The old theory of optical power coupling between two waveguides – introduced in the 1960s – is not supported by any physical process, relying instead on a perturbation of the cladding. This approach leads to physically impossible outcomes – see [IEEE J. Quantum Electron., vol. 54, no. 1, Feb. 2018, article 6300206] and IEEE J. Quantum Electron., vol. 54, no. 2, April 2018, article 6800207]– and prevents development of new functional photonic devices.

A physically meaningful coupled-wave formalism has been developed in the context of quantum Rayleigh conversion of photons for the operation of the optical directional couplers in terms of a phase-sensitive gain coefficient – see following references:

[1] A. Vatarescu, “Photonic Quantum Noise Reduction with Low-Pump Parametric Amplifiers for Photonic Integrated Circuits”, Photonics, vol. 3, article 61, 2016.

[2] A. Vatarescu A, “Phase-Sensitive Amplification with Low Pump Power for Integrated Photonics, “OSA Advanced Photonics Congress, paper ID: IM3A.6., 2016.

[3] A. Vatarescu , “Photonic coupling between quadrature states of light in a homogeneous and optically linear dielectric medium,” J. Opt. Soc. Am. B, vol. 31, no. 7, pp. 1741–1745, July 2014.