Dispersion-Compensated Gain-Clamped 90 nm Wideband

The wideband optical amplifier through the combination of Raman amplifier (RA) and erbium-doped fiber amplifier (EDFA’s) as well as parallel configurations for the three gain-bands of the EDFA’s have been intensively studied for increasing the long- haul transmission capacity in the 1.5-1.6 µm region [61, 62]. On the other hand, the semiconductor optical amplifier (SOA) is promising for in- line amplification of DWDM transmission [63, 64]. However, those experiments have not yet utilized the full gain bandwidth of the SOA to effectively satisfy the urgent need of bandwidth for future metropolitan area network.

In order to utilize the full gain bandwidth of the SOA, we demonstrate a dispersion-compensated gain-clamped wideband (1500-1590 nm) optical amplifier employing a dispersion-compensated- fiber (DCF) based Raman amplifier (RA) and an SOA for wideband amplification with good gain flatness (of 3 dB) as well as dispersion compensation of 10 Gb/s DWDM signals over a 100-km single- mode fiber (SMF) link.

The amplifier design and its system performance are described.

Chapter 3

Fiber Bragg Grating-Based Optical Add-Drop Multiplexing and Cross-Connect Systems

This chapter investigates fiber Bragg grating-based optical add-drop and cross-connect multiplexing systems for WDM long-distance Trunk and Ring Networks.

They are the systems: 1) low-crosstalk and compact optical add-drop multiplexer using a multiport circulator and fiber Bragg gratings [65], 2) Mach-Zehnder fiber Bragg grating-based fixed and reconfigurable multi- channel optical add-drop multiplexers for DWDM networks [66], 3) Mach-Zehnder fiber Bragg grating-based dynamic optical cross-connect [67], respectively.

3.1 Low-Crosstalk and Compact Optical Add-Drop Multiplexer Using a Multi-Port Circulator and Fiber Bragg Gratings

Three kinds of low interferometric-crosstalk optical add-drop multiplexers (OADMs) based on a multiport optical circulator (MOC) with fiber Bragg gratings are proposed and demonstrated [65]. There is a huge intra-band crosstalk level reduction of about 37 dB and 16 dB on the dropped and added channels, respectively, for the proposed MOC-based structure as compared with the conventional structure. Bit-error-rate performance and both intra-band and inter-band crosstalk- induced power penalties of these MOC-based OADMs are examined in a 10 Gb/s system demonstration.

3.1.1 Optical Add-Drop Multiplexing Configurations and Experimental Setup

Figure 3.1 shows the structures of (a) the conventional OADM, (b) the first and second proposed MOC- based OADMs, and (c) the third proposed MOC-based OADM.

The conventional structure (hereafter C-type), consists of two three-port optical circulators (OC1 and OC2), and an FBG with central wavelength matching the ITU DWDM-channel

signal, which will be dropped and added at the OADM node. In the first proposed OADM structure (hereafter, M(i)-type), a six-port optical circulator, instead of two three-port OCs, is used in combination with an FBG. The port connections arranged as indicated in Figure 3.1(b) make the add-drop operation the same as that of Figure 3.1(a). In the second proposed OADM structure (hereafter, M(ii)-type), an additional FBG with the same central wavelength is cascaded in chain to reflect the leakage light (which is due to the insufficient reflectivity) of the first FBG, and thus to reduce the intra-band crosstalk on the added channel at the output port of the OADM. In the third proposed OADM structure (hereafter, M(iii)-type), an additional FBG with the same central wavelength is placed at the fifth port of the MOC to reflect the leakage light of the first FBG. The arrangement and connection in M(iii)-type structure makes it equivalent to the M(ii)-type OADM but with a midway optical isolator in-between two FBGs. The device from port 4 and port 5 is an effective optical isolator (ISO4--5) in MOC as shown in Figure 3.1(c). Such arrangement of M(iii)-type structure provides the capability not only to reduce the intra-band crosstalk of the added channel at the output port, but also to block the leakage of the added channel, therefore completely reduces the intra-band crosstalk of the dropped channel.

