Microlen part II - Classification

Microlens, as discussed in my post “Microlens part I” is one of the most important elements in the field of microoptics. Based on the refraction or diffraction phenomena at the optical interface, it can be classified as refractive microlens or diffractive microlens. In refractive microlens, the light is manipulated by analog phase element of considerable thickness in relation with the optical wavelength. In diffractive microlens light is diffracted at the periodic microstructure of the element.

Fabrication techniques for diffractive microlens have a good control over process parameters, and produce microlenses with good reproducibility and uniformity, which is a difficult task in refractive microlens production. However, from a system point of view, refractive microlens offers several advantages such as: reduced wavelength sensitivity as compared to diffractive microlens (a necessity for broadband applications), possibility of realizing large numerical aperture, and high light efficiency.

Now there is another kind of microlens which is called Graded Index microlens, and is basically a different type of refractive microlens. Rather than being refracted at the surface profile, in GRIN lenses, the light is deflected by a refractive index gradient within the surface. This in general leads to superior aberration characteristics of GRIN microlenses than that of conventional refractive microlenses. However, the major drawback with these kinds of lenses lies in the difficulty in fabrication. The precise formation of pre-specified graded index profile requires a very complex technology. Based on the terminology of GRIN lenses, we can define refractive microlens with a varying surface profile as Graded Thickness Microlenses. The idea seems appropriate since grading of thickness is the most prominent feature here, and the reason for the functionality of these lenses. It also helps to draws an analogy with GRIN lenses.

Both the diffractive microlenses and the graded index microlens have particular benefits, and can be used for certain type of application. However, as a best compromise between simple manufacturing techniques, high speed, low cost and good quality of microlenses produced, gives graded thickness microlens an edge over other kind of microlenses.