Essay/Term paper: Fiber optics

Essay, term paper, research paper:  Science Reports

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 very small diameter about 10 µm. The ends of the bundle are cut
square and polished smooth to prevent distortions. Each fibre transmits a small
element of the image which is seen at the other end of the coherent bundle as a
mosaic. The eye has to 'look through' the fragmented structure to appreciate a
clear image. The image to be transmitted is either in direct contact with the
end of the bundle or focused on to this surface. The image formed at the other
end is viewed using an eyepiece incorporating magnification. One novel method
of magnification is to make one end of the fibres smaller than the other. For
example, if they have an average diameter of 5µm at the image end, and 50µm at
the viewing end, a magnification of x10 is achieved.

In contrast, a bundle of fibres arranged at random is known as an incoherent
bundle, (or sometimes simply a light guide) and is suitable only for the
transport of light not of images. The fibres of such a bundle are relatively
large having diameters of about 50-100µm.

The fibre, must be cabled - enclosed within a protective structure. This usually
includes strength members and an outer buffer. The most common strength member
is Kevlar aramid yarn, which adds mechanical strength. During and after
installation, strength members provide crush resistance and handle the tensile
stresses applied to the cable so that the fibre is not damaged. Steel and
fibreglass rods are also used as strength members in multifibre bundles.

The concentric layers of an optical fibre include the light-carrying core, the
cladding and the protective buffer.

Core : the inner light-carrying member. Cladding : the middle layer, which
serves to confine the light to the core. Buffer : the outer layer which serves
as a 'shock absorber' to protect the core and cladding from damage.

The buffer protects against abrasion, oil, solvents and other contaminates.

The buffer usually defines the cable's duty and flammability rating.

Transmission efficiency and resolution
Light injected into a fibre can adopt any of several zigzag paths, or
modes. When a large number of modes are present they may overlap, for each mode
has a different velocity along the fibre (modal dispersion). The glass fibres
used in present-day fibre-optic systems are based on ultrapure fused silica.
Fibre made from ordinary glass is so dirty that impurities reduce signal
intensity by a factor of on million in only about 5 m (16 ft) of fibre. These
impurities must be removed -- often to the parts-per-billion level – before
useful long-haul fibres can be drawn. But even perfectly pure glass is not
perfectly transparent.

It attenuates, or weakens, light in two ways. One, occurring at shorter
wavelengths, is a scattering caused by unavoidable density fluctuations within
the fibre. The other is a longer wavelength absorption by atomic vibrations
(photons).

For silica, the minimum attenuation, or the maximum transparency, occurs in
wavelengths in the near infrared, at about 1.5 micrometers.

In addition, there are 'end losses' which are light losses at the end faces due
to partial reflection and incidence on the cladding material. Thus, light
sources need to be very powerful, and even then problems can arise when viewing
coloured images since different wavelengths have different transmission
efficiencies.

The thinner and more numerous the fibres, the greater should be the resolution.
However, when the core diameter falls below about 5µm diffraction starts to
occur and transmission efficiency drops. Hence, although fibres with core
diameters down to about 1µm have been used, typical diameters are nearer 10µm. A
deterioration in image quality may occur for a number of reasons, for example
defects in the end faces of the fibres, misalignment of fibres, broken fibres
(causing black spots), or light leakage between adjacent fibres (producing
'cross-talk').

Types of fibres : Singlemode or Multimode ?

In the simplest optical fibre, the relatively large core has uniform optical
properties. Termed a step-index multimode fibre, this fibre supports thousands
of modes and offers the highest dispersion - and hence the lowest bandwidth.

By varying the optical properties of the core, the graded-index multimode fibre
reduces dispersion and increases bandwidth. Grading makes light following longer
paths travel slightly faster than light following a shorter path. Put another
way, light travelling straight down the core without reflecting travels slowest.

The net result is that the light does not spread out nearly as much. Nearly all
multimode fibres used in medical application have a graded index.

