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The maximum reach in a WDM-PON using a broadband light source (BLS) seeded optical source has been experimentally evaluated by taking into account both effects of dispersion-induced pulse broadening and excess intensity noise (EIN) increase. In order to investigate the impact of BLS seed source power on the dispersion-limited performance, the system’s performance has been measured and compared as a function of the spectrum-sliced BLS seed power into a reflective semiconductor optical amplifier (RSOA). From the results, we confirmed that the maximum reach in a RSOA based WDM-PON was mainly degraded by the dispersion-induced EIN increase. Therefore, by mitigating the effect of dispersion-induced EIN increase with a high seed power into a RSOA, the maximum reach in the WDM-PON using a BLS seeded RSOA source could be achieved to be ~60 km of single-mode fiber at the spectrum-sliced BLS seed power of >-10 dBm and a 1.25 Gb/s signal without using any dispersion-compensating techniques.
©2012 Optical Society of America
1. Introduction
Broadband light source (BLS) seeded optical sources, such as a wavelength-locked Fabry-Perot laser diode (FP-LD) and a reflective semiconductor optical amplifier (RSOA) have been considered as attractive solutions for the cost-effective implementation of high-capacity wavelength-division multiplexed passive optical networks (WDM-PONs) [1,2]. In a typical BLS seeded optical source configuration, an array of FP-LDs or RSOAs seeded by a single BLS could generate a number of WDM downstream or upstream signals, and be used for the amplification and direct-modulation of the signals at the same time. Moreover, the gain saturation characteristics of FP-LD or RSOA could suppress the excess intensity noise (EIN) within a spectrum-sliced BLS output, and thus improve the performance of BLS based WDM systems [3–5]. However, due to the inherent wide source bandwidth of the BLS based optical sources; the system’s performance was severely degraded by the chromatic dispersion of transmission fiber [4–6]. Especially, in the WDM-PON using the BLS seeded optical sources, the chromatic dispersion could increase not only the inter-symbol interference (ISI) due to the signal pulse broadening, but also the EIN suppressed within the FP-LD or RSOA [4,5]. In [5], it has been reported that the suppressed EIN with a gain-saturated SOA could be unsuppressed again due to the dispersion-induced temporal walk-off between the frequency components of EIN. This dispersion-induced EIN increase could induce timing jitters and make the “1” rails thicker than before transmission [5]. Therefore, in order to estimate the dispersion-limited performance properly, both effects of a pulse broadening and an EIN increase should be taken into account. Previously, we have reported that a well-known dispersion penalty equation including only the effect of a signal pulse broadening would underestimate the measured system’s penalty in the WDM-PON using BLS seeded optical sources [6]. Thus, we confirmed that the dispersion-limited performance was strongly affected by the dispersion-induced EIN increase. Recently, many studies have also focused on the implementation of cost-effective extended-reach PON architectures in order to reduce the number of network equipments [7–10]. In order to improve the cost-effectiveness of WDM-PONs, it would be worthy of investigation into the maximum transmission distance in an extended-reach PON architecture implemented with the BLS seeded optical sources. Previously, it has been reported that the amount of EIN suppression could be determined with the input power into the FP-LD or RSOA [11]. Thus, we believe that the maximum reach in the WDM-PON using BLS seeded optical sources would be increased by the proper suppression of EIN with a high power of the spectrum-sliced BLS seed output into the FP-LD or RSOA.
In this paper, we have experimentally investigated the effect of seed source power on the maximum achievable reach in the WDM-PON using a BLS seeded optical source. The system’s performance has been measured and compared as a function of a spectrum-sliced BLS seed power into a RSOA. From the measurements, we confirmed that effect of dispersion-induced EIN increase could be mitigated with an enough seed source power into the RSOA. We have also found that the maximum reach in a RSOA based WDM-PON operating at a 1.25 Gb/s could be increased to be > 60 km of conventional single-mode fiber (SMF) without using any dispersion compensating techniques.
