A time compression multiplexing (TCM)-based wavelength division multiplexing passive optical network (WDM-PON) using a reflective semiconductor optical amplifier (RSOA) is proposed, and its feasibility is experimentally demonstrated. In the proposed system, the RSOA pre-amplifies a 10 Gb/s downstream signal and modulates the RSOA output, wavelength-locked to the downstream signal, with a 1.25 Gb/s upstream signal simultaneously. The sensitivity of the downstream signal is improved by about 3 dB through the RSOA. The downstream and upstream signals have power penalties of about 0.1 dB and 1.1 dB, respectively, at bit error rates (BERs) of 10−9 after 20 km transmission.
© 2015 Optical Society of America
With the increased use of broadband Internet, demand for more capacity per customer has become an important business issue for network operators. Current fiber-to-the-home strategies are primarily based on passive optical network (PON) technology but with a diverse range of technical options, such as time division multiplexing (TDM), wavelength division multiplexing (WDM), and optical code division multiplexing (OCDM) [1–3]. It is generally understood that in the future, WDM-PON will be the eventual choice of operators seeking to deliver unlimited bandwidth to customers. However, WDM-PON is an expensive solution. In this context, it is crucial to develop cheaper and more affordable WDM-PON technologies.
In normal WDM-PON, a distinct wavelength is assigned to each optical network unit (ONU). However, this approach places a heavy burden on network operators due to high maintenance and installation expenditures. Thus, many alternative methods have been proposed. As a promising candidate, the concept of colorless ONUs has been conceived, and numerous techniques using reflective semiconductor optical amplifiers (RSOAs) have been reported [4,5]. Re-modulation using RSOA-enabled wavelengths reuses ONUs, thus requiring only a single wavelength per customer [6,7]. Recently, to implement bidirectional WDM-PON for wavelength reuse, WDM-PON based on time compression multiplexing (TCM) using an RSOA was proposed . This approach presented the possibility of using the RSOA as a re-modulator as well as a detector simultaneously, but both have a bandwidth limitation of less than 2 Gb/s due to the capacitance effect, which is increased by the large cavity of the RSOA itself. Furthermore, the cost-effectiveness still needs to be improved considering the traffic patterns between optical line terminals (OLTs) and ONUs. In most cases, as seen in 10G-EPON by IEEE 802.3, demand for large downstream bandwidth is high, while demand for upstream bandwidth is quite low . The asymmetric demand is likely to become symmetric in the future, but it will be a very slow change. This calls for new technologies to strengthen the business case of WDM-PON.
In this paper, we propose and experimentally demonstrate a cost-effective TCM-based WDM-PON system to accommodate asymmetric traffic between ONUs and OLTs. An RSOA at an ONU is used as a re-modulator for a 1.25-Gb/s upstream transmission as in the other approaches. However, we focus on the enhancement of downstream sensitivity to expand the transmission distance at 10 Gb/s by using the RSOA as an optical pre-amplifier. In addition, we modulate a continuous wave (CW) by using TCM instead of re-modulation, and we use a conventional detector instead of RSOA. The relationship between the extinction ratio and side mode suppression ratio (SMSR) of the upstream signal and the optical seed power entering the RSOA together with the applying bias current is investigated to validate the feasibility of the proposed scheme.
2. Experimental setup
To demonstrate the feasibility of the proposed TCM-based WDM-PON, the experimental setup shown in Fig. 1 was constructed. In an OLT, a CW from a direct current (DC)-biased laser diode (LD) with a center wavelength of 1549.3 nm was intensity-modulated through a Mach-Zehnder modulator (MZM), which was fed, in burst mode at 10 Gb/s, by pseudo-random binary sequence (PRBS) signals with a pattern length of 27-1, packet length of 10,240 bits, and packet gap of 15,360 bits. The modulated burst-mode signals were amplified by an Erbium doped fiber amplifier (EDFA) and passed through a circulator. The optical signal was amplified by the EDFA to a level of 3 dBm, and the output power at port 2 of the circulator was 1.5 dBm. The optical power was decreased due to the insertion loss of the circulator.
The wavelength-locked optical signal generated by the RSOA was modulated with a 27-1 PRBS upstream signal at 1.25 Gb/s for bit error rate (BER) measurement. To demonstrate the proposed TCM transmission in the burst-mode transmission, the upstream signal was made up of packets of 1,280 bits, and the packet gap was 1,920 bits long. The RSOA was biased at a DC current of 40 mA to be utilized as a pre-amplifier and a re-modulator simultaneously. At the RSOA output, both the pre-amplified downstream signal and the re-modulated upstream signal existed in turns. Those composite signals were passed through the 1 x 2 coupler again and split into two parts. One was delivered to the ONU and received by an avalanche photo diode (APD) (XL-PHOTONICS, model XAD0250CT-103), while the other went back to the OLT and was received by an APD (Discovery, model DSC-R402APD). Different APDs were chosen for the asymmetric traffic condition. The split ratio of the 1 x 2 coupler was set as 70/30 so that the upstream power was made larger than that entering an optical receiver (RX) to enhance the power margin.
