Equations are given for reflectance, bandwidth and root gain-bandwidth of an active interference filter having unequal mirror reflectances and operating in the reflecting mode. Gain for various configurations is compared keeping the product of the mirror reflectances constant. It is shown that the reflectance gain of an asymmetric configuration can be greater than the transmittance gain while the bandwidths are identical. Specific examples are treated to show how device gain depends on single-pass gain and input and output mirror reflectances.
Vern N. Smiley, "Backward Wave Optical Amplification by an Asymmetric Active Interference Filter," Appl. Opt. 5, 977-980 (1966)