Self-Mixing Laser Distance-Sensor Enhanced by Multiple Modulation Waveforms

Abstract

Optical rangefinders based on Self-Mixing Interferometry are widely described in literature, but not yet on the market as commercial instruments. The main reason is that it is relatively easy to propose new elaboration techniques and get results in controlled conditions, while it is very difficult to develop a reliable instrument. In this paper, we propose a laser distance sensor with improved reliability, realized through a wavelength modulation at a different frequency, able to decorrelate single measurement errors and obtain improvement by averages. A dedicated software is implemented to automatically calculate the modulation pre-emphasis, needed to linearize the wavelength modulation. Finally, data selection algorithms allow to overcome signal fading problems due to the speckle effect. A prototype demonstrates the approach with about 0.1 mm accuracy up to 2 m of distance at 200 measurements per second.

Highly-Linearized Heterodyne Self-Mixing Vibrometer

“…The realized prototype of vibrometer consists of two parts: the analog electronics required to process the fringe signal and introduce a specific current waveform into the laser diode, along with a commercially available data acquisition card (DAQ, Analog Discovery II). The DAQ card was used to generate the modulating wave and capture the signal, with a conceptual arrangement similar to what is depicted in [7]. The schematic representation of the instrument's components is illustrated in Figure 1.…”

“…Due to the compact design and cost-effectiveness solution, the selfmixing effect is employed in widespread applications for high-resolution measurements. In addition to absolute distance measurement [5][6][7][8], the literature reports various SMI sensors, for measuring displacement [9][10][11][12][13], speed [14,15], vibrations [16][17][18][19][20][21], angles [22], the liquid flow and level [23][24][25][26][27][28][29][30], biomedical implementation [31,32], and TeraHertz imaging [33,34] too. A minimal SMI setup, comprises the laser diode, a photodiode positioned inside the laser cavity (referred to as the monitor photodiode, typically located within the laser case), a collimating lens, and the target (see Fig.…”

“…This paper describes the development of a laser instrument for measuring vibrations on a diffusive target placed at a few meters of distance. The proposed sensor is based on a heterodyne self-mixing interferometer (SMI), following the scheme typically used for absolute distance measurement [5][6][7][8].…”

Self-mixing Interferometer: Frequency Modulation Noise Dependence on Laser Source

“…As a result of these changes, the laser output is subject to both amplitude modulation (AM) and frequency modulation (FM). By recovering the target displacement from the signal carried in the output power (AM), SMI has been used as an approach to measure target status: displacement [14][15][16][17], vibration [18][19], speed [20][21], flow [22][23][24][25], distance [26][27][28], and different applications [29][30][31][32][33]. FM is more difficult to read, but it exhibits a sensitivity two orders of magnitude higher than AM [9].…”

Laser Optical Feedback Turns 60

“…Much more difficult for an interferometer is to measure the absolute distance, if not aided by an independent reference signal. The article published in the Special Issue by Cavedo et al [ 16 ] opens a new perspective to cost effective LSM rangefinders, relying on multiple frequency modulation. Their system achieves 0.1 mm accuracy across the one-decade range from 0.2 to 2 m, making a significant step towards commercial application in the consumer market.…”