Optical fiber grating sensors hold massive potential for biomedical applications due to their intrinsic 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 .Optical fiber grating sensors can be used to sense strain, bending, pressure, temperature and refractive index in numerous applications including for medical applications. This paper focuses on the basic working principle of the optical fiber grating, the way it works as a sensor and also the benefits that the optical fiber grating sensor offers to the medical field.
Optical fibers are transparent fibers where they are flexible strands roughly diameter of a human hair. Optical fibers are usually made up of glass or plastics and it is used for transmitting light. During several years, people used optical fibers to broadcast light signals and also audio signals. Moreover, optical fibers are very useful in medical procedures, automobiles and aircraft.
Normally, 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 calledÂ Optical Fiber Gratings because of the refractive index variations scatter light passing through the fiber.
With the increasing interests in the studies of all-fibre systems, optical fibre Bragg grating have been applied in many photonic devices. 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 much 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.
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 fiber 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 fiber and the wavelength encoded, eliminating the problems of amplitude or intensity variations that being the problems to many other types of fiber sensors. Due to their narrow band wavelength reflection they are also can be multiplexed in a fiber optic network.
Principle of the Optical Fiber Grating
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 fiber Bragg grating, the refractive index of the fiber core varies periodically along the length of the fiber.
Schematic diagram of optical fiber Bragg grating
The Reflection and Transmission of Light
An optical fiber 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 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 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.
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 an 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 AS A SENSOR
Typically, optical fibres can be used as sensors to measure strain, temperature, force and other parameters. 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.
OPTICAL FIBER 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 the sixties, when fibre bundles were successfully pioneered in endoscopy, both for illumination and for imaging. Subsequently, 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.
1) Medical Fiber Optics
Fiber optics has already been used in the medical industry for many years. The physical characteristics of the fiber 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. Fiber 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 fiber 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.
2) Fiber Optics for Medical Research
Medical study covers a broad range of applications and areas of study inside the medical field. Often, fiber 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.
2) Fiber Optics for Medical Instruments
Medical Instruments have used 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 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