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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 26 — Dec. 10, 2012
  • pp: B45–B51
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DWDM-PON at 25 GHz channel spacing based on ASE injection seeding

Joon-Young Kim, Sang-Rok Moon, Sang-Hwa Yoo, and Chang-Hee Lee  »View Author Affiliations


Optics Express, Vol. 20, Issue 26, pp. B45-B51 (2012)
http://dx.doi.org/10.1364/OE.20.000B45


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Abstract

We demonstrate a 25 GHz-channel-spaced DWDM-PON based on ASE injection seeding. A 60 km transmission at 1.25 Gb/s per channel is available with a 2nd generation FEC. The major limiting factor is the optical back reflection induced penalty. Thus a high gain reflective modulator and/or relocation of the seed light increase the transmission length. We demonstrated 90 km transmission with relocated seed light to remote node.

© 2012 OSA

1. Introduction

In this paper, we investigate the uppermost limit of the ASE injection seeded DWDM-PON with help of the FEC. The use of FEC enables the reduction of channel spacing and/or the increase in the transmission distance with affordable output power of the seed light at the fixed transmission speed. We analyze the required relative intensity noise (RIN) level when utilizing the FEC. Then, we investigate transmission performance at 25 GHz channel spacing targeting more than 128 channels. We use four different reflective modulators to compare the performances. Transmission distance of 60 km can be achieved with injection seeding at the OLT. The distance can be increased up to 90 km by shifting the seed light position to remote site.

2. RIN requirement

The RIN of the received signal that arises from beating between the uncorrelated frequency components of the spectrum-sliced ASE is a key factor of determining the system performance [7

7. D. M. Baney and W. V. Sorin, “Broadband frequency characterization of optical receivers using intensity noise,” Hewlett-Packard journal 46(1), 6–12 (1995).

]. To investigate the effect of an optical bandwidth on the RIN, we generated an electric field of the ASE based on constant amplitude and uniformly distributed phase over [0~2π] in frequency domain. The spectrum sliced field was obtained by multiplying square root of transfer function of the super-Gaussian filter. The filter order was adjusted to 1.5 for matching a pass band characteristic of the used flat-top type wavelength filter. The RIN spectrum was obtained from the power spectral density of the generated spectrum-sliced ASE. The average RIN of the spectrum sliced ASE within the receiver bandwidth (~750 MHz) is increased as the optical bandwidth narrows as described in Fig. 1(a)
Fig. 1 Simulation and experiment results of (a) RIN as a function of the optical bandwidth and (b) BER penalty by RIN with and without FEC.
. At the 25 GHz channel spacing, the input RIN becomes about −104.4 dB/Hz when the utilized wavelength filter has 3 dB bandwidth of 21.3 GHz. We examined a performance degradation by the increase in the input RIN of the injection seeded DWDM-PON. The simulation was conducted according to Ref [8

8. H.-K. Lee, J.-H. Moon, S.-G. Mun, K.-M. Choi, and C.-H. Lee, “Decision Threshold Control Method for the Optical Receiver of a WDM-PON,” J. Opt. Comm. Netw. 2(6), 381–388 (2010). [CrossRef]

]. In the simulation, the receiver sensitivity, equivalent input noise current, and extinction ratio were 0.8 A/W, 4.5 pA/√Hz, and 10 dB, respectively. When we set target to BER of 10−12, the receiver noise was dominated by the RIN noise in the simulation. Figure 1(b) represents calculated (dashed lines) and measured (hollow circles) power penalties as a function of the RIN at the BER of 10−12 in case of the 1.25 Gb/s transmission. It should be noted that the penalties with FECs are estimated value at the theoretical FEC thresholds for BER of 10−12. The power penalty is dramatically surged as the RIN increases over −110 dB/Hz when we do not utilize the FEC. On the other hand, the easing of the required RIN with the same penalty is possible through using an FEC. In this study, the 1st generation FEC is RS (255, 239) code and the 2nd generation FEC is RS (1901, 1855) with Extended Hamming Product Code (512,502) × (510,500) [10

