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Laser self-mixing interferometry (SMI) has been widely applied in the fields of precision measurement in scientific research, industry and biomedicine, and most researchers preferentially utilize laser diodes (LD) as light sources due to their compact structure and low cost. In most SMI cases, detection sensitivity rather than structure and cost is always the first concern. So in this article, we concentrate on an influencing factor on detection sensitivity named fluorescence to photon lifetime ratio (FPLR) which is an inherent parameter of the active material in a laser cavity. Derivation and simulation based on the time delayed rate equations are followed to validate the significant role played by the FPLR, then experiments according to different laser types including solid state lasers (SSL) and LDs are implemented to further prove the factor mentioned. Results demonstrate that SSLs always have higher detection sensitivity to feedback light than LDs, and is always more suitable to be applied in precision measurement. Additionally, the Yb:YAG laser is more sensitive than the Nd:YAG one. Increasing the pumping level from the threshold will decrease the SMI signal's sensitivity, and reducing laser feedback strength will also decrease the SMI signal's sensitivity. The findings from this article are beneficial to studying laser feedback sensitivity and selecting appropriate laser types in designing SMI sensors or instruments. Though with the disadvantages of large size and high cost, SSLs may be more suitable light sources in the field of high performance SMI sensing if miniaturization and cost are not the significant consideration.

Influence of Fluorescence to Photon Lifetime Ratio on Detection Sensitivity in Laser Self-Mixing Interferometry