Improved wavelength coded optical time domain reflectometry based on the optical switch Ninghua Zhu, Youwan Tong, Wei Chen, Sunlong Wang, Wenhui Sun, and Jianguo Liu* State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China * [email protected]
Abstract: This paper presents an improved wavelength coded time-domain reflectometry based on the 2 × 1 optical switch. In this scheme, in order to improve the signal-noise-ratio (SNR) of the beat signal, the improved system used an optical switch to obtain wavelength-stable, low-noise and narrow optical pulses for probe and reference. Experiments were set up to demonstrate a spatial resolution of 2.5m within a range of 70km and obtain the beat signal with line width narrower than 15MHz within a range of 50km in fiber break detection. A system for wavelength-divisionmultiplexing passive optical network (WDM-PON) monitoring was also constructed to detect the fiber break of different channels by tuning the current applied on the gating section of the distributed Bragg reflector (DBR) laser. ©2014 Optical Society of America OCIS codes: (120.0120) Instrumentation, measurement, and metrology; (060.2370) Fiber optics sensors; (060.5625) Radio frequency photonics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
A. J. Rogers, “Polarization-optical time domain reflectometry: A technique for the measurement of field distributions,” Appl. Opt. 20(6), 1060–1074 (1981). M. K. Barnoski and S. M. Jensen, “Fiber waveguides: a novel technique for investigating attenuation characteristics,” Appl. Opt. 15(9), 2112–2115 (1976). M. Tateda and T. Horiguchi, “Advances in optical time-domain reflectometry,” J. Lightwave Technol. 7(8), 1217–1224 (1989). N. Park, J. Lee, J. Park, J. G. Shim, H. Yoon, J. H. Kim, K. Kim, J.-O. Byun, G. Bolognini, D. Lee, and F. Di Pasquale, “Coded optical time domain reflectometry: Principle and applications,” Proc. SPIE 6781, 678129 (2007). Z. Zhang and X. Bao, “Distributed optical fiber vibration sensor based on spectrum analysis of polarizationOTDR system,” Opt. Express 16(14), 10240–10247 (2008). J. Brendel, “High-resolution photon-counting OTDR for PON testing and monitoring,” in OFC Technical Digest (2008), pp. 945–949. N. H. Zhu, J. H. Ke, H. G. Zhang, W. Chen, J. G. Liu, L. J. Zhao, and W. Wang, “Wavelength coded optical time-domain reflectometry,” J. Lightwave Technol. 28(6), 972–977 (2010). M. Han, Y. Wang, and A. Wang, “Grating-assisted polarization optical time-domain reflectometry for distributed fiber-optic sensing,” Opt. Lett. 32(14), 2028–2030 (2007). N. Linze, P. Mégret, and M. Wuilpart, “Development of an intrusion sensor based on a polarization-OTDR system,” IEEE Sens. J. 12(10), 3005–3009 (2012). K. N. Choi and H. F. Taylor, “Spectrally stable Er-fiber laser for application in phase-sensitive optical timedomain reflectometry,” IEEE Photon. Technol. Lett. 15(3), 386–388 (2003). M. Wegmuller, F. Scholder, and N. Gisin, “Photon-counting OTDR for local birefringence and fault analysis in the metro environment,” J. Lightwave Technol. 22(2), 390–400 (2004). G. Ribordy, N. Gisin, O. Guinnard, D. Stucki, M. Wegmuller, and H. Zbinden, “Photo counting at telecom wavelengths with commercial In-GaAs/InP avalanche photodiodes: Current performance,” J. Mod. Opt. 51, 1381–1398 (2004). P. Eraerds, M. Legre, J. Zhang, H. Zbinden, and N. J. Gisin, “Photon counting OTDR: advantages and limitations,” J. Lightwave Technol. 28(6), 952–964 (2010). B. F. Levine, C. G. Bethea, and J. C. Campbell, “Room‐temperature 1.3‐μm optical time domain reflectometer using a photon counting InGaAs/InP avalanche detector,” Appl. Phys. Lett. 46(4), 333–335 (1985).
