THORLABS EXULUS-4K1 Instructions Manual page 17

calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser
system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed
systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a
Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical
relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These
LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.
CW Laser Example
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Suppose that a CW laser system at 1319 nm produces a 0.5 W Gaussian beam that has a 1/e
diameter of 10 mm. A naive calculation of the average linear power density of this beam would
yield a value of 0.5 W/cm, given by the total power divided by the beam diameter:
However, the maximum power density of a Gaussian beam is about twice the maximum power
density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate A Gaussian beam profile has about twice the maximum
intensity of a uniform beam profile.
determination of the maximum linear power density of the system is 1 W/cm.
An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with
the wavelength of the laser source, so this yields an adjusted LIDT value:
The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser
system, so it would be safe to use this doublet lens for this application.
Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations
Suppose that a pulsed Nd:YAG laser system is frequency tripled to produce a 10 Hz output, consisting of 2 ns output pulses at 355 nm, each with 1 J of energy,
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in a Gaussian beam with a 1.9 cm beam diameter (1/e ). The average energy density of each pulse is found by dividing the pulse energy by the beam area:
As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam
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is ~0.7 J/cm .
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The energy density of the beam can be compared to the LIDT values of 1 J/cm and 3.5 J/cm for a BB1-E01 broadband dielectric mirror and an NB1-
K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz.
Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the
nanosecond pulse regime scale with the square root of the laser pulse duration:
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This adjustment factor results in LIDT values of 0.45 J/cm for the BB1-E01 broadband mirror and 1.6 J/cm for the Nd:YAG laser line mirror, which are to be
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compared with the 0.7 J/cm maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser
line mirror is appropriate for use with this system.
Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths
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Suppose that a pulsed laser system emits 10 ns pulses at 2.5 Hz, each with 100 mJ of energy at 1064 nm in a 16 mm diameter beam (1/e ) that must be
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attenuated with a neutral density filter. For a Gaussian output, these specifications result in a maximum energy density of 0.1 J/cm . The damage threshold of an
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NDUV10A Ø25 mm, OD 1.0, reflective neutral density filter is 0.05 J/cm for 10 ns pulses at 355 nm, while the damage threshold of the similar NE10A absorptive