Expanding a Collimated Beam's Diameter Reduces Beam Divergence
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Can expanding a laser beam effectively create a smaller beam size?
Since a beam expander converts an input beam with a smaller waist and larger divergence into an output beam with a larger waist and smaller divergence, the output beam can have a smaller diameter far from the beam expander when compared with the input beam. The beam diameter does not remain constant with distance due to the effects of diffraction. As the beam divergence is higher for beams with smaller waists, using a beam expander to increase the waist diameter is a technique for reducing the rate at which the beam diameter increases as the beam propagates away from its waist. Beam expanders are often used to reduce beam divergence and ensure the beam diameter does not exceed a specified limit at distances far from the output beam waist.
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Figure 1: The input beam has a smaller waist diameter than the output beam. However, the diameter of the smaller input beam changes at a substantially higher rate than the larger output beam. Within the limited range shown above, the diameter of the input beam exceeds that of the output beam.
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Figure 2: A beam has a nearly constant divergence angle far from the beam waist. As described in the text, this angle
Beam Expanders (and Reducers)
Beam expanders (and beam reducers) accept a collimated beam with one waist diameter and provide a collimated beam with a different waist diameter. The expansion ratio, or magnification (m ),
relates the diameters of the beam waists before
Measuring the beam cross sections at the input and output beam waist locations can provide estimates of the waist diameters. The manufacturer often specifies the output beam waist location, but if it is not known, a measurement of the beam diameter close to the output of the beam expander can be used to approximate the waist diameter.
Beam Divergence
A laser beam's diameter is always smallest at its waist. Away from the waist, the beam diameter increases due to the effects of diffraction, and this rate of increase is the beam's angular divergence. Far from the beam waist, the divergence angle is approximately constant (Figure 2). An estimate of this divergence angle
can be calculated from the wavelength
The difference in the angular divergence of the beams input to and output from a beam expander or reducer can be estimated using the beam expansion ratio
in which θ_{in } and θ_{out} are the divergence angles of the input and output beams, respectively, far from their beam waists.
In the case of a beam expander
A beam reducer
Avoid Beam Clipping
The diameter of the output beam close to the beam expander is often a good approximation of the output beam waist diameter. Farther away, the beam diameter may have increased enough to cause beam clipping or other unintended effects. Calculating estimates of the beam diameter
at critical distances
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Date of Last Edit: Aug. 23, 2021 |
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