The Dissertation Organization - 使用光時域反射儀之高密度分波多工系統的即時監控技術研究

Chapter 1 Introduction

1.5 The Dissertation Organization

There are five chapters organized in this dissertation. In Chapter 2, we demonstrate two improved fiber-Bragg grating based optical add-drop multiplexer structures for support OTDR-monitoring. One is the M-type structure, which is based on a single multiport optical circulator (MOC) and the other is the WDM structure, which is based on a pair of WDM couplers. We demonstrate the function and 10-Gb/s bit-error-rate (BER) system performance of such OADMs for simultaneous add-drop and in-service supervision operations for the first time. The investigation result is important for designing DWDM systems with various OADM nodes to enhance system reliability

In Chapter 3, an in-service supervisory 10-Gb/s DWDM system containing MZ-FBG OADM’s is demonstrated. The link fault location is realized simply by using a conventional OTDR directly through the used MZ-FBG OADM without needs of re-structuring OADM configuration and extra optical components in the OADM node. The 1.65-μm OTDR supervision on such 10-Gb/s 80-km system link is achieved without degrading the system performance.

In Chapter 4, we demonstrate an in-service supervisory 10-Gb/s DWDM system with silica FRAs in forward pump and backward pump schemes. The link fault location is realized simply by using a conventional OTDR and negligible system power penalty, due to the OTDR monitoring, is achieved.

Chapter 5 gives a brief conclusion of this work and research.

Chapter 2 In-Service OTDR-Monitoring-Supported Fiber-Bragg-Grating Optical Add-Drop

Multiplexers

OADMs have been widely deployed to enable greater connectivity and flexibility in DWDM systems and networks. Among them, the FBG based OADMs using a pair of optical circulators [15-19] or employing a single multiport optical circulator [20] seem to be promising, especially for add-drop operation of pre-arranged wavelength channels, since they have the features of simple structure, low loss, and potential low cost. In these systems with OADM nodes, the system should have the in-service fault-location monitoring capability, without sacrificing active traffic, to enhance network reliability and to shorten the network downtime.

Since OTDR is a popular tool to provide the in-service monitoring in fiber-optic transmission systems [21-23], OTDR-monitoring-supported OADM is an interesting and important problem.However, the optical circulators in the FBG-based OADMs may block the propagation of Rayleigh backscattered light of the OTDR probe pulses, and thus they inhibit the OTDR monitoring. Therefore, this problem should be solved.

In this chapter, two OTDR-monitoring-supported FBG-OADMs for in-service fault-location monitoring through a conventional OTDR are investigated. One is the M-type structure, which is based on a single MOC and the other is the WDM structure, which is based on a pair of WDM couplers. The idea of two proposed OADM structures seem straight forward by proper arrangement of port connection through a pair of WDM couplers or a MOC, however, the function demonstration, and 10-Gb/s BER system performance, of such OADMs for simultaneous add-drop and in-service supervision operations are investigated and compared for the first time.

The investigation result is important for designing DWDM systems with various OADM nodes to enhance system reliability.

2.1 Review of Fiber Bragg Grating-Based OADM Systems

The DWDM technique combining with EDFA has shown its capability to cost-effectively, gracefully upgrade the capacity of embedded long-distance transmission systems operating in the 1.55-μm wavelength region [24]. In the network system, the new optical elements are required to provide additional facilities for WDM signals locally transmitting/extraction. Optical add drop multiplexer will play a key role enabling greater connectivity and flexibility in DWDM network [25-27]. The importance of OADM’s is to allow the optical network to transmit/extract on wavelength-by-wavelength basis to optimize traffic, efficient network utilization, network growth, and enhance network flexibility. The fiber Bragg grating-based OADM is the simplest structure for add/drop operation in DWDM system.

2.1.1 Conventional FBG-based OADM

Among FBG-based OADMs, the conventional structure using FBGs sandwiched between a pair of three-port optical circulator [28] is the simplest one.

