Reciprocity in laser ultrasound revisited: Is wavefield characterisation by scanning laser excitation strictly reciprocal to that by scanning laser detection?

TL;DR

This paper challenges the common belief of strict reciprocity in laser ultrasound measurements, clarifies the conditions under which reciprocity holds, and proposes measurement setups that ensure reciprocity for various configurations.

Contribution

It provides a systematic classification of measurement situations and identifies conditions for true reciprocity in laser ultrasound, including a substitute method for laser vibrometer measurements.

Findings

Strict reciprocity between laser detection and excitation is generally invalid.

Correct reciprocal measurement setups depend on the specific probe and excitation configurations.

A substitute measurement method ensures reciprocity with laser excitation.

Abstract

The common believe about strict measurement reciprocity between scanning laser detection and scanning laser excitation is disproved by a simple experiment. Nevertheless, a deeper study based on the reciprocity relation reveals correct reciprocal measurement set-ups for both the probe-excitation / laser-detection and the laser-excitation / probe-detection case. Similarly, the all-laser measurement, that is thermoelastic laser excitation with laser vibrometer detection, is not in general reciprocal with respect to the exchange of excitation and detection positions. Again, a substitute for the laser doppler vibrometer out-of-plane displacement measurement was found which ensures measurement reciprocity together with laser excitation. The apparent confusion in literature about strict validity/non-validity of measurement reciprocity is mitigated by classifying the measurement situations systematically.