Niemeier T, Ulrich R
Opt Lett. 1986 Oct 1;11(10):677-9. doi: 10.1364/ol.11.000677.
A broad stripe waveguide of width W and length L acts as a self-imaging 4 x 4 directional coupler if L = NW(2)/lambda, where N is the effective index. When the arms of a fiber-optic Mach-Zehnder interferometer are connected to two input ports of this coupler, the powers at the four output ports vary in phase quadrature.
如果L = NW²/λ（其中N为有效折射率），那么宽度为W、长度为L的宽条纹波导可作为自成像4×4定向耦合器。当光纤马赫-曾德尔干涉仪的臂连接到该耦合器的两个输入端口时，四个输出端口的功率会以正交相位变化。
