Summary

We have demonstrated C-type and M-type OTDR-monitoring-supported FBG-OADMs. With proper arrangement of port connection through a pair of WDM

couplers or a multiport optical circulator, the in-service OTDR fault-location monitoring has been realized. Negligible OTDR-monitoring-induced system power penalty of the add, drop, and pass-through channels in both OADM structures has been achieved in 10-Gb/s system demonstration. In addition, due to such OTDR-monitoring-supported OADM technique, any fiber faults occurred, whether the reflective breaks or non-reflective faults, in the system link can be observed and identified. The M-type structure is superior to the C-type structure for FBG-OADMs to have compact structure, fewer components required, and potentially low cost.

Such OADMs may find important applications in DWDM networks to implement a centralized automated in-service surveillance system to enhance network reliability.

Chapter 3

In-Service OTDR Supervisory DWDM System Directly Through

Mach-ZehnderFiber-Grating Optical Add-Drop Multiplexers

In this chapter an in-service supervisory 10-Gb/s DWDM system containing MZ-FBG based 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. This technique can be easily to develop to an automated in-service surveillance system, which may provide a in-service monitoring to enhance OADM network reliability

3.1 Review of Mach-Zehnder Fiber Bragg Grating OADM

Compared with the OADM using a FBG sandwiched between a pair of optical circulators in [27], the MZ-FBG four-port devices [17] seem to be promising for constructing either fixed or dynamic add-drop operation of multiple wavelengths without needs of additional wavelength DEMUX and multiplexers [33] at the drop and add ports, respectively. The MZ-FBG device consists of identical FBG’s photo-imprinted in the two arms of a balanced MZ fiber-optic interferometer. Its operating principle and details have been described in [17]. With the growing development of OADM’s in DWDM systems, it is important to use a practical supervisory method which facilitates the in-service monitoring and fault-locating capability without sacrificing active traffic, to enhance network reliability. One work based on monitoring FBG-reflected un-used amplified spontaneous emission (ASE) light for supervising OADM system has been reported recently [34]. The operating status of EDFA and OADM itself can be monitored at each OADM node. However,

the status of inter-span fiber link is unable to supervise, and the centralized surveillance operation is unable to realize. OTDR is a popular tool to provide in-service monitoring in DWDM systems [21, 22]. For OTDR supervision on OADM systems, there are no reports published to date. Each MZ-FBG four-port device has the inherent features of polarization insensitivity, wide spectral operating region, symmetry with the individually drop and add ports, and low insertion loss.

Both symmetry and wide spectral operating region features make it suitable to support the 1.65-μm OTDR monitoring without needs of re-structuring the OADM configuration and extra optical components for propagation of OTDR probe pulses and the Rayleigh backscattering lights in each OADM node. In this chapter, we investigate the in-service OTDR supervision on 10-Gb/s DWDM link through the MZ-FBG OADM. The multi-wavelength add-drop operation and 1.65-μm OTDR supervision are demonstrated and system BER performance is examined. The fault of OADM itself and fiber faults occurred in the system link can be monitored from a centralized transmitting node.

3.2 Demonstration of Short Distance Fiber Link

In this section, we firstly demonstrate feasibility of the proposed directly OTDR-monitoring supported MZ-FBG-based OADM structure in a short SMF link of 16 km with a single OADM.

3.2.1 Experimental Setup

The experimental setup shown in Fig. 3.1 with two short links of 6-km and 10-km conventional SMFs was arranged. At transmitter site, two DFB-LD transmitters (one at 1551.5 nm, and the other at 1555.7 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 1551.5-nm transmitter was equally 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 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 OADM node, we used 200-GHz-grid MZ-FBG device with central wavelengths of 1551.5 was used, and the Tx channel was individually dropped and added by the corresponding MZ-FBG. The insertion loss is about 0.5 dB at 1.55-μm band and about 1.8 dB at 1.65-μm band with a non-adjacent channel rejection ratio of

> 48 dB. The 3-dB and 20-dB bandwidths (BWs) of each MZ-FBG are about 0.82 nm and 1.24 nm, respectively.

At the receiver site, an optical DEMUX with 200-GHz channel spacing was used to block the OTDR light and demultiplex the passed-through channel at 1555.8 nm and the added channel at 1551.5 nm. The DEMUX has a 3-dB BW of 0.88 nm, an insertion loss of 1.5 dB, and a channel isolation of > 40 dB. The PINFET receiver (Rx) with a receiver sensitivity of −17.3 dBm was used for BER measurements.

