An optical flame-sensor includes an optical circulator, an optical-fiber cavity, and a first optical sensor. The optical circulator includes a first port, a second port, and a third port. The first port is configured to receive an optical signal. The second port is configured to output the optical signal received at the first port. The third port is configured to output the input to the second port. The optical-fiber cavity includes a cavity proximal-end optically coupled to the second port, and a mirror at a cavity distal-end, such that a cavity optical signal output by the optical-fiber cavity is the input to the second port. The first optical sensor is optically coupled to the third port to quantify the cavity optical signal.