The state-of-the-art technique for optical vibration analysis of macroscopic structures is laser-Doppler vibrometry in which a single-laser beam measures the motion in the beam direction. Thus, three laser beams are necessary to investigate three-dimensional (3-D) motions. The laser spots can be separated on macroscopic specimens with scattering surfaces to prevent optical crosstalk between the measurement beams, but such separation is impossible for a microscopic scatter point. We demonstrate a solution for this problem: an optical 3-D vibrometer microscope with a single-impinging laser beam, which collects scattered light from at least three directions. We prove that it is possible to realize a small laser focus of <3.5-μm diameter on a proper scatter point such as an etch hole of a microelectromechanical-systems device to obtain real-time, 3-D vibration measurements with megahertz vibration bandwidth and picometer amplitude resolution. A first measurement of operational-deflection shapes is presented.
The real-time measurement of three-dimensional vibrations is currently a major interest of academic research and
industrial device characterization. The most common and practical solution used so far consists of three single-point
laser-Doppler vibrometers which measure vibrations of a scattering surface from three directions. The resulting three
velocity vectors are transformed into a Cartesian coordinate system. This technique does also work for microstructures
but has some drawbacks: (1) The surface needs to scatter light, (2) the three laser beams can generate optical crosstalk if
at least two laser frequencies match within the demodulation bandwidth, and (3) the laser beams have to be separated on
the surface under test to minimize optical crosstalk such that reliable measurements are possible. We present a novel
optical approach, based on the direction-dependent Doppler effect, which overcomes all the drawbacks of the current
technology. We have realized a demonstrator with a measurement spot of < 3.5 μm diameter that does not suffer from
optical crosstalk because only one laser beam impinges the specimen surface while the light is collected from three
The advanced progress in miniaturization technologies of mechanical systems and structures has led to a growing
demand of measurement tools for three-dimensional vibrations at ultra-high frequencies. Particularly radio-frequency,
micro-electro-mechanical (RF-MEM) technology is a planar technology and, thus, the resonating structures are much
larger in lateral dimensions compared to the height. Consequently, most ultra-high-frequency devices have larger inplane
vibration amplitudes than out-of-plane amplitudes. Recently, we have presented a heterodyne interferometer for
vibration frequencies up to 1.2 GHz. In this paper we demonstrate a new method to extract broad-bandwidth spectra of
in-plane vibrations with our new heterodyne interferometer. To accomplish this goal we have combined heterodyne
interferometry, scanning vibrometry, edge-knife technique, amplitude demodulation, and digital-image processing. With
our experimental setup we can realize in-plane vibration measurements up to 600 MHz. We will also show our first
measurements of a broad-bandwidth, in-plane vibration around 200 MHz. Our in-plane and out-of-plane vibration
measurements are phase-correlated and, therefore, our technique is suitable for broad-bandwidth, full-3D vibration
measurements of ultrasonic microdevices.
Several new applications for optical ultra-high frequency (UHF) measurements have been evolved during the last
decade by advancements in ultra-sonic filters and actuators as well as by the progress in micro- and nanotechnology.
These new applications require new testing methods. Laser-based, non-influencing optical testing is the best choice. In
this paper we present a laser-Doppler vibrometer for vibration measurements at frequencies up to 1.2 GHz. The
frequency-shifter in the heterodyne interferometer is a slow-shear-mode Bragg cell. The light source in the
interferometer is a green DPSS (diode pumped solid state) laser. At this wavelength the highest possible frequency shift
between zero and first diffraction order is a few MHz above 300 MHz for a slow shear-mode Bragg cell and, therefore,
the highest possible bandwidth of the laser-Doppler vibrometer should usually be around 300 MHz. A new optical
arrangement and a novel signal processing of the digitized photo-detector signal is employed to expand the bandwidth to
1.2 GHz. We describe the utilized techniques and present the characterization of the new ultra-high-frequency (UHF)
vibrometer. An example measurement on a surface acoustic wave (SAW) resonator oscillating at 262 MHz is also
demonstrated. The light-power of the measurement beam can be switched on rapidly by a trigger signal to avoid thermal
influences on the sample.
The heterodyne interferometer (vibrometer) is a well established technique for measuring all kinds of mechanical
vibrations in a broad range of applications. The non-contact measurement principle relies upon the Doppler (or phase-)
shift that laser light experiences when it is reflected by the vibrating surface.
The speckle nature of the reflected light imposes problems and creates additional measurement noise if the object is
moving transversely through the laser spot or is rotating around an axis perpendicular to the laser direction. Another
implication that can arise is cross coupling from in-plane vibrations into the out-of-plane measurement direction when
small in-plane vibrations are present.
A model is presented in this paper that describes the origin of these disturbances. Using this model it is possible to
quantify the amplitude spectrum of the noise in displacement and velocity measurements. This enables the user to
calculate the limits of resolvable vibration amplitudes when transverse motion is present. The results of the model have
been confirmed well by measurements.
In addition, the influence of the surface roughness and beam inclination on the out-of-plane vibration measurements at a
tilted surface is investigated. The conditions for the measurability of the profile of a transversely moving surface are
derived in this work. It is discussed that the Rq-roughness parameter has to be less than &lgr;/4 to obtain the slope
information in the speckle-perturbed interferometer signal.
In this paper we present an optical derotator for scanning vibrometer measurements on rotating objects. The main part of an optical derotator is a rotating prism. Several concepts are known from literature. We have chosen a Dove prism because it can derotate the rotation of the specimen by simply watching through the prism, which rotates with half the speed. The design of our derotator is presented in this paper as well as a discussion of the system performance. In addition we show experimental measurement results on a fan rotating with 3000 rpm.
Sub-micrometer lateral resolution for the optical vibration measurement can be achieved when the scanned laser beam of a confocal microscope is the measurement beam of a heterodyne laser-Doppler vibrometer. Such a scanning system that allows vibration measurements up to 30 MHz is presented in this paper for the first time. We measured a minimum 1/e2 -power spot diameter of 745 nm and, therefore, the vibration analysis of sub-micrometer mechanical structures is possible with our system. We demonstrate measurements on comb-drive fingers with 2 μm diameter, the tiniest structures available for us.
Laser-Doppler Vibrometry has been proven to be an accurate method to measure vibration amplitudes with a known error budget. Therefore, vibrometer technique is used to realize primary standards in vibration-amplitude measurements at national metrology institutes (e.g. Physikalisch Technische Bundesanstalt PTB in Germany). However, additional error sources emerge for vibrometer measurements at microscopic structures. This paper discusses two error sources that can be neglected for usual macroscopic testing but can become important for microscopic measurements.
To describe experiments on fluorescence excitation of single N-V defect centers in diamond we have used the double well potential (DWP) model which incorporates the possibility of tunneling the nitrogen atom into the vacancy both in ground and excited electronic states of the center. Fluorescence and linear absorption spectra are calculated, DWP's parameter values for N-V centers are determined and manifestations of their variations due to local diamond lattice distortions in fluorescence spectra of various single N-V centers are studied.
Fluorescence microscopy on single light harvesting 2 complexes from photosynthetic bacterium Rhodopseudomonas Acidophila strain 10050 has been carried out. The polarization of the fluorescence emitted by individual complexes immobilized on a surface was detected as a function of time. Abrupt changes in the polarization of the emitted light of single chromophores were observed. These abrupt changes are attributed to a localization of the exciton on a small fraction of the chromophores on the closely coupled ring shaped aggregate of the complex.