2. Results and discussion
Figure 1 shows the apparatus used to measure the performance of a RSOA based WDM-PON system. As a seed source, an amplified spontaneous emission (ASE) from a BLS was spectrally-sliced and launched into a RSOA via a circulator and an arrayed-waveguide grating (AWG) multiplexer/demultiplexer. The AWG had a channel spacing of 100 GHz and a 3-dB bandwidth of 0.4 nm. The RSOA was used for the amplification and direct-modulation of a spectrum-sliced BLS output at a 1.25 Gb/s non-return-to-zero (NRZ) pseudo-random pattern of length 231-1. The bias and modulation currents of RSOA were set to be 27 mA and ±11.2 mA, respectively. The modulated signal at the wavelength of 1550.34 nm was transmitted through a conventional SMF and then demultiplexed by using a second AWG. To avoid the power-budget problem in our dispersion-limited performance evaluation, an erbium-doped fiber amplifier (EDFA) was placed before a second AWG and used to amplify the transmitted signal. Then, the demultiplexed signal was detected by use of a PIN-based receiver having a 940 MHz electrical filter.
Fig. 1 Apparatus used for the performance measurement of a RSOA based WDM-PON system. Acronyms are receiver, Rx; and bit-error-rate test sets, BERTs.
Previously, it has been reported that the dispersion-induced power penalty in a WDM-PON using the spectrum-sliced BLS (or ASE source) without a FP-LD or a RSOA could be estimated with the following equation [6, 12];
where B is the data rate, L is the transmission distance, D is the value of chromatic dispersion, and Bo is the equivalent optical source bandwidth. In [6], we found that the calculated dispersion-induced penalty with the Eq. (1) agreed well with the measured penalty only in case of a spectrum-sliced ASE source without the FP-LD or RSOA, while the calculated penalty for the case of BLS seeded optical sources with the FP-LD or RSOA underestimated the measured penalty. This is because the Eq. (1) did not take into account the effect of dispersion-induced EIN increase in the WDM-PON using BLS seeded optical sources. That is to say, the Eq. (1) could be used to estimate the effect of only dispersion-induced pulse broadening in the BLS based WDM-PON. In order to evaluate the effect of dispersion-induced pulse broadening for our measurements first, we measured the optical spectrum of a BLS seeded RSOA output at the receiver side after passing through a second AWG, as shown in Fig. 2 . The 3-dB bandwidth of our BLS seeded RSOA output was measured to be 0.32 nm, which was a little bit narrower than the 3-dB bandwidth of an AWG due to the spectral filtering effect of two cascaded AWGs. From the measured optical spectrum, we calculated the equivalent optical bandwidth of the BLS seeded RSOA output to be 0.34 nm.Fig. 2 Optical spectrum of a BLS seeded RSOA output measured at the receiver after passing through a second AWG .
Using the equivalent optical source bandwidth estimated from the measured optical spectrum of our BLS seeded RSOA source, the system’s penalty caused by the dispersion-induced pulse broadening could be calculated with the Eq. (1), as shown in Fig. 3 . For comparison, the measured dispersion-induced penalty at a bit-error-rate (BER) of 10−9 with a spectrum-sliced BLS seed power of −15 dBm was also represented in Fig. 3. In our calculation, we assumed that the dispersion value of SMF was −17 ps/nm/km. From the results, we re-confirmed that the calculated pulse broadening induced penalty underestimated the measured penalty. The system’s penalty was dominated by the effect of dispersion-induced pulse broadening, only when the transmission distance was less than 25 km of SMF. With a transmission distance of >25 km of SMF, the significant effect of dispersion-induced EIN increase on the system’s penalty was observed. Therefore, we found that the maximum reach in our RSOA based WDM-PON with the spectrum-sliced BLS seed power of −15 dBm was limited to be <30 km of SMF using a guideline of <1-dB penalty, mainly due to the effect of dispersion-induced EIN increase.
