Transcript Holography
B.SC.II PAPER-B (OPTICS and LASERS) Submitted by Dr. Sarvpreet Kaur Assistant Professor PGGCG-11, Chandigarh Holography 2 History of Holography • Invented in 1948 by Dennis Gabor for use in electron microscopy, before the invention of the laser • Leith and Upatnieks (1962) applied laser light to holography and introduced an important off-axis technique 3 Conventional vs. Holographic photography • Conventional: – 2-d version of a 3-d scene – Photograph lacks depth perception or parallax – Film sensitive only to radiant energy – Phase relation (i.e. interference) are lost 4 Conventional vs. Holographic photography • Hologram: – Freezes the intricate wavefront of light that carries all the visual information of the scene – To view a hologram, the wavefront is reconstructed – View what we would have seen if present at the original scene through the window defined by the hologram – Provides depth perception and parallax 5 Conventional vs. Holographic photography • Hologram: – Converts phase information into amplitude information (in-phase - maximum amplitude, out-ofphase – minimum amplitude) – Interfere wavefront of light from a scene with a reference wave – The hologram is a complex interference pattern of microscopically spaced fringes – “holos” – Greek for whole message 6 Creating Holograms Reconstructing the image Hologram : Direct, object and conjugate waves • Direct wave: corresponds to zeroth order grating diffraction pattern • Object wave: gives virtual image of the object (reconstructs object wavefront) – first order diffraction • Conjugate wave: conjugate point, real image (not useful since image is inside-out due to negative phase angle) – first order diffraction • In general, we wish to view only the object wave – the other waves just confuse the issue 9 Off-axis- Direct, object and conjugate waves Use an off-axis system to record the hologram, ensuring separation of the three waves on reconstruction Reference wave Object wave Direct wave Virtual image 10 Conjugate wave Real image Hologram – Reflection vs. Transmission • Transmission hologram: reference and object waves traverse the film from the same side • Reflection hologram: reference and object waves traverse the emulsion from opposite sides View in Transmission 11 View in reflection Hologram: Wavelength • With a different color, the virtual image will appear at a different angle – (i.e. as a grating, the hologram disperses light of different wavelengths at different angles) • Volume hologram: emulsion thickness >> fringe spacing – Can be used to reporduce images in their original color when illuminated by white light. – Use multiple exposures of scene in three primary colors (R,G,B) 12 Holography application • A telephone credit card used in Europe has embossed surface holograms which carry a monetary value. When the card is inserted into the telephone, a card reader discerns the amount due and deducts (erases) the appropriate amount to cover the cost of the call. • Supermarket scanners read the bar codes on merchandise for the store's computer by using a holographic lens system to direct laser light onto the product labels during checkout. • Holography is used to depict the shock wave made by air foils to locate the areas of highest stress. These holograms are used to improve the design of aircraft wings and turbine blades. Holography application • A holographic lens is used in an aircraft "heads-up display" to allow a fighter pilot to see critical cockpit instruments while looking straight ahead through the windscreen. Similar systems are being researched by several automobile manufactures. • Researchers are developing the sub- systems of a computerized holographic display. • Holography is ideal for archival recording of valuables or fragile museum artifacts. • Optical computers, which use holograms as storage material for data, could have a dramatic impact on the overall holography market. Holography application • To better understand marine phytoplankton, researchers have developed an undersea holographic camera that generates in-line and off-axis holograms of the organisms. A computer controlled stage moves either a video camera or a microscope through the images, and the organisms can be measured as they were in their undersea environment • An interferogram (a sort of hologram) is a technique providing a method of non-destructive analysis that determines structural deformations in objects. Holography application • The using of ultrasound waves as main carriers of the information creates opportunities for holography application in a sound field visualization. This has a great practical importance in: - Undersea acoustics and hydrolocation; - Defectoscopy; - Medical diagnostics; - Biological surveys • The using of X-rays as main carriers of information creates additional opportunities for holography method application in biological, physical and chemical studies Holography application The using of γ –rays allows precise atomic and molecular structural analysis: (a)Holograms of a local structure of crystallic Fe (b) Estimated pictures of a local structure of crystallic Fe 18 History of Fiber Optic Technology • People have used light to transmit info. for hundreds of years • The invention of the laser prompted researchers to study the potential of fiber optics send a much larger amount of data than telephone • first experiment letting the laser beam transmit freely in air & through different types of waveguides • very large losses in the optical fibers prevented coaxial cables from being replaced Decrease in the amount of light reaching the end of the fiber Cont: History of Fiber Optic Technology • • • • • Early fibers had losses around 1,000 dB/km In 1969, several scientists concluded that impurities in the fiber material caused the signal loss Researchers believed it was possible to reduce the losses By removing the impurities researchersobserve the improvement In 1970,(corning glass works)* made