ELECTROMAGNETIC SPECTRUM - CHOICE OF FIBER OPTIC FREQUENCY BASIC INFORMATION

Radio waves and light are electromagnetic waves. The rate at which they alternate in polarity is called their frequency (f ) and is measured in Hertz (Hz), where 1 Hz = 1 cycle per second.

The speed of the electromagnetic wave (v) in free space is approximately 3 X 10^8 ms–1. The term ms–1 means meters per second. The distance traveled during each cycle, called the wavelength ( ) can be calculated by the relationship:

wavelength = speed of light/ frequency


In symbols, this is: alpha =  v/ f


Electromagnetic spectrum
In the early days of radio transmission when the information transmitted was mostly restricted to the Morse code and speech, low frequencies (long waves) were used. The range of frequencies able to be transmitted, called the bandwidth, was very low. This inevitably restricted us to low speed data transmission and poor quality transmission.


As time went by, we required a wider bandwidth to send more complex information and to improve the speed of transmission. To do this, we had to increase the frequency of the radio signal used. The usable bandwidth is limited by the frequency used — the higher the frequency, the greater the bandwidth.


When television was developed we again had the requirement of a wider bandwidth and we responded in the same way — by increasing the frequency. And so it went on.

More bandwidth needed? Use a higher frequency. For something like sixty years this became an established response — we had found the answer! Until fiber optics blew it all away.

The early experiments showed that visible light transmission was possible and we explored the visible spectrum for the best light frequency to use. The promise of fiber optics was the possibility of increased transmission rates.

The old solution pointed to the use of the highest frequency but here we met a real problem. We found that the transmission losses were increasing very quickly. In fact the losses increased by the fourth power. This means that if the light frequency doubled, the losses would increase by a factor of 24 or 16 times.

We quickly appreciated that it was not worth pursuing higher and higher frequencies in order to obtain higher bandwidths if it meant that we could only transmit the data over very short distances. The bandwidth of a light based system was so high that a relatively low frequency could be tolerated in order to get lower losses and hence more transmission range.

So we explored the lower frequency or red end of the visible spectrum and then even further down into the infrared. And that is where we are at the present time. Infrared light covers a fairly wide range of wavelengths and is generally used for all fiber optic communications. Visible light is normally used for very short range transmission using a plastic fiber.


Windows
Having decided to use infrared light for (nearly) all communications, we are still not left with an entirely free hand. We require light sources for communication systems and some wavelengths are easier and less expensive to manufacture than others.

The same applies to the photodetectors at the receiving end of the system. Some wavelengths are not desirable: 1380 nm for example. The losses at this wavelength are very high due to water within the glass. It is a real surprise to find that glass is not totally waterproof.

Water in the form of hydroxyl ions is absorbed within the molecular structure and absorbs energy with a wavelength of 1380 nm. During manufacture it is therefore of great importance to keep the glass as dry as possible with water content as low as 1 part in 109.

It makes commercial sense to agree on standard wavelengths to ensure that equipment from different manufacturers is compatible. These standard wavelengths are called windows and we optimize the performance of fibers and light sources so that they perform at their best within one of these windows

The 1300 nm and 1550 nm windows have much lower losses and are used for long distance communications. The shorter wavelength window centered around 850 nm has higher losses and is used for shorter range data transmissions and local area networks (LANs), perhaps up to 10 km or so. The 850 nm window remains in use because the system is less expensive and easier to install.