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
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.

Coherent and Incoherent Bundles

An ideal fibre transmits light independently of its neighbours, so 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. Such an arrangement is called a coherent bundle, and consists of
fibres of