| CPC G01J 1/0437 (2013.01) [G01J 1/0433 (2013.01)] | 20 Claims |

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1. An apparatus comprising:
a notch filter;
an optical signal detector positioned adjacent to the notch filter; and
a mirror positioned adjacent to the notch filter, wherein the notch filter comprises:
a first distributed Bragg reflector (DBR) stack on a substrate;
a defect structure on the first DBR stack; and
a second DBR stack on the defect structure.
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