Table of Contents
Fiber Optics– Have you ever thought about how you get e-mails or any other information from any corner of the world within a blink of an eye?
This has been made possible by a network of cables which are thin strands of very pure glass about the diameter of a human hair these cables are laid under the ground and below the ocean. The cable which carries most of the world data is optical fiber cables. They are also used in medical equipment. Now we will discuss how the fiber optics work and how they have revolutionized the world around us.
Optical fiber cables are made up of thousands of fiber strands and a single fiber strand is thin as a human hair. Optical fiber carries information in the form of light. We will first learn some fundamental behavior of light to understand the working of optical fibers.
We have been introduced to the ranges of frequency normally associated with electrical signals. The highest frequency was of the order of 1000GHz. If the frequency is increased to about 1015Hz we would find that the signal would literally appear as visible light. Signals in the range 1012 – 1015 Hz appear as infra-red while those in the range 1015 – 1017 Hz appear as ultraviolet light.
This raises the question if we can transmit data using the lower frequency ranges why should we not communicate at the higher light frequencies. In fact, there is no reason why we should not but there are difficulties in taking the light from a source to some form of receiving unit within which the carrier could be separated from the basic information signal.
There are plenty of materials such as glass through which we can pass light. A glass fiber would make a simple conductor of light. The problem is to retain the light within the glass but it is possible by simply placing the fiber within a cladding. It is not quite the same thing as a copper conductor within an insulating sleeve but it has the same effect.
Let us consider a fiber that has been bent as shown in the figure. Light moves in straight lines; therefore when the beam of light arrives at the bend it hits the side and bounces off the side at the same angle as that of impact. This is appropriate so long as the fiber is held in the air but if the fiber is held against another material light is partially if not totally lost.
The need was therefore to introduce a cladding which would prevent fiber touching such unsuitable external materials. The cladding which surrounds the fiber must have a reaction to light similar to that of air i.e an insulator of light. A clad optical fiber is shown and here we see the light continues through the fiber even though it has been bent.
Let us now consider passing light through a pane of glass. We tend to think that all the light has passed through but in fact, there is a small loss. We could demonstrate this by producing thicker and thicker panes of glass because we would not find that the image on the other side would become progressively less easy to see.
Even with a reasonably good form of glass, we would find that after passing through 10m of glass, the signal strength would be down to about 10 percent of the input, and 10 percent means that we would be down to a signal strength of only 1 percent after 20. It follows that we could not transmit a signal along any applicable length of fiber made from ordinary glass.
Therefore we have to look to remove the impurities in the glass rather like removing the impurities in semiconductor materials. Once the impurity level is sufficiently low we find we achieve outcomes that otherwise were impossible. In an optical fiber, the 10 m for a 10 percent signal has been extended to 50Km, and no doubt this will be doubled by the end of the century.
Although we have introduced the light signal in terms of frequency it is also common to define the operation by its wavelength. It will be recalled that the speed of light is given by:
c= 3x 108
The wavelength, frequency, and speed of light are related by:
Usually, we find that the wavelength of light is measured in micrometers.
Most optical fiber system operates with wavelengths in the range of 0.8 to 1.6 µm which are the infra-red wavelengths. The type of glass used in silicon glass.
The speed of the light changes when it passes through a medium and this change in speed is expressed by the refractive index. This variation and the speed of the light lead us to an interesting phenomenon which is refraction. Refraction occurs when light passes from one refractive index to one with another refractive index. The light bends towards the interface when it goes from the medium of high to one of the low refractive index.
We have observed the light which travels in straight lines can pass through fibers that are curved. We were told that losses could occur at the edge of the fiber according to the material which it was touching. To explain these occurrences we need to briefly consider refraction.
The significant measurement of any transparent material is its refractive index which the ratio of the speed of light in free space to speed of light in the transparent material.
The speed of light in a material is always slower than that in free space.
Light rays travel in straight lines through optical materials but when a light ray passes from one material to another, the refractive index changes choice of materials and the angle of incidence as shown. The angle of refraction is that relating to the ray of light after bending.
In the instance shown, we see that the ray of light when entering the glass bends towards the normal. However, if leaving the glass the light bends eventually we reach the point that the angle of refraction is 90. Increase the angle of incidence and the ray is then reflected as shown
This is what we observed when the glass fiber was in space. However when the fiber touched some other material it is possible that refection would not take place and refraction would operate, hence the loss of the light from within the fiber. We, therefore, have to select cladding which has an index of refraction less than that of the fiber. It is not necessary for the differences to be large even 1 percent is sufficient. A difference of 1 percent permits within the limit needed when the fiber is more or less straight but it does limit the extent to which the fiber can be bent around the corners after the fashion indicated. The greatest angle between the ray and the fiber surface without refraction is known as the angle of confinement.
