A fiber optic sensor for sensing pressure, temperature, voltage and other environmental parameters. A fiber is terminated in a series of semiconductor layers, each absorbing light as a function of the sensed parameters at a different wavelength. Light is transmitted down the fiber with plural frequencies and reflected by the termination to provide in the reflected light an attenuation representative of the parameters sensed. The reflected light intensity is detected and electronically processed to provide an indication of the magnitude of the parameters being sensed. With each frequency differently attenuated, the sensed parameters are separately detected. In one implementation two discrete frequencies or wavelengths of light are directed down the unitary fiber to its termination containing first and second semiconductor layers and reflective terminations that have an absorption characteristic which is a function of the sensed parameters such that the first frequency is partially absorbed in the first layer and reflected by its termination while the second frequency is transmitted substantially unaffected through to the second layer where it is partially absorbed and reflected. The two reflected wavelengths of radiation are retransmitted through the optical fiber and coupled into receivers which provide an output signal representing the reflected light magnitude. Parameter sensitive reflective layers may be substituted for the absorptive layers. The semiconductive layers may be employed in a detector configuration.