The optical fibre has been a prerequisite for the extremely rapid development in the field of communications, a development that Charles Kao predicted over 40 years ago.
Just a few years later, Willard Boyle and George Smith radically altered the conditions for the field of photography.
Their invention of digital image sensor – CCD, or charge-coupled device changed the way images were captured by cameras. The CCD became the made it possible for digital transfer of images.
It opened the door to a daily stream of images, which is filling up the optical fibre cables. Only optical fibre is capable of transferring such large quantities of data that electronic image sensor technology yields.
The optical fibre required modern glass technology in order to be developed and manufactured.
A reliable source of light was also needed and this was provided by semiconductor technology. Finally, other essential optical components were required to make optical fibre technology to transfer data.
Optical fibre works on the basis for optical waveguide technology where light is captured inside a fibre with a higher refractive index than its surrounding environment.
A ray of light that is directed into a fibre, bounces against the glass wall and moves forward since the refractive index of glass is higher than the surrounding air.
The next improvement came in the form of coating the bare fibre in a glass cladding with a lower refractive index, which in the 1960s paved the way for industrial manufacturing of instruments for gastroscopy and other medical uses.
For long distance communication, glass fibres appeared less attractive compared to radio waves. However, compared to radio waves, infrared or visible light carries tens of thousands times more information, so the potential of optical light waves could not be disregarded any longer.
The invention of the laser at the beginning of the 1960s was a decisive step forward for fibre optics.
The laser was a stable source of light that emitted an intensive and highly focused beam of light, and could be pumped into a thin optical fibre.
The first lasers
The first lasers emitted infrared light and required cooling. Around 1970 more practical lasers were developed which could work continuously at room temperature. This was a technological breakthrough that facilitated optical communication.
All information could now be coded into an extremely fast flashing light, representing digital ones and zeros. However, how such signals could be transmitted over longer distances was still not known – after just 20 meters, only 1 percent of the light that had entered the glass fibre remained.
Reducing this loss of light became a challenge for a visionary like Charles Kuen Kao. His goal was that at least 1 percent of the light that entered a glass fibre would remain after it had travelled 1 kilometre.
In January 1966, Kao came to the conclusion that it was not imperfections in the fibre thread that was the main problem, instead, it was the glass that had to be purified. He admitted that this would be feasible but very difficult.
The goal was to manufacture glass of a transparency that had never been attained before. In order to produce the purest glass in the world, Kao pointed out that fused silica could be used.
It melts at almost 2,000 degree C, a heat difficult to control but from which one would draw out ultra-thin threads of fibre. After four years, 1971, scientists at the Corning Glass Works in the USA, a glass manufacturer produced a 1 kilometre long optical fibre using chemical processes.
A ray of light that is directed into a glass fibre bounces against the glass wall and moves forward because the refractive index of the glass core is slightly higher than that of the glass cladding.
Ultra-thin fibre made out of glass may seem very fragile. However, when glass is correctly drawn out in a long thread, its properties change. It becomes strong, light and flexible, which is a prerequisite if the fibre is to be buried, drawn under water or bent around corners.
Unlike copper cables, glass fibre is not sensitive to lightning, and unlike radio communication, it is not affected by bad weather. It took a fair share of time to coil the Earth in fibre.
In 1988, the first optical cable was laid out along the bottom of the Atlantic Ocean between the United States and Europe. It was 6,000 km long.
Today, telephone and data communication flows in a network of optical glass fibre, the length of which totals over 1 billion km.
Even in a high purity glass fibre, the signal is slightly reduced along the way and has to be reinforced when it is transmitted over longer distances. This task, which previously required electronics, is today performed by optical amplifiers.
This has brought an end to unnecessary losses that occur when light is transformed to and from electronic signals. Today 95 per cent of the light remains after having been transmitted a full kilometre, a number that should be compared to Kao’s ambition of having 1 per cent left after that same distance.
Semiconductor lasers and light diodes the size of a grain of sand fill networks of optical fibers with light which carries almost all of the telephone and data communication around the world.