10. ITU-T Recommendation G.975.1., Forward error correction for high bit rate DWDM submarine systems.

]. Both FECs require 6.69% overhead for coding. It may be noted that the BER thresholds to get the BER of 10−12 are 1.8 × 10−4 and 4.6 × 10−3 for the 1st generation and the 2nd generation, respectively. When we accept 2 dB penalty compared with clean source (RIN < −120 dB/Hz), the required RIN values of received signal can be reduced to −104.9 and −102.2 dB from −110 dB/Hz for the 1st generation and the 2nd generation FECs, respectively. These values should be compared with the input RIN of −104.4 dB/Hz in Fig. 1(a).

3. Experimental setup and results

Figure 2
Fig. 2 Experimental configuration of 25 GHz-spaced seeded DWDM-PON.
shows the experimental setup for investigating 25 GHz-spaced DWDM-PON based on ASE injection seeding. The 25 GHz-spaced arrayed waveguide grating (AWG) was emulated by combining a 25 GHz-spaced interleaver with a 50 GHz-spaced AWG. The insertion loss of the emulated AWG was 7.5 dB. The pass-band was presented at the inset of the Fig. 2. The pass-band type was flat-top, and its 3-dB bandwidth was 0.17 nm (≈21.3 GHz). Thus, the input RIN was about −104.4 dB/Hz for an unpolarized ASE. An average loss of used single mode fiber was around 0.23 dB/km. And the output power of employed C band broadband light source (BLS) was about 24 dBm. A variable optical attenuator (VOA) was utilized between the BLS and the circulator to adjust the total seed power (Pseed) into the feeder fiber. The spectrum-sliced ASE is injected to the reflective modulator, which would be an RSOA or an F-P LD, at the optical network terminal (ONT). The polarized F-P LD/RSOA were made by a quantum well active medium while the unpolarized ones were made by a tensile strained InGaAsP bulk active medium [9

9. H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express 18(17), 18077–18085 (2010). [CrossRef] [PubMed]

]. A cavity length of the F-P LD was around 600 µm, where its mode spacing (0.56 nm) is 3.3 times wider than the emulated AWG bandwidth (0.17 nm).

To characterize the reflective modulator, we measured the gain as shown in Fig. 3
Fig. 3 Measured fiber-to-fiber gain of reflective modulators as a function of injection power.
. The gain decreases as we increase the injection power, referring to this as gain saturation, which leads to the RIN suppression [8

8. H.-K. Lee, J.-H. Moon, S.-G. Mun, K.-M. Choi, and C.-H. Lee, “Decision Threshold Control Method for the Optical Receiver of a WDM-PON,” J. Opt. Comm. Netw. 2(6), 381–388 (2010). [CrossRef]

,9

9. H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express 18(17), 18077–18085 (2010). [CrossRef] [PubMed]

]. The polarized optical sources have usually higher gain than unpolarized sources. The gain of F-P LD varies with the wavelength detuning (=λInjectionλMode) due to the spectral ripple at the constant injection power. Thus, we represented the minimum gain for the worst case.

The optical output from the reflective modulator gets to the receiver after passing through two AWGs and optical fiber. The RIN of the received signal in back-to-back (B-t-B) condition is represented in Fig. 4
Fig. 4 Measured RIN of received signal and required seed power (Pseed) as a function of injection power (under B-t-B condition).
. In case using F-P LDs, we represented the worst RIN values, because the F-P LD has varied RIN value due to the spectral ripple, like the gain. When the injection power is low, the RSOAs operate in linear region and the F-P LDs operate as lasers. The unpolarized RSOA shows less RIN than polarized RSOA (The difference is not 3 dB due to nonzero polarization dependent gain [9