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15111
15. M. Legré, R. T. Thew, H. Zbinden, and N. Gisin, “High resolution optical time domain reflectometer based on 1.55mum up-conversion photon-counting module,” Opt. Express 15(13), 8237–8242 (2007). 16. M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischka, W. R. Trutna, Jr., and S. Foster, “Realtime long range complementary correlation optical time domain reflectometer,” J. Lightwave Technol. 7(1), 24– 38 (1989). 17. Y. C. Wang, B. J. Wang, and A. B. Wang, “Chaotic correlation optical time domain reflectometer utilizing laser diode,” IEEE Photon. Technol. Lett. 20(19), 1636–1638 (2008). 18. J. Buus and E. J. Murphy, “Tunable lasers in optical networks,” J. Lightwave Technol. 24(1), 5–11 (2006). 19. I. Daubechies, “The wavelet transform, time-frequency localization and signal analysis,” IEEE Trans. Inf. Theory 36(5), 961–1005 (1990).
1. Introduction Distributed optical fiber sensing has been extensively investigated in past decades. And the most widely used approaches are based on the optical time domain reflectometry (OTDR) [1– 7]. It has been a very important diagnostic tool for the testing of fiber transmission systems and components since it was first demonstrated by Barnoski and Jensen [2]. Nevertheless, in order to improve the performance of OTDR, many techniques are still proposed, such as polarization OTDR [1,5,8,9], phase-sensitive OTDR [10], photon-counting OTDR [11–15] and correlation OTDR [16,17]. Besides the new technologies mentioned above, the wavelength coded time-domain reflectometry (OTDR) is a new kind of optical heterodyne technique which is recently developed. The probe and reference optical pulses have different wavelengths to enable the optical heterodyne detection, which can significantly improve the performance of the system. But since the spatial resolution of the system is related with the width of the probe and the reference optical pulse, the width of the probe and reference pulses is usually reduced to 200ns or even shorter [7]. As a result, the distributed Bragg reflector (DBR) laser should be tuned from one wavelength to another wavelength and be stable within 200ns or even shorter. Because of this strict requirement for the DBR laser [18], the application of the wavelength coded OTDR is limited. Furthermore, the line width of the beat signal is also affected by the instability of the probe pulse and reference optical pulse wavelength. In this paper, an improved wavelength coded OTDR based on the 2 × 1 optical switch was proposed. In this scheme, the current applied on the phase section of the DBR laser is modulated by a fixed periodic electrical signal. And one optical switch is adopted to get the narrow probe and reference optical pulses. In every detecting period, the output light of the DBR laser can pass through the optical switch after the DBR laser has established the needed wavelength and has been stable. Therefore, the tuning speed of the DBR laser has no relationship with the line width of the beat signal. Then the system has no requirement for the tuning speed of the DBR laser, and the line width of the beat signal is narrow. Finally, the experiments were set up to demonstrate the ability of the wavelength coded OTDR system. In the first experiment, the fiber breaks at the fiber distance of 50km and 70km at the corresponding to the FC/PC connector at the end of the fiber were detected by the system. In the second experiment, the wavelength coded system was used to monitor the wavelengthdivision-multiplexing passive optical network (WDM-PON) and the fiber breaks at the channels and the COM port of the arrayed waveguide grating (AWG) were successfully detected. 2. Dynamic characteristic of the DBR laser 2.1 Experiment architecture Preparing for the improved wavelength coded OTDR system, an experimental system was setup to demonstrate the dynamic characteristic of the DBR laser used in the system, as shown in Fig. 1. A square waveform voltage was applied to the phase section of the DBR laser. Corresponding to the high voltage VH and the low voltage VL , the laser generates two different light waves at different time. A 50 Ω resistor was used between the waveform
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15112
generator and the laser to limit the current of the phase section. Coupler 1 was employed to separate the light from the DBR laser into two paths, and delayed fiber was inserted into one path, then the light of the two paths was coupled into the photo-detector by coupler 2. The period of the square waveform T0 was set as: T0 = 2Δt = 250μ s (1) where Δt is the delay time of the inserted 25km delay fiber corresponding to a delay time of 125 μ s . As a result, the light waves at different wavelengths from the two paths can fully beat in the photo-detector. The VL was set to 0 V and the VH was set to 6 V, and in this case the beat signal is at 1.06 GHz. The output signal of the photo-detector were then sampled simultaneously and stored by the oscilloscope.
