Optical Data 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.

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 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 fiber losses or various types of dispersion in a longer link, 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.

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. There is, however, a tendency for optical systems to penetrate areas 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 100 Mbit/s, but at rates of e.g. 10 Gbit/s, optical connections will become important, because electrical cables are expensive and lossy at microwave frequencies. Supercomputers may soon extensively use optical interconnects for 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 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.

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