Tag Archives: WDM

Differences between CWDM and DWDM

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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.

Learn more details about CWDM and DWDM SFP+ transceivers at Everything You Need to Know Before Buying CWDM and DWDM SFP+ Transceivers

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

CWDM vs DWDM: What’s the Difference?

Differences Between CWDM and DWDM

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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.

Introduction of EDFA

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Before talking about Erbium-doped fiber amplifier (EDFA), we must have a simple understanding about optical amplifier (OA). OA is a device that amplifiers an optical signal directly, without the need to first convert it to an electrical signal. It is an important component in optical communication. There are usually 3 different types of OA, including erbium-doped fiber amplifier (EDFA), semiconductor optical amplifier (SOA) and Raman amplifier. EDFA is just one type of OA but plays an important role in a long-haul optical fiber communication. Now I’d like to make a detailed introduction of EDFA.

EDFA is an optical repeater device. It is usually used to improve the intensity of optical signals being carried through a fiber optical communication system. An optical fiber is doped with the rare earth element erbium so that the glass fiber could absorb light at one frequency and emit light at another frequency. With its features of high power transfer efficiency and large dynamic range, as well as low noise figure and polarization independent, it is an ideal solution for Wavelength Division Multiplex (WDM) applications and long-haul applications. In addition, a particular advantage of EDFA is its large gain bandwidth, which is typically tens of nanometers and thus actually more than enough to amplify data channels with the highest data rates.

EDFA sample

Now let us learn the basic principle of EDFA. EDFA is a high gain amplifier. It usually has two used pumping bands 980nm and 1480nm. This action amplifies a weak optical signal to a higher power, effecting a boost in the signal strength. The 980nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow so that wavelength stabilised laser sources are typically needed. The 1480nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. In practice, a combination of 980nm and 1480nm pumping bands is usually used in EDFA.

The following picture shows us the working principle of EDFA. In optical fiber communication system, a relatively high-powered beam of light is mixed with the input signal using a wavelength selective coupler. The input signal and the excitation light must be at different wavelengths. The mixed light is guided into a section of fiber with erbium ions included in the core. This high-powered light beam excites the erbium ions to their higher-energy state. When the photons belonging to the signal at a different wavelength from the pump light meet the excited erbium atoms, the erbium atoms give up some of their energy to the signal and return to their lower-energy state. A significant point is that the erbium gives up its energy in the form of additional photons which are exactly in the same phase and direction as the signal being amplified. So the signal is amplified along its direction of travel only. Thus, all of the additional signal power is guided in the same fiber mode as the incoming signal. There is usually an isolator placed at the output to prevent reflections returning from the attached fiber. Such reflections disrupt amplifier operation and in the extreme case can cause the amplifier to become a laser.

principle of EDFA

By this blog, we have learnt that what is EDFA and its basic principle. If you have any requirements, welcome to visit Fiberstore.com or contact us over sales@fiberstore.com. Fiberstore is a professional supplier in this field. It can offer EDFA for you with high quality and competitive price.