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What’s the Difference? Beam Size vs. Clear Aperture

John Hsu

John Hsu | April 24, 2019
Applications Engineer with a drive to stretch the product limits and meet application performance.

Contributing Author: Andy McCarron, Sr. Opto-Mechanical Engineer

Understanding the design of laser beam systems that utilize beam steering technology, more specifically pairing a beam steering set with a laser is important for system set ups. One of the most critical aspects during set up is aligning the clear aperture of the beam steering set with the beam size of the laser. This often raises the question between beam size and clear aperture which I’m going to dissect in this blog, as well as help you with proper calculations.

One of the most common misconceptions in setting up a laser and beam steering system is that the clear aperture value should equal the maximum allowable laser beam size. It’s important to note that clear aperture is a parameter of the optic components, where the optical performances are defined strictly within this aperture. Optics performance outside of the clear aperture are unspecified, where significant illumination beyond this boundary not only causes power loss, but will also cause the optics to do the following:

  1. Deformation of the optic due to excessive heating
  2. Change in index of refraction with temperature (transmissive optics only)
  3. Delamination between the coating and the substrate
  4. Stray light which can damage other components of the system

Typically, lasers emit circular shaped beams with a Gaussian profile. By applying beam shaping technologies, the Gaussian profile can be transformed into a Uniform profile (Top Hat), with the output beam shaped as a square, rectangular, etc. The beam size for a Uniform parameter is straightforward (see figure below). Meanwhile measuring the beam diameter of the Gaussian profile is more subjective and heavily dependent on the methodology being followed. Some of the different criteria used for reporting a Gaussian beam size include; D4σ, 10/90 or 20/80 knife-edge, 1/e2, FWHM, and D86. Where 1/e2 and FWHM (Full-Width at Half-Maximum) are most commonly used. Understanding the laser beam profile and beam size is important as the same laser beam will have a different beam size based on its definition1.

So, what happens when different beam sizes are applied to the same clear aperture? Pretty straightforward as shown in the figure below— when the laser beam overfills the clear aperture, the outer-ring portion of the laser beam will be clipped. Which brings us to the next question, what is the appropriate beam diameter to be applied for a certain clear aperture?

By digging a little deeper into the math equations and using a typical Gaussian beam with a beam size defined by 1/e2 as an example, we can ensure <1% of power loss due to beam clipping. See formulas below:

Therefore, the incident beam diameter on the aperture should be:

And with the same calculation to ensure <10% of power loss, the clear aperture should be at least 1.07 times bigger than the 1/e2 beam size; for <5% power loss, it should be 1.22; and for <0.1% power loss, it should be 1.86.

Important to note is that since a laser beam size value defined by 1/e2 is roughly 1.7 times the diameter defined by FWHM, calculations for the appropriate beam size and clear aperture ratios can be made out of the above examples.

Using the Gaussian beam for the example, but in this case the beam size is defined under FWHM. To ensure <1% of power loss due to beam clipping, the clear aperture should be:

Another point to clear up is the typical misconception of having the input laser beam size exactly the same as the clear aperture. Applying the same formula, and assuming the beam size is defined by 1/e2, you are at risk of clipping 13.5% of the power. It’s critical to note that the amount of power falling off the mirrors will not only cause heat up, delaminate, burn coatings and scatter light as previously mentioned, but will also be deposited onto the internal parts within the scan head enclosure or be absorbed by the DFM lens retaining ring (with glue) and DFM lens holder. In either case, this will cause major damage to current systems during integration and is especially important when working with a high-power laser.

Last but not least, the above calculations are assuming perfect alignment. In case of actual alignment errors, keeping some margins should be considered.

In conclusion, always remember that the definition of the input beam size and the clear aperture size is NOT the same. When this is mistaken, it can cause unexpected power clipping which will negatively affect your system during integration like malfunction or even a system explosion. When users purchase a laser beam steering solution from us, we provide the clear aperture information, whereas the definition of the laser beam size should come with the spec sheet of the laser. In addition, the laser spec sheet might also provide laser alignment specs, such as its laser output position and pointing error. If these aren’t provided, make sure to check with the laser manufacturer to confirm these parameters for best performance purposes. Having tested and successfully integrated different systems across multiple applications, we can guarantee that these recommendations will help clear up confusion and prevent future damages.

 

 

 

 


1. Some lasers may have a central irradiance profile that closely matches a Gaussian profile but also contain additional “lobes” of irradiance in the wings. At high power, these lobes can be significant and must be taken into consideration.