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

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

Proximity Sensor using Self-mixing Effect

This paper is about the utilisation of the well-known self-mixing effect as base for the development of a novel proximity detector. The common used setup for this kind of a sensor is based on two elements: a laser as an emitter and a position-sensitive sensor as a detector. The sensor developed detects the optical power reflected by the object within the laser cavity itself, with no need of any additional detectors. One of the main feature is the ability to measure diffusive target accessible only from one side. A continuous range of measurement starting from 10 mm up to 80 mm is obtained by means of two different physical phenomena: from 0 up to 5mm the detection is only dependent by the level of the optical power returned into the laser cavity, whereas from 5 mm up to 80 mm reading the frequency of the modulation of the interferometric signal. The main advantage of the novel sensor is the elimination of the external detector. In addition, multiple devices configurations can be utilized and there is no need of any optical filters, cause the laser cavity itself works as an optical filter. Background rejection is intrinsically obtained because self-mixing effect shows a sharp cut-off after the focus

Noise Decrease in a Balanced Self-Mixing Interferometer: Theory and Experiments

In a self-mixing interferometer built around a laser diode, the signals at the outputs of the two mirrors are in phase opposition, whereas noise fluctuations are partially correlated. Thus, on making the difference between the two outputs, the useful signal is doubled in amplitude and the signal-to-noise ratio is even more enhanced. Through a second-quantization model, the improvement is theoretically predicted to be dependent on laser facets reflectivity. The results are then validated by experimental measurements with different laser types that show very good agreement with theoretical results. The new technique is applicable to a number of already existent self-mixing sensors, potentially improving significantly their measurement performances

Study of the Errors in Interpolated Fast Fourier Transform for Interferometric Applications

Frequency estimation is often the basis of various measurement techniques, among which optical distance measurement stands out. One of the most used techniques is interpolated fast Fourier transform due to its simplicity, combined with good performance. In this work, we study the lim-its of this technique in the case of real signals, with reference to a particular interferometric tech-nique known as self-mixing interferometry. The aim of this research is the better understanding of frequency estimation performances in real applications, together with guidance on how to im-prove them in specific optical measurement techniques. An optical rangefinder, based on self-mixing interferometry, has been realized and characterized. The simulation results allow us to explain the limits of the interpolated fast Fourier transform applied to the realized instrument. Fi-nally, a method for overcoming them is proposed by decorrelating the errors between the meas-urements, which can provide a guideline for the design of frequency-modulated interferometric distance meters

Evaluation of Self-Mixing Interferometry Performance in the Measurement of Ablation Depth

This paper studies self-mixing interferometry (SMI) for measuring ablation depth during laser percussion drilling of TiAlN ceramic coating. The measurement performance of SMI was investigated in a large processing range producing blind microholes with depths below and beyond the average coating thickness. Signal characteristics of the measurement system were evaluated indicating sources of disturbance. The SMI measurements were compared with a conventional measurement device based on focus variation microscopy to evaluate the measurement error. The measurement error classes were defined, as well as defining the related error sources. The results depict that the measurement error was independent of the processing condition, hence the hole geometry and ablation rate. For 76% of cases, measurement error was below the intrinsic device resolution obtainable by simple fringe counting of half a wavelength (λ/2 = 0.393 μm)

Overview of self-mixing interferometer applications to mechanical engineering

We present an overview of the applications of self-mixing interferometer (SMI) to tasks of interest for mechanical engineering, namely high-resolution measurement of linear displacements, measurements of angles (tilt, yaw, and roll), measurements of subnanometer vibrations, and absolute distance, all on a remote target-representative of the tool-carrying turret of a tool-machine. Along with the advantages of SMI-compactness, low cost, minimum invasiveness, ease of use, and good accuracy, we illustrate the typical performance achieved by the basic SMI sensors, that is, the versions requiring a minimum of signal processing and discuss special features and problems of each approach

Overview of self-mixing interferometer applications to mechanical engineering

We present an overview of the applications of self-mixing interferometer (SMI) to tasks of interest for mechanical engineering, namely high-resolution measurement of linear displacements, measurements of angles (tilt, yaw, and roll), measurements of subnanometer vibrations, and absolute distance, all on a remote target-representative of the tool-carrying turret of a tool-machine. Along with the advantages of SMI-compactness, low cost, minimum invasiveness, ease of use, and good accuracy, we illustrate the typical performance achieved by the basic SMI sensors, that is, the versions requiring a minimum of signal processing and discuss special features and problems of each approach

Performance analysis of an optical distance sensor for roll angle estimation in sport motorcycles

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