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Fault detection technique for wavelength division multiplexing passive optical network using chaotic fiber laser
Optical Fiber Technology 20 (2014) 163–167
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Optical Fiber Technology www.elsevier.com/locate/yofte
Fault detection technique for wavelength division multiplexing passive optical network using chaotic fiber laser Naijun Xu a, Lingzhen Yang a,b,⇑, Juan Zhang a, Xiangyuan Zhang a, Juanfen Wang a, Zhaoxia Zhang a, Xianglian Liu a a b
College of Physics and Optoelectronics, Taiyuan University of Technology, Shanxi, Taiyuan 030024, China Lab of Advanced Transducers and Intelligent Control System, Ministry of Education, Taiyuan University of Technology, Shanxi, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 10 November 2013 Revised 11 January 2014 Available online 11 February 2014 Keywords: Chaos Fault location Wavelength division multiplexing passive optical network Fiber ring laser
a b s t r a c t We propose a fault localization method for wavelength division multiplexing passive optical network (WDM-PON). A proof-of-concept experiment was demonstrated by utilizing the wavelength tunable chaotic laser generated from an erbium-doped fiber ring laser with a manual tunable fiber Bragg grating (TFBG) filter. The range of the chaotic lasing wavelength can cover the C-band. Basing on the TFBG filter, we can adjust the wavelength of the chaotic laser to match the WDM-PON channel with identical wavelength. We determined the fault location by calculating the cross-correlation between the reference and return signals. Analysis of the characteristics of the wavelength tunable chaotic laser showed that the breakpoint, the loose connector, and the mismatch connector could be precisely located. A dynamic range of approximately 23.8 dB and a spatial resolution of 4 cm, which was independent of the measuring range, were obtained. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Wavelength division multiplexing passive optical networks (WDM-PONs) are emerging as next generation access networks because they offer several advantages, such as large capacity, network security, easy management, and upgradability. A WDM-PON system can carry high-capacity data to numerous users, suggesting that fault monitoring is essential to ensuring system reliability. Thus, optical time domain reflectometry (OTDR) is crucial in the communication systems to reduce the downtime caused by a fault in WDM-PON. However, conventional OTDR only operates at a single wavelength and is thus not suitable for WDM-PON because of the arrayed waveguide grating (AWG) placed at the remote node. This unsuitability is caused by the blocking of the OTDR pulse at remote node. If a fault occurs in the drop fiber, the location cannot be identified. Tunable OTDR has been proposed by using wavelength tunable laser pulse to solve this problem [1–3]. However, the tradeoff between dynamic range and spatial resolution exist in pulse OTDR. The utilization of ultrashort pulse in improving spatial resolution requires complicated techniques and increases operational costs. A number of researchers have
⇑ Corresponding author at: College of Physics and Optoelectronics, Taiyuan University of Technology, Shanxi, Taiyuan 030024, China. E-mail addresses: offi[email protected], offi[email protected] (L. Yang). http://dx.doi.org/10.1016/j.yofte.2014.01.012 1068-5200/Ó 2014 Elsevier Inc. All rights reserved.
proposed wavelength tunable sequence OTDR [4,5], which utilizes a wavelength tunable or swept laser modulated by an electrical pseudo-random sequence as probe light. This method can enhance dynamic range without reducing spatial resolution. However, spatial resolution remains limited by the electronic bandwidth. In addition, fault localization by reusing the downstream light sources has been proposed [6]. This method utilizes an OTDR pulse, instead of data, to modulate the downstream light source. However, the dynamic range and the spatial resolution remain dependent on the peak power of the OTDR pulse. A number of researchers proposed the use of broadband light sources in detecting faults in WDM-PON [7–10]. However, optical reflectors, such as mirror, fiber Bragg grating, and reflector of ONU vertical cavity surface-emitting laser (VCSEL) transmitter, are used at each optical network unit (ONU). These solutions require all ONU upgrades to include the optical reflectors and to use VCSELs as upstream transmitters. As the monitoring system of WDM-PON, it should be highly efficient and precise in detecting a fault but should reduce operational costs. Chaotic OTDR has been recently reported [11,12] as a new method for fault detection; however, the longitudinal mode spacing of the laser is unequal to that of the AWG, indicating that the number of match modes is limited and that more channels cannot be detected. The output wavelength tunable chaotic laser of the Fabry–Perot laser is influenced by the feedback strength and the
