Large Mode Area fiber, with cores of roughly 20 to 50 µm and very low NA so that only a few modes are guided. Spreading the power over a bigger area lowers intensity, pushing up the thresholds for self-phase modulation, SBS and facet damage in high-power amplifiers. Coiling the fiber to a set diameter strips the higher-order modes so the output stays close to single-mode.
Move the last amp to the 30 micron LMA and coil it at 10 cm, that buys us the extra peak power without SPM.
Also heard as
- large mode area fiber
- LMA fiber
Related terms
Double-clad fiber
Fiber with a small doped core for the laser light inside a much larger inner cladding that guides multimode pump light.
Self-phase modulation
Spectral broadening of an intense pulse caused by its own intensity changing the refractive index it travels through.
SBS
Stimulated Brillouin Scattering: narrow-linewidth light scattering backward off sound waves it creates in fiber.
M-squared
The beam quality factor: how many times worse a beam focuses and diverges than an ideal Gaussian of the same wavelength.