| With the development of optical manufacturing technology,small aperture optical elements have been widely used.By measuring the wavefront distortion of small aperture optical elements,we can evaluate their optical performance effectively and reduce aberrations of optical system.At present,most methods of measuring wavefront distortion are based on Fizeau interferometer or Twyman-Green interferometer,by which the error introduced by double-pass optical path cannot be avoided.For this reason,a method based on Mach-Zehnder interferometer is proposed in this paper.In this method,the test beam passes through the optical element to be measured only once,so it can eliminate errors introduced by double-pass methods.This method can also be used to reduce diffraction fringes on the edge of optical elements in order that a high precision measurement of small aperture optical elements can be realized.In order to simplify the structure of the interferometer and reduce the cost of production,wavelength-tuning interferometry is used.However,wavelength-tuning interferometry cannot be applied to traditional equal-path Mach-Zehnder interferometer directly.In addition,wavelength-tuning interferometry cannot avoid phase shift error,and the contrast of fringe pattern changes with the injection current of the laser.These issues need to be addressed urgently.In order to develop a set of wavelength-tuning Mach-Zehnder interference system,this paper focuses on the above key technologies.In order to solve the problem that wavelength-tuning interferometry cannot be applied to traditional equal-path Mach-Zehnder interferometer directly,the dynamic range and resolution of tunable semiconductor lasers are studied.Two unequal-path wavelength-tuning MachZehnder optical paths are proposed.A retarder,such as a parallel flat or a right-angle prism,is introduced into the test arm,which increases the optical path difference.A software for interferometric optical testing was developed and applied to the wavelength-tuning MachZehnder interferometer in this project.Anyone can control the hardwares or analyze data easily by the software.The copyright of this software was obtained.This project finally developed a set of wavelength-tuning Mach-Zehnder interference system.The working wavelength is635 nm,the effective aperture is 10 mm,and the maximum length of measurable optical elements is 200 mm.The repeatability of measurement is better than λ100(PV).In order to reduce the phase shift error in wavelength-tuning interferometry,a phase-shift unwrapping algorithm is proposed.The algorithm can unwrap phase-shift by calculating the increment of the phase-shifte between two adjacent frames.Based on this,a wavelength-tuning step current correction algorithm is designed.The step current will be adjusted in real time by calculating phase-shift.Experiments show that the algorithm can calibrate the phase-shift of different phase-shift periods,reducing the phase shift error.In order to solve the problem of contrast variation,the contrast theory of interferogram is studied.The spectrum of the semiconductor laser was measured by spectrometer,and its central wavelength and spectral bandwidth are analyzed.The power of the semiconductor laser was measured.The relationship between power and injected current was analyzed.Fringe pattern intensity normalization algorithms are studied including Max-min algorithm and polynomial fitting algorithm,and the effectiveness of the algorithms are verified by simulations and experiments. |