Fig. 1. Experiment setup for dynamic characteristic measurement of the DBR laser.
2.2 Measurement for dynamic characteristic of DBR laser
Fig. 2. Short time Fourier transform result of the beat signal.
Since the frequency of the beat signal varies with time, it is necessary to observe the dynamic characteristic of the laser in the time-frequency domain. At present, short time Fourier transform and wavelets analysis are the main methods for the time-frequency domain analysis. Comparing with wavelets analysis, the frequency resolution of STFT is fixed and the arithmetic of STFT is simple [19], so it is more specifically suited to our application. Figure 2 shows the STFT result of the beat signal using a 1024 point Hamming window. The overlap was set as 768 points. From Fig. 2, it can be seen that, the wavelength of the output laser slowly increases after the high voltage VH has been applied on the phase section of the DBR laser, and with the increasing of the wavelength, the line width of the beat signal and the noise level decreases together. The output light is stable after 80 μ s , so the DBR laser cannot be used in the classical wavelength coded OTDR which uses 200 ns or even shorter optical pulse as the probe and reference signal in the experiment [7]. Therefore, the improved
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15113
system proposed can avoid the restrict requirement of the laser and expand the application range of the wavelength coded OTDR. 3. System ability 3.1 System architecture
Fig. 3. System architecture (EDFA: erbium-doped fiber amplifier).
The improved wavelength coded OTDR was set up to locate the break of the tested fiber as shown in Fig. 3. A Michelson interferometer was employed in the system. The DBR laser was chosen as the wavelength coded light source. A square waveform provided by waveform generator 1 was applied to the phase section of the DBR laser. The DBR laser was connected to the port 1 of the 2 × 1 optical switch, and a tunable laser was connected to the port 2 of the optical switch. And the optical switch was controlled by waveform generator 2. The output of the optical switch was split into two paths by optical fiber coupler 1. One path was amplified by the EDFA and then launched into coupler 3 as the probe optical signal, and the other path is used as the reference optical pulse. A fraction of the probe signal reflected by the break in the tested fiber was recombined with the reference light by coupler 2, and then launched into a high-speed photo-detector.
Fig. 4. Operation principle of the system.
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15114
The operation principles of the reflectometry are described by the schematic in Fig. 4. The wavelength of the output light from the tunable laser was λ3 and the wavelength of the output light from the DBR laser switched between λ1 and λ2 . The wavelength of DBR laser and the tunable laser were chosen in such a way that the wavelength difference between λ2 and λ3 was so large that the beat signal was far beyond our observing frequency range and the frequency difference between λ1 and λ2 was set as at 1.06 GHz. In addition, the optical signal at λ3 was used to suppress the noise of EDFA [1]. The delay time Ts between the positive edges of waveform 1 and the controlling signal of optical switch P1 must be set larger than 80 μ s , which can enable the DBR to establish the stable wavelength. Using this method, high-speed detection with low tunable speed DBR laser can be achieved. While keeping the period of waveform 1 fixed, vary the delay time T1 between P1 and P2 . If the 1.06 GHz signal appears, then the fiber break is located. It is obvious that the beat signal appears when T1 = 2nl / c
(2)
where, n is the refractive index of the fiber, c is the light speed in free space, and l is the distance from the fiber break to the system. 3.2 Results of fiber break detection and analysis
Fig. 5. Results of fiber break detection. (a) Measured power spectrum of the beat signal at the fiber distance of 50km at the corresponding to the FC/PC connector at the end of the fiber. (b) The line width of the beat signal of Fig. 5(a). (c) Measured power spectrum of the beat signal at the fiber distance of 70km at the corresponding to the FC/PC connector at the end of the fiber. (d) Distance resolution of the beat signal at 70km.
