Plastic Optical Fibre - Essay Example

Plastic optical fibre Table of Contents Plastic optical fibre 3 Types of plastic optic fibres 3 Application of Optical Fibre 5  Attenuation 5  Numerical aperture 6  Dispersion 6  Advantages and disadvantages over the glass optical fibre 6 Reflection 7 Optical sensors 8  Uses of Optic Sensors 8  Natures of fibre optic sensors 9  Advantages of optical sensors and fibres over the other non-optical methods 12 Reflection 14 Plastic optical fibre Plastic optical fibre is also known as polymer optical fibre. It is an optical fibre which has been created from plastic. In most cases, acrylic is usually used as the core material while the cladding material is usually made of fluorinated polymers. There has been, however, an increased use of perfluorinated polymers recently (Bliss, 1994). Data in the optical fibre is transmitted inside the core which allows light transmission. The core makes up about 96% of the diameter of the entire fibre. Compared to the glass fibre, the core of plastic optic fibre can be even up to 100 times bigger. Since the future demands a high speed networking, plastic optical fibre can be a possible option in the next generation. Types of plastic optic fibres  There are different types of plastic optic fibres. These types arise due to the attributes that determine any type of transmission media. The main attributes are; the length over which data will be transmitted and the speed at which the data will be transmitted over a given length. There can be loss of data during the transmission process, and this arises from scattering the light or absorption of the light by impurities in the fibrelike molecules of water or metals, fibre defects like voids, interfaces of core cladding and the end faces. Any one of the loss mechanisms is usually a function of the transmission wave’s wavelength (Weinert, 1999).   One type of a plastic optical fibre is the PMMA. The PMMA loss spectrum has 530 nm, 570 nm and 650 nm transmission windows. All these windows are in the visible range. The 650 nm window is more sensitive because it is narrow. If a 650 nm source shifts with temperature, there could be arousal of problems in this window. On the other hand, the 57 and 530 windows are broader hence less sensitive to source wavelength shifts due to temperature variations. The PMMA plastic fibre is limited to transmitting light of less than 100 m. this is because the losses at 650 nm are 125dB/km while the loses at 570 nm and 530 nm are less than 90dB/km (Plastic Optical Fibres and Applications Conference (Paris)), 1992).   Another type of plastic optical fibre is made from perfluorinated polymers. This newer type of plastic optical fibre exhibits greater light transmission over a wider range of wavelength. There are two notable features that are seen when comparing the loss spectrum of perfluorinated polymer to that of PMMA. One is that the spectrum for the perfluorinated polymer ranges from 650 nm to 1300 nm. This range is notably higher than the range for PMMA. The second outstanding feature is that the loss over this wavelength is below 50bB/km. Due to a reduced rate of loss in this material, fibre links made from perfluorocarbon can be up to several kilometers. Perfluorocarbon therefore overcomes the limitations of distance experienced in the PMMA and can also be operated with the low cost glass optical fibre components at 850 nm to 1300nm (Plastic Optical Fibres and Applications Conference (Paris)), 1992).   The third type of plastic optical fibre is known as the micro structured plastic optical fibre (m POF ). This is the latest development in plastic optical fibre technology. It was reported at first in 2001 by Eijkelenborg. Some features of this type of plastic optical fibre include: the ability to modify the refractive index profile. This is done by modifying the structure of the hole; ability to make high numerical apertures and ability to direct light in materials with low index via photonic band gap guidance. Generally, m POFs are focused on short distance transmission in high bandwidth applications the measured bandwidth over a short distance of 9 meters was 6 GHz. This corresponds to about 2.4 GBps over a distance of 100m with the assumption of a strong mode coupling.   Application of Optical Fibre There are several areas of plastic optical fibre application. Several telecommunication companies use these fibres to transmit signals for cable TV, telephone signals and even for internet communication. It is more advantageous to use optical fibres for this transmission although it can be costly and time consuming especially during installation. Plastic optical fibres are also used in automobiles. They are used for higher GPS speeds, electronic automotive control, anti-skid video brakes, sensory systems and even audio systems. Since a single plastic optical fibre can be used as a replacement of several copper wires, engineers have opted for use of plastic optical fibres hence saving on space and weight of automobiles.   