KEYWORDS: Modulation, Distance measurement, Interferometry, Digital signal processing, Analog electronics, Signal detection, Signal processing, Semiconductor lasers, Phase shift keying, FDA class I medical device development
This paper presents the development of an optical rangefinder based on self-mixing interferometry. The instrument
measures the absolute distance from a remote target, without contact and respecting the Class I safety. A variation of the
laser diode bias current origins a modulation of the wavelength and then, due to the phase shift along the channel path,
the presence of a target generates interferometric fringes. The electrical frequency of the fringes signal is proportional to
the target distance. The realized device consists in analog and digital circuits. The analog circuits drive the laser diode,
detect the interferometric signal and filter it. A Digital Signal Processor is needed to acquire the fringes signal and, by
extracting its frequency, evaluate the absolute distance. The developed rangefinder allows spatial resolution better than
100 μm over a range from 5 cm to 2 m.
KEYWORDS: Digital signal processing, Interferometry, Reconstruction algorithms, Signal processing, Detection and tracking algorithms, Sensors, Algorithm development, Modulation, Vibrometry, Semiconductor lasers
This paper shows the development of an optical instrument for vibrations measurements, without contact. The device is
based on self-mixing interferometry, realized with very low optical complexity and cost. As any interferometer, it detects
the power modulation of the beam laser, due to the remote target displacement. The signal is acquired by a Digital Signal
Processor that provides to reconstruct the target movements, in real-time. Two different algorithms were developed to
cover a great range of optical conditions. The best focus point is determined by an embedded autofocus system that
moves the collimating lens driving a step-by-step engine. The measured distance ranges from 5 cm to 2 m, with a
resolution of about 50 nm.
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