FAQ:

Q: Why do I need a beam expander?

1. To reduce the divergence of a laser beam
2. To increase the beam diameter for better performance from a lens further downstream in the optical system
3. To better protect other optics in the system by reducing the power or energy density of the laser

Q: What types of beam expanders are most common?

1. Galilean
2. Keplerian

Of these two, Galilean is much more commonly used as it allows for a shorter length component.

A Brief Overview of Refractive Beam Expanders

Refractive beam expanders are among the most commonly used optical components.  While their application range is almost as wide as for the laser itself, beam expanders perform multiple important functions:

1. Magnification (or compression when the beam travels in the reverse direction)

2. Reduction of beam divergence

3. Noise reduction with spatial filtering

4. Increased symmetry of diode and excimer lasers

Increased beam diameters are often required to improve beam focusing in material processing applications such as laser marking, engraving, drilling, welding, micromachining and trimming.  Beam magnification also allows for the use of longer focal length lenses - this reduces the possibility of surface contamination from debris in high-volume industrial applications.  Expanders are also vital to metrology, laser ranging and communication applications since beam divergence is inversely proportional to beam magnification.

Common Refractive Beam Expander Designs

Keplerian

Galilean

Definitions

2a           Input Beam Diameter (1/e²)

M            Magnification Ratio

Z              Range over which the output beam diameter is less than or equal to 2aM

Θ             Far-field divergence angle

F₀            Focal length of objective lens

F_E            Focal length of input lens

Z              Z = πM²a²/λ

M            M = f_O/f_E = Øin/Øout