Transforming Gaussian Beams into Uniform, Rectangular Intensity Distributions
- Sunday, 01 January 2012
The majority of laser types in current use produce output beams with circular or elliptical crosssections, with either Gaussian or near- Gaussian intensity profiles. This Gaussian intensity distribution is acceptable, and often beneficial for many applications in which the laser beam is being focused to a small spot. However, there are also many different uses for which a uniform intensity distribution (often referred to as a “flattop”) would be more optimal. For example, in materials processing tasks, a uniform intensity distribution ensures that the entire laser illuminated area is processed evenly. It is also valuable in situations where the laser light is used essentially for illumination. This is because uniform illumination makes identical features that all appear to have the same brightness, regardless of where they are located in the illuminated field, simplifying the image processing task and increasing contrast and resolution. These same benefits apply over a wide range of other applications that can be broadly classed as “illumination,” from machine vision, through flow cytometry, inspection, and even some medical uses.
There are several ways to convert a
Gaussian beam into a uniform intensity
distribution (in both one and two
Achieving Uniform Illumination
The most simple and direct way to transform a Gaussian beam into a uniform intensity distribution is to pass the beam through an aperture which blocks all but the central, and most uniform portion of the beam (Figure 1). There are two disadvantages to this approach. First, a very large fraction of the laser power is discarded, as much as 75%. Second, the resulting beam still has a substantial falloff in intensity from the center to the edge. Additionally, other optics are often needed to clean up the beam by removing stray lobes produced by diffraction from the aperture edge.
Transforming a Gaussian beam to flattop without substantial light loss, therefore, requires some alternative technique which can redirect energy from the center to the edges of the distribution without simply blocking it. This can be accomplished with either diffractive or refractive techniques.
Diffractive optics offer a very powerful means for reshaping the Gaussian intensity distribution. Specifically, they can be used to produce virtually any arbitrary intensity profile, including nearly flattop, as well as a wide variety of patterns. The latter can include arrays of spots and lines, crosshairs, circles, concentric circles, squares, and so on.
Diffractive optics operate by creating interference between various diffracted orders to redistribute the incident intensity distribution. Of course, diffraction effects are by their very nature highly wavelength dependent, so a given component will only work over a narrow wavelength range. This wavelength sensitivity becomes particularly problematic when pairing diffractives with diode lasers because these have a relatively large wavelength bandwidth as compared to other laser types. Also, there are large unit-to-unit variations in the nominal output wavelength of laser diodes.
Diffractive optics also always put at least some light into unwanted diffraction orders. The simplest and lowest cost of diffractive optics for beam shaping is binary, etched gratings. Un fortunately, manufacturing tolerances in the type of optic usually result in a substantial decrease in efficiency due to this phenomenon; an overall efficiency of 70% would be considered excellent for a diffractive beam shaper. Similarly, the large and small scale (ripple) uniformity of the patterns produced with diffractive optics are limited by grating manufacturing tolerances. Finally, diffractive optics for creating two dimensional uniform distributions typically have a relatively limited working distance outside of which the desired intensity pattern will not be produced.