The experimental setup, as shown in Figure 3.2, consists of two sets of DWDM transmitters, an OADM, and related devices was arranged. The first transmitter set was composed of four DFB lasers with central wave lengths of 1550.92-, 1552.52-, 1554.13-, and 1555.75-nm, respectively. The second transmitter set was composed of only one DFB laser with central wavelength of 1554.13 nm, which acted as the add channel. Each transmitter set was modulated by a LiNbO3 modulator with a 2³¹-1 NRZ PRBS data at 10 Gb/s. A PINFET receiver (RX) with a sensitivity of –17.5 dBm at a BER of 1 × 10^-9 was used. Four OADMs including the C-, M(i)-, M(ii)-, and M(iii)-type structures were examined in the experiments separately. The averaged insertion loss and optical isolation between any two ports of the MOC are about 0.9 dB and >45 dB, respectively. The transmission loss, isolation, 3-dB bandwidth, and reflectivity of each FBG are about 0.8 dB, 25 dB, 0.8 nm, and 99 %, respectively.

3.1.2 Experimental Results and Discussion

Figure 3.3 shows the output spectra of the dropped channels in (a) M(i)-type and (b) M(iii)-type OADMs. The upper “dark solid ” spectrum in Figure 3.3(a) corresponds to the 1554.13-nm dropped channel in M(i)-type structure, while the lower “light thin” spectrum in Figure 3.3(a) corresponds the leakage light from the add channel. Note that the intra-band and inter-band crosstalk levels of this dropped channel are about -22 and -25 dB, respectively. Such intra-band crosstalk component of the dropped channel is due to the insufficient reflectivity of the used FBG. However, such intra-band crosstalk component can be drastically reduced as shown in Figure 3.3(b) while using the M(iii)-type OADM. The intra-band crosstalk level is now improved to about -61.5 dB.

Figure 3.4 shows the output spectra of the added channels in (a) M(i)-type and (b) M(iii)-type OADMs. The intra-band crosstalk level of the added channel is about -23.9 dB.

Similarly, such intra-band crosstalk component can be drastically reduced as shown in Figure 3.4(b) while using the M(iii)-type OADM. The intra-band crosstalk level is now improved to about -39.3 dB. For M(ii)-type OADM, the intra-band crosstalk level of the dropped and added channels is –54.5 and -41.5 dB, respectively. The inter-band crosstalk level of the added channel demultiplexed right after the DEMUX is the same as about -28 dB, which is limited by the 28-dB adjacent channel isolation ability of the used DEMUX, in all three OADM cases.

We have measured the 10 Gb/s BER performances of both dropped and added channels operated at 1554.13 nm for these four OADM structures. Fig. 3.5 summarizes the intraband crosstalk levels and power penalty of both 1554.13-nm dropped and added channels for these four OADM structures. The power penalty in all four OADM cases is defined as the degradation of receiving sensitivity at a BER of 10^-9 as compared with the baseline case without OADM. The intra-band crosstalk level of the dropped channel of the C-, M(i)-, M(ii)-, and M(iii)-type structures is -24.7, -22, -54.5, and -61.5 dB, respectively.

The corresponding power penalty of the dropped channel is 0.5, 0.6, 0.4, and 0.3 dB, respectively. The 0.3-dB power penalty of the dropped channel of the M(iii)-type OADM attributes to both the signal-spontaneous beat noise, induced by the amplified spontaneous emission noise power of the EDFA, and the inter-band crosstalk because the intra-band crosstalk level of -61.5 dB can be neglected. Hence, the intra-band crosstalk- induced power penalty of the dropped channel is about 0.3, 0.1, and 0 dB for the M(i)-, M(ii)-, and

M(iii)-type OADMs, respectively. The inter-band crosstalk- induced power penalty can be improved by using a higher adjacent channel isolation of >40 dB of the DEMUX. On the other hand, the intra-band crosstalk level of the added channel of C-, M(i)-, M(ii)-, and M(iii)-type structures is -23, -23.9, -41.5, and -39.3 dB, respectively. The corresponding power penalty of the dropped channel is 0.6, 0.6, 0.5, and 0.5 dB, respectively. The intra-band crosstalk- induced power penalty of the dropped channel is about 0.3, 0.2, and 0.2 dB for the M(i)-, M(ii)-, and M(iii)-type OADMs, respectively. In consequence, these experimental results confirm the feasibility of the proposed MOC-based OADMs. Among them, the M(iii)-type OADM offers lower crosstalk characteristic and better system performance than those of M(i)- and M(ii)-OADMs.

