Fibre Optics & Endoscopy (AQA A Level Physics)

What does 'Fibre Optics' Mean?

• Fibre optics refers to the use of light travelling along a flexible fibre to produce an image
  • This is particularly useful in medicine, as it allows medics to view internal structures with the flexible fibre, without the need for surgery

What is an Endoscope?

• The piece of equipment used to do this is called an endoscope
  • Endoscopes contain bundles of optical fibres along which light is transmitted to an eyepiece

• An optical fibre is a flexible fibre, or core, along which light is transmitted
• The core is surrounded by cladding
  • This protects the core - light escapes if it is unclean or if it makes contact with neighbouring fibres
  • It also has a slightly lower refractive index than the core, allowing light in the core to be totally internally reflected

Cross-section of an optical fibre

Light passing through the core is internally reflected with a large critical angle, θ_c

• The core's refractive index is only just less than the cladding's refractive index
  • This makes the critical angle large
• Therefore, the only light transmitted by the fibre undergoes a small number of reflections and there is a very low amount of loss of information (some energy is absorbed each reflection)
  • Light that would undergo a large number of reflections, and losing information as a result, just passes through the cladding instead

• Optical fibres are utilised in medicine in order to see within the human body
  • The piece of equipment using these optical fibres is called an endoscope

• In an endoscope, fibre optics can be bundled together as
  • Coherent bundles
  • Incoherent bundles

Coherent Bundles

• A coherent bundle of optical fibres is:

A bundle of optical fibres with fixed positions relative to each other at each end and along its length

• This type of bundle is used for transmitting an image to the viewer of the endoscope
• The optical fibres are grouped together in a regular pattern
  • Each fibre is in a fixed position relative to its neighbours
• Each fibre receives and transmits a portion of the image to the endoscope's eyepiece
  • Collectively, the fibres make the whole image
• The fibres in a coherent bundle have a diameter of ~10 μm
  • This is small, for a high resolution
  • Any smaller than this, however, and diffraction affects the image quality

Incoherent Bundles

• An incoherent bundle of optical fibres is:

A bundle of optical fibres grouped together in a random arrangement

• This type of bundle is used to transmit light from a source to the endoscope's target
• The optical fibres do not keep their position relative to their neighbours in the bundle
  • No image needs to be transmitted, this bundle only transmits light to illuminate the target tissue
  • The light reflected by the tissue is transmitted by the coherent bundle to produce an image
• Optical fibres in this bundle have a diameter range of 50 - 100 μm
  • These bundles are cheaper to produce than coherent bundles

Cross-Sections of Coherent and Incoherent Bundles

Fibres in a coherent bundle are regularly spaced. The diameters and spacing of fibres in an incoherent bundle are less crucial, as their role is illumination rather than transmitting a clear image.

Features of an Endoscope

• An endoscope features a long flexible shaft connected to an eyepiece
• Within this shaft is contained:
  • An aperture (hole along the length of the endoscope) through which to operate medical instruments
  • A channel for air or water
  • An incoherent bundle for illumination
  • A coherent bundle for transmitting an image

Uses of an Endoscope

• Endoscopes can be used simply for viewing an internal system, e.g. looking for a tumour in the digestive system
• They can also be used to perform small medical procedures, using tools through the aperture e.g. taking tissue samples for further study
• Some examples of procedures using an endoscope:
  • Gastroscopy (examining the upper digestive system, see the diagram below)
  • Colonoscopy (examining the lower digestive system)
  • Arthroscopy (examining joints for issues such as arthritis, through a small incision)

Basic Structure and Use of an Endoscope

The structure of an endoscope allows internal organs to be viewed or sampled. The lower image shows a gastroscopy, in which the upper digestive system can be viewed.

• Endoscopy is commonly used instead of surgery because:
  • Endoscopy is less painful than surgery
  • There is a lower infection risk than surgery
  • The recovery time is faster than that of surgery
• Of course, endoscopy is limited to small procedures and certain systems - sometimes surgery, magnetic resonance scans or other forms of imaging are more appropriate