Supervisor: Prof K. Abramski States of polarization of chosen fiber elements

Supervisor: Prof K. Abramski

States of polarization of chosen fiber elements

Table of contents Introduction

The Pointcaré sphere

States of polarization

Matrix interpretation of polarization states

Geometrical interpretation of Stokes parameters


(Degree of polatization)

Measurement with Polarimeter

Polarization maintaining fibers

Optimized exctinction ratio measurement

Type’s of polarization controllers

Measurements on polarization controllers

Conclusion

Introduction

Erasmusstudent from Belgium

Finishing my studies Master in electronics

Most interesting parts of my Msc project will be explained


Consider a monochromatic plane wave:

We describe the light by the transverse components of its electric field:


Light is linearly polarized if the field components Ex and Ey oscillate in phase or 180° out of phase.

y

x

E

Ex

Ey

θ


For complex Ex and Ey , the oscillations of the field components along the horizontal and vertical directions are generally not in phase, and we can write:

Ey

Ex

Eεy

εx

Ey

Ex

εy

εx

Ey

Ex

EEy

Matrix representation of polarization states

Matrix approach to describe the polarization of light

The polarization changing characteristics of a device can be represented by a matrix

The Jones vectors

Useful to describe the polarization behavior of coherent light. The matrix form is

Disadvantage: Unpolarized light cannot be characterized in terms of the Jones vectors


The Stokes parameters

Carries complete information on the intensity and state of polarization of a plane wave

For monochromatic light, the amplitude and phase factors are time independent and the Stokes parameters satisfy the condition


The Stokes parameters

S0 measures the total intensity of the beam

S1 gives the extent by which the intensity of horizontal polarization exceeds the intensity of vertical polarization in the beam

S2 determines the excess of the intensity of +45°-polarization over the intensity of -45°-polarization

S3 estimates the excess of the intensity of right circularly polarized light of the intensity of left circularly polarized light


The stokes parameters of completely polarized light can be expressed in a form that makes appear as the Cartesian components of , treated as a polar vector.

The above equations bear close resemblance to the relationships among the Cartesian and spherical polar components of the position vector



It is a sphere of unit radius in a space spanned by the normalized Stokes parameters

Each point on the surface of the Pointcaré sphere represent a unique state of polarization


Points in the equator represent all possible states of linear polarized light

Unpolarized light can be represented by a point inside the sphere


Device that measures the state of polarization

Test set-up:


Result:

Polarization maintaining fibers (PMF) Manufactured with intentionally induced stress

The difference of the effective refractive indices for the two orthogonal field components is high

small changes of the refractive indices can be neglected

Inportant:

Use linear polarized light

Correct azimuth orientation

Polarization maintaining fibers (PMF)

The standard is to align the slow axis of the fiber with the connector key

There are also some other possibilities for alignment:

Slow axisFast axisSpecified by the costumerFree


Extinction ratio

A PMF is only effective if linear polarized light is launched parallel to a main axis

A dimension for the quality of this coupling is the ER

If the ER is poor then either

The PMF has a poor polarization preserving capability

The alignment into the PMF is not optimal.


ER Measurement with Polarimeter

It uses an optimized algorithm

The recorded values during fiber stressing are used to fit a circle on the Poincaré sphere (Pancharatnam theorem)

The smaller the circle the higher is the ER


Measurement in the lab

I used a PMF from Optokon ER in datasheet: 25dB

How to stress the fiber?

By pulling the fiber -> unsuccessful

By heating the fiber -> successful





Polarization controllers

The free-space optics approach

A classic polarization controller consisting of three rotatable wave plates

This approach have produced respectable results.

Polarization controllers

The free-space optics approach

Disadvantages:

Collimating, aligning and refocusing are time consuming and labor intensive.

The wave plates and microlenses are expensive

High insertion loss

Sensitive to wavelength variations

Limited controller speed

Polarization controllers The fiber coil (mickey mouse ears) approach

An all-fiber controller based on this mechanism reduces the insertion loss and cost

Coiling the fiber induces stress, producing birefringence


The amount of birefringence is a function of:

The fiber cladding diameter

The spool diameter (fixed)

The number of fiber loops per spool

The wavelength of the light

Not a function of twisting the fiber paddles!!

The fast axis of the fiber is in the plane of the spool


Disadvantages:

Sensitive to wavelength variations

Limited controller speed

A bulky device (the fiber coils must remain large)

The use is primarily limited to laboratories

Polarization controllers The electro-optic waveguide approach

LiNbO3 based high-speed polarization controllers

Two voltages and the electro-optic effect determine the effective optical axis of each wave plate

Polarization controllers The electro-optic waveguide approach

Disadvantages:

High insertion loss

High polarization-dependent loss

High cost

Expensive and complicated implementation

Measurments on Polarization controllers Polarisazation controller 1 (Thorlabs)

Based on the fiber coil approach

Consist of QWP, a HWP and a QWP

Measurement set-up:

Measurments on Polarization controllers Results:

You can create all type’s of polarizations

Measurments on Polarization controllers Polarisazation controller 2 (Fiberpro)

Based on the fiber coil approach

Consist of two QWP

You can create all type’s of polarizations

Conclusion Msc project is finished

Learned a lot about optics

Thank you for your attention

Documents

Supervisor: Prof K. Abramski States of polarization of chosen fiber elements