Comment and discuss the working principles of an endoscope, uteroscopeor a rectoscope showing the illuminating path, the image path, transmission pathand the liquid transfer or operating instrument ducts, showing the position ofsuitable valves. This will therefore explain how light travels through an optical fibreand show how such fibres are used in medicinal equipment either to transmitlight or to bring back images from within a patient. ContentsFibre OpticsFibre-Optic BundlesCoherent and Incoherent BundlesTransimission efficiency and resolutionTypes of Fibres: Single mode or Multimode ?Fibre PropertiesFibre-Optic EndoscopyIntroductionThe Fibre-Optic EndoscopeSome Applications for Fibre-Optic EndoscopyReferencesFibre OpticsA relatively new technology with vast potential importance, fibre optics, is thechannelled transmission of light through hair-thin glass fibres. The clear advantages of fibre optics are too often obscured by concerns thatmay have been valid during the pioneering days of fibre, but that have sincebeen answered by technical advances.
Fibre is fragileAn optical fibre has greater tensile strength than copper or steel fibres of thesame diameter. It is flexible, bends easily, and resists most corrosiveelements that attack copper cable. Optical cables can withstand pulling forcesof more than 150 pounds. Fibre is hard to work withThis myth derives from the early days of fibre optic connectors. Earlyconnectors where difficult to apply; they came with many small parts that couldtax even the nimble fingered.
They needed epoxy, curing, cleaving and polishing. On top of that, the technologies of epoxy, curing, cleaving and polishing werestill evolving. Today, connectors have fewer parts, the procedures for termination are wellunderstood, and the craftsperson is aided by polishing machines and curing ovensto 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 speedinstallation and reduce a once (but no longer) labour-intensive process.
Fibre Optic BundlesIf light enters the end of a solid glass rod so that the light transmitted intothe 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 internallyreflected back and forth in its passage along the rod, and it emerges from theother end with very little loss of intensity. This is the principle in fibre optics of which long glass fibres of very smallcross-sectional area transmit light from end to end, even when bent, withoutmuch loss of light through their side walls. Such fibres can then be combinedinto ‘bundles’ of dozens to thousands of fibres for the efficient conveyance oflight from one (often inaccessible) point to another. If the glass fibre comes into contact with a substance of equal or higherrefractive index, such as an adjacent glass fibre, dirt or grease, then totalinternal reflection does not occur and light is lost rapidly by transmissionthrough 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 fibrecore. As the angle of incidence I increases, Rincreases and O ( = (n/2) -R)decreases. Eventually, O reaches C, the critical angle, and any furtherreduction in O results in transmission through the side wall. The expression n0 sin Imax is called the numerical aperture of the fibre. Atypical value for this might be 0. 55, making Imax about 33o in air.
SometimesImax is referred to as the half-angle of the fibre, since it describes halfthe field of view acceptably transmitted. The numerical aperture (and henceImax) can be increased by using a core of high refractive index. However, theseglasses have a lower efficiency of transmission, especially at the blue end ofthe 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 lossesoccur if the angles fall below the critical angle.
In practice, a radius ofcurvature down to about twenty times the fibre diameter can be tolerated withoutsignificant losses. Coherent and Incoherent BundlesAn ideal fibre transmits light independently of its neighbours, so if a bundleof fibres is placed together in an orderly manner along its length, with therelative positions remaining unchanged, actual images may be transmitted alongthe fibre. Such an arrangement