Nobel Prize in Physics 2009: Optical Fibres and the CCD Image Sensor

This note covers the Nobel Prize in Physics 2009: who won it, how pure glass fibres carry light signals across continents, how the charge-coupled device (CCD) turns light into a digital picture, how both discoveries unfolded, why they matter and quick facts for exams.

What was the Nobel Prize in Physics 2009 awarded for?

The Royal Swedish Academy of Sciences divided the prize between two separate achievements in the physics of light.

The official citation, split by share, reads: "Kuen Kao "for groundbreaking achievements concerning the transmission of light in fibers for optical communication", while Willard S. Boyle and George E.

Smith "for the invention of an imaging semiconductor circuit - the CCD sensor".

In plain words, the first half rewards the physics that made it possible to send light signals down optical fibres over very long distances without the light fading away.

The second half rewards the invention of a tiny electronic chip, the charge-coupled device, that captures an image by converting light into electrical charge, the technology behind many modern digital cameras.

Both discoveries, taken together, underpin how images, text and voice travel around the globe today. The award is formally called the Nobel Prize in Physics, and it was announced on 6 October 2009.

Who are the laureates?

Charles K. Kao

Charles Kuen Kao was born on 4 November 1933 in Shanghai, China, and died on 23 September 2018 in Hong Kong.

At the time of the award he was affiliated with Standard Telecommunication Laboratories in Harlow, United Kingdom, and with the Chinese University of Hong Kong. He received one half of the prize.

Kao, working with his colleague George A. Hockham, worked out, mathematically, that the huge light losses in the glass fibres of the 1960s were caused mainly by impurities in the glass rather than by flaws in the fibre's shape, and he argued that extremely pure glass could carry light over kilometres rather than metres.

Willard S. Boyle

Willard S. Boyle was born on 19 August 1924 in Amherst, Nova Scotia, Canada, and died on 7 May 2011 in Truro, Nova Scotia. At the time of the award he was at Bell Laboratories, Murray Hill, New Jersey, USA.

He shared one quarter of the prize. Working with George Smith, Boyle sketched out the basic structure of the CCD on a blackboard, turning an idea for a better electronic memory into the design for a light-sensitive imaging chip.

George E. Smith

George E. Smith was born on 10 May 1930 in White Plains, New York, USA, and died on 28 May 2025 in Barnegat Township, New Jersey, USA.

At the time of the award he was also at Bell Laboratories, Murray Hill. He shared one quarter of the prize.

Smith worked alongside Boyle on the same blackboard sketch, and with Boyle on the first theoretical paper describing the CCD; the first experimental paper, also published in 1970, was written by Smith together with G. F. Amelio and M. F. Tompsett.

What problem did optical fibres and digital imaging need to solve?

Long before 2009, people already knew that light could be guided inside a transparent medium. In 1841 D.

Colladon in Geneva showed light following the curve of a water jet, and similar demonstrations followed, including a celebrated light-and-fountain display at the 1889 Paris exhibition.

From the 1930s, doctors used short bundles of glass fibres to peer inside the body, but these bare fibres leaked light wherever they touched each other and wore out quickly.

The deeper problem was attenuation: how much light is lost as it travels along a medium.

In the fibres available in the 1960s, only about 1 percent of the light that entered survived after just 20 metres, far too little for communication over real distances. Meanwhile, the world's communication needs were growing fast.

The first transatlantic telephone cable, laid in 1956, could carry only 36 simultaneous calls, and demand for television and telephony kept rising, so engineers needed a carrier with far higher capacity than copper wire or radio waves.

A separate but related problem existed in photography. Since the 1830s, images had been captured on chemical films that had to be developed, a slow process unsuited to instant transmission.

Capturing an image directly as an electrical signal, one that could be sent down the very same optical fibres being developed for communication, was the unmet need that the CCD eventually answered.

How does light travel down an optical fibre without escaping?

An optical fibre is a very thin thread of glass with two parts: a central core and an outer cladding whose refractive index is slightly lower than the core's.

Because of this difference, light entering the core at a suitable angle keeps bouncing off the boundary and travelling forward instead of escaping, a behaviour based on total internal reflection.

1. Light from a laser or light-emitting diode is coupled into the fibre core as a fast flickering pattern of pulses standing for digital ones and zeros.
2. Each pulse strikes the boundary between the core and the cladding at a shallow angle and, because the core has a higher refractive index, it reflects back into the core instead of leaking out.
3. The pulse keeps bouncing along the length of the fibre, guided purely by this repeated internal reflection.
4. Impurities and scattering in the glass still absorb a little light at every point, so the signal grows weaker with distance, an effect measured as attenuation in decibels per kilometre.
5. Over long routes, optical amplifiers boost the weakening light signal directly, without first converting it back into electricity, so it can continue for thousands of kilometres.