So far we have considered the effect of a ray of light passing along an optical fiber. This assumes that we can take it from a suitable light source and ensure that it passes along the fiber. Ideally, the light source would produce parallel rays of light that move directly into the fiber, but in practice, some of the rays will enter the end of the fiber at an angle. The limiting angle is of the acceptance angle which is illustrated.
In practice, optical fibers are about 0.25-0.5mm in diameter. This includes a plastic protective coating and the cladding so that the light-transmitting core is only about 10-80 µm. this means that the light source has to be very small to remain within the angle of acceptance. Such a source is obtainable from a semiconductor diode laser. If we accept larger core sized say 100µm up to 1 mm, a light-emitting diode can be used. These are cheaper and last longer than the diode lasers but they cannot operate quickly.
When light passes along an optical fiber, losses occur and the light signal is attenuated. There are a variety of reasons for this including absorption of light energy by impurities in the fiber material. Light can also be scattered out of the fiber by impurities. An attenuation wavelength curve for a typical fiber is shown. The unexpected hump in the characteristics is due to traces of water impurity in the fiber. The characteristics are limited by ever-increasing attenuation at a smaller wavelength and by rapidly increasing attenuation due to absorption by the silica in the glass at longer wavelengths. For the characteristics shown we are limited in the choice of the operating condition by the light source available. However, it should be noted that the attenuation values are low when compared with conventional electrical conductor and waveguide systems. This means that we can transmit over relatively long distances without amplifiers or repeaters.
We consider the importance of being able to transmit a number of signals at different frequencies within one system. In the digital system we seek high bandwidth associated with high information capacity.
Let us consider a system capable of handling 100 Mb/s. This is equivalent to 100 systems operating at 1 Mbits/ s and predictably it is cheaper to build the 100 Mbit/s system. The same would be true for analog signals except that we would measure the effect in megahertz.
If for a moment we returned to more conventional electrical transmission cables we find that as frequency or information capacity increases so the transmission losses increase. In optical fibers, the losses are almost independent of frequency or information capacity and this is the significant advantage of using optical fiber systems instead of conventional cable systems. The comparison of loss/frequency characteristics.
In a conventional electrical system, we produce the carrier signal and then modulate it. Semiconductor lasers and LEDs combine the two functions so that they are modulated by the same current which energizes them. It follows that by imposing the signal on the energizing current the light varies in proportion to the signal.
For a digital system involving a diode laser source, the output/input characteristics are shown. Analogue transmission can also take place in which case the light intensity will rise and fall in proportion to the analogue input signal.
Optical fiber system:
In principle, an optical fiber system takes the shown with a light source providing the conversion of an electrical signal into a light signal which passes through the fiber. At the receiving end, we require a detector which will convert the light signal back into an electrical signal.
A receiver is likely to consist of a semiconductor photodetector. This will not produce a particularly strong electrical signal and therefore an electronic amplifier will be incorporated with the photodetector to make up the receiver. The photodetectors can use a variety of materials depending on the system wavelengths. For instance, a silicon diode would be used if the wavelength were about 0.8µm whereas a germanium diode would suitable for 1.2µm. for digital systems, the amplifier will also clean up the signal.
In a certain respect, optical fiber systems are specialized in their applications and for that reason, it is not possible to give them further detailed consideration. However, most communication transmission systems have been converted to optical transmission instead of conductor transmission, and therefore even at this early introductory stage, we need to be aware of the concepts of optical fiber systems.
Before leaving such a system some mention should be made of the variety of optical fibers. Examples of the more common types are shown. The single-mode fibers are now the most favored because of their high information capacity although the multimode fibers are often used in short-distance applications. The choice of the fiber depends on such factor as:
- The need to maximize bandwidth and capacity
- Ease of coupling
- Low attenuation
- Minimize the number of repeaters
- The flexibility of fiber and the ability to be bent
The rate of progress in fiber design in such that the year achievements will be next year rather ordinary performance. By the early 1990s, fibers were transmitting capacities of 400 MB/s over 50Km at 1.3µm wavelength without needing to resort to repeaters. We can look forward to possible capacities of 2000MB/s by the end of the decade.