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wavelength mismatch between the filter and laser modes; thus, the output wavelength of chaotic lasers cannot be continuously tuned [11]. Moreover, the autocorrelation detection of the chaotic signal induces side lobes [12], and the chaotic laser is optical wideband signal, so the dynamic range is limited by the low power of single wavelength after filtering [13]. However, side lobes bring noise into the measurement and affect the dynamic range. A high side lobe may become a ‘‘ghost peak,’’ which causes fault misjudgment. In this paper, we propose a new tunable chaotic OTDR to monitor WDM-PON. We use the wavelength tunable chaotic laser generated from an erbium-doped fiber (EDF) ring laser as a probe light; such laser can provide the identical wavelength of all branches in WDM-PON. In addition, the characteristics of the wavelength tunable chaotic laser are analyzed, and the results show that the performance of the proposed tunable chaotic OTDR has been successfully evaluated. 2. Characteristics of wavelength tunable chaotic OTDR Fig. 1 shows the experimental setup of the proposed tunable chaotic OTDR for monitoring WDM-PON. The wavelength tunable chaotic laser is shown in the dashed box. The length of cavity is about 10 km. The cavity of the chaotic fiber laser comprises a 980 nm laser diode (LD) with the maximum pump power of 250 mW used to pump EDF through a 980/1550 nm wavelength division multiplexer (WDM). A manual tunable fiber Bragg grating filter (OETFG-100, Tunable range: 30 nm, Resolution: 0.1 nm) with 3 dB bandwidth of 0.6 nm is used to satisfy that of the AWG. To confirm that no significant power loss occurs at the AWG because of spectral mismatch, the 3 dB pass bandwidth of AWG is approximately 0.7 nm and the channel spacing of AWG is 100 GHz in our experiments. A polarization controller (PC) changes the polarization states of the light, and an optical isolator ensures the unidirectional transmission of the light in the cavity. The output of the chaotic laser is obtained from the 10% output of the optical coupler (OC1). The chaotic laser is then split into two beams by a 95:5 optical coupler (OC2). The chaotic laser from the 95% port acts as the probe signal, whereas the other acts as the reference signal. For detection, the probe signal is launched into a WDM-PON under test and received by an optical circulator (OC3). The reference signal and the return signal were converted into electrical signals by two identical photoelectric detectors (1 GHz bandwidth). Finally, a real-time oscilloscope (LeCory SDA806Zi-A, 6 GHz bandwidth) records the reference and return signals. We record signals with length of 0.2 ms (2 M points with sampling rate of 10 Gsa/s). A computer is used to process the data in accordance with cross correlation method.
In our experiments, we set the bias current of the 980 nm LD to 400 mA. By adjusting the PC to a proper state and tuning the TFBG filter with the tunable range of 30 nm from 1530 nm to 1560 nm, the wavelength of the chaotic laser can be adjusted to match the channel of the AWG with identical wavelength. An optical spectrum analyzer (Agilent86140B), a radio frequency (RF) spectrum analyzer (AgilentN9020A), and a real time oscilloscope are used to observe the chaotic laser. We use a PD with 12 GHz bandwidth to effectively observe the characteristics of the chaotic laser. The amplified spontaneous emission (ASE) from an EDF has a broad spectral range that covers the entire C-band, as shown in Fig. 2(a). Thus, the wavelength of the chaotic laser can be tuned to match the channel of the WDM-PON with identical wavelength because of this broad spectral range. In our experiments, the number of the chaotic wavelength is limited by the tunable range of the TFBG filter (Fig. 2(b)). In addition, the characteristics of chaotic laser, including time series, RF spectra, and autocorrelation curves, are analyzed at wavelength of 1554.04 nm (Fig. 3.) The time series of the chaotic laser presented noise-like characteristics (Fig. 3(b)). Chaotic signals have a wideband spectrum, which can reach 12 GHz (Fig. 3(c)). The inset in Fig. 3(c) shows the details of the chaotic signal with a span of 300 MHz. Because of the ring cavity in our experiment has its own unique feature, it can be seen from Fig. 3(c) that the spectrum is flat and no information about the cavity. The autocorrelation curves of the chaotic laser signal have a delta-function-like characteristic, which is not related to the change in wavelength (Fig. 3(d)). The comparison shows that the outputs at other wavelengths have similar time and frequency domain characteristics. Therefore, chaotic laser signal can be used as an ideal ranging signal. 3. Experiment results Considering the delta-function-like characteristic of the autocorrelation curve of the chaotic laser, we calculate the cross-correlation between the reference and return signals to locate the fault position. The function of the reference signal is expressed by x(t); the return signal is denoted by k x(t s); k is the loss coefficient; and s is the delay time with respect to the reference signal transmission in the fiber. The cross-correlation function of the two signals can be expressed as follows:
xðtÞ k xðt sÞ k dðsÞ where represent the convolution operation. We can locate the fault position by calculating c s/2n, where c is the light velocity in a vacuum and n is the refractive index of fiber. We obtain the location of fault according to the position of correlation peak.
