Optical system designers often need to evaluate the effect of different laser beam diameters during prototyping. Although it’s possible to do this by introducing several different beam expanders in sequence, or by stopping the beam down with apertures, variable-magnification beam expanders—such as Edmund Optics’ TECHSPEC Variable Beam Expanders—provide flexibility and performance in an easy-to-use package.
Identifying the optimum laser beam propagation characteristics is often an important optical design task. Active sensors, free-space communications systems and other optical instruments require just the right beam profile, size and divergence—parameters that can’t often be found with deterministic modeling alone. So it’s off to the laboratory to do some breadboarding and prototyping.
Designers will usually start with a beam from an off-the-shelf laser and insert a fixed-focus beam expander. The beam expander can be a “homemade” version created from two or more lenses, or a complete integrated unit. In either case, once set, the beam magnification is fixed. That is, the beam diameter, divergence and profile are set once the beam expander is in place. Researchers looking to modify the beam propagation characteristics are faced with the time-consuming task of removing and replacing their beam expanders—and usually living with changes in the beam profile.
Another approach to evaluating different beam diameters is to build the breadboard with a high-magnification beam expander and stop the beam down with fixed apertures or an adjustable iris. Unfortunately, that “simple” solution comes at a price: The aperture introduces diffraction that modifies the beam divergence, unavoidably modifies the beam profile and limits the achievable range of optical power density.
Features and flexibility
In contrast, variable beam expanders (VBEs) give the system designer a quick and easy way to modify the beam diameter. Several manufacturers offer VBEs, each with their own characteristics. Here we’ll introduce the features of VBEs offered by Edmund Optics, which are representative of the class, but also have unique attributes.
The primary characteristic of any beam expander is the magnification. For example, the VBEs described here come in two varieties, 1-3X and 2-8X continuously variable magnifications. Magnification is typically represented as the ratio of the output beam diameter to the input beam diameter, but that’s only half the story. The divergence of the beam is reduced by the same factor by which the diameter increases. If you put a 1-cm-dia, 6-milliradian beam into a 3X beam expander, you’ll get a 2-cm-dia beam with a 2-milliradian divergence.
Refractive beam expander characteristics are driven by the design approach. Keplerian beam expanders produce a real focus between the input and output lenses. The input lens in a Galilean beam expander is a diverging lens, which means there’s no real focus inside the beam expander. Galilean beam expanders are typically shorter than Keplerian, and they’re also often less susceptible to laser damage.
The key to variable beam expanders is, of course, their variability. Designers make the magnification variable by varying the distances between lenses within the beam expander. Some VBEs use two knurled rings to vary the distance between internal lens elements. The overall length of the beam expander remains fixed during adjustment, and the adjustment mechanism doesn’t rotate the lenses, so the beam isn’t inadvertently misaligned when the magnification is changed.
Saving money, time
Product features don’t always translate into customer benefits, but in the case of VBEs there’s a direct connection. Imagine, for example, that you’ve been tasked with developing the design for a free-space optical communications system. You’re given loose requirements that define the transmission distance, the anticipated weather conditions and a performance spec such as a bit-error-rate or signal-to-noise specification. You’ll have to select your laser wavelength and the type of detector, but you’ll also need to identify the optimum beam characteristics.
You’ll be evaluating performance with different beam sizes and divergences, over different beam profiles, and at different power densities.
Consider the situation in the absence of a variable beam expander. You could establish a Gaussian beam profile and put in a fixed high-magnification beam expander, then stop the beam down to smaller diameters. It’s a quick adjustment, but the beam profile is different at every diameter as you cut off different regions of the Gaussian beam, and you’re throwing away energy in the vignetted beam so you can’t evaluate the full trade space for optical power density. Then throw in the divergence at the aperture edge and you’ve now introduced so many uncontrolled variations that your ability to evaluate the trade space is definitively restricted.
If you choose to use several fixed beam expanders your beam quality issues will be addressed, but you’re trading that quality for a big time investment. Removing, replacing, and realigning are all time-consuming steps—and each beam expander is likely to have a different physical footprint, which may present even more of an alignment headache.
Enter the variable beam expander. You establish the beam profile and align the beam expander for one magnification. At that beam size, you can adjust the laser output power to investigate the range of optical power density. Then you turn a couple of knurled rings on the VBE and you modify the beam size and divergence—while maintaining the beam profile and keeping every other element on your bench untouched. At each diameter you can adjust the laser output power so you have access to the full trade space for optical power density.
The value of the variable beam expander
Optical system designers who breadboard or prototype systems frequently need to modify their laser beam propagation characteristics. The traditional options for varying beam diameter waste photons, waste time and waste money. Variable beam expanders present the ease of an adjustable iris combined with the quality of individual fixed beam expanders, and provide excellent value to the optical system engineer.
