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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 11 — May. 28, 2007
  • pp: 6874–6882
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In-service monitoring of 16 port × 32 wavelength bi-directional WDM-PON systems with a tunable, coded optical time domain reflectometry

Jeonghwan Lee, Jonghan Park, Jae Gwang Shim, Hosung Yoon, Jin Hee Kim, Kyoungmin Kim, Jae-Oh Byun, and Namkyoo Park  »View Author Affiliations


Optics Express, Vol. 15, Issue 11, pp. 6874-6882 (2007)
http://dx.doi.org/10.1364/OE.15.006874


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Abstract

We propose a multi-port, multi-wavelength supervisory system for the in-service transmission line monitoring of a bidirectional WDM-PON system. Identifying unique requirements for the performance monitoring of a real field WDM-PON system, we define the architecture for the supervisory system and utilize the most up-to-date technologies (Simplex coding, tunable source, and optical switches) to demonstrate a successful interrogation of a transmission line up to 16 ports × 32 nodes (512 user) capacity. Monitoring of individual branch traces up to 60 km was achieved with the application of a 127-bit simplex code corresponding to a 7.5dB SNR coding gain. In-service transmission experiments showed negligible penalty from the monitoring system to the transmission signal quality, at a 2.5Gbps / 125Mbps (down / up stream) data rate.

© 2007 Optical Society of America

1. Introduction

I: with the large number of users / access lines to monitor, one have to counter the increase in the measurement time. II: with a different wavelength assigned for each user, one would require a means of directing OTDR probe pulses to different user nodes. III: considering cost issues and future expansions, it would be better to secure expansion capability for larger numbers of fiber lines.

Solutions for the above stated problems have existed in the past, but in a very limited form, providing only partial solutions. For example, a cost-effective in-situ OTDR, employing a single-mode 850nm vertical-cavity surface-emitting laser (VCSEL) has been suggested for the access and metropolitan area network [3

3. G. A. Keeler, D. K. Serkland, K. M. Geib, and G. M. Peake, “In situ OTDR for low-cost optical networks using singlemode 850nm VCSEL,” Electron. Lett. 41, 819–820 (2005). [CrossRef]

]. Employing an optical switch, a remote fiber test systems (RFTS), also, have been commercialized in the past to monitor multiple numbers of fibers in a sequential manner [4

4. D. Derickson, Fiber Optic Test and Measurement (Prentice Hall PTR, Upper Saddle River, NJ, 1998), Chap. 11.

]. For the interrogation of a WDM-PON system, a wavelength-tunable OTDR also has been demonstrated [5

5. K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, “In-service individual line monitoring and a method for compensating for the temperature-dependent channel drift of a WDM-PON containing an AWGR using a 1.6um tunable OTDR,” in European Conference on Optical Communications (ECOC’97), Paper 448, pp. 295–298.

, 6

6. U. Hilbk, M. Burmeister, B. Hoen, T. Hermes, J. Saniter, and F. J. Westphal, “Selective OTDR measurements at the central office of individual fiber link in a PON,” in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), Paper Tuk3.

]. Still, with the limited number of ports in the optical switch or arrayed waveguide grating (AWG), employed either in RFTS or a tunable OTDR, the total number of users was limited to usually less than several of 10s. The natural extension for the following evolution will be a multi-port, multi-wavelength system combining the RFTS and an AWG together with a tunable OTDR, which will enable coverage of hundreds of users; sharing the system cost, but at the expense of increased network surveillance time. Seems plausible, but considering a simple math, the use of a conventional 3 minute averaging time for tens / hundreds of user lines would make the measurement system impractical making the purpose of in-line monitoring meaningless.

Even if the simplest resolution for the measurement time reduction can be achieved with the increase of the probe pulse peak power, the peak power cannot be increased indefinitely, due to the concurrent nonlinear penalty for the data and increase in the cost [7

7. P. M. Kjeidsen, M. Obro, J. S. Madsen, and S. K. Nielsen, “Bit-error-rate degradation due to on-line OTDR monitoring above 1.6um,” in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), Paper TuT1.