Fig. 1. Experimental setup of the proposed technique for the localization of the failures in WDM-PON.
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Fig. 2. (a) Output ASE spectrum from erbium-dope fiber with 980 nm laser diode as pump. (b) Optical spectra of 32 channels–the wavelengths of chaotic laser are from 1533.22 nm to 1558.02 nm with spacing of 0.8 nm.
Fig. 3. The characteristics of the wavelength tunable chaotic laser. (a), (b), (c), (d), show the spectrum, time series, RF spectra, and autocorrelation properties of chaotic laser at wavelengths of 1554.04 nm, respectively.
Fig. 4. Experiment results for channels at wavelength of (a) 1553.32 nm, (b) 1554.82 nm, (c) 1556.44 nm, (d) 1557.22 nm, respectively.
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A WDM-PON model is built to evaluate the performance of the proposed tunable chaotic OTDR. The feeder line length is approximately 718 m, followed by a 1 32 AWG with 100 GHz channel spacing. The fault detection for WDM-PON is primarily aimed at the feature of its multidrop and dense network node. A fault that occurs in the feeder line of WDM-PON can be easily detected, as shown in Fig. 4(a). For this reason, the length of the feeder line is kept short in the experiments. The detection of branch fiber is also analyzed. Three AWG channels, namely, 1554.82, 1556.44, and 1557.22 nm, are selected. In the first channel, an optical fiber is inserted after the AWG connect by FC/PC connector, as shown in Fig. 4(b). Two correlation peaks are identified. The first correlation peak represents the AWG location, whereas the second correlation peak at 881.23 m indicates the Fresnel reflection of the unterminated fiber end. In the second channel, two optical fibers are inserted after the AWG and an FC/PC connector is used between the two optical fibers. If the FC/PC connector becomes quite loose, such event can be clearly detected by our proposed tunable chaotic OTDR. As shown in Fig. 4(c), the second correlation peak at 833.23 m is the location of the loose FC/PC connector. At the channel of 1557.22 nm, the two fibers are connected by two mismatch connectors (as shown in the inset), namely, FC/PC connector and angle physical connector (FC/APC). As shown in Fig. 4(d), the short correlation peak after AWG is the location of the two mismatch connectors at 767.32 m. 3.1. Spatial resolution The full width at half maximum (FWHM) of correlation curve determines the spatial resolution (Dz) of chaotic laser ranging. Spatial resolution can be calculated as follows:
Dz ¼ c sFWHM =2n where c is the velocity of light in a vacuum, and n is the refractive index of fiber. However, FWHM is restricted by the acquired bandwidth in the experiment. When we use a PD with bandwidth of 1 GHz, FWHM is approximately 0.4 ns, which corresponds to approximately 4 cm of spatial resolution. When a PD with bandwidth of 12 GHz is used, FWHM is approximately 0.07 ns, and the spatial resolution is approximately 7 mm (Fig. 5(a)). The channel of 1553.32 nm is selected for the experiments to evaluate the spatial resolution. A 3 dB coupler is placed after the branch fiber, and the 3 dB coupler is then connected with a fiber and a tunable fiber delay-line. By tuning the fiber delay-line, we can obtain the spatial resolution through the dual reflection events. Dual reflection events with spacing of approximately 4 cm can be clearly distinguished by tuning the fiber delay-line (Fig. 5(b)). Furthermore, the relationship between spatial resolution and measuring range is analyzed at correlation peaks of 1236.21 and 7855.12 m. The autocorrelation curves of the chaotic laser are compared. The results show that the three FWHM of the correlation
Fig. 6. Relationship between spatial resolution and measuring range, experiment results indicate that spatial resolution is independent of the measuring range.
