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"Lighting a path to the future with nano-optics."

Microlens Arrays

What is a microlens array?   Microlens arrays are simply a piece of glass or plastic with many small lenses placed side by side with lens diameters anywhere between a few microns to about a millimeter. There is no firm cut off diameter, but the term microlens arrays usually stops being used when lens diameters get around a millimeter.  Microlens arrays are a vital optical component in many of today's advanced optical systems and Nalux is an expert in the fabrication of both glass and plastic microlens arrays.  Nalux has made microlens arrays for many different applications including the Subaru astronomical telescope in Japan (see picture above, far right hand side).  There are a number of ways to make these optics with examples being plastic injection molding, lithography and etching, and diamond turning.   Each method has it's advantages and disadvantages (cost, speed, design freedom, surface quality, etc.) and depending on the method used the lenses can be spherical, aspherical, torroidal, or freeform.  

Microlens arrays are used in a wide variety of applications in addition to astronomy.  They are used in the military for tracking systems, they are used in telecommunications for optical switching, they are used in beam splitter applications where a single beam enters the array and each lens creates a separate smaller beam.  Microlens arrays are used to focus the output of an array of laser diodes, they are used in conventional laser systems, some photocopiers, digital projectors, cell phones, embedded computer cameras, and in a host of other applications.  

One of the most common applications for microlens arrays is the homogenization of light.   In other words, if you look at the output of a basic LED, laser diode, or just about any light source you will see that the center is brightest while the intensity decreases the further you get away from the center.   Because of this, if you measure the intensity of the light at the center versus the intensity at the edges the intensity difference can be a factor of 10 times or greater between the edge and the center.   This spells disaster for many applications.   This is where microlens arrays ride to the rescue.   

If you now insert a microlens array in between the light source and the plane where you want the light, each segment of the total light lands in one of the microlenses.   The trick is to make sure that the plane where you want the light is around 10 to 100 times the focal distance of the invidual microlenses.   So, the light is transmitted through the microlens and then reaches its focal plane.   However, the plane where you want the light is far past the focal plane of the lens so the light is now de-focused by the microlens.   So, the light is "smeared" at the plane where you want uniform intensity.   Since this process is happening at all of the individual microlenses, the smeared light from one lens overlaps with the smeared light from all of the other microlenses and this minimizes the localized hot spots and creates uniform intensity.   

Accuracies from one lens to another in microlens arrays can be as close as within 250 nanometers (not cumulative) of it's perfect position and this accuracy can be either in glass or in plastic.  Additionally, if an optic  has microlens arrays both the front surface and the back surface, the center to center alignment of the lenses on the opposing sides can be as close as 1 micron.  

There are several methods for making microlens arrays.  Binary mask lithography and plasma etching can be used to make what is known as reflow microlens arrays.  This method creates tiny pillars of photoresist on a glass wafer.  The photoresist is then heated so that it "reflows" and surface tension causes the pillar of photoresist to flow into a spherical shape, just like a small bead of water sitting on a table.  Plasma etching then transfers this shape into the glass so that you end up with glass microlens arrays.  This method is beneficial when many parts are needed.  Tooling costs and set up costs can be high but the cost for each copy are relatively lower.  

Diamond milling is another method for making microlens arrays.  Using this method the microlens arrays are created by mechanically removing material to shape the lenses into the glass.  

Focused ion beam milling is another method to make microlens arrays using ions to ablate away the material.  This method allows for a great deal of design freedom in microlens arrays because very fine control over surface shape can be achieved using this method.  

Microlens arrays can also be molded out of plastic.  This provides for a very inexpensive way of making these optics (on the order of $0.20 per part in large volumes) although tooling costs can be high.   

So, you can see that these optics are an important part of our everyday life and Nalux is ready to help with your next microlens array project.