Mazen Erfan

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.

On the environmental gas sensing using MEMS FTIR spectrometer in the near-infrared region

Miniaturized MEMS FTIR spectrometers are one of most important candidates for portable environmental sensing due to their compacted and low cost, while leveraging the optical sensing advantages. Spectroscopy-based gas sensing is usually carried out in the mid-infrared taking advantage of the large absorption cross section, where the challenge is the bulky and expensive detector system standing as an obstacle against portability and low cost. In this work, the feasibility of environmental sensing using near-infrared MEMS spectrometer is studied with the aid of computer simulations and experimental measurements. The considered gases in the study are H2O vapor and CO2. The study was carried out considering gas light-gas interaction lengths of 20 cm, 1 m, 6 m and 12 m. The correct detection possibility of CO2 absorption spectrum in the presence of relatively high concentration H2O was studied versus the signal-to-noise ratio (SNR) using principle component regression taking into account the effect of the limited spectrometer resolution. The results indicates that a 40-dB SNR can result in a 5.4% error in the CO2 concentration prediction with 1 m interaction length compared to 1.8% in case of 6 m interaction length. An experimental optical setup was constructed and the absorption spectrum was recorded for different light-air interaction lengths.

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.

High-Q Fabry–Pérot Micro-Cavities for High-Sensitivity Volume Refractometry

This work reports a novel structure for a Fabry–Pérot micro cavity that combines the highest reported quality factor for an on-chip Fabry–Pérot resonator that exceeds 9800, and a very high sensitivity for an on-chip volume refractometer based on a Fabry–Pérot cavity that is about 1000 nm/refractive index unit (RIU). The structure consists of two cylindrical Bragg micromirrors that achieve confinement of the Gaussian beam in the plan parallel to the chip substrate, while for the perpendicular plan, external fiber rod lenses (FRLs) are placed in the optical path of the input and the output of the cavity. This novel structure overcomes number of the drawbacks presented in previous designs. The analyte is passed between the mirrors, enabling its detection from the resonance peak wavelengths of the transmission spectra. Mixtures of ethanol and deionized (DI)-water with different ratios are used as analytes with different refractive indices to exploit the device as a micro-opto-fluidic refractometer. The design criteria are detailed and the modeling is based on Gaussian-optics equations, which depicts a scenario closer to reality than the usually used ray-optics modeling.

Multimode spot-size converter for optical MEMS applications

Efficient power coupling is an unavoidable requirement in optical MEMS systems, either from input multimode optical fiber to the optical MEMS structure or between different multimode waveguides integrated on the same chip. In this work, non-imaging tapered structures and concentrators are proposed and discussed. The optimal transmission condition is deduced for linear tapered structures and compared to industry standard ray tracing simulations, taking into account the three-dimensional structure of the micromachined structures. The study is carried out for different etching depths and account for the practical MEMS mirror reflectivity. In addition, compound parabolic concentrators are discussed and compared to the linear ones. The results show that the linear taper has the advantage of smaller length while the CPC coupling efficiency can be higher. For instance, the optimal taper length is found to be 160 µm converting an input spot size of 100 µm to an output spot size of 40 µm, while about 90 % of the power is delivered for an optical MEMS structure height of 100 µm.

Multi-segment tapered optical mirror for MEMS LiDAR application

In this work, we present a novel and simple optical solution for MEMS LiDARs. The idea is based on increasing the collection optics throughput by removing the MEMS mirror from the path of the collected light, while inserting a multi-segment tapered structure to collect the light from a wide angle. The tapered also converts the large size optical spot captured to a small area compatible with the requirement of low detector noise dimensions. The expected improvement in the collected power is analyzed versus the tapering angle of a single tapered structure. A multi-segment optical system, or multiple tapered structure arranged in parallel, is also introduced allowing for the optimization of the acceptance angle and the power improvement ratio. Using a 3-segment mirror, the expected improvement is about 15x with an acceptance angle of ±30 degrees. The design of a single element taper section is fabricated using aluminum-coated acrylic and tested experimentally showing an improvement of about 7x in the coupled power through an angle of ±10 degrees in good agreement with the theoretical expectations.

