光纤通信(optical fiber communications)中文版

定义：
在光纤中传输信息的技术。

光纤可以将光和信息传输很长的距离。基于光纤的系统已经在长距离光数据传输领域很大程度上取代了无线电通信系统。它们被广泛用于电话、互联网、高速局域网（LANs），有线电视（CATV），并且逐渐用于建筑物之间短距离的数据传输。大多数情况采用二氧化硅光纤，非常短的距离除外，这时更适合采用塑料光纤。 

与采用电缆的系统相比，光纤通信最大的优点包括： 

1. 光纤传输数据容量非常大：单根二氧化硅光纤可以容纳成百上千个电话信道，仅仅占用了理论容量的一小部分。近三十年来，光纤链路的传输容量方面的进展比计算机存储容量和速度的进展还要快很多。 
2. 光在光纤中传输损耗非常小，单模二氧化硅光纤中损耗仅为约0.2 dB/km，因此不对信号放大就可以传输几十千米。 
3. 当需要传输非常长的距离时，可以采用一个光纤放大器放大多个信道。 
4. 由于可以实现非常大的传输速率，传输每个比特的成本非常低。 
5. 光纤光缆比电缆重量轻很多。 
6. 电缆中存在的很多问题在光纤光缆中都不存在，例如，电磁场干扰（EMI）。这在工业环境中是很重要的问题。 

由于光纤传输系统传输容量非常大，所以需要高传输速率时，该系统比同轴铜电缆系统成本低很多。而如果速率较低，无法发挥其传输容量大的优势，光纤系统在经济上的优势很小，甚至可能更贵（不是因为光纤，而是需要附加的收发器）。最主要的原因，目前仍然采用铜电缆作为“最后一英里”（连接家庭用户和办公室）电缆，简单的是因为已经铺设了铜电缆，采用新的系统需要铺设额外的光纤光缆。 

光纤通信已经广泛用于城市区域，甚至光纤入户（FTTH）也逐渐普及，尤其是在日本，因特网用户已经可以得到100 Mbit/s的数据传输速率，高于目前采用电话线路的ADSL系统。有些国家采用例如矢量技术来使现存的铜电缆传输容量变大，为的是省去重新铺设光纤光缆的费用。但是，这仅仅是权宜之计，无法满足用户对增加带宽的需求。 

1.通信窗口 
光纤通信系统的工作波长通常位于某一个“通信窗口”： 

• 第一个窗口是800-900 nm。采用GaAs/AlGaAs激光二极管和发光二极管（LEDs）作为发射器，接收器采用硅光二极管。然而在这一波长区域光纤损耗比较大，也没有合适的光纤放大器工作在这一光谱区域。因此，第一通信窗口仅适宜于短程传输。 
• 第二通信窗口为1300 nm附近的波长，二氧化硅光纤的损耗非常低并且光纤色散非常弱，因此色散展宽最小。这一窗口最早被用于长距离传输。然而，工作在1300 nm的光纤放大器（采用掺镨的玻璃）的性能没有采用掺铒的工作在1500 nm的放大器的性能好。另外，长距离传输通常不需要低色散，因为这样会增强光学非线性效应。 
• 第三通信窗口，是目前最常用的，波长在1500 nm附近。二氧化硅光纤在该波段损耗是最低的，掺铒光纤放大器也可以有很好的性能。光纤色散在该波长区域为反常色散，可以灵活操作（参阅色散位移光纤）。 

第二和第三通信窗口又被细分为下列波段：

最开始第二和第三通信窗口是由1400 nm处存在的损耗峰来划分开的，但是一些高级的光纤不具备这一峰值。 


2.系统设计 
最贱的光纤通信系统类型是光纤链路提供数据信道间点对点的连接。链路包含发送信息的发射器，传输光纤用来传输光波和一个接收器。传输光纤中还可以加入光纤放大器来放大光功率或者采用色散补偿器来消除色散效应。光纤链路词条中有更多的细节描述。 

目前长距离传输中，典型的信道容量为2.5或10 Gbit/s，未来可能可以实现40,100甚至160 Gbit/s。一些先进的系统通过采用多个具有不同波长的信道（粗或者密集波分复用）来显著提高传输容量。遇到的问题是抑制非线性引起的信道间串扰，平衡信道功率（例如，采用增益平坦光纤放大器），以及如何简化系统。另一个方案是采用时分复用技术，在时间域上将不同信道嵌套在一起，通常采用孤子来确保发出的超短脉冲之间完全分离，即使在很小的脉冲间距时也是如此。

