Chapter 1 General Introduction
1.4 The Dissertation Organization
There are six chapters organized in this dissertation. Chapter 2 describes the overview and motivation of these three application areas, and then proposes our system applications for WDM Long-Distance Trunk/Ring Networks.
In Chapter 3, we address the designs of FBG-based optical add-drop multiplexing (OADM) system for DWDM networks, inclusive of using a multi-port circulator and FBGs for low-crosstalk and compact OADMs, and MZ-FBG-based fixed and reconfigurable multi-channel OADMs. Such MZ-OADMs not only can add and drop either single or multiple wavelengths, but also need no additional WDM demultiplexers and multiplexers. We also discuss the designs of fiber Bragg grating (FBG)-based optical cross-connect (OXC) multiplexing system, especially a modified series-type architecture using Mach-Zehnder (MZ) FBGs in combination with 2×2 optical switches (OSWs).
In Chapter 4, we propose simultaneous video and data signals transmission system over multi- mode fiber local area network (MMF-LAN), and the bi-directional transmission systems of multiple amplitude modulation vestigial sideband (AM-VSB) CATV signals, including a repeaterless bi-directional transmission system over 50-km single- mode fiber (SMF), and a bi-directional 100-km SMF and large effective area fiber (LEAF) transmission system.
In Chapter 5, we simulate and demonstrate the optimum configuration and characteristic comparisons of wideband optical amplifier for DWDM system applications.
Various configurations of erbium-doped fiber amplifier (EDFA) for simultaneously amplifying AM-VSB analog CATV signal and multiple optical digital baseband signals in hybrid wavelength division multiplexing (H-WDM) systems are proposed. Furthermore, the demonstration of such wideband amplifier not only carries out dispersion
compensation of the SMF spans, but also offers a good gain flatness of amplification. The nonlinear effect and improved approach within the semiconductor optical amplifier (SOA) are also addressed.
Chapter 6 gives a brief conclusion of this work and research.
Chapter 2
Overview and Motivation
This chapter will describe the overview and motivation in three concern applications:
1) fiber Bragg grating-based OADM systems for WDM Long-Distance Trunk/Ring Networks, 2) multiple AM-VSB signals transmission systems, and 3) wideband optical amplifiers for metropolitan area network as shown in Figure 1.2.
2.1 Fiber Bragg Grating-Based OADM Systems
The dense wavelength-division- multiplexing (DWDM) technique combining with erbium-doped fiber amplifiers (EDFA’s) has shown its capability to cost-effectively, gracefully upgrade the capacity of embedded long-distance transmission systems operating in the 1550-nm wavelength region [12]. The extensive deployments of such WDM amplified point-to-point transmission systems and/or SONET/SDH ring networks open the perspective of efficiently performing network functions in the optical domain. Thus, new optical elements are required to provide additional facilities for WDM signals locally transmitting/extraction (i.e., the add/drop operation), for signal routing and network (re)configuration (i.e., the cross-connect operation) in such DWDM networks. Optical add-drop multiplexer (OADM) and optical cross-connect (OXC), two of the new network elements, will play a key role enabling greater connectivity and flexibility in DWDM networks [13-15]. The importance of OADM’s and OXC’s is that they allow the optical network to transmit/extract and exchange on a wavelength-by-wavelength basis to optimize traffic, efficient network utilization, network growth, and to enhance network flexibility.
2.1.1 Low-Crosstalk and Compact Optical Add-Drop Multiplexer Based on A Multi-Port Circulator and Fiber Bragg Grating
Among FBG-based OADMs, the conventional structure using FBGs sandwiched between a pair of three-port optical circulators [16] is the simplest one. Such structure has been demonstrated over an installed 4 × 2.5 Gb/s optically amplified submarine cable system [17]. Recently, multi-port optical circulators (MOCs) with port number of ≥ 6 are commercially available and are promising for many applications to miniaturize the dimension of fiber-optic device due to the compactness of such MOC. Therefore, we propose and demonstrate three kinds of new and compact OADM structures based on using an MOC with FBGs. The intra-band interferometric crosstalk, which results from the insufficient reflectivity of the used FBG, can be hugely reduced by such MOC-based structure with cascaded FBGs. Bit-error-rate (BER) performance and power penalty, induced by intra-band and inter-band crosstalks, of these OADMs are examined and compared in a 10 Gb/s system demonstration.
2.1.2 Mach-Zehnder Fiber Bragg Grating-based Fixed and Reconfigurable Optical Add-Drop Multiplexers
From the viewpoint of wavelength selecting capability, OADM’s can be classified into fixed and reconfigurable configurations. The basic operation for a fixed OADM is that it can only add and drop the desired single or multiple wavelengths in a pre-assigned pattern at each optical node. So, a fixed OADM does not have any automated wavelength selecting capability. If an OADM is non-reconfigurable, then it may be obsolete or should be modified when the network grows with more WDM channels. Many fixed OADM’s have been implemented based on different technologies such as a pair of back-to-back WDM dielectric-grating multiplexers [18], the fiber Fabry-Perot filters with optical circulators [19], arrayed waveguide grating (AWG) [20, 21], the fiber Bragg gratings (FBG’s) sandwiched between a pair of optical circulators [17, 22], Mach-Zehnder-based (MZ) FBG’s [23, 24], MZ-based grating on silica [25], and FBG’s photoimprinting upon
the 3-dB coupler [26].
