Research On Digital Micro-mirror Devices Based Hadamard Transform Infrared Spectrometer

With the development of chemometrics and computer technology, the application of near-infrared spectrometer has ranged from agriculture to pharmaceutical industry, petrochemical products, textiles and defense fields, which makes it a well-established and important analytical method to achieve quality monitoring and product analysis. Because of its high speed, high signal to noise ratio and high resolution, Hadamard transform spectrometer has become an important research topic among the miniature near-infrared spectrometer area. With the continuous improvement of the demand for research and applications, there’s one problem which needs to be solved urgently is that how to improve the energy efficiency and working spectral range of a near-infrared spectrometer while maintaining the high spectral resolution of the system. The thesis focuses on this issue and aims to achieve high resolution, high energy efficiency and wide spectral wavelength range at the same time by redesigning the structure of the spectrometer, to meet the requirement of the scientific reseach and applications.Since the resolution of the Hadamard transform spectrometer is relatively low at present, the relationship among the incident slit, the collimating lens and the imaging lens are researched. The diffraction efficiency varies with the wavelengths and incident angles of the incident rays are simulated as well. Then by making a trade of the volume and resolution, we specify the optical parameters of the system and optimize the optical system by software. The multi-order diffraction of the grating is studied as well. Then a prototype is designed and constructed to verify the simulations, the resolution measured in the experiment is consistent with the simulation results. The portable Hadamard transform spectrometer is proved to be compact with smaller volume and high resolution.To improve the system flux, two novel Hadamard transform spectrometers are proposed and designed. For the first kind of improvement, a light collecting structure is added in front of the incident slit. The circular incident light beam is redistributed into a stripe beam paralleling with the incident slit by a cylindrical lens. The experiment results show that this device improves the system flux by 14.2% when other parameters are the same. To avoid the block of the incident slit, optical fiber is used to replace the incident slit in the second kind of spectrometer. Cylindrical lens is adopted as the imaging lens, which reshapes the circular beams diffracted by the grating into stripe images on the DMD, to improve the energy efficiency and the resolution at the same time. A M-symmetrical optical design approach is adopted during the design, and the diameter of the spot focused on the single element detector is 180 ?m, which makes the spectrometer smaller and cheaper.To expend the working wavelength range of Hadamard transform spectrometer, two novel spectrum-folded spectrometers are proposed. The first spectrometer folds the broadband spectra from 800 nm to 2000 nm on DMD by using two sub-gratings with different incident angles. The diffraction efficiency of the grating varies with wavelengths and incident angles are simulated to determine the optimum incident angles of the two sub-gratings. The simulation results show that the uniform diffraction efficiency over the entire band is improved by using two sub-gratings. After being optimized in ZEMAX, the resolution of the spectrometer in wavelength of from 800 nm to 2000 nm is better than 5.5 nm. The multi-order diffraction of the two sub-gratings and the 2-demention diffraction of the DMD are studied. The second kind of spectrometer achieves the spectra fold on DMD by using a freeform lens which collimates the light from the source into two beams with different wavelengths and different directions. The working wavelength range is from 800 nm to 2400 nm. The freeform lens is designed with tailoring approach which is usually used in non-imaging optics design. Numerical difference method is adopted to solve the two partial differential equations to get the discrete points of the freeform surface, and the freeform lens is constructed in CAD software. The resolution is about 10 nm over the entire wavelength band. The advantages in performance of the second kind of spectrometer will be much more obvious when the number of the spectrum-folding increases. The results show that, by folding the broadband spectrum on DMD, the limitation of the fixed size of DMD can be avoided and high resolution and broad working wavelength range can be achieved at the same time.