Fig. 2.1 shows the conventional FBG-based sandwiched OADM structure. Such structure has been demonstrated over an installed 4 × 2.5 Gb/s optically amplified submarine cable system [29]. The function of C-FBG OADM is such that the multi-wavelength input enters the OADM at INPUT port and directly into the first optical circulator (OC1) port 1. The signal light is circulated clockwise and exits at the port 2 of the OC1 and into the fiber-Bragg-grating. The grating reflects the light at the center wavelength of λi, which then re-enters the same port of the OC1 and is circulated to the DROP port (OC1 port 3). The added signal enters the port 1 (ADD port) of second optical circulator (OC2) and circulated to port 2 and reflected from the grating to combine it with another channel, that are pass through the FBG , and then circulated to exit the OADM structure at OUTPUT port (port 3 of OC2).

2.1.2 Multi-port Optical Circulator FBG-based OADM

Fig. 2.2 shows the MOC-FBG-based OADM structure [30]. For this structure, a six-port optical circulator is used in combination with an FBG. The port connections

shown in Fig. 2.2 was arranged to simultaneously offer the add-drop operation. The multi-wavelength input enters the OADM at INPUT port and directly into the MOC port 1. The signal light is circulated clockwise and exits at the port 2 of the MOC and into the fiber-Bragg-grating. The grating reflects the light at the center wavelength of λi, which than re-enters the same port of the MOC and is circulated to the DROP port (MOC port 3). The added signal enters the port 4 (ADD port) of MOC and circulated to port 5 and reflected from the grating to combine it with another channel, that are pass through the FBG, and than circulated to exit the OADM structure at OUTPUT propose and demonstrate two fiber-Bragg-grating based OADM structures for in-service fault-location monitoring by using a conventional OTDR.

2.2.1 Improved Conventional FBG-based OADM (C-type)

Fig. 2.3 shows the OTDR-monitoring-supported structure of the improved conventional FBG-based OADM (hereafter C-type). The C-type structure consists of a pair of WDM couplers, an FBG-OADM, and the upper optical path for allowing the propagation of both the OTDR pulses and the Rayleigh backscattered light. The FBG-OADM in the lower optical path is composed of two three-port optical circulators (OC1 and OC2) and an FBG with a central wavelength matching the ITU wavelength, which to be dropped and added at the OADM node. In Fig. 2.3, both the OTDR pulse signal (POTDR, indicated as the solid line) and the corresponding Rayleigh backscattering light (PR,OTDR, indicated as the dashed line) bypass the FBG through the upper optical path between the WDM couplers.

2.2.2 Improved Multi-port OC FBG-based OADM (M-type)

Fig. 2.4 shows the OTDR-monitoring-supported structure of the improved

MOC-FBG-based OADM (hereafter M-type). For this structure, a six-port optical circulator is used in combination with an FBG. This structure is very similar with the reported MOC-based OADM in [3] but with different port arrangement. The port connections shown in Fig. 2.4 was arranged to simultaneously offer the add-drop and in-service OTDR-monitoring operations. The Rayleigh backscattered lights bypass the FBG and erbium-doped fiber amplifier (EDFA) through the MOC from port 3 to port 4. The EDFA and dispersion compensating fiber (DCF) indicated in Fig. 2.3 and 2.4, if required, is used to compensate the losses and chromatic dispersion of the system link.

2.3 Demonstration of Short Distance Fiber Link

In this section, we firstly demonstrate feasibility of the proposed OTDR-monitoring supported OADM structures in a short SMF link of 16 km.

2.3.1 Experimental Setup

For feasibility demonstration, the experimental setup shown in Fig. 2.5 with two short links of 6-km and 10-km conventional single-mode fibers (SMFs) was arranged.