Figure 5(a) shows the measured spectra of the beat signal when the break locates at approximately 50 km, in addition, the delay time T1 has been converted to the fiber distance.
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15115
Figure 5(b) shows the beat signal at 50503.5m from which it can be seen that the line width of the beat signal is 15MHz, narrower than the classical wavelength coded OTDR. And the beat signal with narrower line width can be detected within narrower bandwidth, so the noise level can be reduced and the sensitivity can be improved significantly. Figures 5(c) and 5(d) shows the measured spectra at 70km from which it can be seen that the envelope of the peak values shifted when a 2.5m fiber was inserted at the end of the tested fiber. Therefore a spatial resolution of 2.5m was obtained within the range of 70km, and the 2.5 m is the minimum error value of 70 km fiber in this system. The measurement error mainly results from the fluctuation of the fiber length due to the temperature fluctuation and the optical pulse broadening during the propagation in the 70km tested fiber. In conclusion, comparing with the classical wavelength coded OTDR, both the resolution and the measuring range of the system have been improved [7]. 3.3 WDM-PON monitoring
Fig. 6. Architecture of the WDM-PON monitoring system based on wavelength coded OTDR.
In this section, an experiment was set up to demonstrate the performance of the WDM-PON monitoring system based on the wavelength coded OTDR. The architecture of the experiment is shown as Fig. 6. In Fig. 6(a), the wavelength coded OTDR was connected to the third channel of the AWG for the detection of the fiber break of the COM port. By adjusting the tuning current of applied on the gating section of the DBR laser, the central wavelength of the output laser can be set as 1557.361nm, which is the central wavelength of Channel 3 of AWG. In Fig. 6(b), the wavelength coded OTDR was connected to the COM port of the AWG. Similarly, by adjusting the current applied on the gating section of the DBR, the output laser with the different wavelength can be obtained, and the fiber break detection of the corresponding channel can be realized. Figure 7 shows the measurement results of the WDMPON monitoring experiment, and from Fig. 7 it can be seen that the clear peak on the power spectrum at 15 km on Channel 1, at 16 km on Channel 2 and at 25 km on the COM port respectively. The relationship between the wavelength of the laser and the tuning current is shown as Fig. 7(d).
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15116
Fig. 7. Results of WDM-PON monitoring. (a) The detection of the fiber break at 15km on Channel 1 of the AWG, the wavelength of the output laser was 1555.745nm. (b) The detection of the fiber break at 16 km on Channel 2 of the AWG, the wavelength of the output laser was 1556.553nm. (c) The detection of the fiber break at 25 km on the COM port. (d) The relationship between the wavelength and the tuning current.
4. Conclusion In this paper, an improved wavelength coded optical time domain reflectometry which used a low speed tunable DBR laser to achieve fiber break detection was proposed. In the scheme, 15MHz line width beat signal at 50km and a 2.5m spatial resolution at 70km was obtained by detecting the G605 fiber break. The system can be used to detect the fiber break by observing the beat signal at specified frequency and specified time; therefore it is possible to make further efforts by do some signal processing to improve the ability of the system. And an experiment was also carried to demonstrate the ability of being the tuning-OTDR for WDMPON monitoring. Furthermore, the paper also provides a method to evaluate the dynamic characteristic of the DBR laser based on STFT as described in section 2. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under Grants 61106069, 61021003, 61090391, 61377070, 61307084, 61275079, 61275078, and by the National Basic Research Program of China under Grants 2014CB340102 and 2012CB315702.
