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Recently, the use of optical fibre technology gives numerous advantages that are suitable for clinical use. The optical fibre sensors can be made biologically compatible and they are immune from electromagnetic interference. Optical fibre grating sensors hold massive potential for medical applications due to their internal properties like small in size, biocompatibility, non-toxicity, chemical inertness and electromagnetically inert nature. Grating based sensors are well known technology for structural health monitoring (SHM) in the field of medicine. In this paper, a brief introduction about the optical fibres is presented. This paper gives an overview of the optical fibre grating that includes the principal and the properties of the optical fibre grating. This overview provides essential information for the understanding about this device. This paper also focuses on some general features of fibre grating sensor and also the benefits that the optical fibre grating sensor offers to the medical field.
To understand about the optical fibre and the optical fibre grating sensor.
To study the applications of optical fibre grating sensor in medical applications.
Optical fibres are transparent fibres where they are flexible strands roughly diameter of a human hair. Optical fibres are usually made up of glass or plastics and it is used for transmitting light. During several years, people used optical fibres to broadcast light signals and also audio signals. Moreover, optical fibres are very useful in medical procedures, automobiles and aircraft.
Normally, optical fibres are uniform along their lengths. If a slice is taken from any one point on the fibre, it would look very much like a slice taken from any other part of the fibre, ignoring any tiny imperfections. However, it is possible to make the refractive index of the core glass varies periodically along the length of a fibre where it is rising then falling and then rising again. Such fibres are calledÂ Optical Fibre Gratings because of the refractive index variations scatter light passing through the fibre. (Hecht, 2006)
3.0 LITERATURE REVIEW
3.1 What is Optical Fibre?
Optical fibre is the real media that guides the light by confining it within regions that have different optical indices of refraction. The diameter or thickness of the fibres can be very small, typically from 10Âµm to 1 mm. They are very flexible and can be produced in virtually any desired length. The typical structure of the optical fibres they have a path where most of the light travel called the core which is located at the central region of the fibres. There is also a region which surrounding the core that having a lower index of refraction compared to the core called the cladding. Both the core and the cladding are made up of dielectric materials which conducts no electricity. In order to confine the light in the fibre core, the refractive index of the core should be greater than the cladding.
Light will bends or refractsÂ away from an imaginary line perpendicular to the surface which also called normal line when it passes through one medium with index of refraction n1 to another medium with lower index of refraction n2. Since the angle of the light beam through n1 becomes larger with respect to the normal line, the refracted light through n2 will bends further away from that normal line. In certain condition, at one particular angle called the critical angle, the refracted light will not go into n2, but it will travel along the surface between the two media. If the beam through n1 is greater than the critical angle, then the refracted beam will be reflected entirely back into n1.(HowStuffWorks, 15 November 2012) This phenomenon known as total internal reflection as shown in Figure 1.
Figure 1: Total Internal Reflection in an Optical Fibre
3.2 Introduction to Optical Fibre Gratings
With the increasing interests in the studies of all-fibre systems, optical fibre Bragg grating has been applied in many photonic devices. Optical fibre Bragg grating sensors have been used for years in many demanding environment applications as an alternative to traditional electrical and mechanical sensors. Generally, optical fibre Bragg grating sensors offer higher accuracy, longer stability, smaller size, immunity to electromagnetic interference (EMI) and the ability to measure ultra-high speed events.
Optical fibre Bragg grating sensor technology relies on some advanced technology and physics. Laser light travels through a fibre optic cable core in a much defined area. A Bragg grating is introduced onto the fibre core and the many reflections off of this grating creates a stable sensor. Any strain, such as temperature, pressure and vibration, to the fibre at the Bragg grating will cause a shift and a change of the magnitude of the reflections. This change of reflections allows for very accurate measurements to be performed either over a long period of time or in an ultra-fast event. (Timbercon Inc, 10 October 2012)
Optical fibre gratings make use of the photo-refractive index which is been discovered by Hill et al. in 1978 (Ota, 2000). The refractive index of optical fibre is increased by the exposure to the ultraviolet light. There are many different types of optical fibre grating sensor, working on many different principles includes intensity modulation such as microbending, interferometry, polarization effects, refractive index changes, reflectometry and much more.
