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Chapter 3 Fiber Optics and Integrated Optics Gradient-index optics the refractive index is the function of space Fiber optics Optical wave-guide, tele-communication Integrated optics miniaturized optical system

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True or false statement: The light travels in the straight line in the air. (1) True (2) False n refractive index -- density of the air Ttemperature of the air

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How does light travel? If n=constant; Light travels in straight line If n varying in space; Light travels in curved line! It follows the law of refraction!refraction

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3.1 Gradient Refractive Index 1.Atmospheric refraction The light is bending towards the higher index side Sun rising & setting The true position of the sun is lower than what you see Looming Lift up the image Mirage Images formed as if there is a pool of water!

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2. Gradient index lenses Conventional lens: refraction takes place only at the surface of the lens. Gradient lens: refraction takes place within the lens. Advantages of gradient lens: Correct some aberrationreplace the aspherical lens. Can produce very small lenshard to manufacture in traditional way. Simplify the optical system a gradient lens can replace a number of homogeneous lenses.

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a. Radial gradient lenses The index of refraction varies as a function of distance from the optical axis. Cylindrical symmetry Positive lens: n higher in the center Negative lens: n higher in the periphery The end surface can be : plane or spherical for additional power. r z Optical axis O

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For a positive radial gradient lens, when the shape of the lens is a cylinder, what will be the distribution of the refractive index? What about a negative lens? (2) n higher in the periphery (1) n higher in the center

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b.Axial gradient lenses The surfaces of constant index are planes and normal to the axis. Correction of spherical aberration Conventional lens: marginal ray bends more center ray bends less gradient lens: index is higher near the front higher index material is removed in periphery marginal ray bends less z r Optical axis O

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c. Spherical gradient lens The index of refraction varies symmetrically about a point. The surfaces of content index are spheres. Example: Crystalline lens of the human eyehuman eye

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GRIN is short for graded-index or gradient index. It refers to an optical element in which the refractive index varies. More specifically (from the Photonics Dictionary) a GRIN lens is a lens whose material refractive index varies continuously as a function of spatial coordinates in the medium. Also, a graded-index fiber describes an optical fiber having a core refractive index that decreases almost parabolically and radially outward toward the cladding. GRIN

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GRIN lenses come in two basic flavors: RADIAL or AXIAL which are sometimes refered to as RGRIN and AGRIN respectively. RGRINS are usually used where you want to add optical power to focus light. An RGRIN with flat surfaces can focus light just as a normal lens with curved surfaces does. Thin RGRIN lenses with flat surfaces are known as WOOD lenses, named after the American physisist R.W. Wood who did a lot of experimental work with radial gradients from about 1895 to 1905 and included descriptions of how to make them in his physics text book (available from OSA).

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d.Manufacture of gradients Methods available: neutron irradiation chemical vapor deposition polymerization ion stuffing ion exchange Ag+ diffuse into the glass replace Na+ n(40h) Theoretically: n=0.15 Practically: n=0.05

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What is an Optical Fiber? An optical fiber is a waveguide for light consists of : coreinner part where wave propagates cladding outer part used to keep wave in core bufferprotective coating jacketouter protective shield can have a connector too 3.2 Fiber Optics

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Two types of fiber: step-index; gradient fiber Structure : core(higher n) ; cladding(lower n) Total internal reflection

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Types of Fibers step-index multimode ncnc ncnc nfnf ncnc ncnc nfnf ncnc ncnc nfnf step-index singlemode GRIN

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1. Step-Index fiber NA of a Fiber The NA defines a cone of acceptance for light that will be guided by the fiber

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90- tt max nfnf ncnc must be > critical angle nini NA in air

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NA changes with n air water

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NA is sensitive to n 5% change 1% change

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NA and Acceptance angle ii air water

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Two types of fiber with different propagation modes: single-mode fiber: only single mode is permitted small core diameter: 8.3(core) /125(cladding) m Multi-mode fiber: several modes are permitted large core diameter: 50~62.5(core) /125(cladding) m

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Types of fiber ends beam patterns can be: spherical cylindrical bundles 90 degree

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Fiber-optic Cable Many extremely thin strands of glass or plastic bound together in a sheathing which transmits signals with light beams Can be used for voice, data, and video

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Angle Preservation In an ideal fiber, the angle of incidence will equal the exit angle. 22 22 22

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example: critical bend radius Rough surfaces, bending, and other real-world imperfections will case a change in the exit cone.

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Fiber Tapers 11 22 d1d1 d2d2 way to change the acceptance angles of a fiber sometimes used to collimate light

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2. Gradient-Index Fiber Simplification: continuous n change discrete layers of n From Snells refraction law: At the nth boundary, at the distance R from the axis: Therefore: With n(R) Sin I(R) I(R) Until: Sin I (R ) =1 I(R) =90 , The ray return back to the center( optical axis)

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Additional Fiber Types (All single mode)

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3. Applications A. Transmission of light & image to illuminate hard to reach places; to conduct light out of small places Inside heart, digestive tract, stomach, respiratory tract, lung, etc.

