Attenuation in Optical Fibers

Attenuation/Loss In Optical Fibers

Mechanisms:

Bending loss

Absorption

Scattering loss

dBm refers to a ratio

with respect to a

signal of 1 mW

out in

Power transmission is governed by the following differential equation:

where is the attenuation coefficientand P is the total power.

P (z)=P exp - Z

is usually expressed in dB/km

( / )

dP Pdz

dB km

out

10in

P10 4.343P

Note that positive means loss

LogL

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 times13mm for 125mm claddingRadius>150x Cladding diameter for long times19mm

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

Bending Loss in Single Mode Fiber

Mode Field distributions in straight and bent fibers

Microbending Loss Sensitivity vswavelength

Bending loss for lowest order modes

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

Graded-index Fiber

For r between 0 and a. If α=∞, the formula is that for a step-index fiber

Number of modes is

arnrn 211

212

aknM

Mode number reduction caused by bending

3/2

2232

221

kRnRaNN straightbent

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

Electron on a Spring Model

Mechanical Oscillator Model

Response as a function of Frequency

E-Field of a Dipole

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.

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

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

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

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

Photothermal deflection spectroscopy

HeNe Detector

Arc lamp

Lock-in amplifier

Chopper

Lens

Sample cuvette

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

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)

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

Teng immersion technique

Detector Motor stage

Tunable IR laser

Lock-in Amplifier

Chopper

Intrinsic Material Loss for Silica

Rayleigh Scattering ~ (1/l)4

Due to intrinsic index variations in amorphous silica

Spectral loss profile of a Single Mode fiber

Fundamentals of Photonics - Saleh and Teich

Spectral loss of single and Multi-modesilica fiber

Intrinsic and extrinsic loss components for silica fiber