Wanping Zheng

Multichannel laser vibrometer and its applications

A multi-channel laser measurement system based on a commercially available single-channel laser vibrometer has been developed to measure the vibrations of low mass flexible space structures. The first generation of the multi-channel laser vibrometer is bulk-optic based system with laser being switched by an acousto-optical modulator. To overcome some of its drawbacks, for instance, being physically large and cumbersome, requiring tedious alignment, the second generation of the system was developed which employs fiber-optic technique to distribute the laser signal to the desired fiber-optic laser head. The design of the fiber-optic switch minimizes the power loss of laser beam in such a way that the vibrometer can work in its optimal range. Both bulk-optic and fiber-optic multi-channel vibrometers were verified using accelerometers on a light weight communication satellite antenna reflector. The modal parameters of the reflector were identified using measured frequency response functions both from laser vibrometer and from accelerometers. The result show that the laser system gives virtually the same result as the accelerometers.

Advanced MEMS and Integrated-Optic Components for Multifunctional Integrated Optical Micromachines

Optical technologies can play a strategic role in improving the performance, functionality, and reducing the mass of various spacecraft technologies, such as true time-delay T/R modules for phased-array antennas and optical sensor systems for satellite navigation and systems status. However, current photonic and fiber-optic systems tend to be bulky relative to the requirements for space applications. Micro integrated-optic circuits increase the integration of optical components on a single substrate, to provide multi-function optical processing and switching similar to electronic integrated circuits. This minimizes the number of external optical interconnections required and sensitivity to external vibrations; maximizing the system information capacity, optical throughput, and reliability, while minimizing the overall system size and weight. This paper considers a systematic development of MEMS integrated-optic circuits on SOI for various space application. A unique blend of MEMS, smart-material and photonic technologies is employed to miniaturize the size of the basic components, while improving on the attainable performance.

Performance of a 512/spl times/3 pixel microbolometer detector for space imaging applications

A 512/spl times/3 pixel microbolometer detector array has been developed for space imaging applications. Each pixel includes thermally sensitive and reference microbolometer detectors fabricated by surface micromachining and arranged in a Wheatstone bridge configuration. The detector array is sensitive to thermal infrared radiation and early characterisation shows good uniformity of response between the three lines and among pixels in a given line. The pixel bridge configuration reduces the impact of die temperature drift on pixel output to approximately 40 /spl mu/V/K, a three hundred fold improvement over a single microbolometer detector.

Miniature high-performance infrared spectrometer for space applications

Infrared spectroscopy probes the characteristic vibrational and rotational modes of chemical bonds in molecules to provide information about both the chemical composition and the bonding configuration of a sample. The significant advantage of the Infrared spectral technique is that it can be used with minimal consumables to simultaneously detect a large variety of chemical and biochemical species with high chemical specificity. To date, relatively large Fourier Transform (FT-IR) spectrometers employing variations of the Michelson interferometer have been successfully employed in space for various IR spectroscopy applications. However, FT-IR systems are mechanically complex, bulky (> 15 kg), and require considerable processing. This paper discusses the use of advanced integrated optics and smart optical coding techniques to significantly extend the performance of miniature IR spectrometers by several orders of magnitude in sensitivity. This can provide the next generation of compact, high-performance IR spectrometers with monolithically integrated optical systems for robust optical alignment. The entire module can weigh under 3 kg to minimize the mass penalty for space applications. Miniaturized IR spectrometers are versatile and very convenient for small and micro satellite based missions. They can be dedicated to the monitoring of the CO2 in an Earth Observation mission, to Mars exobiology exploration, as well as to vital life support in manned space system; such as the cabin air quality and the quality of the recycled water supply.

Advanced microphotonic and MEMS technologies for the MEOS microsatellite Earth observation mission

Infrared spectroscopy is a vital component of various Earth observation and planetary exploration space missions. It probes the characteristic vibrational modes of chemical bonds in molecules to provide information about not only the chemical composition but also the local bonding configuration and environment of the chemical bond. In particular, infrared spectroscopy has been employed to study atmospheric processes and various gas distributions. To date, mainly large bulk-optic Fourier transform (FT-IR) spectrometers employing variations of the Michelson interferometer, and massive high-order dispersive spectrometers have been employed in space for atmospheric studies on large and costly satellite platforms. In the following paper, we discuss the use of advanced optical coding and signal processing techniques to enable high-performance infrared spectral measurements of atmospheric components using MPBs miniature IR waveguide spectrometer. The miniature size of the spectrometer enables several dedicated spectrometers to be accommodated on a single microsatellite to extend the measurements that are feasible. In particular, these advanced technologies are being employed as the basis for the development of the use of the MEOS micro Earth observation mission.

