Modeling lidt using the stochastic model within this

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Modeling LIDT using the stochastic model Within this discussion of the mechanisms of laser damage and the models for deter- mining LIDT, it is easy to overlook an es- sential point: it is very unlikely that the laser-damage measurement will overesti- mate the LIDT. Exposing an optic to twice the fluence of its LIDT will almost cer- tainly damage it. The uncertainty arises at and below the specified LIDT. There is some finite probability that an optic will be damaged at levels well below the LIDT. At this point a system designer has de- fined the laser, determined the tolerable level of damage, and calculated the accept- able risk. So how can the designer then determine if a given optic with a specified LIDT will meet the system’s needs? The starting point is to understand exactly how the vendor has determined the LIDT. What sampling protocol was used? What laser, with what intensity profile? How did the vendor characterize the defects? Knowing that, there are some rules of thumb that can be used to extrapolate ven- dor test results into predictions of perfor- mance in a given system. Given an LIDT determined with a test at a wavelength of λ 1 with a pulse length of τ 1 and a beam di- ameter of 1 , the LIDT at a wavelength of λ 2 with a pulse length of τ 2 and a beam diameter of 2 can be estimated as: LIDT 2 = LIDT 1 × λ 2 / λ 1 × τ 2 × ( 1 ) 2 τ 1 2 However, there is a caveat to this equa- tion—this expression only works if the parameters of the two lasers are close to one another. The best solution is to test for laser dam- age under the exact conditions in which the optic will be used. Barring that, a sys- tem designer should demand exact infor- mation about the test conditions under which LIDT was determined. In addition, designers should also have some degree of confidence that their optics fabrication partners have their processes under con- trol so that defects are well understood. Understanding LIDT and how your op- tical components were tested will allow you to know that you are selecting the proper optics for your application. With the variables defined, the defects charac- terized, and the stochastic model well es- tablished, system designers can be confi- dent they are moving their own process from art into science. Nathan Carlie is R&D manager and Cory Boone is technical marketing engineer, both at Edmund Optics, Barrington, NJ; e-mails: ncar- [email protected] and [email protected]; . FIGURE 2. Sample data showing the different fluences used during a multishot LIDT test and the resulting damage frequencies. In this test, a linear regression of the data found that the optic’s LIDT was around 7.5 J/cm 2 .
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  • Fall '19
  • Photonics, LIDT

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