An optical system may have an optical transmitter including a digital signal processor to receive a signal channel, add data corresponding to a pilot tone, generate a digital signal associated with the signal channel and including the pilot tone, and output the digital signal. At least some embodiments are flame retardant. The power transistor also includes at least one diode formed in the semiconductor body. a detector for detecting the filtered radiation, the detector being responsive to the applied signal to determine the relative intensities of the maxima and minima in the absorption spectra of the substance, characterized in that the radiation source is selectively activated in response to the applied signal.