3.2.2 Experimental Results

Fig. 3.2 illustrates evolution of optical spectra of the transmitter and OTDR channels in MZ-FBG-based OADM in short distance fiber link at (a) the input port, (b) the drop port, and (c) the output port of the C-type OADM, and (d) the 1551.5-nm output port of the DEMUX. The instantaneous spectral components of OTDR pulses at 1.65-μm as shown in Fig. 3.2 (a) were completely rejected at the drop port as shown in Fig. 3.2 (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 1556.4-nm channel (λS2) and the OTDR channel are about −38 and less than −58 dB, respectively. The corresponding inter-band crosstalk levels of the added channel (λ^’S1) are about −47 and less than −49 dB as shown in Fig.

3.2 (d).

Fig. 3.3 shows the OTDR trace of MZ-FBG OADM system link. Fig. 3.3 (a) is the healthy OTDR trace of the MZ-FBG 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 MZ-FBG OADM at 6-km position was about 1.8 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. 3.3 (b) illustrates an abnormal condition when there was a fiber cut occurred at the OADM node.

We have measured the 10-Gb/s BER performances of the dropped, added and, passed-through channels of the OADM system. There were no OTDR-pulse-induced burst noises observed in the eye diagrams while operating the OTDR for in-service monitoring. Fig. 3.4 shows the 10-Gb/s BER performances of the DROP port of MZ-FBG and output of DEMUX at 16-km system link with (a) OTDR on and (b) off operations. The resultant chromatic-dispersion (CD) induced power penalty, 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 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-induced and the OTDR-supervision induced power penalty of the dropped and added channels for both 1551.5-nm and 1555.8-nm signals was negligible (≤ 0.1 dB).

3.3 Demonstration of Long Distance Fiber Link

In a long distance fiber link experiment system with SMF of 80 km, we used two MZ-FBGs with reflection central wavelengths of 1551.5 nm and 1554.6 nm to demonstrate the in-service OTDR monitoring in MZ-FBG-based OADM network in long distance fiber link system.

3.3.1 Experimental Setup

The experimental setup of the proposed OTDR supervisory OADM transmission system is shown in Fig. 3.5. At transmitter (Tx) node, two DFB laser diodes (λ1 = 1551.5 nm and λ2 = 1554.6 nm) were used. A LiNbO3 modulator was used to externally modulate the CW light of each DFB laser diode with a 2³¹-1 NRZ

PRBS data at 10-Gb/s. The output power of each Tx was amplified by an EDFA and then split by a 3-dB coupler. Each EDFA has an output power of about +15 dBm and noise figure of 5 dB. Due to the shortage of 10-Gb/s optical transmitters, the data signal and the adding channel shared a single common transmitter source, each channel with an output power of +7 dBm. The data signals of both Tx’s were pre-compensated by using a 7-km dispersion compensating fiber module with a total dispersion and transmission loss of about -638 ps/nm and 4 dB, respectively. The 1.65-μm OTDR pulses of Anritsu MW9060A OTDR combined with two Tx signals through a 1.55/1.65-μm WDM coupler, and then launched into the system link, which consists of two 40-km conventional single-mode fiber spans and an OADM node. The insertion loss of the WDM coupler is about 1 dB at 1.55-μm and 2 dB at 1.65-μm. The channel isolation of the WDM coupler is > 25 dB for 1.65-μm port and

> 33 dB for 1.55-μm port.

In the OADM node, two 200-GHz-grid MZ-FBG devices with central wavelengths of 1551.5 nm and 1554.6 nm were used, and thus each Tx channel was individually dropped and added by the corresponding MZ-FBG. Each MZ-FBG has an averaged insertion loss of 0.5 dB at 1.55-μm and a non-adjacent channel rejection ratio of > 48 dB. The 3-dB and 20-dB bandwidths of each MZ-FBG are about 0.82 nm and 1.24 nm, respectively. The EDFA3 was used to compensate the losses of both OADM and the second 40-km link. A pair of optical circulators (OC1 and OC2) bypasses the EDFA to provide the propagation path of the Rayleigh backscattering light of OTDR. At receiver (Rx) node, an optical DEMUX with 200-GHz channel spacing was used to block the OTDR light and demultiplex the added channels for detection. The DEMUX has a 3-dB BW of 0.88 nm, an insertion loss of 1.5 dB, and a channel isolation of > 40 dB. The PINFET receiver with a receiver sensitivity of -17.3 dBm was used for BER measurements.