Plastic optical fibres are also used in the medicine field. This fibre can bend to 25 mm and therefore doctors use it to view internal organs of the body. The same fibre cable has a high tolerance when it comes to contamination or scratching. This optimizes its performance. The heavy copper wires and silica fibres in aerospace can be replaced by plastic optical fibres. Efforts are also being made to make use of fibre optics in channelling light to instrumentation panels hence increasing the brightness and reducing the weight.  Attenuation Since polymers are organic materials, they possess several rotational and vibrational bonds which can soak electromagnetic radiations. The effect becomes more severe and acute at wavelengths on the side of the red visible spectrum. The best use of plastic optical fibre is with the visible wavelengths. An efficient way of measuring attenuation of spectrums for PMMA is truncation. The maximum length of data transmission is determined by attenuation.  Numerical aperture For any fibre optic, the numerical aperture is an important parameter although sometimes it can be over emphasized and misunderstood. Scientifically, the numerical aperture is the Sine of one half of the angle of the light acceptance cone of the fibre. For plastic optical fibres, it is important to know the numerical aperture of the fibre because plastic optical fibres usually have a bigger core diameter compared to the glass optical fibres. Stringent adjustment of the properties for the optical systems is therefore required so as to avoid an increase in losses due to strong absorption. It is therefore sufficient to input light with a lower aperture than the normal theoretical aperture. The fibre’s length and the varying numerical aperture with the injection aperture should therefore be known.  Dispersion Dispersion refers to the scattering of the signal’s travel time inside the fibre optic cable. Dispersion in plastic optical fibres is brought about by mode dispersion which comes by as a result of different travel times for the different beams of light. Critical properties for transmission are determined by dispersion. Such conditions include: the maximum bit rate, the cut off frequency and the bandwidth as well.  Advantages and disadvantages over the glass optical fibre Plastic optical fibre has several advantages and disadvantages over the glass optical fibre. One advantage of plastic optical fibre is that it is pliable. This means that they bend easily compared to the glass fibre.  Plastic optical fibre’s bend radius is shorter than that of the glass optical fibre hence they are more resilient to abuse and damage. The glass fibre can easily be damaged due to the intrinsic characteristics of its material. It is also easier to connect, polish and terminate the plastic optical fibre which makes it cost effective with reference to installations and maintenance. Plastic optical fibre also has a higher numerical aperture compared to the glass optical fibre.   One major disadvantage of plastic optical fibre is that it has a low bandwidth. The low bandwidth limits the rate at which data is being transmitted. This makes the glass optical fibre transmit data at a higher rate compared to the plastic optical fibre. The other disadvantage of this cable is the high cost of installation and maintenance. Its performance is also limited to certain temperatures.   Plastic optical fibre is mainly made from organic material. These materials are drawn from preforms with a low temperature like 200 0C. A cladding material is made in to a hollow shape and then filled with a mixture of the monomer and reactive chemicals which polymerize and form the core. Reflection In conclusion, plastic optical fibres have revolutionized the transfer of data in the modern world. Given its flexibility and durability, plastic optical fibres can be used in any environment to save on space. Although their bandwidth is small, they can be used instead of copper wires and are even more efficient. This is one of the high speed optical fibre whose future is very promising because it can be adopted in any field of technology for data transmission.   As a reflection, this report focuses on plastic optical fibre and covers several aspects. Among the areas covered include: the different types of plastic optical fibres which include the PMMA, the perfluorinated polymers and the micro structured plastic optical fibre. The different bit rates of the different types of plastic optical fibre were also examined in this report. This piece illustrates several ways of application of these optic fibre cables which include screening in the hospitals, application in the aerospace and automotive industries and most importantly in the data communication field. The numerical aperture and attenuation have been well explained in this report. Finally, this report illustrates what dispersion is in plastic optical fibre, compares the advantages and disadvantages of plastic optic fibre to the glass optic fibre and describes the materials used and how they are used to create the fibre cable.   