peaks were similar (approximately 0.4 ns) (Fig. 6), indicating that the spatial resolution is independent of the measuring range. 3.2. Dynamic range Chaotic OTDR utilizes the cross-correlation between the reference and return signals to locate the fault. The dynamic range is defined as ratio (in dB) of the correlation peak of reflects point to the system noise; this ratio is the peak–noise ratio (PNR). We treat the launch port as the reflected point to evaluate the dynamic range of the proposed chaotic OTDR. The PNR in Fig 7(a) is approximately 26.8 dB.A tunable optical attenuator is added after the launch port to simulate the transmission loss in the fiber. Fig. 7(b) shows the PNR change by tuning the optical attenuator. The PNR drops almost linearly with increasing transmission loss. The insertion loss of AWG, which is approximately 3 dB in the experiment, is then deducted. The dynamic range is approximately 23.8 dB, which corresponds to approximately 119 km of measurable range. 4. Discussion The main advantages of the proposed method are enumerated as follows: The wavelength of chaotic laser can be continuously tuned: In our experiments, the ASE from an EDF has a broad spectral range whether the channel spacing of AWG is 100 GHz or 50 GHz. The wavelength of the chaotic laser can be adjusted to match the channel of the WDM-PON with identical wavelength, and a wavelength resolution of 0.1 nm is achieved. The range of chaotic lasing wavelength can cover the C-band and L-band working region: A widely tunable EDF ring laser has been reported [14]. Chaotic oscillation of EDF ring laser
Fig. 5. (a) Relationship between FWHM and bandwidth. (b) Spatial resolution of 4 cm.
N. Xu et al. / Optical Fiber Technology 20 (2014) 163–167
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Fig. 7. The dynamic range of tunable chaotic OTDR. (a) PNR is about 26.8 dB. (b) Relationship between PNR and transmission loss.
arises from the nonlinear Kerr effect. In our experiments, the tunable range of TFBG filter is approximately 30 nm, and the number of chaotic wavelengths is limited. In principle, if a widely tunable range of filter is used, the output wavelength of chaotic laser can cover C-band and L-band. High dynamic range: One of the important features of chaotic fiber laser is high output power. In principle, the output power can be more effectively enhanced by using a higher power pump LD so that higher dynamic range can be achieved. 5. Conclusion A WDM-PON monitoring technique based on EDF ring laser is experimentally demonstrated using wavelength tunable chaotic laser. Each branch link can be detected in accordance with correlation method, which detects not only fiber breakpoint but also loose and mismatched connectors. We believe this technique has extensive application potential for the miniaturization of WDM-PON with the development of the fiber to the home. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant 61107033 and the program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi Province, China. References [1] M. Thollabandi, T.Y. Kim, S. Hann, C.S. Park, Tunable OTDR based on direct modulation of self-injection-locked RSOA for in-service monitoring of WDMPON, IEEE Photon. Technol. Lett. 20 (15) (2008) 1323–1325.
[2] J.H. Park, J.S. Baik, C.H. Lee, Fault-localization in WDM-PON, Opt. Exp. 15 (4) (2007) 1461–1466. [3] K. Tanaka, M. Tateda, Y. Inoue, Measuring the individual attenuation distribution of passive branched optical networks, IEEE Photon. Technol. Lett. 8 (7) (1996) 915–917. [4] J. Lee, J. Park, J.G. Shim, H. Yoon, J.H. Kim, K. Kim, J.-O. Byun, N. Park, In-service monitoring of 16 port 32 wavelength bidirectional WDM-PON systems with a tunable, coded optical time domain reflectometry, Opt. Exp. 15 (11) (2007) 6874–6882. [5] W. Shin, B.A. Yu, Y.L. Lee, Y.C. Noh, D.K. Ko, K. Oh, Wavelength tunable optical time domain reflectometry based on wavelength swept fiber laser employing two-dimensional digital micro-mirror array, Opt. Commun. 