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Wavelength division multiplexing (WDM) is a technology that multiplexes optical signals of different wavelengths into one optical fiber for transmission.

·CWDM (sparse wavelength Division Multiplexing): large wavelength interval, usually 20nm

·DWDM (dense wavelength Division Multiplexing): small wavelength interval, usually at 0.8-2nm

Now I will mainly introduce DWDM dense wavelength Division Multiplexing (Dense Wavelength Division Multiplexing) in WDM.

DWDM combines optical carriers to transmit on one optical fiber, which improves the transmission capacity of each optical fiber. DWDM can carry SDH service, IP service and ATM service.

ITU ITU standard DWDM wavelength is 1528.77nm-1563.86nm, which mainly has low attenuation and dispersion in C band. 100GHz (0.8nm) wavelength spacing can have 40 channels, and 50GHz (0.4nm) wavelength spacing can have 80 channels.

DWDM unit structure

ZR transponder: complete the conversion of G.957 optical signal to G.692 fixed-wavelength optical signal; zugphotosynthetic filter and optical divider: complete the combination and division of G.962 fixed-wavelength optical signal

Angular optical amplifier (OLA): located in the middle of the optical transmission section, the optical signal is amplified by OLA

Zugt optical monitoring channel: used to carry the management and monitoring of DWDM system, so that the network management system can effectively manage DWDM system.

How the DWDM system works

One-way transmission of two-fiber:

Unidirectional wavelength division multiplexing system uses two optical fibers, one optical fiber only completes the transmission of optical signal in one direction, and the transmission of reverse optical signal is completed by another optical fiber.

Advantages: each signal is carried by different light wavelengths, which will not be affected, and the same wavelength can be reused in both directions.

Disadvantages: the resource utilization of optical fiber and optical devices is not high.

Two-way transmission of single fiber:

The bidirectional wavelength division multiplexing system only needs one optical fiber to transmit in two different directions at the same time, and the wavelengths used are separated from each other in order to realize the full-duplex communication between the two sides.

Advantages: it can reduce the number of optical fiber and line amplifiers and save costs.

Disadvantages: high requirements, the need to solve multi-channel interference, to extend the transmission distance requires optical amplification.

Classification of DWDM system

An open DWDM system

OTU is used in the transmitter to convert the non-standard wavelength into the standard wavelength. The main function of this device is to convert the non-standard wavelength into the standard wavelength regulated by ITU-T to meet the wavelength compatibility of the system.

The integrated DWDM system

The service signal itself has met the standard wavelength, and the receiver and sender do not need OTU.

Key components of DWDM

ZJ light source

The function of light source is to produce laser or fluorescence, which is an important device in optical fiber communication system.

The light source of DWDM system has relatively large dispersion capacity and standard and stable wavelength.

The laser has two modulation modes: direct modulation and indirect modulation.

The wavelength of the laser is stable.

PV photodetector

The function of the photodetector is to convert the received optical signal into the corresponding electrical signal. Because the optical signal transmitted from the optical fiber is generally very weak, it puts forward a very high requirement for the photodetector.

ZOA optical amplifier

Optical amplifiers are used to enhance optical signals, including erbium-doped optical tension amplifier EDFA (Erbium Doped Fiber Amplifier) and Raman fiber amplifier.

Dead optical multiplexer and optical demultiplexer

The core components of wavelength division multiplexing system are wavelength division multiplexer, that is, optical multiplexer and optical demultiplexer, which are actually optical filters, and their characteristics determine the performance of the whole system to a great extent. its requirements are sufficient number of multiplexing channels, low insertion loss, wide passband range and so on.

There are many kinds of optical wavelength division multiplexer, which can be divided into four types: interference filter type, optical fiber coupler type, raster type and arrayed waveguide grating type.

Networking type

Point-to-point networking

Zigzag network

Ad ring networking

Advantages of DWDM technology

Large capacity: because DWDM technology makes full use of optical fiber bandwidth resources, dozens or even hundreds of channel signals are multiplexed in one optical fiber, so the capacity of a single optical fiber is greatly improved.

"transparent" data transmission: DWDM systems are transparent to "data", regardless of the rate of the signal and the mode of electrical modulation. Therefore, a variety of service signals with different rates, formats and characteristics can be transmitted at the same time.

It is convenient and flexible to upgrade and expand the system: it can introduce new services by adding wavelengths without interrupting existing services, thus protecting the existing investment to the maximum extent.

Network economy and reliability: the new communication network composed of DWDM technology is much simpler than that of traditional TDM technology, and the network is hierarchical. Because of the simplified network structure, clear hierarchy and convenient service scheduling, the economy and reliability of the network are obvious.

The DWDM technology will be one of the key technologies to realize the all-optical network, and the DWDM system can be compatible with the future all-optical network. It is possible to realize a transparent and highly survivable all-optical network on the basis of the built DWDM network in the future.

Comparison between DWDM and CWDM

DWDM technology and CWDM technology are two different products of WDM technology, which have their own advantages in different network levels.

CWDM technology has a good application prospect in the edge access layer of man because of its low cost and simple structure. DWDM technology has become the first choice of backbone network, core metropolitan area network and local network backbone transmission equipment because of its large capacity and long-distance transmission.

When choosing the CWDM/DWDM scheme, we should comprehensively consider the needs and budget of the project, and choose the best scheme according to their characteristics and differences.

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