This report presents recent advances in the design and fabrication of a tunable Fabry-Pérot interferometer (FPI) with subwavelength grating (SWG) reflectors, as well as measurement results and applications. The FPI is designed as wavelength selecting element for highly miniaturized mid-wave infrared spectrometers. The optical resonator of the FPI is built between two highly reflecting mirrors. The mirrors are integrated in a supporting MEMS structure with one electrostatically movable and one fixed mirror carrier. The FPI is fabricated in a bulk micromachining batch process on wafer level from two silicon substrates. The substrates are bonded together with an intermediate SU-8 layer. The reflectors are made of aluminum subwavelength gratings, structured on a thin LP-Si3N4 membrane by nanoimprint lithography. The subwavelength structures build a frequency selective surface with high reflectance and low absorbance in a defined spectral range. Simulations and optimization of the design were done using finite element method with a 3D EM frequency domain solver. Comparison of simulation results and measurements of fabricated reflectors and FPIs are in very good agreement. The FPIs are used in the 5th interference order and can be tuned from 3.5 μm to 2.9 μm electrically. The measured maximum transmittance is between 70 % and 50 % and the measured FWHM bandwidth is lower than 50 nm. The new subwavelength grating reflectors can be integrated in a MEMS batch process more cost-efficient than previously used reflectors of dielectric layer stacks
As part of a future optical platform on-chip, we present a waveguide integrated tunable Fabry-Pérot Interferometer (FPI) for the long infrared wavelength range. The FPI consists of two parallel Bragg reflectors that are located at the ends of two waveguides facing each other. The waveguides are made of silicon and are suspended in air. The reflectors are realized as an alternating stack of silicon and air layers with high (H) and low (L) refractive index. The filter transmittance is evaluated by analytic calculations and electromagnetic finite difference time domain simulations. Filters with (HL)² layer stack show a full width half maximum of 270 nm and a peak transmittance of more than 25% at a wavelength of 9.4 µm at the first interference order in the simulation. It is evaluated by measurements.A MEMS actuator is used to tune the filter wavelength by changing the distance between both reflectors. A digital electrostatic actuator concept with a linear drive characteristic, designed for a large travel range up to 4 µm with a driving voltage of less than 30 V, is presented and evaluated together with the filter.The MEMS fabrication process for the structures is based on bonding and deep reactive ion etching (DRIE). The DRIE etch process was optimized, hereafter achieving a reduced roughness of less than 3 nm of the waveguide sidewalls.For transmission measurements the silicon waveguides are coupled to a laser source and a detector using optical fibers together with optical couplers on the chip. The filter performance was characterized in the range from 9μm to 9.4 µm.
Oxygen shows significant absorption lines in the millimeter wave spectrum. Resonators are widely used to achieve a strong absorption even with a short absorption paths length for concentration measurements. A sensor system based on a Fabry-Pérot resonator for oxygen measurements at ambient pressure is presented here. The Fabry-Pérot resonator consists of two metal mirrors with a diameter of 50 mm. For purpose of oxygen detection the resonator covers a frequency range between 55 GHz and 65 GHz with a resonant peak density between 1 GHz and 1.5 GHz, depending on the mirror distance, and a quality factor of approximately 7000. To achieve a compact sensor system the concept envisages two integrated transceiver circuits directly coupling to coaxial ports in the metal mirrors of the resonator. The integrated SiGe front-end addresses a frequency band from 50 GHz to 75 GHz. They are realized as heterodyne structures with integrated directional couplers, thus it is possible to measure scattering parameters. For first oxygen concentration measurements, the resonator sample was coupled to a commercially available Vector Network Analyzer. The cavity was filled with oxygen concentrations of 0% vol. and 20% vol. at ambient pressure and temperature resulting in a significant change of the quality factor for frequencies close to the oxygen absorption line at 60.6 GHz. The sensor does not contain hot components. This is an advantage compared to other oxygen sensors, like electrochemical or metal-oxide sensors
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