定义：
采用光束来传输信息，例如在光纤中

光用于数据传输中具有很大的潜力实现非常高的数据速率。这主要是光波频率高的结果，因此也可以常用非常宽的光学带宽。例如，在光纤中传输波长范围为1300 nm到1600 nm的光，对应的带宽为43 THz，这比采用电缆高了几个数量级。尽管现在还无法完全达到理论上如此高的带宽，光链路（自由空间链路或者光纤链路）的传输容量远远大于电缆或者无线电通信链路。 
对于有限光带宽情况，容许的数据传输的比特率依赖于传输系统的信噪比。根据Shannon-Hartley理论，实际的无误比特率与1的对数值与信噪比的和成正比。信噪比很高时，光纤传输系统也无法达到理论预测的水平，因为非线性效应会引起信号畸变。实际中，可以得到的光谱效率约为光带宽每赫兹为1 bit/s，依赖于调制格式。高级调制格式，例如差分正交相移键控（DQPSK），可以得到更高的光谱效率。 

波导传输与自由空间传输对比 
大多数情况下，光数据传播采用光纤作用传输介质（光纤通信），因为在光在光纤中由于损耗极低可以传输非常长的距离，不存在对准问题，不存在大气扰动以及类似效应。当然自由空间光通信也具有许多应用，例如地球轨道卫星间，遥远的宇宙飞船与地面站之间，或者城市写字楼间的短距离之间。 

比特率问题 
即使数字传输系统也不是无误的，统计的涨落是受噪声（激光器噪声，放大器噪声，散粒噪声，接收器的附加噪声）影响的，是一小部分传输的比特是缺损的。比特误码率（也就是不正确的传输比特所占的比例）强烈依赖于传输功率，而传输功率需要足够高使比特误码率低于一个可接受的水平（例如，地球上的通信系统要求为10−12，而控制卫星需要10−6）。实际上误码可以被检测出来（采用一些校验方法）并且纠正。误码纠正机制可以利用传输数据一定程度的冗余度或者重新传输被破坏的数据包。其它的一些有害的效应，例如光纤损耗或者长链路中存在的各种色散效应，或者自由空间中传输的背景光，可以通过提高传输功率进行补偿。为了保持一定的比特误码率需要提高光功率称为功率损耗，更具体的也可以叫做色散损耗，如果这是考虑的因子是色散。 

长距离到短距离 
光数据传输被应用到很多领域，例如，电话，互联网流量，有线电视，主要是一些至少为几千米的长距离传输。但是，光系统有渗透到越来越短距离应用领域的趋势。尤其是在日本，网络连接已经通过光纤传输到用户家庭中（光纤入户）。现今局域网（LANs）在数据速率为100 Mbit/s时采用电缆是可以正常工作的，但是当速率为10 Gbit/s时，需要采用光连接器，因为电缆在微波频段非常昂贵，损耗很大。不久的超级计算机会在短距离内采用光互联。 
光板对板连接，光芯片对芯片连接甚至芯片间连接很受重视，并且已经有了一定的发展。例如，现在CPU芯片中电数据传输线所占的比重在增加。在高数据速率时，采用光学方法实现上述数据传输线功能会在空间和能量消耗更小。而面临的问题就是需要发展合适的微激光器或者纳米激光器，可以工作在非常小的功率水平，同时可以制作在二氧化硅衬底上。
 

Definition: the transmission of information using light beams, e.g. in fibers

More specific terms: free-space optical communications, optical fiber communications

Light has an enormous potential for data transmission with very high data rates. This is basically a consequence of the high optical frequencies, which also make it possible to utilize very broad optical bandwidths. For example, the wavelength range from 1.3 to 1.6 μm, which may be transmitted in an optical fiber, corresponds to a bandwidth as large as 43 THz, which is orders of magnitude higher than that of any electrical cable. Even though the theoretical potential of this bandwidth can so far not be fully utilized, an optical link (either a free-space link or a fiber-optic link) can have a capacity far beyond that of an electrical cable, or of a radio frequency link. There have already been demonstrations of fiber-optical data transmission systems with far over 1 Tbit/s, using a single fiber.

Laser-based communications is clearly the most important laser application in terms of global laser sales, and in addition still exhibits strong growth.

