Applications of Interferometric Measurements and Photoacoustic Detection in Optical Metrology

Measurement loops and detectors are irreplaceable constituents of an experimental process in optical metrology. Interferometry and photoacoustic methods can be applied as key techniques for developing such measurement and detection systems. In this thesis, two interferometric measurement arrangements are presented. The phase modulation process in interferometry is important for the measurement speed. Mechanical phase modulation, in such cases, suffers from limitations with regard to driving voltage amplitude, increased modulation frequency and system noise. In this work, the potential of electro-optic phase modulation has been assessed for developing a multi-wavelength interferometric sensor to replace the mechanical phase modulation system. The results not only show the suitability of the electro-optic sensor to improve the measurement speed of the multi-wavelength interferometer, but the sensor is also able to operate at considerably low driving voltage. Another interferometric method driven by an optical system consisting of a mirror array has been presented in this thesis to measure the surface parallelism of step gauge faces. Contact methods are commonly used for this purpose. However, the research gap lies in the available options for non-contact methods to carry out such measurements. The alignment sensitivity is a major factor that controls the accuracy of the measurement with the presented array of mirrors which are arranged as a periscope and a triangular prism reflector-type configuration. Hence, in this work, methods to monitor the alignment sensitivity and estimate corresponding corrections have been presented. The theoretical and experimental studies on the performance of the custom-built optical system show that the system has the potential to serve as a suitable tool for non-contact surface parallelism measurement of step gauge faces. For the development of detection instruments, a silicon cantilever-based photoacoustic radiation detector is presented in this work. Photoacoustic detectors are widely employed for power meter applications, however, in most available cases, the pressure sensors have limitations on the highest detectable radiation power. The photoacoustic detector presented in this work, has shown radiation detection capability with a linear dynamic range of nearly six orders of magnitude and highest detectable power in the milliwatt level which can be further extended with suitable adjustments of the detection parameters. The spectral coverage of the system was initially tested from ultraviolet to infrared region, and in the latest work the radiation sensitivity has been successfully demonstrated in the terahertz range with proper absorber materials. A numerical model for designing the cantilever pressure sensor has also been developed to improve the detection sensitivity. It is concluded that the cantilever-based photoacoustic detector can be a good solution for power measurement applications which require a broad spectral coverage and large dynamic range with robust pressure sensing element offering high damage threshold.