This document discusses techniques for measuring birefringence in optical fibers. Birefringence is the difference in refractive indices for different polarization states. The beat length is the distance over which a state of polarization repeats and can be used to characterize birefringence. Several methods are presented: Rayleigh scattering detects polarization-dependent scattering patterns; magneto-optic modulation applies a magnetic field to induce birefringence; prism coupling couples light out of the fiber at angles related to propagation constants; and wavelength scanning analyzes modulation patterns with wavelength to determine beat length and birefringence. Each technique is described along with relevant setup diagrams.

1. Birefringence Measurements
A Class Seminar in Fiber Optics
PHY904
Shuvan Prashant T.

2. Outline
• Birefringence in fibers
• Beat Length
• Rayleigh Scattering Technique
• Magneto Optic Modulation
• Prism Coupling Technique
• Wavelength Scanning

3. Birefringence
 Definition: Perception of two refractive indices for different
states of polarization
 Double refraction
 ∆n = ne - no
 Anisotropy, Ordinary ray, extraordinary ray
 E and D are non parallel
 Examples : uniaxial crystals

4. Beats
• Birefringence in fibers : difference in
propagation constants(or refractive indices)
of the orthogonal polarisation modes:
randomisation
• B = ∆n = ∆(β/k0) = (λ/2π)*∆β
• Special fibers with built-in birefringence
– Elliptic core fibers and stress induced
birefringence
• Example of Elliptic Core Fiber

5. Beats
z1
z3
z2
Beat Length : Distance after which SOP repeats itself
L = 2π/∆β = 4z1

6. Rayleigh Scattering technique
• Light gets scattered out of fiber: Rayleigh
Scattering
• Concept: For a linearly polarized light
undergoing scattering, there is no scattered
wave along the axis of the dipole along the
direction of polarization. Maximum scattering
takes place in plane perpendicular to the
direction of polarization.

7. Setup
Laser Polariser QWP Lens
Cladding
Mode
Stripper
Index
Matching
Liquid

8. Limitations
• Best Suited for highly birefringent fibers with a
beat length of few mm
• Rayleigh Scattering decreases as the fourth
order of wavelength, method is convenient to
be used in the visible region alone.

9. Magneto optic Modulation
• Faraday Effect
• Application of longitudinal magnetic field on a
material induces circular birefringence in the
medium.
• Scan an electromagnet along the fiber 
• Within the fiber
– Circular SOP  No effect
– Linear SOP Maximum Effect
• Modulation of SOP  periodicity of modulation
( Beat Length)

10. Setup
SOLENOID
QWP
LASER
SBC
LOCKIN Amplifier
QWP
POL
PD
Lens Lens
Lens

11. Prism Coupling Technique
• The leaky waveguide mode couples into a
prism placed just above it due to higher index
of the prism.
• Phase matching condition ensures that
direction of out coupled light is characteristic
of the propagation constant of the mode of
the waveguide.
• Different angles for modes with different
betas Modal Birefringence

12. Setup
θ
ψ A/
n
θ1

13. Limitations
• Suitable for high birefringence fibers(Beat
Length < few mm)
• Local measurements of birefringence of
polished fiber : Used for fiber directional
couplers
• Polishing and removal of cladding do change
the birefringence of the fiber

14. Wavelength Scanning
• High Birefringence
• Modulation of spectrum
• Spacing of modulation (∆λ)
• Beat Length = (∆λ/λ)*L
• λ Center Wavelength between
two peaks
• L Sample Length of fiber

15. Wavelength Dependence of Group Beat
length and birefringence
• Perform a windowed Fourier transformation
of the data to retrieve the dominant fringe
frequency around each wavelength.
• Actual period is obtained by dividing the data
spacing by the normalized frequency.
• A linear fitting of the fiber birefringence as a
function of wavelength is conducted.

16. Windowed Fourier Transformation of OSA trace

17. To sum it up
• Birefringence : The random effect is made the
property to control the polarisation. (High
Birefringence Fiber)
• Different techniques used to extract the modal
birefringence or beat length:
– Rayleigh Scattering
– Magneto Optic Modulation
– Prism Coupling
– Wavelength Scanning
• Low Birefringence Measurements

18. References
• Introduction to Fiber Optics, Ajoy Ghatak & K
Thyagarajan, 1st Edition, pg 441-449.
• Wavelength Scanning: “Highly Birefringent
Hollow-Core Photonic Bandgap Fiber”,
Technical Paper, Corning Inc.