The WDM System

Introduction
In fiber optic communications, Wavelength Division Multiplexing (WDM) is a technology that multiplexes a number of optical carrier signals into a single optical fiber by using different wavelengths of laser light. This technique allows bi-directional communications on a fiber strand, as well as capacity multiplication. A WDM system (Figure 1) uses a multiplexer at the transmitter to link the signals and a demultiplexer at the receiver to separate them. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an add-and-drop optical multiplexer. The concept was first published in 1978, and in 1980 WDM systems were being made in the laboratory. As a system concept, WDM forms include coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM).

The CWDM system
In simple terms, the CWDM team performs two functions: segregate light to ensure that only the desired combination of wavelengths are used, multiplexing and demultiplexing the signal over a single fiber link.

CWDM solutions typically provide the capacity of 8 wavelengths, separated by 20 nm, from 1470 nm to 1610 nm, allowing the transport of 8 client interfaces through the same fiber, as shown in the Figure 2. Additionally, CWDM has the ability to transport up to 16 channels (wavelengths) on the spectrum grid from 1270nm to 1610nm with a channel spacing of 20nm. Each channel can operate at 2.5, 4, or 10 Gbit / s. CWDM cannot be amplified since most 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 global range of approximately 100 kilometers. However, due to the larger channel spacing in CWDM, cheaper uncooled lasers are used, giving a cost advantage over DWDM systems.

CWDM proves to be the initial entry point for many organizations due to its lower cost. Each CWDM wavelength generally supports up to 2.5 Gbps and can be expanded to 10 Gbps. 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 to cost-effectively transport large amounts of data traffic in telecommunications or business networks. Optical networks and especially the use of CWDM technology have proven to be the most cost-effective way to address 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 headend (HE) or center, or central office (CO), where individual signals at discrete wavelengths are multiplexed or combined into one fiber for downstream transmission. The multiplexing function is performed by means of a passive CWDM (Mux) multiplexer module that uses a sequence of specific wavelength filters. Filters are connected in series to combine the various specific wavelengths into a single fiber for transmission to the field. On the outside floor, a CWDM demultiplexer module (Demux), essentially a Mux mirror, is employed to extract each specific wavelength of the feeder fiber for distribution to individual FTTX applications.

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

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

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber, as 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 load capacity of that fiber from 2.5 Gb / s to 20 Gb / s. Currently, due to DWDM, individual fibers have been able to transmit data at rates of up to 400 Gb / s.

A basic DWDM system contains five main components: a DWDM terminal multiplexer, an intermediate line repeater, an optical add-and-drop multiplexer (OADM), a DWDM terminal demultiplexer, and an optical monitoring channel (OSC). A DWDM terminal multiplexer contains a wavelength conversion 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. A drop-and-drop optical multiplexer is a remote amplification site that amplifies the multiple wavelength signal that may have traveled 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 conversion transponders separates the multiple wavelength optical signal into individual data signals and outputs them on separate fibers for client layer systems (such as SONET / SDH). An Optical Monitoring Channel (OSC) is a data channel that uses an additional wavelength, usually outside the EDFA amplification band (at 1,510nm, 1,620nm, 1,310nm, or other proprietary wavelength).

DWDM is designed for long distance transmission where the wavelengths are close together and do not suffer from scattering and attenuation. When erbium doped fiber amplifiers (EDFAs), a kind of performance enhancer for high-speed communications, these systems can operate for thousands of kilometers. DWDM is widely used for the 1550nm band to take advantage of EDFA’s capabilities. EDFAs are commonly used for 1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L band).

A key advantage of DWDM is its protocol and bit rate independence. DWDM-based networks can transmit data over 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 through an optical channel. From a QOS standpoint, DWDM-based networks create a lower cost way to quickly respond to bandwidth demands and protocol changes from clients.

DWDM DeMultiplexer. Transmitters. 200 km. Each wavelength behaves as if it has it own virtual fibre Optical amplifiers needed to overcome losses in mux/demux and long fibre spans.

