Radio, wire, and cable transmission systems use frequency to describe the operational region of the radio-frequency spectrum to be used. Frequency is measured in Hertz. However, since fiber optic transmission was researched by physicists, wavelength is used to describe the location of operation in the spectrum.
Light is an extension of the radio-frequency spectrum at the high end. This concept of continuous spectrum is shown in this figure. The notation commonly used for wavelength is λ. It is length, and its basic measurement unit is the meter.
We can relate frequency F and wavelength λ by this formula:
F times λ = 3 x 108 m/s, where 3 x 108 meters/second is the velocity of light in a vacuum. F has the unit of Hertz, and wavelength has the unit of meters.
Here is an example. One FM radio station for music is at 104 MHz. What is the equivalent wavelength?
So let’s follow the formula, here F is 104 MHz, so we get 104 x 106 which is 104 MHz, times λ equals 3 x 108. Thus we get λ equals 2.8846 meters.
The operational wavelengths in fiber optics are commonly given in nanometers (nm). A nanometer is 1 x 10-9 meters, or 0.000000001 meters.
A popular wavelength for fiber optic operation is 1310nm. So what is its equivalent frequency. Let’s follow the same formula:
1310 x 10-9 x F equals 3 x 108. After calculation, this gets us F = 2.29 x 1014 Hz or 2.29 x 105 GHz, or 229 THz.
This figure shows the three basic windows of operation for optical fiber. These are:
- 820 – 900 nm
- 1280 – 1350 nm
- 1528 – 1620 nm
We should pay attention to the so-called “water” absorption line at about 1400nm. Water is the cause of fiber impurity, but it is actually the OH- radical absorption line. There is high attenuation in the region around 1400nm as a result.
So let’s look at the bandwidth for the third window. Following the same formulas, we can get 1528nm is equivalent to 196 THz, and 1620 nm is equivalent to 185 THz. So the bandwidth is 196 THz – 185 THz = 11 THz or 11,000 GHz. This is about 110 times the useful radio-frequency spectrum!