9. H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express 18(17), 18077–18085 (2010). [CrossRef] [PubMed]

].). However, the F-P LDs show higher RIN arisen from mode partition noise. The RINs were decreased as we increased the injection power because of the gain saturation. It may be noted that the effects of gain saturation are more pronounced in F-P LDs due to decrease of both the mode partition noise and the intensity noise of the injected light. At high injection power, difference of RIN between RSOA and F-P LD becomes very small. It can be explained from the deeply saturated gain. In other words, the F-P LD operates as a saturated regenerative amplifier [9

9. H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express 18(17), 18077–18085 (2010). [CrossRef] [PubMed]

]. As seen in Fig. 4, the RSOA shows less RIN over a wide range of injection power. Thus we selected RSOA for transmission experiment. Nevertheless, it should be noted that the RIN performance of the F-P LD will be enhanced when the cavity length increases so that the mode spacing is comparable to or narrower than the AWG bandwidth as seen in Ref [9

9. H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express 18(17), 18077–18085 (2010). [CrossRef] [PubMed]

]. It may be noted that if we match the input wavelength to the lasing wavelength, the RIN of F-P LD becomes less than the ROSA [11

11. J.-Y. Kim, H.-S. Cho, S.-G. Mun, H.-K. Lee, and C.-H. Lee, “High-Capacity DWDM-PON using Triple-Contact F-P LDs,” IEEE Photon. Technol. Lett. 23(2), 127–129 (2011). [CrossRef]

].

We measured BER curves in B-t-B configuration as seen in Fig. 5(a)
Fig. 5 (a) Measured BER curves in B-t-B condition and (b) calculated extinction ratio penalty.
. The RSOAs were modulated by non-return-to-zero (NRZ) signal where data rate is 1.25 Gb/s and pattern length of pseudorandom binary sequence (PRBS) is 231-1. In the unpolarized RSOA case, we obtain the BER that is better than the 1st generation FEC threshold at both −20 and −25 dBm injection power. However, in case of the polarized RSOA, the BER was worse than the 1st generation FEC threshold. This can be explained from the higher RIN and poor extinction ratio. The extinction ratios of polarized RSOA and unpolarized RSOA were 7 dB and 10 dB, respectively. The bad extinction ratio is attributed to the limited modulation bandwidth of the polarized RSOA.

To investigate upstream transmission performance, we measured the power and BER of the received optical signal as a function of the length of the transmission fiber, as shown in Fig. 6
Fig. 6 (a) Measured received power and (b) BER curves as a function of transmission length.
. When the transmission length is increased the seed power is increased to maintain the constant injection power. As the received power is decreased by the increase in the fiber length (Fig. 6(a)), the BER is degraded (Fig. 6(b)). The inclination of BER degradation by the fiber transmission is more moderate when the polarized RSOA is employed. This is because the polarized RSOA features about 3-4 dB higher power due to the higher gain than the unpolarized one. The BER threshold of the 2nd generation FEC was achieved up to 60 km transmission when the injection power was −20 dBm with both polarized and unpolarized RSOAs. For the 60 km upstream transmission with the −20 dBm injection power, the seed power at OLT was 24 dBm which is commercially available. It should be commented that the 24 dBm seed power is beyond the range of the eye-safety that is 21.34 dBm without fast power shut down given the class 1M laser. Therefore, the fast power shut down is required in this case. But, 5 dB reduction of seed power is possible when the transmission length is 50 km.

Since the back reflection is the main factor of performance degradation for the upstream transmission, we investigate the effect of back reflection on the system performance. In Fig. 7(a)
Fig. 7 (a) Back-reflection induced crosstalk as a function of fiber length and (b) measured power penalties as a function of the crosstalk.
, the crosstalk (ratio of back reflection power to signal power) level by back reflection varies with the transmission length and the injection power [12

12. J.-Y. Kim, H.-K. Lee, S.-H. Yoo, S.-R. Moon, H.-Y. Rhy, B. S. Kim, H.-K. Lee, and C.-H. Lee, “Impairments and design of WDM-PON based on injection seeding,” in Proceedings of FTTH conference and Expo (Orlando, FL, 2011), Page 2 of 12.

