A proximity detector based on self-mixing technique, well suited for different industrial applications, is demonstrated. Instead of using a light-source plus a detector, the proposed sensor is realized by a single laser source. Two different physical effects in the laser diode allow for a continuous detecting range, from 10 mm up to 80 mm. The main advantages of the sensor are target detection from just one point of view; multiple sensors configuration does not need optical filters; separation of source and detector is eliminated; and background rejection is intrinsically given by the self-mixing effect, which shows a sharp cut-off after the focus.
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.
We apply balanced detection to self-mixing interferometry for displacement and vibration measurement, using two photodiodes for implementing a differential acquisition. The method is based on the phase opposition of the self-mixing signal measured between the two laser diode facet outputs. The balanced signal obtained by enlarging the self-mixing signal, also by canceling of the common-due noises mainly due to disturbances on laser supply and transimpedance amplifier. Experimental results demonstrate the signal-to-noise ratio significantly improves, with almost twice signals enhancement and more than half noise decreasing. This method allows for more robust, longer-distance measurement systems, especially using fringe-counting.
Proc. SPIE. 8542, Electro-Optical Remote Sensing, Photonic Technologies, and Applications VI
KEYWORDS: Digital signal processing, Detection and tracking algorithms, Modulation, Sensors, Interferometry, Semiconductor lasers, Vibrometry, Signal processing, Reconstruction algorithms, Algorithm development
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.
Proc. SPIE. 8542, Electro-Optical Remote Sensing, Photonic Technologies, and Applications VI
KEYWORDS: Digital signal processing, FDA class I medical device development, Modulation, Interferometry, Phase shift keying, Semiconductor lasers, Signal processing, Distance measurement, Analog electronics, Signal detection
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.
An optical sensor for measuring the moisture level of clay has been realized by a couple of telecommunications lasers at 1300 and 1550 nm. The sensor can operate directly during building material production. The measurement principle is based on the measurement of the optical reflection at different wavelengths in the infrared region. Custom low-noise electronics allows rejecting disturbances of ambient light, and a digital processing makes the system independent on the clay distance. By means of a proper calibration, the sensor can monitor the moisture level during brick production, without moving parts or optical filters.
To measure blood flow rate in ex-vivo circulation, we propose an optical Doppler flowmeter
based on the self-mixing effect within a laser diode (SM-LD). Advantages in adopting SM-LD
techniques derive from reduced costs, ease of implementation and limited size. Moreover, the
provided contactless sensing allows sensor reuse, hence further cost reduction. Preliminary measurements performed on bovine blood are reported, thus demonstrating the applicability of the proposed measurement method.
The design, realization and characterization of a new type of laser instrument for non-contact measurement of differential
displacements are presented. The operating principle is based on two distinct vibrometers that use semiconductor lasers
in the self-mixing interferometric configuration. The differential vibration is obtained by electronic subtraction of the
vibration signals supplied by the two channels of the vibrometers. The prototype instrument is made by two compact
optical heads (5 cm length, 1 cm diameter) and an electronic unit. The working distance can be varied between 10 and 50
cm, and operation is guaranteed on any diffusive surface. The maximum measurable vibration is in excess of 100 μm,
and noise-equivalent differential vibration equals 20 nm. The instrument can be used to measure the differential
vibrations of two metal samples kept into contact, revealing the hysteresis cycle in the micro-slip and gross-slip regimes.
We present a new method for the measurement of the absolute distance of a remote target based on the laser diode self-mixing interferometry technique, assisted by an electronic feedback loop capable of increasing the measurement accuracy. In this method, we use an electronic feedback loop to generate a wavelength change that exactly corresponds to one single interferometric fringe. This allows to measure the target distance with a higher accuracy, in principle limited only by detection shot-noise, and not by the fringe quantization error typical of conventional fringe counting approaches. The target distance can be measured with 0.3 mm accuracy, in the 0.2-3 m range.
In this work we demonstrate that the normal photomixing scheme, i.e. one built around a two-mode laser (or a mode-locked laser) as the source, and a fast photodiode acting as the optical mixer of the two modes, can be used also to perform the electrical demodulation of an incoming weak signal at the same frequency.
In particular, we consider a mode spacing c/2L in the range of mm-waves, typically 60GHz for a practical WLAN communication system. With optical powers in the range of mW’s (or 0dBm) for each mode, and an optical power amplifier boosting powers up to 8-10 dBm, the two modes can be photomixed on a high frequency photodiode and obtain an electrical signal with power of about 0dBm at the carrier frequency of 60GHz. Now, if an electrical signal, with a frequency slightly different from 60GHz, is applied to the photodiode output, electrical mixing with the photomixing carrier takes place and demodulation of the weak signal is performed, down to the baseband.
In this work we report on a reliable and low-cost
frequency-response characterization method for high-bandwidth
photodiodes. Using the photomixing technique, we were able to
experimentally characterize the electrical response of commercial
devices up to 60GHz using both DFB and low-cost Fabry-Perot laser
In this paper, semiconductor laser feedback interferometry is applied to the characterization of vibrating mass microsensors, such as gyroscopes and accelerometers. Complete characterization of such devices with this technique includes the identification of the vibration modes and the measurement of the resonance curves of the different axes, the determination of resonance frequency, quality factor and the actuation efficiency as functions of different parameters such as pressure. In the case of a gyroscope, tuning of the driving and of the sensing axes can be also performed, as well as the measurement of the Coriolis force. Thanks to its very simple optical implementation, feedback interferometry provides a viable alternative to the standard electrical measurements, and is especially useful for the characterization of prototypes, for which a dedicated electronics circuit is not yet available.
We have developed a fiberoptics setup which can be easily specialized with minor changes to implement different schemes of optical chaos generation and synchronization using semiconductor lasers. Long and short cavity, open and closed loop configurations have been compared, as well as various encoding/decoding methods for secure transmission based on chaotic carriers, such as CSK (Chaotic Shift Keying), ACM (Additive Chaotic Masking), CM (Chaos Modulation). Different transmission media, possibly including optical amplifiers, have been also tested.
In this paper, we report on feedback interferometric measurements on a micromachined gyroscope and on a micromachined linear accelerometer. Characterization has been performed for different values of pressure and of other parameters using a laser diode. Resonance frequencies and quality factors have been measured. Moreover, hysteresis and other nonlinear phenomena on specific samples have also been detected. The proposed method is based on optical injection and represents an efficient alternative to the standard electrical measurements, which actually shows some limitations for bare prototype testing.