Optical grating-based interferometric sensors have been the subject of prior investigations, with recent work focused on micromachined microphone applications. The silicon structure is similar in construction to capacitive microelectromechanical-system microphones, with the exception that the microphone backplate contains an optical-diffraction grating at the center. The grating serves as a beam splitter in this system, allowing only a portion of the incident light to pass to the diaphragm and back, enabling interferometric readout of diaphragm displacements. A cited advantage of this system is the ability to design highly perforated backplates with low mechanical damping and with the ability to realize low thermal-mechanical noise. Grating backplates, however, have their own unique optical design constraints different from capacitive sensors. This paper details a rigorous finite element computational fluid dynamics model for flow resistance of a grating backplate. The model is validated for a case study backplate fabricated in the epitaxial layer of a 2-μm silicon-on-insulator wafer. The dynamics of the backplate are studied in isolation from other microphone elements by mounting the backplate in close proximity to a rigid optical-reflector and using electrostatic actuation to vibrate the backplate for extraction of compliance, resonance frequency, and quality factor.

The Department of Energy (DOE) and the National Nuclear Security Administration (NNSA) seek revolutionary sensing innovations for the monitoring of nuclear detonations. Performance specifications are to be consistent with those obtainable by only an elite few products available today, but with orders of magnitude reduction in size, weight, power, and cost. The proposed commercial innovation calls upon several technologies including the combination of meso-scale fabrication and assembly, photonics-based displacement / motion detection methods, and the use of digital control electronics . Early Phase II development has demonstrated verified and repeatable sub 2ng noise floor from 3Hz to 100Hz, compact integration of 3-axis prototypes, and robust deployment exercises. Ongoing developments are focusing on low frequency challenges, low power consumption, ultra-miniature size, and low cross axis sensitivity. We are also addressing the rigorous set of specifications required for repeatable and reliable long-term explosion monitoring, including thermal stability, reduced recovery time from mass re-centering and large mechanical shocks, sensitivity stability, and transportability. Successful implementation will result in small, hand-held demonstration units with the ability to address national security needs of the DOE/NNSA. Additional applications envisioned include military/defense, scientific instrumentation, oil and gas exploration, inertial navigation, and civil infrastructure monitoring.

Most surface mount microelectromechanical system (MEMS) microphone packages are similar in construction, consisting of a printed circuit board with sound inlet, a MEMS die with a through-wafer etch aligned over the sound inlet, and cap which serves to protect the structure and render an enclosed back volume. From a lumped modeling perspective, this system is a network of acoustical and mechanical elements. Network models (i.e., equivalent circuit models) have proven to be the most common modeling technique for simulating important features of these microphones, including frequency response functions and internal noise floors. While these models have many advantages including their ability to be solved efficiently using modern circuit simulation software, they do not lend themselves well to an understanding of system dynamics as a decomposition of the fundamental mechanical modes of the packaged system. We present a state space model for complete MEMS microphone packages and present frequency response simulations as a superposition of the system's eigenmodes. In addition to offering insight into package behavior, we believe these models are better equipped to address advanced features such as feedback altered dynamics using internal actuation capabilities. Simulations are compared with measurements on a surface mount optical MEMS microphone discussed prior.

A microelectromechanical systems (MEMS) optical microphone that measures the interference of light resulting from its passage through a diffraction grating and reflection from a vibrating diaphragm is described ( JASA, v. 122, no. 4, 2007). In the present embodiment, both the diffractive optical element and the sensing diaphragm are micromachined on silicon. Additional system components include a semiconductor laser, photodiodes, and required readout electronics. Advantages of this optical detection technique have been demonstrated with both omnidirectional microphones and biologically inspired directional microphones. In efforts to commercialize this technology for hearing aids and other applications, a goal has been set to achieve a microphone contained in a small surface-mount package (occupying 2 × 2 mm × 1 mm volume), with ultralow noise (20 dBA) and a broad frequency response (20 Hz-20 kHz). Such a microphone would be consistent in size with the smallest MEMS microphones available today but would have noise performance characteristics of professional-audio microphones significantly larger in size and more expensive to produce. This paper will present several unique challenges in our effort to develop the first surface-mount packaged optical MEMS microphone. The package must accommodate both optical and acoustical design considerations. Dynamic models used for simulating frequency response and noise spectra of fully packaged microphones are presented and compared with measurements performed on prototypes.

We investigate experimentally the interference in far-field radiation of two contra-propagating evanescent fields using a conventional optical microscope. A laser beam illuminates a glass-air interface under total internal reflection condition and through the proper setup a double evanescent illumination was produced. The evanescent fields radiate from the surface into the far-field domain due to small surface scatterers. Thus, coherent interference is produced in the far-field region which is correlated with the relative positions of the evanescent illumination sources. Finally, the above-described could be considered as a device for high accuracy micro-scale measurements as well as a direct visualization method of evanescent phenomena.

