Optical techniques developed for sensing purposes proved to be essential in many application ï¬elds, ranging from aerospace, industry, process control, to security, and also medicine. The capabilities of these sensors are generally enhanced when a bulk-optical conï¬guration is replaced by optical ï¬bre technology. In the past few years, research programmes and also the market for ï¬bre sensors have assumed a relevant role. This is undoubtedly due to the growing interest in optoelectronics, but also to the very satisfactory performance and reliability that optical ï¬bre sensors are now able to provide. This paper focuses on the advantages that optical ï¬bre sensors offer to the biomedical ï¬eld, recalls the basic working principles of sensing, and discusses some examples.
Optical fiber Bragg grating sensors can be used to sense strain, bending, pressure, temperature and refractive index in numerous applications including for medical applications.
INTRODUCTION
Normally, an optical fibers are uniform along their lengths. If a slice is taken from any one point on the fiber, it would look very much like a slice taken from any other part of the fiber, ignoring any tiny imperfections. However, it is possible to make the refractive index of the core glass varies periodically along the length of a fiber where it is rising then falling and then rising again. Such fibers are called Optical Fiber Gratings because of the refractive index variations scatter light passing through the fiber.
Optical fiber gratings make use of the photo-refractive index which is been discovered by Hill et al. in 1978. The refractive index of optical fiber 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 fiber 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.
Optical fiber Bragg grating sensors have been used for years in many demanding environment applications as an alternative to traditional electrical and mechanical sensors. Generally, optical fiber 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 fiber Bragg grating sensor technology relies on some advanced technology and physics. Laser light travels through a fiber optic cable core in a very defined area. A Bragg grating is introduced onto the fiber core and the many reflections off of this grating creates a stable sensor. Any strain, such as temperature, pressure and vibration, to the fiber 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.
PRINCIPLE OF THE OPTICAL FIBER GRATING
The optical fiber grating is a device that creates a periodic disturbance in the refractive index of the fiber core, reflecting only the specific wavelength corresponding to that period. This periodic disturbance in the refractive index is brought about by forming an interference fringe on the fiber by means of lateral irradiation with UV light, and by writing that pattern in the fiber by the photo-refractive effect. Generally, in a simple optical fiber Bragg grating, the refractive index of the fiber core varies periodically along the length of the fiber.
C:\Users\Aspire\Downloads\cutmypic.png
Schematic diagram of optical fiber Bragg grating
The Reflection and Transmission of Light
A fiber Bragg grating consists of many reflection points that reflect particular wavelengths of incident light and the point is created by intense UV light affecting the fiber core. This process is also called "writing" where by writing a lot of such reflection points into the fiber at regular intervals create a grating.
There is a distance between the reflection points of a fiber Bragg grating that is always equal. The wavelength that precisely matches with the distance between two reflection points is reflected by the grating while all other wavelengths are transmitted through the grating without being reflected or damped.
Light Reflection and Transmission in a Fiber Bragg Grating
A light spectrum is transmitted into a fiber containing a fiber Bragg grating
The refractive index of the fiber core is modulated with a period of . When a light with a broad spectrum is transmitted into one end of fiber that contains a fiber 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.
Properties of Optical Fiber Bragg Gratings
For a optical fiber Bragg grating that consists of a periodic modulation of the refractive index in the core of an optical fiber, the phase fronts are perpendicular to the fiber longitudinal axis and the grating planes are of a constant period. Light that propagates along the fiber 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. 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 fiber core at the free space center wavelength.
OPTICAL FIBER GRATING SENSOR IN MEDICAL FIELD
1) Medical Fiber Optics
Fiber optics have been used in the medical industry for years. The physical characteristics of fiber make it a natural choice for many different applications. Commonly used for illumination, flexible image bundles, light conductors, flexible light guides, laser delivery systems, and equipment interconnects, fiber optics provide a very compact, flexible conduit for light or data delivery in equipment, surgical, and instrumentation applications.
Traditional medical fiber optic applications 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 manufactures multiple glass and plastic optical fiber products for use in a myriad of medical applications.
2) Fiber Optics for Medical Research
Medical research covers a wide range of applications and areas of study within the medical field. Often, fiber optic products in this area are designed to be very application specific as each products requirement is intended to support and/or test a theory, procedure, or instrument. While some applications share various product attributes with another product, the vast majority require precise and unique characteristics achieved through specialty product design.
Timbercon specializes in application specific and custom product design, development, and manufacturing. Our medical fiber optic technologies have been designed to allow a high degree of customization and specialty product development. This balance of technology and customization capability enables Timbercon to create the exact product with the precise tolerance that your application requires.
2) Fiber Optics for Medical Instruments
Medical Instruments utilize fiber optics for a variety of applications including illumination, image transfer, and laser signal delivery.
A large portion of the fiber used in these applications support site illumination either as an integrated component of an instrument or as an individual light source.
Examination lights
FO headlight
Vet otoscope
Laryngoscope (blade illumination)
Anoscope (with annular illumination)
Otoscope
Binocular indirect ophthalmoscope
Amnioscope
Microscope illumination
Heart catheter
Fiber Bragg Grating Sensor Technology
http://www.timbercon.com/fiber-bragg-grating-sensor-technology/
Medical Fiber Optics
http://www.timbercon.com/medical-fiber-optics/
Application of Optical Fibre Sensors to Measuring the Mechanical Properties of Composite Materials and Structures http://cdn.intechopen.com/pdfs/14299/InTech-Application_of_optical_fibre_sensors_to_measuring_the_mechanical_properties_of_composite_materials_and_structures.pdf
Fibre Bragg Grating Sensors:An Introduction to Bragg gratings and interrogation techniqueshttp://www.smartfibres.com/Attachments/Smart%20Fibres%20Technology%20Introduction.pdf
Andreas Othonos, Kyriacos Kalli, David Pureur and Alain Mugnier-Fibre Bragg Gratings http://123seminarsonly.com/Seminar-Reports/025/49180978-Fiber-bragg-gratings-Andreas-Othaonos.pdf
What is Fiber Bragg Grating - FBG
http://www.fiberoptics4sale.com/wordpress/what-is-fiber-bragg-grating/
Fiber Bragg Gratings And Their Applications As Temperature And Humidity Sensors http://www.physics.mun.ca/~pinglu/Dr.Chen/publication/BookChapter.pdf