3.3.2 Experimental Results

In the experiments, both EDFA3 and OC pair in OADM node were omitted for simplifying the demonstration. The input power level at input and add ports of each MZ-FBG device was arranged at about −11 and +7 dBm, respectively. Such high power level of each adding channel is required to satisfy the 20-dB attenuation of the

second span link and the required receiver sensitivity. Fig. 3.6 shows the observed 1.65-μm OTDR traces of the in-service transmission system under normal and fault conditions. Since the single-way dynamic range (SWDR) of the used OTDR with 10-μs pulsewidth was about 20 dB, it was hard to diagnose the system link with a total loss of about 30 dB at 1.65-μm. The 30-dB attenuation resulted from the losses of the 1.55/1.65-μm coupler (1 dB), each 40-km SMF span (10 dB), each MZ-FBG (2.4 dB), and the 7-km DCF module (4.5 dB). Due to the insufficient SWDR, the healthy OTDR trace (not shown) revealed the first 40-km span with correct trace slope but with the noise floor (not shown) for the corresponding second 40-km span.

For observing the OTDR trace of the second 40-km link, we have to decrease the attenuation of the electrical signal of the detected Rayleigh backscattering light, and thus making the operational amplifiers in OTDR receiving circuit to operate in the saturation region. Therefore, the flat OTDR trace as shown in Fig. 3.6 (a), (b), and (c) of the first 40-km link occurred. In addition, the trace slope of the second 40-km span does not reveal the SMF loss coefficient because the detected backscattering signals were too weak (near the noise floor) to have an accurate signal processing for the used OTDR.

Even so, the OTDR supervision still worked successfully. Note that the last reflection spike at 87 km in Fig. 3.6 (a) coincides with the total link length of the system with the segment of 7-km DCF in Rx node. Fig. 3.6 (b) and (c) illustrate the abnormal conditions while a fiber break occurred at 80-km and 40-km positions, respectively. Both events can be identified. For Fig. 3.6 (c), when we attenuated the electrical signal of the detected backscattering signals through the GPIB interface to let the operational amplifiers operate in linear region. The flat trace of the first 40-km link will recover to the healthy trace with a correct trace slope (not shown here). On the other hand, the flat and distorted trace problems can be solved when an OTDR with a large SWDR of > 32 dB is used. Then, any fiber faults occurred in the fiber link whether before or after the OADM node could be located and identified.

Consequently, the demonstrations confirm the feasibility of 1.65-μm OTDR supervision directly through the OADM itself without needs of re-structuring OADM configuration.

Fig. 3.7 illustrates the evolution of signal spectra at (a) the IN1 port, (b) the

DROP1 port, (c) the DROP2 port, and (d) the OUT2 port of the OADM. The instantaneous spectral components of the OTDR channel in Fig. 3.7 (a) were completely rejected by the MZ-FBG’s at the DROP1 and DROP2 ports as shown in Fig. 3.7 (b) and (c), respectively. The ASE hump around 1.56-μm in Fig. 3.7 (b) and (c) was due to the leakage light of the corresponding ADD channel contaminated by the ASE light generated from the corresponding EDFA at Tx node. Low inter-band crosstalk levels of the DROP channel, about –40 dB resulted from both the adjacent data channel and the ASE light, and about −62 dB (for DROP1 channel) and −57 dB (for DROP2 channel) resulted from the OTDR channel were obtained due to the high non-adjacent channel isolation characteristic of MZ-FBG’s. The residual light of either the λ2-channel in Fig. 3.7 (b) or the λ1-channel in Fig. 3.7 (c) was now laid beneath the ASE light. The intra-band crosstalk level of the DROP channel resulted from the leakage light of the ADD channel is around < –27 dB as shown in the insets of Fig. 3.7 (b) and (c). Similarly, due to the symmetry feature of MZ-FBG, the intra-band crosstalk level of the ADD channel at OUT2 port resulted from the leakage light of the corresponding IN channel, due to the insufficient reflectivity of the FBG’s in MZ-FBG, was also around −27 dB (not shown here). Note that the OTDR pulses passed directly through the MZ-FBG1 and MZ-FBG2. Fig. 3.7 (d) shows the spectral components of such passed-through OTDR channel and two add channels at OUT2 port of the OADM node. The inset in Fig. 3.7 (d) illustrated the inter-band crosstalk level of the ADD2 channel at the output port of DEMUX of

≤ –50 dB relative to the ADD1 channel, and ≤ −57 dB relative to the OTDR channel.

