Birefringence Measurements

A Class Seminar in Fiber Optics PHY904

Shuvan Prashant T.

Outline

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

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

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

Beats

z1

z3

z2

Beat Length : Distance after which SOP repeats itself

L = 2π/∆β = 4z1

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.

Setup

Laser Polariser QWP LensCladding

Mode Stripper

Index Matching

Liquid

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.

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)

Setup

SOLENOIDQWP

LASERSBC

LOCKIN Amplifier

QWP

POLPD

Lens Lens

Lens

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

Setup

\θ

\ψ A/

n

\θ1

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

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

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.

Windowed Fourier Transformation of OSA trace

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

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.