The Introduction to AON

1.Background
With the rapid development and globalization of the modern society, a large quantity of data needs to be transmitted, thus resulting in the explosive growth of information content. The explosive growth of information content enables people to places a higher demand on bandwidth, which is a symbol of communication content. However, the electronic bottleneck of photoelectric conversion has restricted the high-speed transmission of data, giving rise to the failure of optical communication network to meet the requirements of high-speed, large-capacity and long-haul transmission. In order to make full use of the potential bandwidth of fiber, continuously improve the transmission rate of fiber and accommodate the explosive growth of communication services, all-optical network (AON) is proposed.

2.What is AON?
All-optical network (AON) is emerging as a promising network for very high data rates, flexible switching and broadband application support. In principle, all-optical network is founded on the premise of keeping the transmission and exchange of data signals entirely in the optical domain from source to destination, thus removing the intermediate electronics to eliminate the so-called electronic bottleneck and allow arbitrary signal formats, bit-rates, and protocols to be transported. In an all-optical network, data signals are always maintained in the optical domain except when they enter or exit the network, as shown in Figure 1. It means that there is no electrical signal processing in the entire transmission, so various transmission modes (PDH, SDH, ATM, etc.) can be applied in the AON to significantly improve the utilization of network resources. Being equipped with excellent transparency, survivability, scalability and compatibility, AON can achieve the data transmission of ultra-long haul, ultra-large capacity and ultra-high speed to become the preferred choice of the future high-speed broadband network.

An all-optical network

Figure 1: An all-optical network

3.Properties over the current optical communication network
AONs are able to arm the communication network with better manageability, flexibility and transparency. Compared with the traditional communication networks and the current optical communication networks, AONs are equipped with the following advantages that they don’t possess.
(1) AON provides huge bandwidth. Because the transmission and exchange of signals in AON entirely operate in the optical domain, AON can make the best use of the transmission capacity of fiber.
(2) AON achieves the transparent transmission. Adopting optical circuit switching to choose the routing according to wavelengths, AON is transparent to signal formats, bit-rates and modulation modes. That is to say, AON allows arbitrary signal formats, bit-rates, and protocols to be transported.
(3) AON has nice compatibility. Not only can AON be compatible with the current networks, but also AON is able to support the future broadband integrated services digital network (ISDN) as well as the network upgrade.
(4) AON possesses excellent scalability. Adding new nodes to the network has no effect on the original network architecture and node devices.
(5) AON is equipped with good reconfigurability. According to the requirements of communication capacity, AON can dynamically change the network architecture. AON is capable of recovering, building and removing the wavelength link.
(6) AON adopts lots of passive components to take place of the large photoelectric conversion equipments. Possessing simple configuration, AON is easy to maintain. At the same time, the overall exchange rate of AON can be greatly lifted to improve the reliability of network.

4.Key technologies
The key technologies applied in AONs fall into four categories: all-optical switching technology, optical cross connection (OXC) technology, optical add-drop multiplexing (OADM) technology, all-optical relay technology and optical amplifier technology.

4.1 All-optical switching technology
All-optical switching is the directly switching process which omits the OEO conversion to make full use of optical communication bandwidth. All-optical switching technology contains light-path switching technology and packet switching technology. The light-path switching can be divided into three types: space-division switching, time-division switching, wavelength/frequency-division switching. Asynchronous transfer mode (ATM), belonging to the packet switching technology, has been extensively studied.

4.2 OXC technology
OXCs are the devices applied in the optical network nodes to flexible and effectively manage the fiber transmission network by cross-connecting the optical signals. OXC technology is an important means of achieving the reliable network protection and recovery as well as automatic wiring and monitoring.

4.3 OADM technology
OADM, utilized in the optical network nodes, is able to selectively add or drop some wavelength signals as well as directly pass some wavelength signals without affecting other wavelength channel transmission. That is to say, OADM in the optical domain accomplishes the functions that SDH ADM does in time domain. OADM technology possesses transparency, thus able to deal with the signals of arbitrary formats and rates.