Sometimes inventions appear totally unanticipated. The image sensor, CCD, or charge-coupled device, is such an invention. Without the image sensor, CCD, or charge-coupled device, CCD, the development of digital cameras would have taken a slower course.
Without CCD we would not have seen the astonishing images of space taken by the Hubble space telescope, or the images of the red desert on our neighbouring planet Mars.
This was not what the inventors of the CCD, Willard Boyle and George Smith, had imagined when they began their work.
One day in September 1969, they outlined the basis of an image sensor on a blackboard in Boyle’s office. At that time they did not have photographic images in mind. Their aim with the CCD was to create a better electronic memory.
The CCD is yet another success story of our electronic era. Just like many other devices in the electronics industry, a digital image sensor, CCD, is made out of silicon. The size of a stamp, the silicon plate holds millions of photocells sensitive to light. Photoelectric effect occurs when light hits the silicon plate and knocks out electrons in the photocells.
The liberated electrons are gathered in the cells which become small wells for them. The larger the amount of light, the larger the number of electrons that fills these wells. When voltage is applied to the CCD array, the content of the wells can be progressively read out; row by row, the electrons slide off the array. So for example, an array of 10 x 10 image points is transformed into a 100 points long chain.
In this manner the CCD transforms the optical image into electric signals that are subsequently translated into digital ones and zeros. Each cell can then be recreated as an image point, a pixel. When the width of a CCD, expressed in pixels, is multiplied with its height, the image capacity of the sensor is obtained.
Thus a CCD with 1280 x 1024 pixels yields a capacity of 1.3 megapixels (1.3 million pixels). The CCD renders an image in black and white, so various filters have to be used in order to obtain the colours of light.
One kind of filter that contains one of the base colours red, green or blue, is placed over every cell in the image sensor. An array is read out row by row. The image sensor, CCD, is the advanced digital camera’s electronic eye.
The advantages of the electronic image sensor quickly became evident. In 1970, just about a year after the invention, Smith and Boyle could demonstrate a CCD in their video camera for the first time.
In 1972, the American company Fairchild constructed the first image sensor with 100 x 100 pixels, which entered production a few years later.
The early years
In 1975, Boyle and Smith themselves constructed a digital video camera of a sufficiently high resolution to manage television broadcasts.It would not be until 1981 before the first camera with built-in CCD appeared on the market.
Not withstanding its bulky and primitive characteristics, when compared to contemporary cameras, it initiated a more commercially oriented digitalization in the field of photography.
Five years later in 1986, the first 1.4 megapixel image sensor (1.4 million pixels) arrived, and a further nine years on in 1995, the world’s first fully digital photographic camera appeared.
Camera manufacturers around the world quickly caught on, and soon the market was flooded with ever smaller and cheaper products.
With cameras equipped with image sensors instead of film, an era in the history of photography had ended. When it comes to everyday photography, digital cameras have turned out to be a commercial success.
Entry of CMOS
Lately the CCD has been challenged by another technology, CMOS, or Complementary Metal Oxide Semiconductor; a technology that was invented at about the same time as CCD.
Both make use of the photoeffect, but while the electrons gathered in a CCD march in line in order to be read out, every photocell in a CMOS is read out on site.
However, one also has to take into account its higher noise levels and the loss of image quality, and consequently CMOS is not sufficiently sensitive for many advanced applications.
It is precisely thanks to digital technology that the wide-angle camera on the Hubble space telescope can send the most astonishing images back to Earth.
The camera’s sensor initially consisted of only 0.64 megapixels. However, as four sensors were interconnected, they provided a total of 2.56 megapixels. This was a big thing in the 1980s when the space telescope was designed.
Today the Kepler satellite has been equipped with a mosaic sensor of 95 megapixels, and the hope is that it will discover Earth-like planets around stars other than the sun. Early on, astronomers realized the advantages of the digital image sensor.
It spans the entire light spectrum, from X-ray to infrared. It is a thousand times more sensitive than photographic film. Out of 100 incoming light particles a CCD catches up to 90, whereas a photographic plate or the human eye will only catch one.
In a few seconds, light from distant objects is gathered — a process that previously would have taken several hours.
(Edited excerpts from public information available at www.nobelprize.org)