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Highly Birefringent Fibre Based Polarisation Modulated Ellipsometry and Sensor Applications

Gebremichael, Y. (2000). Highly Birefringent Fibre Based Polarisation Modulated Ellipsometry and Sensor Applications. (Unpublished Doctoral thesis, City, University of London)


This work discusses polarisation modulated HiBi fibre based ellipsometry. The polarisation of light is discussed with mathematical representation of polarised light and propagation of polarised light through optical components. Such mathematical representation allows convenient analysis of the interaction of light with matter. The history and evolution ellipsometry as a measuring tool through various optical configurations is reviewed. This gives an insight in to the existing ellipsometric configurations and point out important features such as operation procedure, type of modulation, resolution and speed of measurement, optical components used and application limitations etc. The review concludes polarisation modulation as being an obvious choice for fast real time ellipsometric applications.

High biréfringent (HiBi) fibre based polarisation modulated ellipsometry is discussed in which rotating polarisation is generated by stretching a HiBi fibre. Mathematical analysis of the optical set up is given using Muller and Jones matrix methods. The performance of the system is studied thoroughly and applied for refractive index and film thickness measurements both on bulk (absorbing or non-absorbing) optical samples. The potential for sensor use is also demonstrated with dynamic measurements carried out on oil films and sol-gel based films. The system is simulated to identify sources of errors and their effect of the ellipsometric parameters, Ψ and ∆. The conclusions drawn from the error analysis identify the quarter wave plates as being major sources of errors due to azimuth misalignment and component imperfection. This finding lead to an alternative approach for the ellipsometric configuration.

A modified ellipsometric configuration that does not employ any use of quarter wave plates is introduced. The new configuration enables a measurement technique that is simpler (ratio of intensities) and more accurate with faster data update rates. Polarisation modulation again involves stretching a HiBi fibre longitudinally. The output beam is a phase-generated carrier and is analysed with Bessel functions. Lock-in amplifier feedback control system is implemented to control the polarisation. The new configuration is applied for ellipsometric measurements and results obtained are presented and discussed.

The main advantages of this system include improved accuracy, more compact system with smaller number of optical components and thus cheaper system. The ease of signal processing as simple ratio of intensities is also an important advantage for high bandwidth measurements.

Starting with polarisation theory of light, the thesis discusses the present state of ellipsometry and its limited application arising mainly due to the bulky nature of existing systems and mechanically moving nature of operation. This also limits the bandwidth of the measurement. An all fibre ellipsometer should overcome most of the limitations of existing ellipsometers and with a wider scope of application.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: School of Science & Technology > Engineering > Electrical & Electronic Engineering
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Gebremichael thesis 2000 PDF-A.pdf]
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