DOEs for Fractional Laser Aesthetic Treatments
Background: Laser fractional treatment
Fractional laser aesthetic treatments are a family of laser processes conducted with Energy- Based Devices (EBDs) that rejuvenate the skin by careful, controlled application of laser energy over thousands of individual spots, often called micro-thermal treatments zones. To achieve this pattern of spots on the skin, the laser energy needs to be either rapidly scanned from spot to spot, divided into an array of beams, or some combination of the two.
Fractional laser treatment was first developed in 2004 and has since become one of the most sought after aesthetic treatments. Various laser wavelengths are used in fractional treatments, from MIR (CO2 lasers and erbium-YAG) to short pulse NIR and even VIS (Nd-YAG lasers at nano or pico-second regime pulses at 1064nm or 532nm).
Laser beam shaping approaches for fractional treatment
As described above, there are three main approaches to achieving the pattern of spots on the patients’ skin:
- Using a micro-scanner mounted in the handle to steer the beam spot by spot over the entire field (that is both in X and Y direction). The handle is moved over the skin with the scanner generating the pattern, but stitching of the treated areas is done manually.
- Splitting the beam into an array of spots is often done using a diffractive optical element, called a diffractive beam splitter. This device splits the laser beam into an array of spots with well-defined separations. This array is often symmetrical (such 9X9 spots, 7X7 spots and others) and covers a small area. The treatment is performed by moving the hand piece to cover new areas and then stitching the treated areas manually. Another option, which is sometimes used, is to generate the pattern of spots with a micro-lens array that works by a refractive rather than a diffractive method.
- Using a combination of a one-dimensional (1D) diffractive beam splitter and a 1D scanner to generate the matrix of dots combines the flexibility of the scanner approach (where the treatment area shape can be changed) with the robustness of the diffractive beam splitter approach (where scanning can be done only in a single axis, making the scanner simpler and cheaper and the process more robust).
Advantages of using diffractive optical elements in fractional laser skin treatments
The splitting of a beam into an array of spots can be done by either a micro-lens array or a diffractive optical element (DOE). The DOE optics approach has several advantages:
- Uniformity: The type of DOE used in fractional treatments is designed using digital diffractive optics optimization methods to assure a uniform distribution of the spots, regardless of the laser input distribution. Each spot is exactly the same as the input spot. That is not the case with a micro-lens array, where each spot is created by a sub-aperture, and the spot intensities have the same Gaussian envelope as the input beam profile. Often this means that when using a micro-lens array, the laser must be homogenized and passed through an aperture or multimode fiber, which reduces efficiency.
- Accuracy: Diffractive optical components produced by Holo/Or have no tolerance on their separation angle, as they are produced by lithographic methods. All spots are focused by a single external lens to the same plane. In contrast, a micro-lens array has a tolerance on lens radius of convergence (ROC), resulting in changes to spot diameters due to different defocus.
- Shaping flexibility: A diffractive beam splitter can be combined with other shaping functions, such as homogenization, to reduce hot spots which can harm a patient’s skin and other optical components in the device (multi-mode lasers used in aesthetic laser treatments often have less stable beam profiles).
For more information about this or other Holo/Or products, contact Holo/Or.