Optical sensors   An optical fibre sensor is a sensing device that makes use of an optical fibre either intrinsically (as a sensing element) or extrinsically (as a way of sending signals to from a sensor to the devices that receive and decode the sent signals). It is possible to multiplex several sensors in one fibre cable. This can be done by shifting the wavelength for each sensor lightly (Narayanaswamy & Wolfbeis, 2004). Fibre optic sensors do not require electric power at the remote point and they are also not affected by electromagnetic interference. This makes them possible to be used on places with flammable material or high voltage electricity. It is possible to design fibre optic sensors which can withstand high temperatures (Haus, 2010).  Uses of Optic Sensors There is a range of parameters which can be measured by fibre optic sensors. One such parameter is the measurement of time delay. A device which is known as optical time domain reflectometer can be used to determine the time delay. Another instrument that implements optical frequency domain reflectometry can then be used to calculate the wavelength shift (López-Higuera, 1998).   As intrinsic sensors, the optic fibre sensors can be used to determine and measure strain. Strain can be defined as a normalized measurement of deformation that represents a displacement between particles with relativity to a given length in a body. Another parameter that can be measured by an optic fibre sensor is temperature (B.D.Gupta, et al., 2006). Temperature is defined as the numerical measurement of hotness or coldness. The ratio force per unit area over which the force acts upon is known as pressure. With the use of fibre optic sensors, pressure can also be determined (B.D.Gupta, et al., 2006).   The above quantities are measured by modifying the optic fibre sensor so that the quantity can modulate polarization, intensity, wavelength, phase and even the time of transit of light in the given fibre. Light varying sensors are the simplest sensors since they only require a detector and a resource. Intrinsic fibre optic sensors are advantageous in that they can provide sensing which is distributed over large areas.   With temperature, measurement can be done with a fibre that has evanescent loss varying with temperature. The Raman scattering of the fibre optical can also be analysed to measure temperature. Electrical voltage is another quantity that can also be measured with fibre due to a non-linear optical effect by using a fibre which has been specially doped. The doped fibre alters light polarization as an electric voltage function (Haus, 2010).  Natures of fibre optic sensors There are different natures of fibre optic sensors. The most common one is based on fibre Bragg gratings. The Bragg which is the maximum reflexivity wavelength does not depend only on the Bragg grating period but also on the mechanical strain as well as the temperature. In the case of silica fibres, the Bragg wavelength’s fractional response is about 20% lower than its strain. This is because the strain effect is usually lowered by a refractive index decrease. In this type of fibre optic sensor, the effect of temperature is usually close to the effect expected from thermal expansion. The resolution is usually a few με for pure strain sensing. In the acoustic phenomena where dynamic measurements are required, it is possible to achieve sensitivities which are better than 1 nε in a one hertz bandwidth (López-Higuera, 1998).   Another nature of fibre optic sensors uses the distributed sensing mechanism rather than the Bragg gratings. This type of fibre sensor uses the fibre itself for sensing. The principles of sensing are then based on Raman scattering, Rayleigh scattering or even Brillouin scattering (Narayanaswamy & Wolfbeis, 2004).   Rayleigh scattering is an optical phenomenon which was named after Lord Rayleigh who was a British physicist. Basically, this technique involves the scattering of light at centres which are lesser than the wavelength of the light. In this case, the light is scattered with amplitude which is proportional to the amplitude which is incoming. This is done up to the 4th power of the inverse wavelength and up to 1 + cos2 θ. The θ is usually the scattering angle. Forward scattering (where θ = 0) and backward scattering (where θ =π) are usually equal. At larger scattering centres, scattering is usually described by the theory of Mie scattering. Theses scattering centres can either be molecules or atoms (Haus, 2010). Raman scattering on the other hand is a process of non-linear scattering which involves optical phonons. Particularly, a response which is non-instantaneous is usually caused by crystal lattice vibrations. The effect of the association of these vibrations and optical phonons is the one referred to as Raman scattering. When two beams of lasers having a different wavelength but the same direction of polarization are propagated via a Raman active medium together, the beam with the longer wavelength (it is known as the Stokes wave) experiences an optical amplification at the short wavelength beam’s expense. As a result, the lattice vibrations get excited which leads to an increase in temperature. Exploitation of the Raman gain can be done by use of Raman lasers and Raman amplifiers. For the gain to be substantial, the Stokes shift should correspond to a difference in frequency of several terahertzes. During the process of Raman scattering, a pump photon gets converted to a less energy signal photon. The photon energies’ difference gets carried away by a lattice vibrations quantum.   