Nowadays, fiber optic network is gaining its popularity because it has high speed, high density and high bandwidth, etc. Compared with traditional copper cable, the fiber optic cable could support much further distance although the exact distance is limited by many factors. For the super fast optical communication, transmission distance has already become the most vital issue. The optical signal may become weak over long distance. Thus, many components and methods have been adopted to break the limitations of the optical transmission distance. This article will emphasize the factors that limit optical transmission distance.
Typically, the dispersion in the fiber optic cable could have a great impact on the transmission distance. There are two types of dispersion—chromatic dispersion and modal dispersion. Chromatic dispersion is the spreading of the signal over time resulting from the different speeds of light rays, while modal dispersion is the spreading of the signal over time resulting from the different propagation mode.
As it is known to all, optical fiber cable could be divided into single-mode fiber cable and multimode fiber cable. For the single-mode fibers, transmission distance is affected by chromatic dispersion, because the core of single-mode fibers is much smaller than that of multimode fibers. And this is the main reason why single-mode fiber can have longer transmission distance than multimode fiber. For the multimode fibers, transmission distance is largely affected by the modal dispersion. Due to the fiber imperfections, the optical signals of multimode fibers cannot arrive simultaneously and there is a delay between the fastest and the slowest modes, which causes the dispersion and limits the performance of multimode fibers (see the following picture).
Fiber optic cable is the path sending the optical signals. However, most of the terminals are electronic based. The conversions between electrical signals and optical signals are necessary. Fiber optic transceivers are widely used in today’s optical network to achieve this purpose. The conversion of signals depends on a LED (light emitting diode) or a laser diode inside the transceiver, which is the light source of fiber optic transceiver. The light source can also affect the transmission distance of a fiber optic link.
LED diode based transceivers can only support short distances and low data rate transmission. Thus, they cannot satisfy the increasing demand for higher data rate and longer transmission distance. For longer and higher transmission data rate, laser diode is used in most of the modern transceivers. The most commonly used laser sources in transceivers are Fabry Perot (FP) laser, Distributed Feedback (DFB) laser and Vertical-Cavity Surface-Emitting (VCSEL) laser. The following chart shows the main characteristics of these light sources.
As is shown in the above chart, different laser sources support different frequencies. The maximum distance of fiber optic transmission system can support is affected by the frequency at which the fiber optic signal will be transmitted. Generally the higher the frequency, the longer distance the optical system can support. So it is essential to select the right frequency to transmit optical signals. Typically single-mode fibers use frequencies of 1300 nm and 1550 nm, while multimode fibers use frequencies of 850 nm and 1300 nm.
Another factor influencing the transmission distance is the bandwidth of fiber optic cables. Generally, the transmission distance decreases proportionally, as the bandwidth increases. For example, a fiber that can support 500 MHz bandwidth at a distance of one kilometer will only be able to support 250 MHz at 2 kilometers and 100 MHz at 5 kilometers. Single-mode fibers have an inherently higher bandwidth than multimode fibers due to the way in which light passes through them.
Splices and connectors in most fiber optic system are inevitable. Signal loss can be caused when the optical signal passes through each splice and connector. The total amount of the loss depends on the types, quality and number of connectors and splices.
According to the above statement, the optical transmission distance is affected by various factors including the fiber type, light source of transceiver, frequency of transmission, bandwidth as well as splices and connectors. So it is necessary to consider these factors to minimum the limitations on transmission distance when deploying the fiber optic network.