SPONSORED: LASER DAMAGE TESTING
as a damage percentage. As the pulse duration is fixed, it is actually the beam energy which is varied, increasing row by row until a damage pattern is obtained. This is displayed on a graph of damage probability versus the peak power density. The critical factor in laser
damage is the power level. ‘Often a numerical result is what’s required, however if you are developing a new product, graphical analysis can help establish your route to success,’ Dr Belford added. This is an important point. While a pass/fail test is effective to test an established component, using established materials, the LIDT test provides a wealth of information. You can effectively see when
the material starts to break down under the action of a laser. This is not only highly useful information for the end user, it’s also a fascinating area to work in. ‘It’s exciting work where you can actually see the effect of a laser in action, disintegrating
the sample. I find it interesting, in terms of how the materials age and how the combination of elements or coating with the sample can make a good sample or not,’ said Dr Belford.
Material effects BRL conducts a range of specialised laser damage tests, including longevity and durability certifications, to test how a material reacts to a wide range of wavelengths, repetition rates and pulse durations. During these extensive laser
damage tests, BRL has noticed some interesting material effects for femtosecond and continuous wave (CW) laser sources. Dr Belford explained: ‘When
we move to the femtosecond regime, because the laser pulse is a very, very short duration, the actual energy going through the component isn’t too high. And the sample doesn’t react to heat. ‘Yet, the damage to the optical component is often worse than expected, because of electronic breakdown.
‘Although breakdown mechanisms are fairly well understood for small optics, very large optics show additional interactions to the established knowledge, and this is an area which we are looking at in depth,’ she said. CW laser damage is another
area of interest, which has a different dominant damage
“You can overtest an optic. You can give it too many shots, and that changes the sample under test”
mechanism, compared to a pulsed laser. Here, we know CW power resistance is dependent on the spot size. ‘But, in transmitting optics, the damage does not seem to be as dependent on surface defects, they are more dependent on antireflective coatings and the material itself. If it’s a dielectric coating, for example, the optical components under test tend to group themselves into the type
of substrate used when you’re conducting an LIDT test,’ Dr Belford added. This is where the lines
between physics and chemistry start to blur, and a multidisciplinary approach is needed to investigate further. ‘We need to not be afraid to skip between physics and chemistry and engineering, because that’s what we’re doing in laser damage testing – we’re breaking bonds in a material. We’re using a laser to achieve that effect. Unless you have some inkling about why that happens, then it’s a difficult problem to solve.’ BRL is continuing to apply its expertise to many of these areas, shining a light on the effects of laser damage at the material level. ‘There are many interesting
developments and trends within the field of laser damage testing at the moment. ‘Going forward, we need to understand more about the effects of laser damage at the material level,’ Dr Belford concluded. EO
New White Paper now online
High power laser damage:
By Dr Rona Belford, Kieran Ross, Belford Research (BRL Laser Damage)
VIEW FOR FREE*
Threshold elevation Why some optics damage below their published threshold values
www.electrooptics.com/white-papers
Electro Optics
Shutterstock.com
*Registration required
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