3.1.3 Summary

We have demonstrated the MOC-based OADMs with fiber Bragg gratings for DWDM networks. Three kinds of MOC-based OADMs have been proposed and their add-drop operations have been investigated. Compared with the conventional structure, there is a huge reduction of the intra-band crosstalk level of about 37 dB and 16 dB for the dropped and added channels, respectively, for the proposed M(iii)-type structure. Among them, the M(iii)-type OADM offers lower interferometric crosstalk characteristics and better system performance than those of M(i)- and M(ii)-type OADMs. Satisfied BER performance and low power penalty of ≤ 0.2 dB, induced by the intra-band crosstalk, of the M(iii)-type OADM have been achieved in 10 Gb/s system demonstration. The experimental results confirm the feasibility of the proposed MOC-based OADMs. Such a low-crosstalk, miniaturized, and compact OADM may find important applications in DWDM networks.

3.2 Mach-Zehnder Fiber Bragg Grating-based Fixed and Reconfigurable Multi-Channel Optical Add-Drop Multiplexers for DWDM Networks

We first utilize the Mach- Zehnder fiber-Bragg-grating (MZ-FBG) based devices with associated optical switches to construct large-dimension OADM’s without needs of wavelength demultiplexers and multiplexers at the drop and add ports, respectively [66].

The single- and cascaded-stage MZ-FBG’s as the add-drop elements separately in a total of one fixed and two reconfigurable OADM architectures are considered. We investigate the required constitutive elements, and examine the relevant characteristics of system insertion loss and differential system loss for the added, dropped, and passed-through channels. The system power penalty induced by both inter- and intra-band crosstalks, and the required spectral characteristics for the MZ-FBG devices are analyzed. The add-drop dimension limits of these MZ-FBG-based OADM’s are estimated. Other related issues such as multiwavelength amplification, and bandwidth narrowing for the cascaded OADM chain in DWDM networks are also addressed.

3.2.1 Basic MZ-FBG Add-Drop Devices

FBG’s have a variety of lightwave applications due to their inherent features of all- fiber geometry, low insertion loss, low back-reflection, and low cost. But the most distinguishing feature of FBG’s is the flexibility they offer for achieving desired sharp spectral characteristics at WDM-grid wavelengths with high reflectivity and flat narrow passband.

A. Single-stage MZ-FBG Device

Figure 3.6(a) shows the single-stage MZ-FBG device for constructing fixed and reconfigurable OADM’s. This MZ-FBG device, first proposed by Johnson and Hill et al.

[1], consists of identical Bragg gratings photo- imprinted in the two arms of a balanced MZ interferometer. Compared with the OADM using FBG sandwiched between a pair of optical circulators in [16, 17], lower loss, lower cost, and allowable higher input power are expected in the MZ-FBG, because it is formed only by optical fiber. Compared with another OADM circuit using AWG technology, the undesirable bandwidth narrowing problem [39] can be eliminated and a flattened- filter response can be obtained in the MZ-FBG, because the apodized Bragg grating is applicable to the MZ-FBG [68]. Today’s commercial MZ-FBG device provides quite low loss characteristics of 0.5 dB for the INPUT to DROP, INPUT to OUTPUT, and ADD to OUTPUT ports. Temperature coefficient of reflection wavelength is about 0.015 nm/^oC. High channel rejection ratio of

<-40 dB at the DROP port for adjacent channels can be obtained, however, interferometric intraband crosstalk of up to –20 dB, induced by the non- infinite Bragg reflectivity, for the ADD channel, may cause severe system power penalty. Mizuochi et al, [69] demonstrated a cascaded MZ-FBG scheme to solve the interferometric crosstalk recently.

B. Cascaded-stage MZ-FBG Device

Figure 3.6(b) shows the polarization- insensitive cascaded MZ-FBG device [69] for constructing both fixed and reconfigurable OADM’s. The cascaded MZ-FBG device includes a pair of single-stage devices (MZ-FBG-A and MZ-FBG-B) with a midway FBG.

