Abstract: An optical-fiber-based spectrometer utilizes a compact, P-C-board-mountable optical multiplexer which permits multiple channels of reference and measurement data from remote and hostile environments to be analyzed in rapid sequence utilizing synchronized computer data-storage and comparison to give rapid-fire answers to the question of the presence or absence of species of interest in a sample or process being analyzed, the relative light level in the reference and data channels being balanced by an optical attenuator having only an air path, attenuation being effected by controlled misalignment of optical fiber ends.