4.4 All-optical repeater technology
All-optical repeater technology is to directly amplify the optical signals in the optical path. Replacing the traditional OEO repeaters with the all-optical transmission repeaters, we can settle the problems of the repeater intricacy and electronic bottleneck to achieve the all-actinic signal transmission. The all-optical transmission repeaters include semi-conductor optical amplifier (SOA), Praseodymium-doped fiber amplifier (PDFA) and erbium-doped fiber amplifier (EDFA).

5.Main Components
In all-optical networks, a large quantity of optical components, which include active components and passive components. We will discuss five main components applied in all-optical networks.

5.1 Optical connectors
Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. The connectors mechanically couple and align the cores of fibers so light can pass. Fiber Optic Connectors according to connector structure can be divided into: FC,SC, ST, LC, D4, DIN, MU, MTP, MPO and so on in various forms. The optical interface is the physical interface used to connect fiber optic cable.

Optical connectors

5.2 WDM multiplexer/demultiplexer
In a WDM system, multiplexers at the transmitter are used to join the signals together, and demultiplexers at the receiver are utilized to split them apart. According to different wavelength patterns, WDM multiplexer/demultiplexer can be divided into CWDM multiplexer/demultiplexer and DWDM multiplexer/demultiplexer.

Multiplexer Demultiplexer

5.3 OADM
An optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing systems for multiplexing and routing different channels of light into or out of asingle mode fiber (SMF). CWDM OADM is designed to optical add/drop one multiple CWDM channels into one or two fibers. DWDM OADM is designed to optical add/drop one multiple DWDM channels into one or two fibers.

OADM

5.4 Optical amplifiers
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal.

Optical amplifiers

5.5 Optical switches
An optical switch is a device used to open or close an optical circuit which enables signals in optical fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to another in telecommunication. In a network system, optical switch plays an important role in protecting the path.

Optical switches

6.Development prospects
All-optical network is the developing goal of the optical communication networks. To achieve the integrated all-optical network, we will experience two phases of development. The first phase is to develop the optical communication network into the all-optical transmission network. During the whole point-to-point fiber transmission process, the photoelectric conversion is not required. The second phase is to achieve the integrated all-optical network. After fulfilling the whole point-to-point transmission, lots of functions, such as signal processing, signal storing, signal exchanging, signal multiplexing/demultiplexing and so on, needs to be completed by the photonic technology. Fulfilling the functions of transmitting, exchanging and processing the end-to-end optical signals is the second developing phase—-the integrated AON.

The Application of EDFA

1.Introduction
Optical amplifiers is the critical technology for the optical communication networks, enabling the transmission of many terabits of data over distances from a few hundred kilometers to thousands of kilometers by overcoming the fiber loss limitation. As the first optical amplifier commonly used in optical communications systems, EDFA has resulted in a dramatic growth in transmission capacity with the deployment of WDM systems. Be equipped with the features of high output power, high gain, wide bandwidth, polarization independence and low noise figure, EDFAs have become one of the key components used in the new-generation optical communication system. This article is intended to give an overview of EDFA application to help you better understand EDFA.

2.What is EDFA?
Erbium-doped fiber amplifier (EDFA) is an optical repeater device that is utilized to boost the intensity of optical signals being carried through a fiber optic communications system. An optical fiber is doped with the rare earth element erbium so that the glass fiber can absorb light at one frequency and emit light at another frequency.