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

Fiber Optical Technology Comprehensive Summary and Guide

How does it work?

We may think that digitized information is “encoded” or placed in pulses of light for transmission. The information travels along the fiberglass at the speed of light (186,000 miles / second). When it reaches its destination, a decoder converts the light information into an image, audio.

Fiber optic cable

Fiber optic cables consist of the following components:

Core: clear plastic or glass through which light travels
Cladding: glass cover that surrounds the core that acts as a mirror to reflect light back into the core. This is called total internal reflection.
Buffer coating: covers and protects the fiber
Aramid Wire Resistance Member – Strengthens the integrity of data transmission through the optical fibers in the cable.
Protective outer shell – Extruded PVC is typical


Fiber Cable Designs

There are two basic types of cable design. They are: Loose tube (typically used for “OSP” outdoor plant installations) and with tight protection (typically used for indoor installations).

Loose tube fiber cable consists of: (shown as correct figure)。

Multiple 250 µm coated fibers
One or more loose tubes holding those fibers
Gel filling to block moisture and protect the movement of the fibers.
Central Force Member
Aramid yarn strength member
Outer jacket

Strongly shielded fiber cable consists of (as shown in the figure on the left):

900 µm tight buffer around a 250 µm fiber
Central Force Member
Aramid yarn strength member
Outer jacket

Fiber Cable Types

There are two basic types of cable. Simplex and Duplex. Both types of cable come: singlemode and multimode. Singlemode is for long distance cable runs and Multimode is for shorter cable runs.

Fiber optic tools and test equipment

With each fiber installation, special tools and equipment are required to complete the job. Welink recommends the following Fusion Splicers fiber optic tools and equipment for test kits.

Fiber Optic Splitters

A fiber optic splitter combines light signals and divides them into single or multiple outputs. Welink dividers are immune to electromagnetic interference (EMI), consume no electrical power, and add no noise to the system design. Welink dividers can be manufactured in custom fiber lengths and with any type of connector.

Welink is a professional fiber product store in the fiber optic industry, it supplies all kinds of above fiber optic products in the item, and there is good news that it is doing 30% discount from above price, if there are any needs. Welink is a very good choice.

Present Great Developments and Expections for FTTH

FTTH, fiber for the home, you know. Provides access technology to the end customer. There is a situation that fiber optic cables extend to the ONU (Optical Network Unit) depending on the customer’s facilities, they also provide the customer with virtually unlimited bandwidth for all applications such as video, voice and data High speed, and the speed can reach up to 1G per customer. Related passive optical product: PLC splitter. And FTTH is a test of the future, it becomes the only technology that can meet the requirements for such a high bandwidth, the FTTH architecture is shown in Figure.

We know that ONU is required for each client rather than for a group of up to several hundred clients. FTTH is not profitable at the moment, and depends on technological advances to provide more cost-effective bandwidth in fiber optic cables and effective UN technology.

Well, FTTH will be the preference of many people. After a thorough understanding, we can know that the easiest way to provide FTTH is to use a passive 1: N optical splitter to divide the optical bandwidth approximately equally among the N clients. Alternatively, a single fiber can also be applied for both transmission directions using Wavelength Division Multiplexing (WDM). The division of optical power among many consumers in a PON (passive optical network) has a significant optical power budget.

In recent years, FTTH has some developments, such as a proposal for the creation of a GNDG (National Gigabit Data Network), it may exceed the bandwidth communication requirements for the future and the proposal also allows the use of technologies Access alternatives, already discussed, such as XDSL, coaxial or wireless to provide service at lower data rates. However, the interface with such solutions can cost almost as much as the final FTTH infrastructure. In fact, WDM with optical splitters or optical amplifiers are the main technologies necessary to implement such an FTTH network. Now, WDM systems can provide 40G bandwidths per fiber, use 16 wavelengths at 2.5G each, and hope it can reach 100 rerabits in the next 5 years.