Solutions to Achieve Long-haul Transmission With DWDM Systems

In order to increase the transmission distance of optical signals, many technologies, like the TDM (time division multiplexing) and WDM (wavelength division multiplexing), have been used. Except for that, several optical components like single mode fiber optic cables, optical amplifiers and dispersion compensating modules (DCMs) are also put into use to realize the goal. Today, this article intends to illustrate the solutions to achieve longer transmission distances with DWDM technology.

Solutions to Extend Transmission Distances

When it comes to long-haul optical transmissions, DWDM (dense wavelength division multiplexing) is a topic that cannot be ignored. DWDM technology enables different wavelengths to transmit over a single optical fiber. Different wavelengths are combined in a device—Mux/Demux which is short of multiplexer/demultiplexer. The DWDM Mux/Demux provides low insertion loss and low polarization-dependent loss for optical links. Here take a 8CH DWDM Mux/Demux for example to illustrate how to extend distance in long haul transmission.

Solution One

The first solution is suitable for applications that are less than 50km. The picture below shows a unidirectional application with 8CH DWDM Mux/Demux. As we can see, in this links, the DWDM Mux/Demux transmits 1550nm signal over one single mode fiber. The eight different signals from the transmitters are multiplexed into 1550nm signal by the 8CH DWDM Mux. Then they go through the single mode fiber and are separated into the original wavelengths by the DWDM Demux. The use of DWDM Mux/Demux and single mode fiber allows the system to transmit over 50km without optical amplifier or DCM.

8 channel mux demux in long haul transmission

Notes: this solution is the basic application of DWDM Mux/Demux in a relative long distance comparing to CWDM technology which suits short distance deployment.

Solution Two

Different from the first solution, if the link distance is longer than 50km, this solution can be taken into account. Optical signal loss will become greater as the links are getting longer, which means an optical amplifier module or dispersion compensator is needed. Therefore, to achieve a satisfying signal quality in long-distance transmission, an EDFA which can boost the weakened optical signals is added in this solution (as shown in the picture below).

edfa in long haul transimission

Solution Three

This DWDM configuration is similar to the former one, but with the EDFA, the link distance on the single mode fiber is up to 200km. However, sometimes an EDFA is not enough to achieve a quality signal, especially in some long haul systems like CATV system. Because these systems often have a high requirement for the quality of optical signal. Therefore, as we can see in the following picture, except for the DWDM Mux/Demux and EDFA, there is also a DCM.

edfa and dcm in long haul transmission

This solution is a point-to-multipoint long haul system deploying a DCM to extend the transmission distance. From the picture, the EDFA is placed midway between the transmitter and receiver in the transmission path. And in order to ensure the quality of the whole transmission, a DCM module is added in this link to deal with the accumulated chromatic dispersion without dropping and regenerating the wavelengths on the link.

Notes: all the three solutions are unidirectional transmission on single mode fiber cables. If a network requires bidirectional transmission to transfer eight signals, you can use a 16CH DWDM Mux/Demux over single fiber or a 8CH DWDM Mux/Demux over dual fiber.

Summary

WDM technology, especially the DWDM, is the critical step to go into the super-long distance transmission in optical communication. This post mainly introduces three basic solutions to realize long haul transmission with DWDM Mux/Demux. All the components including the DWDM Mux/Demux (both 8 channels and 16 channels), EDFA, DCM and optical modules are available in FS.COM. If you have any needs, please contact us via sales@fs.com.

Differences between CWDM and DWDM

In fiber-optic communications, WDM (wavelength-division multiplexing) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber as well as multiplication of capacity. Generally, WDM technology is applied to an optical carrier which is typically described by its wavelength.

WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split the signals apart (see Figure 1). WDM system is very popular in the telecommunication industry because it allows the capacity of the network to be expanded without laying more fiber. By utilizing WDM and optical amplifiers, users can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Moreover, the capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.

WDM operating principle

Figure 1

WDM could be divided into CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). DWDM and CWDM are based on the same concept of using multiple wavelengths of light on a single fiber but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. Below part will introduce some differences between CWDM and DWDM system.

Wavelength Spacing

CWDM provides 8 channels with 8 wavelengths (from 1470nm through 1610nm) with a channel spacing of 20nm. While DWDM can accommodate 40, 80 or even 160 wavelengths with narrower wavelength spans which are as small as 0.8nm, 0.4nm or even 0.2nm (see Figure 2).

CWDM-VS-DWDM

Figure 2

Transmission Distance

DWDM multiplexing system is capable of having a longer haul transmittal by keeping the wavelengths tightly packed. It can transmit more data over a larger run of cable with less interference than CWDM system. CWDM system cannot transmit data over long distance as the wavelengths are not amplified. Usually, CWDM can transmit data up to 100 miles (160km).

Power Requirements

The power requirements for DWDM are significantly higher. For instance, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4W per wavelength. Meanwhile, an uncooled CWDM laser transmitter uses about 0.5W of power.