Next generation microbolometers for high resolution remote sensing

We describe our recent work on improving the time constant and NETD of uncooled microbolometer detector. Such improvement would enable infrared remote sensing with a spatial resolution better than 150 m from low earth orbits. Radiometric simulation of the new microbolometer designs shows that reduction of time constant by a factor of 3 and reduction of NETD by a factor of 1.5 can be achieved. Linear arrays of 512/spl times/3 microbolometers embedding the new designs have been fabricated for experimental validation.

Microbolometer instrument payload design for a microsatellite mission

This paper describes the design of the mission and the instrument. It compares the use of microsatellites, ground and air based platforms. An Earth observation microsatellite mission is based on microbolometer technology. The detectors are linear arrays, filtered to be sensitive in five bands: 3 bands between 8.5 and 12.5 microns, and 2 bands between 3.0 and 4.5 microns. It is expected that this Earth observation satellite will produce useful dates like ice-shelf formation/melt, cloud monitoring, oil spills, forest fire tracking and damage assessment, Kyoto Accord compliance monitoring, city heat bloom, temperatures of oceans, land, and ice with the focus on northern coastal waters, Arctic and sub-Arctic land areas. Hence, satellite-based sensors have more advantages over terrestrial or airborne sensors.

Advanced MEMS/Smart-Material Coding and Filtering Technologies for High-Performance Miniature Integrated IR Spectrometers

Infrared spectroscopy can be a vital component of various Earth observation and planetary exploration space missions. It probes the characteristic vibrational modes of chemical bonds in molecules to provide information about not only the chemical composition but also the local bonding configuration and environment of the chemical bond. The IR spectral technique can be used with minimal consumables to simultaneously detect large variety of chemical and biochemical species. To date, mainly large bulk-optic Fourier Transform (FT-IR) spectrometers employing variations of the Michelson interferometer have been successfully employed in space due to the attainable performance. However, they typically require costly, large spacecraft platforms and complex environmental controls that limit the deployment of IR spectroscopy. In the following paper, we discuss the use of advanced optical coding and signal processing techniques, as facilitated using MEMS multi-channel optical signal processors, to significantly extend the performance limitations of miniature integrated-optic IR spectrometers. This technology can provide high-performance broad-band IR spectrometers suitable for passive monitoring in a compact module weighing under 2 kg.

Closed Loop Micromanipulators for Optical Metrology

The use of light beam micromanipulators in space subsystems has received particular attention in recent years. To compensate for changes in system parameters during the flight, these devices require closed loop control. This paper reports on an integrated feedback mechanism devised for flexural tortional devices. It consists in using monolithically embedded photodetectors to measure the portion of incident light crossing an aperture created in the manipulator. The manipulator position is determined from the detector section shadowed by the displaced aperture. A model is presented for the electrostatically actuated manipulator, predicting effects of structural parameters on angular displacement and resolution. As proof of concept, photodiode assisted closed loop manipulators were microfabricated in varying configurations on Si wafer. Their characteristics of deflection versus actuation voltage were better than those predicted by the model. The feedback mechanism was validated in light of the good agreement between differential photocurrents measured from contiguous photodiodes and theoretical prediction. Details on device modeling, microfabrication, and characterization are reported.

Multichannel fiber optic laser vibrometer

A non-contact nine channel fiber optic laser vibrometer has been developed to measure the vibrations of low mass flexible space structures. The Multi-Channel Fiber optic VIBrometer (MCFVIB) system is based on a commercial single channel laser vibrometer and a fiber-optic distribution system. This entailed the development of appropriate systems architecture for the optical signals, active 1 X 9 optical switch, and optical sensor heads suitable for coupling the reflected test signal back into the fiber optic system. MCFVIB employs single-mode, polarization maintaining fibers to maintain the signal coherence. The performance of the MCFVIB was tested at various vibrational frequencies using a shaker. Correlation of the measurements against those measured by an accelerometer indicate excellent linearity and accuracy for the fiber optic vibrometer system.