Hey guy,
I am stuck at problem of coupling light from a MMF to an SMF. I do expect loss, but what is the best way to couple light into an SMF, I plan to use a lens from Thorlabs, however, i need to find an analytical solution for the problem. i.e what is the best i can do! if anyone could refer to me a paper would be really great as would be your advice!
Thanks
Multimode fibre has on the order of 100 modes. Coupling that into single-mode fibre will give you a loss of about 20 dB, with a *huge* amount of drift and fluctuation due to the instability of the interference pattern between the modes.
If the SMF and MMF have similar NA, lenses are not going to be better than just butt-coupling the two.
Cheers,
Phil Hobbs
Something that might be important in comparing the two fibers: is the multi-mode fiber a very low NA? What are the core diameters? Basic optics comes in here. If the two NAs are basically the same, the source "spot" of the large core fiber can't be focused down more than the original core size.
The clearest way I know of to explain this is by phase space volume.
Waves have a fundamental tradeoff between angular spread and areal spread. The product of the equivalent solid angle of a single mode and its equivalent area is given by
n**2*A*Omega = lambda**2/2.
A 100-mode fibre has a phase space volume of ~50*lambda**2, and you just can't squeeze that 50 pounds into a half-pound can.
A mismatched NA will make it impossible to reach even that efficiency, so using a lens can help with that in general, but only up to the limit set by the phase space volume.
This assumes that the modes are mutually incoherent and uniformly excited, which isn't always true. If your MM fibre is illuminated by a spatially coherent monochromatic source (i.e. a decent laser), it is possible in principle to corral all the light into a single mode at the output of the fibre. It's very difficult in practice, due to (a) the sensitivity of mode coupling to small bends and strains, and (b) the complexity of the custom optical elements required.
Cheers,
Phil Hobbs
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