Miniature fiber-optic-coupled sensors based on optically excited, self-resonant microbeams have been proposed for measuring temperatures within muscle fascicles and tendons. The proposed sensors could be used in medical and biological research on humans and other animals. The proposed sensors would be variants of those described in several previous articles in NASA Tech Briefs: "Proximity Measurement of Pressure and Temperature" (NPO-20223), Vol. 22, No. 1 (January 1998), page 48; and "Measurement of Stresses and Strains in Muscles and Tendons" elsewhere in this issue.
Each sensor would be made of electrically nonconductive materials that are chemically and galvanically inert with respect to living tissue. Typical sensor dimensions would be about 0.5 by 0.5 by 0.1 mm. These dimensions are suitable for surgical implantation in muscle and tendon tissues; these dimensions are also comparable to diameters of cores of multimode optical fibers, making the sensors amenable to fiber-optic coupling.
Each sensor would contain a cantilever microbeam located in a vacuum cavity in a housing. The beam would serve as an optomechanical resonator (as explained below) with a resonance frequency of the order of a megahertz and a resonance quality factor (Q) of the order of 105. The beam would face an integral, embedded photodiode that would be coupled to external instrumentation via a multimode optical fiber. Taken together, the beam and vacuum cavity would constitute a quarter-wave Fabry-Perot interferometric structure.
Optical excitation supplied via the optical fiber would cause the photodiode to generate a voltage that would bend the beam via electrostatic attraction. Following initial bending, the beam would vibrate at its natural resonance frequency. Because the beam and vacuum cavity would constitute a quarter-wave Fabry-Perot interferometric structure, the vibrations would give rise to modulation of the incident light reflected from the beam. The modulation of the light would alter the photovoltage and thereby contribute to a feedback mechanism that would sustain the resonant vibration. In addition, the modulated light reflected into the fiber would travel to the far end of the fiber, where a photodetector would convert the modulation to a quasi-digital stream of electrical pulses. The pulse stream would be fed to a counting circuit to determine the frequency of vibration.
The beam would be made of polycrystalline silicon doped to have a high thermal coefficient of stiffness, so that its resonance frequency would vary appreciably with temperature. Typically, the temperature coefficient would be such that measurements of frequency could be converted to temperature measurements with resolutions as small as a millidegree. The dynamic range of the sensor would be of the order of 108.
This work was done by Frank Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.