282 (6) (2009) 1191–1195. [6] K.W. Lim, E.S. Son, K.H. Han, Y.C. Chung, Fault localization in WDM passive optical network by reusing downstream light sources, IEEE Photon. Technol. Lett. 17 (12) (2005) 2691–2693. [7] S.B. Park, D.K. Jung, H.S. Shin, D.J. Shin, S. Hwang, Y. Oh, C. Shin, Optical fault monitoring method using broadband light source in WDM-PON, Electron. Lett. 42 (4) (2006) 239–241. [8] K. Lee, S.B. Kang, D.S. Lim, H.K. Lee, W.V. Sorin, Fiber link loss monitoring scheme in bidirectional WDM transmission using ASE-injected FP-LD, IEEE Photon. Technol. Lett. 18 (3) (2006) 523–525. [9] S.C. Lin, S.L. Lee, C.K. Liu, C.L. Yang, S.C. Ko, T.W. Liaw, G. Keiser, Design and demonstration of REAM-based WDM-PONs with remote amplification and channel fault monitoring, J. Opt. Commun. Netw. 4 (4) (2012) 336–343. [10] E. Wong, X. Zhao, C.J. Chang-Hasnain, Upstream vertical cavity surfaceemitting laser for fault monitoring and localization in WDM passive optical networks, Opt. Commun. 281 (8) (2008) 2218–2226. [11] A. Wang, N. Wang, Y. Yang, B. Wang, M. Zhang, Y. Wang, Precise fault location in WDM-PON by utilizing wavelength tunable chaotic laser, J. Lightw. Technol. 30 (21) (2012) 3420–3426. [12] Z. Xie, L. Xia, Y. Wang, C. Yang, C. Cheng, D. Liu, Fiber fault detection with high accuracy using chaotic signal from an SOA ring reflectometry, IEEE Photon. Technol. Lett. 25 (8) (2013) 709–712. [13] L. Xia, D. Huang, J. Xu, D. Liu, Simultaneous and precise fault locating in WDMPON by the generation of optical wideband chaos, Opt. Lett. 38 (19) (2013) 3762–3764. [14] S. Yamashita, M. Nishihara, Widely tunable erbium-doped fiber ring laser covering both C-band and L-band, IEEE J. Sel. Top. Quant. 7 (1) (2001) 41–43.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Fault detection technique for wavelength division multiplexing passive optical network using chaotic fiber laser Naijun Xu a, Lingzhen Yang a,b,⇑, Juan Zhang a, Xiangyuan Zhang a, Juanfen Wang a, Zhaoxia Zhang a, Xianglian Liu a a b
College of Physics and Optoelectronics, Taiyuan University of Technology, Shanxi, Taiyuan 030024, China Lab of Advanced Transducers and Intelligent Control System, Ministry of Education, Taiyuan University of Technology, Shanxi, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 10 November 2013 Revised 11 January 2014 Available online 11 February 2014 Keywords: Chaos Fault location Wavelength division multiplexing passive optical network Fiber ring laser
a b s t r a c t We propose a fault localization method for wavelength division multiplexing passive optical network (WDM-PON). A proof-of-concept experiment was demonstrated by utilizing the wavelength tunable chaotic laser generated from an erbium-doped fiber ring laser with a manual tunable fiber Bragg grating (TFBG) filter. The range of the chaotic lasing wavelength can cover the C-band. Basing on the TFBG filter, we can adjust the wavelength of the chaotic laser to match the WDM-PON channel with identical wavelength. We determined the fault location by calculating the cross-correlation between the reference and return signals. Analysis of the characteristics of the wavelength tunable chaotic laser showed that the breakpoint, the loose connector, and the mismatch connector could be precisely located. A dynamic range of approximately 23.8 dB and a spatial resolution of 4 cm, which was independent of the measuring range, were obtained. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Wavelength division multiplexing passive optical networks (WDM-PONs) are emerging as next generation access networks because they offer several advantages, such as large capacity, network security, easy management, and upgradability. A WDM-PON system can carry high-capacity data to numerous users, suggesting that fault monitoring is essential to ensuring system reliability. Thus, optical time domain reflectometry (OTDR) is crucial in the communication systems to reduce the downtime caused by a fault in WDM-PON. However, conventional OTDR only operates at a single wavelength and is thus not suitable for WDM-PON because of the arrayed waveguide grating (AWG) placed at the remote node. This unsuitability is caused by the blocking of the OTDR pulse at remote node. If a fault occurs in the drop fiber, the location cannot be identified. Tunable OTDR has been proposed by using wavelength tunable laser pulse to solve this problem [1–3]. However, the tradeoff between dynamic range and spatial resolution exist in pulse OTDR. The utilization of ultrashort pulse in improving spatial resolution requires complicated techniques and increases operational costs. A number of researchers have
⇑ Corresponding author at: College of Physics and Optoelectronics, Taiyuan University of Technology, Shanxi, Taiyuan 030024, China. E-mail addresses: offi[email protected], offi[email protected] (L. Yang). http://dx.doi.org/10.1016/j.yofte.2014.01.012 1068-5200/Ó 2014 Elsevier Inc. All rights reserved.