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15117
Abstract: This paper presents an improved wavelength coded time-domain reflectometry based on the 2 × 1 optical switch. In this scheme, in order to improve the signal-noise-ratio (SNR) of the beat signal, the improved system used an optical switch to obtain wavelength-stable, low-noise and narrow optical pulses for probe and reference. Experiments were set up to demonstrate a spatial resolution of 2.5m within a range of 70km and obtain the beat signal with line width narrower than 15MHz within a range of 50km in fiber break detection. A system for wavelength-divisionmultiplexing passive optical network (WDM-PON) monitoring was also constructed to detect the fiber break of different channels by tuning the current applied on the gating section of the distributed Bragg reflector (DBR) laser. ©2014 Optical Society of America OCIS codes: (120.0120) Instrumentation, measurement, and metrology; (060.2370) Fiber optics sensors; (060.5625) Radio frequency photonics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
A. J. Rogers, “Polarization-optical time domain reflectometry: A technique for the measurement of field distributions,” Appl. Opt. 20(6), 1060–1074 (1981). M. K. Barnoski and S. M. Jensen, “Fiber waveguides: a novel technique for investigating attenuation characteristics,” Appl. Opt. 15(9), 2112–2115 (1976). M. Tateda and T. Horiguchi, “Advances in optical time-domain reflectometry,” J. Lightwave Technol. 7(8), 1217–1224 (1989). N. Park, J. Lee, J. Park, J. G. Shim, H. Yoon, J. H. Kim, K. Kim, J.-O. Byun, G. Bolognini, D. Lee, and F. Di Pasquale, “Coded optical time domain reflectometry: Principle and applications,” Proc. SPIE 6781, 678129 (2007). Z. Zhang and X. Bao, “Distributed optical fiber vibration sensor based on spectrum analysis of polarizationOTDR system,” Opt. Express 16(14), 10240–10247 (2008). J. Brendel, “High-resolution photon-counting OTDR for PON testing and monitoring,” in OFC Technical Digest (2008), pp. 945–949. N. H. Zhu, J. H. Ke, H. G. Zhang, W. Chen, J. G. Liu, L. J. Zhao, and W. Wang, “Wavelength coded optical time-domain reflectometry,” J. Lightwave Technol. 28(6), 972–977 (2010). M. Han, Y. Wang, and A. Wang, “Grating-assisted polarization optical time-domain reflectometry for distributed fiber-optic sensing,” Opt. Lett. 32(14), 2028–2030 (2007). N. Linze, P. Mégret, and M. Wuilpart, “Development of an intrusion sensor based on a polarization-OTDR system,” IEEE Sens. J. 12(10), 3005–3009 (2012). K. N. Choi and H. F. Taylor, “Spectrally stable Er-fiber laser for application in phase-sensitive optical timedomain reflectometry,” IEEE Photon. Technol. Lett. 15(3), 386–388 (2003). M. Wegmuller, F. Scholder, and N. Gisin, “Photon-counting OTDR for local birefringence and fault analysis in the metro environment,” J. Lightwave Technol. 22(2), 390–400 (2004). G. Ribordy, N. Gisin, O. Guinnard, D. Stucki, M. Wegmuller, and H. Zbinden, “Photo counting at telecom wavelengths with commercial In-GaAs/InP avalanche photodiodes: Current performance,” J. Mod. Opt. 51, 1381–1398 (2004). P. Eraerds, M. Legre, J. Zhang, H. Zbinden, and N. J. Gisin, “Photon counting OTDR: advantages and limitations,” J. Lightwave Technol. 28(6), 952–964 (2010). B. F. Levine, C. G. Bethea, and J. C. Campbell, “Room‐temperature 1.3‐μm optical time domain reflectometer using a photon counting InGaAs/InP avalanche detector,” Appl. Phys. Lett. 46(4), 333–335 (1985).