a multimode fiber with losses under 20 dB/km. in 1972, the company made a high silica-core multimode optical fiber with 4dB/km. Nowadays, multimode fibers can have losses as low as 0.5 dB/km at wavelengths around 1300 nm *company Introduction • Optical fiber has a number of advantages over the copper wire since it is made glass or plastic • light has a much higher frequency than any radio signal we can generate, fiber has a wider bandwidth To carry more information at one time Fiber Optics as a Transmission Medium • • • • • • • • • Information is carried through a fiber optic cable by transmitting pulses of light (which is also an EM wave)! A fiber optic cable is a coaxial arrangement of glass or plastic material of immense clarity (i.e., highly transparent) A clear cylinder of optical material called the core is surrounded by another clear wrapper of optical material called the cladding These two materials are selected to have different indices of refraction The fiber is surrounded by a plastic or teflon jacket to protect and stiffen the fiber Light is guided through the optical fiber by continual reflection from the core-cladding boundary This is made possible due to the different refractive indices of the core and cladding materials The index of refraction (n) of a material affects the angle by which a light ray is bent while passing through the material If the light incident on the core-cladding boundary is at a suitable angle, then the light will be totally reflected from the boundary. This is called total internal reflection Beginning Of Fiber Optics • Swiss Physicist Daniel Colladon Recognized Light Guiding(1841) How do They Work? • Snell’s Law n1 sin(1)=n2 sin(2) The speed and wavelength of light change when entering a different medium • Index of Refraction (n=c/v c=speed of light in vacuum) Total Internal Reflection c=arcsin(n2/n1) light is to enter the second medium at an angle greater than critical angle for TIR to occur acceptance angle max=sin-1(n12-n22)1/2 Numerical Aperture(NA)=sin(acceptance angle) cladding index of refraction lower than core index of refraction Cross section of optical fiber cable Core and cladding with different indices of refraction Core-cladding boundary Optical Fiber • Fiber is the medium to guide the light form the transmitter to the receiver. • There are two types: – Multimode Fiber – Single-Mode Fiber Multimode Fiber In multimode fibers more than one light transmitted at a time. Fiber diameter ranges from 50-to-100 micron. 31 Multimode Fiber Multimode Fiber is divided into two types: – Multimode Step-index Fiber – Multimode Graded-index Fiber Multimode Step-index Fiber • Lights are sent at angles lower than the critical angle or straight • Any light angle exceed the critical angle will cause it to penetrate through cladding. • Obviously light with lower angle will reach the end faster than others. Multimode Step-index Fiber • The difference in signals receiving time result in unstable wave light at the receiver. • To avoid this problem there should be spacing between the light pulses but this will limit the bandwidth. • Used for very short distance Multimode Graded-index Fiber • In this mode reduce the problem with Multimode Step-Index. • All the beams reaching the receiver almost at the same time. • This can be done by slowing down the ones with shorter distance. • This is done in fiber implementation by increasing its refractive index at the center and gradually decreases it toward the edges Single-Mode Fiber • • • • Only one light is transmitted in the fiber. Diameter ranges from 8.3 to 10 microns. It has Higher transmission rate. it can be used for longer distance. Fiber Optics in The 20th Century • • • • • Telephone Systems Internet Video Feeds Medical Operations Home Theater Systems Losses in optical fibres The predominant losses in optic fibres are: –absorption losses due to impurities in the fibre material –material or Rayleigh scattering losses due to microscopic irregularities in the fibre –chromatic or wavelength dispersion because of the use of a non-monochromatic source –radiation losses caused by bends and kinks in the fibre –modal dispersion or pulse spreading due to rays taking different paths down the fibre –coupling losses caused by misalignment & imperfect Summer Presentation 38 surface finishes Advantages of fiber optics Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber Immunity to Noise - Immune to electromagnetic interference (EMI). Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline. High Security - Impossible to “tap into.” Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation) Reliability - More resilient than copper in extreme environmental conditions. Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is physically very flexible. Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult. Disadvantages • Different Signals • New system required for Fiber Optics Thank you Any Question please 41 Thank you! SOURCES: http://bsfp.media-security.ru/school2/10.htm http://www.100top.ru/encyclopedia/article/?articleid=28743 http://www.holophile.com/history.htm http://www.britannica.com/ http://www.litiholographics.com/technology/tech_producing.htm http://vcs.abdn.ac.uk/ENGINEERING/lasers/laser1.jpg http://www.holo.com/holo/book/book1.html http://www.hmt.com/holography/ http://www.art-in-holography.org/ http://inventors.about.com/library/inventors/blholography.htm Reference http://www.wordiq.com/definition/Laser http://www.asamnet.de/~birners/public_html/laser.html http://vcs.abdn.ac.uk/ENGINEERING/lasers/lasers.html Some links: lasers http://www.lasersurplus.com/lasers.htm http://www.midwest-laser.com/html/ilt5490a___ilt5470k.html http://www.hhr-lasers.com/ http://www.coherentinc.com/ http://www.spectra-physics.com/ http://www.continuumlasers.com/ How lasers work http://technology.niagarac.on.ca/courses/tech238g/Lasers.html http://www.clf.rl.ac.uk/Public/index.htm http://www.m2m.ecs.soton.ac.uk/Default.asp?id=283