MEMS FTIR spectrometer with enhanced resolution for low cost gas sensing in the NIR

In this work, we report the detection of C2H2 and CO2 in the NIR range using a MEMS Fourier Transform Infrared (FTIR) spectrometer. For this purpose, a super resolution autoregressive (AR) algorithm is used. The spectrometer is working in the wavelength range 1300–2500 nm while its core engine is a scanning Michelson interferometer micromachined using deep reactive ion etching (DRIE) technology on SOI wafer. The interferometer scanning mirror is driven by a MEMS electrostatic actuator with travel range corresponding to a resolution of about 30 cm-1.The spectrometer with the algorithm are used for measuring a standard optical filter with line width of 1 nm and measured line width is 1.7 nm that corresponds to 7.5 cm-1. The spectrum of a mixture of C2H2 and CO2 is measured using the MEMS spectrometer and a gas cell with 10cm light-gas interaction length. The AR model is applied on the interferogram. The resulting spectrum after the AR application shows an enhanced resolution of 15cm-1 that led to better identification of the absorption peaks.

Optical MEMS notch filter based on the multi-mode interference in a butterfly metallic waveguide

In this work, we report a novel notch optical filter based on the imaging properties of a MEMS-based Multimode Interference (MMI) waveguide. The concept is based on the dependence of the imaging lengths on the different wavelengths, where each wavelength exits the waveguide at a different lateral position. Thus, by properly choosing the output waveguide position, it is possible to have a good selective optical filter as well as a good notch optical filter (the complementary response). To validate this concept an MMI structure is fabricated using Deep Reactive Ion Etching (DRIE) technology on a silicon-on-insulator (SOI) wafer. The walls of the waveguide are metalized with Aluminum to decrease the insertion loss. The design makes use of the compactness of the parabolic butterfly shape to reduce the MMI length. The structure is fed by a 9/125 single-mode fiber and the Amplified Spontaneous Emission ASE out of a Semiconductor Optical Amplifier is used as a wideband source for the optical response characterization. The output is measured on an optical spectrum analyzer demonstrating a notch filter response around 1550 nm with about 20-dB rejection ratio. The reported results open the door for integrated, low-cost and fabrication insensitive optical MEMS notch filter.

NOVEL DESIGN OF FABRY–PÉROT CAVITIES ACHIEVING SUPERIOR SENSITIVITY FOR VOLUME REFRACTOMETRY OF HOMOGENEOUS LIQUIDS

This paper reports a new design of optofluidic Fabry–Perot (FP) micro cavity that combines the highest reported quality factor for an on-chip FP resonator that exceeds 3600, and the highest reported sensitivity for an on-chip volume refractometer based on a FP cavity that is about 1000 nm/RIU. For using the optical resonator as a refractometer, it is convenient to have sharp and highly selective resonance peaks for accurate measurements; thereby the quality factor (Q) of the resonator is preferred to be high. The highest reported Q factor reported by other groups is only 400 [1]. This limitation comes from using straight mirror for the FP, which causes high diffraction loss due to beam expansion after few round trips. Our group has previously reported a cavity employing curved mirrors and a micro-tube in-between holding the analyte [2]. The curvature of the mirrors and the micro-tube achieved better light confinement and hence high Q factors up to 2,800. On the other hand, the sensitivity was only 428 nm/RIU since the analyte doesn’t fill the entire cavity. The highest reported sensitivity by literature was 907 nm/RIU in case of the analyte occupying the whole cavity [1]. In this work, a novel structure for FP micro-cavity is reported, achieving both high Q factor resonator and high sensitivity refractometer. The proposed structure is schematically depicted in Fig. 1. It employs cylindrical Bragg mirrors forming the FP cavity to confine light in the in-plane direction, while an external cylindrical lens - implemented by a fiber rod lens (FRL) - is used to confine light in the out-of-plane direction before it enters the FP cavity and after it exits to be efficiently collected by the collecting fiber. The cavities are fabricated from silicon by Deep Reactive Ion Etching (DRIE) process, and then capped by a PDMS cover. The FRLs are placed later after micro fabrication. A photo of the chip combining several cavities with different lengths is shown in Fig. 2. The analyte is passed between the mirrors enabling its detection from the shift of resonance peaks of the transmission spectra. The spectra are obtained by recording the output from an optical detector while varying the input light wavelength from a tunable laser in the near infrared band. The spectrum of a cavity of 318 µm physical length filled with DI-water is presented in Fig. 3 showing the narrow line widths of the resonance peaks. The peak has a linewidth of 0.44 nm, which provides a Q factor of 3649. Mixtures of ethanol and DI-water with different ratios are used as analytes with different refractive indices to exploit the device as a micro-opto-fluidic refractometer, which are plotted in Fig. 4. The sensitivity is obtained from the slope of the linear plot in Fig. 5 of the resonance wavelength shift versus the difference in refractive index between the analytes and the DI-water, which is taken as a reference. The cavity used has a physical length of 256 µm and the obtained sensitivity is 1000 nm/RIU.