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A laser sensor technique for nm-scale measurements using a single laser diode, applied to acoustic vibrations for NDT

Ajit Ha ha defendido hoy su tesis doctoral titulada "Continuous Wave-Frequency Modulated optical Feedback: Theory and Applications" en el auditorio Joan Salvador de la Facultad de Ópticas y Optometría de Terrassa (FOOT). La tesis, dirigida por Santiago Royo Royo del CD6, se ha centrado en el uso de técnicas de interferometría òptica de realimentación con el objetivo de desarrollar un sensor láser con capacidad de medir vibraciones en el rango de los nanómetros para aplicaciones de ensayos no destructivos (NDT).


Abstract de la tesis:
Paradoxically optical feedback (OF) (the phenomenon in which a fraction of the optical field emitted by a laser is fed back into its cavity), that was once considered an undesired phenomenon in the context of optical fibre communications because it introduces instabilities in the laser, has proven to be extremely useful in different real world photonic sensing applications. The optical feedback affects the internal parameters of the laser, such as the facet reflection coefficient, the laser gain, the photon and carrier density, and ultimately the emitted intensity from the laser. This direct relationship among the OF, intensity and frequency modulation is appealing because it transfigures the laser into a complete interferometric set-up where the laser itself acts as source, detector and interfering media all in one, suited for non-contact, non-destructive photonic sensing.

The study in this Thesis contributes to different areas of optical feedback interferometry (OFI): from the theoretical formalization of  understanding the transfer function of the laser under OF based on scattering theory, to signal processing of the optical feedback signal(OFS) in time and frequency domain simultaneously using the wavelet transform for combating noise, speckle management and extraction of the vibration related parameters of periodic and transient vibrations all in a single processing step. Furthermore, based on the Lang-Kobayashi formulation, experimental evidences are shown of the ability of injection modulation to stabilize the laser even under strong feedback conditions, taking advantage of the fact that the frequency deviation produced in the laser emission by OF is in opposite direction to that of injection modulation. To add on, however, the main contribution of this Thesis has been to develop yet another variant of OFI, continuous
wave frequency modulated differential optical feedback (CWFM-DOF), that combines the non-linear dynamics due to OF and that due to injection modulation to measure the optical path difference (OPD) below half the emission wavelength with resolution way below the classical optical feedback (C-OF), while keeping the experimental setup the same as that of C-OF.

Among the broad range applications which would benefit from such a sensor, such as Photo-acoustic tomography (PAT) or laser ultrasonic (LUS), the proposed methodology is tested to characterize the vibration of membrane of an acoustic transducer and to measure the displacement of a metallic target due to the acoustic pressure. Thus, the Thesis extends the performance of OFI sensors and covers in detail the theoretical and experimental aspects of CWFM-DOF, and its application to the detection of very small perturbations.