Thoughts About Fiber Optics
Light is a form of electromagnetic radiation. It exists at many wavelengths from the longer (730 nanometers or more) infrared light, through the visible color spectrum, to the shorter wavelength ultra-violet light. When a short pulse of light is sent down a narrow optical fiber, it will be changed (degraded) by its passage down the fiber. It will emerge as a weaker signal with various distortions. The physical characteristics of the optical fiber, the thickness of the actual optical core, and the wavelength of light being used must be properly matched to assure proper signal carrying capability.
The fundamental guideline is that equipment manufacturers have provided specifications that must be followed when implementing fiber optic cables, transmitters, and receivers. The only two variables that are under our direct control are the transmission speed and the distance. For any particular network design there will be a limited set of possible product solutions that provide the necessary speed and distance. These solutions can then be individually assessed based on two factors: cost and future growth. For example, a particular fiber optic cable may be physically capable of carrying signals at a rate as high as 655 Mbits/sec. Today we could decide to use this type of cable although the transmitting and receiving equipment may only be capable of 100 Mbits/sec. We could make this decision based on our assumption that in the future we will replace the transmitting and receiving equipment but we don’t want to have to dig up a roadway or a building foundation to lay new cable. We must, therefore, weigh cost against future growth requirements when we compare and contrast the set of possible product solutions that meet our current distance and speed requirements.
There are three fundamental types of fiber optic cable in use. These are called Multimode Step-Index, Multimode Graded-Index, and Single-Mode Step Index. In general conversation these are grouped by their general characteristics and simply called Multimode Fiber (MMF) and Single-Mode Fiber (SMF).
SMF cable allows communication distances of between 40km and 200km as compared with as little as 2km for MMF. The superior capabilities of single-mode fiber results from very precise manufacturing techniques. Interestingly, because the telephone companies use SMF in great quantities, the cost per foot for SMF is actually lower than the less capable MMF. If there were no other considerations then, clearly, SMF would be the cable of choice for all applications. There is a significant requirement that is associated with SMF, however.
The beam of light used with SMF must be from high quality laser light source. Multimode fiber, in spite of its shorter distance capabilities, is much better suited to carrying the less focused light from a low-cost light emitting diode (LED). True, the LED is not as powerful as the laser, and the multimode fiber is potentially more expensive than the SMF used with the laser – but the LED transmitter device may be significantly less expensive than the laser transmitter device. These issues become part of the cost analysis process.
Detailed Technology Discussion:
Light is degraded during transmission due to attenuation, polarization, and dispersion.
Attenuation results from the glass fiber absorbing the energy of the light. The rate at which light is absorbed is dependent on the wavelength of the light and the characteristics of the particular glass. Glass is a silicon compound; silicon dioxide, SiO2). Optical engineers have found that adding different additional chemicals to the basic silicon dioxide they can change the optical properties of the glass. By adding roughly 4% germanium dioxide (GeO2), for example, they can create a glass that has much less attenuation, and much ‘flatter’ attenuation across various frequencies of light, than silicon dioxide by itself. This process is called ‘doping’ the glass and the germanium dioxide is referred to as a ‘dopant’. The dopant modifies the way that the glass refracts light (the ‘refractive index’) and, hence, improves the light carrying capabilities of the fiber.
The absorption of light in the fiber results from minute variations (less than 1/10th of the wavelength) in the density or composition of the glass. This is called “Rayleigh scatter” and it’s the same optical property that makes the sky look blue. We said in the previous paragraph that the addition of dopants modified the refractive index and improved capacity. The addition of dopants also increased the density variation in the cable, which increases absorption and reduces capacity. We will trust the optical engineers to have found the point of diminishing returns in their manufacture of fiber optic cable.
