Very effective is the method of compaction of optical carriers - WDM (Wavelength Division Multiplexing). The essence of this method lies in the fact that a number of information flows, each transported on its optical carrier, with the help of special devices - optical multiplexers - is combined into one optical signal, which is introduced into the optical fiber. On the receiving side, the reverse operation of demultiplexing is performed.

Advantages of CWDM technology:

• Transmission of 16 independent services over two pairs of optical fibers.

• Low cost compared to DWDM.

• Flexibility in the implementation of different topologies.

• Data transmission over long distances.

• Unified management system for all nodes of the CWDM network

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What is CWDM?

Coarse Wavelength Division Multiplexing (CWDM) is a data transmission technology that allows simultaneous transmission of different protocols over a single pair of optical fibers. CWDM is based on the use of optical channels separated from each other at a distance of 20 nm. These optical channels, which lie in the range from 1310 to 1610 nm, are specified by recommendation G-694.2 of the International Telecommunication Union (ITU). When the range is extended down to 1270 nm, the number of possible transmission channels increases to 18. However, in this scenario, two problems arise. First, at shorter wavelengths, the radiation loss is almost twice as great, and therefore the maximum allowable transmission distance is markedly reduced; secondly, it is necessary to use special fibers.

Therefore, in practice, the number of possible transmission channels does not exceed 16.

Why is CWDM?

CWDM technology extends the "lifetime" of existing fiber optic networks by using a frequency grid not used by traditional transceivers. The technology is invariant to information transfer protocols, which makes it possible to organize various telecommunication services in one transport environment. The increase in the frequency distance between the channels leads to a noticeable reduction in the cost of active and passive components compared to DWDM technology - Dense Wavelength Division Multiplexing (dense spectral multiplexing with a distance between channels of 0.8 nm). In addition, coarse spectral multiplexing provides flexibility of the information transmission system and the possibility of implementing various topologies.

What technologies can operators use to increase the capabilities of existing optical networks?

There are three easily accessible and easy-to-install and use spectral compaction or wavelength division multiplexing technology:

• 2-channel WDM;

• rough spectral multiplexing (CWDM);

• Dense spectral compaction (DWDM).

These technologies can offer the operator one additional wavelength (or virtual fiber), 18 additional wavelengths, or up to 160 additional wavelengths. All these technologies use the existing fiber in the operator's network.

What is WDM (Wavelength Division Multiplexing)?

Technology for adding two or more optical signals with different wavelengths, transmitted simultaneously by one fiber and separated at the far end by wavelengths. The most typical applications (2-channel WDM) combine wavelengths of 1310 nm and 1550 nm in a single fiber.

What is DWDM (Dense Wavelength Division Multiplexing)?

Technology for combining up to 160 wavelengths, transmitting them simultaneously in one fiber, followed by separation at the far end. DWDM uses distances between wavelengths up to 25GHz and requires the use of lasers with very strict tolerances and radiation stability. The DWDM wavelength band is rounded from 1530 nm to 1565 nm. In the same band, erbium-doped optical signal amplifiers (EDFA) work.

What is the main difference between WDM, CWDM & DWDM applications?

In most cases, WDM is the most economical solution when there is a lack of fiber in the cable, giving a fiber gain of 2 to 1 or 3 to 1 by combining the wavelengths of 1310 nm, 1550 nm and 1490 nm in one fiber. In the case where more channels are required to expand the capacity of the existing fiber-optic infrastructure, CWDM provides an effective solution for optical spans of short length (up to 80 km). For a low cost, CWDM can provide an increase in the capacity of the existing fiber of 18 to 1. With current optical signal loss characteristics in 1310 nm and 1490 nm transparency windows, WDM and CWDM applications are best suited for short distances. Where high capacitance or long-distance transmission is required, DWDM solutions are the preferred method for increasing fiber capacity. With its high-precision lasers optimized for 1550nm window operation (to reduce losses), DWDM systems are the ideal solution for more demanding networks. DWDM systems can use EDFA to amplify all wavelengths in a DWDM window and increase the transmission length to 500 km.

What are the benefits of each of these three WDM technologies?

Two-channel WDM (and three-channel) can be used to quickly and easily add additional (or two additional) wavelengths. It is very easy to install and connect and very inexpensive.