3. Results and discussion
The downstream and upstream signals existed in turns after passing through the RSOA. Figure 2 shows a screenshot taken using a Tektronix digital phosphor oscilloscope (DPO) at the ONU receiver, which was connected to the 1 x 2 coupler. Both downstream and upstream signals are distinctly seen to be carried.
Each downstream packet consisted of 10,240 bits, and each upstream packet consisted of 1,280 bits. The guard time between packets in burst-mode transmissions using a bit discrimination circuit (BDC) is typically assigned to be 100 ns [9,10]. In our experiment, a guard time of about 256 ns was used to simplify the investigation of transmission performance. For smaller packet gaps, a commercial burst-mode BDC can be utilized to make the power of the downstream and upstream signals uniform and help distinguish each signal. Moreover, the power level difference (the power of the upstream signal is larger than that of the downstream signal by about 1.4 dB) can be ignored by using the limiting amplifier inside the BDC.
The downstream transmission experiment was done with and without the RSOA to demonstrate the improvement of optical link performance by the amplification of the signal. Figure 3 shows the eye patterns of those two cases that were measured at the ONU receiver. The downstream signal with the RSOA provided the eye diagram with a clearer eye opening than that of the signal without the RSOA. From the eye pattern, the signal power with the RSOA was amplified by 4.5 dB, and it is easily seen in the figure.
Figure 4 shows the measured BER curves of the downstream signal. In real environments, the proposed system should be run with the burst-mode signals, but in this experiment, BERs were measured with the continuous mode signal to present the optimized performance. The received power was measured at the ONU front-end to compare the downstream performance with and without the RSOA. At a BER of 10−9, the power penalty was about 0.1 dB after 20-km transmission, and the power margin was increased by about 3 dB owing to the pre-amplification by the RSOA.
The RSOA has a broadband spectrum, which means that the spectral power density is very low. Therefore, it needs to be wavelength-locked by the wavelength of the downstream signal for upstream transmission. To investigate the seed light power at the RSOA for wavelength locking, the SMSR and the peak power (PKP) from the RSOA were measured as a function of the seed light power. The SMSR is defined as the ratio of the PKP to the ground noise floor. Figure 5 shows the measured SMSR and PKP of the wavelength-locked RSOA. Up to –10 dBm, the SMSR and PKP were increased with seed light power. However, the gap between the SMSR and PKP for the given seed light power was decreased with an increase in seed power, because the main mode was increased by the increasing seed power, but the side mode was relatively decreased by more phase locking. Conclusively, the SMSR appeared to be more steeply increased . A better SMSR means narrower spectral width. Therefore, the inter-symbol interference on the fiber dispersion of the upstream signal can be alleviated. In the inset, the spectrum of the wavelength-locked RSOA was compared with that of the wavelength-unlocked RSOA. To obtain sufficient seed light power, the output of the OLT should be set high to overcome the link loss. However, it requires high power in the OLT.
Figure 6 shows the back-to-back sensitivity of the upstream signal with the seed light power. Beyond 0 dBm, the increasing seed power enhances the bias level optically inside the RSOA. Even though we obtain a better SMSR, the system performance is degraded by the extinction ratio-induced power penalty. As seen in Figs. 5 and 6, the tolerable seed light power was in the range of −13 dBm to 0 dBm. In our experiment, the seed light power was set to −11 dBm with an SMSR of >40 dB without modulation.
The measured BER after 20-km upstream transmission is shown in Fig. 7 together with back-to-back performance. Although the seed light power was set below the MZM quadrature point due to the link loss (12 dB between CIR port 2 and RSOA in Fig. 1), the re-modulation worked out successfully, showing good performance. The power penalty in the BER curves was measured to be about 1.1 dB at a BER of 10−9 after 20-km transmission.
A cost-effective TCM-based transmission system was demonstrated as an application to asymmetric traffic distribution (10 Gb/s for downstream and 1.25 Gb/s for upstream) between an ONU and OLT. The RSOA at the ONU acted as a re-modulator and an optical pre-amplifier simultaneously. In an attempt to use the same wavelength as that of the 10 Gb/s downstream signal in the upstream signal, the RSOA was wavelength locked with precise consideration of the seed light power from the downstream signal, and the downstream power was re-modulated for the 1.25 Gb/s upstream transmission. Due to the optical amplification of the RSOA at 10 Gb/s, the downstream power margin was improved by about 3 dB after 20-km transmission. The downstream and upstream signals had power penalties of approximately 0.1 dB and 1.1 dB, respectively, at BERs of 10−9.
This work was supported in part by ICT R&D program of MSIP/IITP [B0101-15-1360, Loudness Based Broadcasting Loudness and Stress Assessment of Indoor Environment Noises].
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