2.1 Working Principles of EDFA
The erbium-doped fiber (EDF) is at the core of EDFA technology, which is a conventional silica fiber doped with Erbium. When the Erbium is illuminated with light energy at a suitable wavelength (either 980 nm or 1480 nm), it is motivated to a long-lifetime intermediate state, then it decays back to the ground state by emitting light within the 1525-1565 nm band. The Erbium can be either pumped by 980 nm light, in which case it passes through an unstable short lifetime state before rapidly decaying to a quasi-stable state, or by 1480 nm light in which case it is directly excited to the quasi-stable state. Once in the quasi-stable state, it decays to the ground state by emitting light in the 1525-1565 nm band. This decay process can be stimulated by pre-existing light, thus resulting in amplification. The working principles of EDFA are shown in the Figure 1.

working-principle-of-EDFA

Figure 1: The working principles of EDFA

2.2 Baisc configuration of EDFA
EDFA configuration is mainly composed of an EDF, a pump laser, and a component (often referred to as a WDM) for combining the signal and pump wavelength so that they can propagate simultaneously through the EDF. In principle, EDFAs can be designed such that pump energy propagates in the same direction as the signal (forward pumping), the opposite direction to the signal (backward pumping), or both direction together. The pump energy may either be 980 nm pump energy, 1480 nm pump energy, or a combination of both. Practically, the most common EDFA configuration is the forward pumping configuration using 980 nm pump energy, as shown in the Figure 2.

EDFA-configuration

Figure 2: The EDFA configuration with 980 nm pump energy

3.Application of EDFA
As the most advanced technology of optical amplifiers, EDFA is extensively in the optical communication networks. Next we’ll discuss application forms and application fields of EDFA.

3.1 Forms of application

3.1.1 booster amplifier
When used as the booster amplifier, EDFA is deployed in the output of an optical transmitter to improve the output power of the multi-wavelength signal having been multiplexed, as shown in the Figure 3. In this way, distances of optical communication transmission can be extended. This application form places a demand of higher output power on EDFA.

The booster amplifier

Figure 3: The booster amplifier

3.1.2 preamplifier
When used as the preamplifier, EDFA needs the features of low noise and high gain. Being equipped with these features, EDFA can significantly improve the sensitivity of an optical receiver when deployed in the input of an optical receiver, as shown in the Figure 4.

The preamplifier

Figure 4: The preamplifier

3.1.3 line amplifier
When used as the line amplifier, EDFA is able to periodically compensate the transmission loss of lines. As a substitute for OEO repeater, EDFA can directly amplify the optical signals transmitted in lines. In this way, we solve the bottleneck problems of photoelectric interchange to lay a foundation for all-optical network (AON). Figure 5 shows this application of EDFA.

The line amplifier

Figure 5: The line amplifier

3.2 Fields of application
EDFA has the following fields of application:
(1) EDFA can be employed in the high-capacity and high-speed optical communication system. The application of EDFA is very constructive to deal with the problems of low sensitivity of receivers and short transmission distances owing to a lack of OEO repeater.
(2) EDFA can be utilized in long-haul optical communication system. By utilizing EDFA, we can dramatically lower construction cost by increasing the repeater spacing to reduce the quantity of regenerative repeaters. The long-haul optical communication system mainly includes the land trunk optical transmission system and the submarine optical fiber cable transmission system.
(3) EDFA can be used in the optical fiber subscriber access network system. If the transmission distances are too long, EDFA will function as the line amplifier to compensate for the transmission losses of lines, thus greatly increasing the number of subscribers.
(4) EDFA can be employed in wavelength-division multiplexing (WDM) system, especially dense wavelength-division multiplexing (DWDM) system. Utilization of EDFA in WDM system is able to solve the problems of insertion loss and reduce the influences of chromatic dispersion.
(5) EDFA can be utilized in community antenna television (CATV) system. In CATV system, EDFA functions as the booster amplifier to greatly improve the input power of an optical transmitter. Utilizing EDFA to compensate for the insertion loss of optical power splitters can significantly enlarge the scale of the distribution network and increase the number of subscribers.

