Overview of DWDM System Components of the Campus Network and Metropolitan Network

Optical technology has been widely used in the current telecommunication field and is now quite mature. As we all know, optical signals are propagated in the form of carriers, and wave modulation allows the transmission of analog or digital signals on carriers at high frequencies (typically 186 to 196 Thz), usually up to several gigahertz (GHz) or gigabits per seconds (Gbps). 

 

If you want to increase the bitrate even further, you can use several characteristic carrier waves, which are specific carriers that transmit on a single fiber without significant mutual interference. Each frequency corresponds to a different wavelength. Dense Wavelength Division Multiplexing (DWDM) is a communication technique that uses quite small frequency intervals. This article will describe in detail DWDM technology and DWDM components.

 

Introduction of DWDM Technology

 

DWDM technology is a type of Frequency Division Multiplexing (FDM), which utilizes different wavelengths to carry independent signals on a single fiber. By using DWDM, up to 80 channels can be multiplexed onto a single fiber strand. DWDM technology features include low cost, long distance, high bandwidth, and scalable capacity. DWDM devices use optical filters to separate the different wavelengths of light being carried on the fiber. Each wavelength is amplified by an erbium-doped fiber amplifier (EDFA) and sent to its destination. A DWDM equipment has two components: an Optical Line Terminal (OLT) at the centralized office and an Optical Network Unit (ONU) or Optical Network Terminal (ONT) at the customer premises. 

 

DWDM optical modules are used in the DWDM system for multiplexing and de-multiplexing these different wavelength signals. The DWDM device generally includes an arrayed waveguide grating (AWG) multiplexer/demultiplexer and two sets of input/output optical coupling devices. DWDM Mux/Demux is short for Dense Wavelength Division Multiplexing Multiplexer/Demultiplexes. It's one kind of special WDM device used in the WDM network for splitting and combining signals with different wavelengths. A DWDM Mux combines or multiplexes up to 80 channels onto a single fiber while a DWDM Demux separates or de-multiplexes those channels back out again. One advantage of using DWDM is that its protocol and bitrate are independent, thus DWDM-based networks can transmit data in IP, ATM, SONET, SDH, and Ethernet. Thanks to these technical advantages, the DWDM component has become one of the key technologies in today’s high-speed data communications networks.

 

DWDM System Components

 

DWDM systems are generally composed of the following 5 key components containing Optical transmitter/receiver, DWDM multiplexer/multiplexer filter, optical division and multiplexing (OADM), optical amplifier, and transponder (wavelength converter).

 

Optical Transmitters/Receivers

 

The transmitter in a DWDM component is an essential device in a DWDM system, and its function is to provide source signals and then multiplex these signals. It can be said that the system design of DWDM depends heavily on the characteristics of the optical transmitter. In DWDM equipment, the source consists of multiple optical transmitters. Incoming electronic data bits (0 or 1) trigger the modulation of the optical flow e.g. flash = 1 when there is no light = 0. In the module, the lasers generate precise optical pulses. These light pulses are characterized by a precise wavelength. In optical carrier-based systems, the digital information stream is routed to a physical layer device whose output is a light source (LED or laser) connected to a fiber optic cable. Transmitters/Receivers can convert the input digital signal from an electrical signal to an optical signal, completing the conversion of electrons to photons. The electrical signal triggers the light source and transmits the optical signal into the fiber, inputting code 1 when there is light, and code 0 when there is no light or very little light signal. In this way, the electrical-to-optical conversion is completed without changing the digital signal format of the earth city. The pulsed light is transmitted through the fiber in the form of total internal reflection. At the receiving end, a photodiode detects the incoming light signal as 1 or 0 and thus converts it to an electrical signal. A pair of optical fibers is usually connected to two different devices (one for the transmitting fiber and the other for the receiving fiber)

 

In DWDM systems, the wavelength of light is exact, which allows the optical signal to be transmitted without interchannel distortion or crosstalk. Several individual lasers are usually used to set up the different channels in a DWDM signal. The operating wavelength of each laser is different. Current DWDM devices generally operate using 200, 100, and 50-GHz spacings, and new systems supporting 25-GHz spacings and 12.5-GHz spacings are being investigated in the industry. Currently, QSFPTEK can provide DWDM equipment operating at 100 and 50-GHz including DWDM SFP SFP+ QSFP28, etc.

