Guido Giuliani

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%.

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 velocimetry: application to vibration and velocity measurement

A review of recent experimental and theoretical results concerning laser diode self-mixing velocimetry is presented, showing that this technique can be deployed to measure velocity and vibration of solid targets with an extremely simple optical setup. This technique reduces optical alignment problems and achieves results comparable to those obtained by the conventional laser Doppler velocimetry (LDV) approach. It is demonstrated that the self-mixing signal can be processed to recover the target velocity and vibration by applying the same analysis method used for LDV. An optimal signal processing method is then proposed to recover the target velocity with good accuracy, also in the presence of relevant speckle disturbance. Application to the measurement of sub-micron vibrations is also demonstrated, using a self-mixing vibrometer instrument capable of 5-nm accuracy. As an example, the characterization of response and hysteresis of piezoceramic transducers (PZTs) is carried out. These results illustrate the effectiveness of the self-mixing technique in the field of laser velocimetry, opening the way to new applications where compactness and low cost of the measuring apparatus are essential.

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.

Absolute Distance Measurement With Improved Accuracy Using Laser Diode Self-Mixing Interferometry in a Closed Loop

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, which is assisted by an electronic feedback loop that is capable of improving the measurement accuracy. The feedback loop supplies a periodic change of the emitted wavelength that exactly corresponds to a single interferometric fringe. This allows the measurement of the target distance with higher accuracy, which, in principle, is limited only by the detection shot noise and not by the fringe quantization error that is typical for the conventional fringe-counting approaches. We developed a prototype that is capable of measuring the target distance with 0.3-mm accuracy in the 0.2- to 3-m range.

Alternate oscillations in semiconductor ring lasers

We report on fabrication and characterization of single-longitudinal- and transverse-mode semiconductor ring lasers. A bifurcation from bidirectional stable operation to a regime with alternate oscillations of the counterpropagating modes was observed experimentally and is theoretically explained through a two-mode model. Analytical expressions for the onset and the frequency of the oscillations are derived, and L-I curves numerically evaluated. Good quantitative agreement between theory and measurements made over a large number of tested devices is obtained.

Noise analysis of conventional and gain-clamped semiconductor optical amplifiers

We present a numerical study of the noise of conventional and gain-clamped semiconductor optical amplifiers (SOAs), using a detailed device model. The model makes use of a density-matrix gain calculation, and takes into account the forward and backward amplified spontaneous emission (ASE) spectra and the spatial carrier hole-burning. The device is longitudinally divided into M sections and a rate equation for averaged photon and carrier densities is used for each section. We demonstrate that the accuracy on the calculated noise figure strictly depends on the number of sections M. We obtain a good tradeoff between the results accuracy and the computational complexity with M=8. The model is then applied to study the noise in a distributed Bragg reflector (DBR)-type gain-clamped SOA for varying signal power, pump current, and lasing wavelength. We show that changes in the spatial carrier profile caused by the input signal significantly affect the noise figure, even when the gain is constant. A slight dependence of the noise figure on lasing wavelength is also foreseen, while the dependence on the pump current is negligible. A new method for gain-clamped SOA noise figure reduction is proposed, based on unbalanced Bragg reflectors. An improvement of noise figure (NF) as large as 2 dB is devised.