At transmitter site, two DFB-LD transmitters (one at 1554.13 nm, and the other at 1555.75 nm) and one 1.65-μm OTDR operated with 1-μs pulse having a peak power of about -15 dBm were used. The output power of 1554.13-nm transmitter was split to offer the optical signal for the transmitting channel and the add channel. Each DFB-LD was externally modulated (EM) by a LiNbO3 modulator with a 2²³-1 NRZ PRBS data at 10-Gb/s. The EDFA with an output power of about 16 dBm and a noise figure of 5 dB was used. The OTDR pulses, through a 1.55/1.65-μm WDM coupler, combined with two transmitter channels, and then launched into the 6-km SMF link.

The optical attenuators (VOA1 and VOA2) were used to control the power level. The input power level of each transmitter channel at input and add ports of the OADM was about −4 and −6.8 dBm, respectively.

optical isolation of the MOC are about 1.1 dB and > 45 dB, respectively, at 1.55-μm band, and about 1.5 dB and > 35 dB, respectively, at 1.65-μm band. The 3-dB bandwidth, reflectivity, adjacent channel rejection ratio, and non-adjacent channel rejection ratio of the FBG are about 0.8 nm, 99.98 %, 25 dB, and 39 dB, respectively.

At the receiving site, an optical demultiplexer (DEMUX) with a 3-dB bandwidth of 0.88 nm, an averaged insertion loss of 1.5 dB, and a channel isolation of 40 dB was used to demultiplax the passed-through channel at 1555.75 nm and the added channel at 1554.13 nm. The PINFET receiver (RX) with a receiver sensitivity of −17.5 dBm was used for BER measurement.

The measured transmission and reflection spectra of the 1554.13-nm FBG are shown in the Fig. 2.6 (a) and (b), respectively. The 3-dB bandwidth, reflectivity, adjacent (0.8 nm separation) channel rejection ratio, and non-adjacent (≥ 1.6 nm separation) channel rejection ratio of the FBG are about 0.8 nm, 99.98 %, 25 dB, and 39 dB, respectively.

2.3.2 Experimental Results and Discussions

Fig. 2.7 illustrates the evolution of optical spectra of the transmitter and OTDR channels in C-type OADM link at (a) the input port, (b) the drop port, and (c) the output port of the C-type OADM, and (d) the 1554.13-nm output port of the DEMUX. The instantaneous spectral components of OTDR pulses at 1.65-μm as shown in Fig. 2.7 (a) were completely rejected at the drop port as shown in Fig. 2.7 (b) due to the excellent channel rejection ratio characteristic of the used FBG. Here, the inter-band crosstalk levels of the drop channel (λS1) resulted from the 1555.75-nm channel (λS2) and the OTDR channel are about −37 and less than −58 dB, respectively. The corresponding inter-band crosstalk levels of the added channel (λ^’S1) are about −35 and less than −51 dB as shown in Fig. 2.7 (d).

Fig. 2.8 illustrates the evolution of optical spectra of the transmitter and OTDR channels in M-type OADM link at (a) the input port, (b) the drop port, and (c) the output port of the M-type OADM, and (d) the 1554.13-nm output port of the DEMUX. The instantaneous spectral components of OTDR pulses at 1.65-μm as shown in Fig. 2.8 (a) were completely rejected at the drop port as shown in Fig. 2.8

(b) due to the excellent channel rejection ratio characteristic of the used FBG. Here, the inter-band crosstalk levels of the drop channel (λS1) resulted from the 1555.75-nm channel (λS2) and the OTDR channel are about −39 and less than −54 dB, respectively. The corresponding inter-band crosstalk levels of the added channel (λ^’S1) are about –35 and less than −46 dB as shown in Fig. 2.8 (d). The intra-band crosstalk levels, resulted from the leakage power of the FBG, of both drop and add channels is less than −40 dB.

