定义：
光纤通信链路，其中信号光在光纤中传输。

光纤链是光纤通信系统的一部分，用来为两个点（点对点连接）提供数据连接。它通常包含一个数据发射器，传输光纤（通常具有内置的光纤放大器）和一个接收器。这些器件大多数都是基于光纤光学，会在后面先采用一个简单的单通道系统中进行介绍。接着会介绍更加复杂一些的方案。 

图1：光纤链路的示意图，包含一个数据发射器，包含一些放大器的长传输光纤和一个接收器。放大器中还可以加入附加的器件用来进行色散补偿或者信号正反馈。 

目录 

1. 传输格式
2. 发射器
3. 传输光纤
4. 接收器
5. 双向传输
6. 复用技术
7. 有源光缆
8. 噪声和串扰限制

传输格式 
许多情况下，数据传输是数字化的，这样系统会非常通用并且相对对非线性畸变不那么敏感。存在多种不同的调制格式，也就是说可以采用不同的方法来编码信息。例如，简单的不归零（NRZ）格式传输比特是采用发送一个高的或者低光功率值，比特之间没有间隙，采用其它的方法进行合成。与之相比，归零（RZ）格式每发送一个比特返回一个安静状态，很容易实现自合成，但是它在相同的数据速率下需要要更高的光传输带宽。除了装置的细节和光带宽要求（与调制效率有关），不同的传输格式对噪声和串扰的敏感程度是不同的。 

发射器 
发射器将入射的电子信号转化成被调制的光束。信息可以通过，例如光功率（强度），光相位或者偏振，来进行编码；强度调制是最常见的。光波长一般是处于通信窗口中（参阅词条光纤通信）。 
典型的发射器是采用单模激光二极管（一般为VCSEL或者DFB激光器），或者直接采用驱动电流进行调制（DML，直接调制激光器），或者采用外置的光调制器（例如电吸收或者Mach-Zehnder调制器）。直接调制是更简单的选择，当数据速率为10Gbit/s或者更高时可以工作。然而，激光二极管中载流子浓度的变化会引起瞬时频率的变化，因此以啁啾的形式产生信号畸变。尤其当传输距离很长时，信号光对色散更加敏感。因此，外置调制器更适宜于高数据传输速率（10或者40Gbit/s）和长传输距离（千米以上）的情况。激光器可以工作在连续波模式，此时信号畸变最小。 
当信道数据速率更高时，可以采用时分复用，例如，在时间上交叉的四个速度为40 Gbit/s的通道可以得到总的速率为160 Gbit/s。归零格式的高数据速率情况，最好采用脉冲光源（例如产生孤子脉冲的锁模激光器）与强度调制器组合在一起。这会减小对调制器带宽的要求，因为在各脉冲之间调制器的透射率如何变化是没有影响的。 
在高数据速率情况下，发射器需要满足一些条件。特别是，需要达到很高的消光比，低的定时抖动，低强度噪声以及准确控制的时钟频率。当然，数据发射器还需要在最小人工参与的情况下工作稳定和可靠。 
在一些简单的情况下，发光二极管（LED）可以被用作发射器，由于其空间相干性很弱需要采用多模光纤。传输速率或者距离则被模间色散所限制，对于更长的带宽距离积情况，需要采用单模光纤。在短距离情况下，几百 Mbit/s是可能的。 

传输光纤 
长距离传输或者介质采用的传输光纤通常是单模光纤，当然在短距离传输时也可以采用多模光纤。后面的情况下，模间色散会限制传输距离或者比特速率。 
在长程宽带的光纤通道中某些特定位置包含光纤放大器来防止光功率下降太多。还可以采用分布放大器，其中注入的泵浦光束到传输光纤本身，然后产生拉曼增益来实现放大。另外，还需采取措施进行色散补偿（与光纤中的色散相互作用）和信号正反馈。后者需要信号的功率和光束质量（例如，脉冲宽度和长度）同时被储存。既可以通过单纯的光信号处理来实现，也可以采用电子方法先探测信号，接着进行光信号处理，再重新发送信号。 

接收器 
接收器需要快的光探测器，通常是光二极管，还需要包含来放大弱信号和提取数字信号的快速的电子学装置。在高速数据情况时，还需要电子色散补偿的电路。 
雪崩二极管灵敏度很高。接收器的灵敏度受限于噪声，通常是电子噪声。但是信号光本身也包含光噪声，例如放大器噪声。这些光学噪声不能通过改善接收器设计来消除。下面会对噪声效应进行更详细的描述。 

