定义：
在波长域工作的波分技术
波分复用是指不同波长的信号合在一起传播，并且再次分开的技术。最多是被应用在光纤通信中用来在多个波长略有不同的信道中传输数据。采用这种方法可以极大的提高光纤链路的传输容量，可以通过结合使用有源器件例如光纤放大器可以提高使用效率。除了应用到电信中，波分复用还可以应用到单根光纤控制多个光纤传感器的情况。
电信系统中的WDM
理论上，采用单个信道中极其高的数据传输速率就可以达到单根光纤能承担的数据传输容量极限，这是对应的信道带宽非常大。然而由于二氧化硅单模光纤的低损耗传输窗口的带宽非常大（几十THz），这时的数据速率远远大于光电发射器和接收器能接受的数据速率。并且，传输光纤中存在的各种色散对宽带很宽的信道带来非常不利的效应，这样会极大的限制传输距离。波分复用技术可以解决这一问题，保持每个信号的传输速率在一个合适的水平（10 Gbit/s）的同时，通过多个信号的结合可以实现非常高的数据传输速率。
根据国际电信联盟（ITU）的标准可以将WDM分为两种：
- 粗波分复用（CWDM，ITU标准G.694.2）中信道数目较小，例如四个或者八个，比较大的20 nm的信道间距。标称波长范围是从1310nm到1610nm。发射器的波长容差相对较大，为±3 nm，因此未加稳定措施的分布反馈激光器可以使用。单信道的传输速率通常从1到3.125 Gbit/s。因此得到的总的数据速率在光纤到户没有实现的都市区域是有用的。
- 密集波分复用（DWDM，ITU标准G.694.1）是拓展到非常大数据容量的情况，同常在互联网骨干网络中需要使用。它包含很多的信道数目（40,80,160），因此对应的信道间距很小分别为12.5,50,100 GHz.所有通道的频率都以一个特定的193.10 THz（1552.5 nm）为参考标准。发射器需要满足很窄的波长容差的要求。通常发射器是采用温度稳定的分布反馈激光器。单信道的传输速率在1到10 Gbit/s之间，今后有望达到40 Gbit/s.
由于掺铒光纤放大器的放大带宽很大，所有的信道可以在同一个装置中被放大（应用满标度的CWDM波长范围时除外）。然而，当增益与波长有关或者当存在光纤非线性数据信道间相互作用（串扰，信道干涉）时，就会出现问题。结合不同的技术，例如发展宽带（双波段）光纤放大器，增益平坦滤波器，非线性数据反馈等，该问题已经有了很大的改进。信道带宽，信道间距，传输功率，光纤和放大器类型，调制格式以及色散补偿机制等系统参数需要统筹考虑来达到最佳的总体性能水平。
尽管目前的光纤链路中单根光纤中只包含较少的信道数目，也需要更换可以满足多个信道同时工作的发射器和接收器，这比更换整个系统来得到更高的数据容量要便宜很多。这一方案虽然极大的提高的数据传输容量，却不需要添加额外的光纤。
除了提高传输容量，波分复用也使复杂的通信系统更加灵活。不同的数据信道可以存在与系统中不同的位置，其它的信道也很灵活的可以被提取出来。在这种情况下需要用到分插复用器，这一期间可以根据数据信道的波长被插入信道或者从信道中提取出来。分插复用器可以灵活的重新配置系统，从而为大量的处于不同位置的用户提供数据连接。
在许多情况下，可以用时分复用（TDM）代替波分复用。时分复用是不同信道根据到达时间的不同而不是波长不同而被区分开。
Acronym: WDM
Definition: a multiplexing technique working in the wavelength domain
More general term: optical multiplexing
Opposite term: time division multiplexing
Wavelength division multiplexing is a kind of frequency division multiplexing – a technique where optical signals with different wavelengths are combined, transmitted together, and separated again. It is mostly used for optical fiber communications to transmit data in several (or even many) channels with slightly different wavelengths. In this way, the transmission capacities of fiber-optic links can be increased strongly, so that most efficient use is made not only of the fibers themselves but also of the active components such as fiber amplifiers. Apart from telecom, wavelength division multiplexing is also used for, e.g., interrogating multiple fiber-optic sensors within a single fiber.
