EE 230: Optical Fiber Communication Lecture 5

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

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 • 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 125mm 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 Fiber Optics Communication Technology-Mynbaev & Scheiner

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

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 Response as a function of Frequency Mechanical Oscillator Model

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. Arc lamp Lock-in amplifier Chopper Lens HeNe Detector Sample cuvette Photothermal deflection spectroscopy

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. Tunable IR laser Chopper Lock-in Amplifier Detector Motor stage Teng immersion technique

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

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