In a new study, researchers have shown that 3D printing can be used to make highly precise
and complex miniature lenses with sizes of just a few microns.
The microlenses can be used to correct color distortion during imaging, enabling small
and lightweight cameras that can be designed for a variety of applications.
A member of the research team from University of Stuttgart in Germany says that the ability
to 3D print complex micro-optics means that they can be fabricated directly onto many
different surfaces such as the CCD or CMOS chips used in digital cameras.
The micro-optics can also be printed on the end of optical fibers to create very small
medical endoscopes with excellent imaging quality.
In The Optical Society (OSA) journal Optics Letters, researchers detail how they used
a type of 3D printing known as two-photon lithography to create lenses that combine
refractive and diffractive surfaces.
They also show that combining different materials can improve the optical performance of these
lenses.
3D printing of micro-optics has improved drastically over the past few years and offers a design
freedom not available from other methods.
The researcher says that their optimized approach for 3D printing complex micro-optics opens
many possibilities for creating new and innovative optical designs that can benefit many research
fields and applications.
Two-photon lithography uses a focused laser beam to solidify, or polymerize, a liquid
light-sensitive material known as photoresist.
The optical phenomenon known as two-photon absorption allows cubic micrometer volumes
of the photoresist to be polymerized, which enables fabrication of complex optical structures
on the micron scale.
The research team has been investigating and optimizing micro-optics made with two-photon
lithography for the past 10 years.
The researcher says they noticed that color errors known as chromatic aberrations were
present in some of the images created with their micro-optics, so they set out to design
3D printed lenses with improved optical performance to reduce these errors.
Chromatic aberrations occur because the way that light bends, or refracts, when it enters
a lens depends on the color, or wavelength, of the light.
This means that without correction, red light will focus to a different spot than blue light,
for example, causing fringes or color seams to appear in images.
The researchers designed miniature versions of lenses traditionally used to correct for
chromatic aberrations.
They began with an achromatic lens, which combines a refractive and diffractive component
to limit the effects of chromatic aberration by bringing two wavelengths into focus on
the same plane.
The researchers used a commercially available two-photon lithography instrument to add a
diffractive surface to a printed smooth refractive lens in one step.
They then took this a step further by designing an apochromatic lens by combining the refractive-diffractive
lens with another lens made from a different photoresist with different optical properties.
Topping the two-material lens with the refractive-diffractive surface reduces chromatic aberrations even
more, thus improving imaging performance.
To show that the new apochromatic lens could reduce chromatic aberration, the researchers
measured the focal spot location for three wavelengths and compared them to a simple
refractive lens with no color correction.
While the reference lens with no chromatic correction showed focal spots separated by
many microns, the apochromatic lenses exhibited focal spots that aligned within 1 micron.
The researchers also used the lenses to acquire images.
Images taken using the simple reference lens showed strong color seams.
Although the 3D printed achromat reduced these drastically, only images taken with the apochromat
completely eliminated the color seams.
The test results showed that the performance of 3D printed micro-optics can be improved
and that two-photon lithography can be used to combine refractive and diffractive surfaces
as well as different photo resists.
The researchers point out that fabrication time will become faster in the future, which
makes this approach more practical.
It currently can take several hours to create one micro-optical element, depending on size.
As the technology continues to mature, the researchers are working to create new lens
designs for different applications.