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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 3, FEBRUARY 1, 2006
A Novel Hybrid WDM/SCM-PON Sharing Wavelength for Up- and Down-Link Using Reflective Semiconductor Optical Amplifier Jeung-Mo Kang, Student Member, IEEE, and Sang-Kook Han, Member, IEEE
Abstract—A hybrid wavelength-division multiplexed/subcarrier multiplexed passive optical network (WDM/SCM-PON) which shares the same wavelength both up-link and down-link is presented by using a reflective semiconductor optical amplifier (RSOA) as a modulator. We investigated down-link of 622 Mb/s using distributed feedback laser diode with direct modulation and 100-Mb/s up-link using an RSOA as a modulator with 900-MHz SCM signal. In our scheme, a novel efficient and cost-effective WDM/SCM-PON using the same wavelength for down/up-link at each optical network unit is proposed and experimentally demonstrated. Index Terms—Passive optical network (PON), reflective semiconductor optical amplifier (RSOA), subcarrier multiplexing (SCM), wavelength-division multiplexing (WDM).
I. INTRODUCTION
W
AVELENGTH-DIVISION multiplexed-passive optical network (WDM-PON) is a promising solution for the future high-speed access networks such as fiber-to-the-home or fiber-to-the-office by reasons of large capacity, network security, protocol transparency, and upgradability [1]. However, because of relatively expensive WDM components, the WDM-PON has been considered as a next-generation solution, especially, high-cost optical source in the optical network units (ONUs). Recently, to overcome this problem, there have been several proposals which can be classified into two groups. One approach is remodulation method of down-link signal at each ONU. A saturated semiconductor optical amplifier (SOA) modulator [2] or injection-locked Fabry–Pérot laser diode (F-P LD) was used to remodulate the high-speed modulated down-link signal [3], or using optical frequency-shift keying modulation format in down-link and using ON–OFF keying modulation format in up-link [4]. However, in remodulation methods, additional high-cost devices are required in each ONU such as external modulator and unnecessary high-speed modulation in down-link signal with sacrifice of extinction ratio (ER) to satisfy remodulation conditions. The other approach tends to control the wavelength source in the central office (CO) rather than ONU. For example, they used additional devices in CO and/or each ONU for the up-link wavelength source such as the efforts of employing the spectrum-sliced light–emitting diode
Manuscript received August 23, 2005; revised November 16, 2005. This work was supported by the MIC, Korea, under the ITFSIP supervised by the IITA. The authors are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul, Korea (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2005.863632
Fig. 1.
Schematic diagram of the proposed hybrid WDM/SCM-PON.
[5], spectrum-sliced amplified spontaneous emission (ASE) of erbium-doped fiber amplifier (EDFA) [6], the ASE injection-locked F-P LD [7], and the wavelength-seeded reflective SOAs (RSOAs) [8]. However, in using the spectrum-slicing methods, the maximum data rate per channel and transmission distance were limited due to the beat noise and fiber dispersion. In this letter, we propose a novel hybrid WDM/subcarrier multiplexed (SCM)-PON taking the advantages of WDM-PON as mentioned above and the SCM technique. In the proposed scheme, the down-link signal was modulated directly using distributed-feedback laser diode (DFB-LD) and the up-link signal was remodulated using an RSOA with SCM technique. No additional high-cost devices are required such as external modulator and EDFA. II. PRINCIPLES OF OPERATION WDM-PON employing RSOA as a modulator offers some advantages. First, RSOA can be used as a modulator which accomplishes both modulation and amplification functions. Second, this amplification function gives additional gain for incident optical power to overcome device and transmission losses. So, it is possible to accommodate amplifier-less PON. Lastly, RSOA operating in gain saturation region can reduce the intensity noise of the optical signal [9]. We utilized these advantages of RSOA in the proposed hybrid WDM/SCM-PON optical link. Fig. 1 shows the proposed novel hybrid WDM/SCM-PON scheme. A CO consists of DFB-LD array which offers the wavelength from to for down-link, receiver array to detect up-link signal from each ONU, and arrayed waveguide grating (AWG) which is operated as wavelength multiplexer/demultiplexer (MUX/DEMUX). The circulators are located between AWG and DFB-LD/receiver array to separate down-link and up-link signals. In remote node (RN), another AWG is used as DEMUX for the down-link signal and as MUX for the up-link signal. Each ONU is composed of RSOA as a modulator and a receiver. As mentioned above, normally, two wavelength
1041-1135/$20.00 © 2006 IEEE
KANG AND HAN: NOVEL HYBRID WDM/SCM-PON SHARING WAVELENGTH FOR UP- AND DOWN-LINK
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Fig. 2. Experimental setup of proposed hybrid WDM/SCM PON link.
sources are required in each ONU for both up- and down-link transmission. But, here, we have used a simple concept of SCM remodulation technique of down-link signal for up-link transmission. The down-link signal from CO to ONU was directly modulated with a low-pass filter (LPF), then the up-link modulation was achieved using SCM technique with the radio-frequency (RF) signal which apart from the baseband signal spectrum at each ONU. Due to the mixture of WDM and SCM techniques, a simple ONU which shares the same wavelength both up- and down-link transmission is possible.
