan (Department of Physics and Institute of Photonics Technologies, National Tsing-Hua University, Hsinchu, Taiwan) References
1 Chow, C.W., Yeh, C.H., Wang, C.H., Shih, F.Y., and Chi, S.: ‘Signal
remodulation high split-ratio hybrid WDM-TDM PONs using RSOA-based ONUs’, Electron. Lett., 2009, 45, pp. 903 – 905
2 Duong, T., Genay, N., Chanclou, P., Charbonnier, B., Pizzinat, A., and
Brenot, R.: ‘Experimental demonstration of 10 Gbit/s upstream transmission by remote modulation of 1 GHz RSOA using adaptively modulated optical OFDM for WDM-PON single fiber architecture’. Proc. ECOC, Brussels, Belgium, 2008 (paper Th.3.F.1)
3 Omella, M., Papagiannakis, I., Schrenk, B., Klonidis, D., Birbas, A.N.,
Kikidis, J., Prat, J., and Tomkos, I.: ‘Full-duplex bidirectional transmission at 10 Gbps in WDM PONs with RSOA-based ONU using offset optical filtering and electronic equalization’. Proc. OFC, San Diego, CA, USA, 2009 (paper OThA7)
4 Schrenk, B., de Valicourt, G., Omella, M., Lazaro, J.A., Brenot, R., and
Prat, J.: ‘Direct 10-Gb/s modulation of a single-section RSOA in PONs with high optical budget’, IEEE Photonics Technol. Lett., 2010, 22, pp. 392 – 394
5 Kim, H.: ‘10-Gb/s operation of RSOA using a delay interferometer’,
IEEE Photonics Technol. Lett., 2010, 22, pp. 1379– 1381
6 Chow, C.W., Yeh, C.H., Wang, C.H., Wu, C.L., Chi, S., and Lin, C.:
‘Studies of OFDM signal for broadband optical access networks’, IEEE J. Sel. Areas Commun., 2010, 28, pp. 800 – 807
7 Lu, J., Letaief, K.B., Chuang, J.C.-I., and Liou, M.L.: ‘M-PSK and
M-QAM BER computation using signal-space concepts’, IEEE Trans. Commun., 1999, 47, pp. 181 – 184
8 Peng, W.W., Takahashi, H., Morita, I., and Tanaka, H.: ‘Transmission of
a 213.7-Gb/s single-polarization direct-detection optical OFDM
superchannel over 720-km standard single mode fiber with EDFA-only amplification’. Proc. ECOC, Torino, Italy, 2010 (paper PD2.5)
