Optical Displacement Measurements

A powerful method to characterize CMUTs is to measure the membrane displacement of single cells by using optical interferometry. This method enables the measurement of displacements which are much smaller than the wavelength of light. The motion of a reflective surface of interest modulates the path lengths of laser beams, which are made to coincide, i.e. to interfere. This operating principle is well known as the Mach-Zehnder interferometer.

Since 2003 we have an optical fiber interferometer OFV-511 (Polytec GmbH, Waldbrinn, Germany) in our lab, which is based on the idea of the Mach-Zehnder interferometer. The interferometer is attached to a common microscope by a microscope adapter OVF-072 (Polytec), see Fig.1. The microscope is required to position the laser beam at the spot of interest at the CMUT cell. The interferometer itself is connected to an ultrasonic vibrometer controller OFV-2700/2 (Polytec) that contains a modified wide-band displacement decoder OVD-30 (Polytec) with an extended frequency range. Thus, this system enables to measure in a frequency range 50 kHz to 30 MHz, which covers the most of our CMUT devices.

 

FIGURE 1. Microscope and microscope adapter with camera for the optical interferometer OFV-511.

 

 

Table 1 gives a detailed overview over the main specifications of this system:

 

TABLE 1. Main specifications of our optical measurement system.

Max. Displacement	± 75 nm (50 kHz - 20MHz)    ± 50 nm (20MHz - 30MHz)
Output Level	50 nm per Volt @ 50
Max. Velocity	6 m/s
Linearity Error	< - 1% in ± 25 nm    < - 5% in ± 50 nm
Calibration Error	< ± 3 % @ 100 kHz / 50nm peak to peak
Resolution	Noise limited, typically 0.25 nm

 

 

 

FIGURE 2. CMUT array attached in a PCB board, immersed in soybean oil below the microscope lense.

 

To capture the output of the interferometer decoder, we use a digital oscilloscope (Infiniium 500 MHz, 2GS/s, Agilent Technologies Inc., Palo Alto, CA). The data are then transferred over a GPIB—IEEE 488 bus to a common personal computer (PC). To move the CMUT under the lens of the microscope (20× and 100×, respectively) to position the laser of the interferometer (see Figure 2), we use a common xy-stage, which is controlled by a software written in LabViewTM Version 6.0 (National Instruments, Austin, TX).

Fig. 3 gives an overview of our common setup for experiments for CMUT characterization in immersed operation, i.e. the CMUT is immersed in soybean oil or water, with the top surface parallel to the air-oil interface (see also Fig. 2).

 

FIGURE 3. Overview of the different components when a CMUT in immersion is characterized.

 

For air-coupled CMUTs the measurement is simple to do. However, when measurements are done in immersed operation, besides the refractive index of the liquid, also the effect of acoustic-optic interaction needs to be considered. By using a correction method, good agreement between optical and electrical measurements could be found, i.e. electrical measurements were used to verify this correction technique. More details of these direct comparisons can be found in [1].

Instead of measuring directly in the surface of the CMUT, the system also can be used to measure the displacement of the liquid-air interface above the CMUT. Because the system also enables the acquisition of transient pulses, this is a powerful method to characterize a CMUT. The advantages are that no correction for of acoustic-optic interaction and refractive index are required. In [2] we used this idea to verify finite element results for both conventional and collapse operation mode of a CMUT. A good match was observed.

Further, we used this system (Fig. 3) to visualize the membrane movement of a CMUT single cell, when a pulse excitation is used. Link to MembraneScan.avi.

 

 

More details about our work, were we used optical displacement measurements to characterize CMUTs, to investigate acoustic cross-talk, and to verify our finite element models can be found in [3, 4, 5]. Further details about our work about acoustic cross-talk investigations in CMUT arrays are described in link to acoustic cross talk section.

 

References

 

[1] B. Bayram, M. Kupnik, G. Yaralioglu, Ö. Oralkan, D. Lin, X. Zhuang, A. Ergun, A. Sarioglu, S. Wong, and B. T. Khuri-Yakub, “Characterization of Cross-Coupling in Capacitive Micromachined Ultrasonic Transducers,” in Proc. IEEE Ultrason. Symp., 2005, pp. 601–604.

[2] B. Bayram, G. Yaralioglu, M. Kupnik, A. Ergun, Ö. Oralkan, A. Nikoozadeh, and B. T. Khuri-Yakub, “Dynamic analysis of capacitive micromachined ultrasonic transducers,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 52, pp. 2270–2275, 2005.

[3] O. Oralkan, B. Bayram, G. Yaralioglu, A. Ergun, M. Kupnik, D. T. Yeh, I. O. Wygant, and B. T. Khuri-Yakub, “Experimental characterization of collapse-mode CMUT operation,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 53, pp. 1513–1523, 2006.

[4] B. Bayram, M. Kupnik, G. Yaralioglu, Ö. Oralkan, D. Lin, A. Ergun, S. Wong, and B. T. Khuri-Yakub, “Finite element modeling and experimental characterization of crosstalk in 1-D CMUT arrays,” submitted to IEEE Trans. Ultrason., Ferroelect., Freq. Contr., 2006.

[5] G. Yaralioglu, A. Ergun, and B. T. Khuri-Yakub, “Finite Element Analysis of Capacitive Micromachined Ultrasonic Transducers,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 52, pp. 2185–2198, 2005.