THORLABS EXULUS-4K1 Instructions Manual page 16

adjust for changes in spot size. This calculation assumes a uniform beam intensity
profile. You must now adjust this energy density to account for hotspots or other
nonuniform intensity profiles and roughly calculate a maximum energy density. For
reference a Gaussian beam typically has a maximum energy density that is twice that
2
of the 1/e beam.
Now compare the maximum energy density to that which is specified as the LIDT for
the optic. If the optic was tested at a wavelength other than your operating wavelength,
the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with
wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm
You now have a wavelength-adjusted energy density, which you will use in the following step.
Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot
size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1
mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the
larger size beam exposing more defects.
The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an
approximation is as follows:
Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT
maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10
-7
s. For pulses between 10 s and 10
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation
between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.
[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).
L I D T C A L C U L A T I O N S
In order to illustrate the process of determining whether a given laser system will damage an optic, a number of
example calculations of laser induced damage threshold are given below. For assistance with performing similar
calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the
-4
s, the CW LIDT must also be checked before deeming the optic appropriate for your application.
LIDT in energy density vs. pulse length and spot size. For short pulses,
energy density becomes a constant with spot size. This graph was
obtained from [1].
2
at 1064 nm scales to 0.7 J/cm
2
at 532 nm):
-9 -7
s and 10