Each FBG has the same Bragg wavelength and spectral characteristics. When several down-stream WDM channels are launched into the INPUT port, Bragg gratings formed in the two arms of the MZ-FBG-A reflect the Bragg wavelength λB to be dropped out from the DROP port. The remaining wavelengths pass through the midway FBG, and then enter the MZ-FBG-B to emerge from the OUTPUT port. Even if the reflectivity of the MZ-FBG-A is imperfect, the midway FBG intercepts the residual power at λB. Since the reflected leakage component is thrown away to the termination port, no multiple reflection occurs between the midway FBG and the MZ-FBG-A. Moreover, the MZ-FBG-B can eliminate a very weak leakage from the midway FBG, thus the intraband crosstalk is no longer detectable. On the other hand, the wavelength λB inserted from the ADD port is reflected by the MZ-FBG-B to emerge from the OUTPUT port. Even if the reflectivity of the second MZ-FBG-B is imperfect, almost most of the residual power at λB is thrown away to the termination port. Similarly, the weak leakage of the ADD signal can be eliminated by the midway FBG and the MZ-FBG-A.

For this cascaded-stage MZ-FBG device, low loss characteristics can be maintained, for example, 0.5-0.8 dB for the INPUT to DROP port, 1.2-1.5 dB for the INPUT to OUTPUT port, and 0.5-0.8 dB for the ADD to OUTPUT port. Furthermore, high channel rejection ratio of <-40 dB for adjacent channels and low intraband crosstalk of –50 dB for the ADD signal and –71 dB for the DROP signal have been realized. The feasibility of this cascaded-stage device to add-drop a single wavelength has been demo nstrated in an 8 × 10 Gb/s system experiment with crosstalk-free performance [69].

For WDM systems, there are N wavelengths per fiber link with a channel spacing of 200 GHz (1.6 nm), 100 GHz (0.8 nm), or 50 GHz (0.4 nm). The N may be 2, 4, 8, 16, or more wavelengths, each carrying 2.5 or 10 Gb/s traffic. At each OADM node, only m optical channels may be dropped and added from the corresponding DROP and ADD ports, where m is typically in the range of 0 (which means all optical channels are passing through the OADM) to N/2 for most OADM’s operated in the networks. In the section, we employ the four-port MZ-FBG device, either in single-staged or cascaded-stage scheme,

to construct both fixed and reconfigurable OADM’s. For a fixed OADM providing add-drop capability of m pre-arranged wavelengths, the m optical channels in a fiber link, in which there are N optical channels and m ≤ N/2, will be dropped and added sequentially according to the Bragg wavelengths of the MZ-FBG devices from the corresponding DROP and ADD ports. In contrast, for a reconfigurable OADM providing selective add-drop capability of m wavelengths, the m optical channels will be dropped and added dynamically according to the reconfigured pattern, which is detected by the optical modem in reconfigurable OADM. Therefore, m = N for reconfigurable OADM’s.

3.2.2 Fixed MZ-OADM Architecture

Figure 3.7 shows the fixed OADM (hereinafter, F-OADM) providing add-drop capability of m pre-arranged wavelengths, which consists of m pieces of the four-port polarization- insensitive MZ-FBG devices. There are one input port, one common output port, and m separate DROP and ADD ports. The m channels will be dropped and added sequentially according to the Bragg wavelengths of the MZ-FBG devices from the corresponding DROP and ADD ports. Each MZ-FBG device has different Bragg wavelength, λi, which is designed to separately match the desired WDM channel wavelength. For each MZ-FBG device, all required FBG’s have the same Bragg wavelengths. The 3-dB bandwidth of each FBG included within a single-stage or cascaded-stage MZ-FBG device should be large enough to cover the corresponding channel signal with high reflectivity and low out-of-band transmission loss.

