Is a wavelength division multiplexed WDM optical signals with different wavelengths are multiplexed into an optical fiber transmission technology. 
Ø CWDM (Coarse Wavelength Division Multiplexing): wavelength interval large, typically 20 nm
Ø DWDM (Dense Wavelength Division Multiplexing): wavelength interval is small, usually in 0.8-2nm
now mainly to introduce you to the WDM DWDM Dense Wavelength Division Multiplexing (Dense Wavelength Division Multiplexing).
The DWDM optical carrier composite in an optical fiber transmission, increased transmission capacity per fiber. DWDM can carry SDH services, IP services, ATM services. 
ITU standards ITU DWDM wavelength 1528.77nm-1563.86nm, predominantly low attenuation and dispersion in the C-band. 100GHz (0.8nm) wavelength spacing may be 40 channels, 50GHz wavelength spacing may be 80 channels (0.4nm).
DWDM cell structure 
Ø repeater: complete conversion G.692 to G.957 optical signal of fixed wavelength light; Ø optical multiplexer and the optical demultiplexer: complete multiplexer G.962 and a fixed-wavelength optical signal demultiplexed;
Ø optical amplifier (OLA): located in the middle section of the optical transmission, the optical signal amplified by the OLA;
Ø optical supervisory channel: for carrying DWDM systems management and monitoring, network management system to effectively manage the DWDM system.
DWDM系统的工作方式
Ø 双纤单向传输：
单向波分复用系统采用两根光纤，一根光纤只完成一个方向光信号的传输，反向光信号的传输由另一根光纤来完成。
优点：各信号通过不同光波长携带，不会影响，同一波长可以在两个方向上重复利用。
缺点：光纤以及光器件资源利用率不高。
Ø 单纤双向传输：
双向波分复用系统看只需要一根光纤，在一跟光纤上同时向两个不同的方向传输，所用的波长相互分开，以实现彼此双方全双工的通信联络。
优点：可以减少使用光纤和线路放大器的数量，节约成本。
缺点：要求较高，需要解决多通道干扰，延长传输距离需要进行光放大。
DWDM系统的分类
Ø 开放式DWDM系统
在发送端采用OTU将非标准的波长转换为标准波长，该器件的主要作用在于把非标准波长转换为ITU-T所规范的标准波长，以满足系统的波长兼容性。
Ø 集成式DWDM系统
业务信号本身已经满足标准波长，收端发端都不需要OTU。
DWDM关键部件
Ø 光源
光源的作用是产生激光或荧光，是组成光纤通信系统的重要器件。
DWDM系统的光源具有比较大的色散容纳值和标准而稳定的波长。
激光器有直接调制和间接调制两种调制方式。
激光器的波长的稳定。
Ø 光电检测器
光电检测器的作用是把接收到的光信号转换成相应的电信号。由于从光纤传送过来的光信号一般是非常微弱的，因此对光检测器提出了非常高的要求。
Ø 光放大器
光放大器用来增强光信号，主要有掺铒光张放大器EDFA（Erbium Doped Fiber Amplifier）、拉曼光纤放大器。
Ø 光复用器和光解复用器
波分复用系统的核心部件是波分复用器，即光复用器和光解复用器，实际均为光学滤波器，其特性好坏在很大程度上决定了整个系统的性能，其要求是复用信道数量足够、插入损耗小、通带范围宽等。
光波分复用器的种类有很多，大致可以分为四类：干涉滤光器型、光纤耦合器型、光栅型、阵列波导光栅型。
Network Type
Ø networking point 
Ø a chain network 
Ø ring networking 
DWDM technology advantages
Ø Large Capacity: Because DWDM technology makes full use of bandwidth resources of optical fiber, the optical fiber in a multiplexed channels of dozens or even hundreds signal, greatly improving the capacity of a single optical fiber;
Ø data "transparent" transmission: DWDM systems 'data' is transparent, and electrically independent of the rate of modulation signal. Thus simultaneous transmission rate, format, features a variety of different traffic signals.
Ø system upgrades, convenient and flexible expansion: can without disrupting existing business by adding the wavelength introduction of new services, the maximum protection of existing investments.
Ø Networking economy and reliability: the use of new communications network consisting of DWDM technology than with conventional electricity network consisting of time-division multiplexing technology to greatly simplified and structured network. Due to simplify the network structure, hierarchy and service scheduling convenience, thus bringing the economy and reliability of the network is obvious.
Ø may constitute all-optical network: DWDM technology will be one of the key technologies to realize all-optical networks, and DWDM system compatible with future all-optical networks, may be achieved on the basis of transparent DWDM network has been built on in the future, with high degree of all-optical network survivability.
DWDM and CWDM comparison
of DWDM and CWDM wavelength division multiplexing technology is the product of two different technologies, each with advantages in different network hierarchy.
CWDM technology because of its low cost, simple structure and characteristics of multi-service access layer on the edge of the MAN better application prospect .DWDM technology is due to the characteristics of its large capacity, long-distance transmission network and the backbone core MAN, the preferred local network backbone transmission equipment.
In selecting CWDM / DWDM solutions, and comprehensive consideration the needs and budget of the project, combined with their characteristics and differences, choose the best solution.