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Nanometer-scale displacement sensing using self-mixing interferometry with a correlation-based signal processing technique

Hast J., Okkonen M., Heikkinen H., Krehut L., Myllylä R.

Opto-Electronics Review

2006

Vol. 14, No. 2

129-134

A self-mixing interferometer is proposed to measure nanometre-scale optical path length changes in the interferometer's external cavity. As light source, the developed technique uses a blue emitting GaN laser diode. An external reflector, a silicon mirror, driven by a piezo nanopositioner is used to produce an interference signal which is detected with the monitor photodiode of the laser diode. Changing the optical path length of the external cavity introduces a phase difference to the interference signal. This phase difference is detected using a signal processing algorithm based on Pearson's correlation coefficient and cubic spline interpolation techniques. The results show that the average deviation between the measured and actual displacements of the silicon mirror is 3.1 nm in the 0-110 nm displacement range. Moreover, the measured displacements follow linearly the actual displacement of the silicon mirror. Finally, the paper considers the effects produced by the temperature and current stability of the laser diode as well as dispersion effects in the external cavity of the interferometer. These reduce the sensor's measurement accuracy especially in long-term measurements.

Low cost velocity sensor based on the self-mixing effect in a laser diode

Krehut L., Hast J., Alarousu E., Myllyla R.

Opto-Electronics Review

2003

Vol. 11, No. 4

313-319

In this paper, a low cost velocity sensor based on the self-mixing effect in a laser diode is described. Theory of self-mixing effect in the laser diode is shortly presented. Experimental velocity measurements are presented in order to evaluate the operation of the velocity sensor. In the design, the attention is focused to develop a budget sensor, which frequency response of the detection electronics is up to 85 MHz. This limits the maximum measurable velocity to 27.5 m/s. The total material costs of the velocity sensor were 234 euros. The experimental measurements conducted so far show that the linearity of the developed velocity sensor is at least as good of a mirror moved by a translation stage with velocities ranging from 1.0 to 48.5 mm/s. The velocity of the translation stage was controlled by a computer. When the mirror velocity is lower than 20 mm/s, the maximum relative presision with the mirror velocity is less than 3.5%. When the mirror velocity is higher than 20 mm/s the relative precision with the mirror velocity is below 0.5%. In an additional experiment with a vibrating loudspeaker's membrane, it is also demonstrated that a maximum Doppler frequency is clearly detectable over the noise level at 12.5 MHz.