A holographic diffuser is a type of diffractive optical element that performs a radiance profile shaping operation on an incoming beam. Diffusers, of any type, tend to homogenise the radiance profile of the beam traversing the element. They scramble the incoming light in such a way that any non-uniformity is averaged out. One added feature of a holographic diffuser that makes it stand out from the rest of diffusers is that the shape of the resulting beam can be designed to be of any desired form. Thus, the output beam can be round, rectangular or any other geometrical or freeform shape. Moreover, the target radiance profile could also have areas with different levels of radiance.
Another remarkable feature of holographic diffusers is the fact that the output beam can be characterised by sharp edges as opposed to the smooth edges that are inherent to Gaussian beams. The radiation emission pattern from almost all lasers can be accurately described in terms of Gaussian modes. But this type of beam profile is suboptimal for most applications. With a Gaussian beam, the sample being illuminated, for whatever purpose, does not receive a uniform irradiance and, perhaps most critical of all, a significant percentage of light leaks outside the target area due to the aforementioned smooth edges of Gaussian beams. For these reasons, the inclusion of a holographic diffuser along the optical path effectively increases the throughput of the laser delivery system.
These remarkable capabilities of holographic diffusers are due to the fact that the internal structure of the element can be very accurately designed to meet a certain target. To start with, a holographic diffuser works by the principle of light diffraction which in turn can only be analysed in terms of wave optics. A holographic diffuser consists of a series of discrete modulating elements that can be simply referred to as pixels. Each pixel imparts a local phase delay to a small portion of the incoming beam. Then, simply by interference among the many small beam elements, a new beam envelope starts to emerge. After some propagation distance, or at the focus of a lens, the final beam envelope is formed. In most cases of practical interest, the relation between the output and input beams is given by a spatially scaled Fourier transform operation. Thus, by using specialised optimisation algorithms, the phase values required at every specific pixel location that will in turn generate a desired radiance output pattern can be calculated.