  Optical Feshbach resonances (OFRs) allow one to control cold atomic
scattering, produce ultracold molecules and study atomic interactions via
photoassociation spectroscopy. In the limit of ultracold s-wave collisions the
strength of an optical Feshbach resonance can be expressed via an
energy-independent parameter called the optical length. Here we give fully
analytic approximate expressions for its magnitude applicable to near-threshold
bound states of an excited molecular state dominated by a single
resonant-dipole or van der Waals interaction. We express these magnitudes in
terms of intuitive quantities, such as the laser intensity, excited state
binding energy, the s-wave scattering length and the Condon point.
Additionally, we extend the utility of the optical length to associative STIRAP
in 3D optical lattices by showing that the free-bound Rabi frequency induced by
a laser coupling a pair of atoms in an optical lattice site can be
approximately related to the trap frequency and the optical length.