What we should realize from the discussion of attenuation and absorption is that different types of fiber optic cable, manufactured to different specifications, can make a significant difference in the capabilities of that cable. Attenuation, for a given wavelength of light, is measured in dB/km (decibels per kilometer). Since we will know the type of cable being used and the type of light source (LED, laser, power output, color) we can calculate the specific dB loss for any length of cable. A fiber optic link will have a specified attenuation budget. That is, for any given implementation, there is a maximum amount of degradation (loss; measured in decibels; dB) that can be tolerated before the link fails to operate properly. In general, we will be presented with distance limitations and not dB measurements. If we design a fiber link that is within the specified distance limitations then we should be able to trust the fiber manufacture and equipment vendor concerning the constraints of the attenuation budget.
Polarization refers to a very specific engineering property of light. The property refers to an analysis of light energy that treats light as an electromagnetic wave that is vibrating in a single plane. We can understand this by the example of ‘polarized’ sunglasses that reduce glare by cutting out light that is reflected off roads, windows, or lakes. This light is reflecting in a basically horizontal plane. The polarized glasses cut out light that is vibrating in this horizontal plane. Light vibrates in different planes. When a particular plane of vibration is introduced to one end of a fiber optic cable (a light beam is shined into the cable) there is no maintenance of the orientation of the plane of the light from one end of the cable to the other. Suffice it to say that the cylindrically symmetric glass cable provides no capability to act in on this engineering property. Fiber optic communications in the 1990’s do not utilize any aspects of light polarization in the transmission of data and, hence, the issue is currently irrelevant. It is mentioned here only to prepare for some future time when engineers may utilize the polarization angle of a transmitted light plane to encode some type of signal.
The third degrading property of a fiber optic cable is dispersion. This is the effect of ‘spreading out’ that occurs as the light travels down the cable. A short pulse of light gradually becomes longer and eventually falls back far enough that the next pulse catches up with it. The most significant types of dispersion in fiber optic cable are called chromatic, modal, and waveguide.
Chromatic dispersion results from the combination of different wavelengths of light being produced at the light source and different refractive indicies in the transmission medium. The light source isn’t 100% accurate and the cable isn’t 100% pure. The result is that the transmitted pulse doesn’t remain precise. An extreme example of the effect of chromatic dispersion can be seen when light passes through a glass pyramid and is spread out into the colors of the rainbow. Chromatic dispersion is reduced by improving the quality of the light source and by increasing the purity of the glass material and by increasing the accuracy of the manufacturing process.
Modal dispersion results from the fact that a transmitted light pulse is not one single beam (mode) of light. Light is being transmitted in a beam that is not perfectly focused. Consequently, some modes of light (rays of light) travel a slightly longer path from one end of the cable to the other, and other modes of light travel a slightly shorter path. Consequently, some components of a light pulse will arrive before others. The difference between the arrival time of light taking the shortest mode versus the longest increases as the length of the cable increases. Improving the quality of the light source reduces modal dispersion.
Waveguide dispersion is a complex effect caused by the shape of the fiber core and by its chemical composition. The specific way that light is dispersed as it travels down the fiber cable can be predicted and controlled by modifying the composition of the glass. Engineers actually use the effects of waveguide dispersion to counteract other dispersion effects in cable design. The balance between the waveguide dispersion effects and the other dispersion effects in a cable become limiting factors in determining which wavelengths of light can be used to create a signal pulse. This type of engineering aspect of fiber optic cable helps to account for the selection options that are available in the commercial marketplace.
A common misconception regarding the propagation of light is that a dispersed beam of light splits up into an infinite number of angles and propagates through an infinite number of paths as it travels from one point to another. Let’s apply this fault thinking to a fiber optic cable. We could mistakenly conclude that when an LED light source produces a pulse of light that the light enters the fiber optic cable at an infinite number of angles and propagates down the cable using an infinite number of paths. This is not true. Light travels down a finite number of ‘paths’. When a light source is dispersed there are a finite number of ‘rays’ of light produced. These paths are called MODES.
