Christian Rembe

Heterodyne Laser-Doppler Vibrometer with a Slow-Shear-Mode Bragg Cell for Vibration Measurements up to 1.2 GHz

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.

Heterodyne laser-Doppler interferometric characterization of contour-mode resonators above 1 GHz

This paper reports the design and optical characterization of PZT transduced high-overtone width-extensional mode resonators with resonance frequency above 1 GHz. For the first time, a novel technique to measure resonance frequencies and vibrations of contour-mode resonators optically up to 1.2 GHz using only a 618 MHz carrier frequency is demonstrated. The output signal from the photo detector of the vibrometer is digitized by a fast digital oscilloscope and demodulated off-line in a computer. A signal processing algorithm has been developed to acquire frequency components from the photo detector which are higher than the carrier frequency to construct I and Q (in-phase and quadrature) signals from a virtual carrier signal with frequency 2x of the original carrier frequency. This method enabled us to realize an algorithm to extend the measurement bandwidth by a factor of 2 for small vibration amplitudes (≪ 20 nm).

Optical derotator for scanning vibrometer measurements on rotating objects

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.

The ultra fine dynamics of MEMS as revealed by the Polytec micro system analyzer

Polytec presents its latest Micro System Analyzer for dynamic characterization of MEMS. Polytec continues to advance laser Doppler vibrometry since its introduction as a MEMS characterization tool in the 1990s and has introduced the first confocal vibrometer microscope with the Micro System Analyzer in 20051. Laser vibrometer out-of-plane resolution down to 0.2 pm / root 15.6 Hz is achieved by combination of highly sensitive Doppler shift measurement, digital decoding techniques and FFT signal analysis. Laser spot sizes less than 750 nm are measured for a high magnification 100X microscope objective and compared to theoretical limitations. The theoretical determination of the lateral resolution limit is discussed in detail with the implication that measurement of objects a couple orders of magnitude smaller (<10nm) can be measured. Example measurements that illustrate the unique measurement capabilities are performed on Sandia comb drive resonators and RF switches. New measurements on the Sandia comb drive show clear advances in resolution, including the ability to place and focus the measurement beam on very tiny structures such as a 2 micron comb finger. High Frequency measurements of comb drive deflection are made out to 2MHz. Furthermore, examples of stroboscopic in-plane response measurements show resolution better than 0.01 pixel. Transient response measurements are taken to determine critical performance parameters on an RF MEMS device developed by XComWireless. This includes measurements of snap down voltage, settling time, resonant frequency and three dimensional deflection shapes of transient time response.

Positioning Errors in Coherence Scanning Interferometers: Determination of Measurement Uncertainties with Novel Calibration Artifacts

Coherence scanning interferometry (CSI) [1][2] is a non-contact versatile and fast technology for precision measurements of samples with lateral dimensions of a few micrometers up to a few centimeters. CSI can even measure on rough surfaces and have a large vertical measurement range which is only limited by the maximum displacement of the employed mechanical scanning stage. The resolution and accuracy in vertical direction can be in the nanometer range but are strongly dependent on the positioning noise and accuracy of the scanning stage.

Approaching attometer laser vibrometry

The heterodyne two-beam interferometer has been proven to be the optimal solution for laser-Doppler vibrometry (LDV) regarding accuracy and signal robustness. The theoretical resolution limit for a two-beam interferometer of laser class 3R (up to 5 mW visible measurement-light) is in the regime of a few femtometer per square-root Hertz and well suited to study vibrations in microstructures. However, some new applications of radio-frequency microelectromechanical (RF-MEM) resonators, nanostructures, and surface-nano-defect detection require resolutions beyond that limit. The resolution depends only on the photodetector noise and the sensor sensitivity to specimen displacements. The noise is already defined in present systems by the quantum nature of light for a properly designed optical sensor and more light would lead to an inacceptable influence like heating of the tiny specimen. Noise can only be improved by squeezed-light techniques which require a negligible loss of measurement light which is impossible to realize for almost all technical measurement tasks. Thus, improving the sensitivity is the only path which could make attometer laser vibrometry possible. Decreasing the measurement wavelength would increase the sensitivity but would also increase the photon shot noise. In this paper, we discuss an approach to increase the sensitivity by assembling an additional mirror between interferometer and specimen to form an optical cavity. A detailed theoretical analysis of this setup is presented and we derive the resolution limit, discuss the main contributions to the uncertainty budget, and show a first experiment proving the sensitivity and resolution improvement of our approach.

