Astrid Aksnes

Wide frequency range measurements of absolute phase and amplitude of vibrations in micro- and nanostructures by optical interferometry.

A heterodyne interferometer has been built in order to characterize vibrations on micro- and nanostructures. The interferometer offers the possibility of both absolute phase and high resolution absolute amplitude vibrational measurements. By using two acousto-optic modulators (AOMs) in one of the interferometer arms and varying the frequency inputs of both, the setup is designed to measure vibrations in the entire frequency range 0 - 1.2GHz. The system is here demonstrated on Capacitor Micromachined Ultrasonic Transducers (CMUTs) and a PZT transducer to show measurements from 5kHz up to 35MHz. We have measured absolute amplitudes with picometer resolution.

Compound Speckle Characterization Method and Reduction by Optical Design

Speckle and the compound speckle can be reduced by angle diversity. In laser projection displays, simple, low cost and efficient speckle reduction techniques require smart optical design . Using a MEMS scanner together with a condenser lens, laser beams with different illumination angles are obtained on the diffuser surface with low speckle contrast ratio (CR). After homogenizing within a rod integrator, the speckle field illuminates a display panel, and is projected onto the screen which forms the compound speckle. Characterization method to evaluate the compound speckle reduction efficiency is analyzed and discussed in a simplified optical system. The preliminary speckle reduction is demonstrated in a commercial projector where a 600 mW green laser has been used as the illumination source, and the compound speckle CR is brought down from 0.38 to 0.14.

Hyperspectral imaging of atherosclerotic plaques in vitro.

Vulnerable plaques constitute a risk for serious heart problems, and are difficult to identify using existing methods. Hyperspectral imaging combines spectral- and spatial information, providing new possibilities for precise optical characterization of atherosclerotic lesions. Hyperspectral data were collected from excised aorta samples (n = 11) using both white-light and ultraviolet illumination. Single lesions (n = 42) were chosen for further investigation, and classified according to histological findings. The corresponding hyperspectral images were characterized using statistical image analysis tools (minimum noise fraction, K-means clustering, principal component analysis) and evaluation of reflectance/fluorescence spectra. Image analysis combined with histology revealed the complexity and heterogeneity of aortic plaques. Plaque features such as lipids and calcifications could be identified from the hyperspectral images. Most of the advanced lesions had a central region surrounded by an outer rim or shoulder-region of the plaque, which is considered a weak spot in vulnerable lesions. These features could be identified in both the white-light and fluorescence data. Hyperspectral imaging was shown to be a promising tool for detection and characterization of advanced atherosclerotic plaques in vitro. Hyperspectral imaging provides more diagnostic information about the heterogeneity of the lesions than conventional single point spectroscopic measurements.

Sinusoidal rotating grating for speckle reduction in laser projectors: feasibility study

This paper describes a novel idea for reduction of speckle contrast in laser display projectors using the rotation of a diffraction pattern whose zeroth order has been canceled out without loosing power. The feasibility of the proposed method was investigated by illuminating gratings with a sinusoidal phase on two spatial light modulators (SLMs) in series for minimal intensity modulation, where the phase grating pattern was rotated with respect to the previous one on both SLMs. Two series of measurements were done with different periods of the sinusoidal grating. For each series, an image of the speckle pattern was recorded at discrete rotation angles of the phase grating, and then an average image was calculated. Experimental results were compared with a new theoretical model for speckle contrast of N partially correlated speckle patterns. The experimental measurement results compare well with the theoretical predictions resulting in a minimum speckle contrast of 0.36, with further reduction possible. Parameters necessary to achieve target contrast (0.08 or less) are discussed.

Heterodyne Interferometry for High Sensitivity Absolute Amplitude Vibrational Measurements

A heterodyne interferometer has been built in order to characterize vibrations on Micro-Electro-Mechanical Systems (MEMS). The interferometer offers the possibility of both phase and high resolution absolute amplitude vibrational measurements, which is of great importance. A frequency shift is achieved by introducing acoustooptic (AO) modulation in one of the interferometer arms. By using a lock-in amplifier a narrow bandwidth detection regime is achieved. This factor improves the amplitude resolution. By using two AO-modulators and varying the frequency inputs of both, the setup is designed to measure vibrations in the entire frequency range 0 - 1.2GHz. The absolute amplitude is obtained by performing two measurements at each sample point. The first step is to measure the first harmonic of the object vibration. The second step is to measure the frequency components of the light reflected from the test device corresponding to the frequency without object modulation. This is obtained by mixing the detector signal with an external signal generator, and adjusting the frequency of the latter. By combining these two measurements we are able to determine the absolute amplitude of the vibration. The interferometric setup can be used to characterize various kinds of micro- and nanostructures. The system is here demonstrated on a Surface Acoustic Wave (SAW) device and on Capacitor Micromachined Ultrasonic Transducers (CMUTs). We have measured absolute amplitudes with picometer resolution.