Fig. 3. RF spectrum of the proposed hybrid WDM/SCM-PON: (a) RF spectrum of 622-Mb/s down-link digital base-band signal with/without LPF at Point a; (b) 100-Mb/s up-link SCM modulated signal with/without down-link signal at Point b; and (c) recovered 100-Mb/s up-link signal at Point c. (d) HPF effects of a gain-saturated RSOA.
III. EXPERIMENTS AND RESULTS Fig. 2 shows the experimental setup of the proposed hybrid WDM/SCM-PON optical link. A directly modulated DFB-LD (1551.08 nm) with an injection current of 30.5 mA was used for downstream transmission from CO to ONU. We used an LPF (cutoff frequency: 550 MHz) between pulse pattern generator and DFB-LD to cutoff sidelobes of 622-Mb/s (pseudorandom binary sequence (PRBS): ) digital signal. The optical circulator was inserted between the DFB-LD and the AWG to separate down-link and up-link signals. The AWG (temperature sensitivity: 0.002 nm C) performed wavelength MUX/DEMUX function as an RN. The down-link signal routed to a proper ONU by AWG and then the signal is divided by a 3-dB coupler at ONU. One divided signal entered into the photodetector (PD) for down-link detection and the other divided signal entered into RSOA and remodulated with 100-Mb/s (PRBS: ) up-link signal mixed with 900-MHz subcarrier signal. The injection current of RSOA was 32 mA and the temperature was controlled as 25 C. The mixed signal was filtered by a bandpass filter (BPF: MHz GHz) before it entered into the RSOA to prevent the overlapping with the baseband down-link signal. The up-link signal modulated at ONU passed through the AWG (RN) again then routed to PD by optical circulator in CO. After PD, the detected SCM signal entered the BPF ( MHz GHz) then down-converted with 900-MHz local oscillator (CO) signal. The received SCM signal in PD passed through an LPF (cutoff frequency: 100 MHz) and then entered into the low noise amplifier to amplify to a proper power level for detection. The output power of DFB-LD in CO was about 0 dBm and ER was about 8 dB. The input power of RSOA in ONU was set to about 15 dBm by considering the losses of transmission and devices. We have measured the RF spectrum of the down-link 622-Mb/s baseband signal and the up-link SCM signal at three points ( , , and ) as shown in Fig. 2. Fig. 3(a) shows the
Fig. 4. Basic characteristics of RSOA: (a) RSOA performances of current versus gain and (b) RSOA performances of gain saturation.
RF spectrum of the baseband signal with and without LPF at Point for down-link transmission. Using an LPF, the sidelobes of the baseband signal removed effectively while the main lobe was maintained. Fig. 3(b) shows the RF spectrum of the up-link signal after PD at Point with and without the down-link signal. Lastly, Fig. 3(c) shows the recovered 100-Mb/s up-link signal at Point . In case of using an RSOA as a modulator, we can observe the well known high-pass filter (HPF) characteristics for amplification function as shown in Fig. 3(d). So, the unnecessary residual down-link signal at each ONU was suppressed by the HPF effect of RSOA as shown in Fig. 3(b) and (d). Moreover, this residual baseband down-link was removed in case of using a BPF after the PD and then the residual down-link signal does not seriously affect the SCM up-link signal detection. In the proposed scheme, the performances of RSOA become critical since an RSOA offers modulation function in ONU as well as amplification function. We have analyzed the characteristics of RSOA and showed in Fig. 4. Fig. 4(a) shows the gain profile of RSOA for various injection currents from 8 to 60 mA with an input power of 20 dBm. The RSOA gives about 23-dB gain for the wavelength range from 1540 to 1570 nm with an injection current of 30 mA. This gain is high enough to overcome the device insertion and transmission losses in the proposed scheme. As mentioned earlier, an operation of RSOA in
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 3, FEBRUARY 1, 2006
( dB ) of the RSOA which was used in the experiments was about 1.1 GHz with input power of 15 dBm. In the case when higher frequency response RSOA are available, we can expect the increased transmission capacity of both up- and down-link transmissions. IV. CONCLUSION
Fig. 5. BER and eye diagram of 622-Mb/s down-link and 100-Mb/s up-link at 900-MHz subcarrier.
