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EE 230: Optical Fiber Communication Lecture 5 From the movie Warriors of the Net Attenuation in Optical Fibers.

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Presentation on theme: "EE 230: Optical Fiber Communication Lecture 5 From the movie Warriors of the Net Attenuation in Optical Fibers."— Presentation transcript:

1 EE 230: Optical Fiber Communication Lecture 5 From the movie Warriors of the Net Attenuation in Optical Fibers

2 Attenuation/Loss In Optical Fibers Mechanisms: Bending loss Absorption Scattering loss dBm refers to a ratio with respect to a signal of 1 mW

3 Bending Loss Fiber Optics Communication Technology-Mynbaev & Scheiner Example bending loss 1 turn at 32 mm diameter causes 0.5 db loss Index profile can be adjusted to reduce loss but this degrades the fibers other characteristics Rule of thumb on minimum bending radius: Radius>100x Cladding diameter for short times 13mm for 125  m cladding Radius>150x Cladding diameter for long times 19mm This loss is mode dependent Can be used in attenuators, mode filters fiber identifier, fiber tap, fusion splicing Microbending loss Property of fiber, under control of fabricator, now very small, usually included in the total attenuation numbers

4 Bending Loss in Single Mode Fiber Mode Field distributions in straight and bent fibers Microbending Loss Sensitivity vs wavelength Bending loss for lowest order modes

5 Bending Loss Outside portion of evanescent field has longer path length, must go faster to keep up Beyond a critical value of r, this portion of the field would have to propagate faster than the speed of light to stay with the rest of the pulse Instead, it radiates out into the cladding and is lost Higher-order modes affected more than lower-order modes; bent fiber guides fewer modes

6 Graded-index Fiber For r between 0 and a. If α=∞, the formula is that for a step-index fiber Number of modes is

7 Mode number reduction caused by bending

8 Absorption In the telecom region of the spectrum, caused primarily by excitation of chemical bond vibrations Overtone and combination bands predominate near 1550 nm Low-energy tail of electronic absorptions dominate in visible region Electronic absorptions by color centers cause loss for some metal impurities

9 Electron on a Spring Model Mechanical Oscillator Model Response as a function of Frequency

10 E-Field of a Dipole

11 Vibrational absorption When a chemical bond is dipolar (one atom more electronegative than the other) its vibration is an oscillating dipole If signal at telecom wavelength is close enough in frequency to that of the vibration, the oscillating electric field goes into resonance with the vibration and loses energy to it Vibrational energies are typically measured in cm -1 (inverse of wavelength). 1550 nm = 6500 cm -1.

12 Overtones and combination bands Harmonic oscillator selection rule says that vibrational quantum number can change by only ±1 Bonds between light and heavy atoms, or between atoms with very different electronegativities, tend to be anharmonic To the extent that real vibrations are not harmonic, overtones and combination bands are allowed (weakly) Each higher overtone is weaker by about an order of magnitude than the one before it

13 Overtone absorptions in silica Si-O bond fairly polar, but low frequency 0→1 at 1100 cm -1 ; would need six quanta (five overtones) to interfere with optical fiber wavelengths OH bonds very anharmonic, and strong 0→1 at 3600 cm -1 ; 0→2 at 7100 cm -1 ; creates absorption peak between windows

14 Attenuation in plastic fibers C-H bonds are anharmonic and strong, about 3000 cm -1 First overtone (0→2) near 6000 cm -1 Combination bands right in telecom region Polymer fiber virtually always more lossy than glass fiber

15 Absorptive Loss Hydrogen impurity leads to OH bonds whose first overtone absorption causes a loss peak near 1400 nm Transition metal impurities lead to broad absorptions in various places due to d-d electronic excitations or color center creation (ionization) For organic materials, C-H overtone and combination bands cause absorptive loss

16 Photothermal deflection spectroscopy HeNeDetector Arc lamp Lock-in amplifier Chopper Lens Sample cuvette

17 Scattering loss: from index discontinuity Scatterers are much smaller than the wavelength: Rayleigh and Raman scattering Scatterers are much bigger than the wavelength: geometric ray optics Scatterers are about the same size as the wavelength: Mie scattering Scatterers are sound waves: Brillouin scattering

18 Raman scattering A small fraction of Rayleigh scattered light comes off at the difference frequency between the applied light and the frequency of a molecular vibration (a Stokes line) In addition, some scattered light comes off at the sum frequency (anti-Stokes)

19 Mie scattering from dimensional inhomogeneities Similar effect to microbending loss Mie scattering depends roughly on λ -2 ; scattering angle also depends upon λ In planar waveguide devices, roughness on side walls leads to polarization- dependent loss

20 Teng immersion technique DetectorMotor stage Tunable IR laser Lock-in Amplifier Chopper

21 Intrinsic Material Loss for Silica Rayleigh Scattering ~ (1/ ) 4 Due to intrinsic index variations in amorphous silica

22 Spectral loss profile of a Single Mode fiber Fundamentals of Photonics - Saleh and Teich Spectral loss of single and Multi-mode silica fiber Intrinsic and extrinsic loss components for silica fiber

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