Lidt testing laser damage testing is inherently a de

  • No School
  • AA 1
  • 4

This preview shows page 4 out of 4 pages.

LIDT testing Laser-damage testing is inherently a de- structive test. An optic is exposed to a given level of laser fluence and then is ex- amined, typically with a differential inter- ference contrast microscope. The fluence is then increased, and the exposure and examination steps are repeated. This pro- cess continues until damage is observed on the optic. Conceptually, this is a sim- ple process, but it hides several levels of complexity (see Fig. 2). In most cases, the laser used in LIDT testing has a Gaussian intensity profile, so the central portion of the exposed area on the optic will see a much high- er fluence than the surrounding regions. Even defining the area of the Gaussian beam is problematic. If the energy den- sity at the 1/e point is 50 J/cm 2 and a de- fect is detected, there are three possible explanations. First, the defect could be right at that 1/e position and the damage threshold for that particular test would be 50 J/cm 2 . Second, the damage could have been outside the 1/e point, in which case the measured threshold would be <50 J/ cm 2 . Finally, the damage could have oc- curred under the central portion of the beam, at a fluence level higher than 50 J/ cm 2 . Some of this uncertainty is reduced if the testing beam is conditioned into a top-hat profile with a more uniform in- tensity profile out to a well-defined outer diameter—but even then, shot-to-shot in- tensity variations still introduce uncertain- ty. Clearly, a single test will not serve to provide a proper measurement of LIDT. This uncertainty is mitigated by mak- ing many measurements. However, not every possible use scenario can be tested because high-energy lasers are expensive, and it is unrealistic to expect a vendor to have a stable of high-energy lasers cover- ing every wavelength range and every pos- sible temporal characteristic. Vendors can test at high fluence levels much more easily by interrogating an op- tic with a small-diameter beam. However, this can lead to the surface being under- sampled, possibly missing some defects on the sample. It would take 400 exposures to cover a 40-mm-diameter optic with a 2-mm-diameter beam. Factor in the sev- eral different exposures at difference flu- ence levels and then add in some reason- able lot sampling, and it becomes clear that some statistical sampling and simulation is necessary for practical testing. The goal of this simulation is to answer questions such as, “given that damage was not de- tected at 40 J/cm 2 with 60% of the sur- face sampled by one pass of a 2-mm-di- ameter top-hat beam, how likely is it that the damage threshold for this full optic is actually below 40 J/cm 2 ?” The answer can’t be determined from that set of measurements alone, as it re- quires a full set of exposures to different fluence levels and a stochastic model of the damage processes. A stochastic mod- el of LIDT will include the beam size and intensity profile, the areal coverage, and some information about the nature and distribution of the defects.
Image of page 4

You've reached the end of your free preview.

Want to read all 4 pages?

  • Fall '19
  • Photonics, LIDT

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes
A+ icon
Ask Expert Tutors