A planar lightwave circuit comprising: a first input port; a second input port; a first output port; a second output port and a third output port; a tunable coupler, wherein the second input port is optically coupled to a first end of the tunable coupler and the second output port, wherein the first input port is optically coupled to a second end of the tunable coupler and the third output port; and a resonant mirror assembly optically coupling the first input port to the first output port, the resonant mirror assembly defining a reflector and including: n cascaded optical resonators (R1 through Rn); and n+1 bus waveguides (312 through 318), each having a first end and a second end; wherein each of the resonators is optically coupled to a pair of bus waveguides according to its pair of coupling coefficients, thereby defining 2n coupling coefficients (K1 through K 2n); wherein at least a portion of light received at the first input port is reflected back to that port through the effect of the resonant mirror assembly; the resonant mirror assembly further includes: (i) a first coupler that optically couples the first input port to the first end of the first one of the n bus waveguides and to the first end of the nth of the n bus waveguides; (ii) a second coupler that optically couples the first output port to the second end of the first one of the n bus waveguides and to the second end of another one of the n bus waveguides; the 2n coupling coefficients are configured such that a first coupling coefficient of the 2n coupling coefficients is independently controllable and is different in value than the other coupling coefficients (K 2 -K 6); and the resonant mirror assembly exhibits a tunable reflectivity/transmissivity. a first input port; a second input port; a first output port; a second output port and a third output port; a tunable coupler, wherein the second input port is optically coupled to a first end of the tunable coupler and the second output port, wherein the first input port is optically coupled to a second end of the tunable coupler and the third output port; and a resonant mirror assembly optically coupling the first input port to the first output port, the resonant mirror assembly defining a reflector and including: n cascaded optical resonators (R1 through Rn); and n+1 bus waveguides (312 through 318), each having a first end and a second end; wherein each of the resonators is optically coupled to a pair of bus waveguides according to its pair of coupling coefficients, thereby defining 2n coupling coefficients (K1 through K 2n); n cascaded optical resonators (R1 through Rn); and n+1 bus waveguides (312 through 318), each having a first end and a second end; wherein each of the resonators is optically coupled to a pair of bus waveguides according to its pair of coupling coefficients, thereby defining 2n coupling coefficients (K1 through K 2n); wherein at least a portion of light received at the first input port is reflected back to that port through the effect of the resonant mirror assembly; the resonant mirror assembly further includes: (i) a first coupler that optically couples the first input port to the first end of the first one of the n bus waveguides and to the first end of the nth of the n bus waveguides; (ii) a second coupler that optically couples the first output port to the second end of the first one of the n bus waveguides and to the second end of another one of the n bus waveguides; (i) a first coupler that optically couples the first input port to the first end of the first one of the n bus waveguides and to the first end of the nth of the n bus waveguides; (ii) a second coupler that optically couples the first output port to the second end of the first one of the n bus waveguides and to the second end of another one of the n bus waveguides; the 2n coupling coefficients are configured such that a first coupling coefficient of the 2n coupling coefficients is independently controllable and is different in value than the other coupling coefficients (K 2 -K 6); and the resonant mirror assembly exhibits a tunable reflectivity/transmissivity.