Proceedings Article | 14 April 2020
KEYWORDS: Monochromators, Luminescence, Digital micromirror devices, Spectroscopy, Mirrors, Sensors, Time resolved spectroscopy, Fluorescence spectroscopy, Spherical lenses, Spatial light modulators
Fluorescence-lifetime spectroscopy has been recently used in different applications in rapid medical diagnoses for different diseases, e.g. cancer, oral carcinoma, actinic cheilitis and tissue diagnosis. In addition, it has various applications in biology, chemistry analysis, pharmaceutical applications and physics. The existing fluorescence spectrometer systems are bulky, contain complex optical setups, large and use either monochromators with movable parts, hence less mechanically stable, or use non-tunable bandpass filters. Therefore, the user has to adjust the filters while changing the fluorophore. In this work, we propose a spectrometer setup that can be tuned for different emission wavelength using spatial light modulators (SLM). SLMs use highly stable MEMS micromirrors in their architecture, hence, the system has no large moving parts and is mechanically stable. In addition, the system is suitable for more fluorophores and thus widens the application potentials of the system. Digital micromirror devices (DMD) are widely used in optical projection systems. However, some recent developments were made to use them in conventional optical spectroscopy which enabled using single point detectors to collect the spectrum instead of using linear or area camera sensor arrays. This is expected to have a big effect on the cost of the spectrometer. Using similar techniques, in this work, a light source, e.g. a supercontinuum laser was used along with dichroic filters to fix the excitation wavelength at a certain value. After that, the selected wavelength(s) pass through a beam splitter towards the sample. The beam splitter is installed to separate the emission and excitation light. The fluorescence emission is reflected from the beam splitter and is focused on a diffraction grating. The different wavelengths are projected to the DMD. The used DMD has a VGA resolution with a mirror pitch of 5.4µm. Each micromirror could be controlled individually into two states, on and off at ±17 degrees. The setup ensured that different wavelengths of light always fall on different mirror(s) and hence the desired wavelength(s) could be selected at once. A high-speed-gated single pixel CMOS detector with a large diameter was used to capture the reflected light with the desired wavelengths from the DMD. The system scans the mirrors and hence the wavelengths to readout the spectrum. In addition, the system measures the fluorescence lifetime of the used fluorophore at the emission wavelength. The proposed setup is compact and less complicated than conventional systems. Furthermore, the spectrometer, apart from the light source, has a great potential to be miniaturized. Thus, it can be used as hand-held system. The proposed system was characterized and the proof of concept of the setup was verified. Due to the small pitch of the DMD and the large number of mirrors, it was found that it has a high resolution especially in the VIS spectral range. The system is considered as semi-tunable as it can be tuned for different emission wavelengths and thus, can be used with different fluorophores. However, the light source along with the filters should be changed for different excitation wavelengths. or pulsed laser diodes can be used.
