HEDS operate with a sub-aperture approach,
and are therefore sensitive to centering and have an effect on the spot shape.
The spot size in the splitting axis is increased (spot becomes slightly elliptical),
and this effect is felt only for single-mode lasers (low M2
Spot size is defined by the formula:
- beam size for M2
EFL - Effective focal length
λ - Wavelength
D - Input beam size
value of input laser beam (beam quality)
For low M2
(<~4) the beam in focus will have some ellipticity because of added 0.75.
For higher M2
values the spots will become symmetrical.
HEQS 2x2 splitters increase M2
symmetrically, so no ellipticity will appear,
but spot size will increase for low M2
(single mode) lasers.