Mohan Sanghadasa

Vibration Modeling of Arc-Based Cantilevers for Energy Harvesting Applications

Abstract Cantilever beams are widely used for designing transducers for low-frequency vibration energy harvesting. However, in order to keep the dimensions within reasonable constraints, a large tip mass is generally required for reducing the resonance frequency below 100 Hz which has adverse effect on the reliability. This study provides a breakthrough toward realizing low-frequency micro-scale transduction structures. An analytical out-of-plane vibration model for standalone arc-based cantilever beams was developed that includes provisions for shear and rotary inertia, multidirectional arcs, and multiple layers. The model was applied to a multilayered cantilever beam (10-mm wide and 0.1-mm thick) composed of three arcs, and the results indicate that the fundamental bending mode of the beam was 38 Hz for a silicon substrate thickness of 100 μm. The model was validated with modal experimental results from an arc-based cantilever made out of aluminum.

A simplified technique for efficient fiber-polymer-waveguide power coupling using a customized cladding with tunable index of refraction

A novel technique to minimize the mode mismatch between the fiber and waveguide modes in integrating optical fiber to polymer waveguides is presented. The mode fields at the facets of the waveguides were tailored, optimizing the waveguide geometry as well as tuning the index of refraction of the lower cladding near the facets, by chemical composition. No additional processes were required after the core was deposited. The modified lower cladding can be integrated efficiently with any other lower-cladding material used in the remaining portion of the waveguide, resulting in hybrid waveguides. A power-coupling loss as low as 0.46 dB at the fiber-waveguide interfaces was demonstrated with the available fiber

Backscatter Compensation in IFOG Based Inertial Measurement Units With Polymer Phase Modulators

Precision guidance in navigation systems requires highly accurate, compact, and low cost inertial measurement units (IMUs). The key active guided-wave component of the IMU is the phase modulator. In our approach, electro-optic polymers have been utilized in fabricating low loss phase modulators with low half-wave drive voltage using advanced hybrid waveguide fabrication processes and novel optical integration techniques. However, the interference between the primary wave and the backscatter waves generated by the phase modulator and/or the interference between the two counter-propagating backscatter waves at the detector of the IMU has been a major source of error in this approach. A novel technique was introduced in assessing the error caused by backscatter and an offset waveguide design was developed to suppress the interference of backscatter light. The novel design not only preserved the miniaturization, but also improved the insertion loss with the use of a shorter waveguide. The gyro level tests performed with the backscatter compensated modulators showed about 5 times improvement of the average bias uncertainty over gyros integrated with a standard symmetric phase modulator.

Applications of SLDs in fiber optical gyroscopes

The performance of interferometric fiber optical gyroscopes (IFOGs) has continued to advance resulting in expanded applications for both commercial and military inertial sensors. The primary advantages of the IFOG technology for inertial systems are the high reliability and lower cost. Most IFOG designs incorporate a fiber light source, a fiber sensing coil with discrete components connected, typically, with optical fiber pigtails. Fiber light sources require several optical components and are expensive to produce as well as bulky to package. The use of superluminescent diodes (SLDs) as a light source provides a much smaller, less expensive alternative and provides more flexibility in the integration architecture. The challenge for SLD development is the achievement of high power while maintaining the spectral quality and long lifetime. Presented here are the source requirements and the performance achieved for SLDs designed for these applications.

Components for IFOG based inertial measurement units using active and passive polymer materials

Highly accurate, compact, and low cost inertial measurement units (IMUs) are needed for precision guidance in navigation systems. Active and passive polymer materials have been successfully used in fabricating two of the key guided-wave components, the phase modulator and the optical transceiver, for IMUs based on the interferometric fiber optic gyroscope (IFOG) technology. Advanced hybrid waveguide fabrication processes and novel optical integration techniques have been introduced. Backscatter compensated low loss phase modulators with low half-wave drive voltage (Vπ) have been fabricated with CLD- and FTC- type high performance electro-optic chromophores. A silicon-bench architecture has been used in fabricating high gain low noise transceivers with high optical power while maintaining the spectral quality and long lifetime. Gyro bias stability of less than 0.02 deg/hr has been demonstrated with these components. A review of the novel concepts introduced, fabrication and integration techniques developed and performance achieved are presented.

