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.

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 24.0 dB, respectively, at 1.55-μm band, and about 1.2 dB and 20.0 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 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.