Micromachined tunable diffraction grating for diverse imaging in integral field spectroscopy
Sanathanan, Muttikulangara Swaminathan
Date of Issue2018-12-31
School of Mechanical and Aerospace Engineering
This report presents the design and development of micro-optical components for a computational imaging spectrometer. The main elements that were involved in the optical design include a microlens array that samples the spatial information and a diffraction grating that provides spectral information. One of the significant drawbacks of scanning based imaging systems is the low light collection efficiency. The new technique proposed in this study used a quasi-snapshot technology that significantly improves the optical throughput. In a quasi-snapshot system, multiple images are captured with the optical system that can tune its optical transfer function (OTF). In this work, tunable micro-optics were investigated for the realization of the OTF modulation. In snapshot imaging spectrometers, there is a trade-off between spatial sampling (x, y) and the measured wavelength band (∆λ). This system attempted to overcome this trade-off by using computational methods and measurement diversity. In this technology, a microlens array was tightly packed to improve the spatial resolution. The close packing of the microlens array resulted in cross-talk of spectral information between the neighboring spatial pixels in the image sensor. An inverse algorithm was employed to retrieve the spectral information of the corresponding spatially separated microlenses. Optical diversity was incorporated using dispersive tuning elements, such as diffraction grating, diffractive optical element, or prism. Multishot images were captured by tuning, which changes the OTF enabling to obtain spectral-spatial data with a small image sensor. Optical simulations were carried out using ray tracing for a set of wavelengths, obtaining a multiplexed spatial-spectral image. The architecture allowed mixing of the spectral data with the neighboring microlenses, dramatically improved camera pixel utilization. By appropriate mixing and prior knowledge of the signal, information can be extracted. The output of the optical image captured by tuning the diffraction grating changes the mixing that allows measurement diversity. Tuning the diffraction grating by modulating the pitch was realized using microelectromechanical systems (MEMS) technology. The tuning of the pitch was designed and modeled by MEMS comb drive based electrostatic actuation with silicon beams as the grating grooves. Tuning introduces an angular dependent shift in the dispersion based on diffraction relation. Both, theoretical and finite element method analyses were carried out for the mechanical mass-spring design of the suspended grating structure. The device was fabricated using surface micromachining technology on an silicon-on-insulator (SOI) wafer. The expected performance of the device was compared with the tested results to investigate fabrication tolerance and feasibility. Optical experiments were carried out to observe the shift in diffraction angle with electrostatic actuation. Further, a reliability study was conducted on the device by subjecting it to mechanical vibrations. An alternative approach of tuning the diffraction grating is by rotation, and was implemented by MEMS technology. The rotation of the diffraction grating introduces changes to the direction of dispersion. To improve the optical performance, the rotary actuator and the diffraction grating were fabricated separately. The rotary actuator was realized by SOI technology, and the actuation through electrostatic three-phase stepper motor principle. The rotating actuator works with the move and hold mechanism that allows precise angular movement with high repeatability. The stepping motion mechanism allows the device to work in a non-resonant mode, that benefits the usage of conventional cameras such as CCD/CMOS, where the frame capture speed is limited. Diffraction gratings were fabricated using negative photoresist, SU-8 by replication technique allowed realization of a blazed angle profile, resulted in high diffraction efficiency. Finally, the technology for bonding the replicated diffraction grating onto the electrostatic stepper motor using UV curable glue was proposed. The multishot imaging technique is a middle ground between scanning technology and snapshot imaging, preserving the advantage of light collection efficiency. The tunable diffraction gratings to implement diversity of optical transfer function in imaging systems opened a new pathway to carve full fledge use of MEMS devices with optics than the traditional scanning based systems.
DRNTU::Engineering::Electrical and electronic engineering::Microelectromechanical systems