We have measured the 10-Gb/s BER performances of the dropped, added and, passed-through channels of both C-type and M-type OADMs. Fig. 2.9 shows the 10-Gb/s BER performances of the DROP, ADD, and passed-through channels of the C-type FBG-based OADM in 16-km system link with OTDR (a) off and (b) on operations. Fig. 2.10 shows the 10-Gb/s BER performances of the DROP, ADD, and passed-through channels of the M-type FBG-based OADM in 16-km system link with OTDR (a) off and (b) on operations. There were no OTDR-pulse-induced burst noises observed in the eye diagrams while operating the OTDR for in-service monitoring. Table 2.1 summarizes the resultant chromatic-dispersion (CD) and OTDR-monitoring induced power penalty of the 16-km system links. The CD-induced power penalty (δPCD), due to the used short SMF link, is defined as the degradation of receiver sensitivity at BER of 10^-9 as compared with the baseline case without OADM. The OTDR-induced power penalty (δPOTDR) is defined as the degradation of receiver sensitivity at BER of 10^-9 in presence of OTDR monitoring as compared with the case while switching the OTDR off. Both CD- and OTDR-induced power penalties of each channel for either M-type or C-type OADM is almost the same of ≤ 0.1 dB, and these penalties can be negligible.

Fig. 2.11 and Fig. 2.12 illustrate the OTDR trace of C-type and M-type OADM system link, respectively. Fig. 2.11 (a) is the healthy OTDR trace of the C-type OADM system link and Fig. 2.11 (a) is the healthy OTDR trace of the M-type OADM system link. Note that the Fresnel reflection is observed at 16 km, which coincides with the total link length. The main influence of OADM on the OTDR channel is the attenuation of the OTDR channel, and hence limiting the OTDR’s diagnosing capability of the system link. The OTDR monitored insertion loss of the M-type OADM at 6-km position was about and 4.5 dB. Similar OTDR monitoring

for C-type OADM system link has been achieved with a lower monitored insertion loss of 1.9 dB. Any fiber faults occurred, whether the reflective breaks or non-reflective faults (due to fiber crush and bending), in the system link can be observed and the events can be identified. Fig. 2.11 and Fig. 2.12 (b) illustrates an abnormal condition when there was a fiber cut occurred at the input end of the FBG in the C-type and M-type OADM, respectively.

2.4 Demonstration of Long Distance Fiber Link

In the short distance fiber link experiment system, we used the Anritsu MW9060A OTDR [31] operated at a repetition rate of about 300 Hz with a pulsewidth of 1-μs. The peak wavelength of the OTDR signal is about 1659 nm. The peak power of the OTDR 1-μs pulse is about −15 dBm. The single-way dynamic range of this 1-μs operating OTDR is about 17 dB and the event resolution is about 100 m. While the OTDR operating with a pulsewidth of 10-μs, the single-way dynamic range of this OTDR is about 23 dB and the event resolution is about 1 km..

In the long distance fiber link experiment system with SMF of 85 km, we used a new Anritsu MW9076 OTDR [32] and the peak wavelength of the OTDR signal is about 1625 nm.

2.4.1 Experimental Setup

For long distance fiber link demonstration, the experimental setup shown in Fig.

2.13 with two links of 45-km and 40-km conventional single-mode fibers (SMFs) was arranged. At transmitter site, two DFB-LD transmitters (one at 1554.13 nm, and the other at 1555.75 nm) and one 1.65-μm OTDR operated with 10-μs pulse having a peak power of about +5 dBm were used. The dispersion compensating fibers (7-km DCF and 14-km DCF) were used to compensate the related SMF links. The dispersion of 7-km DCF and 14-km DCF is −638 and −1276 ps/nm, respectively. The output power of 1554.13-nm transmitter was split to offer the optical signal for the transmitting channel and the add channel. Each DFB-LD was externally modulated by a LiNbO3 modulator with a 2²³-1 NRZ PRBS data at 10-Gb/s. The EDFA with an output power of about +20 dBm and a noise figure of 5 dB was used. The OTDR

pulses, through a 1.55/1.65-μm WDM coupler, combined with two transmitter channels, and then launched into the 45-km SMF link. The optical attenuators (VOA1, VOA2) were used to control the power level. The input power level of each transmitter channel at input and add ports of the OADM was about −1.2 and −4.8 dBm, respectively.