the gain saturation region is needed for intensity noise reduction. Fig. 4(b) shows the gain saturation characteristics of RSOA with the wavelength of 1550 nm for various injection currents. The gain of RSOA begins to saturate with an input power of about 30 dBm. When the input power of 15 dBm entered into RSOA in ONU, the gain is already saturated. So, we have both the intensity noise reduction and enough gain using RSOA as shown in Fig. 4(a) and (b). We have investigated the performance of the proposed WDM/SCM-PON link by means of bit-error rates (BERs) and eye-diagram measurements. Fig. 5 shows the measured BERs of up- and down-link transmission for various cases. In the first case, we measured the influence of LPF on the baseband signal which was used at CO in case of down-link transmission (622 Mb/s). As shown in Fig. 5, the usage of LPF in CO caused about 2-dB power penalty but it was not serious since 23 dBm of the received power would be enough to detect the down-link signal at ONU even if 10-km transmission was considered. For brevity of results, we did not annex the performances of the down-link signal which has other wavelength. And it did not restrict the applications of the proposed system. Then, we have measured the up-link signal (100 Mb/s) by changing the wavelength by 100-GHz spacing. Four wavelengths (1540.56, 1550.12, 1560.61, and 1570.42 nm) were used in experiments. Comparing the BER of back-to-back (BTB) to 10-km transmission, the maximum power penalty was about 2.5 dB in case of 1570.42 nm. In the case of 20-km transmission, the power penalty was increased about 4 dB. The inset in Fig. 5 shows the eye diagram of the 1550.12-nm up-link signal of the BTB case. Even in the worst case of 1570.42 nm, the required minimum received power at CO was about 14 dBm. And this value means that the proposed scheme would not need an additional high-cost amplification devices such as EDFA or SOA because the RSOA which used as a modulator in the proposed scheme gives enough gain to reach this power margin in ONU even if including the AWG loss (about 4 dB). Since we have used different modulation formats and frequency range in up-link and down-link, the possible Rayleigh backscattering effect was not considered in our work. The main critical issue of RSOA is a frequency response limitation due to resistance–capacitance (RC)-time constant [10]. Modulation frequency response
We have proposed and demonstrated a novel hybrid WDM/SCM-PON optical link which shares the same wavelength both 622-Mb/s down-link and SCM-based 100-Mb/s up-link. The required received power ( BER) for 622-Mb/s down-link signal was about 23 dBm and for 100-Mb/s up-link signal was about 14 dBm at the worst case after 10-km transmission. The power penalty for low-pass filtering of the baseband signal was about 2 dB. However, it was not serious since we had enough power budgets due to RSOA. Using RSOA as a modulator, we can obtain an additional gain as well as the reduction of unnecessary residual baseband signal which acts as a noise in up-link transmission during SCM modulation in ONU by taking high-pass filtering characteristics of RSOA. ACKNOWLEDGMENT The authors would like to thank Samsung Electronics for providing the RSOA module used in this work. REFERENCES [1] R. D. Feldman, E. E. Harstead, S. Jiang, T. H. Wood, and M. Zirngibl, “An evaluation of architectures incorporating wavelength division multiplexing for broad-band fiber access,” J. Lightw. Technol., vol. 16, no. 9, pp. 1546–1559, Sep. 1998. [2] H. Takesue and T. Sugie, “Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized light sources,” J. Lightw. Technol., vol. 21, no. 11, pp. 2546–2556, Nov. 2003. [3] L. Y. Chan, C. K. Chan, D. T. K. Tong, F. Tong, and L. K. Chen, “Upstream traffic transmitter using injection-locked Fabry–Pérot laser diode as modulator for WDM access networks,” Electron. Lett., vol. 38, no. 1, pp. 43–45, 2002. [4] N. Deng, C. K. Chan, L. K. Chen, and F. Tong, “Data remodulation on downstream OFSK signal for upstream transmission in WDM passive optical network,” Electron. Lett., vol. 39, no. 24, pp. 1741–1743, 2003. [5] M. H. Reeve, A. R. Hunwicks, S. G. Methley, L. Bickers, and S. Hornung, “LED spectral slicing for single-mode local loop applications,” Electron. Lett., vol. 24, no. 7, pp. 389–390, 1988. [6] D. K. Jung, S. K. Shin, C. H. Lee, and Y. C. Chung, “Wavelength-division-multiplexed passive optical network based on spectrum-slicing techniques,” IEEE Photon. Technol. Lett., vol. 10, no. 9, pp. 1334–1336, Sep. 1998. [7] H. D. Kim, S. G. Kang, and C. H. Lee, “A low-cost WDM source with an ASE injected Fabry–Pérot semiconductor laser,” IEEE Photon. Technol. Lett., vol. 12, no. 8, pp. 1067–1069, Aug. 2000. [8] P. Healey, P. Townsend, C. Ford, L. Johnston, P. Townley, I. Lealman, L. Rivers, S. Perrin, and R. Moore, “Spectral slicing WDM-PON using wavelength-seeded reflective SOAs,” Electron. Lett., vol. 37, no. 19, pp. 1181–1182, 2001. [9] K. Sato and H. Toba, “Reduction of mode partition noise by using semiconductor optical amplifiers,” IEEE J. Sel. Topics Quantum Electron., vol. 7, no. 2, pp. 328–333, Mar./Apr. 2001. [10] H. C. Shin, J. S. Lee, I. K. Yun, S. W. Kim, H. I. Kim, H. S. Shin, S. T. Hwang, Y. J. Oh, and C. S. Shim, “Reflective SOAs optimized for 1.25 Gbit/s WDM-PONs,” in Proc. 17th Annu. Meeting IEEE LEOS (LEOS 2004), Puerto Rico, Nov. 2004, pp. 308–309.
