Defect-selective photothermal imaging: a sensitive tool for damage mapping in optical coatings

Sreekumar, K (2009) Defect-selective photothermal imaging: a sensitive tool for damage mapping in optical coatings. In: Journal Of Optics A-Pure And Applied Optics, 11 (4). 045506-1-045506-10.

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Abstract

A defect-selective photothermal imaging system for the diagnostics of optical coatings is demonstrated. The instrument has been optimized for pump and probe parameters, detector performance, and signal processing algorithm. The imager is capable of mapping purely optical or thermal defects efficiently in coatings of low damage threshold and low absorbance. Detailed mapping of minor inhomogeneities at low pump power has been achieved through the simultaneous action of a low-noise fiber optic photothermal beam defection sensor and a common-mode-rejection demodulation (CMRD) technique. The linearity and sensitivity of the sensor have been examined theoretically and experimentally, and the signal to noise ratio improvement factor is found to be about 110 compared to a conventional bicell photodiode. The scanner is so designed that mapping of static or shock sensitive samples is possible. In the case of a sample with absolute absorptance of 3.8 x 10(-4), a change in absorptance of about 0.005 x 10(-4) has been detected without ambiguity, ensuring a contrast parameter of 760. This is about 1085% improvement over the conventional approach containing a bicell photodiode, at the same pump power. The merits of the system have been demonstrated by mapping two intentionally created damage sites in a MgF2 coating on fused silica at different excitation powers. Amplitude and phase maps were recorded for thermally thin and thick cases, and the results are compared to demonstrate a case which, in conventional imaging, would lead to a deceptive conclusion regarding the type and location of the damage. Also, a residual damage profile created by long term irradiation with high pump power density has been depicted.

Item Type:	Journal Article
Publication:	Journal Of Optics A-Pure And Applied Optics
Publisher:	Institute of Physics
Additional Information:	Copyright of this article belongs to Institute of Physics.
Keywords:	photothermal imaging;defect mapping;optical coating;optical beam deflection;fiber sensor;lock-in amplification;common-mode-rejection demodulation (CMRD).
Department/Centre:	Division of Physical & Mathematical Sciences > Instrumentation Appiled Physics
Date Deposited:	30 Apr 2009 04:33
Last Modified:	19 Sep 2010 05:29
URI:	http://eprints.iisc.ac.in/id/eprint/19642

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