Optical fiber is a form of waveguide. The lateral dimension of a waveguide depends upon the wavelength of the signal conveyed. A waveguide is not capable of carrying a wavelength much larger than its diameter.
Optical fibers are characterized by their core diameters. In single-mode fiber, the standard core diameter is 9 μm. Standard multi-mode diameters are 50 and 62.5 μm. The different core diameters give single- and multi-mode optical fiber different parameters, applications and installation methods. Multi-mode fiber is capable of simultaneously conveying numerous propagation modes. As the name implies, single-mode fiber can convey only one propagation mode.
Multi-mode optical fiber, with its larger acceptance angle, has greater light-gathering ability. Also, the splicing of separate fiber segments is easier and more forgiving. Because the single-mode core is smaller, light is reflected fewer times for a given length, so there is less attenuation.
The lower-cost LED can be used as the light source for multi-mode optical fiber, whereas single-mode requires the more expensive laser light source. These different light sources limit the bandwidth multi-mode fibers can support.
Because of the different core sizes, single- and multi-mode fiber have different length limits, which also vary with the data rate. For example, multi-mode OMI with a 62.5/125-μm core diameter, 100BASE–SX is rated for 275 m. 10GBASE multi-mode is rated for 33 m. Single-mode 9/125-μm, 100BASE–SX is rated for 5 km. Single-mode 10GBASE is rated for 10 km.
The bottom line is that multi-mode optical fiber is used primarily in moderate-sized buildings, while single-mode is mostly seen in utility-scale projects, usually in underground applications. Different core sizes cannot be spliced. To a limited extent, however, they are compatible with different types of network equipment as outlined in manufacturers’ specifications.
Splicing is critical, so much so that multi-mode optical fibers in short runs within buildings are not normally spliced. Instead, the entire run is usually replaced if there’s physical damage or performance is unsatisfactory. Long single-mode runs are generally spliced. Special skills and elaborate equipment including a temperature-controlled van may be required.
Optical fiber that has been damaged can be rejoined by means of splicing or connectors. Of the two, splicing is the more efficient and reliable, but demands more elaborate equipment. There are two methods of splicing, fusion and mechanical. Loss in either method is low compared to optical fiber connectors. In a mechanical splice, two fibers are aligned and the alignment is maintained using an index matching fluid. After alignment, the fiber ends are held in place by mechanical means. Rather than being permanently joined, the fiber ends are held together so the light beam can cross from one to the other with less than 0.5-dB loss.
To perform a mechanical splice, the jacket and any other coverings are cut back, exposing the core and cladding. The ends are cleaved as in fusion splicing, but less precision is required. Then, the fibers are aligned inside the mechanical splice unit. Index matching gel allows the technician to align the ends so the light beam crosses the joint with minimal attenuation. In this method, the ends are not fused. The mechanical splice unit protects the splice, maintains alignment and becomes a permanent part of it.
Fusion splicing introduces less loss, typically less than 0.1 dB. It is more expensive and more reliable than mechanical splicing. It involves precisely aligning the fiber ends, then fusing them using an electric arc or other heat source.
To make a fusion splice, the technician first removes any protective coatings, jackets and tubes, then cleaves the fiber. The result should be a smooth surface perpendicular to the fiber axis. The process typically involves polishing and inspecting the ends with a microscope. Next, the fiber ends are fused. They are precisely aligned and heat is applied to permanently join the fiber ends. A good fusion splice will not easily break, but should be protected from excessive bending or tensile force. Heat-shrink tubing and gel in conjunction with mechanical protection completes the process.
The initial investment for mechanical splicing ranges from $1,000 to $2,000 with a $12 to $40 cost per splice. For fusion splicing, the cost per splice ranges from $0.50 to $1.50 with an initial investment of $15,000 to $50,000, not including a temperature-controlled van, earth-moving equipment, a truck with flatbed trailer, and worker training. Then, particularly in long underground installations, there is the whole matter of locating the fault.
By way of comparison, first consider the conventional time-domain reflectometer (TDR). The conventional TDR resembles radar. One end of the conductor under investigation is connected to the instrument, which generates a pulse and injects it into the conductor. If there is a fault (open or short) consisting of even a slight change in impedance, its presence reflects the pulse back to the instrument, which records and displays the time interval and calculates the distance to the anomaly.
If the conductor is of uniform impedance and properly terminated, the pulse will not reflect, and the entire signal will be absorbed into the equipment connected at the far end. This reflection quality is a great help in locating a fault, especially in an underground installation where the ability to locate a fault avoids extensive digging. The discontinuity is not only located, but also characterized by measuring the amplitude of the reflected signal.
The optical time-domain reflectometer (OTDR) is similar to the TDR, the difference being that it is optimized to characterize and locate anomalies that may exist in an optical fiber link. The instrument injects light pulses into optical fiber. The phenomenon known as Rayleigh backscattering causes these pulses to reflect back from any discontinuity into the instrument, which measures the strength of the return pulses, integrates them as a function of time, and displays them on the flat screen.
Like the conventional TDR, the OTDR displays the distance to the anomaly, so the problem can be located without extensive digging when underground. Long single-mode optical fiber segments, still on a reel or in the warehouse, can be verified prior to installation or backfilling.
OTDRs are available in various form factors. The ac bench model has a large, clear display and advanced analytical features, so it is suitable for laboratory work and in the field when difficult challenges arise.
The hand-held, battery-powered OTDR is widely used for field work because of its rugged, environmentally-tolerant case, easy portability and ac independence except for occasional battery charges. While lacking some features of the bench model and having a smaller display, it has the essential analytical capabilities and can accurately locate optical fiber discontinuities.
The fiber break locator is a far less costly, smaller instrument that can locate major optical fiber faults such as breaks, strong reflections and significant signal loss. The difference between an OTDR and a fiber break locator is that that the break locator doesn’t give a reading of where the problem resides in the cable. Instead, there is just a visual indication. The instrument puts out a visible laser light meant to reflect from anomalies. Breaks and micro-bends in the fiber deflect the laser light into the fiber jacket, producing a red glow at the point of the fault. (Thus break locators aren’t of much use for underground or otherwise inaccessible cables.) There are also infrared versions which require special goggles but which can be used in bright room light.