Modeling superresolution reflection microscopy via absorbance modulation (Conference Presentation)

Absorbance modulation enables lateral superresolution in optical lithography and transmission microscopy by generating a dynamic aperture within a photochromic absorbance-modulation layer (AML) coated on a substrate or a specimen. The absorbance-modulation is the property of photochromic molecules modulated between two states. The process is therefore solely controlled by far-field radiation at different wavelengths.nThe applicability of this concept to reflection microscopy has not been addressed so far, although reflection imaging exhibits the important ability to image a wide range of samples, transparent or opaque, dielectric or metallic. We will present a simulation model for absorbance-modulation imaging (AMI) in confocal reflection microscopy and it is shown that imaging well beyond the diffraction limit is feasible. Our model includes the imaging properties of confocal microscopy, reflections at the boundaries, the photochromic process and diffraction due to propagation through a subwavelength aperture.nWe derive an analytical design equation which estimates the dependence of the achievable resolution on relevant parameters, such as the AML properties and the applied light powers. This equation is very similar to the corresponding equation for STED (Stimulated emission depletion) microscopy and it is helpful for a fast design of the arrangement of optical setup and AML. As rapid scanning is relevant for a short imaging duration, we further derived an estimation for the pixel dwell time. We prove the validity of these equations by comparing the estimations with the complex numerical simulations. In addition, we show that a resolution enhancement down to 1/5 of the diffraction limit is possible.

Laser-Doppler vibrometry with variable GHz heterodyne carrier via frequency-offset lock

The generation of a heterodyne carrier frequency via offset-lock in an optical phase-locked loop (OPLL) is a widespread technique in communication, spectroscopy and other fields. Commercial state-of-the-art laser-Doppler vibrometers (LDV) generate heterodyne frequency carrier by acoustooptic devices (Bragg cells) efficiently with the slow shear mode up to 409 MHz. Therefore, these LDVs are limited in measurement bandwidth and it is impossible to adjust the heterodyne carrier frequency to the optimal value in respect to the requested demodulation bandwidth. For RF-MEMS (radio-frequency microelectromechanical systems) testing, carrier frequencies in heterodyne LDVs have to exceed 1 GHz to enable the unambiguous reconstruction of the measured vibration, which is restricted by the conventional heterodyning techniques. Recently, we demonstrated a LDV microscope with the generation of a variable heterodyne carrier frequency up to 200 MHz by offset-lock in an OPLL with visible DBR semiconductor lasers. In this paper, we demonstrate the increase of heterodyne-frequency-carrier generation by conventional RF electronics up to 1.4 GHz and discuss the decisive OPLL parameters for the application of this technique to ultra-high-frequency laser-Doppler vibrometry. Our LDV microscope shows an (out-of-plane) vibration amplitude sensitivity of less than 1 pm/ √ Hz for vibration frequencies higher than 50 MHz, which enables the vibration measurement of most RF-MEMS. First measurements of resonances of a piezoelectric transducer are presented.

Optical vibration analysis of MEMS devices with pm-resolution in x, y, and z directions

Laser-Doppler vibrometry has become the state-of-the-art technique for broadband vibration analysis with picometer resolution in microelectromechanical systems (MEMS). Displacement or velocity is detected only in direction of the measurement beam and, thus, three impinging laser beams are necessary to investigate all components of a threedimensional (3D) motion. This requirement is not problematic for 3D-vibration measurements on macroscopic objects with scattering surfaces but for reflective microstructures. A general problem of measuring 3D vibrations with three laser beams is optical crosstalk. This problem is especially critical for MEMS applications because the three beams have to be positioned closely to achieve high lateral resolution. In this paper, we prove that it is possible to impinge the small laser focus of a single laser beam with 3.3 μm diameter on a proper edge, corner or etch hole of a MEMS device to obtain real-time, 3D-vibration measurements with picometer amplitude resolution without optical crosstalk. We present the first measurements of the 3D-vibrations in MEMS devices. We prove that our method can meet the requirement of the MEMS community for fast, full-3D, broad-bandwidth, vibration measurements with picometer amplitude resolution and micrometer spatial resolution.