Spectral broadening caused by dynamic speckle in self-mixing velocimetry sensors

Self-mixing laser sensors require few components and can be used to measure velocity. The self-mixing laser sensor consists of a laser emitting a beam focused onto a rough target that scatters the beam with some of the emission re-entering the laser cavity. This ‘self-mixing’ causes measurable interferometric modulation of the laser output power that leads to a periodic Doppler signal spectrum with a peak at a frequency proportional to the velocity of the target. Scattering of the laser emission from a rough surface also leads to a speckle effect that modulates the Doppler signal causing broadening of the signal spectrum adding uncertainty to the velocity measurement. This article analyzes the speckle effect to provide an analytic equation to predict the spectral broadening of an acquired self-mixing signal and compares the predicted broadening to experimental results. To the best of our knowledge, the model proposed in this article is the first model that has successfully predicted speckle broadening in a self-mixing velocimetry sensor in a quantitative manner. It was found that the beam spot size on the target and the target speed affect the resulting spectral broadening caused by speckle. It was also found that the broadening is only weakly dependent on target angle. The experimental broadening was consistently greater than the theoretical speckle broadening due to other effects that also contribute to the total broadening

RoSco Schematics

<p>This zip file contains the complete schematics for RoSco. RoSco - Rodent Scope - A user-configurable digital wireless telemetry system for freely behaving animals.</p

RoSco Firmware and Documents

<p>This zip file contains the source code, development manual and user manual for the RoSco module. RoSco - Rodent Scope - A user-configurable digital wireless telemetry system for freely behaving animals.</p

Self-mixing laser velocimetry: a realistic model

The self-mixing laser sensor makes compact, low-cost velocimetry sensing possible. In this work we describe a process for modelling the velocimetry signal that includes the dynamic speckle effect. We give results showing a good match between experimentally acquired signals and the model

A novel self-mixing sensor architecture using a PLL for noise immunity

The accuracy and maximum range of a self-mixing laser range finder are limited by the SNR of the self-mixing signal. In this work we have proposed a PLL sweep technique that was demonstrated to dramatically improve the noise immunity of a self-mixing range finder. It allowed operation with a non-cooperative target over a range of 200–900 mm and improved the self-mixing SNR by more than 20 dB

Parallel self-mixing flow sensor using monolithic VCSEL array

This paper describes the first multi-channel velocity sensor based on a self mixing effect in a monolithic VCSEL array. The self-mixing signals are obtained directly via the feedback caused variation in laser junction voltage, omitting the need for a photodetector. This solution, enabling concurrent acquisition of a plurality of signals, offers significant technological advantage over single channel implementation as it removes the need to scan the target in a raster fashion. In comparison with the spot-raster imaging sensors, the acquisition time is significantly shortened - the mechanical scanning process in is replaced by concurrent acquisition of several channels enabling flow velocity to be sensed on a 2D grid by performing a single sweep of the area. The system was used to measure the velocity profile of fluid flow resembling that of blood circulation in peripheral blood vessels

Thermal dependence of the signal to noise ratio of self-mixing sensors based on multimode VCSELs

We investigate experimentally the signal-to-noise ratio (SNR) of the self-mixing signal from a Vertical-Cavity Surface-Emitting Laser (VCSEL) based sensor as a function of laser driving current and the ambient temperature. The maximum SNR in the current-temperature space can be well approximated by the model related to the temperature dependence of the threshold current for individual VCSEL transverse modes