Price

The DWDM price is typically four or five times higher than that of the CWDM counterparts. The higher cost of DWDM is attributed to the factors related to the lasers. The manufacturing wavelength tolerance of a DWDM laser die compared to a CWDM die is a key factor. Typical wavelength tolerances for DWDM lasers are on the order of ±0.1 nm, while tolerances for CWDM laser die are ±2-3 nm. Lower die yields also drive up the costs of DWDM lasers relative to CWDM lasers. Moreover, packaging DWDM laser die for temperature stabilization with a Peltier cooler and thermister in a butterfly package is more expensive than the uncooled CWDM coaxial laser packing.

To sum up, CWDM and DWDM have different features. Choosing CWDM or DWDM is a difficult decision. We should first understand the differences between them. Fiberstore has various kinds of WDM products, such as 10GBASE DWDM, 40 channel DWDM Mux, CWDM Mux/Demux module and so on. It is an excellent option for choosing CWDM and DWDM equipment.

Related Article: The Advantages and Disadvantages of Multimode and Single-mode Fiber

Differences Between CWDM and DWDM

Wavelength division multiplexing (WDM) is a technology or technique modulating numerous data streams, i.e. optical carrier signals of varying wavelengths (colors) of laser light, onto a single optical fiber. The goal of WDM is to have a signal not to interfere with each other. It is usually used to make data transmission more efficiently. It has also been proven more cost effective in many applications, such as WDM network applications, broadband network application and fiber to the home (FTTH) applications and so on. According to channel spacing between neighbored wavelengths, there are two main types of WDM: Coarse WDM (CWDM) and Dense WDM (DWDM). Though both of them belong to WDM technology, they are quite different. We can differentiate them from the definition, data capacity, cable cost and transmission distance.

Definition
CWDM is defined by wavelengths and has wide-range channel spacing. DWDM is defined by frequencies and has narrow channel spacing.

  • CWDM is a method of combining multiple signals on laser beams at various wavelengths for transmission along fiber optic cables, such that the number of channels is fewer than in DWDM but more than in standard WDM. “Course” means the channel spacing is 20 nm with a working channel passband of +/-6.5 nm from the wavelengths center. From 1270 nm to 1610 nm, there are 18 individual wavelengths separated by 20nm spacing.
  • DWDM is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. “Dense” refers to the very narrow channel spacing measured in Gigahertz (GHz) as opposed to nanometer (nm). DWDM typically uses channel spacing of 100 GHz with a working channel passband of +/-12.5 GHz from the wavelengths center. It uses 200GHz spacing essentially skipping every other channel in the DWDM grid. And it has also gone one step further using an Optical Interleaver to get down to 50GHz spacing doubling the channels’ capacity from 100GHz spacing.
Data Capacity

In fiber optic network system, DWDM system could fit more than 40 different data streams in the same amount of fiber used for two data streams in a CWDM system. In some cases, CWDM system can perform many of the same tasks compared to DWDM. Despite the lower transmission of data through a CWDM system, these are still viable options for fiber optic data transmission.

Cable Cost

CWDM system carries less data, but the cabling used to run them is less expensive and less complex. A DWDM system has much denser cabling and can carry a significantly larger amount of data, but it can be cost prohibitive, especially where there is necessary to have a large amount of cabling in an application.

Transmission Distance

DWDM system is used for a longer-haul transmission through keeping the wavelengths tightly packed. It can transmit more data over a significantly larger run of cable with less interference. However, CWDM system cannot travel long distances because the wavelengths are not amplified, and therefore CWDM is limited in its functionality over longer distances. If we need to transmit the data over a very long range, DWDM system solution may be the best choice in terms of functionality of the data transmission as well as the lessened interference over the longer distances that the wavelengths must travel. As far as cost is concerned, when required to provide signal amplification about 100 miles (160 km), CWDM system is the best solution for short runs.

What Is The DWDM Mux Demux Working Principle?

From the wikipedia we know that“ In electronics, a Mux Demux is a device that selects one of several analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output. Multiplexers are mainly used to increase the amount of data that can be sent over the network within a certain amount of time and bandwidth .A mux is also called a data selector.”

An electronic multiplexer makes it possible for several signals to share one device or resource, for example one A/D converter or one communication line, instead of having one device per input signal.

Conversely, a demultiplexer (or demux) is a device taking a single input signal and selecting one of many data-output-lines, which is connected to the single input. A multiplexer is often used with a complementary demultiplexer on the receiving end.

DWDM Data Transmission Technology

Dense wavelength-division multiplexing (DWDM) revolutionized data transmission technology by increasing the capacity signal of embedded fiber. This increase means that the incoming optical signals are assigned to specific wavelengths within a designated frequency band, then multiplexed onto one fiber. This process allows for multiple video, audio, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems. This technology responds to the growing need for efficient and capable data transmission by working with different formats, such as SONET/SDH, while increasing bandwidth.

DWDM Fiber Working Principle

DWDM fiber works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. 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.As for DWDM mux/demux, the common configuration is 2CH, 4CH, 8CH, 16 CH(16-Channel DWDM Mux/Demux), 32CH, 40CH channels.They are available in the form of Plastic ABS module cassette, 19” rack mountable box or standard LGX box. And no matter what kind of connectors, like FC, ST, SC, LC etc, all are available on our website: http://www.fs.com, and we also can mix connector on one device. We can provide you with the most suitable DWDM equipment.