When we construct a fiber optic cable with a core diameter large enough so that light entering one end will be able to find multiple paths through the cable we say that the cable is a MULTIMODE cable. There are multiple paths (modes) by which a light ‘ray’ may be propagated. For a fiber with a core diameter of 62.5 microns (a standard size) a light with a wavelength of 1300 nanometers (a standard wavelength) will find roughly 228 modes for propagation.
This core will be covered with another layer of glass, called the cladding layer. The cladding will have a lower refractive index than the core. Consequently, light that strikes the cladding, coming from the core, will be reflected back into the core. This effect (of reflection from a surface with a different refractive index) can be seen when we observe a reflection in a smooth body of water. The same principle is used to make stray light bounce back into the cable core.
The problem with multimode cable is that some of the modes are longer than others. This means that a pulse of light will be ‘spread out’ due to the modal dispersion. This causes an effect referred to as ‘intersymbol interference’, which restricts the distance that a pulse can be usefully sent over multimode fiber. Remember that as cable length increases the ‘spread’ of a pulse increases. The light on the longer path falls further and further behind the light on the shorter path as the overall distance traveled increases.
Multimode Graded Index Fiber
One way to reduce the modal dispersion in multimode fiber is to change the fiber’s optical characteristics to compensate for the problem. By modifying the refractive index of the glass through very precise manufacturing techniques, the light can be made to travel more slowly than light that is bouncing around in modes near the outside of the core. The shorter paths are made slower so that they are held back to the pace of the faster (but longer) paths. The light that travels farther travels faster and the light that travels less travels slower. The net effect is that the light pulse stays together and doesn’t spread out in the way that it would with non-Graded Index fiber. A graded index fiber typically transmits roughly 800 modes. The core fiber has a varying refractive index. The core is clad with a glass with a lower refractive index, just like non-GI multimode, to cause stray light to bounce back into the cable.
In this design, the core fiber is very narrow compared to the wavelength of light being used. Instead of the 62.5 microns used in a multimode cable, single mode cable may be only 8 microns in diameter. The result is that only a single path exists through the cable core through which light can travel. The cable has a single mode.
In the real world there are some additional issues for the cable designer to consider. As much as 20% of the light in a single-mode cable actually travels down the cladding. The effective diameter of the cable is a blend of the single-mode core and the degree to which the cladding carries light. This is referred to as the “mode field diameter” and it may be larger than the physical diameter of the core. The diameter of the mode field is determined by the refractive indices of the core and cladding.
Core diameter is a compromise. If the core is too narrow, then the degree of signal loss that occurs when the cable bends becomes too high. If the core is made smaller (of if the wavelength of light is made longer) then the ratio of diameter to wavelength gets smaller. At very small ratios, a sharp bend in the cable will cause the light so simply
Comparison Of Light Sources
Both lasers and LEDs are used as light sources. Laser light sources are significantly more expensive than LED light sources however they produce a light that can be precisely controlled and which has a high power. Because the LED light sources produce a more dispersed light source (many modes of light) these light sources are used with multi-mode cable. When a laser source is used (which produces close to a single mode of light) a single-mode cable is used.
The design of a fiber optic transmission system is dominated by two factors:
- The signal level
The receiver must be sensitive enough to receive the transmitted signal after it has been reduced by the transmission systems attenuation factors.
- The control of dispersion
The wavelength of the transmitted light must be consistent with the optical properties of the cable being used.
When specifying equipment for use in a fiber optic connection system the following factors must be taken into consideration:
- The type of fiber cable being used
- Refractive index
- Absorption spectrum
- The wavelength of light being used
- The characteristics of the light source
- The spectral line width (frequency range) of the source
- The characteristics of the receiver devices
- The modulation method 1. The method of varying the light to represent data bits
When installing any fiber system be sure to avoid excessive bends in the cable. We often see the cable pulled at almost a 90 degree bend as it leaves the connector in the equipment rack. DON’T DO THIS! Let the cable gradually arc away from its connecting point.