Background: Laser skin treatment with a diffractive element
The use of diffractive optical elements (DOEs) in aesthetic laser treatments is well established in procedures such as fractional skin treatment and tattoo removal. A diffractive element has no angular tolerances and is a flat and compact passive component, making it ideal for integration into aesthetic laser systems.
Diffractive optics have many advantages over the alternatives, including their absolute angular accuracy, compactness and robustness, and comparatively low costs. However, diffractive elements are chromatic components, i.e their splitting or diffusion angles depend on the wavelength of light. In most laser based applications, this characteristic is a non issue, as a specific wavelength is required to be able to perform the treatment or process in a specific sample material however, in some hand delivered treatments where accuracy is of the essence, there is a requirement for an additional visible light beam to be added in order to guide the physician in the process.
In such treatments and applications, the chromatic operation of the diffractive element may present an issue as the invisible wavelength, typically in the IR range, and the visible guide laser that should mark the treated area will not cover the same area, as the DOE will generate a different angle for each of the wavelengths. This becomes an even greater issue in treatments where achieving an accurate stitching of the treated areas is crucial, and when trying to improve the treatment rate and skin coverage.
Special diffractive optical element design combining both wavelengths
In order to offer an appropriate solution for such types of treatments, Holo/or needed to develop a special DOE design that will both shape or split the treatment beam at the target plane, and at the same time enable marking the edges of the treated field with the guide beam. For example – shaping the treating beam to a rectangular spot and the guide beam to either a frame with the same angles as the treated field, or four visible markers that define the corners of the treated field. To understand how this was done, one must review how digital diffractive optics are designed.
Diffractive optical elements, or DOE, are designed by a method called Iterative Fourier Transform Algorithm (IFTA), or the Gerchberg-Saxton algorithm. In this method, an initial phase is propagated to the far field by Fourier transform, and the resulting intensity is compared to a target intensity distribution (for example 9X9 split orders of equal intensity). The target intensity is the inverse-transformed with a compensation function that is proportional to the difference between the intensities in that iteration and the desired ones. This way, iteratively, the phase is modified until an optimal distribution is achieved, that is as close as possible to the targeted intensity.
Now, a diffractive element can only be designed with a certain angle at a single wavelength for the same diffracted order. However, a specific definition of the target intensity can be used to create a diffractive element where the corners of the treated area will coincide with other orders at the guide laser wavelength, so that the result is a marking of the corners of the treated area. Such an element has lower efficiency at the guide laser wavelength, but that has no effect on the ability to find the corners of the treated area. The effect on the treatment wavelength is minor.
This sort of diffractive element can be combined with a diffractive lens, creating a single component solution that will generate both patterns (treatment and marking) at focus, without any need for an external lens. Such diffractive elements can be tailored to any pair of wavelengths – treatment at wavelengths 1064nm, 1550nm, 2940nm, or any other, and guide lasers at any visible wavelength.
