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.

Microlens part I - Introduction

Warning: It's a technical post

In my article, “what are POEMS, technically?” I discussed about the different emerging fields (some of them are already commercially viable) based on the miniaturization of the various mechanical, electronic and optical component, along with the changes in approach in terms of processes, techniques and materials. Albeit superficially we mange to get some idea about the vast (but very small) fields of MEMS, MOEMS and POEMS.

Today I will try to enlighten you about microlens, which can be considered as one of the most important components in the field of microoptics.

Starting with the definition: Simply speaking microlens is a lens whose geometrical size or aperture is less than 1 mm. So, most of the time when we say microlens, we mean lens which is small...smaller than the conventional lens. That’s that for the time being. I will introduce complexities in the definition later.

Having understood about the basic defining criteria for a lens to be known as microlens, let me throw some light on how it all started. I mean the background: Conventional lenses have been an area of interest for long. Even in the mid of 17th century, geniuses like Robert Hooke had already produced their own lenses. However, it was not until the arrival of fiber optics that brought with itself the trend of miniaturization for optical components. The important fabrication techniques, nevertheless, were still based on classical way of grinding and polishing. More recently (1970s), the development of IC fabrication techniques and its adaptation in the field of optics, has helped the idea of miniaturization to be realised, which in turn helped to generate optical components in the micrometer range.

The synergy of fiber optics with IC fabrication techniques, along with their potential applications, produced the basic thrust that lead to idea of producing smaller dimension optical components, (microlens being most important of them).