For the reconfigurable OADM’s, it can add and drop either single or multiple wavelength channels dynamically at each optical node according to the network’s management. Therefore they offer more flexibility for efficient network utilization and customer control and management in DWDM networks. Several reconfigurable OADM’s have been proposed and demonstrated recently. One was implemented by using a pair of N
× N AWG sandwiched between a pair of 1 × N mechanical optical switch (OSW) [27], and another one was realized by actively controlling the MZ-based FBG filter [28]. However, both of them only can only add-drop a single wavelength. For reconfigurably add-dropping either single or multiple desired wavelengths, some OADM’s were realized by introduction of OSW’s in such OADM architectures. For example, one was implemented by inserting the 2 × 2 mechanical OSW within each optical path of a pair of AWG multiplexers [29, 30], or by fabricating the thermo-optic switches in between the silica-based AWG multiplexers [31]. Some OADM’s were realized by introduction of OSW’s in the parallel [32] or series [33] FBG-based architectures. On the other hand, the acousto-optic tunable filter has also been shown the capability for acting as a reconfigurable OADM with its fast µs-switching speed but with undesired side lobes [34].
However, for these OADM’s reported in [35-38], additional WDM demultiplexer and multiplexer are required at the drop and add ports, respectively, to implement full add-drop functionality. This drawback makes them unpractical and expensive to be used.
Recently, an InP-based OADM [39] consisting of a loop-back AWG demultiplexer integrated with MZ interferometer electro-optic switches has been demonstrated without the need of additional WDM multiplexer and multiplexer. However, the poor features of high intraband crosstalk of –20 dB, high on-chip insertion loss of 7-11 dB, and polarization-sensitive property imply that more improvements are required for such device to be capable of add-dropping multiple WDM channels. With the motivation of investigating both fixed and reconfigurable polarization- insensitive OADM’s, we first, to the best of our knowledge, utilize the Mach-Zehnder fiber Bragg grating-based (MZ-FBG) [40] devices with the associated mechanical OSW’s to construct large-dimension OADM’s without the needs of additional WDM demultiplexers and multiplexers.
2.1.3 Mach-Zehnder Fiber Bragg Grating-Based Dynamic Optical Cross-Connect
Fiber Bragg grating (FBG) as a cross-connecting element in OXCs is very attractive recently [41]. On the other hand, the FBG-based Mach-Zehnder Interferometer (MZ-FBGs) device [42] has sho wn its capability for optical add-drop multiplexers.
Recently, a dynamic 2 × 2 OXC employing MZ-FBGs and optical space switches (OSWs) was reported [43]. However, the number of required MZ-FBGs in this structure is too large, due to the used parallel-type architecture, to limit its usage for practical application.
A series-type 2 × 2 MZ-FBG-based OXC was reported [44]. It has strictly non-blocking operation at the expense of serious circulated and accumulated high-order intra-band crosstalks accompanied with basic intra-band crosstalks. These crosstalks limit its performance.
Therefore, we propose a modified series-type architecture using MZ-FBGs in combination with 2 × 2 OSWs for constructing a rearrangeably non-blocking 2 ×2 OXC with not only a huge reduction of four order of required MZ-FBGs as compared with the work in [44], but also eliminating the circulated and accumulated high-order intra-band crosstalks as compared with the work in [44]. Dynamic single- and multi- channel cross-connections can be realized according to the control of OSW arrangements.
Network demonstration and bit-error-rate (BER) performance of the proposed OXC are examined. The loss and crosstalk characteristics of the OXC, and thus the allowable number of DWDM channels are investigated.
2.2 Multiple AM-VSB Signals Transmission Systems
A combination of EDFA and WADM technology can provide unprecedented ability to expand transmission capacity in long-distance transmission systems or local area networks. Bi-directional transmission over a single fiber can further double the transmission capacity. However, the amplified spontaneous emission (ASE) of EDFA, crosstalk from Rayleigh backscattering, and fiber nonlinear effects in the transmission
process give many constraints to system performance, especially those employing analog bi-directional transmission. In this section, we begin with simultaneous video and data signals transmission system and then propose bi-directional AM-VSB transmission links.
2.2.1 Simultaneous Video and Data Signals Transmission System over Multi-Mode Fiber Local Area Network
Delivery of AM-VSB CATV video signals through standard single- mode fiber (SMF) link using the 1.3-µm or 1.55-µm opto-electronic technology to increase the transmission distance, the number of distribution nodes, and to maintain high video quality is the trend for CATV operators in the worlds [40–44]. On the other hand, numerous high-speed lightwave local-area networks (LANs) using multi- mode fibers (MMFs) have been installed in the campus. Beside the data service, new service such as video broadcasting to buildings and classrooms is expected to implement through the existed LAN networks. However, the delivery of CATV video signals over an MMF LAN has not yet reported. The 1.55-µm video performance degraded by the intermodal dispersion of MMF and the power penalty of 1.3-µm data transmission due to the on- line video delivery are needed to study. We investigate the transmission of AM CATV video signal at 1.55-µm and the 155-Mb/s data signal at 1.3-µm simultaneously over a graded-index MMF LAN. System designs and network size for this MMF LAN are also described and investigated.