 系统的另一个重大进展是采用光纤网络将不同位置的用户连接起来。这一方案非常灵活，也带来了很多技术问题，例如需要添加或者分离出波长信道，并且理想情况下是采用可重构的方式，或者不断重新调整连接拓扑从而能得到最有性能，还需要恰当处理错误信息，使它们对整个系统的影响最小。由于不断涌现和发展一些新的概念（例如，考虑拓扑，调制格式，色散控制，非线性控制和软件等）和新型装置（发射器，接收器，光纤，光纤器件，电子回路），目前何种系统会占据光纤通信的主导地位还不十分明朗。 

对于比特误码率和功率损耗问题的讨论，参考词条光数据传输。 

3.光纤的传输容量 
近三十年来，光纤的传输容量被大大的提高。每根光纤允许的传输带宽比电子存储芯片的存储容量或者微处理器的计算能力等增长的更快。 光纤的传输容量取决于其长度。长度越长，存在的有害效应例如模间色散等更多，因此能实现的传输速率更低。 

在几百米或者更短距离传输时（在存储区域网络内），采用多模光纤更加方便，因为安装更加便宜（由于大的纤芯面积，更易切割）。根据发射器技术和光纤长度，数据速率可以在几百 Mbit/s与10 Gbit/s之间。

 单模光纤用于传输更长距离时，几千米或者更长的情况下。目前商用电信系统传输几十千米或者更长时的单信道传输速率为2.5或者10 Gbit/s。未来系统可能会采用单信道速率高达40 Gbit/s，甚至160 Gbit/s，而目前得到需要的传输容量是采用具有不同波长的多个信道来实现的；这就是波分复用（WDM）。总的数据速率为几个太比特每秒，足以同时传输几百万个电话线路。这一容量还没有达到光纤的极限。另外，一个光纤光缆包含很多条光纤。 

总的来说，在可预见的未来，无需担心光纤数据传输会达到技术上的极限。相反的，数据传输容量的发展甚至比数据存储以及计算能力的发展更加迅速，这一事实使人们预测任何传输极限很快都回国时，超强的计算能力和存储设备会在高容量数据网络中广泛运用，类似于从发电站产生的电能在很大的电网中普遍应用。这一发展会更多的受限于软件和安全问题，而不是受限于数据传输。 

4.光纤通信的关键器件 
光纤通信系统依赖于一些关键器件： 

1. 光发射器，通常采用半导体激光器（VCSELs），光纤激光器，以及光调制器 
2. 光接收器，采用光二极管（雪崩二极管） 
3. 光纤，对损耗，导波性质，色散和非线性等因素进行了优化 
4. 色散补偿模块 
5. 半导体和光纤放大器（主要是掺铒光纤放大器，有时是拉曼放大器）来保证在长光纤中传输时保持足够的信号功率，或者在信号被探测之间先放大 
6. 光滤波器（采用光纤布拉格光栅）和耦合器 
7. 光开关和复用器（采用阵列波导光栅），例如，光分/插复用器（OADMs）可以在WDM系统中分离出或者插入波长信道 
8. 电控光开关 
9. 信号再生装置（电或者光再生器），时钟恢复 
10. 各种电子学器件，用作信号处理或者控制 
11. 计算机和软件来控制系统工作 

光纤通信中，光学和电学器件通常集成在光子集成回路上。这一技术领域的进步会使光纤通信拓展到千家万户（光纤入户）以及办公室中。

Definition: the technology of transmitting information through optical fibers

More general terms: optical data transmission

Optical fibers can be used to transmit light and thus information over long distances. Fiber-based systems have largely replaced radio transmitter systems for long-haul optical data transmission. They are widely used for telephony, but also for Internet traffic, long high-speed local area networks (LANs), cable TV (CATV), and increasingly also for shorter distances within buildings. In most cases, silica fibers are used, except for very short distances, where plastic optical fibers can be advantageous.

Compared with systems based on electrical cables, the approach of optical fiber communications (lightwave communications) has advantages, the most important of which are:

• Due to the large optical bandwidth, the transmission capacity of fibers for data transmission is huge: a single silica fiber can carry millions of telephone channels, utilizing only a small part of the theoretical capacity. In the last decades, the progress concerning transmission capacities of fiber links has been significantly faster than e.g. the progress in the speed or storage capacity of computers.
• The propagation losses of light in fibers are amazingly small: ≈ 0.2 dB/km for modern single-mode silica fibers, so that many tens of kilometers can be bridged without amplifying the signals.
• A large number of channels can be reamplified in a single fiber amplifier, if required for very large transmission distances.

See also our useful tutorial "Passive Fiber Optics"! This explains many aspects of fiber optics using interesting simulations.

