Multi-channel averaging detection for fast imaging of weakly absorbing defects in surface thermal lensing

Abstract

Surface thermal lensing (STL) is a high sensitive pump-probe technique for photothermal characterization of weakly absorbing defects in optical materials. Commonly, the frequency modulation is applied in the pump beam, and the STL signal that appears as small modulation on top of an intense background is then detected on the probe beam by lock-in amplification. However, the lock-in detection by filtering the signal in the frequency domain results in low efficiency in the STL, preventing investigations of the large area surfaces. Here it is shown that the multi-channel averaging, which corresponds to temporal integration, is better suited for detection of the STL signal. It is demonstrated that by converting the conventional single-point scanning STL to the line scanning STL and employing suitable width of the time bins and the number of records, the imaging speed of the multi-channel averaging detection can be improved by a factor of 3.7 relative to the lock-in detection at the same signal-to-noise ratio of 6 dB and imaging resolution of 20 μm. Therefore, high-quality photothermal images of weakly absorbing defects can be recorded with higher efficiency and lower laser irradiance, making it possible to investigate large samples.

Multipass lock-in thermography for the study of optical coating absorption

“…Another way to determine the absorption is to detect effects related to the temperature rise. Photothermal deflection (PDT) [21][22][23][24] or surface thermal lensing (STL) [25] use the modification of the surface and of the refractive index (i.e., the optical path) that is probed by a laser beam and linked to the absorption through a calibration procedure. Interferometric techniques can also be used to measure temperature-induced modifications.…”

Low absorption metrology of thin film optical components for high power continuous wave lasers

“…A strong advantage of this method is that absorption can be dissociated from scattering, as opposed to other photometric techniques based on measurements of losses, such as cavity ring down [7], [8]. We can split photothermal measurements in two families: a first directly measures the temperature as laser calorimetry [9], [10] or photothermal radiometry [11], [12] and a second detects side effects of heating as photothermal deflection [13]- [16], surface thermal lensing [17] or common-path interferometry [18]. Other strategies are based on self-phase modulation, photoacoustic spectroscopy (Self-phase modulation [19], [20], photoacoustic spectroscopy [21]- [23]).…”