]. To be specific, since the polarized RSOA has a higher gain than that of the unpolarized RSOA, the crosstalk level is lower even after transmission of the same distance. Also the lower injection power results in the higher gain and consequently, the lower crosstalk. Then the crosstalk by back reflections with 60 km fiber were −7.4 (polarized RSOA) and −4.2 dB (unpolarized RSOA) as seen in Fig. 7 (a). Figure 7(b) shows the measured power penalties by back reflection as a function of the crosstalk. For the same devices, the performance degradation was higher at low injection power on account of the worse RIN. The unpolarized RSOA provides the better back reflection immunity since it features better noise characteristic and extinction ratio. The measured penalties at 60 km fiber transmission were 1.5 (polarized) and 2 dB (unpolarized), respectively. We expect that the distance would be extended by around 10 km if the polarized RSOA has a sufficient modulation bandwidth.

4. Conclusion

References and links

1.

O. Kipouridis, C. M. Machuca, A. Autenrieth, and K. Grobe, “Cost assessment of next-generation passive optical networks on real-street scenario,” in Proceedings of the Optical Fiber Communication Conference (Los Angeles, CA, 2012), Paper NTu2F.4.

2.

W. R. Lee, M. Y. Park, S. H. Cho, J. Lee, C. Kim, G. Jeong, and B. W. Kim, “Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers,” IEEE Photon. Technol. Lett. 17(11), 2460–2462 (2005). [CrossRef]

3.

F. Payox, P. Chanclou, and N. Genay, “WDM-PON with colorless ONUs,” in Proceedings of the Optical Fiber Communication Conference (Anaheim, CA, 2007), Paper OTuG5.

4.

H. D. Kim, S. G. Kang, and C. H. Lee, “A low-cost WDM source with an ASE injected Fabry-Perot semiconductor laser,” IEEE Photon. Technol. Lett. 12(8), 1067–1069 (2000). [CrossRef]

5.

ITU-T Recommendation G.698.3, Multichannel seeded DWDM applications with single-channel optical interfaces.

6.

J.-Y. Kim, H.-K. Lee, S.-R. Moon, and C.-H. Lee, “25-GHz-channel-spaced DWDM-PON based on ASE injection with reduced filtering effect,” in Proceedings of the Conference on Lasers and Electro-Optics (Baltimore, MD, 2011), Paper JWA1.

7.

D. M. Baney and W. V. Sorin, “Broadband frequency characterization of optical receivers using intensity noise,” Hewlett-Packard journal 46(1), 6–12 (1995).

8.

H.-K. Lee, J.-H. Moon, S.-G. Mun, K.-M. Choi, and C.-H. Lee, “Decision Threshold Control Method for the Optical Receiver of a WDM-PON,” J. Opt. Comm. Netw. 2(6), 381–388 (2010). [CrossRef]

9.

H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express 18(17), 18077–18085 (2010). [CrossRef] [PubMed]

10.

ITU-T Recommendation G.975.1., Forward error correction for high bit rate DWDM submarine systems.

11.

J.-Y. Kim, H.-S. Cho, S.-G. Mun, H.-K. Lee, and C.-H. Lee, “High-Capacity DWDM-PON using Triple-Contact F-P LDs,” IEEE Photon. Technol. Lett. 23(2), 127–129 (2011). [CrossRef]

12.

J.-Y. Kim, H.-K. Lee, S.-H. Yoo, S.-R. Moon, H.-Y. Rhy, B. S. Kim, H.-K. Lee, and C.-H. Lee, “Impairments and design of WDM-PON based on injection seeding,” in Proceedings of FTTH conference and Expo (Orlando, FL, 2011), Page 2 of 12.

13.