For fixed add-drop operation of m optical channels, the number of required MZ-FBG’s, Z, of an F-OADM are Z = m for the OADM using single-stage MZ-FBG devices, and Z = 2m with additional m midway FBG’s for the OADM using cascaded-stage MZ-FBG devices. In addition, the differential system loss, to be discussed in sub-section 3.2.4, between the WDM channels at the OUTPUT port will be increased, especially for the rear WDM wavelengths, which are close to the output port of an F-OADM. Thus, the optical attenuators (hereinafter, ATT’s) with different losses as the values indicated at the right hand side of each ATT, located at the ADD ports, are used to eliminate the differential system loss. Here, the total number of required ATT’s, T, for reducing differential system insertion loss of an F-OADM, in spite of using single- or cascaded-stage MZ-FBG devices, is T = m.

3.2.3 Reconfigurable MZ-OADM Architectures A. Reconfigurable MZ-FBG-based OADM Type 1

There are two types of reconfigurable MZ-FBG-based OADM’s. Figure 3.8 shows the first type of reconfigurable OADM (hereinafter, R1-OADM), providing dynamically selective add-drop capability of m wavelengths. The R1-OADM is constructed by using m pieces of four-port MZ-FBG devices, m + 1 pieces of 2 × 2 mechanical OSW’s, optical supervisory channel circuits, and the associated EDFA and optical limiting amplifier (OLA). The control signal of this R1-OADM is realized by means of the supervisory optical channel λSV, in which the add-drop arrangement instructions of this node and also of other destination nodes are included. The supervisory optical channel is selected to have a wavelength (usually in the 1620-1650 nm or 1300-1310 nm range) different from the wavelengths selected for the WDM payload optical channels (usually in the 1520-1590 nm range). At each R1-OADM node, the supervisory channel is first demultiplexed by an appropriate WDM device, then received by the optical modem and converted to electrical signal that containing the add-drop arrangement instructions. The dotted path lines downstream of the modem indicate electrical signal paths, in contrast to the solid lines indicating optical signal paths. The EDFA located after the WDM device is used to amplify all WDM channels to provide sufficient power levels and optical signal-to-noise ratio levels. On the other hand, the OLA is employed to compensate both system insertion loss and differential system insertion loss. The optical delay line is required for the supervisory channel to have an appropriate time delay to let switch controller activate the desired OSW’s.

To perform the optic al switching, the node control processor signals the switch controller to position the 2 × 2 OSW’s in appropriate states to select the desired optical channels to be add-dropped. Each 2 × 2 OSW has one of the “cross” and “bar” operation states at one time. While setting all m + 1 2 × 2 OSW’s in the “bar” states, all N WDM channels will pass this OADM directly through the upper optical paths in all 2 × 2 OSW’s.

Thus there is no added and dropped channels happened in this OADM. Whilst setting both the first and the last OSW’s in the “cross” states and other m-1 OSW’s in the “bar” states, then m WDM channels will be dropped simultaneously and other N minus m channels will pass through this OADM. To select the first channel, setting two nearest adjacent 2 × 2

OSW’s, one in front of and the other right after the first MZ-FBG, both in “cross” states, then the add-drop of λ1 can be implemented. Similarly, setting two nearest adjacent 2 × 2 OSW’s of the last MZ-FBG both in “cross” states, then the add-drop of λm can be implemented. Therefore, by control of the desired 2 × 2 OSW(s), dynamic add-drop control of any desired single or multiple channel(s) can be realized. The 2 × 2 OSW used for this OADM has a switching response time of about 0.3 ms. Again, a supervisory channel at the output port of the OADM, provided by a new optical transmitter at λSV, which carries the control signal of next and the rest OADM nodes in the networks.

For dynamic add-drop operation of 0 - m optical channels, the number of required MZ-FBG’s, Z, and the total number of required midway FBG’s, V, of an R1-OADM are the same as that of the F-OADM. The number of required control devices (here, i.e., 2 × 2 OSW’s), C, of an R1-OADM is C = m + 1 in spite of using single-stage and cascaded-stage MZ-FBG devices. Similarly, the ATT’s with different losses located at the

For dynamic add-drop operation of 0 - m optical channels, the number of required MZ-FBG’s, Z, and the total number of required midway FBG’s, V, of an R1-OADM are the same as that of the F-OADM. The number of required control devices (here, i.e., 2 × 2 OSW’s), C, of an R1-OADM is C = m + 1 in spite of using single-stage and cascaded-stage MZ-FBG devices. Similarly, the ATT’s with different losses located at the