Laser induced purely-thermal-wave interferometry (PTWI) using a novel photopyroelectric (PPE) instrument

A novel purely thermal-wave interferometric technique and its applications to non-contact and non-destructive evaluation of Ti:sapphire laser crystals, high-precision measurement of thermal diffusivity of gases, and high-sensitivity gas (hydrogen) sensors have been successfully developed both theoretically and experimentally. A comprehensive theoretical and experimental analysis of the system noise and detectivity has been conducted to consolidate the basis of the technique.; Unlike the conventional single-ended photopyroelectric(PPE) technique, different thermal-wave interference patterns can be obtained by adjusting two incident beams (relative intensity and phase shift) and two thermal-wave cavities on both sides of a pyroelectric detector. It is found that the large base-line signal and large optical noise, which are encountered in the single-ended PPE scheme, can be coherently and completely suppressed in the fully destructive interferometric measurement.; Differential surface absorptance, differential and absolute bulk absorption coefficient of Ti:sapphire laser crystals have been separately measured using an extended PPE-interference (PPEI) theory. Unlike the single-ended PPE method, in which thermal contributions from several optical parameters are always coupled together, the destructive interferometric: method provides a unique method for extracting precise values of one of these coupled parameters, without the need of equally precise knowledge of the values of others.; The comparison measurement of thermal diffusivity of air using the single-ended PPE method and the PPEI method shows that the PPEI method enhances the measuring precision by one significant figure when compared with the single-beam method. The conventionally used concept of "thermal-wave reflection coefficient" has been extended to a more general case that is sample-thickness dependent.; A novel hydrogen gas sensor has been initialized and developed based on the PPEI technique. It is shown that the detectivity of the PPEI hydrogen sensor is much enhanced when compared with the single-ended PPE method. The correlation study of the PPEI signal and the direct optical reflecting signal verifies the theory and mechanism of the PPEI sensor.; A comprehensive theoretical and experimental analysis of the PPEI system noise has been made to explain why the PPEI technique is inherently superior to the conventional PPE technique in terms of overall noise level, signal dynamic range and detectivity.