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15111
15. M. Legré, R. T. Thew, H. Zbinden, and N. Gisin, “High resolution optical time domain reflectometer based on 1.55mum up-conversion photon-counting module,” Opt. Express 15(13), 8237–8242 (2007). 16. M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischka, W. R. Trutna, Jr., and S. Foster, “Realtime long range complementary correlation optical time domain reflectometer,” J. Lightwave Technol. 7(1), 24– 38 (1989). 17. Y. C. Wang, B. J. Wang, and A. B. Wang, “Chaotic correlation optical time domain reflectometer utilizing laser diode,” IEEE Photon. Technol. Lett. 20(19), 1636–1638 (2008). 18. J. Buus and E. J. Murphy, “Tunable lasers in optical networks,” J. Lightwave Technol. 24(1), 5–11 (2006). 19. I. Daubechies, “The wavelet transform, time-frequency localization and signal analysis,” IEEE Trans. Inf. Theory 36(5), 961–1005 (1990).
1. Introduction Distributed optical fiber sensing has been extensively investigated in past decades. And the most widely used approaches are based on the optical time domain reflectometry (OTDR) [1– 7]. It has been a very important diagnostic tool for the testing of fiber transmission systems and components since it was first demonstrated by Barnoski and Jensen [2]. Nevertheless, in order to improve the performance of OTDR, many techniques are still proposed, such as polarization OTDR [1,5,8,9], phase-sensitive OTDR [10], photon-counting OTDR [11–15] and correlation OTDR [16,17]. Besides the new technologies mentioned above, the wavelength coded time-domain reflectometry (OTDR) is a new kind of optical heterodyne technique which is recently developed. The probe and reference optical pulses have different wavelengths to enable the optical heterodyne detection, which can significantly improve the performance of the system. But since the spatial resolution of the system is related with the width of the probe and the reference optical pulse, the width of the probe and reference pulses is usually reduced to 200ns or even shorter [7]. As a result, the distributed Bragg reflector (DBR) laser should be tuned from one wavelength to another wavelength and be stable within 200ns or even shorter. Because of this strict requirement for the DBR laser [18], the application of the wavelength coded OTDR is limited. Furthermore, the line width of the beat signal is also affected by the instability of the probe pulse and reference optical pulse wavelength. In this paper, an improved wavelength coded OTDR based on the 2 × 1 optical switch was proposed. In this scheme, the current applied on the phase section of the DBR laser is modulated by a fixed periodic electrical signal. And one optical switch is adopted to get the narrow probe and reference optical pulses. In every detecting period, the output light of the DBR laser can pass through the optical switch after the DBR laser has established the needed wavelength and has been stable. Therefore, the tuning speed of the DBR laser has no relationship with the line width of the beat signal. Then the system has no requirement for the tuning speed of the DBR laser, and the line width of the beat signal is narrow. Finally, the experiments were set up to demonstrate the ability of the wavelength coded OTDR system. In the first experiment, the fiber breaks at the fiber distance of 50km and 70km at the corresponding to the FC/PC connector at the end of the fiber were detected by the system. In the second experiment, the wavelength coded system was used to monitor the wavelengthdivision-multiplexing passive optical network (WDM-PON) and the fiber breaks at the channels and the COM port of the arrayed waveguide grating (AWG) were successfully detected. 2. Dynamic characteristic of the DBR laser 2.1 Experiment architecture Preparing for the improved wavelength coded OTDR system, an experimental system was setup to demonstrate the dynamic characteristic of the DBR laser used in the system, as shown in Fig. 1. A square waveform voltage was applied to the phase section of the DBR laser. Corresponding to the high voltage VH and the low voltage VL , the laser generates two different light waves at different time. A 50 Ω resistor was used between the waveform
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15112
generator and the laser to limit the current of the phase section. Coupler 1 was employed to separate the light from the DBR laser into two paths, and delayed fiber was inserted into one path, then the light of the two paths was coupled into the photo-detector by coupler 2. The period of the square waveform T0 was set as: T0 = 2Δt = 250μ s (1) where Δt is the delay time of the inserted 25km delay fiber corresponding to a delay time of 125 μ s . As a result, the light waves at different wavelengths from the two paths can fully beat in the photo-detector. The VL was set to 0 V and the VH was set to 6 V, and in this case the beat signal is at 1.06 GHz. The output signal of the photo-detector were then sampled simultaneously and stored by the oscilloscope.