2.2.2 Repeaterless Bi-directional Transmission of Multiple AM-VSB CATV Signals
Lightwave subcarrier
- multiplexed systems ha ve been widely used for cable television (CATV) networks to deliver amplitude-modulated vestigial-sideband (AM-VSB) analog video signals [50-53]. Repeaterless long distance fiber transmission systems using erbium-doped fiber amplifiers (EDFAs) have many applications, in which cases it is infeasible or impossible to have an in- line amplifier. Bi-directional transmission over asingle- fiber using wavelength-division- multiplexing (WDM) technique offers the advantages of capacity doubling. Several repeaterless bi-directional digital transmission systems have been reported recently [54-57], but the repeaterless bi-directional transmission of multiple AM-VSB analog signals has not yet been studied.
In addition, in some countries, telecommunication operator should put out its optical cabled fiber with associated transmitters and receivers to lease to the CATV multiple system operators (MSOs) for trunk delivery of 80 channels of analog CATV signals between any two MSOs (MSO1 and MSO2) across another different CATV-operator service area. Hence, when the dummy fiber is exhausted in the fiber cable, the fiber- leased company has to face the question that is it feasible to transport multiple AM-VSB video signals bi-directionally over a single leased conventional single- mode fiber (SMF) link.
Therefore, in such case, bi-directional transmission over a single- fiber using WDM technique is the simple method to solve the problem. We investigate and demonstrate bi-directional transmission of multiple AM-VSB channels over convent ional SMF. Two kinds of multiplexers, the optical circulator (OC) and the optical multiplexer (MUX) configurations, for supporting bi-directional operation are studied and compared.
Extending the repeaterless bi-directional transmission distance is also addressed.
2.2.3 Bi-directional CATV 100 km Transmission Systems
When the dummy fiber is exhausted in the fiber cable, the fiber- leased company has to face the question: Is it feasible to transport multiple AM-VSB video signals bi-directionally over a single leased fiber link? Therefore, in such case, bi-directional transmission over a single- fiber is a simple method to solve the problem. In this sub-section, we demonstrate bi-directional transmission of multiple AM-VSB channels over 100-km fiber link. The conventional single- mode fiber (SMF) and large effective area fiber (LEAF) were acted as the leased fiber link separately for comparison.
2.3 Wideband Optical Amplifier for Metropolitan Area Network
With the unprecedented growth of Internet network, many large capacity WDM transmission experiments with aggregate capacities of hundreds of Gb/s or Tb/s have been demonstrated using several types of wideband optical amplifiers. The gain bandwidths of EDFAs have been enlarged through the use of gain-equalizing filters, new host materials (fluoride and tellurite glasses), and two gain band configurations [58, 59] in the 1.5- to 1.6-µm region. In this section, we investigate the configurations and amplification characteristics of wideband optical amplifier.
2.3.1 Erbium-Doped Fiber Amplifier for Hybrid WDM Systems
The implementation of Hybrid WDM systems with different signal formats is still a challenge, especially for long-distance applications using erbium-doped fiber amplifiers (EDFA’s) [60]. Because different signal formats require different sensitivities for a given quality of service, for example, around 0 dBm for AM-VSB SCM signal and about –30 and –20 dBm, respectively, for an STM-16 (2.5 Gbit/s) and STM-64 (10 Gbit/s) SONET/SDH signals. Due to the difference in the required sensitivity among the various signal formats, the optical signal levels in such systems can vary greatly, up to several tens of decibels. In a typical H-WDM system employing in- line EDFA’s for AM-VSB/QAM and baseband digital signal trans missions, the input power to the EDFA for the analog channel needs to be greater than 0 dBm to operate the EDFA in deep saturation, owing to the stringent system carrier-to-noise ratio (CNR) requirement. In contrast, the input power for the digital channels is usually less than –10 dBm due to the low output power used in digital lightwave transmitters.
Here we theoretically investigate and compare various EDFA configurations to simultaneously amplify hybrid digital/analog WDM signals. The configurations considered include single-stage and two-stage designs in dual- forward, dual-backward, and various bi-directional pumping schemes, each with and without the midway optical isolator. For bi-directional pumping case, the pump-power passed and un-passed
arrangements are also considered. For the two-stage design with the midway isolator cases, the optimum position and the isolation effect of midway isolator on EDFA performance are studied. A total of nine EDFA configurations are examined and compared. The investigation result provides the best EDFA configuration to design both power and in-line amplifiers for hybrid WDM lightwave systems.
2.3.2 Dispersion-Compensated Gain-Clamped 90 nm Wideband Optical Amplifier
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.
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.