{"id":3592,"date":"2014-01-28T17:08:45","date_gmt":"2014-01-28T22:08:45","guid":{"rendered":"http:\/\/labrigger.com\/blog\/?p=3592"},"modified":"2014-03-18T14:43:53","modified_gmt":"2014-03-18T18:43:53","slug":"damage-thresholds-and-tisapph-lasers","status":"publish","type":"post","link":"http:\/\/labrigger.com\/blog\/2014\/01\/28\/damage-thresholds-and-tisapph-lasers\/","title":{"rendered":"Damage thresholds and Ti:Sapph lasers"},"content":{"rendered":"

\"d\"<\/a><\/p>\n

Recently, a moderately expensive optical element got fried at Labrigger. During the postmortem, we revisited our power and energy calculations, and our LIDT (laser intensity damage threshold) calculations. <\/p>\n

LIDT calculations remind me of radiation-health physics (e.g., how much radiation can a human be safely exposed to?). My nuclear physics professor, the excellent R.T. Carpenter, included a lecture on nuclear health physics in one of his courses. I suspect this was a requirement of some accreditation body, because it didn’t really fit the style he maintained over the other lectures. This is the man who famously repaired a neutron howitzer<\/a> with his own hand while peaking over a radiation shield. <\/p>\n

One of the themes of the lecture was how purely empirical the whole field is. How much radiation of a particular type can a living thing stand? Unfortunately, we can’t usually start from fundamental principles, as physicists usually like to do. However, some basic trends are clear, so by taking a few data points, we can interpolate for the intermediate values.<\/p>\n

Thorlabs has a nice tutorial<\/a> on this stuff, but it’s pretty general. Let’s go over the numbers for a bog-standard Ti:Sapph (e.g., a MaiTai or Chameleon<\/a>):<\/p>\n

Using round numbers<\/h3>\n

Rep rate: 80 MHz
\nPulse width: 100 fs
\nAverage power: 3 W<\/p>\n

Peak power<\/a>: 375 kW
\nEnergy<\/a>: 37.5 nJ
\n(calculation notes<\/a>, more info<\/a>)<\/p>\n

If the laser intensity damage threshold (LIDT) is given as:
\n2 J\/cm^2 at 1064 nm, 10 ns, 10 Hz<\/p>\n

And we’re using, say 800 nm, then we first need to adjust for the wavelength:<\/strong>
\nAdjusted LIDT = LIDT Energy * sqrt(laser wavelength\/LIDT wavelength)
\nAdjusted LIDT = 2 J\/cm^2 * sqrt(800\/1064) = 2 J\/cm^2 * 0.867 = 1.73 J\/cm^2<\/strong>
\nNot such a big deal.<\/p>\n

Now we need to adjust for pulse duration:<\/strong>
\nAdjusted LIDT = LIDT Energy * sqrt(laser pulse duration\/LIDT pulse duration)
\nAdjusted LIDT = 1.73 J\/cm^2 * sqrt(100 fs\/ 10 ns) = 1.73 J\/cm^2 * 0.00316 = 5.47 mJ\/cm^2<\/strong>
\nThe ultrafast pulses have a much larger effect on LIDT.<\/p>\n

If the energy, 37.5 nJ, is spread over about 1 mm^2, then that’s about 3.75 uJ\/cm^2. Technically, that’s below the LIDT, but only in the domain of ns pulses.<\/p>\n

In our case, multiphonon<\/a> ionization can become a dominant process, then LIDT is proportional to<\/a> (Energy density) * (1\/e^2 diameter) * (pulse duration)^-1. Unfortunately, there are no helpful graphs from Thorlabs to refer to for this regime. (Labrigger can provide one data point, based on the one result pictured above.)<\/p>\n

\"t\"<\/a><\/p>\n

Corrected 18 Mar 2014. Thanks, Tony!<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

<\/a><\/p>\n

Recently, a moderately expensive optical element got fried at Labrigger. During the postmortem, we revisited our power and energy calculations, and our LIDT (laser intensity damage threshold) calculations. <\/p>\n

LIDT calculations remind me of radiation-health…<\/p>\n

Read More<\/a><\/div><\/p>\n","protected":false},"author":1,"featured_media":3596,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[32,15],"class_list":["post-3592","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-hardware","tag-laser","tag-optics"],"_links":{"self":[{"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/posts\/3592","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/comments?post=3592"}],"version-history":[{"count":8,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/posts\/3592\/revisions"}],"predecessor-version":[{"id":3682,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/posts\/3592\/revisions\/3682"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/media\/3596"}],"wp:attachment":[{"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/media?parent=3592"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/categories?post=3592"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/labrigger.com\/blog\/wp-json\/wp\/v2\/tags?post=3592"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}