Today there exist different commercial micro-optics measurement instruments for the characterization of micro-optical
components and microlenses in particular. However there is often a lack of a complete quantitative optical
characterization of the latter components. Therefore we will focus in this paper on the optical characterization of all
types of refractive microlenses, more in particular spherical and aspherical microlenses. Moreover the results of the
performed round robin within the European 6th FP Network of Excellence on Micro-optics "NEMO" will allow us to
select the most appropriate instrumentation tools for characterizing refractive microlenses.
The active Twyman-Green laser interferometer for MEMS measurement equipped with Spatial Light Modulator (SLM)
as a reference element is reported. The SLM is electrically addressed, reflective (made in Liquid Crystal on Silicone
technology) and phase-only device which allows to actively shape of the reference beam wavefront in the
interferometer. The proper use of the SLM in interferometric MEMS measurement is possible after opto-mechanical
modification of the interferometer, performed calibration procedures and special interferogram processing. All these
aspects are described. The use of such device benefits extension of measurement range and simplification testing
procedures. Usefulness of the SLM is shown at the examples of active microelements testing. Advantages and
disadvantages of SLM application are described and potential of this device for interferometry is discussed.
Today there exist different commercial and proprietary micro-optics measurement instruments for the characterization of micro-optical components and microlenses in particular. However there is often a lack of a complete quantitative optical characterization of the latter components. Therefore we will focus in this paper on the optical characterization of spherical microlenses. Moreover the results of the performed round robin within the European 6th FP Network of Excellence on Micro-optics "NEMO" will allow us to select the most appropriate instrumentation tools for characterizing refractive spherical microlenses.
Micro-Electro-Mechanical Systems are nowadays frequently used in many fields of industry. The number of their applications increases and their functions become more responsible, therefore precise knowledge about their properties is necessary. Due to its fragility and small sizes non-contact and high sensitive measurement method is required. Two-beam laser interferometry is one of the most popular testing methods of microelements. Such method implemented in Twyman-Green interferometer allows for full-field shape determination and out-of-plane displacement measurement. However the elements under test may bring additional challenges: their surfaces may have complicated shape or large shape gradients which prohibit their testing by means of interferometer with a flat reference mirror. To overcome such problems we propose to use LCOS (Liquid Crystal On Silicon) - phase, reflective SLM as an active reference element. LCOS serves as an adaptive reference mirror and phase shifter. The use of such element allows increasing measurement range of the interferometer and simplifies out-of-plane displacement measurement through object wavefront compensation. The applicability of the modified Twyman-Green interferometer will be shown at the examples of active micromembranes testing.
Micro-Electro-Mechanical Systems are nowadays frequently used in many fields of industry. The number of their applications increase and their functions became more complex and demanding. Therefore precise knowledge about their static (shape, deformations, stresses) and dynamic (resonance frequencies, amplitude and phase of vibration) properties is necessary. Two beam laser interferometry is one of the most popular testing methods of micromechanical elements as a non-contact, high-accurate method allowing full-field measurement. First part of the paper present microbeam actuators designed for MEMS/MOEMS applications. The proposed structures are the straight silicon microbeams formed by KOH etching of Si wafer. Aluminium nitride (AlN) thin films are promising materials for many acoustic and optic applications in MEMS field. In the proposed architecture the actuation layer is sandwiched between two metal electrodes on the top of beam. In the second part we describe the methodology of the actuator characterization. These methods applied are: stroboscopic interferometry and active interferometry (LCOS SLM is used as a reference surface in Twyman-Green interferometer). Moreover some results of FEM analysis of the sample are shown and compared with experimental results. Dynamic measurements validate the design and simulations, and provide information for optimization of the actuator manufacturing process.
Increasing technological capabilities to produce microelements (for example: microbeams, micromembranes and micromirrors) and their expanding areas of application introduce unprecedented requirements concerning their design and testing. Conventional two beam interferometry is one of the most popular testing methods of microelements that have reflecting surface. However the elements under test may bring additional challenges: their surface finish may be mixed i.e. reflective-diffusive which restrict their analysis by conventional interferometry; their surfaces may have complicated shape or large shape gradients which restrict their testing by means of interferometer with flat reference mirror. In this paper we propose to solve these problems by converting conventional Twyman-Green interferometer into multifunctional measurement platform by introducing different reference surfaces including: mirror (for conventional two beam interferometer); diffuser (for ESPI); Liquid Crystal On Silicon (LCOS) phase spatial light modulator (for active interferometer). Diffuser allows to implement ESPI in the same system configuration. Special software enables to combine the results of measurement by conventional interferometry (mirror-like surface) and ESPI (diffuse surface). LCOS serves as an adaptive reference mirror and phase shifter. The use of such element allows to increase measurement range of the interferometer and simplifies out-of-plane displacement measurement through object wavefront compensation. The
applicability of the platform will be shown at the examples of active micromembranes testing in static and dynamic modes of their work.