CWDM can easily and quickly add up to 18 additional wavelengths at ITU standardized frequencies. It is ideal for networks of moderate sizes with transverse dimensions of up to 100 km. Since the distance between wavelengths is 20 nm, less expensive lasers can be used, which provides a very low cost for solutions with moderate capacitance.

DWDM offers high-capacity and long-range solutions for FOCL sections with a high growth in fiber requirements and where long-distance transmission is required. DWDM systems can be deployed at a relatively low initial cost and channels (wavelengths) are easily added as they grow. EDFA amplifiers, together with dispersion compensators, can increase the range of systems to several thousand kilometers.

What are the limitations of each of these technologies?

Two (or three) channel WDM is limited to one or two channels, which can be added to the 1310 nm channel. The range of the system is usually limited to losses in the 1310 nm channel.

CWDM systems, although multichannel, do not have any optical amplification mechanisms and range limitations are determined by the channel with maximum attenuation. Moreover, channels from the range from 1360 nm to 1440 nm may experience the greatest attenuation (from 1 to 2 dB/km) due to the water peak in this area for some types of optical cable.

DWDM systems are typically limited in range to 4-5 amplification sites due to amplified Spontaneous Emissions (ASE) noises in EDFA. Simulation tools are available to determine exactly how much EDFA can be installed. On long sections (> 120 km) dispersion problems can create problems, which requires the installation of dispersion compensation modules. The DWDM band is limited by wavelengths ranging from 1530 nm to 1565 nm by the EDFA gain range.

What is reach extension and how can I use it?

Reach extension is a common term for amplifying or recreating a signal to allow it to travel a greater distance. Due to the analog nature of transmission, the optical signal, when transmitted through an optical connection, degrades due to dispersion, power loss, crosstalk, and nonlinear effects in the fiber and optical components. Two common approaches are used to combat these undesirable effects: Regeneration and Enhancement. Regeneration – recreating a signal by converting an optical signal to an electrical signal, processing it, and then converting it back to an optical signal. Amplification is an increase in the amplitude (power dB) of an optical signal without converting to an electrical signal.

What is 1R, 2R and 3R regeneration?

There are three different levels of optical regeneration that can be applied to increase transmission range.

• 1R-amplification: This regeneration technique adds optical power to a signal without affecting its shape or synchronicity. EDFA simply adds photons to an incoming optical signal at a specific wavelength and phase of that signal. This does not restore or resynchronize the incoming signal. A side effect of EDFA is the creation of amplified spontaneous emission noise, which accumulates with each EDFA in the line and can only be "purified" by converting the optical signal to an electrical form and back again. A typical amount of EDFA in a cascade connection is not more than 4 or 5.
  
   

• 2R-amplification and reshaping: This technique amplifies and restores the shape of the degraded signal. The shape of the recreated signal is close to the original signal, but the duration of time cycles (synchronicity) is not restored. The accumulation of jitter leading to a loss of synchronization will limit the number of cascade-installed 2R regenerators.
  
   

• 3R-regeneration, reshaping and re-timing: Together with the amplification and recovery of 3R, regeneration also recreates the original cycle duration (synchronicity) of the original signal, thus creating an ideal opportunity to increase the life of synchronous and asynchronous signals. An almost unlimited number of 3R regenerators can be installed along the path of the signal.

What is wavelength conversion and why is it necessary?

Wavelength conversion is the conversion from one wavelength to another for transportation. Due to the attenuation characteristics of the 1310 nm and 850 nm signals, it is sometimes necessary to convert these signals to a wavelength of 1550 nm to transmit them over the long spans of the optical fiber, benefiting from low losses at 1550 nm. Wavelength conversion is also used to convert wideband optical signals such as 1310nm or 1550nm to discrete ITU CWDM or DWDM wavelengths, allowing multiple wavelengths to be combined when transmitted over a single fiber.

If I convert my 1310nm signal to the wavelength xWDM, do I need to convert it back to 1310nm before receiving it at the far end?

No, it is usually not required. Most optical equipment manufactured in the last 10 years most likely has a broadband receiver that will operate in the range from ~1260nm to ~1620nm. This means that an interface that transmits to 1310nm is likely to receive a signal that has been converted for DWDM or for CWDM applications.