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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
The Pointcaré sphere
(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
States of polarization
Consider a monochromatic plane wave:
We describe the light by the transverse components of its electric field:
States of polarization
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
States of polarization
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
Matrix representation of polarization states
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
Matrix representation of polarization states
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
Geometrical interpretation of Stokes parameters
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
Geometrical interpretation of Stokes parameters
The Pointcaré sphere
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
The Pointcaré sphere
Points in the equator represent all possible states of linear polarized light
Unpolarized light can be represented by a point inside the sphere
Measurement with Polarimeter
Device that measures the state of polarization
Test set-up:
Measurement with Polarimeter
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
Polarization maintaining fibers (PMF)
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.
Polarization maintaining fibers (PMF)
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
Polarization maintaining fibers (PMF)
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 maintaining fibers (PMF)
Measurement in the lab
Polarization maintaining fibers (PMF)
Measurement in the lab
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
Polarization controllers The fiber coil (mickey mouse ears) approach
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
Polarization controllers The fiber coil (mickey mouse ears) approach
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