Guided versus Free-space Transmission

In most cases, optical data transmission uses optical fibers as the transmission medium (→ optical fiber communications), because light can be guided in fibers over very long distances with very low losses, also avoiding alignment issues, atmospheric influences and the like. However, there are also applications for free-space optical communications, e.g. between Earth-orbiting satellites, between a remote spacecraft and an Earth-based station, or over short distances between metropolitan buildings.

Bit Error Issues

For a limited optical bandwidth, the maximum bit rate for data transmission depends on the signal-to-noise ratio of the transmission system. According to the Shannon–Hartley theorem, the possible virtually error-free bit rate scales with the logarithm of 1 plus the signal-to-noise ratio. Particularly for high signal-to-noise ratios, fiber-optical transmission systems (see below) can not fully realize that theoretical potential, since nonlinearities lead to signal distortions. In practice, the achievable spectral efficiency is roughly 1 bit/s per Hertz of optical bandwidth, and depends on the modulation format. Advanced modulation formats, such as differential quadrature phase shift keying (DQPSK), promise somewhat higher values of spectral efficiency.

Even a digital transmission system is not totally error-free – statistical fluctuations related to noise influences (e.g. laser noise, amplifier noise, shot noise, or excess noise of a receiver) cause a small fraction of the transmitted bits to be defective. Typically, the bit error rate (i.e., the fraction of incorrectly transmitted bits) is strongly dependent on the transmitted optical power, and the latter must be high enough to keep the bit error rate below a certain acceptable level (e.g. 10⁻¹² for Earth-based telecommunication systems, or 10⁻⁶ for satellite control). Practically all of the remaining bit errors can be detected (e.g. using some kind of checksums) and corrected. The error correction scheme can use some level of redundancy in the transmitted data or involve retransmitting corrupted data packets. Additional detrimental influences such as propagation losses or various types of dispersion in a longer link (mainly chromatic dispersion), or background light in free-space transmission, can often be compensated by somewhat increasing the transmitted power. The increase of optical power required to maintain a given bit error rate is called a power penalty, or more specifically e.g. a dispersion penalty if dispersion is the considered factor.

From Larger to Smaller Distances

Optical data transmission is increasingly used in various areas, such as telephony, Internet traffic, cable TV – mostly for larger transmission distances of at least a few kilometers, while shorter distances (e.g. from roadside cabinet to a home installation) are often treated with electrical cables (e.g. telecom or TV cables). There is, however, a tendency for also using optical systems with smaller and smaller transmission distances. Particularly in Japan, many Internet connections are already delivered to homes with optical fibers (→ fiber to the home). Current local area networks (LANs) work well with electrical cables for data rates of 1 Gbit/s, and 10 Gbit/s are also possible, but only over quite limited distances; electrical cables are expensive and lossy for use at microwave frequencies. Therefore, optical connections start to become more important even within buildings.

Supercomputers may soon extensively use optical interconnects even for quite short distances. Optical board-to-board connections, optical chip-to-chip and even intra-chip connections are being seriously considered, and are partly already developed. For example, an increasing portion of the area of current CPU chips is occupied by electrical data transmission lines. At high data rates, it is expected to be favorable in terms of space and also of energy consumption to realize such transmission lines with optical means. A challenge, however, is still to develop suitable micro- or nano-lasers, which can be operated with very low power levels, and can be fabricated on silicon wafers.

Another possible application (yet at the R & D stage) is short-distance free-space data transmission within rooms, e.g. from an LED-based transmitter at the ceiling to a PC, a notebook or a smaller mobile device. The transmitter does not send out a directed beam, but illuminates a large area. Due to the relatively small distances, one can still achieve a reasonably high signal-to-noise ratio for realizing high data rates. This approach is competing with traditional wireless communications e.g. based on Wi-Fi (WLAN). Possible advantages of optical transmission include higher data rates and avoiding interference or interception problems, as the sent-out light will usually not be detectable in other rooms. A problem, however, is the back channel, particularly for small mobile device which cannot use a powerful light emitter for reasons of power consumption. One possibility is to use radio transmission for the back channel. For many applications such as video streaming this can work well, since download data rates are far higher than upload rates.

Bibliography