Self-mixing interferometry with a laser diode: experimental considerations for sensing applications

An investigation of the effects of external reflectance and pump current on the steady state current threshold and the output power of a laser diode are presented. Back-coupled external reflectance is achieved by placing a mirror in front of the laser diode cavity. Furthermore, by using a rotating wheel, the effect of external reflectance, pump current and combination of individual contributions to frequency shifting on the laser intensity power spectrum are explored. Feedback power ratios below 0.05 and pump currents up to 2I(sth) (I-sth, solitary threshold current) are used in these experiments. The power emitted by the two laser facets are measured through an internal (built-in package) and an external photodiode; both observations are compared. Surprisingly, when evaluating the power-current (P-I) curve for the feedback and solitary cases, a cross over is noticed at approximate to 1.54I(sth) only through the external photodiode. Nonetheless, measurements obtained with the internal photodiode agree with the theoretical steady state analysis. Moreover, it is proved that a linear combination of the individual frequency shifts holds in the power spectrum; in addition, a modulation of the carrier density by frequency-shifted optical feedback is also observed. The results from these experiments provide a practical background that ensures a correct interpretation of measurements when using this system for sensing applications.