• Due to the huge transmission rate achievable, the cost per transported bit can be extremely low.
• Compared with electrical cables, fiber-optic cables are very lightweight.
• Fiber-optic cables are immune to problems that arise with electrical cables, such as ground loops or electromagnetic interference (EMI). Such issues are important, for example, for data links in industrial environments.

Mostly due to their very high data transmission capacity, fiber-optic transmission systems can achieve a much lower cost than systems based on coaxial copper cables, if high data rates are needed. For low data rates, where their full transmission capacity cannot be utilized, fiber-optic systems may have less of an economic advantage, or may even be more expensive (not due to the fibers, but the additional transceivers). The primary reason, however, for the still widespread use of copper cables for the “last mile” (the connection to the homes and offices) is simply that copper cables are already laid out, whereas new digging operations would be required to lay down additional fiber cables.

Fiber communications are already extensively used within metropolitan areas (metro fiber links), and even fiber to the home (FTTH) spreads more and more – particularly in Japan, where private Internet users can already obtain affordable Internet connections with data rates of hundreds of Mbit/s – well above the performance of current ADSL systems, which use electrical telephone lines. In other countries, one often tries to squeeze out higher transmission capacities from existing copper cables, e.g. with the technique of vectoring, in order to avoid the cost of laying down fiber cables to the premises. This, however, is more and more seen only as a temporary solution, which cannot satisfy further growth of bandwidth demand.

It is also possible to transmit analog signals through fibers; that technology is called radio and microwave over fiber.

Telecom Windows

Optical fiber communications typically operate in a wavelength region corresponding to one of the following “telecom windows”:

• The first window at 800–900 nm was originally used. GaAs/AlGaAs-based laser diodes and light-emitting diodes (LEDs) served as transmitters, and silicon photodiodes were suitable for the receivers. However, the propagation losses are relatively high in this region, and fiber amplifiers are not well developed for this spectral region. Therefore, the first telecom window is suitable only for short-distance transmission (often through plastic optical fibers).
• The second telecom window utilizes wavelengths around 1.3 μm, where the loss of silica fibers is much lower and the fibers' chromatic dispersion is very weak, so that dispersive broadening is minimized. This window was originally used for long-haul transmission. However, fiber amplifiers for 1.3 μm (based on, e.g. on praseodymium-doped glass) are not as good as their 1.5-μm counterparts (erbium-doped fiber amplifiers). Also, low dispersion has been found not to be necessarily ideal for long-haul transmission, as it increases the effect of optical nonlinearities.
• The third telecom window, which is now very widely used, utilizes wavelengths around 1.5 μm. The losses of silica fibers are lowest in this region, and erbium-doped fiber amplifiers are available which offer very high performance. Fiber dispersion is usually anomalous but can be tailored with great flexibility (→ dispersion-shifted fibers).
• The utilization of even longer-wavelength bands has been considered, but there has not been much progress. In principle, one could achieve still substantially lower propagation losses, but only in non-silica fibers with very low infrared absorption e.g. between 2 μm and 3 μm. It would be challenging to develop and commercially introduce such new fiber technologies, together with adapted transmitters, receivers and amplifiers.

The second and third telecom windows are further subdivided into the following wavelength bands:

The second and third telecom windows were originally separated by a pronounced loss peak around 1.4 μm, but they can effectively be joined with advanced fibers with low OH content which do not exhibit that peak.

The use of the spectral bands is still limited by the availability of suitable fiber amplifiers. Erbium-doped fiber amplifiers can be optimized for the C band and/or the L band. Various other variants have been considered for shorter and longer wavelengths – for example, thulium-doped fiber amplifiers for the S band, praseodymium-doped amplifier (with fluoride glass) for the O band, or amplifiers with bismuth-doped silica for the O, E and S band. However, these are still not used to a substantial extent.

System Design

The simplest type of fiber-optic communication system is a fiber-optic link providing a point-to-point connection with a single data channel. Such a link essentially contains a transmitter for sending the information optically, a transmission fiber for transmitting the light over some distance, and a receiver. The transmission fiber may be equipped with additional components such as fiber amplifiers for regenerating the optical power or dispersion compensators for counteracting the effects of chromatic dispersion. The article on fiber-optic links gives more details.

