定义：
光数据在自由空间中传输，通常在空气或者真空中。

通常的光数据传输是在光纤中传播的，因为采用光纤可以传输很长的距离，而且损耗很少，不存在对准问题以及大气中的干扰。但是，不采用任何波导结构也可以在自由空间中（或者类似的在水中）传输光数据。光通信的起源为Alexander Graham Bell在1870s的专利“光电话机”， 后来不仅在太空中还是地球上，光电报用户都逐渐增多。这需要在发送者和接受者之间存在畅通无阻的视线，还有一些自由空间光学器件，例如望远镜。现在采用的光源通常是某种激光器（通常与放大器结合使用），因为激光光束的高方向性是高性能通信系统中至关重要的一个因素。在激光通信中强调了激光器突出的作用。 

传输问题 
尤其是在长距离传输中，发射器中发出的光需要是对准程度很好的激光光束，这样可以减小发射器和接收器之间比较大的功率损耗。为了减小光束发散程度，可以采用具有大的光束半径且光束质量很好的光源。理想情况下，采用的为衍射极限光源，再加上高质量的光望远镜来对准光束。由于光波长很短，因此在光发射器中的光束发散是远远小于小尺寸的无线电光源或者微波光源中。在无线电传输中常用的一个参数为天线增益，光望远镜的天线增益远远高于任何有限尺寸微波天线，例如，对于中等尺寸的直径为25cm的望远镜的天线增益已经超过了100dB. 

图1：自由空间光通信简单的装置示意图。尽管发射器信号是对准的，仍有一部分的传输功率不能到达探测器。 
在接收器方面也需要具有很好的方向性，不仅要将发射器发出的功率尽可能的接收，还需要使干扰因素最小化，例如，背向散射光会产生噪声，因此会减小数据传输容量。如果在接收端采用一个大的望远镜可以同时得到高灵敏度和高方向性。 

高方向性需要发射器和接收器之间严格对准。可以采用自动反馈系统来稳定对准后的器件。而当地面接收器需要从遥远的卫星接收信号时，还可以利用自适应光学来进一步提高方向性来减小大气中的干扰。 

还有一个需要考虑的重要问题是自由空间链路中的功率分配问题，包含发射器功率和所有的功率损耗。接收器能接收到的功率很大一部分取决于数据传输速率，而数据传输速率则会受调制格式，可接受的比特误码率和各种噪声（激光器噪声，放大器噪声），接收器的额外噪声（例如，雪崩二极管）和背景光等的影响。通过采用窄带的光滤波器，可以有效抑制背景噪声，因为背景噪声通常带宽很宽而信号的带宽则是确定的，有限的。 

大气中的扰动存在时，例如晕，灰尘和雾，会引起强信号的衰减同时也会造成信号间的干扰。为了解决这一问题，发展了很多复杂的数值信号处理技术，即使需要穿过厚的云层也可以保证很高容量的光学链路。 

空间应用 
一些空间应用需要传输很大量的数据。例如，地球轨道卫星之间的数据传输（卫星间通信），2001年ESA已经做了首次示范。采用平均功率为几个瓦特的中等强度的激光光源是可以实现在几千公里距离上每秒传输几十兆比特或者更高的速率的。 

在更远的宇宙飞船与地球上或者近地球的空间站之间也可以传输数据。例如行星探测器会产生大量的虚拟数据，怎样将这些大量的数据传送到地球上是面临挑战。目前只有工作在X波段或者Ka波段的无线电通信线路能够完成这项工作。而光数据链路仅能作为下行链路，需要的数据容量远远大于上行链路，光学通信可以将传输容量拓展到几百Kbit/s，甚至几Mbit/s。宇宙飞船上有一个脉冲激光器光源（采用脉冲位置调制），还有对准接收器的中等尺寸的光望远镜。后者可以是一个大型地面望远镜或者地球轨道上的接收器。

 光学技术相比于无线电通信线路的优势在于光波的波长更短，因此可以更有方向性的发送和接收信息，因此需要更低的功率同时数据传输速率更高。从技术层面上看，天线增益更高。这对于星际间距离的应用非常重要。但是，光学链路也对气候条件更加敏感。 

