Optical fiber certification is still being improved upon regularly, so if you are in the business of optical fiber installation then you have to be sure you are keeping up with the latest standards and certification methods. First, let’s quickly review the types of fibers, their characteristics, and the light sources they each use before getting too involved. Don’t worry, there won’t be a test later.
Multimode fiber is used for short-haul, lower bandwidth connections using 850 nm or 1300 nm light wavelengths (but is typically optimized for 850 nm so our info below will be for 850 nm) and can be divided into 4 types:
OM1 – 62.5 micron glass core, LED light source, 200 MHz/km
OM2 – 50 micron glass core, LED light source, 500 MHz/km
OM3 – 50 micron laser-optimized glass core, usually a VCSEL light source, 2000 MHz/km (typically used for 10Gb connections up to 300 meters)
OM4 – 50 micron laser-optimized glass core, usually a VCSEL light source, 4700 MHz/km (typically used for 10 Gb connections up to 550 meters, or 40/100 Gb connections up to 150 meters)
Singlemode fiber is used for long-haul, high bandwidth connections using 1310 nm or 1550 nm light wavelengths, has an 8 micron glass core, uses a laser light source, and has an unlimited bandwidth (which is limited practically to about 100,000 GHz).
Now, when testing singlemode fiber you would use a laser as your light source during testing because the network equipment to be connected to that fiber will use a laser, and you want your testing to mimic the network equipment and deliver accurate results. When testing OM1 or OM2 multimode fiber (which typically will use an LED), you will use an LED during testing to again mimic the network equipment and deliver accurate results.
However, when testing OM3 or OM4 fiber which would often be connected to network equipment that uses a VCSEL, you would still use an LED light source on your tester. This goes against the previous trend, so many contractors get this wrong. Why is it different?
An LED has many modes of light (which is the opposite of a laser, which is very focused and has very few modes of light) while a VCSEL has less modes of light. The idea is that an LED will give you a worst case scenario result when testing while a VCSEL will give you a very optimistic result, and the standards committees have stated that VCSELs should not be used in testing of fiber to avoid overly-optimistic results.
The bottom line is you should be sure you are using a contractor who is keeping up with standards, has the proper equipment to ensure a good installation, and cares about your network as if it was their own.