For almost a year, I’ve been covering the expansion—explosion?—of optical fiber’s applications. Data speed is a factor in almost all of these networks; networks are getting progressively faster. Network backbones and data center connections, for example, have outpaced copper cabling technology, and now many have exceeded the capabilities of multimode (MM) fiber, prompting a move to single-mode (SM) fiber.


In addition, applications such as fiber to the home (FTTH), optical local area networks and distributed antenna systems for wireless are derived from outside plant (OSP) telco applications that have used nothing but SM fiber for 30 years.


Contractors familiar with OSP installation will be familiar with SM fiber, but many premises cabling contractors will not. They probably know SM fiber has virtually infinite bandwidth but fear it’s harder to install and test.


The most obvious difference between the two fiber types is that SM fiber has a much smaller core than MM fiber, although they share the same outside (cladding) diameter. But SM also has some major differences in associated components and installation techniques that every contractor needs to know.


Let’s start with connectors. While SM and MM fiber have the same 125-micron outside diameter and use the same styles of connectors, the connector dimensions are different. SM connectors have a slightly smaller hole in the ferrule (by just a few microns) to maintain tighter tolerances in fiber alignment. This is necessary to keep connection loss as low as possible.


MM fiber has looser manufacturing tolerances, so it needs a bigger hole in the connector ferrule. The minute differences mean using MM connectors on SM fiber will cause fiber alignment errors and high loss. Trying to use SM connectors with MM fiber means sometimes the fiber won’t fit in the ferrule hole at all.


In addition, the termination process is different for SM and MM connectors. The tighter tolerances required in SM connectors is only part of the problem. SM links use lasers that are sensitive to reflections, requiring a special polishing process to minimize connector reflectance. SM polishing requires using expensive diamond polishing film and a polishing slurry plus careful procedures to get an even polish.


In other words, forget field polishing SM connectors. Field termination of SM fiber is done using factory-made pigtails that are fusion-spliced onto the fibers or prepolished/splice-on connectors using fusion or mechanical splices. That means you may need to add a fusion splicer to your fiber optic equipment kit.


If you often deal with SM fiber, you will probably need a fusion splicer anyway. Splicing MM fiber is rare; connectors are relatively low loss, cheap and more flexible to make connections. SM connectors are more expensive and more susceptible to airborne dirt contamination, so splicing is often the preferred method of joining fibers. Some fusion splicers work with bare fibers and the fusion splice-on connectors, a major advantage to the contractor faced with buying new equipment.


Speaking of new equipment, there is, of course, a need for SM test equipment. A video microscope is useful to inspect connectors for cleanliness, since dirt is a bigger problem with the small core of SM fiber. MM optical loss test sets use light-emitting diode (LED) test sources that are relatively cheap, while SM tests require laser sources at 1,310 nanometers (nm) and 1,550 nm that are more costly.


If you already have an optical time-domain reflectometer (OTDR), you can probably upgrade to an SM module with 1,310-nm and 1,550-nm lasers. If you don’t have an OTDR, you will probably need one for testing splices and reflectance at connections. New OTDRs also have higher resolution capability for use with short links like FTTH and premises cabling, a feature you will need.


Incorporating new equipment means learning how to use it, so your installers will need some training. Fortunately, most of the equipment is automated, and many manufacturers will send someone to provide basic training. Then the installers can practice on the equipment until they are confident enough to use it in the field. One warning: OTDRs still require a competent operator to analyze traces; you cannot just push a button and trust the instrument’s results.


In case you’re adding up the costs, don’t panic. SM installation costs more, but it also pays more, which will compensate for your investments. In addition, it widens your market to some new applications that are growing quickly. So, it may be worth getting into.