Heterodyne interferometer for absolute amplitude vibration measurements with femtometer sensitivity

A heterodyne interferometer for highly sensitive vibration measurements in the range 100 kHz - 1.3GHz is presented. The interferometer measures absolute amplitude and phase. The signal processing of the setup is analyzed and described in detail to optimize noise suppression. A noise floor of 7.1 fm/Hz(1/2) at 21 MHz was achieved experimentally where the bandwidth is the inverse of all time needed for filter settling and signal sampling. To demonstrate the interferometer, measurements up to 220 MHz were performed on arrays of capacitive micromachined ultrasonic transducers (CMUTs). The measurements provided detailed information e.g. about the frequency response, vibration patterns and array uniformity. Such measurements are highly valuable in the design process of ultrasonic transducers.

Inspection of processes during silicon wafer sawing using low coherence interferometry in the near infrared wavelength region

Multi-wire sawing of silicon wafers is a tribological process. Slurry consisting of small silicon carbide particles embedded in polyethyleneglycol carries out the abrasive material removal process. During this process small silicon chips are removed from the bulk material. Low coherence interferometry (LCI) is widely used for high accuracy surface topography measurements of materials. This paper presents an application of LCI where the surface of a material (silicon) is inspected from the inside. Light in the near infrared (NIR) wavelength region is used. High spatial resolution is necessary to be able to observe the processes on the micro scale. Therefore a modified solid immersion approach is suggested. That makes it possible to reach a spatial resolution in the range of the illumination wavelength. The topography changes produced by the chippings are in the range of some micrometers. To be able to estimate the volumes of the Si chippings interferometric phase measurements are applied.

Characterization of acoustic vibrations on micro- and nanostructures with picometer sensitivity

A heterodyne interferometer with picometer sensitivity for non-destructive characterization of micro- and nanostructures has been built. The setup is designed to measure phase and amplitude in the entire frequency range 0-1.2GHz. The object can be scanned in the x- and y-direction with sub-micrometer precision. Absolute amplitude of vibration is determined by combining separate measurements of the carrier and sideband frequency of the detected signal. The detector signal is mixed with a signal from a generator. By adjusting the frequency of the signal generator, we can choose the carrier or sideband frequency. We have performed measurements on capacitor micro-machined ultrasound transducers (CMUTs) which are being developed for diagnostic imaging of vulnerable plaques in arteries. Arrays of ~7500 CMUTs with a total area of 1.3mm x 0.9mm are planned used in an intravascular catheter. The CMUTs studied have typical radii of 5.7-12.5μm, membrane thickness of 100nm, and center frequencies 10-35MHz. Characterization of both single and arrays of CMUTs is important to optimize the manufacturing process and the design. Quality control during manufacture is also important to identify imperfect elements. Other structures have been characterized such as a piezoelectric element with excitation frequencies from a few kHz to several hundreds of kHz and a LiNbO3 surface acoustic wave (SAW) transducer with excitation frequencies from 20MHz to 30MHz. We have performed initial measurements of absolute amplitudes with picometer resolution. Theoretical calculations agree well with the measurements. The setup can be used to characterize a large range of micro- and nanostructures.

Optical vibration measurements of cross coupling effects in capacitive micromachined ultrasonic transducer arrays

Optical vibration measurement systems are excellent tools for characterizing ultrasonic transducers. This paper presents measurements on immersed arrays of capacitive ultrasonic transducers (CMUTs) using a heterodyne interferometer. The interferometer allows measurements of vibrations from DC up to 1 GHz with a noise floor of ~1pm/√Hz. Previously CMUTs have been characterized in air. The transducer is intended for intravascular use. Therefore the CMUTs were characterized in the transparent fluids kerosene and rapeseed oil that have acoustic properties closer to blood. The optical measurements on immersed CMUTs were validated by assessing the measurement errors caused by the acousto optic effects in the fluid. When immersed there is significant cross coupling between individual CMUTs within an array. Simulations presented here indicate that this causes an acoustic wave mode that is bound to the interface between the CMUTs and the fluid. This is confirmed by measurements of the phase velocity and attenuation coefficient of this wave. The measurement results indicate that the wave exists up to a maximum frequency and that the attenuation constant increases with increasing frequency. Rapeseed oil causes a significantly larger attenuation coefficient than kerosene, which most probably is due to a considerable difference in fluid viscosities. There was a mismatch between the simulated and measured phase velocity for low frequencies. It is likely that the cause of this is coupling between the fluid CMUT interface waves and Lamb waves in the substrate of the CMUT array. Measurements performed with the heterodyne interferometer have confirmed the presence of dispersive waves bound to the surface of the transducer by directly showing their propagation along the array. The setup has also characterized the bound waves by measuring dispersion relations.

High sensitivity vibration measurements with absolute calibration

A heterodyne interferometer has been built to characterize surface vibrations between 0 - 600 MHz. The system has been used to measurements of high resolution absolute amplitude and phase on a SAW device at 25 MHz.