Optically Excited And Interrogated Micromechanical Silicon Cantilever Structure

Abstract

Micromechanical silicon cantilever structures of typical dimensions 1 mm x 80 pm x 5 pm have been excited by absorption of pulsed light from diode laser at 790 nm or a LED at 830 nm respectively. The excitation motion of the cantilevers was measured by means of a fiber -optic Michelson interferometer as well as a reflective multimode fiber optic pick up. In addition to the optical signal the electrical signal of a thin film piezoresistive transducer localized at the base of the cantilever has been measured. The optimal fiber position with respect to the cantilever was determined experimentally. At this position a signal to noise ratio of about 6 could be realized for the detected signal at 5 kHz bandwidth for optical power levels of 56 pW. Using a 125 pm diameter optical fiber this corresponds to an optical power density of 4.6 mW /mm2 which is below the critical value of 5 mW /mm2 suggested for explosive environments.

Optical microresonator sensor systems

“…Wolfelschneider et al, showed that the cantilevers, typically 1 mm long, could be driven into vibration by as little as 22 microwatts of modulated optical power, aimed at the optimal site about 50 microns along from the cantilever base. 30 Multimode fibres were used to bring the activating and the detecting optical beams to the sensor head, with the detecting fibre collecting light reflected back by a large paddle at the cantilever's free end. Incoherent detection was found to be possible, allowing LEDs to be used for both sources if desired; the quality factors of the vibrations were reasonably high, at around 50, and the system obviously has the potential to become an optical microresonator sensor.…”