]. The other approach that can be taken is the use of the recently developed OTDR coding technology [8–11

8. M. D. Jones, “Using simplex codes to improve OTDR sensitivity,” IEEE Photon. Technol. Lett. 5, 822–824 (1993). [CrossRef]

]. For this work, as much as 7.5dB coding gains in the SNR were achieved with the application of 127bit Simplex coding to the OTDR. Considering that the coding gain also can be utilized to reduce the measurement time rather than the dynamic range improvement, significant reductions in the measurement time can be obtained (for example, from 3 minutes to 5.6 seconds with a 127 bit code, ignoring the associated code processing time). Assuming a 16 port × 32 node WDM-PON architecture which we illustrate as an example in this work, this corresponds to a measurement time of less than an hour for the whole network, including 512 users, instead of a day.

2. System description: WDM-PON

Fig. 1. Schematic of the supervisory system and experimental setup for in-service monitoring of fiber link faults in a bidirectional WDM-PON system (down-stream signal at the L-band, upstream signal at the C-band, and supervisory system at the S-band)

For the generation of the probe pulse (isolated, or coded) in the supervisory system, an S-band tunable laser (wavelength tuning speed for the worst case : from the last (32) to first channel ~ 0.3 second) and a modulated C/S-band semiconductor optical amplifier (SOA) were used in combination, at a wavelength of 1458.15~1481.76 nm, utilizing the cyclic property of the AWG (FSR of ~36nm, 3dB BW of 0.4nm, and maximum integrated crosstalk of -23dB).

A C-L/S band WDM coupler (C/L→ S isolation of >12dB and S→C/L isolation of >30dB) was employed in the FDP to couple the probe pulse and multiplexed downstream signals. WDM couplers were also placed after all the distribution fibers, to suppress the out-band crosstalk from probe pulses to subscriber receiver units.

3. Construction of the Surveillance System

Fig. 2. Schematic diagram of the board and picture of the constructed surveillance system

3.1 Hardware

Rayleigh backscattered lights from the transmission line when launching the simplex coded probe pulses were fed to the avalanche photo diode (APD) in the OTDR unit. After the decoding procedures on the back-scattered coded traces, a final OTDR trace in a conventional format was obtained [9

9. D. Lee, H. Yoon, P. Kim, J. Park, and N. Park, “Optimization of SNR Improvement in the Non-coherent OTDR based on Simplex Codes,” J. Lightwave Technol. 24, 322–328 (2006). [CrossRef]

]. To resolve the added computational complexity from the decoding procedure involving matrix operations (compared to the simple averaging process in a conventional OTDR), a single board computer (SBC) unit was employed with a high-speed interface to the on-board DSP. After the averaging process in the DSP for traces of each corresponding code-word, the averaged coded traces were then sent to an advanced RISC machine (ARM) processor through a dual port random access memory (DPRAM). The traces were then sent to a SBC (using an Ethernet interface) where the decoding operation finally took place.

Figure 2 shows the schematic diagram of the hardware board layout and photographs of the constructed hardware including the case and display unit. The tunable coded-OTDR, power source, the ARM board, C-band TLS (wavelength tuning speed for the worst case : from the last (32) to first channel ~ 0.6 second), single board computer, and a 1×16 optical switch were mounted in the 3.5U case equipped with an LCD monitor, constituting an independent, stand-alone surveillance system. The connection for the surveillance system to the higher hierarchy management system was provided with a USB/Ethernet port along with a PS/2 port placed in the back panel of the system. Also placed in the back panel were the SC/APC output ports of the 1×16 switch, for the connection of WDM PON links. It is worth mentioning that a C-band TLS module was integrated in the board also with a C/S band WDM coupler, to support external sources (such as S-band tunable laser) operating at other surveillance wavelengths.

3.2 Firmware

Fig. 3. Main window of the PC Graphic User Interface
Fig. 4. Control and measurement option windows

3.3 Software

Programming on the SBC was carried out with Visual C++ to support the graphic user interface (GUI) for: the display of the decoded trace, including the event list, control of the measurement options, communication with the ARM board, the decoding option for the received data, control of the TLS, and control of the optical switch.

