Fiber Optics Assignment: Many modern medical materials and equipment work on a principle that is beyond the capacity of human transducers.
Comment and discuss the working principles of an endoscope, uteroscope, or rectoscope. Show the illuminating path, image path, transmission path, and liquid transfer or operating instrument ducts, as well as the position of suitable valves. This will explain how light travels through an optical fibre and how such fibres are used in medicinal equipment to transmit light or bring back images from within a patient.
Contents
- Fibre Optics
- Fibre-Optic Bundles
- Coherent and Incoherent Bundles
- Transmission Efficiency and Resolution
- Types of Fibres: Single mode or Multimode?
- Fibre Properties
- Fibre-Optic Endoscopy
- Introduction
- The Fibre-Optic Endoscope
- Some Applications for Fibre-Optic Endoscopy
References
A relatively new technology with vast potential importance, fibre optics is the channelled transmission of light through hair-thin glass fibres. The clear advantages of fibre optics are too often obscured by concerns that may have been valid during the pioneering days of fibre but have since been answered by technical advances.
Fibre is fragile. An optical fibre has greater tensile strength than copper or steel fibres of the same diameter. It is flexible, bends easily, and resists most corrosive elements that attack copper cable. Optical cables can withstand pulling forces of more than 150 pounds. However, working with fibre can be challenging. This myth derives from the early days of fibre optic connectors. Early connectors were difficult to apply and came with many small parts that could tax even the nimble-fingered.
They needed epoxy, curing, cleaving, and polishing. The technologies of epoxy, curing, cleaving, and polishing were still evolving. Today, connectors have fewer parts, and the procedures for termination are well understood. The craftsperson is aided by polishing machines and curing ovens to make the job faster and easier. Even better, epoxyless connectors eliminate the need for the messy and time-consuming application of epoxy. Polishing is an increasingly simple and straightforward process. Pre-terminated cable assemblies also speed installation and reduce a once labor-intensive process.
Fibre Optic Bundles
If light enters the end of a solid glass rod and is transmitted into the rod at an angle exceeding the critical angle, total internal reflection occurs. The light continues to be internally reflected back and forth in its passage along the rod, and it emerges from the other end with very little loss of intensity. This is the principle of fibre optics, where long glass fibres of very small cross-sectional area transmit light from end to end, even when bent, without much loss of light through their side walls. Such fibres can be combined into bundles of dozens to thousands of fibres for the efficient conveyance of light from one point to another, often in inaccessible locations. If the glass fibre comes into contact with a substance of equal or higher refractive index, such as an adjacent glass fibre, dirt or grease, then total internal reflection does not occur and light is lost rapidly by transmission through the area of contact.
To avoid leakage” and protect the fibers, they are clad in “glass skins” with a refractive index lower than that of the fiber core. As the angle of incidence (I) increases, the reflection coefficient (R) increases and the angle of refraction (O = (n/2) – R) decreases. Eventually, O reaches the critical angle (C), and any further reduction in O results in transmission through the side wall. The expression n0 sin Imax is called the numerical aperture of the fiber. A typical value for this might be 0.55, making Imax about 33 degrees in air.
Sometimes Imax is referred to as the half-angle of the fibre, since it describes half the field of view acceptably transmitted. The numerical aperture (and hence Imax) can be increased by using a core of high refractive index. However, these glasses have a lower efficiency of transmission, especially at the blue end of the spectrum, and are not commonly used. The above analysis applies only to a straight line fibre. If the fibre is curved, the angles of incidence vary as the light travels along the fibre and losses occur if the angles fall below the critical angle.
In practice, a radius of curvature down to about twenty times the fibre diameter can be tolerated without significant losses. An ideal fibre transmits light independently of its neighbours. If a bundle of fibres is placed together in an orderly manner along its length, with the relative positions remaining unchanged, actual images may be transmitted along the fibre.
