Interferometry is a powerful and versatile tool for active MEMS characterisation. The high accuracy measurement of
deformations and vibrations of MEMS structures is an important application and well described by classical
interferometry. Deformation measurements in multi-layered structures requires a more sophisticated approach. All
phase changes along the optical path of the object light influence the measurements. Thus the shape and the
displacement of obstacles (like glass cover plates) must be included to quantify the measurement results.
The paper presents numerical simulations of the light path in an interferometric deformation measurement. A ray
tracing program is developed that keeps track of the optical path length and can thus be used to analyse disturbances
along the optical path. The simulations show how the deformation of more than one interface influences the phase
measurement. The phase errors are quantified and the reliability of the deformation measurements is evaluated.
Different interface geometries are examined. The simulations are compared to measurements on a MEMS pressure
MEMS characterisation is an important application area for interferometry. In this paper a Mach-Zehnder
interferometer configuration is presented that combines both coherent and low coherent techniques in one setup. It
incorporates the application of classical Laser Interferometry (LI) and Electronic Speckle Pattern Interferometry as well
as classical Low Coherence Interferometry (LCI), full-field Optical Coherence Tomography and Low Coherence
Speckle Interferometry. Digital Holography can be applied by minor modifications of the setup.
The setup, working principle, and applications of the interferometer will be described. Measurements on a MEMS-based
pressure sensor are presented. The sensor consists of a glass wafer attached to a silicon membrane. A cavity is
etched into the glass wafer. The wafers are bonded and form a vacuum cavity. Membrane deformations are measured
through the window using LI and LCI. LCI provides information about the shape of the glass window. Results from
speckle techniques are compared with similar results from plane wave techniques. The influence of the glass window
and the illumination of the object are investigated.
The use of liquid crystal spatial light modulators in applications, require good characterization of phase, polarization and amplitude shifting properties. This report presents a new approach for simultaneously characterizing the depolarization and controlling the polarization properties of a reflective twisted nematic liquid crystal spatial light modulator (LC SLM). The SLM was set up as a part of a Michelson interferometer. The phase response was determined by using a piezo-electric actuator for phase stepping in the reference arm. During the polarization measurement the reference beam was removed and the polarization state of the input and output was determined by a polarization state generator (PSG) and a polarization state analyzer (PSA), each consisting of a polarizer and a quarter-wave plate. Hereby, both phase response and polarization control properties could be determined independently in the same measurement configuration simply by changing static polarization components. The systematic rotation of the quarter wave plates of the PSG and the PSA using stepper motors gives out-put data whose Fourier transform in terms of angular frequency components can be used to determine all the elements of the Mueller matrix. The Mueller matrix of a commercial SLM (Holoeye LC-2500) was determined for 17 evenly spaced voltage levels addressed to the SLM.
The cut-off effect in slightly asymmetric systems prevent the wider practical usage of long range surface polaritons (LRSP) in the infrared region. Due to asymmetry caused by the presence of even a few nanometers wide air gap the LRSP mode disappear. We propose a solution of this problem by using an effective medium approach. The basic idea is to compensate for the air gap induced asymmetry by introducing an additional layer into the system. That allows us to shift the cut-off to values sufficiently for experimental observation of LRSP. FTIR spectra of end-fire coupled LRSPs in the GaAs/Au/GaAs multilayer system were measured in the (800-3000) cm-1 range. Various geometric and polarization sensitive effects of the infrared system capable of influencing the LRSP coupling is discussed. A pronounced increase in the p and s polarized light transmission ratio at long-wavelengths is explained as an increase in the propagation distance L of LRSPs. A fairly good fit between experimental and calculated curves was achieved at long wavelengths. The width of the air gap was used as a parameter in the calculations.
We consider a scattering system consisting of a dielectric film deposited on a semi-infinite metal, and focus on the wavelength dependence of the total integrated scattering and angle resolved scattering from such a system. In particular we study theoretically by a large scale rigorous numerical simulation approach the reflectivity, R((lambda) ), as well as the total scattered energy, U((lambda) ), of such systems as functions of the wavelength of the incident light. The scattering system consists of vacuum in the region x3 $GTR d1+(zetz) 1(x1), a dielectric film in the region, d2+(zetz) 1(x1), and a metal in the region x3 < d2 + (zetz) (x1). This system is illuminated from the vacuum side by p-polarized light whose wavelength is allowed to vary from 0.2micrometers to 1.2micrometers . The film is assumed to have a dielectric function that is insensitive to the wavelength of the incident light. In obtaining the numerical results reported here the metal substrate is taken to be silver. The dielectric function of silver for a given wavelength is obtained by interpolation from experimental values. The surface profile functions,(zetz) 1,2(x1), are assumed to be either zero or single-valued functions of x1 that are differentiable as many times as is necessary, and to constitute zero-mean, stationary Gaussian random processes. Their surface height auto-correlation function is characterized by a Gaussian power spectrum. We study and discuss the wavelength dependence of R((lambda) ) and U((lambda) ) for several scattering systems obtained by turning on and off the surface profile functions (zetz) 1,2(x1) and/or the correlation between these two surface profile functions.
Real time monitoring and control, by multiwavelength phase modulated ellipsometry of the growth of plasma deposited optical structures is presented. The transparent layers consists of SiO2 SiNx and oxynitrides. We present an efficient method for the estimation of the optical parameters based on a sliding window containing the last acquired measurements. This method is used to monitor the deposition of multilayer coatings with homogenous deposition conditions, and is also used in feedback control of such coatings. The sliding window method is further developed to follow slowly time-varying parameters such as the deposition rate. A preliminary study of a fast novel method based on the same principle for real time monitoring of refractive index gradients is described. As an example, the real time monitoring of the growth of a linear gradient index is demonstrated.