A unified approach for the coupled-mode analysis of nonlinear optical
couplers is proposed. This approach is basically an extension of the w
ork of [17] to include optical nonlinearity. After the nonlinear coupl
ed-mode theory is established, various basis functions are used as tri
al fields to derive coupled-mode equations. It is found that two publi
shed coupled-mode theories can in fact be deduced from the proposed on
e with individual linear or nonlinear waveguide modes serving as trial
fields, thus making the coupled-mode equations from variational princ
iple and reciprocity theorem equivalent. Coupled-mode equations based
on system modes are also presented. Furthermore, analytical and/or num
erical methods for solving coupled-mode equations are included. More r
esearch and discussion can be conducted based on the knowledge address
ed in this paper.