One of the types which appear to be attractive in many applications is the optical fibre Bragg grating sensor. They are formed by the light guiding core of the fibre and the wavelength encoded, eliminating the problems of amplitude or intensity variations that being the problems to many other types of fibre sensors. Due to their narrow band wavelength reflection they are also can be multiplexed in a fibre optic network.
3.3 Principle of Optical Fibre Gratings
An optical fibre Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fibre that reflects specific wavelengths of light and transmits all the other components. An optical fibre Bragg grating can also be regarded as a fibre device with a periodic variation of the refractive index of the fibre core along the length of the fibre. Generally, in a simple optical fibre Bragg grating, the refractive index of the fibre core varies periodically along the length of the fibre. Figure 2 shows the schematic diagram of optical fibre Bragg grating.
Figure 2: Schematic diagram of optical fibre Bragg grating
3.4 The Reflection and Transmission of Light
An optical fibre Bragg gratingÂ consists of many reflection points that reflect particular wavelengths of incident light and the point is created byÂ intense UV lightÂ affecting the fibre core. This process is also called "writing" where by writing a lot of such reflection points into the fibre at regular intervals create a grating.
The distance between the reflection points of a fibre Bragg grating are always equal. The wavelengthÂ that precisely matches with the distance between two reflection points is reflected by the grating whileÂ all other wavelengthsÂ are transmitted through the grating without being reflected or damped (Optical Sensing, 11 November 2012) as shown in Figure 3.
Figure 3: A light spectrum is transmitted into a fibre containing a fibre Bragg grating
Light Reflection and Transmission in a Fiber Bragg Grating
Source: National Instruments 2006
The refractive index of the fibre core is modulated with a period of . When a light with a broad spectrum is transmitted into one end of fibre that contains a fibre Bragg grating, the part of the light with wavelength matching the Bragg grating wavelength will be reflected back to the input end while the rest of the light passing through to the other end.(Fibre Optic Training & Tutorials, 2 October 2012)
3.5 Properties of Optical Fibre Bragg Gratings
For an optical fibre Bragg grating that consists of a periodic modulation of the refractive index in the core of an optical fibre, the phase fronts are perpendicular to the fibre longitudinal axis and the grating planes are of a constant period. Light that propagates along the fibre core will interact with each grating plane, in which the Bragg condition is used for the discussion of the light propagation,
Where is the spacing between the grating planes, is the angle between the incident light and the scattering planes, is the wavelength of the light and n is an integer.
If the Bragg condition is not satisfied, the light reflected from each of the subsequent planes becomes progressively out of phase and will finally disappear. When the Bragg condition is satisfied, the contributions of reflected light from each grating plane add constructively in the backward direction to form a back-reflected peak with a center wavelength is defined by the grating parameters that is the Bragg wavelength.
The Bragg grating condition is the requirement that satisfies the principles of energy conservation and also the principles momentum conservation so that the center wavelength reflected by a uniform Bragg wavelength can be determined.
Energy conservation: The frequency of the incident radiation is equal to the frequency of the reflected radiation.
Momentum conservation: The incident wave vector, , plus the grating wave vector, K , is equal to the wave vector of the scattered radiation, .
Where K (the grating vector) has a direction normal to the grating planes with a magnitude 2/. The diffracted wave vector is equal in magnitude but opposite in direction to the incident wave vector. (Othonos, 2011)Thus the momentum conservation condition becomes
2 ) =
Which can be simplifies and becomes the first order Bragg condition:
Where the Bragg grating wavelength, , is the center wavelength of the input light in the free space that will be back-reflected from the Bragg grating and is the effective reflective index of the fibre core at the free space center wavelength.
4.0 OPTICAL FIBRE GRATING AS A SENSOR
Typically, optical fibres can be used as sensors to measure strain, temperature, force and other parameters by modifying the fibres so that the property to measure modulates the phase, intensity, wavelength, polarization or transit time of light in the fibre. The small size and the fact that no electric power is required at the remote location give the fibre optic sensor benefits to conventional electric sensor in certain applications.(School of Computer Science, 26 October 2012)
The optical fibre gratings has been used widely in medical field since it can be configured to sense strain, bending, pressure, temperature and refractive index. Below are some examples where the optical fibre grating sensors is being used in medical applications.