Fig. 3 Calculated and measured dispersion-induced power penalties as a function of SMF transmission length. In this measurement, the spectrum-sliced BLS seeded power was set to be −15 dBm.
In order to evaluate the effect of the spectrum-sliced BLS seed power on the maximum reach in the WDM-PON using a RSOA based optical source, the system’s performance was measured by adjusting the launched seed power into a RSOA. First, the back-to-back BER curves were measured with five different levels of the spectrum-sliced BLS seed power into the RSOA, as shown in Fig. 4(a) . No BER floor in a 1.25 Gb/s signal transmission was observed even when the spectrum-sliced BLS seed power into the RSOA was as low as −16 dBm. Then, the improvement of receiver sensitivity in our system was measured to be 0.5 dB at a BER of 10−9, when the spectrum-sliced BLS seed power was changed from −16 dBm to −7 dBm. From this sensitivity improvement, we confirmed that a high seed power into the RSOA could suppress EIN efficiently. Using these back-to-back BER curves as references, we have measured the power penalties at a BER of 10−9 as a function of SMF transmission distance with five different levels of the spectrum-sliced BLS seed power, as shown in Fig., 4(b). For comparison, the calculated power penalty using the Eq. (1) was represented again in Fig. 4(b). No significant differences in the optical spectra of the RSOA output measured with the different levels of the spectrum-sliced BLS seed power were observed. Therefore, we could assume that the dispersion-induced pulse broadening was not affected by the spectrum-sliced BLS seed power. Then, as it can be seen in Fig. 4(b), we found that the higher spectrum-sliced BLS seed power launched into the RSOA, the longer transmission distance could be achieved in the WDM-PON using a BLS seeded RSOA source. For example, in case of the spectrum-sliced BLS seed power of −16 dBm, a BER floor at 10−9 with a 1.25 Gb/s signal was observed at the SMF transmission distance of ~30 km. With the spectrum-sliced BLS seed power of −7 dBm, the SMF transmission distance where the BER floor occurred increased to be ~63 km. Moreover, owing to the suppression of the dispersion-induced EIN increase, the measured system’s penalty agreed well with the calculated penalty at a spectrum-sliced BLS seed power of −7 dBm and <60 km of SMF transmission distance. However, with >60 km of SMF transmission distance, the dispersion-induced EIN increase induced a BER floor even at the high seed power of > −10 dBm. From the results, we found that the maximum reach in the WDM-PON using a BLS seeded RSOA source could be increased to be ~60 km when the spectrum-sliced BLS seed power into a RSOA was higher than −10 dBm with a 1-dB penalty guideline and a 1.25 Gb/s NRZ signal. In addition, we confirmed that the maximum reach in the WDM-PON using a BLS seeded optical sources would be strongly dependent on the suppression of dispersion-induced EIN increase with a high spectrum-sliced BLS seed power level into a RSOA or a FP-LD.
Fig. 4 (a) Back-to-back BER curves and (b) power penalties as a function of SMF transmission distance, which were measured with five different levels of spectrum-sliced BLS seed power.
Finally, we also analyzed the power-budget limited reach without an EDFA which enabled us to evaluate the system’s performance with a high dispersion-induced penalty, as shown in Fig. 1. The RSOA output in our measurement was measured to be + 5 dBm with a BLS seed power of >-15 dBm. As can be seen in Fig. 4(a), the receiver sensitivity was about −23.5 dBm with a BLS seed power of >-10 dBm. The insertion losses of an AWG and a 3-port circulator were also measured to be 4 dB and 1 dB, respectively. Therefore, the power-budget limited reach could be estimated to be ~76 km with a fiber loss of 0.23 dB/km and a 1-dB power penalty guideline. From the results, we confirmed that the maximum reach in our BLS seeded WDM-PON was mainly limited by the chromatic dispersion of transmission fiber.
