Technical notes

Fiber and collimation lens selection

Single-mode (SM) fibers have small core diameter which allow only propagation of a Gaussian (TEM00) laser mode. This makes the output beam a high-quality Gaussian beam well suited for light-sheet excitation. The SM fibers are characterized by their (beam) NA and operating wavelength range (e.g. 400-680 nm). To create a collimated beam of required diameter, the collimator lens needs to be matched to the beam NA and required beam diameter D (Figure).

Lens can be characterized by f-number $f_n=f/D$ (f is focal length, D is the effective aperture diameter), which relates to NA as $NA=1/(2f_n)$. Rearranging these equations and assuming that lens NA is equal to fiber NA, we obtain formula for collimator lens focal length $f=D/(2NA)$.

The Optotune ETL has maximum aperture 16 mm, so we would like to create a collimated beam of diameter D=15 mm. For SM fibers (Thorlabs) NA is typically 0.10-0.14, so the collimator lens focal length has to be f=50-75 mm. A lens of a middle f-value, 60-mm achromatic doublet (Thorlabs AC254-060-A) is therefore a suitable choice. If you are using SM fibers that have different NA, choose your collimator lens accordingly.