Kao's key insight was that the attenuation of the glass fibres of his time was dominated by chemical impurities, especially iron, rather than by the shape of the waveguide.

He calculated that fibres drawn from extremely pure fused silica glass could, in principle, let 1 percent of the light survive after travelling a full kilometre, roughly a fifty-fold improvement on the fibres then available.

Four years after his 1966 paper, a team at Corning Glass Works in the United States manufactured the first fibre pure enough to approach that goal.

Draw a long thin cylinder labelled "core" surrounded by a slightly thicker layer labelled "cladding".

Draw a zig-zag arrow entering one end of the core and bouncing repeatedly off the core-cladding boundary as it travels to the far end, with an angle label at one bounce point to show the light staying inside rather than crossing into the cladding.

How does a CCD sensor turn light into a digital picture?

A charge-coupled device is a small silicon chip divided into a grid of light-sensitive cells called pixels. Each pixel works through the photoelectric effect, the phenomenon (first explained by Albert Einstein, for which he won the 1921 Nobel Prize in Physics) in which light striking a material knocks electrons loose.

1. Light falls on the silicon surface of the chip, and in each pixel the photoelectric effect knocks loose a number of electrons proportional to how much light that pixel received.
2. These freed electrons collect in a tiny "potential well" beneath each pixel, created by applying a voltage to an electrode above it, so each pixel stores its own private pool of charge.
3. When read-out begins, a changing voltage pattern applied across the chip shifts the stored charge, row by row, from one pixel to its neighbour, rather like a bucket brigade passing buckets along a line.
4. The shifted charge reaches the edge of the chip, where it is read out one row at a time and converted into an electrical signal.
5. That electrical signal is converted into a digital number for every pixel, and the complete set of numbers is reassembled by a computer into the final digital image.

Because the chip itself can only tell how much light fell on each pixel, not what colour it was, colour cameras place small red, green or blue filters over the pixels in a repeating pattern, with twice as many green filters as red or blue because the human eye is more sensitive to green light.

Draw a grid of small squares representing pixels.

Draw arrows showing one row of charge sliding sideways, pixel by pixel, towards an output amplifier at the edge of the grid, and label the amplifier as the point where charge becomes a digital number.

A rival chip technology, CMOS, reads every pixel in place instead of shifting charge along a chain, which uses less power but can be noisier for demanding work. The table below compares the two.

How did the discovery unfold?

Both strands of this prize built on decades of earlier work before reaching their breakthroughs in the 1960s and bearing fruit soon after.

Why does it matter?

Optical fibres now form what the prize committee called the circulatory system of the communication society: a single global thread of glass fibre over a billion kilometres long, long enough to encircle the Earth more than 25,000 times, carrying almost all internet and telephone traffic.

Without Kao's insight into glass purity, such long-distance, high-capacity communication would not exist in its present form.

The CCD, meanwhile, replaced photographic film with an electronic eye. It made modern digital cameras and video possible, and its sensitivity, far higher than the human eye or photographic film, proved essential for astronomy, giving instruments such as the Hubble Space Telescope their detailed images of distant galaxies.

CCD technology is also used in medicine, for example in endoscopes that let doctors see inside the body during diagnosis or minimally invasive surgery.

The sources also note an open, ongoing question: whether the cheaper, lower-power CMOS sensor will eventually replace CCD in most applications, or whether the two technologies will continue to serve different needs side by side.

How does this connect to what you study?

School physics courses that cover total internal reflection and refractive index in the chapter on optics describe exactly the principle that keeps light trapped inside an optical fibre's core. A fibre's core has a slightly higher refractive index than its surrounding cladding, and that single difference, which looks like a small textbook fact, is the entire reason a light pulse can travel for kilometres without escaping through the sides.

The same courses that explain the photoelectric effect, the emission of electrons when light of sufficient energy strikes a material, describe the physics at the heart of every pixel in a CCD sensor. This is the same effect for which Albert Einstein won the 1921 Nobel Prize in Physics, so a single idea from a physics textbook connects two Nobel Prizes separated by nearly ninety years.

Chapters on semiconductors and electronics also help here, because a CCD is built from silicon using the same kind of metal-oxide-semiconductor structure used in other electronic devices, with a voltage applied to an electrode to create the "well" where charge collects.

Seeing how total internal reflection and the photoelectric effect, two ideas usually tested with simple diagrams and short definitions, became the basis of the global internet and the modern camera is a useful reminder that textbook physics often sits directly behind everyday technology that students use every day without noticing the science inside it.