S.-M. Lee, M.-H. Kim, and C.-H. Lee, “Demonstration of a bidirectional 80-km-reach DWDM-PON with 8-Gb/s capacity,” IEEE Photon. Technol. Lett. 19(6), 405–407 (2007). [CrossRef]

OCIS Codes
(060.4250) Fiber optics and optical communications : Networks
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Access Networks and LAN

History
Original Manuscript: October 1, 2012
Revised Manuscript: October 19, 2012
Manuscript Accepted: October 22, 2012
Published: November 28, 2012

Virtual Issues
European Conference on Optical Communication 2012 (2012) Optics Express

Citation
Joon-Young Kim, Sang-Rok Moon, Sang-Hwa Yoo, and Chang-Hee Lee, "DWDM-PON at 25 GHz channel spacing based on ASE injection seeding," Opt. Express 20, B45-B51 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-26-B45


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References

  1. O. Kipouridis, C. M. Machuca, A. Autenrieth, and K. Grobe, “Cost assessment of next-generation passive optical networks on real-street scenario,” in Proceedings of the Optical Fiber Communication Conference (Los Angeles, CA, 2012), Paper NTu2F.4.
  2. W. R. Lee, M. Y. Park, S. H. Cho, J. Lee, C. Kim, G. Jeong, and B. W. Kim, “Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers,” IEEE Photon. Technol. Lett.17(11), 2460–2462 (2005). [CrossRef]
  3. F. Payox, P. Chanclou, and N. Genay, “WDM-PON with colorless ONUs,” in Proceedings of the Optical Fiber Communication Conference (Anaheim, CA, 2007), Paper OTuG5.
  4. H. D. Kim, S. G. Kang, and C. H. Lee, “A low-cost WDM source with an ASE injected Fabry-Perot semiconductor laser,” IEEE Photon. Technol. Lett.12(8), 1067–1069 (2000). [CrossRef]
  5. ITU-T Recommendation G.698.3, Multichannel seeded DWDM applications with single-channel optical interfaces.
  6. J.-Y. Kim, H.-K. Lee, S.-R. Moon, and C.-H. Lee, “25-GHz-channel-spaced DWDM-PON based on ASE injection with reduced filtering effect,” in Proceedings of the Conference on Lasers and Electro-Optics (Baltimore, MD, 2011), Paper JWA1.
  7. D. M. Baney and W. V. Sorin, “Broadband frequency characterization of optical receivers using intensity noise,” Hewlett-Packard journal46(1), 6–12 (1995).
  8. H.-K. Lee, J.-H. Moon, S.-G. Mun, K.-M. Choi, and C.-H. Lee, “Decision Threshold Control Method for the Optical Receiver of a WDM-PON,” J. Opt. Comm. Netw.2(6), 381–388 (2010). [CrossRef]
  9. H.-K. Lee, H.-S. Cho, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with an 80 Gb/s capacity based on wavelength-locked Fabry-Perot laser diode,” Opt. Express18(17), 18077–18085 (2010). [CrossRef] [PubMed]
  10. ITU-T Recommendation G.975.1., Forward error correction for high bit rate DWDM submarine systems.
  11. J.-Y. Kim, H.-S. Cho, S.-G. Mun, H.-K. Lee, and C.-H. Lee, “High-Capacity DWDM-PON using Triple-Contact F-P LDs,” IEEE Photon. Technol. Lett.23(2), 127–129 (2011). [CrossRef]
  12. J.-Y. Kim, H.-K. Lee, S.-H. Yoo, S.-R. Moon, H.-Y. Rhy, B. S. Kim, H.-K. Lee, and C.-H. Lee, “Impairments and design of WDM-PON based on injection seeding,” in Proceedings of FTTH conference and Expo (Orlando, FL, 2011), Page 2 of 12.
  13. S.-M. Lee, M.-H. Kim, and C.-H. Lee, “Demonstration of a bidirectional 80-km-reach DWDM-PON with 8-Gb/s capacity,” IEEE Photon. Technol. Lett.19(6), 405–407 (2007). [CrossRef]

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