In photonics since 1999. Benelux technical support, answer within one working day.
Reading a damage threshold specification
A laser damage threshold is only meaningful together with the wavelength, the pulse duration and the number of pulses it was measured at. A figure in joules per square centimetre for a 10 ns pulse at 1064 nm says almost nothing about your femtosecond system.
THE SHORT ANSWER
A laser damage threshold is only meaningful together with the wavelength, the pulse duration and the number of pulses it was measured at. A figure in joules per square centimetre for a 10 ns pulse at 1064 nm says almost nothing about your femtosecond system.
| Pulsed unit | J/cm2, with pulse length |
|---|---|
| Continuous unit | W/cm2 |
| Scaling | Roughly with the square root of pulse duration |
| Dependence | Wavelength, pulse length, pulse count, spot size |
| Test basis | Often 1-on-1 or 100-on-1 |
| Margin | Design well below the quoted figure |
Damage threshold is the specification most often quoted and least often read properly.
The conditions are the number
A threshold is measured under stated conditions: wavelength, pulse duration, repetition rate, spot size and the number of shots per test site. Change any of them and the figure changes. Thresholds scale roughly with the square root of pulse duration, so a coating rated for nanosecond pulses offers far less margin under picosecond illumination, and essentially none of the quoted value under femtosecond pulses.
One shot is not a lifetime
A 1-on-1 test fires a single pulse at each fresh site and reports where damage first appears. That is a useful physical limit and a poor predictor of how the optic behaves after ten million pulses. Where a lifetime matters, ask for multi-shot data and expect a lower number.
Your beam is not uniform
Energy density means peak local density, not average power divided by beam area. A beam with structure in it can carry local peaks several times the average, and the coating fails at the peak. Measure the profile if the margin is thin.
Working practice
Calculate the energy density in your actual beam, compare it with a threshold measured under conditions close to yours, and design with real margin. Where the margin is uncomfortable, expand the beam. Doubling the beam diameter divides the energy density by four, and it is nearly always cheaper than the alternative.
When to use it
- Specifying any optic for a pulsed laser
- Comparing two coatings that quote different test conditions
- Deciding whether a cemented component is acceptable in the beam
- Sizing the beam to keep energy density inside the limit
What to watch out for
- Thresholds scale approximately with the square root of pulse duration; a nanosecond figure does not transfer to picoseconds
- Specifications tested one pulse per site are not lifetime figures
- Cement and absorbing substrates fail long before dielectric coatings do
- A local hot spot in a non-Gaussian beam can be several times the average energy density
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.