proposed wavelength tunable sequence OTDR [4,5], which utilizes a wavelength tunable or swept laser modulated by an electrical pseudo-random sequence as probe light. This method can enhance dynamic range without reducing spatial resolution. However, spatial resolution remains limited by the electronic bandwidth. In addition, fault localization by reusing the downstream light sources has been proposed [6]. This method utilizes an OTDR pulse, instead of data, to modulate the downstream light source. However, the dynamic range and the spatial resolution remain dependent on the peak power of the OTDR pulse. A number of researchers proposed the use of broadband light sources in detecting faults in WDM-PON [7–10]. However, optical reflectors, such as mirror, fiber Bragg grating, and reflector of ONU vertical cavity surface-emitting laser (VCSEL) transmitter, are used at each optical network unit (ONU). These solutions require all ONU upgrades to include the optical reflectors and to use VCSELs as upstream transmitters. As the monitoring system of WDM-PON, it should be highly efficient and precise in detecting a fault but should reduce operational costs. Chaotic OTDR has been recently reported [11,12] as a new method for fault detection; however, the longitudinal mode spacing of the laser is unequal to that of the AWG, indicating that the number of match modes is limited and that more channels cannot be detected. The output wavelength tunable chaotic laser of the Fabry–Perot laser is influenced by the feedback strength and the
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wavelength mismatch between the filter and laser modes; thus, the output wavelength of chaotic lasers cannot be continuously tuned [11]. Moreover, the autocorrelation detection of the chaotic signal induces side lobes [12], and the chaotic laser is optical wideband signal, so the dynamic range is limited by the low power of single wavelength after filtering [13]. However, side lobes bring noise into the measurement and affect the dynamic range. A high side lobe may become a ‘‘ghost peak,’’ which causes fault misjudgment. In this paper, we propose a new tunable chaotic OTDR to monitor WDM-PON. We use the wavelength tunable chaotic laser generated from an erbium-doped fiber (EDF) ring laser as a probe light; such laser can provide the identical wavelength of all branches in WDM-PON. In addition, the characteristics of the wavelength tunable chaotic laser are analyzed, and the results show that the performance of the proposed tunable chaotic OTDR has been successfully evaluated. 2. Characteristics of wavelength tunable chaotic OTDR Fig. 1 shows the experimental setup of the proposed tunable chaotic OTDR for monitoring WDM-PON. The wavelength tunable chaotic laser is shown in the dashed box. The length of cavity is about 10 km. The cavity of the chaotic fiber laser comprises a 980 nm laser diode (LD) with the maximum pump power of 250 mW used to pump EDF through a 980/1550 nm wavelength division multiplexer (WDM). A manual tunable fiber Bragg grating filter (OETFG-100, Tunable range: 30 nm, Resolution: 0.1 nm) with 3 dB bandwidth of 0.6 nm is used to satisfy that of the AWG. To confirm that no significant power loss occurs at the AWG because of spectral mismatch, the 3 dB pass bandwidth of AWG is approximately 0.7 nm and the channel spacing of AWG is 100 GHz in our experiments. A polarization controller (PC) changes the polarization states of the light, and an optical isolator ensures the unidirectional transmission of the light in the cavity. The output of the chaotic laser is obtained from the 10% output of the optical coupler (OC1). The chaotic laser is then split into two beams by a 95:5 optical coupler (OC2). The chaotic laser from the 95% port acts as the probe signal, whereas the other acts as the reference signal. For detection, the probe signal is launched into a WDM-PON under test and received by an optical circulator (OC3). The reference signal and the return signal were converted into electrical signals by two identical photoelectric detectors (1 GHz bandwidth). Finally, a real-time oscilloscope (LeCory SDA806Zi-A, 6 GHz bandwidth) records the reference and return signals. We record signals with length of 0.2 ms (2 M points with sampling rate of 10 Gsa/s). A computer is used to process the data in accordance with cross correlation method.