短程自由空间光数据链路 
在技术上都市中写字楼间的数据传输（局域网间）所需要的难度要小一些，这时采用自由空间激光数据链路传输几百米或者几千米比其它任何的电缆安装更加简单，并且性价比更好，尤其是需要穿过道路或者其它障碍物的时候或者在有限时间内就需要一个连接点的情况。采用这种链路可以得到非常快的网络连接，尽管可能只有一个链路与光纤网络直接连接。 

因为几乎所有发射的功率都能到达接收器（光电二极管），所以需要的激光功率适中即可。因此也不会出现激光安全的隐患，尤其是当采用人眼安全激光器时，它辐射的是1500nm光谱区域的光。然而它比电缆的服务效力要差，因为链路很容易受空气扰动（大雨，雾，雪或者强风等）和飞行物体的干扰。从这个角度看，自由空间传输比其它无线技术更加脆弱，但是它还具有潜力达到很大的传输容量，抗电磁干扰，因此不用担心所谓的电子烟雾。另外，在不同数据链路之间不存在相互干扰，而且它在数据安全上也占优势，因为激光光束是严格对准的，很难截取信息。最后一点，通过一些方法可以增强可靠性，例如，采用多光束结构，更大的功率余量，采用辅助系统，并且利用量子编码技术可以极大的提高安全性。

而当传输距离不是很远（例如，几千米），传输数据速率也适中的情况下，甚至不需要激光发射器，只需要采用发光二极管（LEDs）就够了。 

在不存在直接光视线时也可以实现短程光数据连接。当采用紫外光时，它在大气中存在强烈的散射，只能接受到部分的光。出现了辐射深紫外（UV-C）的发光二极管以及合适的半导体光探测器之后，这项技术的实现存在了可能。 

激光数据链路相比于无线电或者微波链路主要的优势在于其能达到的很高的数据传输速率，低功率需求，小尺寸以及信息被截取的几率更低。并且，无需采用政府分配的频率，也不存在不同数据链路之间相互干扰的影响。
 

Definition: optical data transmission through free space, usually through air or vacuum

More general terms: optical communications, optical data transmission

Optical data transmission on Earth is in most cases done via optical fibers, because these allow the transmission over relatively large distances without excessive power losses, alignment issues, and disturbing influences of the atmosphere. However, it is also possible to transmit data optically via free space (or similarly, through water), not exploiting any kind of waveguide structure. This kind of optical communications has early origins, e.g. the “photo phone” patent by Alexander Graham Bell in the 1870s and the optical telegraph and is now increasingly used, both in space and on Earth. Usually, it requires an unobstructed line of sight between sender and receiver, and normally also some special free-space optics such as telescopes. The light source used is nowadays virtually always some kind of laser (possibly combined with an amplifier), because the high directionality of a laser beam is obviously a vital ingredient for high-performance communications. The prominent role of a laser is emphasized by the term laser communications.

Transmission Issues

Particularly for large transmission distances, it is essential to direct the energy of the sender accurately in the form of a well-collimated laser beam in order to limit the often still very large loss of power between the sender and the receiver. In order to limit the beam divergence, it is necessary to arrange for a large beam radius from an optical source with high beam quality. Ideally, one uses a diffraction-limited source and a large high-quality optical telescope for collimating a large beam. Due to the short wavelength of light, the beam divergence of an optical transmitter can be much smaller than that of a radio or microwave source of similar size. Using a frequently used term in the area of radio transmission, the antenna gain of an optical telescope can be very high – well over 100 dB even for moderate telescope diameters of e.g. 25 cm – and thus much higher than for any microwave antenna of limited size.

Figure 1: Simple setup for free-space optical communications. Although the transmitter signal is approximately collimated, part of the transmitted power may miss the detector.

It is also advantageous to have a high directionality on the side of the receiver: it is essential not only to collect as much of the sender's power as possible, but also to minimize disturbing influences, e.g. from background light, which introduces noise and thus reduces the data transmission capacity. Both high sensitivity and high directionality can be achieved by using a large telescope at the receiver end.

Of course, high directionality also requires high precision in the alignment of the sender and receiver. It may then be necessary to stabilize the alignment with an automatic feedback system. For ground-based receivers of signals from remote satellites (see below), it is considered to use adaptive optics to increase the directionality further by reducing the influence of atmospheric disturbances.

