Silvano Donati

Laser diode self-mixing technique for sensing applications

The laser diode self-mixing (or feedback) interferometric technique is reviewed as a general tool for remote sensing applications. The operating principle is outlined, and the attainable performance is compared to conventional coherent detection. Applications to metrology and to new sensing schemes are described, experimental results are reported and the overall performance of the sensors are assessed.

Laser diode feedback interferometer for measurement of displacements without ambiguity

We report what, to our knowledge, is the first example of laser feedback interferometer capable of measuring displacements of arbitrary form using a single interferometric channel. With a GaAlAs laser diode we can measure 1.2-m displacements, with interferometric resolution, simply by means of the backreflection from the surface (reflective or diffusive) under test. The operation is performed at moderate (i.e., not very weak) levels of feedback, such that a two-level hysteresis is found in the amplitude modulated signal. This is shown to allow the recovery of displacement without sign ambiguity from a single interferometric signal. Experimental results are reported, which are found to be in good agreement with the underlying theory. Performances of the developed feedback interferometer are finally presented. >

Operating regimes of GaAs-AlGaAs semiconductor ring lasers: experiment and model

Theory and experiments of single-mode ridge waveguide GaAs-AlGaAs semiconductor ring lasers are presented. The lasers are found to operate bidirectionally up to twice the threshold, where unidirectional operation starts. Bidirectional operation reveals that just above threshold, the lasers operate in a regime where the two counterpropagating modes are continuous wave. As the injected current is increased, a new regime appears where the intensities of the two counterpropagating modes undergo alternate sinusoidal oscillations with frequency in the tens of megahertz range. The regime with alternate oscillations was previously observed in ring lasers of the gas and dye type, and it is here reported and investigated in semiconductor ring lasers. A theoretical model based on a mean field approach for the two counterpropagating modes is proposed to study the semiconductor ring laser dynamics. Numerical results are in agreement with the regime sequence experimentally observed when the injected current is increased (i.e., bidirectional continuous-wave, bidirectional with alternate oscillations, unidirectional). The boundaries of the different regimes are studied as a function of the relevant parameters, which turn out to be the pump current and the conservative and dissipative scattering coefficients, responsible for an explicit linear coupling between the two counterpropagating field modes. By a fitting procedure, we obtain good numerical agreement between experiment and theory, and also an estimation for the otherwise unknown scattering parameters.

Measurement of the linewidth enhancement factor of semiconductor lasers based on the optical feedback self-mixing effect

A new method for the measurement of the linewidth enhancement factor of semiconductor lasers is presented, based on the interferometric self-mixing effect. It is a fast and easy to perform method that does not require radio frequency nor optical spectrum measurements. A small fraction of the emitted light is backreflected into the laser cavity by a remote target driven by a sine waveform. The mixing of the returned and the lasing fields generates a modulation of the optical output power in the form of an interferometric waveform, with a shape that depends on the optical feedback strength and the linewidth enhancement factor /spl alpha/, according to the well-known Lang-Kobayashi theory. We show that the value of /spl alpha/ can be retrieved from a simple measurement of two characteristic time intervals of the interferometric waveform. Experimental results obtained on different laser diodes show an accuracy of /spl plusmn/6.5%.

Synchronization of chaotic injected-laser systems and its application to optical cryptography

We demonstrate that two chaotic systems, each made by two coupled semiconductor lasers, can be synchronized using direct-optical feedback. The robustness of the proposed synchronization scheme against mismatch of source parameters and difference in starting conditions is tested by numerical simulations. Applications to secure data transmission are proposed, namely chaotic masking and chaotic shift keying (CSK).

Unidirectional bistability in semiconductor waveguide ring lasers

Large-diameter ridge-guided semiconductor lasers weakly coupled to a straight output waveguide show unidirectional operation and directional bistability at currents up to about twice the threshold. The direction of lasing in the ring may be controlled by biasing contacts at either end of the coupled guide.

Self-mixing laser diode vibrometer

The principle and the experimental realization of a new type of laser vibrometer based on the self-mixing interference effect in a laser diode are presented. The self-mixing configuration allows for a practical set-up that is simpler by far than conventional laser vibrometer schemes. The vibrometer relies on locking of the system to half the interferometric fringe, and on active phase-nulling by wavelength modulation. This allows an extended dynamic range to be achieved, whilst retaining a good sensitivity to sub-wavelength vibrations. We have designed and built a prototype of the vibrometer that can operate on nearly any kind of rough surface, covering the 0.1 Hz–70 kHz frequency range of vibration. The noise floor is less than 100 pm Hz−1/2, and the maximum measurable vibration amplitude is 180 µm peak to peak. The proposed method can find application in modal analysis and noise and vibration measurements in industrial and scientific environments.

Optical feedback interferometry for sensing application

We review laser diode feedback interferometry as a general tool for sensing applications. After outlining the basic principles and the theoretical approaches used to describe the phenomenon, we present a few selected examples of applications in interferometry, as developed by various groups in recent years, such as a displacement sensor, a veloci- meter or vibration sensor, and an absolute distance meter or range finder and angle sensor. Experimental results are also reported as an illustration.

Chaos and locking in a semiconductor laser due to external injection

We have analyzed the behavior of a semiconductor laser subjected to increasing external injection. Numerical simulations show the well-known nonlinear modulation and locking regimes, followed by an intermediate chaotic region that precedes definitive locking to the external source at significantly higher injection levels. >

Dynamic behavior and locking of a semiconductor laser subjected to external injection

In this paper, we analyze the phenomena arising when a monomode semiconductor laser is subjected to external injection from another laser. The system stability is investigated as a function of detuning and of the relative injected power. Different regimes, spanning from phase locking to chaos and coherence collapse, are described by analytical and numerical methods for weak and moderate injection. Previous theoretical studies are extended by describing the inverse transition from chaos to stability and by deriving the final locking condition. Also, further investigation on the coherence collapse regime has been performed. Besides contributing to the exploration of an interesting fundamental phenomenon, the results of this analysis are useful for different applications, including coherent detection and chaotic cryptography.