Electronic Military & Defense Annual Resource

4th Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

Issue link: https://electronicsmilitarydefense.epubxp.com/i/350588

Contents of this Issue


Page 29 of 59

device leads to periodic bending, and resonance is excited by matching the heating source frequency to that of the natural frequency of the NEMS device. The optically excited mechanical motion of the sensor array is detected using a holography-based optical inter- ferometry system. Schematics of this system are shown in Figure 3 on page 29. The output of a 200 mW frequency doubled Nd:YAG laser operating at 532 nm is split into two paths, forming the signal and reference beam. The signal beam passes through a lens (point to image switching lens, or PISL) and a 50 X objective to illuminate the sample which is mounted on a piezotransducer. The transducer allows ultra- sonic actuation of the sensor in underwater and fluid-based environments. For remote actuation in ground and air-based surveillance, the sensor is optically driven using a laser. The PISL and the 50 X objective create an array of detection points on the sensor array, facilitating multiple point detection. The position and the focal length of the PISL determine the area of the sample that is illuminated. The reflected signal beam from the sensor array interferes with the reference beam at the photorefractive crystal (PRC). The PRC acts as a dynamic hologram, capturing the time-varying surface motion of the sensor array. The reference beam is phase modulated using an electro- optic phase modulator (PM). The function generators driving the piezotransducer and the phase modulator through a radio frequency (RF) power amplifier are synchronized using a 10 MHz reference clock. The polarization of the interfering beams is selected using half-wave plates (HWP) placed in their respective paths before the PRC crystal. A 10 by 10 by 2.25 mm bismuth silicon oxide (BSO) is used as the photore- fractive crystal. The response time of the crystal is ~100 ms. The external angle between the interfering beams is set at 50 degrees in order to maximize the response of the crystal for operation in the diffusive regime. The diffracted reference beam reads out the strength of the diffraction grating. The polarization of the diffracted reference beam is 90 degrees rotated with respect to the transmitted signal beam due to the anisotropic self-diffraction in optically active BSO crystals. A quarter-wave plate (QWP) and a linear polarizer (LP) isolate the diffracted reference beam from the signal beam. A CCD records the displacement image containing quantitative infor- mation about the sensor vibrations. A 16-bit Pixis CCD (512 by 512 pixel) captures the displacement images. The camera is externally triggered using a function generator, which allows the custom timing of the camera for measurement purposes. In the full-field optical excitation of NEMS, a laser beam of an elliptical cross-section of 160 by 180 µm 2 with an optical power of 3.75 W is incident on the NEMS array. A sample containing 60 doubly clamped beams with a length of 10 µm, a width of 1 µm, and a thickness of 200 nm (150 nm of silicon and 50 nm of Cr) is fabricated in a configuration as shown in Figure 4(a). For the full-field displacement measurements, the drive fre- quency of the excitation laser is swept from 20.0 to 24.2 MHz in discrete steps of 0.1 MHz, and the mix-down frequency for detection is fixed at 5 Hz. The exposure time of the camera is set at 20 ms, and the camera is externally triggered to take four frames per cycle. At each drive frequency, 250 cycles of vibrations at the mix-down frequency of 5 Hz are recorded. Figure 4(b) shows a full-field displacement image of the NEMS array obtained at 21.6 MHz. An array of 3 by 3 pixels is selected at the middle of each beam to determine its displace- ment response. Figures 4(c) and 4(d) show the displacement and resonance frequency distribution of the NEMS array. In biological and chemical sensing applications, these quantita- tive maps can be used to monitor the presence and concentra- tion of various analytes. These parameters have a direct effect on the resonance and thus will shift the resonant frequency, which can be easily monitored by tuning the frequency of the laser until the sensors start vibrating again. This shift is directly proportional to the concentration of the analytes, and functionalizing the sensors can provide positive identification of various analytes in the environment. Application In The Field The long-term vision of this research is to provide real-time reconnaissance and threat assessment of biological and chemical hazards to ground-based troops. Ground person- nel can wear these sensors to assess uncharted territory for biological and chemical threats. In addition, these sensors can be deployed using air reconnaissance drones and monitored using multiplexed optical LIDAR systems that incorporate the holography detection technique described here. Successful implementation of this technology will result in expandability of the technology into CBRNE (chemical, biological, radio- logical, nuclear, and explosives) handhelds, integration with electro-optic (EO) sensors in human-machine interface (HMI), and battle management systems (BMS). Technology 30 Electronic Military & Defense Annual Resource, 4th Edition Dr. Ashwin Sampathkumar is a member of the research staff at Riverside Research where he is currently developing novel tech- nologies for the remote, long-range characterization and imaging of materials at the micro- and nanoscale using photoacoustics and hyperspectral methods. Sampathkumar received a B.E. in mechanical engineering from University of Madras in 2003 and his Ph.D. in mechanical engineering from Boston University in 2010. Figure 4 (a,b,c, and d) a b c d

Articles in this issue

Archives of this issue

view archives of Electronic Military & Defense Annual Resource - 4th Edition