4. Experimental results

Fig. 5. OTDR traces measured to show multi-port function

Fig. 6. Measured loss traces using conventional OTDR (a, b) and 31-bit coded-OTDR (c, d) (Inset: the first codeword of the 31-bit simplex code pattern as SOA output)

Fig. 7. Link loss traces using a) the conventional OTDR and b) the 127-bit Simplex coded-OTDR for long reach (40km + 20km) application

To investigate the feasibility of including users with long length of distribution fiber, trace acquisition was tested using longer codewords (127 bit, thus better SNR gain = 7.5dB) and higher probe pulse peak power (12.5dBm). Figure 7 compares the OTDR traces of Channel 25 with and without the coding technology. The pulse width of the OTDR pulse was set at 1,000ns to achieve the spatial resolution of 100m. Total of 60km fiber (40km and 20km of SMF, as the feeder and distribution fiber respectively) was used. The averaging number for the OTDR trace in the conventional single-pulse / 127-bit coded pulse mode was 127,000 times / 1,000 times (per each codeword), respectively. As expected, 7.5dB of the SNR enhancement was achieved with the coded OTDR, when compared to the conventional OTDR. Worth to note, as users with such a long span length will be very rare in a conventional access WDM-PON network, we believe that the addition of few users (out of 512 lines) with a long span length (and associated interrogation time) would not cause serious trouble for the total network interrogation time. Successful interrogation of the feeder and distribution fiber of up to 60km was achieved with the long-reach application of the coded OTDR. In contrast, it was difficult to distinguish any event from the trace of distribution fiber with the OTDR in an average mode, due to the high noise level. It is important to mention again, even if this amount of SNR gain in theory can be achieved with the conventional OTDR (with 7.5dB increase in pulse power or orders of magnitude increase in the averaging time), the associated system cost - in the form of signal degradation due to the nonlinearity, or unrealistic interrogation time to cover hundreds of subscriber lines - makes the conventional OTDR inapplicable for our WDM-PON application.

Fig. 8. BER characteristics of a) downstream (2.5Gbps) and b) upstream (125Mbps) signals. No transmission penalty was observed with the OTDR operation

5. Conclusion

References and links

1.

K. S. Kim, “On the evolution of PON-based FTTH solutions,” Inform. Sci. 149, 21–30 (2003). [CrossRef]

2.

S. J. Park, C. H. Lee, K. T. Jeong, H. J. Park, J. G. Ahn, and K. H. Song, “Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network,” J. Lightwave Technol. 22, 2582–2591 (2004). [CrossRef]

3.

G. A. Keeler, D. K. Serkland, K. M. Geib, and G. M. Peake, “In situ OTDR for low-cost optical networks using singlemode 850nm VCSEL,” Electron. Lett. 41, 819–820 (2005). [CrossRef]

4.

D. Derickson, Fiber Optic Test and Measurement (Prentice Hall PTR, Upper Saddle River, NJ, 1998), Chap. 11.

5.

K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, “In-service individual line monitoring and a method for compensating for the temperature-dependent channel drift of a WDM-PON containing an AWGR using a 1.6um tunable OTDR,” in European Conference on Optical Communications (ECOC’97), Paper 448, pp. 295–298.

6.

U. Hilbk, M. Burmeister, B. Hoen, T. Hermes, J. Saniter, and F. J. Westphal, “Selective OTDR measurements at the central office of individual fiber link in a PON,” in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), Paper Tuk3.

7.

P. M. Kjeidsen, M. Obro, J. S. Madsen, and S. K. Nielsen, “Bit-error-rate degradation due to on-line OTDR monitoring above 1.6um,” in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), Paper TuT1.

8.

M. D. Jones, “Using simplex codes to improve OTDR sensitivity,” IEEE Photon. Technol. Lett. 5, 822–824 (1993). [CrossRef]

9.

D. Lee, H. Yoon, P. Kim, J. Park, and N. Park, “Optimization of SNR Improvement in the Non-coherent OTDR based on Simplex Codes,” J. Lightwave Technol. 24, 322–328 (2006). [CrossRef]

10.

J. Park, G. Bolognini, D. Lee, P. Kim, P. Cho, F. D. Pasaquale, and N. Park, “Raman-based distributed Temperature Sensor with Simplex Coding and Link Optimization,” IEEE Photon. Technol. Lett. 18, 1879–1881 (2006). [CrossRef]

11.