Another principle of optical fibre sensor operation is the Brillouin scattering. This is an effect which is caused by a medium’s non-linearity and it involves scattering of acoustic phonons. A photon for instance, is converted into a low energy photon and a phonon. The low energy photon normally propagates in a backward direction. The process of optical fields coupling with acoustic waves occurs through electrostriction. This effect is spontaneous and it can occur even at lower optical powers hence reflecting the phonon field which was generated thermally. In high optical powers situations, a stimulated effect can arise in which the optical fields aid in contribution towards the population of phonons. In Brillouin reflection, the reflected beam’s frequency is comparatively lower than the frequency of the incident beam. The difference in in frequency however is equal to the emitted phonons’ frequency. It is possible to calculate Brillouin shift from the acoustic velocity, the effective refractive index (in fibres) and the vacuum wavelength. Brillouin shift is dependent on the composition of the material and sometimes the medium’s pressure and temperature (B.D.Gupta, et al., 2006).   In optical fibres with Brillouin scattering, there is frequent encounter of stimulated Brillouin scattering. This happens due to amplification of narrow band optical signals in a fibre amplifier like those from single frequency lasers. Stimulated Brillouin can also occur as a result of propagation of the narrow band optical signals through a passive fibre. Brillouin threshold for narrow band light of the optical fibres is in the correspondence of a Brillouin gain of order 90 db. Stimulated Brillouin scattering brings in strict power limitations for the propagation and amplification of optical narrow band signals in fibres. Brillouin gain can hence be used in the operation of Brillouin fibre lasers.  Advantages of optical sensors and fibres over the other non-optical methods There are several advantages of optical sensors and fibres over the other non-optical methods. Optical fibre does not have any moving parts or electrical parts. It is for this reason that they are immune to electrical interference. It is therefore possible to separate the remote sensor which is the sensing system from electrical interference sources which are known. Optical fibre sensors cannot spark and this means that they are safe and can be used I hazardous environments like mining areas, oil refineries, pharmaceutical manufacturing plants and even chemical processing plants. They are safe to maintenance and repair personnel since the danger of electrical shock is eliminated. Another advantage of these sensors over the conventional non optical means is that they have a broad bandwidth. With the use of optical sensors, video, text, microwave, audio and data signals can be modulated on a light-carrier wave over long distances and demodulated at the receiving end. Their attenuation rate over long distances is also very low compared to non-optical means. The attenuation in optical fibres is in the range of 0.2bD/km. optical means does not require the costly metal conductors which have recently become target for metal theft due to their value in the scrap metal industry.   On the other hand, there are several disadvantages associated with optical sensors and fibre optics. The biggest disadvantage is the high cost of installation. Purchasing the materials required for installation is expensive as well as the installation itself. The optical transmitters and receivers are very expensive since it is a new technology. Splicing the wires used in the optic fibre can be challenging and expensive as well. Another disadvantage is the susceptibility to fibre fuse especially at very high optical powers. The meeting of too much light with an imperfection can damage up to 1.5 kilometers at a rate of several meters per second. To prevent a fibre fuse, a protection device should be installed at the transmitter and it should be used to break the circuit hence preventing further damage. It is not possible to carry electrical power with fibre optic cables for the operation of the terminal devices.   