Quick facts for exams

The Nobel Prize in Physics 2009 was announced on 6 October 2009 by the Royal Swedish Academy of Sciences. It was divided between two achievements: one half to Charles K.

Kao for his work on transmitting light through pure glass fibres for optical communication, and the other half jointly to Willard S. Boyle and George E. Smith for inventing the charge-coupled device (CCD) image sensor.

Kao was born in Shanghai, China, and worked in the United Kingdom and Hong Kong; Boyle was born in Nova Scotia, Canada;

Smith was born in New York, USA; both Boyle and Smith worked at Bell Laboratories in New Jersey, USA. The prize fund that year was 10,000,000 Swedish kronor.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Optical fibre — a thin thread of glass with a core and cladding that guides light along its length by repeated internal reflection.
• Attenuation — the loss of signal strength, in this case light intensity, as it travels along a fibre or medium.
• Total internal reflection — the complete bouncing back of light at a boundary when it travels from a medium of higher refractive index towards one of lower refractive index, at a sufficiently shallow angle.
• Refractive index — a number describing how much a material slows down and bends light compared with a vacuum.
• Cladding — the outer layer of an optical fibre, with a lower refractive index than the core, that keeps light trapped inside.
• Charge-coupled device (CCD) — a semiconductor chip that records an image by storing and shifting electrical charge produced by light in each pixel.
• Pixel — one of the many individual light-sensitive points on an image sensor, each corresponding to one point in the final picture.
• Photoelectric effect — the release of electrons from a material when light of sufficient energy strikes it.
• CMOS — Complementary Metal Oxide Semiconductor, a rival image sensor technology in which each pixel is read out individually.
• Bucket brigade — the descriptive name for the way charge is shifted step by step across a CCD during read-out, likened to passing buckets along a line.
• Fused silica — extremely pure glass made chiefly from silicon dioxide, used to manufacture low-loss optical fibres.
• Optical amplifier — a device that strengthens a weakening light signal directly, without converting it back into electricity first.

Common errors and misconceptions

• Misconception: The whole 2009 Physics prize went to one discovery. Correct: It was split between two separate achievements, fibre optics for Kao and the CCD for Boyle and Smith.
• Misconception: Optical fibres work because the glass is perfectly transparent. Correct: They work because of total internal reflection at the core-cladding boundary; purity of the glass reduces losses, it does not eliminate the reflection principle.
• Misconception: Boyle and Smith set out to invent a camera sensor. Correct: They were originally trying to design a better electronic memory to compete with magnetic bubble memory at Bell Laboratories.
• Misconception: A CCD stores a finished colour image directly. Correct: The chip records only light intensity per pixel; colour filters placed over the pixels are needed to recover colour information.
• Misconception: CMOS and CCD are the same technology under different names. Correct: They read out charge in different ways, CCD shifts charge to the edge, CMOS reads each pixel in place, giving each technology different strengths.
• Misconception: Kao personally built the first low-loss fibre. Correct: Kao calculated and predicted that pure glass fibres would work; a team at Corning Glass Works first manufactured a fibre pure enough to match his prediction, four years later.

Exam-style questions with model answers

Q1. Who shared the Nobel Prize in Physics 2009, and in what proportions? [2 marks]

1. Charles K. Kao received one half of the prize, while Willard S. Boyle and George E. Smith shared the other half between them.

Q2. State the official citation for Charles K. Kao's share of the prize. [2 marks]

1. His citation reads "for groundbreaking achievements concerning the transmission of light in fibers for optical communication".

Q3. Explain how total internal reflection allows an optical fibre to carry light over long distances. [4 marks]

1. An optical fibre has a core with a higher refractive index than its surrounding cladding. When light inside the core strikes the core-cladding boundary at a shallow enough angle, instead of passing through into the cladding, it is completely reflected back into the core. This repeated bouncing, rather than any single straight path, is what guides the light pulse along the whole length of the fibre, even around gentle bends, letting the signal travel for kilometres with only slow, gradual loss of intensity due to absorption and scattering in the glass.

Q4. Why did Kao argue that purifying the glass, rather than redesigning the fibre's shape, was the key problem to solve? [4 marks]

1. Kao's detailed calculations showed that the heavy light losses measured in 1960s glass fibres were caused mainly by absorption and scattering linked to chemical impurities such as iron ions in the glass, rather than by imperfections in the waveguide's shape or bending. He concluded that fibres drawn from very pure fused silica glass could, in principle, let far more light survive over much longer distances than existing fibres allowed, setting a target of 1 percent of light remaining after one kilometre, a target met within a few years once sufficiently pure glass could be manufactured.

