An optical diffuser or homogenizer helps modify a single-mode or multi-mode incident light beam into a well-structured output beam with a pre-designed shape. A holographic diffuser is a type of optical diffuser that involves a unique type of diffractive optical element or DOE to shape the radiance profile of an incoming beam. While any type of diffuser aims to scramble the incident light beam to eliminate non-uniformity and achieve homogeneity in the beam’s radiance profile, holographic diffusers do this differently and offer an extra feature. Holographic diffusers use holographic patterns to modify the input laser beams and the advantage of holographic diffusers is that they can shape the resulting output beam in any desired form, such as rectangular, round, or other geometrical shapes. More importantly, you can create different levels of radiance in the targeted radiance profile.
Another important characteristic of holographic diffusers is that the resulting beam profile has sharp edges, unlike the Gaussian beam profile which has smooth edges. The radiation emission pattern of almost every laser beam follows the mathematical expression of Gaussian modes. However, when it comes to various application fields that use laser beams, you can not rely on this type of beam profile. The reason behind this is that the illuminated sample of a Gaussian beam does not display a uniform irradiance pattern. Along with this, the smooth edges of the Gaussian beam result in a significant loss of light energy. As a result, we need to modify the input laser beam by using optical elements, such as a holographic diffuser. Adding a holographic diffuser to the laser delivery system helps more laser light pass through effectively.
If you are wondering why the holographic diffusers are effective in modifying the input beam, then the answer lies in the internal structure of the diffractive optical element. One can precisely design the element’s internal structure to get a certain result. The working principle of holographic diffusers is the principle of diffraction. Due to the diffraction, the input beam spreads out and the beam’s wavefront gets modified.
A holographic diffuser involves pixels or a series of discrete modulating elements. Every pixel introduces a slight delay in the phase of a small part of the incident beam. When these delayed beams interact through interference, they collectively form a new beam pattern. After traveling a certain distance or reaching the focal point of a lens, the ultimate beam shape is created. In most practical cases, a spatially scaled Fourier transform operation can help us understand the relation between the input and output beam. To find the required phase value at each pixel, we can use specialized optimization algorithms. Calculating the required phase values accurately is crucial to producing a desired radiance output pattern.
