A laser diode's emitting facet is modelled as a slit of half-width at
PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Interference and diffraction · Interference and diffraction
What an Instrument Can See, Ch Scalar diffraction theory, derived
Problem
A laser diode's emitting facet is modelled as a slit of half-width a=5.0μm at λ=850nm. Take z=10mm as a typical distance to a collimating lens placed just outside the diode package.
((a)) Compute the Fresnel number NF=a2/(λz) at this distance.
((b)) State whether the field at the lens is better described by the Fresnel or the Fraunhofer approximation, and explain in one sentence why laser diodes are known to diverge rapidly.