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B. Tele-communication

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advantages: a.light in weight efficient use of space in conduits b.less expensive c.Free from electrical interference aircraft, military, security d.Flexible e.Secure to interception f.Low power lost g.enormous capacity of transmission: WDM/ DWDM(Dense Wavelength Division Multiplexing) Higher data rates over longer distances-- more bandwidth for internet traffic Problem remained: Attenuation: power lost ( minimum at 1.55 m) Dispersion: modal, material (minimum at 1.31 m),

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Types of Dispersion in Fibers modal- time delay from path length differences - usually the biggest culprit in step-index material - n( ) : different times to cross fiber -(note: smallest effect ~ 1.3 m) waveguide - changes in field distribution -(important for SM) non-linear - n can become intensity-dependent NOTE:GRIN fibers tend to have less modal dispersion because the ray paths are shorter

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Effect of Modal Dispersion time modal example:step index~ 24 ns km -1 GRIN~ 122 ps km -1 initial pulsefarther downfarther still

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4. Bel & Decibel (dB): Comparative unit Input: 1 output: 2 Attenuation Bel Decibel (dB) 2= 10 1 1Bel 10dB 2= 100 1 2Bel 20dB 2= 1 0Bel 0dB 2= 1/2 10lg0.5 -3dB

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Fibers are made of glass - commonly high-quality fused silica (SiO2) - some trace impurities (usually controlled) Losses due to: - Rayleigh scattering (~ -4 ) - absorption - mechanical stress - coatings

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Attenuation Profiles absorption and scattering in fiber page 297 Rayleigh Scattering IR absorption 89% transmission

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Fiber loss: (dB/km) Where, L1,L2 distance from the start of the fiber, L1 1, L2 2 20years ago: -20dB/km was thought to be the limit Now: -0.2dB/km fiber is commonly used Single-mode fiber: 50~100KM Multi-mode fiber: 2~4KM

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Dispersion: The Basics Light propagates at a finite speed fastest ray slowest ray slowest ray: one entering at highest angle (high order mode) fastest ray: one traveling down middle (axial mode) there will be a difference in time for these two rays

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Coupling with Lenses n 3 n 1 n 2 Edge coupling using a lens. i /2 o /2 n 2 n 3 n 1

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Coupling with Prisms Commercial applications? (sensors) Research labs Optical fiber tap p n p Prism Field Prism coupling. The n3n3 region is typically air. n 3 n 2 n 1 Z Film Field

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Review 1.Optical fibers carry modes of light 2.Step-index, GRIN, single mode & multimode 3.NA is related to acceptance cone and ns. 4.How Step-index and GRIN fibers propagate light. 5.Factors that change light propagation in fibers: a.mechanical aspects (bending, tapers, etc) b.attenuation c.dispersion

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3.3 Integrated Optics Integrated optics Integrated circuit

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Integrated optics offers a particularly interesting candidate for implementing parallel, reversible computing structures Integrated optics (integrated wave-guide): Miniature dimension of fiber optics usually manufactured in the way of thin film ( thickness in the order of wavelength) planar guideswide strip guidesnarrow Beam couplers: guide light to enter the thin film

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PLC Planar Lightwave Circuits Si Wafer Bottom Cladding Top Cladding Waveguide CoreGuided Light Si SiO 2

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Coupling with Prisms

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Grating Couplers (Input and Output)

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1.Integrated prisms Thin film prisms Thin film prisms thinner film effective velocity of light thicker filmeffective velocity of light Refractive gradient prisms light bend towards the high index side.

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2. Thin film lenses Luneburg lens a flat circular mound Index being highest in the center, decreasing towards periphery Geodesic lens dome shaped film: uniform thickness rays follow the shortest path between two points on a surface

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3.Other Integrated Elements Light modulators: operate on amplitude,phase, frequency,state of polarization Electrical signal change Light direction change Light switches, deflectors, Light scanners 4. Manufacture Earlier way: vacuum depositionTaO, LiNb coated on a substrate Modern ways: diffusion techniques ion implantation proton bombardment electron or laser writing

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Monolithic integrated optics Light source, light guiding, modulating, detection are performed in a single crystal GaAs gallium arsenide semiconductor material Fiber optical gyroscope

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Advantages of Integrated-Optic Circuits: Small size, low power consumption Efficiency and reliability of batch fabrication Higher speed possible (not limited by inductance, capacitance) parallel optical processing possible (WDM) Substrate platform type: Hybrid -- (near term, use existing technology) Monolithic -- (long term, ultimately cheaper, more reliable) quartz, LiNbO, Si, GaAs, other III-V semiconductors

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homework 2,3,5 Translation(E to C)