Bassem Mortada

Fully Integrated Mach-Zhender MEMS Interferometer With Two Complementary Outputs

In this paper, a novel Mach-Zhender interferometer for spectroscopy applications is presented. The interferometer is fully integrated on an silicon on insulator wafer using deep reactive ion etching technology, the moving mirror is coupled to a comb drive microelectromechanical systems (MEMS) actuator. Optical propagation inside the MEMS structure is modeled and the diffraction effect is studied. Practical results show the complementary nature of the two outputs and a resolution of 25 nm at 1.55 μm is reported when using the interferometer as an Fourier transform infrared spectrometer. The complementary nature of the interferometer can be further used for source noise reduction.

On-Chip Micro–Electro–Mechanical System Fourier Transform Infrared (MEMS FT-IR) Spectrometer-Based Gas Sensing:

In this work, we study the detection of acetylene (C2H2), carbon dioxide (CO2) and water vapor (H2O) gases in the near-infrared (NIR) range using an on-chip silicon micro-electro-mechanical system (MEMS) Fourier transform infrared (FT-IR) spectrometer in the wavelength range 1300–2500 nm (4000–7692 cm−1). The spectrometer core engine is a scanning Michelson interferometer micro-fabricated using a deep-etching technology producing self-aligned components. The light is free-space propagating in-plane with respect to the silicon chip substrate. The moving mirror of the interferometer is driven by a relatively large stroke electrostatic comb-drive actuator corresponding to about 30 cm−1 resolution. Multi-mode optical fibers are used to connect light between the wideband light source, the interferometer, the 10 cm gas cell, and the optical detector. A wide dynamic range of gas concentration down to 2000 parts per million (ppm) in only 10 cm length gas cell is demonstrated. Extending the wavelength range to the mid-infrared (MIR) range up to 4200 nm (2380 cm−1) is also experimentally demonstrated, for the first time, using a bulk micro-machined on-chip MEMS FT-IR spectrometer. The obtained results open the door for an on-chip optical gas sensor for many applications including environmental sensing and industrial process control in the NIR/MIR spectral ranges.

Wideband Optical MEMS Interferometer Enabled by Multimode Interference Waveguides

A novel optical MEMS interferometer is proposed based on spatial splitting and combining of optical beams using the imaging properties of multimode interference (MMI) waveguides. The light propagates in air, allowing operation over wide spectral range covering both the infrared and the visible ranges. The optical propagation in the structure is modeled and the interferometer is analyzed by incorporating the modal analysis technique for the waveguides and the angular spectrum approach for free-space propagation. The beam splitter and the overall interferometer are fabricated using deep reactive ion etching technology on silicon-on-insulator wafer. The MMI waveguide sidewalls are aluminum metalized to improve the insertion loss of the interferometer. The fabricated splitter and interferometer are characterized in the visible and near-infrared spectral ranges. The splitter output intensity profile is recorded to verify its wideband proper operation. The interferometer is characterized versus the wavelength and tested as a Fourier transform spectrometer, thanks to a monolithically integrated corner mirror driven by a comb-drive actuator. The spectral resolution obtained is 2.5 nm at 635-nm wavelength.

High-throughput deeply-etched scanning Michelson interferometer on-chip

A miniaturized scanning Michelson interferometer is demonstrated on-chip using a deep-etching process. Etching depths larger 300 μm were obtained with side-wall angle better than 0.1 degree and scalloping depth smaller than 60 nm. Multi-mode optical fibers with core diameters of 62.5 μm and 200 μm were used for delivering the white light to SOI chips with device layer heights of 90 μm and 200 μm for evaluating the improvement with larger depth. The resulting interferograms were compared showing 12-dB increase in the signal, which is a significant boost for the signal-to-noise ratio. The presented interferometer opens the door for the use of miniaturized instruments in practical applications.

Mid infrared MEMS FTIR spectrometer

In this work we report, for the first time to the best of our knowledge, a bulk-micromachined wideband MEMS-based spectrometer covering both the NIR and the MIR ranges and working from 1200 nm to 4800 nm. The core engine of the spectrometer is a scanning Michelson interferometer micro-fabricated using deep reactive ion etching (DRIE) technology. The spectrum is obtained using the Fourier Transform techniques that allows covering a very wide spectral range limited by the detector responsivity. The moving mirror of the interferometer is driven by a relatively large stroke electrostatic comb-drive actuator. Zirconium fluoride (ZrF4) multimode optical fibers are used to connect light between the white light source and the interferometer input, as well as the interferometer output to a PbSe photoconductive detector. The recorded signal-to-noise ratio is 25 dB at the wavelength of 3350 nm. The spectrometer is successfully used in measuring the absorption spectra of methylene chloride, quartz glass and polystyrene film. The presented solution provides a low cost method for producing miniaturized spectrometers in the near-/mid-infrared.

Environmental mid-infrared gas sensing using MEMS FTIR spectrometer

In this work, we report carbon dioxide gas sensing in the ambient air in the mid-infrared range around 4250 nm using MEMS FTIR spectrometer. The core engine of the spectrometer is a scanning Michelson interferometer fabricated using deep etching technology on silicon-on-insulator wafer. The measured Signal-to-Noise Ratio (SNR) is 24 dB at a wavelength of 4250 nm and the spectral resolution is about 60 cm-1. A free-space gas cell using CaF2 lenses with lightgas interaction lengths of 12 cm and 120 cm is demonstrated. The results demonstrate about 400 ppm concentration detection in the ambient air. The theoretical sensitivity limit based on the achieved SNR and resolution is about 15 ppm.