3. Summary
The maximum reach in an extended-reach WDM-PON using a spectrum-sliced BLS seeded RSOA source has been experimentally evaluated by taking into account both effects of dispersion-induced pulse broadening and EIN increase. In our measurements, we confirmed that the system’s performance was mainly degraded by the dispersion-induced EIN increase especially at a long transmission distance region. Thus, in order to increase the maximum reach in the WDM-PON using a BLS seeded RSOA source, the effect of dispersion-induced EIN increase should be suppressed with a high seed source power into a RSOA. From the results, we found that the maximum reach in the WDM-PON using a BLS seeded RSOA source could be increased to be ~60 km of conventional SMF at the spectrum-sliced BLS seed power of >-10 dBm and a 1.25 Gb/s NRZ signal.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (NRF-2010-0009356).
References and links
1. C.-H. Lee and S.-G. Mun, “WDM-PON based on wavelength-locked Fabry-Perot LDs,” J. Opt. Soc. Korea 12(4), 326–336 (2008). [CrossRef]
2. B. W. Kim, “RSOA-based wavelength-reuse gigabit WDM-PON,” J. Opt. Soc. Korea 12(4), 337–345 (2008). [CrossRef]
3. A. D. McCoy, P. Horak, B. C. Thomsen, M. Ibsen, and D. J. Richardson, “Noise suppression of incoherent light using a gain-saturated SOA: implications for spectrum-sliced WDM systems,” J. Lightwave Technol. 23(8), 2399–2409 (2005). [CrossRef]
4. G. J. Pendock and D. D. Sampson, “Transmission performance of high bit rate spectrum-sliced WDM systems,” J. Lightwave Technol. 14(10), 2141–2148 (1996). [CrossRef]
5. H. Kim, S. Kim, S. Hwang, and Y. Oh, “Impact of dispersion, PMD, and PDL on the performance of spectrum-sliced incoherent light sources using gain-saturated semiconductor optical amplifiers,” J. Lightwave Technol. 24(2), 775–785 (2006). [CrossRef]
6. C. H. Kim, J. H. Lee, D. K. Jung, Y.-G. Han, and S. B. Lee, “Performance comparison of directly-modulated, wavelength-locked Fabry-Perot laser diode and EAM-modulated spectrum-sliced ASE source for 1.25 Gb/s WDM-PON,” presented at OFC2007/NFOEC Mar. 2007, JWA82.
7. R. P. Davey, P. Healey, I. Hope, P. Watkinson, D. B. Payne, O. Marmur, J. Ruhmann, and Y. Zuiderveld, “DWDM reach extension of a GPON to 135 km,” J. Lightwave Technol. 24(1), 29–31 (2006). [CrossRef]
8. I. T. Monroy, R. Kjaer, B. Palsdottir, A. M. J. Koonen, and P. Jeppesen, “10 Gb/s bidirectional single fibre long reach PON link with distributed Raman amplification,” presented at Eur. Conf. Optical Communication (ECOC2006) Sep. 2006, We3.P.166.
9. H. H. Lee, K. C. Reichmann, P. P. Iannone, X. Zhou, and B. Palsdottir, “A hybrid-amplified PON with 75-nm downstream band-with, 60 km reach, 1:64 split and multiple video services,” presented at OFC2007/NFOEC Mar. 2007, OWL2.
10. C. H. Kim, J. H. Lee, and K. Lee, “Analysis of maximum reach in WDM PON architecture based on distributed Raman amplification and pump recycling technique,” Opt. Express 15(22), 14942–14947 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-15-22-14942. [CrossRef] [PubMed]
11. K. Sato and H. Toba, “Reduction of mode partition noise by using semiconductor optical amplifiers,” IEEE J. Sel. Top. Quantum Electron. 7(2), 328–333 (2001). [CrossRef]
12. K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, “Bidirectional WDM PON using light-emitting diodes spectrum-sliced with cyclic arrayed-waveguide grating,” IEEE Photon. Technol. Lett. 16(10), 2380–2382 (2004). [CrossRef]