4.Conclusion
Of the various technologies available for optical amplifiers, EDFA technology is by far the most advanced. Nowadays EDFA is extensively in the optical fiber communication networks. As communication technologies continue to be developed, EDFA will become the preferred choice for the future optical amplifiers. Being equipped with the features of flat gain over a large dynamic gain range, low noise, high saturation output power and stable operation with excellent transient suppression, EDFA will play a more and more important role in optical communication system to better serve subscribers.

The WDM System

Introduction
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals into a single optical fiber by using different wavelengths of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. A WDM system (Figure 1) uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer. The concept was first published in 1978, and by 1980 WDM systems were being realized in the laboratory. As a system concept, the ways of WDM includes coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM).

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Figure 1: The WDM system

The CWDM System
In simple terms, CWDM equipment performs two functions: segregating the light to ensure only the desired combination of wavelengths are used, multiplexing and demultiplexing the signal across a single fiber link.

Typically CWDM solutions provide 8 wavelengths capability, separated by 20nm, from 1470nm to 1610nm, enabling the transport of 8 client interfaces over the same fiber, as is shown in Figure 2. What’s more, CWDM has the capability to transport up to 16 channels (wavelengths) in the spectrum grid from 1270nm to 1610nm with a 20nm channel spacing. Each channel can operate at either 2.5, 4 or 10Gbit/s. CWDM can not be amplified as most of the channels are outside the operating window of the erbium doped fiber amplifier (EDFA) used in Dense Wavelength Division Multiplexing (DWDM) systems. This results in a shorter overall system reach of approximately 100 kilometers. However, due to the broader channel spacing in CWDM, cheaper un-cooled lasers are used, giving a cost advantage over DWDM systems.

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Figure 2:The CWDM system

CWDM proves to be the initial entry point for many organizations due to its lower cost. Each CWDM wavelength typically supports up to 2.5Gbps and can be expanded to 10Gbps support. This transfer rate is sufficient to support GbE, Fast Ethernet or 1/2/4/8/10GFC, STM-1/STM-4/STM-16/OC3/OC12/OC48, as well as other protocols.

CWDM is the technology of choice for cost efficiently transporting large amounts of data traffic in telecoms or enterprise networks. Optical networking and especially the use of CWDM technology has proven to be the most cost efficient way of addressing this requirement.

In CWDM applications, a fiber pair (separate transmit and receive) is typically used to serve multiple users by assigning a specific wavelength to each subscriber. The process begins at the head end (HE) or hub, or central office (CO), where individual signals at discrete wavelengths are multiplexed, or combined, onto one fiber for downstream transmission. The multiplexing function is accomplished by means of a passive CWDM multiplexer (Mux) module employing a sequence of wavelength-specific filters. The filters are connected in series to combine the various specific wavelengths onto a single fiber for transmission to the field. In the outside plant a CWDM demultiplexer (Demux) module, essentially a mirror of the Mux, is employed to pull off each specific wavelength from the feeder fiber for distribution to individual FTTX applications.

CWDM is suitable for use in metropolitan applications, also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example, the downstream signal might be at 1310 nm while the upstream signal is at 1550nm. CWDM can also be used in conjunction with a fiber switch and network interface device to combine multiple fiber lines from the switch over one fiber. CWDM is optimized for a cost conscience budgets in mind, with low-cost, small-powered laser transmitters enabling deployments to closely match guaranteed revenue streams.

The DWDM System
DWDM stands for Dense Wavelength Division Multiplexing. Here “dense” means the wavelength channels are very narrow and close to each other. DWDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber, as is shown in Figure 3. In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex eight OC -48 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s.

