This paper presents the fabrication of a tunable Fabry-Perot (F-P) filter array using microelectromechanical systems (MEMS) technology for infrared multispectral imaging and infrared communication. The primary focus of the study is on the design and fabrication process of the F-P filter structure, resulting in the successful creation of a tunable 12×14 MEMS F-P filter array. The experimental findings demonstrate a tunable spectral range spanning from 3.78 μm to 4.89 μm, with a maximum wavelength tuning amplitude of 1.11 μm and a maximum spectral transmittance of 72%. The fabrication procedure demonstrates compatibility with the current CMOS process, hence enabling the incorporation of a tunable MEMS F-P filter module via MOEMS-CMOS heterogeneous integration technology. The MEMS F-P tunable filter, functioning as a dispersive component, exhibits promising potential for various applications such as wavelength division multiplexing, multispectral imaging, and gas sensing spectroscopy.
The tunability of optical transmittance spectra can be available by mounting one of the mirrors of the Fabry-Perrot cavity on a movable structure. The F-P filter prepared by adopting MEMS process can realize the advantages of miniaturization, array, and high output. The size of the MEMS F-P filter can be reduced to a few hundred micrometers. This feature introduces a new problem for the characterization of optical performance, that is, the incident light needs to be focused onto the mirror with a size of a few hundred micrometers. However, in the actual test, the incident light with a hundred-micron spot is usually a convergent beam with a certain cone angle. It is found that through theoretical analysis, compared to parallel incident light, the convergent light beam passed through the F-P cavity leads to the decrease at peak transmittance and the broadening of full width at half maximum. The reason for that was the converging light with a cone angle passing through the F-P cavity had different incident angles and caused diverse optical path difference. As a result, the light emitting from the cavity with various wavelength would appear in the transmission spectra. In summary, the test results under the converging light could not truly reflect the performance of the F-P cavity and the influence of the cone angle of incident light beam on the performance characterization of MEMS F-P filter was analyzed by theoretical arithmetic and simulation.
A sensitive structure with built-in T-shaped beams was studied in the paper to achieve high fill-factor and high sensitivity for Micro-Optical-Electro-Mechanical-Systems (MOEMS) application. The silicon proof mass in the structure was supported by four identical T-shaped beams, which were distributed symmetrically and orthogonally in the plane to suppress the in-plane cross coupling. In particular, the T-shaped beam was composed of three parts: stress releasing structure, cantilever and flexible linking structure. The stress releasing structure was used to avoid torsion or warpage caused by residual stress and improve the sensitivity at the same time. The mechanical properties were studied systematically by finite element simulation. The stiffness in z-axis direction was much lower than the in-plane stiffness of the structure, indicating high z-axis sensitivity and small cross coupling error. The reason for high sensitivity of the sensitive structures was fully illustrated by comparing the displacement responses for different beams. The simulation results indicated that the sensitivity was improved more than twice because the stress releasing structure and flexible linking structure reduced the axial stress caused by deflection. Finally, the optical performances were also evaluated in terms of bandwidth and tuning range when used for MEMS Fabry Perot Optical Tunable Filter. The wavelength tuning range achieved about 1.8μm in long-wave infrared waveband by controlling the applied voltage.
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