Free-space Optical Communications

In most cases, optical data transmission on Earth is done using fiber optics, because these allow transmission over relatively long distances without excessive power loss, alignment problems, and interference from the atmosphere. However, it is also possible to transmit data optically through free space (or similarly through water) without using any kind of waveguide structure. This type of optical communication has early origins, e.g. Alexander Graham Bell's “photophone” patent in the 1870s and the optical telegraph, and is now increasingly used both in space and on Earth. It usually requires an unobstructed line of sight between sender and receiver, and usually also some special free-space optics such as telescopes. The light source used nowadays is almost always some kind of laser (possibly combined with an amplifier), because the high directionality of a laser beam is obviously an essential ingredient for high-performance communication. The prominent role of a laser is emphasized by the term laser communications.

Transmission Issues

Especially for large transmission distances, it is essential to direct the energy of the transmitter in the form of a well-collimated laser beam to limit the often still very large power loss between transmitter and receiver. To limit the beam divergence, it is necessary to provide a large beam radius from an optical source with high beam quality. Ideally, one uses a diffraction-limited source and a large, high-quality large optical telescope to collimate the beam. Due to the short wavelength of the light, the beam divergence of an optical transmitter can be much smaller than that of a radio or microwave source of similar size. To use a term commonly used in the field of radio communications, the antenna gain of an optical telescope can be very high – well over 100 dB even for moderate telescope diameters of, say, 25 cm – and thus much higher than for any microwave antenna of limited size.

It is also advantageous to have a high directionality on the receiver side: it is important not only to collect as much of the transmitter power as possible, but also to minimize interference, 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 implies the requirement of high precision in the alignment of the transmitter and receiver. It may be necessary to stabilize the alignment with an automatic feedback system. For ground-based receivers of signals from remote satellites (see below), one may use adaptive optics to further increase the directionality by reducing the influence of atmospheric disturbances.

An important issue is the power budget of a free-space link, including the transmitter power and all power losses. The remaining power at the receiver largely is a key factor for the possible data rate, although 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 effectively suppressed with additional narrowband optical filters, since the optical bandwidth of the signal is quite limited, while 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 digital signal processing techniques have been developed that, amazingly, allow reliable high-capacity optical links even through dense clouds. In some situations, adaptive optics can also be very helpful.

Space Applications

Some space applications require large amounts of data to be transmitted. An example is the transmission between different satellites in Earth orbit (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 average laser power on the order of a few watts.

Data can also be exchanged between a more distant spacecraft and a station on or near Earth. For example, planetary probes can generate large amounts of image data, and a major challenge is to send large amounts of data back to Earth. Until recently, the only available technology was radio links operating in the X- or Ka-band. Currently, optical data links are being considered primarily for the downlink, where the desired data volumes are much larger than for the uplink, and optical communications could greatly increase the transmission capacity to many megabits per second. The spacecraft then has a pulsed laser source (e.g. using pulse position modulation) and a moderate-sized optical telescope aimed at the receiver. The latter may be a large ground-based telescope or an Earth-orbiting transceiver.

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

Short-range Free-space Optical Data Links

Much less technologically challenging are data links between buildings in large cities (LAN-to-LAN connections), where a free-space laser data link over distances of hundreds of meters or even a few kilometers can be much easier and less expensive to install than any kind of cable, especially if a road or other barrier has to be crossed, or if the connection is only needed for a limited period of time. It is then possible, for example, to provide fast Internet access to all buildings involved, even if only one of them has direct access to a fiber optic network.

The required laser powers are very moderate, since a significant part of the transmitted power can hit the receiver (e.g. a photodiode). Therefore, there are usually no significant laser safety issues, especially if eye-safe lasers emitting in the 1.5μm spectral range are used. However, the availability of services is less than with a cable, as the link can be disturbed by either atmospheric influences (e.g. heavy rain, fog, snow, or strong wind) or flying objects such as birds and drones. 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 to electromagnetic interference, and does not raise concerns about electro smog. It also does not cause interference between different data links, so it does not require a license to operate, 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, reliability can be improved in several ways, e.g. with multi-beam architectures, larger power margins, and backup systems, and 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 need a laser transmitter, because light-emitting diodes can be used.

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

The main 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 eavesdropping by unauthorized parties. In addition, there is no need for government frequency allocation and no risk of mutual interference between different laser data links.

More to Learn

Encyclopedia articles:

• optical data transmission
• quantum cryptography

Blog articles:

• The Photonics Spotlight 2009-11-13: “Photodetection: Optical and Electrical Powers”