If you’re accustomed to certifying copper cable, you’ll be surprised that how easy it is to certify fiber optic cable because it’s immune to electrical interference. You only need to check a few measurements.

Attenuation (or decibel loss) – Measured in decibels/kilometer (db/km), this is the decrease of signal strength as it travels through on multimode fiber optic cables.

Return loss – This is the amount of light reflected from the far end of the cable back to the source. The lower the number, the better.  For example, a reading of -60 decibels is better than -20 decibels. Like attenuation, return loss is usually greater with multimode cable.

Graded refractive index – This measures how the light is sent down the fibre cable. It’s commonly used with 850- and 1300-nm wavelengths. Compared to other operating frequencies, these two ranges yield the lowest intrinsic power loss. (This is valid for multimode fiber only.)

Propagation delay – This is the time it takes a signal to travel from one point to another over a transmission channel.

Optical Time Domain (OTDR) – This enables you to isolate cable faults by transmitting high-frequency pulses onto a cable and examining their reflections along the cable. With OTDR, you can also determine the length of a fibre optic cable because the OTDR value includes the distance the optical signal travels.

 

Choosing a Fibre tester

There are many fibre optic testers on the market today. Basic fibre optic testers function by shining a light down one end of the cable. At the other end, there’s a receiver calibrated to the strength of the light source. With this test, you can measure how much light is going to the other end of the cable. Generally, these testers give you the results in dB lost, which you then compare to the loss budget. If the measured loss is less than the number calculated by your loss budget, your installation is good.

Newer fiber optic testers have a broad range of capabilities. They can test both 850- and 1300-nm signals at the same time and can even check your cable for compliance with specific standards.

Test sets commonly use two types of optical transmitters:

LEDs – Most popular for short-haul applications because of their low cost, LEDs provide a typical power output of -18 dBm (12.5 microwatts) into a multimode cable.

Laser diodes – For increased distances, laser diodes are best, because they can transmit more power into the cable. The diodes are well suited to single-mode applications because of the narrow yet powerful beam but are often used to increase distances over multimode cable. Power output can be higher than -12 dBm (50 microwatts) – four times more powerful than the equivalent LED.