Fiber Optics

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Fiber Optics

Assignment

Many modern medical materials and equipment work on a principle which is beyond the capacity of human transducers.

Comment and discuss the working principles of an endoscope, uteroscope or a rectoscope showing the illuminating path, the image path, transmission path and the liquid transfer or operating instrument ducts, showing the position of suitable valves.

This will therefore explain how light travels through an optical fibre and show how such fibres are used in medicinal equipment either to transmit light or to bring back images from within a patient.

Contents

Fibre Optics

Fibre-Optic Bundles

Coherent and Incoherent Bundles

Transimission 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

Fibre Optics

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 that 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.

Fibre is hard to work with

This myth derives from the early days of fibre optic connectors. Early connectors where difficult to apply; they came with many small parts that could tax even the nimble fingered. They needed epoxy, curing, cleaving and polishing. On top of that, the technologies of epoxy, curing, cleaving and polishing were still evolving.

Today, connectors have fewer parts, the procedures for termination are well understood, and 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, straightforward process. Pre-terminated cable assemblies also speed installation and reduce a once (but no longer) labour-intensive process.

Fibre Optic Bundles

If light enters the end of a solid glass rod so that the light transmitted into the rod strikes the side of the rod at an angle O, exceeding the critical angle, then 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 in fibre optics of which 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 then be combined into 'bundles' of dozens to thousands of fibres for the efficient conveyance of light from one (often inaccessible) point to another.

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 such 'leakage' and to protect the fibres, they are clad in 'glass skins' of refractive index lower than that of the fibre core.

As the angle of incidence I increases, R increases and O ( = (n/2) -R) decreases. Eventually, O reaches C, the critical angle, 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 fibre. A typical value for this might be 0.55, making Imax about 33o in air. Sometimes...
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