Anyone who reads my articles knows that sooner or later I would write an article on this topic. This is a quick primer on how different optical fiber connections work. Optical fiber connections are going to end up in your home far sooner than holographic or first surface optical storage. I am not going to give a review of individual standards in this article but instead an overview of the functioning of the major technologies used today.
The first and most simple type of optical connection uses simple LED light.
By switching the LED on and off, you can produce the 1s and 0s of machine language. The problem with this mechanism is it takes a long time to switch the state of the light (at least, a long time by computer standards). This is still used in the optical connection for audio systems. The S/PDIF / TOSLINK on the back of audio equipment sends information by pulsing an LED on and off and sending the light via a plastic optical fiber to the receiver.
It is possible to get a glass optical fiber for this purpose but this is somewhat akin to using single malt scotch to run your car. It will work but it’s an unnecessary expense. The bandwidth on this type of system has long since been surpassed by copper wire. The reason it is still used in audio equipment is the fact fiber does not receive electromagnetic interference.
Next up is single channel un-amplified coherent light fiber optics.
This is the most common type of system in use today. A laser is used to transfer information over glass fiber. Rather than pulsing the laser electrically, these systems use an optical switch.
Lasers use coherent light which holds some advantages for switching. As you may remember from my holographic storage article, coherent light waves can be used to interfere with one another. This phenomenon can be used to make an optical switch.
First, split the light beam into two paths using a beam splitter of your choice. Recombine the two such that the two beams are in phase. This will be an ‘on’ signal – a digital ‘1’. Now extend or shorten one of the two paths by half of one wave length. The two paths will destructively interfere, effectively turning the laser off.
Using a MEMS (micro electro mechanical system), this can be done faster than switching the laser electronically. Furthermore, it is possible to place multiple switches in a single path. By staggering the timing at which they operate, it is possible to send more information than a single copper circuit can handle.
Thanks to the wave nature of light, it is possible to multiplex many signals to the same fiber. For instance, start with one single channel signal at 1500 nm. There is nothing stopping you from adding another signal – 1000nm for instance. Still more channels can be added at 656nm and 502nm etc.
At the other end, you can divide the signal out into its component channels with a diffraction grating. The number of channels you can add to a single fiber is a function of the quality diffraction grating and the bandpass window of your fiber (see first surface optical storage for a definition of ‘bandpass window’).
The quality of the grating determines how narrow a range of frequencies you can differentiate one from the other. It is simply a matter of dividing the bandpass window by the number of frequencies you can differentiate. With good fiber and a good grating, the theoretical limit can range into the thousands.
My favorite technology is by no means the fastest but it is by far the most elegant. An un-amplified signal is fine over a few kilometers, but if you want to cross an ocean, the glass will absorb your signal. The solution is to include a number of amplifier-repeaters. The punch line is you can’t use electricity. Copper absorbs electricity faster than glass absorbs light. You could use electrical amplifiers but most of your energy would go into boiling water of the coast of Labrador.
The solution is to use an all-light system.
Two lasers, one is an amplifier beam the other is the information beam. There are a number of amplifiers along the line. The amplifier beam powers these. The amplifiers use the energy from the amplifier beam to amplify the information beam. This raises the obvious question: Why not just use the amp beam to carry the information?
The simple answer is the amplifier beam is more than powerful enough to cut steel plate. It takes little imagination to know what will happen to the precision micro electronics of a switch.
The amplifiers themselves are light pumped solid-state lasers (not to be confused with semiconductor lasers). For the sake of non-physicists like myself, I will back up a little. Imagine a single neon atom in a neon sign. An electron will hit it and it will become energized. Something will then cause it to release its energy in the form of a photon. The quantum mechanics of neon atoms determine the exact frequency or wavelength of this photon. The cause of this could be another photon. This is called stimulated emission, the “SE” of laser.
If you have a population of atoms in an energized state, all it takes is one photon to cause a chain reaction releasing the energy of the whole population. One photon hits one atom and the atom releases the photon and each continues on to stimulate another atom and so on. The beauty of this is all the atoms release exactly the same frequency of light perfect lock step (in ‘Phase’ as physicists like to say). This light amplification is the LA of Laser. For sake of those interested, the R is for radiation.
In this case, that just means the stimulated emission produces light rather than something else.
This is important to all-light amplifiers because it is possible to use light of any frequency to energize the population. Once the population is energized, it takes a certain specific frequency to cause it to release its energy. When this happens, it releases its energy at the same frequency.
Thus, one frequency can be used to power the amplifier to the energized state without causing it to release its charge. When the carrier signal hits the amplifier with an ‘on’ signal, the amplifier releases an ‘on’ signal of many times that strength. Thus, we can send information across oceans with a readable signal strength at the other end.
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