Abstract: A method for measuring the secondary path intensity of an optical unit, such as an optical amplifier, is provided. The method includes applying a pulse of light to a first end of the unit and detecting the light exiting from the second end of the unit. The exiting light is analyzed to determine a primary pulse intensity and a secondary pulse intensity which preferably includes the effects of Rayleigh backscattering (RBS). The ratio of the secondary pulse intensity to the primary pulse intensity provides an accurate measure of the unit's secondary path intensity.

Abstract: Disclosed is a fiber amplifier including a gain fiber and a source of pump power having a wavelength .lambda..sub.p coupled to one end thereof. A signal of wavelength .lambda..sub.s is introduced into a first end of the gain fiber, and an amplified signal of wavelength .lambda..sub.s is coupled from the second end thereof. The signal can traverse an undesired secondary path as well as a preferred primary path to the amplifier output. To prevent light of wavelength .lambda..sub.s from traversing the secondary path and thus increasing the amplifier noise figure, the amplifier includes attenuating fiber means for attenuating that portion of the signal of wavelength .lambda..sub.s that propagates in the secondary path.

Abstract: Apparatus for controlling the temperature of an open liquid body when actual measurement of that temperature is not possible. A thermal analog of the liquid body, which is exposed to the same heating or cooling source as the liquid body, is incorporated into a sensor which provides controlling feedback to the heating or cooling source. The sensor, which is disposed in the atmosphere which surrounds or flows by the liquid body, includes a dry element and a wet wick, body of which are in thermal contact with a temperature sensing element. The dry element senses the temperature of the atmosphere and the wet wick has a cooling effect on the sensing element that is related to the rate of evaporation of liquid from the body. This sensor is advantageously employed in a system for regulating the temperature of a reagent film on a microscope slide.