A microelectromechanical systems (MEMS) optical microphone has been presented that measures the interference of light resulting from its passage through a diffraction grating and reflection from a vibrating diaphragm. [J. Acoust. Soc. Am. 122, (2007).] In this embodiment, both the diffractive optical element and the sensing diaphragm are micromachined on silicon. Additional system components include a semiconductor laser, photodiodes, and required readout electronics. In our efforts to commercialize this technology for hearing-aids and other applications, a goal has been set to achieve a microphone contained in a small surface mount package (occupying 2 × 2 × 1 mm3 volume), with ultra-low noise (15 dBA) and broad frequency response (20 Hz–20 kHz). Such a microphone would be consistent in size with the smallest MEMS microphones available today, but would have the noise performance characteristics of professional-audio microphones at least 10× larger in size and 10× more expensive to produce. This paper will present several unique challenges in our effort to develop the first surface mount packaged optical MEMS microphone, including the optical and acoustic design of the package. Dynamic models used for simulating frequency response and noise spectra of complete capsules will be presented and compared with measurements performed on recent prototypes.

In this paper, integration and packaging of directional biomimetic microphones using a diffraction-based optical displacement detection method is described. The optical detection method senses the displacement of the microphone diaphragm by monitoring the change in the intensity of a diffracted laser beam. A detailed optical model of the integrated optical detection scheme is developed and used to guide the package design. Measurement results with microphone packages suitable for hearing aid and acoustic measurement systems show that the noise and sensitivity performances of these small-sized microphones are comparable with commercial miniature directional microphones. These microphones incorporate integrated electrostatic actuators which can be used for active feedback control. This capability is also demonstrated to improve the frequency response of the microphone without degrading its noise performance.

A micromachined accelerometer device structure with diffraction-based optical detection and integrated electrostatic actuation is introduced. The sensor consists of a bulk silicon proof mass electrode that moves vertically with respect to a rigid diffraction grating backplate electrode to provide interferometric detection resolution of the proof-mass displacement when illuminated with coherent light. The sensor architecture includes a monolithically integrated electrostatic actuation port that enables the application of precisely controlled broadband forces to the proof mass while the displacement is simultaneously and independently measured optically. This enables several useful features such as dynamic self-characterization and a variety of force-feedback modalities, including alteration of device dynamics in situ. These features are experimentally demonstrated with sensors that have been optoelectronically integrated into sub-cubic-millimeter volumes using an entirely surface-normal, rigid, and robust embodiment incorporating vertical cavity surface emitting lasers and integrated photodetector arrays. In addition to small form factor and high acceleration resolution, the ability to self-characterize and alter device dynamics in situ may be advantageous. This allows periodic calibration and in situ matching of sensor dynamics among an array of accelerometers or seismometers configured in a network.

A micromachined optical microphone structure using a grating based interferometer has been presented previously and is undergoing continued development (JASA, vol. 118, pp 3000-3009, November 2005). Two advantages of the approach that have been highlighted in prior work are high displacement resolving capability of the microphone diaphragm vibration (2 pm rms over the audio bandwidth) and a flexible mechanical design space for achieving broad bandwidth and low thermal noise designs. Here, we summarize a variety of structures we are fabricating using Sandia National Laboratories silicon-based microfabrication technology to explore this versatile design space. These structures are being packaged to resemble instrumentation-type microphones with approximately 1 cm2 form factor in order to facilitate rigorous acoustic evaluation in the Micromachined Sensors and Transducers Laboratory (MiST) and anechoic test facilities at Georgia Tech.

A diffraction-based optical detection method for microphone applications has been demonstrated previously [Hall et al., J. Acoust. Soc. Am. 118, 3000–3009 (2005)]. This method, coupled with proper integration techniques can produce precision measurement microphones with 24 dBA noise levels and suitable bandwidths. Thus far, these characterization studies have been performed using experimental setups, which would disturb the acoustic field due to size and non-symmetric features. In these regards, previous optical microphone test beds have been inadequate experimental platforms. This has motivated the development of a more robust integrated instrumentation microphone package for future testing and characterization. In order to meet the size restrictions for such an optical microphone platform, vertical cavity surface emitting lasers are used as light sources and small photodiode arrays are used to detect intensity variations in refracted orders of the optical detection method. The overall dimensions and shape of the package are comparable to commercially available half-inch calibration microphones and impose minimal sound field disturbance. The design is adapted to allow simple replacement and remounting for multiple microphone testing including biomimetic directional microphones [Miles et al., J. Acoust. Soc. Am. 98, 3059–3070 (1995)]. [Work partially supported by NIH Grant 5R01DC005762-03 and the Catalyst Foundation.]