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Figure 3: The DWDM system

A basic DWDM system contains five main components: a DWDM terminal multiplexer, an intermediate line repeater, an optical add-drop multiplexer (OADM), a DWDM terminal demultiplexer and an Optical Supervisory Channel (OSC). A DWDM terminal multiplexer contains a wavelength-converting transponder for each data signal, an optical multiplexer and an optical amplifier (EDFA). An intermediate line repeater is placed approximately every 80–100 km to compensate for the loss of optical power as the signal travels along the fiber. An optical add-drop multiplexer is a remote amplification site that amplifies the multi-wavelength signal that may have traversed up to 140 km or more before reaching the remote site. A DWDM terminal demultiplexer consisting of an optical demultiplexer and one or more wavelength-converting transponders separates the multi-wavelength optical signal back into individual data signals and outputs them on separate fibers for client-layer systems (such as SONET/SDH). An Optical Supervisory Channel (OSC) is a data channel which uses an additional wavelength usually outside the EDFA amplification band (at 1,510nm, 1,620nm, 1,310nm or another proprietary wavelength).

DWDM is designed for long-haul transmission where wavelengths are packed tightly together and do not suffer the effects of dispersion and attenuation. When boosted by erbium doped fiber amplifiers (EDFAs)—a sort of performance enhancer for high-speed communications—these systems can work over thousands of kilometers. DWDM is widely used for the 1550nm band so as to leverage the capabilities of EDFA. EDFAs are commonly used for the 1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L Band).

A key advantage to DWDM is that it’s protocol and bit rate independence. DWDM-based networks can transmit data in IP, ATM, SONET/SDH, and Ethernet, and handle bit rates between 100Mb/s and 2.5Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel. From a QOS standpoint, DWDM-based networks create a lower cost way to quickly respond to customers’ bandwidth demands and protocol changes.

Conclusion
WDM, as a multiplexing technology in optical field, can form a optic-layer network called “all-optic network”, which will be the most advanced level of optical communications. It will be the future trend of optical communications to build a optical network layer based on WDM and OXC to eliminate the bottleneck of photoelectric conversion with a pure all-optic network. As the first and most important step of all-optic network communications, the application and practice of WDM is very advantageous to developing the all-optic network and pushing forward optical communications!

Typical CWDM Optical Elements and Features

CWDM VS DWDM differ noticeably in the spacing between adjacent wavelengths. DWDM packs many channels into a small usable spectrum, spacing them 1 to 2 nm apart; DWDM systems support a high channel count, but also require expensive cooling equipment and independent lasers and modulators to ensure that adjacent channels do not interfere. CWDM systems, on the other hand, use 10 to 25 nm spacing, with 1300 or 850 nm lasers that drift less than 0.1 nm/c. This low drift eliminates the need for cooling equipment, which, in turn, reduces the total system cost. As a result, CWDM systems support less total bandwidth than DWDM systems, but with 8 to 16 channels, each operating between 155 Mbps and 3.125 Gbps to over 100 Gbps. Typical systems support eight wavelengths, data rates up to 2.5 Gbps per wavelength, and distances up to 50 km.

CWDM uses lasers with a wide channel CWDM wavelength spacing. In contrast, DWDM, which is widely used in long-haul networks and some metro core networks (particularly those with large diameters), uses lasers with much narrower wavelength spacing, typically 0.8 or 0.4 nm. The wide channel spacing of CWDM means a lower system cost can be achieved. This lower equipment cost is the result of a lower optical CWDM mux/demux cost (due to wider tolerance on the wavelength stability and bandwidth).

CWDM represents significant costs savings-from 25% to 50% at the component level over DWDM, both for equipment OEMs and service provides. CWDM products cost about 3500 dollars per wavelength. Traditional CWDM only scale to about eight wavelengths, but for metro access applications, this may be adaquare. Also, mux demux manufacturer china have found ways to combine CWDM with its regular DWDM blades that allow the systems to scale up to 20 wavelengths. CWDM system architecture can benefit the metro access market because it takes advantage of the inherent natural properties of the optical devices and eliminates the need to artificially control the component characteristics.