Many optical components can be directly made from fibers we make components using optical fiber the question is why we want to make components from the optical fibers and what kind of components do we require. we are going to use the optical fiber in the telecommunication system or we can also it in the sensing system. So when we use fiber optics in a system we require several components and if we have the components which are all-optical than the data rate is not compromised also the complexity of the system decreases. So we would like to have as many components as possible in the domain itself and preferably in optical fiber because when we make components out of the optical fiber then it is easy to splice them with the transmission fiber or sensing fiber the insertion loss is low compatibility is better. What kind of the components or devices we want to study or make are switches and then we have power splitters, wavelength filters, multiplexers and de-multiplexers, polarization controllers, fiber gratings, fiber amplifier, and dispersion component.
A very basic component and important component that are using in the fiber optic is the directional coupler. It is essentially a port device so we have fiber coming in at 1 and 4; while the fiber coming out from the 2 and 3.
What we can have using this will if we launch power P in port 1, depending upon the parameter of this device or this component the entire power may come out of port 3, then it is basically a switch. You are switching from port 1 to port 3. It may also happen that if we launch power in port 1 then 50% power comes out of port 2 and 50 % power comes out of port 3 then it is power splitter or we have the desired ratio of power here. If we launch two wavelengths λ1 and λ2 in port 1 then it may happen that the λ1 comes out of the port 2 and λ2 comes out of the port 3. Then such a component is de-multiplexer. We can also input λ1 from port 2 and λ2 from port 3 and both the wavelength may come out of port 1 then it is a wavelength multiplexer or WDM coupler all these components can be made out of the directional coupler. So what is a directional coupler if we have 2 cores which are put together very close to each other and we launch light in core 1 then as the light propagates through the optical fiber this composite system then there is redistribution of power between the two cores and after a certain length, all the power will come out of the other core. So there is a switching of the power from core 1 to core 2. So if we look at a four-port device and if we launch light in core 1 then the light may come out of core 2 in the output end.
How does it work?
If we have two well-separated course there is a cladding. So If we have two fibers that are well separated and these are single-mode fibers then this fiber supports this mode and propagation of light in the first fiber is not affected by the propagation of the light in the second fiber, because they are very separated.
But if we bring it close to one another then what may happen that now this core will be combined if the separation between the two cores is not very large. So there is a possibility of combining these two fields.
- Fiber couplers allow to couple light typically between two fibers or transfer the same amount of power from one fiber to another fiber, typically with the coupling coefficient depending on the optical wavelength. Couple ratio is the output to input power ratio. With the help of a fiber coupler, we can mix two signals in one fiber. Using a coupler we can send signals from one fiber to the multiple fibers.
Types of optical couplers:
There are three major types of couplers.
- Diffusion coupler
- Area splitting coupler
- Beam splitting coupler
- Fiber Bragg gratings provide strongly wavelength-dependent reflection and transmission properties normal fibers provide a uniform refractive index while in fiber Bragg the refractive index change periodically along the short length. They can be used by introducing chromatic dispersion into a system or optical filters.
- Fiber amplifiers can amplify light in certain wavelength regions it can amplify the optical signal. In the case of the optical amplifier, it is not necessary to convert the optical signals into electrical signals and then provide the amplification. The optical amplifier performs amplification without any conversion. The optical amplifier can be placed anywhere on optical length to provide amplification. Optical amplifiers are bidirectional and optical multiplexing is also possible. That means optical amplifiers accept many signals at a different wavelength and provide amplification. Fiber polarizers can be made in which two different polarization states of light travel at a slightly different velocity that causes. The random spreading of the optical signal occurs due to the cause of the imperfection and asymmetries in the glass fiber core itself.
Single polarization fibers can guide only light with a certain polarization direction which in turn compromised of two orthogonal polarization modes. Asymmetric differences in fiber introduce small refractive index variations between the two states. This is known as birefringence or double refraction. Birefringence, in turn, causes slight differences in group velocity and phase which is referred to as PMD (Polarization mode dispersion).
Mathematically PMD occurs as a statistically random measurement along with the fiber which is to say that both polarized states exhibit some variable value of PMD at any point in time.
Types of the optical amplifier
- Semiconductor optical amplifier
- Raman optical amplifier
Semiconductor optical amplifiers can provide amplification in a semiconductor waveguide which is small in size and provides amplification by using stimulated emission which is similar to a laser.
Raman optical amplifier uses Reiman scatting principle to obtain amplify the signal.