With the continuous development of technology and industrial automation, new sensing devices are being manufactured daily. The new sensors in production require very small wiring. It is possible for instance to configure sixteen sensors and make them share a single source of power. This is done by connecting a master connector to the master sensor. This master connector hence distributes power to the slave sensors therefore eliminating the need for power lines for each sensor. The only wiring required for the slave sensors is the output wiring. These new sensors are also easy to install and maintain. Some connector designs can allow the detaching of the sensor without necessarily tampering with the output wiring or cable installation.   There are new optical sensors that incorporate the performance of a two optical sensors in a single package. The output by certain models is a combination of digital and analogue or double digital output. There is an incorporation of a 12 bit CPU or a 16 bit CPU and a 12 bit A/D converter in most sensors that are being developed. The converter is responsible for providing high resolution and fast response time. New types of sensors that support remote settings functionality have been developed specifically for use in hazardous environments. A single sensor has its own remote control with which a 5 feet standard cable comes along. There are several factors which drive the development of fibre optic sensors and technology. The need for more compact machinery and lighter products especially in the aviation industry is one such factor. Compared to the conventional non optical ways of data transmission and communication, fibre optic technology is relatively fast and it is due to this factor that the use of fibre optic technology is being taken up at a very high rate. It is estimated that in the next five years, optical fibre technology will have replaced the conventional copper wire technology in the communication industry. Although it will take some time for fibre technology to completely replace the non-optic technology in some fields like aviation and auto mobiles, fibre optic technology holds the ultimate future for such fields given its flexibility, low weight and less space occupation. Reflection In conclusion, it is evident that fibre optic sensors are economically efficient data transducers. Since optical data transmission does not require electricity, it is eventually a cheap and efficient method of data transmission especially in this age where demand for information and networking is increasing. Fibre optic technology is however relatively expensive during installation as compared to non-optic methods of data transmission. In reflection, this report looks at fibre optic sensors in details. It starts by explaining the parameters which are measurable by use of fibre optic sensors and the optical technology. One parameter is time delay which is measured by a device known as optical time domain reflectometer. Strain and temperature can also be measured by fibre technology. Temperature is measured using the Raman scattering principle. Another quantity discussed in this report is electrical voltage which is measured with fibre due to a non-linear optical effect by using a fibre which has been specially doped. The nature of the sensors and principle of operation of the sensor is also discussed in this report. There are different natures of fibre optic sensors. The most common one is based on fibre Bragg gratings and the other principle is distributed sensing mechanism. There are also principles of sensing for the fibre optic sensor. These are based on Raman scattering, Rayleigh scattering or even Brillouin scattering. This report the outlines the advantages and disadvantages of fibre optic sensors and the fibre optic technology as compared to other non-optic methods. It touches slightly on the different types of fibre technology that are being manufactured currently in different technological fields. Finally, this report gives a slight indication of what factors may be driving the use optic technology and the estimated time-scales for the fibre technology to take over in communication and other industrial fields. References B.D.Gupta, Gupta & Das, B., 2006. Fiber Optic Sensors: Principles and Applications. 2nd ed. New Delhi: New India Publishing. Bliss, J., 1994. Plastic Optical Fibers and Applications. 1st ed. Boston: Information Gatekeepers Inc. Haus, J., 2010. Optical Sensors: Basics and Applications. 1st ed. Germany: John Wiley & Sons. López-Higuera, J. M., 1998. Optical Sensors. 1st ed. Ramaier Narayanaswamy, Otto S. Wolfbeis: Ed. Universidad de Cantabria. Narayanaswamy, R. & Wolfbeis, O. S., 2004. Optical Sensors: Industrial, Environmental and Diagnostic Applications. 1st ed. Berlin: Springer. Plastic Optical Fibres and Applications Conference (Paris)), 1992. Plastic Optical Fibres and Applications Conference. 1st ed. Boston: Plastic Optical Fibres and Applications Conference (Paris)). Weinert, A., 1999. Plastic optical fibers: principles, components, installation. 1st ed. Nw York: Publicis MCD Verlag. Read More