Nanotechnology research and development for military and industrial applications

Researchers at the Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC) have initiated multidiscipline efforts to develop nano-based structures and components for insertion into advanced missile, aviation, and autonomous air and ground systems. The objective of the research is to exploit unique phenomena for the development of novel technology to enhance warfighter capabilities and produce precision weapons. The key technology areas that the authors are exploring include nano-based microsensors, nano-energetics, nano-batteries, nano-composites, and nano-plasmonics. By integrating nano-based devices, structures, and materials into weaponry, the Army can revolutionize existing (and future) missile systems by significantly reducing the size, weight and cost. The major research thrust areas include the development of chemical sensors to detect rocket motor off-gassing and toxic industrial chemicals; the development of highly sensitive/selective, self-powered miniaturized acoustic sensors for battlefield surveillance and reconnaissance; the development of a minimum signature solid propellant with increased ballistic and physical properties that meet insensitive munitions requirements; the development of nano-structured material for higher voltage thermal batteries and higher energy density storage; the development of advanced composite materials that provide high frequency damping for inertial measurement units packaging; and the development of metallic nanostructures for ultraviolet surface enhanced Raman spectroscopy. The current status of the overall AMRDEC Nanotechnology research efforts is disclosed in this paper. Critical technical challenges, for the various technologies, are presented. The authors approach for overcoming technical barriers and achieving required performance is also discussed. Finally, the roadmap for each technology, as well as the overall program, is presented.

Miniaturization of Piezoelectric Microphones

Miniaturization of acoustic sensors offers improved measurement capability to the aeroacoustics community, such as surface mounting with minimal flow perturbation, large bandwidth and improved spatial resolution for measurements in the turbulent boundary layer. Furthermore, reductions in sensor size enable higher-density acoustic arrays, reduced spatial averaging and increased spatial resolution. Regardless of transduction method, fundamental noise mechanisms, however, place a lower bound on this miniaturization in order to maintain a desired minimum detectable signal (MDS). This paper presents the results of a theoretical and experimental study into the scaling effects of piezoelectric microphones. The ultimate goal is to drastically reduce sensor size while maintaining a desired MDS. To progress towards that goal, a series of piezoelectric microphones with a range of geometries were designed and fabricated to serve as “test structures”. Finally, acoustical and electrical characterization was performed to confirm the scaling.

Polyimides with attached chromophores for improved performance in electro-optical devices

A method of chemical synthesis that allows for the facile attachment of a wide variety of chemical compounds, including highly active nonlinear optical chromophores, to polyimides has been developed recently at the Naval Air Warfare Center, Weapons Division. The synthesis of these compounds is presented, along with a discussion of their relevant physical and chemical properties, alone and in comparison to equivalent guest/host materials. Examples of attached chromophores include the well-known Disperse Red 1, along with high-activity chromophores of more recent interest such as FTC and CLD. The synthesis of structures that contain both attached chromophores and chemical functionalities that enable thermal cross-linking of the polyimides is also discussed.

Overview of EO Polymers and Polymer Modulator Stability

This is a brief overview of the technology of nonlinear optical polymers (NLOP) and their use in electro-optic (EO) modulators. This paper also covers preliminary results from the authors laboratories on highly active CLD- and FTC-type chromophores in guest-host films of APC amorphous polycarbonate. Emphasis will be given to thermal stability and long-term EO modulator aging.

A low frequency MEMS vibration sensor for low power missile health monitoring

This paper addresses the design, fabrication and characterization of a first-generation, low frequency MEMS vibration sensor. The sensor is designed specifically for applications requiring extremely low power vibration detection at only targeted frequencies. For development, lumped element and finite element modeling was performed, driving the design towards a realizable geometry that addresses the targeted performance specs. The sensors were microfabricated using conventional surface micromachining, sol-gel PZT (lead zirconate titanate) thin films, and bulk silicon etching techniques. The completed sensors were then characterized to determine electrical, mechanical and piezoelectric properties at the material and device level. Results demonstrate functional operation with performance close to predicted specifications.