双向传输 
全双工连接可以提供双向的数据传输。这可以采用分立的光纤，或者采用单根光纤。后者情况可以在每个端口放置光纤分束器来连接发射器和接收器。然而，双向工作也需要权衡，因此在有些情况下（例如，非常高的数据速率）采用两根分立的光纤更好。 

复用技术 
典型的单通道系统的传输容量约为2.5或者10 Gbit/s；以后可能会采用更高的比如40 Gbit/s甚至160 Gbit/s的高传输速率。对于高数据速率情况，而已采用多个通道进行复用，在光纤中同时传输，然后分离开进行探测。 
常用的复用技术是波分复用（WDM）。这时不同的中心波长对应不同的信道。可以采用这种方法复用几百个信号（DWDM，密集波分复用），而为了使系统不至于太复杂，最好采用普通的波分复用器复用一定数目的信道。最大的问题就是消除非线性引起的信道间串扰，平衡信道间的功率（例如采用增益平坦光纤放大器），和简化系统。 
另一个技术是时分复用，其中不同的信道在时域上分离的，需要采用孤子来确认发射的超短脉冲是完全分离的。 
传输距离短时，例如数据中心之间的联络情况，可以简单的采用包含许多光纤的带状光缆以及对应数目的发射器和接收器。然而，这种方案的缺点就是光缆比较庞大。 

有源光缆 
传输距离短的情况可以采用有源光缆（AOC），其中光缆的终端放置了发射器和接收器（以及对应的电子装置）。通常的电子接口为USB或者HDMI，于是采用这种有源光缆是和电缆的用法完全一样的，并且还包含注入小直径、轻便以及可传输距离长的优点。 

噪声和串扰限制 
系统的数据传输容量最终是由噪声限制的。在光纤放大系统中，例如，光纤放大器中的自发辐射引起的量子噪声就是无法避免的（参阅放大器噪声）。噪声会以许多形式来影响系统性能，例如定时抖动（参阅Gordon-Haus抖动，尤其在孤子系统中）或者强度噪声会影响光探测。 
除了噪声，系统中的信号失真也会限制传输距离或者误码率。色散和传输光纤的非线性会引起严重的信号的失真。图2给出了一张眼图。信号可以很好的被探测到，而当光纤长度增加一倍后（这里未显示），情况就有不同了。 

图2：通信信号的眼图。这是RP光纤功率软件中的一个演示例子。 
发射器对信号探测有很大的影响。例如，简单调制的发射器会产生不需要的频率啁啾，这回增加传输光纤中的色散效应，因此传输一段距离后更难得到准确的信号。 
一个相关但是更复杂的问题是复用系统中不同信道之间的串扰问题。如果系统中信道间距是固定的，信道间会相互影响，影响的形式为当一个信号被放大是因另一个信号的功率减小（参阅四波混频）。这一效应与系统结构非常相关，包括发射器类型，调制格式，光纤参数，探测技术等。将这些效应建立模型然后优化传输系统是非常复杂的。 
噪声及其它的效应会引起误码率，也就是传输的比特中的一部分不能被完全探测到。假设比特误码率很低，可以采用一些技术来探测失误的比特并且纠正它们（例如通过重新发送存在误码的数据包）。随着传输距离和数据速率的增大，误码率存在一些极限。在这种情况下，比较不同光线链路的一个常用参数为带宽距离积。

Definition: optical communication links where the signal light is transported in fibers

Alternative term: fiber channels

A fiber-optic link (or fiber channel) is usually a part of an optical fiber communications system which provides a data connection between two points (point-to-point connection). It essentially consists of a data transmitter, a transmission fiber (in some cases with built-in fiber amplifiers), and a receiver. Even for very long transmission distances, extremely high data rates of many Gbit/s or even several Tbit/s can be achieved.

The used components, which are mostly based on fiber optics, are explained in the following, beginning with a simple single-channel system. More sophisticated approaches are discussed thereafter.

Figure 1: Schematic of a fiber-optic link, with a data transmitter, a long transmission fiber with several fiber amplifiers, and a receiver. The amplifiers can be supplemented with additional components for dispersion compensation or signal regeneration.