Fig. 1. Experiment setup for dynamic characteristic measurement of the DBR laser.
2.2 Measurement for dynamic characteristic of DBR laser
Fig. 2. Short time Fourier transform result of the beat signal.
Since the frequency of the beat signal varies with time, it is necessary to observe the dynamic characteristic of the laser in the time-frequency domain. At present, short time Fourier transform and wavelets analysis are the main methods for the time-frequency domain analysis. Comparing with wavelets analysis, the frequency resolution of STFT is fixed and the arithmetic of STFT is simple [19], so it is more specifically suited to our application. Figure 2 shows the STFT result of the beat signal using a 1024 point Hamming window. The overlap was set as 768 points. From Fig. 2, it can be seen that, the wavelength of the output laser slowly increases after the high voltage VH has been applied on the phase section of the DBR laser, and with the increasing of the wavelength, the line width of the beat signal and the noise level decreases together. The output light is stable after 80 μ s , so the DBR laser cannot be used in the classical wavelength coded OTDR which uses 200 ns or even shorter optical pulse as the probe and reference signal in the experiment [7]. Therefore, the improved
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15113
system proposed can avoid the restrict requirement of the laser and expand the application range of the wavelength coded OTDR. 3. System ability 3.1 System architecture
Fig. 3. System architecture (EDFA: erbium-doped fiber amplifier).
The improved wavelength coded OTDR was set up to locate the break of the tested fiber as shown in Fig. 3. A Michelson interferometer was employed in the system. The DBR laser was chosen as the wavelength coded light source. A square waveform provided by waveform generator 1 was applied to the phase section of the DBR laser. The DBR laser was connected to the port 1 of the 2 × 1 optical switch, and a tunable laser was connected to the port 2 of the optical switch. And the optical switch was controlled by waveform generator 2. The output of the optical switch was split into two paths by optical fiber coupler 1. One path was amplified by the EDFA and then launched into coupler 3 as the probe optical signal, and the other path is used as the reference optical pulse. A fraction of the probe signal reflected by the break in the tested fiber was recombined with the reference light by coupler 2, and then launched into a high-speed photo-detector.
Fig. 4. Operation principle of the system.
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15114
The operation principles of the reflectometry are described by the schematic in Fig. 4. The wavelength of the output light from the tunable laser was λ3 and the wavelength of the output light from the DBR laser switched between λ1 and λ2 . The wavelength of DBR laser and the tunable laser were chosen in such a way that the wavelength difference between λ2 and λ3 was so large that the beat signal was far beyond our observing frequency range and the frequency difference between λ1 and λ2 was set as at 1.06 GHz. In addition, the optical signal at λ3 was used to suppress the noise of EDFA [1]. The delay time Ts between the positive edges of waveform 1 and the controlling signal of optical switch P1 must be set larger than 80 μ s , which can enable the DBR to establish the stable wavelength. Using this method, high-speed detection with low tunable speed DBR laser can be achieved. While keeping the period of waveform 1 fixed, vary the delay time T1 between P1 and P2 . If the 1.06 GHz signal appears, then the fiber break is located. It is obvious that the beat signal appears when T1 = 2nl / c
(2)
where, n is the refractive index of the fiber, c is the light speed in free space, and l is the distance from the fiber break to the system. 3.2 Results of fiber break detection and analysis
Fig. 5. Results of fiber break detection. (a) Measured power spectrum of the beat signal at the fiber distance of 50km at the corresponding to the FC/PC connector at the end of the fiber. (b) The line width of the beat signal of Fig. 5(a). (c) Measured power spectrum of the beat signal at the fiber distance of 70km at the corresponding to the FC/PC connector at the end of the fiber. (d) Distance resolution of the beat signal at 70km.