 

DWDM Mux/Demux Filters

 

Optical filters also called Mux filters can combine signals from multiple transmitters emitting multiple wavelengths (all in the 1550nm band) operating on different fibers on a single fiber. The signal output by the optical multiplexer is called a composite signal. On the receiving terminal, an optical drop filter (Demux filter) splits all the separate wavelength signals of the composite signal onto different fibers. Different fibers transmit the demultiplexed wavelengths down as many fiber optic receivers as possible. Generally, the Mux and Demux (transmit and receive) assemblies are installed in the same chassis. 

 

Optical Mux/Demux devices can both be passive devices. Component signals are multiplexed and demultiplexed optically rather than electronically, so they do not require an external power supply. The following picture illustrates bidirectional DWDM operation. N optical light pulses of different wavelengths transmitted by N separate fibers are merged by a DWDM Mux. Each of these N signals is then multiplexed into a pair of fibers. The composite signal is received by the DWDM demultiplexer, which separates each of the N component signals and passes each signal to a single fiber. The transmitted and received signals are indicated by arrows representing client devices. It requires a pair of fibers to be used; one for transmitting and the other for receiving.

 

 

Optical Add/Drop Multiplexers

 

The optical add/drop multiplexers perform a special function different from the Mux/Demux filters and are responsible for the "Add/Drop" function. The image below shows a single channel DWDM OADM in operation. This OADM is only responsible for adding or dropping light signals of a specific wavelength. A composite signal is split into two parts, drop and pass, from left to right, and the OADM only drops the red light signal stream. The dropped signal flows are delivered to the receiver of the client equipment. The rest of the optical signal passing through the OADM is multiplexed with the newly added signal stream. the OADM adds a new red optical signal stream that operates at the same wavelength as the dropped signal. By combining the new optical signal stream with the passed signal, a new composite signal is formed. 

 

 

Optical Amplifiers

 

Optical amplifiers work by using additional energy to stimulate photons in the signal to increase the amplitude, or by increasing the gain of the optical signal on the fiber to strengthen the optical signal. The optical amplifiers are in-fiber devices. It can amplify optical signals over a very wide range of wavelengths, a feature that has led to its widespread use in DWDM equipment. The most common type of in-fiber optical amplifier is the erbium-doped fiber amplifier (EDFA). To extend the transmission distance of DWDM equipment, many different types of optical amplifiers are available, including DWDM EDFA, CATV EDFA, SDH EDFA, EYDFA, and Raman amplifier. The following figure shows a DWDM EDFA in operation.

 

 

Transponders (Wavelengths Converters)/OEO

 

Optical transponders can convert optical signals from one incoming wavelength to a different outgoing wavelength, a feature that makes them suitable for DWDM components. A transponder is an optical-electrical-optical (O-E-O) wavelength converter. The transponder performs O-E-O operation, converting the wavelengths of light, hence the name OEO. in DWDM devices, the transponder converts the customer's optical signal into an electrical signal and then rewinds, reshapes, or performs replay, reshaping, and retiming functions. The picture below shows a bi-directional transponder.

 

 

Conclusion

 

This article describes in detail DWDM technology and DWDM components. DWDM device is widely used in voice transmission, e-mail, video, multimedia data, etc. Using DWDM technology can bring higher bandwidth. It also mentions the DWDM system components, including Optical Transmitters/Receivers, DWDM Mux/DeMux Filters, Optical Add/Drop Multiplexers (OADMs), Optical Amplifiers, and Transponders. If you would like to learn more about DWDM systems, you can contact QSFPTEK via sales@qsfptek.com. QSFPTEK can provide you with a wide range of optical equipment including DWDM Mux/Demux, 10G/100G Transponders, and many other DWDM devices.