The Source Two Angle object can be used to simulate the output of a fiber.

The spatial shape parameter controls whether the emitter is rectangular (0) or elliptical (1).

The angular shape parameter controls whether the angular distribution is rectangular (0) or elliptical (1).

Nicholas Herringer demonstrated the difference between angular distribution settings.  This is important because if the half angles are set to ArcSin(NA) and the angular distribution is set to 0, rays will be launched with angles as large as Sqrt(2) ArcSin(NA).  Setting angular distribution = 1 will constrain the rays to angles <= ArcSin(NA).