What’s So Good About Diffractive Optics?

It’s important to keep in mind that the benefits of a DOE are not free. Also, DOEs have their own restrictions. They are harder to produce, and generally produce the anticipated outcome only under specific conditions.

For instance, let’s say you want to produce a circular laser beam with a very uniform intensity profile. What are your options for achieving this?

Most laser sources produce a roughly Gaussian beam, so you could expand this beam heavily with a refractive expander, and then mask out everything but the center of the beam. This will give you a relatively uniform beam, but you will waste a lot of light.

You could use a more complicated refractive design, like a micro-lens array. This is difficult to engineer, and won’t give perfect results, but it can do a very good job under a variety of conditions. The beam intensity can be made uniform over a large distance, and the input beam to the micro-lens array will not need to be perfectly collimated. It will also work across a relatively broad wavelength range.

Finally, you could design a DOE beam shaper. These can be designed to give any intensity profile you like, but it will be expensive to produce. It may (depending on what it is doing) have certain flaws characteristic of DOEs, like a strong zero-order beam (where a large fraction of input to the DOE passes through without being shaped). It may be very sensitive to errors in the input beam wavelength or collimation, and it may only produce the desired intensity profile over a short range of distances.

Here is a modern practical application of diffractive optics to solve a surgical problem.

When cataract surgery is done by an ophthalmologist (like myself), the cloudy lens of the eye is replaced by an artificial lens called an intra-ocular lens or an “implant” for short.

The standard implant, with its refractive optical design like a magnifying lens, creates chromatic aberration because its edges act as prisms. We know that prisms bend short wavelength light (e.g. blue) MORE than long wavelength light (e.g. red). Therefore a pinpoint light will be seen to have a halo of chromatic aberration with a red outer edge.

A lens designed with diffractive optics also creates chromatic aberration, but in reverse: It bends short wavelength light LESS, etc. In this case a pinpoint light will be seen to have a halo of chromatic aberration with a blue outer edge.

Since chromatic aberration of either kind interferes with ideal sharp vision, the obvious solution is to use an implant designed with a combination of refractive and diffractive optics; the two kinds of chromatic aberration can be made to cancel each other.

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