Figure 5(a) shows the measured spectra of the beat signal when the break locates at approximately 50 km, in addition, the delay time T1 has been converted to the fiber distance.
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15115
Figure 5(b) shows the beat signal at 50503.5m from which it can be seen that the line width of the beat signal is 15MHz, narrower than the classical wavelength coded OTDR. And the beat signal with narrower line width can be detected within narrower bandwidth, so the noise level can be reduced and the sensitivity can be improved significantly. Figures 5(c) and 5(d) shows the measured spectra at 70km from which it can be seen that the envelope of the peak values shifted when a 2.5m fiber was inserted at the end of the tested fiber. Therefore a spatial resolution of 2.5m was obtained within the range of 70km, and the 2.5 m is the minimum error value of 70 km fiber in this system. The measurement error mainly results from the fluctuation of the fiber length due to the temperature fluctuation and the optical pulse broadening during the propagation in the 70km tested fiber. In conclusion, comparing with the classical wavelength coded OTDR, both the resolution and the measuring range of the system have been improved [7]. 3.3 WDM-PON monitoring
Fig. 6. Architecture of the WDM-PON monitoring system based on wavelength coded OTDR.
In this section, an experiment was set up to demonstrate the performance of the WDM-PON monitoring system based on the wavelength coded OTDR. The architecture of the experiment is shown as Fig. 6. In Fig. 6(a), the wavelength coded OTDR was connected to the third channel of the AWG for the detection of the fiber break of the COM port. By adjusting the tuning current of applied on the gating section of the DBR laser, the central wavelength of the output laser can be set as 1557.361nm, which is the central wavelength of Channel 3 of AWG. In Fig. 6(b), the wavelength coded OTDR was connected to the COM port of the AWG. Similarly, by adjusting the current applied on the gating section of the DBR, the output laser with the different wavelength can be obtained, and the fiber break detection of the corresponding channel can be realized. Figure 7 shows the measurement results of the WDMPON monitoring experiment, and from Fig. 7 it can be seen that the clear peak on the power spectrum at 15 km on Channel 1, at 16 km on Channel 2 and at 25 km on the COM port respectively. The relationship between the wavelength of the laser and the tuning current is shown as Fig. 7(d).
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15116
Fig. 7. Results of WDM-PON monitoring. (a) The detection of the fiber break at 15km on Channel 1 of the AWG, the wavelength of the output laser was 1555.745nm. (b) The detection of the fiber break at 16 km on Channel 2 of the AWG, the wavelength of the output laser was 1556.553nm. (c) The detection of the fiber break at 25 km on the COM port. (d) The relationship between the wavelength and the tuning current.
4. Conclusion In this paper, an improved wavelength coded optical time domain reflectometry which used a low speed tunable DBR laser to achieve fiber break detection was proposed. In the scheme, 15MHz line width beat signal at 50km and a 2.5m spatial resolution at 70km was obtained by detecting the G605 fiber break. The system can be used to detect the fiber break by observing the beat signal at specified frequency and specified time; therefore it is possible to make further efforts by do some signal processing to improve the ability of the system. And an experiment was also carried to demonstrate the ability of being the tuning-OTDR for WDMPON monitoring. Furthermore, the paper also provides a method to evaluate the dynamic characteristic of the DBR laser based on STFT as described in section 2. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under Grants 61106069, 61021003, 61090391, 61377070, 61307084, 61275079, 61275078, and by the National Basic Research Program of China under Grants 2014CB340102 and 2012CB315702.
#209356 - $15.00 USD (C) 2014 OSA
Received 2 Apr 2014; revised 2 Jun 2014; accepted 3 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015111 | OPTICS EXPRESS 15117
