Optics / 04 / Damage check

How hard is the beam
on your optic?

Turn a beam into the numbers a coating data sheet quotes: peak fluence, peak irradiance and the margin against the stated damage threshold, scaled to your pulse.

Independently check before important decisions. This calculator can make mistakes. Do not use its results for important decisions without independently double-checking the calculations, inputs, assumptions and manufacturer’s specifications. These estimates do not establish a safe operating limit.

A comparison, not a prediction. Real thresholds depend on the coating run, the surface, cleanliness, the number of shots and the test method. A margin above your safety factor is not a guarantee. Runs in your browser; nothing is sent or stored.

Beam and optic

Light

Pulsed Continuous (CW)

Vacuum wavelength

Pulse energy

Pulse duration, FWHM of the power

Pulse shape (for the peak irradiance only)

Gaussian sech² Rectangular

Beam cross-section

Round Elliptical

Beam radius w at the optic (1/e² intensity)

Radius, not diameter: 1/e² diameter ÷ 2.

Angle of incidence 0° = normal incidence; tilt in the plane of the first radius

Data sheet

Damage threshold (fluence)

… measured with pulses of

… at the test wavelength

The data-sheet value was measured at this angle of incidence Leave unticked for a value measured at normal incidence (it is then compared per area of the surface).

Safety factor 2 to 3 on a single-shot data-sheet value is usual; more for many shots or a beam with hot spots.

Result

Verify this estimate. This tool can make mistakes. Independently double-check the calculations before using a result for any important decision. A calculated margin is not a guarantee against damage.

Peak fluence on the surface: —
Margin: scaled threshold ÷ fluence: —
Threshold scaled to your pulse: —
Data-sheet value needed for your safety factor: —
Peak irradiance on the surface: —

Results to 4 significant figures. Displayed digits are not a measurement uncertainty.

Model and limits

F = \frac{2 E}{π w_{t} w_{s}} \cos α

peak fluence on the surface; α is the angle of incidence

I = \frac{2 P}{π w_{t} w_{s}} \cos α, P = \frac{k E}{τ}

peak irradiance; for a pulse, P is its peak power

F_{th} = F_{ref} \sqrt{\frac{τ}{τ_{ref}}} \cdot \frac{λ}{λ_{ref}}

data-sheet threshold scaled to your pulse (about 20 ps to 100 ns)

margin = \frac{F_{th}}{F}

with F / cos α instead of F when the data sheet was measured at your angle

F_{needed} = s \frac{F F_{ref}}{F_{th}}

data-sheet value needed (at its test conditions) for safety factor s

\frac{P}{d},, d = 2 \sqrt{w_{t} w_{s}}

CW linear power density, d the 1/e² diameter

The peak on the axis of a Gaussian beam is twice the average over π w_t w_s. A flat-top or multimode beam, or a hot spot, has a different peak: enter the radius of a Gaussian with the same peak, or raise the safety factor.
A surface tilted by AOI spreads the beam over 1/cos(AOI); the radii are those of the beam itself. The fluence is compared per area of the surface, right for a data-sheet value measured at normal incidence; tick the option when the value was measured at your angle, and the beam fluence is compared instead. The CW linear power density uses the beam diameter without an angle factor.
The √τ and λ scaling are empirical supplier rules for roughly 20 ps to 100 ns and moderate changes. Below 20 ps the value for 20 ps is shown and flagged; far from the test conditions the result is marked as a guess.
Many pulses on one site lower the threshold (S-on-1). Inside a standing-wave cavity the two directions add, and the fringes can double the peak again: neither is applied here.
CW thresholds are thermal and depend on absorption at your wavelength; they are compared without scaling.

Sources