A special kind of fiber-optic links is used for timing distribution and synchronization, and is explained in the last paragraph of this article.

Transmission Formats

In most cases, the data transmission is digital, making the system very versatile and relatively insensitive, e.g. to nonlinear distortions. There are various different modulation formats, i.e., different methods for encoding the information. For example, a simple non-return-to-zero (NRZ) format transmits subsequent bits by sending a high or low optical power value, with no gaps between the bits, and extra means for synchronization. In contrast, a return-to-zero (RZ) format is easily self-synchronizing by returning to a rest state after each bit, but it requires a higher optical transmission bandwidth for the same data rate. Apart from details of the equipment and the optical bandwidth required (related to the modulation efficiency), different transmission formats also differ in terms of their sensitivity e.g. to noise influences and cross-talk.

Transmitter

The transmitter converts the electronic input signal into a modulated light beam. The information may be encoded e.g. via the optical power (intensity), optical phase or polarization; intensity modulation is most common. The optical wavelength is typically in one of the so-called telecom windows (see the article on optical fiber communications).

A typical transmitter is based on a single-mode laser diode (normally a VCSEL or a DFB laser), which may either be directly modulated via its drive current (DML = directly modulated laser), or with an external optical modulator (e.g. an electroabsorption or Mach–Zehnder modulator). Direct modulation is the simpler option, and can work at data rates of 10 Gbit/s or even higher. However, the varying carrier density in the laser diode then leads to a varying instantaneous frequency and thus to signal distortions in the form of a chirp. Particularly for long transmission distances, this makes the signal more sensitive to the influence of chromatic dispersion. Therefore, external modulation is usually preferred for the combination of high data rates (e.g. 10 or 40 Gbit/s) with long transmission distances (many kilometers). The laser can then operate in continuous-wave mode, and signal distortions are minimized.

For even higher single-channel data rates, time division multiplexing may be employed, where e.g. four channels with 40 Gbit/s are temporally interleaved to obtain a total rate of 160 Gbit/s. For high data rates with return-to-zero formats, it can be advantageous to use a pulsed source (e.g. a mode-locked laser emitting soliton pulses) combined with an intensity modulator. This reduces the bandwidth demands on the modulator, as it does not matter how the modulator's transmittance evolves between the pulses.

For high data rates, the transmitter needs to meet a number of requirements. In particular, it is important to achieve a high extinction ratio (low pedestal pulses), a low timing jitter, low intensity noise, and a precisely controlled clock rate. Of course, a data transmitter should operate stably and reliably with minimum operator intervention.

In simple cases, a light-emitting diode (LED) is used in the transmitter, but due to the poor spatial coherence this requires the use of multimode fibers. The transmission rate or distance is then restricted due to intermodal dispersion; for a longer bandwidth–distance product, single-mode fibers are required. For short distances, several hundred Mbit/s are possible.

Transmission Fiber

The transmission fiber is usually a single-mode fiber in the case of medium or long-distance transmission, but can also be a multimode fiber for short distances. In the latter case, intermodal dispersion can limit the transmission distance or bit rate.

Long-range broadband fiber channels can contain fiber amplifiers at certain points (lumped amplifiers) to prevent the power level from dropping to too low a level. Alternatively, it is possible to use a distributed amplifier, realized with the transmission fiber itself, by injecting an additional powerful pump beam (typically from the receiver end) which generates Raman gain. In addition, means for dispersion compensation (counteracting the effects of chromatic dispersion of the fiber) and for signal regeneration may be employed. The latter means that not only the power level but also the signal quality (e.g. pulse width and timing) is restored. This can be achieved either with purely optical signal processing, or by detecting the signal electronically, applying some optical signal processing, and resending the signal.

Receiver

The receiver contains some type of fast photodetector, normally a photodiode, and suitable high-speed electronics for amplifying the weak signal (e.g. with a transimpedance amplifier) and extracting the digital (or sometimes analog) data. For high data rates, circuitry for electronic dispersion compensation may be included.

Avalanche photodiodes can be used for particularly high sensitivity. The sensitivity of the receiver is limited by noise, normally of electronic origin. Note, however, that the optical signal itself is accompanied by optical noise, such as amplifier noise. Such optical noise introduces limitations which can not be removed with any receiver design. Noise effects are discussed below in more detail.

