Helen L. Kung

Standing-wave Fourier transform spectrometer based on integrated MEMS mirror and thin-film photodetector

We report a novel, miniature Fourier transform spectrometer with a linear architecture that works by sampling a standing wave. The spectrometer consists of an electrostatically actuated microelectromechanical mirror with on-resonance displacement of up to 65 /spl mu/m, a thin-film photodetector, and an electrical back plane for actuating the mirror. The integrated device offers mirror stability and fixed relative alignment of the three components. The spectrometer has better than 32-nm resolution at 633 nm.

Silicon-based micro-Fourier spectrometer

A novel Fourier spectrometer based on a partly transparent thin-film detector in combination with a tunable silicon micromachined mirror was developed. The operation principle based on the detection of an intensity profile of a standing-wave by introducing a partly transparent detector in the standing-wave. Varying the position of the mirror results in a phase shift of the standing-wave and thus in a change of the optical intensity profile within the detector. The photoelectric active region of the sensor is thinner than the wavelength of the incoming light, so that the modulation of the intensity leads to the modulation of the photocurrent. The spectral information of the incoming light can be determined by the Fourier transform of the sensor signal. Based on the linear arrangement of the sensor and the mirror, the spectrometer facilitates the realization of one- and two-dimensional arrays of spectrometers combining spectral and spatial resolution. The operation principle of the spectrometer will be described and the influence of the detector design on the spectrometer performance will be discussed. A spectral resolution of down to 6 nm was achieved under real-time imaging conditions.

Transform spectrometer based on measuring the periodicity of Talbot self-images

We demonstrate a compact transform spectrometer based on measuring the periodicity of Talbot self-images. The system has no moving parts; it contains only a tilted absorption grating that is imaged onto a CCD camera. The linear architecture of the system makes it possible to use this design in imaging arrays of spectrometers. Unlike other transform spectrometers, its resolution is independent of wavelength.

Adaptive time-domain filtering for real-time spectral discrimination in a Michelson interferometer

We present a method of spectral discrimination that employs time-domain processing instead of the typical frequency-domain analysis and implement the method in a Michelson interferometer with a nonlinear mirror scan. The technique yields one analog output value per scan instead of a complete interferogram by directly filtering a measured scan with a reference function in the time domain. Such a procedure drastically reduces data-processing requirements downstream. Additionally, using prerecorded interferograms as references eliminates the need to compensate for scan nonlinearities, which broadens the field of usable components for implementation in miniaturized sensing systems. With our efficient use of known spectral signatures, we demonstrate real-time discrimination of 633- and 663-nm laser sources with a mirror scan length of 1 microm , compared with the Rayleigh criterion of 7 microm.

Wavelength monitor based on two single-quantum-well absorbers sampling a standing wave pattern

We demonstrate a wavelength monitor and a two-wavelength detector based on two single-quantum-well absorbers that sample a standing wave created by a distributed Bragg reflector. As a wavelength monitor, our device is power independent over a 15 dB range. Wavelength discrimination is linear over a 12 nm range.

Adaptive imaging spectrometer in a time-domain filtering architecturedaptive Imaging Spectrometer in a Time-Domain Filtering Architecture.

We demonstrate an imaging spectrometer with 30nm resolution that utilizes a novel time-domain filtering architecture. The architecture is based on a pixel by pixel integration of the interferogram signal mixed with reference waveforms. The system can be adapted in real time to discriminate between LED sources of different wavelengths, perform signal processing on the spectra, as well as discriminate between highly overlapping, broadband spectral features in a scene illuminated by a tungsten lamp. Unlike a conventional spectral signature discrimination system, which needs a dedicated computation subsystem running a discrimination algorithm, the time-domain filtering architecture embeds much of the computation in the filtering, which will aid the design of integrated miniaturized spectral signature discrimination systems.

Parallel-plate MEMS mirror design for large on-resonance displacement

We present an electrostatically actuated MEMS mirror with 65 /spl mu/m of displacement. This design provides a 2 mm square reflective surface and allows for easy fabrication, making it suitable for a wide range of applications.

Compact Fourier transform spectrometer based on sampling a standing wave

We demonstrate a Fourier-transform spectrometer based on a large-displacement MEMS mirror and sampling an optical standing wave with a thin photoconductor. The 1D design should permit integration of many spectrometers into an imaging array.

Thin-film-technology-based micro-Fourier spectrometer

A novel Fourier spectrometer using thin film technology was developed. The spectrometer based on a semi transparent thin film detector in combination with a tunable micro machined mirror. The semi transparent detector is introduced into a standing wave created in front of the mirror to sample the profile of the standing wave. Varying the position of the mirror results in a shift of the phase of the standing waves and thus in a change of the optical generation profile within the semi transparent detector. The active region of the sensor (thickness-absorption) is thinner than the wavelength of the incoming light, so that the modulation of the intensity results in a modulation of the overall photocurrent. The spectral information of the incoming light can be determined by the Fourier transformation of the sensor signal. Based on the linear arrangement of the sensor and the mirror, the spectrometer facilitates the realization of 1D and 2D arrays of spectrometers combining medium range spectral resolution with medium range spatial resolution. The novel device is filling the gap between solid-state camera technology with only three-color channels (red, green and blue) but high spatial resolution on one hand and precision spectrometers with high spectral resolution but no spatial resolution on the other hand. An analytical optical model of the spectrometer was applied to evaluate different detector concepts. The model was used to study the performance of different device designs regarding the spectral resolution of the spectrometer, the spectral range and the linearity of the response. The calculations will be compared with experimental results of semi transparent amorphous silicon detectors.

Integrated standing-wave transform spectrometer for near infrared optical analysis

We have presented an integrated standing-wave spectrometer operating in the near a mirror flexure uniformity allows for increased mirror scan length without tip-tilt problems, which improves spectral resolution. The GaAs detector provides excellent near IR responsivity. The integrated design allows the mirror to move within a few gm of the detector. The simple operation of this compact device, with a low power continuous-scan mirror, permanent alignment, and only three contacts, should enable implementation in a wide variety of miniaturized sensing systems.