End week 3

 I didn't get a chance to do any lab work in the past two days. It's difficult to calibrate your setup and run diagnostics when one has no laser beam. I also found out that we're going to be getting very little laser time. Apparently we'll have time in the morning after Rod's people get here and before lunch, so that's probably 1030-noon.

 I've read a few articles in the past few days which I'll summarize shortly. There was also a picnic outside the main building at ILE today (i.e. directly outside Bianca's/Johanna's/my office as well). The food was quite good (as was the wine) but they set up speakers and played Latin music, the volume of which made it a little hard to concentrate, for most of the afternoon.

Laser Technology for Graffiti Removal

Sasha Chapman, J. Cult. Heritage 1 (2000)

 Two historically significant monuments at Stonehenge and Avebury were subject to graffiti attacks in 1998 and 1996, respectively.

 Eight of the standing stones of West Kennet Avenue were "daubed" with paint. Some of the stones are apparently also a site of scientific interest due to the existence of old lichen colonies. In any case, 2 of the stones had been daubed with white emulsion paint and 6 with black gloss. Conservators used an Nd:YAG laser at 1064nm at a "high" (i.e. unspecified) fluence and monitored the damage threshold acoustically.   The stones consisted of quartz, silica, and ferrous oxides, and the surfaces varied enormously from being "dense and glass-like" to "sugary". The areas of stone which were more porous responded badly to laser treatment, turning either a dark grey or brown as a result of change in the oxides. The authors found that applying a solvent to the stone to remove the majority of the paint and using the laser to remove the remainder was most effective.

 The Heel stone at Stonehenge was attacked with spray-paint, but one of the utilized paint cans was found at the sight, so chemical tests were run with the paint and small quantities of sarsen (sandstone which comprises Stonehenge). In said tests/trials, it was found that acetone successfully removed most of the paint without damaging the stone. On sight, however, removal of paint was more difficult due to heavy lichen growth. The conservators (including M. Cooper, the author of a paper previously summarized in this blog) attempted 532nm as well, and found that it cleaned more effectively than 1064nm at cleaning paint from areas where acetone had been applied. Some residues were left.

 This article was remarkably data-free, but it does illustrate another application of laser cleaning.

Laser Beam Width, Divergence, and Propagation Factor: Status and Experience with the Draft Standard

John M. Fleischer, SPIE Vol. 1414 Laser Beam Diagnostics (1991)

 This article begins by defining several Gaussian beam parameters unambiguously (i.e. beam width is full width, divergence is the full angle and not the half angle). They also define a propagating factor, M^2, as

M^2 = pi * D(o)*theta /4

Where D(o) is the smallest beam width and theta is the full angular divergence.

 The author then describes several practical methods for characterizing Gaussian beam parameters. The first method they describe is the slit-scan method, in which one scans a narrow slit across the beam. After finding the maxima, the user attempts to find the 2 locations where 13.5% of the peak irradiance are incident. The difference between those two is the 1/e^2 beam width. The disadvantage of this technique is that it is impractical for very small beams.

 The next described method is the knife-edge test, wherein one traces a knife edge across a beam and measures transmitted power. The distance between the 10% and 90% points multiplied by 1.56 is the 1/e^2 diameter. This test is less precise than the slit scan and is inaccurate for higher order modes, but is simply to implement.

 The author next describes a diagnostic utilizing a scanning pinhole to map out the transverse beam profile. The diagnostic features large error for higher order modes.

 The final test the author discusses is the "encircled energy" test, wherein one aligns an aperture with the center of the laser beam and varies the area of the aperture. The area at which 86.5% of the power or energy is allowed through has the 1/e^2 diameter. However, the problems with this are:

1. Difficulty and uncertainty in aligning the aperture with the center of the beam

2. Assumption of a symmetric beam, often untrue.

 I also attempted to read "Interaction of Femtosecond Laser Pulses with Tempera Paints" by Gaspard et al, but found myself distracted by the rather loud music outside my office (as well as the US/Slovenia game) and also found the chemistry portion of the article rather difficult and tedious to read. I'll give it another go over the weekend.