Fibre Properties

Numerical aperture (NA) of the fibre defines which light will be propagated and
will not. NA defines the light-gathering ability of the fibre. Imagine a cone
coming from the core. Light entering the core from within this cone will be
propagated by total internal reflection. Light entering from outside the cone
will not be propagated.

NA has an important consequence. A large NA makes it easier to inject more light
into a fibre, while a small NA tends to give the fibre a higher bandwidth. A
large NA allows greater modal dispersion by allowing more modes in which light
can travel. A smaller NA reduces dispersion by limiting the number of modes.

Fibre-optic endoscopy

Introduction

An endoscope is an instrument designed to provide a direct view of an internal
part of the body, and possibly to perform tasks such as the removal of samples,
injection of fluids and diathermy. Fibre optics has extended the scope of the
instrument considerably by permitting the transmission of images from
inaccessible areas such as the oesophagus, stomach, intestines, heart and lungs.

The fibre-optic endoscope

The long flexible shaft of the instrument is usually constructed of steel mesh,
often with a crush-resistant covering of a bronze or steel spiral, it is then
sheathed with a protective, low-friction covering of PVC or some other
impervious material, which forms a hermetic seal around the instrument. The
shaft is about 10mm in diameter; about 0.6-1.8 m long (depending on the
application) and has a short deflectable section about 50-85mm long leading to
its distal tip.

Within the shaft lie:

(a) at least one non-coherent fibre-optic bundle to transmit light from the
distant light source to the distal tip;

(b) a coherent fibre-optic bundle transmitting the image from the objective lens
at the distal tip;

(c) an irrigation channel through which water can be pumped to wash the
objective lens;

(d) an operations channel for the Performance of tasks;

(e) control cables.

The viewing end of the endoscope contains:

(a) an eyepiece, with focus controls and camera attachment;

(b) distal tip deflection controls, giving polydirectional control up to about
200o, plus locking capability;

(c) objective lens control which may be a push-pull wire effecting focusing;

(d) valve controls for air aspiration, (suctioned withdrawal of body fluids
through the operations channel) and lens washing and air insufflation
(application of water or air jet through the irrigation channel);

(d) operating channel valve, which controls the entry of catheters, electrodes,
biopsy forceps and other flexible devices;

(e) connection with the umbilical tube, providing light through a non-coherent
fibre-optic bundle and water or air from the pump or aspirator system.
A typical Micro-video Endoscopy Unit would contain: (a)Optical catheter
system as described above,

(b)Colour video monitor

(c)CO2 and fluid insufflation,

(d)Instrumentation,

(e)Disposables,

(f)Miscellaneous accessories.

Some applications of Fibre-optic endoscopy

Endoscopic examination of the gastrointestinal tract has proved especially
successful with the diagnosis and treatment of ulcers, cancers, constrictions,
bleeding sites, and so on. The heart, respiratory system and pancreas have also
been investigated.

Another application is the measurment of the proportion of haemoglobin in the
blood which is combined with oxygen using an oximeter. Two incoherent bundles
are introduced into the blood stream: one is used to illuminate a sample of
blood and the other to assess the absorption of light by the blood.

An endoscope can also be equipped with a laser that can vaporize, coagulate, or
cut structures, often with more ease and flexibility than a more rigid cutting
knife. It is a less invasive method that causes less scarring and a quicker
recovery time than other surgical techniques.

Common types of endoscopes are the cytoscope to view the bladder, the
bronchoscope to view the lungs, the otoscope to view the ear, the arthroscope to
view the knee and other joints, and the laparascope to view the female
reproductive structures. The most common surgery performed through endoscopy is
biopsy, the removal of tissue for microscopic study to detect a malignancy.
Diagnostic hysteroscopy with directed biopsy and dilatation and curettage,
removal of polyps, and removal of foreign objects and Cystourethroscopy are
other important fields which endoscopy makes possible.

References:

Pope, Jean A.; Medical Physics 2nd r.e. Heinemann Educational Printers 1973.

Brown, B. H. , Smallwood R. H.; Medical Physics and Physiological Measurement;

Blackwell Scientific Publications, Billing and Son's Publishers; 1981.