An important issue is the power budget of a free-space link, including the transmitter's power and all power losses. The remaining power at the receiver largely determines the possible data transmission rate, even though this is also influenced by the modulation format, the acceptable bit error rate, and various noise sources, in particular laser noise, amplifier noise, excess noise in the receiver (e.g. an avalanche photodiode), and background light. The latter can often be efficiently suppressed with additional narrow-band optical filters, since the optical bandwidth of the signal is fairly limited, whereas background light is usually very broadband.

Severe challenges can arise from the effects of atmospheric disturbances such as clouds, dust and fog, which can cause not only strong signal attenuation but also intersymbol interference. To solve this problem, sophisticated techniques of digital signal processing have been developed, which amazingly allow for reliable high-capacity optical links even through thick clouds.

Space Applications

Some space applications require large amount of data to be transferred. An examples is the transmission between different Earth-orbiting satellites (inter-satellite communications), which was first demonstrated by ESA in 2001 (ESA). It is possible to transmit tens of megabits per second or more over many thousands of kilometers, using moderate laser average powers of the order of a few watts.

Data can also be exchanged between a more remote spacecraft and a station on or near Earth. For example, planetary probes can generate a lot of image data, and a major challenge is to send large amount of data back to Earth. Until recently, radio links operating e.g. in the X band or Ka band were the only available technology. Currently, optical data links are considered particularly for the downlink, where the desired data volumes are much larger than for the uplink, and optical communications could greatly expand the transmission capacity to hundreds of kbit or even several megabits per second. The spacecraft then has a pulsed laser source (employing pulse position modulation, for example) and an optical telescope of moderate size targeting the receiver. The latter can be a large ground-based telescope or a transceiver in an Earth orbit.

The basic advantage of optical technology over radio links is that the much shorter wavelength allows for a much more directional sending and receiving of information, resulting in much lower power requirements and higher data rates. In technical terms, the antenna gain can be much higher. This is particularly important for bridging interplanetary distances. On the other hand, optical links are more sensitive to weather conditions.

Short-range Free-space Optical Data Links

Technologically much less challenging are data links between metropolitan buildings (LAN-to-LAN connections), where a free-space laser data link over distances of hundreds of meters or even over a few kilometers can be much simpler and more cost-effective to install than any kind of cable, particularly if a road or some other kind of barrier has to be crossed, or if a connection is required for only a limited time. It is then possible e.g. to obtain fast Internet access for all buildings involved, even if only one of them has direct access to a fiber network.

The laser powers required are very moderate, as a significant fraction of the sent power can hit the receiver (e.g. a photodiode). Therefore, there are usually no significant laser safety issues, particularly if eye-safe lasers emitting in the 1.5-μm spectral region are used. However, the availability of services is smaller than with a cable, because the link may be disturbed either by atmospheric influences (e.g. heavy rain, fog, snow, or strong wind) or by flying objects such as birds. In this respect, free-space transmission is less robust than other wireless technologies such as radio links, but it has a higher potential for transmission capacity, is immune against electromagnetic interference, and does not raise concerns in the context of electro-smog. Also, it does not lead to interference between different data links, so it does not need a license to be operated, and it is superior in terms of data security, since it is more difficult to intercept a tightly collimated laser beam than a radio link. Finally, the reliability can be enhanced in various ways, e.g. with multibeam architectures, larger power margins, and backup systems, and the security can be extremely high with certain schemes of quantum cryptography.

For not too long distances (e.g., up to a few kilometers) and moderate data rates, one does not even require a laser transmitter, because light-emitting diodes (LEDs) can be used instead. However, high performance requires laser transmitter; transmission capacities can then be above 1 Tbit/s.

It is even possible to establish short-range optical data connections without a direct line of sight. When ultraviolet light is used, this is strongly scattered in the atmosphere, and it is possible to receive some of that light. That technology has become more interesting with the advent of light-emitting diodes (LEDs) emitting in the deep UV (UV-C), and also of suitable semiconductor photodetectors.

Essential advantages of laser data links over radio frequency (RF) or microwave links are the possible high data rate, low power requirements, compact size and lower probability of signal interception by unauthorized parties. Also, there is no need for governmental frequency assignment and no risk of mutual interference of different laser data links.

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