Bidirectional Transmission

So-called full duplex links provide a data connection in both directions. These may simply be based on separate optical fibers, or work with a single fiber. The latter can be realized e.g. by using fiber-optic beam splitters at each end to connect a transmitter and a receiver, which may preferably work at different wavelengths. However, the need for bidirectional operation introduces various trade-offs, which in some cases (e.g. for very high data rates) make a system with two separate fibers preferable.

Multiplexing

A typical single-channel system for long-haul transmission has a transmission capacity of e.g. 2.5, 10 Gbit/s, 40 Gbit/s or even 160 Gbit/s. For higher data rates, several data channels can be multiplexed (combined), transmitted through the fiber, and separated again for detection.

The most common technique is wavelength division multiplexing (WDM). Here, different center wavelengths are assigned to different data channels. It is possible to combine even hundreds of channels in that way (DWDM = dense WDM), but coarse WDM with a moderate number of channels is often preferred in order to keep the system simpler. 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 may employ space division multiplexing in various forms. For example, one can use multi-core fibers or few-mode fibers. For short distances, for example for connections within data centers, it can be simpler to use ribbon fiber cables with multiple fibers. However, a disadvantage of that approach is that the cables become bulkier.

Active Optical Cables

For short transmission distances, so-called active optical cables (AOC) can be used, where a transmitter and a receiver (together with corresponding electronics) are rigidly attached to the ends of an optical fiber cable. Common electrical interfaces such as Ethernet, USB or HDMI ports are available, so that the use of such an active optical cable is essentially the same as that of an electrical cable, while offering advantages like reduced diameter and weight and also a larger possible transmission distance.

Fiber to the Home

It is possible to use optical links even to supply data over the “last mile” to single homes and offices. This technology is called fiber to the home (FTTH). In many cases, however, the last mile is still bridged with copper cables, and fiber-optic transmission occurs only up to some small stations close to the users.

Limitations via Noise and Cross-talk

Ultimately, the data transmission capacity of any system is limited by noise. In amplified optical systems, quantum noise e.g. in the form of spontaneous emission in fiber amplifiers (→ amplifier noise) is not avoidable. It can impact the system performance in different forms, such as timing jitter (→ Gordon–Haus jitter, particularly in soliton systems) or intensity noise affecting the photodetection.

Apart from noise, certain systematic signal distortions can also limit the transmission distance or bit error rate. In particular, chromatic dispersion and nonlinearities of the transmission fiber can cause severe signal distortions. As an example, Figure 2 shows a so-called eye diagram. Here, the “eye” is wide open, so that the signal could still be well detected. For twice the fiber length (not shown here), this would be different.

Figure 2: Eye diagram for the telecom signal after the fiber. This has been taken from a demo case for the RP Fiber Power software.

Note that the transmitter also has an important impact on signal detection issues. For example, a simple directly modulated transmitter may produce some unwanted frequency chirp, which increases the effect of chromatic dispersion in the transmission fiber and thus makes it more difficult to receive a clean signal after some propagation distance.

A related and even more sophisticated topic is cross-talk between the different channels e.g. of a WDM system. In systems with constant channel spacing, the channels can also influence each other in the form that one channel is amplified at the expense of the power in another channel (→ four-wave mixing). The impact of such effects can depend strongly on the system architecture, including the transmitter type, modulation format, fiber parameters, detection techniques, etc. The modeling of these effects and the subsequent optimization of communication systems are complex tasks.

Noise and related influences always cause some bit error rate, i.e., some portion of the transmitted bits will not be correctly detected. Provided that the bit error rate is at a sufficiently low level, occasional bit errors can be detected with certain techniques and corrected (e.g. by resending of defective data packets). For increasing transmission distances and/or data rates, the bit error rate finally sets some limits. In that context, the bandwidth–distance product is often used in a comparison of different fiber-optic links.

Fiber-optic Links for Timing Distribution and Timing Synchronization

While most fiber-optic links are used for transmitting data, there are also similar systems for the distribution of ultra-precise timing signals. Even with quite simple technology, the ultra-precise pulse-to-pulse spacing of a mode-locked laser is quite well preserved even over a link length of tens of kilometers. Advanced technology can also largely suppress effects of varying group delays, e.g. as a result of temperature changes of the fiber. It has become possible, e.g., to synchronize laser oscillators with very high precision through such links.

Bibliography