Single mode fiber-optic cable carries a single intense beam of parallel light rays usually emitted by an injection laser diode (ILD) through a ray-straightening collimating lens; the ray then travels down a narrow optical channel for long distance digital signal transmission. Typically made from glass, these fibers offer greater bandwidths and less signal loss than multimode fibers. Their cores are thinner than a human hair, measuring about eight microns (µm) in diameter, wrapped in cladding of about 125 µm, and transmit laser light at higher wavelengths of about 1,300 or 1,550 nanometers (nm). With less index refraction than multimode fibers, the intense laser light travels parallel to the axis of the fiber, minimizing pulse dispersions and other signal degradations that occur in the interweaving waves of multimode fiber. These costlier single mode fibers serve long distance needs in telecommunications and cable television networks.
A laser injects the high-bandwidth single mode fiber-optic cable with light of a narrow spectral width. A long strand of glass fiber typically propagates laser transmission with the use of wave division multiplexing (WDM), which divides signals by different wavelengths to increase transmission throughput. This greatly improves the transmission rate of single-mode over multimode fiber, at up to 50 times the potential distance.
The single light wave in the tighter core virtually eliminates distortions from light interference or loss. This generates the highest transmission speeds from transmitter to receiver of any fiber. It functions regardless of electromagnetic interference (EMI) and prohibits eavesdropping by eliminating signal leakage. Light wavelengths around 1,300 nm serve for shorter distances and 1,500 nm serve longer distances.
A transmitting laser diode sends the light signal down the single mode fiber-optic cable. Like ping pong balls through a slightly larger diameter pipe, the light, unable to stop, bounce, escape, or turn back, travels forward through a core surrounded by non-absorptive cladding that is ten times thicker. The wavelength propagates continuously forward with an inability to refract, reflect, or disperse as heat within the waveguide. It has nowhere else to go, except if it encounters absorptive manufacturing flaws or installation or connecting errors.
The signal pulses may travel through regenerators or attenuators until reaching a receiver. There, a photodiode decodes the waveforms about 8,000 times a second, converting them back to electronic computer signals as digital data and audio/video information. This is like reading an entire 24-volume set of encyclopedias in one second.
In single mode fiber-optic cable, this form of low-loss, lowest-order propagation can only work above a certain cutoff wavelength. This is known as single mode (SM) step index. This means only straight light beams are selected for single mode transmission; they don't intermingle or bounce at different rates in mulitmode wave propagations, as through the wider cores of multimode fibers.
Different types of single mode fiber-optic cable include cutoff or dispersion-shifted fiber, non-zero dispersion-shifted, low water peak fiber, and others. Also known as mono-mode or uni-mode fiber, it is primarily used for wide area networks (WAN); however, it has received increased attention from local area networks (LAN), which extend their reach over greater distances in settings like university or corporate campuses. These higher-cost cables have limiting factors such as bending radius, so they must be planned out carefully before installation by a skilled technician.