A vest that capable forÂ monitoring the recruitment of various muscle groups to the breathing process is made by using fibre Bragg grating bend sensors.
It is also possible to monitor the mechanical vibrations together with cardiac activity by using long period grating sensor.
The multiplexed fibre Bragg grating sensors has been used to monitor temperature profiles.
The MHz frequency strains together with medical ultrasound can be detected by the fibre Bragg grating.
By using the combination of refractive index sensitivity with a smart fibre coating, the label-free detection of specific biochemical species and DNA are allowed.
4.1 General Features of Optical Fibre Grating Sensors
1) Small size(Aston University Engineering and Applied Science)
-The diameter is only 125 microns which it is suitable for minimally invasive surgery.
2) Multiple sensors in a single fibre
- There are possibly hundreds of sensors are able to be integrated into a single fibre in
certain cases so that the sensors can have an operative separation of less than a
3) Made up of glass
- As glass is an insulator, the sensors will immune to the electromagnetic interface
therefore it can be used in a MRI environment.
5.1 Optical Fibre Grating Sensor in Medical Field
In the medical field, the opportunities offered by optical fibres have always been advantageously exploited. In fact, the use of optical fibres in medicine goes back to 1960s, when a bundle of fibres were successfully pioneered in endoscopy, both for illumination and for imaging. Afterwards, cavitational laser surgery and therapy also benefited from fibres, which proved to be the most flexible, and a low-attenuation delivery system inside the ancillary channel of endoscopes, and inside the natural channels of the human body as well. More recently, and especially since 1980, a great deal of research in optical fibres has been dedicated to sensing, and again the medical field found good opportunities for developing very promising sensors.(10:10 Computer Services Ltd., 15 November 2012)
5.1.1 Medical Fibre Optics
Fibre optics has already been used in the medical industry for many years. The physical characteristics of the fibre make it a natural choice for many different applications. Usually it is used for illumination, flexible image bundles, light conductors, flexible light guides, laser delivery systems, and equipment interconnects. Fibre optics provides a very compact, flexible conduit for light or data delivery in equipment, surgical and also for instrumentation applications. The applications of traditional medical fibre optic include light therapy, x-ray imaging, ophthalmic lasers, lab and clinical diagnostics, dental hand pieces, surgical and diagnostic instrumentation, endoscopy, surgical microscopy and a wide range of equipment and instrument illumination.(Timbercon Inc, 8 October 2012)
5.1.2 Fibre Optics for Medical Research
Medical study covers a broad range of applications and areas of study inside the medical field. Often, fibre optic products in this area are conceived to be very application specific as each products requirement is proposed to support and/or test a theory, method, or equipment. While some applications share various product attributes with another product, the vast majority need precise and exclusive characteristics achieved through specialty product design.
5.1.3 Fibre Optics for Medical Instruments
Medical Instruments have used fibre optics for a variety of applications including illumination, image transfer, and laser signal delivery. A large portion of the fibre used in these applications assist site illumination either as an integrated component of an instrument or as an individual light source.
Laryngoscope (blade illumination)
Anoscope (with annular illumination)
Binocular indirect ophthalmoscope
6. 0 CONCLUSIONS
A brief introduction on the optical fibres is discussed. Since many years the optical fibres are fabricated to transmit light or audio signal from one point to another by using the principle of total internal reflection. An overview of optical fibre grating, its role as a sensor and their applications in medical field also has been presented. The optical fibre grating works when the refractive index of the fibre core varies periodically along the fibres. It also can be used as a sensor where it can detects strain, pressure, temperature and other perimeters. Lastly, it can be concluded that the optical fibre grating sensor had its role in medical field since it really gives benefit in many applications.
The author is highly indebted to the supervisor, Assoc. Prof. Dr. Awangku Abdul Rahman bin Awangku Yussuf for his many helpful discussions and constructive suggestions. Thanks are also due to Dr. Khairi Abdul Rahim for his guidance, helpful and detail explanations and continuous assistance. Sincere thanks apply to the group members, Mohd Hasif Juhan, Afiqah Musa, Susilawati Yaso and all friends for the continuous encouraged cooperation, discussions, and suggestions. Last but not least, special thanks also to Mr. Zahri Zakaria and Mrs. Norma Saleh for their constant advice, support and understanding.