In our experiments, we set the bias current of the 980 nm LD to 400 mA. By adjusting the PC to a proper state and tuning the TFBG filter with the tunable range of 30 nm from 1530 nm to 1560 nm, the wavelength of the chaotic laser can be adjusted to match the channel of the AWG with identical wavelength. An optical spectrum analyzer (Agilent86140B), a radio frequency (RF) spectrum analyzer (AgilentN9020A), and a real time oscilloscope are used to observe the chaotic laser. We use a PD with 12 GHz bandwidth to effectively observe the characteristics of the chaotic laser. The amplified spontaneous emission (ASE) from an EDF has a broad spectral range that covers the entire C-band, as shown in Fig. 2(a). Thus, the wavelength of the chaotic laser can be tuned to match the channel of the WDM-PON with identical wavelength because of this broad spectral range. In our experiments, the number of the chaotic wavelength is limited by the tunable range of the TFBG filter (Fig. 2(b)). In addition, the characteristics of chaotic laser, including time series, RF spectra, and autocorrelation curves, are analyzed at wavelength of 1554.04 nm (Fig. 3.) The time series of the chaotic laser presented noise-like characteristics (Fig. 3(b)). Chaotic signals have a wideband spectrum, which can reach 12 GHz (Fig. 3(c)). The inset in Fig. 3(c) shows the details of the chaotic signal with a span of 300 MHz. Because of the ring cavity in our experiment has its own unique feature, it can be seen from Fig. 3(c) that the spectrum is flat and no information about the cavity. The autocorrelation curves of the chaotic laser signal have a delta-function-like characteristic, which is not related to the change in wavelength (Fig. 3(d)). The comparison shows that the outputs at other wavelengths have similar time and frequency domain characteristics. Therefore, chaotic laser signal can be used as an ideal ranging signal. 3. Experiment results Considering the delta-function-like characteristic of the autocorrelation curve of the chaotic laser, we calculate the cross-correlation between the reference and return signals to locate the fault position. The function of the reference signal is expressed by x(t); the return signal is denoted by k x(t s); k is the loss coefficient; and s is the delay time with respect to the reference signal transmission in the fiber. The cross-correlation function of the two signals can be expressed as follows:
xðtÞ k xðt sÞ k dðsÞ where represent the convolution operation. We can locate the fault position by calculating c s/2n, where c is the light velocity in a vacuum and n is the refractive index of fiber. We obtain the location of fault according to the position of correlation peak.
Fig. 1. Experimental setup of the proposed technique for the localization of the failures in WDM-PON.
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Fig. 2. (a) Output ASE spectrum from erbium-dope fiber with 980 nm laser diode as pump. (b) Optical spectra of 32 channels–the wavelengths of chaotic laser are from 1533.22 nm to 1558.02 nm with spacing of 0.8 nm.
Fig. 3. The characteristics of the wavelength tunable chaotic laser. (a), (b), (c), (d), show the spectrum, time series, RF spectra, and autocorrelation properties of chaotic laser at wavelengths of 1554.04 nm, respectively.
Fig. 4. Experiment results for channels at wavelength of (a) 1553.32 nm, (b) 1554.82 nm, (c) 1556.44 nm, (d) 1557.22 nm, respectively.
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A WDM-PON model is built to evaluate the performance of the proposed tunable chaotic OTDR. The feeder line length is approximately 718 m, followed by a 1 32 AWG with 100 GHz channel spacing. The fault detection for WDM-PON is primarily aimed at the feature of its multidrop and dense network node. A fault that occurs in the feeder line of WDM-PON can be easily detected, as shown in Fig. 4(a). For this reason, the length of the feeder line is kept short in the experiments. The detection of branch fiber is also analyzed. Three AWG channels, namely, 1554.82, 1556.44, and 1557.22 nm, are selected. In the first channel, an optical fiber is inserted after the AWG connect by FC/PC connector, as shown in Fig. 4(b). Two correlation peaks are identified. The first correlation peak represents the AWG location, whereas the second correlation peak at 881.23 m indicates the Fresnel reflection of the unterminated fiber end. In the second channel, two optical fibers are inserted after the AWG and an FC/PC connector is used between the two optical fibers. If the FC/PC connector becomes quite loose, such event can be clearly detected by our proposed tunable chaotic OTDR. As shown in Fig. 4(c), the second correlation peak at 833.23 m is the location of the loose FC/PC connector. At the channel of 1557.22 nm, the two fibers are connected by two mismatch connectors (as shown in the inset), namely, FC/PC connector and angle physical connector (FC/APC). As shown in Fig. 4(d), the short correlation peak after AWG is the location of the two mismatch connectors at 767.32 m. 3.1. Spatial resolution The full width at half maximum (FWHM) of correlation curve determines the spatial resolution (Dz) of chaotic laser ranging. Spatial resolution can be calculated as follows:
