In my October web exclusive, I used some animated graphics to explain how fiber works to transmit light and the different types of fiber. This article has more animations to see how fiber optics transmits data.

I like to demonstrate sending signals over fiber with a laser pointer and large-core 1 mm plastic optical fiber (POF), as shown here. This is a simple demo I recommend for teachers in science classes and instructors in technical training.

Signal degradation from attenuation

When I hit the button on the laser pointer, light is coupled into the plastic fiber and comes out the far end. If you look closely at the loop of fiber you can see a faint glow, accentuated at the taped points, which is the light loss in the fiber. In the fiber loops, it’s the attenuation of the fiber along the length of the fiber and at the tape; it’s from the additional stress on the fiber at that point. 

That’s exactly what is happening in a fiber optic data link.

A transmitter sends an electrical pulse from the connected electronic system to a source such as the laser in the laser pointer above, which converts the electrical signal to light. The light is coupled into the fiber and transmitted down its length. As the light travels down the fiber, it is attenuated by absorption and scattering in the core of the fiber, shown by the pulse height getting smaller as it traverses the length of the fiber. Most links will have splices or connectors like the one shown in the middle of the fiber above. Look closely and you will see the pulse height drop over the splice.

In the demo with the laser pointer, our eyes are the detector. In a real data link, there is a semiconductor detector that converts the light pulse to an electrical pulse to connect to the electronics at the receiving end.

Remember the fiber optic link uses light, but the equipment it connects is all electronics. Fiber optics is another communications medium like copper wire cable or radio waves for wireless communications that transmit signals in different ways, but all begin and end as electronic signals.

The link animation shows the signal loss (in decibels, dB) in the link caused by the attenuation of the fiber (dB/km times length) and loss caused by the joints in the fiber made with splices or connectors (dB). If the signal attenuation is too high, the receiver will have problems separating the signal from electronic noise in the receiver, which will cause a high bit error rate (BER) and poor signal transmission.

Signal degradation from dispersion

The other signal degradation is caused by dispersion, illustrated below.

As the signal travels down the fiber, it may suffer dispersion or broadening of the pulse caused by the interaction of the light with the fiber. Dispersion has several causes. Modal dispersion is prevalent in multimode fiber where the light travels in different modes with different lengths, causing differences in the speeds of the pulse in each mode. Chromatic dispersion is caused by the different speeds of light in the glass core for the different wavelengths of light in the source and can affect signals in multimode and single-mode fiber. Polarization mode dispersion is an issue in single-mode fiber where the signal may be transmitted at different speeds depending on its plane of polarization—it’s complicated.

The issue of dispersion is simple; it broadens pulses such that they may become wide enough to merge with the pulses before and after it, making it impossible for the receiver to distinguish the two separate signals, causing high BER.

Attenuation and dispersion are the two issues that can affect fiber optic link performance. Techs should measure the loss caused by fiber attenuation and splice or connector loss in every link. Premises links using multimode fiber are not tested for dispersion but are limited in length by the typical dispersion of the fiber specified in standards. Long single-mode links may be tested for chromatic and polarization mode dispersion to qualify them for high-speed transmission.