D. Lee, H. Yoon, P. Kim, J. Park, N. Y. Kim, and N. Park, “SNR Enhancement of OTDR Using Biorthogonal Codes and Generalized Inverses,” IEEE Photon. Technol. Lett. 17, 163–165 (2005). [CrossRef]

12.

H. Izumita, S. Furukawa, Y. Koyamada, and I. Sankawa, “Fading noise reduction in coherent OTDR,” IEEE Photon. Technol. Lett. 4, 201–203 (1992). [CrossRef]

OCIS Codes
(060.2300) Fiber optics and optical communications : Fiber measurements
(060.2330) Fiber optics and optical communications : Fiber optics communications
(120.4820) Instrumentation, measurement, and metrology : Optical systems
(120.5820) Instrumentation, measurement, and metrology : Scattering measurements
(290.1350) Scattering : Backscattering
(290.5870) Scattering : Scattering, Rayleigh

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 4, 2007
Revised Manuscript: May 14, 2007
Manuscript Accepted: May 14, 2007
Published: May 18, 2007

Citation
Jeonghwan Lee, Jonghan Park, Jae Gwang Shim, Hosung Yoon, Jin Hee Kim, Kyoungmin Kim, Jae-Oh Byun, and Namkyoo Park, "In-service monitoring of 16 port x 32 wavelength bi-directional WDM-PON systems with a tunable, coded optical time domain reflectometry," Opt. Express 15, 6874-6882 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-11-6874


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References

  1. K. S. Kim, "On the evolution of PON-based FTTH solutions," Inform. Sci. 149, 21-30 (2003). [CrossRef]
  2. S. J. Park, C. H. Lee, K. T. Jeong, H. J. Park, J. G. Ahn, and K. H. Song, "Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network," J. Lightwave Technol. 22, 2582-2591 (2004). [CrossRef]
  3. G. A. Keeler, D. K. Serkland, K. M. Geib, and G. M. Peake, "In situ OTDR for low-cost optical networks using singlemode 850nm VCSEL," Electron. Lett. 41, 819-820 (2005). [CrossRef]
  4. D. Derickson, Fiber Optic Test and Measurement (Prentice Hall PTR, Upper Saddle River, NJ, 1998), Chap. 11.
  5. K. Tanaka, H. Izumita, N. Tomita and Y. Inoue, "In-service individual line monitoring and a method for compensating for the temperature-dependent channel drift of a WDM-PON containing an AWGR using a 1.6um tunable OTDR," in European Conference on Optical Communications (ECOC’97), Paper 448, pp. 295-298.
  6. U. Hilbk, M. Burmeister, B. Hoen, T. Hermes, J. Saniter, and F. J. Westphal, "Selective OTDR measurements at the central office of individual fiber link in a PON," in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), Paper Tuk3.
  7. P. M. Kjeidsen, M. Obro, J. S. Madsen, and S. K. Nielsen, "Bit-error-rate degradation due to on-line OTDR monitoring above 1.6um," in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), Paper TuT1.
  8. M. D. Jones, "Using simplex codes to improve OTDR sensitivity," IEEE Photon. Technol. Lett. 5, 822-824 (1993). [CrossRef]
  9. D. Lee, H. Yoon, P. Kim, J. Park, and N. Park, "Optimization of SNR Improvement in the Non-coherent OTDR based on Simplex Codes," J. Lightwave Technol. 24, 322-328 (2006). [CrossRef]
  10. J. Park, G. Bolognini, D. Lee, P. Kim, P. Cho, F. D. Pasaquale, and N. Park, "Raman-based distributed Temperature Sensor with Simplex Coding and Link Optimization," IEEE Photon. Technol. Lett. 18, 1879-1881 (2006). [CrossRef]
  11. D. Lee, H. Yoon, P. Kim, J. Park, N. Y. Kim, and N. Park, "SNR Enhancement of OTDR Using Biorthogonal Codes and Generalized Inverses," IEEE Photon. Technol. Lett. 17, 163-165 (2005). [CrossRef]
  12. H. Izumita, S. Furukawa, Y. Koyamada, and I. Sankawa, "Fading noise reduction in coherent OTDR," IEEE Photon. Technol. Lett. 4, 201-203 (1992). [CrossRef]

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