Dz ¼ c sFWHM =2n where c is the velocity of light in a vacuum, and n is the refractive index of fiber. However, FWHM is restricted by the acquired bandwidth in the experiment. When we use a PD with bandwidth of 1 GHz, FWHM is approximately 0.4 ns, which corresponds to approximately 4 cm of spatial resolution. When a PD with bandwidth of 12 GHz is used, FWHM is approximately 0.07 ns, and the spatial resolution is approximately 7 mm (Fig. 5(a)). The channel of 1553.32 nm is selected for the experiments to evaluate the spatial resolution. A 3 dB coupler is placed after the branch fiber, and the 3 dB coupler is then connected with a fiber and a tunable fiber delay-line. By tuning the fiber delay-line, we can obtain the spatial resolution through the dual reflection events. Dual reflection events with spacing of approximately 4 cm can be clearly distinguished by tuning the fiber delay-line (Fig. 5(b)). Furthermore, the relationship between spatial resolution and measuring range is analyzed at correlation peaks of 1236.21 and 7855.12 m. The autocorrelation curves of the chaotic laser are compared. The results show that the three FWHM of the correlation
Fig. 6. Relationship between spatial resolution and measuring range, experiment results indicate that spatial resolution is independent of the measuring range.
peaks were similar (approximately 0.4 ns) (Fig. 6), indicating that the spatial resolution is independent of the measuring range. 3.2. Dynamic range Chaotic OTDR utilizes the cross-correlation between the reference and return signals to locate the fault. The dynamic range is defined as ratio (in dB) of the correlation peak of reflects point to the system noise; this ratio is the peak–noise ratio (PNR). We treat the launch port as the reflected point to evaluate the dynamic range of the proposed chaotic OTDR. The PNR in Fig 7(a) is approximately 26.8 dB.A tunable optical attenuator is added after the launch port to simulate the transmission loss in the fiber. Fig. 7(b) shows the PNR change by tuning the optical attenuator. The PNR drops almost linearly with increasing transmission loss. The insertion loss of AWG, which is approximately 3 dB in the experiment, is then deducted. The dynamic range is approximately 23.8 dB, which corresponds to approximately 119 km of measurable range. 4. Discussion The main advantages of the proposed method are enumerated as follows: The wavelength of chaotic laser can be continuously tuned: In our experiments, the ASE from an EDF has a broad spectral range whether the channel spacing of AWG is 100 GHz or 50 GHz. The wavelength of the chaotic laser can be adjusted to match the channel of the WDM-PON with identical wavelength, and a wavelength resolution of 0.1 nm is achieved. The range of chaotic lasing wavelength can cover the C-band and L-band working region: A widely tunable EDF ring laser has been reported [14]. Chaotic oscillation of EDF ring laser
Fig. 5. (a) Relationship between FWHM and bandwidth. (b) Spatial resolution of 4 cm.
N. Xu et al. / Optical Fiber Technology 20 (2014) 163–167
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Fig. 7. The dynamic range of tunable chaotic OTDR. (a) PNR is about 26.8 dB. (b) Relationship between PNR and transmission loss.
arises from the nonlinear Kerr effect. In our experiments, the tunable range of TFBG filter is approximately 30 nm, and the number of chaotic wavelengths is limited. In principle, if a widely tunable range of filter is used, the output wavelength of chaotic laser can cover C-band and L-band. High dynamic range: One of the important features of chaotic fiber laser is high output power. In principle, the output power can be more effectively enhanced by using a higher power pump LD so that higher dynamic range can be achieved. 5. Conclusion A WDM-PON monitoring technique based on EDF ring laser is experimentally demonstrated using wavelength tunable chaotic laser. Each branch link can be detected in accordance with correlation method, which detects not only fiber breakpoint but also loose and mismatched connectors. We believe this technique has extensive application potential for the miniaturization of WDM-PON with the development of the fiber to the home. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant 61107033 and the program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi Province, China. References [1] M. Thollabandi, T.Y. Kim, S. Hann, C.S. Park, Tunable OTDR based on direct modulation of self-injection-locked RSOA for in-service monitoring of WDMPON, IEEE Photon. Technol. Lett. 20 (15) (2008) 1323–1325.
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