Significant progress in MEMS/MOEMS development requires new measurement methods. The pulse interferometry is one of the widely used technique for oscillating object analyses. It is based on object observation during the short laser pulse illumination. In the paper a new method for electronic time synchronization of oscillating objects, laser pulses and camera registration is proposed. The system is developed to measure the silicon micromembrane surface shape. Laser pulse mode properties and camera adaptation to a microinterferometer setup are considered in the paper. A complementary system oriented on the investigation of the relationship between object transient shape changes and oscillation phases is shown. Preliminary results of measurement of silicon membrane shape changes during oscillations are presented.
The visualization and analysis methods for studying vibration modes of macro and micro scale objects with spectrally reflecting and scattering surfaces, developed at the Institute of Micromechanics and Photonics of the Warsaw University of Technology, are presented. Silicon technology prepared microspecimens (AFM cantilevers and active PZT micromembranes) are investigated using two-beam time-average interferometry. Vibration modes of flat and non-flat surface microelements are displayed using four and five-frame temporal phase stepping methods. The calculated contrast of vibrating object interferograms provides the information on the vibration amplitude. Scattering surface objects are studied by time-average fiber-optics digital speckle pattern interferometer (DSPI) with heterodyning. Sinusoidal phase modulation introduced at the object vibration frequency enables quantitative analysis of the amplitude and phase of sinusoidal type vibrations. Laser diode modulation and/or single mode fiber stretching is applied for that purpose. For low vibration amplitudes (a0 < l/20) the method using linear approximation of the zero order Bessel function provides an automatic analysis tool for quantitative estimation of the vibration modes.
Stroboscopic interferometry is the most popular method for investigation of active, vibrating elements. The interferograms obtained in measurement steps may be analysed by temporal phase shifting method or by spatial carrier frequency methods. The first one requires sequential capturing of phase-shifted interferograms which complicates the measurement system and introduces high stability requirements for the setup. The spatial methods need a single interferogram with a proper spatial carrier frequency (SCF), so they are more suitable for dynamic events analysis. The most frequently used spatial method is based on Fourier transform of an interferogram with linear SCF and can be applied to analysis of restricted class of elements represented by quasi-linear fringes. This can be easily expanded by considering elements with circular carrier fringes (CCF). In the paper two approaches to analysis of interferograms with CCF, namely: coordinate transform Fourier transform technique and direct filtering Fourier transform technique are explained. The error analysis of both techniques applied for different classes of interferograms is presented. The methodology of CCF interferogram analysis based on FT methods applied for micromembranes is presented and several exemplary results are given.
We present a methodology for static and dynamic testing of mechanical properties of microelements. The measurement path includes temporal phase shifting interferometry for quantitative static shape elements analysis. This is followed by determination of the resonance frequency by means of modified time average interferometry and transient amplitude and phase maps of vibrating micromembrane capturing and evaluation by phase shifting stroboscopic interferometry. Proper application of combination of these methods allows for quick and accurate analysis of micromembranes and optimization of their manufacturing conditions.
Increasing technological capabilities to produce active microelements (incl. microbeams, micromembranes and micromirrors) and their expanding areas of application introduce unprecedented requirements concerning their design and testing. Conventional two beam interferometry is one of the most popular testing method of microelements that have reflecting surface. Sometimes elements under test have complicated shape or shape gradients which restricts their testing by means of interferometer with flat reference mirror. In this paper we propose to use Liquid Crystal On Silicon (LCOS) spatial light modulator which serves as an adaptive reference mirror and phase shifter in Twyman-Green interferometer applied for microelements measurement. Initial tests have been performed and results confirming applicability of LCOS in active interferometer system are presented.
The automated system for microelement testing, which consists of Twyman-Green interferometer and waveguide grating interferometer integrated with optical microscope is presented. The microsocpe is additionally equipped with mechanisms to fix, microposition and load a specimen under study. Full-field analysis of out-of-plane and in-plane displacements of microelements is performed at this study. The applicability of the system is proven by performing test of silicon membrane and silicon beam.
Increasing technological capabilities to produce active microelements (incl. microbeams, micromembranes and micromirrors) and their expanding areas of application introduce unprecedented requirements concerning their design and testing. The paper presents a concept of an optical measurement system and methodology for out-of-plane displacement testing of such active microelements. The system is based on Twyman-Green microinterferometer. It gives the possibility to combine the capabilities of time average and quasi-stroboscopic interferometry methods to find dynamic behavior of active microelements (e.g., resonance frequencies and amplitude distributions in vibration modes). For mapping the zero-order Bessel function modulating the contrast of two-beam interference fringes the four-frame technique is applied. The calibration of the contrast variation in time-averaged interferograms enables quantitative evaluation of the vibration amplitude encoded in the argument of the Bessel function. For qualitative estimation of the vibration amplitude sign a simple quasi-stroboscopic technique is proposed. In this technique, laser pulses have the same frequency as the signal activating the microelement under test. This self-synchronous system enables to visualize the shape of the tested element at maximum deflection. Exemplary results of measurements performed with active micromembranes are presented.
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