8 channels, 1RU Rack Mount, Duplex, CWDM Mux  Demux

The typical CWDM optical elements are as follows:

CWDM Uncooled Coaxial Lasers: Distributed-feedback multiquantum well (DFB/MQW) lasers are often used in CWDM systems. These lasers typically come in eight wavelengths and feature a 13 nm bandwidth. Wavelength drift is typically only 5 nm under normal office conditions (say, with a 50℃ total temperature delta), making temperature compensation unnecessary. For additional cost savings, the lasers do not require external gratings or other filters to achieve CWDM operation. They are available with or without an integral isolator.

CWDM Transmitters/Receives: OC-48 CWDM transmitters typically use an uncooled DFB laser diode and are pigtailed devices in a standard 24-pin DIP package. Six to eight channels are supported (six channels: 1510 to 1610 nm; two additional channels are located at 1470 and at 1490 nm.) The OC-48 receiver typically uses an APD photodetector, has a built-in DC-DC converter, and employs a PLL for clock recovery. Transmission distances of up to 50 km are achievable with these modules.

CWDM Multiplexers/Demultiplexer: These come in 4 or 8 channel module, just 4 channel CWDM Multiplexer or 8 channel CWDM Multiplexer, typically use thin -film filters optimized for CWDM applications, with filtering bands matched t other CWDM wavelengths. Filters need to feature low insertion loss and high isolation between adjacent channels.

CWDM Optical ADD/Drop Modules (OADMS): These are available in various configurations with one, two, or four add and drop channels using the same thin-film filters as the CWDM mux and demux modules.

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CWDM Technology VS DWDM Technology

WDM is a technology that is achieved using a multiplexer to combine wavelengths traveling through different fibers into a single fiber. The space between the individual wavelengths transmitted through the same fiber are the basis for differentiating the CWDM and DWDM.

CWDM- Coarse wavelength division multiplexing. WDM systems with fewer than eight active wavelengths per fiber. DWDM – Dense wavelength division multiplexing. WDM systems with more than eight active wavelengths per fiber.

CWDM is defined by wavelengths. DWDM is defined in terms of frequencies. DWDM’s tighter wavelength spacing fit more channels onto a single fiber, but cost more to implement and operate. CWDM match the basic capacities of DWDM but at lower capacity and lower cost. CWDM enable carriers to respond flexibly to divers customers needs in metropolitan regions where fiber may be at a premium. The point and purpose of CWDM is short-range communications. It uses wide-range frequencies and spreads wavelengths far apart from each other. DWDM is designed for long-haul transmission where wavelengths are packed tightly together. Vendors have found various techniques for cramming 32, 64, or 128 wavelengths into a fiber. DWDM system is boosted by Erbium-Doped Fiber Amplifier, so that to work over thousands of kilometers for high-speed communications.

Hardware Cost
The cost difference between CWDM and DWDM systems can be attributed to hardware and operational costs. Despite the superiority in terms of cost of DWDM laser with respect to the CWDM DFB laser chilled provide cost effective solutions for long haul and metro rings large capacity demanding. In both applications, the cost of DWDM system is set off by the large number of customers who use this system. Except for encapsulation, the DWDM laser for stabilizing the temperature with a cooler and a thermistor, it is more costly than an uncooled laser coaxial CWDM.

Power Consumption
The energy requirements for DWDM are significantly higher. For example:DWDM laser temperature stabilized through coolers integrated modules encapsulation, These devices together with the associated PIN and the control circuit consumes approximately 4 W of power per wavelength monitor. However, an uncooled CWDM laser transmitter consumers about 0.5w. The transmitter of 8 channel CWDM system consume about 4W of power, while the same functionality in a DWDM system can consume up to 30W. As the number of wavelengths in DWDM systems with increased transmission speed, power and thermal management associated with them becomes a critical issue for the designers.

Because DWDM doesn’t span long distance as its light signal isn’t amplified, which keeps costs down but also limits maximum propagation distances. Manufacturers may cite working ranges of 50 to 80 kilometers, and by signal amplifiers to achieve 160 kilometer. CWDM supports fewer channels and that may be adequate for carrier who would like to start small but expand later when demand increases.