A typical channel capacity for long-haul transmission for older systems is 2.5 or 10 Gbit/s; more advanced systems offer 40, 100 or 160 Gbit/s or even more. The transmission capacity can be further multiplied by simultaneously using several, dozens or even hundreds of different wavelength channels (coarse or dense wavelength division multiplexing). Overall transmission capacities of many dozens of Tbit/s can be reached that way. The main challenges are to suppress channel cross-talk via nonlinearities, to balance the channel powers (e.g. with gain-flattened fiber amplifiers), and to simplify the systems. Another approach is time division multiplexing, where several input channels are combined by nesting in the time domain, and solitons are often used to ensure that the sent ultrashort pulses stay cleanly separated even at small pulse-to-pulse spacings. Finally, one can employ space division multiplexing where different spatial channels are used – either with multi-core fibers or with multimode (few-mode) fibers. Hundreds of Tbit/s over thousands of kilometers are possible with such techniques.

Another important development is that of systems which link many different stations with a sophisticated fiber-optic network. This approach can be very flexible and powerful, but also raises a number of non-trivial technical issues, such as the need for adding or dropping wavelength channels, ideally in a fully reconfigurable manner, or to constantly readjust the connection topology so as to obtain optimum performance, or to properly handle faults so as to minimize their impact on the overall system performance. As many different concepts (e.g. concerning topologies, modulation formats, dispersion management, nonlinear management, and software) and new types of devices (senders, receivers, fibers, fiber components, electronic circuits) are constantly being developed, it is not clear so far which kind of system will dominate the future of optical fiber communications.

For a discussion of aspects such as bit error rates and power penalties, see the article on optical data transmission.

Transmission Capacity of Optical Fibers

Within the last 30 years, the transmission capacity of optical fibers has been increased enormously. The rise in available transmission bandwidth per fiber is even significantly faster than e.g. the increase in storage capacity of electronic memory chips, or in the increase in computation power of microprocessors.

The transmission capacity of a fiber depends on the fiber length. The longer a fiber is, the more detrimental certain effects such intermodal or chromatic dispersion are, and the lower is the achievable transmission rate.

For short distances of a few hundred meters or less (e.g. within storage area networks), it is often more convenient to utilize multimode fibers, as these are cheaper to install (for example, due to their large core areas, they are easier to splice). Depending on the transmitter technology and fiber length, they achieve data rates between a few hundred Mbit/s and ≈ 10 Gbit/s.

Single-mode fibers are typically used for longer distances of a few kilometers or more. Currently used commercial telecom systems typically transmit between 10 Gbit/s and 160 Gbit/s per data channel over distances of ten kilometers or more. The required total capacity is usually obtained by transmitting many channels with slightly different wavelengths through fibers; this is called wavelength division multiplexing (WDM). Total data rates can be many dozens of Tbit/s or even >100 Tbit/s, sufficient for transmitting many millions of telephone channels simultaneously. Even such a huge capacity does by far not reach the physical limit of an optical fiber. In addition, note that a fiber-optic cable can contain multiple fibers; it is also possible to utilize multi-core fibers, where multiple fiber cores are contained in a single fiber. Alternatively, space division multiplexing can also be realized with multimode fibers, using multiple-input multiple-output receiver technology.

In conclusion, there should be no concern that technical limitations to fiber-optic data transmission could become severe in the foreseeable future. On the contrary, the fact that data transmission capacities can evolve faster than e.g. data storage and computational power, has inspired some people to predict that any transmission limitations will soon become obsolete, and large computation and storage facilities within high-capacity data networks will be extensively used, in a similar way as it has become common to use electrical power from many power stations within a large power grid. Such developments may be more severely limited by software and security issues than by the limitations of data transmission.

Key Components for Optical Fiber Communications

Optical fiber communication systems rely on a number of key components:

• optical transmitters, based mostly on semiconductor lasers (often VCSELs), fiber lasers, and optical modulators
• optical receivers, mostly based on photodiodes (often avalanche photodiodes)
• optical fibers with optimized properties concerning propagation losses, guiding properties, dispersion and nonlinearities
• dispersion-compensating modules
• semiconductor and fiber amplifiers (mostly erbium-doped fiber amplifiers, sometimes Raman amplifiers) for maintaining sufficient signal powers over long lengths of fibers, or as preamplifiers before signal detection
• optical filters (e.g. based on fiber Bragg gratings) and couplers
• optical switches and multiplexers (e.g. based on arrayed waveguide gratings); for example, optical add/drop multiplexers (OADMs) allow wavelength channels to be added or dropped in a WDM system
• devices for signal regeneration (electronic or optical regenerators), clock recovery and the like
• various kinds of electronics e.g. for signal processing and monitoring
• computers and software to control the system operation

In many cases, optical and electronic components for fiber communications are combined on photonic integrated circuits. Further progress in this technological area will help optical fiber communications to be extended to private households (→ fiber to the home) and small offices.

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