Fiber coupler is a great utilization to combine and to split one or more optical power input and outputs. Power transfer in fiber coupler depends on the geometry of the waveguides and the coupling coefficient. Power transfers between the waveguides are calculated using matrix transfer method. By introducing power input to the one of the waveguides, assuming cross section, power propagation, and fiber axis are held constants at coupling constant coupling coefficient, it shows that coupling length of NX2, linear order NX3, and triangular NX3 fiber couplers are odd-multiple of z=π/(2κ), z=π/((√2)κ), z=π/((2√2)κ). The calculation of power has been shown in 3D of coupling coefficient from 0.1-1.5/mm . It describes that the higher the coupling coefficient, the shorter the coupling length or phase required to transfer power completely to the other waveguides. The minimum coupling length or phase of NX2 and NX3 shows that the coupling length of linear order NX3 is larger with a factor √2 than the coupling length of NX2. However, the coupling length of triangular order NX3 is shorter with a factor 1/√2 than the coupling length of NX2 fiber couplers.