Laser Ablation – Peak Power and Energy Considerations
April 3, 2020
Precision Laser Ablation:
Precise removal of thin layers requires tight control of the laser parameters of energy, energy stability and spatial uniformity. When specifying an ablation system, it is useful to determine the “process window” for the materials to be ablated.
The process window is the energy range which produces appropriate material removal, i.e. acceptable results. Energy below this range does not ablate adequately and energy above this range damages or ablates excessive material. The sum of the energy stability and spatial uniformity must not exceed the process window in order to achieve good results. In addition, accurate control of the nominal energy, which can be provided by a digitally controlled attenuator, is critical for predictable results. For these applications, more energy is not necessarily better due to the fact that excess energy decreases the effective resolution.
A Customer Case Study:
We received feedback from a customer recently who was having difficulty adjusting to the correct energy. The nominal energy setting was 3% of the max energy. The attenuator provided a very linear adjustment range from 0 to 100%, with 0.1% resolution. One step therefore changed the energy from 3.0% to 3.1%, a change of 3.3% in the nominal energy.
The process window was rather tight so this made optimizing the energy difficult. Had the max energy been only twice the process window, the result would be much more accurate. If the nominal setting was 50%, then one step would be 50.1%, which is only a 0.2% change in the nominal energy. The effective resolution would then be over 16 times higher. For optimum results, systems should be specified with a maximum energy of no more than twice the expected process window.
Energy (joules) = average power (watts) / repetition rate (hertz) Laser Fluence = energy / area
For pulsed lasers, pulse repetition rate is a determinant of energy. Laser fluence, or target energy density, is the optical energy delivered per unit area of your work surface.
Laser ablation offers performance and productivity advantages in a range of life science and industrial applications, but there are energy considerations unique to your application:
Energy requirements vary widely depending on material, melt vs. vaporization and spot diameter of the work surface. Material removal begins at the ‘ablation threshold’, and as fluence increases, more material is removed per volume until it reaches a threshold, at which point additional energy transforms to heat, which can cause secondary thermal effects without additional removal of material.
Longer wavelengths present challenges that can require additional components such as wave guides in the handpiece.
Energy requirements depend on treatment of hard or soft tissue. Energy requirements for removal of sulcular epithelium in treatment of periodontal disease requires less than ½ the energy needed for surgical excision.
The therapeutic window is the ratio of threshold power required to create a rupture to that required for treatment (producing a mild lesion). The width of the therapeutic window must be sufficient to accommodate for variations in pigmentation.
If you know your process window parameters, give us a call so we can help you select one of our Jewel series lasers or systems to give you the best results.