Q5. Describe, step by step, how a CCD sensor converts incoming light into a digital image. [6 marks]

1. A CCD chip is covered by a grid of light-sensitive pixels, each one a tiny region of silicon beneath an electrode. When light falls on a pixel, the photoelectric effect releases electrons, and the number released is proportional to the amount of light that pixel received. A voltage applied to the electrode creates a potential well that traps these electrons directly beneath that pixel, so each pixel builds up its own separate pool of charge representing the brightness at that point in the image. To read out the picture, a changing three-phase voltage pattern is applied across the chip, which shifts the stored charge sideways from pixel to pixel, row by row, towards the edge of the chip, in a manner often described as a bucket brigade. At the edge, each row of charge packets is amplified and converted into a digital number proportional to its size. Collecting all these numbers for every pixel in the array lets a computer reconstruct the complete image, with colour obtained by using red, green and blue filters placed over the individual pixels.

Q6. Give two applications of CCD technology outside ordinary photography. [3 marks]

1. CCD sensors are used in astronomy, for example aboard the Hubble Space Telescope, to capture highly sensitive images of distant objects in space, and in medicine, for example in endoscopes used for imaging inside the body during diagnosis and minimally invasive surgery.

Q7. What problem were Boyle and Smith originally trying to solve when they invented the CCD? [2 marks]

1. They were trying to design a better electronic memory device at Bell Laboratories to compete with magnetic bubble memory technology, and their sketch for this memory instead became the basis of the CCD.

Q8. Discuss why the Nobel Prize in Physics 2009 is described as rewarding two different but related technologies rather than one. [5 marks]

1. The prize honours two separate inventions that both concern the physics of light but solve different problems. Charles Kao's work addressed how to send a light signal over long distances without it fading away, by showing that extremely pure glass fibres could transmit light with far less loss than previously thought possible, a breakthrough in optical communication. Boyle and Smith's work, in contrast, addressed how to capture and store an image as an electrical signal using the charge-coupled device, a breakthrough in imaging rather than transmission. The two achievements are related because they both depend on controlling light with physics and because, together, they make possible the modern flow of digital images and data: a CCD can capture an image, and an optical fibre network can then carry that image around the world almost instantly. The Royal Swedish Academy of Sciences recognised this connection by dividing a single prize between them, calling the laureates, in effect, masters of light for shaping the information technology of our daily lives.

Key takeaways

• The 2009 Physics Nobel was split between fibre-optic communication (Kao) and the CCD image sensor (Boyle and Smith).
• Kao showed that impurities, not waveguide shape, caused most light loss in 1960s glass fibres.
• Pure fused silica fibres, first made in 1970, let light travel far longer distances with much lower loss.
• Optical fibres use total internal reflection at the core-cladding boundary to guide light.
• Boyle and Smith sketched the CCD design in 1969 while aiming to build a better electronic memory.
• A CCD converts light into charge using the photoelectric effect, then shifts that charge row by row to be read out.
• CCD sensors enabled modern digital cameras, astronomy instruments and medical imaging devices.
• CMOS is a rival sensor technology that reads each pixel in place rather than shifting charge.

Test yourself

What citation did Charles K. Kao receive for his share of the 2009 Physics prize?

He was cited for groundbreaking achievements concerning the transmission of light in fibers for optical communication.

Where were Willard S. Boyle and George E. Smith both working when they invented the CCD?

Willard S. Boyle and George E. Smith both worked at Bell Laboratories, Murray Hill, New Jersey, USA, at the time.

What physical principle lets light stay trapped inside an optical fibre's core?

Total internal reflection at the boundary between the higher-index core and the lower-index cladding keeps light trapped inside the fibre.

What physical effect produces electrons inside a CCD pixel when light strikes it?

The photoelectric effect releases electrons from the silicon when light strikes each pixel of the CCD sensor.

What were Boyle and Smith originally trying to build when they sketched the CCD?

They were trying to design a better electronic memory to compete with magnetic bubble memory technology at Bell Laboratories.

Approximately how long is the total length of optical fibre installed worldwide?

The total length is over a billion kilometres, enough to encircle the globe more than 25,000 times.

How does a CCD read out the charge stored in its pixels?

Changing voltages shift the stored charge row by row towards the edge of the chip, where it is digitised, in a bucket-brigade-like process.

Name one scientific application of CCD sensors.

CCD sensors are used in astronomy, for instance aboard the Hubble Space Telescope, to capture highly sensitive images of distant space.