Ultra-Narrow Bandwidth Fabry-Perot Optical Filter Illuminated by a Gaussian Beam

We implement an ultra-narrow bandwidth (UNB) Fabry-Perot (FP) optical filter that has two flat mirrors illuminated by a normally incident Gaussian beam from an optical fiber. Owing to the finite waist radius of the Gaussian beam, the UNB FP optical filter’s transmittance curve deviates significantly from that of the plane wave incidence case. Applying an amplified spontaneous emission to the UNB FP optical filter, we find the waist radius of the Gaussian beam from the beat noise spectrum of the UNB FP optical filter output. Then, the transmittance of the UNB FP optical filter is evaluated by expanding the Gaussian beam as infinitely many plane waves. With this measurement method, we implement the UNB FP optical filter having its optical bandwidth down to 270 MHz.