  When two nonorthogonal resonances are coupled to the same radiation channel,
avoided crossing arises and a bound state in the continuum (BIC) appears in
parametric space. This paper presents numerical and analytical results on the
properties of avoided crossing and BIC due to the coupled guided-mode
resonances in one-dimensional leaky-mode photonic lattices with slab geometry.
In symmetric photonic lattices with up-down mirror symmetry, Friedrich-Wintgen
BICs with infinite lifetime are accompanied by avoided crossings due to the
coupling between two guided modes with the same transverse parity. In
asymmetric photonic lattices with broken up-down mirror symmetry, quasi-BICs
with finite lifetime appear with avoided crossings because radiating waves from
different modes cannot be completely eliminated. We also show that
unidirectional-BICs are accompanied by avoided crossings due to guided-mode
resonances with different transverse parities in asymmetric photonic lattices.
The Q factor of a unidirectional-BIC is finite, but its radiation power in the
upward or downward direction is significantly smaller than that in the opposite
direction. Our results may be helpful in engineering BICs and avoided crossings
in diverse photonic systems that support leaky modes.
