Huseyin R. Seren

Lamellar grating optimization for miniaturized fourier transform spectrometers

Microfabricated Lamellar grating interferometers (LGI) require fewer components compared to Michelson interferotemeters and offer compact and broadband Fourier transform spectrometers (FTS) with good spectral resolution, high speed and high efficiency. This study presents the fundamental equations that govern the performance and limitations of LGI based FTS systems. Simulations and experiments were conducted to demonstrate and explain the periodic nature of the interferogram envelope due to Talbot image formation. Simulations reveal that the grating period should be chosen large enough to avoid Talbot phase reversal at the expense of mixing of the diffraction orders at the detector. Optimal LGI grating period selection depends on a number of system parameters and requires compromises in spectral resolution and signal-to-bias ratio (SBR) of the interferogram within the spectral range of interest. New analytical equations are derived for spectral resolution and SBR of LGI based FTS systems.

Lamellar grating based MEMS Fourier Transform Spectrometer

Design, fabrication, and characterization of a high-performance micromachined lamellar-grating-interferometer-based Fourier transform spectrometer are presented. The device is designed to give high deflections with very low dynamic deformation and good mode separation. Mechanical self-stoppers are introduced to withstand accelerations larger than 500 g due to shock. The clear aperture area of the grating is about 10 mm2. The maximum deflection while electrostatically actuated at ambient conditions is ±356 μm at 71.2 V and 340 Hz, setting a record for comparable devices. At a pressure of 8.6 Pa, the same deflection is reached at 4.3 V. Six hundred eighty spectra per second can be recorded with a resolution of 14 cm-1. With a HeNe laser at 633 nm, a spectral resolution of 0.54 nm (22 cm-1) is reached using electrostatic actuation. The microelectromechanical systems device is integrated into a compact Fourier transform spectrometer setup including a blackbody source, an infrared (IR) detector, and a visible laser using the device back side for reference. Early results with IR interferograms are also reported. In addition, the devices are actuated with pressure waves in the ambient air to reach deflections up to ±700 μm. With this setup, the spectrum of a red laser is measured with a resolution of 0.3 nm (12.4 cm-1).

MEMS Fourier transform IR spectrometer

A comb-actuated MEMS lamellar grating FTIR spectrometer with maximum OPD of 652μm and clear aperture area of 9.6mm² is developed. Laser and IR interferograms in 2.5–16μm wavelength band are acquired at ambient pressure.

Miniaturized FR4 spectrometers

A miniaturized and electromagnetically driven FR4 based moving platform is developed for Fourier Transform spectrometer applications. Both Michelson interferometer and Lamellar Grating interferometer configurations are demonstrated. ±500 µm translational motion (corresponding to 5 cm−1 spectral resolution) is demonstrated with the moving platform. Two methods are proposed and partially demonstrated for pure translational motion: (1) integrated control system using a quad photo detector feedback and (2) corner cube retroreflector. The fundamental advantages and the limits of the lamellar grating interferometers are also discussed.

2D scanning MEMS stage integrated with microlens arrays for high-resolution beam steering

A novel MEMS stage using one set of comb fingers, capable of 2-axis motion is designed and developed. With an integrated 1.1mm square microlens-array it deflects 40um in-plane at 60V and 95um out-of-plane at 100V.

MEMS Fourier Transform Spectrometer

A comb actuated lamellar grating interferometer based MEMS Fourier Transform Infrared (FTIR) Spectrometer device is designed, fabricated and characterized. The device operates at out-of-plane resonant mode which will allow ultra miniaturized, sensitive, robust, and fast spectrometers. As a novel approach pantograph type springs are used in the mechanical design to achieve high deflections. The dynamic deformation on the gratings is minimized using additional suspension springs. Optical simulations are conducted to extensively analyze the device performance in terms of spectral resolution and signal-to-bias ratio (SBR). In the light of simulations and experiments, the grating geometry is optimized for the region of wavelengths of interest (2.5–16 μm). Comb structures are designed and placed around pantograph springs for low voltage operation. The fabrication process is developed based on CMOS compatible bulk micromachining of a silicon-on-insulator wafer. A maximum peak to peak mechanical deflection of 478 μm is acquired with 50 V p-p input voltage in ambient pressure.

Lamellar grating interferometer based compact FT spectrometers

Lamellar grating interferometers (LGI) offer compact spectrometer architecture with high spectral resolution and large clear aperture. This study investigates the diffraction based inherent limitations of LGI spectrometers in contrary to conventional Michelson type spectrometer architecture. Simulations and experiments were conducted to demonstrate and explain periodic nature of the interferogram envelope due to Talbot image formation. Simulations reveal that grating period should be chosen large enough to avoid Talbot phase reversal at the expense of mixing diffraction orders. Overall optimization effort on the LGI system reveals that it is possible to build compact spectrometers to be used directly in the field without any performance degradation in contrary to bulky FTIR systems.

MEMS scanners for display, imaging, and spectroscopy and their dynamic characterization

Moving micro-mechanical structures combined with laser light sources and micro-optics enable a number of powerful applications in display, imaging, and spectroscopy. Examples of systems developed in our laboratory are: rotational scanners developed for micro-projectors, dynamic diffraction gratings with large out-of-plane motion developed for Fourier Transform spectrometers, and 2 degree-of-freedom MEMS stages that carry micro-lens arrays for laser beam steering and imaging applications. Precise control of motion is critical in all those applications. We developed a number of optical characterization tools for point-based and fullfield dynamic characterization of micro and nano mechanical structures. In this paper, we first briefly discuss the applications and then describe the details of the optical characterization tools. First setup is a stroboscopic interferometry for dynamic deformation analysis. Second setup is a simple technique for simultaneous in-plane and out-of-plane measurement with nanometric precision. The setup is constructed using one photo detector and a Mirau-type interference objective. For out-of-plane measurements, interference fringes are used to compute the the deflection amount. For in-plane measurements, knife edge technique is used to modulate the reflected beam intensity using a sharp edge in the object. Third setup is a simple optical angle sensor for rotational mirrors, which uses only one bi-cell photo detector. The setup is able to measure amplitude, phase, and quality factor of torsional devices.