| CPC G01N 15/1434 (2013.01) [G01N 21/7746 (2013.01); G01N 2021/655 (2013.01); Y10S 977/88 (2013.01)] | 17 Claims |

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1. An add-drop filter for transmitting at least one optical signal, the add-drop filter comprising:
a first optical waveguide and a second optical waveguide, the first optical waveguide defining a through channel from an input port to an output port configured to carry the at least one optical signal; and
at least one active resonator coupled between the first and second optical waveguides, the at least one active resonator comprising a passive resonator and a gain medium, the gain medium configured to provide an optical gain to the at least one active resonator when the gain medium is excited by light received from a pump laser;
wherein the first and second optical waveguides and the at least one active resonator define:
an add channel configured to insert an add optical signal from an add port in the second optical waveguide through the at least one active resonator and into the at least one optical signal in the through channel; and
a drop channel configured to remove a drop optical signal from the at least one optical signal in the through channel through the active resonator and out a drop port of the second optical waveguide; and
wherein the at least one active resonator is configured by the gain medium and the power of the light received from the pump laser to provide an add efficiency of the add channel essentially equal to a drop efficiency of the drop channel.
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