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Quantifying alignment of particle accelerator discs using Fourier domain short coherence interferometry

We apply Fourier domain short coherence interferometry (FDSCI) to quantify alignment of particle accelerating structures (AS) of the future compact linear collider (CLIC). Even micrometer-level misalignments inside the AS, composed of stacked copper discs, reduce the CLIC performance. Thus submicron accuracy across at least 8.6 mm measurement range is required from the quality assurance tester. To measure the internal alignment we insert a fiber-optic probe into the AS to illuminate and collect reflected light from the internal walls, Fig. 1. Our measurement technique bases on detecting a spectral interferogram that reveals after Fourier transform the distance between the wall and the probe end. The internal shape is then scanned point-by-point by rotating and pulling back the AS. In Fig. 2 we show preliminary data by measuring mirror position from several mirror-to-probe distances.fdsci

Fig. 1. Fiber-optic FDSCI setup.


Fig. 2. Mirror position measured from several mirror-to-probe distances.

Scanning White Light Interferometry

The short coherence length of the white light can be used to find the surface profile of various objects: silicon wafers, micromechanical components, ceramics, biological samples, thin films etc. The optical system can be scaled to meet the needs of the particular application by adjusting the magnification, the light source wavelength etc. The resolution is in nanometer range (1-2 nm) and with phase-shifting (and smaller measurement range) the resolution in the range of 0.1 nm is achieved.



For more information, please contact:
Ivan Kassamakov, Ph. D.


Stroboscopic white light interferometry

Micro (Opto) Electro Mechanical System (M(O) EMS ) devices are produces in increasingly large quantities. A reliable method for dynamic 3D profiling is required for a quality control (QC) process. For static 3D profilometry, scanning white light interferometry (SWLI) is a well-established method (see above). However for dynamic measurements at frequencies higher than about 1 kHz, conventional CCD-cameras used for SWLI are not fast enough to acquire clean profile data. If the vibration frequency of the sample is higher than frame rate of the camera, the image will appear to be blurred. Using stroboscopic illumination synchronized with the periodic oscillation of the device under test, the problem can be circumvented (Fig. 1).

In practice a short light pulse is produced at a certain phase angle of oscillation. While integrating over number of periods, information is acquired only during the light pulses. If the pulse length is short compared to the oscillation period (less than 10%), and the synchronization is stable, a quasi-static image is acquired from the phase angle in question. Changing the relative phase of illumination effectively allows measurement of arbitrary part of oscillatory motion.

Fig. 1 – Relationship between stroboscopic signal, sample drive voltage and camera exposure

Our objective is to modify an existing white light interferometer for dynamic 3D profile measurement of MEMS devices. By using a stroboscopic LED setup (Fig. 2), the oscillation can effectively be frozen. The two-channel function generator is used to provide voltage for both the sample under study and the LED used for the illumination. The channels are synchronized together, and by adjusting the inter-channel delay, stroboscopic imaging of arbitrary phase angle of the oscillation period is possible.

Fig. 2 – Schematic of the stroboscopic SWLI setup

Fig. 3 – Quasi-static 3D profile of MEMS sample driven by sine wave voltage; Left: 0° phase angle; Right 90° phase angle

Fig. 4 – 2D profiles of MEMS sample at different phase angle settings

For more information, please contact:

Kalle Hanhijärvi, Ivan Kassamakov, Ph. D.


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