History lesson: The birth of fibre optics

The actual start/discovery of fibre optics is like most things in history, a little hazy. It is better to first look at the first use/discovery of total internal reflectance, the property on which fibre optics were first based the history of which is also a little hazy. You just can’t win with poorly documented historical facts.

Total internal reflection is where the angle of the light reflected within a medium is such that all of the light is reflected back into the medium. Only light of a particular angle will be reflected back, any other light will be refracted out of the medium. This can be used to make light travel round corders via a suitable guide such as glass or water (more on that later).

Good bouncy light vs bad bouncy light

Diagram showing how light can be totally internally reflected in a glass fibre

The earliest reference to the use of total internal reflectance to ‘pipe’ light was in a book I have now lost which said that tubes of glass had been discovered in Egyptian tombs that appeared to direct light from the exterior into the chambers. However, after days of research I could find no evidence of these glass tubes, I couldn’t even find any evidence of them having used skylights! So I pretty sure that ‘fact’ was at best wishful romantic thinking. The ancient Egyptians contributed much to science (for example glass) but not total internal reflectance.

So getting back to documented knowledge.

Daniel Colladon and Jacques Babinet, (working independently) are generally thought of as being the first to discover the use of total internal reflection to guide light. Both scientists produced demonstrations of light being ‘carried’ through a stream of water pouring out of a hole into a bucket in around 1841.

While Colladon had originally produced the flowing water experiment around the same time as Babinet, Babinet had taken the experiment further and shown that the same effect was possible in bent glass rods and quartz fibres. Initially this ‘trick’ was considered an unarguably attractive but essentially irrelevant property of light. Indeed Colladon spent most of his time shortly after discovering the effect of light on water helping to design special effects for the theatre and producing magnificently lit fountains. A more modern version of this effect can be seen in the work of the artists Jeppe Hein who make huge installation fountains magnificently lit through a combination of lighting tricks.

Over the next 60 years, development in the uses of total internal reflectance was slow and focused mainly on the use of fibres to guide light to specific locations. For example, in 1898 David Smith pioneered the use of a bent glass rod for use in dental operations. The limiting factor in the further development of light guiding was not due to a lack of creativity or scientific understanding of total internal reflectance; the problem lay with the poor quality of glass available at the time. Many of the early fibre technologies developed in this period were limited by the large loss of light over relatively short distances of glass or quartz rods.

Fibre optic light guides had been produced by Charles Vernon Boys in 1887 and involved firing a bow with an arrow trailing molten quartz across his lab. As the arrow pulled the fibre out across the lab, it pulled out very thin fibres that could be easily bent. I did want to try and repeat this in my lab so I could make a video of it, but I don’t think anyone would be very impressed if I put a hole in the side of my environmentally controlled lab, or for that matter covered it in molten quartz. So instead enjoy this cartoon of me doing it….

No fictional lab partners were harmed in the making of this cartoon

See, this cartoon shows it would have been absolutely fine!

Obviously making fibres via this arrow technique was not reliable enough to do much with. Like the light guiding from buckets experiment it was at best a nice trick. It then wasn’t until almost 50 years later in 1938  when, Owens‑Illinois demonstrated a repeatable way of mass‑producing fibres by pulling continuous strands of glass. At this stage of fibre optic production however the fibres were still made from just a single glass core with no coatings or protection. This was important because initially it greatly reduced their use.

Going back to the TIR explanation earlier, I said that TIR could only be sustained if the angle of the light was right. The angle at which TIR occurs is determined by the refractive index of the materials involved. The impact of the refractive index makes bare fibres very vulnerable to leaking light when used. If for example you handle the newly made fibre you will coat the outside in grease from your skin, this would change the refractive index causing the TIR light to be lost to the coating.

Not everything is better Naked

TIR in a fibre optic with and without cladding

Any similarity to our logo is just an optical illusion created by me copying and pasting the image

A simple schematic of a modern fibre optic

This problem was eventually solved in the 1950s by Brian O’Brian, the then president of the Optical Society of America, who proposed that the best way to prevent this loss was to the coat the fibres in a uniform low index cladding material. This use of low index cladding material ensures that the high index core of the fibre, where the light is confined by TIR, can remain insulated from any contact with other surfaces.

Things have changed a bit for various applications but this basic structure of a fibre is now equivalent to most modern uses (e.g. broadband). In a future post I’ll explain a little more of the history of fibre optics post 1950 and go into why a CIA project was critical to their early development.

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