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.

In this review, self-mixing interferometry (SMI), a new configuration of interferometry, is discussed. SMI has practical advantages compared to standard interferometry, for example SMI does not require any optical part external to the laser chip and can be employed in a variety of measurements. Applications range from the traditional measurements related to optical pathlength -like displacement, small-amplitude vibrations, velocityto sensing of weak optical echoes -for return loss and isolation factor measurements, CD readout and scroll sensing -and also, a special feature because of the interaction with the medium, measurements of physical parameters, like the laser linewidth, coherence length, and the alfa factor. Because it is also a coherent detection scheme, the SMI works close to the quantum limit of the received field, typically 90 dBm, so that minimum detectable amplitudes of 100 pm/ Hz are currently achieved upon operation on diffusive targets, whereas a corner cube allows half-wavelength counting mode -or 0.5 μm resolution -on a dynamic range up to 2 m and more. With its compact setup, the SMI is easy to deploy in the field and can interface a variety of experiments -from MEMS testing to rotating machines vibration testing to pickup of biological motility. The illustration shows a double-channel, differential SMI incorporated in a thermomechanical test equipment to trace the mechanical hysteresis cycle of the beads of a motor-engine brake. : Self-mixing interferometry Figure 2 (online color at: www.lpr-journal.org) Self-coupling interaction can be represented by a rotating-vector addition, with a small returned component a E 0 exp iϕ dynamically added to the cavity field E 0 : the in-phase component cos ϕ generates AM modulation, whereas the in-quadrature component sin ϕ is responsible for FM.2ks (k = wavevector, s = distance) external to the perturbed laser. This is the case studied by Spencer and Lamb in [2]. In applications, we will readily take advantage of weak coupling in self-coupled systems by making out of it very sensitive echo sensors and interferometers.Before proceeding, let us explain the self-mixing modulations as the result of rotating-vector addition [5] as in Fig. 2. Let E 0 be the unperturbed cavity field, and aE 0 expi2ks the field back from the target, a being the attenuation and 2ks the phase delay of propagation. As is well known from communication theory, rotating vector addition generates an AM modulation driven by the in-phase component of the modulating term, that is aE 0 cos 2ks, and an FM by the in-quadrature component, or aE 0 sin 2ks.In applications, while the AM is readily available from the intensity (or power) detected by a photodiode, FM is difficult to retrieve because is impressed onto the optical frequency (it requires frequency downconversion, see Sect. 3.2.3).Also in the weak regime of mutual-coupling we find AM and FM in each of the two interacting lasers, and the driving terms are now the ratio of amplitudes and the frequency difference of the two waves [6]. The...

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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.