Before the advent of optical fiber communication, copper-wired telecommunication systems were already deployed. However, copper wires had several drawbacks, including limited bandwidth, significant signal loss over long distances, and being expensive and heavy. The laser’s invention in 1960 sparked curiosity among researchers in the communication industry: could lightwaves serve as the ideal carrier?
The advantages of light as a carrier were evident. Visible light, with a wavelength of 600 nm corresponds to a frequency of around 500 THz, offered a bandwidth five orders higher than a 5 GHz microwave carrier. The higher the frequency, the greater the potential bandwidth. Yet, a critical challenge remained - finding the right material capable of guiding and confining lightwave. At that time, even the most transparent glass exhibited a loss of about 200 dB/km, with dB being the power ratio between output and input. This means that after traveling just 100 meters through the glass, only 1% of the light remained. such high losses posed a major obstacle to practical optical communication.
The breakthrough came in 1966, when Charles Kao, along with his colleague, published a paper titled, “Dielectric-fiber surface waveguides for optical frequencies”.The term “dielectric” means non-conductive, and “dielectric-fiber surface waveguides’ is what we call optical fiber today. Optical fiber is a type of waveguide, essentially a tube or pipe, used to confine and guide light, like water pipe to confine and guide water.
In this paper, Charles Kao analyzed the loss mechanisms that plagued early optical fibers. He identified several key types of losses:
- Intrinsic loss: this loss resulted from light being absorbed by silicon dioxide, an unavoidable consequence. However, Charles Kao identified a relatively low absorption region in the glass, specifically between \(1 \mu m\) to \(0.3 \mu m\).
- Extrinisc loss: this loss was caused by light being absorbed by impurities within the fiber. Charles Kao’s analysis suggested that by reducing impurities to levels as low as 1 part per million (1 ppm), the extrinsic loss could be brought down to 20 dB/km.
- Rayleigh scattering loss: Charles Kao’s insight on rapidly cooling the liquid state glass enabled a significant reduction in Rayleigh scattering loss to the order of 1 dB/km.
In this paper, Charles Kao also analyzed the bending loss and how dispersion will affect the bandwidth of the optical fiber. His pioneering work on optical fiber communication earned him the title of “the father of fiber optics”.
Only four years after the paper’s publication, in 1970, three scientists at Corning, successfully invented the world’s first low-loss optical fiber, with an impressive attenuation rate of 17 dB/km. Over time, they further pushed the boundaries and reduced the attenuation rate to 4 dB/km. The breakthrough showed the immense potential of optical fibers as a reliable medium for transmitting information across vast distances. In 1976, a 45 Mb/s optical fiber communication system was demonstrated. This demonstration marked a critical milestone for optical fiber communication and set the stage for the rapid advancement of this transformative technology and laid the infrastructure foundation of the Internet revolution.
If you are interested in Charles Kao’s life, his autobiography, ‘潮平岸阔’ (Chinese version) or ‘A Time and a Tide’ (English version), provides a journey through his childhood in Shanghai during WWII, his educational journey, and his career as a researcher at Standard Telecommunication Lab, and how he was appointed as Vice-Chancellor of The Chinese University of Hong Kong.
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Reference
- Kumar S, Deen M J. Fiber optic communications: fundamentals and applications. John Wiley & Sons, 2014.
- Keiser G. Optical fiber communications. New York: McGraw-Hill, 2000.
- 潮平岸阔：高锟自述