In the experiment, the C- and M-type structures were examined separately. The averaged insertion loss and channel isolation of each 1.55/1.65-μm WDM coupler are about 0.9 dB and 24 dB, respectively, at 1.55-μm band, and about 1.2 dB and 20 dB, respectively, at 1.65-μm band. The averaged one-way insertion loss and optical isolation of the MOC are about 1.1 dB and > 45 dB, respectively, at 1.55-μm band, and about 1.5 dB and > 35 dB, respectively, at 1.65-μm band. The 3-dB bandwidth, reflectivity, adjacent channel rejection ratio, and non-adjacent channel rejection ratio of the FBG are about 0.8 nm, 99.98 %, 25 dB, and 39 dB, respectively. At the receiving site, an optical DEMUX with a 3-dB bandwidth of 0.88 nm, an averaged insertion loss of 1.8 dB, and a channel isolation of 40 dB was used to demultiplex the passed-through channel at 1555.75 nm and the added channel at 1554.13 nm. The excellent channel rejection ratio characteristic of the used FBG. Here, the inter-band crosstalk levels of the drop channel (λS1) resulted from the 1555.75-nm channel (λS2) and the OTDR channel are about −38 and less than −59 dB in C-type OADM link, about −42 and less than −57 dB in M-type OADM link, respectively. The corresponding inter-band crosstalk levels of the added channel (λ^’S1) are about −39 and less than −53 dB in C-type OADM as shown in Fig. 2.14 (d), about −39 and less

than −55 dB in M-type OADM as shown in Fig. 2.15 (d).

In long-link experimental system, we also have measured the 10-Gb/s BER performances of the dropped, added and, passed-through channels of both C-type and M-type OADMs. There were no OTDR-pulse-induced burst noises observed in the eye diagrams while operating the OTDR for in-service monitoring. Fig. 2.16 shows the 10-Gb/s BER performances of the DROP, ADD, and passed-through channels of of C-type FBG-based OADM in 85-km system link with OTDR (a) off and (b) on operations. Fig. 2.17 shows the 10-Gb/s BER performances of the DROP, ADD, and passed-through channels of M-type FBG-based OADM in 85-km system link with OTDR (a) off and (b) on operations. The resultant chromatic-dispersion induced power penalty (δPCD), due to the used SMF link, is defined as the degradation of receiver sensitivity at BER of 10^-9 as compared with the baseline case without OADM. The OTDR-induced power penalty (δPOTDR) is defined as the degradation of receiver sensitivity at BER of 10^-9 in presence of OTDR monitoring as compared with the case while switching the OTDR off. Table 2.2 summarizes the resultant chromatic-dispersion and OTDR-monitoring induced power penalty of the 85-km system links. Both CD-induced and the OTDR-induced power penalty of each channel for either M-type or C-type OADM is almost the same of ≤ 0.1 dB, and these penalties can be negligible.

Fig. 2.18 (a) illustrates the healthy OTDR trace of the C-type OADM system link. Note that the Fresnel reflection is observed at 85 km, which coincides with the total link length. Fig. 2.19 (a) illustrates the healthy OTDR trace of the M-type illustrates an abnormal condition when there was a fiber cut occurred at the input end

of the FBG in the M-type OADM.

2.5 Discussion

Consequently, the experimental results confirm the feasibility of simultaneous add-drop and fault-location operations for both OADM structures without degrading the system performance. For FBG-based OADMs, the M-type structure has the advantages of compact structure, fewer components required, and potentially low cost as compared with the C-type structure. Although the total insertion loss of the M-type OADM encountered by the OTDR signal is larger than the C-type structure, the break faults of the FBG path and the connecting fibers within the OADM is also able to be monitored in M-type OADM, but it is unable in C-type OADM. The maximum diagnosing link distance, without intermediate amplification, by the

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