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The Nobel Prize in Physics 2009 "for groundbreaking achievements concerning the transmission of light in fibers for optical communication" "for the invention of an imaging semiconductor circuit – the CCD sensor" Charles K. Kao 高錕 Willard S. Boyle George E. Smith 1/2 of the prize 1/4 of the prize •Standard Telecommunication Laboratories Harlow, UK; Chinese Univ of Hong Kong Hong Kong, China 1/4 of the prize Bell Laboratories Murray Hill, NJ, USA b. 1933 b. 1924 b. 1930 (in Shanghai, China) (in Amherst, NS, Canada) How fiber optics impacts you This is reason for development of global all-optical communication networks. This is your Internet, Email, … Orders of magnitude more information capacity than is available through electrical systems The Light Guide: Guiding light from one location to another • Guiding light by (a) lenses; (b) mirrors • Guiding distance is limited because of losses due to partially reflective lenses or absorptive mirrors. Alternative: the fiber light guide • Feasibility of transmitting optical radiation through glass fiber waveguides was first suggested by KC Kao and E Hockham in 1966. • Treats the fiber as a dielectric waveguide in which the light is guided down the fiber by reflections at the surface of the glass. • “Total internal reflection” (TIR) guides light down the fiber. Total Internal Reflection (TIR) • • Total internal reflection occurs at the boundary between two media of different refractive indices – when light travelling from the medium of higher refractive index to the lower one strikes the interface at the critical angle. – For angles less than this critical angle, the ray is partially transmitted into the second medium energy is lost. – For angles greater than the critical angle the ray is totally reflected back into the original medium. In practice, the core is surrounded by a glass cladding of lower refractive index to protect the core and to prevent changes in the critical angle due to contamination on the surface. Typical dimensions are 10 or 50 micrometers for the core and 125 micrometers for the fiber. In addition, a protecting plastic “buffer” is placed around the fiber. Types of optical fibers Geometry, refractive index profile and typical rays in a step-index multi-mode fiber (MMF), single-mode fiber (SMF) and graded index (GRIN) MMF. • Light propagates in the form of modes, with a distinct propagation constant and group velocity, maintaining its transverse spatial distribution and polarization. • Each mode is described as the sum of the multiple reflections of a TEM wave bouncing within the guide, in the direction of an optical ray at a certain bounce angle. Charles K. Kao and optical fiber Kao at work in his laboratory at Harlow, England in 1966. “[...] a fibre of glassy material constructed in a cladded structure [...] represents a possible practical optical Waveguide with important potential as a new form of communication medium […] Compared with existing coaxial-cable and radio systems, this form of waveguide has a larger information capacity and possible advantages in basic material cost.” K.C. Kao and G.A. Hockham, “Dielectric-Fibre Surface Waveguides for optical frequencies” Proc. IEEE, 113, 1151 (1966). Attenuation in optical fibers • • • Of particular importance in the application of fibers is the amount of light that can be transmitted down the fibre. – The first optical fibres had an attenuation of around 1000 dB/km. – Fibres are now available with attenuation < 0.1 dB/km. Several factors contribute to the attenuation of light through fibers, including absorption, scattering and bending effects. – Absorption occurs at impurities in the fiber such as beads of water which become imbedded in the glass during manufacture and absorb the light preferentially at certain wavelengths. – Electronic and vibrational resonances Scattering is the change in direction of propagation of a ray by collision with an inhomogeneity or irregularity in the glass. – This effect is known as Rayleigh scattering and is proportional to λ-4. – The overall effect is that the total attenuation is wavelength dependent. Attenuation (dB)= 10 log(Pin/Pout). Attenuation in optical fibers Absorption by electronic band silicon-oxygen (Si-O) bonds • The variation of attenuation with wavelength of a typical modern fiber (close to fundamental limit). • Kao and Hockham pointed out that the attenuation of glass fiber (in 1966) was largely caused by the presence of impurity metal ions, such as iron, copper, vanadium, and chromium. • If a glass could be developed with attenuation of only 20 dB/km, then optical communication could become a reality. With such a fiber, 1% of the input light reaches the far end of a 1 km length of fiber. Rayleigh Scattering • The scattering of light off of the molecules of the air, or • from particles up to about a tenth of the wavelength of the light. • It is Rayleigh scattering off the molecules of the air which gives us the blue sky. • In optical fibers, Rayleigh scattering is caused by density fluctuations or non-uniformity during the manufacturing process, dominant for 800 nm band. TAT-8 • In 1988 AT&T laid the first fiber-optic transatlantic telephony cable 3,148 miles long • Connected North America to France • Repeaters every 40 miles • 565 Mbps bandwidth • Used 1300 nm light • Attenuation 0.4 dB/km Fiber Amplifier • Special fiber with Erbium atoms in it is used to amplify light without changing it to an electrical signal first • Uses stimulated emission, the same principle that makes lasers work Freeway Analogy to the fiber-based network • TAT-8 in 1980 – 565 Mbps – Electro-optical repeaters • TAT-12/13 in 1996 – 2.5 Gbps – Optical amplifiers • 1998 – 20 Gbps – WDM with 8 wavelengths Charge-coupled Devices or CCDs CCD, the electronic image sensor, converts the optic image to electronic signals that are translated into digital ones and zeros. The galaxy cluster Abell 2218. Image: WFPC2, Hubble Space Telescope, NASA. CCD (Charge-Coupled Devices) Bucket Brigade • Integration • Charge Shift and Read-out •Charge Amplifier Charge-coupled Devices or CCDs Basic Building Block: the MOS capacitor The charge genera- ted by photons is forced to move one step at a time through the application of voltage pulses on the electrodes. color imaging by generally use a Bayer mask over the CCD. Each square of four pixels has one filtered red, one blue, and two green (the human eye is more sensitive to green than either red or blue). Charge-coupled Devices or CCDs Willard Boyle (left) and George Smith (right) with the CCD Camera. Original notes from the Boyle and Smith’s brainstorm meeting on September 8 1969, when they made the first sketch of a CCD. Further Reading • Web site of Nobel Foundation http://nobelprize.org/nobel_pri zes/physics/laureates/2009/in dex.html (See the Info and Scientific Background). • Blog on Nobel Prize by Physics Today http://blogs.physicstoday.org/ newspicks/2009/10/nobelprize-in-physics-awarded.html • Sec. 4.7 and 5.2 of Hecht