Fig. 3.8 shows the 10-Gb/s BER performances of the (a) 1551.5-nm and (b) 1554.6-nm channels of the 80-km system link with OTDR on and off operations. The BER floor was not observed below BER of 1 × 10^-9. The BER of the DCF-compensated system link without the OADM node has also been measured (not shown here), and is almost the same as the back-to-back case, where all EDFA’s, SMF links, OADM node, and DCF’s were excluded. While inserting the MZ-FBG OADM in system link, the OADM-induced power penalty was about 0.1 dB for both dropped channels, and was about 0.5 dB for both added channels. Note that the OTDR-supervision induced power penalty of the dropped and added channels for both 1551.5-nm and 1554.6-nm signals was negligible (≤ 0.1 dB). Consequently, the experimental results confirmed the feasibility of this technique for simultaneous

add-drop operation and fault location without degrading system performance.

3.4 Summary

We have presented an in-service 1.65-μm OTDR supervisory DWDM transmission system directly through MZ-FBG OADM itself without needs of re-structuring OADM configuration and extra optical components in the OADM node. OTDR-supervision induced power penalty of the added and dropped channels in 10-Gb/s 80-km system is negligible. Any fiber faults occurred in the system link can be observed and identified. When the OTDR trace recognition technique is employed, this supervisory technique can be easily to develop to an automated in-service surveillance system to provide a in-service monitoring to enhance OADM network reliability.

Chapter 4

In-service 1.65-μm OTDR Monitoring on Distributed Fiber Raman Amplifier System

An in-service supervisory 10-Gb/s 1.55-μm dense wavelength division multiplexing (DWDM) system using distributed fiber Raman amplifier (FRA) is demonstrated. The link fault location is realized simply by using a conventional 1.65-μm OTDR. Negligible system power penalty due to the in-service OTDR monitoring is achieved. This technique can be easily to develop to an automated in-service surveillance system, which may provide a real-time monitoring to enhance the reliability of distributed FRA system.

4.1 Introduction

Silica fiber Raman amplifiers (FRAs) are particularly attractive for broadband application in DWDM systems [35-39] since they offer the advantages of greatly extended bandwidth, distributed amplification, and low noise performance with the installed fibers as the gain media. With the growing deployment of distributed FRA, the system should facilitate the in-service fault-location monitoring capability to enhance system reliability. In general, the 1.65-µm optical time domain reflectometer (OTDR) is a popular tool to provide in-service monitoring of fibers carrying live signals in 1.55 µm DWDM transmission systems [40], [22]. Since OTDR operates with high peak powers, the stimulated Raman scattering (SRS) effect in the conventional transmission fiber gives rise to power depletion [41], [42] of the data channel, and may degrade the bit-error-rate (BER) performance. However, such problem, which maybe more seriously in distributed FRA link, and the in-service monitoring impact on distributed FRA systems has not yet reported. In this work, we investigate the in-service 1.65-µm OTDR monitoring on the 1.55-µm distributed FRA transmission link. The system power penalty due to the OTDR monitoring and the residual pump light in a 10 Gb/s dispersion shifted fiber (DSF) link is examined.

The distorted OTDR trace, which resulted from the backscattered Raman amplified

spontaneous emission light due to the insufficient spectral filtering of the used WDM coupler, was also observed.

4.2 Experimental Setup

Fig. 4.1 shows the experimental setup of the proposed OTDR-monitored distributed FRA system, in which the forward pumping and backward pumping schemes are separately examined. At transmitter site, for feasibility study, a 1550-nm DFB laser diode (DFB-LD) with a CW output power of 7 dBm was externally modulated by a LiNbO3 modulator with 2³²-1 NRZ pseudo-random-bit-sequence (PRBS) data at 10 Gb/s. The modulated signal was amplified by a conventional-wavelength band (C-band) erbium doped fiber amplifier (EDFA) with an output power of about +17 dBm and a noise figure of 5 dB. The power level of data channel was adjusted by the variable optical attenuator (VOA1) to 0 dBm at position A. The data channel combined with the 1656-nm OTDR probe channel that operated in 10-μs pulse width, and than launched into the distributed FRA link through a 1.55/1.65-μm coupler (WDM1). The peak power of OTDR pulses at the output of OTDR was about +10.5 dBm.

Although it seems to allocate the data channel at 1583-nm to catch the maximum gain of this distributed FRA in practical application, the data channel with central wavelength of 1550-nm was arranged for giving a channel separation of 106

Although it seems to allocate the data channel at 1583-nm to catch the maximum gain of this distributed FRA in practical application, the data channel with central wavelength of 1550-nm was arranged for giving a channel separation of 106