Artists often come to resent their greatest hits, and while inventor Alexander Graham Bell didn’t hate his most famous creation, the telephone, it was far from his only priority and passion. An inveterate tinkerer, throughout his long life Bell pursued hundreds of projects across dozens of fields, inventing early versions of the metal detector and iron lung, improving Thomas Edison’s phonograph, building hydrofoil boats and giant tetrahedral kites, serving as President of the National Geographic Society, and building the Silver Dart – the first aircraft to fly in the British Empire. But the invention Bell considered his most important was so advanced that it would take nearly 100 years for technology to catch up with the concept. This is the story of the photophone.

In 1878, two years after patenting the telephone, Bell was honeymooning in Europe with his wife Mabel Hubbard when he read an article by scientist Robert Sabine on how the electrical resistance of the element selenium changes with exposure to light. Bell immediately realized that this effect could be used to transmit the human voice via beams of light. On his return to his home in Washington, D.C, Bell and his assistant Charles Sumner Tainter as his assistant immediately began work on such a device, which he dubbed the ‘photophone,’

The photophone transmitter consisted of a mirror and lens which focused a beam of sunlight through a fine grating attached to a thin metal diaphragm. The speaker’s voice caused the diaphragm to vibrate and the grating to distort, modulating the amount of light transmitted through it. This was later changed to a thin diaphragm of silvered glass or mica, off which the light beam was reflected. The receiver was also originally non-electronic, relying on the photoacoustic effect to translate the light beam back into sound. In this design, a thin diaphragm was coated with a layer of carbon black; light falling on the carbon caused it to heat up, distorting the diaphragm and producing sound. While this design worked, it was not very sensitive, forcing Bell and Tainter to switch to a powered Selenium photocell. In this new design, a parabolic mirror like a satellite dish gathered and focused the light beam from the transmitter onto a piece of selenium, which was connected to a battery and telephone headset. The light falling on the selenium changed its resistance, creating a modulated current that the headset converted into sound.

On February 19, 1880, Bell and Tainter conducted the first successful indoor test of the photophone, with Bell clearly hearing Tainter sing Auld Lang Syne across the laboratory. On April 1, they communicated over 79 meters down an alleyway behind the building, while on June 3 they transmitted a message from the roof of the Franklin School to the window of the laboratory some 213 metres away. This marked the first successful demonstration of wireless communication, predating the development of radio by over 20 years. Bell was ecstatic, writing in a letter to his father:

“I have heard articulate speech by sunlight! I have heard a ray of the sun laugh and cough and sing! …I have been able to hear a shadow and I have even perceived by ear the passage of a cloud across the sun’s disk. You are the grandfather of the Photophone and I want to share my delight at my success.”

Such was Bell’s exuberance that he even suggested naming his second daughter “Photophone”. Thankfully cooler heads prevailed, and Mabel convinced him to go for the more sensible “Marian.”

Nature magazine had high hopes for the potential of the photophone, writing in an article on September 23, 1880:

“…the distance to which sounds have been actually transmitted by the Photophone is less than a quarter of a mile, but there is no reason to doubt that the method can be applied to much greater distances, and that sounds can be transmitted from one station to another wherever a beam of light can be flashed; hence we may expect the slow spelling out of words in the flashing signals of the heliograph to be superseded by the more expeditious whispers of the Photophone.”

Unfortunately, however, Bell’s creation was far ahead of its time, and the technology simply did not exist to make the photophone into a feasible means of communication. With electric lighting still in its infancy, the photophone was dependent on sunlight and would have been useless on cloudy days or at night. Furthermore, its light beam dissipated quickly in the air, limiting its range to a few hundred meters, and could only travel in straight lines. Consequently, Bell eventually sold the patent to the National Bell Telephone Company, which continued to tinker with the idea over the next few decades. Infrared versions of the photophone were used from the 1930s to the 1950s by various navies for ship-to-ship communication over ranges of up to 14 kilometres, but it would not be until the 1960s and 70s that a pair of new technologies finally made long-distance optical communication a practical reality.

The first of these was the laser, invented in 1960 by Theodore Maiman of Hughes Research Laboratories in Malibu, California. Lasers allowed the production of straight, coherent beams of light that did not dissipate over long distances. The second was the introduction of ultra-pure, flexible glass fibres by the Corning Glass Works of Corning, New York in 1970. Such fibres allowed for the transmission of light signals over long distances – and around curves – via a phenomenon known as total internal reflection. This occurs when a beam of light hits the interface of two transparent materials of different refractive indices – say, glass and air – at a particular angle. Instead of refracting into the air, the light beam is instead completely reflected back into the glass. As almost none of the beam’s energy is lost in the process, it can continue reflecting back and forth over vast distances. This even works in curved conductors; indeed, in a famous demonstration of the phenomenon before the Royal Institution in 1870, scientist John Tyndall created a curved arc of water by draining a tank through a horizontal pipe, then shone a light through the pipe. Thanks to total internal reflection, the light was entirely contained within the water arc, with none leaking out the sides. This is the same principle used in the transmission of signals through fibre optic cables.

The idea of routing light around curves and corners using internally-reflective pipes is not a new one, with American engineer William Wheeler patenting such a system for lighting homes in 1881. However, Wheeler’s system used pipes with mirrored walls which absorbed large amounts of light, making it useful only over short distances. Only the total internal reflection made possible by ultra pure optical fibres provides the low losses required to reliably transmit signals across oceans and continents.

Today, more than 4 billion kilometres of optical fibre have been laid around the globe – nearly the distance from the earth to the planet Neptune – carrying everything from telephone calls to high-speed internet. But as recent as this optical revolution might seem, its technological seeds were laid all the way back in the 1870s by the endlessly fertile mind of Alexander Graham Bell. And given how integral fibre optics are to our modern ultra-connected way of life, Bell was perhaps not exaggerating when he declared:

“In the importance of the principles involved, I regard the photophone as the greatest invention I have ever made; greater than the telephone.”

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