WDM in Telecom Systems
Theoretically, the full data transmission capacity of a fiber could be exploited with a single data channel of very high data rate, corresponding to a very large channel bandwidth. However, given the enormous available bandwidth (tens of terahertz) of the low-loss transmission window of silica single-mode fibers, this would lead to a data rate which is far higher than what can be handled by optoelectronic senders and receivers. Also, various types of dispersion in the transmission fiber would have very detrimental effects on such wide-bandwidth channels, so that the transmission distance would be strongly restricted. Wavelength division multiplexing solves these problems by keeping the transmission rates of each channel at reasonably low levels (e.g. 10 Gbit/s or 100 Gbit/s) and achieving a high total data rate by combining several or many channels.
Two different versions of WDM, defined by standards of the International Telecommunication Union (ITU), are distinguished:
Course Wavelength Division Multiplexing
Coarse wavelength division multiplexing (CWDM, ITU standard G.694.2 [11]) uses a relatively small number of channels, e.g. four or eight, and a large channel spacing of 20 nm. The nominal wavelengths range from 1310 nm to 1610 nm. The wavelength tolerance for the transmitters is fairly large, e.g. ±3 nm, so that unstabilized DFB lasers can be used. The single-channel bit rate is usually between 1 and 3.125 Gbit/s. The resulting total data rates are useful e.g. within metropolitan areas, as long as broadband technologies are not widespread in households (→ fiber to the home).
Dense Wavelength Division Multiplexing
Dense wavelength division multiplexing (DWDM, ITU standard G.694.1 [10]) is the extended method for very large data capacities, as required e.g. in the Internet backbone. It uses a large number of channels (e.g. 40, 80, or 160), and a correspondingly small channel spacing of 12.5, 25, 50 or 100 GHz. All optical channel frequencies refer to a reference frequency which has been fixed at 193.10 THz (1552.5 nm). The transmitters have to meet tight wavelength tolerances. Typically, they are temperature-stabilized DFB lasers. The single-channel bit rate can be between 1 and 100 Gbit/s, and in the future even higher.
Even much higher channel counts (>1000) have become practical by using novel methods of producing the channels. One such technique is spectral slicing of broadband femtosecond pulses [8]. While such a femtosecond laser is substantially more expensive than a DFB laser as usually used for one channel, the femtosecond laser can replace a large number of such lasers and at the same time substantially reduce the efforts required for signal multiplexing.
Common Fiber Amplifiers
Due to the wide amplification bandwidth of erbium-doped fiber amplifiers, all channels can often be amplified in a single device (except in cases where e.g. the full range of CWDM wavelengths is used). However, problems can arise from the variation of gain with wavelength or from interaction of the data channels (crosstalk, channel interference) e.g. via fiber nonlinearities. Enormous progress has been achieved with a combination of various techniques, such as the development of very broadband (double-band) fiber amplifiers, gain flattening filters, nonlinear data regeneration and the like. The system parameters such as channel bandwidth, channel spacing, transmitted power levels, fiber and amplifier types, modulation formats, dispersion compensation schemes, etc., need to be well balanced to achieve optimum overall performance.
Upgrade of Existing Fiber Links
Even for existing fiber links with only one or a few channels per fiber, it can make sense to replace senders and receivers for operation with more channels, as this can be cheaper than replacing the whole system with a system with a higher transmission capacity. In fact, this approach often eliminates the need to install additional fibers, even though the demand on transmission capacities is increasing enormously.
Increased Flexibility
Apart from increasing the transmission capacity, wavelength division multiplexing also adds flexibility to complex communication systems. In particular, different data channels can be injected at different locations in a system, and other channels can be extracted. For such operations, add–drop multiplexers can be used, which allow one to add or drop data channels based on their wavelengths. Reconfigurable add–drop multiplexers make it possible to reconfigure the system flexibly so as to provide data connections between a large number of different stations.
The optimization of the key parameters such as number of optical channels, their frequency spacing, their optical bandwidth and the used modulation format is a complex task, involving various kinds of trade-offs.
Alternative or Addition: Time Division Multiplexing
Time division multiplexing (TDM) can be an alternative to wavelength division multiplexing. Here, different channels are distinguished by arrival time rather than by wavelength.
It is also possible to combine both techniques, which can lead to highest overall bit rates above 1 Tbit/s.
