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Controlling the characteristic response of the ring resonator through adjustable coupling can be useful in variety of applications, such as add-drop filters, modulators, lasers, switches, sensors and others. Coupled mode theory (CMT) is effectively utilized for modeling of a generic micro-resonant cavity with two independently adjustable add/drop couplers and integrated phase-shifter(s). It is shown that for a lossy cavity the non-identical input/output coupling values are required to satisfy a resonance condition. The critical coupling and coupling/loss optimization is discussed for the regular and δβ-reversal tunable couplers. Resonance coupling diagram is derived for passive and perturbed resonator structure for these types of tunable couplers.